173 - Pathogenesis and pathology of osteoarthritis · Pathogenesis and pathology . of...

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Pathogenesis and pathology of osteoarthritis

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progression, research in OA pathogenesis, biomarkers, and treatment has broadened immensely and many new potential therapeutic targets have emerged over the past years.

THE NORMAL JOINT: ANATOMY— PHYSIOLOGY—FUNCTIONUnderstanding of joint dysfunction requires some knowledge of nor-mality and the comparison of the diseased state to the physiologic situ-ation (Fig. 173.2). The most important requirements for normal joint function are the freedom of the opposed articular surfaces to move pain free—and largely frictionlessly—over each other within the required range of motion as well as the correct distribution of load across joint tissues. Thus, pathology in many cases might be caused by acute or chronic mechanical overloading, systematic mal-loading, or habitual underloading, leading to disuse atrophy.

The correct functioning of the joint is largely dependent on its design. Joints are highly specialized organs whose properties are pro-vided by the articular cartilage and its extracellular matrix that, under physiologic conditions, is capable of sustaining high cyclic loading. Articular cartilage covers the joint surfaces and is mainly responsible for the unique biomechanical properties of the joints. Joints are, however, complex composites of different types of connective tissue, including subchondral bone, cartilage surfaces, ligaments, and the joint capsule. The bony backbone defines the shape of the joint and the articulating surfaces—in combination with the articular cartilage—determine the absorption properties during movement. Also, the syno-vial capsule and, in particular, the synovial membrane (i.e., the synovial lining cell layer) vastly contribute to the physiologic functioning of the articulating joints. It is the synovial capsule together with the liga-ments that provides the mechanical stability of the joints and ulti-mately determines their flexibility and range of motion. The synovial membrane, containing high metabolically active surface cells, the synoviocytes, plays a crucial role in nourishing the chondrocytes as well as maintaining the normal metabolic milieu within the joints by removing metabolites and matrix degradation products from the syno-vial space. Also, synoviocytes produce large amounts of important mediators (cytokines and growth factors), matrix degrading enzymes, as well as hyaluronic acid and other factors such as lubricin/superficial zone protein, which provide the joint surfaces with its lubrication capacity. All these factors enable maintaining the local milieu of the synovioarticular joint organ.

Together all different tissues with their own functional capacities permit correct functioning and integrity of the joints.

PATHOLOGYMacroscopically, normal hyaline articular cartilage is a rather unruffled white to yellowish overlay coating the articulating joint surface (Figs. 173.3e and 173.4a). The synovial fluid makes it to appear slippery and provides its gliding properties. Microscopically, hyaline cartilage con-sists of evenly stained (“hyaline”) collagen- and proteoglycan-rich extra-cellular matrix with sparsely distributed cartilage cells (“chondrocytes”). The cells represent less than 5% of the total volume of articular carti-lage but are of obvious importance for the maintenance of the tissue. Chondrocytes are surrounded in most parts by a specialized pericellular matrix forming a biomechanical and biochemical interface between the rigid interterritorial matrix and the cells. The mechanical properties of

Pathogenesis and pathology of osteoarthritisThomas Aigner and Nicole Schmitz

Osteoarthritis (OA) is pathologically primarily characterized by focal cartilage damage, bone sclerosis, and some sort of synoviopathy.

Osteoarthritic changes within the articular cartilage can be categorized by typing, staging, and grading of the lesions.

OA is strongly associated with age, but bone and cartilage changes in OA are different from those of normal aging.

There are many different hypotheses trying to explain cartilage and joint degeneration, including chronic mechanical (over)load, matrix proteolysis, age-induced changes of the cartilage matrix and the chondrocytes, as well as increasing damage to the genomic DNA of the chondrocytes, leading to a deranged cellular phenotype.

Proinflammatory cytokines as well as the activation of cellular inflammatory signaling pathways including interleukin-1 and the MAP kinases likely play an important role in OA pathogenesis.

Biomechanical factors are essential in the pathogenesis of OA. Altered joint biomechanics are generated by joint incongruity, laxity, muscle weakness, and impaired proprioception in addition to trauma and heavy physical load.

Despite the fact that presumably a high variety of different phenomena contribute to the pathogenesis of OA, premature aging of the chondrocytes and the matrix presumably plays a crucial role in its initiation and progression; thus, OA might be in analogy the “Morbus Alzheimer” of the joint.

INTRODUCTIONOsteoarthritis (OA), also known as degenerative joint disease or osteo-arthrosis, is the most common form of arthritis and the leading source of physical disability with severely impaired quality of life in people in industrialized nations. Although derived from the Greek words osteon for bone, arthron for joint, and the suffix -itis for inflammation, the site of most pronounced structural alterations is not the bone but the joint cartilage, and severe inflammation is seen in only few patients. OA is generally considered as a disease of the elderly, progressively causing loss of joint function, but although it is true that OA preva-lence increases sharply with age it is not part of normal aging. Very little is known about the underlying causes of OA, and many hypoth-eses regarding its pathogenesis have been proposed over time. Genetic defects causing malfunction of structural genes give rise to premature and often severe OA in certain families, but in the majority of OA cases no such defects have been identified. A number of risk factors are known that apart from age include heredity, malalignment of the articulating surfaces, obesity, metabolic diseases, and joint trauma. Each can make contributions to the initiation and progress of the disease in different compartments of the joint. Biochemical processes involving cartilage, bone, and synovium eventually intertwine and col-lectively damage all three joint compartments. This results in articular cartilage breakdown, osteophyte formation, subchondral bone sclerosis, bone marrow lesions, and alterations of the synovium on both mor-phologic and biochemical levels, often causing, for example, episodic synovitis (Fig. 173.1). Advances in molecular biology raise hopes that new therapeutic targets will be identified that will allow more than just symptomatic therapy. Joint replacement is still the unsurpassed therapy for advanced and incapacitating OA. However, with increasing appre-ciation of the contribution of all three joint compartments to disease

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Articular cartilage

SCHEMATIC VIEW OF THE MAIN STRUCTURES OF A HEALTHY (LEFT) AND DEGENERATE OA (RIGHT) JOINT

Joint capsule

Synovialmembrane

(Subchondral) bone

Destructed cartilage

Capsular fibrosis

Synovial hyperplasia

Osteophyteformation

(Subchondral)bone remodellingand sclerosis

Fig. 173.1 Schematic view of the main structures of a healthy (left) and degenerated OA (right) joint. In OA the articular cartilage is lost or severely thinned, the (subchondral) bone is sclerotic, the joint capsule is thickened, and the synovial membrane is activated. (Courtesy of E. Bartnik, Frankfurt.)

Fig. 173.2 The images show coronal magnetic resonance imaging datasets of the knee acquired using a fast low-angle shot (FLASH) or spoiled gradient-recalled-echo sequence (SPGR). This imaging sequence has been extensively validated for detecting cartilage lesions and for performing quantitative measures of cartilage volume and thickness. The left image shows a healthy knee with normal cartilage thickness, the right image a knee with OA. Note the osteophytes and the extensive cartilage loss in the lateral femorotibial compartment. (Courtesy of Felix Eckstein, Salzburg, Austria.)

articular cartilage largely depend on the biochemical composition of the extensive interterritorial (extracellular) cartilage matrix.

Macroscopically, OA cartilage is often yellowish or brownish, is typically soft, and is often swollen. The surface shows roughening in the early stages and overt fibrillation and matrix loss in the later stages until the eburnated subchondral bone plate is visible (see Fig. 173.3f, g). These changes can be seen and graded radiographically (see Fig. 173.3a-d) and can be visualized in more detail on the histologic level (see Fig. 173.4). Thus, microscopically, the surface shows undulations (roughening) in the early and overt fissures and splits as well as matrix loss in the later disease stages (see Fig. 173.4b) until the subchondral bone plate becomes visible (see Fig. 173.4e). Besides the total destruc-tion of matrix areas, also the degradation of matrix molecules plays an important role preceding and driving the final loss of the respective

matrix areas (see Fig. 173.4d, e: loss of toluidine blue staining reflecting the loss of proteoglycans in damaged cartilage areas). Apart from the degradation of molecular components, destabilization of supramolecu-lar structures also takes place. For example, destabilization of the col-lagen network results in microscopically and, finally, macroscopically visible matrix destruction. Both mechanical wear and enzymatic deg-radation appear to play a pivotal role during the disease process. Together, these cause the destruction of the cartilage matrix on the molecular (e.g., proteoglycan depletion) and the macromolecular (e.g., network loosening), explaining the changes observed on the micro-scopic (e.g., fissures) and the macroscopic level (e.g., cartilage tear).

At the margins of joints frequently (osteo)cartilaginous outgrowths appear (chondro-osteophytes). They are best considered as a process of secondary chondroneogenesis in the adult. Osteophytes derive from mesenchymal precursor cells within periosteal or synovial tissue and often merge with or overgrow the original articular cartilage. Thus, in this process, mesenchymal precursor cells differentiate into chondro-cytes. A similar, but less structured process is observed in the areas of the eburnated bone, in which the articular cartilage is completely torn off. Here, mesenchymal multipotential stem cells of the bone marrow undergo also chondrogenic differentiation: metaplastic cartilage in forms of nodules or “tufts” is found either within the bone marrow or at the naked bone surface.

Osteophytes could be considered as endogenous repair attempts in degenerating joints and might be a physiologic response to mechanical overloading by increasing the articulating joint surface, thus having a supportive function. However, they are mainly found in non–weight-bearing areas and their mechanical stability and biologic benefit are questionable. To date, the molecular mechanisms in the development of osteophytes are largely unknown. Mechanical or biochemical stimuli could play a central role. However, most osteophytes do not take part in the articulating process and are subsequently not exposed to major mechanical load. Thus, it is more likely that growth factors play a dominant role in the induction and promotion of osteophyte forma-tion. For example, the exogenous application of transforming growth factor-β (TGF-β) and bone morphogenetic protein-2 (BMP-2) into knee joints of adult mice leads to significant osteophyte formation.

TYPING, STAGING, AND GRADING OF JOINT CARTILAGE ALTERATIONS IN OSTEOARTHRITISOverall, the classification of OA cartilage degeneration is rather complex because all patients present with at least to some extent dif-ferent histories, symptoms, and morphologic changes. Common to all

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Fig. 173.3 Knee: (a) grade 0 normal, (b) grade 1 lateral tibiofemoral narrowing, (c) grade 3 lateral tibiofemoral narrowing, and (d) grade 3 lateral tibiofemoral narrowing. (e, f ) Macroscopic appearance of femoral condyles of a normal (e) and severely damaged (f ) knee. (g) Arthroscopic image of a cartilage defect of the femoral condyle within the knee joint. (a-d, from Altman RD, Gold GE. Atlas of individual radiographic features in osteoarthritis, revised. Osteoarthritis Cartilage 2007;15[Suppl A]:A1-A56; g, courtesy of Dr. W. Eger, Rummelsberg.)

of them is some sort of structural joint (cartilage) damage, pain, and limitation in joint movement.

Obviously, many other tissues apart from the articular cartilage are involved in this process, but, traditionally, the cartilage has been used to score OA severity (at least as long as structural changes are assessed). In general, the process of joint destruction can always be evaluated for the pathogenesis (“typing”), for its extent (“staging”), and for the degree of the most extensive focal damage (“grading”).

“Typing” is mostly related to “primary” (i.e., idiopathic) and “sec-ondary” (i.e., “caused by”) OA. Primary OA is most frequently observed. Whereas the addition “primary” implies that there is no obvious cause, still minor preexisting conditions also exist in this condition (i.e., “pre-conditions” or “risk factors”). The major causes leading to secondary OA joint degeneration are listed in Table 173.1. “Grading” and “staging” have been much more under debate, also regarding the basic meaning of both words: “grading” should refer to the evaluation of histologic changes at one (or the worst) site of joint destruction, whereas “staging” should refer to the overall disease process (in analogy to “grading” and “staging” in tumor pathology). Both represent an attempt to score processes relevant to the disease.

The grading system most commonly used (partly with minor modi-fications) is the histochemical-histologic grading system by Mankin and coworkers in 1971 (Table 173.2; Fig. 173.5).1 Despite repetitive criticism that the Mankin score shows a high interindividual variabil-ity, this might be related to the training status of the involved scoring people. However, clearly some of the subcategories of the Mankin score do not belong to primary cartilage degeneration but describe features observed in secondary cartilage formation (i.e., osteophyte formation: see Table 173.3) and should be excluded in future scoring attempts. A staging system used internationally is that of Outerbridge (Table 173.4), while another has been established in Germany by Otte (Table 173.5; see Fig. 173.5).2 The Outerbridge system was primarily described for the patella but later successfully applied to other joints. Whereas Mankin addresses the piece of cartilage under the microscope, the

TABLE 173.1 TYPING OF JOINT DESTRUCTION

PrimaryNo (major) causative reason known

SecondaryArticular goutBone infarctionEndocrine disorders (e.g., hyperparathyroidism)HemophiliaIntra-articular infectionsJoint instability (e.g., meniscus lesions)Neuropathy (e.g., Charcot’s joint)Overload causing excessive wear (work, sport, varus or valgus deformity)Paget’s diseasePsoriatic arthritisRheumatic diseaseTrauma

staging systems look at the whole joint surface mostly macroscopically (but if needed the worst lesion can be evaluated histologically). At the site of the highest cartilage damage, grading and staging are closely correlated. Clearly, a pure macroscopic staging system is too rough for scientific purposes and, thus, a new staging system has been proposed by Pritzker and colleagues (Table 173.6)3 that combines histopatho-logic grading parameters with the extension of the lesions. Doubtless, along with new scientific insights and more extensive and specified medical options, we will need more elaborated and validated “grading” and “scoring” systems, and this will be a major task in the near future.

Of crucial importance will be the use of a defined and unified nomenclature to make studies, descriptions, and results comparable, whereas at the moment many similar-sounding terms are used for partly different phenomena and differently sounding words for the

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NormalMankin 0–2 Mankin 3–5

OAMankin 6–7 Mankin 8–10 Mankin >10

Stage 0 Stage I Stage II Stage III Stage IV

Grading according to Mankin

Staging according to Otte

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Fig. 173.5 The grading system according to Mankin and colleagues (1971)1 compared with the staging system according to Otte (1969).2

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Fig. 173.4 Conventional histology shows fibrillation and matrix loss in OA cartilage (b) compared with normal cartilage (a). In severely damaged areas nearly all articular cartilage is destroyed (e). Also a moderate (d) to severe (e) loss of proteoglycans is found, as visualized by toluidine blue staining. Besides changes in articular cartilage, also changes in the subchondral bone are prominent, namely, thickening of the subchondral bone plate (f, OA; c, normal).

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TABLE 173.2 GRADING OF OSTEOARTHRITIS ACCORDING TO MANKIN AND COLLEAGUES (1971)

Feature Score Histologic feature

Cartilage structure 0 Normal

1 Superficial fibrillation

2 Pannus and superficial fibrillation*

3 Fissures to the middle zone

4 Fissures to the deep zone

5 Fissures to the calcified zone

Chondrocytes 0 Normal

1 Diffuse hypercellularity

2 Cell clusters

3 Hypocellularity

Safranin-O staining 0 Normal

1 Slight reduction

2 Moderate reduction

3 Severe reduction

4 No staining

5 Total disorganization*

Tidemark 0 Intact

1 Tidemark penetrated by vessels†

*Might best be removed (relates to osteophyte formation).†Might best be supplemented with “or duplicated tidemark.”From Mankin HJ, Dorfman H, Lippiello L, et al. Biochemical and metabolic abnormalities in articular cartilage from osteoarthritic human hips. J Bone Joint Surg Am 1971;53:523-537.

TABLE 173.3 STAGING OF OSTEOPHYTE DEVELOPMENT ACCORDING TO GELSE AND AIGNER (2003)

Stage 0 (normal)

Normal periosteum

Stage I Slight thickening of the periosteumIncipient formation of fibrocartilage (some round cells, some

metachromatic tissue staining of the extracellular matrix)No/slight active bone formationMolecular markers:

Focal collagen type II expressionNo collagen type X

Stage II Pronounced thickening of the periosteal layersWell-established formation of fibrocartilage (many round cells,

strong metachromatic tissue staining of the extracellular matrix)Some/moderate bone formationMolecular markers:

Distinct collagen type II expressionNo collagen type X

Stage III Pronounced thickening of the periosteal layersWell-established formation of fibrocartilage (many round cells,

strong metachromatic tissue staining of the extracellular matrix, formation of lacunae)

Strong active bone formationMolecular markers:

Distinct collagen type II expressionCollagen type X expression in basal areasCollagen type VI: intermixed with collagen types I, III, and V in

the intercellular matrix

Stage IV Significant thickening of the periosteal layerApparent formation of fibrocartilage with partial hyalinization of

the extracellular matrix (chondrocyte-like cells in lacunae, strong metachromatic tissue staining of the extracellular matrix)

Some active bone formationMolecular markers:

Ubiquitous presence of collagen type IICollagen type X in basal areasCollagen type VI: mostly pericellular

From Gelse K, Soeder S, Eger W, et al. Osteophyte development-molecular characterization of differentiation stages. Osteoarthritis Cartilage 2003;11:141-148.

TABLE 173.4 GRADING SCHEME ORIGINALLY SUGGESTED BY OUTERBRIDGE FOR MACROSCOPIC CHANGES SEEN ON THE PATELLA

Grade I Softening and swelling of the cartilage

Grade II Fragmentation and fissuring in an area ≤ 0.5 inch in diameter

Grade III Area more than half an inch in diameter is involved

Grade IV Erosion of cartilage down to bone

From Outerbridge RE. The etiology of chondromalacia patellae. J Bone Joint Surg Br 1961;43:752-757.

TABLE 173.5 STAGING OF JOINT DESTRUCTION ACCORDING TO OTTE (1969)

Grade Morphology

0 Normal

I Superficial fibrillation, no cartilage loss

II Cartilage lesions (without full-thickness defects): deep fibrillation, fissures to middle zone and/or partial cartilage matrix loss

III Cartilage lesions (without full thickness defects): fissures to deep zone and partial cartilage matrix loss

IV Complete cartilage loss (at least focally)

From Otte P. Die konservative Behandlung der Hüft-und Kniearthrose und ihre Gefahren. Dtsch Med Jahresschr 1969;20:604-609.

TABLE 173.6 SCORING OF OSTEOARTHRITIS ACCORDING TO PRITZKER AND COLLEAGUES (2006)

Grade Histologic properties

0 Matrix: surface intact (normal architecture)Cells: intact, appropriate orientation

1 Matrix: superficial zone intact, edema and/or superficial fibrillation (abrasion), focal superficial matrix condensation

Cells: cell death, proliferation (cluster formation), hypertrophy

2 As above:Matrix: discontinuity at superficial zone (deep fibrillation)± Loss of proteoglycan staining in upper third of cartilage± Focal perichondral increased proteoglycan stain in middle zone± Disorientation of chondron columns

3 As above:Matrix: vertical fissures into middle zone and branched fissures± Loss of proteoglycan staining into lower two thirds of cartilage± New collagen formation cells: cell death, regeneration,

hypertrophy in cartilage domains adjacent to fissures

4 As above:Cartilage matrix loss with delamination of superficial zoneExcavation with matrix loss from superficial to middle zone± Formation of cysts in the middle layer

5 Complete matrix loss with denudation of the sclerotic subchondral bone or fibrocartilage

± Microfracture with repair limited to bone surface

6 Bone remodeling (more than osteophyte formation only) with microfracture, fibrocartilage, and osseous repair above the previous surface

From Pritzker KP, Gay S, Jimenez SA, et al. Osteoarthritis cartilage histopathology: grading and staging. Osteoarthritis Cartilage 2006;14:13-29.

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TABLE 173.7 OARSI TERMINOLOGY OF OSTEOARTHRITIC JOINT DEGENERATION—A PROPOSAL FOR A CONSENSUS

Preferred term Definition Synonym

Superficial zone Includes the surface and upper zones

Surface zone Cartilage zone immediately subjacent to the joint surface.Collagen fibers are aligned parallel to surface (without cells)This zone might be not present in normal cartilage samples/in some species.

Lamina splendens

Upper zone Collagen fibers are aligned parallel to surface. Chondrocytes, elongated and flattened, are aligned parallel to collagen fibers and to joint surface.

Horizontal zone/layerTangential zone/layerSuperficial zone/layer

Middle zone Zone subjacent to upper zone.Collagen fibers are aligned intermediately between upper and deep zone alignments. Chondrocytes present in groups (chondrons) aligned parallel to collagen fibers.

Transitional zone/layerIntermediate zone/layerMiddle zone

Deep zone Zone subjacent to mid zone and above calcified cartilage. Radial zone/layer

Upper deep Collagen fibers are aligned predominantly perpendicular to joint surface.

Lower deep Chondrocytes within chondrons are aligned parallel to collagen fibers and perpendicular to joint surface.

TidemarkPenetrationAdvancementDuplication/multiplication

Zone of increased calcification at border of uncalcified and calcified cartilage Calcification frontLigne bordant (increased basophilic stain)

Calcified cartilage Zone between tidemark and the subarticular bone plate Calcified zone

Surface undulations Surface irregularities that do not involve discontinuity of articular surface RougheningUnevenness

Fissure Vertical cracks, irregularities or discontinuities of cartilage matrix CleftCrack

Superficial Restricted to surface and the superficial zone FlakingFraying

Middle Restricted down to the middle zone Fibrillation

Deep Restricted down to the deep zone Fissuring

Simple Unbranched fissure

Complex Branched fissure

Split/Splitting Horizontal (0°-60° to cartilage surface) matrix cleft/separation

ErosionSurfaceInto the superficial zoneInto the middle zoneInto the deep zoneFull depth

Loss of articular cartilage tissue including superficial and at least portions of deeper cartilage layers

LossUlcerationAbrasionDelaminationFlakingSpallationExcavation

Eburnation Smooth shiny bone surface indicative of exposed bone at articular surface Denudation

From Pritzker K, Aigner T, in press.

same (e.g., fissuring, clefting, flaking). Therefore, a consensus nomen-clature is proposed by the Osteoarthritis Research Society International (OARSI) (Table 173.7).4

TYPING AND GRADING OF SYNOVIAL MEMBRANE ALTERATIONS IN OSTEOARTHRITISClinically relevant OA joint disease is invariably associated with some sort of synovial pathology. This reflects the notion that there is a direct relation between clinical symptoms and the synovial reaction in OA and most likely these changes in the synovial membrane are at least partly involved in the progression of the disease. In OA synovial speci-mens, in principle, four different types of OA synoviopathies are found: hyperplastic, inflammatory, fibrotic, and detritus-rich synoviopathy (Table 173.8).5

Detritus-rich synovitis, which is found in end-stage OA disease, is due to abundant macromolecular cartilage and bone detritus (i.e., bone and cartilage fragments attached to or incorporated into the synovial

membrane; Fig. 173.6h, i) in addition to abundant molecular debris that is not visible microscopically. Besides the debris, a significant amount of fibrinous exudate is found at the surface of the synovial membrane. This exudate may be combined with incorporated fibrin, reflecting longer ongoing fibrinous exudation already being organized (i.e., resorbed). Detritus-rich synoviopathy usually contains a minor inflammatory cell infiltrate consisting of lymphocytes and granulocytes as well as some foreign-body giant cells.

Another form of OA synoviopathy found in late-stage disease, fibrotic OA synoviopathy (capsular fibrosis) (see Fig. 173.6e),5 is mainly characterized by the shortening and thickening of the joint capsule, which is partly responsible for some symptoms, in particular joint stiffness, seen in OA patients.

The most interesting of the OA synoviopathies in terms of patho-genesis is the inflammatory OA synoviopathy, which displays moder-ately extensive lymphocytic infiltrates (see Fig. 173.6f, g). It is intriguing to speculate whether this condition reflects some kind of autoimmune aspect that may be occurring, at least in this subset of OA patients. Interestingly, the lymphocytic infiltrate in the subsynovial stroma

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although it is clear that osteophyte development is a continuous process and many osteophytes show different steps in various portions at the same time, one can define basic steps based on the cellular phenotype and the matrix composition of the predominating tissue (Fig. 173.7; see Table 173.3).7,8 Initially, mesenchymal precursor cells derived either from periosteum or synovium initiate chondrogenic dif-ferentiation. This results in fibrocartilage composed of both fibrous and cartilaginous matrix components. In early osteophytes, endochondral ossification is initiated. The deepest cell layer becomes hypertrophic and resembles very much the lowest cells found in the fetal growth plate. Mature osteophytes are characterized by the predominance of a hyaline-cartilage-like extracellular matrix. At a first glance, mature osteophytes can, macroscopically and histologically, easily be mistaken for original articular cartilage. Although hyaline zones in osteophytes resemble articular cartilage in terms of structural composition, there are, nevertheless, certain differences such as a more random cellular arrangement, the lack of a distinct tidemark, and a missing linear subchondral bone plate.

Understanding osteophyte formation and classifying its maturation stage is on the one hand interesting per se for understanding changes going on in the chondro-osseous department in OA joints, but addi-tionally osteophyte formation represents an interesting in-vivo model system to understand and evaluate processes occurring after many modern cartilage repair strategies (e.g., transplantation of mesenchy-mal precursor cells for filling up cartilage defects).

ANIMAL MODELS OF OSTEOARTHRITISAnimal model systems represent an important adjunct and substitute for studies of OA in humans. They provide means to study OA patho-physiology as well as assist in the development of disease-modifying therapeutic agents and biologic markers for diagnosing and construct-ing a prognosis for the disease. OA is a heterogeneous condition leading to pain and reduced joint function due to a structurally damaged joint. Not surprisingly, for such a heterogeneous disorder, identification of an optimal model system for the human disease is difficult or impos-sible and a number of models employing various species are currently in use. These include spontaneous as well as induced (surgically, enzy-matically/chemically, mechanically, and genetically) models (see Chapter 172). Unfortunately, all the models differ somehow and no gold standard has yet been identified. Different subsets of human patients have disease etiologies that vary, for instance, genetic versus traumatic causes, and, in this regard, can manifest different mecha-nisms of disease. Given this heterogeneity of the OA disease process, identification of an appropriate disease-mechanism–oriented model may be a more realistic goal and better suited to a particular investiga-tion than the “universal model” that has not yet been identified. Rather, as a consequence of this disease heterogeneity in the human, a plethora of models is required.

appears to correlate directly with interleukin (IL)-1β in the synovial fluid as well as matrix metalloproteinase-1 (MMP-1) expression by synoviocytes, suggesting a direct stimulatory role of the inflammatory cells on the activity of the synovial lining cells. In any case, the pres-ence of inflammation in a significant portion of OA patients clearly points to the option of anti-inflammatory therapy at least for some subsets of OA patients.

In early OA, mostly hyperplastic OA synoviopathy is found (see Fig. 173.6d). This pattern shows only moderate synovial hyperplasia with or without cellular activation but without significant capsular fibrosis or thickening and without significant inflammatory infiltrates or mac-romolecular detritus. Overall, three forms of alterations of the synovial surface can be observed:

1. Increased cytoplasmic volume of the usually flat synovial lining cells. These cells may even become cuboidal or even cylindrical, suggesting that they have been activated in some way (see Fig. 173.6c).

2. The under normal conditions single (flat) cell layer of synovial lining cells (see Fig. 173.6a, b) can proliferate to form as many as five cell layers

3. The whole synovial surface, including the underlying stroma, can become hyperplastic and form the classic synovial villi.

Synovial hyperplasia per se can be found in all forms of OA synovi-opathy and in chronic synovitis. Thus, villous hyperplasia is largely a non-specific feature of chronic synovial alteration and activation.

So far, no well-established scoring system is available for human OA synoviopathy. Recently, a simple scoring system was proposed by Krenn and colleagues to separate inflammatory and non-inflammatory synovial alterations mainly based on the intensity of inflammatory infiltrates, synovial and stromal activation.6 In 2002 we proposed a scoring system specifically for OA synoviopathy basically dividing the OA-associated synoviopathies into four categories (see Table 173.7): hypertrophic, fibrotic, inflammatory, and detritus-rich. These can always be subdivided into mild, moderate, and strong depending on the intensity of changes present.5 This presumably reflects the different roles of OA synoviopathy and its implications for the clinical picture. Whatever scoring system is used, importantly one should average the changes present in the overall joint and not just rely on one particular region, because synovial changes are notoriously heterogenous within affected joints.

EVALUATION OF REGENERATIVE CARTILAGE FORMATION IN OSTEOARTHRITIC JOINTS (CHONDRO-OSTEOPHYTE FORMATION)Central for the basic understanding of osteophytic tissue is the analysis of the developmental steps during osteophyte formation. Thus,

TABLE 173.8 MAJOR HISTOPATHOLOGIC FEATURES OF THE FOUR PATTERNS OF OA-ASSOCIATED SYNOVIOPATHY IN COMPARISON TO EACH OTHER AND TO NORMAL SYNOVIAL MEMBRANE

NormalHyperplastic synoviopathy

Inflammatory synoviopathy

Fibrotic synoviopathy

Detritus-rich synoviopathy

Villous hyperplasia − ++(+) ++(+) ++(+) ++(+)

Synovial lining—proliferation − + ++ ++ ++(+)

Synovial lining—activation − + ++ + +

Fibrinous exudate − − (+) + ++(+)

Capsular fibrosis − − (+) +++ +++

(Macromolecular) cartilage and bone debris − − (+) − +++

Granulocytic infiltrate − − − − +

Lymphoplasmocellular infiltrate—diffuse − − ++ (+) +(+)

Lymphoplasmocellular infiltrate—Aggregates/follicles − − ++ (+) (+)

Bold italics = key diagnostic criteria.From Oehler S, Neureiter D, Meyer-Scholten C, et al. Subtyping of osteoarthritic synoviopathy. Clin Exp Rheumatol 2002;20:633-640.

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Fig. 173.6 (a, b) Normal synovial membrane shows a rather flat surface with a flat layer of inactive, non-proliferated layer of synoviocytes. In contrast, OA synoviocytes are at least in some cases severely activated and proliferated (c) similar to the situation found in rheumatoid arthritis. Most cases of late-stage OA synovial specimens show a moderate to abundant synovial hyperplasia (d, e) and often some sort of capsule thickening (e). A minority of cases of OA synovial membranes show mild to moderate (f, g) inflammatory infiltrates usually lying in aggregates around blood vessels (f ). In part of the cases lymphoid follicles also are found (g). End-stage rapid progressive cartilage destruction leads to detritus-rich synovitis with cartilage and bone fragments incorporated in fibrinous exudate (i, van Gieson stain) or the synovial stroma (h). (Reprinted with permission from Aigner T, van der Kraan P, van den Berg W. Osteoarthritis and inflammation—inflammatory changes in osteoarthritic synoviopathy. In: Buckwalter JA, Lotz M, Stoltz JF, eds. Osteoarthritis, inflammation and degradation: a continuum. Amsterdam: IOS Press, 2007:219-230.)

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a b c d e

f

GAGs

Stage 0 Stage I Stage II

OsteophyteNormal

Stage IIIArticularcartilage

GrowthplateStage IV

Aggrecan

Col1

Col2a

Col2B, Col11

Col 10

Col6pericellular

Col6interterritorial

g h i j

Fig. 173.7 Osteophyte development can be subdivided into four stages with different structural organization although many osteophytes show different stages simultaneously in different areas. Stage I (early chondrophytes) shows first chondrocytic differentiation of previously undifferentiated mesenchymal precursor cells (b, g, k). Stage II (chondrophytes) shows extensive areas of newly formed cartilage, but no (endochondral) bone formation is observed (c, h, k). Stage III (early osteophytes) shows an arrangement as the fetal growth plate cartilage with hypertrophic differentiation in the deepest cartilage layers and active bone formation underneath (d, i, k). Stage IV (mature osteophytes) shows a structure most resembling hyaline articular cartilage physiologically covering the joint surfaces (e, j, k). Normal periosteum is shown in a and f (a to e, H&E; f to j, toluidine blue staining). (Reprinted with permission from Gelse K, Soeder S, Eger W, et al. Osteophyte development-molecular characterization of differentiation stages. Osteoarthritis Cartilage 2003;11:141-148.)

Extracellular matrixfunctional element

Chondracytesreactive elementArticular cartilage

Fig. 173.8 Articular cartilage mainly consists of extracellular matrix (more than 95% of tissue volume), its functional element. Interspersed in between the abundant matrix are the cells, the chondrocytes, which are, however, the living (i.e., reacting) element of the articular cartilage tissue. (Reprinted with permission from Aigner T, Sachse A, Gebhard PM, et al. Osteoarthritis: pathobiology—targets and ways for therapeutic intervention. Adv Drug Deliv Rev 2006;58:128-149.)

PATHOGENETIC CONCEPTS OF OSTEOARTHRITISOA is a heterogeneous condition and most likely many different causes exist that initiate or at least promote the disease process. Conse-quently, many different hypotheses for the pathogenesis of OA have been brought forward. Presumably, many of them reflect part of the mechanisms important for the initiation and progression of joint degeneration. Here a rough overview of the relevant pathogenetic con-cepts is provided.

The articular cartilage and the extracellular matrixArticular cartilage is a highly specialized and uniquely designed bioma-terial (see Chapter 8). It is largely an avascular, aneural, and alymphatic matrix that is synthesized by the sparsely distributed resident cells—the chondrocytes. The cartilage matrix can be subdivided according to different cartilage zones based on the arrangement of the cells and the matrix fibrils (i.e., superficial, radial, deep, and calcified). Also, the cartilage matrix can be split up in different compartments depending on its relationship to the cells: whereas the pericellular matrix is imme-diate to the cells, the interterritorial matrix compartment represents the major portion of the cartilage matrix far off the cells and the territo-rial matrix the (not really well defined and characterized) cell-associ-ated compartment in between (Fig. 173.8).

At the supramolecular level, the interterritorial cartilage matrix consists of two basic components: a fibrillar and an extrafibrillar matrix (see Fig. 173.8). The fibrillar matrix is a network consisting mainly of collagen II together with other collagens, predominantly IX, XI, and XVI (Fig. 173.9). Collagen XI is located in the core of the collagen II fibrils and is thought to be involved in fibril initiation and limiting fibril diameter. Collagen IX is located periodically along the surface of collagen II fibrils in antiparallel direction and might be responsible for crosslinking the collagen network with itself but also to the noncol-lagenous matrix. The function of collagen XVI, which is also present in articular cartilage, is so far unknown. Of note, the so-called type

collagen II fibrils also contain many non-collagenous protein compo-nents such as small proteoglycans and cartilage matrix proteins. The non-fibrillar component of hyaline articular cartilage consists predomi-nantly of highly sulfated aggrecan monomers (Fig. 173.10), which are attached to hyaluronic acid and form very large aggregates. In terms of the physical properties of the cartilage matrix, tensile strength comes from the collagen network, which hinders expansion of the viscoelastic aggrecan component and, thus, provides compressive stiffness of the tissue. On the other hand, the aggrecan-hyaluronan aggregates bind high amounts of intercellular water owing to their extensive fixed

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COOH

Type II

Type XI

Type IX

Type X

Type VI

NH2Chondrocyte

CARTILAGE COLLAGENS

Fig. 173.9 Cartilage collagens. The different collagen types synthesized by chondrocytes. The chondrocyte, depicted on the left side of the figure, constitutively produces three collagen types: type II, type XI, and type IX. These three collagens, shown inside the bracket, are incorporated into the same collagen fibril. The proportions of these collagens in the fibrils change with age. The other two collagen types, type X and type VI (marked with dashed arrows), are not made by all chondrocytes. Type X collagen is synthesized only by the hypertrophic chondrocytes in growth-plate cartilage or in articular cartilage near the tidemark and through the calcified cartilage. Type VI collagen is not synthesized in embryonic cartilage but appears in mature cartilage. The levels of both type X and type VI collagen in articular cartilage appear to increase in OA. All the cartilage collagens are synthesized as molecules containing at least two different kinds of domains, triple helical and non-helical. The helical regions are depicted as three-chained coils, and the non-helical regions are contiguous small boxes. In some collagen types (II and XI) the non-helical regions (yellow) are removed as fibrils are formed. The other collagen types (IX, X, and VI) retain their non-helical regions and are shown as one solid color through the entire molecule. Type IX collagen is also a proteoglycan and contains one glycosaminoglycan chain (small orange kinked chain). Disulfide bonds between two collagen chains are shown as red boxes. All the molecules and their domains are drawn approximately to scale as a linear representation of their respective molecular weights.

THE STRUCTURE OF AGGRECAN

Proteoglycantandem repeat

G1 E1 G2KS- richdomain E2 (C5- rich domain) G3

Link protein

Keratan sulfate O-linked oligosaccharides

N-linked oligosaccharides Chondroitin sulfate

Lectinbinding

NH2

NH2 COOH

Ig fold

Fig. 173.10 The structure of aggrecan. In aggrecan the three globular domains (G1, G2, and G3) are separated by two extended segments (E1 and E2), which carry the glycosaminoglycans chondroitin sulfate (CS, in the CS-rich domain) and keratan sulfate (KS, in the KS-rich domain, but some also in the E1 segment and within the CS-rich domain). Furthermore, the core protein is substituted with N- and O-linked oligosaccharides. The G1 and G2 domains, as well as the link protein (LP), contain a double loop structure (proteoglycan tandem repeat [PTR]). In addition, both G1 and LP show an additional loop structure (immunoglobulin fold [Ig fold]) that can selectively interact with H hyaluronic acid to form aggregates. The G3 domain contains a lectin-binding region.

charges and are responsible for the elasticity of the tissue. Thus, under compression, the cartilage matrix is compliant but rapidly regains its elasticity as water molecules are drawn back into the matrix on unload-ing by the strongly hydrophilic aggrecan aggregates.

The territorial matrix is defined as the cell-associated matrix located between the pericellular and the interterritorial matrix compartments, but no real specific biochemical characterization is available so far.

Clearly, it shares most of its basic composition with the interterritorial matrix to which it shows no clear border separating them from each other.

The pericellular cartilage matrix demonstrates in many respects a very distinct composition compared with the territorial and interter-ritorial cartilage matrices. In terms of structural collagens, type VI collagen (see Fig. 173.9) is the predominant collagen present, which in hyaline articular cartilage is concentrated within the pericellular matrix. Ultrastructural studies have shown a physical overlap of the type VI collagen network with the type II collagen positive matrix, which supports the concept that type VI collagen is a central molecular component forming a mechanical interface between the rigid type II matrix and the (softer) cells. Additionally, type VI collagen presumably plays some role in cell anchoring and cell-matrix interaction and signal-ing together with other molecules present in the pericellular cartilage matrix.

Most of the cartilage matrix is formed during fetal development and the phase of skeletal growth until the closure of the growth plates at the end of adolescence. In fact, the collagen backbone appears to show virtually no turnover during life at least in the (inter)territorial matrix compartments. However, other matrix components, namely, the large aggregating proteoglycan aggrecan and the small proteoglycans as well as some collagen types (e.g., types VI and IX) show a significant turn-over throughout life. This physiologic turnover is highly relevant for the maintenance of the cartilage matrix integrity on the molecular, and in particular also the macromolecular, level.

Pathologic matrix degradationOne major threat to the cartilage matrix and thus to the integrity of articular cartilage are matrix-degrading enzymes destroying the colla-gen network as well as the interlying proteoglycans (Fig. 173.11). Besides direct degradation of molecular components, destabilization of the supramolecular structures also takes place and plays an important role in the loosening of the overall matrix architecture.

The destruction of articular cartilage and the loss of its biomechani-cal function is largely due to the destruction and loss of the (inter)territorial cartilage matrix. So far, our knowledge focuses on degrada-tion processes of the two major components of the interterritorial cartilage matrix, the collagen network and the interwoven proteoglycan aggregates. Loss of aggrecan and its fixed (negative) charges is charac-teristic of the early stages of cartilage degeneration, whereas the overall content of collagen remains rather constant nearly throughout the disease process. Still, loosening of the collagen network is a major feature also in early cartilage degeneration. So far, it is unknown what happens first—the loss of proteoglycans or the loosening of the collagen network—because both do eventually influence the other as well. Loos-ening of the collagen network leads to a loss of proteoglycans, and a loss of proteoglycans leads to a mechanical overload and, thus, damage and loosening of the collagen network. In particular, the latter appears to be responsible for the hyperhydration of articular cartilage in the early phases of the disease process, macroscopically visible as softening

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microfracturing during compression and, thus, molecular disintegra-tion. Also, the aggrecan molecules change over age. Aggrecan mono-mers and polymers get smaller and have fewer sugar side chains. Interestingly, this is not only related to accumulating molecular deg-radation but also newly synthesized aggrecan aggregates appear to be smaller in the aged tissue, thus carrying less fixed charges. Although the molecular mechanisms driving this decline are so far unclear, severely reduced sugar side chains significantly limit the ability of aggrecan to bind water and, thus, to maintain the elasticity of the articular cartilage matrix.

The time-dependent formation of advanced glycation end products is another interesting phenomenon involving post-translational, non-enzymatic protein and lipid modifications that contribute to changes in matrix biochemistry and chondrocyte biology in aging cartilage. The accumulation of advanced glycation end products has been shown to increase the stiffness of the collagen network and can downregulate the anabolic activity of chondrocytes, further imbalancing cartilage tissue homeostasis.

Crystal deposition diseaseOne interesting phenomenon frequently observed within the articular tissues of degenerated joints is the deposition of anorganic crystals. In general, there are two major forms of crystal deposition disease in the articulating joints: deposition of sodium urate crystals (i.e., gout [see Chapter 183]) and the deposition of basic calcium phosphate (BCP) or calcium pyrophosphate dihydrate (CPPD) (so-called pseudogout or chondrocalcinosis [see Chapters 186 and 187]). Both forms can cause significant symptomatic disease, but gout is clearly usually sympto-matic and pseudogout, in most cases, a clinically silent process. Pseu-dogout is strongly associated with (OA) cartilage degeneration and can be detected on routine radiography and by conventional polarizing microscopy. Several studies have demonstrated a relation between the prevalence of crystal occurrence and severity as well as progression of OA. So far four factors have been identified that play a role in this type of crystal formation: (1) overproduction of the anionic component pyrophosphate by the chondrocytes, (2) increased calcium concentra-tion in the cartilage of OA patients, (3) changes in the pericellular matrix milieu, and (4) the involvement of matrix vesicles that have been shown to produce CPPD crystals in vitro.10 Overall, the cause and the relevance of chondrocalcinosis continue to be ambiguous: clearly, it is well correlated to the degeneration of the articular cartilage and it is thought to be related to a metabolic imbalance within the cells and the tissue, most likely the articular cartilage and the chondrocytes. However, so far it remains largely unclear what comes first—the cell and matrix degradation or the crystal formation. Most likely they promote each other with metabolic disturbance leading to cellular degeneration and vice versa.

Role of biochemical differencesAn interesting feature of OA is that not all joints are affected equally. Although knee and hip joints are most often involved, ankles are gener-ally spared in symptomatic OA. Although it is obvious that there are anatomic differences between the ankle and knee joint, this alone does not explain why the knee is more susceptible to OA. Studies comparing knee and ankle cartilage have identified several biochemical differences that might be of additional relevance (Table 173.9): (1) differences in biochemical composition and biomechanical properties of the matrix resulting in higher dynamic stiffness of the ankle cartilage, (2) decreased response to catabolic factors such as interleukin-1 (IL-1), and (3) more efficient synthetic matrix repair with an increase in collagen type II synthesis and aggrecan turnover seen in ankle lesions.11 Taken together it seems that there are differences not only in the anatomy and mor-phology of the joints and its cartilage but also in the cellular phenotype chondrocytes themselves, which may explain why some joints are less prone to develop OA than others.

The articular cartilage and chondrocytesThe articular cartilage consists mostly of extracellular matrix. This matrix is the functional element of the cartilage tissue, that is, the

and swelling of the OA articular cartilage. Degradation processes appear to be specifically prominent in the surface zone and around the chondrocytes in OA cartilage. Enhanced levels of many metalloprotein-ases including matrix metalloproteinases (MMPs), as well as adamaly-sins such as ADAMs (a disintegrin and metalloproteinase) and ADAMTSs (a disintegrin and metalloproteinase with thrombospondin type-1 motifs) are the most likely candidate enzymes responsible for the increased matrix degradation in OA cartilage.9 So far it is rather enigmatic which proteases are really crucial for the degradation of the various cartilage matrix components, although MMP-13 is certainly a top candidate for primary collagen type II fibril degradation, MMP-2 (gelatinase A) is a good candidate for subsequent cleavage of denatured collagen fibrils (“gelatins”), whereas ADAMTS-4 and ADAMTS-5 are favored to be the major aggrecanases responsible for the proteoglycan breakdown.

Age-induced degenerative changes of the cartilage matrixClearly, the extracellular cartilage matrix is different depending on the age of the individual. One obvious reason for this is the continuous loading and intermittent overloading during life. Thus, damaged matrix molecules due to continuous mechanical forces, but also degradative enzymatic activity, which are not sufficiently replaced as part of a permanent physiological turnover, accumulate over time in any tissue, but in particular in the mechanically heavily challenged articular car-tilage. These damaged components threaten the functional integrity of the extracellular matrix and the articular cartilage, in particular if chal-lenged by further mechanical stress. However, beyond the classic wear and tear concept, which certainly holds true for some aspects of OA, a second theory for explaining the association between cartilage degen-eration and aging of its matrix focuses on well-known age-related changes in the extracellular matrix of articular cartilage, modulating its biomechanical properties and integrity.

Besides pure degradation of matrix components, also molecular modifications are of high relevance for the functional integrity of the cartilage matrix: thus, the collagen network stiffens due to increased covalent cross-linking of the single collagen chains (pyridinium cross-linking). This makes the fibrillar network more rigid and less flexible for physiologic deformation occurring during (physiologic) joint loading and movement. In consequence, the collagen network is prone to

THE HALLMARK OF OA CARTILAGE DEGENERATION ISA LOSS OF CARTILAGE MATRIX HOMEOSTASIS

Osteoarthritis:Imbalance of cartilage matrix turnover

IGFBMP...

Il-2βTNFa...

AnabolismAggrecan(collagen type II)Collagen type VICollagen type IXLink protein...

CatabolismCollagenasesMMP-1(MMP-8)MMP-13

AggrecanasesMMP-3MMP-14ADAMTS-1ADAMTS-4ADAMTS-5

GelatinaseMMP-2MMP-9

Fig. 173.11 The hallmark of OA cartilage degeneration is a loss of cartilage matrix homeostasis. The insufficiency of anabolic factors such as insulin-like growth factor-I (IGF-I) and BMPs in combination with an increased influence of catabolic factors such as IL-1β and TNF-α result in an overexpression of matrix degrading proteases (collagenases, aggrecanases, and gelatinases). The catabolic activity of these enzymes cannot be compensated by the concurrent increase in anabolic activity (i.e., expression of aggrecan and collagen types II, VI, and others).

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TABLE 173.9 DIFFERENCES BETWEEN KNEE AND ANKLE JOINT STABILITY, MOTION, AND CARTILAGE

Feature Knee Ankle

Joint stability Relatively unstable Highly stable

Non-congruent Highly congruent

Joint motion Flexion/extension Flexion/extensor

Rotation

Cartilage

Cellularity Same Same

Cartilage thickness 2-6 mm 1-1.5 mm

Superficial chondrons Single cells Clusters of 2-4 cells

Sulfated glycosaminoglycan content

Lower Higher

Water content Higher Lower

Collagen content Same Same

Dynamic stiffness Lower Higher

Hydraulic permeability Higher Lower

Peak stress with 65% final strain

11 MPs 16 MPs

Glycosaminoglycan synthesis

In explants Higher Lower

In alginate Same Same

Proteoglycan half-life 22.68 days 16.58 days

Protein synthesis Lower Higher

IC30 for IL-1 reduction of proteoglycan synthesis

In alginate 11 pg/mL 56 pg/mL

In explants 6 pg/mL 35 pg/mL

Influence of Fn-fs on

Proteoglycan synthesis (anti-anabolic)

Low dose (1 nM) High dose (100 nM)

Proteoglycan loss (catabolic)

Significant at 7 days Not significant after 28 days

Attempted repair No significant rebound Significant rebound

(Response to anabolic factors after catabolic stimulation)

Response to degeneration Upregulation of collagen degradation

Upregulation of matrix synthesis

From Eger W, Schumacher BL, Mollenhauer J, et al. Human knee and ankle cartilage explants: catabolic differences. J Orthop Res 2002;20:526-534.

matrix provides the mechanical properties of the cartilage tissue. However, the cells (i.e., the chondrocytes) represent the only vital element of the cartilage tissue, although they represent only about two to three volume percent of the articular cartilage in the adult. Thus, besides changes in the extracellular matrix changes within the cells also are obviously potential causes of the OA disease process and the study of the chondrocyte cell phenotype/behavior (Fig. 173.12) can provide substantial scientific insights into the disease mechanisms of OA.

The developmental history as a model of chondrocyte reactivity in the adultA very important and interesting aspect of cellular behavior in the adult organism is the recapitulation of molecular mechanisms that occurred during fetal development (Fig. 173.13). This phenomenon is also true

for OA chondrocytes. In fact, many of the biologic changes that occur in OA cells mimic a differentiation pattern characteristic of fetal skel-etogenesis. This includes changes not only in cellular phenotypes and in anabolic and catabolic events but also in other basic mechanisms during the disease process such as matrix calcification, apoptosis, and proliferation. Thus these (evolutionary and developmental) compari-sons are attractive for explaining chondrocyte behavior and disease pathways in the adult, but uncoordinated degenerative events should not be mistaken for tightly regulated developmental processes (Fig. 173.14). Both scenarios presumably involve similar molecular and regulatory events, but just as in a jigsaw puzzle the assembly is the challenge.

The OA chondrocyte phenotypeChondrocytes in normal adult articular cartilage are stable, postmi-totic, differentiated cells that maintain tissue homeostasis by synthe-sizing very low levels of extracellular matrix components to replace damaged molecules, thus preserving the structural integrity of the cartilage matrix. The cells are the major regulators of matrix anabolism and catabolism of articular cartilage. One well-documented change in OA cartilage is the induction of an activated cellular phenotype within the chondrocytes whereby matrix anabolism is strongly stimulated. Nevertheless, the chondrocytes fail to compensate for matrix damage induced externally (e.g., by mechanical stress or enzymatic degradation through synovial proteases). Additionally, the chondrocytes do play an active role in the degradative process themselves, a phenomenon termed chondrocytic chondrolysis.12 During chondrocytic chondrolysis, OA chondrocytes activate or upregulate the expression of many matrix-degrading proteases such as the MMPs, which are largely responsible for the breakdown of the collagenous and non-collagenous cartilage matrix components. This elevated proteolytic activity is not sufficiently counterbalanced by an increase of the chrondrocyte anabolic activity. This is particularly true for the upper zones of damaged cartilage (the “progression zone”), in which the anabolic activity even drops again severely.13

Activation of inflammatory signalingIn addition to local and/or general inflammatory responses within the synovial membrane, the activation of inflammatory (signaling) path-ways within the chondrocytes themselves appears to play a crucial role in OA disease progression. Activation of such processes is independent of direct inflammatory cell infiltrates (i.e., lymphocytes, granulocytes, plasma cells), which are not present in OA articular cartilage. Activated inflammatory signaling pathways have been shown to induce catabolic responses in chondrocytes, namely, matrix degrading proteases such as MMP-13, MMP-1, and others. One of the most prominent catabolic cytokines in OA is the proinflammatory cytokine IL-1. Elevated levels of IL-1 are found in synovial fluids of patients suffering from rheuma-toid arthritis and, to a lesser extent, in synovial fluid from OA patients. Although polymerase chain reaction (PCR)-based studies could not confirm an increased expression of IL-1 mRNA in OA chondrocytes,14 there might still be increased levels of IL-1 protein diffused into carti-lage from the synovial space. IL-1 significantly affects gene expression patterns within articular chondrocytes via multiple intracellular path-ways, particularly the MAP kinases and NF-κB pathways (Fig. 173.15).15 IL-1 downregulates the expression of the major cartilage matrix com-ponents, aggrecan and collagen type II, and, thus, counteracts the effects of anabolic factors on matrix synthesis. Additionally, IL-1 induces the expression of matrix degrading enzymes such as MMP-1, MMP-3, MMP-13, or ADAMTS-4, which are all potential major players in the destruction of cartilage matrix components. Besides these direct effects, IL-1 also induces other cytokines with synergistic (catabolic) effects such as IL-6 and leukemia inducing factor (LIF), further expanding its versatile effects on cartilage tissue homeostasis.

Oxygen, reactive oxygen species, and reactive nitrogen speciesArticular cartilage is an avascular tissue and its nutrition is mainly supplied by the synovial fluid. Because of the rather long diffusion

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CELL BIOLOGY OF OA

Synovial factors Matrix alterations

• Dedifferentiated cells• Hypertrophic cells• Precursor cells

Changes in phenotype to:

Autocrine andparacrine factors

Proliferation and/or(apoptotic) death

(Pre)senescence

Anabolicactivation

Catabolic activation

Chondrocyte

Aggrecan

S–S

Link proteinFibronectin

Type II collagen

Fig. 173.12 Cell biology of OA: how do chondrocytes react? OA chondrocytes are exposed to severely abnormal extracellular stimuli, including autocrine and paracrine factors, synovial factors, and altered matrix constituents, that induce a plethora of abnormal cellular responses made apparent by the changes in anabolism, catabolism, and phenotype that have been demonstrated in the cells. Also, chondrocyte numbers are modified by proliferation or apoptosis. In addition, cells might become presenescent, leading to an overall loss of chondrocyte function. In this schematic, an OA chondrocyte is embedded in a cartilaginous extracellular matrix of type II collagen, aggrecan, and fibronectin, for simplicity. Other collagens, proteoglycans, and noncollagenous proteins are also present at varying levels. (Reprinted with permission from Aigner T, Soder S, Gebhard PM, et al. Mechanisms of disease: role of chondrocytes in the pathogenesis of osteoarthritis—structure, chaos and senescence. Nat Clin Pract Rheumatol 2007;3:391-399.)

THE DEVELOPMENTAL MODEL OF CHONDROCYTE BEHAVIORAPPLIED TO OA IN THE ADULT

Osteoarthritis Developmental model: endochondral ossification

Pathomechanisms Development steps Marker genes

Differentiation Chondrogenesis

Proliferation Proliferation

Catabolism Matrix degradation

Calcification Calcification

Cell death/apoptosis Cell death/apoptosis

Hypertrophy

AnabolismMatrix synthesis

COL2AEpichondral

Resting

Proliferative

Hypertrophic

Bone

COL10

SOX9

COL2/9/11aggrecan

Ki-67ssDNA

MMP-13

Fig. 173.13 The developmental model of chondrocyte behavior applied to OA in the adult. One way to interpret cellular behavior in adult disease is to investigate whether it shows similarities to developmental or evolutionary processes. Several processes that occur in OA are also known to have occurred during fetal chondroneogenesis, including changes in the chondrocytic phenotype (differentiation), matrix anabolism and catabolism, (apoptotic) cell death, proliferation, and matrix calcification. The analysis of events during fetal development allows us to identify marker genes that can assist in the identification of the molecular context of a gene in the adult chondrocyte. For example, expression of Sox9 indicates differentiation to the chondrocyte phenotype, type IIA collagen (COL2A) is a chondroprogenitor cell marker, and type X collagen (COL10) is a marker of hypertrophic chondrocytes. Ki-67 indicates cell proliferation whereas the onset of MMP-13 expression suggests increased matrix catabolism potentially linked to hypertrophic differentiation. ssDNA indicates apoptotic DNA fragmentation, whereas aggrecan, COL2, COL9, and COL11 indicate anabolic cell activity (the synthesis of new cartilage matrix). Despite the appeal of a comparative approach, one should be cautious not to mistake uncoordinated degenerative processes for highly structured developmental processes. COL2/2A/9/10/11, collagen type II/IIA/IX/X/XI; MMP, matrix metalloproteinase; ssDNA, single-stranded DNA. (Reprinted with permission from Aigner T, Soder S, Gebhard PM, et al. Mechanisms of disease: role of chondrocytes in the pathogenesis of osteoarthritis-structure, chaos and senescence. Nat Clin Pract Rheumatol 2007;3:391-399.)

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still hypothetical. NO inhibits actin polymerization, which affects cell adhesion, signaling from extracellular matrix, and phagocytosis. Fur-thermore, it has been found that NO can inhibit matrix synthesis and promote cell death of chondrocytes mediated by caspase-3 and tyrosine kinase activation. However, the concept that NO is a promoter of cell death itself so far remains unproven. Thus, RNS as well as ROS are exciting areas of future research in the pathogenesis and molecular biology of OA.

Obesity and adipokinesBeing overweight is a strong risk factor for the development of knee OA and less so for the hand and the hip. Two main theories have been proposed to explain this association between obesity and OA: the bio-mechanical and the systemic/metabolic.

The biomechanical hypothesis proposes that obesity leads to an increased loading of the (knee) joints beyond their capabilities (due to the increased body weight). Although it is known that moderate loading is beneficial for chondrocyte physiology and cartilage (matrix) integrity, excessive stress disrupts the homeostasis of the cartilage matrix. Obvi-ously, mechanical overload represents a direct physical insult to the cartilage matrix. Additionally, mechanical forces are transmitted to the cells and transformed into intracellular signals. Sensitive mechanore-ceptors such as integrins initiate intracellular signaling cascades, trig-gering a variety of cellular responses, including the release of paracrine or autocrine factors. With increased mechanical stress through, for example, being overweight, cells are overstrained and fail to perform adequately. Although this theory sounds like a straightforward explana-tion, epidemiologic studies have also shown a significant correlation between hand OA and obesity, which cannot be completely explained by mechanical stress. Therefore, the systemic/metabolic hypothesis proposes that metabolic factors related to obesity act directly or indi-rectly on chondrocytes leading to the increased risk for developing OA.19 Several studies suggest that so-called adipokines, which are pro-teins synthesized and secreted mostly by adipocytes, are the major factors linking obesity to OA. Leptin, the prototypic adipokine, has been found in cartilage of OA patients and shows biologic activity on chondrocytes. It has been shown to act as a proinflammatory cytokine and a catabolic factor in cartilage metabolism via induction of MMPs. Conversely, it might also demonstrate anabolic effects through the

RNA IN SITU HYBRIDIZATION FOR MARKER GENES ANDOA RELEVANT PROTEASES IN THE TIBIA OF NEWBORN MICE

Cartilagesynthesis

Cartilagematurationand degradation

Restingchondrocytes

Proliferatingchondrocytes

Hypertrophicchondrocytes

Bone

Sox9 Ihh Col10a1 MMP13 MMP9

Fig. 173.14 RNA in-situ hybridization for marker genes and OA relevant proteases in the tibia of newborn mice. Anabolic and catabolic events in the growth plate of the primary ossifying skeleton are at least in part separated. Sox9 mRNA expression marks the zone of proliferation, which differs from the region of terminal chondrocyte maturation characterized by the expression of Ihh as a marker for the prehypertrophic chondrocyte and Col10A1 as a specific marker for the entire hypertrophic cartilage. (Reprinted with permission from Aigner T, Gerwin N. Growth plate cartilage as developmental model in osteoarthritis research—potentials and limitations. Curr Drug Targets 2007;8:377-385.)

distance the partial pressure of oxygen is very low in healthy cartilage and presumably even further decreased in OA. Thus, chondrocytes live in a hypoxic environment with an O2 tension around 6% at the joint surface to as low as 1% in the deep layers of the articular cartilage. Even though the oxygen level in articular cartilage is physi-ologically very low, a certain level of oxygen availability appears to be essential also for chondrocytes. One major factor in the chondrocytes’ adaption to hypoxia has been found to be the transcription factor hypoxia-inducible factor-1α (HIF-1α), which has key functions in con-trolling energy generation, cell survival and even influences matrix synthesis.16

During normal (oxygen) metabolism so-called reactive oxygen specias (ROS) are formed as natural byproducts.17 They are involved in the control of various aspects of biologic processes, including cell acti-vation, proliferation, and (apoptotic) cell death. Especially, low levels of ROS have been reported to act as a second messenger in (physiologic) intracellular cell signaling involved in the regulation of the expression of a wide variety of gene products, including cytokines, MMPs, adhe-sion molecules, and matrix components. However, in pathologic condi-tions, including inflammatory joint diseases, elevated production of ROS in combination with depletion of antioxidants has been observed within the cells and causally implicated in the progression of these diseases. Such an imbalance between oxidants and antioxidants leading to cellular or tissular structural and/or functional changes is referred to as “oxidative stress.” At this time, our knowledge on the redox state of cartilage in pathologic circumstances remains fragmented. However, ROS have been implicated—besides metalloproteinases—in the process of matrix and cell component degradation in OA. ROS may directly oxidize nucleic acids, transcriptional factors, membrane phospholipids, intracellular and extracellular components leading to impaired biologic activity, cell death, and breakdown of matrix components. Perhaps most importantly, ROS are the major cause of DNA damage within the genome.

Besides ROS, reactive nitrogen species (RNS) also might be impor-tant in the pathogenesis of OA.18 RNS are derived from nitric oxide (NO), which is produced in small amounts by nitric oxide synthase (NOS) and performs important functions in many physiologic proc-esses. Human chondrocytes cultured from OA patients express induc-ible NOS (iNOS) and produce significant amounts of NO. The mechanisms by which NO could contribute to OA pathogenesis are

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emptying of cellular lacunae (within the cartilage matrix) is a seemingly obvious histologic feature of OA cartilage.21 However, opinions on the prevalence and importance of chondrocyte death for OA pathology differ widely,22 and most likely apoptotic cell death is a rather rare event.23,24 Also, lacunar emptying appears to be largely a technical artifact (mainly due to cell shrinkage) except in late-stage disease when in fact there is enhanced cell loss.23

Apoptosis is a complex cellular process, and the factors responsible for apoptotic cell death in articular cartilage are largely unknown.25 Because β1-integrin–mediated cell-matrix interactions provide survival signals for chondrocytes, reduction in extracellular matrix integrity due to degradation may be partly responsible for chondrocyte death in vivo. In cultured chondrocytes, treatment with Fas ligand (or anti-Fas), NO donors, tumor necrosis factor-α (TNF-α) or IL-1, taurosporin, ceramide, or retinoic acid has been shown to induce apoptosis, but it is uncertain to which of these factors articular chondrocytes are sufficiently exposed to in the body. Certainly, chondrocytes become somewhat fragile if the (peri-)cellular matrix is removed or deranged26 as in OA cartilage.27,28 In fact, degradation products of pericellular matrix components such as fibronectin might directly induce cellular death programs in chondrocytes.

Altogether, the experimental evidence clearly suggests that apopto-sis occurs in OA cartilage, but at a very low rate at least in the earlier stages. The relative contribution of apoptotic cell death to the patho-genesis of OA is difficult to assess because of the chronic nature of the disease process. Also, it is difficult to assess whether apoptosis is primary or secondary to cartilage matrix destruction. There is a good likelihood that, at least in later-stage disease, chondrocyte death and matrix loss form a vicious cycle, the progression of one having promot-ing effects on the other.

The aging chondrocyte: genomic integrity and the chaotic phenotypeIt has been known for a very long time that age is the most prominent risk factor for OA, but the explanations for this clear and strong association have changed over time.29 Besides the classic hypothesis of continuing wear and tear, the aging of matrix and cells are pre-sumably very important etiopathogenetic factors for explaining this relationship.

The senescence theory of OA postulates that the chondrocytes become senescent due to cellular stress and/or (focal) proliferation, finally leading to a failure of the cells to fulfill their essential functions (e.g., in maintaining proper matrix turnover). In general, cellular aging is associated with a number of changes that may undermine the ability of cells to maintain tissue homeostasis and culminate in cellular senescence. Senescence occurs in cultures of continuously dividing somatic cell populations, including chondrocytes, after a limited number of population doublings and is presumably due to telomere erosion (replicative senescence). In post-mitotic cells such as chondro-cytes in vivo obviously classic cellular (replicative) senescence does not play a role in general. More attractive, however, is the concept of “pro-gressive” cellular senescence, which is precipitated by steadily increas-ing damage to the genomic DNA mostly due to oxidative stress. Thus, oxidatively damaged molecules (DNA, proteins, lipids) accumulate with aging and are thought to gradually derange cellular functions. Accumulation of damaged molecules is usually of only limited rele-vance for cells if they are proliferating, because a large portion of these molecules are re-synthesized during replication. However, this con-stant molecular renewal is missing in post-mitotic cells. Thus, post-mitotic cells accumulate (oxidatively) damaged molecules significantly with time.

Substantial DNA damage in addition to other cellular degenerative alterations is known to occur in OA chondrocytes.30 These effects would be expected to lead to apoptotic cell death in most cell types. As alluded to earlier, however, apoptosis appears to occur rather rarely in OA cartilage. Instead, the chondrocytes remain in a pre-apoptotic or para-apoptotic state with an uncoordinated pattern of gene expression, as shown in many in-vivo studies of chondrocyte behavior (Fig. 173.16). In this respect, the downregulation of molecules that are usually responsible for regulating cell integrity and/or removal after non-accept-able cell damage may be an important permissive factor at this stage.

stimulation of proteoglycan and collagen synthesis and the induction of growth factors.

A third (indirect) effect of obesity is certainly also the induction of a (latent) diabetic metabolic state in the obese patients over time, enhancing, for example, advanced glycation end products formation within the cartilage matrix and leading to all their detrimental effects on matrix mechanoproperties and cell behavior as discussed earlier.

The concept of progressive (apoptotic) cell lossOne of the most simple explanations for OA cartilage degeneration would be a mere loss of viable chondrocytes due to cell death during the disease process. Because the chondrocytes are the only source of matrix component synthesis in articular cartilage, any significant cell loss would immediately result in a distortion of cartilage matrix homeostasis. Cell death can, in principle, be divided into apoptosis and necrosis. Apoptosis has evolved as a mechanism to eliminate surplus, abnormal, or dysfunctional cells whose survival and proliferation would be detrimental. Apoptosis is thus normally a beneficial process, although aberrant apoptosis can occur in pathologic states and apop-tosis is clearly disadvantageous when it leads to the elimination of healthy cells.

Many studies have addressed whether cell death plays a role in the pathology of OA,20 because articular chondrocytes cannot self-renew and cell loss would therefore be permanent and detrimental. Also

SCHEMATIC REPRESENTATION OF THE INTERLEUKIN-1 (IL-1)SIGNALING PATHWAY

MAPKKKK

NIK

Catabolism

Catabolic genes

MAPKKK/TAK1/TAB1

NFΚB p38

c-jun, ATF, SAP-1, ELK, ...

ERKJNK

IL 1

IL 1-R

MAPKK

Fig. 173.15 Schematic representation of the interleukin-1 (IL-1) signaling pathway. IL-1 signals through four major cellular signaling pathways including the three major MAP kinases (ERK, JUN, and P38) as well as the NF-κB cascade. This explain the high pleomorphism of genes influenced by IL-1 stimulation in chondrocytes. (Reprinted with permission from Aigner T, van der Kraan P, van den Berg W. Osteoarthritis and inflammation—inflammatory changes in osteoarthritic synoviopathy. In: Buckwalter JA, Lotz M, Stoltz JF, eds. Osteoarthritis, inflammation and degradation: a continuum. Amsterdam: IOS Press, 2007:219-230.)

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The synovial membraneThe two main clinical symptoms of OA, pain and joint stiffness, are both significantly related to synovial inflammation and capsular fibro-sis. However, although its clinical importance is clear, the role of synovial inflammation in the pathogenetic process of cartilage destruc-tion remains largely unknown (Fig. 173.17).32

The synovial (inflammatory) reaction observed in OA joint disease has been primarily considered to be a secondary effect resulting from the release of cartilage debris from the damaged articular cartilage. This is in contrast to the situation found for example in rheumatoid arthri-tis, which is considered to originate from a synovial inflammatory autoimmune reaction with secondary cartilage destruction. However, inflammatory reactions in the synovial membrane do occur to some degree in all OA joints. Also, the fact that most OA patients display a minor elevation of C-reactive protein within the serum suggests that the inflammatory component plays some role within the disease process.

Synoviocyte activation and proliferation as well as synovial hyper-plasia presumably all represent reactive changes responding to increased demands for clearance of molecular debris in the synovial fluid of the joint. This also explains the increase in the amount of CD68-positive type A synoviocytes, which have phagocytic capacity, in the synovial lining layer.

Although a cellular inflammatory component is missing in particu-lar cases of early OA, synovial hyperplasia and activation is likely to generate significant problems for the articular cartilage homeostasis. Synoviocytes are able to secrete not only matrix-degrading proteases (e.g., MMPs) but also catabolic cytokines (e.g., IL-1, TNF-α), inducing inflammatory signaling pathways within the chondrocytes themselves (see Fig. 173.15).

Many studies have found elevated MMP levels in synovial fluid of OA patients, namely, collagenase and stromelysin. Davidson and asso-ciates33 showed upregulation in OA synovium compared with synovium from patients with fracture of the femoral neck of MMP-9, MMP-11, MMP-13, MMP-16, and MMP-28 and ADAMTS-2, ADAMTS-10, and ADAMTS-16.

Not only proteases, cytokines, and growth factors but also other factors are expressed by inflamed OA synovium. OA synovium pro-duces increased amounts of ROS, such as NO, peroxynitrite, and superoxide anion.

IL-1 is also synthesized in substantial quantities in OA synovial tissues, and this may be a major source of the increased IL-1 levels in

For example, the expression of the small GTPase RhoB, a molecule that is constitutively expressed by normal articular chondrocytes, is significantly downregulated in OA cartilage.31 RhoB is involved in cytoskeletal organization, cell transformation, and survival, but, most importantly, appears to be required for the apoptotic response at least in some cell types. One intriguing speculation is that the downregula-tion of RhoB in OA chondrocytes might be a prerequisite for the sus-tained pre-apoptotic or para-apoptotic phenotype of OA chondrocytes despite the substantial DNA damage that in normal cells would lead to apoptotic cell death.

Clearly, aging chondrocytes differ from normal cartilage cells, and, to a greater degree, chondrocytes from OA cartilage are likely to show signs of degeneration. However, aging does not inevitably lead to OA and not all aged chondrocytes show losses of function. On the other hand, even in “normal” joints of elderly people the cartilage no longer looks juvenile. The major difference between normal aged cartilage and OA cartilage is that lesions do not progress and do not result in symp-tomatic disease as in OA cartilage. Although all individuals are sus-ceptible to the same age-related changes, these appear to progress faster in some individuals (i.e., patients with primary OA) than others. Thus, OA shows “premature” or accelerated degeneration of articular carti-lage due to a premature senescence of the chondrocytes that maintain its structural integrity. By analogy to neurodegenerative disorders one could name OA the “Morbus Alzheimer” of articular cartilage and chondrocytes.29

This analogy is particularly intriguing as OA, and to a lesser extent aged chondrocytes show discoordinate reaction patterns (see Fig. 173.16), which are most likely to be related to a disturbance of the “cellular brain,” the gene regulation machinery. Also, cartilage and brain share an important similarity: both have “(very) old” largely postmitotic cells (i.e., basically no cell supply and proliferation after puberty) and, thus, show hardly any regeneration capacity. However, cartilage has an additional problem in that it lacks the plasticity of the neuronal network. As discussed earlier, a functionally intact chondro-cyte cannot adequately replace a dysfunctional chondrocyte located at a distance from it. Obviously, additional research will be needed to determine whether accelerated cell aging processes account for the phenotype of the disease or, as is the case in Alzheimer’s disease, there are additional features that would allow one to take new therapeutic approaches. Even if cell aging is an inevitable feature of OA (e.g., for limiting tumorigenic capacity), these processes can be used to identify and manipulate the causes of premature chondrocyte degeneration.

CHONDROCYTE BEHAVIOR

Coordinated processes

Functional Dysfunctional

Matrixturnover – repair

Cellular dysfunctionCell death/apoptosis

Un-coordinated

Fig. 173.16 Chondrocyte behavior. Articular chondrocytes in the normal joint behave in a very structured manner: they react to extracellular stimuli (e.g., joint loading, matrix changes, and exposure to cytokines and growth factors) according to their internal, predetermined program. In OA cartilage degeneration, chondrocytes are exposed to abnormal stimuli such as non-physiologic loading conditions, byproducts of matrix destruction (e.g., fibronectin and collagen fragments), and cytokines and growth factors that are not normally expressed in normal cartilage. This exposure leads to structured/deterministic cellular reactions, some of which are functionally positive for the tissue (e.g., anabolism), others of which are dysfunctional/detrimental (e.g., increased matrix catabolism and cell death). Potentially even more problematic for preserving tissue homeostasis are the unstructured/stochastic reaction patterns typically seen in OA chondrocytes, which lead to a significant microheterogeneity of cellular reaction patterns.

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formation as well as subchondral stiffening, which as such has the potential to enhance the progression of cartilage destruction.

One interesting notion that would fit the mechanical interrelation between cartilage and subchondral bone outlined earlier is the inverse correlation of osteoporotic bone changes and OA. Whereas this was supported by data of several initial studies, more recent work reported partly contradicting (supporting and rejecting) data. Therefore, future more extensive studies have to be done to further elucidate this phenomenon.

Continuous loading and mechanic stress?The most long-standing theory in the pathogenesis of primary OA involves the cumulative (detrimental) effects of continuous mechanical wear and tear on articular cartilage. Joints and, in particular, the articu-lar cartilage are always exposed to mechanical stress from loading, shearing, stretching, or hydrostatic pressure. This results in continu-ous microtrauma to the cartilage and repetitive damage to the cartilage extracellular matrix (see Chapter 6). Actually, pathologically increased mechanical stress has been linked to decreased matrix synthesis and the induction of proinflammatory genes. This might be explained by the fact that mechanical signals are directly transmitted to the chon-drocytes via mechanoreceptors (e.g., integrins) and thus transmitted to the intracellular compartment. Here they can trigger a variety of cel-lular responses by modulation of gene expression. One factor in the pathogenesis might be oxidants produced by chondrocytes in that process that causes oxidative damage accumulating over a lifetime. Thus, either cellular overstress or just continuous loading cycles might result in the loss of extracellular matrix integrity and function and in slowly progressing destruction of the tissue and the cells.

A more sophisticated explanation of the involvement of loading in the degeneration process is based on the fact that joints and joint geometry are remodeled over one’s lifetime and a redistribution of load might lead to increased stress in formerly unloaded and, therefore, atrophic cartilage areas. This age-related load redistribution could also explain why cartilage in the elderly is incapable of withstanding mechanical forces.

Neuromuscular function and proprioception—roles in joint homeostasisJoint stability is dependent on several neuromuscular factors, including strength and coordination of the joint-related muscles as well as the ability to sense the position and movement of the limb, the so called proprioception.35 The quadriceps femoris is one of the major muscles

OA synovial fluid. The fact that TNF-α is less abundant is in line with the observation that TNF-α can be found only in a limited number of OA cases. Also, members of the TGF-β superfamily are found in OA synovium. Synovial tissues from patients with OA express and secrete TGF-β, mainly TGF-β1. Expression of BMP-2 and BMP-4 was reduced in OA synovial tissue compared with controls. Vascular endothelial growth factor (VEGF) as well as basic fibroblast growth factor (bFGF) have been detected in OA synovium, and immunoreactivity increased with higher histologic inflammation grade.

Altogether, the synovial reaction is clearly of major importance to the symptoms of OA but also involved in its progression. The latter effect is presumably most of all mediated by the secretion of cartilage matrix degrading proteases as well as chondrocyte-modulating cata-bolic cytokines.

The subchondral boneAnother important tissue, which is often neglected in OA research, is the subchondral bone,34 which undergoes severe thickening (sclerosis), in particular in the subchondral bone plate (compare Fig. 173.4f with normal bone shown in Fig. 173.4c). Although it is not yet clear whether changes within this tissue precede changes in the articular cartilage (i.e., increased subchondral bone mass or stiffness as a risk factor for OA) or whether subchondral bone changes are secondary adaptation processes after changes in the biomechanical properties of the overlying articular cartilage. That both are closely related is suggested by the fact that the cartilage marker cartilage oligomeric matrix protein (COMP) and the bone marker bone sialoprotein (BSP) increased concomitantly in persons with early stages of what later developed into radiographic OA. Already in early stages this tissue compartment shows significant changes in terms of increased thickness of the subchondral bone plate as well as of adjacent bone trabeculae. In later stages, severe remodeling processes take place in particular in areas of advanced cartilage destruc-tion: apart from extensive bone sclerosis, significant aseptic bone necrosis is a common feature of late-stage OA joints. In areas of total cartilage destruction (i.e., the eburnated bone plate), synovial fluid gets access to the bone marrow and presumably leads to the bone cysts frequently seen in late stage disease. Growth factors from the synovial fluid are probably involved in inducing fibrocytic and even chondro-metaplastic changes, which lead to the “cartilage nodules” or “tufts” characteristic for late-stage disease. At least in moderate to advanced lesions, the changes in the subchondral bone represent one tissue responsible for the joint pain and, thus, are an interesting target tissue for symptomatic treatment in these patients. Also, modification of bone remodeling might be an interesting way to prevent osteophyte

INTERACTION ACTION BETWEEN SYNOVIUM AND CARTILAGE IN OA

Synovium

Cytokines

Growthfactors

e.g., IL-1, TNF

e.g., TGF betaBMP

TIMP

MMP, ADAMTS

Activation

Degradation products

Activeenzymes

Latentenzymes

Synovialactivation

Joint space Cartilage

Chondrocyte

Matrixdegradation

Early Late

Synthesisof matrix

molecules

Synthesisof matrix

molecules↑ ↓

Fig. 173.17 Interaction action between synovium and cartilage in OA. Molecular detritus from the cartilage activates the synovial lining cells. The synovial lining cells produce cytokines, growth factors, and (latent) enzymes. Synoviocyte-derived cytokines and growth factors further activate the chondrocytes. Enzymes produced by the synovial lining cells can directly degrade matrix molecules if not inactivated by inhibitors in the synovial fluid. Latent enzymes can be activated in the milieu of the OA cartilage. (Reprinted with permission from Aigner T, van der Kraan P, van den Berg W. Osteoarthritis and inflammation—inflammatory changes in osteoarthritic synoviopathy. In: Buckwalter JA, Lotz M, Stoltz JF, eds. Osteoarthritis, inflammation and degradation: a continuum. Amsterdam: IOS Press, 2007:219-230.)

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process. It is the molecular phenotype of the resident chondrocytes that determines the homeostasis of the cartilage matrix. The cellular reac-tion pattern of the chondrocytes in OA cartilage degeneration, however, is poorly understood, mainly because many of the involved genes are not yet identified and characterized. This exactly is one of the strengths of the gene expression chip technology.41,42 In fact, a lot of studies have been performed during the past decade using the array-chip technology and quite a few interesting genes and gene clusters were found42: this included in addition to known candidate gene groups such as anabolic and catabolic genes also new gene networks, such as a cluster of oxida-tive defense genes (e.g., superoxide dismutase-2 [SOD2]—the major mitochondrial ROS scavenger) and others. Further studies have to validate the relevance of these findings for understanding and manipu-lating these molecular networks in the context of OA.

EpigeneticsClearly, one major issue during disease progression is a severe altera-tion of the gene expression phenotype of the articular chondrocytes. Besides gene regulation by ordinary transcription factors, epigenetic gene regulation may play an important role in determining gene expres-sion levels, namely, methylation of genes coding for cytokines, growth factors, and so on.43 In fact, first experimental data indicate that dif-ferences in the methylation status within disease-relevant promoters are likely to induce/repress respective gene expression.44,45 This is, however, not true for all genes. Thus, for example, no changes in the methylation levels of the aggrecan gene in aged and diseased chondro-cytes were found.46 Also, no de novo methylation of the p21(WAF1/CIP1)-promoter-CpG island is involved in this process, although p21(WAF1/CIP1) is known to be regulated by methylation, for example, in oncogenesis.47 The overall genome-wide methylation level remains unchanged between normal and diseased and aged chondrocytes, although this does not exclude differences in methylation levels for selected promoter regions. Altogether data are sparse so far and epi-genetic disregulation in OA chondrocytes is clearly one potentially important new research topic for understanding the cellular (dis)behav-ior during the disease process.

CONCLUSIONThe most common and generally accepted theory of the pathogenic mechanisms of primary OA involves the cumulative effects of continu-ous mechanical wear and tear on articular cartilage. In the model of biochemical cartilage degeneration, the initiation and progression of primary OA is linked to time/age-related modifications of resident cartilage matrix components as well as age-dependent changes in the properties of newly synthesized and secreted matrix components, which together culminate in a structurally and functionally inferior cartilage matrix. In addition to the extracellular cartilage matrix, the chondrocytes are viewed as major contributors to disease, progression and premature aging of the chondrocytes appears to be important in the pathogenesis of OA. Unfortunately, the underlying causes of pre-mature aging are largely unknown at the moment and are therefore important areas for future research. Of interest, modern aging research points out that aging is not an inevitable event, at least not with respect to the period between 50 and 70 years of age, but rather an interesting target for therapeutic intervention. Thus, anti-aging strategies might well complement present therapeutic approaches related to anabolism, catabolism, apoptosis, and inflammation processes, all of which are known to be relevant in OA.

involved in providing knee joint stability. An association of weak quad-riceps and radiographic as well as symptomatic OA has been demon-strated, which most likely results from increased load being applied to articular cartilage in case of muscular weakening. Thus, muscle-strengthening seems to have a preventive effect for OA. Whether the knee joint also benefits from quadriceps strengthening after the onset of OA remains so far unclear. Another important factor for joint stabil-ity is the proprioception. It is based on specialized nerve endings known as mechanoreceptors, which are located in the muscles and the liga-ments and are essential for fine tuning of muscular movement. Pro-prioception declines with age, and a further decrease is seen in patients with OA. However, it is unclear whether impaired proprioception in OA contributes or results from the disease.

GENETICS, FUNCTIONAL GENOMICS, AND EPIGENETICSGeneticsNo doubt, OA, like nearly all other diseases, is initiated and progresses dependent on the genetic background of the individual. Therefore, the potential of genetics for elucidating the pathogenesis of OA is the subject of intensive investigation at the moment (see Chapter 174). Clearly, tools have been emerging rapidly in this area of research and the first hot candidate genes have been identified, such as frizzled related protein 336 and asporin.37 However, clear-cut pathogenetic con-cepts have not emerged for any of the suggested genes. Methods for dissecting the complex interplay between genes and environment are still to be developed and refined. One major difficulty is to separate genes that influence the development of the joints (thus leading, for example, to a mechanical weakness) from genes leading to an insuffi-ciency of the cells to maintain adequate repair and joint homeostasis later in life, which are finally relevant for preventing and treating OA. Another reason for the complexity of the interpretation of genetic data is that many of the genes detected are likely to be linked to other organ systems such as neuronal crosslinking, muscle strength, and mental perception. All these will have roles in OA development and disease manifestation without being related to cartilage physiology and pathobiology.

It has been known for some time that “OA runs in families,” but to what extent this is due to shared genetic influences or shared family environment is still uncertain. The disease is clearly multifactorial and polygenetic, that is, it results from the interaction of several, possibly many, genes. This fact, combined with the late onset of the disease, which makes linkage studies almost impossible, has made the task of identifying susceptibility genes very difficult. Classic twin studies have estimated the influence of genetic factors to be 39% to 65% for radio-graphic OA of the hand and knee, about 60% for OA of the hip, and up to 70% for OA of the spine.38 In contrast, the Farmington study, a multigenerational cohort study of hand OA, estimated heritability to be only 28% to 34%.39 Overall, the strongly varying results of these studies point to a considerable heterogeneity of the genetics of OA. Also, it seems that different combinations of different susceptibility genes may apply to different forms of OA.40

Functional genomicsOne major change in OA research in recent years has been the shift from investigating primarily biochemical aspects of articular cartilage matrix destruction to studying the molecular aspects of the disease

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REFERENCESFull references for this chapter can be found on www.expertconsult.com.

http://www.expertconsult.com

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