Osteoclast Formation

187

Miep H. Helfrich and Stuart H. Ralston (eds.), Bone Research Protocols, Methods in Molecular Biology, vol. 816,DOI 10.1007/978-1-61779-415-5_13, Springer Science+Business Media, LLC 2012

Chapter 13

RANKL-Mediated Osteoclast Formation from Murine RAW 264.7 cells

Patricia Collin-Osdoby and Philip Osdoby

Abstract

Extensive research efforts over the years have provided us with great insights into how bone-resorbing osteoclasts (OCs) develop and function and, based on such work, valuable antiresorptive therapies have been developed to help combat the excessive bone loss that occurs in numerous skeletal disorders. The RAW 264.7 murine cell line has proven to be an important tool for in vitro studies of OC formation and function, having particular advantages over the use of OCs generated from primary bone marrow cell populations or directly isolated from murine bones. These include their ready access and availability, simple culture for this pure macrophage/pre-OC population, sensitive and rapid development into highly bone-resorptive OCs expressing hallmark OC characteristics following their RANKL stimulation, abundance of RAW cell-derived OCs that can be generated to provide large amounts of study material, relative ease of transfection for genetic and regulatory manipulation, and close correlation in characteristics, gene expres-sion, signaling, and developmental or functional processes between RAW cell-derived OCs and OCs either directly isolated from murine bones or formed in vitro from primary bone marrow precursor cells. Here, we describe methods for the culture and RANKL-mediated differentiation of RAW cells into bone-resorptive OCs as well as procedures for their enrichment, characterization, and general use in diverse analytical assays.

Key words: Osteoclast , Osteoclast development , Bone resorption , RANKL , Mouse macrophage , RAW 264.7 cells

Osteoclasts (OCs) are cells uniquely responsible for dissolving the organic and inorganic components of bone during bone develop-ment and remodeling throughout life. They originate from hematopoietic precursors of the monocyte/macrophage lineage present in the bone marrow and peripheral circulation, and their numbers and/or activity frequently increase in a wide array of

1. Introduction

188 P. Collin-Osdoby and P. Osdoby

clinical disorders associated with excessive bone loss ( 1 ) . For many years, investigations into how OCs develop and function were hampered by considerable diffi culties associated with isolating and culturing these normally rare cells. Whereas cell lines have fre-quently provided an invaluable research tool and are widely used to decipher mechanisms involved in osteoblast (OB) differentiation and bone formation, no immortalized cell lines for mature OCs exist and the few pre-OC cell lines that were reported either did not undergo full OC differentiation ( 2, 3 ) or involved coculture systems and cells that were not readily available to all researchers ( 4 6 ) . To further compound the problem, it was diffi cult to reli-ably generate bone-resorptive OCs expressing mature OC charac-teristics from primary bone marrow or circulating precursor cells in vitro. This all changed with the breakthrough discovery of the key OC differentiation signal, receptor activator of nuclear factor B ligand (RANKL), that triggers the full development and activa-tion of OCs both in vitro and in vivo ( 7 9 ) . During OB develop-ment or in response to specifi c hormonal or local signals, RANKL becomes highly expressed on the surface of OB/stromal cells and interacts with a receptor, RANK, upregulated by macrophage colony-stimulating factor (M-CSF) on the surface of pre-OCs to stimulate their fusion, differentiation, and resorptive function. Many researchers now routinely form OCs in vitro through the exogenous addition of soluble recombinant RANKL (in combina-tion with M-CSF to stimulate pre-OC proliferation, survival, and RANK expression) to cultures of primary bone marrow cells or peripheral blood monocytes derived from various species (e.g., human, mouse, rat, rabbit, or chicken, as discussed in other chapters in this volume). However, such procedures still require the isola-tion of primary precursor populations, and in suffi cient numbers, to provide enough in vitro generated OCs for experimentation or characterization.

In addition to primary cells, at least one pre-OC cell line, murine macrophage RAW 264.7 cells, responds to RANKL stim-ulation in vitro to generate bone resorbing multinucleated OCs (RAW-OCs) with the hallmark characteristics expected for fully differentiated OCs ( 10 12 ) . RAW cells have been extensively employed in macrophage studies for >30 years and were origi-nally established from the ascites of a tumor induced in a male mouse by intraperitoneal injection of Abelson leukemia virus (although RAW cells do not secrete detectable virus particles) ( 13 ) . RAW cells express the c-fms receptor for M-CSF ( 14 ) as well as M-CSF, perhaps explaining why they also express high levels of RANK ( 10 ) and do not require M-CSF as a permissive factor in their RANKL-induced formation into RAW-OCs. RAW cells are often used in studies of OC differentiation and function, in parallel or as a prelude to studies with OCs formed from primary cells. There are many advantages of this system over the

18913 RANKL-Mediated Osteoclast Formation from Murine

generation of OCs from primary cell populations, including the following: (1) ready access (making it unnecessary to schedule experiments around when primary cells may become available) and widespread availability of this cell line to most researchers, (2) easy culture and homogeneous nature of the pre-OC popula-tion (devoid of OBs, stromal, lymphocytes, or other cell types), (3) sensitive and very rapid RANKL-mediated formation of RAW-OCs (within days), (4) very large number of RAW-OCs that can be generated (and, thus, RNA or protein for study), (5) high bone pit resorptive capability and expression of OC characteristics exhibited by RAW-OCs, (6) relative ease of transfection for genetic and regulatory manipulation, and (7) close correlation in characteristics, gene expression, signaling, and developmental or functional processes between RAW-OCs, OCs formed from primary precursor cells in vitro, and isolated in vivo formed OCs. In this chapter, we describe methods for the culture and RANKL-mediated differentiation of RAW cells into bone-resorptive RAW-OCs, the preparation of RAW-OC enriched populations by serum density gradient fractionation, and the culture and characteriza-tion of RAW-OCs. Such in vitro generated OCs can be analyzed using biochemical, immunological, physiological, molecular, functional, or other assays according to commonly employed procedures; see also various other chapters on osteoclasts in this volume.

All media and solutions are prepared with glass distilled water.

1. Culture medium: mix 90 ml of sterile Dulbeccos modifi ed Eagle medium (DMEM) supplemented with 4 mM L -glutamine, 1.5 g/l sodium bicarbonate, 4.5 g/l glucose, and 1.0 mM sodium pyruvate with 10 ml of fetal bovine serum (FBS, Invitrogen-Gibco) and 1 ml of a 100 stock of antibiotic/anti-mycotic (a/a, Invitrogen-Gibco); store at 4C and prewarm to 37C for use with cells.

2. Phosphate buffered saline, pH 7.2 (PBS). 3. RANKL (Enzo Life Sciences, PeproTech, EMD4Biosciences,

R&D Systems, or homemade): reconstitute and store as a con-centrated stock solution (typically 100 g/ml in PBS) in ali-quots (~1050 l) at 80C as recommended by the manufacturer, briefl y thaw and dilute into culture medium (to 35 ng/ml fi nal concentration for murine recombinant soluble RANKL) immediately before use with RAW cells, and refreeze remaining RANKL (and aim to thaw individual vials no more than three times to retain optimal bioactivity).

2. Materials

2.1. Tissue Culture Medium, Solutions, and Supplies


4. Mosconas high bicarbonate (MHB): add 8 g of NaCl, 0.2 g of KCl, 50 mg of NaH 2 PO 4 , 1.0 g of NaHCO 3 , 2 g of dextrose, 10 ml of a/a, and 990 ml of water; check pH is 7.2 and sterile-fi lter.

5. Hanks balanced salt solution (HBSS, Invitrogen-Gibco), pH 7.2.

6. Collagenase (Type 3): prepare a 3% stock (3 g in 100 ml) solu-tion in HBSS; store in aliquots (0.51.0 ml) at 20C.

7. Trypsin: 1% stock (1 g in 100 ml) solution in HBSS; store in aliquots (1.0 ml) at 20C.

8. Collagenasetrypsin digestion solution: briefl y thaw and add 71 l of 3% collagenase solution and 141 l of 1% trypsin solu-tion to 3 ml of MHB (per dish) immediately before use with cells.

9. Protease (EC 3.4.24.31, Sigma P-8811): 0.1% (100 mg in 100 ml) stock solution in PBS; store at 4C for up to several months or in aliquots (0.5 ml) at 20C for long-term storage.

10. EDTA: 2% (2 g in 100 ml) stock solution (using EDTA sodium salt) in PBS; store at 4C.

11. ProteaseEDTA digestion solution: briefl y thaw and add 50 l of 0.1% protease solution and 50 l of 2% EDTA solution to 5 ml of PBS (per dish) immediately before use with cells.

12. Supplies: sterile bottles, fl asks, and tissue culture dishes; rubber cell scrapers (Fisher); hemocytometer.

13. Devitalised bone or dentine slices, prepared as described (see Section 3.1 ). Ivory is obtained through donation from a local zoo or, in the USA, the Federal Department of Fish and Wildlife Services (or similar Department in other countries). Bovine cortical bone is obtained from a local slaughterhouse.

1. Segments of ivory and bovine cortical bone are thoroughly cleaned and washed (multiple HBSS and 70% ethanol rinses), sliced into small chunks and then reduced to rectangular 0.4-mm thick sheets using a low-speed Isomet saw (Buehler, Lake Bluff, IL).

2. The sheets are rinsed three times with 70% ethanol, incubated in 70% ethanol overnight, and then washed for several hours in HBSS before circular disks are cut using a 5-mm paper punch.

3. Methods

3.1. Preparation of Devitalized Bone or Dentine Slices


3. The disks are soaked repeatedly in 70% ethanol in sterile 50-ml tubes (alcohol changes can be gently poured off because the disks tend to stick to the side of the tube), and stored in 70% ethanol at 20C.

4. For experimental use, the required number of disks are removed from the tube using alcohol-presoaked tweezers (to maintain sterility) in a tissue culture hood, transferred to a new sterile 50-ml polypropylene tube, rinsed extensively by inversion and mild shaking at least three times with ~40 ml sterile HBSS per wash, and the disks transferred using sterile tweezers into culture wells or dishes containing sterile HBSS for 324 h of preincubation in a tissue culture incubator prior to the plating of cells. HBSS is removed only immediately before the disks are used so that they do not dry prior to RAW cell or RAW-OC seeding.

RAW 264.7 cells are obtained from the ATCC or similar cell repos-itory. They represent a murine macrophage cell line that has the capability to be grown indefi nitely as an OC precursor population or can be differentiated by treatment with RANKL into multinu-cleated bone resorptive OCs expressing the hallmark characteristics of in vivo formed OCs (see Subheading 3.3 ).

All work should be performed in a sterile hood using sterile solutions and supplies.

1. If starting from a frozen (liquid nitrogen) vial of RAW cells, quickly (


with additional medium as needed to yield 8 ml per 100-mm dish or 0.5 ml (or 1.0) per well of a 24-well dish, and then place the cells into a tissue culture incubator.

7. To grow the RAW cells for an extended period of time, refeed the cultures every 23 days and subculture when they reach confl uency as in steps 4 6 .

This method is based on the published procedure of Hsu et al. ( 10 ) .

1. Culture RAW cells to confl uency (see Subheading 3.2 , steps 1 3 ).

2. Subculture confl uent RAW cells into 24-well dishes as described in Subheading 3.1 , steps 4 6 (see Notes 3 6 ). If the cells are to be used for cytochemical or immunological staining, replate the RAW cell suspension into 24-well dishes that contain a sterile glass coverslip in each well. If bone resorption is to be evaluated in parallel with OC development in the RAW cell cultures, replate the RAW cell suspension into 24-well dishes that contain 24 small disks of bone or ivory (see Subheading 3.1 ) per well (with or without a glass coverslip under the disks).

3. Immediately add soluble recombinant RANKL to the dishes at a fi nal concentration of 35 ng/ml to initiate OC development (day 0) and increase the volume in the wells to 0.5 ml (or 1.0) with additional culture medium (see Note 7).

4. Culture to day 3. Briefl y examine the cells under a microscope for evidence that RAW cells are beginning to fuse into multi-nucleated RAW-OCs. Refeed the developing RAW-OC cell cultures with 0.5 ml (or 1.0) of fresh medium containing 35 ng/ml RANKL.

5. Culture until day 5 or 6 when many multinucleated RAW-OCs have formed but have not completely covered the dish (see Note 8). The day 5 or 6 RAW-OC populations may be imme-diately fi xed and used for cytochemical or immunological stain-ing, harvested for biochemical or molecular studies, or analyzed for bone resorption (see other Chapters in this volume). For greater bone resorption, such cultures may be incubated until days 79. Alternatively, RAW-OCs can be purifi ed further by serum gradient density fractionation (see Subheading 3.4 ).

Because not all RAW cells fuse into multinucleated RAW-OCs by day 5 or 6, those that have can be purifi ed from the remaining mononuclear cells using serum density gradient fractionation (see Note 9). This procedure is a modifi cation of the one we routinely use to purify in vitro formed OCs or OC-like cells from chick or human origin ( 15 ) . Directions are provided for RAW-OCs formed on 100-mm tissue culture dishes. All steps are conducted at room

3.3. RAW-OC Formation ( See Note 4 )

3.4. Serum Gradient Purifi cation of RAW-OC


temperature, unless otherwise noted, and are performed in a sterile hood using sterile solutions and supplies.

1. Remove the spent culture medium from two 100-mm dishes of day 5 or 6 RANKL-generated RAW-OCs.

2. Gently add 10 ml of MHB to each 100-mm dish to wash the cells. Remove and discard the washes.

3. Repeat step 2 to wash the RAW-OCs twice more with MHB. 4. Add 10 ml of MHB to each dish and place them into a tissue

culture incubator at 37C for 15 min. 5. Remove and discard the MHB solution from each dish. 6. Add 5 ml of freshly prepared collagenasetrypsin digestion

solution to each dish and incubate at 37C for 5 min. 7. Remove the dishes from the incubator and shake the plates

gently by hand back and forth (e.g., slide the dish on a fl at surface) for ~30 s to detach and loosen the interaction of cells with extracellular matrix produced by the cells.

8. Completely remove the collagenasetrypsin solution contain-ing the released matrix material from each dish and discard (see Note 10).

9. Gently wash the adherent cells on each dish by releasing 10 ml of PBS slowly against the side wall of the dish. Completely remove and discard the washes.

10. Repeat step 9 to wash the cells on each dish with PBS twice more.

11. Add 5 ml of proteaseEDTA digestion solution to each dish. Incubate at 37C for 1015 min (see Note 11).

12. Loosen the adherent cells on each dish by fl ushing the proteaseEDTA incubation solution with a pipette gently over the sur-face of the cell layer to free the cells (see Note 12).

13. Transfer the cell suspensions from two 100-mm dishes into one 50-ml sterile centrifuge tube containing 1.0 ml FBS (to inhibit further protease action).

14. Centrifuge the cells at 100 g for 5 min. 15. Remove and discard the supernatant. Gently resuspend the cell

pellet in 15 ml of MHB by repeatedly drawing up and releasing from a pipette (not too vigorously, see Note 12).

16. Prepare 16 ml of 70% FBS in MHB (11.2 ml FBS plus 4.8 ml of MHB) in a 50-ml centrifuge tube, and 16 ml of 40% FBS in MHB (6.4 ml FBS plus 9.6 ml MHB) in another 50-ml tube.

17. Prepare an FBS gradient in a 50-ml round-bottom centrifuge tube. To do this, carefully dispense 15 ml of the 70% FBS-MHB solution (from step 16) into the bottom of the tube. Very slowly overlay this with 15 ml of the 40% FBS-MHB solution (from


step 16), using a pipette held at a 45 angle against the side of the tube just above the 70% FBS layer and slowly releasing the 40% FBS solution in a thin stream so as not to deform the sur-face of the 70% FBS layer.

18. Let the tube stand undisturbed for 30 min (at room tempera-ture) to allow the larger multinucleated RAW-OCs to settle under normal gravity and penetrate the FBS layers (see Note 13).

19. Carefully take off the top 17 ml which contains mononuclear cells, and transfer it into a 50-ml tube.

20. Then, remove the 16 ml middle layer, which contains primarily mononuclear cells and some small multinucleated RAW-OCs, and transfer this into another 50-ml tube.

21. The bottom 12 ml fraction contains predominantly large multinucleated RAW-OCs.

22. Centrifuge the purifi ed RAW-OC bottom fraction (and the other fractions if they are also to be cultured and/or analyzed) at 100 g for 5 min.

23. Gently resuspend the RAW-OC pellet in culture medium, count an aliquot in a hemocytometer, and plate 1,0004,000 cells per well of a 24-well dish. Typically, the purifi ed RAW-OCs from two 100-mm dishes can be cultured in 210 wells of a 24-well dish (with 0.51.0 ml medium per well) for 624 h (see Note 14). The top and middle fractions from the serum gradient fractionation are typically cultured in 2040 wells and 1530 wells of a 24-well dish, respectively. Alternatively, RAW-OCs (and the top and middle fractions, if desired) may be used immediately for analysis (see Subheading 3.2 , step 5).

Serum gradient fractionation routinely provides 4,00010,000 purifi ed RAW-OCs from one 100-mm dish (this depends on the effi ciency of ones technique and, more importantly, on the exact stage of RAW-OC used to purify the cells; see Notes 6, 8, 13, and 14). In unfractionated RANKL-generated RAW-OC cultures, multinucleated (more than three nuclei) RAW-OCs typically rep-resent ~1% on a per cell basis and 25% on a per nuclear basis of the total cell population (Fig. 1 , left panel). By contrast, serum gradi-ent purifi ed RAW-OCs (with more than three nuclei) typically comprise 6090% on a per cell basis and 96% on a per nuclear basis of the total cell population in the bottom serum fraction (Fig. 1 , lower right panel). On average, RAW-OCs in this bottom serum fraction contain 1530 nuclei per cell.

Standard protocols can be used to evaluate the morphological (light, scanning electron microscopy), ultrastructural (transmission elec-tron microscopy), histochemical (general or enzymatic activity stains), or immunocytochemical staining (e.g., for OC developmen-tal markers) characteristics of RAW cells representing pre-OCs and

3.5. Phenotypic and Functional Characterization of RAW-OCs


in vitro RANKL-formed RAW-OCs (see Chapter 9 , this volume ). Whereas untreated RAW cells do not stain for tartrate resistant acid phosphatase (TRAP) activity, a key marker and enzyme involved in OC bone resorption, RANKL-differentiated RAW cell cultures develop both TRAP+ mononuclear and multinucleated cells (Fig. 2a, c ). The RAW-OCs formed by cell fusion contain multiple nuclei clustered together, and the cells may appear either spread out or partially elongated when cultured on plastic (Fig. 2a ). RAW-OCs cultured on bone or ivory (either during RANKL development or following replating of the differentiated cells) frequently display a more compact and highly motile elongated shape with numerous pseudopodial extensions (Fig. 2c ). Resorption pits formed by RAW-OCs are typifi ed by clusters of multilobulated excavation cavities or long resorption tracks (which may also be multilobulated) adjacent to or underlying RAW-OCs actively engaged in bone resorption (Fig. 2c ). Molecular, immunological, and/or biochemical analyses have shown that RAW-OCs express the key hallmark properties of OCs including TRAP, calcitonin receptor, cathepsin K, matrix

Fig. 1. RANKL-mediated RAW-OC formation and serum gradient purifi cation. ( Left ) RAW cells were cultured with 35 ng/ml murine recombinant RANKL for 6 days and then subjected to serum gradient fractionation. A well cultured in parallel was fi xed and stained for TRAP activity to show the proportion of mononuclear and multinucleated TRAP+ cells that arise by day 6 in RANKL-differentiated RAW cell cultures. The cells were viewed using a light microscope and images were captured with a computer-linked digital camera. (Reduced from original magnifi cation, 100.) ( Right ) The top , middle , and bottom fractions from the serum gradient fractionation were replated and cultured on plastic for several hours, after which the cells were fi xed and stained for TRAP activity. ( Upper right ) The top fraction consists entirely of mononuclear cells, some of which are TRAP+ (in contrast to untreated RAW cells which are all TRAP-, not shown). (Reduced from original magnifi ca-tion, 200.) ( Middle right ) The middle fraction primarily contains mononuclear cells, a portion of which are TRAP+, and some small multinucleated RAW-OC. (Reduced from original magnifi cation, 200.) ( Lower right ) The bottom fraction con-sists primarily of large multinucleated RAW-OC, although a few mononuclear cells may still be present. (Reduced from original magnifi cation, 100).


metalloproteinase-9, integrin v 3, and c-src (refs. 10, 16 , our unpublished data ). Both the phenotypic and functional characteris-tics of RANKL-differentiated RAW-OCs resemble those of in vivo formed isolated murine OCs or RANKL-differentiated OCs (MA-OC) formed from murine bone marrow cells in the presence of M-CSF (Fig. 2b, d ). Thus, like RAW-OCs, TRAP+ MA-OCs exhibit a well-spread morphology on plastic (Fig. 2b ) and a more compact, motile phenotype on bone or ivory (Fig. 2d ). Multilobulated resorp-tion pits and tracks formed by MA-OCs (Fig. 2c ) also resemble those formed by RAW-OCs, with well defi ned margins and deep resorption lacunae (Fig. 2d ). Resorption pit formation by RAW-

Fig. 2. RANKL-mediated RAW-OC or MA-OC formation and bone pit resorption. ( a , c ) RAW cells were cultured with 35 ng/ml murine recombinant RANKL for 6 days on plastic ( a ) or ivory ( c ), and then fi xed and stained for TRAP activity. Note the well spread morphology of RAW-OCs on plastic ( a ) compared with the more compact and motile phenotype of such cells actively engaged in bone resorption on ivory ( c ). Abundant resorption pits and tracks were evident that were frequently composed of connecting excavation cavities. These represent periods of RAW-OC attachment and pit formation, followed by RAW-OC movement to an adjacent area of ivory for further resorption. ( a ) and ( b ) reduced from original magnifi cation, 200). ( b , d ) Murine bone marrow cells were isolated and cultured at 5.6 10 5 cells per well of a 24-well dish (1.9 cm 2 per well) with 25 ng/ml of murine M-CSF and 35 ng/ml of murine RANKL for 6 days on plastic ( c ) or ivory ( d ), after which the cells (MA-OCs) were fi xed and stained for TRAP activity. Like RAW-OCs, the TRAP+ MA-OCs were well spread on plastic ( b ) and more compact on ivory ( d ). Resorption pits and tracks formed by MA-OCs ( d ) were indistinguishable from those formed by RAW-OCs ( b ). ( b ) and ( d ) reduced from original magnifi cation, 100 and 200, respectively).


OCs, in the presence or absence of modulators, can be quantifi ed as for other OCs (see Chapters 8 12 , this volume). In addition to these phenotypic and functional analyses, RAW-OCs provide abun-dant material (protein, RNA, etc.) for investigations of gene or pro-tein expression or microarray profi ling, receptors and signal transduction pathways, production of various factors (cytokines, chemokines, growth factors, and arachidonic acid metabolites), release of other substances (free radicals and enzymatic activities), cell and matrix interactions, and diverse regulatory mechanisms (particularly since RAW cells are more easily transfected than pri-mary bone marrow cells) (see Note 15).

1. Some DMEM formulations may produce a visible dark precipi-tate that causes rapid cell death of RAW cells during culture. In such cases, we fi nd it best to obtain new DMEM lacking Fe(NO 3 ) 3 along with a separate stock (1,000) of Fe(NO 3 ) 3 that is stored in aliquots (1 ml) at 20 C. This iron stock is only thawed and added to DMEM at the time that complete medium is prepared (containing FBS and a/a) for current use. Typically, this complete medium is usable for 10 days to 2 weeks for cell refeeding before evidence of precipitation occurs, at which time any remaining medium should be discarded. Some inves-tigators have reported that RAW cells also grow and form OCs well in -MEM medium, so this may be an alternative to DMEM if problems are encountered with the latter.

2. We fi nd that a rubber-tipped cell scraper works best because it completely contacts the surface of the tissue culture fl ask (or dish) and causes the least cell damage.

3. In general, RAW cells should be subcultured at a ratio of 1:31:6.

4. If more cells will be needed than are provided by a reasonable number of T25 fl asks, the confl uent RAW cells can be subcul-tured into T75 fl asks (at a 1:31:6 ratio) and then grown to confl uency.

5. The number of RAW cell passages affects RANKL-mediated OC formation. In our hands, RAW-OCs seem to better form after passage 4 and will no longer form in response to RANKL stimulation once they have undergone 1820 passages from the time that they were received from the ATCC repository. The reason for this is not fully clear, although other researchers have similarly noted that not all RAW 264.7 cell lines (or passages?) will form OCs after RANKL treatment, and subclones of RAW 264.7 cells can be derived that are more or less effi cient at

4. Notes


RANKL-mediated OC formation ( 17, 18 ) . It is also possible that particular lots of FBS may differentially infl uence RANKL-mediated RAW-OC formation. Therefore, different lots and sources of FBS may be tested if there are diffi culties encoun-tered in trying to form RAW-OCs, although RAW-OCs have been formed even in serum-deprived conditions ( 19 ) .

6. The density of RAW cells plated affects the rate and yield of RAW-OC development, as well as the subsequent analysis of RAW-OCs formed. Too low a cell density (100500 cells/cm 2 ) delays RAW-OC formation and decreases the fi nal yield. For most purposes (e.g., testing the effects of various agents on RAW-OC development), the plating density for RAW cells should be in the range of 10 3 3 10 4 /cm 2 to facilitate count-ing or characterization of the RAW-OCs formed and still gen-erate a suffi cient number for analysis. If RAW-OCs are to be purifi ed by serum density gradient fractionation, the initial plating density of RAW cells should be considerably higher (1.5 10 5 /cm 2 ) so that enough RAW-OCs are obtained fol-lowing their purifi cation (see Subheadings 3.1 and 3.2 ). However, too high a density of RAW cells (4.57.5 10 5 /cm 2 ) inhibits RAW-OC formation.

7. In our experience, the potency of recombinant soluble RANKL for inducing OC formation is dependent upon its source (not only for RAW cells, but also for human monocyte, mouse bone marrow, or chicken bone marrow preparations). Thus, differ-ent commercial RANKL preparations vary signifi cantly in the dose required, kinetics of OC formation, and fi nal yield of bone pit resorptive OCs obtained. This is not strictly due to species compatibility issues because human or murine recombinant RANKL are similarly effi cient for inducing OCs from murine RAW or bone marrow-derived cells, chicken bone marrow cells, or human peripheral blood monocytes (although all but RAW cells require M-CSF costimulation). Although we have successfully used various commercial RANKL preparations, we typically prepare soluble recombinant mouse RANKL in our own lab that exhibits high osteoclastogenic activity with murine RAW or bone marrow cells, chicken bone marrow cells, or human monocytes. At 35 ng/ml, this mouse RANKL induces multinucleated TRAP+ cell formation that is fi rst apparent on days 34 of RAW cell culture. Lower RANKL concentrations delay the kinetics and fi nal yield (and size) of RAW-OC forma-tion. Others have used recombinant RANKL preparations in the range of 20100 ng/ml (depending upon its source and bioactivity) to form multinucleated TRAP+ cells that usu-ally fi rst appear on days 34 of RAW cell culture ( 11, 18, 20, 21 ) . However, certain recombinant mouse RANKL prepara-tions appear to require an additional anti-RANKL antibody


cross-linking step to induce osteoclastogenesis ( 16 ) . Therefore, it is recommended that pilot studies be performed with each new source and preparation of recombinant RANKL to ascertain an appropriate dose to achieve the level of RAW-OC formation needed (see also the discussion in Chapter 7 , this volume).

8. In our model of RANKL-mediated RAW-OC development, TRAP+ cells fi rst become apparent on day 2 of culture, and multinucleated TRAP+ cells appear on days 34 and nearly reach a peak on days 56 of culture. It is important to either use the cells or subject them to serum gradient purifi cation at this point (and not wait another day until the full peak of RAW-OC formation has occurred) because if the cells become overconfl uent and overfused, they die very rapidly (


that few viable RAW-OCs will be recovered following the serum gradient purifi cation.

14. Even under controlled conditions of RANKL-mediated RAW-OC formation as discussed in this chapter, once such cells have formed they tend to apoptose very rapidly and the cells can be lost within 24 h if allowed to develop too long (see also discussion in Chapter 8 , this volume). Short survival after formation in culture is specifi cally a problem with mouse osteo-clasts and not seen with for example human osteoclasts. Addition of 10 ng/ml IL-1 to promote RAW-OC survival on plastic only slows the apoptotic process slightly, and a recent report indicates that it preferentially activates larger over smaller RAW-OCs ( 22 ) . Therefore, we typically use RAW-OCs formed in tissue culture dishes by days 56 for analysis within 624 h (e.g., staining, RNA or protein extraction, etc.). If RAW-OCs have been formed on bone or ivory, resorption pits are usually evident by day 4 and maximal by day 6 or 7; modu-lators can be added at appropriate times to observe stimulatory or inhibitory effects on resorption. When RAW-OCs are puri-fi ed by serum gradient fractionation and replated onto tissue culture dishes, their viability is usually extended for an addi-tional 24 h. Alternatively, purifi ed RAW-OCs can be replated onto bone or ivory (~800 cells per well of a 48-well dish con-taining one piece of ivory or bone) and cultured with 35 ng/ml RANKL and 10 ng/ml IL-1 , in the presence or absence of other modulators, for 56 days to ascertain effects primarily on preformed RAW-OCs (although some additional RAW-OC development also occurs during this time period since the 70% serum purifi ed fraction still contains some mononuclear cells). Purifi ed RAW-OCs typically do not exhibit pit formation within the fi rst 24 h after replating onto bone or ivory.

15. Although many of the phenotypic and functional characteris-tics of RAW-OCs match those of RANKL-differentiated pri-mary murine bone marrow-derived OCs or isolated in vivo formed murine OCs, this cannot automatically be assumed to be true for any particular property being evaluated. The most obvious difference is the requirement for M-CSF in RANKL-stimulated OC formation from bone marrow cells (which have relatively low RANK prior to M-CSF exposure) but not for transformed RAW cells (which already make M-CSF and express high RANK levels). In addition, apoptosis/survival pathways (including ERK) may differ between primary bone marrow cells and transformed RAW cells, and various other differences have been noted. Therefore, it is important to consider that the specifi c attribute under study in the RAW-OC cell system may not necessarily refl ect that of normal murine OC formation or function. However, because RAW cells are


easier to obtain and culture than primary bone marrow cells, represent a pure population of pre-OCs (defi cient in osteo-blasts, stromal cells, lymphocytes, etc.), are more readily trans-fected, and provide abundant material for study, they provide a highly valuable resource for rapidly and effi ciently screening and determining mechanisms underlying OC-related processes. Therefore, we recommend that RAW cell studies are subse-quently followed by at least a limited number of experiments using primary murine OCs (directly isolated and/or RANKL-generated in vitro) to confi rm that these processes are likewise observed in normal murine OCs and are not unique to trans-formed RAW cells or RAW-OCs.

Acknowledgments

We are greatly indebted to the Drs. Xuefeng Yu and Hong Zheng for their advice and many valuable contributions to an earlier version of this chapter. This work was supported by NIH Grants AR32927, AG15435, and AR32087 to P.O.

References

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Chapter 13: RANKL-Mediated Osteoclast Formation from Murine RAW 264.7 cells1. Introduction2. Materials2.1. Tissue Culture Medium, Solutions, and Supplies

3. Methods3.1. Preparation of Devitalized Bone or Dentine Slices3.2. RAW 264.7 Cell Culture3.3. RAW-OC Formation ( See Note 4)3.4. Serum Gradient Purification of RAW-OC3.5. Phenotypic and Functional Characterization of RAW-OCs

4. NotesReferences

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Osteoclast Formation

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