Sensory Neuropathies

755 Brain in Health and Disease

Sean Sweeney

Reading Material for the Lysosomal Storage Disease lecture:

Haltia, M. (2006) The Neuronal ceroid-lipofuscinoses: from pastto present. Biochim. Biophys. Acta - Molecular Basis of DiseaseVol. 1762 p850-856

Jeyakumar et al., (2005) Storage Solutions: Treating LysosomalDisorders of the Brain. Nature Reviews Neuroscience. 6. 1-12

Beutler, E. (2006) Lysosomal Storage Diseases: Natural Historyand Ethical and Economic Aspects. Molecular Genetics andMetabolism. 88, 208-215

Pain:Sensation of pain: nociception (latin: ‘nocere’ - to hurt)

Role: to alert to impending injury or to trigger appropriateprotective response

Transduction of noxious stimuli (thermoceptive pain, mechanoreceptive pain) BUT ALSO cognitive and emotional processing

Sensory modality, in PNS, CNS and ANS

A class of neuron (global?) proposed by Sherrington, activated by stimuli capable of causing tissue damage.(Sherrington, C.S. The Integrative Action of the NervousSystem (Scribner, New York, 1906)

Nociception mediated by diverse sensory neurons in periphery.(innervating the skin): majority of this lecture.

Others Tissues:Corneal afferents: sensitive to capsaicin and inflammatory mediators BUT, mostly pain produced in response to smallstimuli.

Teeth: any stimuli produces pain.

Visceral Pain: poorly localised, deep and dull. Tissue damagenot required, other stimuli (distension)

Nociception is a strong stimulus with resultant strong responsesRequires modulation (sensitisation) at cellular, neuronal and circuit level.(Psychosomatic (!?))

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Nociceptive neurons arise in the Dorsal Root Ganglion (DRG)Noxious stimuli transduced into neuronal activity by molecular triggers responsive to various

stimuli

1st response, reflex withdrawal, followed by higher order behavioural responses.

MDEG = BNC1Na+ channel (TTX insensitive)aka ‘acid sensingion channel

TREK1 = two poreK+ channel

endogenous vanilloids

Examining structure of natural and synthetic receptor agonists illustrates structuralsimilarities.

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The ‘inflammatory soup’: components released in responseto damage or inflammationpotentiate or maintain the initial nociceptive signal.

Protons, ATP, neurotransmittersalter neuronal excitability directly

Bradykinin, NGF bind tometabotropic receptors (longer signal!)

Local tissue acidosis: hallmark physiologicalresponse to injury. Pain correlated to degreeof acidosis.

Nociceptive sensory neurons respond to stimuli with subcellular modulation:threshold for stimulation is reduced. (hyperalgesia, peripheral sensitisation)Modification of TRPV1 (and others) results in lowered threshold activation.

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Nociceptive sensory dendrite terminal

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Damage to nociceptor neurons increases in transcription or trafficking of Na+ channelswith a reduction in K+ channels results in ‘spontaneous’ or ‘ectopic’ activation.

Long term activity of nociceptor neurons results in longer term changes in activitymediated by transcription (mediated also by cytokines) : leads to long term changes inactivity (positive and negative).

The circuitry:

primary sensory neuronsin the DRG project dendritesto peripheral tissue.

Major types: C Fibres (SLOW!!)A∂ Fibres (FAST!!)

Morphological and physiologicaldifferences

Diameter: linked to speed of conduction

A∂Fibres: ca. 20m/s

C Fibres: ca. 2m/s

(ALL relatively slow, but C much slower than A∂)

A∂: two classes, mechanosensitive and mechanothermal

C: polymodal

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Sensory integration in the dorsal hornis subject to activity dependentcentral sensitisation (a)

and longer term transcription dependentcentral sensitisation (b).

Cox2 induction acts to reduce inhibitoryinput

DREAM= inhibitory transcription factor

The Ascending Pain Pathway

Secondary neurons and processing by higher order relays of neurons: modulation can occur at higher levelsof communication between second order neurons or feed down through descending inhibitory pathwaysto affect local circuits in made by the primary neurons.

Descending system alters responses of reflex circuits.

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Modulation of nociceptive response at 1st point of sensory integration (in dorsal horn) : Melzack and Wall’s Gate Theory of Pain.

Pain ameliorated by mechanosensitive input. Highlights synaptic interactions in dorsal horn.

Descending inputs

Natural opoid peptides present in periaqueductal gray matter and spinal cord regions involved in modulation of pain:enkephalins, endorphins, dynorphins

Enkephalin receptors on presynapticsite of nociceptive neuron respondsto enkephalin to inhibit releaseof Glutamate and substance P.

(enkephalin projections also controledby descending projections)

Many points of control have evolved:Complexity offers many alternative strategies and targets for therapeutic intervention:

Transduction: TRPV1, TRPV2, TRPV3, TRPM8, ASIC, DRASIC, MDEG, TREK-1BK1, BK2, P2X3

Peripheral Sensitisation: NGF, TrkA, TRPV1, Nav1.8, PKA, PKC isoforms, CaMK IVErk1/2, p38, JNK, IL-1ß, cPLA2, COX2, EP1, EP3, EP4, TNF-alpha

Membrane excitibility of primary afferents: Nav1.8, Nav1.9, K+ channel

Synaptic Transmission:Presynaptic: VGCC, adenosine-R, (mGluR)Postsynaptic: AMPA/kainate-R, NMDA-R, mGlu-R, NK1, Nav1.3, K+ channels

Central Inhibition: GABA, GABAA-R, GABAB-R, Glycine-R, NE, 5HT, opoid receptorsCB1

Signal Transduction: PKA, PKC isoforms, ERK, p38, JNKGene Expression: c-fos, c-jun, CREB, DREAM

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Pain is a strong stimulus.

Many points of control have evolved, therefore many points of control can become defective.

Gives rise to the generation of non-nociceptor mediated pain: For example……

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Sprouting after peripheral nerve injury

Disinhibition (after excitotoxic shock?)

Disease related Peripheral Pain:

Definitions and classificationNeuralgia: Pain in the distribution of a nerve or nerves.

The term should be used primarily to refer to non paroxysmal pains.

Neuropathy: A disturbance of function or a pathologic change in the nerves.

mononeuropathy: involving a single nerve

mononeuropathy multiplex: involving several nerves

Polyneuropathy: involving symmetric or bilateral nerves

neuritis: special type of neuropathy with an inflammatory process.

allodynia: condition where normally non-painful stimuli become painful

Classification:

can be by cause: (diabetic, entrapment)by anatomic site (intercostal neuralgia)

Etiology based classification of peripheral neuropathies:

Focal or multifocal lesions of PNSEntrapment syndromes, Phantom limb, stump pain, Post-traumatic neuralgiaPostherpetic neuralgia (shingles), Diabetic mononeuropathy, Ischemic neuropathyPolyarteritis nodosa

Generalised Lesions of the PNS (polyneuropathies)Diabetes mellitus, Alcohol, Plasmocytoma, HIV neuropathy, HSNs, Fabry’s diseaseLeukodystrophies, vitamin B deficiency, Bannworth’s syndrome(neuroborreliosis)Toxic neuropathies: arsenic, thallium, chloramphenicol, vinca alkaloids, gold,isoniazid, nitrofurantoin (antibiotic), metronidazole (anti-protist)

Lesions of the CNSspinal cord injury, brain infarction (esp. thalamus and brainstem), spinal infarctionsyringomyelia, MS

Complex neuropathic disordersComplex regional pain syndromes type I and II (reflex sympathetic dystrophy,causalgia).

Demyelinating neuropathies.

Charcot-Marie-Tooth Disease

(aka peroneal muscular atrophy and hereditary motor sensory neuropathy)

Most commonly inherited neurological disorder: estimated 2.6million affectedworldwide.

Discovered 1886: Jean-Martin Charcot, Pierre Marie and Howard Henry Tooth

Sensory neuropathy with progressive loss of use of feet and hands:Nerves to extremities degenerate with muscle weakness (and degeneration)through loss stimulation. ‘Hammer toes’, ‘foot drop’ (loss of tendon/muscle tensionbalance).

Diagnosed with electrophysiology (conduction velocity tests) followed by genetic testing

Bilateral and length dependent.

No current cure (therapy, physical and occupational)

Does not affect life expectancy

No ethnic association (except CMT4 - Spanish gypsies)

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CMT: 18 types can be identified by genetic testing

Two main classifications:CMT type 1 - demyelinating diseaseCMT type 2 - diminished responses in sensory neurons (six subtypes)

Demyelinating CMT (types 1 and 4)

Disease Locus Gene Function

CMT1A duplication of PMP22 myelin protein 22HNPP loss of PMP22 “CMT1B dominant MPZ myelin protein zeroCMT1C dominant LITAF LPS-induced TNF-alpha factorCMT1D dominant EGR2 Early Growth Response 2 (with sox10!)CMTX1 dominant GJB1 connexin 32

CMT4A recessive GDAP1 ganglioside induced diff. ass. proteinCMT4B1 recessive MTMR2 myotubularin related protein 2 (phosphatase)CMT4B2 recessive SBF2 myotubularin related protein 13CMT4C recessive SH3TC2 SH3 and TPR adaptor moleculeCMT4D recessive NDRG1 N-myc downstream related geneCMT4F recessive PRX Periaxin (acts with dystroglycan/dystrophin)CMT4 recessive EGR2 Early Growth Response 2

Presence of myelin sheath critical for nerve conduction velocity. Loss of myelin sheath results in reduced andless efficient transmission of action potentialsAlso: loss of myelin can result in spontaneous production of action potentials

A∂ fibres aremyelinated

Axonal Forms of CMT

Disease Locus Protein

CMT1F NFL Neurofilament light chain proteinCMT2A mfn2A mitofusin 2ACMT2B rab7 endosomal trafficking protein Rab7CMT2C NFL Neurofilament light chain proteinCMT2D GARS Glycyl tRNA synthetaseCMT2E NFL NFLCMT2F HSPB heat shock protein 27 (allelic to dHMN)CMT2I ?CMT2J ?CMT2K GDAP1 ganglioside induced diff. ass. protein

DI-CMT B DNM2 dynamin related protein 2DI-CMT CYARS tyrosine tRNA synthetase

Theme emerges: Demyelinating - loss of myelinAxonal and demyelinating - axonal traffic and mitochondrialfission/fusion (would fit with ‘length dependency’)

Enlarged mitochondria seen in many forms of neuropathy!

CMT disease Genes CMT pathology diagnostic criteria

PMP22, MPZGJB1, EGR2MTMR2/13

Schwann cellsmyelinate poorly

Reduced NCV definesCMT1 and CMT4

Schwann cellsfail to supportaxons

MFN2, Rab7aNEFL, DMN2

Axonal transportdefects

Progressive axonalloss

Muscle denervationSensory losses

Normal NCV andreduced currentamplitudesdefine CMT2

Final common pathway

Hereditary, Sensory and Autonomic Neuropathy type 1 (HSAN1)

synonyms:Hereditary sensory neuropathy type 1 (HSN1)Charcot-Marie Tooth type 2B syndrome (HMSN 2B)Hereditory sensory radicular neuropathyThevenard syndromeFamilial trophoneurosisMal perforant du piedFamilial syringomyelia

Slowly progressive characterised by distal sensory loss, occasional lancinating pain, autonomic disturbance (often seen in sweating), juvenile or adult onset. Variable motor involvement, slow healing wounds, chronicskin ulcers. Often results in amputation (of legs).

autosomal dominant inheritance, (types II to V are recessive)

clinically and genetically heterogenous: Found in occasional families (England, founder populations in Canada and Austrailia, also China)

Sural nerve biopsy reveals demyelination

(Auer-Grumbach (2008) Orphanet Journal of Rare Diseases, 3:7

Hereditary, Sensory and Autonomic Neuropathy type 1 (HSAN1) Contd.

Mutations mapped to SPTLC1 gene. (Dawkins et al., (2001) Nat. Genet. 27; 309-312

SPTLC1 a subunit of the Serine palmitoyl-transferase (SPT)enzyme (spt1 and spt2 comprise the heterodimer).

SPT: a pyridoxal-5’-phosphate dependent enzyme, 1st step in the de novo synthesis ofsphingolipids

Dominant mutation suggests C133W/Ycreates a dominant negative (?)

Structure from sphingomonaspaucimobilis (a homodimer) wouldsuggest C133W/Y would be dominantly inactivating

Why would a loss of sphingolipidsynthesis particularly affect the sensorysystem in a length dependent manner?

Loss of spt2 (second subunit of SPT) in Drosophila results in enlarged mitochondria inneuronal tissue

Mitochondria in neurons are essential for ATP production, but also for Ca2+ homeostasis and apoptosis

Mitochondrial dysfunction predominant in many neuropathic and neurodegenerativeconditions

e.g. Hereditary spastic paraplegia (paraplegin, HSP60), amyotrophic lateral sclerosis (SOD1)Familial Parkinson’s disease (PINK1, parkin), Friedreich’s Ataxia (Frataxin),mitochondrial encephalomyopthies.

Mitochondrial fission/fusion related genes prevalent in neuropathies:

GDAP1mitofusin2dynamin related protein 2

Enlarged mitochondria present in

HSAN1Diabetic Sensory Neuropathy

How important is mitochondrial fission/fusion to sensory dendritic function?

Mitochondria:

Mitochondria highly dynamic and undergo continual fusion and fissionThis controls overall morphology AND proper function (allows turnover via autophagy (rab7?)).

opa-1: dominant mutation inDominant Optic Atrophy(a sensory structure)

FusionMfns mediate tethering ofpre-fusogenic mitochondria(Mfns are GTPases)

OPA1 on inner membrane

FissionFis1 cover outer membraneDrp coalesces in spotsof constrictionGDAP1 promotes fission(on outer membrane)Ceramide?

Do fusion/fission defects affect respiratory capacity? (usually only seen with complete arrestof fusion)

Do fusion/fission defects alter calciumhomeostasis?

Do fusion/fission defects alter mitochondrialtransport along axons? (mitochondrialaggregation?)

Do fusion/fission defects alter responses to apoptosis?

a) In normal cells mitochondria transported todendritic extensions, soma, hillock, nodes of Ranvierand synaptic regions.• In CMT, DOA, HSAN1 heterogeneity of mitochondrial population - mitochondria with poor function• Mitochondrial aggregation - ineffective mitochondriadistribution, inneffective autophagic turnover? Chen and Chan (2006) 18, 453-459

Current Opinion in Cell biologyRequirement for more mitochondria in absence of myelin?

Mitochondrial fission and fusion and neurological function

Summary

Pain is a powerful stimuli and perception is regulated at many levels, molecular, cellular, synaptic and systems.

The complexity of pain perception is reflected in the variety of diseases where pain is a prominent outcome(neuropathies, neuralgias, neuritis etc).

The genetic condition Charcot-Marie Tooth disease resultsfrom the loss of myelin in peripheral nerves and alsoloss of mitochondrial fission/fusion dynamics (other conditionsmay share this etiology)

Mitochondrial fission/fusion is essential to neuronal functionthough the mechanism remains unlcear

Reading Material:

Julius, D. and Basbaum, A.I. (2001) Molecular mechanisms of nociception. Nature413, 203- 210

Nave, K.-A., Sereda, M.W. and Ehrenreich (2007) Mechanisms of Disease: inheritedDemyelinating neuropathies - from basic to clinical research. Nature Cilincal PracticeNeurology 3, 453-464

Scholz, J. and Woolf, C.J. (2002) Can we conquer pain? Nature Neuroscience. 5, 1062- 1067

Detmer, S.A. and Chan, D.C. (2007) Functions and Dysfunctions of mitochondrialDynamics. Nature Reviews Molecular Cellular Biology. 8. 870-879