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Pathophysiology of Myasthenia GravisBenjamin W. Hughes, Ph.D.,1 Maria Luisa Moro De Casillas, M.D.,1

and Henry J. Kaminski, M.D.1,2

ABSTRACT

Myasthenia gravis (MG) is arguably the best understood autoimmune disease,and its study has also led to fundamental appreciation of mechanisms of neuromusculartransmission. MG is caused by antibodies against the acetylcholine receptor (AChR),

which produce a compromise in the end-plate potential, reducing the safety factor foreffective synaptic transmission. It is clear that AChR antibody destruction of the

postsynaptic surface is dependent on complement activation. A muscle-specific kinasehas been recently found to be an antigenic target in MG patients without antibodies againstthe AChR. Autoantibody production in MG is a T-cell-dependent process, but how abreakdown in tolerance occurs is not known. In MG there is an interesting differentialinvolvement of muscle groups, in particular, the extraocular muscles. This article reviewsnormal neuromuscular transmission, mechanisms of the autoimmune process of MG, anddifferential susceptibility of eye muscles to MG.

KEYWORDS: Myasthenia gravis, neuromuscular transmission, acetylcholine receptor,

neuromuscular junction

Objectives: On completion of this article, the reader will be able to summarize the anatomy and physiology of the neuromuscularjunction, the immunopathogenesis of myasthenia gravis, and the differential susceptibility of eye muscles to myasthenia gravis.

Accreditation: The Indiana University School of Medicine is accredited by the Accreditation Council forContinuing Medical Education to

provide continuing medical education for physicians.

Credit: The Indiana University School of Medicine designates this educational activity for a maximum of 1 Category 1 credit toward the

AMAPhysicians Recognition Award.Each physicianshould claim only those hours of creditthat he/she actually spent in the educational

activity.

Disclosure: Statements have been obtained regarding the authors relationships with financial supporters of this activity, use of trade

names, investigational products, and unlabeled uses that are discussed in the article. The authors have nothing to disclose.

To understand the pathophysiology of myasthe-nia gravis (MG), a thorough appreciation of normalneuromuscular transmission and the underlying anatomyof the neuromuscular junction (NMJ) is imperative.15

Although basic mechanisms of how the junction func-tions to transmit signals from nerve to muscle have been

well established for decades, the last 10 years has seen anexplosion of information regarding the molecular under-

pinning of transmission. In concert with these develop-

ments, appreciation of the intricacies of the autoimmunereaction underlying destruction of the postsynaptic sur-face of the muscle has expanded. This review will discussthe structure of the NMJ, how neuromuscular transmis-sion occurs, and the pathogenesis of MG, as well as asummary of why certain muscle groups may be differ-entially involved by the disease.

Myasthenia Gravis; Editor in Chief, Karen L. Roos, M.D.; Guest Editor, Robert M. Pascuzzi, M.D. Seminars in Neurology, Volume 24, Number 1,2004. Address for correspondence and reprint requests: Henry J. Kaminski, M.D., Department of Neurology, University Hospitals of Cleveland,11100 Euclid Avenue, Cleveland, OH 44106. Departments of 1Neurology and 2Neurosciences, Case Western Reserve University, Louis StokesCleveland DVA Medical Center, University Hospitals of Cleveland, Cleveland, Ohio. Copyright# 2004 by Thieme Medical Publishers, Inc., 333Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662. 0271-8235,p;2004,24,01,021,030,ftx,en;sin00285x.

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ANATOMY AND PHYSIOLOGY OF

THE NEUROMUSCULAR JUNCTION

Presynaptic SurfaceAs the motor nerve approaches a muscle it branches,innervating many muscle fibers and providing a single,unmyelinated nerve terminal to each of the fibers. The

NMJ is a specialized synapse designed to transmitnerve impulses from the nerve terminal to muscle, viathe chemical transmitter, acetylcholine (ACh), which isstored at the terminal in synaptic vesicles (Fig. 1). These

vesicles are aligned near release sites called active zoneswhere calcium channels are arranged in parallel doublerows. When an action potential reaches the nerve term-inal, calcium channels of the P/Q-type (N-type channelsalso are localized to the presynaptic membrane) areactivated, calcium enters the presynaptic terminal, andthe local calcium concentration rises significantly, trig-gering the vesicles to release their contents. In the

vertebrate NMJ, a typical nerve terminal action potentialdoes not fully activate nerve terminal calcium channels,as the duration of the action potential is 1 ms, whilenerve terminal calcium channels are activated with a timeconstant of1.3 ms.6 Therefore, enhancement of AChrelease from the presynaptic nerve terminal can beachieved by increasing the nerve terminal action poten-tial duration via blockade of delayed rectifier potassium

channels with 3,4-diaminopyridine or inhibiting calciumuptake with quanidine, agents used in treatment ofLambert-Eaton syndrome.

The mechanism of relaying the calcium signal tosynaptic vesicle fusion involves conformational changesin multiple proteins on the synaptic vesicle membraneand the plasma membrane of the nerve terminal.4,7,8The

events leading to release of synaptic vesicle contents arebeing increasingly defined. Vesicles initially undergo aprocess called docking, in which they come into closeproximity with the nerve terminal membrane and thenundergo priming that allows them to respond to thecalcium signal. During docking, munc18 dissociatesfrom syntaxin and synaptophysin from synaptobrevinallowing the synaptic core complex to form. Three pro-teins, two on the plasma membrane (syntaxin and syn-aptic vesicle associated protein 25 or SNAP 25) and oneon the synaptic vesicle membrane (synaptobrevin), arethought to form the docking complex. N-ethylmalei-

mide sensitive factor (NSF) and soluble NSF attachmentprotein (SNAP) associate to form a fusion complex withthe docking proteins. N-ethylmaleimide sensitive factor,

which is an ATPase, cross-links multiple core complexesinto a network, and ATP hydrolysis leads to hemifusionof the vesicle and presynaptic nerve terminal membranes.Synaptotagmin likely acts as the calcium sensor, butother synaptic proteins may act in concert to perform

Figure 1 Schematic of the mammalian neuromuscular junction, illustrating the major proteins located at the mammalian neuromus-

cular junction. The presynaptic nerve terminal contains synaptic vesicles (striped ovals), voltage-gated potassium (VGKC, triangles), and

calcium channels (VGCC, gray rectangles). Acetylcholine receptor illustrated here in its pentameric structure. Voltage-gated sodium

channels (striped rectangles), muscle-specific kinase (MuSK), rapsyn, utrophin, and the sarco-dystroglycan complex are located at the

postsynaptic membrane. Details are discussed throughout the article. Objects are not drawn to scale.

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this function. The cytoplasmic portion of synaptotagminincludes two regions with high homology to the calciumand phospholipid-binding domains of a proteinkinase. Synaptotagmin likely binds phospholipid of theplasma membranes and syntaxin. Binding of calcium tosynaptotagmin may alter interactions of synaptotagmin

with membrane lipids and syntaxin, allowing the mem-

branes to fully fuse. Botulinum toxins are proteasesthat target several of the synaptic vesicle proteins andtheir investigation has greatly enhanced understandingof transmitter release.9 How the synaptic release me-chanism is capable of discharging synaptic vesiclecontents in the extremely rapid fashion that it does isnot known.

After releasing their contents to the synaptic cleft,synaptic vesicle membrane is recycled by a clathrin-mediated mechanism. After reuptake of the vesicles,the clathrin-coated vesicles shed their coats and translo-cate into the interior. The vesicle membranes fuse with

endosomes in the nerve terminal and new vesicles budfrom the endosome. The new vesicles package ACh andother chemicals by active transport and translocate backto the active zone either by diffusion or by a cytoskeletaltransport process.

Synaptic CleftBetween the nerve and muscle plasma membranes lies aspace of50 nm called the synaptic cleft (Fig. 1). Theextracellular matrix of the synaptic cleft is a complexcollection of proteins that regulate the synthesis ofpostsynaptic proteins and the concentration of acetyl-choline esterase (AChE). The basement membrane isenriched with collagen and contains several forms oflaminin, all of which bind to a-dystroglycan in thepostsynaptic membrane. The laminins form a networkin the synaptic space that anchors other extracellularmatrix proteins.10

Once released from the synaptic vesicles, diffusionof ACh across the cleft is rapid because of the smalldistance to cross and the high diffusion rate of ACh.2

AChE is concentrated in the basal lamina of the post-synaptic membrane, and its action in hydrolyzing ACh,as well as the diffusion of ACh out of the cleft, leads to a

rapid decline in ACh concentration.11This prevents theAChR from being activated more than once in responseto ACh. AChE inhibitors, such as pyridostigmine andedrophonium, prolong the duration of action of ACh onthe postsynaptic membrane, slowing the decay of theACh-induced endplate current.12 In postsynaptic dis-eases of neuromuscular transmission, AChE inhibition

will serve to enhance ACh action and promote achieve-ment of an end-plate potential that will lead to actionpotential generation. Schwann cells surrounding thenerve terminal have been found to secrete an acetylcho-line binding protein, which may reduce the effective

concentration of ACh in the synaptic cleft, and play arole in modulation of neuromuscular transmission.13,14

Postsynaptic Surface

Once ACh traverses the synaptic cleft it binds the AChRconcentrated across from the nerve terminal active zones

(Fig. 1). This binding results in opening of the AChRion channel and the entry of cations, mainly sodium, intothe muscle, leading to end-plate potential generation.

When a certain threshold depolarization is achieved,voltage-gated sodium channels, at the bottom of thepostsynaptic folds, open, allowing the entry of moresodium ions and generating the muscle action potentialand contraction.4

The postsynaptic skeletal muscle surface is char-acterized by invaginations of the plasma membrane,termed secondary synaptic folds.2 The folds increasethe surface area of the postsynaptic membrane and

their architecture allows end-plate depolarization tobe focused on their depths.15 AChR concentrated atthe tops of the secondary synaptic folds are anchored tothe dystrophin-related protein complex through rapsyn,a protein involved in clustering of AChR at the NMJduring synapse development.2 AChR clusters are con-nected to the cytoskeletal elements via associations withthe dystroglycan and sarcoglycan protein complexes andutrophin.16 Mice deficient in rapsyn do not effectivelycluster AChR at the synapse. Some congenital myasthe-nia patients have been found to have rapsyn mutationsor alterations in utrophin expression.1719 In addition tothe anchoring system serving to concentrate AChR,muscle fiber nuclei near the NMJ preferentially expressAChR subunit genes.20

In addition to AChR, other proteins are differ-entially expressed at the NMJ. Na channels are highlyconcentrated in the membrane at the base of the synapticfolds by interaction with ankyrin, the sarcoglycan com-plex, the dystroglycan complex, dystrobrevins, and dys-trophin/utrophin (Fig. 1). Utrophin and dystrobrevinalso connect to a1 syntrophin and a2 syntrophin, whichin turn associate with nitric oxide synthetase.21,22 Nitricoxide produced locally on the postsynaptic surfacecould serve a signaling function and has been shown to

influence synaptogenesis.23

The Acetylcholine Receptor

The AChR is composed of four subunits and exists intwo isoforms.2426The adult AChR is composed of twoa-subunits and one copy of each of the b-, d-, ande-subunits, while the fetal AChR has a g-subunit inplace of the e-subunit. High amino acid sequence homo-logy exists among the subunits of the AChR with eachcontaining four a-helices, designated M1 to M4, thatspan the plasma membrane. The extracellular portions of

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the subunits consisting of the N- and C-terminal regionsand the region between M2 and M3 form a large extra-cellular vestibule that surrounds the channel extracellularorifice. The regions between M1 and M2 and betweenM3 and M4 form a smaller vestibule around the in-tracellular orifice of the ion channel.27,28 The skeletalmuscle nicotinic AChR has a large extracellular surface

with the potential to form many epitopes for autoanti-body binding and accommodate simultaneous binding ofdifferent antibodies.

In developing muscle, the fetal AChR is ex-pressed along the entire fiber surface and contributes tothe muscle being able to produce spontaneous contrac-tions, which is critical to development.29 With innerva-tion fetal AChR is down-regulated and the adultreceptor is expressed at the synapse. Only mammalianextraocular muscle is known to express fetal AChRprotein at the synapse in adulthood30,31; however, reex-pression of the fetal AChR in certain patients with

congenital myasthenia prevents lethality of some muta-tions of AChR subunit genes.32 Adult and fetal AChRcan be distinguished by their electrophysiological char-acteristics. Adult channels have shorter mean open timesand a single channel conductance that is 50% largerthan that found with fetal channels. The differencesbetween adult and fetal AChR channels are due toreplacement of the g- with the e-subunit. Phosphoryla-tion and other forms of posttranslational modificationcan alter properties of the AChR, and in particular,subunit phosphorylation appears to regulate agonist-induced desensitization.24

Neuromuscular Junction Formation

and Muscle-Specific Kinase

The development of the NMJ requires a complex seriesof interactions between developing motor neurons andmuscle fibers.2,11 Present understanding indicates thatagrin, a protein synthesized by motor neurons and stablydeposited into the synaptic basal lamina, stimulates amuscle-specific kinase (MuSK), a receptor tyrosinekinase, that is expressed selectively in skeletal muscle.

This signal is thought to cluster significant postsynapticproteins, including AChRs, at the NMJ (Fig. 1).33This

formulation is based on data showing that agrin canstimulate multiple aspects of postsynaptic differentiationin cultured myotubes, including the clustering andtyrosine phosphorylation of AChR.3436 Mice lackingagrin or MuSK fail to form neuromuscular synapses, andas a result, die at birth because of a failure to move orbreathe.37,38 MuSK mutant mice do not concentrateAChR or other synaptic proteins at the NMJ, but ratherthese proteins are instead uniformly expressed along themuscle fiber.

The precise mechanism by which agrin activatesMuSK is poorly understood.39,40 Agrin stimulates the

rapid tyrosine phosphorylation of MuSK in myotubes,consistent with the idea that MuSK is a receptor,or a component of a receptor complex, for agrin.On the other hand, forced expression of MuSK infibroblasts or myoblasts does not lead to its phosphor-

ylation by agrin.38 Also, recombinant agrin and MuSKin vitro do not bind, indicating that additional compo-

nents that may be expressed in myotubes and skeletalmuscle fibers, and not in myoblasts or fibroblasts, areessential for agrin to activate MuSK. As will be describedlater, MuSK has been identified as an antigenic target inMG patients seronegative for antibodies against theAChR.

SAFETY FACTOR FOR

NEUROMUSCULAR TRANSMISSIONCompromise of the safety factor for neuromusculartransmission is common to all neuromuscular transmis-

sion disorders.15

The safety factor is defined as the ratioof the end-plate potential amplitude to the differencebetween the membrane potential and the thresholdpotential for initiating an action potential. Normally,the nerve terminal releases enough ACh to induce anexcitatory end-plate potential, which is greater than thethreshold needed to initiate an action potential, andtherefore, the safety factor is quite large. Quantal release,AChR conduction properties, AChR density, andAChE activity contribute to the end-plate potentialand the safety factor. Sodium channel concentration atthe postsynaptic surface affects the safety factor bymaking the action potential threshold easier to achieve.In addition, the synaptic folds architecture, which allowscurrent to be concentrated on the sodium channels,enhances the end-plate potential. AChE activity termi-nates the action of ACh and its inhibition will enhanceAChR activation. Postsynaptic factors also influence thesafety factor. Repetitive stimulation reduces transmitterrelease, which under normal conditions will reduce theend-plate potential but not enough to prevent actionpotential generation. In pathological situations, such asMG, this reduction may be enough to produce transmis-sion failure.

IMMUNOPATHOLOGYMG meets strict criteria that define antibody-mediated,autoimmune disorders: (1) antibodies against the AChRcan be detected in 80 to 90% of patients41; (2) immu-noglobulin G (IgG) is deposited at the NMJ, the siteof pathology42; (3) administration of IgG from MGpatients to experimental animals reproduces the char-acteristic clinical features of the disease43; (4) therapiesthat reduce the serum concentration of anti-AChR anti-bodies improve weakness44; and (5) MG can be inducedin animals by immunization with purified AChR,



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further supporting the concept that an immune responsedirected against the AChR is responsible for the diseasein patients.45

Autoantibodies in MGAChR antibodies are polyclonal, vary in light chain

composition and subclass structure, and identify aheterogeneous epitope repertoire that varies amongpatients.25,26,46,47 AChR antibodies produce deficientneuromuscular transmission by three different mechan-isms: (1) they bind to the AChR and alter function; (2)they promote endocytosis and accelerate the degradationrate of AChR; the effect of these antibodies in accel-erating degradation is thought to result from their abilityto cross-link the receptors48; and (3) antibodies activatecomplement leading to destruction of the postsynapticsurface.

The most important mechanism of neuromuscu-

lar transmission compromise is antibody-mediated com-plement activation, and this statement is supported byseveral experimental observations. C3 activation frag-ments, C9, and the membrane attack complex can bedetected at the NMJ in patients and experimental auto-immune MG of animals.49,50 Methods that depletecomplement, block activation, or engineer defects incomplement components protect animals against diseaseinduction.5155 Skeletal muscle is protected from auto-logous complement-mediated injury by cell surface reg-ulators located on their plasma membranes.56,57 Theseregulators are three proteins: the decay acceleratingfactor, the membrane cofactor protein, and the mem-brane inhibitor of reactive lysis. Collectively, thesecontrol proteins accelerate the decay of autologous C3convertases and membrane attack complex that inappro-priately assembles on self cell surfaces, promote thecleavage of uncomplexed autologous cell-bound comple-ment fragments, and inhibit the formation of autologousattack complex, which brings about lysis of the post-synaptic membrane. Deficiency of decay acceleratingfactor in mice increases the severity of experimentallyinduced MG, suggesting that these complement regu-lators could modulate disease severity in humans.58

Most pathogenic antibodies bind epitopes formed

by the a-subunit in a region termed the main immuno-genic region (MIR).59 The most critical immunogenicarea within the MIR is located on the extracellularN-terminal segment of the a-subunit. The MIRmay be a particularly pathogenic region because (1) itslocation on surface of the AChR is easily accessed byantibody, (2) its amino acid structure is highly immuno-genic, and (3) its location in densely packed AChR ofthe NMJ allows cross-linking and antigenic modula-tion.25,26 The immunodominance of this region mightalso be facilitated by the molecular mimicry between theMIR and epitopes on certain retroviruses.60

In general, direct effects on AChR function arenot considered to contribute significantly to MG pathol-ogy, but some patients have antibodies that bind theACh binding site. These antibodies can contribute tomyasthenic weakness and could cause acute myastheniccrisis. In animals, they produce a unique form of severeexperimental autoimmune MG.61

The first autoantibodies detected in MG patientswere the striational antibodies, when MG patient serumwas found to bind skeletal muscle in immunohisto-chemical studies.62 The majority of these striationalantibodies have been found to bind the structural proteintitin.63,64 Antibodies against other skeletal muscle pro-teins, including the ryanodine receptor, myosin, tropo-myosin, troponin, a-actinin, and actin, have been foundamong patients.65 The pathogenic significance of theseantibodies is not known, although ryanodine antibodiescan compromise in vitro muscle contractility. The anti-body profile of MG patients suggests differences in im-

munopathogenesis. Older-onset patients have higherfrequencies of antibodies against the structural proteintitin, and the disease is more severe.64 Patients withthymoma also tend to be positive for not only titinantibodies but also ryanodine receptor antibodies, andsuch patients have been found to have associated myo-carditis or myositis.66

Up to 20% of patients with generalizedMG are seronegative for AChR antibodies with30% of these patients having autoantibodies againstMuSK.6769 Ocular MG patients who are seronegativefor the AChR are also negative for MuSK. MuSK-positive patients exhibit more severe disease than theAChR-positive patients, and some have been foundto have muscle atrophy. Some MuSK antibodies com-promise AChR channel function, but specific patho-genic mechanisms have not been defined. One wouldpredict that MuSK antibodies compromise neuromus-cular transmission by affecting AChR clustering at theNMJ.

Cellular Autoimmunity

Autoantibody production in MG is a T-cell-dependentprocess and a breakdown in tolerance toward self-anti-

gens is the primary abnormality in MG. CD4T-helpercells that are AChR-specific are crucial for the activationof B cells and for the synthesis of high-affinity IgGautoantibodies.46 CD4 T cells in patients with MGregulate the production of anti-AChR antibodies, andsuppressor T cells are present, which can decrease anti-body production.

Several properties of the anti-AChR CD4 Tcells are fundamental for understanding the pathogenesisof MG. CD4 T-helper cell receptors respond to anti-gens processed by antigen-presenting cells (APC)and associate with self major histocompatibility



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complex (MHC) class II molecules. T-cell receptorbinds AChR only after it is processed by proteolyticcleavage and association with a MHC molecule byAPC. The T-cell receptor grants antigen specificityto CD4T cells. The requirement for MHC associationrestricts the number of epitopes that a T cell canrecognize. Differences in MHC class II molecules

have been shown to affect the severity of symptomsin experimental autoimmune MG and could be true inhumans.70

The AChR a-subunit contains the majority ofT-cell recognition sites, although other subunits arerecognized by T cells from patients. The epitopes onthe a-subunit recognized by the T cell are distinct fromthe MIR recognized by antibodies. Similar to antibodyproduction, the T-cell response is polyclonal and variesamong MG patients. The T-cell-dependent nature ofMG explains its association with human leukocyte anti-gen (HLA) antigens.7173 An increased association of

HLA-A1, -B8, and -DRw3 (and -B12 antigen in Japan)occurs among MG women under 40 without thymoma.HLA-A3, -B7, and -DRw2 (and -A10 in Japan) occur

with greater frequency in men after 40 without athymoma. Restriction fragment length polymorphismanalysis has demonstrated a link between MG andHLA-DQ, suggesting that HLA site is close to thecoding region for the epitope of the AChR binding site.

With increasing disease duration, CD4 sensitizationspreads to other parts of the AChR. The epitoperepertoire of anti-AChR CD4 cells in MG patients iscomplex and characteristic of an individual patient.46 Afew sequence regions of anti-AChR CD4 cells arerecognized in most or all MG patients. These sequencesform CD4 epitopes that are both universal and im-munodominant. These universal epitopes sensitizepathogenic CD4 cells and drive the synthesis ofAChR antibodies.47 CD4 T cells from MG patientsrespond in a remarkably heterogeneous fashion74 andrecognize epitopes from each of the AChR subunitsincluding the adult e-subunit, the d-subunit, and thefetal g-subunit. Patients who have generalized weaknessfor greater than 5 years usually have CD4 T cells thatrecognize all subunits, whereas patients with weaknessfor shorter periods will only recognize few subunits

of the AChR. These findings suggest enlargement ofthe CD4 epitope repertoire with time in generalizedMG.

Studies of anti-AChR CD4 cells T-cellreceptors from MG patients do not allow distinctionbetween an initial pathogenic mechanism, whichentails molecular mimicry, or the intervention of asuperantigen. Superantigens are proteins found on

viruses or bacteria that stimulate particularly powerfulproliferative responses. Superantigens are not processedbut directly bind to the class II MHC and to the T-cellreceptor.

Cytokine Influences

Cytokines are peptides with intercellular signaling prop-erties that regulate local and systemic immune responses,as well as other biological processes. Cytokines, secretedby different CD4 T-helper cell subsets after antigen-stimulated activation, appear to play an important rolein the pathogenesis of MG.46 T-helper cells can be

distinguished on the basis of the array of cytokinessecreted. Th1 cells secrete proinflammatory cytokinessuch as interferon-g, interleukin-2, and tumor necrosisfactor (TNF)-a. Th2 cells express anti-inflammatory/regulatory cytokines including interleukin-4 and inter-leukin-10. Both Th1 and Th2 cell groups induce anti-body synthesis by secreting cytokines that induceproliferation and differentiation of B cells. Some studiesof cellular cytokine secretion in MG patients have shownthat MG is associated with involvement of both Th1 and

Th2 cells.75,76 Th1 cells are involved in the antibodyresponse in MG and have an intricate epitope repertoire.

In MG, the AChR is the sensitizing antigen and thetarget of the autoimmune Th1 cells. However, molecularmimicry between one AChR epitope and a microbialantigen might also play a role in the launching of theautoimmune response. Interleukin-4, secreted by Th2cells, may be protective in experimental MG of animalsas interleukin-4 has anti-inflammatory effects on both

Th1 cells and APCs and acts as a growth-factor fortransforming-growth factor (TGF)-b secreting cells.

TGF-b, which inhibits both cellular and humoralimmunity, is also up-regulated in MG patients. Othercytokines that are being investigated in MG pathogen-esis include TNF-a and TNF-b. Their function stillneeds further clarification; most likely they participate

within the cytokine network, modulating the immuneresponse.46

The Thymus in MG PathogenesisThe thymus plays a key role in tolerance induction toself-antigens and in responsiveness of lymphocytes toforeign antigens.46,74 During development bone marrowstem cells appear in the thymic subcapsular epithelium,

where a random process of gene rearrangements occursin the regions that will code for the T-cell antigen

receptor. The immature T cells pass through the thymiccortex, and those that recognize MHC antigens passthrough to the medulla. The majority of immature Tcells that do not recognize self MHC antigens areeliminated. During this stage in the thymic cortex, Tcells, which would react toward self-antigens, are alsoremoved; however, the mechanisms by which this occursare not known. Once in the medulla, the T cellsdifferentiate into helper and suppressor cells and even-tually are released to the periphery.

Several findings indicate that the thymus is in-volved in MG pathogenesis. Pathological changes,



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specifically lymphoid follicular hyperplasia and thymo-mas, are frequent in patients with MG.77 MG thymictissue contains abnormally elevated amounts of mature Tcells. Thymomas and myasthenic thymus are enrichedfor AChR-reactive T cells.78 When myasthenic thymictissue fragments are transplanted to severe combinedimmunodeficiency mice, they produce human antibo-

dies, which bind to the AChR.79

Most myasthenicthymuses contain B cells capable of producing AChRantibodies, particularly those hyperplastic thymuses withgerminal centers. Normal and myasthenic human thy-muses express gene transcripts and epitopes of all AChRsubunits.8082 The expression of antigenically similarproteins to the AChR in the thymus would provide asource of antigen for autosensitization of T cells towardthe skeletal muscle AChR. Further, thymectomy appearsto improve the clinical course of patients with MG.83

THE DIFFERENTIAL INVOLVEMENTOF MUSCLE GROUPS BY MGMG demonstrates a remarkable variation in musclegroup involvement among patients. There is no betterexample of this than the involvement of ocular muscles.

Weakness of the extraocular muscle (EOM) and thelevator palpebrae are the initial signs of MG in themajority, ultimately occurs in nearly all, and may remainrestricted to these muscles in 10 to 15% of patients.84

One of the authors (H.J.K.) has recently provideddetailed reviews of the preferential involvement of theEOM by MG,85,86 and the arguments are briefly sum-marized here.

The most simple, and least scientifically exciting,is that ocular muscle dysfunction leads to dramatic sym-ptoms of double or blurred vision, which brings thepatient to medical attention early.87 Because of the pre-cision required to maintain alignment of the visual axes,even a minor reduction of force generation produces

visual compromise, in contrast to a small reduction inforce generation of a limb muscle, which may not bereadily appreciated or be ignored by a patient. Also, inextremity muscle, proprioceptive input may compensateto some extent by recruitment of additional motorunits to perform a task. In ocular motor control, pro-

prioception may not compensate in the same manner forreductions of force generation.

Several factors specific to the EOM place them atrisk for MG involvement. Unique physiological proper-ties of the oculomotor system may be of paramountimportance in targeting of EOM by neuromusculartransmission abnormalities. The ocular motor neuronfiring frequencies are extremely high, at times 400 to500 Hz, which would exacerbate a transmission defect.EOM singly innervated fibers possess anatomical char-acteristics that would be predicted to make them suscep-tible to neuromuscular transmission block. The fibers

have less prominent synaptic folds, and therefore onewould predict fewer AChRs and sodium channels on thepostsynaptic membrane.88 A reduction in AChR andsodium channels would reduce the safety factor predis-posing EOM to neuromuscular transmission failure.Na channel density varies between fast-twitch andslow-twitch fibers with fast-twitch fibers having higher

densities in the region of the NMJ.89

These differencescould contribute to clinical involvement of musclegroups other than EOM by MG.90 EOM also containsmulti-innervated fibers that are similar to reptile tonicand intermediate fibers that have tonic contractile char-acteristics with the functional consequence that forcegeneration is directly proportional to the membranedepolarization caused by the end-plate potential. There-fore, a safety factor does not exist for multi-innervatedfibers, and any reduction of AChR would decreasecontractile force of these fibers.85,86,88 Also, the neuro-muscular junctions have a scarcity (to complete absence)

of junctional folds.91

The autoimmune process of patients with ocularmyasthenia does differ from patients with generalizeddisease. Ocular myasthenia patients have lower levels ofAChR antibodies.72The intensity of T-cell responses toAChR epitopes of ocular myasthenia patients are lowerthan that of generalized patients, and they fluctuate overtime.92 Complement regulatory proteins are expressed atlower levels in EOM; therefore, a complement-mediateddisease, such as MG, could be more likely to inducedamage of these muscles.93 The combination of mildautoimmune disease and physiologic susceptibility maybe the explanation for the existence of purely ocularmyasthenia.

The reasons for levator palpebrae involvement arenot as clearly understood. The muscle is more similar toother skeletal muscle than to the EOM. The levatormuscle fibers have twitch characteristics and some arehighly fatigue-resistant, based on histological appear-ances, but there are no multi-innervated fibers. Non-immune disorders of neuromuscular transmission alsohave a predilection for producing ptosis, suggesting thatphysiological reasons predispose them to neuromusculartransmission failure. The levator fibers are under con-stant neuronal stimulation during eye opening. This

makes neuromuscular fatigue more likely to occur com-pared with other skeletal muscles. The junctional folds oflevator NMJ are also sparse as in EOM and suggest alower AChR number and a reduction in safety factor.85

SUMMARYThis review discussed the basic physiology of neuromus-cular transmission and the structure that underliescommunication between the nerve and muscle, theNMJ. Diseases of the NMJ and ion channels areof general interest because an understanding of their



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pathophysiology may provide insight into disorders atother neural synapses, such as epileptic or psychiatricdiseases. MG serves as a model for the investigation ofall autoimmune disorders.

ACKNOWLEDGMENTS

This work was supported by NIH grants EY013238 andEY014837 to Dr. Kaminski, who is also supported bythe Department of Veterans Affairs Medical ResearchService.

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