Myasthenia gravis pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Overview
Pathophysiology
Myasthenia gravis is a neuromuscular disease caused by an autoimmune reactions. The main problem in this disease is the abnormal transmission of nerve impulses to muscle fibers in NMJ.[1] In the nerve terminals of alpha motor neurons, there are lots of vesicles containing ACh. When the action potential reaches the synaptic end, voltage gated Ca channels will open and trigger the release of these vesicles. ACh will diffuse into synaptic cleft and binds to AChR. The action of ACh will end with the work of AChE. ACh receptors consist of 5 subunits and are transmembrane proteins. There are other proteins which help AChR clustering and signal transduction including MuSK. It is the receptor of a protein named agrin. When these two bind to each other, the result is maintaining the clustering of AChRs.[2][1][3]
Not all of the MG patients share the same auto antibodies. One of these autoantibodies is antibody against AChR. They will destruct AChR by 3 mechanisms. First they will activate the complement system, second they will increase the degradation of AChR by Ab binding and third by blocking AChR’s function.[4] The other type of autoantibody in MG patients are antibody against MsUK protein (muscle-specific receptor tyrosine kinase).[1][5] AChR antibodies are IgG1 and IgG3 and can bind to complement and activates them, but in contrast antibodies against MuSK are IgG4 and cannot activate complement system.(14-15-10) the function of the MuSK starts with the binding of agrin and LRP4. Activated MuSK cause recruitment and clustering of AChRs.(16_17_18) There are a group of MG patients which are seronegative for both AChR and MuSK antibodies.(31-32) About 50 percent of them turn out to be positive for clustered AChR antibodies after cell-based immunofluorescence. (15-33-34) The other half may be positive for other antibodies including antibody against LRP4 (which are IgG1)(35), cortactin (which help AChR clustering)(38), ryanodine receptor, titin, myosin, alpha actin, rapsyn and gravin.(40-41-42) Other than B cells, T cells have a role in the pathology on MG too. They will not act as the effector cells but stimulate B cells to produce more antibodies.(43)
Thymus abnormalities including thymic hyperplasia and thymoma are very common in myasthenia gravis and thymectomy is one of the treatment of this disease.(1-2) In thymus, we have myoid cells which present intact AChR on their surface. On the other hand thymic epithelial cells produce AChR subunits which activate helper T cells. These T cells attack AChR on the myoid cells and the cascade of antibody production and complement activation will begin.(45-46-47)
References
- ↑ 1.0 1.1 1.2 Hoch W, McConville J, Helms S, Newsom-Davis J, Melms A, Vincent A (March 2001). "Auto-antibodies to the receptor tyrosine kinase MuSK in patients with myasthenia gravis without acetylcholine receptor antibodies". Nat. Med. 7 (3): 365–8. doi:10.1038/85520. PMID 11231638.
- ↑ Horton RM, Manfredi AA, Conti-Tronconi BM (May 1993). "The 'embryonic' gamma subunit of the nicotinic acetylcholine receptor is expressed in adult extraocular muscle". Neurology. 43 (5): 983–6. PMID 7684117.
- ↑ Ruegg MA, Bixby JL (January 1998). "Agrin orchestrates synaptic differentiation at the vertebrate neuromuscular junction". Trends Neurosci. 21 (1): 22–7. PMID 9464682.
- ↑ Sahashi K, Engel AG, Lambert EH, Howard FM (March 1980). "Ultrastructural localization of the terminal and lytic ninth complement component (C9) at the motor end-plate in myasthenia gravis". J. Neuropathol. Exp. Neurol. 39 (2): 160–72. PMID 7373347.
- ↑ Vincent A, McConville J, Farrugia ME, Bowen J, Plested P, Tang T, Evoli A, Matthews I, Sims G, Dalton P, Jacobson L, Polizzi A, Blaes F, Lang B, Beeson D, Willcox N, Newsom-Davis J, Hoch W (September 2003). "Antibodies in myasthenia gravis and related disorders". Ann. N. Y. Acad. Sci. 998: 324–35. PMID 14592891.