重症筋無力症に関する最近の知見—骨格筋アセチルコリン受容体mRNAの定量

Recent advances in the study of myasthenia gravis—Determination of muscle mRNA encoding the α-subunit of the nicotinic acetylcholine receptor.

はじめに

　神経筋シナプスの構成要素の一つであるニコチン性アセチルコリン受容体（nicotinic acetylcholine receptor；nAChR）は，運動神経終末から遊離されるアセチルコリンの結合に呼応して筋細胞膜のイオン透過性を亢進させる。すなわち，受容体はアセチルコリン化学伝達を電気的変化に導く変換器として作動する。重症筋無力症（myasthenia gravis；MG）はnAChRが抗体により破壊され神経筋伝達が障害される結果，筋力低下をきたす自己免疫性受容体疾患である。

　1973年Patrick, J. らは，電気ウナギ発電器官から精製したnAChR蛋白を家兎に接種して受容体抗体を作製する過程で，血中に抗体が産生されると家兎が筋脱力を発症することを発見し^1），MG研究に新たな展開をもたらした。MGが自己免疫性受容体疾患であり，神経筋伝達異常は前シナプスではなく，後シナプス膜側の異常に起因することが明らかになり，永年の論争に終止符が打たれた。そして今日，MGにおける免疫異常の主座が受容体であることが判明したことから，疾患研究とnAChRの分子生物学的研究とが密接な関わりをもちながら発展する状況に至った。ここでは免疫異常によってnAChRが破壊されるとき，その急速な崩壊が受容体遣伝子の発現にどのような影響を及ぼすかについて述べる^2）。

　In adult mammalian skeletal muscles, nicotinic acetylcholine receptors (nAChR) are packed at the muscle endplates. When motor nerves are dissected, nAChRs appear over the entire surface of the muscle fibers. The close analysis of this phenomenon "denervation supersensitivity" would provide information for understanding nerve regulation of nAChR gene expression both in the peripheral and central nervous systems. In the former part of the present study, we analyzed expression of the nAChR mRNA in experimental autoimmune myasthenia gravis (EAMG).The latter part was focused on the effect of the deafferentation of the nerve over the neuronal cells in the cerebral cortex.

　1) The denervation supersensitivity based on the muscle contents of nAChR determined by thetoxin binding assay method was not observed in rats injected with purified nAChR. The lack of the increase of nAChR after denervation in EAMG may be explained as follows. Newly formed receptors after denervation have the same metabolic characteristics as extrajunctional nAChR and they replaced the original receptors. The synthesized receptors are metabolically unstable, and anti nAChR anti-bodies accelerated the degradation of the original junctional and newly formed receptors. Under these conditions, the increase of the nAChR protein was not detected. Alternatively we made a hybridization analysis using α-subunit cDNA clone to assess mRNA specific to the receptor in the hindlimb muscles. Rats immunized with Torpedo califorrzica nAChR protein showed cholinesterase inhibitor responsive muscular weakness, and decreased level of the muscle contents of the receptors. Serum antibodies were shown to accelerate the degradation of the receptors of cultured muscle cells. The rats received the surgical denervation of the ischiadic nerve of the unilateral hind limb, and we aimed to find how the myasthenic muscles respond to the denervation. In the innervated-hind limb muscle of EAMG rats, the nAChR levels in muscle were reduced to 56% of the normal control levels. The level of muscle contents of mRNA encoding α-subunit was about 4 times more than those from normal control rats. In the denervated hind limb of the EAMG rats, the nAChR protein was only 5 % of the control denervated muscles. However the mRNA level in EAMG rats increased to the same level of the control. The data indicated that antibody-mediated reduction of nAChR enhanced its synthesis by increasing the production of nAChR specific mRNA in EAMG, and that the EAMG muscles, upon the denervation, were not affected at the level of gene transcription of nAChR mRNA.