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はじめに
2006年,筆者ら1,2)およびNeumannら3)のグループは,前頭側頭葉変性症(frontotemporal lobar degeneration: FTLD)および筋萎縮性側索硬化症(amyotrophic lateral sclerosis: ALS)患者脳に認められるユビキチン陽性細胞内凝集体(封入体)の主要構成蛋白質として,TAR DNA-binding protein of 43 kDa(TDP-43)を同定した。2008年,家族性および孤発性ALSの患者に,TDP-43遺伝子変異が相次いで発見され4-11),FTLDおよびALSの発症とTDP-43の異常が直接関連することが判明した。現在TDP-43は,タウ,αシヌクレインに次いで,神経変性疾患に出現する封入体を構成する第3の主要な蛋白質として注目を集めている。本稿では,TDP-43に関する最新の知見を筆者らの研究成果とともに紹介し,FTLDやALSの発症メカニズムとTDP-43の異常との関連性について考察したい。
Abstract
In most neurodegenerative disorders,distinctive intracellular inclusion bodies are found in degenerative neurons,which are known to be neuropathological hallmarks of diseases. Recently,TAR DNA-binding protein of 43 KDa (TDP-43) has been identified as a major constituent protein of ubiquitin-positive inclusions in brains with frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). These disorders are now referred to as TDP-43 proteinopathy. TDP-43 deposited in brains with FTLD and ALS was found to be phosphorylated and ubiquitinated. To study the role of these post-translational modifications in the formation of TDP-43 aggregates,we have produced polyclonal and monoclonal antibodies specific for TDP-43 phosphorylated at Ser409 and Ser 410. These antibodies specifically recognized abnormally phosphorylated TDP-43,but not normal TDP-43 in immunohistochemical analyses of brains of FTLD and ALS patients. Immunoblot analyses using these antibodies showed that phosphorylated and fragmented TDP-43 was deposited in diseased brains. Furthermore,we identified casein kinase 1 as a candidate protein kinase,which was responsible for abnormal phosphorylation of TDP-43. Phosphorylated recombinant TDP-43 proteins were demonstrated to be easier to fibrillate than wild-type TDP-43 in vitro. Recent discoveries of the missense mutations in the TDP-43 gene in familial or sporadic ALS cases prove a direct link between altered TDP-43 function and neurodegeneration. Elucidating the biochemical processes responsible for phosphorylation,fragmentation,and intracellular aggregation of TDP-43 may provide important insights into the pathogenesis of TDP-43 proteinopathy.
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