細胞内TDP-43蓄積のメカニズム

The Molecular Mechanisms of Intracellular TDP-43 Aggregates

はじめに

　2006年，筆者らのグループ^1）および米国Virginia M-Y. Leeのグループ^2）は，それぞれ独立に，ユビキチン陽性封入体を伴う前頭側頭葉変性症（frontotemporal lobar degenaration with tau-negative, ubiquitin-positive inclusion：FTLD-U）および筋萎縮性側索硬化症（amyotrophic lateral sclerosis：ALS）の患者脳に蓄積する新しい細胞内蓄積蛋白質としてTDP-43を同定した。

　TDP-43は，核に局在する蛋白質で，転写・翻訳の制御に関与すると考えられている。患者脳において，TDP-43は核内だけでなく細胞質においても蓄積することから，TDP-43の局在変化がその蓄積と密接に関連していることが推測できる。現在，筆者らも含めていくつかのグループが，酵母・培養細胞を用いてTDP-43の細胞内蓄積を再現することに成功しており，その蓄積メカニズムが解明されつつある。本稿では，筆者らの結果と共に最近の報告を紹介し，細胞内TDP-43蓄積メカニズム，およびそれに伴う細胞死誘導メカニズムについて考察する。

Abstract

　TAR DNA binding protein of 43 kDa (TDP-43) was identified as a major component of the ubiquitin-positive inclusions found in frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS).　To investigate the molecular mechanisms underlying the formation of intracellular aggregates of TDP-43,we have established cellular models of intracellular TDP-43 aggregates similar to the pathological inclusions found in the brain FTLD and ALS patients.　Deletion of the sequences near the nuclear localization signal resulted in cytoplasmic localization of TDP-43,whereas deletion mutants that lack the region around the RNA recognition motif localized in the nuclei,forming unique dot-like structures.　Proteasome inhibition caused these structures to assemble into round aggregates that are reactive with anti-phosphorylated TDP-43 and anti-ubiquitin antibodies.　Green fluorescent protein (GFP)-tagged N-terminal or C-terminal fragments of TDP-43 also promoted the formation of the abnormal intracellular inclusions.　Co-expression of DsRed-tagged full-length TDP-43 with GFP-tagged C-terminal fragments of TDP-43 causes the formation of cytoplasmic inclusions that are positive for both GFP and DsRed.　Cells with GFP and DsRed positive inclusions were nagative for normal nuclear staining for endogenous TDP-43.　These results suggest that GFP-tagged C-terminal fragments of TDP-43 are bound not only to transfected DsRed-full-length TDP-43 but also to endogenous TDP-43.　Endogenous TDP-43 may also be included in the cytoplasmic aggregates of TDP-43 C-terminal fragments,which results in the failure of its nuclear localization and therefore inhibits its function.　We propose that the generation and aggregation of phosphorylated C-terminal fragments of TDP-43 play a primary role in the formation of inclusions,and the resultant loss of normal TDP-43 localization leads to neuronal degeneration in TDP-43 proteinopathy.