精神分裂症致病基因DISC1變異對α7 nicotinic receptor表現之探討

Abstract

Disrupted-in-Schizophrenia 1 (DISC1)是精神分裂症和其他精神疾病中被認為最常見的致病基因，它在一個蘇格蘭家族中首度被發現有balanced chromosomal translocation (1; 11) (q42.1; q14.3)而使DISC1發生斷裂。另一方面，α7 nicotinic receptor (α7 nAChR)已知和很多行為有關，包括認知方面的功能。在精神分裂症死後病人的腦部一些區域已發現α7 nAChRs的表現有缺陷，基因的研究結果也顯示α7 nAChRs和精神分裂症有關。因此本篇論文將探討DISC1 (1-597 a.a.)和α7 nAChR之間的關係。我們在人類神經瘤細胞株SH-SY5Y轉殖mutant DISC1 (C-terminal truncation, 1-597 a.a.)，發現mutant DISC1可以減少α7 nAChR mRNA和蛋白的表現。另外，C57BL/6J;129S6/SvEv (B6/Sv129)交配之小鼠已被發現在單股染色體之DISC1有發生變異，同時我們也發現將B6/Sv129胎鼠(E18)的cortex急性取下，其α7 nAChRs表現量減少。先前我們已有實驗證實α7 nAChRs會透過autophagosome的途徑參與Aβ的清除，因此我們透過B6/Sv129小鼠的primary cortical culture去研究DISC1變異對Aβ產生的神經毒性有無影響，在此我們發現給予Aβ1-42之後，heterozygous (+/-) B6/Sv129小鼠的primary culture發現有較少的NeuN (+) 細胞，此結果顯示有DISC1變異會導致Aβ1-42的神經毒性增加，也暗示DISC1變異可能在精神分裂症中對於認知功能障礙扮演一個重要的角色。我們更進一步觀察B6/Sv129成鼠的各個區域受體的表現，發現和wild-type小鼠相比，在heterozygous (+/-) B6/Sv129小鼠的striatum有表現較少的α7 nAChRs；在amygdale和striatum有表現較少的ChAT。此結果也可以部分解釋在shuttle avoidance test中heterozygous (+/-) B6/Sv129小鼠和wild-type小鼠相比有較差的恐懼學習的表現。此外，在EPM test中，我們也發現急性給予nicotine對wild-type小鼠有anxiogenic的作用，但是在heterozygous (+/-) B6/Sv129小鼠anxiogenic作用減低。在PPI test中，我們發現不管是公鼠或母鼠，wild-type和heterozygous (+/-) B6/Sv129小鼠的驚嚇反應都沒有顯著差異。 此結果顯示DISC1可以影響cholinergic system，進而影響情緒、認知及精神疾病相關症狀。

Disrupted-in-Schizophrenia 1 (DISC1) is one of the most promising risk genes for schizophrenia and other major mental disorders, which is disrupted by a balanced chromosomal translocation (1; 11) (q42.1; q14.3) in a large Scottish family. On the other hand, α7 nicotinic receptor (α7 nAChR) is well-known to be involved in a wide variety of behavioral functions including cognition. Post-mortem studies have revealed a disturbance of α7 nAChRs expression in various cerebral areas in schizophrenia patients. Genetic linkage studies have also shown that the α7 subunit is involved in schizophrenia. In this study, we investigated the relationship between DISC1 (1-597 a.a.) and α7 nAChR. It was found that transfection of mutant DISC1 (C-terminal truncation, 1-597 a.a.) in human neuroblastoma cells, SH-SY5Y, significantly inhibited the mRNA and protein expression of α7 nAChR. In addition, the acutely isolated cortex from fetal heterozygous (+/-) C57BL/6J; 129S6/SvEv (B6/Sv129) mice (E18), which have DISC1 gene mutation, had less expression of α7 nAChRs. Previously, we have found that α7 nAChR is involved in the clearance of Aβ via autophagosome process. We thus examined the effect of mutant DISC1 on the Aβ-induced neuronal death by using primary cortex neuronal culture. It was found that there were more NeuN (+) cells in primary cortical culture derived from wild-type mice than in heterozygous (+/-) B6/Sv129 mice. This result indicated that DISC1 mutation is more sensitive to Aβ1-42–induced neuronal toxicity and may play a role in cognition impairment in schizophrenia. Furthermore, we also found that there were more α7 nAChRs in the striatum, more ChAT protein expression in the amygdale and striatum from adult wild-type than in heterozygous (+/-) B6/Sv129 mice. This result may partially explain that the heterozygous (+/-) B6/Sv129 mice had impairment in fear-memory of shuttle avoidance test. In addition, we also found that acute administration of nicotine exerted anxiogenic effect and the effect was reduced in heterozygous (+/-) B6/Sv129 mice. There was no significant difference of startle response in response to prepulse inhibition test between wild-type and heterozygous (+/-) B6/Sv129 mice. In conclusion, our results demonstrated that influence on cholinergic system may be involved in the impairment of emotion, cognition following the defect of DISC1.