Joubert綜合征（Joubert syndrome， JBTS）是一種十分少見(jiàn)的常染色體隱性遺傳神經(jīng)系統(tǒng)發(fā)育遲滯疾病,。主要是小腦蚓部發(fā)育不良加上其他異常,，常見(jiàn)癥狀是發(fā)作性氣喘，在新生兒期出現(xiàn)發(fā)作性呼吸急促或呼吸暫停,。眼球常有急促運(yùn)動(dòng),，智力發(fā)育遲鈍，由于小腦蚓部發(fā)育不良而致共濟(jì)失調(diào)和平衡障礙,，某些患者的視網(wǎng)膜發(fā)育不良或缺失,，導(dǎo)致先天性失明,，也有脈絡(luò)膜脫失者。此外,，尚可有多指(趾)畸形,、舌部腫瘤等。研究發(fā)現(xiàn),，Abelson helper integration site-1 (AHI1)基因突變可導(dǎo)致N-端Ahi1蛋白片段形成,，并導(dǎo)致Joubert綜合征。但是AHI1基因突變引起發(fā)育遲滯的機(jī)制尚不明確,。
本研究發(fā)現(xiàn),，亨廷頓-相關(guān)蛋白1（Hap1），一種對(duì)出生后幼鼠存活率起關(guān)鍵作用的蛋白,，可與全長(zhǎng)Ahi1蛋白相結(jié)合,，而不能與N-端Ahi1蛋白片段結(jié)合。這一Ahi1-Hap1結(jié)合在神經(jīng)分化過(guò)程中受到神經(jīng)生長(zhǎng)因子（NGF）的調(diào)控,。NGF誘導(dǎo)Hap1A的去磷酸化,，并降低其與Ahi1的結(jié)合，同時(shí)Hap1A在神經(jīng)突起末端的分布增多,。對(duì)小鼠腦組織蛋白的分析也表明,，Ahi1在胞漿而非突觸部分與磷酸化的Hap1A結(jié)合，提示Ahi1主要在神經(jīng)元胞體發(fā)揮生理作用,。對(duì)小鼠腦組織胞漿成分進(jìn)行抗Ahi1抗體免疫共沉淀后的質(zhì)譜分析發(fā)現(xiàn),，除Hap1外，Ahi1還與CEND1/BM88蛋白結(jié)合,。CEND1/BM88是一種特異表達(dá)于神經(jīng)元的蛋白,，參與調(diào)控神經(jīng)分化和神經(jīng)發(fā)生，在出生后小鼠的腦組織中高度表達(dá),。在Ahi1基因敲除小鼠的下丘腦組織中,，Ahi1蛋白的缺失導(dǎo)致CEND1/BM88水平下降，小鼠發(fā)育遲滯,。而在N2a細(xì)胞中過(guò)表達(dá)Ahi1則能穩(wěn)定CEND1/BM88,。此外，對(duì) Ahi1基因敲除小鼠下丘腦神經(jīng)元進(jìn)行原代培養(yǎng)發(fā)現(xiàn),，神經(jīng)元的突起較野生型減少,，長(zhǎng)度較短。而在該原代神經(jīng)元中過(guò)表達(dá)CEND1/BM88則可促進(jìn)其神經(jīng)元突起生長(zhǎng),。
本研究的發(fā)現(xiàn)提示,，在早期發(fā)育過(guò)程中，CEND1/BM88參與Ahi1相關(guān)的下丘腦神經(jīng)元分化,。上述發(fā)現(xiàn)對(duì)于研究Jourbert綜合征中出現(xiàn)的發(fā)育遲滯的相關(guān)病理機(jī)制提供了新的思路,。
該文章于2013年5月發(fā)表在journal Neuroscience 雜志上,，文章的的第一作者是湘雅醫(yī)學(xué)院和美國(guó)emory大學(xué)聯(lián)合培養(yǎng)的的博士研究生翁翎，李曉江研究員是本文章的通訊作者,，負(fù)責(zé)實(shí)驗(yàn)的設(shè)計(jì)、數(shù)據(jù)結(jié)果的分析,、文章的撰寫(xiě)和修改,。（生物谷Bioon.com）
doi:10.1523/JNEUROSCI.0119-13.2013
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Loss of Ahi1 Affects Early Development by Impairing BM88/Cend1-Mediated Neuronal Differentiation
Ling Weng1,2, Yung-Feng Lin3, Alina L. Li2, Chuan-En Wang2, Sen Yan2, Miao Sun2, Marta A. Gaertig2, Naureen Mitha2, Jun Kosaka4, Taketoshi Wakabayashi5, Xingshun Xu6, Beisha Tang1, Shihua Li2, and Xiao-Jiang Li2,7
Mutations in the Abelson helper integration site-1 (AHI1) gene result in N-terminal Ahi1 fragments and cause Joubert syndrome, an autosomal recessive brain malformation disorder associated with delayed development. How AHI1 mutations lead to delayed development remains unclear. Here we report that full-length, but not N-terminal, Ahi1 binds Hap1, a huntingtin-associated protein that is essential for the postnatal survival of mice and that this binding is regulated during neuronal differentiation by nerve growth factor. Nerve growth factor induces dephosphorylation of Hap1A and decreases its association with Ahi1, correlating with increased Hap1A distribution in neurite tips. Consistently, Ahi1 associates with phosphorylated Hap1A in cytosolic, but not in synaptosomal, fractions isolated from mouse brain, suggesting that Ahi1 functions mainly in the soma of neurons. Mass spectrometry analysis of cytosolic Ahi1 immunoprecipitates reveals that Ahi1 also binds Cend1 (cell cycle exit and neuronal differentiation protein 1)/BM88, a neuronal protein that mediates neuronal differentiation and is highly expressed in postnatal mouse brain. Loss of Ahi1 reduces the levels of Cend1 in the hypothalamus of Ahi1 KO mice, which show retarded growth during postnatal days. Overexpressed Ahi1 can stabilize Cend1 in cultured cells. Furthermore, overexpression of Cend1 can rescue the neurite extension defects of hypothalamic neurons from Ahi1 KO mice. Our findings suggest that Cend1 is involved in Ahi1-associated hypothalamic neuronal differentiation in early development, giving us fresh insight into the mechanism behind the delayed development in Joubert syndrome.
