Joubert综合征（Joubert syndrome， JBTS）是一种十分少见的常染色体隐性遗传神经系统发育迟滞疾病。主要是小脑蚓部发育不良加上其他异常，常见症状是发作性气喘，在新生儿期出现发作性呼吸急促或呼吸暂停。眼球常有急促运动，智力发育迟钝，由于小脑蚓部发育不良而致共济失调和平衡障碍，某些患者的视网膜发育不良或缺失，导致先天性失明，也有脉络膜脱失者。此外，尚可有多指(趾)畸形、舌部肿瘤等。研究发现，Abelson helper integration site-1 (AHI1)基因突变可导致N-端Ahi1蛋白片段形成，并导致Joubert综合征。但是AHI1基因突变引起发育迟滞的机制尚不明确。
本研究发现，亨廷顿-相关蛋白1（Hap1），一种对出生后幼鼠存活率起关键作用的蛋白，可与全长Ahi1蛋白相结合，而不能与N-端Ahi1蛋白片段结合。这一Ahi1-Hap1结合在神经分化过程中受到神经生长因子（NGF）的调控。NGF诱导Hap1A的去磷酸化，并降低其与Ahi1的结合，同时Hap1A在神经突起末端的分布增多。对小鼠脑组织蛋白的分析也表明，Ahi1在胞浆而非突触部分与磷酸化的Hap1A结合，提示Ahi1主要在神经元胞体发挥生理作用。对小鼠脑组织胞浆成分进行抗Ahi1抗体免疫共沉淀后的质谱分析发现，除Hap1外，Ahi1还与CEND1/BM88蛋白结合。CEND1/BM88是一种特异表达于神经元的蛋白，参与调控神经分化和神经发生，在出生后小鼠的脑组织中高度表达。在Ahi1基因敲除小鼠的下丘脑组织中，Ahi1蛋白的缺失导致CEND1/BM88水平下降，小鼠发育迟滞。而在N2a细胞中过表达Ahi1则能稳定CEND1/BM88。此外，对 Ahi1基因敲除小鼠下丘脑神经元进行原代培养发现，神经元的突起较野生型减少，长度较短。而在该原代神经元中过表达CEND1/BM88则可促进其神经元突起生长。
本研究的发现提示，在早期发育过程中，CEND1/BM88参与Ahi1相关的下丘脑神经元分化。上述发现对于研究Jourbert综合征中出现的发育迟滞的相关病理机制提供了新的思路。
该文章于2013年5月发表在journal Neuroscience 杂志上，文章的的第一作者是湘雅医学院和美国emory大学联合培养的的博士研究生翁翎，李晓江研究员是本文章的通讯作者，负责实验的设计、数据结果的分析、文章的撰写和修改。（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.
