Jenkins Paul M, Kim Namsoo, Jones Steven L, Tseng Wei Chou, Svitkina Tatyana M, Yin Henry H, Bennett Vann
Howard Hughes Medical Institute and Department of Biochemistry.
Department of Psychology and Neuroscience.
Proc Natl Acad Sci U S A. 2015 Jan 27;112(4):957-64. doi: 10.1073/pnas.1416544112. Epub 2014 Dec 31.
Axon initial segments (AISs) and nodes of Ranvier are sites of clustering of voltage-gated sodium channels (VGSCs) in nervous systems of jawed vertebrates that facilitate fast long-distance electrical signaling. We demonstrate that proximal axonal polarity as well as assembly of the AIS and normal morphogenesis of nodes of Ranvier all require a heretofore uncharacterized alternatively spliced giant exon of ankyrin-G (AnkG). This exon has sequence similarity to I-connectin/Titin and was acquired after the first round of whole-genome duplication by the ancestral ANK2/ANK3 gene in early vertebrates before development of myelin. The giant exon resulted in a new nervous system-specific 480-kDa polypeptide combining previously known features of ANK repeats and β-spectrin-binding activity with a fibrous domain nearly 150 nm in length. We elucidate previously undescribed functions for giant AnkG, including recruitment of β4 spectrin to the AIS that likely is regulated by phosphorylation, and demonstrate that 480-kDa AnkG is a major component of the AIS membrane "undercoat' imaged by platinum replica electron microscopy. Surprisingly, giant AnkG-knockout neurons completely lacking known AIS components still retain distal axonal polarity and generate action potentials (APs), although with abnormal frequency. Giant AnkG-deficient mice live to weaning and provide a rationale for survival of humans with severe cognitive dysfunction bearing a truncating mutation in the giant exon. The giant exon of AnkG is required for assembly of the AIS and nodes of Ranvier and was a transformative innovation in evolution of the vertebrate nervous system that now is a potential target in neurodevelopmental disorders.
轴突起始段(AISs)和郎飞结是颌口脊椎动物神经系统中电压门控钠通道(VGSCs)聚集的部位,有助于快速的长距离电信号传导。我们证明,近端轴突极性以及AIS的组装和郎飞结的正常形态发生均需要锚蛋白G(AnkG)的一个迄今未被描述的可变剪接巨型外显子。该外显子与I-连接蛋白/肌联蛋白具有序列相似性,是在早期脊椎动物髓鞘形成之前,由祖先的ANK2/ANK3基因在第一轮全基因组复制后获得的。这个巨型外显子产生了一种新的神经系统特异性480 kDa多肽,它将ANK重复序列和β-血影蛋白结合活性的已知特征与一个长度近150 nm的纤维结构域结合在一起。我们阐明了巨型AnkG以前未被描述的功能,包括将β4血影蛋白招募到AIS,这可能受磷酸化调节,并证明480 kDa的AnkG是通过铂复型电子显微镜成像的AIS膜“底层”的主要成分。令人惊讶的是,完全缺乏已知AIS成分的巨型AnkG基因敲除神经元仍然保留远端轴突极性并产生动作电位(APs),尽管频率异常。巨型AnkG缺陷小鼠能活到断奶,这为患有严重认知功能障碍且巨型外显子发生截短突变的人类的存活提供了一个解释。AnkG的巨型外显子是AIS和郎飞结组装所必需的,是脊椎动物神经系统进化中的一项变革性创新,现在是神经发育障碍的一个潜在靶点。