Synaptic vesicle proteins and ATG9A self-organize in distinct vesicle phases within synapsin condensates

Ectopic expression in fibroblasts of synapsin 1 and synaptophysin is sufficient to generate condensates of vesicles highly reminiscent of synaptic vesicle (SV) clusters and with liquid-like properties. We show that unlike synaptophysin, other major integral SV membrane proteins fail to form condensates with synapsin, but coassemble into the clusters formed by synaptophysin and synapsin in this ectopic expression system. Another vesicle membrane protein, ATG9A, undergoes activity-dependent exo-endocytosis at synapses, raising questions about the relation of ATG9A traffic to the traffic of SVs. We have found that both in fibroblasts and in nerve terminals ATG9A does not coassemble into synaptophysin-positive vesicle condensates but localizes on a distinct class of vesicles that also assembles with synapsin but into a distinct phase. Our findings suggest that ATG9A undergoes differential sorting relative to SV proteins and also point to a dual role of synapsin in controlling clustering at synapses of SVs and ATG9A vesicles

在成纤维细胞中异位表达突触蛋白1（synapsin 1）与突触泡蛋白（synaptophysin），即可生成与突触囊泡（synaptic vesicle, SV）簇高度相似且具备类液体特性的囊泡聚集体。研究显示，与突触泡蛋白不同，其余主要突触囊泡整合膜蛋白无法与突触蛋白1形成聚集体，但可在该异位表达系统中，共组装进入突触泡蛋白与突触蛋白1所构建的簇结构内。另一囊泡膜蛋白ATG9A可在突触部位发生活性依赖的胞吞胞吐循环，这引发了关于其运输通路与突触囊泡运输通路之间关联的相关疑问。本研究发现，无论是在成纤维细胞还是神经末梢中，ATG9A均不会与突触泡蛋白阳性的囊泡聚集体发生共组装，而是定位于一类独特的囊泡群体中；该类囊泡同样可与突触蛋白1结合，但会形成不同的相结构。本研究结果表明，相较于突触囊泡蛋白，ATG9A存在差异化分选机制，同时也揭示了突触蛋白1在突触处可同时调控突触囊泡与ATG9A囊泡的聚集过程，发挥双重调控作用。