Neuroligin 2 Governs Synaptic Morphology and Function Through RACK1-Cofilin Signaling in Drosophila

Neuroligins are transmembrane cell adhesion proteins well-known for their genetic links to autism spectrum disorders. Neuroligins can function by regulating the actin cytoskeleton, however the factors and mechanisms involved are still largely unknown. Here, using the Drosophila neuromuscular junction as a model, we reveal that F-Actin assembly at the Drosophila NMJ is controlled through Cofilin signaling mediated by an interaction between DNlg2 and RACK1, factors not previously known to work together. The deletion of DNlg2 displays disrupted RACK1-Cofilin signaling pathway with diminished actin cytoskeleton proteo-stasis at the terminal of the NMJ, aberrant NMJ structure, reduced synaptic transmission, and abnormal locomotion at the third-instar larval stage. Overexpression of wildtype and activated Cofilin in muscles are sufficient to rescue the morphological and physiological defects in dnlg2 mutants, while inactivated Cofilin is not. Since the DNlg2 paralog DNlg1 is known to regulate F-actin assembly mainly via a specific interaction with WAVE complex, our present work suggests that the orchestration of F-actin by Neuroligins is a highly dynamic and complex process critical for neural connectivity.

神经连接蛋白（Neuroligins）是一类跨膜细胞黏附蛋白，因其与孤独症谱系障碍（autism spectrum disorders）存在明确遗传关联而被广泛关注。神经连接蛋白可通过调控肌动蛋白细胞骨架发挥功能，但其中涉及的具体调控因子与分子机制仍未完全阐明。本研究以果蝇神经肌肉接头（Drosophila neuromuscular junction, 简称NMJ）为模型，揭示了果蝇NMJ处的F-肌动蛋白组装是通过由DNlg2（果蝇神经连接蛋白2）与RACK1（激活蛋白激酶C受体1）相互作用介导的丝切蛋白（Cofilin）信号通路调控的——这两种因子此前从未被报道存在协同作用。敲除DNlg2会破坏RACK1-丝切蛋白信号通路，导致NMJ末端的肌动蛋白细胞骨架蛋白质稳态失衡、NMJ结构异常、突触传递功能减弱，并引发三龄幼虫期运动行为异常。在肌肉细胞中过表达野生型与激活型丝切蛋白，足以挽救dnlg2突变体的形态与生理缺陷，而失活型丝切蛋白则无此挽救效果。鉴于其旁系同源基因果蝇神经连接蛋白1（DNlg1）主要通过与WAVE复合物（WAVE complex）的特异性相互作用调控F-肌动蛋白组装，本研究表明，神经连接蛋白对F-肌动蛋白的调控编排是一个高度动态且复杂的过程，对神经连接（neural connectivity）至关重要。