Department of Molecular and Cellular Physiology
Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305.
J Neurosci. 2020 Oct 14;40(42):8088-8102. doi: 10.1523/JNEUROSCI.0454-20.2020. Epub 2020 Sep 24.
Emerging evidence supports roles for secreted extracellular matrix proteins in boosting synaptogenesis, synaptic transmission, and synaptic plasticity. SPARCL1 (also known as Hevin), a secreted non-neuronal protein, was reported to increase synaptogenesis by simultaneously binding to presynaptic neurexin-1α and to postsynaptic neuroligin-1B, thereby catalyzing formation of trans-synaptic neurexin/neuroligin complexes. However, neurexins and neuroligins do not themselves mediate synaptogenesis, raising the question of how SPARCL1 enhances synapse formation by binding to these molecules. Moreover, it remained unclear whether SPARCL1 acts on all synapses containing neurexins and neuroligins or only on a subset of synapses, and whether it enhances synaptic transmission in addition to boosting synaptogenesis or induces silent synapses. To explore these questions, we examined the synaptic effects of SPARCL1 and their dependence on neurexins and neuroligins. Using mixed neuronal and glial cultures from neonatal mouse cortex of both sexes, we show that SPARCL1 selectively increases excitatory but not inhibitory synapse numbers, enhances excitatory but not inhibitory synaptic transmission, and augments NMDAR-mediated synaptic responses more than AMPAR-mediated synaptic responses. None of these effects were mediated by SPARCL1-binding to neurexins or neuroligins. Neurons from triple or from quadruple conditional KO mice that lacked all neurexins or all neuroligins were fully responsive to SPARCL1. Together, our results reveal that SPARCL1 selectively boosts excitatory but not inhibitory synaptogenesis and synaptic transmission by a novel mechanism that is independent of neurexins and neuroligins. Emerging evidence supports roles for extracellular matrix proteins in boosting synapse formation and function. Previous studies demonstrated that SPARCL1, a secreted non-neuronal protein, promotes synapse formation in rodent and human neurons. However, it remained unclear whether SPARCL1 acts on all or on only a subset of synapses, induces functional or largely inactive synapses, and generates synapses by bridging presynaptic neurexins and postsynaptic neuroligins. Here, we report that SPARCL1 selectively induces excitatory synapses, increases their efficacy, and enhances their NMDAR content. Moreover, using rigorous genetic manipulations, we show that SPARCL1 does not require neurexins and neuroligins for its activity. Thus, SPARCL1 selectively boosts excitatory synaptogenesis and synaptic transmission by a novel mechanism that is independent of neurexins and neuroligins.
越来越多的证据表明，细胞外基质蛋白在促进突触发生、突触传递和突触可塑性方面发挥着作用。SPARCL1（也称为 Hevin）是一种分泌型非神经元蛋白，据报道，它通过同时与突触前神经连接蛋白 1α和突触后神经黏附素 1B 结合，从而催化形成跨突触神经连接蛋白/神经黏附素复合物，来促进突触发生。然而，神经连接蛋白和神经黏附素本身并不能介导突触发生，这就提出了一个问题，即 SPARCL1 如何通过与这些分子结合来增强突触形成。此外，目前尚不清楚 SPARCL1 是作用于所有含有神经连接蛋白和神经黏附素的突触，还是仅作用于部分突触，以及它是否除了促进突触发生外，还增强突触传递，或者诱导沉默突触。为了探讨这些问题，我们研究了 SPARCL1 的突触效应及其对神经连接蛋白和神经黏附素的依赖性。我们使用来自雄性和雌性新生小鼠大脑的混合神经元和神经胶质培养物，结果表明 SPARCL1 选择性地增加兴奋性突触但不增加抑制性突触的数量，增强兴奋性突触但不增强抑制性突触传递，并增强 NMDAR 介导的突触反应超过 AMPAR 介导的突触反应。这些效应都不是由 SPARCL1 与神经连接蛋白或神经黏附素结合介导的。缺乏所有神经连接蛋白或所有神经黏附素的三 或四 条件性 KO 小鼠的神经元对 SPARCL1 完全有反应。总之，我们的研究结果表明，SPARCL1 通过一种独立于神经连接蛋白和神经黏附素的新机制，选择性地促进兴奋性突触但不促进抑制性突触的发生和传递。越来越多的证据表明细胞外基质蛋白在促进突触形成和功能方面发挥着作用。先前的研究表明，分泌型非神经元蛋白 SPARCL1 促进了啮齿动物和人类神经元的突触形成。然而，目前尚不清楚 SPARCL1 是否作用于所有突触或仅作用于部分突触，诱导功能性或主要无功能的突触，以及通过桥接突触前神经连接蛋白和突触后神经黏附素来产生突触。在这里，我们报告 SPARCL1 选择性地诱导兴奋性突触，增加其功效，并增强其 NMDAR 含量。此外，我们使用严格的遗传操作，表明 SPARCL1 不需要神经连接蛋白和神经黏附素来发挥作用。因此，SPARCL1 通过一种独立于神经连接蛋白和神经黏附素的新机制，选择性地促进兴奋性突触的发生和传递。
