A presynaptic spectrin network controls active zone assembly and neurotransmitter release

Abstract

We have previously reported that Drosophila Tenectin (Tnc) recruits PS2/PS integrin to ensure structural and functional integrity at larval NMJs (Wang et al., 2018). In muscles, Tnc/integrin engages the spectrin network to regulate the size and architecture of synaptic boutons. In neurons, Tnc/integrin controls neurotransmitter release. Here we show that presynaptic Tnc/integrin modulates the synaptic accumulation of key active zone components, including the Ca 2+ channel Cac and the active zone scaffold Brp. Presynaptic α-Spectrin appears to be both required and sufficient for the recruitment of Cac and Brp. We visualized the endogenous α-Spectrin and found that Tnc controls spectrin recruitment at synaptic terminals. Thus, Tnc/integrin anchors the presynaptic spectrin network and ensures the proper assembly and function of the active zones. Since pre-and postsynaptic Tnc/integrin limit each other, we hypothesize that this pathway links dynamic changes within the synaptic cleft to changes in synaptic structure and function. The synaptic extracellular matrix (ECM) contains various synaptic organizers and diffusible molecules that establish and modify synaptic connections during development. ECM receptors, including integrins, transduce these signals by sensing the changes in the synaptic environment and orchestrating the appropriate changes in synapse structure and function (Chavis and Westbrook, 2001;Huang et al., 2006). Disruption of integrins impair synapse development and maturation, neurotransmission, and long-term synaptic plasticity (reviewed in (Park and Goda, 2016)). Integrins together with their ECM ligands form complexes with cytoskeletal proteins and active zone (AZ) components (Carlson et al., 2010;Nishimune et al., 2004;Sunderland et al., 2000). However, it remains unclear how these interactions could influence neurotransmitter release.Spectrin, a principal component of the membrane skeleton, is critical for synaptic transmission (Featherstone et al., 2001;Goodman, 1999). Spectrin subunits function as docking platforms for membrane channels, neurotransmitter receptors and adhesion molecules (reviewed in (Machnicka et al., 2014)). For example, spectrins associate with presynaptic Ca 2+ channels in synaptosome lysates (Carlson et al., 2010;Khanna et al., 2007a;Khanna et al., 2007b). Also, a spectrin-containing presynaptic web enables AZ function in the rodent CNS (Phillips et al., 2001). Recent super-resolution fluorescence microscopy uncovered periodic actin/spectrin lattices along axons, dendrites and spines in a broad range of neuronal cell types (He et al., 2016;Qu et al., 2017;Xu et al., 2013); however, at presynaptic and postsynaptic sites, these patterns were absent (Sidenstein et al., 2016).We have previously found that Drosophila Tenectin (Tnc), an integrin ligand that accumulates in the synaptic cleft at the larval NMJ, modulates synapse development and function (Wang et al., 2018). Tnc recruits αPS2/βPS integrin and forms cis active complexes with distinct pre-and post-synaptic functions....