Postsynaptic Control of Trans-synaptic Nanostructure

Abstract

Many synaptic proteins form subsynaptic nanoclusters to execute their vital functions in information transmission in the central nervous system. Nanoclusters are dynamic, with properties that change over multiple timescales and in response to specific stimuli, supporting a wide range of functional states in the synapse. We still lack a complete accounting of the factors shaping properties of synaptic nanostructure, especially the trans-synaptic alignment of nanoclusters, which strengthens communication between neurons. I tested how the nanostructure of the synapse overall is influenced by the postsynaptic compartment. First, I tested if postsynaptic cell identity results in distinct organization of core, ubiquitous excitatory synapse proteins by comparing excitatory synapses forming onto principal cells with those forming onto parvalbumin expressing interneurons (PV-INs). PV-INs contained larger synapses with larger, denser nanoclusters of postsynaptic scaffold protein PSD-95, which were also fewer in number when accounting for synapse size. Postsynaptic cell identity also impacted presynaptic organization, as Munc13-1, vital for neurotransmitter vesicle docking and priming, formed larger but less dense and more numerous nanoclusters at excitatory synapses forming onto PV-INs. Trans-synaptic alignment also differed at PV-INs, occurring further away and over a wider distance than at excitatory synapses forming onto principal cells. These results demonstrate how postsynaptic cellular contexts yield distinct trans-synaptic protein organization. Second, I tested if postsynaptic ionotropic glutamate receptors (iGluRs) influence synaptic nanostructure. Knockout of GluN1, the NMDAR obligate subunit, resulted in profound pre- and postsynaptic reorganization. The resulting synapses were larger and contained denser, more numerous nanoclusters of PSD-95 and Munc13-1. Intriguingly, more Munc13-1 nanoclusters were enriched with PSD-95, suggesting enhanced trans-synaptic alignment. Prolonged NMDAR activity block did not fully mimic the changes to Munc13-1, suggesting that NMDAR presence regulates presynaptic nanostructure. Furthermore, mutations to the GluA2 AMPAR subunit differentially altered PSD-95 nanocluster number and size. These data demonstrate that iGluRs have the underappreciated capacity to shape the organization of other synaptic proteins. Overall, my findings show both cis- and trans-synaptic nanostructure is influenced by postsynaptic factors. Specific protein expression profiles of postsynaptic cells, including variability in glutamate receptor subtypes, may provide a basis for circuit-specific diversity of excitatory synapse function and organization.