Synapse type-specific proteomic dissection identifies IgSF8 as a hippocampal CA3 microcircuit organizer

Nuno Apóstolo,Sybren Portegies,Giuseppe Condomitti,Samuel N Smukowski,Jolijn Ten Bos,Keimpe D Wierda,Natalia V Gounko,Jeffrey N Savas,Davide Comoletti,Jeroen Vanderlinden,Joris De Wit,Vasily Rybakin,Laura Trobiani,Kristel M Vennekens

doi:10.1038/s41467-020-18956-x

Abstract

Excitatory and inhibitory neurons are connected into microcircuits that generate circuit output. Central in the hippocampal CA3 microcircuit is the mossy fiber (MF) synapse, which provides powerful direct excitatory input and indirect feedforward inhibition to CA3 pyramidal neurons. Here, we dissect its cell-surface protein (CSP) composition to discover novel regulators of MF synaptic connectivity. Proteomic profiling of isolated MF synaptosomes uncovers a rich CSP composition, including many CSPs without synaptic function and several that are uncharacterized. Cell-surface interactome screening identifies IgSF8 as a neuronal receptor enriched in the MF pathway. Presynaptic Igsf8 deletion impairs MF synaptic architecture and robustly decreases the density of bouton filopodia that provide feedforward inhibition. Consequently, IgSF8 loss impairs excitation/inhibition balance and increases excitability of CA3 pyramidal neurons. Our results provide insight into the CSP landscape and interactome of a specific excitatory synapse and reveal IgSF8 as a critical regulator of CA3 microcircuit connectivity and function.

Highlights

Excitatory and inhibitory neurons are connected into microcircuits that generate circuit output
To isolate mossy fiber (MF) synapses and start uncovering their cell-surface protein (CSP) composition, we relied on two key features of the MF synapse: its large size and the presence of puncta adherentia (PA), a specialized type of adhesive junction found at large synapses[33] that is morphologically and molecularly distinct from the synaptic junction
Electron microscopy (EM) analysis showed the presence of large synaptosomes with a presynaptic compartment packed with synaptic vesicles engulfing a postsynaptic compartment (Supplementary Fig. 1b)

Summary

Introduction

Excitatory and inhibitory neurons are connected into microcircuits that generate circuit output. Central in the hippocampal CA3 microcircuit is the mossy fiber (MF) synapse, which provides powerful direct excitatory input and indirect feedforward inhibition to CA3 pyramidal neurons. Our results provide insight into the CSP landscape and interactome of a specific excitatory synapse and reveal IgSF8 as a critical regulator of CA3 microcircuit connectivity and function. Each MF bouton contains a large vesicle pool and multiple release sites capable of providing powerful excitatory input to CA3 pyramidal neurons. This robust excitation is controlled by strong feedforward inhibition of CA3 pyramidal neurons[8], mediated by filopodia extending from the MF bouton that form excitatory synapses onto interneurons in stratum lucidum (SL)[9] (Fig. 1a). Feedforward inhibition decreases with aging, resulting in CA3 hyperactivity[16]

Methods

Results

Conclusion