Dynamic mechanisms of neuroligin-dependent presynaptic terminal assembly in living cortical neurons

Shasta L Sabo,Luke AD Bury

doi:10.1186/1749-8104-9-13

Shasta L Sabo, Luke AD Bury

Open Access

https://doi.org/10.1186/1749-8104-9-13

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Abstract

BackgroundSynapse formation occurs when synaptogenic signals trigger coordinated development of pre and postsynaptic structures. One of the best-characterized synaptogenic signals is trans-synaptic adhesion. However, it remains unclear how synaptic proteins are recruited to sites of adhesion. In particular, it is unknown whether synaptogenic signals attract synaptic vesicle (SV) and active zone (AZ) proteins to nascent synapses or instead predominantly function to create sites that are capable of forming synapses. It is also unclear how labile synaptic proteins are at developing synapses after their initial recruitment. To address these issues, we used long-term, live confocal imaging of presynaptic terminal formation in cultured cortical neurons after contact with the synaptogenic postsynaptic adhesion proteins neuroligin-1 or SynCAM-1.ResultsSurprisingly, we find that trans-synaptic adhesion does not attract SV or AZ proteins nor alter their transport. In addition, although neurexin (the presynaptic partner of neuroligin) typically accumulates over the entire region of contact between axons and neuroligin-1-expressing cells, SV proteins selectively assemble at spots of enhanced neurexin clustering. The arrival and maintenance of SV proteins at these sites is highly variable over the course of minutes to hours, and this variability correlates with neurexin levels at individual synapses.ConclusionsTogether, our data support a model of synaptogenesis where presynaptic proteins are trapped at specific axonal sites, where they are stabilized by trans-synaptic adhesion signaling.

Highlights

Synapse formation occurs when synaptogenic signals trigger coordinated development of pre and postsynaptic structures
Our data support a model of synaptogenesis in which presynaptic proteins are trapped at, but not actively attracted to, specific axonal sites, where they are stabilized by trans-synaptic adhesion signaling
We developed a strategy for long-term imaging of synaptic protein recruitment to nascent sites of presynaptic terminal formation, initiated by contact with neuroligin-1expressing HEK293 cells as a proxy for postsynaptic dendrites

Summary

Introduction

Synapse formation occurs when synaptogenic signals trigger coordinated development of pre and postsynaptic structures. It is unknown whether synaptogenic signals attract synaptic vesicle (SV) and active zone (AZ) proteins to nascent synapses or instead predominantly function to create sites that are capable of forming synapses It is unclear how labile synaptic proteins are at developing synapses after their initial recruitment. A number of studies have focused on understanding the functions and mechanisms of trans-synaptic adhesion molecules in recent years, it remains unclear how synaptogenic adhesion results in presynaptic protein recruitment and synaptogenesis [16,17] It is not known whether trans-synaptic adhesion actively attracts synaptic proteins to sites of signaling or acts primarily to stabilize proteins at nascent terminals. It is unclear if trans-synaptic adhesion regulates synaptic protein levels at individual synapses or primarily creates sites in the axon that are capable of synapse formation while other cellular processes regulate levels of recruitment

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