Postsynaptic Neuroligin-1 Mediates Presynaptic Endocytosis During Neuronal Activity.

Jiaqi Keith Luo,Holly Melland,Jess Nithianantharajah,Sarah L Gordon

doi:10.3389/fnmol.2021.744845

Abstract

Fast, high-fidelity neurotransmission and synaptic efficacy requires tightly regulated coordination of pre- and postsynaptic compartments and alignment of presynaptic release sites with postsynaptic receptor nanodomains. Neuroligin-1 (Nlgn1) is a postsynaptic cell-adhesion protein exclusively localised to excitatory synapses that is crucial for coordinating the transsynaptic alignment of presynaptic release sites with postsynaptic AMPA receptors as well as postsynaptic transmission and plasticity. However, little is understood about whether the postsynaptic machinery can mediate the molecular architecture and activity of the presynaptic nerve terminal, and thus it remains unclear whether there are presynaptic contributions to Nlgn1-dependent control of signalling and plasticity. Here, we employed a presynaptic reporter of neurotransmitter release and synaptic vesicle dynamics, synaptophysin-pHluorin (sypHy), to directly assess the presynaptic impact of loss of Nlgn1. We show that lack of Nlgn1 had no effect on the size of the readily releasable or entire recycling pool of synaptic vesicles, nor did it impact exocytosis. However, we observed significant changes in the retrieval of synaptic vesicles by compensatory endocytosis, specifically during activity. Our data extends growing evidence that synaptic adhesion molecules critical for forming transsynaptic scaffolds are also important for regulating activity-induced endocytosis at the presynapse.

Highlights

Activity-dependent neural signalling and plasticity relies on the coordination of pre- and postsynaptic compartments
The pHluorin moiety is tagged to the lumenal domain of a synaptic vesicle protein; its fluorescence is quenched at rest by the acidic intravesicular pH and increases upon exocytosis as the vesicle fuses with the plasma membrane and the pHluorin is exposed to the neutral extracellular environment (Figure 1A). sypHy fluorescence decreases as the vesicle membrane and protein cargo are retrieved by compensatory endocytosis and vesicles reacidify, but this can be blocked by the use of the vATPase inhibitor bafilomycin (Figure 1A)
Through specific examination of presynaptic activity independent of postsynaptic signalling, we have revealed that the loss of Nlgn1 has transsynaptic impacts on synaptic vesicle cycling

Summary

Introduction

Activity-dependent neural signalling and plasticity relies on the coordination of pre- and postsynaptic compartments. The alignment of presynaptic release sites with postsynaptic receptor nanodomains is important for tightly regulating synaptic efficacy and for fast, high fidelity neurotransmission This critically relies on transsynaptic organisation by adhesion molecules such as the neurexin–neuroligin complex (Sudhof, 2008; Haas et al, 2018). Nlgn is a single pass transmembrane protein with an extracellular catalytically inactive acetylcholine esterase domain and an intracellular c-terminus which interacts with scaffolding proteins such as PSD-95 (Ichtchenko et al, 1995; Irie et al, 1997) It forms a constitutive homo- or heterodimer (with its family member neuroligin-3, Nlgn3), and via its extracellular N-terminal domain binds to two presynaptic neurexin molecules in a calcium-dependent manner to mediate transsynaptic cell adhesion (Nguyen and Sudhof, 1997; Arac et al, 2007). NMDAR-mediated EPSC amplitude and NMDA/AMPA ratio are robustly reduced in both in vitro and in vivo Nlgn knockdown or knockout systems (Chubykin et al, 2007; Kim et al, 2008; Blundell et al, 2010; Jung et al, 2010; Kwon et al, 2012; Budreck et al, 2013; Espinosa et al, 2015; Jiang et al, 2017)

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