Physiological Perspectives on Molecular Mechanisms and Regulation of Vesicular Glutamate Transport: Lessons From Calyx of Held Synapses.

Tetsuya Hori,Shigeo Takamori

doi:10.3389/fncel.2021.811892

Abstract

Accumulation of glutamate, the primary excitatory neurotransmitter in the mammalian central nervous system, into presynaptic synaptic vesicles (SVs) depends upon three vesicular glutamate transporters (VGLUTs). Since VGLUTs are driven by a proton electrochemical gradient across the SV membrane generated by vacuolar-type H+-ATPases (V-ATPases), the rate of glutamate transport into SVs, as well as the amount of glutamate in SVs at equilibrium, are influenced by activities of both VGLUTs and V-ATPase. Despite emerging evidence that suggests various factors influencing glutamate transport by VGLUTs in vitro, little has been reported in physiological or pathological contexts to date. Historically, this was partially due to a lack of appropriate methods to monitor glutamate loading into SVs in living synapses. Furthermore, whether or not glutamate refilling of SVs can be rate-limiting for synaptic transmission is not well understood, primarily due to a lack of knowledge concerning the time required for vesicle reuse and refilling during repetitive stimulation. In this review, we first introduce a unique electrophysiological method to monitor glutamate refilling by VGLUTs in a giant model synapse from the calyx of Held in rodent brainstem slices, and we discuss the advantages and limitations of the method. We then introduce the current understanding of factors that potentially alter the amount and rate of glutamate refilling of SVs in this synapse, and discuss open questions from physiological viewpoints.

Highlights

Upon the arrival of action potentials at presynaptic sites, synaptic vesicles (SVs) that store neurotransmitters in the lumen undergo exocytic fusion with the presynaptic plasma membrane, thereby releasing their contents to neighboring neurons
Secretory carrier membrane protein 5 (SCAMP5), one of the genuine SV residents (Takamori et al, 2006), is responsible for proper sorting of NHE6 to fusion-competent SVs, and secretory carrier membrane protein 5 (SCAMP5) knock-down results in a similar reduction in mEPSCs. Since both NHE6 and SCAMP5 are associated with autism spectrum disorder (ASDs; Morrow et al, 2008; Castermans et al, 2010; Kondapalli et al, 2014; Schwede et al, 2014), regulation of vesicular glutamate content by NHE6 and SCAMP5 may be implicated in the pathogenesis of ASDs
The notion that vesicular glutamate transporters (VGLUTs) levels dictate the speed of refilling, but not the steady-state levels of glutamate content observed in the calyx of Held synapses is somewhat contradictory to earlier biochemical transport assays using pharmacological manipulations, arguing that the number of available VGLUTs on SVs influences the magnitude and to a lesser extent, the rate of glutamate uptake (Wilson et al, 2005), the reason remains enigmatic

Summary

Introduction

Upon the arrival of action potentials at presynaptic sites, synaptic vesicles (SVs) that store neurotransmitters in the lumen undergo exocytic fusion with the presynaptic plasma membrane, thereby releasing their contents to neighboring neurons. At calyx of Held synapses, Cl− concentration at presynaptic terminals was ∼21 mM (Price and Trussell, 2006), which seems to be optimal for glutamate transport measured in vitro.

Results

Conclusion