Identification of a stereotypic molecular arrangement of endogenous glycine receptors at spinal cord synapses.

Stephanie A Maynard,Antoine Triller,Olivier Gemin,Carmen Villmann,Philippe Rostaing,Christian G Specht,Natascha Schaefer,Andréa Dumoulin,Adrien Candat

doi:10.7554/elife.74441

Abstract

Precise quantitative information about the molecular architecture of synapses is essential to understanding the functional specificity and downstream signaling processes at specific populations of synapses. Glycine receptors (GlyRs) are the primary fast inhibitory neurotransmitter receptors in the spinal cord and brainstem. These inhibitory glycinergic networks crucially regulate motor and sensory processes. Thus far, the nanoscale organization of GlyRs underlying the different network specificities has not been defined. Here, we have quantitatively characterized the molecular arrangement and ultra-structure of glycinergic synapses in spinal cord tissue using quantitative super-resolution correlative light and electron microscopy. We show that endogenous GlyRs exhibit equal receptor-scaffold occupancy and constant packing densities of about 2000 GlyRs µm-2 at synapses across the spinal cord and throughout adulthood, even though ventral horn synapses have twice the total copy numbers, larger postsynaptic domains, and more convoluted morphologies than dorsal horn synapses. We demonstrate that this stereotypic molecular arrangement is maintained at glycinergic synapses in the oscillator mouse model of the neuromotor disease hyperekplexia despite a decrease in synapse size, indicating that the molecular organization of GlyRs is preserved in this hypomorph. We thus conclude that the morphology and size of inhibitory postsynaptic specializations rather than differences in GlyR packing determine the postsynaptic strength of glycinergic neurotransmission in motor and sensory spinal cord networks.

Highlights

Synaptic transmission relies on the integration of spatially and temporally controlled signals by neurotransmitter receptors in the postsynaptic membrane
We show that Glycine receptors (GlyRs) are packed at a constant density of about 2000 receptor complexes per μm2 at mature synapses, suggesting that they are assembled in a stereotypic fashion
Combining single molecule PALM imaging, molecular counting and 3D electron microscopy we have shown that glycinergic synapses in different regions of the spinal cord follow the same structural principle, insofar as their receptor-scaffold occupancy and packing densities are the same

Summary

Introduction

Synaptic transmission relies on the integration of spatially and temporally controlled signals by neurotransmitter receptors in the postsynaptic membrane. Glycinergic neurons arise from different embryonic origins, with specific types of neurons residing in characteristic layers of the spinal cord (Lu et al, 2015). Depending on their location, glycinergic neurons mediate sensory and motor information in the dorsal and ventral spinal cord, respectively, which requires high reliability and fidelity of transmission (Alvarez, 2017). The electrophysiological properties of glycinergic currents indicate that only a limited number of receptors are activated by the release of a single synaptic vesicle (Oleskevich et al, 1999, Singer & Berger, 1999), suggesting that the nanoscale organization of the receptors determines signal amplitude

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