Novel Strategies for Glutamate Clearance in the Glia-Deprived Synaptic Hub of C. elegans

Abstract

Brain function requires the ability to form neuronal circuits that mediate focused and accurate communication. Since the vast majority of brain synapses use Glutamate (Glu) as their neurotransmitter, unintended spillover of Glu between adjacent synapses is a critical challenge. To ensure accurate neurotransmission and avert synaptic mix-up, specialized Glu Transporters (GluTs) clear the synapse of released Glu. While classical views of neuronal morphology and physiology depict isolated spiny synapses enwrapped by GluT-expressing glia, in reality, a considerable portion of synapses are flat, glial coverage in some parts of the brain is rather sparse, and extracellular space is larger than previously estimated. This suggests that diffusion in interstitial fluids might have an important role in Glu clearance in these synapses. To understand basic principles of Glu clearance in flat-, glia-deprived synapses, we study the physiology of neuronal circuits in the C. elegans nerve ring, the nematodes aspiny synaptic hub. We use behavioral assays, Ca2+ imaging, and iGluSnFR to follow synaptic activity in intact animals. We find that synapses in a nociceptive avoidance circuit are dramatically affected by distal GluTs, while an adjacent chemoattraction circuit is controlled by proximal GluTs. We also find that pharyngeal pulsatility and mobility, which could agitate interstitial fluids, are critical for synaptic physiology. We therefore conclude that robust Glu clearance in the nematode is provided differentially by distal and proximal GluTs, aided by agitation of interstitial fluids. Such principles might be informative in determining additional factors that contribute to robust Glu clearance in other neuronal systems.\n\nSignificance StatementThe nervous system depends on faithful relay of information without inadvertent mixing of signals between neuronal circuits. Classical views of the nervous system depict isolated synapses, enwrapped by glia that express neurotransmitter-transporters. However, this view is incomplete, since many synapses are flat, deprived of glia, and exposed to a larger-than-expected extracellular space. We use optogenetic tools to investigate glutamate clearance strategies in the aspiny and glia-deprived synaptic hub of intact nematodes. We find a division of labor among Glutamate transporters: while some transporters display classical localization near the synapses, others are distal, and cooperate with agitation of interstitial fluids to prevent glutamate accumulation. These novel principles might contribute to synaptic clearance in higher animals, affecting normal neuronal physiology and disease.