Molecular Dissection of Synaptic Remodeling in GABAergic Neurons

Abstract

Synaptic circuits are dynamically refined during development as synapses are either stabilized or eliminated. This process requires both neuronal activity and genetic programming; however, the molecules that mediate this interaction are poorly understood. Here, I identify a Degenerin/Epithelial Sodium Channel (DEG/ENaC) protein, UNC-8, as a regulator of synapse removal in C. elegans. UNC-8 is transcriptionally-regulated to promote synapse disassembly in an activity-dependent pathway that requires calcium influx through voltage-gated calcium channels and activation of the neuronal phosphatase calcineurin. Activation of the canonical apoptotic protein CED-4 also promotes removal of the presynaptic density through the UNC-8 pathway. We propose a model in which voltage-gated calcium channels activate calcineurin to promote UNC-8 channel activity. Sodium influx through UNC-8 may act as a molecular trigger, depolarizing the presynaptic membrane to enhance activity of the local calcium channels. We propose that intracellular calcium then exceeds a critical threshold that activates a downstream pathway including the cell death pathway components CED-3 and CED-4 and the F-actin severing protein, gelsolin. Previous work has shown that the apoptotic pathway stimulates gelsolin to physically dismantle an actin network that stabilizes the presynaptic active zone. In addition to defining a mechanism for the remodeling role of UNC-8, this work demonstrates that efficient elimination of remodeling GABAergic synapses also depends on a parallel-acting pathway regulated by the homeodomain transcription factor IRX-1/Iroquois. Removal of the synaptic vesicle priming protein UNC-13 is dependent on IRX-1 activity, but does not require UNC-8 function, suggesting that these pathways can differentially regulate the turnover of specific active zone components. Additionally, we find that GABAergic signaling is required for proper synapse removal in the UNC-8 pathway, and that ionotropic and metabotropic GABA receptors adopt opposing roles in presynaptic disassembly. Our results provide the first example of a presynaptic DEG/ENaC protein that promotes synapse elimination. Neurotransmission and genetic programming both converge on the UNC-8-dependent pathway; therefore, providing a link between transcriptional regulation and neuronal activity. This work advances our understanding of synapse disassembly, and thus may eventually reveal therapeutic targets against diseases that arise from synaptic dysfunction.