Molecular Dissection of Synaptic Remodeling in GABAergic Neurons
Miller-Fleming, Tyne Whitney
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.