Synaptic networks are extensively remodeled in the developing brain by mechanisms that require neural activity. Members of the DEG/ENaC (Degenerin/Epithelial sodium channel) family are known to modulate plasticity in the mammalian brain, but the molecular events that regulate this effect are poorly defined. Here we describe an activity-dependent mechanism in which the DEG/ENaC protein, UNC-8, promotes synaptic remodeling in C. elegans. GABAergic Dorsal D (DD) motor neurons reverse polarity by relocating synapses from ventral to dorsal muscles in the first larval stage. This pathway is blocked by the UNC-55/COUP-TF transcription factor in Ventral D (VD) GABAergic neurons, which ectopically remodel in an unc-55 mutant. We exploited this mutant phenotype in a cell-specific profiling strategy to identify UNC-55-regulated transcripts. This approach revealed fifty UNC-55 targets that are required for GABA neuron remodeling, including UNC-8. We find that UNC-8 functions in DD neurons where it is localized near ventral DD synapses. Our results show that UNC-8 is required for removing ventral synapses in a mechanism that depends on GABA. A necessary role for neurotransmitter release is also suggested by our finding that a voltage-gated calcium channel subunit, UNC-2, promotes remodeling. In vitro reconstitution of an UNC-8 channel results in robust cation transport activity that is strongly inhibited by extracellular calcium. The negative effect of calcium on UNC-8 function is consistent with a model in which depletion of extracellular calcium by UNC-2 at active GABAergic synapses effectively relieves the calcium block and thereby induces UNC-8 activation. Our results support a model in which UNC-8 functions as an activity sensor in GABAergic DD motor neurons to trigger the deconstruction of ventral synapses, thus promoting the remodeling process.

Authors: Miller, Tyne W, Petersen, Sarah C, Gornet, Megan E, Wang, Ying, Lu, Han, Matthewman, Cristina, Bianchi, Laura, Richmond, Janet E, Mitani, Shohei, Hori, Sayaka, Miller, David M

Affiliations:
- Department of Biological Sciences, University of Illinois at Chicago, Chicago, IL.
- Department of Cell & Developmental Biology, Vanderbilt University, Nashville, TN.
- Department of Physiology, Tokyo Women's Medical University, School of Medicine, Tokyo, Japan.
- Department of Physiology & Biophysics, University of Miami, Miami, FL.