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.
<p> 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.
<p> 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.
| Identifer | oai:union.ndltd.org:VANDERBILT/oai:VANDERBILTETD:etd-01262017-122140 |
| Date | 09 February 2017 |
| Creators | Miller-Fleming, Tyne Whitney |
| Contributors | Kendal S. Broadie, David M. Miller, III, Donna J. Webb, Randy D. Blakely, Christopher V. Wright |
| Publisher | VANDERBILT |
| Source Sets | Vanderbilt University Theses |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.library.vanderbilt.edu/available/etd-01262017-122140/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to Vanderbilt University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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