In various systems, activity of neurons or muscle leads to various forms of plasticity, thus shaping network connectivity and regulating synaptic strength. In C. elegans, activity was demonstrated in a few systems to play a role in synapse formation and function. We chose to analyze the development of the 3 cholinergic SAB motor neurons that are innervating the head muscles. In this system, electrical silencing of the muscle cells during development was demonstrated to regulate SAB morphology (Zhao and Nonet, 2000). Using fluorescently-tagged acetylcholine receptors (AChR), we observed SAB overgrowth and ectopic synapse formation in
unc-13 and
unc-18 mutant worms in which neuromuscular transmission was disrupted. We could confirm that this effect is not due to the loss of movement because there is no SAB overgrowth in the
unc-54 myosin mutants that are paralyzed. To silence the electrical activity of muscle cells, we specifically expressed in muscles the Drosophila HisCl1 histamine-gated chloride channel and the TWK-18 temperature-dependent potassium channel. In both conditions, inhibition of muscle cell activity causes SAB overgrowth, suggesting that a retrograde factor(s) controls SAB development. We could further pinpoint a critical developmental window at the L1 stage during which SAB development is plastic. In addition, we demonstrated that chronic - but not acute - increase of synaptic transmission through acetylcholinesterase inhibition leads to a decrease in the number of synaptic AChRs, suggesting an activity-dependent regulation of AChR number during development. Through a transcriptomic approach, we expect to find genes involved in the overgrowth of the SAB and the regulation of AChR number. We are using RNA-Seq to detect genes differentially expressed upon electrical manipulation of the muscle cells. In parallel, we are using the tools that we developed to better define the conditions leading to SAB overgrowth and AChR downregulation, as well as testing a number of candidate genes. References: Zhao, H., and Nonet, M.L. (2000). A retrograde signal is involved in activity-dependent remodeling at a C. elegans neuromuscular junction. Development 127, 1253-1266.