Molecular Regulators of Synaptic Specificity

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Project Summary Specific circuits in the brain determine how we sense and respond to our environment. These highly connected networks emerge during development as neurons extend projections to defined meeting sites, identify partners, and begin synaptogenesis. Although initial connections may be modified later, the overall pattern of connectivity is predictable thus suggesting that partner selection can be encoded by the genome. It follows that genetic analysis can be powerfully employed to find synaptic specificity genes by identifying mutants with altered patterns of connectivity. With the goal of identifying novel, conserved target selection proteins, I will screen for mutations that disrupt distinctive behaviors that depend on neuron-specific synapses in C. elegans. This work focuses on the PVD sensory neuron and its synaptic targets, PVC and AVA. PVD stimulation activates PVC, its dominant partner, and triggers forward movement. If the PVC connection is removed, however, AVA is activated instead, resulting in reverse locomotion. Thus, mutants that selectively disrupt either PVD→PVC or PVD→AVA connections can be identified from readily distinguished behaviors (e.g., forward vs reverse movement). With its short life cycle and powerful genetic tools, C. elegans is especially useful for unbiased genetic screens. In Aim 1 I will use an optogenetic strategy to activate PVD in a forward genetic EMS mutagenesis screen that uses a high-throughput video recording system (WormLab) to identify mutants with selectively altered locomotion. Behavioral mutants with these specific locomotory phenotypes will be screened with GRASP (GFP Reconstitution Across Synaptic Partners) markers to confirm that either PVD→PVC or PVD→AVA synapses are disrupted during synaptogenesis. Molecular cloning methods will be used to identify the affected synaptic specificity genes. Aim 2 adopts an independent approach that stems from the expectation that synaptic specificity genes in PVD should be regulated by cell autonomous transcription factors (TFs). My strategy exploits a list of 35 PVD-enriched TFs previously derived from RNA-Seq profiling. I will use RNAi and available genetic mutants in the GRASP marker assay to test each of these TFs for potential roles in either PVD→PVC or PVD→AVA synaptogenesis. This TF screen has the advantage of dysregulating multiple target genes simultaneously for a robust synaptic specificity phenotype. I will use PVD-specific RNA-Seq to identify the targets of the synapse-specific TFs and then test them individually for roles in PVD synaptic specificity using the behavioral assay and GRASP markers. Together, these approaches in C. elegans are expected to reveal key determinants of synaptic specificity that can be tested for conserved roles in more complex nervous systems and for links to neurological disorders associated with altered synaptogenesis such as Autism Spectrum Disorder (ASD).