Cyfip2 controls the acoustic startle threshold through FMRP, actin polymerization, and GABA_Breceptor function

Abstract

Animals process a constant stream of sensory input, and to survive they must detect and respond to dangerous stimuli while ignoring innocuous or irrelevant ones. Behavioral responses are elicited when certain properties of a stimulus such as its intensity or size reach a critical value, and such behavioral thresholds can be a simple and effective mechanism to filter sensory information and determine if a response is appropriate. For example, the acoustic startle response is a conserved and stereotyped defensive behavior induced by sudden loud sounds, but dysregulation of the threshold to initiate this behavior can result in startle hypersensitivity that is associated with sensory processing disorders including schizophrenia and autism. Through a previous forward genetic screen for regulators of the startle threshold a nonsense mutation inCytoplasmic Fragile X Messenger Ribonucleoprotein (FMRP)-interacting protein 2(cyfip2) was found that causes startle hypersensitivity in zebrafish larvae, but the molecular mechanisms by which Cyfip2 establishes the acoustic startle threshold are unknown. Here we use conditional transgenic rescue and CRISPR/Cas9 gene knockdown approaches to determine that Cyfip2 requires both Rac1 and FMRP pathways, but not the closely related FXR1 or FXR2, to regulate the acoustic startle threshold in early neurodevelopment. Using a candidate-based drug screen we find that Cyfip2 also acts acutely to maintain the startle threshold through Arp2/3-mediated branched actin polymerization and N-methyl D-aspartate receptors (NMDARs). To identify proteins and pathways that may be targets of Cyfip2-FMRP-mediated translational regulation, we then performed discovery proteomics and determined that loss of Cyfip2 alters cytoskeletal and extracellular matrix components and disrupts oxidative phosphorylation and GABA receptor signaling. Finally, we validated our proteomics findings by showing that the GABABreceptor agonist baclofen, but not the GABAAagonist muscimol, restores normal startle sensitivity incyfip2mutants. Together, these data reveal that Cyfip2 acts through multiple pathways to regulate excitatory/inhibitory balance in the startle circuit to control the processing of acoustic information.