Tuning the Circuit, Steering the Swarm: CASY-1 in Synaptic and Behavioral Modulation

Abstract

Disruptions in neuromodulatory signaling are a hallmark of several neurodevelopmental and psychiatric disorders. However, the molecular mechanisms that fine-tune neuromodulation across circuits and behaviors remain poorly understood. Calsyntenins, a family of transmembrane vesicular cargo proteins implicated in autism and Alzheimer’s disease, have emerged as critical regulators of synaptic and behavioral plasticity. In Caenorhabditis elegans, we uncover a novel role for the calsyntenin ortholog casy-1 in regulating cholinergic synaptic transmission and collective swarming behavior. We show that the heightened excitation–inhibition (E–I) balance at the neuromuscular junction (NMJ) in casy-1 mutants is likely due to disrupted neuropeptide signaling. casy-1 genetically interacts with the FLP-21/NPR-1 pathway, reducing their signaling in sensory neurons and ultimately altering motor circuit dynamics. Through genetic, pharmacological, and bioimaging-based approaches, we propose that CASY-1 modulates synaptic transmission by regulating the trafficking of neuropeptide-containing vesicles. The role of CASY-1 in neuromodulation extends beyond synaptic transmission to behavioral plasticity. casy-1 mutants exhibit reduced pigment-dispersing factor 1 (PDF-1) signaling, which contributes to a striking collective swarming phenotype. Despite the presence of abundant food, casy-1 mutants aggregate into dense feeding groups, failing to disperse like wild-type animals. This defect phenocopies mutants with impaired neuropeptide processing and release and is rescued by restoring casy-1 expression in sensory neurons. Using domain-specific CRISPR knockouts, we demonstrate that the C-terminal domain of CASY-1, which is required for its vesicular trafficking function, is responsible for swarm formation in casy-1 mutants. Furthermore, we identify disrupted serotonin and reciprocal PDF-1 signaling in casy-1 mutants, with PDF-1 overexpression fully rescuing the swarming phenotype. Together, these findings establish CASY-1 as a neuron-specific regulator that links intracellular vesicle dynamics with circuit-level neuromodulation and complex behavior. By connecting synaptic transmission with neuromodulatory control of collective behavior, this study provides mechanistic insight into how disruptions in a conserved trafficking protein may underlie neuromodulatory imbalance and associated behavioral anomalies.