PROJECT SUMMARY/ABSTRACT Synaptic dysfunction is a major contributor towards the development of a large range of neurological disorders. For proper formation, synapses require a series of signaling pathways to guide them, but not all of those pathways are well understood. Rap Guanine Exchange Factors, or RapGEFs, are signaling proteins that act to accelerate the rate of activation of GTPases that achieve downstream functions. RAPGEF subfamily members are associated with multiple neurological disorders, including schizophrenia. In murine models, disruption of RAPGEF6 function results in reduced anxiety behaviors, like that observed in patients with schizophrenia, and increased long term potentiation. In addition, they found no changes in gross brain morphology. Although these studies suggest that RAPGEFs modulate synaptic function, the exact mechanism is currently unknown. To address whether loss of RapGEF function influences synapses, mutations within pxf-1, a C. elegans RapGEF orthologue were studied. When exposed to the acetylcholinesterase inhibitor, aldicarb, pxf-1 mutants displayed decreased sensitivity indicating altered synaptic function. Additionally, pxf-1 and rac-2 GTPase mutants display decreased synaptic vesicle intensity in cholinergic synapses. Based on these preliminary findings, the central hypothesis is that PXF-1 alters cytoskeletal reorganization within neurons to influence synaptic formation and accomplishes this through activation of Rac GTPase signaling. To test this hypothesis, this study will use the C. elegans neuromuscular junction. C. elegans are an advantageous model system for the study of neuronal function due to their well- defined genome, translucent body for imaging techniques, and overall ease of genetic manipulation. The goal of this project is to determine the mechanism of PXF-1 function within the neuromuscular junction through the following aims. Aim 1. To identify the downstream GTPase that PXF-1 is activating in this pathway. Aim 2. To investigate whether inhibiting a GAP protein in the PXF-1 pathway can restore synaptic development and function. Overall, this research will provide insight into the molecular mechanisms that govern synaptic formation and how their dysregulation may lead to development of disorders. This research will support the development of future treatments for synaptopathologies.