PROJECT SUMMARY The thematic focus of this Center is to understand how a signaling hub of molecules that regulate small GTPases contribute to the precise developmental formation and maturation of inter-connectivity between neuronal components of cortical microcircuits. The Center will address this goal with interrelated projects that will provide an in-depth examination of the function of Trio and SynGAP; two potent regulators of GTPase signaling in the developing brain. Multiple signaling molecules are involved in the organization and refinement of cortical microcircuits that are required for correct information processing in the cortex. Trio and Syngap1 are known to regulate synaptic connections but how they contribute to precise arrangements of cortical microcircuits through neuronal migration, synaptic morphogenesis and refinement of connections during development is not known. Pathogenic variants in these genes are causally linked to neurodevelopmental disorders that, while characterized by heterogeneous genetic etiology, have shared symptoms. Convergence in symptoms suggests that disorders with diverse molecular causes can be understood by defining the pathophysiological consequences on the synapses and circuits that underlie the disrupted behaviors. In Project 3, we will address the broader goals of The Center by analyzing the roles of these complex signaling molecules in the formation and refinement of cortical microcircuits. Using ex vivo functional electrophysiological and optogenetic mapping approaches we will dissect the effect of disrupted Trio and SynGAP signaling on cortical architecture. The project will assess (1) the spatial cell-type and temporal functions of Trio and SynGAP, (2) the importance of enzymatic and non-enzymatic functional domains to their roles in cortical connectivity, and (3) the action of novel small molecule inhibitors of these GTPase regulators in synaptic function and connectivity at acute and chronic timescales. These studies will transform our understanding of how Trio and SynGAP function in developing cortical circuits and along with collaborative and integrated efforts in the Center will create a foundational understanding of their roles in neurodevelopment.