PROJECT SUMMARY/ABSTRACT Ongoing systematic human genetic studies of neurodevelopmental disorders continue to uncover pathogenic mutations in genes encoding synaptic proteins, demonstrating the importance of these proteins for neurological and neuropsychiatric functions. Although the molecular and cellular functions of many such synaptic proteins have been studied to various extents, the functional roles of these proteins in neural circuits and behaviors are poorly understood because in-depth neurological and behavioral studies in animal models are often lacking. Consequently, the pathological mechanisms underlying these synaptic disorders remain elusive. This knowledge gap can be significantly narrowed by studying a few prioritized genes that are highly penetrant and affect a broad spectrum of neurological and neuropsychiatric features common among neurodevelopmental disorders. The gene encoding syntaxin-binding protein 1 is one such exciting example because it is one of a few genes most frequently mutated in neurodevelopmental disorders. The absence of syntaxin-binding protein 1 abolishes neurotransmitter release. This essential function is well understood at the molecular level, yet it remains unknown how its haploinsufficiency in humans causes a range of neurological impairments including epileptic seizures and intellectual disabilities. Thus, the overall goal of this project is to decipher the synaptic dysfunction of neural circuits in the mouse models of this disorder and understand their relevance to disease pathogenesis at the whole-organism level. The apparently paradoxical effects of syntaxin-binding protein 1 deletion and haploinsufficiency lead to the central hypothesis that its haploinsufficiency preferentially impairs GABAergic inhibitory transmitter release and causes an imbalance between excitation and inhibition, resulting in hyperexcitable neural circuits and neurological deficits, which can be reversed upon restoring protein function in adulthood. We propose to combine genetic manipulations with optogenetic, physiological, and behavioral methods to delineate synapse-specific alterations of neurotransmission in cortical circuits (Aim 1), to determine the contributions of specific cell types to the pathogenesis of the disorder (Aims 2 and 3), and to test the reversibility of the disease phenotypes in adulthood (Aim 3). This project shifts the research focus of syntaxin-binding protein 1 from neurotransmitter release per se to its function at the levels of neural circuitry and behavior. The proposed research will provide mechanistic insights into the pathogenesis of this devastating neurodevelopmental disorder. Beyond this particular disorder, understanding how disruption of excitation-inhibition balance affects neural circuits and behaviors will have ramifications for a growing list of neurodevelopmental disorders caused by mutations that alter synaptic transmission.