PROJECT SUMMARY Synaptic organizers form macromolecular bridges that span the cleft of synapses, the contact and communication points between neurons. Defects in many different synaptic organizers are implicated in neuropsychiatric disorders like schizophrenia (SZ), autism spectrum disorder (ASD), and intellectual disability (ID), illnesses in dire need of better medications. Synaptic organizers regulate the formation and function of synapses, but their molecular mechanisms are not well understood. They likely produce a code that helps wire billions of neurons together into neural circuits that are discrete and specific, yet also plastic, such as those regulating human behavior. We are leveraging the neurexin (NRXN) family of synaptic organizers, and their many partners, as prototypes to understand how trans-synaptic bridges are formed and shape synaptic connections. Defects in NRXNs and their partners are implicated in SZ, ASD and ID. Working model: When NRXN trans-synaptic bridges malfunction, fundamental synaptic processes change and lead to the pathology of severe mental illness. Mounting evidence suggests that the trans-synaptic bridges formed between NRXNs and their partners are dynamically controlled via a portfolio of different mechanisms that either boost them or suppress them. It is not known on a molecular level how these different regulatory mechanisms exactly work to control the formation and/or function of NRXN-mediated trans-synaptic bridges, or the bases of their dynamic action. We hypothesize that NRXNs and their partners form a molecular platform that is targeted by a set of fundamentally different and dynamic regulatory mechanisms, exploiting: 1) protein 3D conformational changes; 2) regulators with opposite functions; 3) the oligomeric status of partners; and 4) factors secreted by astrocytes. In this proposal, we will determine the underpinnings of key mechanisms, first establishing the molecular ground rules using structural, biophysical, and biochemical techniques, and then strategically interrogating these rules in a cellular context and in in vivo studies. Collectively, our results will reveal how regulatory mechanisms mold NRXN trans-synaptic bridges, impacting their ability to regulate synapse formation and synaptic communication. This proposal is significant because in addition to revealing molecular mechanisms that regulate a very large portfolio of trans-synaptic bridges and their functions, it will also reveal protein interactions that can be targeted to manipulate specific synaptic connections for therapeutic purposes and address specific CNS disorders. This proposal is conceptually innovative because it propels an emerging paradigm shift that trans-synaptic bridges, like those formed between NRXNs and their partners, are in fact subject to intense regulation by other interacting proteins, including other synaptic organizers, and these synergize together to shape synaptic connections. This proposal is also technica...