The role of the protocadherin gene cluster in neurodevelopment and the implications for neurodevelopmental disorders

PROJECT SUMMARY During development, individual neurons extend highly branched neurites that innervate the surrounding territory with minimal overlap. In mammals, the clustered protocadherins (Pcdh) provide each neuron with a unique cell specific identity required for self-identification and self-avoidance in the brain. Individual PCDHa/b/g protein isoforms cis-dimerize, and engage in homophilic trans interactions to create a lattice-like structure at contacting membrane surfaces, providing the diversity required for self-recognition. Deletion of Pcdh genes in mice results in deficits in neuronal wiring and altered behaviors. Biophysical and structural data, cellular assays, and functional studies support a model in which perfect homophilic matching of PCDH isoforms leads to the assembly of an extended PCDH lattice structure on sister neurites, triggering intracellular signaling, and a series of yet to be identified steps leading to self-avoidance. Many neuronal transmembrane proteins execute neurodevelopmental processes through both nuclear and membrane associated signaling mechanisms; however, the mechanisms regulating PCDH mediated self-avoidance remain unknown. My recent work in the Maniatis lab has demonstrated that the PCDHa/g intracellular domain (ICDs) are required for interactions with protein partners at the membrane (e.g. WAVE complex and WNT signaling modulators) and in the nucleus (e.g. chromatin and transcriptional regulators). In the nucleus, the PCDH-ICD interacts with and may regulate the expression of genes involved in axon guidance, synapse formation and participate in an auto-regulatory feedback loop. During the K99 phase, I will leverage these preliminary findings to assess the need for each downstream signaling mechanism for proper self-avoidance in vitro (Aim 1) and in vivo (Aim 2). During the independent R00 phase, I will implement knowledge and techniques acquired in the mentored phase to investigate whether PCDH SNVs from ASD cases disrupt the identified downstream signaling mechanisms, proper self-avoidance and neural circuitry during development (Aim 3). Understanding the mechanisms of self-avoidance is a fundamental step toward revealing how neural circuits are formed during development in vertebrates and could have important implications for neurodevelopmental disorders. The work described in the mentored portion of this proposal will be conducted under the mentorship of Dr. Maniatis, who has a strong record of mentoring postdoctoral research scientists who have transitioned into their own laboratories. Ultimately, this award will allow me to accomplish three broad training goals including; (1) learning to address biological questions using in vivo models, (2) developing and extending my knowledge of molecular and genetic approaches to probe gene function, and (3) expanding on my background in iPSC based disease modeling by probing PCDH function in cortical organoids. Achieving these goals and training will provide me with the strong foundation required to pursue a faculty position at a research institution where I can integrate in vitro and in vivo approaches to investigate novel questions in the field of neurodevelopment.