Corey Harwell - US grants

DESCRIPTION (provided by applicant): During development neurons in the cortex must navigate through a sea of potential partners in order make the proper synaptic connections required for functional circuitry. Understanding the cellular and molecular mechanisms involved in the specificity of connections is crucial to our ability address a myriad of neurological disorders such as autism, schizophrenia, and epilepsy, in which specific neuronal connections are altered in ways that prevent normal network function. The objective of this study is to understand how the secreted molecule Sonic Hedgehog functions to convey synaptic preferences during cortical development. Sonic Hedgehog (SHH) is a secreted molecule that has numerous critical functions during nervous system development. First as a morphogen regulating proliferation and dorsoventral patterning of the nervous system, and later as an axon guidance cue in the developing spinal cord and retina. Mutations of SHH in humans are associated with a broad range of clinical symptoms, ranging from severe malformation of the brain (holoprosencephaly) to milder learning disabilities and delays in speech acquisition. These genetic studies suggest that Shh function may not only be critical for the patterning of the nervous system, but also may have roles in human cortical circuit formation, and highlight our need to understand Shh function in the cortex during circuit development. Previously I've shown that Shh is expressed in specific populations of subcortical projection neurons located primarily in cortical layers V and VI, while the Sonic Hedgehog receptor Boc is expressed in a complementary population of local and colossal projection neurons. We further showed that Boc and Shh expression is required for the development of layer II/III to layer V synaptic connections. These results have lead us to hypothesize that neural cell type specific expression of Shh signaling components in the developing cortex is required for the development of specific cortical circuits. We propose to characterize the cell type specificity and cellular localization o Shh signaling components Shh, Ptch1, Smo, Boc, in the developing cortex. We will then assess the mechanism by which these components contribute to the development of cortical circuitry, and their dependence on noncanonical Shh signaling. Finally, we will test if expression of Shh and its cognate receptors are sufficient to alter the synaptic preferences of cortical neurons. In order to reach my ultimate goal of achieving tenure in the Department of Neurobiology at Harvard Medical School, I have developed a career development plan and formed a committee of mentors, consisting of tenured faculty in the department. Dr. Rosalind Segal will serve as my primary mentor, with the department chair, Dr. Michael Greenberg and tenured professor Dr. Wade Regehr serving as co-mentors. I have chosen each member of my mentoring committee because of their extensive track records of mentorship both at the laboratory and departmental level. I have also chosen this particular group because of their areas of scientific expertise, and the technical and scientific advice that I stand to gain from our mentoring relationship. My career development activities will be focused on three major aspects to my career success. 1) Mentorship and guidance focused on laboratory management and organization. 2) The development and growth of my independent research program. 3) Navigating institutional responsibilities and fulfilling requirements for promotion and tenure and expanding my scientific network and profile.

Cortical radial glia are neural stem cells that self renew and produce all cortical neuron cell types in an orderly sequential fashion. There is a fundamental gap in understanding the molecular mechanism that underlies the orderly production of neuronal cell types. Our overall goal is to understand the intrinsic timing mechanism that regulates cell fate transitions during cortical neurogenesis. We have identified the transcriptional regulator Prdm16 (Positive Regulatory Domain-containing 16) as being a critical component for regulating precisely timed cell fate transitions during cortical neurogenesis. Our studies of Prdm16 serve as an entry point to understanding the relevant genetic and epigenetic programs regulating the mode of radial glia cell division, and its relationship to neuronal fate potential. In this proposal we plan to utilize MARIS (Method for Analyzing RNA following Intracellular Sorting) to define the molecular programs regulated by Prdm16 in a cell type and stage specific manner. We will also determine the temporal pattern of PRDM16 binding and regulation of cis- regulatory elements during cortical neurogenesis. The finding from our studies will have a direct impact on increasing our understanding of how expression of early transcriptional programs and chromatin modifications can contribute to neurodevelopmental disorders.

The septum is a structure in the basal forebrain that has critical roles in regulating emotional states including fear, anxiety and depression. Despite our growing appreciation for the importance of lateral septal circuit function in modulating emotional states, we know very little about the specific functions of the diverse septal neuron cell types and even less about the developmental mechanisms that create this diversity. We have identified the transcriptional regulator Prdm16 (Positive Regulatory Domain-containing 16), as being a critical component for regulating the production of distinct subtypes of neurons in the lateral septum. Our studies of Prdm16 serve as an entry point to understanding the relevant genetic programs regulating the development and specification of diverse septal neuron subtypes. In this proposal we plan to utilize single-cell RNA sequencing to map the developmental trajectories and overall diversity of neuronal cell types in the septum. The results from these studies will provide a foundation for understanding the development and function of temporally specified septum neuron types, and their contribution to behavioral circuits.