PROJECT SUMMARY Despite increasing diagnoses, gastroenteropancreatic neuroendocrine tumors (GEP-NETs) remain poorly understood, both in relation to their molecular pathogenesis and cells-of-origin. My recent work using transgenic mice demonstrated, for the first time, the involvement of enteric neural crest-derived cells (ENCCs) in neuroendocrine differentiation and tumor development driven by Men1 gene deletion. Consistent with its role in cell fate patterning, Hedgehog signaling was implicated in neuroendocrine reprogramming of ENCCs. Menin, encoded by the MEN1 gene, has been shown to regulate Hedgehog signaling in mouse models of pancreatic NET development, however reciprocity between menin and Hedgehog has yet to be studied in ENCCs. This proposal aims to address whether loss of menin in ENCCs drives acquisition of the neuroendocrine cell fate with lineage commitment being actively modulated by Hedgehog signaling. My goals are to define the cells-of- origin for MEN1 GEP-NETs and to decipher the role of Hedgehog in driving neuroendocrine cell patterning. I will combine my experience using novel transgenic mouse models and ex vivo tissue culture techniques with state-of-the-art single cell and spatial transcriptome profiling to define the contribution of ENCCs to neuroendocrine differentiation and GEP-NET development. I hypothesize that reciprocal signaling by menin and Hedgehog drives ENCC reprogramming and gives rise to hyperplastic neuroendocrine cells with tumorigenic potential. This Research Plan will determine whether MEN1-associated GEP-NETs originate from reprogrammed ENCC populations (Aim 1) and decipher the role of Hedgehog signaling in neuroendocrine reprogramming of MEN1 GEP-NETs (Aim 2). During the mentored K01 award period, I will work closely with my primary mentor Dr. Juanita Merchant and co-mentor Dr. Megha Padi, distinguished experts in gastrointestinal biology and single cell analysis respectively, to develop the skillset to accomplish my research and career objectives. My Career Development Plan will facilitate my goal of becoming a productive independent investigator by combining rigorous didactic training and formal mentorship under a team of faculty who bring established expertise in cell and molecular biology, gastrointestinal physiology, large genomic data analysis, and 3-D organoid systems. Finally, the breadth of career development resources and extensive shared research facilities at the University of Arizona make it an ideal environment for me to carry out the proposed research project and achieve my career objectives. By leveraging innovative GEP-NET models and cutting-edge sequencing methods, this award will enable me to establish a state-of-the-art research program with the long- term goal of defining the cellular signals that govern enteric neuroendocrine cell fate.