Proposal Summary The enteric nervous system (ENS) is a vital part of the peripheral nervous system and is responsible for the control of critical gut functions like peristalsis and gastrointestinal secretion. Abnormal development of the ENS can lead to life threatening disorders like Hirschsprung’s disease, characterized by the absence of innervating neurons and glia in the gut. Seminal experiments in chick have shown that two distinct neural crest cell populations innervate the gut and give rise to the ENS: the vagal and the sacral neural crest. Although extensive research has been done on the vagal neural crest’s contribution to the ENS, very little is known about the role of the sacral neural crest in the post-umbilical gut. We aim to address this gap in knowledge by using modern-day molecular biology techniques to gain granular understanding of sacral neural crest- derivatives in the post-umbilical gut, characterize their unique gene signatures, and determine the requirement of sacral-specific transcriptional regulators in ENS development. We hypothesize that the sacral neural crest contributes to unique derivatives within the post-umbilical gut, distinct from vagal-derived structures, and that these derived-cells are under the regulation of a novel gene regulatory scheme. Ultimately, this work will address a lack of understanding of the sacral neural crest in ENS development and shed light on the etiology of ENS-derived congenital disorders. Aim 1: Retroviral mediated lineage analysis of the chick sacral neural crest: Previous work in quail-chick chimeras had multiple disadvantages like traumatic surgery and cross-species artifacts. Here we will implement our technique of replication incompetent avian (RIA) retroviruses for comparative cell lineage analysis of vagal and sacral-derived cells in the post-umbilical gut, visualize interactions between the two populations, and perform clonal analysis of sacral neural crest cells. Aim 2: Transcriptional profiling of sacral neural crest-derived cells in the post-umbilical gut: In order to identify the gene signature of sacral-derived structures of the ENS and regulators of their differentiation, we propose single cell RNA sequencing of sacral neural crest derivatives across multiple time points to characterize the transcriptional profile of sacral neural crest entry into the gut and neuronal differentiation. Upon identification of transcriptional regulators, we will perform conditional loss-of-function analysis to examine their role in regulating the sacral neural crest’s neuronal differentiation.