FUCCI tracking shows cell-cycle-dependent Neurog3 variation in pancreatic progenitors

During pancreas organogenesis, Neurog3^HI endocrine-committing cells are generated from a population of Sox9⁺ mitotic progenitors with only a low level of Neurog3 transcriptional activity (Neurog3^TA.LO). Low-level Neurog3 protein, in Neurog3^TA.LO cells, is required to maintain their mitotic endocrine-lineage-primed status. Herein, we describe a Neurog3-driven FUCCI cell-cycle reporter (Neurog3^P2A.FUCCI) derived from a Neurog3 BAC transgenic reporter that functions as a loxed cassette acceptor (LCA). In cycling Sox9⁺ Neurog3^TA.LO progenitors, the majority of cells in S-G₂-M phases have undetectable levels of Neurog3 with increased expression of endocrine progenitor markers, while those in G₁ have low Neurog3 levels with increased expression of endocrine differentiation markers. These findings support a model in which variations in Neurog3 protein levels are coordinated with cell-cycle phase progression in Neurog3^TA.LO progenitors with entrance into G₁ triggering a concerted effort, beyond increasing Neurog3 levels, to maintain an endocrine-lineage-primed state by initiating expression of the downstream endocrine differentiation program prior to endocrine-commitment.