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

During pancreas organogenesis, Neurog3 endocrine-committing cells are generated from a population of Sox9 mitotic progenitors with only a low level of Neurog3 transcriptional activity (Neurog3 ). Low-level Neurog3 protein, in Neurog3 cells, is required to maintain their mitotic endocrine-lineage-primed status. Herein, we describe a Neurog3-driven FUCCI cell-cycle reporter (Neurog3 ) derived from a Neurog3 BAC transgenic reporter that functions as a loxed cassette acceptor (LCA). In cycling Sox9 Neurog3 progenitors, the majority of cells in S-G -M phases have undetectable levels of Neurog3 with increased expression of endocrine progenitor markers, while those in G 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 progenitors with entrance into G 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.