Precommitment low-level neurog3 expression defines a long-lived mitotic endocrine-biased progenitor pool that drives production of endocrine-committed cells

The current model for endocrine cell specification in the pancreas invokes high-level production of the transcription factor Neurogenin 3 (Neurog3) in Sox⁹⁺ bipotent epithelial cells as the trigger for endocrine commitment, cell cycle exit, and rapid delamination toward proto-islet clusters. This model posits a transient Neurog3 expression state and short epithelial residence period. We show, however, that a Neurog3^TA.LO cell population, defined as Neurog3 transcriptionally active and Sox⁹⁺ and often containing nonimmunodetectable Neurog3 protein, has a relatively high mitotic index and prolonged epithelial residency. We propose that this endocrine-biased mitotic progenitor state is functionally separated from a pro-ductal pool and endows them with long-term capacity to make endocrine fate-directed progeny. A novel BAC transgenic Neurog3 reporter detected two types of mitotic behavior in Sox⁹⁺ Neurog3^TA.LO progenitors, associated with progenitor pool maintenance or derivation of endocrine-committed Neurog3^HI cells, respectively. Moreover, limiting Neurog3 expression dramatically increased the proportional representation of Sox9⁺ Neurog3^TA.LO progenitors, with a doubling of its mitotic index relative to normal Neurog3 expression, suggesting that low Neurog3 expression is a defining feature of this cycling endocrine-biased state. We propose that Sox9⁺ Neurog3^TA.LO endocrine-biased progenitors feed production of Neurog3^HI endocrine-committed cells during pancreas organogenesis.