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

Quantification of factors influencing fluorescent protein expression using RMCE to generate an allelic series in the ROSA26 locus in mice

Fluorescent proteins (FPs) have great utility in identifying specific cell populations and in studying cellular dynamics in the mouse. To quantify the factors that determine both the expression and relative brightness of FPs in mouse embryonic stem cells (mESCs) and in mice, we generated eight different FP-expressing ROSA26 alleles using recombinase-mediated cassette exchange (RMCE). These alleles enabled us to analyze the effects on FP expression of a translational enhancer and different 3′-intronic and/or polyadenylation sequences, as well as the relative brightness of five different FPs, without the confounding position and copy number effects that are typically associated with randomly inserted transgenes. We found that the expression of a given FP can vary threefold or more depending on the genetic features present in the allele. The optimal FP expression cassette contained both a translational enhancer sequence in the 5′-untranslated region (UTR) and an intron-containing rabbit β-globin sequence within the 3′-UTR. The relative expressed brightness of individual FPs varied up to tenfold. Of the five different monomeric FPs tested, Citrine (YFP) was the brightest, followed by Apple, eGFP, Cerulean (CFP) and Cherry. Generation of a line of Cherry-expressing mice showed that there was a 30-fold variation of Cherry expression among different tissues and that there was a punctate expression pattern within cells of all tissues examined. This study should help investigators make better-informed design choices when expressing FPs in mESCs and mice

Nkx2.2 repressor complex regulates islet β-cell specification and prevents β-to-α-cell reprogramming

In order to determine how cell fates are determined and maintained, Sussel and colleagues studied the endogenous regulatory pathway by which β-cell identity is maintained. They show that pancreatic β cells can be reprogrammed by a mutation in a single transcription factor, Nkx2.2, leading to repression of Arx, an α-cell-specific factor. This paper provides novel insight into an in vivo link between cell-specific DNA methylation events, gene regulation, and cellular reprogramming