Abstract
The transcription factor Neurogenin3 functions as a master regulator of endocrine pancreas formation, and its deficiency leads to the development of diabetes in humans and mice. In the embryonic pancreas, Neurogenin3 is transiently expressed at high levels for a narrow time window to initiate endocrine differentiation in scattered progenitor cells. The mechanisms controlling these rapid and robust changes in Neurogenin3 expression are poorly understood. In this study, we characterize a Neurogenin3 positive autoregulatory loop whereby this factor may rapidly induce its own levels. We show that Neurogenin3 binds to a conserved upstream fragment of its own gene, inducing deposition of active chromatin marks and the activation of Neurog3 transcription. Additionally, we show that the broadly expressed endodermal forkhead factors Foxa1 and Foxa2 can cooperate synergistically to amplify Neurogenin3 autoregulation in vitro. However, only Foxa2 colocalizes with Neurogenin3 in pancreatic progenitors, thus indicating a primary role for this factor in regulating Neurogenin3 expression in vivo. Furthermore, in addition to decreasing Neurog3 autoregulation, inhibition of Foxa2 by RNA interference attenuates Neurogenin3-dependent activation of the endocrine developmental program in cultured duct mPAC cells. Hence, these data uncover the potential functional cooperation between the endocrine lineage-determining factor Neurogenin3 and the widespread endoderm progenitor factor Foxa2 in the implementation of the endocrine developmental program in the pancreas.
Highlights
Neurogenin3 is essential for pancreatic endocrine differentiation, but the mechanisms regulating its expression are poorly understood
Neurogenin3 is transiently expressed at high levels for a narrow time window to initiate endocrine differentiation in scattered progenitor cells
Despite that Neurog3 expression peaks at E15.5 [6], it should be noted that endocrine progenitor cells represent a minor fraction of total pancreatic cells, explaining the modest enrichment levels observed in endocrine-specific genes relative to Actb
Summary
Neurogenin is essential for pancreatic endocrine differentiation, but the mechanisms regulating its expression are poorly understood. Neurogenin is transiently expressed at high levels for a narrow time window to initiate endocrine differentiation in scattered progenitor cells The mechanisms controlling these rapid and robust changes in Neurogenin expression are poorly understood. In addition to decreasing Neurog autoregulation, inhibition of Foxa by RNA interference attenuates Neurogenin3-dependent activation of the endocrine developmental program in cultured duct mPAC cells These data uncover the potential functional cooperation between the endocrine lineage-determining factor Neurogenin and the widespread endoderm progenitor factor Foxa in the implementation of the endocrine developmental program in the pancreas. Another mechanism used by transcription factors to control their protein levels is self-regulation In this regard, exogenous Neurog has been shown to induce the endogenous mouse Neurog gene in pancreatic duct-like mPAC cells [8], revealing positive autoregulation as a potential mechanism that may contribute to the rapid accumulation of Neurog protein in endocrine progenitors. We reveal that Foxa is instrumental for Neurog autoregulation but it is required for the activation of other Neurog target genes, indicating that Neurog and Foxa functionally cooperate to switch on the endocrine differentiation program in the pancreas
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