Cooperative Transcriptional Regulation of the Essential Pancreatic Islet Gene NeuroD1 (Beta2) by Nkx2.2 and Neurogenin 3

Abstract

Nkx2.2 and NeuroD1 are two critical regulators of pancreatic ␤ cell development. Nkx2.2 is a homeodomain transcription factor that is essential for islet cell type specification and mature ␤ cell function. NeuroD1 is a basic helix-loop-helix transcription factor that is critical for islet ␤ cell maturation and maintenance. Although both proteins influence ␤ cell development directly downstream of the endocrine progenitor factor, neurogenin3 (Ngn3), a connection between the two proteins in the regulation of ␤ cell fate and function has yet to be established. In this study, we demonstrate that Nkx2.2 transcriptional activity is required to facilitate the activation of NeuroD1 by Ngn3. Furthermore, Nkx2.2 is necessary to maintain high levels of NeuroD1 expression in developing mouse and zebrafish islets and in mature ␤ cells. Interestingly, Nkx2.2 regulates NeuroD1 through two independent promoter elements, one that is bound and activated directly by Nkx2.2 and one that appears to be regulated by Nkx2.2 through an indirect mechanism. Together, these findings suggest that Nkx2.2 coordinately activates NeuroD1 with Ngn3 within the endocrine progenitor cell and also plays a role in the maintenance of NeuroD1 expression to regulate ␤ cell function in the mature islet. Collectively, these findings further define the conserved regulatory networks involved in islet ␤ cell formation and function.The pancreas is an intricate organ composed of exocrine tissue that secretes digestive enzymes into pancreatic ducts, and the endocrine islets of Langerhans that produce the metabolic hormones insulin, glucagon, somatostatin, pancreatic polypeptide, and ghrelin. The different pancreatic cell lineages arise during the critical developmental events referred to as the primary and secondary transitions (Ref. 1; reviewed in Ref. 2). The primary transition occurs between embryonic day (e) 8.5 and e11.5 2 and encompasses the initial patterning and specification of the pancreatic endoderm, which originates from the foregut. The secondary transition is the critical stage between e12.5 and e15.5 when endocrine and exocrine progenitors expand and a large second wave of differentiation is initiated. The secondary transition also marks an increase in the expression and/or relocalization of a number of transcription factors that are important in pancreatic development, including Pdx1, Ptf1a, Ngn3, Nkx2.2, NeuroD1, Pax4, Pax6, and Nkx6.1 (reviewed in Ref. 3). A large number of studies of these transcription factors have yielded a timeline of gene expression and determined which cell lineages are regulated by each transcription factor (2-7). Notably, temporal and spatial changes in many of the transcription factor expression profiles can re-program a progenitor or precursor cell to alter, prevent, or initiate endocrine differentiation (3, 8 -12). The cumulative findings of these studies have illustrated that the regulation of islet cell differentiation depends on complex relationships between the transcriptional regulatory cascades. These regulato...

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