Genetic evidence that Nkx2.2 acts primarily downstream of Neurog3 in pancreatic endocrine lineage development

Many pancreatic transcription factors that are essential for islet cell differentiation have been well characterized; however, because they are often expressed in several different cell populations, their functional hierarchy remains unclear. To parse out the spatiotemporal regulation of islet cell differentiation, we used a Neurog3-Cre allele to ablate Nkx2.2, one of the earliest and most broadly expressed islet transcription factors, specifically in the Neurog3(+) endocrine progenitor lineage (Nkx2.2(Delta endo)). Remarkably, many essential components of the beta cell transcriptional network that were down-regulated in the Nkx2.2(KO) mice, were maintained in the Nkx2.2(Delta endo) mice - yet the Nkx2.2(Delta endo) mice displayed defective beta cell differentiation and recapitulated the Nkx2.2(KO) phenotype. This suggests that Nkx2.2 is not only required in the early pancreatic progenitors, but has additional essential activities within the endocrine progenitor population. Consistently, we demonstrate Nkx2.2 functions as an integral component of a modular regulatory program to correctly specify pancreatic islet cell fates.