Novel roles for Pdx1 in the endocrine pancreas

Abstract

Inadequate β-cell function and numbers contribute to the progression of all major forms of diabetes, a highly prevalent disorder with significant morbidity and mortality, resulting in intense focus in recent years on developing alternate sources of β-cells for cell replacement therapies. Recent advances in generating β-like cells from stem/precursor cells or mature related cell-types have been directly guided by our understanding of transcription factors and signaling pathways that regulate embryonic development of insulin-producing β-cells. Pancreatic duodenal homeobox 1 (Pdx1) is a homeodomain transcription factor that governs both early pancreatic organogenesis and the later differentiation of endocrine β-cells. Homozygous mutations in humans and mice lead to pancreatic agenesis. In contrast, Pdx1 heterozygous mutations lead to glucose intolerance with age due to defects in β-cell function and survival. Similarly, heterozygous mutations in humans result in autosomal dominant Maturity Onset Diabetes of the Young (MODY) as well as type-2 diabetes. Human Pdx1 mutations are located throughout protein: in the N-terminal transactivation domain, the central DNA-binding homeodomain and the evolutionary conserved but poorly understood C-terminal region. To determine the role of C-terminal domain in vivo as well as to gain new insights into specific roles for Pdx1, I characterized a Pdx1 hypomorphic allele, Pdx1ΔC/ΔC , that prevents translation of the C-terminus. Pdx1 ΔC/ΔC animals display a global reduction in all endocrine lineages during development resulting from decreased numbers of Neurogenin 3 (Ngn3)-expressing endocrine progenitors. Pdx1 occupies a conserved Ngn3 enhancer at embryonic day 13.5 and physically interacts with the one-cut transcription factor Hnf6 to directly regulate Ngn3 expression. Moreover, the absence of the C-terminus impairs Pdx1 transactivation of Ngn3. Pdx1ΔC/ΔC mice also have reduced Hnf6, Hnf1β, Foxa2, and Sox9 transcripts, suggesting that Pdx1 contributes to specification of Ngn3+ endocrine progenitors by directly regulating Ngn3 expression and by participating in a recently described transcription factor co-regulatory network regulating the endocrine progenitor. Postnatally, Pdx1ΔC/ΔC mice develop overt diabetes by 3-4 weeks of age, due to defects in both β-cell mass and function. Further studies will determine the mechanisms whereby Pdx1 regulates postnatal function, which may reveal additional insight into how Pdx1 mutations lead to diabetes