The transcription factor Pax6 is required for pancreatic beta cell identity, glucose-regulated ATP synthesis, and Ca(2+) dynamics in adult mice
Mitchell RK., Nguyen-Tu MS., Chabosseau P., Callingham RM., Pullen TJ., Cheung R., Leclerc I., Hodson DJ., Rutter GA.
Heterozygous mutations in the human paired box gene PAX6 lead to impaired glucose tolerance. Although embryonic deletion of the Pax6 gene in mice leads to loss of most pancreatic islet cell types, the functional consequences of Pax6 loss in adults are poorly defined. Here we developed a mouse line in which Pax6 was selectively inactivated in beta cells by crossing animals with floxed Pax6 alleles to mice expressing the inducible Pdx1CreERT transgene. Pax6 deficiency, achieved by tamoxifen injection, caused progressive hyperglycemia. Although beta cell mass was preserved 8 days post-injection, total insulin content and insulin:chromogranin A immunoreactivity were reduced by approximately 60%, and glucose-stimulated insulin secretion was eliminated. RNA sequencing and quantitative real-time PCR analyses revealed that, although the expression of key beta cell genes, including Ins2, Slc30a8, MafA, Slc2a2, G6pc2, and Glp1r, was reduced after Pax6 deletion, that of several genes that are usually selectively repressed ("disallowed") in beta cells, including Slc16a1, was increased. Assessed in intact islets, glucose-induced ATP:ADP increases were significantly reduced (p < 0.05) in betaPax6KO versus control beta cells, and the former displayed attenuated increases in cytosolic Ca(2+) Unexpectedly, glucose-induced increases in intercellular connectivity were enhanced after Pax6 deletion, consistent with increases in the expression of the glucose sensor glucokinase, but decreases in that of two transcription factors usually expressed in fully differentiated beta-cells, Pdx1 and Nkx6.1, were observed in islet "hub" cells. These results indicate that Pax6 is required for the functional identity of adult beta cells. Furthermore, deficiencies in beta cell glucose sensing are likely to contribute to defective insulin secretion in human carriers of PAX6 mutations.