MafB Is Critical for Glucagon Production and Secretion in Mouse Pancreatic α Cells In Vivo .

The MafB transcription factor is expressed in pancreatic alpha and beta cells during development but becomes exclusive to alpha cells in adult rodents. Mafb-null (Mafb(-/-)) mice were reported to have reduced alpha- and beta-cell numbers throughout embryonic development. To further analyze the postnatal function of MafB in the pancreas, we generated endocrine cell-specific (Mafb(Delta Endo)) and tamoxifen-dependent (Mafb(Delta TAM)) Mafb knockout mice. Mafb(Delta Endo) mice exhibited reduced populations of insulin-positive (insulin(+)) and glucagon(+) cells at postnatal day 0, but the insulin(+) cell population recovered by 8 weeks of age. In contrast, the Arx(+) glucagon(+) cell fraction and glucagon expression remained decreased even in adulthood. Mafb(Delta TAM) mice, with Mafb deleted after pancreas maturation, also demonstrated diminished glucagon(+) cells and glucagon content without affecting beta cells. A decreased Arx(+) glucagon(+) cell population in Mafb(Delta Endo) mice was compensated for by an increased Arx(+) pancreatic polypeptide(+) cell population. Furthermore, gene expression analyses from both Mafb(Delta Endo) and Mafb(Delta TAM) islets revealed that MafB is a key regulator of glucagon expression in alpha cells. Finally, both mutants failed to respond to arginine, likely due to impaired arginine transporter gene expression and glucagon production ability. Taken together, our findings reveal that MafB is critical for the functional maintenance of mouse alpha cells in vivo, including glucagon production and secretion, as well as in development.