PDGF signalling controls age-dependent proliferation in pancreatic beta-cells
In addition to this summary, please see the accompanying commentary provided by BCBC Investigator Andrew Stewart, M.D.
Pdgfr and ligand levels were assessed in the islets and beta cells of mice at various developmental stages (6 weeks, 6 months, neonatal). Both homologs of Pdgfr (α and β) exhibited an age-dependent decrease in protein and mRNA during the progression from neonate to adulthood. A similar reduction was observed in Ezh2 and Pdgf ligand mRNA. In contrast, no age-associated decrease was observed in other known regulators of beta cell proliferation (Pdx1, Foxo1/3, Rb1, p130, E2f1). This suggests that an age-dependent decline in Ezh2 levels is likely correlated with a decline in Pdgfr.
A Pdgfra conditional knockout mouse (βPdgfrαKO) was generated to assess how premature loss of Pdgfr-α in islet beta cells would affect neonatal Ezh2 expression and subsequent beta cell proliferation (figure 1). At 2-3 weeks of age, mutant mice exhibited lower islet protein and mRNA levels of Ezh2 and higher levels of p16INK4a (cyclin dependent kinase inhibitor - higher levels are associated with decreased beta cell proliferation) mRNA compared with wild type controls. These mice demonstrated a 50% decrease in beta cell mass, a 3-fold reduction in beta cell proliferation, and a diminished glucose tolerance. The authors concluded that early loss of Pdgfra causes an Ezh2-mediated deficiency in both beta cell proliferation and glucose regulation. Beta cell regeneration in response to Pdgfr was further studied in mice with chemically induced diabetes. Following streptozotocin treatment, Pdgfra and Pdgfrb were induced in βPdgfrαKO and wild type mice. In wild type mice, induction of Pdgfr signalling led to increased beta cell Ezh2 expression, a gradual increase in beta cell proliferation/mass, and eventual restoration of glucose regulation (3-4 weeks after challenge). In contrast, βPdgfrαKO mice showed no recovery of beta cell mass and no increase in Ezh2 expression. These mice developed severe diabetes indicating that Pdgfr-α is necessary for restoration of beta cell mass in an adult model of chemically induced diabetes.
Pdgfr signalling and beta cell proliferation were assessed in juvenile and adult mice treated with platelet-derived growth factor-AA (PDGF-AA). In juvenile islets (3 weeks) exposed to PDGF-AA an increase was observed in Ezh2 and phospho-Pdgfr-α protein. Adult islets (7-9 months) treated with PDGF-AA showed no change in Pdgfr signalling or Ezh2 levels, thereby indicating that activation of Pdgfr signalling via PDGF-AA promotes beta cell proliferation only in juvenile islets and that adult islets have lost their competence for Pdgfr signalling. To extend these studies, a mouse model was generated in which human PDGFR mRNA and PDGFR-α(D842V) were expressed constitutively in beta cell islets (βPdgfrαTg). During early development (neonatal and juvenile) there was no difference between mutant and control islets in terms of Pdgfr mRNA levels, beta cell expansion, or glucose regulation. In adult wild type mice (3 months) there was a significant decrease in beta cell Ezh2 expression while mutant mice had elevated beta cell Ezh2 expression at 8-14 months. βPdgfrαTg mice demonstrated a decrease in beta cell levels of p16INK4a and p19Arf mRNA (Ezh2 targets that restrict proliferation), had greater than 9-fold higher levels of beta cell proliferation (at 14 months), and continued to maintain normal glucose regulation. These experiments confirm that activation of Pdgfr-α can sustain in vivo beta cell expansion into and during adulthood without loss of normal metabolic control.