Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC100
An inducible mouse model of neonatal diabetes
K. Shimomura1, M. Iberl1, P. Proks1, F. Ashcroft1
1. Physiology Anatomy and Genetics, Oxford University, Oxford, Oxford, United Kingdom.
Neonatal diabetes (ND) is a rare disease, commonly caused by gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of the KATP channel. In wild-type pancreatic β-cells, glucose metabolism stimulates insulin secretion by closing KATP channels, thereby inducing membrane depolarization, calcium influx and insulin exocytosis. Gain-of-function mutations in the KATP channel impair channel inhibition by metabolically generated ATP, thereby reducing channel closure and causing ND. We have generated an inducible mouse model of ND using the Cre-lox system. We targeted the Kir6.2-V59M mutation, which causes ND in human patients, selectively to pancreatic β-cells by crossing floxed Kir6.2-V59M mice with Rip-Cre-ER mice. Gene expression was induced at 12 weeks by tamoxifen injection. Within 3-5 days of induction of the Kir6.2-V59M gene, animals were hyperglycaemic (>25mM glucose; for these tests blood was withdrawn from tail under local anaesthesia using EMLA cream) and glucose-stimulated insulin secretion from isolated islets was almost completely suppressed, due to a marked reduction in ATP inhibition of the KATP channel blocker (IC50=275±67µM, n=26 versus 12±2µM, n=9 for uninduced controls). Longer-term hyperglycaemia (>2 weeks), led to a reduction in islet density, the percentage of β-cells per islet, and insulin content. Islet morphology was also disturbed. Subsequent treatment with the sulphonylurea glibenclamide (the glibenclamide pellet was implanted subcutaneously under general anaesthesia; 2% isoflurane for induction, 1% for maintenance), which blocks KATP channels, restored free-fed blood glucose levels to <10mM and improved glucose tolerance. After 2 weeks of glibenclamide therapy, insulin content, the percentage of β-cells/islet and islet morphology were similar to that of uninduced islets. However, islet density did not recover. Treatment with glibenclamide within 1 day of elevated blood glucose (>20mM), however, protected the islets from all damage. We conclude that long-term KATP channel activation can have both reversible and irreversible effects on pancreatic islets, either directly due to the increased K+ efflux, or indirectly as a result of the hyperglycaemia. These data may help explain why some ND patients fail to respond to sulphonylurea therapy (their islets may have been damaged) and why the dose of drug required to treat many patients declines with time (some recovery of islet function may take place). Our results also suggest that early treatment is advisable.
Where applicable, experiments conform with Society ethical requirements