The long-term objective of this program is to understand the regulation of ATP-sensitive K+, KATP, channels by adenine nucleotides; the short-term goal is to challenge the prevailing regulatory model. The levels of adenine nucleotides, ATP and ADP, in pancreatic ?-cells vary with the rate of glucose metabolism. KATP channels respond to these variations and are key players in the normal control of insulin secretion by blood glucose. These channels are the targets for sulfonylureas, hypoglycemic drugs used to treat type 2 diabetes. Mutations in the ABCC8/SUR1 or KCNJ11/Kir6.2 channel components are causes of neonatal diabetes (ND) and neonatal hyperinsulinism (HI) while polymorphisms in both subunits confer increased risk for type 2 diabetes. The prevailing regulatory hypothesis, used to interpret how mutations and polymorphisms alter channel activity, is that ATP hydrolysis at SUR1 is required to counteract or antagonize the inhibitory action of ATP on the Kir6.2 pore. Thus an overactive SUR1 produces ND by 'hyperactivating' the Kir6.2 pore. Altered SUR1 ATPase activity is proposed to underlie hyperactivation and the increased risk posed by ABCC8 polymorphisms. A recent study from our laboratory challenges the prevailing model (Ortiz et al, JBC, 2012). This study used two ND mutant SURs, without an associated Kir6.2, to define the allosteric relations between ATP and sulfonylurea binding with changes in SUR1 conformation. We established that ATP hydrolysis is not essential to switch SUR1 into a stimulatory conformation and proposed that an increased affinity for ATP is the underlying cause of the disorder. This study has now been extended to show there is a direct relation between the affinity of SUR1 for ATP and clinical phenotype; SURs with greater than normal affinity for ATP correlate with neonatal diabetes, those with lower affinity correlate with congenital hyperinsulinism. The negative allosteric relation between ATP and sulfonylurea interactions with SUR1 underlies the known need for higher doses of sulfonylureas to achieve metabolic control in ND patients. The proposed work will extend the analysis to full channels using pharmacologic and electrophysiologic studies on additional ND SUR1 mutants, polymorphisms, and on SURs with substitutions that inhibit ATPase activity. Previous studies on the action of ATP analogs on KATP channel function are used to support the prevailing model, thus additional work is proposed to define their action on conformational switching of SUR1. The overall objective of the project is to develop the data required to modify the prevailing model so that it can adequately explain regulation of KATP channels under normal physiologic conditions and has predictive value for understanding how mutations, particularly ABCC8 mutations, affect channel function.
Diabetes is a multifactorial disease. Monogenic forms of diabetes, secondary to mutations in key genes, provide insight into the control of glucose homeostasis under normal and pathologic conditions. Mutations in ABCC8, the gene that encodes SUR1, the regulatory subunit of ?-cell KATP channels, cause neonatal diabetes by hyperactivating the KCNJ11/Kir6.2 pore, while polymorphisms in both subunits confer an increased risk for type 2 diabetes. This application is focused on using mutations and polymorphisms in ABCC8/SUR1 to establish the mechanism by which adenine nucleotides regulate ?-cell channel activity and thus insulin secretion during changes in glucose metabolism. We have discovered a direct correlation between the affinity of SUR1 for ATP and clinical phenotype; receptors with higher affinity than wildtype correlate with neonatal diabetes, while those with reduced affinity correlate with congenital hyperinsulinism. The proposed research directly challenges the prevailing hypothesis for how nucleotides regulate KATP channels and aims to define a model to explain channel activity in normal and pathologic states.
