Diabetes is characterized by ‘glucotoxic’ loss of pancreatic β-cell function and insulin content, but underlying mechanisms remain unclear. A mouse model of insulin-secretory deficiency induced by β-cell inexcitability (KATP gain-of-function) demonstrates development of diabetes and reiterates the features of human neonatal diabetes. In the diabetic state, β-cells lose their mature identity and dedifferentiate to neurogenin3-positive and insulin-negative cells. Lineage-tracing experiments show that dedifferentiated cells can subsequently re-differentiate to mature neurogenin3-negative, insulin-positive, β-cells after lowering of blood glucose by insulin therapy. We demonstrate here that β-cell dedifferentiation, rather than apoptosis, is the main mechanism of loss of insulin-positive cells, and re-differentiation accounts for restoration of insulin content and antidiabetic-drug responsivity in these animals. These results may help explain gradual decrease in β-cell mass in long-standing diabetes, and recovery of β-cell function and drug responsivity in type-2 diabetic patients following insulin therapy, and suggest an approach to rescuing ‘exhausted’ β-cells in diabetes.

ATP-insensitive KATP channel mutations cause neonatal diabetes mellitus (NDM). To explore the mechanistic etiology, we generated transgenic mice carrying an ATP-insensitive mutant KATP channel subunit. Constitutive expression in pancreatic β-cells caused neonatal hyperglycemia and progression to severe diabetes and growth retardation with loss of islet insulin content and β-cell architecture. Tamoxifen-induced expression in adult β-cells led to diabetes within 2-weeks, with similar secondary consequences. Diabetes was avoided by transplantation of normal islets under the kidney capsule, before induction. Moreover, the endogenous islets maintained normal insulin content and secretion in response to sulfonylureas, but not glucose, consistent with reduced ATP sensitivity of β-cell KATP channels. In NDM, transfer to sulfonylurea therapy is less effective in older patients. This may result from poor glycemic control or lack of insulin, since glibenclamide treatment prior to tamoxifen-induction prevented diabetes and secondary complications in mice, but failed to halt disease progression after diabetes had developed.

Pancreatic β-cells secrete insulin in response to closure of ATP-sensitive K+ (KATP) channels, which causes membrane depolarization and a concomitant rise in intracellular Ca2+ (Cai). In intact islets, β-cells are coupled by gap junctions, which are proposed to synchronize electrical activity and Cai oscillations after exposure to stimulatory glucose (>7 mM). To determine the significance of this coupling in regulating insulin secretion, we examined islets and β-cells from transgenic mice that express zero functional KATP channels in approximately 70% of their β-cells, but normal KATP channel density in the remainder. We found that KATP channel activity from approximately 30% of the β-cells is sufficient to maintain strong glucose dependence of metabolism, Cai, membrane potential, and insulin secretion from intact islets, but that glucose dependence is lost in isolated transgenic cells. Further, inhibition of gap junctions caused loss of glucose sensitivity specifically in transgenic islets. These data demonstrate a critical role of gap junctional coupling of KATP channel activity in control of membrane potential across the islet. Control via coupling lessens the effects of cell–cell variation and provides resistance to defects in excitability that would otherwise lead to a profound diabetic state, such as occurs in persistent neonatal diabetes mellitus.

SUMMARY Carrier-facilitated pyruvate transport across the inner mitochondrial membrane plays an essential role in anabolic and catabolic intermediary metabolism. The mitochondrial pyruvate carrier 2 (Mpc2) is believed to be a component of the complex that facilitates mitochondrial pyruvate import. Complete MPC2 deficiency resulted in embryonic lethality in mice. However, a second mouse line expressing an N-terminal truncated MPC2 protein (Mpc2Δ16) was viable, but exhibited reduced capacity for mitochondrial pyruvate oxidation. Metabolic studies demonstrated exaggerated blood lactate concentrations after pyruvate, glucose, or insulin challenge in Mpc2Δ16 mice. Additionally, compared to WT controls, Mpc2Δ16 mice exhibited normal insulin sensitivity, but elevated blood glucose after bolus pyruvate or glucose injection. This was attributable to reduced glucose-stimulated insulin secretion and was corrected by sulfonylurea KATP channel inhibitor administration. Collectively, these data are consistent with a role for MPC2 in mitochondrial pyruvate import and suggest that Mpc2 deficiency results in defective pancreatic beta cell glucose sensing.

The prediction that overactivity of the pancreatic ATP-sensitive K(+) channel (K(ATP) channel) underlies reduced insulin secretion and causes a diabetic phenotype in humans has recently been borne out by genetic studies implicating "activating" mutations in the Kir6.2 subunit of K(ATP) as causal in both permanent and transient neonatal diabetes. Here we characterize the channel properties of Kir6.2 mutations that underlie transient neonatal diabetes (I182V) or more severe forms of permanent neonatal diabetes (V59M, Q52R, and I296L). In all cases, the mutations result in a significant decrease in sensitivity to inhibitory ATP, which correlates with channel "overactivity" in intact cells. Mutations can be separated into those that directly affect ATP affinity (I182V) and those that stabilize the open conformation of the channel and indirectly reduce ATP sensitivity (V59M, Q52R, and I296L). With respect to the latter group, alterations in channel gating are also reflected in a functional "uncoupling" of sulfonylurea (SU) block: SU sensitivity of I182V is similar to that of wild-type mutants, but the SU sensitivity of all gating mutants is reduced, with the I296L mutant being resistant to block by tolbutamide (