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ATP-Sensitive K⁺ Channel Signaling in Glucokinase-Deficient Diabetes

From the Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri

As the rate-limiting controller of glucose metabolism, glucokinase represents the primary ß-cell "glucose sensor." Inactivation of both glucokinase (GK) alleles results in permanent neonatal diabetes; inactivation of a single allele causes maturity-onset diabetes of the young type 2 (MODY-2). Similarly, mice lacking both alleles (GK^–/–) exhibit severe neonatal diabetes and die within a week, whereas heterozygous GK^+/– mice exhibit markedly impaired glucose tolerance and diabetes, resembling MODY-2. Glucose metabolism increases the cytosolic [ATP]-to-[ADP] ratio, which closes ATP-sensitive K⁺ channels (K_ATP channels), leading to membrane depolarization, Ca²⁺ entry, and insulin exocytosis. Glucokinase insufficiency causes defective K_ATP channel regulation, which may underlie the impaired secretion. To test this prediction, we crossed mice lacking neuroendocrine glucokinase (nGK^+/–) with mice lacking K_ATP channels (Kir6.2^–/–). Kir6.2 knockout rescues perinatal lethality of nGK^–/–, although nGK^–/–Kir6.2^–/– animals are postnatally diabetic and still die prematurely. nGK^+/– animals are diabetic on the Kir6.2^+/+ background but only mildly glucose intolerant on the Kir6.2^–/– background. In the presence of glutamine, isolated nGK^+/–Kir6.2^–/– islets show improved insulin secretion compared with nGK^+/–Kir6.2^+/+. The significant abrogation of nGK^–/– and nGK^+/– phenotypes in the absence of K_ATP demonstrate that a major factor in glucokinase deficiency is indeed altered K_ATP signaling. The results have implications for understanding and therapy of glucokinase-related diabetes.

Abbreviations: DMEM, Dulbecco’s modified Eagle medium; GSIS, glucose-stimulated insulin secretion; K_ATP channel, ATP-sensitive K⁺ channel; MODY-2, maturity-onset diabetes of the young type 2

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