Sur1 Knockout Mice

Abstract

Sur1 knockout mouse ␤-cells lack K ATP channels and show spontaneous Ca 2؉ action potentials equivalent to those seen in patients with persistent hyperinsulinemic hypoglycemia of infancy, but the mice are normoglycemic unless stressed. Sur1 -/-islets lack first phase insulin secretion and exhibit an attenuated glucose-stimulated second phase secretion. Loss of the first phase leads to mild glucose intolerance, whereas reduced insulin output is consistent with observed neonatal hyperglycemia. Loss of K ATP channels impairs the rate of return to a basal secretory level after a fall in glucose concentration. This leads to increased hypoglycemia upon fasting and contributes to a very early, transient neonatal hypoglycemia. Whereas persistent hyperinsulinemic hypoglycemia of infancy underscores the importance of the K ATP -dependent ionic pathway in control of insulin release, the Sur1 -/-animals provide a novel model for study of K ATP -independent pathways that regulate insulin secretion.ATP-sensitive potassium channels (K ATP channels) 1 are a unique combination of a K ϩ inward rectifier (either K IR 6.1 or K IR 6.2) and a sulfonylurea receptor (SUR1 or SUR2) transport ATPase superfamily members (1-3). These channels respond to changes in ATP/ADP and can couple metabolism to membrane electrical activity. SUR1 and K IR 6.2 comprise the K ATP channels in pancreatic ␤-cells that regulate the ionic pathway mediating glucose-stimulated insulin secretion by setting the resting membrane potential below the activation threshold for voltage-gated Ca 2ϩ channels (4). Mutations in human Sur1 or K IR 6.2 cause a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) characterized by oversecretion of insulin despite severe hypoglycemia (1, 5). Surprisingly, two recent studies involving disruption of K ATP channels in mice produced a quite different picture. Targeted overexpression of a dominant-negative K IR 6.2 subunit, K IR 6.2 G132S , in ␤-cells reduced channel activity producing animals that were hypoglycemic at birth but became increasingly hyperglycemic secondary to ␤-cell death (6).K IR 6.2 null mice, K IR 6.2 -/-, on the other hand, completely lack ␤-cell K ATP channels but exhibit a less severe phenotype (7). The K IR 6.2 -/-animals have nearly normal blood glucose levels, showing mild glucose intolerance when challenged with glucose. These animals are reported to release a small amount of insulin in response to glucose, whereas isolated, perifused islets show a small first phase of glucose-stimulated insulin secretion and no second phase. The normal blood glucose levels have been attributed to insulin hypersensitivity secondary to the loss of SUR2A/K IR 6.2 K ATP channels in skeletal muscle.SUR1 null mice, Sur1 -/-, unlike their K IR 6.2 -/-counterparts, are not insulin-hypersensitive. Isolated Sur1 -/-islets exhibit a pattern of glucose-stimulated insulin release consistent with regulation by an underlying K ATP -independent pathway (or pathways), the nature of which is unknown (8 -12). The Sur...

Resveratrol Binds to the Sulfonylurea Receptor (SUR) and Induces Apoptosis in a SUR Subtype-specific Manner

“…However, when pancreatic islets from SUR1KO mice were treated with resveratrol, no such alterations in islet morphology were observed (2.28 Ϯ 0.67% apoptotic nuclei/total number of nuclei). When islets from normal or knock-out mice were treated only with solvent, no visible apoptotic changes were observed (2.03 Ϯ 0.58 and 2.27 Ϯ 0.47% apoptotic nuclei/total number of nuclei, respectively), which was in agreement with the observations of Seghers et al (35) in their first characterization of SUR1KO mice.…”

“…Preparation of Pancreatic Islets-Experiments were performed on pancreatic islets of Langerhans isolated from wild-type C57BL/6 mice (Charles River GmbH, Sulzfeld, Germany) or from SUR1 knock-out (SUR1KO) mice (35). Islets were obtained from male or female mice (age, 20 -25 weeks; and weight, 25-35 g) by collagenase IV digestion of the pancreas.…”

Hyperinsulinism induced by targeted suppression of beta cell K ATP channels

“…Whereas fura-4F fluorescence is largely targeted to the cytosol, cytosolic fluorescence of EGFP-tagged K ATP channels, measured separately under the same conditions, was relatively low (J.D.J., unpublished observations). [4][5][6][7][8][9][10][11][12][13][14], there was also no difference in serum insulin levels between AAA-TG and control littermates (8). However, analysis of serum insulin levels of fasted and fed adult mice (4-8 months) indicated consistently 2.5-fold higher levels in AAA-TG than in control littermates (Table 1, Fig.…”

“…In the previously characterized K ATP -deficient transgenic and knockout animals, abnormal appearance of peripherally distributed alpha cells among the core of insulin-containing beta cells (5-7) and a decrease in beta cell mass were observed in adult islets (6,7). This loss of beta cells likely underlies the overt hyperglycemia observed beginning at 4 weeks of age in Kir6.2[G123S] mice.…”

“…A similar phenotype is observed with both Kir6.2-and SUR1-knockout mice (6, 7), neither of which express any beta cell K ATP channels. Again, these mice show transient hypoglycemia as neonates, but glucose-induced insulin secretion is abolished or highly attenu- ated in adults, and the mice are normoglycemic (6,7), suggesting a compensatory nonelectrical regulation of secretion.…”