Sur1 knockout mice. A model for K(ATP) channel-independent regulation of insulin secretion

Sur1 knockout mouse beta-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.     

A signaling role of glutamine in insulin secretion

Children with hypoglycemia due to recessive loss of function mutations of the beta-cell ATP-sensitive potassium (K(ATP)) channel can develop hypoglycemia in response to protein feeding. We hypothesized that amino acids might stimulate insulin secretion by unknown mechanisms, because the K(ATP) channel-dependent pathway of insulin secretion is defective. We therefore investigated the effects of amino acids on insulin secretion and intracellular calcium in islets from normal and sulfonylurea receptor 1 knockout (SUR1-/-) mice. Even though SUR1-/- mice are euglycemic, their islets are considered a suitable model for studies of the human genetic defect. SUR1-/- islets, but not normal islets, released insulin in response to an amino acid mixture ramp. This response to amino acids was decreased by 60% when glutamine was omitted. Insulin release by SUR1-/- islets was also stimulated by a ramp of glutamine alone. Glutamine was more potent than leucine or dimethyl glutamate. Basal intracellular calcium was elevated in SUR1-/- islets and was increased further by glutamine. In normal islets, methionine sulfoximine, a glutamine synthetase inhibitor, suppressed insulin release in response to a glucose ramp. This inhibition was reversed by glutamine or by 6-diazo-5-oxo-l-norleucine, a non-metabolizable glutamine analogue. High glucose doubled glutamine levels of islets. Methionine sulfoximine inhibition of glucose stimulated insulin secretion was associated with accumulation of glutamate and aspartate. We hypothesize that glutamine plays a critical role as a signaling molecule in amino acid- and glucose-stimulated insulin secretion, and that beta-cell depolarization and subsequent intracellular calcium elevation are required for this glutamine effect to occur.     

A unique allosteric insulin receptor monoclonal antibody that prevents hypoglycemia in the SUR-1−/− mouse model of KATP hyperinsulinism

Loss-of-function mutations of the ß-cell ATP-sensitive potassium channels (K_ATP) cause the most common and severe form of congenital hyperinsulinism (K_ATPHI), a disorder of ß-cell function characterized by severe hypoglycemia. Children with K_ATPHI are typically unresponsive to medical therapy and require pancreatectomy for intractable hypoglycemia. We tested the hypothesis that inhibition of insulin receptor signaling may prevent hypoglycemia in K_ATPHI. To test this hypothesis, we examined the effect of an antibody allosteric inhibitor of the insulin receptor, XMetD, on fasting plasma glucose in a mouse model of K_ATPHI (SUR-1^{? / ?} mice). SUR-1^{? / ?} and wild-type mice received twice weekly intraperitoneal injections of either XMetD or control antibody for 8 wks. Treatment with XMetD significantly decreased insulin sensitivity, and increased hepatic glucose output and fasting plasma glucose. These findings support the potential use of insulin receptor antagonists as a therapeutic approach to control the hypoglycemia in congenital hyperinsulinism.     

Glucose stimulates Ca2+ influx and insulin secretion in 2-week-old beta-cells lacking ATP-sensitive K+ channels

In adult beta-cells glucose-induced insulin secretion involves two mechanisms (a) a K(ATP) channel-dependent Ca(2+) influx and rise of cytosolic [Ca(2+)](c) and (b) a K(ATP) channel-independent amplification of secretion without further increase of [Ca(2+)](c). Mice lacking the high affinity sulfonylurea receptor (Sur1KO), and thus K(ATP) channels, have been developed as a model of congenital hyperinsulinism. Here, we compared [Ca(2+)](c) and insulin secretion in overnight cultured islets from 2-week-old normal and Sur1KO mice. Control islets proved functionally mature: the magnitude and biphasic kinetics of [Ca(2+)](c) and insulin secretion changes induced by glucose, and operation of the amplifying pathway, were similar to adult islets. Sur1KO islets perifused with 1 mm glucose showed elevation of both basal [Ca(2+)](c) and insulin secretion. Stimulation with 15 mm glucose produced a transient drop of [Ca(2+)](c) followed by an overshoot and a sustained elevation, accompanied by a monophasic, 6-fold increase in insulin secretion. Glucose also increased insulin secretion when [Ca(2+)](c) was clamped by KCl. When Sur1KO islets were cultured in 5 instead of 10 mm glucose, [Ca(2+)](c) and insulin secretion were unexpectedly low in 1 mm glucose and increased following a biphasic time course upon stimulation by 15 mm glucose. This K(ATP) channel-independent first phase [Ca(2+)](c) rise was attributed to a Na(+)-, Cl(-)-, and Na(+)-pump-independent depolarization of beta-cells, leading to Ca(2+) influx through voltage-dependent calcium channels. Glucose indeed depolarized Sur1KO islets under these conditions. It is suggested that unidentified potassium channels are sensitive to glucose and subserve the acute and long-term metabolic control of [Ca(2+)](c) in beta-cells without functional K(ATP) channels.     

5-amino-imidazole carboxamide riboside acutely potentiates glucose-stimulated insulin secretion from mouse pancreatic islets by KATP channel-dependent and -independent pathways

AMP-activated protein kinase (AMPK) is an important signaling effector that couples cellular metabolism and function. The effects of AMPK activation on pancreatic beta-cell function remain unresolved. We used 5-amino-imidazole carboxamide riboside (AICAR), an activator of AMPK, to define the signaling mechanisms linking the activation of AMPK with insulin secretion. Application of 300 microM AICAR to mouse islets incubated in 5-14 mM glucose significantly increased AMPK activity and potentiated insulin secretion. AICAR inhibited ATP-sensitive K(+) (K(ATP)) channels and increased the frequency of glucose-induced calcium oscillations in islets incubated in 8-14 mM glucose. At lower glucose concentration (5mM) AICAR did not affect K(ATP) activity or intracellular ([Ca(2+)](i)). AICAR also did not inhibit (86)Rb(+) efflux from islets isolated from Sur1(-/-) mice that lack K(ATP) channels yet significantly potentiated glucose stimulated insulin secretion. Our data suggest that AICAR stimulates insulin secretion by both K(ATP) channel-dependent and -independent pathways.     

Glucagon-like peptide-1 and islet lipolysis.

A role for glucagon-like peptide 1 (GLP-1) has been suggested in stimulating beta-cell lipolysis via elevation of cAMP and activation of protein kinase A, which in turn may activate hormone-sensitive lipase (HSL), thereby contributing to fatty acid generation (FFA) from intracellular triglyceride stores. FFAs may then be metabolized to a lipid signal, which is required for optimal glucose-stimulated insulin secretion. Since HSL is expressed in islet beta-cells, this effect could contribute to the stimulation of insulin secretion by GLP-1, provided that a lipid signal of importance for insulin secretion is generated. To examine this hypothesis, we have studied the acute effect of GLP-1 on isolated mouse islets from normal mice and from mice with high-fat diet induced insulin resistance. We found, however, that although GLP-1 (100 nM) markedly potentiated glucose-stimulated insulin secretion from islets of both feeding groups, the peptide was not able to stimulate islet palmitate oxidation or increase lipolysis measured as glycerol release. This indicates that a lipid signal does not contribute to the acute stimulation of insulin secretion by GLP-1. To test whether lipolysis might be involved in the islet effects of long-term GLP-1 action, mice from the two feeding groups were chronically treated with exendin-4, a peptide that lowers blood glucose by interacting with GLP-1 receptors, in order to stimulate insulin secretion, for 16 days before isolation of the islets. The insulinotropic effects of GLP-1 and forskolin were exaggerated in isolated islets from exendin-4 treated mice given a high-fat diet, with a augmented palmitate oxidation as well as islet lipolysis at high glucose levels in these islets. Exendin-4 treatment had less impact on mice fed a normal diet. From these results we conclude that while GLP-1 does not seem to induce beta-cell lipolysis acutely in mouse islets, the peptide affects beta-cell fat metabolism after long-term adaptation to GLP-1 receptor stimulation.     

Exendin-4 and exercise promotes beta-cell function and mass through IRS2 induction in islets of diabetic rats

Not only exendin-4 but also exercise has been reported to improve glucose homeostasis by enhancing insulinotropic action, but the nature of its molecular mechanism has not been clarified. We investigated a mechanism to promote insulinotropic action by means of exendin-4 and exercise training in 90% pancreatectomized (Px) rats fed 40% energy fat diets. Px diabetic rats were divided into 4 groups: 1) exendin-4, 2) exendin-4 plus exercise, 3) saline (control), and 4) exercise. During the 8-week experimental period, rats in the exendin-4 groups were subcutaneously administered with 150 pmol/kg exendin-4 twice a day, while those in the exercise groups ran on an uphill treadmill with a 15 degree incline at 20 m/min for 30 min 5 days a week. First phase insulin secretion was elevated by both the administration of exendin-4 and exercise training during hyperglycemic clamp. However, second phase insulin secretion did not differ among the groups. Individual treatment of exendin-4 and exercise expanded beta-cell mass by increasing its proliferation and reducing its apoptosis, but the administration of exendin-4 plus exercise training did not produce any additional, positive effects. Both exendin-4 and exercise enhanced insulin receptor substrate (IRS)-2 expression through the activation of cAMP responding element binding protein in the islets, which potentiated their insulin/insulin like growth factor-1 signaling. The potentiation of the signaling increased the expression of pancreas duodenum homeobox-1, involved in beta-cell proliferation. In conclusion, exendin-4 and exercise equivalently improved glucose homeostasis due to the induction of IRS-2 in the islets of diabetic rats through a cAMP dependent common pathway.     

Insulinotropic effect of the non-steroidal compound STX in pancreatic β-cells

The non-steroidal compound STX modulates the hypothalamic control of core body temperature and energy homeostasis. The aim of this work was to study the potential effects of STX on pancreatic β-cell function. 1-10 nM STX produced an increase in glucose-induced insulin secretion in isolated islets from male mice, whereas it had no effect in islets from female mice. This insulinotropic effect of STX was abolished by the anti-estrogen ICI 182,780. STX increased intracellular calcium entry in both whole islets and isolated β-cells, and closed the K(ATP) channel, suggesting a direct effect on β-cells. When intraperitoneal glucose tolerance test was performed, a single dose of 100 μg/kg body weight STX improved glucose sensitivity in males, yet it had a slight effect on females. In agreement with the effect on isolated islets, 100 μg/kg dose of STX enhanced the plasma insulin increase in response to a glucose load, while it did not in females. Long-term treatment (100 μg/kg, 6 days) of male mice with STX did not alter body weight, fasting glucose, glucose sensitivity or islet insulin content. Ovariectomized females were insensitive to STX (100 μg/kg), after either an acute administration or a 6-day treatment. This long-term treatment was also ineffective in a mouse model of mild diabetes. Therefore, STX appears to have a gender-specific effect on blood glucose homeostasis, which is only manifested after an acute administration. The insulinotropic effect of STX in pancreatic β-cells is mediated by the closure of the K(ATP) channel and the increase in intracellular calcium concentration. The in vivo improvement in glucose tolerance appears to be mostly due to the enhancement of insulin secretion from β-cells.     

Relative hypoglycemia and hyperinsulinemia in mice with heterozygous lipoprotein lipase (LPL) deficiency. Islet LPL regulates insulin secretion.

Lipoprotein lipase (LPL) provides tissues with fatty acids, which have complex effects on glucose utilization and insulin secretion. To determine if LPL has direct effects on glucose metabolism, we studied mice with heterozygous LPL deficiency (LPL+/-). LPL+/- mice had mean fasting glucose values that were up to 39 mg/dl lower than LPL+/+ littermates. Despite having lower glucose levels, LPL+/- mice had fasting insulin levels that were twice those of +/+ mice. Hyperinsulinemic clamp experiments showed no effect of genotype on basal or insulin-stimulated glucose utilization. LPL message was detected in mouse islets, INS-1 cells (a rat insulinoma cell line), and human islets. LPL enzyme activity was detected in the media from both mouse and human islets incubated in vitro. In mice, +/- islets expressed half the enzyme activity of +/+ islets. Islets isolated from +/+ mice secreted less insulin in vitro than +/- and -/- islets, suggesting that LPL suppresses insulin secretion. To test this notion directly, LPL enzyme activity was manipulated in INS-1 cells. INS-1 cells treated with an adeno-associated virus expressing human LPL had more LPL enzyme activity and secreted less insulin than adeno-associated virus-beta-galactosidase-treated cells. INS-1 cells transfected with an antisense LPL oligonucleotide had less LPL enzyme activity and secreted more insulin than cells transfected with a control oligonucleotide. These data suggest that islet LPL is a novel regulator of insulin secretion. They further suggest that genetically determined levels of LPL play a role in establishing glucose levels in mice.     

Overnight culture unmasks glucose-induced insulin secretion in mouse islets lacking ATP-sensitive K+ channels by improving the triggering Ca2+ signal

A current model ascribes glucose-induced insulin secretion to the interaction of a triggering pathway (K(ATP) channel-dependent Ca(2+) influx and rise in cytosolic [Ca(2+)](c)) and an amplifying pathway (K(ATP) channel-independent augmentation of secretion without further increase of [Ca(2+)](c)). However, several studies of sulfonylurea receptor 1 null mice (Sur1KO) failed to measure significant effects of glucose in their islets lacking K(ATP) channels. We addressed this issue that challenges the model. Compared with controls, fresh Sur1KO islets showed slightly elevated basal [Ca(2+)](c) and insulin secretion. In 15 mm glucose, the absolute rate of secretion was approximately 3-fold lower in Sur1KO than control islets, with only poor increase above base line. Overnight culture of Sur1KO islets in 10 mm glucose (not in 5 mm) augmented basal insulin secretion and considerably improved the response to 15 mm glucose, which reached higher values than in control islets, in which culture had little impact. Glucose stimulation during KCl depolarization showed that the amplifying pathway is functional in fresh and cultured Sur1KO islets. The differences in insulin secretion between fresh and cultured Sur1KO islets and between Sur1KO and control islets were not attributable to differences in insulin content, glucose oxidation rate, or synchronization of [Ca(2+)](c) oscillations. The unmasking of glucose-induced insulin secretion in beta-cells lacking K(ATP) channels is paradoxically due to improvement in the production of a triggering signal (elevated [Ca(2+)](c)). The results show that K(ATP) channels are not the only transducer of glucose effects on [Ca(2+)](c) in beta-cells. They explain controversies in the literature and refute arguments raised against the model implicating an amplifying pathway in glucose-induced insulin secretion.     

Fasting-induced suppression of LH secretion does not require activation of ATP-sensitive potassium channels

Reproductive hormone secretions are inhibited by fasting and restored by feeding. Metabolic signals mediating these effects include fluctuations in serum glucose, insulin, and leptin. Because ATP-sensitive potassium (K(ATP)) channels mediate glucose sensing and many actions of insulin and leptin in neurons, we assessed their role in suppressing LH secretion during food restriction. Vehicle or a K(ATP) channel blocker, tolbutamide, was infused into the lateral cerebroventricle in ovariectomized mice that were either fed or fasted for 48 h. Tolbutamide infusion resulted in a twofold increase in LH concentrations in both fed and fasted mice compared with both fed and fasted vehicle-treated mice. However, tolbutamide did not reverse the suppression of LH in the majority of fasted animals. In sulfonylurea (SUR)1-null mutant (SUR1(-/-)) mice, which are deficient in K(ATP) channels, and their wild-type (WT) littermates, a 48-h fast was found to reduce serum LH concentrations in both WT and SUR(-/-) mice. The present study demonstrates that 1) blockade of K(ATP) channels elevates LH secretion regardless of energy balance and 2) acute fasting suppresses LH secretion in both SUR1(-/-) and WT mice. These findings support the hypothesis that K(ATP) channels are linked to the regulation of gonadotropin-releasing hormone (GnRH) release but are not obligatory for mediating the effects of fasting on GnRH/LH secretion. Thus it is unlikely that the modulation of K(ATP) channels either as part of the classical glucose-sensing mechanism or as a component of insulin or leptin signaling plays a major role in the suppression of GnRH and LH secretion during food restriction.     

Increased expression of GAD65 and GABA in pancreatic beta-cells impairs first-phase insulin secretion

The functional role of glutamate decarboxylase (GAD) and its product GABA in pancreatic islets has remained elusive. Mouse beta-cells express the larger isoform GAD67, whereas human islets express only the smaller isoform GAD65. We have generated two lines of transgenic mice expressing human GAD65 in pancreatic beta-cells (RIP7-hGAD65, Lines 1 and 2) to study the effect that GABA generated by this isoform has on islet cell function. The ascending order of hGAD65 expression and/or activity in beta-cells was Line 1 heterozygotes < Line 2 heterozygotes < Line 1 homozygotes. Line 1 heterozygotes have normal glucose tolerance, whereas Line 1 homozygotes and Line 2 heterozygotes exhibit impaired glucose tolerance and inhibition of insulin secretion in vivo in response to glucose. In addition, fasting levels of blood glucose are elevated and insulin is decreased in Line 1 homozygotes. Pancreas perfusion experiments suggest that GABA generated by GAD65 may function as a negative regulator of first-phase insulin secretion in response to glucose by affecting a step proximal to or at the K(ATP)(+) channel.     

Both triggering and amplifying pathways contribute to fuel-induced insulin secretion in the absence of sulfonylurea receptor-1 in pancreatic beta-cells

In normal beta-cells glucose induces insulin secretion by activating both a triggering pathway (closure of K(ATP) channels, depolarization, and rise in cytosolic [Ca(2+)](i)) and an amplifying pathway (augmentation of Ca(2+) efficacy on exocytosis). It is unclear if and how nutrients can regulate insulin secretion by beta-cells lacking K(ATP) channels (Sur1 knockout mice). We compared glucose- and amino acid-induced insulin secretion and [Ca(2+)](i) changes in control and Sur1KO islets. In 1 mm glucose (non-stimulatory for controls), the triggering signal [Ca(2+)](i) was high (loss of regulation) and insulin secretion was stimulated in Sur1KO islets. This "basal" secretion was decreased or increased by imposed changes in [Ca(2+)](i) and was dependent on ATP production, indicating that both triggering and amplifying signals are involved. High glucose stimulated insulin secretion in Sur1KO islets, by an unsuspected, transient increase in [Ca(2+)](i) and a sustained activation of the amplifying pathway. Unlike controls, Sur1KO islets were insensitive to diazoxide and tolbutamide, which rules out effects of either drug at sites other than K(ATP) channels. Amino acids potently increased insulin secretion by Sur1KO islets through both a further electrogenic rise in [Ca(2+)](i) and a metabolism-dependent activation of the amplifying pathway. After sulfonylurea blockade of their K(ATP) channels, control islets qualitatively behaved like Sur1KO islets, but their insulin secretion rate was consistently lower for a similar or even higher [Ca(2+)](i). In conclusion, fuel secretagogues can control insulin secretion in beta-cells without K(ATP) channels, partly by an unsuspected influence on the triggering [Ca(2+)](i) signal and mainly by the modulation of a very effective amplifying pathway.     

GABAB receptors and glucose homeostasis: evaluation in GABAB receptor knockout mice

GABA has been proposed to inhibit insulin secretion through GABAB receptors (GABABRs) in pancreatic beta-cells. We investigated whether GABABRs participated in the regulation of glucose homeostasis in vivo. The animals used in this study were adult male and female BALB/C mice, mice deficient in the GABAB1 subunit of the GABABR (GABAB(-/-)), and wild types (WT). Blood glucose was measured under fasting/fed conditions and in glucose tolerance tests (GTTs) with a Lifescan Glucose meter, and serum insulin was measured by ELISA. Pancreatic insulin content and islet insulin were released by RIA. Western blots for the GABAB1 subunit in islet membranes and immunohistochemistry for insulin and GABAB1 were performed in both genotypes. BALB/C mice preinjected with Baclofen (GABABR agonist, 7.5 mg/kg ip) presented impaired GTTs and decreased insulin secretion compared with saline-preinjected controls. GABAB(-/-) mice showed fasting and fed glucose levels similar to WT. GABAB(-/-) mice showed improved GTTs at moderate glucose overloads (2 g/kg). Baclofen pretreatment did not modify GTTs in GABAB(-/-) mice, whereas it impaired normal glycemia reinstatement in WT. Baclofen inhibited glucose-stimulated insulin secretion in WT isolated islets but was without effect in GABAB(-/-) islets. In GABAB(-/-) males, pancreatic insulin content was increased, basal and glucose-stimulated insulin secretion were augmented, and impaired insulin tolerance test and increased homeostatic model assessment of insulin resistance index were determined. Immunohistochemistry for insulin demonstrated an increase of very large islets in GABAB(-/-) males. Results demonstrate that GABABRs are involved in the regulation of glucose homeostasis in vivo and that the constitutive absence of GABABRs induces alterations in pancreatic histology, physiology, and insulin resistance.     

Secretory and electrophysiological characteristics of insulin cells from gastrectomized mice: evidence for the existence of insulinotropic agents in the stomach

Mice were subjected to gastrectomy (GX) or sham operation (controls). Four to six weeks later the pancreatic islets were isolated and analysed for cAMP or alternatively incubated in a Krebs-Ringer based medium in an effort to study insulin secretion and cAMP accumulation in response to glucose or the adenylate cyclase activator forskolin. Freshly isolated islets from GX mice had higher cAMP content than islets from control mice, a difference that persisted after incubation for 1 h at a glucose concentration of 4 mmol/l. Addition of forskolin to this medium induced much greater cAMP and insulin responses in islets from GX mice than in islets from control mice. In contrast, the insulin response to high glucose (16.7 mmol/l) was much weaker in GX islets than in control islets. Glucose-induced insulin release was associated with a 2-fold rise in the cAMP content in control islets. Surprisingly no rise in cAMP was noted in GX islets incubated at high glucose. Capacitance measurements conducted on isolated insulin cells from GX mice revealed a much lower exocytotic response to a single 500 ms depolarisation (from -70 mV to zero) than in control insulin cells. Addition of cAMP to the cytosol enhanced the exocytotic response in insulin cells from control mice but not from GX mice. The depolarisation-triggered inward Ca(2+) current in insulin cells from GX mice did not differ from that in control mice, and hence the reduced exocytotic response following GX cannot be ascribed to a decreased Ca(2+) influx. Experiments involving a train of ten 500 ms depolarisations revealed that the exocytotic response was prominent in control insulin cells but modest in GX insulin cells. It seems that cAMP is capable of eliciting insulin release from insulin cells of GX mice only when cAMP is generated in a specific microdomain conceivably through the intervention of membrane-associated adenylate cyclases that can be activated by forskolin. The GX-evoked impairment of depolarisation-induced exocytosis and glucose-stimulated insulin release may reflect the lack of a gastric agent that serves to maintain an appropriate insulin response to glucose and an appropriate exocytotic response to depolarisation by raising cAMP in a special glucose-sensitive compartment possibly regulated by a soluble adenylate cyclase.     

β-Cells with relative low HIMP1 overexpression levels in a transgenic mouse line enhance basal insulin production and hypoxia/hypoglycemia tolerance

Rodent pancreatic β-cells that naturally lack hypoglycemia/hypoxia inducible mitochondrial protein 1 (HIMP1) are susceptible to hypoglycemia and hypoxia influences. A linkage between the hypoglycemia/hypoxia susceptibility and the lack of HIMP1 is suggested in a recent study using transformed β-cells lines. To further illuminate this linkage, we applied mouse insulin 1 gene promoter (MIP) to control HIMP1-a isoform cDNA and have generated three lines (L1 to L3) of heterozygous HIMP1 transgenic (Tg) mice by breeding of three founders with C57BL/6J mice. In HIMP1-Tg mice/islets, we performed quantitative polymerase chain reaction (PCR), immunoblot, histology, and physiology studies to investigate HIMP1 overexpression and its link to β-cell function/survival and body glucose homeostasis. We found that the HIMP1 level increased steadily in β-cells of L1 to L3 heterozygous HIMP1-Tg mice. HIMP1 overexpression at relatively lower levels in L1 heterozygotes results in a negligible decline in blood glucose concentrations and an insignificant elevation in blood insulin levels, while HIMP1 overexpression at higher levels are toxic, causing hyperglycemia in L2/3 heterozygotes. Follow-up studies in 5-30-week-old L1 heterozygous mice/islets found that HIMP1 overexpression at relatively lower levels in β-cells has enhanced basal insulin biosynthesis, basal insulin secretion, and tolerances to low oxygen/glucose influences. The findings enforced the linkage between the hypoglycemia/hypoxia susceptibility and the lack of HIMP1 in β-cells, and show a potential value of HIMP1 overexpression at relatively lower levels in modulating β-cell function and survival.