Diabetes and insulin secretion: whither KATP?

Glycolytic enzyme Enolase-1 regulates insulin gene expression in pancreatic β-cell

Enolase-1 (Eno1) plays a critical role in regulating glucose metabolism; however, its specific impact on pancreatic islet β-cells remains elusive. This study aimed to provide a preliminary exploration of Eno1 function in pancreatic islet β-cells. The findings revealed that the expression of ENO1 mRNA in type 2 diabetes donors was significantly increased and positively correlated with HbA1C and negatively correlated with insulin gene expression. A high level of Eno1 in human insulin-secreting rat INS-1832/13 cells with co-localization with intracellular insulin proteins was accordingly observed. Silencing of Eno1 using siRNA or inhibiting Eno1 protein activity with an Eno1 antagonist significantly reduced insulin secretion and insulin content in β-cells, while the proinsulin/insulin content ratio remained unchanged. This reduction in β-cells function was accompanied by a notable decrease in intracellular ATP and mitochondrial cytochrome C levels. Overall, our findings confirm that Eno1 regulates the insulin secretion process, particularly glucose metabolism and ATP production in the β-cells. The mechanism primarily involves its influence on insulin production, suggesting that Eno1 represents a potential target for β-cell protection and diabetes treatment.

KATP Channel Blocker Glibenclamide Prevents Radiation-Induced Lung Injury and Inhibits Radiation-Induced Apoptosis of Vascular Endothelial Cells by Increased Ca2+ Influx and Subsequent PKC Activation

Radiation-induced lung injury (RILI) is a common and severe side effect of thoracic radiotherapy, which compromises patients' quality of life. Recent studies revealed that early vascular injury, especially microvascular damage, played a central role in the development of RILI. For this reason, early vascular protection is essential for RILI therapy. The ATP-sensitive K⁺ (K_ATP) channel is an ATP-dependent K⁺ channel with multiple subunits. The protective role of the K_ATP channel in vascular injury has been demonstrated in some published studies. In this work, we investigated the effect of K_ATP channel on RILI. Our findings confirmed that the K_ATP channel blocker glibenclamide, rather than the K_ATP channel opener pinacidil, remitted RILI, and in particular, provided protection against radiation-induced vascular injury. Cytology experiments verified that glibenclamide enhanced cell viability, increased the potential of proliferation after irradiation and attenuated radiation-induced apoptosis. Involved mechanisms included increased Ca²⁺ influx and PKC activation, which were induced by glibenclamide pretreatment. In conclusion, the K_ATP channel blocker glibenclamide remitted RILI and inhibited the radiation-induced apoptosis of vascular endothelial cells by increased Ca²⁺ influx and subsequent PKC activation.

Kcne2 deletion impairs insulin secretion and causes type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) represents a rapidly increasing threat to global public health. T2DM arises largely from obesity, poor diet, and lack of exercise, but it also involves genetic predisposition. Here we report that the KCNE2 potassium channel transmembrane regulatory subunit is expressed in human and mouse pancreatic β cells. Kcne2 deletion in mice impaired glucose tolerance as early as 5 wk of age in pups fed a Western diet, ultimately causing diabetes. In adult mice fed normal chow, skeletal muscle expression of insulin receptor β and insulin receptor substrate 1 were down-regulated 2-fold by Kcne2 deletion, characteristic of T2DM. Kcne2 deletion also caused extensive pancreatic transcriptome changes consistent with facets of T2DM, including endoplasmic reticulum stress, inflammation, and hyperproliferation. Kcne2 deletion impaired β-cell insulin secretion in vitro up to 8-fold and diminished β-cell peak outward K⁺ current at positive membrane potentials, but also left-shifted its voltage dependence and slowed inactivation. Interestingly, we also observed an aging-dependent reduction in β-cell outward currents in both Kcne2^+/+ and Kcne2^-/- mice. Our results demonstrate that KCNE2 is required for normal β-cell electrical activity and insulin secretion, and that Kcne2 deletion causes T2DM. KCNE2 may regulate multiple K⁺ channels in β cells, including the T2DM-linked KCNQ1 potassium channel α subunit.-Lee, S. M., Baik, J., Nguyen, D., Nguyen, V., Liu, S., Hu, Z., Abbott, G. W. Kcne2 deletion impairs insulin secretion and causes type 2 diabetes mellitus.

Islet Hypersensitivity to Glucose Is Associated With Disrupted Oscillations and Increased Impact of Proinflammatory Cytokines in Islets From Diabetes-Prone Male Mice

Pulsatile insulin release is the primary means of blood glucose regulation. The loss of pulsatility is thought to be an early marker and possible factor in developing type 2 diabetes. Another early adaptation in islet function to compensate for obesity is increased glucose sensitivity (left shift) associated with increased basal insulin release. We provide evidence that oscillatory disruptions may be linked with overcompensation (glucose hypersensitivity) in islets from diabetes-prone mice. We isolated islets from male 4- to 5-week-old (prediabetic) and 10- to 12-week-old (diabetic) leptin-receptor-deficient (db/db) mice and age-matched heterozygous controls. After an overnight incubation in media with 11 mM glucose, we measured islet intracellular calcium in 5, 8, 11, or 15 mM glucose. Islets from heterozygous 10- to 12-week-old mice were quiescent in 5 mM glucose and displayed oscillations with increasing amplitude and/or duration in 8, 11, and 15 mM glucose, respectively. Islets from diabetic 10- to 12-week-old mice, in contrast, showed robust oscillations in 5 mM glucose that declined with increasing glucose. Similar trends were observed at 4-5-weeks of age. A progressive left shift in maximal insulin release was also observed in islets as db/db mice aged. Reducing glucokinase activity with 1 mM D-mannoheptulose restored oscillations in 11 mM glucose. Finally, overnight low-dose cytokine exposure negatively impacted oscillations preferentially in high glucose in diabetic islets compared with heterozygous controls. Our findings suggest the following: 1) islets from frankly diabetic mice can produce oscillations, 2) elevated sensitivity to glucose prevents diabetic mouse islets from producing oscillations in normal postprandial (11-15 mM glucose) conditions, and 3) hypersensitivity to glucose may magnify stress effects from inflammation or other sources.

Diazoxide, a K(ATP) channel opener, prevents ischemia-reperfusion injury in rodent pancreatic islets

Diazoxide (DZ) is a pharmacological opener of ATP-sensitive K(+) channels that has been used for mimicking ischemic preconditioning and shows protection against ischemic damage. Here we investigated whether diazoxide supplementation to University of Wisconsin (UW) solution has cellular protection during islet isolation and improves in vivo islet transplant outcomes in a rodent ischemia model. C57/B6 mice pancreata were flushed with UW or UW + DZ solution and cold preserved for 6 or 10 h prior to islet isolation. Islet yield, in vitro and in vivo function, mitochondrial morphology, and apoptosis were evaluated. Significantly higher islet yields were observed in the UW + DZ group than in the UW group (237.5 ± 25.6 vs. 108.7 ± 49.3, p < 0.01). The islets from the UW + DZ group displayed a significantly higher glucose-induced insulin secretion (0.97 ng/ml ± 0.15 vs. 0.758 ng/ml ± 0.21, p = 0.009) and insulin content (60.96 ng/islet ± 13.94 vs. 42.09 ng/islet ± 8.15, p = 0.002). The DZ-treated islets had well-preserved mitochondrial morphology with superior responses of mitochondrial potentials, and calcium influx responded to glucose. A higher number of living cells and less late apoptotic cells were observed in the UW + DZ group (p < 0.05). Additionally, the islets from the UW + DZ group had a significantly higher cure rate and improved glucose tolerance. This study is the first to report mitoprotective effects of DZ for pancreas preservation and islet isolation. In the future, it will be necessary to further understand the underlying mechanism for the mitoprotection and to test this promising approach for pancreas preservation and the islet isolation process in nonhuman primates and ultimately humans.

KATP channels and cardiovascular disease: suddenly a syndrome

ATP-sensitive potassium (KATP) channels were first discovered in the heart 30 years ago. Reconstitution of KATP channel activity by coexpression of members of the pore-forming inward rectifier gene family (Kir6.1, KCNJ8, and Kir6.2 KCNJ11) with sulfonylurea receptors (SUR1, ABCC8, and SUR2, ABCC9) of the ABCC protein subfamily has led to the elucidation of many details of channel gating and pore properties. In addition, the essential roles of Kir6.x and SURx subunits in generating cardiac and vascular KATP(2) and the detrimental consequences of genetic deletions or mutations in mice have been recognized. However, despite this extensive body of knowledge, there has been a paucity of defined roles of KATP subunits in human cardiovascular diseases, although there are reports of association of a single Kir6.1 variant with the J-wave syndrome in the ECG, and 2 isolated studies have reported association of loss of function mutations in SUR2 with atrial fibrillation and heart failure. Two new studies convincingly demonstrate that mutations in the SUR2 gene are associated with Cantu syndrome, a complex multi-organ disorder characterized by hypertrichosis, craniofacial dysmorphology, osteochondrodysplasia, patent ductus arteriosus, cardiomegaly, pericardial effusion, and lymphoedema. This realization of previously unconsidered consequences provides significant insight into the roles of the KATP channel in the cardiovascular system and suggests novel therapeutic possibilities.

ABCC8 polymorphisms are associated with triglyceride concentration in type 2 diabetics on sulfonylurea therapy

BACKGROUND

The failure of therapy with oral hypoglycemic drugs leads to not only poorly regulated glycemic status, but also dyslipidemia and increased body weight and body mass index (BMI). Sulfonylureas act as insulin secretagogues by binding to the sulfonylurea receptor (SUR-1) encoded by the gene ABCC8. The aim of this study was to explore whether there is an association of ABCC8 polymorphisms SUR1 exon 16 (-3C/T), SUR-1 exon 31 (Arg1273Arg), and SUR-1 exon 33 (S1369A) with lipid concentration and BMI in type 2 diabetics on sulfonylurea therapy.

MATERIALS AND METHODS

This study included 251 unrelated type 2 diabetics on sulfonylurea therapy. Height and weight were measured for BMI calculation. All polymorphisms were detected by polymerase chain reaction-restriction fragment length polymorphism methods. Lipid concentrations and BMI were measured at inclusion into the study and after 6 months of follow-up.

RESULTS

Wild-type allele carriers for the SUR-1 exon 31 polymorphism (Arg1273Arg) had a significantly higher triglyceride (TG) concentration when compared with the carriers of two variant alleles (p=0.023). Polymorphic allele carriers of the SUR-1 exon 16 (-3C/T) polymorphism were more frequent in the subgroup of patients with the TG concentration increase after 6 months (p for genotype and allelic differences: 0.024 and 0.015, respectively).

CONCLUSION

ABCC8 polymorphisms in exon 16 and 31 are associated with the TG concentration in type 2 diabetics on sulfonylurea therapy.

Accounting for near-normal glucose sensitivity in Kir6.2[AAA] transgenic mice

K(ir)6.2[AAA] transgenic mouse islets exhibit mosaicism such that approximately 70% of the beta-cells have nonfunctional ATP-sensitive potassium (K(ATP)) channels, whereas the remainder have normal K(ATP) function. Despite this drastic reduction, the glucose dose-response curve is only shifted by approximately 2 mM. We use a previously published mathematical model, in which K(ATP) conductance is increased by rises in cytosolic calcium through indirect effects on metabolism, to investigate how cells could compensate for the loss of K(ATP) conductance. Compensation is favored by the assumption that only a small fraction of K(ATP) channels are open during oscillations, which renders it easy to upregulate the open fraction via a modest elevation of calcium. We show further that strong gap-junctional coupling of both membrane potential and calcium is needed to overcome the stark heterogeneity of cell properties in these mosaic islets.

Molecular biology of K(ATP) channels and implications for health and disease

The ATP-sensitive potassium (K(ATP)) channel is expressed in most excitable tissues and plays a critical role in numerous physiological processes by coupling intracellular energetics to electrical activity. The channel is comprised of four Kir6.x subunits associated with four regulatory sulfonylurea receptors (SUR). Intracellular ATP acts on Kir6.x to inhibit channel activity, while MgADP stimulates channel activity through SUR. Changes in the cytosolic [ATP] to [ADP] ratio thus determine channel activity. Multiple mutations in Kir6.x and SUR genes have implicated K(ATP) channels in various diseases ranging from diabetes and hyperinsulinism to cardiac arrhythmias and cardiovascular disease. Continuing studies of channel physiology and pathology will bring new insights to the molecular basis of K(ATP) channel function, leading to a better understanding of the role that K(ATP) channels play in both health and disease.

Hyperinsulinism and diabetes: genetic dissection of beta cell metabolism-excitation coupling in mice

The role of metabolism-excitation coupling in insulin secretion has long been apparent, but in recent years, in parallel with studies of human hyperinsulinism and diabetes, genetic manipulation of proteins involved in glucose transport, metabolism, and excitability in mice has brought the central importance of this pathway into sharp relief. We focus on these animal studies and how they provide important insights into not only metabolic and electrical regulation of insulin secretion, but also downstream consequences of alterations in this pathway and the etiology and treatment of insulin-secretion diseases in humans.

Genetic basis of beta-cell dysfunction in man

Although the genetic causes of monogenic disorders have been successfully identified in the past, the success in dissecting the genetics of complex polygenic diseases has until now been limited. With the introduction of whole genome wide association studies (WGAS) in 2007, the picture has been dramatically changed. Today we know of about 20 genetic variants increasing the risk of type 2 diabetes (T2D). Most of them seem to influence the capacity of beta-cells to increase insulin secretion to meet the demands imposed by an increase in body weight and insulin resistance. This probably represents only the tip of the iceberg, and over the next few years refined tools will provide a more complete picture of the genetic complexity of T2D. This will not only include the current dissection of common variants increasing the susceptibility of the disease but also rare variants with stronger effects, copy number variations and epigenetic effects like DNA methylation and histone acetylation. For the first time, we can anticipate with some confidence that the genetics of a complex disease like T2D really can be dissected.

Genes and type 2 diabetes mellitus

In 2007, five whole genome-wide association studies were published on the genetics of type 2 diabetes mellitus (T2DM), followed by the discovery of 11 genes consistently associated with T2DM. This breakthrough provided the first glimpses of a complete picture of the disease's genetic complexity. Currently, we are only beginning to understand how DNA methylation, histone acetylation, and deacetylation may introduce epigenetic changes throughout one's lifetime. Such changes may influence age-related modifications in gene-expression that contribute to age-related diseases. In the future, the possibility of whole-genome DNA methylation studies may elucidate the extent of these epigenetic effects. This article reviews genes that have recently been determined to be associated with T2DM.

beta-cell hyperexcitability: from hyperinsulinism to diabetes

Nutrient oxidation in beta cells generates a rise in [ATP]:[ADP] ratio. This reduces K(ATP) channel activity, leading to depolarization, activation of voltage-dependent Ca(2+) channels, Ca(2+) entry and insulin secretion. Consistent with this paradigm, loss-of-function mutations in the genes (KCNJ11 and ABCC8) that encode the two subunits (Kir6.2 and SUR1, respectively) of the ATP-sensitive K(+) (K(ATP)) channel underlie hyperinsulinism in humans, a genetic disorder characterized by dysregulated insulin secretion. In mice with genetic suppression of K(ATP) channel subunit expression, partial loss of K(ATP) channel conductance also causes hypersecretion, but unexpectedly, complete loss results in an undersecreting, mildly glucose-intolerant phenotype. When challenged by a high-fat diet, normal mice and mice with reduced K(ATP) channel density respond with hypersecretion, but mice with more significant or complete loss of K(ATP) channels cross over, or progress further, to an undersecreting, diabetic phenotype. It is our contention that in mice, and perhaps in humans, there is an inverse U-shaped response to hyperexcitabilty, leading first to hypersecretion but with further exacerbation to undersecretion and diabetes. The causes of the overcompensation and diabetic susceptibility are poorly understood but may have broader implications for the progression of hyperinsulinism and type 2 diabetes in humans.

Association of sulfonylurea receptor 1 genotype with therapeutic response to gliclazide in type 2 diabetes

To investigate the effects of sulfonylurea receptor 1 (SUR1) exon 33 (TCC-->GCC, S1369A) polymorphism on responsiveness to gliclazide. About 115 patients with type 2 diabetes were treated with gliclazide for 8 weeks. SUR1 genotypes were tested by Taqman-PCR. After gliclazide treatment, there was association between T/G polymorphism and decrease of HbA1c. G carriers were more sensitive to gliclazide and the decrease of HbA1c was more significant than TT genotype (TT, 0.76%+/-1.70%; TG+GG, 1.60%+/-1.39%, P=0.044). The polymorphism of SUR1S1369A was associated with the therapeutic efficacy of gliclazide.

Distinct modulation of Kv1.2 channel gating by wild type, but not open form, of syntaxin-1A

SNARE proteins, syntaxin-1A (Syn-1A) and SNAP-25, inhibit delayed rectifier K(+) channels, K(v)1.1 and K(v)2.1, in secretory cells. We showed previously that the mutant open conformation of Syn-1A (Syn-1A L165A/E166A) inhibits K(v)2.1 channels more optimally than wild-type Syn-1A. In this report we examined whether Syn-1A in its wild-type and open conformations would exhibit similar differential actions on the gating of K(v)1.2, a major delayed rectifier K(+) channel in nonsecretory smooth muscle cells and some neuronal tissues. In coexpression and acute dialysis studies, wild-type Syn-1A inhibited K(v)1.2 current magnitude. Of interest, wild-type Syn-1A caused a right shift in the activation curves of K(v)1.2 without affecting its steady-state availability, an inhibition profile opposite to its effects on K(v)2.1 (steady-state availability reduction without changes in voltage dependence of activation). Also, although both wild-type and open-form Syn-1A bound equally well to K(v)1.2 in an expression system, open-form Syn-1A failed to reduce K(v)1.2 current magnitude or affect its gating. This is in contrast to the reported more potent effect of open-form Syn-1A on K(v)2.1 channels in secretory cells. This finding together with the absence of Munc18 and/or 13-1 in smooth muscles suggested that a change to an open conformation Syn-1A, normally facilitated by Munc18/13-1, is not required in nonsecretory smooth muscle cells. Taken together with previous reports, our results demonstrate the multiplicity of gating inhibition of different K(v) channels by Syn-1A and is compatible with versatility of Syn-1A modulation of repolarization in various secretory and nonsecretory (smooth muscle) cell types.

Two populations of pancreatic islet α-cells displaying distinct Ca2+ channel properties

In low or absence of glucose, alpha-cells generate rhythmic action potentials and secrete glucagon. alpha-Cell T-type Ca(2+) channels are believed to be pacemaker channels, which are expected to open near the resting membrane potential (around -60 mV) to initiate a small depolarization. A previous publication, however, showed that alpha-cell T-type Ca(2+) channels have an activation threshold of -40 mV, which does not appear to fulfill their role as pacemakers. In this work, we investigated the Ca(2+) channel characteristics in alpha-cells of mouse-insulin-promoter green-fluorescent-protein (MIP-GFP) mouse. The beta-cells of MIP-GFP were conveniently distinguished as green cells, while immunostaining indicated that the majority of non-green cells were alpha-cells. We found that majority of alpha-cells possessed T-type Ca(2+) channels having an activation threshold of -40 mV; these cells also had high-voltage-activated (HVA) Ca(2+) channels (activation threshold of -20 mV). A novel finding here is that a minority of alpha-cells had T-type Ca(2+) channels with an activation threshold of -60 mV. This minor population of alpha-cells was, surprisingly, devoid of HVA Ca(2+) channels. We suggest that this alpha-cell subpopulation may act as pacemaker cells in low or absence of glucose.

Insulin regulates islet alpha-cell function by reducing KATP channel sensitivity to adenosine 5'-triphosphate inhibition

Glucose regulates pancreatic islet alpha-cell glucagon secretion directly by its metabolism to generate ATP in alpha-cells, and indirectly via stimulation of paracrine release of beta-cell secretory products, particularly insulin. How the cellular substrates of these pathways converge in the alpha-cell is not well known. We recently reported the use of the MIP-GFP (mouse insulin promoter-green fluorescent protein) mouse to reliably identify islet alpha- (non-green cells) and beta-cells (green cells), and characterized their ATP-sensitive K(+) (K(ATP)) channel properties, showing that alpha-cell K(ATP) channels exhibited a 5-fold higher sensitivity to ATP inhibition than beta-cell K(ATP) channels. Here, we show that insulin exerted paracrine regulation of alpha-cells by markedly reducing the sensitivity of alpha-cell K(ATP) channels to ATP (IC(50) = 0.18 and 0.50 mM in absence and presence of insulin, respectively). Insulin also desensitized beta-cell K(ATP) channels to ATP inhibition (IC(50) = 0.84 and 1.23 mM in absence and presence of insulin, respectively). Insulin effects on both islet cell K(ATP) channels were blocked by wortmannin, indicating that insulin acted on the insulin receptor-phosphatidylinositol 3-kinase signaling pathway. Insulin did not affect alpha-cell A-type K(+) currents. Glutamate, known to also inhibit alpha-cell glucagon secretion, did not activate alpha-cell K(ATP) channel opening. We conclude that a major mechanism by which insulin exerts paracrine control on alpha-cells is by modulating its K(ATP) channel sensitivity to ATP block. This may be an underlying basis for the proposed sequential glucose-insulin regulation of alpha-cell glucagon secretion, which becomes distorted in diabetes, leading to dysregulated glucagon secretion.

KATP channels in mouse spermatogenic cells and sperm, and their role in capacitation

Mammalian sperm must undergo a series of physiological changes after leaving the testis to become competent for fertilization. These changes, collectively known as capacitation, occur in the female reproductive tract where the sperm plasma membrane is modified in terms of its components and ionic permeability. Among other events, mouse sperm capacitation leads to an increase in the intracellular Ca(2+) and pH as well as to a hyperpolarization of the membrane potential. It is well known that ion channels play a crucial role in these events, though the molecular identity of the particular channels involved in capacitation is poorly defined. In the present work, we report the identification and potential functional role of K(ATP) channels in mouse spermatogenic cells and sperm. By using whole-cell patch clamp recordings in mouse spermatogenic cells, we found K(+) inwardly rectifying (K(ir)) currents that are sensitive to Ba(2+), glucose and the sulfonylureas (tolbutamide and glibenclamide) that block K(ATP) channels. The presence of these channels was confirmed using inhibitors of the ATP synthesis and K(ATP) channel activators. Furthermore, RT-PCR assays allowed us to detect transcripts for the K(ATP) subunits SUR1, SUR2, K(ir)6.1 and K(ir)6.2 in total RNA from elongated spermatids. In addition, immunoconfocal microscopy revealed the presence of these K(ATP) subunits in mouse spermatogenic cells and sperm. Notably, incubation of sperm with tolbutamide during capacitation abolished hyperpolarization and significantly decreased the percentage of AR in a dose-dependent fashion. Together, our results provide evidence for the presence of K(ATP) channels in mouse spermatogenic cells and sperm and disclose the contribution of these channels to the capacitation-associated hyperpolarization.

The effects of NN414, a SUR1/Kir6.2 selective potassium channel opener, in healthy male subjects

The aim of the present study was to investigate the effect of a single dose of NN414 (a selective SUR1/Kir6.2 potassium channel opener). Sixty-four healthy male subjects were enrolled at 8 dose levels (0.625-12.5 mg/kg or placebo). The study consisted of a baseline day and a dosing day. NN414 or placebo was administered in the evening about 10 pm. On both study days, an oral glucose tolerance test (OGTT) was performed following an overnight fast (corresponding to 9 hours postdose), and glucose, insulin, glucagon, and growth hormone concentrations were determined. NN414 was well tolerated, with no clinically relevant changes in safety parameters, although there was an increase in gastrointestinal side effects. NN414 treatment lowered glucose during the OGTT and 24-hour insulin and glucose levels. In conclusion, a single dose of NN414 is associated with improvements in glucose-related parameters in healthy male subjects.

Diabetes and insulin secretion: the ATP-sensitive K+ channel (K ATP) connection

The ATP-sensitive K+ channel (K ATP channel) senses metabolic changes in the pancreatic beta-cell, thereby coupling metabolism to electrical activity and ultimately to insulin secretion. When K ATP channels open, beta-cells hyperpolarize and insulin secretion is suppressed. The prediction that K ATP channel "overactivity" should cause a diabetic state due to undersecretion of insulin has been dramatically borne out by recent genetic studies implicating "activating" mutations in the Kir6.2 subunit of K ATP channel as causal in human diabetes. This article summarizes the emerging picture of K ATP channel as a major cause of neonatal diabetes and of a polymorphism in K ATP channel (E23K) as a type 2 diabetes risk factor. The degree of K ATP channel "overactivity" correlates with the severity of the diabetic phenotype. At one end of the spectrum, polymorphisms that result in a modest increase in K ATP channel activity represent a risk factor for development of late-onset diabetes. At the other end, severe "activating" mutations underlie syndromic neonatal diabetes, with multiple organ involvement and complete failure of glucose-dependent insulin secretion, reflecting K ATP channel "overactivity" in both pancreatic and extrapancreatic tissues.

Genetic prediction of future type 2 diabetes

BACKGROUND

Type 2 diabetes (T2D) is a multifactorial disease in which environmental triggers interact with genetic variants in the predisposition to the disease. A number of common variants have been associated with T2D but our knowledge of their ability to predict T2D prospectively is limited.

METHODS AND FINDINGS

By using a Cox proportional hazard model, common variants in the PPARG (P12A), CAPN10 (SNP43 and 44), KCNJ11 (E23K), UCP2 (-866G>A), and IRS1 (G972R) genes were studied for their ability to predict T2D in 2,293 individuals participating in the Botnia study in Finland. After a median follow-up of 6 y, 132 (6%) persons developed T2D. The hazard ratio for risk of developing T2D was 1.7 (95% confidence interval [CI] 1.1-2.7) for the PPARG PP genotype, 1.5 (95% CI 1.0-2.2) for the CAPN10 SNP44 TT genotype, and 2.6 (95% CI 1.5-4.5) for the combination of PPARG and CAPN10 risk genotypes. In individuals with fasting plasma glucose > or = 5.6 mmol/l and body mass index > or = 30 kg/m(2), the hazard ratio increased to 21.2 (95% CI 8.7-51.4) for the combination of the PPARG PP and CAPN10 SNP43/44 GG/TT genotypes as compared to those with the low-risk genotypes with normal fasting plasma glucose and body mass index < 30 kg/m(2).

CONCLUSION

We demonstrate in a large prospective study that variants in the PPARG and CAPN10 genes predict future T2D. Genetic testing might become a future approach to identify individuals at risk of developing T2D.

Open form of syntaxin-1A is a more potent inhibitor than wild-type syntaxin-1A of Kv2.1 channels

We have shown that SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor) proteins not only participate directly in exocytosis, but also regulate the dominant membrane-repolarizing Kv channels (voltage-gated K+ channels), such as Kv2.1, in pancreatic beta-cells. In a recent report, we demonstrated that WT (wild-type) Syn-1A (syntaxin-1A) inhibits Kv2.1 channel trafficking and gating through binding to the cytoplasmic C-terminus of Kv2.1. During beta-cell exocytosis, Syn-1A converts from a closed form into an open form which reveals its active H3 domain to bind its SNARE partners SNAP-25 (synaptosome-associated protein of 25 kDa) and synaptobrevin. In the present study, we compared the effects of the WT Syn-1A and a mutant open form Syn-1A (L165A, E166A) on Kv2.1 channel trafficking and gating. When co-expressed in HEK-293 cells (human embryonic kidney-293 cells), the open form Syn-1A decreased Kv2.1 current density more than (P<0.05) the WT Syn-1A (166+/-35 and 371+/-93 pA/pF respectively; control=911+/-91 pA/pF). Confocal microscopy and biotinylation experiments showed that both the WT and open form Syn-1A inhibited Kv2.1 expression at the plasma membrane to a similar extent, suggesting that the stronger reduction of Kv2.1 current density by the open form compared with the WT Syn-1A is probably due to a stronger direct inhibition of channel activity. Consistently, dialysis of the recombinant open form Syn-1A protein into Kv2.1-expressing HEK-293 cells caused stronger inhibition of Kv2.1 current amplitude (P<0.05) than the WT Syn-1A protein (73+/-2 and 82+/-3% of the control respectively). We found that the H3 but not H(ABC) domain is the putative active domain of Syn-1A, which bound to and inhibited the Kv2.1 channel. When co-expressed in HEK-293 cells, the open-form Syn-1A slowed down Kv2.1 channel activation (tau=12.3+/-0.8 ms) much more than (P<0.05) WT Syn-1A (tau=7.9+/-0.8 ms; control tau=5.5+/-0.6 ms). In addition, only the open form Syn-1A, but not the WT Syn-1A, caused a significant (P<0.05) left-shift in the steady-state inactivation curve (V(1/2)=33.1+/-1.3 and -29.4+/-1.1 mV respectively; control V(1/2)=-24.8+/-2 mV). The present study therefore indicates that the open form of Syn-1A is more potent than the WT Syn-1A in inhibiting the Kv2.1 channel. Such stronger inhibition by the open form of Syn-1A may limit K+ efflux and thus decelerate membrane repolarization during exocytosis, leading to optimization of insulin release.

Diet-induced glucose intolerance in mice with decreased beta-cell ATP-sensitive K+ channels

ATP-sensitive K+ channels (K(ATP) channels) control electrical activity in beta-cells and therefore are key players in excitation-secretion coupling. Partial suppression of beta-cell K(ATP) channels in transgenic (AAA) mice causes hypersecretion of insulin and enhanced glucose tolerance, whereas complete suppression of these channels in Kir6.2 knockout (KO) mice leads to hyperexcitability, but mild glucose intolerance. To test the interplay of hyperexcitability and dietary stress, we subjected AAA and KO mice to a high-fat diet. After 3 months on the diet, both AAA and KO mice converted to an undersecreting and markedly glucose-intolerant phenotype. Although Kir6.2 is expressed in multiple tissues, its primary functional consequence in both AAA and KO mice is enhanced beta-cell electrical activity. The results of our study provide evidence that, when combined with dietary stress, this hyperexcitability is a causal diabetic factor. We propose an "inverse U" model for the response to enhanced beta-cell excitability: the expected initial hypersecretion can progress to undersecretion and glucose-intolerance, either spontaneously or in response to dietary stress.

Pattern of genes influenced by conditional expression of the transcription factors HNF6, HNF4alpha and HNF1beta in a pancreatic beta-cell line

Using the rat insulinoma cell line INS-1 we generated beta-cell clones that are most efficient for gene transfer, as they contain an FRT site for Flp recombinase-mediated, site-directed integration of a single copy transgene. Therefore, the gene-of-interest can be introduced by DNA transfection without the need to select individual cell clones. Additionally, the clones contain the tetracycline repressor allowing tetracycline induction of the transgene. By oligonucleotide microarray we define the beta-cell specific phenotype of the Flp-In T-REx cell clones. Using a clone expressing the HNF6, HNF4alpha and HNF1beta transcription factors at a limited level, we introduced the expression vectors encoding these factors. We show efficient tetracycline induction of these transcription factors by western blots and immunocytochemistry. Microarrays reveal that these three factors affect a similar number of genes with only few genes regulated in common. Statistical analysis reveals that the three transcription factors affect genes categorized to different biological processes. Furthermore, we document the usefulness of these Flp-In T-REx cells for the functional analysis of mutated HNF1beta transcription factors found in human MODY5 patients. We show that the expression of the mutant P328L329del and A263insGG affects only very few transcripts and these are predominantly distinct from those induced by wild-type HNF1beta.

ATP-sensitive K(+) channels regulate the concentrative adenosine transporter CNT2 following activation by A(1) adenosine receptors

This study describes a novel mechanism of regulation of the high-affinity Na(+)-dependent adenosine transporter (CNT2) via the activation of A(1) adenosine receptors (A(1)R). This regulation is mediated by the activation of ATP-sensitive K(+) (K(ATP)) channels. The high-affinity Na(+)-dependent adenosine transporter CNT2 and A(1)R are coexpressed in the basolateral domain of the rat hepatocyte plasma membrane and are colocalized in the rat hepatoma cell line FAO. The transient increase in CNT2-mediated transport activity triggered by (-)-N(6)-(2-phenylisopropyl)adenosine is fully inhibited by K(ATP) channel blockers and mimicked by a K(ATP) channel opener. A(1)R agonist activation of CNT2 occurs in both hepatocytes and FAO cells, which express Kir6.1, Kir6.2, SUR1, SUR2A, and SUR2B mRNA channel subunits. With the available antibodies against Kir6.X, SUR2A, and SUR2B, it is shown that all of these proteins colocalize with CNT2 and A(1)R in defined plasma membrane domains of FAO cells. The extent of the purinergic modulation of CNT2 is affected by the glucose concentration, a finding which indicates that glycemia and glucose metabolism may affect this cross-regulation among A(1)R, CNT2, and K(ATP) channels. These results also suggest that the activation of K(ATP) channels under metabolic stress can be mediated by the activation of A(1)R. Cell protection under these circumstances may be achieved by potentiation of the uptake of adenosine and its further metabolization to ATP. Mediation of purinergic responses and a connection between the intracellular energy status and the need for an exogenous adenosine supply are novel roles for K(ATP) channels.

Activating mutations in the gene encoding the ATP-sensitive potassium-channel subunit Kir6.2 and permanent neonatal diabetes

BACKGROUND

Patients with permanent neonatal diabetes usually present within the first three months of life and require insulin treatment. In most, the cause is unknown. Because ATP-sensitive potassium (K(ATP)) channels mediate glucose-stimulated insulin secretion from the pancreatic beta cells, we hypothesized that activating mutations in the gene encoding the Kir6.2 subunit of this channel (KCNJ11) cause neonatal diabetes.

METHODS

We sequenced the KCNJ11 gene in 29 patients with permanent neonatal diabetes. The insulin secretory response to intravenous glucagon, glucose, and the sulfonylurea tolbutamide was assessed in patients who had mutations in the gene.

RESULTS

Six novel, heterozygous missense mutations were identified in 10 of the 29 patients. In two patients the diabetes was familial, and in eight it arose from a spontaneous mutation. Their neonatal diabetes was characterized by ketoacidosis or marked hyperglycemia and was treated with insulin. Patients did not secrete insulin in response to glucose or glucagon but did secrete insulin in response to tolbutamide. Four of the patients also had severe developmental delay and muscle weakness; three of them also had epilepsy and mild dysmorphic features. When the most common mutation in Kir6.2 was coexpressed with sulfonylurea receptor 1 in Xenopus laevis oocytes, the ability of ATP to block mutant K(ATP) channels was greatly reduced.

CONCLUSIONS

Heterozygous activating mutations in the gene encoding Kir6.2 cause permanent neonatal diabetes and may also be associated with developmental delay, muscle weakness, and epilepsy. Identification of the genetic cause of permanent neonatal diabetes may facilitate the treatment of this disease with sulfonylureas.

Pore loop-mutated rat KIR6.1 and KIR6.2 suppress KATP current in rat cardiomyocytes

Cardiomyocytes express mRNA for all major subunits of ATP-sensitive potassium (K(ATP)) channels: KIR6.1, KIR6.2, SUR1A, SUR2A, and SUR2B. It has remained controversial as to whether KIR6.1 may associate with KIR6.2 to form the tetrameric pore of K(ATP) channels in cardiomyocytes. To explore this possibility, cultured rat cardiomyocytes were examined for an inhibition of K(ATP) current by overexpression of pore loop-mutated (inactive) KIR6.x. Bicistronic plasmids were constructed encoding loop-mutated (AFA or SFG for GFG) rat KIR6.x followed by EGFP. In ventricular myocytes, the overexpression of KIR6.1SFG-pIRES(2)-EGFP or KIR6.2AFA-pIRES(2)-EGFP DNA caused, after 72 h, a major decrease of K(ATP) current density of 85.8% and 82.7%, respectively (P < 0.01), relative to EGFP controls (59 +/- 9 pA/pF). In atrial myocytes, overexpression of these pore-mutated KIR6.x by 6.0-fold and 10.6-fold, as assessed by quantitative immunohistochemistry, caused a decrease of K(ATP) current density of 73.7% and 58.5%, respectively (P < 0.01). Expression of wild-type rat KIR6.2 increased the ventricular and atrial K(ATP) current density by 58.3% and 42.9%, respectively (P < 0.01), relative to corresponding EGFP controls, indicating a reserve of SUR to accommodate increased KIR6.x trafficking to the sarcolemma. The results favor the view that KIR6.1 may associate with KIR6.2 to form heterotetrameric pores of native K(ATP) channels in cardiomyocytes.

Functional properties of four splice variants of a human pancreatic tandem-pore K+ channel, TALK-1

TALK-1a, originally isolated from human pancreas, is a member of the tandem-pore K+ channel family. We identified and characterized three novel splice variants of TALK-1 from human pancreas. The cDNAs of TALK-1b, TALK-1c, and TALK-1d encode putative proteins of 294, 322, and 262 amino acids, respectively. TALK-1a and TALK-1b possessed all four transmembrane segments, whereas TALK-1c and TALK-1d lacked the fourth transmembrane domain because of deletion of exon 5. Northern blot analysis showed that among the 15 tissues examined, TALK-1 was expressed mainly in the pancreas. TALK-1a and TALK-1b, but not TALK-1c and TALK-1d, could be functionally expressed in COS-7 cells. Like TALK-1a, TALK-1b was a K+-selective channel that was active at rest. Single-channel openings of TALK-1a and TALK-1b were extremely brief such that the mean open time was <0.2 ms. In symmetrical 150 mM KCl, the apparent single-channel conductances of TALK-1a and TALK-1b were 23 +/- 3 and 21 +/- 2 pS at -60 mV and 11 +/- 2 and 10 +/- 2 pS at +60 mV, respectively. TALK-1b whole cell current was inhibited 31% by 1 mM Ba2+ and 71% by 1 mM quinidine but was not affected by 1 mM tetraethylammonium, 1 mM Cs+, and 100 microM 4-aminopyridine. Similar to TALK-1a, TALK-1b was sensitive to changes in external pH. Acid conditions inhibited and alkaline conditions activated TALK-1a and TALK-1b, with a K1/2 at pH 7.16 and 7.21, respectively. These results indicate that at least two functional TALK-1 variants are present and may serve as background K+ currents in certain cells of the human pancreas.

Insulin granule dynamics in pancreatic beta cells

Glucose-induced insulin secretion in response to a step increase in blood glucose concentrations follows a biphasic time course consisting of a rapid and transient first phase followed by a slowly developing and sustained second phase. Because Type 2 diabetes involves defects of insulin secretion, manifested as a loss of first phase and a reduction of second phase, it is important to understand the cellular mechanisms underlying biphasic insulin secretion. Insulin release involves the packaging of insulin in small (diameter approximately 0.3 micro m) secretory granules, the trafficking of these granules to the plasma membrane, the exocytotic fusion of the granules with the plasma membrane and eventually the retrieval of the secreted membranes by endocytosis. Until recently, studies on insulin secretion have been confined to the appearance of insulin in the extracellular space and the cellular events preceding exocytosis have been inaccessible to more detailed analysis. Evidence from a variety of secretory tissues, including pancreatic islet cells suggests, however, that the secretory granules can be functionally divided into distinct pools that are distinguished by their release competence and/or proximity to the plasma membrane. The introduction of fluorescent proteins that can be targeted to the secretory granules, in combination with the advent of new techniques that allow real-time imaging of granule trafficking in living cells (granule dynamics), has led to an explosion of our knowledge of the pre-exocytotic and post-exocytotic processes in the beta cell. Here we discuss these observations in relation to previous functional and ultra-structural data as well as the secretory defects of Type 2 diabetes.