Molecular basis of Kir6.2 mutations associated with neonatal diabetes or neonatal diabetes plus neurological features

Transient neonatal diabetes due to a missense mutation (E227K) in the gene encoding the ATP-sensitive potassium channel (KCNJ11)

Neonatal diabetes is a monogenic form of diabetes. Herein, we report on a newborn presenting diabetic ketoacidosis at 17 days of life. A KCNJ11 mutation was identified. In such cases, insulin can be replaced by sulfonylurea with a successful metabolic control, as an example of how molecular diagnosis may influence the clinical management of the disorder.

Complex Membrane Channel Blockade: A Unifying Hypothesis for the Prodromal and Acute Neuropsychiatric Sequelae Resulting from Exposure to the Antimalarial Drug Mefloquine

The alkaloid toxin quinine and its derivative compounds have been used for many centuries as effective medications for the prevention and treatment of malaria. More recently, synthetic derivatives, such as the quinoline derivative mefloquine (bis(trifluoromethyl)-(2-piperidyl)-4-quinolinemethanol), have been widely used to combat disease caused by chloroquine-resistant strains of the malaria parasite, Plasmodium falciparum. However, the parent compound quinine, as well as its more recent counterparts, suffers from an incidence of adverse neuropsychiatric side effects ranging from mild mood disturbances and anxiety to hallucinations, seizures, and psychosis. This review considers how the pharmacology, cellular neurobiology, and membrane channel kinetics of mefloquine could lead to the significant and sometimes life-threatening neurotoxicity associated with mefloquine exposure. A key role for mefloquine blockade of ATP-sensitive potassium channels and connexins in the substantia nigra is considered as a unifying hypothesis for the pathogenesis of severe neuropsychiatric events after mefloquine exposure in humans.

The effect of early, comprehensive genomic testing on clinical care in neonatal diabetes: an international cohort study

BACKGROUND

Traditional genetic testing focusses on analysis of one or a few genes according to clinical features; this approach is changing as improved sequencing methods enable simultaneous analysis of several genes. Neonatal diabetes is the presenting feature of many discrete clinical phenotypes defined by different genetic causes. Genetic subtype defines treatment, with improved glycaemic control on sulfonylurea treatment for most patients with potassium channel mutations. We investigated the effect of early, comprehensive testing of all known genetic causes of neonatal diabetes.

METHODS

In this large, international, cohort study, we studied patients with neonatal diabetes diagnosed with diabetes before 6 months of age who were referred from 79 countries. We identified mutations by comprehensive genetic testing including Sanger sequencing, 6q24 methylation analysis, and targeted next-generation sequencing of all known neonatal diabetes genes.

FINDINGS

Between January, 2000, and August, 2013, genetic testing was done in 1020 patients (571 boys, 449 girls). Mutations in the potassium channel genes were the most common cause (n=390) of neonatal diabetes, but were identified less frequently in consanguineous families (12% in consanguineous families vs 46% in non-consanguineous families; p<0·0001). Median duration of diabetes at the time of genetic testing decreased from more than 4 years before 2005 to less than 3 months after 2012. Earlier referral for genetic testing affected the clinical phenotype. In patients with genetically diagnosed Wolcott-Rallison syndrome, 23 (88%) of 26 patients tested within 3 months from diagnosis had isolated diabetes, compared with three (17%) of 18 patients referred later (>4 years; p<0·0001), in whom skeletal and liver involvement was common. Similarly, for patients with genetically diagnosed transient neonatal diabetes, the diabetes had remitted in only ten (10%) of 101 patients tested early (<3 months) compared with 60 (100%) of the 60 later referrals (p<0·0001).

INTERPRETATION

Patients are now referred for genetic testing closer to their presentation with neonatal diabetes. Comprehensive testing of all causes identified causal mutations in more than 80% of cases. The genetic result predicts the best diabetes treatment and development of related features. This model represents a new framework for clinical care with genetic diagnosis preceding development of clinical features and guiding clinical management.

FUNDING

Wellcome Trust and Diabetes UK.

Permanent neonatal diabetes by a new mutation in KCNJ11: unsuccessful switch to sulfonylurea

Permanent neonatal diabetes (PNDM) can result from activating heterozygous mutations in KCNJ11 gene, encoding the Kir6.2 subunit of the pancreatic ATP-sensitive potassium channels (KATP). Sulfonylureas promote KATP closure and stimulate insulin secretion, being an alternative therapy in PNDM, instead of insulin. Male, 20 years old, diagnosed with diabetes at 3 months of age. The genetic study identified a novel heterozygous mutation in exon 1 of the KCNJ11 gene - KCNJ11:c1001G>7 (p.Gly334Val) - and confirmed the diagnosis of PNDM. Therefore it was attempted to switch from insulin therapy to sulfonylurea. During glibenclamide institution C-peptide levels increased, however the suboptimal glycemic control lead us to restart an intensive insulin scheme. This new variant of KCNJ11 mutation had a phenotypic lack of response to sulfonylurea therapy. Age, prior poor metabolic control and functional change of KATP channel induced by this specific mutation may explain the observed unsuccessful switch to sulfonylurea. Interestingly, C-peptide levels raise during glibenclamide administration support some degree of improvement in insulin secretory capacity induced by the treatment. Understanding the response to sulfonylurea is crucial as successful treatment may be life-changing in these patients.

Phosphorylation-dependent changes in nucleotide binding, conformation, and dynamics of the first nucleotide binding domain (NBD1) of the sulfonylurea receptor 2B (SUR2B)

The sulfonylurea receptor 2B (SUR2B) forms the regulatory subunit of ATP-sensitive potassium (KATP) channels in vascular smooth muscle. Phosphorylation of the SUR2B nucleotide binding domains (NBD1 and NBD2) by protein kinase A results in increased channel open probability. Here, we investigate the effects of phosphorylation on the structure and nucleotide binding properties of NBD1. Phosphorylation sites in SUR2B NBD1 are located in an N-terminal tail that is disordered. Nuclear magnetic resonance (NMR) data indicate that phosphorylation of the N-terminal tail affects multiple residues in NBD1, including residues in the NBD2-binding site, and results in altered conformation and dynamics of NBD1. NMR spectra of NBD1 lacking the N-terminal tail, NBD1-ΔN, suggest that phosphorylation disrupts interactions of the N-terminal tail with the core of NBD1, a model supported by dynamic light scattering. Increased nucleotide binding of phosphorylated NBD1 and NBD1-ΔN, compared with non-phosphorylated NBD1, suggests that by disrupting the interaction of the NBD core with the N-terminal tail, phosphorylation also exposes the MgATP-binding site on NBD1. These data provide insights into the molecular basis by which phosphorylation of SUR2B NBD1 activates KATP channels.

Age at the time of sulfonylurea initiation influences treatment outcomes in KCNJ11-related neonatal diabetes

AIMS/HYPOTHESIS

Individuals with heterozygous activating mutations of the KCNJ11 gene encoding a subunit of the ATP-sensitive potassium channel (KATP) can usually be treated with oral sulfonylurea (SU) pills in lieu of insulin injections. The aim of this study was to test our hypothesis that younger age at the time of initiation of SU therapy is correlated with lower required doses of SU therapy, shorter transition time and decreased likelihood of requiring additional diabetes medications.

METHODS

We performed a retrospective cohort study using data on 58 individuals with neonatal diabetes due to KCNJ11 mutations identified through the University of Chicago Monogenic Diabetes Registry ( http://monogenicdiabetes.uchicago.edu/registry ). We assessed the influence of age at initiation of SU therapy on treatment outcomes.

RESULTS

HbA1c fell from an average of 8.5% (69 mmol/mol) before transition to 6.2% (44 mmol/mol) after SU therapy (p < 0.001). Age of initiation of SU correlated with the dose (mg kg(-1) day(-1)) of SU required at follow-up (r = 0.80, p < 0.001). Similar associations were observed across mutation subtypes. Ten participants required additional glucose-lowering medications and all had initiated SU at age 13 years or older. No serious adverse events were reported.

CONCLUSIONS/INTERPRETATION

Earlier age at initiation of SU treatment is associated with improved response to SU therapy. Declining sensitivity to SU may be due to loss of beta cell mass over time in those treated with insulin. Our data support the need for early genetic diagnosis and appropriate personalised treatment in all cases of neonatal diabetes.

Neonatal diabetes with intractable epilepsy: DEND syndrome

Permanent Neonatal Diabetes Mellitus (PNDM), is a rare monogenic disorder, caused by activating mutations of the KATP channel. The most severe clinical form of PNDM presents as Developmental delay, Epilepsy and Neonatal Diabetes (DEND) syndrome. Diagnosis is confirmed by genetic mutation testing. Oral sulfonylurea therapy improves neurological outcome.

Decreasing cx36 gap junction coupling compensates for overactive KATP channels to restore insulin secretion and prevent hyperglycemia in a mouse model of neonatal diabetes

Mutations to the ATP-sensitive K(+) channel (KATP channel) that reduce the sensitivity of ATP inhibition cause neonatal diabetes mellitus via suppression of β-cell glucose-stimulated free calcium activity ([Ca(2+)]i) and insulin secretion. Connexin-36 (Cx36) gap junctions also regulate islet electrical activity; upon knockout of Cx36, β-cells show [Ca(2+)]i elevations at basal glucose. We hypothesized that in the presence of overactive ATP-insensitive KATP channels, a reduction in Cx36 would allow elevations in glucose-stimulated [Ca(2+)]i and insulin secretion to improve glucose homeostasis. To test this, we introduced a genetic knockout of Cx36 into mice that express ATP-insensitive KATP channels and measured glucose homeostasis and islet metabolic, electrical, and insulin secretion responses. In the normal presence of Cx36, after expression of ATP-insensitive KATP channels, blood glucose levels rapidly rose to >500 mg/dL. Islets from these mice showed reduced glucose-stimulated [Ca(2+)]i and no insulin secretion. In mice lacking Cx36 after expression of ATP-insensitive KATP channels, normal glucose levels were maintained. Islets from these mice had near-normal glucose-stimulated [Ca(2+)]i and insulin secretion. We therefore demonstrate a novel mechanism by which islet function can be recovered in a monogenic model of diabetes. A reduction of gap junction coupling allows sufficient glucose-stimulated [Ca(2+)]i and insulin secretion to prevent the emergence of diabetes.

Clinical utility of next-generation sequencing for the molecular diagnosis of monogenic diabetes

Monogenic diabetes resulting from mutations that primarily reduce insulin-secreting pancreatic β-cell function accounts for 1-2% of all cases of diabetes, and is genetically and clinically heterogeneous. Currently, genetic testing for monogenic diabetes relies on selection of the appropriate gene for analysis based on the availability of comprehensive phenotypic information, which can be time consuming, costly and can limit the differential diagnosis to a few selected genes. In recent years, the exponential growth in the field of high-throughput capture and sequencing technology has made it possible and cost effective to sequence many genes simultaneously, making it an efficient diagnostic tool for clinically and genetically heterogeneous disorders such as monogenic diabetes. Making a diagnosis of monogenic diabetes is important as it enables more appropriate treatment, better prediction of disease prognosis and progression, and counseling and screening of family members. We provide a concise overview of the genetic etiology of some forms of monogenic diabetes, as well as a discussion of the clinical utility of genetic testing by comprehensive multigene panel using next-generation sequencing methodologies.

Pharmacological rescue of trafficking-impaired ATP-sensitive potassium channels

ATP-sensitive potassium (K_ATP) channels link cell metabolism to membrane excitability and are involved in a wide range of physiological processes including hormone secretion, control of vascular tone, and protection of cardiac and neuronal cells against ischemic injuries. In pancreatic β-cells, K_ATP channels play a key role in glucose-stimulated insulin secretion, and gain or loss of channel function results in neonatal diabetes or congenital hyperinsulinism, respectively. The β-cell K_ATP channel is formed by co-assembly of four Kir6.2 inwardly rectifying potassium channel subunits encoded by KCNJ11 and four sulfonylurea receptor 1 subunits encoded by ABCC8. Many mutations in ABCC8 or KCNJ11 cause loss of channel function, thus, congenital hyperinsulinism by hampering channel biogenesis and hence trafficking to the cell surface. The trafficking defects caused by a subset of these mutations can be corrected by sulfonylureas, K_ATP channel antagonists that have long been used to treat type 2 diabetes. More recently, carbamazepine, an anticonvulsant that is thought to target primarily voltage-gated sodium channels has been shown to correct K_ATP channel trafficking defects. This article reviews studies to date aimed at understanding the mechanisms by which mutations impair channel biogenesis and trafficking and the mechanisms by which pharmacological ligands overcome channel trafficking defects. Insight into channel structure-function relationships and therapeutic implications from these studies are discussed.

Molecular mechanism of sulphonylurea block of K(ATP) channels carrying mutations that impair ATP inhibition and cause neonatal diabetes

Sulphonylurea drugs are the therapy of choice for treating neonatal diabetes (ND) caused by mutations in the ATP-sensitive K(+) channel (KATP channel). We investigated the interactions between MgATP, MgADP, and the sulphonylurea gliclazide with KATP channels expressed in Xenopus oocytes. In the absence of MgATP, gliclazide block was similar for wild-type channels and those carrying the Kir6.2 ND mutations R210C, G334D, I296L, and V59M. Gliclazide abolished the stimulatory effect of MgATP on all channels. Conversely, high MgATP concentrations reduced the gliclazide concentration, producing a half-maximal block of G334D and R201C channels and suggesting a mutual antagonism between nucleotide and gliclazide binding. The maximal extent of high-affinity gliclazide block of wild-type channels was increased by MgATP, but this effect was smaller for ND channels; channels that were least sensitive to ATP inhibition showed the smallest increase in sulphonylurea block. Consequently, G334D and I296L channels were not fully blocked, even at physiological MgATP concentrations (1 mmol/L). Glibenclamide block was also reduced in β-cells expressing Kir6.2-V59M channels. These data help to explain why patients with some mutations (e.g., G334D, I296L) are insensitive to sulphonylurea therapy, why higher drug concentrations are needed to treat ND than type 2 diabetes, and why patients with severe ND mutations are less prone to drug-induced hypoglycemia.

K(ATP) channels and islet hormone secretion: new insights and controversies

ATP-sensitive potassium channels (K(ATP) channels) link cell metabolism to electrical activity by controlling the cell membrane potential. They participate in many physiological processes but have a particularly important role in systemic glucose homeostasis by regulating hormone secretion from pancreatic islet cells. Glucose-induced closure of K(ATP) channels is crucial for insulin secretion. Emerging data suggest that K(ATP) channels also play a key part in glucagon secretion, although precisely how they do so remains controversial. This Review highlights the role of K(ATP) channels in insulin and glucagon secretion. We discuss how K(ATP) channels might contribute not only to the initiation of insulin release but also to the graded stimulation of insulin secretion that occurs with increasing glucose concentrations. The various hypotheses concerning the role of K(ATP) channels in glucagon release are also reviewed. Furthermore, we illustrate how mutations in K(ATP) channel genes can cause hyposecretion or hypersecretion of insulin, as in neonatal diabetes mellitus and congenital hyperinsulinism, and how defective metabolic regulation of the channel may underlie the hypoinsulinaemia and the hyperglucagonaemia that characterize type 2 diabetes mellitus. Finally, we outline how sulphonylureas, which inhibit K(ATP) channels, stimulate insulin secretion in patients with neonatal diabetes mellitus or type 2 diabetes mellitus, and suggest their potential use to target the glucagon secretory defects found in diabetes mellitus.

Decomposition of slide helix contributions to ATP-dependent inhibition of Kir6.2 channels

Regulation of inwardly rectifying potassium channels by intracellular ligands couples cell membrane excitability to important signaling cascades and metabolic pathways. We investigated the molecular mechanisms that link ligand binding to the channel gate in ATP-sensitive Kir6.2 channels. In these channels, the "slide helix" forms an interface between the cytoplasmic (ligand-binding) domain and the transmembrane pore, and many slide helix mutations cause loss of function. Using a novel approach to rescue electrically silent channels, we decomposed the contribution of each interface residue to ATP-dependent gating. We demonstrate that effective inhibition by ATP relies on an essential aspartate at residue 58. Characterization of the functional importance of this conserved aspartate, relative to other residues in the slide helix, has been impossible because of loss-of-function of Asp-58 mutant channels. The Asp-58 position exhibits an extremely stringent requirement for aspartate because even a highly conservative mutation to glutamate is insufficient to restore normal channel function. These findings reveal unrecognized slide helix elements that are required for functional channel expression and control of Kir6.2 gating by intracellular ATP.

Pathogenesis of the metabolic syndrome: insights from monogenic disorders

Identifying rare human metabolic disorders that result from a single-gene defect has not only enabled improved diagnostic and clinical management of such patients, but also has resulted in key biological insights into the pathophysiology of the increasingly prevalent metabolic syndrome. Insulin resistance and type 2 diabetes are linked to obesity and driven by excess caloric intake and reduced physical activity. However, key events in the causation of the metabolic syndrome are difficult to disentangle from compensatory effects and epiphenomena. This review provides an overview of three types of human monogenic disorders that result in (1) severe, non-syndromic obesity, (2) pancreatic beta cell forms of early-onset diabetes, and (3) severe insulin resistance. In these patients with single-gene defects causing their exaggerated metabolic disorder, the primary defect is known. The lessons they provide for current understanding of the molecular pathogenesis of the common metabolic syndrome are highlighted.

A Kir6.2 pore mutation causes inactivation of ATP-sensitive potassium channels by disrupting PIP2-dependent gating

In the absence of intracellular nucleotides, ATP-sensitive potassium (KATP) channels exhibit spontaneous activity via a phosphatidylinositol-4,5-bisphosphate (PIP2)-dependent gating process. Previous studies show that stability of this activity requires subunit-subunit interactions in the cytoplasmic domain of Kir6.2; selective mutagenesis and disease mutations at the subunit interface result in time-dependent channel inactivation. Here, we report that mutation of the central glycine in the pore-lining second transmembrane segment (TM2) to proline in Kir6.2 causes KATP channel inactivation. Unlike C-type inactivation, a consequence of selectivity filter closure, in many K(+) channels, the rate of inactivation in G156P channels was insensitive to changes in extracellular ion concentrations or ion species fluxing through the pore. Instead, the rate of G156P inactivation decreased with exogenous application of PIP2 and increased when PIP2-channel interaction was inhibited with neomycin or poly-L-lysine. These findings indicate the G156P mutation reduces the ability of PIP2 to stabilize the open state of KATP channels, similar to mutations in the cytoplasmic domain that produce inactivation. Consistent with this notion, when PIP2-dependent open state stability was substantially increased by addition of a second gain-of-function mutation, G156P inactivation was abolished. Importantly, bath application and removal of Mg(2+)-free ATP or a nonhydrolyzable analog of ATP, which binds to the cytoplasmic domain of Kir6.2 and causes channel closure, recover G156P channel from inactivation, indicating crosstalk between cytoplasmic and transmembrane domains. The G156P mutation provides mechanistic insight into the structural and functional interactions between the pore and cytoplasmic domains of Kir6.2 during gating.

Functional characterization of a novel KCNJ11 in frame mutation-deletion associated with infancy-onset diabetes and a mild form of intermediate DEND: a battle between K(ATP) gain of channel activity and loss of channel expression

ATP-sensitive potassium (K_ATP) channels are widely distributed in various tissues and cell types where they couple cell metabolism to cell excitability. Gain of channel function (GOF) mutations in the genes encoding Kir6.2 (KCNJ11) or the associated regulatory ssulfonylurea receptor 1 subunit (ABCC8), cause developmental delay, epilepsy and neonatal diabetes (DEND) due to suppressed cell excitability in pancreatic β-cells and neurons. The objective of this study was to determine the molecular basis of infancy-onset diabetes and a mild form of intermediate DEND, resulting from a novel KCNJ11 in frame mutation plus deletion. The naturally occurring Kir6.2 mutation plus deletion (Ser225Thr, Pro226_Pro232del) as well as the isolated S225T mutation or isolated del226–232 deletion were coexpressed with SUR1 in COS cells in homozygous or heterozygous states. The protein expression and gating effects of the resulting channels were assessed biochemically and electrophysiologically. For both the deletion and point mutations, simulated heterozygous expression resulted in overall increased conductance in intact cells in basal conditions and rightward shifted ATP dose-response curves in excised patches, due to increased intrinsic open probability. Interestingly, homomeric channels for the combined deletion/mutation, or for the deletion alone, showed dramatically reduced channel expression at the cell membrane, which would underlie a reduced function in vivo. These results demonstrate that both the mis-sense mutation and the deleted region in the Kir6.2 subunit are important for control of the intrinsic channel gating and suggest that the clinical presentation could be affected by the competition between loss-of-function by reduced trafficking and enhanced channel gating.

Amino acid properties may be useful in predicting clinical outcome in patients with Kir6.2 neonatal diabetes

BACKGROUND

Mutations in the KCNJ11 gene, which encodes the Kir6.2 subunit of the β-cell K(ATP) channel, are a common cause of neonatal diabetes. The diabetes may be permanent neonatal diabetes mellitus (PNDM) or transient neonatal diabetes mellitus (TNDM), and in ≈ 20% of patients, neurological features are observed. A correlation between the position of the mutation in the protein and the clinical phenotype has previously been described; however, recently, this association has become less distinct with different mutations at the same residues now reported in patients with different diabetic and/or neurological phenotypes.

METHODS

We identified from the literature, and our unpublished series, KCNJ11 mutations that affected residues harbouring various amino acid substitutions (AAS) causing differences in diabetic or neurological status. Using the Grantham amino acid scoring system, we investigated whether the difference in properties between the wild-type and the different AAS at the same residue could predict phenotypic severity.

RESULTS

Pair-wise analysis demonstrated higher Grantham scores for mutations causing PNDM or diabetes with neurological features when compared with mutations affecting the same residue that causes TNDM (P=0.013) or diabetes without neurological features (P=0.016) respectively. In just five of the 25 pair-wise analyses, a lower Grantham score was observed for the more severe phenotype. In each case, the wild-type residue was glycine, the simplest amino acid.

CONCLUSION

This study demonstrates the importance of the specific AAS in determining phenotype and highlights the potential utility of the Grantham score for predicting phenotypic severity for novel KCNJ11 mutations affecting previously mutated residues.

Engineered interaction between SUR1 and Kir6.2 that enhances ATP sensitivity in KATP channels

The ATP-sensitive potassium (K_ATP) channel consisting of the inward rectifier Kir6.2 and SUR1 (sulfonylurea receptor 1) couples cell metabolism to membrane excitability and regulates insulin secretion. Inhibition by intracellular ATP is a hallmark feature of the channel. ATP sensitivity is conferred by Kir6.2 but enhanced by SUR1. The mechanism by which SUR1 increases channel ATP sensitivity is not understood. In this study, we report molecular interactions between SUR1 and Kir6.2 that markedly alter channel ATP sensitivity. Channels bearing an E203K mutation in SUR1 and a Q52E in Kir6.2 exhibit ATP sensitivity ∼100-fold higher than wild-type channels. Cross-linking of E203C in SUR1 and Q52C in Kir6.2 locks the channel in a closed state and is reversible by reducing agents, demonstrating close proximity of the two residues. Our results reveal that ATP sensitivity in K_ATP channels is a dynamic parameter dictated by interactions between SUR1 and Kir6.2.

Mice expressing a human K(ATP) channel mutation have altered channel ATP sensitivity but no cardiac abnormalities

AIMS/HYPOTHESIS

Patients with severe gain-of-function mutations in the Kir6.2 subunit of the ATP-sensitive potassium (K(ATP)) channel, have neonatal diabetes, muscle hypotonia and mental and motor developmental delay-a condition known as iDEND syndrome. However, despite the fact that Kir6.2 forms the pore of the cardiac K(ATP) channel, patients show no obvious cardiac symptoms. The aim of this project was to use a mouse model of iDEND syndrome to determine whether iDEND mutations affect cardiac function and cardiac K(ATP) channel ATP sensitivity.

METHODS

We performed patch-clamp and in vivo cine-MRI studies on mice in which the most common iDEND mutation (Kir6.2-V59M) was targeted to cardiac muscle using Cre-lox technology (m-V59M mice).

RESULTS

Patch-clamp studies of isolated cardiac myocytes revealed a markedly reduced K(ATP) channel sensitivity to MgATP inhibition in m-V59M mice (IC(50) 62 μmol/l compared with 13 μmol/l for littermate controls). In vivo cine-MRI revealed there were no gross morphological differences and no differences in heart rate, end diastolic volume, end systolic volume, stroke volume, ejection fraction, cardiac output or wall thickening between m-V59M and control hearts, either under resting conditions or under dobutamine stress.

CONCLUSIONS/INTERPRETATION

The common iDEND mutation Kir6.2-V59M decreases ATP block of cardiac K(ATP) channels but was without obvious effect on heart function, suggesting that metabolic changes fail to open the mutated channel to an extent that affects function (at least in the absence of ischaemia). This may have implications for the choice of sulfonylurea used to treat neonatal diabetes.

Compound heterozygous mutations in the SUR1 (ABCC 8) subunit of pancreatic K(ATP) channels cause neonatal diabetes by perturbing the coupling between Kir6.2 and SUR1 subunits

KATP channels regulate insulin secretion by coupling β-cell metabolism to membrane excitability. These channels are comprised of a pore-forming Kir6.2 tetramer which is enveloped by four regulatory SUR1 subunits. ATP acts on Kir6.2 to stabilize the channel closed state while ADP (coordinated with Mg(2+)) activates channels via the SUR1 domains. Aberrations in nucleotide-binding or in coupling binding to gating can lead to hyperinsulinism or diabetes. Here, we report a case of diabetes in a 7-mo old child with compound heterozygous mutations in ABCC8 (SUR1[A30V] and SUR1[G296R]). In unison, these mutations lead to a gain of KATP channel function, which will attenuate the β-cell response to increased metabolism and will thereby decrease insulin secretion. (86)Rb(+) flux assays on COSm6 cells coexpressing the mutant subunits (to recapitulate the compound heterozygous state) show a 2-fold increase in basal rate of (86)Rb(+) efflux relative to WT channels. Experiments on excised inside-out patches also reveal a slight increase in activity, manifested as an enhancement in stimulation by MgADP in channels expressing the compound heterozygous mutations or homozygous G296R mutation. In addition, the IC 50 for ATP inhibition of homomeric A30V channels was increased ~6-fold, and was increased ~3-fold for both heteromeric A30V+WT channels or compound heterozygous (A30V +G296R) channels. Thus, each mutation makes a mechanistically distinct contribution to the channel gain-of-function that results in neonatal diabetes, and which we predict may contribute to diabetes in related carrier individuals.

Genetic defects in the hotspot of inwardly rectifying K(+) (Kir) channels and their metabolic consequences: a review

Inwardly rectifying potassium (Kir) channels are essential for maintaining normal potassium homeostasis and the resting membrane potential. As a consequence, mutations in Kir channels cause debilitating diseases ranging from cardiac failure to renal, ocular, pancreatic, and neurological abnormalities. Structurally, Kir channels consist of two trans-membrane domains, a pore-forming loop that contains the selectivity filter and two cytoplasmic polar tails. Within the cytoplasmic structure, clusters of amino acid sequences form regulatory domains that interact with cellular metabolites to control the opening and closing of the channel. In this review, we present an overview of Kir channel function and recent progress in the characterization of selected Kir channel mutations that lie in and near a C-terminal cytoplasmic 'hotspot' domain. The resultant molecular mechanisms by which the loss or gain of channel function leads to organ failure provide potential opportunities for targeted therapeutic interventions for this important group of channelopathies.

Channeling dysglycemia: ion-channel variations perturbing glucose homeostasis

Maintaining blood glucose homeostasis is a complex process that depends on pancreatic islet hormone secretion. Hormone secretion from islets is coupled to calcium entry which results from regenerative islet cell electrical activity. Therefore, the ionic mechanisms that regulate calcium entry into islet cells are crucial for maintaining normal glucose homeostasis. Genome-wide association studies (GWAS) have identified single-nucleotide polymorphisms (SNPs), including five located in or near ion-channel or associated subunit genes, which show an association with human diseases characterized by dysglycemia. This review focuses on polymorphisms and mutations in ion-channel genes that are associated with perturbations in human glucose homeostasis and discusses their potential roles in modulating pancreatic islet hormone secretion.

Neonatal diabetes mellitus due to L233F mutation in the KCNJ11 gene

BACKGROUND

Neonatal diabetes mellitus (NDM) due to KCNJ11 gene mutation presents with diabetes in the first 3 months of life and sometimes with neurological features like developmental delay, muscle weakness and epilepsy.

METHODS

A 5-week-old boy presented with diabetic ketoacidosis. Molecular genetic analysis of the patient revealed heterozygous missense mutation, L233F in the KCNJ11 gene, while his mother was mosaic for the same mutation.

RESULTS

The treatment strategy was changed from insulin injections to oral glibenclamide and with a better glycemic control.

CONCLUSION

The patient with NDM due to mutation L233F (not reported till date) in the KCNJ11 gene can be successfully treated with oral glibenclamide therapy.

Mechanism of KATP hyperactivity and sulfonylurea tolerance due to a diabetogenic mutation in L0 helix of sulfonylurea receptor 1 (ABCC8)

Activating mutations in different domains of the ABCC8 gene-coded sulfonylurea receptor 1 (SUR1) cause neonatal diabetes. Here we show that a diabetogenic mutation in an unexplored helix preceding the ABC core of SUR1 dramatically increases open probability of (SUR1/Kir6.2)(4) channel (KATP) by reciprocally changing rates of its transitions to and from the long-lived, inhibitory ligand-stabilized closed state. This kinetic mechanism attenuates ATP and sulfonylurea inhibition, but not Mg-nucleotide stimulation, of SUR1/Kir6.2. The results suggest a key role for L0 helix in KATP gating and together with previous findings from mutant KATP clarify why many patients with neonatal diabetes require high doses of sulfonylureas.

Long-term response to sulfonylurea in a patient with diabetes due to mutation in the KCNJ11 gene

OBJECTIVE

To report the long-term (30-month) effect of the switch from insulin to sulfonylurea in a patient carrying the p.G53D (c.158G>A) mutation in KCNJ11 gene.

SUBJECT AND METHOD

A 29-year-old male patient was diagnosed with diabetes in the third month of life and after identification of a heterozygous p.G53D mutation in the KCNJ11 gene, the therapy was switched from insulin to sulfonylurea.

RESULTS

Long-term follow-up (30 months) showed that good metabolic control was maintained (HbA1c: 6.6%) and the glibenclamide dose could be reduced.

CONCLUSION

Long-term therapy with sulfonylureas in patients with neonatal diabetes due to mutation in the KCNJ11 gene is safe and promotes sustained improvement of glycemic control.

A conserved tryptophan at the membrane-water interface acts as a gatekeeper for Kir6.2/SUR1 channels and causes neonatal diabetes when mutated

We identified a novel heterozygous mutation, W68R, in the Kir6.2 subunit of the ATP-sensitive potassium (KATP) channel, in a patient with transient neonatal diabetes. This tryptophan is absolutely conserved in mammalian Kir channels. The functional effects of mutations at residue 68 of Kir6.2 were studied by heterologous expression in Xenopus oocytes, and by homology modelling. We found the Kir6.2-W68R mutation causes a small reduction in ATP inhibition in the heterozygous state and an increase in the whole-cell KATP current. This can explain the clinical phenotype of the patient. The effect of the mutation was not charge or size dependent, the order of potency for ATP inhibition being W Collapse

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MESH Headings

• ATP-Binding Cassette Transporters/genetics
• ATP-Binding Cassette Transporters/metabolism
• Amino Acid Sequence
• Animals
• Cell Membrane/genetics
• Cell Membrane/metabolism
• Child
• Conserved Sequence/genetics
• Diabetes Mellitus/genetics
• Diabetes Mellitus/metabolism
• Female
• Genetic Carrier Screening
• Humans
• Infant, Newborn
• Infant, Newborn, Diseases/genetics
• Infant, Newborn, Diseases/metabolism
• Molecular Sequence Data
• Mutation/genetics
• Oocytes/metabolism
• Phenotype
• Potassium Channels, Inwardly Rectifying/genetics
• Potassium Channels, Inwardly Rectifying/metabolism
• Rats
• Receptors, Drug/genetics
• Receptors, Drug/metabolism
• Sulfonylurea Receptors
• Tryptophan/genetics
• Xenopus laevis

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Grants

• Wellcome Trust
• 084655 Wellcome Trust
• 089795 Wellcome Trust
• 076436 Wellcome Trust

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Affiliation(s)

Roope Männikkö Henry Wellcome Centre for Gene Function, Department of Physiology, Anatomy and Genetics, University of Oxford, Parks Road, Oxford OX1 3PT, UK
Phillip J Stansfeld
Alexandra S Ashcroft
Andrew T Hattersley
Mark S P Sansom
Sian Ellard
Frances M Ashcroft

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Collaborators Collapse

Current understanding of K ATP channels in neonatal diseases: focus on insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are cell metabolic sensors that couple cell metabolic status to electric activity, thus regulating many cellular functions. In pancreatic beta cells, K(ATP) channels modulate insulin secretion in response to fluctuations in plasma glucose level, and play an important role in glucose homeostasis. Recent studies show that gain-of-function and loss-of-function mutations in K(ATP) channel subunits cause neonatal diabetes mellitus and congenital hyperinsulinism respectively. These findings lead to significant changes in the diagnosis and treatment for neonatal insulin secretion disorders. This review describes the physiological and pathophysiological functions of K(ATP) channels in glucose homeostasis, their specific roles in neonatal diabetes mellitus and congenital hyperinsulinism, as well as future perspectives of K(ATP) channels in neonatal diseases.

Defects in beta cell Ca²+ signalling, glucose metabolism and insulin secretion in a murine model of K(ATP) channel-induced neonatal diabetes mellitus

AIMS/HYPOTHESIS

Mutations that render ATP-sensitive potassium (K(ATP)) channels insensitive to ATP inhibition cause neonatal diabetes mellitus. In mice, these mutations cause insulin secretion to be lost initially and, as the disease progresses, beta cell mass and insulin content also disappear. We investigated whether defects in calcium signalling alone are sufficient to explain short-term and long-term islet dysfunction.

METHODS

We examined the metabolic, electrical and insulin secretion response in islets from mice that become diabetic after induction of ATP-insensitive Kir6.2 expression. To separate direct effects of K(ATP) overactivity on beta cell function from indirect effects of prolonged hyperglycaemia, normal glycaemia was maintained by protective exogenous islet transplantation.

RESULTS

In endogenous islets from protected animals, glucose-dependent elevations of intracellular free-calcium activity ([Ca(2+)](i)) were severely blunted. Insulin content of these islets was normal, and sulfonylureas and KCl stimulated increased [Ca(2+)](i). In the absence of transplant protection, [Ca(2+)](i) responses were similar, but glucose metabolism and redox state were dramatically altered; sulfonylurea- and KCl-stimulated insulin secretion was also lost, because of systemic effects induced by long-term hyperglycaemia and/or hypoinsulinaemia. In both cases, [Ca(2+)](i) dynamics were synchronous across the islet. After reduction of gap-junction coupling, glucose-dependent [Ca(2+)](i) and insulin secretion was partially restored, indicating that excitability of weakly expressing cells is suppressed by cells expressing mutants, via gap-junctions.

CONCLUSIONS/INTERPRETATION

The primary defect in K(ATP)-induced neonatal diabetes mellitus is failure of glucose metabolism to elevate [Ca(2+)](i), which suppresses insulin secretion and mildly alters islet glucose metabolism. Loss of insulin content and mitochondrial dysfunction are secondary to the long-term hyperglycaemia and/or hypoinsulinaemia that result from the absence of glucose-dependent insulin secretion.

Permanent neonatal diabetes mellitus--the importance of diabetes differential diagnosis in neonates and infants

BACKGROUND

The differential diagnosis of various types and forms of diabetes is of great practical importance. This is particularly true for monogenic disease forms, where some spectacular applications of pharmacogenetics have recently been described.

DESIGN

For many years the distinct character of diabetes diagnosed in the first weeks and months of life remained unnoticed. The results of the search for type 1 diabetes-related autoantibodies, description of the HLA haplotypes distribution and analysis of clinical features in patients diagnosed in the first 6 months of life provided the initial evidence that the etiology of their disease might be different from that of autoimmune diabetes.

RESULTS

Over the last decade, mutations in about a dozen of genes have been linked to the development of Permanent Neonatal Diabetes Mellitus (PNDM). The most frequent causes of PNDM are heterozygous mutations in the KCNJ11, INS and ABCC8 genes. Although PNDM is a rare phenomenon (one case in about 200,000 live births), this discovery has had a large impact on clinical practice as most carriers of KCNJ11 and ABCC8 gene mutations have been switched from insulin to oral sulphonylureas with an improvement in glycemic control. In this review we summarize the practical aspects of diabetes differential diagnosis in neonates and infants.

CONCLUSIONS

Genetic testing should be advised in all subjects with PNDM as it may influence medical care in subjects with these monogenic forms of early onset diabetes.

KCNJ11 activating mutations cause both transient and permanent neonatal diabetes mellitus in Cypriot patients

Heterozygous mutations of the KCNJ11 gene encoding the Kir6.2 subunit of the ATP-sensitive potassium channel (K(ATP) channel) of the pancreatic β-cell cause diabetes in about 30-60% of all permanent neonatal diabetes mellitus cases diagnosed before 6 months of age. The K(ATP) channel plays an essential role in the regulation of the electrical status of the membrane through which the secretion of insulin is activated. Transient neonatal diabetes mellitus due to KCNJ11 mutations is less frequent than abnormalities affecting the imprinted region of chromosome 6q24. We studied the genetic basis of two Cypriot patients who developed diabetes before 6 months of age. They both carried mutations of the KCNJ11 gene. The R201H mutation was identified in a patient who developed hyperglycemia and ketoacidosis at the age of 40 d and was successfully transferred to sulphonylureas which activate the channel through an ATP independent route. The R50Q mutation was identified in a child diagnosed at day 45 after birth with remission of his diabetes at 9 months of age. The same defect was identified both in his asymptomatic mother and the recently diagnosed 'type 2' diabetic maternal grandmother. The remission-relapse mechanism in cases of transient neonatal diabetes is not known. Nevertheless, it is possible that the residue of the mutation within the Kir6.2 molecule is associated with the sensitivity to ATP reflecting to the severity of the diabetic phenotype.

Glucose metabolism and neurological outcome in congenital hyperinsulinism

Advances in imaging and surgical techniques allow a complete cure for children with focal-type congenital hyperinsulinism (CHI). In contrast, management of diffuse-type CHI remains a matter of controversy. To prevent hypoglycemic brain damage, extensive surgery has been recommended in the past, resulting in diabetes. On the basis of 2 data sets of patients with congenital hyperinsulinism, the German registry for CHI with 235 patients (ages 1 day to 19 years) and the diabetes treatment register (Diabetes Patienten-Verlaufsdokumentationssystem initiative), a follow-up study was initiated for diabetes mellitus and the intellectual and physical development as well as motor function. In our ongoing study, we investigated 20 patients with CHI (12 male, mean ages 9.9 years). Six of 20 patients had undergone subtotal pancreatectomy. In early infantile development (0-3 years) we observed a trend to motor and speech delay. In early childhood (2.5-7 years) there appeared a trend to an advantage of results of nonverbal tasks compared with verbal tasks. Before 1990 most patients (∼75%) were treated by subtotal pancreatectomy; since 2000, a more conservative approach is obvious (4/68). All patients with diabetes (n = 25) developed the condition after undergoing subtotal pancreatectomy. No spontaneous manifestation of diabetes was noted before adulthood. There was a wide range of age (0-17.7 years) at manifestation indicating a long period during which glucose tolerance is compensated. Compared with >40.000 children with type 1 diabetes mellitus from the Diabetes Patienten-Verlaufsdokumentationssystem registry, we found significant differences with a tendency for being overweight as well as small stature. Mean daily insulin dose and HbA1c was comparable in both groups.

Neurological channelopathies

Inherited ion channel mutations can affect the entire nervous system. Many cause paroxysmal disturbances of brain, spinal cord, peripheral nerve or skeletal muscle function, with normal neurological development and function in between attacks. To fully understand how mutations of ion channel genes cause disease, we need to know the normal location and function of the channel subunit, consequences of the mutation for biogenesis and biophysical properties, and possible compensatory changes in other channels that contribute to cell or circuit excitability. Animal models of monogenic channelopathies increasingly help our understanding. An important challenge for the future is to determine how more subtle derangements of ion channel function, which arise from the interaction of genetic and environmental influences, contribute to common paroxysmal disorders, including idiopathic epilepsy and migraine, that share features with rare monogenic channelopathies.

AV59M KCNJ11 gene mutation leading to intermediate DEND syndrome in a Chinese child

Heterozygous activating mutations in the KCNJ11 gene can cause permanent and transient neonatal diabetes. In the present study, we sequenced the KCNJ11 gene in a Chinese boy diagnosed with permanent neonatal diabetes mellitus (PNDM) and also in his parents. A heterozygous 175G > A (V59M) mutation was identified in the patient, while no KCNJ11 gene mutations were found in his parents, indicating that this mutation is de novo. The patient with the V59M mutation successfully switched from insulin injections to oral glibenclamide; 2 years of follow-up revealed that the patient had intermediate developmental delay, epilepsy and neonatal diabetes (DEND) syndrome. This is the first patient who is reported to have iDEND syndrome due to KCNJ11 V59M mutation in China.

Functional complementation and genetic deletion studies of KirBac channels: activatory mutations highlight gating-sensitive domains

The superfamily of prokaryotic inwardly rectifying (KirBac) potassium channels is homologous to mammalian Kir channels. However, relatively little is known about their regulation or about their physiological role in vivo. In this study, we have used random mutagenesis and genetic complementation in K(+)-auxotrophic Escherichia coli and Saccharomyces cerevisiae to identify activatory mutations in a range of different KirBac channels. We also show that the KirBac6.1 gene (slr5078) is necessary for normal growth of the cyanobacterium Synechocystis PCC6803. Functional analysis and molecular dynamics simulations of selected activatory mutations identified regions within the slide helix, transmembrane helices, and C terminus that function as important regulators of KirBac channel activity, as well as a region close to the selectivity filter of KirBac3.1 that may have an effect on gating. In particular, the mutations identified in TM2 favor a model of KirBac channel gating in which opening of the pore at the helix-bundle crossing plays a far more important role than has recently been proposed.

Genetic polymorphisms in diabetes: influence on therapy with oral antidiabetics

Due to new genetic insights, etiologic classification of diabetes is under constant scrutiny. Hundreds, or even thousands, of genes are linked with type 2 diabetes. Three common variants (Lys23 of KCNJ11, Pro12 of PPARG, and the T allele at rs7903146 of TCF7L2) have been shown to be predisposed to type 2 diabetes mellitus across many large studies. Individually, each of these polymorphisms is only moderately predisposed to type 2 diabetes. On the other hand, monogenic forms of diabetes such as MODY and neonatal diabetes are characterized by unique clinical features and the possibility of applying a tailored treatment.Genetic polymorphisms in drug-metabolizing enzymes, transporters, receptors, and other drug targets have been linked to interindividual differences in the efficacy and toxicity of a number of medications. Mutations in genes important in drug absorption, distribution, metabolism and excretion (ADME) play a critical role in pharmacogenetics of diabetes.There are currently five major classes of oral pharmacological agents available to treat type 2 diabetes: sulfonylureas, meglitinides, metformin (a biguanide), thiazolidinediones, and α-glucosidase inhibitors. Other classes are also mentioned in literature.In this work, different types of genetic mutations (mutations of the gene for glucokinase, HNF 1α, HNF1β and Kir6.2 and SUR1 subunit of KATP channel, PPAR-γ, OCT1 and OCT2, cytochromes, direct drug-receptor (KCNJ11), as well as the factors that influence the development of the disease (TCF7L2) and variants of genes that lead to hepatosteatosis caused by thiazolidinediones) and their influence on the response to therapy with oral antidiabetics will be reviewed.

The molecular mechanisms and pharmacotherapy of ATP-sensitive potassium channel gene mutations underlying neonatal diabetes

Neonatal diabetes mellitus (NDM) is a monogenic disorder caused by mutations in genes involved in regulation of insulin secretion from pancreatic β-cells. Mutations in the KCNJ11 and ABCC8 genes, encoding the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel Kir6.2 and SUR1 subunits, respectively, are found in ∼50% of NDM patients. In the pancreatic β-cell, K(ATP) channel activity couples glucose metabolism to insulin secretion via cellular excitability and mutations in either KCNJ11 or ABCC8 genes alter K(ATP) channel activity, leading to faulty insulin secretion. Inactivation mutations decrease K(ATP) channel activity and stimulate excessive insulin secretion, leading to hyperinsulinism of infancy. In direct contrast, activation mutations increase K(ATP) channel activity, resulting in impaired insulin secretion, NDM, and in severe cases, developmental delay and epilepsy. Many NDM patients with KCNJ11 and ABCC8 mutations can be successfully treated with sulfonylureas (SUs) that inhibit the K(ATP) channel, thus replacing the need for daily insulin injections. There is also strong evidence indicating that SU therapy ameliorates some of the neurological defects observed in patients with more severe forms of NDM. This review focuses on the molecular and cellular mechanisms of mutations in the K(ATP) channel that underlie NDM. SU pharmacogenomics is also discussed with respect to evaluating whether patients with certain K(ATP) channel activation mutations can be successfully switched to SU therapy.

Pancreatic β-cell KATP channels: Hypoglycaemia and hyperglycaemia

The pancreatic β-cell ATP-sensitive K(+) channel (K(ATP) channel) plays a critical role in glucose homeostasis by linking glucose metabolism to electrical excitability and insulin secretion. Changes in the intracellular ratio of ATP/ADP mediate the metabolic regulation of channel activity. The β-cell K(ATP) channel is a hetero-octameric complex composed of two types of subunits: four inward-rectifying potassium channel pore-forming (Kir6.2) subunits and four high-affinity sulfonylurea receptor 1 (SUR1) subunits. Kir6.2 and SUR1 are encoded by the genes KCNJ11 and ABCC8, respectively. Mutations in these genes can result in congenital hyperinsulinism and permanent neonatal diabetes. This review highlights the important role of the β-cell K(ATP) channel in glucose physiology and provides an introduction to some of the other review articles in this special edition of the Reviews in Endocrine and Metabolic Disorders.

Permanent neonatal diabetes due to activating mutations in ABCC8 and KCNJ11

The ATP-sensitive potassium (K(ATP)) channel is composed of two subunits SUR1 and Kir6.2. The channel is key for glucose stimulated insulin release from the pancreatic beta cell. Activating mutations have been identified in the genes encoding these subunits, ABCC8 and KCNJ11, and account for approximately 40% of permanent neonatal diabetes cases. The majority of patients with a K(ATP) mutation present with isolated diabetes however some have presented with the Developmental delay, Epilepsy and Neonatal Diabetes syndrome. This review focuses on mutations in the K(ATP) channel which result in permanent neonatal diabetes, we review the clinical and functional effects as well as the implications for treatment.

Down-regulation of Kir6.2 affects calcium influx and insulin secretion in HIT-T15 cells

In pancreatic beta cells, ATP-sensitive potassium (K(ATP)) channels are metabolic sensors that couple cell metabolism to electrical activity, and therefore K(ATP) channels regulate insulin secretion. We assume that down-regulating the expression of Kir6.2 subunits of K(ATP) channels may change calcium influx induced by glucose and insulin secretion regulated by K(ATP) channels. In our study, we employ Kir6.2-shRNA plasmid to downregulate Kir6.2 expression in HIT-T15 cells. Then, we research the effect of downregulation of Kir6.2 on K(ATP) current, cytoplasmic free Ca2+ concentration and insulin secretion. All results illustrate that downregulation of Kir6.2 subunits of K(ATP) channels in HIT-T15 cells affects K(ATP) current and insulin secretion, and fails to promote calcium influx. The results demonstrate the function of Kir6.2 subunits in electrophysiology characteristic, insulin secretion and calcium influx, and RNA interference provides a feasible alternative to study the function of Kir6.2 subunits in K(ATP) channels in different kinds of diabetes.

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

The first clinical case of a mutation at residue K185 of Kir6.2 (KCNJ11): a major ATP-binding residue

BACKGROUND

Closure of the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel plays a key role in insulin secretion from the pancreatic beta-cells. Many mutations in KCNJ11 and ABCC8, which respectively encode the pore-forming (Kir6.2) and regulatory (SUR1) subunits of the K(ATP) channel, cause neonatal diabetes. All such mutations impair the ability of metabolically generated ATP to close the channel. Although lysine 185 is predicted to be a major contributor to the ATP-binding site of Kir6.2, no mutations at this residue have been found to cause neonatal diabetes to date.

METHODS

We report a 3-year-old girl with permanent neonatal diabetes (PNDM) caused by a novel heterozygous mutation (K185Q) at residue K185 of KCNJ11. The patient presented with marked hyperglycaemia and ketoacidosis at 70 days after birth, and insulin therapy was commenced.

RESULTS

Wild-type and mutant K(ATP) channels were expressed in Xenopus oocytes and the effects of intracellular ATP on macroscopic K(ATP) currents in inside-out membrane patches were measured. In the simulated heterozygous state, the K185Q mutation caused a substantial reduction in the ability of MgATP to inhibit the channel. Heterozygous K185Q channels were still blocked effectively by the sulphonylurea tolbutamide.

CONCLUSIONS

We report the first clinical case of a PNDM caused by a mutation at K185. Functional studies indicate that the K185Q mutation causes PNDM by reducing the ATP sensitivity of the K(ATP) channel, probably via a reduction in ATP binding to Kir6.2. Based on the experimental data, the patient was successfully transferred to sulphonylurea therapy.

ATP-sensitive potassium channels in health and disease

The ATP-sensitive potassium (K(ATP)) channel plays a crucial role in insulin secretion and thus glucose homeostasis. K(ATP) channel activity in the pancreatic beta-cell is finely balanced; increased activity prevents insulin secretion, whereas reduced activity stimulates insulin release. The beta-cell metabolism tightly regulates K(ATP) channel gating, and if this coupling is perturbed, two distinct disease states can result. Diabetes occurs when the K(ATP) channel fails to close in response to increased metabolism, whereas congenital hyperinsulinism results when K(ATP) channels remain closed even at very low blood glucose levels. In general there is a good correlation between the magnitude of K(ATP) current and disease severity. Mutations that cause a complete loss of K(ATP) channels in the beta-cell plasma membrane produce a severe form of congenital hyperinsulinism, whereas mutations that partially impair channel function produce a milder phenotype. Similarly mutations that greatly reduce the ATP sensitivity of the K(ATP) channel lead to a severe form of neonatal diabetes with associated neurological complications, whilst mutations that cause smaller shifts in ATP sensitivity cause neonatal diabetes alone. This chapter reviews our current understanding of the pancreatic beta-cell K(ATP) channel and highlights recent structural, functional and clinical advances.

Human K(ATP) channelopathies: diseases of metabolic homeostasis

Assembly of an inward rectifier K+ channel pore (Kir6.1/Kir6.2) and an adenosine triphosphate (ATP)-binding regulatory subunit (SUR1/SUR2A/SUR2B) forms ATP-sensitive K+ (KATP) channel heteromultimers, widely distributed in metabolically active tissues throughout the body. KATP channels are metabolism-gated biosensors functioning as molecular rheostats that adjust membrane potential-dependent functions to match cellular energetic demands. Vital in the adaptive response to (patho)physiological stress, KATP channels serve a homeostatic role ranging from glucose regulation to cardioprotection. Accordingly, genetic variation in KATP channel subunits has been linked to the etiology of life-threatening human diseases. In particular, pathogenic mutations in KATP channels have been identified in insulin secretion disorders, namely, congenital hyperinsulinism and neonatal diabetes. Moreover, KATP channel defects underlie the triad of developmental delay, epilepsy, and neonatal diabetes (DEND syndrome). KATP channelopathies implicated in patients with mechanical and/or electrical heart disease include dilated cardiomyopathy (with ventricular arrhythmia; CMD1O) and adrenergic atrial fibrillation. A common Kir6.2 E23K polymorphism has been associated with late-onset diabetes and as a risk factor for maladaptive cardiac remodeling in the community-at-large and abnormal cardiopulmonary exercise stress performance in patients with heart failure. The overall mutation frequency within KATP channel genes and the spectrum of genotype-phenotype relationships remain to be established, while predicting consequences of a deficit in channel function is becoming increasingly feasible through systems biology approaches. Thus, advances in molecular medicine in the emerging field of human KATP channelopathies offer new opportunities for targeted individualized screening, early diagnosis, and tailored therapy.

Interaction between mutations in the slide helix of Kir6.2 associated with neonatal diabetes and neurological symptoms

ATP-sensitive potassium (K(ATP)) channels regulate insulin secretion from pancreatic beta-cells. Gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of this channel cause neonatal diabetes. We report two novel mutations on the same haplotype (cis), F60Y and V64L, in the slide helix of Kir6.2 in a patient with neonatal diabetes, developmental delay and epilepsy. Functional analysis revealed the F60Y mutation increases the intrinsic channel open probability (Po(0)), thereby indirectly producing a marked decrease in channel inhibition by ATP and an increase in whole-cell K(ATP) currents. When expressed alone, the V64L mutation caused a small reduction in apparent ATP inhibition, by enhancing the ability of MgATP to stimulate channel activity. The V64L mutation also ameliorated the deleterious effects on the F60Y mutation when it was expressed on the same (but not a different) subunit. These data indicate that F60Y is the pathogenic mutation and reveal that interactions between slide helix residues can influence K(ATP) channel gating.

K(ATP) channelopathies in the pancreas

Adenosine-triphosphate-sensitive potassium channels (KATP) are regulated by adenosine nucleotides, and, thereby, couple cellular metabolism with electrical activity in multiple tissues including the pancreatic beta-cell. The critical involvement of KATP in insulin secretion is confirmed by the demonstration that inactivating and activating mutations in KATP underlie persistent hyperinsulinemia and neonatal diabetes mellitus, respectively, in both animal models and humans. In addition, a common variant in KATP represents a risk factor in the etiology of type 2 diabetes. This review focuses on the mechanistic basis by which KATP mutations underlie insulin secretory disorders and the implications of these findings for successful clinical intervention.

Neonatal diabetes in a child positive for islet cell antibodies at onset and Kir6.2 activating mutation

In contrast to the autoimmune type 1 diabetes, patients with monogenic diabetes due to KCNJ11 mutations do not have pancreatic auto-antibodies at onset. Here we describe a patient diagnosed at the age of 12 weeks that showed ICA at diagnosis, yet has been tested with positive result for KCNJ11 mutation.

Glibenclamide unresponsiveness in a Brazilian child with permanent neonatal diabetes mellitus and DEND syndrome due to a C166Y mutation in KCNJ11 (Kir6.2) gene

Heterozygous activating mutations of KCNJ11 (Kir6.2) are the most common cause of permanent neonatal diabetes mellitus (PNDM) and several cases have been successfully treated with oral sulfonylureas. We report on the attempted transfer of insulin therapy to glibenclamide in a 4-year old child with PNDM and DEND syndrome, bearing a C166Y mutation in KCNJ11. An inpatient transition from subcutaneous NPH insulin (0.2 units/kg/d) to oral glibenclamide (1 mg/kg/d and 1.5 mg/kg/d) was performed. Glucose and C-peptide responses stimulated by oral glucose tolerance test (OGTT), hemoglobin A1c levels, the 8-point self-measured blood glucose (SMBG) profile and the frequency of hypoglycemia episodes were analyzed, before and during treatment with glibenclamide. Neither diabetes control nor neurological improvements were observed. We concluded that C166Y mutation was associated with a form of PNDM insensitive to glibenclamide.

Kir6.2 variant E23K increases ATP-sensitive K+ channel activity and is associated with impaired insulin release and enhanced insulin sensitivity in adults with normal glucose tolerance

OBJECTIVE

The E23K variant in the Kir6.2 subunit of the ATP-sensitive K(+) channel (K(ATP) channel) is associated with increased risk of type 2 diabetes. The present study was undertaken to increase our understanding of the mechanisms responsible. To avoid confounding effects of hyperglycemia, insulin secretion and action were studied in subjects with the variant who had normal glucose tolerance.

RESEARCH DESIGN AND METHODS

Nine subjects with the E23K genotype K/K and nine matched subjects with the E/E genotype underwent 5-h oral glucose tolerance tests (OGTTs), graded glucose infusion, and hyperinsulinemic-euglycemic clamp with stable-isotope-labeled tracer infusions to assess insulin secretion, action, and clearance. A total of 461 volunteers consecutively genotyped for the E23K variant also underwent OGTTs. Functional studies of the wild-type and E23K variant potassium channels were conducted.

RESULTS

Insulin secretory responses to oral and intravenous glucose were reduced by approximately 40% in glucose-tolerant subjects homozygous for E23K. Normal glucose tolerance with reduced insulin secretion suggests a change in insulin sensitivity. The hyperinsulinemic-euglycemic clamp revealed that hepatic insulin sensitivity is approximately 40% greater in subjects with the E23K variant, and these subjects demonstrate increased insulin sensitivity after oral glucose. The reconstituted E23K channels confirm reduced sensitivity to inhibitory ATP and increase in open probability, a direct molecular explanation for reduced insulin secretion.

CONCLUSIONS

The E23K variant leads to overactivity of the K(ATP) channel, resulting in reduced insulin secretion. Initially, insulin sensitivity is enhanced, thereby maintaining normal glucose tolerance. Presumably, over time, as insulin secretion falls further or insulin resistance develops, glucose levels rise resulting in type 2 diabetes.

An in-frame deletion in Kir6.2 (KCNJ11) causing neonatal diabetes reveals a site of interaction between Kir6.2 and SUR1

CONTEXT

Activating mutations in genes encoding the Kir6.2 (KCNJ11) and SUR1 (ABCC8) subunits of the pancreatic ATP-sensitive K(+) channel are a common cause of permanent neonatal diabetes (PNDM). All Kir6.2 mutations identified to date are missense mutations. We describe here a novel in-frame deletion (residues 28-32) in Kir6.2 in a heterozygous patient with PNDM without neurological problems that are detectable by standard evaluation.

OBJECTIVE

The aim of the study was to identify the mutation responsible for neonatal diabetes in this patient and characterize its functional effects.

DESIGN

Wild-type and mutant Kir6.2/SUR1 channels were examined by heterologous expression in Xenopus oocytes.

RESULTS

The Kir6.2-28Delta32 mutation produced a significant decrease in ATP inhibition and an increase in whole-cell K(ATP) currents, explaining the diabetes of the patient. Tolbutamide block was only slightly reduced in the simulated heterozygous state, suggesting that the patient should respond to sulfonylurea therapy. The mutation decreased ATP inhibition indirectly, by increasing the intrinsic (unliganded) channel open probability. Neither effect was observed when Kir6.2 was expressed in the absence of SUR1, suggesting that the mutation impairs coupling between SUR1 and Kir6.2. Coimmunoprecipitation studies further revealed that the mutation disrupted a physical interaction between Kir6.2 and residues 1-288 (but not residues 1-196) of SUR1.

CONCLUSIONS

We report a novel KCNJ11 mutation causing PNDM. Our results show that residues 28-32 in the N terminus of Kir6.2 interact both physically and functionally with SUR1 and suggest that residues 196-288 of SUR1 are important in this interaction.