Potassium Channels, Sulphonylurea Receptors and Control of Insulin Release

maturity-onset diabetes of the young. Diabetologia 40, 859–862 40 Fajans, S.S., Bell, G.I., Bowden, D.W., Halter, J.B. and Polonsky, K.S. (1994) Maturity-onset diabetes of the young. Life Sci. 55, 413–422 41 Herman, W.H. et al. (1994) Abnormal insulin secretion, not insulin resistance, is the genetic or primary defect of MODY in the RW pedigree. Diabetes 43, 40–46 42 Byrne, M.M. et al. (1995) Altered insulin secretory responses to glucose in subjects with a mutation in the MODY1 gene on chromosome 20. Diabetes 44, 699–704

43 Hertz, R., Magenheim, J., Berman, I. and Bar-Tana, J. (1998) Fatty acyl-CoA thioesters are ligands of hepatic nuclear factor 4 alpha. Nature 392, 512–516 44 Nishigori, H. et al. (1998) Frameshift mutation, A263fsinsGG, in the hepatocyte nuclear factor-1 beta gene associated with diabetes and renal dysfunction. Diabetes 47, 1354–1355 45 Jonsson, J., Carisson, L., Edlund, T. and Edlund, H. (1994) Insulin-promoterfactor 1 is required for pancreas development in mice. Nature 371, 606–609 46 Iwasaki, N. et al. (1997) Mutations in the hepatocyte nuclear factor-1 alpha/

Potassium Channels, Sulphonylurea Receptors and Control of Insulin Release Mark J. Dunne, Karen E. Cosgrove, Ruth M. Shepherd and Carina Ämmälä

Clinical profiles of the glucose regulation disorders persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI) and diabetes mellitus are diametrically opposed: unregulated insulin secretion versus insulin insufficiency. Yet, despite this, recent studies of PHHI and other rare neonatal conditions have revealed common pathways of cellular dysfunction relevant to our understanding of diabetes. Such work has been based upon integration of the genetics of these diseases with the cellular and molecular biology of a potassium channel known to play a major role in the ‘glucosesensing apparatus’ of the pancreatic β cell – the ATP-sensitive K1 (KATP) channel. The structure of this protein complex is unique among ion channel families, because it is composed partly of a K1 channel and partly of an ATP-binding cassette protein that has an extraordinarily high affinity for sulphonylurea compounds. Here, we describe how defects in KATP channel genes give rise to insulin hypersecretion, and may also predispose to the onset of Type 2 diabetes, and how acquired losses of function of these channels have been implicated in maturity onset diabetes of the young and reactive hyperinsulinaemia-induced hypoglycaemia. Pancreatic β cells are electrically active and, in the presence of stimulatory concentrations of glucose (typically M.J. Dunne, K.E. Cosgrove and R.M. Shepherd are at the Institute of Molecular Physiology and Department of Biomedical Science, The University of Sheffield, Western Bank, Sheffield, UK S10 2TN; and C. Ämmälä is at the Department of Physiology and Neuroscience, Division of Molecular and Cellular Physiology, Sölvegatan 19, SE-223 62 Lund, Sweden.

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>5.5 mM), they will release insulin. This is principally dependent upon membrane depolarization, which leads to the activation of voltage-dependent Ca2+ channels. The resulting accelerated influx of Ca2+ ions allows Ca2+regulated exocytosis to occur, whereby insulin-containing secretory granules fuse with the plasma membrane to release their contents (Fig. 1). Carbohydrate metabolism is needed to initiate

MODY3 gene in Japanese subjects with early- and late-onset NIDDM. Diabetes 46, 1504–1508 47 Urhammer, S.A. et al. (1997) Genetic variation in the hepatocyte nuclear factor-1 alpha gene in Danish Caucasians with late-onset NIDDM. Diabetologia 40, 473–475 48 Hani, E.H. et al. (1998) A missense mutation in the hepatocyte nuclear factor 4-alpha, resulting in a reduced transactivational activity, in human late-onset non insulin-dependent diabetes mellitus. J. Clin. Invest. 101, 521–526

the key change in cell membrane potential and this is facilitated through the closure of K+ ion channels – the ATP-sensitive K+ (KATP) channels. The molecular details of ‘depolarizationresponse coupling’ have now been elucidated (for a review see Ref. 1) (Fig. 1). (1) The Na+–K+-ATPase pump and open KATP channels maintain the resting membrane potential. (2) KATP channels act as ‘metabolic sensors’ and close as a consequence of the increase in the intracellular ATP:ADP ratio produced by glucose metabolism. (3) A depolarization of the cell membrane leads to the activation of voltage-dependent Ca2+ channels, and (4) an ensuing rise in the free intracellular concentration of Ca2+ ([Ca2+]i) close to the membrane then initiates the release of insulin by exocytosis. In this scheme (Fig. 1), voltagedependent Ca2+ influx is shown as the dominant influence upon the release of insulin-containing granules. As KATP channels have a pivotal role in depolarization-response coupling, defects in their operation will have dramatic consequences for the control of insulin release. The model also documents how pharmacological regulation of these channels has important therapeutic implications. Thus, selective inhibition of KATP channels with drugs such as the antidiabetic sulphonylureas (for example, glibenclamide, tolbutamide, etc.) will mimic the actions of glucose and promote insulin release. Conversely, KATP channel ‘openers’, such as the hyperglycaemia-inducing compound diazoxide, and voltage-gated Ca2+ channel blockers, such as nifedipine, exert the opposite effect and inhibit secretion by preventing Ca2+ entry1.

1043-2760/99/$ – see front matter © 1999 Elsevier Science. All rights reserved. PII: S1043-2760(98)00135-0

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Two exciting areas of molecular endocrinology are providing important, additional insights into the significance of K+ channels and the control of insulin release in health and disease: the structural resolution of KATP channels and the identification of ‘KATP channelopathies’. • K+ Channels and Sulphonylurea Receptors Insulin-secreting cells express a KATP channel complex formed by the interaction of subunits belonging to at least two distinct families of proteins (for a review see Ref. 2) (Fig. 2). The K+-selective pore or tunnel is formed by the Kir6.2 subunit, a member of the inward rectifier K+ channel family. Kir6.2 shares ~70% homology with Kir6.1, consists of 390 amino acids, and has a predicted membrane topology with two a-helical transmembrane domains linked by a highly conserved sequence of amino acids. As this linking region shares sequence homology with the Por H5-region of voltage-gated K+ channels, it is thought to be important for the control of K+ selectivity through the pore (Fig. 2B). The other subunit is a larger protein – a receptor with high affinity for sulphonylureas, designated SUR1 (Ref. 3). Two closely related genes encode two sulphonylurea receptors, SUR1 and SUR2 (Ref. 4). Three splice variants of the SUR2 protein have been described and designated, SUR2A, SUR2B and SUR2C (Ref. 2). Human SUR1 is 1581 amino acids in length and, like the cystic fibrosis transmembrane conductance regulator (CFTR), it is a member of the superfamily of ATP-binding cassette (ABC) proteins. These proteins share common structural features with numerous predicted transmembrane sequences and nucleotidebinding domains (Fig. 2C)5. Neither the native Kir6.2 subunit, nor the SUR1 subunit, will form operational K+ channels independently. However, when co-expressed, K+ channel currents are generated that closely resemble those of the native b-cell KATP channel complex6,7. KATP channels in other tissues are heteromultimeric complexes of other Kir6.x and SUR proteins: cardiac Kir6.2 + SUR2A, smooth muscle TEM Vol. 10, No. 4, 1999

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Figure 1. The ionic control of insulin release from human pancreatic b cells. In normal b cells, at non-stimulatory glucose concentrations, the resting membrane potential (approximately –70 mV) is determined by open KATP channels. When the extracellular glucose concentration is elevated, glucose is taken up by the b cell and glucose metabolism is initiated. The rate-limiting step in this process is dependent upon the activity of glucokinase (1) and the formation of glucose-6-phosphate. Subsequent metabolic events lead to an increase in the cytosolic ATP:ADP ratio (2) and the closure of KATP channels (3). This leads to membrane depolarization (4) and the opening of voltage-dependent Ca2+ channels (5). The increased Ca2+ influx then initiates the release of insulin through exocytosis of secretory granules (6). These events account for first phase insulin release in response to glucose stimulation. In human b cells, second phase insulin release is thought to occur independently of KATP channel function, through a route that is still largely dependent upon the elevation of cytosolic Ca2+ levels46,47.

Kir6.2 + SUR2B, and the smooth muscle nucleotide-activated channel Kir6.1 + SUR2B (Ref. 2). The putative topological organization of the KATP channel suggests an obligatory octameric complex formed by four Kir6.2 subunits lining the pore coupled to four SUR1 subunits8,9 (Fig. 2D). SUR1 affects the trafficking and distribution of Kir6.2. Kir6.2 determines biophysical properties such as ion selectivity, rectification and gating of the complex. However, channel sensitivity to physiological regulators, such as the adenine and guanosine nucleotides, is a complex process involving both subunits2. Kir6.2 confers the ATP sensitivity of the

complex, whereas ADP (and GDP) binds to one of the nucleotide-binding folds of SUR1 and antagonizes the effects of ATP. Therefore, SUR1 serves as a transmembrane conductance regulator of Kir6.2 by conferring high sensitivity to metabolically derived signals and to key pharmacological agents, such as diazoxide and the sulphonylureas (Fig. 2A). Sulphonylurea compounds have other effects on insulin-secreting cells in addition to closure of KATP channels. These include inhibition of the Na–KATPase10 and both the inhibition and activation of swell-activated Cl– channels (for review see Ref. 11). In 147

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Figure 2. The architecture of ATP-sensitive K+ (KATP) channels in pancreatic b cells. (A) The key regulatory and pharmacological influences on KATP channels in pancreatic b cells: (i) a decreased ATP:ADP ratio will sustain channel open events in intact b cells; (ii) activation of channels by diazoxide and inhibition by tolbutamide. (B) The predicted topology of the K+ channel subunit Kir6.2. (C) The predicted topology of the b-cell SUR1 gene product – an ATP-binding cassette protein. Note the characteristic high number of transmembrane spanning domains 1–17, two extracellular glycosylation sites, and the presence of two intracellularly disposed nucleotide-binding domains (NBD) with Walker A and B binding motifs (•). (D) The heteromultimeric KATP channel complex is an obligatory octameric structure composed of (Kir6.2–SUR1)4.

addition, sulphonylureas have also been shown to potentiate the process of Ca2+-dependent exocytosis in b cells12,13. This is thought to occur through a mechanism that is independent of KATP channel closure, but dependent upon direct interaction with the proteins associated with exocytosis. Whether each of these additional actions of sulphonylureas also involve SUR1 or related SURs has yet to be determined, but it is interesting to note that >90% of all sulphonylurea-binding proteins are localized to the granule membranes14. Finally, a putative endogenous ligand of sulphonylurea receptors has been cloned. Human a-endosulfine, is a 13-kDa peptide expressed in a range of tissues including muscle, brain and endocrine cells15. The protein displaces the binding of sulphonylureas to b-cell membranes, and initiates insulin release through the inhibition of KATP channels. Therefore, a-endosulfine may act as an endogenous peptide modulator of KATP channels through interactions with SUR1. The gene that encodes SUR1 consists of 39 exon boundaries and is clustered with the KIR6.2 gene – a single open reading frame lying immediately 3’ of the SUR1 gene, separated by only

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4.5 kilobase pairs16. Their location in the human genome on the short arm of chromosome 11 corresponds to a genetic locus linked to familial forms of a potentially lethal childhood disorder associated with persistent hypoglycaemia as a result of hyperinsulinism16,17. The evidence is now convincing that gene defects encoded within SUR1 or KIR6.2 can be correlated with the functional loss of KATP channels in b cells, and that this leads to the loss of regulated insulin secretion and the pathogenesis of this neonatal disease. • K+ Channels and Unregulated Insulin Secretion Hypoglycaemia is a relatively common childhood metabolic abnormality and when persistent or recurrent is most frequently a consequence of hyperinsulinism; neonatal hyperinsulinism (OMIM: 256450) (for a recent review see Ref. 18). Until recently, the pathophysiology of this group of conditions was not understood, although a defect in b-cell function was first suggested in 1981, when it was shown that glucose failed to promote a concentrationdependent release of insulin in tissue isolated from a child with hyperinsulinaemia-induced hypoglycaemia19.

Persistent Hyperinsulinaemic Hypoglycaemia of Infancy (PHHI) PHHI is a severe condition that is usually unresponsive to medical treatment and is associated with profound hypoglycaemia in the early neonatal condition. Sporadic cases of PHHI are thought to be rare in the general population (~1:45000), but the disease has an incidence in communities with high rates of consanguinity approaching that of cystic fibrosis in European Caucasians; that is, ~1:2500 live births (for a review see Ref. 20). Severe forms usually present within the first few hours or days of birth, as the result of extreme and sustained low blood glucose levels owing to unregulated secretion of insulin. Failure to recognize and to treat hypoglycaemia promptly carries a substantial risk of severe brain damage and mental retardation because of a lack of fuels to sustain normal brain metabolism. Medical treatment for the disorder involves an increased carbohydrate intake to meet the elevated requirement, and usually one or more drugs that inhibit insulin secretion. These agents include diazoxide, which was first introduced to treat hyperinsulinism in the 1960s, and somatostatin, both of which will activate TEM Vol. 10, No. 4, 1999

KATP channels in normal b cells and inhibit insulin release20,21. This occurs as the result of increased K+ channel openings causing a hyperpolarization of the membrane and closure of voltage-dependent Ca2+ channels. Unfortunately, however, the responsiveness of children with PHHI to these agents is highly variable, and patients who do not show an adequate and immediate response require surgery to remove their pancreas – usually a 95% pancreatectomy, to prevent recurrent hypoglycaemia. The variability in sensitivity to medical therapy has until now been unexplained (see below). Pancreatectomy usually induces a clinical remission of symptoms, but this procedure will invariably predispose the PHHI patient to the development of diabetes mellitus in later life. The Correlation of KATP Channel Defects with PHHI The genetic basis of PHHI has not been determined conclusively. In those cases where mutations have been linked to the disease22, defects in the SUR1 (>25 reported) and KIR6.2 (three reported) genes are mainly inherited in an autosomal recessive manner. Gene defects in SUR1 can also be inherited in a different manner. In these cases, the patient inherits a single paternal recessive SUR1 gene mutation, and a portion of the pancreas is reduced to hemizygosity because of the loss of maternal imprinted genes23,24. Loss of heterozygocity in the affected b cells results in insulin hypersecretion, but because of the loss of other maternally expressed imprinted genes (such as the tumour supressor genes H19 and p57KIP2), the pancreata of patients with this condition also have a morphologically distinctive appearance, with focal regions of b-cell hyperproliferation. This has given rise to the terminology of ‘focal PHHI’ (Fo-PHHI) as distinct from ‘diffuse PHHI’, which is not associated with the loss of imprinted genes18. For a number of the SUR1 and KIR6.2 mutations that have been described22,23, it has been demonstrated in non-b-cell expression systems that these gene defects produce a variety of abnormalities in recombinant KATP TEM Vol. 10, No. 4, 1999

channel function, including trafficking defects, assembly defects and regulatory problems25. However, only the R1437Q(23)X (exon 35)26 and V187D (exon 4)27 mutations have been studied in b cells isolated from patients with the disease, and it is these studies that have provided the link between how variations in the SUR1 gene can lead to the clinical manifestation of hyperinsulinaemia-induced hypoglycaemia. In these and other experiments, insulin-secreting cells were prepared from several children with sporadic and familial disease after pancreatectomy. With the use of patch-clamp techniques under a number of experimental conditions, it was revealed quite remarkably that b cells from these children were spontaneously electrically active, even in the presence of low glucose concentrations, and that the appearance of action potentials was brought about in association with the absence of functional KATP channels26,28,29 (Fig. 3A). The exon 35 mutation causes a reading frame shift and a SUR1 gene product truncated by 200 amino acids. This results in complete loss of KATP channel function26. The exon 4 mutation appears only in the Finnish cohort of PHHI patients, and causes a valine to aspartic acid change at amino acid 187, which is located within either transmembrane domain 4 or 5. Patch-clamp studies of acutely isolated b cells once again demonstrated a loss of KATP channel function, and subsequent expression studies, using SUR1 mRNA engineered to contain the V187D mutation, showed that the latter did not reconstitute channels when co-injected with KIR6.2 mRNA into the Xenopus laevis expression system27. In isolated PHHI b cells, the resting cell membrane potential is close to the threshold for the activation of voltagedependent Ca2+ channels, and it is activity from these channels that leads to the appearance of spontaneous, regenerating action potentials in the unstimulated PHHI b cell29. Because Ca2+ influx is a key determinant of insulin secretion under normal conditions, inappropriate Ca2+ channel activity readily accounts for the unregulated secretion of insulin1,20. Furthermore, PHHI is

caused by the selective loss of KATP channels, because the major regulatory and pharmacological properties of delayed-rectifier K+ channels, Ca2+- and voltage-gated K+ channels and nonselective cation channels are unaffected by the disease phenotype in PHHI b cells30. Similar conclusions were also reached when the consequences of KATP channel dysfunction were examined using transgenic mice engineered to express a ‘dominant-negative’ form of Kir6.2 in b cells31. Substitution of a glycine residue for serine at position 132 within the pore-forming region of Kir6.2 terminates K+ channel activity. Affected mice developed hyperinsulinaemic hypoglycaemia during the neonatal period, and b cells isolated from the animals had: (1) impaired KATP channel function, (2) depolarized resting membrane potentials, and (3) elevated levels of cytosolic [Ca2+]i. Therefore, these alterations mimic symptoms of the human disease PHHI. As the animals develop, the period of hyperinsulinaemic hypoglycaemia is followed in older rodents by hypoinsulinaemia and hyperglycaemia – the onset of diabetes. A high frequency of apoptotic b cells in the pre-diabetic period suggests that unregulated Ca2+ influx carries a compound pathology – underpinning inappropriate insulin secretion and then leading to the induction of premature apoptosis. The concept of b-cell ‘burn-out’ as a direct result of apoptosis has a clinical correlation. In a recent review, Glaser and colleagues18 discuss how long-term, intensive treatment of PHHI patients with somatostatin leads to remission of symptoms without the need for surgery. In such cases it seems likely that this is caused by destruction of b cells through programmed cell death. Whether this is a result of unregulated Ca2+ entry, or as a direct consequence of somatostatin, which is known to be apoptogenic32, remains to be determined. In a separate series of transgenic animal studies, a severe loss of b-cell function was also found in tissue isolated from ‘Kir6.2 knockout’ animals33. However, quite surprisingly these b cells were less reminiscent of human PHHI b cells than the insulin-secreting 149

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Figure 3. ATP-sensitive K+ (KATP) channelopathies and unregulated insulin secretion. (A) Persistent hyperinsulinaemic hypoglycaemia of infancy (PHHI) arises from defects (*) in either the KIR6.2 or SUR1 genes. Because these b cells lack operational KATP channels, the membrane potential remains depolarized (approximately –30 mV) in the absence of glucose metabolism. This leads to the persistent activation of voltage-dependent Ca2+ channels, causing unregulated entry of Ca2+ and persistent release of insulin as a consequence. Because functional KATP channels are absent, patients with PHHI are unresponsive to medical treatment with the KATP channel agonist diazoxide. (B) Hyperinsulinaemic hypoglycaemia (HH) arises from acquired loss of channel function. HH b cells do not express defects in KATP channel genes, but owing to defects (*) in either glucokinase (GK) or glutamate dehydrogenase (GDH) augmented metabolism leads to excessive ATP production and the enforced closure of KATP channels. This causes inappropriate membrane depolarization and unregulated Ca2+ entry. Because KATP channels are functional, diazoxide is effective in the clinical management of the HH syndromes, GK-HH and GDH-HH.

cells isolated from dominant-negative mice. Another interesting feature of the knockout mouse model is that despite having impaired glucose-dependent insulin release, both in vitro and in vivo, they present normal postprandial blood glucose values33. This might suggest a negative correlation between Kir6.2 operation and insulin receptor expression in skeletal muscle tissue, which will tend to improve peripheral glucose clearance, despite impaired insulin secretion. Hyperinsulinaemic Hypoglycaemia (HH) Some children with congenital hyperinsulinism present with milder symptoms of hyperinsulinaemic-induced hypoglycaemia than described for PHHI, often with episodes of hypoglycaemia that are sporadic and occur postprandially20. In addition, some patients present symptoms much later in life (even in adulthood) and when treated with diazoxide they are invariably medically responsive. In these cases, hyperinsulinism can be inherited in an autosomally dominant fashion and is not linked to defects in

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the SUR1 or KIR6.2 genes34. So far, mutations in two other genes, both associated with glucose homeostasis in b cells, have now been shown to give rise to clinically distinct forms of HH: GKHH and GDH-HH (Fig. 3B). GK-HH arises from defects in the glucokinase gene (GK)35. Glucokinase is a key rate-limiting enzyme in glucose metabolism and thereby determines the level of glucose at which the secretion of insulin is terminated (Fig. 1). Gene mutations that decrease the sensitivity of the enzyme for glucose are known to give rise to maturityonset diabetes of the young (MODY), an autosomal dominant condition. By contrast, the mutations described in GK-HH patients result in the generation of an ‘activated’ gene product with a markedly increased sensitivity to glucose. It has been suggested that these mutations result in excessive ATP production in b cells and the inappropriate closure of KATP channels35. GDH-HH has been described in patients who present with a clinical phenotype of hyperinsulinism associated with hyperammonaemia. Several defects in the glutamate dehydrogenase

(GLUD1) gene have been described36. Some of the cases were found to be sporadic, with new mutations identified in the proband whereas, in other families, autosomal dominant inheritance was demonstrated. GTP normally acts as an allosteric inhibitor of glutamate dehydrogenase. GLUD1 gene mutations result in decreased sensitivity of the mature enzyme to inhibition by GTP and produces an ‘activated enzyme’. In the liver, this results in excessive ammonia production, while in the b cells, following dietary intake leucine-induced stimulation of glutamate dehydrogenase is presumed to increase the flux of glutamine into the Krebs cycle. As in the case of GKHH, the predicted b-cell response to these mutations is an increase in intracellular ATP concentration and the enforced closure of KATP channels (Fig. 3B). This results in a constant membrane depolarization and uncontrolled voltage-gated Ca2+ channel activity. However, in contrast to PHHI, because the KATP channels are intact, patients with HH are clinically responsive to diazoxide and therefore do not require surgery. TEM Vol. 10, No. 4, 1999

Diabetes Diabetes mellitus is a group of diseases characterized by high levels of blood glucose resulting from defects in insulin secretion, insulin action, or both. Type 1 disease is a polygenic condition resulting from autoimmune destruction of pancreatic b cells. The insulin insufficiency is therefore absolute and treatment requires exogenous administration of insulin. Type 2 disease is more likely to be associated with defects in depolarizationresponse coupling, as the condition arises from both insufficient insulin production and target organ insulin mishandling. This polygenic disease is often associated with obesity and other lifestyle factors and accounts for some 75% of all cases of diabetes. Finally, Type 3 disease or MODY is a condition that accounts for 10% of all cases of Type 2 diabetes. Unlike the other forms of diabetes, MODY is a monogenic condition with at least five different candidate genes (including GK) linked to the onset of the disorder, which is inherited in an autosomal dominant manner with high penetrance. Despite intensive investigation of the candidate genes of glucose metabolism, few gene variations have been identified that have an important role in the onset of Type 1 diabetes. A pathogenic role for the KATP channel is implicated by virtue of the important role these channels play in depolarization-response coupling, and because diabetes is commonly associated with insulin hypersecretion in the early stages. Genomic screening of white Caucasians of American, British, French and Danish origins have revealed that genetic defects in SUR1 are indeed associated with impaired insulin secretion37–39. However, these gene polymorphisms appear to have little functional effect on KATP channel activity, and occur at a similar frequency in the non-diabetic population. KIR6.2 was also thought to be an unlikely major diabetogene40–42, but a recent study of Type 2 disease in French Caucasians suggests a positive role of KIR6.2 in the polygenetic basis of this condition43. TEM Vol. 10, No. 4, 1999

In contrast to our understanding of K+ channel dysfunction and PHHI, in vitro studies of human diabetic b cells are rare, as access to isolated, functional tissue is exceptional. Nevertheless, two studies have reported defects in K+ channel operation in disease tissue. However, as the clinical information was limited in both of these studies, it is not possible to deduce whether defects such as the appearance of a novel ATP-activated K+ channel44 or the loss of KATP channels (M.J. Dunne et al., unpublished) are the result of genetic predisposition, changes in the cell phenotype in association with the diabetic state, or indeed both. • Future Directions Progress in the areas covered in this review has been impressive over the past few years, yet many key questions remain. Are there additional KATP channel subunits to be discovered? How do nucleotides bring about opening and closure of channels after interactions with the subunits? How are the subunits associated and targeted to the cell membrane? The genetics of PHHI are still far from resolved; autosomal recessive and dominant inheritance have been described, as well as loss of maternal imprinted genes. How well does this complex genetic heterogeneity relate to the clinical phenotype? As more SUR1 and KIR6.2 gene defects are described, how do these mutations influence the production of KATP channels? Is PHHI simply loss of KATP channels in b cells, or is it a systemic disorder? Are defects in other aspects of b-cell function associated with the onset of HH? And finally, is it possible to integrate a greater understanding of the biology of these rare conditions with the establishment of new and better means of treatment45? Our greatest challenges, however, lie with diabetes. One and a half million people in the UK (3% of the population) have diabetes, and treatment regimens give only partial protection against disabling late complications, including blindness, renal failure and coronary heart disease. Diabetes is an incurable disease, and its incidence is rising. Is it possible, therefore, to resolve the genetic basis of this

disease and fully understand the relationship between genes and cellular dysfunction? For monogenic diseases this process has begun, but for diabetes the challenges are even greater, considering the far greater complexities of genetic and ethnic origin, environmental influences and individual host factors. • Acknowledgements Work in our laboratories is supported by the British Diabetic Association (MJD), the Medical Research Council (MJD) and the Economic Union (MJD, CÄ); and by Stiftelsen Clas Groschinskys minnesfond, Tore Nilssons Stiftelse, Åke Wibergs Stiftelse, Wilhelm och Martina Lundgrens vetenskapsfond, Swedish Medical Research Council (CÄ). KEC was supported by a Medical Research Council Studentship. References 1 Dunne, M.J., Aynsley-Green, A. and Lindley, K.J. (1997) Nature’s KATP channel knockout; PHHI a K+ channel disorder of pancreatic b-cells leading to hypersecretion of insulin. News Physiol. Sci. 12, 197–203 2 Ashcroft, F.M. and Gribble, F.M. (1998) Correlating structure and function in ATP-sensitive K+ channels. Trends Neurosci. 21, 288–294 3 Aguilar-Bryan, L. et al. (1995) Cloning of the beta cell high-affinity sulphonylurea receptor: a regulator of insulin secretion. Science 268, 423–426 4 Inagaki, N. et al. (1995) A family of sulphonylurea receptors determines the pharmacological properties of ATP-sensitive K+ channels. Neuron 16, 1011–1017 5 Aguilar-Bryan, L., Clement, J.P., IV, Gonzalez, G., Kunjilwar, K., Babenko, A. and Bryan, J. (1998) Toward understanding the assembly and structure of KATP channels. Physiol. Rev. 78, 227–245 6 Inagaki, N. et al. (1995) Reconstitution of IKATP: an inward rectifier subunit plus the sulphonylurea receptor. Science 270, 1165–1170 7 Sakura, H., Ammala, C., Smith, P.A., Gribble, F.M. and Ashcroft, F.M. (1995) Cloning and functional expression of the cDNA encoding a novel ATP-sensitive potassium channel expressed in pancreatic b cells, brain, heart and skeletal muscle. FEBS Lett. 377, 338–344 8 Clement, J.P., IV et al. (1997) Association and stoichiometry of K(ATP) channel subunits. Neuron 18, 827–838 9 Shyng, S.L. and Nichols, C.G. (1997) Octameric stoichiometry of the KATP channel complex. J. Gen. Physiol. 110, 655–664 10 Ribalet, B., Mirell, C.J., Johnson, D.G. and Levin, S.R. (1996) Sulfonylurea binding

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Correspondence The articles in Trends in Endocrinology and Metabolism cover recent progress in the field of endocrinology and often include novel ideas that challenge existing paradigms. Although articles are peer reviewed, considered speculation is encouraged, with a view to stimulating debate centred on new observations and techniques. Naturally you may not always agree with the points of view expressed in TEM. If so, we would be happy to receive your comments for potential publication as Letters to the Editor. Please send all correspondence to: Helen Carroll, Editorial Assistant, Elsevier Trends Journals, 68 Hills Road, Cambridge, UK CB2 1LA Tel: +44 1223 315961 Fax: +44 1223 464430 E-mail: [email protected]

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