Type 2 diabetes mellitus not quite exciting enough.pdf

8/13/2019 Type 2 diabetes mellitus not quite exciting enough.pdf

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Type 2 diabetes mellitus: not quiteexciting enough?

Frances Ashcroft1,* and Patrik Rorsman2

1University Laboratory of Physiology, Parks Road, Oxford OX1 3PT, UK and 2Oxford Centre for Diabetes,

Endocrinology and Metabolism, Churchill Hospital, Oxford OX3 7LJ, UK

Received December 12, 2003; Revised and Accepted January 12, 2004

Type 2 diabetes mellitus is a serious metabolic disease that afflicts around 5% of the population in Westernsocieties and over 150 million people worldwide. It is characterized by elevation of the blood glucoseconcentration, usually presents in middle age, and is exacerbated by obesity. Both genetic and environ-

mental factors contribute to the disease but in the vast majority of cases the aetiology is still not understood.Here we present a novel hypothesis for the aetiology of type 2 diabetes. We postulate that the electrical

activity of the insulin-secreting b-cells of the pancreas acts to integrate the genetic and environmentalfactors that predispose to disease risk. Our hypothesis is supported by a substantial amount of data gathered

from a range of different disciplines and makes predictions that can be tested experimentally both in vitroand in man.

INTRODUCTION

Natural history of type 2 diabetes

Diabetes mellitus refers to a range of conditions that are allcharacterized by elevation of the blood glucose level. It may beroughly divided into two principal varieties, type 1 and type 2

diabetes, which have very different aetiologies (1). Type 1diabetes presents in childhood as an autoimmune attack on thepancreatic b-cells that results in their complete destruction:consequently, the patient must take insulin for the rest of their life(2). It accounts for


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a decrease in b-cell mass of>50% is require to cause diabetes(25) and hyperglycaemia does not occur in type-1 diabetics aslong as b-cell mass remains above 3050% (26). It thereforeseems that the insulin secretory defect in type 2 diabetes doesnot result primarily from insufficient b-cell mass but ratherfrom impaired insulin secretion.

Electrical activity and insulin release

Electrical activity plays a critical role in the regulation of insulinsecretion (27). No insulin is secreted in the absence of b-cellelectrical activity, and there is a direct correlation between theextent ofb-cell electrical activity, once initiated, and the amountof insulin secretion (28). Thus changes in electrical activity areimmediately mirrored by changes in insulin secretion. Thestimulatory actions of almost all insulin secretagogues, includingglucose, neurotransmitters and hormones, converge at the levelof electrical activity, which serves to integrate the effects ofall these agents. Although most secretagogues have additionaldownstream effects on insulin secretion (29,30), none are effec-

tive in the absence of electrical activity. Thus, b-cell electricalactivity is an essential step in insulin release.In the b-cell, electrical activity is determined by a complex

interplay between several different types of ion channels (27).Tiny changes in the activity of these ion channels can havedramatic effects on electrical activity and thus on insulin sec-retion (Fig. 1). This is especially the case around the thresholdfor electrical activity, where almost undetectable changes incurrent amplitude can make the difference between firing andnon-firing, and consequently between insulin secretion and nosecretion at all. Thus, changes in ionic currents switch electricalactivity (and insulin release) on and off, as the blood glucoseconcentration fluctuates around the fasting blood glucose level.Because electrical activity is so responsive to tiny changes in

current amplitude, it also serves as an exquisitely sensitivedetector of small changes in b-cell metabolism and bloodhormone levels (which regulate ion channel activity).

Here, we propose that the insulin secretory defect that occursin type 2 diabetes is the result of inadequate b-cell electricalactivity in response to secretagogues. This reduction inelectrical activity may result from polymorphisms in genesencoding ion channels, or in genes that regulate ion channelfunction (e.g. metabolic genes), membrane targeting orexpression (e.g. transcription factors), as well as environmentalfactors such as age and obesity. Evidence in support of thishypothesis is accumulating, and includes much recent data,which are summarized here.

Mechanism of insulin release

Insulin is stored in secretory vesicles, which are released inresponse to elevation of intracellular Ca2. This resultsprincipally from Ca2 influx across the plasma membranethrough voltage-gated Ca2 channels (27). Opening of thesechannels is triggered by the electrical activity of the cell.Glucose modulates electrical activity principally via metabo-lically induced changes in the activity of ATP-sensitive K

(KATP) channels (31). In the absence of glucose (Fig. 2A), thesechannels are open and the ensuing K efflux keeps themembrane hyperpolarised so that voltage-gated Ca2 channels

remain shut. Glucose metabolism closes KATP channels and

depolarizes the b-cell (31). In turn, this leads to opening ofvoltage-gated Ca2 channels and initiation of electrical activity(Fig. 2B). The mechanism by which KATP channel closure iscoupled to glucose metabolism remains unclear, although it isbelieved to involve changes in the intracellular concentrationsof the adenine nucleotides ATP and ADP (32). The KATPchannel is also the molecular target for common antidiabeticdrugs, such as the sulfonylureas (33), which have been widelyused to treat type 2 diabetes for more than half a century (34).Sulfonylureas close these channels independently of glucosemetabolism, and thereby elicit electrical activity and insulinsecretion (32,35).

Figure 1.The input resistance of the b-cell membrane determines the ease withwhich electrical activity can be initiated. It is largely determined by the activityof the KATPchannel and is low when KATP channels are open and high whenthey are shut. From Ohms Law (VIR), it is evident that the same magnitudeof current (I) will produce a much greater change in membrane potential (DV),and thus in insulin secretion, when KATP channels are closed (R is high) thanwhen they are open (R is low). At low glucose (blue trace) the input resistanceof the membrane (Ri) is low, so that injection of a small current (I, black traceabove) only depolarizes the membrane by a small amount (DV1). At high glu-cose (red trace), the input resistance is higher (5Ri), so that the same currentdepolarizes the cell by a much larger amount (DV2), and may be sufficient totrigger an action potential (dotted line). Potentiators of insulin secretion suchas acetylcholine and arginine produce small inward currents (126,127). Theyare therefore ineffective in the absence of glucose, when the activity of the

KATP-channels is high (i.e. R is low), but are able to stimulate electrical activityand insulin secretion at glucose concentrations that shut most KATPchannels(i.e. R is high) (125128). In vivo, blood glucose levels rarely fall below34mM in man and thus the b-cell input resistance is always relatively high.This explains why acetylcholine, which is released in response to the sightand smell of food, is able stimulate insulin secretion even before blood glucoselevels rise. Likewise, the presence of food in the gut releases incretins, such asGLP-1 and GIP, which also initiate insulin secretion prior to elevation of bloodglucose. Because arginine and GLP-1 can stimulate insulin secretion even intype 2 diabetes, the input resistance must already be relatively high at fasting

blood glucose concentrations. However, the fact that these agents are less effec-tive in diabetes also suggests that, at the same glucose concentration, the inputresistance is not as high as that in non-diabetic b-cells.

R22 Human Molecular Genetics, 2004, Vol. 13, Review Issue 1


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Glucose not only initiates b-cell electrical activity: it alsomodulates action potential frequency, and thereby Ca2-influxand insulin secretion. Electrical activity exhibits a characteristicbursting pattern, which consists of slow oscillations in mem-brane potential between a depolarized plateau, on which Ca2-dependent action potentials are superimposed, and a hyperpolar-ized electrically silent interval (Fig. 2C) (27,28). As glucose isincreased, the duration of the plateau phase increases and that ofthe silent interval decreases so that electrical activity becomescontinuous at 20 mM glucose (36). This produces a graded

increase in action potential firing and Ca2

influx that accountsfor the glucose-dependence of insulin secretion (EC50, 1015 mM glucose). A complex interaction between KATPchannels,voltage-gated Ca2 channels, Ca2-activated Kchannels, Ca2

pumps and metabolism is responsible for this bursting pattern ofb-cell electrical activity and its modulation by glucose (27,37).

Theb-cells are electrically coupled to one another, so that theislet functions as an electrical syncytium (3840). This servesto coordinate electrical activity and insulin secretion across theislet and accounts for the fact that glucose-stimulated insulinsecretion from a single islet is pulsatile and mirrors the slowwaves of electrical activity (41).

In addition to glucose, insulin secretion is stimulatedby amino acids (e.g. arginine), incretins [e.g. glucagon-likepeptide 1 (GLP-1)], neurotransmitters (e.g. acetylcholine) anddrugs (e.g. sulfonylureas) (27). Some of these agents, likesulfonylureas, can stimulate release in the absence of glucosebecause they close KATP channels, depolarize the b-cellmembrane and elicit electrical activity. In contrast, incretinsand neurotransmitters are ineffective in the absence of glucose,but are able to amplify insulin secretion produced by aninitiator and can even initiate insulin secretion at glucose

concentrations just below threshold. Their action is dependenton the ambient level of KATP channel activity, which conferstheir dependence on the glucose concentration (Fig. 1).

WHY IS ELECTRICAL ACTIVITY USED TO

CONTROL INSULIN RELEASE?

Why is electrical activity used to control insulin release? Afterall, it would be possible to achieve a graded response to glucosesimply by direct metabolic regulation of exocytosis. Indeed,glucose is still able to elicit a graded increase in insulin

Figure 2. Role of ion channels in regulating b-cell electrical activity and insulin secretion. Insulin secretion is triggered by an increase in intracellular calcium([Ca

2]i), which is, in turn, regulated by the electrical activity of the b-cell. When metabolism is low (A), KATPchannels are open keeping the membrane hyper-polarized and Ca2 channels closed, so that [Ca2]iremains low. When metabolism increases (B), KATPchannels shut depolarizing the b-cell membrane potential,thereby eliciting electrical activity and opening voltage-gated Ca2channels. This leads to Ca2 influx and an increase in [Ca2]ithat triggers exocytosis of insulingranules. (C) Membrane potential recorded from a b-cell within a mouse islet at 5 mMand 10 mM glucose, concentrations above (10 mM) and below (5 mM) thethreshold for electrical activity and insulin release. Electrical activity consists of bursts (slow waves) of Ca

2action potentials during which Ca2enters the cell and

triggers insulin secretion (see text for further details).

Human Molecular Genetics, 2004, Vol. 13, Review Issue 1 R23


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secretion when changes in electrical activity are bypassed: forexample, by continuous depolarization of the b-cell (42,43).The answer seems to be that electrical activity enables insulinsecretion to be switched on and off quickly and precisely. Thisis particularly important in the case of insulin, as inappropriaterelease of the hormone can lead to life-threatening hypogly-

caemia. Hypoglycaemia is a common complication in diabeticpatients taking sulfonylureas, or patients with congenitalhyperinsulinaemia, where insulin secretion cannot be switchedoff rapidly when blood glucose levels fall. The rapid onoffswitch of insulin secretion provided by electrical activity alsoenables insulin secretion to be pulsatile (44), and so avoidsdown-regulation of insulin receptors in target tissues. This mayexplain why metabolic regulation of insulin secretion alone isnot sufficient to control blood glucose and underscores theimportance of electrical activity in insulin release.

DO CHANGES IN ELECTRICAL ACTIVITY

UNDERLIE THE IMPAIRED INSULINSECRETION IN TYPE 2 DIABETES?

b-Cell dysfunction in type 2 diabetes manifests as a reductionin the insulin secretory response not only to glucose, but alsoto potentiators of insulin release such arginine, hormones,incretins and sulfonylureas (4548). Any mechanistic explana-tion of the disease must also account for all these secretoryabnormalities. The permissive role of electrical activity inmodulating insulin secretion to both initiators and potentiatorsof release suggests that the abnormalities in hormone releasefound in type 2 diabetes might be secondary to changes inelectrical activity.

The most direct way to test our hypothesis would be to

compare electrical activity in non-diabetic and diabetic humanb-cells. Unfortunately, such in vitro data are extremely limited,because of the difficulty in obtaining viable islets from type 2diabetics. However, we recently found that glucose fails toelicit electrical activity, Ca2 influx and insulin secretionin b-cells and islets isolated from a type 2 diabetic (unpub-lished data). This is consistent with earlier reports that glucosedid not elevate intracellular Ca2 (49), and that glucose-stimulated insulin secretion was impaired (50), in type 2diabetic islets.

Clinical studies are also consistent with the idea thatb-cellelectrical activity may be impaired in type 2 diabetes. The factthat sulfonylureas and glinides, which close KATP channels(32), are effective in the treatment of type 2 diabetes suggests

that KATP-channel closure is not complete in diabetes. On theother hand, the finding that arginine and GLP-1 stimulateinsulin secretion in type 2 diabetic patients (4547) argues thateven in diabetic b-cells most KATP channels must be closed atstimulatory glucose concentrations. This is because theseagents are only effective when the membrane resistance ishigh (Fig. 1). As neither potentiator is as potent in type 2diabetics as in healthy individuals (4547), however, membraneresistance and electrical activity are likely to be reduced. Thus,we hypothesize that glucose intolerance and type 2 diabetes areassociated with a lower electrical resistance of the b-cellmembrane in the presence of stimulatory glucose concentra-

tions (compared with the non-diabetic b-cell), which leads toreduced Ca2-dependent electrical activity and insulin release.

DIABETES CORRELATES WITH MUTATIONS/

POLYMORPHISMS IN GENES REGULATING

ISLET CELL ELECTRICAL ACTIVITY

It is well established that type 2 diabetes is a polygenic disease(7). It has been linked to polymorphisms in many genes, buthow these polymorphisms relate to one another, and to thedisease phenotype, has not been established. We postulate thatthe collective action of several common gene variants, andlifestyle factors, combine to produce a small decrease in b-cellelectrical activity, and thus a reduction in insulin secretion. Weanticipate that the functional consequences of each individualgene variant will be small, so that a single polymorphism, byitself, is unlikely to result in diabetes. However, the cumulativeeffect of several such polymorphisms will increase disease risk,and in combination with age and/or obesity lead to overt

disease. In effect, electrical activity serves as a bottleneck atwhich the effects of many different genes and lifestyle factorsconverge.

Impaired ion channels

Polymorphisms in genes encoding ion channels are obviouscausative candidates for the reduction in electrical activity thatwe propose underlies type 2 diabetes. Importantly, a numberof such polymorphisms have been identified. One that hasreceived much recent attention is a polymorphism (E23K) inthe Kir6.2 subunit of the KATP channel, with a prevalence of34% in Caucasians (51). Although early studies failed to showa significant association with type 2 diabetes (51,52), meta-

analysis and more recent studies using larger sample sizes haverevealed that the K allele accounts for between 11 and 15% ofthe population risk of type 2 diabetes in Caucasians, with anodds ratio of around 1.5 (5359). When heterologouslyexpressed in mammalian cells, the E23K polymorphism leadsto a 2-fold reduction in the ATP sensitivity of the KATPchannel(54) and enhanced activation by MgGDP (55,60). It is thereforeexpected to reduce b-cell electrical activity and insulinsecretion. That such small changes in KATP-channel regulationcan indeed exert a marked effect on b-cell function isexemplified by the fact that a 4-fold reduction in KATPchannelATP sensitivity causes severe neonatal diabetes in transgenicmice (61). The functional effects of the E23K polymorphismin man are controversial. Some studies report that insulin

secretion is impaired (56), whereas others have failed to find adifference in insulin secretion (62,63), but report that the abilityof glucose to inhibit glucagon secretion is impaired (63). Itis possible that these differences relate to the genetic back-ground of the patients examined, but more studies are clearlyneeded. Polymorphisms in SUR1 (e.g. S1369A; 59) have alsobeen linked to type 2 diabetes, but as yet no functionalconsequences of these polymorphisms have been identified.

Overactivity of KATP-channels is not the only mechanism thatmay be expected to impair electrical activity. Polymorphisms ingenes encoding the voltage-gated Ca2- and K -channelsinvolved in generating b-cell action potentials that lead to



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decreased or increased channel activity, respectively, could alsoreduce b-cell electrical activity. Indeed, overexpression of thevoltage-gated K-channel Kv1.5 decreases the secretoryresponse of islets (64), and deletion of the L-type Ca2-channelin mice results in reduced insulin secretion and glucoseintolerance (65). Furthermore, a 7 mV shift in the threshold

for L-type Ca

2

-channel activation towards more negativemembrane potentials enhanced b-cell electrical activity (66),and led to increased serum insulin levels and hypoglycaemiain mice (67). This suggests that a small positive shift inthe activation threshold for these channels might increase thethreshold for glucose-dependent electrical activity and thusimpair insulin secretion. There is also some evidence thatvoltage-gated Ca2 channels are influenced by b-cell metabo-lism (68), so that it is possible that polymorphisms in metabolicgenes may modulate electrical activity and insulin release viachannels other than the KATP channel. It is of interest thatthe b-subunit of the voltage-gated calcium channel (CACNB2)lies with the region of chromosome 10p that was linked topermanent neonatal diabetes in a genome-wide linkage analysis

of a large family (69). Finally, cytoplasmic Ca

2

influencesthe activity of small conductance Ca2-activated K channels,providing a feedback mechanism that links changes in Ca2

handling by the b-cell with electrical activity and insulinsecretion (7072). However, with the exception of the KATPchannel subunits Kir6.2 and SUR1, the association of poly-morphisms in genes encoding b-cell ion channels with type 2diabetes has not yet been widely studied.

Polymorphisms in genes encoding proteins that regulate ionchannel transcription, membrane targeting or activity may alsobe expected to influence b-cell electrical activity and therebyinsulin secretion. In this context it is interesting that anothercandidate gene within the chromosome 10p locus is PIP5K2A(59), a phosphatidyl 4-phosphate 5-kinase type 2, which may

be expected to influence the level of PIP2in the membrane andthereby KATP channel activity (73).

Impaired metabolic regulation of ion channels

Because metabolism regulates b-cell electrical activity, poly-morphisms in metabolic genes may also be expected toinfluence the ability of glucose to stimulate electrical activityand insulin secretion. Support for the idea that impairedmetabolic regulation of electrical activity occurs in type 2diabetes comes from a number of rare (15%) monogenicforms of diabetes (7). These are collectively referred to asmaturity-onset diabetes of the young (MODY) because theypresent early in life. The first of these to be identified

(MODY2) results from inactivating mutations in glucokinase,the high Kmenzyme that phosphorylates glucose in b-cells andis rate-limiting for glucose metabolism (10) (Fig. 3). AllMODY2 patients are heterozygotes: permanent neonataldiabetes results from homozygous mutations (74). Other formsof MODY are due to mutations in genes (e.g. HNF1a, HNF4a,IpF1) encoding a transcriptional network that regulates theexpression of several genes critical for glucose sensing (7580). Studies on knock-out animals have shown the reducedb-cell glycolytic metabolism caused by MODY mutations leadsto a decrease in KATP channel closure and electrical activity inresponse to glucose, which in turn causes less insulin release

(81,82). Mutations in mitochondrial DNA (most frequentlyA3243G in the leucine tRNA gene) cause maternally inheriteddiabetes, probably by impairing b-cell metabolism and soreducing electrical activity (8385). These account for a further12% of diabetic cases.

There is a demonstrable overlap between MODY and

multifactorial type 2 diabetes, and mutations in IpF-1(MODY4) have been associated with type 2 diabetes (86). Asyet, there is no evidence that polymorphisms in other MODYgenes contribute to type 2 diabetes. It is pertinent, however, thata mutation in the HNF-1a(MODY3) gene accelerates the onsetof type 2 diabetes in the OjiCree population by 7 years, and isfound in 40% of diabetic OjiCree patients (87). Likewise, a30G/A variant in the b-cell promoter of the glucokinase(MODY2) gene has been implicated in impaired glucosetolerance (88). Furthermore, a polymorphism in HNF4a hasrecently been found to associate with reduced risk of diabetes(59).

Mitochondrial metabolism generates substantially more ATPthan glycolysis and the production of mitochondrial ATP is

critical for both glucose-dependent insulin secretion and KATPchannel closure (85,89). It is therefore pertinent that poly-morphisms in genes that regulate mitochondrial ATP pro-duction are associated with type 2 diabetes. UCP2 is anuncoupling protein that resides in the inner mitochondrialmembrane and uncouples electron transport from ATPsynthesis (90) (Fig. 3). Thus it tends to lower cytosolic ATPlevels. Deletion of the UCP2 gene in mice enhances islet ATPproduction and insulin secretion in response to glucose (91).Conversely, overexpression of UCP2 in b-cells attenuates ATPgeneration and insulin secretion during glucose stimulation(92). This suggests that the level of UCP2 expression mayinfluence insulin secretion in man. Consistent with this idea, acommon polymorphism in the UCP2 promoter (866G/A)

causes a 2-fold increase in the risk of type 2 diabetes in obesewhite Europeans (93). The b-cell transcription factor PAX6preferentially binds to, and trans activates, the 866A variantin insulin-secreting (INS-1) cells and is expected to increaseUCP2 mRNA expression in isletb-cells, thereby reducing ATPlevels, electrical activity and insulin release. The frequency ofthe 866A variant in the European population is 37% (94),suggesting it may make a significant contribution to type 2diabetes. Recent studies further suggest that the 866A variantis associated with differences in glucose-stimulated insulinsecretion in glucose-tolerant human subjects both in vivoand inisolated islets (95).

A common variant in mitochondrial DNA itself (16189) isalso associated with type 2 diabetes (96). This variant causes

a T to C transition in a region of mtDNA that lies close tocontrol sequences governing replication and transcription. It isassociated with increased fasting plasma insulin in severalpopulations, maternal restraint of fetal growth and thinness atbirth (97). Consequently, it has been suggested that it mayserve as (one of) the gene(s) underlying the thrifty phenotypethat is thought to predispose to type 2 diabetes (98,99). It isestimated that about 4% of diabetes in the UK diabeticpopulation is attributable to this gene variant. Because itsprevalence is higher in Polynesians (93%) and Pima Indians(50%) the 16189 variant may have a substantially larger effectin these populations (100).



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Loss-of-function mutations in the transcription factor peroxi-some proliferator-activated receptor (PPARg) cause severeinsulin resistance and type 2 diabetes, but are very rare (101).A polymorphism in this gene (P12A) is found in manyethnic groups. The more common P allele (frequency 85%)is associated with a 1.25-fold increase in diabetes risk and apopulation risk of 25% (102). Although the less common A12allele is associated with a reduced risk of diabetes in thegeneral population (103), in diabetics it is associated withdecreasedb-cell function and increased disease severity (104).

In b-cells, PPARg enhances the expression (and activity) ofa number of genes involved in glucose sensing, includingglucokinase (105) and GLUT2 (106). However, it also upreg-ulates UCP2 (107). These data suggest that the A12 allele,which has reduced transcriptional activity (102), may haveopposing effects on b-cell metabolism, acting to reduce ATPlevels by decreasing glycolytic activity and at the same timetending to enhance ATP levels by decreasing expression ofUCP2. Whether or not the overall result is impaired b-cellmetabolism (and thus diabetes) will depend on the relativestrengths of these effects, and may also be influenced by thepresence of other gene variants or environmental factors.

Whatever the underlying mechanism, however, the data supportthe view that defective metabolic regulation ofb-cell electricalactivity is involved in type 2 diabetes.

THE INSIDIOUS INFLUENCE OF AGE

AND OBESITY

Any explanation of type 2 diabetes must account for the factthat disease develops with age and that it is enhanced by

obesity. There is evidence that the interaction of such life stylefactors with genetic ones may also occur at the level ofb-cellelectrical activity. In mice, for example, ageing causes areduction in the sensitivity of the KATP channel, and therebyelectrical activity, to glucose, an effect that appears to be medi-ated by decreasedb-cell metabolism rather than changes in theKATPchannel itself (81). This may reflect the well-documenteddecline in mitochondrial function with age, that is believedto result from accumulating mutations in mitochondrialDNA (84,108). A recent in vivo study showed a 40% declinein mitochondrial oxidative and phosphorylation function inmuscle between 27 and 70 years (109). In addition, ageing

Figure 3.b-Cell metabolism and the role of UCP2. Glucose enters the b-cell via the high capacity glucose transporter GLUT2 (Km50 mM) which ensures that theintracellular glucose concentration tracks the plasma glucose level. Glucose is then phosphorylated in a reaction catalysed by glucokinase (Gk), which serves as therate-limiting step in glycolysis. The importance of Gk in glucose metabolism is illustrated by the fact that mutations in Gk cause MODY2. Glycolysis convertsglucose-6-phosphate to pyruvate, which enters the mitochondria where it is metabolized within the TCA cycle and the respiratory chain, leading to the establish-ment of a proton gradient across the mitochondrial membrane. ATP is generated by the F1F0-ATPase, using energy derived from this proton gradient, and is trans-

ported to the cytoplasm. The importance of mitochondrial ATP generation in insulin secretion is illustrated by the fact that mutations in mitochondrial DNA causematernally inherited diabetes with deafness. UCP2 is a proton channel that dissipates the mitochondrial proton gradient and so reduces ATP synthesis. Changes inthe expression levels of UCP2 influence ATP levels and thereby electrical activity and insulin secretion (the higher the expression, the less ATP, electrical activity

and secretion). UCP2, uncoupling protein 2. ANT, adenine nucleotide transferase.



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reduces insulin sensitivity (8), thereby placing a greatersecretory demand on the b-cell. In part, this may also berelated to the decline in mitochondrial function (109).

Obesitypromotes insulin resistance, which can lead to insulininsufficiency if the secretory capacity of the b-cell is alreadylower than normal. However, obesity may also reduce insulin

secretion via changes in b-cell electrical activity. Obeseindividuals (110) and type-2 diabetics (111) have highercirculating levels of free fatty acids (FFAs), which are taken upand metabolized by b-cells. Chronic exposure to FFAs leads tothe accumulation of long chain acyl CoAs (LC-CoAs) withinthe b-cell. LC-CoAs both enhance KATP channel activity andreduce its ATP sensitivity (112,113), thereby reducing glucose-dependent closure of KATP channels, electrical activity andinsulin secretion. KATP channel activity may also be enhancedby up-regulation of UCP2 in obesity and a consequent decreasein ATP synthesis (91,114,115). This is mediated by FFAstimulation of UCP2 expression (116), probably via PPARg(107). Mice that lack UCP2 show an enhanced insulin secretorycapacity compared with wild-type mice when maintained on a

high-fat diet (114).Changes inb-cell metabolism as a consequence of age and/orobesity will translate into reducedb-cell electrical activity andinsulin secretion, not only to glucose but also to incretins andsulfonylureas.

THE YIN AND YANG OF b-CELL

ELECTRICAL ACTIVITY

Finally, we point out that diabetes may not only result frommutations that reduce electrical activity. It is also possible formutations that lead to excessive b-cell electrical activity to

cause diabetes, albeit by a different mechanism. Congenitalhyperinsulinism (CHI) is associated with constant b-celldepolarization due to permanent KATP channel closure (117).This results from loss-of-function mutations in genes encodingthe b-cell KATP channel subunits Kir6.2 and SUR1, or acti-vating mutations in the metabolic enzymes glucokinase andpyruvate dehydrogenase (118,119). The unregulated insulinsecretion characteristic of CHI necessitates subtotal pancrea-tectomy early in life in most patients. However, a dominantmutation in SUR1 produces a milder disease that can bemanaged without surgery (120). Interestingly, many of thesepatients develop a progressive insulin insufficiency anddiabetes in later life. A similar pattern is seen in mice inwhich the Kir6.2 gene has been disrupted in b-cells (121).

These cells are continuously electrically active and showenhanced apoptosis, probably as a consequence of increasedCa2 influx. It therefore seems possible that b-cell lossunderlies the diabetes of CHI patients, an idea that is supportedby studies showing increased b-cell apoptosis in pancreatictissue removed from patients with focal CHI (122,123). Thus,although it is clear that common type 2 diabetes is notassociated with a reduced b-cell mass (see earlier), in someunusual forms of the disease it appears possible that excessiveelectrical activity leads to b-cell destruction and therebyreduced insulin secretion. However, not all dominant CHImutations result in diabetes in later life (124).

A MODEL FOR TYPE 2 DIABETES

We propose that individuals at risk of type 2 diabetes carry one ormore polymorphisms in ion channel genes, or in genes regulatingtheir activity, membrane targeting or transcription. The functionaleffect of these gene variants is a small reduction in b-cellelectrical activity, which results in decreased Ca2 influx and

insulin secretion (Fig. 4). This does not cause diabetes in earlylife, because glucose homeostasis is tightly controlled andfeedback loops ensure that the b-cell secretory output is adjusted.With age, b-cell metabolic function declines, leading to areduction in glucose sensitivity, electrical activity and insulinsecretion. In non-diabetics this does not pose a problem, as theb-cell adjusts its secretory output. However, in individualswho already have reducedb-cell function, the further reductionin electrical activity means that b-cell is no longer able tocompensate, so that insulin secretion declines and glucoseintolerance, and subsequently overt diabetes, develop. Obesityexacerbates the situation both by causing insulin resistance(increasing the demand on the b-cell) and by further decreasinginsulin secretion from the b-cell. This is mediated, at least in

part, by a reduction in b-cell electrical activity. The cellulardefects culminating in reduced electrical activity and insulinsecretion have been analysed by mathematical modelling (125).It seems possible that, once sufficient experimental data havebeen accumulated, such models may help to determine in silicohow combinations of several gene variants, that individually onlyhave minor effects on b-cell metabolism, electrical activity orsecretion, can collectively lead to type 2 diabetes.

Predictions

To be of value, a hypothesis should make a number of testablepredictions. Our hypothesis predicts that:

The risk of type 2 diabetes should be increased in individualswho carry disease-promoting polymorphisms in one or moregenes. The more polymorphisms an individual carries, thegreater the risk of type 2 diabetes and the earlier the onset ofthe disease.

These polymorphisms will be concentrated in genes that,directly or indirectly, regulate b-cell electrical activity. Thisis not limited to genes encoding ion channels but includesgenes that regulate ion channel activity (e.g. metabolicgenes, kinases), membrane trafficking and expression (e.g.transcription factors).

Detailed analysis of genes associated with permanentneonatal diabetes, or MODY, will be of value, as is itspossible that mutations producing severe functional effects

cause neonatal diabetes, whereas polymorphisms havingonly mild effects enhance susceptibility to type 2 diabetes.

Glucose will stimulate electrical activity less effectively, if atall, in type 2 diabetic b-cells than in non-diabetic b-cells.Likewise, glucose will cause less elevation of [Ca2]i.

Although sulfonylureas, arginine and incretins will stillstimulate electrical activity in diabetic b-cells, they will beless effective than in non-diabetic b-cells (at the sameglucose concentration).

The ability of glucose to close KATP channels and stimulateelectrical activity will decline with age in both diabeticand non-diabetic b-cells, as a consequence of increasing



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impairment of mitochondrial function. At the same age,diabetes-prone individuals may have a lower mitochondrialfunction than those not at risk of the disease.

Obesity will result in a reduced ability of glucose tostimulate electrical activity in b-cells. Cells that possess similar glucose-sensing properties to b-

cells, such as the GLP-1 producing L-cells and (probably)glucose-sensing neurones in the brain, will also showimpaired electrical activity and disturbed hormonal secretionin type 2 diabetes. Similar candidate genes will be involved.

When assessing the effects of gene polymorphisms on diabetesrisk, it is important to be aware that large sample sizes will beessential, as gene variants are likely to have small effects onb-cell function. This holds for functional as well as geneticstudies. Furthermore, some gene variants will enhance diseaserisk, while others may reduce it. Indeed, a single gene variantmay have both beneficial and deleterious effects on risk,resulting from its expression in different tissues. Such multi-plicity of actions may help explain why the aetiology ofdiabetes has proved so difficult to sort out.

CONCLUDING REMARKS

The data discussed here are consistent with the idea thatimpaired regulation of b-cell electrical activity explains thedefective insulin secretion found in type 2 diabetes. Electricalactivity serves as a common conduit through which effects ofpolymorphisms in genes involved in metabolism, and lifestylefactors such as age and obesity are integrated and translatedinto changes in insulin release. Changes in electrical activity

can account not only for impaired insulin secretion in responseto glucose, but also for the reduced response to incretins,neurotransmitters and sulfonylureas that occurs in type 2diabetes. We hope that our hypothesis will provoke debate andexperiment, and provide increased impetus for the study of type2 diabetic islets. It will stand or fall on the fruits of thisresearch.

ACKNOWLEDGEMENTS

We thank our colleagues in Oxford and Lund for muchstimulating debate and criticism. The work of our groups is

supported by the Wellcome Trust, the Royal Society, DiabetesUK, the European Union, Juvenile Diabetes ResearchFoundation, the Swedish Research Council, the Swedish

Diabetic Association and the Goran Gustafsson Foundation inNatural Sciences and Medicine. F.M.A. is a Royal SocietyResearch Professor.

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