The HNF-1alpha-SNARE connection

Maturity-onset diabetes of the young (MODY) is a monogenic form of type 2 diabetes mellitus that is characterized by impairment of glucose-stimulated insulin secretion from pancreatic beta-cells. We previously reported that heterozygous mutations of the hepatocyte nuclear factor (HNF)-1alpha gene cause a form of MODY (MODY3). We have subsequently found that collectrin, a recently cloned kidney-specific gene of unknown function, is a novel target of HNF-1alpha in pancreatic beta-cells. In addition, we have demonstrated that collectrin forms a complex with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex by direct interaction with snapin, a protein that is thought to be involved in neurotransmission by binding to synaptosomal-associated protein, 25 KD (SNAP25). Collectrin favours the formation of SNARE complexes and controls insulin exocytosis.     

New insights into the molecular mechanisms of priming of insulin exocytosis

Exocytosis of insulin vesicles in the pancreatic β-cell involves a sequence of regulated events, whose normal function and efficient adaptation to increased demand are essential for the maintenance of glucose homeostasis. These exocytotic events comprise the trafficking and docking of vesicles to the plasma membrane, followed by fusion triggered by Ca²⁺. Recent studies have unravelled post-docking steps mediated by novel factors, which, by their interactions with soluble N -ethylmaleimide-sensitive factor attachment protein receptor (SNARE)- and SNARE-associated proteins, confer the docked vesicles fusion competence. These priming steps define the releasable pool of insulin vesicles, which accounts for the first phase of insulin secretion, and controls the rate at which vesicles are replenished for the second phase of secretion. This article aims to summarize what is currently known about the mechanisms that underlie the priming activity of these proteins, focusing on Munc13, a topic to which we have made some recent contributions. Abnormal glucose homeostasis in type 2 diabetes is because of the failure of islet β-cells to augment insulin secretion sufficiently to compensate for reduced insulin sensitivity. A better understanding of the priming steps may help develop novel approaches to increase insulin secretory capacity and thereby prevent the progression to type 2 diabetes.     

Clinical and genetic features of childhood-onset Type 2 diabetes in Japan

We aimed to define the detailed clinical features of Japanese childhood-onset Type 2 diabetes mellitus (T2DM) patients who were followed-up, and to determine whether discernable characteristics were dissimilar or not from those of adult- and childhood-onset T2DM in other countries. Subjects were 22 patients (10 males and 12 females) under treatment without HNF-1α or mitochondrial gene mutations, and who were apparently diagnosed as diabetic when less than 15 years of age. Body mass indexes at onset in boys and girls were 25.8 ± 6.3 and 24.7 ± 3.6, respectively, with mean ages 13.3 ± 1.7 and 12.8 ± 2.0 years, respectively. Most patients had a short diabetic duration that required insulin treatment. One or both parents of 18 of the 22 T2DM subjects were diabetic and 7 subjects had a history of diabetes in their family across three generations. We demonstrated that a relatively large number of Japanese childhood-onset T2DM cases have a strong genetic factor, and are not necessarily related to excessive obesity. Furthermore, most required insulin therapy in the initial stages because of insufficient pancreatic β-cell reserves. This suggests that malfunction of pancreatic β-cells triggers hyperglycemia resulting in the requirement for insulin in Japanese some childhood-onset T2DM patients.     

Towards better understanding of the contributions of overwork and glucotoxicity to the β-cell inadequacy of type 2 diabetes

Type 2 diabetes (T2D) is characterized by reduction of β-cell mass and dysfunctional insulin secretion. Understanding β-cell phenotype changes as T2D progresses should help explain these abnormalities. The normal phenotype should differ from the state of overwork when β-cells compensate for insulin resistance to keep glucose levels normal. When only mild hyperglycaemia develops, β-cells are subjected to glucotoxicity. As hyperglycaemia becomes more severe, so does glucotoxicity. β-Cells in all four of these situations should have separate phenotypes. When assessing phenotype with gene expression, isolated islets have artefacts resulting from the trauma of isolation and hypoxia of islet cores. An advantage comes from laser capture microdissection (LCM), which obtains β-cell-rich tissue from pancreatic frozen sections. Valuable data can be obtained from animal models, but the real goal is human β-cells. Our experience with LCM and gene arrays on frozen pancreatic sections from cadaver donors with T2D and controls is described. Although valuable data was obtained, we predict that the approach of taking fresh samples at the time of surgery is an even greater opportunity to markedly advance our understanding of how β-cell phenotype evolves as T2D develops and progresses.     

Role of the M3 muscarinic acetylcholine receptor in β-cell function and glucose homeostasis

The release of insufficient amounts of insulin in the presence of elevated blood glucose levels is one of the key features of type 2 diabetes. Various lines of evidence indicate that acetylcholine (ACh), the major neurotransmitter of the parasympathetic nervous system, can enhance glucose-stimulated insulin secretion from pancreatic β-cells. Studies with isolated islets prepared from whole body M₃ muscarinic ACh receptor knockout mice showed that cholinergic amplification of glucose-dependent insulin secretion is exclusively mediated by the M₃ muscarinic receptor subtype. To investigate the physiological relevance of this muscarinic pathway, we used Cre/loxP technology to generate mutant mice that lack M₃ receptors only in pancreatic β-cells. These mutant mice displayed impaired glucose tolerance and significantly reduced insulin secretion. In contrast, transgenic mice overexpressing M₃ receptors in pancreatic β-cells showed a pronounced increase in glucose tolerance and insulin secretion and were resistant to diet-induced glucose intolerance and hyperglycaemia. These findings indicate that β-cell M₃ muscarinic receptors are essential for maintaining proper insulin secretion and glucose homeostasis. Moreover, our data suggest that enhancing signalling through β-cell M₃ muscarinic receptors may represent a new avenue in the treatment of glucose intolerance and type 2 diabetes.     

Mutations of maturity-onset diabetes of the young (MODY) genes in Thais with early-onset type 2 diabetes mellitus

Objective  Six known genes responsible for maturity-onset diabetes of the young (MODY) were analysed to evaluate the prevalence of their mutations in Thai patients with MODY and early-onset type 2 diabetes.
Patients and methods  Fifty-one unrelated probands with early-onset type 2 diabetes, 21 of them fitted into classic MODY criteria, were analysed for nucleotide variations in promoters, exons, and exon–intron boundaries of six known MODY genes, including HNF-4α , GCK , HNF-1α , IPF-1 , HNF-1β , and NeuroD1/β2 , by the polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) method followed by direct DNA sequencing. Missense mutations or mutations located in regulatory region, which were absent in 130 chromosomes of non-diabetic controls, were classified as potentially pathogenic mutations.
Results  We found that mutations of the six known MODY genes account for a small proportion of classic MODY (19%) and early-onset type 2 diabetes (10%) in Thais. Five of these mutations are novel including GCK R327H, HNF-1α P475L, HNF-1α G554fsX556, NeuroD1 –1972 G > A and NeuroD1 A322N. Mutations of IPF-1 and HNF-1β were not identified in the studied probands.
Conclusions  Mutations of the six known MODY genes may not be a major cause of MODY and early-onset type 2 diabetes in Thais. Therefore, unidentified genes await discovery in a majority of Thai patients with MODY and early-onset type 2 diabetes.     

A missense mutation in the hepatocyte nuclear factor 4 alpha gene in a UK pedigree with maturity-onset diabetes of the young

Summary Maturity-onset diabetes of the young (MODY) is a monogenic subgroup of non-insulin dependent diabetes mellitus (NIDDM) characterised by an early age of onset (< 25 years) and an autosomal dominant mode of inheritance. MODY is genetically heterogeneous with three different genes identified to date; hepatocyte nuclear factor 4 alpha (HNF-4α) [MODY1], glucokinase [MODY2] and hepatocyte nuclear factor 1 alpha (HNF-1α) [MODY3]. A nonsense mutation in the HNF-4α gene has recently been shown to cause MODY in a single large North American pedigree (RW). We screened a large UK Caucasian MODY family which showed weak evidence of linkage to the MODY1 locus on chromosome 20q (lod score for ADA 0.68 at θ = 0) for mutations in the coding region of the HNF-4α gene by direct sequencing. A missense mutation resulting in the substitution of glutamine for glutamic acid was identified in exon 7 (E276Q). The mutation was present in all of the diabetic members of the pedigree plus two unaffected subjects and was not detected in 75 normal control subjects or 95 UK Caucasian subjects with late-onset NIDDM. This is the first missense mutation to be described in the HNF-4α gene. [Diabetologia (1997) 40: 859–862] Received: 7 March 1997 and in revised form: 16 April 1997     

Mutation screening in 18 Caucasian families suggest the existence of other MODY genes

Summary Maturity-onset diabetes of the young (MODY) is a heterogeneous subtype of non-insulin-dependent diabetes mellitus characterised by early onset, autosomal dominant inheritance and a primary defect in insulin secretion. To date five MODY genes have been identified: hepatocyte nuclear factor-4 alpha (HNF-4 α/MODY1/TCF14) on chromosome 20 q, glucokinase (GCK/MODY2) on chromosome 7 p, hepatocyte nuclear factor-1 alpha (HNF-1 α/MODY3/TCF1) on chromosome 12 q, insulin promoter factor-1 (IPF1/MODY4) on chromosome 13 q and hepatocyte nuclear factor-1 beta (HNF-1 β/MODY5/TCF2) on chromosome 17cen-q. We have screened the HNF-4 α, HNF-1 α and HNF-1 β genes in members of 18 MODY kindreds who tested negative for glucokinase mutations. Five missense (G31D, R159W, A161T, R200W, R271W), one substitution at the splice donor site of intron 5 (IVS5nt + 2T→A) and one deletion mutation (P379fsdelT) were found in the HNF-1 α gene, but no MODY-associated mutations were found in the HNF-4 α and HNF-1 β genes. Of 67 French MODY families that we have now studied, 42 (63 %) have mutations in the glucokinase gene, 14 (21 %) have mutations in the HNF-1 α gene, and 11 (16 %) have no mutations in the HNF-4 α, IPF1 and HNF-1 β genes. Eleven families do not have mutations in the five known MODY genes suggesting that there is at least one additionnal locus that can cause MODY. [Diabetologia (1998) 41: 1017–1023] Received: 5 January 1998 and in revised form: 8 April 1998     

Insights on pathogenesis of type 2 diabetes from MODY genetics

Maturity-onset diabetes of the young (MODY) is a type of non-insulin-dependent diabetes mellitus caused by rare autosomal-dominant mutations. MODY genes play key biochemical roles in the pancreatic β cell; therefore, common variants of MODY genes are excellent candidate genes for type 2 diabetes. We review recent studies that suggest that common MODY gene variation contributes modestly to the heritability of type 2 diabetes.     

Diabetes mellitus and impaired pancreatic β-cell proliferation

The factors that normally regulate the proliferation of the insulin-producing pancreatic β-cell largely remain elusive although several factors have been identified that influence β-cell growth in vitro. The adult β-cell is normally virtually quiescent, but its replicatory activity can be enhanced in vitro by certain nutrients and growth factors, and long-term alterations in β-cell mass constitute an important means to accommodate an increased demand for insulin. Likewise, expansion of the β-cell mass by recruitment of β-cells to proliferate may constitute a means by which the organism can compensate for the loss or dysfunction of β-cells occurring in diabetes. However, neither in human or animal models for type-1 diabetes, nor in type-2 diabetes, is β-cell regeneration a noteworthy feature. Thus, if β-cells could be induced to replicate at a higher rate, this may prove beneficial in maintaining normoglycaemia, since the β-cell mass is a major determinant of the total amount of insulin that can be secreted by the pancreas. The present review will focus on the normal regulation of β-cell mitogenesis and hormones production in vitro and in vivo, and furthermore, will present evidence for an insufficient extent of β-cell regeneration in different forms of diabetes mellitus. Additionally, the possibility of manipulating β-cell proliferation by peptides and genetic engineering, and the significance of β-cell mitogenesis in islet transplantation will be discussed in relation to treatments of diabetes mellitus.     

Roles of cAMP signalling in insulin granule exocytosis

Insulin secretion is regulated by a series of complex events generated by various intracellular signals including Ca²⁺, ATP, cAMP and phospholipid-derived signals. Glucose-stimulated insulin secretion is the principal mode of insulin secretion, and the mechanism potentiating the secretion is critical for physiological responses. Among the various intracellular signals involved, cAMP is particularly important for amplifying insulin secretion. Recently, glucagon-like peptide-1 (GLP-1) analogues and dipeptidyl peptidase-IV (DPP-IV) inhibitors have been developed as new antidiabetic drugs. These drugs all act through cAMP signalling in pancreatic β-cells. Until recently, cAMP was generally thought to potentiate insulin secretion through protein kinase A (PKA) phosphorylation of proteins associated with the secretory process. However, it is now known that in addition to PKA, cAMP has other targets such as Epac (also referred to as cAMP-GEF). The variety of the effects mediated by cAMP signalling may be linked to cAMP compartmentation in the pancreatic β-cells.     

Thiazolidinediones and the preservation of beta-cell function, cellular proliferation and apoptosis

The thiazolidinediones (TZDs) or glitazones are pharmaceutical agents that have profound effects on energy expenditure and conservation. They also exert significant anti-inflammatory effects and influence cell proliferation and cell death. The drugs are primarily used in clinical practice in the treatment of patients with type 2 diabetes mellitus, a disorder of insulin resistance that occurs when the pancreatic β-cells are unable to produce adequate amounts of insulin to maintain euglycaemia. Loss of pancreatic β-cell function in type 2 diabetes is progressive and often precedes overt diabetes by 10 years or more, as was shown by the United Kingdom Prospective Diabetes Study. Any therapeutic or preventive approach that would limit or reverse loss of β-cell function in diabetes would have profound effects on the morbidity associated with this widespread disease. Evidence suggesting a potential role of TZDs in preserving β-cell function in type 2 diabetes as well as the ability of these agents to exert anti-inflammatory and proapoptotic anticancer effects, and their ability to promote cellular proliferation in various organs is reviewed.     

Decreased insulin content and secretion in RIN 1046-38 cells overexpressing α2-Adrenergic receptorsreceptors

Several G_i-protein-coupled receptors normally expressed in islet β-cells inhibit insulin secretion on binding of their respective agonists. To study the effect of supraphysiologic expression of such a receptor in insulin-secreting β-cells, we stably transfected cDNA encoding the mouse α_2a-adrenergic receptor into RIN 1046-38 cells. Four different cell lines were selected, each overexpressing the α_2a-adrenergic receptor to varying degrees. Cell lines showing the highest level of receptor expression showed significantly reduced insulin content, and reduced basal and stimulated insulin secretion. Pertussis toxin (PTX) treatment of cells was able to reverse partially the reduced insulin secretory response. Our results suggest that overexpression of a G_i-protein-coupled receptor in β-cells causes tonic inhibition of both insulin synthesis and secretion. Abnormalities in expression or function of such receptors could be a contributory factor in the impaired insulin secretion present in type II diabetes.     

Mutations in the hepatocyte nuclear factor-1α gene in Caucasian families originally classified as having Type I diabetes

Summary Mutations in the hepatocyte nuclear factor-1α (HNF-1α) gene are the cause of maturity-onset diabetes of the young type 3 (MODY3), which is characterised by a severe impairment of insulin secretion and an early onset of the disease. Also at onset of diabetes some MODY patients show similar clinical symptoms and signs as patients with Type I (insulin-dependent) diabetes mellitus. The objective of this study was to estimate the prevalence of MODY3 patients misclassified as Type I diabetic patients. From a large population-based sample of unrelated Danish Caucasian Type I diabetic patients with an affected first degree relative, 39 patients (6.7 %) who did not carry any high-risk HLA-haplotypes, i. e. DR3 or DR4 or both were examined by single-strand conformational polymorphism scanning and direct sequencing of the coding region and the minimal promoter of the HNF-1α gene. Four of the 39 Type I diabetic patients (10 %) were identified as carrying mutations in the HNF-1α gene. One patient carried a missense mutation (Glu48Lys) in exon 1, two patients carried a missense mutation (Cys241Gly) in exon 4 and one patient carried a frameshift mutation (Pro291fsdelA) in exon 4. The mutations were all identified in heterozygous form, segregated with diabetes, and were not identified in 84 unrelated, healthy subjects. Furthermore, family history in three of the four families showed diabetes in four consecutive generations, suggestive of an autosomal dominant inheritance. In conclusion, about 10 % of Danish diabetic patients without a high-risk HLA-haplotype, originally classified as having Type I diabetes could have diabetes caused by mutations in the HNF-1α gene. Clinical awareness of family history of diabetes and mode of inheritance might help to identify and reclassify these diabetic subjects as MODY3 patients. [Diabetologia (1998) 41: 1528–1531] Received: 25 May 1998 and in revised form: 13 July 1998     

Pioglitazone attenuates fatty acid-induced oxidative stress and apoptosis in pancreatic beta-cells

Aims:  Thiazolidinediones (TZDs), ligands for peroxisome proliferator–activated receptor γ, are antidiabetic agents that improve hyperglycemia by decreasing insulin resistance in obese diabetic animal models and patients with type 2 diabetes. We have studied whether pioglitazone, a TZD, can exert a direct effect against pancreatic β-cell lipoapoptosis.
Methods:  MIN6 cells were cultured in medium containing either 5.6 (low glucose) or 25 mM glucose (high glucose) in the presence or absence of 0.5 mM palmitate for 48 h. We examined the effect of 10 μM pioglitazone on MIN6 cells on glucose-stimulated insulin secretion, cellular ATP, uncoupling protein-2 (UCP-2) mRNA expression, intracellular triglyceride content, reactive oxygen species production, the number of apoptotic cells and nuclear factor-κB (NF-κB) activity.
Results:  Pioglitazone recovered partly impaired glucose-stimulated insulin secretion and cellular ATP in MIN6 cell exposed to high glucose with 0.5 mM palmitate. Pioglitazone suppressed intracellular triglyceride accumulation in cells exposed to high glucose with 0.5 mM palmitate. Palmitate-induced upregulation of UCP-2 mRNA levels was suppressed by pioglitazone in a dose-dependent manner. Pioglitazone decreased palmitate-induced reactive oxygen species production in MIN6 cells by 24% and in mouse islet cells by 53%. Pioglitazone also decreased palmitate-induced NF-κB activity by 40% and protected β-cells from palmitate-induced apoptosis by 22% in MIN6 cell.
Conclusions:  Pioglitazone attenuated fatty acid–induced oxidative stress and apoptosis in pancreatic β-cells. TZDs might be used as a mean for maintaining β-cell survival and preserving capacity of insulin secretion in patients with diabetes mellitus.     

Short- and long-term effects of beta-cellulin and transforming growth factor-alpha on beta-cell function in cultured fetal rat pancreatic islets

The polypeptide β-cellulin, identified in conditioned media from insulinoma cell cultures and produced by pancreatic islet cells, was recently identified as a possible autocrine growth factor for the pancreatic islet β-cell. In this study, we investigated the short- and long-term actions of β-cellulin, and the structurally related transforming growth factor-α (TGF-α), on β-cell function in fetal rat pancreatic islets in vitro. We found that neither β-cellulin nor TGF-α (10 nMeach), in contrast to glucose (20 mM), acutely influenced β-cell levels of cytosolic-free Ca²⁺. Additionally, whereas glucose markedly increased short-term (60-min) insulin release, neither β-cellulin nor TGF-α (10nM each) influenced the rate of hormone secretion at basal (3 mM) or stimulatory (20 mM) concentrations of glucose. Likewise, long-term (24-h) exposure of islets to a high glucose concentration significantly augmented the secretion of insulin. This effect was slightly potentiated by TGF-α (10 nM), but not β-celluin (10 nM), at high (but not low) glucose concentrations. Conversely, the islet insulin content was not significantly affected by β-cellulin or TGF-α at any glucose concentration tested. We conclude that, although β-cellulin is produced by islet cells, the peptide does not seem to be of importance for the regulation of insulin production by isolated pancreatic β-cells.     

Genetic and biochemical pathways of β-cell failure in type 2 diabetes

We review mechanisms of β-cell failure in type 2 diabetes. A wealth of information indicates that it is caused by impaired insulin secretion and decreased β-cell mass. Interestingly, there appears to be a link between these two mechanisms. The earliest reaction to peripheral insulin resistance is an increase in insulin production, owing primarily to increased secretion, and to a lesser extent to decreased clearance. Experimental animal models indicate that hyperinsulinaemia promotes an increase in β-cell mass, largely via increased β-cell replication. In contrast, following the onset of overt diabetes, there is a slowly progressive loss of β-cell function and mass, both in animal models and in diabetic humans. It is of great interest that most diabetes-associated genes identified in genome-wide association studies appear to be enriched in the β-cell and to have the potential to regulate mass and/or function. Here, we review evidence derived from experimental animal models to unravel the mechanisms underlying β-cell dysfunction. We focus primarily on signalling pathways, as opposed to nutrient sensing, and specifically on the notion that insulin and growth factor signalling via Foxo1 in pancreatic β-cells links insulin secretion with cellular proliferation and survival.     

Maturity-onset diabetes of the young: clinical heterogeneity explained by genetic heterogeneity

Maturity-onset diabetes of the young (MODY) can be defined by the clinical characteristics of early-onset Type 2 (non-insulin-dependent) diabetes and autosomal dominant inheritance. Mutations in four genes have been shown to cause MODY: glucokinase, hepatic nuclear factor 1 alpha (HNF1α), hepatic nuclear factor 4 alpha (HNF4α) and insulin protein factor 1 (IPF1). In white Caucasians it is now possible to define the gene in most patients with a clinical diagnosis of MODY. Each gene involved in MODY has its own specific clinical and physiological characteristics. Patients with mutations of the glucokinase gene have mild fasting hyperglycaemia throughout life, and rarely require medication or develop microvascular complications. The principle pathophysiology is stable beta-cell dysfunction characterized by reduced sensing of glucose by the pancreas. Patients with mutations in HNF1α have normal glucose tolerance in early childhood and usually present with symptomatic diabetes in their late teens or early adulthood. They show increasing hyperglycaemia and treatment requirements with frequent microvascular complications. The underlying defect is progressive beta-cell failure, with the early lesion characterized by failure to increase insulin secretion with increasing glucose levels. Patients with HNF4α and IPF1 mutations show a similar clinical picture to HNF1α although diabetes may be diagnosed later. There are other patients with MODY in whom the genetic defect is still unknown. Molecular genetic testing in patients with diabetes offers the possibility of making a firm diagnosis of MODY and allows prediction of the future clinical course. The role of predictive testing in non-diabetic subjects within families is uncertain at present. Preliminary evidence suggests that maintaining insulin sensitivity by avoiding obesity and regular physical exercise may help delay the onset of diabetes © 1998 John Wiley & Sons, Ltd.