Association and haplotype analysis of the insulin-degrading enzyme (IDE) gene, a strong positional and biological candidate for type 2 diabetes susceptibility

Pharmacological activation of insulin-degrading enzyme improves insulin secretion and glucose tolerance in diet-induced obese mice

AIM

To investigate the use of synthetic preimplantation factor (sPIF) as a potential therapeutic tool for improving glucose-stimulated insulin secretion (GSIS), glucose tolerance and insulin sensitivity in the setting of diabetes.

MATERIALS AND METHODS

We used a preclinical murine model of type 2 diabetes (T2D) induced by high-fat diet (HFD) feeding for 12 weeks. Saline or sPIF (1 mg/kg/day) was administered to mice by subcutaneously implanted osmotic mini-pumps for 25 days. Glucose tolerance, circulating insulin and C-peptide levels, and GSIS were assessed. In addition, β-cells (Min-6) were used to test the effects of sPIF on GSIS and insulin-degrading enzyme (IDE) activity in vitro. The effect of sPIF on GSIS was also tested in human islets.

RESULTS

GSIS was enhanced 2-fold by sPIF in human islets ex vivo. Furthermore, continuous administration of sPIF to HFD mice increased circulating levels of insulin and improved glucose tolerance, independently of hepatic insulin clearance. Of note, islets isolated from mice treated with sPIF exhibited restored β-cell function. Finally, genetic (shRNA-IDE) or pharmacological (6bK) inactivation of IDE in Min-6 abolished sPIF-mediated effects on GSIS, showing that both the protein and its protease activity are required for its action.

CONCLUSIONS

We conclude that sPIF is a promising secretagogue for the treatment of T2D.

Common variants in genes involved in islet amyloid polypeptide (IAPP) processing and the degradation pathway are associated with T2DM risk: A Chinese population study

AIM

To explore the genetic effects of SLC30A8, IAPP, PCSK1, PCSK2, CPE, PAM and IDE, key genes involved in IAPP processing and degradation pathway on T2DM risk and metabolic traits in Chinese population.

METHODS

Common variants were genotyped in 10936 Chinese subjects by Asian Screening Array and Multi-Ethnic Global Array. Associations of SNPs with occurrences of T2DM and related traits were evaluated through logistic and multiple linear regression. Genetic risk score (GRS) model was constructed based on 6 T2DM-variants, and its relationship with T2DM and related traits was assessed.

RESULTS

SLC30A8-rs13266634, PCSK1-rs155980, PCSK2-rs6136035, CPE-rs532192464, PAM-rs7716941, and IDE-rs117929184 were the top SNPs significantly associated with T2DM after adjusting for age, sex, and BMI, associated with blood glucose level, insulin secretion, and insulin sensitivity (all FDR p < 0.05). GRS calculated based on the above SNPs was remarkably correlated with T2DM, blood glucose, and insulin secretion. Furthermore, there was a significant interaction between SLC30A8 and IAPP in patients with T2DM (P = 0.0083).

CONCLUSION

Our study showed that common variants in genes involved in IAPP processing and the degradation pathway were associated with T2DM in Chinese population. Subjects with high GRS exhibited poorer glucose metabolism and insulin secretion.

Loss of postprandial insulin clearance control by Insulin-degrading enzyme drives dysmetabolism traits

Systemic insulin availability is determined by a balance between beta-cell secretion capacity and insulin clearance (IC). Insulin-degrading enzyme (IDE) is involved in the intracellular mechanisms underlying IC. The liver is a major player in IC control yet the role of hepatic IDE in glucose and lipid homeostasis remains unexplored. We hypothesized that IDE governs postprandial IC and hepatic IDE dysfunction amplifies dysmetabolic responses and prediabetes traits such as hepatic steatosis. In a European/Portuguese population-based cohort, IDE SNPs were strongly associated with postprandial IC in normoglycemic men but to a considerably lesser extent in women or in subjects with prediabetes. Liver-specific knockout-mice (LS-IDE KO) under normal chow diet (NCD), showed reduced postprandial IC with glucose intolerance and under high fat diet (HFD) were more susceptible to hepatic steatosis than control mice. This suggests that regulation of IC by IDE contributes to liver metabolic resilience. In agreement, LS-IDE KO hepatocytes revealed reduction of Glut2 expression levels with consequent impairment of glucose uptake and upregulation of CD36, a major hepatic free fatty acid transporter. Together these findings provide strong evidence that dysfunctional IC due to abnormal IDE regulation directly impairs postprandial hepatic glucose disposal and increases susceptibility to dysmetabolic conditions in the setting of Western diet/lifestyle.

Targeting Insulin-Degrading Enzyme in Insulin Clearance

Hepatic insulin clearance, a physiological process that in response to nutritional cues clears ~50–80% of circulating insulin, is emerging as an important factor in our understanding of the pathogenesis of type 2 diabetes mellitus (T2DM). Insulin-degrading enzyme (IDE) is a highly conserved Zn²⁺-metalloprotease that degrades insulin and several other intermediate-size peptides. Both, insulin clearance and IDE activity are reduced in diabetic patients, albeit the cause-effect relationship in humans remains unproven. Because historically IDE has been proposed as the main enzyme involved in insulin degradation, efforts in the development of IDE inhibitors as therapeutics in diabetic patients has attracted attention during the last decades. In this review, we retrace the path from Mirsky’s seminal discovery of IDE to the present, highlighting the pros and cons of the development of IDE inhibitors as a pharmacological approach to treating diabetic patients.

Vitamin E supplementation and caloric restriction promotes regulation of insulin secretion and glycemic homeostasis by different mechanisms in rats

Vitamin E and caloric restriction have antioxidant effects in mammals. The aim of this study was to evaluate effects of vitamin E supplementation and caloric restriction upon insulin secretion and glucose homeostasis in rats. Male Wistar rats were distributed among the following groups: C, control group fed ad libitum; R, food quantity reduction of 40%; CV, control group supplemented with vitamin E [30 mg·kg^-1·day^-1]; and RV, food-restricted group supplemented with vitamin E. The experiments ran for 21 days. Glucose tolerance and insulin sensitivity was higher in the CV, R, and RV groups. Insulin secretion stimulated with different glucose concentrations was lower in the R and RV groups, compared with C and CV. In the presence of glucose and secretagogues, insulin secretion was higher in the CV group and was lower in the R and RV groups. An increase in insulin receptor occurred in the fat pad and muscle tissue of groups CV, R, and RV. Levels of hepatic insulin receptor and phospho-Akt protein were higher in groups R and RV, compared with C and CV, while muscle phospho-Akt was increased in the CV group. There was a reduction in hepatic RNA levels of the hepatocyte growth factor gene and insulin degrading enzyme in the R group, and increased levels of insulin degrading enzyme in the CV and RV groups. Thus, vitamin E supplementation and caloric restriction modulate insulin secretion by different mechanisms to maintain glucose homeostasis.

Vagotomy Reduces Insulin Clearance in Obese Mice Programmed by Low-Protein Diet in the Adolescence

The aim of this study was to investigate the effect of subdiaphragmatic vagotomy on insulin sensitivity, secretion, and degradation in metabolic programmed mice, induced by a low-protein diet early in life, followed by exposure to a high-fat diet in adulthood. Weaned 30-day-old C57Bl/6 mice were submitted to a low-protein diet (6% protein). After 4 weeks, the mice were distributed into three groups: LP group, which continued receiving a low-protein diet; LP + HF group, which started to receive a high-fat diet; and LP + HFvag group, which underwent vagotomy and also was kept at a high-fat diet. Glucose-stimulated insulin secretion (GSIS) in isolated islets, ipGTT, ipITT, in vivo insulin clearance, and liver expression of the insulin-degrading enzyme (IDE) was accessed. Vagotomy improved glucose tolerance and reduced insulin secretion but did not alter adiposity and insulin sensitivity in the LP + HFvag, compared with the LP + HF group. Improvement in glucose tolerance was accompanied by increased insulinemia, probably due to a diminished insulin clearance, as judged by the lower C-peptide : insulin ratio, during the ipGTT. Finally, vagotomy also reduced liver IDE expression in this group. In conclusion, when submitted to vagotomy, the metabolic programmed mice showed improved glucose tolerance, associated with an increase of plasma insulin concentration as a result of insulin clearance reduction, a phenomenon probably due to diminished liver IDE expression.

Acute Exercise Improves Insulin Clearance and Increases the Expression of Insulin-Degrading Enzyme in the Liver and Skeletal Muscle of Swiss Mice

The effects of exercise on insulin clearance and IDE expression are not yet fully elucidated. Here, we have explored the effect of acute exercise on insulin clearance and IDE expression in lean mice. Male Swiss mice were subjected to a single bout of exercise on a speed/angle controlled treadmill for 3-h at approximately 60-70% of maximum oxygen consumption. As expected, acute exercise reduced glycemia and insulinemia, and increased insulin tolerance. The activity of AMPK-ACC, but not of IR-Akt, pathway was increased in the liver and skeletal muscle of trained mice. In an apparent contrast to the reduced insulinemia, glucose-stimulated insulin secretion was increased in isolated islets of these mice. However, insulin clearance was increased after acute exercise and was accompanied by increased expression of the insulin-degrading enzyme (IDE), in the liver and skeletal muscle. Finally, C2C12, but not HEPG2 cells, incubated at different concentrations of 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) for 3-h, showed increased expression of IDE. In conclusion, acute exercise increases insulin clearance, probably due to an augmentation of IDE expression in the liver and skeletal muscle. The elevated IDE expression, in the skeletal muscle, seems to be mediated by activation of AMPK-ACC pathway, in response to exercise. We believe that the increase in the IDE expression, comprise a safety measure to maintain glycemia at or close to physiological levels, turning physical exercise more effective and safe.

Acute exercise restores insulin clearance in diet-induced obese mice

The aim of this study was to investigate the insulin clearance in diet-induced obese (DIO) mice submitted to acute endurance exercise (3h of treadmill exercise at 60-70% VO2max). Glucose-stimulated insulin secretion in isolated islets; ipGTT; ipITT; ipPTT; in vivo insulin clearance; protein expression in liver, skeletal muscle, and adipose tissue (insulin degrading enzyme (IDE), insulin receptor subunitβ(IRβ), phospho-Akt (p-Akt) and phospho-AMPK (p-AMPK)), and the activity of IDE in the liver and skeletal muscle were accessed. In DIO mice, acute exercise reduced fasting glycemia and insulinemia, improved glucose and insulin tolerance, reduced hepatic glucose production, and increased p-Akt protein levels in liver and skeletal muscle and p-AMPK protein levels in skeletal muscle. In addition, insulin secretion was reduced, whereas insulin clearance and the expression of IDE and IRβ were increased in liver and skeletal muscle. Finally, IDE activity was increased only in skeletal muscle. In conclusion, we propose that the increased insulin clearance and IDE expression and activity, primarily, in skeletal muscle, constitute an additional mechanism, whereby physical exercise reduces insulinemia in DIO mice.

Assignment of Functional Relevance to Genes at Type 2 Diabetes-Associated Loci Through Investigation of β-Cell Mass Deficits

Type 2 diabetes (T2D) has been associated with a large number of genomic loci, many of which encompass multiple genes without a definitive causal gene. This complexity has hindered efforts to clearly identify functional candidate genes and interpret their role in mediating susceptibility to disease. Here we examined the relevance of individual genes found at T2D-associated loci by assessing their potential contribution to a phenotype relevant to the disease state: production and maintenance of β-cell mass. Using transgenic zebrafish in which β-cell mass could be rapidly visualized in vivo, we systematically suppressed the expression of orthologs of genes found at T2D-associated genomic loci. Overall, we tested 67 orthologs, many of which had no known relevance to β-cell mass, at 62 human T2D-associated loci, including eight loci with multiple candidate genes. In total we identified 25 genes that were necessary for proper β-cell mass, providing functional evidence for their role in a physiological phenotype directly related to T2D. Of these, 16 had not previously been implicated in the regulation of β-cell mass. Strikingly, we identified single functional candidate genes at the majority of the loci for which multiple genes were analyzed. Further investigation into the contribution of the 25 genes to the adaptive capacity of β-cells suggested that the majority of genes were not required for glucose-induced expansion of β-cell mass but were significantly necessary for the regeneration of β-cells. These findings suggest that genetically programmed deficiencies in β-cell mass may be related to impaired maintenance. Finally, we investigated the relevance of our findings to human T2D onset in diabetic individuals from the Old Order Amish and found that risk alleles in β-cell mass genes were associated with significantly younger age of onset and lower body mass index. Taken together, our study offers a functional approach to assign relevance to genes at T2D-associated loci and offers experimental evidence for the defining role of β-cell mass maintenance in genetic susceptibility to T2D onset.

Candidate genes for Alzheimer's disease are associated with individual differences in plasma levels of beta amyloid peptides in adults with Down syndrome

We examined the contribution of candidates genes for Alzheimer's disease (AD) to individual differences in levels of beta amyloid peptides in adults with Down syndrom, a population at high risk for AD. Participants were 254 non-demented adults with Down syndrome, 30-78 years of age. Genomic deoxyribonucleic acid was genotyped using an Illumina GoldenGate custom array. We used linear regression to examine differences in levels of Aβ peptides associated with the number of risk alleles, adjusting for age, sex, level of intellectual disability, race and/or ethnicity, and the presence of the APOE ε4 allele. For Aβ42 levels, the strongest gene-wise association was found for a single nucleotide polymorphism (SNP) on CAHLM1; for Aβ40 levels, the strongest gene-wise associations were found for SNPs in IDE and SOD1, while the strongest gene-wise associations with levels of the Aβ42/Aβ40 ratio were found for SNPs in SORCS1. Broadly classified, variants in these genes may influence amyloid precursor protein processing (CALHM1, IDE), vesicular trafficking (SORCS1), and response to oxidative stress (SOD1).

Reduced insulin clearance and lower insulin-degrading enzyme expression in the liver might contribute to the thrifty phenotype of protein-restricted mice

Nutrient restriction during the early stages of life usually leads to alterations in glucose homeostasis, mainly insulin secretion and sensitivity, increasing the risk of metabolic disorders in adulthood. Despite growing evidence regarding the importance of insulin clearance during glucose homeostasis in health and disease, no information exists about this process in malnourished animals. Thus, in the present study, we aimed to determine the effect of a nutrient-restricted diet on insulin clearance using a model in which 30-d-old C57BL/6 mice were exposed to a protein-restricted diet for 14 weeks. After this period, we evaluated many metabolic variables and extracted pancreatic islet, liver, gastrocnemius muscle (GCK) and white adipose tissue samples from the control (normal-protein diet) and restricted (low-protein diet, LP) mice. Insulin concentrations were determined using RIA and protein expression and phosphorylation by Western blot analysis. The LP mice exhibited lower body weight, glycaemia, and insulinaemia, increased glucose tolerance and altered insulin dynamics after the glucose challenge. The improved glucose tolerance could partially be explained by an increase in insulin sensitivity through the phosphorylation of the insulin receptor/protein kinase B and AMP-activated protein kinase/acetyl-CoA carboxylase in the liver, whereas the changes in insulin dynamics could be attributed to reduced insulin secretion coupled with reduced insulin clearance and lower insulin-degrading enzyme (IDE) expression in the liver and GCK. In summary, protein-restricted mice not only produce and secrete less insulin, but also remove and degrade less insulin. This phenomenon has the double benefit of sparing insulin while prolonging and potentiating its effects, probably due to the lower expression of IDE in the liver, possibly with long-term consequences.

Cafeteria diet inhibits insulin clearance by reduced insulin-degrading enzyme expression and mRNA splicing

Insulin clearance plays a major role in glucose homeostasis and insulin sensitivity in physiological and/or pathological conditions, such as obesity-induced type 2 diabetes as well as diet-induced obesity. The aim of the present work was to evaluate cafeteria diet-induced obesity-induced changes in insulin clearance and to explain the mechanisms underlying these possible changes. Female Swiss mice were fed either a standard chow diet (CTL) or a cafeteria diet (CAF) for 8 weeks, after which we performed glucose tolerance tests, insulin tolerance tests, insulin dynamics, and insulin clearance tests. We then isolated pancreatic islets for ex vivo glucose-stimulated insulin secretion as well as liver, gastrocnemius, visceral adipose tissue, and hypothalamus for subsequent protein analysis by western blot and determination of mRNA levels by real-time RT-PCR. The cafeteria diet induced insulin resistance, glucose intolerance, and increased insulin secretion and total insulin content. More importantly, mice that were fed a cafeteria diet demonstrated reduced insulin clearance and decay rate as well as reduced insulin-degrading enzyme (IDE) protein and mRNA levels in liver and skeletal muscle compared with the control animals. Furthermore, the cafeteria diet reduced IDE expression and alternative splicing in the liver and skeletal muscle of mice. In conclusion, a cafeteria diet impairs glucose homeostasis by reducing insulin sensitivity, but it also reduces insulin clearance by reducing IDE expression and alternative splicing in mouse liver; however, whether this mechanism contributes to the glucose intolerance or helps to ameliorate it remains unclear.

Genetic associations of type 2 diabetes with islet amyloid polypeptide processing and degrading pathways in asian populations

Type 2 diabetes (T2D) is a complex disease characterized by beta cell dysfunctions. Islet amyloid polypeptide (IAPP) is highly conserved and co-secreted with insulin with over 40% of autopsy cases of T2D showing islet amyloid formation due to IAPP aggregation. Dysregulation in IAPP processing, stabilization and degradation can cause excessive oligomerization with beta cell toxicity. Previous studies examining genetic associations of pathways implicated in IAPP metabolism have yielded conflicting results due to small sample size, insufficient interrogation of gene structure and gene-gene interactions. In this multi-staged study, we screened 89 tag single nucleotide polymorphisms (SNPs) in 6 candidate genes implicated in IAPP metabolism and tested for independent and joint associations with T2D and beta cell dysfunctions. Positive signals in the stage-1 were confirmed by de novo and in silico analysis in a multi-centre unrelated case-control cohort. We examined the association of significant SNPs with quantitative traits in a subset of controls and performed bioinformatics and relevant functional analyses. Amongst the tag SNPs, rs1583645 in carboxypeptidase E (CPE) and rs6583813 in insulin degrading enzyme (IDE) were associated with 1.09 to 1.28 fold increased risk of T2D (P_Meta = 9.4×10⁻³ and 0.02 respectively) in a meta-analysis of East Asians. Using genetic risk scores (GRS) with each risk variant scoring 1, subjects with GRS≥3 (8.2% of the cohort) had 56% higher risk of T2D than those with GRS = 0 (P = 0.01). In a subcohort of control subjects, plasma IAPP increased and beta cell function index declined with GRS (P = 0.008 and 0.03 respectively). Bioinformatics and functional analyses of CPE rs1583645 predicted regulatory elements for chromatin modification and transcription factors, suggesting differential DNA-protein interactions and gene expression. Taken together, these results support the importance of dysregulation of IAPP metabolism in T2D in East Asians.

Diabetes genes identified by genome-wide association studies are regulated in mice by nutritional factors in metabolically relevant tissues and by glucose concentrations in islets

Background

Genome-wide association studies (GWAS) have recently identified many new genetic variants associated with the development of type 2 diabetes. Many of these variants are in introns of known genes or between known genes, suggesting they affect the expression of these genes. The regulation of gene expression is often tissue and context dependent, for example occurring in response to dietary changes, hormone levels, or many other factors. Thus, to understand how these new genetic variants associated with diabetes risk may act, it is necessary to understand the regulation of their cognate genes.

Results

We identified fourteen type 2 diabetes-associated genes discovered by the first waves of GWAS for which there was little prior evidence of their potential role in diabetes (Adam30, Adamts9, Camk1d, Cdc123, Cdkal1, Cdkn2a, Cdkn2b, Ext2, Hhex, Ide, Jazf1, Lgr5, Thada and Tspan8). We examined their expression in metabolically relevant tissues including liver, adipose tissue, brain, and hypothalamus obtained from mice under fasted, non-fasted and high fat diet-fed conditions. In addition, we examined their expression in pancreatic islets from these mice cultured in low and high glucose. We found that the expression of Jazf1 was reduced by high fat feeding in liver, with similar tendencies in adipose tissue and the hypothalamus. Adamts9 expression was decreased in the hypothalamus of high fat fed mice. In contrast, the expression of Camk1d, Ext2, Jazf1 and Lgr5 were increased in the brain of non-fasted animals compared to fasted mice. Most notably, the expression levels of most of the genes were decreased in islets cultured in high glucose.

Conclusions

These data provide insight into the metabolic regulation of these new type 2 diabetes genes that will be important for determining how the GWAS variants affect gene expression and ultimately the development of type 2 diabetes.

Han ethnicity-specific type 2 diabetic treatment from traditional Chinese medicine?

Insulin-degrading enzyme (IDE) gene is one of the type 2 diabetes mellitus susceptibility genes specific to the Han Chinese population. IDE, a zinc-metalloendopeptidase, is a potential target for controlling insulin degradation. Potential lead compounds for IDE inhibition were identified from traditional Chinese medicine (TCM) through virtual screening and evaluation of their pharmacokinetic properties of absorption, distribution, metabolism, excretion, and toxicity. Molecular dynamics (MD) simulation was performed to validate the stability of complexes from docking simulation. The top three TCM compounds, dihydrocaffeic acid, isopraeroside IV, and scopolin, formed stable H-bond interactions with key residue Asn139, and were linked to active pocket residues His108, His112, and Glu189 through zinc. Torsion angle trajectories also indicated some stable interactions for each ligand with IDE. Molecular level analysis revealed that the TCM candidates might affect IDE through competitive binding to the active site and steric hindrance. Structural feature analysis reveals that high amounts of hydroxyl groups and carboxylic moieties contribute to anchor the ligand within the complex. Hence, we suggest the top three TCM compounds as potential inhibitor leads against IDE protein to control insulin degradation for type 2 diabetes mellitus. An animated interactive 3D complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:JBSD:29.

Variations in/nearby genes coding for JAZF1, TSPAN8/LGR5 and HHEX-IDE and risk of type 2 diabetes in Han Chinese

Several genetic loci (JAZF1, CDC123/CAMK1D, TSPAN8/LGR5, ADAMTS9, VEGFA and HHEX-IDE) were identified to be significantly related to the risk of type 2 diabetes and quantitative metabolic traits in European populations. Here, we aimed to evaluate the impacts of these novel loci on type 2 diabetes risk in a population-based case-control study of Han Chinese (1912 cases and 2041 controls). We genotyped 13 single-nucleotide polymorphisms (SNPs) in/near these genes and examined the differences in allele/genotype frequency between cases and controls. We found that both IDE rs11187007 and HHEX rs1111875 were associated with type 2 diabetes risk (for both variants: odds ratio (OR)=1.15, 95% confidence interval (CI) 1.04-1.28, P=0.009). In a meta-analysis where we pooled our data with the three previous studies conducted in East Asians, we found that the variants of JAZF1 rs864745 (1.09 (1.03-1.16); P=3.49 × 10(-3)) and TSPAN8/LGR5 rs7961581 (1.11(1.05-1.17); P=1.89 × 10(-4)) were significantly associated with type 2 diabetes risk. In addition, the meta-analysis (7207 cases and 8260 controls) also showed that HHEX rs1111875 did have effects on type 2 diabetes in Chinese population (OR=1.15(1.10-1.21); P=1.93 × 10(-8)). This large population-based study and meta-analysis further confirmed the modest effects of the JAZF1, TSPAN8/LGR5 and HHEX-IDE loci on type 2 diabetes in Chinese and other East Asians.

Yeast Ste23p shares functional similarities with mammalian insulin-degrading enzymes

The S. cerevisiae genome encodes two M16A enzymes: Axl1p and Ste23p. Of the two, Ste23p shares significantly higher sequence identity with M16A enzymes from other species, including mammalian insulin-degrading enzymes (IDEs). In this study, recombinant Ste23p and R. norvegicus IDE (RnIDE) were isolated from E. coli, and their enzymatic properties compared. Ste23p was found to cleave established RnIDE substrates, including the amyloid-beta peptide (Abeta1-40) and insulin B-chain. A novel internally quenched fluorogenic substrate (Abz-SEKKDNYIIKGV-nitroY-OH) based on the polypeptide sequence of the yeast P2 a-factor mating propheromone was determined to be a suitable substrate for both Ste23p and RnIDE, and was used to conduct comparative enzymological studies. Both enzymes were most active at 37 degrees C, in alkaline buffers and in high salt environments. In addition, the proteolytic activities of both enzymes towards the fluorogenic substrate were inhibited by metal chelators, thiol modifiers, inhibitors of cysteine protease activity and insulin. Characteristics of STE23 expression were also evaluated. Our analysis indicates that the 5' terminus of the STE23 gene has been mischaracterized, with the physiologically relevant initiator corresponding to residue M53 of the publicly annotated protein sequence. Finally, we demonstrate that, unlike haploid-specific Axl1p, Ste23p is expressed in both haploid and diploid cell types. Our study presents the first comprehensive biochemical analysis of a yeast M16A enzyme, and provides evidence that S. cerevisiae Ste23p has enzymatic properties that are highly consistent with mammalian IDEs and other M16A enzymes.

Polymorphisms within insulin-degrading enzyme (IDE) gene determine insulin metabolism and risk of type 2 diabetes

Insulin-degrading enzyme (IDE) is the ubiquitously expressed major enzyme responsible for insulin degradation. Insulin-degrading enzyme gene is located on chromosome region 10q23-q25 and exhibits a well-replicated peak of linkage with type 2 diabetes (T2DM). Several genetic association studies examined IDE gene as a susceptibility gene for T2DM with controversial results. However, pathophysiological mechanisms involved have remained elusive. We verified associations of two IDE polymorphisms (rs1887922 and rs2149632) with T2DM risk in two independent German cohorts and evaluated in detail the association of common variants with insulin metabolism and glycemic traits. We confirmed previously published findings for diabetes-associated rs1887922 and rs2149632 in the European Prospective Investigation into Cancer and Nutrition-Potsdam cohort (n = 3049; RR 1.26, p = 0.003 and RR 1.33, p < 0.0001 for additive model). Haplotypes which carried one risk allele of rs2149632 or two risk alleles of both studied IDE SNPs also demonstrated a strong association with increased T2DM risk in this cohort (p = 0.001 and p < 0.0001, respectively). However, we found no significant T2DM association in the cross-sectional metabolic syndrome Berlin-Potsdam cohort (n = 1026). In nondiabetic subjects (NGT+IFG/IGT; n = 739), we found an association of rs2149632 with impaired glucose-derived insulin secretion and a trend to decreased insulin sensitivity for rs1887922. In the NGT subjects (n = 440), the association with decreased insulin secretion for rs2149632 remain significant, and the association with decreased hepatic insulin degradation for rs1887922 were observed additionally. This study validates and confirms the association of IDE polymorphisms with T2DM risk in the prospective German cohort and provides novel evidence of influences of IDE genetic variants on insulin metabolism.

Variants in the insulin-degrading enzyme gene are associated with metabolic syndrome in Chinese elders

Metabolic syndrome (MetS) is a cluster of metabolic abnormalities sharing potential common underlying mechanisms. Insulin-degrading enzyme (IDE) plays a primary role in insulin degradation and cellular insulin processing and therefore affects glucose and lipid metabolism. Genetic association studies have been focused on the relationship between the IDE gene and the development of MetS. To identify specific genetic risks for MetS associated with IDE gene, a case-control association study was performed on 563 Chinese elders in Shanghai, China. Cases were those with MetS (n = 241), and controls were those without MetS (n = 219). Five unrelated genetic markers (single nucleotide polymorphisms) at the IDE gene were used for association analyses. The single-locus association analysis revealed that the A/T allele of rs11187033 was associated with MetS (odds ratio = 0.698; 95% confidence interval, 0.526-0.928; P = .013). Patients with MetS had more haplotype G-T-Ts than controls (P = .008). None of the other 4 single nucleotide polymorphisms was significantly associated with MetS. This result suggests that the rs11187033 at IDE gene might contribute to MetS susceptibility in Chinese elders.

QTL for several metabolic traits map to loci controlling growth and body composition in an F2 intercross between high- and low-growth chicken lines

Quantitative trait loci (QTL) for metabolic and body composition traits were mapped at 7 and 9 wk, respectively, in an F2 intercross between high-growth and low-growth chicken lines. These lines also diverged for abdominal fat percentage (AFP) and plasma insulin-like growth factor-I (IGF-I), insulin, and glucose levels. Genotypings were performed with 129 microsatellite markers covering 21 chromosomes. A total of 21 QTL with genomewide level of significance were detected by single-trait analyses for body weight (BW), breast muscle weight (BMW) and percentage (BMP), AF weight (AFW) and percentage (AFP), shank length (ShL) and diameter (ShD), fasting plasma glucose level (Gluc), and body temperature (Tb). Other suggestive QTL were identified for these parameters and for plasma IGF-I and nonesterified fatty acid levels. QTL controlling adiposity and Gluc were colocalized on GGA3 and GGA5 and QTL for BW, ShL and ShD, adiposity, and Tb on GGA4. Multitrait analyses revealed two QTL controlling Gluc and AFP on GGA5 and Gluc and Tb on GGA26. Significant effects of the reciprocal cross were observed on BW, ShD, BMW, and Gluc, which may result from mtDNA and/or maternal effects. Most QTL regions for Gluc and adiposity harbor genes for which alleles have been associated with increased susceptibility to diabetes and/or obesity in humans. Identification of genes responsible for these metabolic QTL will increase our understanding of the constitutive “hyperglycemia” found in chickens. Furthermore, a comparative approach could provide new information on the genetic causes of diabetes and obesity in humans.

PPARgamma transcriptionally regulates the expression of insulin-degrading enzyme in primary neurons

Insulin-degrading enzyme (IDE) is a protease that has been demonstrated to play a key role in degrading both Abeta and insulin and deficient in IDE function is associated with Alzheimer's disease (AD) and type 2 diabetes mellitus (DM2) pathology. However, little is known about the cellular and molecular regulation of IDE expression. Here we show IDE levels are markedly decreased in DM2 patients and positively correlated with the peroxisome proliferator-activated receptor gamma (PPARgamma) levels. Further studies show that PPARgamma plays an important role in regulating IDE expression in rat primary neurons through binding to a functional peroxisome proliferator-response element (PPRE) in IDE promoter and promoting IDE gene transcription. Finally, we demonstrate that PPARgamma participates in the insulin-induced IDE expression in neurons. These results suggest that PPARgamma transcriptionally induces IDE expression which provides a novel mechanism for the use of PPARgamma agonists in both DM2 and AD therapies.

Association between TCF7L2 gene polymorphisms and susceptibility to type 2 diabetes mellitus: a large Human Genome Epidemiology (HuGE) review and meta-analysis

BACKGROUND

Transcription factor 7-like 2 (TCF7L2) has been shown to be associated with type 2 diabetes mellitus (T2MD) in multiple ethnic groups in the past two years, but, contradictory results were reported for Chinese and Pima Indian populations. The authors then performed a large meta-analysis of 36 studies examining the association of type 2 diabetes mellitus (T2DM) with polymorphisms in the TCF7L2 gene in various ethnicities, containing rs7903146 C-to-T (IVS3C>T), rs7901695 T-to-C (IVS3T>C), a rs12255372 G-to-T (IVS4G>T), and rs11196205 G-to-C (IVS4G>C) polymorphisms and to evaluate the size of gene effect and the possible genetic mode of action.

METHODS

Literature-based searching was conducted to collect data and three methods, that is, fixed-effects, random-effects and Bayesian multivariate mete-analysis, were performed to pool the odds ratio (OR). Publication bias and study-between heterogeneity were also examined.

RESULTS

The studies included 35,843 cases of T2DM and 39,123 controls, using mainly primary data. For T2DM and IVS3C>T polymorphism, the Bayesian OR for TT homozygotes and TC heterozygotes versus CC homozygote was 1.968 (95% credible interval (CrI): 1.790, 2.157), 1.406 (95% CrI: 1.341, 1.476), respectively, and the population attributable risk (PAR) for the TT/TC genotypes of this variant is 16.9% for overall. For T2DM and IVS4G>T polymorphism, TT homozygotes and TG heterozygotes versus GG homozygote was 1.885 (95%CrI: 1.698, 2.088), 1.360 (95% CrI: 1.291, 1.433), respectively. Four ORs among these two polymorphisms all yielded significant between-study heterogeneity (P < 0.05) and the main source of heterogeneity was ethnic differences. Data also showed significant associations between T2DM and the other two polymorphisms, but with low heterogeneity (P > 0.10). Pooled ORs fit a codominant, multiplicative genetic model for all the four polymorphisms of TCF7L2 gene, and this model was also confirmed in different ethnic populations when stratification of IVS3C>T and IVS4G>T polymorphisms except for Africans, where a dominant, additive genetic mode is suggested for IVS3C>T polymorphism.

CONCLUSION

This meta-analysis demonstrates that four variants of TCF7L2 gene are all associated with T2DM, and indicates a multiplicative genetic model for all the four polymorphisms, as well as suggests the TCF7L2 gene involved in near 1/5 of all T2MD. Potential gene-gene and gene-environmental interactions by which common variants in the TCF7L2 gene influence the risk of T2MD need further exploration.

Association between TCF7L2 gene polymorphisms and susceptibility to type 2 diabetes mellitus: a large Human Genome Epidemiology (HuGE) review and meta-analysis

Background

Transcription factor 7-like 2 (TCF7L2) has been shown to be associated with type 2 diabetes mellitus (T2MD) in multiple ethnic groups in the past two years, but, contradictory results were reported for Chinese and Pima Indian populations. The authors then performed a large meta-analysis of 36 studies examining the association of type 2 diabetes mellitus (T2DM) with polymorphisms in the TCF7L2 gene in various ethnicities, containing rs7903146 C-to-T (IVS3C>T), rs7901695 T-to-C (IVS3T>C), a rs12255372 G-to-T (IVS4G>T), and rs11196205 G-to-C (IVS4G>C) polymorphisms and to evaluate the size of gene effect and the possible genetic mode of action.

Methods

Literature-based searching was conducted to collect data and three methods, that is, fixed-effects, random-effects and Bayesian multivariate mete-analysis, were performed to pool the odds ratio (OR). Publication bias and study-between heterogeneity were also examined.

Results

The studies included 35,843 cases of T2DM and 39,123 controls, using mainly primary data. For T2DM and IVS3C>T polymorphism, the Bayesian OR for TT homozygotes and TC heterozygotes versus CC homozygote was 1.968 (95% credible interval (CrI): 1.790, 2.157), 1.406 (95% CrI: 1.341, 1.476), respectively, and the population attributable risk (PAR) for the TT/TC genotypes of this variant is 16.9% for overall. For T2DM and IVS4G>T polymorphism, TT homozygotes and TG heterozygotes versus GG homozygote was 1.885 (95%CrI: 1.698, 2.088), 1.360 (95% CrI: 1.291, 1.433), respectively. Four ORs among these two polymorphisms all yielded significant between-study heterogeneity (P < 0.05) and the main source of heterogeneity was ethnic differences. Data also showed significant associations between T2DM and the other two polymorphisms, but with low heterogeneity (P > 0.10). Pooled ORs fit a codominant, multiplicative genetic model for all the four polymorphisms of TCF7L2 gene, and this model was also confirmed in different ethnic populations when stratification of IVS3C>T and IVS4G>T polymorphisms except for Africans, where a dominant, additive genetic mode is suggested for IVS3C>T polymorphism.

Conclusion

This meta-analysis demonstrates that four variants of TCF7L2 gene are all associated with T2DM, and indicates a multiplicative genetic model for all the four polymorphisms, as well as suggests the TCF7L2 gene involved in near 1/5 of all T2MD. Potential gene-gene and gene-environmental interactions by which common variants in the TCF7L2 gene influence the risk of T2MD need further exploration.

Structure, function, and regulation of insulin-degrading enzyme

The short half-life of insulin in the human body (4-6 min) prompted the search and discovery of insulin-degrading enzyme (IDE), a 110-kDa metalloprotease that can rapidly degrade insulin into inactive fragments. Genetic and biochemical evidence accumulated in the last sixty years has implicated IDE as an important physiological contributor in the maintenance of insulin levels. Recent structural and biochemical analyses reveal the molecular basis of how IDE uses size and charge distribution of the catalytic chamber and structural flexibility of substrates to selectively recognize and degrade insulin, as well as the regulatory mechanisms of this enzyme. These studies provide a path for potential therapeutics in the control of insulin metabolism by the degradation of insulin.

IDE Gene Polymorphism Influences on BPSD in Mild Dementia of Alzheimer's Type

Insulin degrading enzyme (IDE) degrades amyloid beta (Abeta), which may inhibit the accumulation of Abeta in a brain affected with dementia of Alzheimer's type (DAT). A decrease in the activity of IDE results in changes in glucose utilization in the brain, which could affect the cognitive and psychiatric symptoms of DAT. We investigated a possible association of IDE gene polymorphism and the behavioral and psychological symptoms of dementia (BPSD) in mild DAT. The genotyping for IDE and apolipoprotein E (ApoE) was determined in 207 patients with mild DAT and 215 controls. The occurrence of BPSD was demonstrated using the Behavioral Pathology in Alzheimer's Disease Rating Scale (BEHAVE-AD). IDE gene polymorphism is unlikely to play a substantial role in conferring susceptibility to DAT, but it may be involved in the development of affective disturbance through the course of mild DAT, regardless of the presence of an ApoE epsilon4 allele. The present data could be the result of a small sample size. Further investigations using larger samples are thus required to clarify the correlation between IDE gene polymorphism, susceptibility to DAT, and emergence of BPSD.

Lymphocytes from patients with type 1 diabetes display a distinct profile of chromatin histone H3 lysine 9 dimethylation: an epigenetic study in diabetes

OBJECTIVE

The complexity of interactions between genes and the environment is a major challenge for type 1 diabetes studies. Nuclear chromatin is the interface between genetics and environment and the principal carrier of epigenetic information. Because histone tail modifications in chromatin are linked to gene transcription, we hypothesized that histone methylation patterns in cells from type 1 diabetic patients can provide novel epigenetic insights into type 1 diabetes and its complications.

RESEARCH DESIGN AND METHODS

We used chromatin immunoprecipitation (ChIP) linked to microarray (ChIP-chip) approach to compare genome-wide histone H3 lysine 9 dimethylation (H3K9me2) patterns in blood lymphocytes and monocytes from type 1 diabetic patients versus healthy control subjects. Bioinformatics evaluation of methylated candidates was performed by Ingenuity Pathway Analysis (IPA) tools.

RESULTS

A subset of genes in the type 1 diabetic cohort showed significant increase in H3K9me2 in lymphocytes but not in monocytes. CLTA4, a type 1 diabetes susceptibility gene, was one of the candidates displaying increased promoter H3K9me2 in type 1 diabetes. IPA identified two high-scoring networks that encompassed genes showing altered H3K9me2. Many of them were associated with autoimmune and inflammation-related pathways, such as transforming growth factor-beta, nuclear factor-kappaB, p38 mitogen-activated protein kinase, toll-like receptor, and interleukin-6. IPA also revealed biological relationships between these networks and known type 1 diabetes candidate genes.

CONCLUSIONS

The concerted and synergistic alteration of histone methylation within the identified network in lymphocytes might have an effect on the etiology of type 1 diabetes and its complications. These studies provide evidence of a novel association between type 1 diabetes and altered histone methylation of key genes that are components of type 1 diabetes-related biological pathways and also a new understanding of the pathology of type 1 diabetes.

Association of polymorphisms in the insulin-degrading enzyme gene with type 2 diabetes in the Korean population

Insulin-degrading enzyme (IDE) is a metalloproteinase which degrades insulin and terminates its action. Homologous deletion of IDE gene resulted in hyperinsulinemia and glucose intolerance in a rat model of type 2 diabetes mellitus. Several genetic association studies examined IDE as a susceptibility gene for type 2 diabetes in European descents. Here we investigated the genetic association of IDE polymorphisms with the risk of type 2 diabetes and its related phenotypes in the Korean population. Among six single nucleotide polymorphisms analyzed, g.-179T>C (OR=1.73, P=0.04), and g.IVS18+99G>A (OR=1.23, P=0.02) revealed borderline association with increased risk of type 2 diabetes. Combining our results with previous data obtained from the European population, g.-179T>C (OR=1.11, P=0.03), and g.IVS24-64A>T (OR=1.18, P=0.005) showed significant association with type 2 diabetes. Haplotype consisting of common alleles of the six polymorphisms was associated with decreased risk of type 2 diabetes (OR=0.82, P=0.02). However, none of the polymorphisms was significantly associated with metabolic phenotypes. We can conclude that variations in IDE might contribute to diabetes susceptibility in the Korean population.

Genome-wide search for susceptibility genes to type 2 diabetes in West Africans: potential role of C-peptide

C-peptide is a substance that the pancreas releases into the circulation in equimolar amounts to insulin and has demonstrated important physiological effects which relate to the vascular field, in particular the microcirculation. For this analysis, we included 321 full and 36 half sibling pairs affected with type 2 diabetes (T2D) from West Africa. A genome-wide panel of 390 tri-nucleotide and tetra-nucleotide repeats with an average distance of 8.9 cM was performed on a total of 691 persons. Variance components based on multipoint linkage approach as implemented in SOLAR were performed for log C-peptide. Significant linkage evidences were observed on 10q23 at D10S2327 with a LOD score of 4.04 (nominal p-value=0.000008, empirical p-value=0.0004); and on 4p15 at D4S2632 with a LOD score of 3.48 (nominal p-value=0.000031, empirical p-value=0.0013). Other suggestive evidence of linkage were observed on 15q14 at D15S659 with a LOD score 2.41 (nominal p-value=0.000435, empirical p-value=0.0068), and on 18p11 near D18S976 with a LOD score 2.18 (nominal p-value=0.000771 and empirical p-value=0.0094). Interestingly, five positional candidate genes for diabetes and related complications are located in our linkage region (the pituitary adenylate cyclase activating polypeptide (PACAP in 18p11); the peroxisome proliferator-activated receptor gamma coactivator 1 (PPARGC1 in 4p15); PTEN, PPP1R5, and IDE located in 10q23. In conclusion, we identified four major genetic loci (10q23, 4p15, 15q14, and 18p11) influencing C-peptide concentration in West Africans with T2D.

Genome-wide association scans for Type 2 diabetes: new insights into biology and therapy

Type 2 diabetes is a complex, multifactorial disease, for which genetic and environmental factors jointly determine susceptibility. Disentangling the genetic aetiology of Type 2 diabetes has proven a challenging task, rewarded, until recently, with only limited success. However, the field of Type 2 diabetes genetics has been transformed over the past few months, with the publication of six genome-wide association scans, leading to the establishment of novel genomic regions that harbour disease susceptibility loci. Here, we provide an overview of the main recent findings and discuss their significance in providing biological insights and their translational implications.

A new era for Type 2 diabetes genetics

Diabet. Med. (2007)

Association of genetic variants of insulin degrading enzyme with metabolic features in women with polycystic ovary syndrome

OBJECTIVE

To evaluate the influence of the four single nucleotide polymorphisms of the insulin-degrading enzyme (IDE) gene on metabolic features in women with polycystic ovary syndrome (PCOS) in a Chinese population.

DESIGN

Prospective, case-control study.

SETTING

University-based hospital.

PATIENT(S)

Three hundred fifteen patients with PCOS and 327 healthy controls.

INTERVENTION(S)

Peripheral venous puncture, ultrasonography, oral glucose tolerance test (OGTT).

MAIN OUTCOME MEASURE(S)

Genotype analysis of four single nucleotide polymorphisms in the IDE gene, hormonal and metabolic phenotypes.

RESULT(S)

No significant differences in genotypes of these polymorphisms were found between PCOS patients and healthy controls. But the frequency of the C allele of rs2209972 was significantly higher in the PCOS group than that in the control group. The single nucleotide polymorphisms rs4646953, rs1887922, and rs1544210 had no impact on clinical and biochemical characteristics of women with PCOS. There were significant differences in body mass index (BMI) and insulin level in the rs2209972 genotype of women with PCOS. The women with PCOS with the CC genotype of rs2209972 had statistically significantly higher fasting insulin level and homeostasis model assessment for insulin resistance than the women with PCOS with the TT genotype.

CONCLUSION(S)

The single nucleotide polymorphism rs2209972 in the human IDE gene is associated with metabolic features of PCOS women in a Chinese population.

Structure of substrate-free human insulin-degrading enzyme (IDE) and biophysical analysis of ATP-induced conformational switch of IDE

Insulin-degrading enzyme (IDE) is a zinc metalloprotease that hydrolyzes amyloid-beta (Abeta) and insulin, which are peptides associated with Alzheimer disease (AD) and diabetes, respectively. Our previous structural analysis of substrate-bound human 113-kDa IDE reveals that the N- and C-terminal domains of IDE, IDE-N and IDE-C, make substantial contact to form an enclosed catalytic chamber to entrap its substrates. Furthermore, IDE undergoes a switch between the closed and open conformations for catalysis. Here we report a substrate-free IDE structure in its closed conformation, revealing the molecular details of the active conformation of the catalytic site of IDE and new insights as to how the closed conformation of IDE may be kept in its resting, inactive conformation. We also show that Abeta is degraded more efficiently by IDE carrying destabilizing mutations at the interface of IDE-N and IDE-C (D426C and K899C), resulting in an increase in Vmax with only minimal changes to Km. Because ATP is known to activate the ability of IDE to degrade short peptides, we investigated the interaction between ATP and activating mutations. We found that these mutations rendered IDE less sensitive to ATP activation, suggesting that ATP might facilitate the transition from the closed state to the open conformation. Consistent with this notion, we found that ATP induced an increase in hydrodynamic radius, a shift in electrophoretic mobility, and changes in secondary structure. Together, our results highlight the importance of the closed conformation for regulating the activity of IDE and provide new molecular details that will facilitate the development of activators and inhibitors of IDE.

A genome-wide association study identifies novel risk loci for type 2 diabetes

Type 2 diabetes mellitus results from the interaction of environmental factors with a combination of genetic variants, most of which were hitherto unknown. A systematic search for these variants was recently made possible by the development of high-density arrays that permit the genotyping of hundreds of thousands of polymorphisms. We tested 392,935 single-nucleotide polymorphisms in a French case-control cohort. Markers with the most significant difference in genotype frequencies between cases of type 2 diabetes and controls were fast-tracked for testing in a second cohort. This identified four loci containing variants that confer type 2 diabetes risk, in addition to confirming the known association with the TCF7L2 gene. These loci include a non-synonymous polymorphism in the zinc transporter SLC30A8, which is expressed exclusively in insulin-producing beta-cells, and two linkage disequilibrium blocks that contain genes potentially involved in beta-cell development or function (IDE-KIF11-HHEX and EXT2-ALX4). These associations explain a substantial portion of disease risk and constitute proof of principle for the genome-wide approach to the elucidation of complex genetic traits.

The genetic landscape of type 2 diabetes in mice

Inbred mouse strains provide genetic diversity comparable to that of the human population. Like humans, mice have a wide range of diabetes-related phenotypes. The inbred mouse strains differ in the response of their critical physiological functions, such as insulin sensitivity, insulin secretion, beta-cell proliferation and survival, and fuel partitioning, to diet and obesity. Most of the critical genes underlying these differences have not been identified, although many loci have been mapped. The dramatic improvements in genomic and bioinformatics resources are accelerating the pace of gene discovery. This review describes how mouse genetics can be used to discover diabetes-related genes, summarizes how the mouse strains differ in their diabetes-related phenotypes, and describes several examples of how loci identified in the mouse may directly relate to human diabetes.

Plaque-Associated Overexpression of Insulin-Degrading Enzyme in the Cerebral Cortex of Aged Transgenic Tg2576 Mice With Alzheimer Pathology

It was proposed that insulin-degrading enzyme (IDE) participates in the clearance of amyloid beta (Abeta) in the brain, and its low expression or activity may be relevant for the progression of Alzheimer disease. We performed a longitudinal study of brain level, activity, and distribution of IDE in transgenic mice (Tg2576) expressing the Swedish mutation in human Abeta precursor protein. At 16 months of age, Tg2576 showed a significant 2-fold increment in IDE protein level as compared with 4.5- and 11-month-old animals. The peak of IDE was in synchrony with the sharp accumulation of sodium dodecyl sulfate-soluble Abeta and massive Abeta deposition into plaques. At this stage, IDE appeared surrounding Abeta fibrillar deposits within glial fibrillar acidic protein-positive astrocytes, suggesting that it was locally overexpressed during the Abeta-mediated inflammation process. When primary astrocytes were exposed to fibrillar Abeta in vitro, IDE protein level increased as compared with control, and this effect was reduced by the addition of U0126, a specific inhibitor of the ERK1/2 mitogen-activated protein kinase cascade. We propose that in Tg2576 mice and in contrast to its behavior in Alzheimer brains, active IDE increases with age around plaques as a component of astrocyte activation as a result of Abeta-triggered inflammation.

Insulin signaling in the central nervous system: Learning to survive

Insulin is best known for its role in peripheral glucose homeostasis. Less studied, but not less important, is its role in the central nervous system. Insulin and its receptor are located in the central nervous system and are both implicated in neuronal survival and synaptic plasticity. Interestingly, over the past few years it has become evident that the effects of insulin, on neuronal survival and synaptic plasticity, are mediated by a common signal transduction cascade, which has been identified as "the PI3K route". This route has turned out to be a major integrator of insulin signaling in the brain. A pronounced feature of this insulin-activated route is that it promotes survival by directly inactivating the pro-apoptotic machinery. Interestingly, it is this same route that is required for the induction of long-term potentiation and depression, basic processes underlying learning and memory. This leads to the hypothesis that the PI3K route forms a direct link between learning and memory and neuronal survival. The implications of this hypothesis are far reaching, since it provides an explanation why insulin has beneficial effects on learning and memory and how synaptic activity can prevent cellular degeneration. Applying this knowledge may provide novel therapeutic approaches in the treatment of neurodegenerative diseases such as Alzheimer's disease.

The C-terminal domain of human insulin degrading enzyme is required for dimerization and substrate recognition

Insulin degrading enzyme (IDE), a zinc metalloprotease, can specifically recognize and degrade insulin, as well as several amyloidogenic peptides such as amyloid beta (Abeta) and amylin. The disruption of IDE function in rodents leads to glucose intolerance and cerebral Abeta accumulation, hallmarks of type 2 diabetes and Alzheimer's disease, respectively. Using limited proteolysis, we found that human IDE (113kDa) can be subdivided into two roughly equal sized domains, IDE-N and IDE-C. Oligomerization plays a key role in the activity of IDE. Size-exclusion chromatography and sedimentation velocity experiments indicate that IDE-N is a monomer and IDE-C serves to oligomerize IDE-N. IDE-C alone does not have catalytic activity. It is IDE-N that contains the crucial catalytic residues, however IDE-N alone has only 2% of the catalytic activity of wild type IDE. By complexing IDE-C with IDE-N, the activity of IDE-N can be restored to approximately 30% that of wild type IDE. Fluorescence polarization assays using labeled insulin reveal that IDE-N has reduced affinity to insulin relative to wild type IDE. Together, our data reveal the modular nature of IDE. IDE-N is the catalytic domain and IDE-C facilitates substrate recognition as well as plays a key role in the oligomerization of IDE.

Genetic variations in the gene encoding TFAP2B are associated with type 2 diabetes mellitus

To search a gene(s) conferring susceptibility to type 2 diabetes mellitus, we genotyped nearly 60,000 gene-based SNPs for Japanese patients and found evidence that the gene at chromosome 6p12 encoding transcription-factor-activating protein 2beta (TFAP2B) was a likely candidate in view of significant association of polymorphism in this gene with type 2 diabetes. Extensive analysis of this region identified that several variations within TFAP2B were significantly associated with type 2 diabetes [a variable number of tandem repeat locus: chi(2)=10.9, P=0.0009; odds ratio=1.57, 95% CI 1.20-2.06, intron 1+774 (G/T); chi(2)=11.6, P=0.0006; odds ratio=1.60, 95% CI 1.22-2.09, intron 1+2093 (A/C); chi(2)=12.2, P=0.0004; odds ratio=1.61, 95% CI 1.23-2.11]. The association of TFAP2B with type 2 diabetes was also observed in the UK population. These results suggest that TFAP2B might be a new candidate for conferring susceptibility to type 2 diabetes and contribute to the pathogenesis of type 2 diabetes.

Genetics of the APM1 locus and its contribution to type 2 diabetes susceptibility in French Caucasians

We have carried out a detailed reexamination of the genetics of the APM1 locus and its contribution to the genetic basis of type 2 diabetes susceptibility in the French Caucasian population. The G allele of single nucleotide polymorphism -11426 in the APM1 promoter showed modest association with type 2 diabetes (odds ratio 1.44 [95% CI 1.04-1.98]; P = 0.03), providing corroborative evidence that single nucleotide polymorphisms in the APM1 promoter region contribute to the genetic risk of type 2 diabetes. A "sliding window" analysis identified haplotypes 1-1-1, 1-1-1-1, and 1-1-1-1-1 as being strongly protective against type 2 diabetes (P </= 0.0001). Evidence is presented that the APM1 gene is a locus of low linkage disequilibrium, high haplotype diversity, and high recombination. We were unable to obtain data to support the hypothesis that genetic variation in the APM1 gene is a major contributor to the type 2 diabetes linkage result at chromosome 3q27. Finally, in families with early-onset type 2 diabetes, we obtained suggestive evidence of a linkage peak for serum adiponectin levels (logarithm of odds = 2.1) that closely matched the position of the type 2 diabetes linkage peak. This result indicated that the type 2 diabetes susceptibility locus at 3q27 influences both genetic predisposition to type 2 diabetes and serum adiponectin levels in patients with type 2 diabetes.

Quantitative trait locus dissection in congenic strains of the Goto-Kakizaki rat identifies a region conserved with diabetes loci in human chromosome 1q

Genetic studies in human populations and rodent models have identified regions of human chromosome 1q21–25 and rat chromosome 2 showing evidence of significant and replicated linkage to diabetes-related phenotypes. To investigate the relationship between the human and rat diabetes loci, we fine mapped the rat locus Nidd/ gk2 linked to hyperinsulinemia in an F2 cross derived from the diabetic (type 2) Goto-Kakizaki (GK) rat and the Brown Norway (BN) control rat, and carried out its genetic and pathophysiological characterization in BN.GK congenic strains. Evidence of glucose intolerance and enhanced insulin secretion in a congenic strain allowed us to localize the underlying diabetes gene(s) in a rat chromosomal interval of ∼3–6 cM conserved with an 11-Mb region of human 1q21–23. Positional diabetes candidate genes were tested for transcriptional changes between congenics and controls and sequence variations in a panel of inbred rat strains. Congenic strains of the GK rats represent powerful novel models for accurately defining the pathophysiological impact of diabetes gene(s) at the locus Nidd/ gk2 and improving functional annotations of diabetes candidates in human 1q21–23.

Quantitative trait loci near the insulin-degrading enzyme (IDE) gene contribute to variation in plasma insulin levels

Insulin-degrading enzyme (IDE) plays a principal role in the proteolysis of several peptides in addition to insulin and is encoded by IDE, which resides in a region of chromosome 10q that is linked to type 2 diabetes. Two recent studies presented genetic association data on IDE and type 2 diabetes (one positive and the other negative), but neither explored the fundamental question of whether polymorphism in IDE has a measurable influence on insulin levels in human populations. To address this possibility, 14 single nucleotide polymorphisms (SNPs) from a linkage disequilibrium block encompassing IDE have been genotyped in a sample of 321 impaired glucose tolerant and 403 nondiabetic control subjects. Analyses based on haplotypic genotypes (diplotypes), constructed with SNPs that differentiate common extant haplotypes extending across IDE, provided compelling evidence of association with fasting insulin levels (P = 0.0009), 2-h insulin levels (P = 0.0027), homeostasis model assessment of insulin resistance (P = 0.0001), and BMI (P = 0.0067), with effects exclusively evident in men. The strongest evidence for an effect of a single marker was obtained for rs2251101 (located near the 3' untranslated region of IDE) on 2-h insulin levels (P = 0.000023). Diplotype analyses, however, suggest the presence of multiple interacting trait-modifying sequences in the region. Results indicate that polymorphism in/near IDE contributes to a large proportion of variance in plasma insulin levels and correlated traits, but questions of sex specificity and allelic heterogeneity will need to be taken into consideration as the molecular basis of the observed phenotypic effects unfolds.