Glucocorticoid-mediated induction of ZBTB16 affects insulin secretion in human islets and EndoC-ßH1 ß-cells.

Glucocorticoid use is associated with steroid-induced diabetes mellitus and impaired pancreatic ß-cell insulin secretion. Here, the glucocorticoid-mediated transcriptomic changes in human pancreatic islets and the human insulin-secreting EndoC-ßH1 cells were investigated to uncover genes involved in ß-cell steroid stress-response processes. Bioinformatics analysis revealed glucocorticoids to exert their effects mainly on enhancer genomic regions in collaboration with auxiliary transcription factor families including AP-1, ETS/TEAD, and FOX. Remarkably, we identified the transcription factor ZBTB16 as a highly confident direct glucocorticoid target. Glucocorticoid-mediated induction of ZBTB16 was time- and dose-dependent. Manipulation of ZBTB16 expression in EndoC-ßH1 cells combined with dexamethasone treatment demonstrated its protective role against glucocorticoid-induced reduction of insulin secretion and mitochondrial function impairment. In conclusion, we determine the molecular impact of glucocorticoids on human islets and insulin-secreting cells and investigate the effects of glucocorticoid targets on ß-cell function. Our findings can pave the way for therapies against steroid-induced diabetes mellitus.

Type 2 diabetes candidate genes, including PAX5, cause impaired insulin secretion in human pancreatic islets.

Type 2 diabetes (T2D) is caused by insufficient insulin secretion from pancreatic ß cells. To identify candidate genes contributing to T2D pathophysiology, we studied human pancreatic islets from approximately 300 individuals. We found 395 differentially expressed genes (DEGs) in islets from individuals with T2D, including, to our knowledge, novel (OPRD1, PAX5, TET1) and previously identified (CHL1, GLRA1, IAPP) candidates. A third of the identified expression changes in islets may predispose to diabetes, as expression of these genes associated with HbA1c in individuals not previously diagnosed with T2D. Most DEGs were expressed in human ß cells, based on single-cell RNA-Seq data. Additionally, DEGs displayed alterations in open chromatin and associated with T2D SNPs. Mouse KO strains demonstrated that the identified T2D-associated candidate genes regulate glucose homeostasis and body composition in vivo. Functional validation showed that mimicking T2D-associated changes for OPRD1, PAX5, and SLC2A2 impaired insulin secretion. Impairments in Pax5-overexpressing ß cells were due to severe mitochondrial dysfunction. Finally, we discovered PAX5 as a potential transcriptional regulator of many T2D-associated DEGs in human islets. Overall, we have identified molecular alterations in human pancreatic islets that contribute to ß cell dysfunction in T2D pathophysiology.

Human pancreatic islet miRNA-mRNA networks of altered miRNAs due to glycemic status.

MicroRNAs (miRNAs) are short non-coding RNAs that regulate gene expression via mRNA targeting, playing important roles in the pancreatic islets. We aimed to identify molecular pathways and genomic regulatory regions associated with altered miRNA expression due to glycemic status, which could contribute to the development of type 2 diabetes (T2D). To this end, miRNAs were identified by a combination of differential miRNA expression and correlation analysis in human islet samples from donors with normal and elevated blood glucose levels. Analysis and clustering of highly correlated, experimentally validated gene targets of these miRNAs revealed two islet-specific clusters, which were associated with key aspects of islet functions and included a high number of T2D-related genes. Finally, cis-eQTLs and public GWAS data integration uncovered suggestive genomic signals of association with insulin secretion and T2D. The miRNA-driven network-based approach presented in this study contributes to a better understanding of impaired insulin secretion in T2D pathogenesis.

Ribosomal biogenesis regulator DIMT1 controls ß-cell protein synthesis, mitochondrial function, and insulin secretion.

We previously reported that loss of mitochondrial transcription factor B1 (TFB1M) leads to mitochondrial dysfunction and is involved in the pathogenesis of type 2 diabetes (T2D). Whether defects in ribosomal processing impact mitochondrial function and could play a pathogenetic role in ß-cells and T2D is not known. To this end, we explored expression and the functional role of dimethyladenosine transferase 1 homolog (DIMT1), a homolog of TFB1M and a ribosomal RNA (rRNA) methyltransferase implicated in the control of rRNA. Expression of DIMT1 was increased in human islets from T2D donors and correlated positively with expression of insulin mRNA, but negatively with insulin secretion. We show that silencing of DIMT1 in insulin-secreting cells impacted mitochondrial function, leading to lower expression of mitochondrial OXPHOS proteins, reduced oxygen consumption rate, dissipated mitochondrial membrane potential, and a slower rate of ATP production. In addition, the rate of protein synthesis was retarded upon DIMT1 deficiency. Consequently, we found that DIMT1 deficiency led to perturbed insulin secretion in rodent cell lines and islets, as well as in a human ß-cell line. We observed defects in rRNA processing and reduced interactions between NIN1 (RPN12) binding protein 1 homolog (NOB-1) and pescadillo ribosomal biogenesis factor 1 (PES-1), critical ribosomal subunit RNA proteins, the dysfunction of which may play a part in disturbing protein synthesis in ß-cells. In conclusion, DIMT1 deficiency perturbs protein synthesis, resulting in mitochondrial dysfunction and disrupted insulin secretion, both potential pathogenetic processes in T2D.

Human Islet MicroRNA-200c Is Elevated in Type 2 Diabetes and Targets the Transcription Factor ETV5 to Reduce Insulin Secretion.

MicroRNAs (miRNAs) are part of deregulated insulin secretion in type 2 diabetes (T2D) development. Rodent models have suggested miR-200c to be involved, but the role and potential as therapeutic target of this miRNA in human islets are not clear. Here we report increased expression of miR-200c in islets from T2D as compared with nondiabetic (ND) donors and display results showing reduced glucose-stimulated insulin secretion in EndoC-ßH1 cells overexpressing miR-200c. We identify transcription factor ETV5 as the top rank target of miR-200c in human islets using TargetScan in combination with Pearson correlation analysis of miR-200c and mRNA expression data from the same human donors. Among other targets were JAZF1, as earlier shown in miR-200 knockout mice. Accordingly, linear model analysis of ETV5 and JAZF1 gene expression showed reduced expression of both genes in islets from human T2D donors. Western blot analysis confirmed the reduced expression of ETV5 on the protein level in EndoC-ßH1 cells overexpressing miR-200c, and luciferase assay validated ETV5 as a direct target of miR-200c. Finally, LNA knockdown of miR-200c increased glucose-stimulated insulin secretion in islets from T2D donors approximately threefold. Our data reveal a vital role of the miR-200c-ETV5 axis in ß-cell dysfunction and pathophysiology of T2D.

TIGER: The gene expression regulatory variation landscape of human pancreatic islets.

Genome-wide association studies (GWASs) identified hundreds of signals associated with type 2 diabetes (T2D). To gain insight into their underlying molecular mechanisms, we have created the translational human pancreatic islet genotype tissue-expression resource (TIGER), aggregating >500 human islet genomic datasets from five cohorts in the Horizon 2020 consortium T2DSystems. We impute genotypes using four reference panels and meta-analyze cohorts to improve the coverage of expression quantitative trait loci (eQTL) and develop a method to combine allele-specific expression across samples (cASE). We identify >1 million islet eQTLs, 53 of which colocalize with T2D signals. Among them, a low-frequency allele that reduces T2D risk by half increases CCND2 expression. We identify eight cASE colocalizations, among which we found a T2D-associated SLC30A8 variant. We make all data available through the TIGER portal (http://tiger.bsc.es), which represents a comprehensive human islet genomic data resource to elucidate how genetic variation affects islet function and translates into therapeutic insight and precision medicine for T2D.

Differential DNA Methylation and Expression of miRNAs in Adipose Tissue From Twin Pairs Discordant for Type 2 Diabetes.

The prevalence of type 2 diabetes (T2D) is increasing worldwide, but current treatments have limitations. miRNAs may play a key role in the development of T2D and can be targets for novel therapies. Here, we examined whether T2D is associated with altered expression and DNA methylation of miRNAs using adipose tissue from 14 monozygotic twin pairs discordant for T2D. Four members each of the miR-30 and let-7-families were downregulated in adipose tissue of subjects with T2D versus control subjects, which was confirmed in an independent T2D case-control cohort. Further, DNA methylation of five CpG sites annotated to gene promoters of differentially expressed miRNAs, including miR-30a and let-7a-3, was increased in T2D versus control subjects. Luciferase experiments showed that increased DNA methylation of the miR-30a promoter reduced its transcription in vitro. Silencing of miR-30 in adipocytes resulted in reduced glucose uptake and TBC1D4 phosphorylation; downregulation of genes involved in demethylation and carbohydrate/lipid/amino acid metabolism; and upregulation of immune system genes. In conclusion, T2D is associated with differential DNA methylation and expression of miRNAs in adipose tissue. Downregulation of the miR-30 family may lead to reduced glucose uptake and altered expression of key genes associated with T2D.

Replication study reveals miR-483-5p as an important target in prevention of cardiometabolic disease.

BACKGROUND: Alterations in levels of circulating micro-RNAs might reflect within organ signaling or subclinical tissue injury that is linked to risk of diabetes and cardiovascular risk. We previously found that serum levels of miR-483-5p is correlated with cardiometabolic risk factors and incidence of cardiometabolic disease in a case-control sample from the populations-based Malmö Diet and Cancer Study Cardiovascular Cohort (MDC-CC). We here aimed at replicating these findings and to test for association with carotid atherosclerosis. METHODS: We measured miR-483-5p in fasting serum of 1223 healthy subjects from the baseline examination of the population-based, prospective cohort study Malmö Offspring Study (MOS) and correlated miR-483-5p to cardiometabolic risk factors and to incidence of diabetes mellitus and coronary artery disease (CAD) during 3.7 (± 1.3) years of follow-up using logistic regression. In both MOS and MDC-CC we related mir-483-5p to carotid atherosclerosis measured with ultrasound. RESULTS: In cross-sectional analysis miR-483-5p was correlated with BMI, waist circumference, HDL, and sex. After adjustment for age and sex, the association remained significant for all risk factors except for HDL. Logistic regression analysis showed significant associations between miR-483-5p and new-onset diabetes (OR = 1.94, 95% CI 1.06-3.56, p = 0.032) and cardiovascular disease (OR = 1.99, 95% CI 1.06-3.75, p = 0.033) during 3.7 (± 1.3) years of follow-up. Furthermore, miR-483-5p was significantly related with maximum intima-media thickness of the carotid bulb in MDC-CC (p = 0.001), but not in MOS, whereas it was associated with increasing number of plaques in MOS (p = 0.007). CONCLUSION: miR-483-5p is related to an unfavorable cardiometabolic risk factor profile and predicts diabetes and CAD, possibly through an effect on atherosclerosis. Our results encourage further studies of possible underlying mechanisms and means of modifying miR-483-5p as a possible interventional target in prevention of cardiometabolic disease.

Transcriptional analysis of islets of Langerhans from organ donors of different ages.

Insulin secretion is impaired with increasing age. In this study, we aimed to determine whether aging induces specific transcriptional changes in human islets. Laser capture microdissection was used to extract pancreatic islet tissue from 37 deceased organ donors aged 1-81 years. The transcriptomes of the extracted islets were analysed using Ion AmpliSeq sequencing. 346 genes that co-vary significantly with age were found. There was an increased transcription of genes linked to senescence, and several aspects of the cell cycle machinery were downregulated with increasing age. We detected numerous genes not linked to aging in previous studies likely because earlier studies analysed islet cells isolated by enzymatic digestion which might affect the islet transcriptome. Among the novel genes demonstrated to correlate with age, we found an upregulation of SPP1 encoding osteopontin. In beta cells, osteopontin has been seen to be protective against both cytotoxicity and hyperglycaemia. In summary, we present a transcriptional profile of aging in human islets and identify genes that could affect disease course in diabetes.

Diagnostic potential of miR-483 family for IGF-II producing non-islet cell tumor hypoglycemia.

OBJECTIVE: In insulin-like growth factor II (IGF-II) producing non-islet cell tumor hypoglycemia (NICTH), high molecular weight forms of IGF-II (big IGF-II) are produced as a cause of spontaneous hypoglycemia. MicroRNA (miRNA)-483 family, encoded in an intron lesion of IGF2 gene, is suggested to be co-expressed with IGF-II. Here, we tested whether serum miR-483-5p and -3p levels are associated with the presence of big IGF-II in NICTH. DESIGN: Serum samples from patients who were suspected to have IGF-II producing NICTH (n = 42) were tested. MiR-483-5p and -3p levels were evaluated using quantitative PCR. IGF-II level was analyzed using ELISA. The presence of big IGF-II was identified by Western blotting. RESULTS: Big IGF-II was detected in the sera of 32 patients. MiR-483-5p (P = 0.0015) and -3p (P = 0.027) levels were significantly higher in sera with big IGF-II (n = 32) than in those without (n = 10), whereas serum IGF-II level (P = 0.055) was not significantly different between the groups. The median serum concentration of miR-483-5p was ~10 times higher than that of miR-483-3p. Although a strong correlation was observed between the two miRNAs (r = 0.844, P < 0.0001), but neither of which was correlated with serum IGF-II level. The areas under the receiver operating characteristic curves of miR-483-5p (0.853) and -3p (0.722) were higher than that of IGF-II (0.694) for detecting the presence of big IGF-II. CONCLUSION: The associations of serum miR-483-5p and -3p levels with the presence of big IGF-II suggest the diagnostic potential of these miRNAs for IGF-II producing NICTH.

A circular RNA generated from an intron of the insulin gene controls insulin secretion.

Fine-tuning of insulin release from pancreatic ß-cells is essential to maintain blood glucose homeostasis. Here, we report that insulin secretion is regulated by a circular RNA containing the lariat sequence of the second intron of the insulin gene. Silencing of this intronic circular RNA in pancreatic islets leads to a decrease in the expression of key components of the secretory machinery of ß-cells, resulting in impaired glucose- or KCl-induced insulin release and calcium signaling. The effect of the circular RNA is exerted at the transcriptional level and involves an interaction with the RNA-binding protein TAR DNA-binding protein 43 kDa (TDP-43). The level of this circularized intron is reduced in the islets of rodent diabetes models and of type 2 diabetic patients, possibly explaining their impaired secretory capacity. The study of this and other circular RNAs helps understanding ß-cell dysfunction under diabetes conditions, and the etiology of this common metabolic disorder.

MicroRNA Networks in Pancreatic Islet Cells: Normal Function and Type 2 Diabetes.

Impaired insulin secretion from the pancreatic ß-cells is central in the pathogenesis of type 2 diabetes (T2D), and microRNAs (miRNAs) are fundamental regulatory factors in this process. Differential expression of miRNAs contributes to ß-cell adaptation to compensate for increased insulin resistance, but deregulation of miRNA expression can also directly cause ß-cell impairment during the development of T2D. miRNAs are small noncoding RNAs that posttranscriptionally reduce gene expression through translational inhibition or mRNA destabilization. The nature of miRNA targeting implies the presence of complex and large miRNA-mRNA regulatory networks in every cell, including the insulin-secreting ß-cell. Here we exemplify one such network using our own data on differential miRNA expression in the islets of T2D Goto-Kakizaki rat model. Several biological processes are influenced by multiple miRNAs in the ß-cell, but so far most studies have focused on dissecting the mechanism of action of individual miRNAs. In this Perspective we present key islet miRNA families involved in T2D pathogenesis including miR-200, miR-7, miR-184, miR-212/miR-132, and miR-130a/b/miR-152. Finally, we highlight four challenges and opportunities within islet miRNA research, ending with a discussion on how miRNAs can be utilized as therapeutic targets contributing to personalized T2D treatment strategies.

Selectively Bred Diabetes Models: GK Rats, NSY Mice, and ON Mice.

The polygenic background of selectively bred diabetes models mimics the etiology of type 2 diabetes. So far, three different rodent models (Goto-Kakizaki rats, Nagoya-Shibata-Yasuda mice, and Oikawa-Nagao mice) have been established in the diabetes research field by continuous selective breeding for glucose tolerance from outbred rodent stocks. The origin of hyperglycemia in these rodents is mainly insulin secretion deficiency from the pancreatic ß-cells and mild insulin resistance in insulin target organs. In this chapter, we summarize backgrounds and phenotypes of these rodent models to highlight their importance in diabetes research. Then, we introduce experimental methodologies to evaluate ß-cell exocytosis as a putative common defect observed in these rodent models.

Potential Protection Against Type 2 Diabetes in Obesity Through Lower CD36 Expression and Improved Exocytosis in ß-Cells.

Obesity is a risk factor for type 2 diabetes (T2D); however, not all obese individuals develop the disease. In this study, we aimed to investigate the cause of differential insulin secretion capacity of pancreatic islets from donors with T2D and non-T2D (ND), especially obese donors (BMI ≥30 kg/m2). Islets from obese donors with T2D had reduced insulin secretion, decreased ß-cell exocytosis, and higher expression of fatty acid translocase CD36. We tested the hypothesis that CD36 is a key molecule in the reduced insulin secretion capacity. Indeed, CD36 overexpression led to decreased insulin secretion, impaired exocytosis, and reduced granule docking. This was accompanied by reduced expression of the exocytotic proteins SNAP25, STXBP1, and VAMP2, likely because CD36 induced downregulation of the insulin receptor substrate (IRS) proteins, suppressed the insulin-signaling phosphatidylinositol 3-kinase/AKT pathway, and increased nuclear localization of the transcription factor FoxO1. CD36 antibody treatment of the human ß-cell line EndoC-ßH1 increased IRS1 and exocytotic protein levels, improved granule docking, and enhanced insulin secretion. Our results demonstrate that ß-cells from obese donors with T2D have dysfunctional exocytosis likely due to an abnormal lipid handling represented by differential CD36 expression. Hence, CD36 could be a key molecule to limit ß-cell function in T2D associated with obesity.

Glucocorticoid induces human beta cell dysfunction by involving riborepressor GAS5 LincRNA.

OBJECTIVE: A widely recognized metabolic side effect of glucocorticoid (GC) therapy is steroid-induced diabetes mellitus (DM). However, studies on the molecular basis of GC-induced pancreatic beta cell dysfunction in human beta cells are lacking. The significance of non-coding RNAs in various cellular processes is emerging. In this study, we aimed to show the direct negative impact of GC on beta cell function and elucidate the role of riborepressor GAS5 lincRNA in the GC signaling pathway in human pancreatic beta cells. METHODS: Patients undergoing two weeks of high-dose prednisolone therapy were monitored for C-peptide levels. Human pancreatic islets and the human beta cell line EndoC-ßH1 were incubated in pharmacological concentrations of dexamethasone. The GAS5 level was modulated using anti-sense LNA gapmeR or short oligonucleotides with GAS5 HREM (hormone response element motif). Immunoblotting and/or real-time PCR were used to assess changes in protein and RNA expression, respectively. Functional characterization included glucose-stimulated insulin secretion and apoptosis assays. Correlation analysis was performed on RNAseq data of human pancreatic islets. RESULTS: We found reduced C-peptide levels in patients undergoing high-dose GC therapy. Human islets and the human beta cell line EndoC-ßH1 exposed to GC exhibited reduced insulin secretion and increased apoptosis. Concomitantly, reduced expression of important beta cell transcription factors, PDX1 and NKX6-1, as well as exocytotic protein SYT13 were observed. The expression of the glucocorticoid receptor was decreased, while that of serum and glucocorticoid-regulated kinase 1 (SGK1) was elevated. The expression of these genes was found to significantly correlate with GAS5 in human islet transcriptomics data. Increasing GAS5 levels using GAS5 HREM alleviated the inhibitory effects of dexamethasone on insulin secretion. CONCLUSIONS: The direct adverse effect of glucocorticoid in human beta cell function is mediated via important beta cell proteins and components of the GC signaling pathway in an intricate interplay with GAS5 lincRNA, a potentially novel therapeutic target to counter GC-mediated beta cell dysfunction.

Defective exocytosis and processing of insulin in a cystic fibrosis mouse model.

Cystic fibrosis-related diabetes (CFRD) is a common complication for patients with cystic fibrosis (CF), a disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). The cause of CFRD is unclear, but a commonly observed reduction in first-phase insulin secretion suggests defects at the beta cell level. Here we aimed to examine beta- and alpha-cell function in the Cftrtm1EUR/F508del mouse model (C57BL/6J), which carries the most common human mutation in CFTR, the F508del mutation. CFTR expression, beta cell mass, insulin granule distribution, hormone secretion and single cell capacitance changes were evaluated using islets (or beta cells) from F508del mice and age-matched wild-type mice aged 7-10 weeks. Granular pH was measured with DND-189 fluorescence. Serum glucose, insulin and glucagon levels were measured in vivo, and glucose tolerance was assessed using IPGTT. We show increased secretion of proinsulin and concomitant reduced secretion of C-peptide in islets from F508del mice compared to WT mice. Exocytosis and number of docked granules was reduced. We confirmed reduced granular pH by CFTR stimulation. We detected decreased pancreatic beta cell area, but unchanged beta cell number. Moreover, the F508del mutation caused failure to suppress glucagon secretion leading to hyperglucagonemia. In conclusion, F508del mice have beta cell defects resulting in 1) reduced number of docked insulin granules and reduced exocytosis, and 2) potential defective proinsulin cleavage and secretion of immature insulin. These observations provide insight into the functional role of CFTR in pancreatic islets and contribute to increased understanding of the pathogenesis of CFRD.

In Vivo Silencing of MicroRNA-132 Reduces Blood Glucose and Improves Insulin Secretion.

Dysfunctional insulin secretion is a hallmark of type 2 diabetes (T2D). Interestingly, several islet microRNAs (miRNAs) are upregulated in T2D, including miR-132. We aimed to investigate whether in vivo treatment with antagomir-132 lowers expression of miR-132 in islets thereby improving insulin secretion and lowering blood glucose. Mice injected with antagomir-132 for 24 h, had reduced expression of miR-132 expression in islets, decreased blood glucose, and increased insulin secretion. In isolated human islets treated with antagomir-132, insulin secretion from four of six donors increased. Target prediction coupled with analysis of miRNA-messenger RNA expression in human islets revealed DESI2, ARIH1, SLC25A28, DIAPH1, and FOXA1 to be targets of miR-132 that are conserved in both species. Increased expression of these targets was validated in mouse islets after antagomir-132 treatment. In conclusion, we identified a post-transcriptional role for miR-132 in insulin secretion, and demonstrated that systemic antagomir-132 treatment in mice can be used to improve insulin secretion and reduce blood glucose in vivo. Our study is a first step towards utilizing antagomirs as therapeutic agents to modulate islet miRNA levels to improve beta cell function.

miR-483-5p associates with obesity and insulin resistance and independently associates with new onset diabetes mellitus and cardiovascular disease.

Our aim was to identify serum microRNAs (miRNAs) in healthy humans which associate with future onset of both diabetes mellitus and cardiovascular disease. We performed global profiling of 753 mature human miRNAs in serum of 12 pilot subjects followed by measurement of 47 consistently expressed miRNAs in fasting serum of 553 healthy subjects from the baseline exam (1991-1994) of the population based Malmö Diet and Cancer Study Cardiovascular Cohort (MDC-CC), of whom 140 developed diabetes, and 169 cardiovascular diseases during follow-up. We used multivariate logistic regression to test individual miRNAs for association with incident diabetes and cardiovascular disease as compared to control subjects (n = 259). After Bonferroni correction and adjustment for age and sex, each SD increment of log-transformed miR-483-5p was significantly associated with both incident diabetes (OR = 1.48; 95% CI 1.18-1.84, P = 0.001) and cardiovascular disease (OR = 1.40; 95% CI 1.15, 1.72, P = 0.001). In cross sectional analysis, miR-483-5p was correlated with BMI (r = 0.162, P = 0.0001), fasting insulin (r = 0.156, P = 0.0002), HDL (r = -0.099, P = 0.02) and triglycerides (r = 0.11, P = 0.01). Adjustment for these metabolic risk factors, as well as traditional risk factors attenuated the miR-483-5p association with incident diabetes (OR = 1.28 95% CI 1.00-1.64, P = 0.049) whereas its association with incident cardiovascular disease remained virtually unchanged (OR = 1.46 95% CI, 1.18-1.81, P = 0.0005). In conclusion, miR-483-5p associates with both diabetes and cardiovascular disease. The association with diabetes seems partly mediated by obesity and insulin resistance, whereas the association with incident cardiovascular disease is independent of these metabolic factors and traditional cardiovascular disease risk factors.

MicroRNAs in islet hormone secretion.

Pancreatic islet hormone secretion is central in the maintenance of blood glucose homeostasis. During development of hyperglycaemia, the ß-cell is under pressure to release more insulin to compensate for increased insulin resistance. Failure of the ß-cells to secrete enough insulin results in type 2 diabetes (T2D). MicroRNAs (miRNAs) are short non-coding RNA molecules suitable for rapid regulation of the changes in target gene expression needed in ß-cell adaptations. Moreover, miRNAs are involved in the maintenance of α-cell and ß-cell phenotypic identities via cell-specific, or cell-enriched expression. Although many of the abundant miRNAs are highly expressed in both cell types, recent research has focused on the role of miRNAs in ß-cells. It has been shown that highly abundant miRNAs, such as miR-375, are involved in several cellular functions indispensable in maintaining ß-cell phenotypic identity, almost acting as "housekeeping genes" in the context of hormone secretion. Despite the abundance and importance of miR-375, it has not been shown to be differentially expressed in T2D islets. On the contrary, the less abundant miRNAs such as miR-212/miR-132, miR-335, miR-130a/b and miR-152 are deregulated in T2D islets, wherein the latter three miRNAs were shown to play key roles in regulating ß-cell metabolism. In this review, we focus on ß-cell function and describe miRNAs involved in insulin biosynthesis and processing, glucose uptake and metabolism, electrical activity and Ca2+ -influx and exocytosis of the insulin granules. We present current status on miRNA regulation in α-cells, and finally we discuss the involvement of miRNAs in ß-cell dysfunction underlying T2D pathogenesis.

Islet microRNAs in health and type-2 diabetes.

Failure of the ß-cell to secrete enough insulin is a major contributing factor in the pathogenesis of type-2 diabetes (T2D). MicroRNAs provide an extra layer in the regulation of protein expression, and are thus involved in ß-cell compensation during development of the disease. In this review, we discuss how microRNAs can regulate their target protein expression and phenotypic output, present the status of nutritional regulation of microRNA expression, and summarize work on microRNA expression in human islets. In conclusion, current data lend support to microRNAs being essential regulators of insulin secretion. Future work will describe microRNAs in α-cell function, details of the microRNA-mRNA network, and possibilities to use microRNAs as biomarkers and in therapeutic treatment of T2D and complications.