Contribution of Intronic miR-338-3p and Its Hosting Gene AATK to Compensatory β-Cell Mass Expansion

Circular RNAs in human diseases

Circular RNAs (circRNAs) are a unique class of RNA molecules formed through back-splicing rather than linear splicing. As an emerging field in molecular biology, circRNAs have garnered significant attention due to their distinct structure and potential functional implications. A comprehensive understanding of circRNAs' functions and potential clinical applications remains elusive despite accumulating evidence of their involvement in disease pathogenesis. Recent research highlights their significant roles in various human diseases, but comprehensive reviews on their functions and applications remain scarce. This review provides an in-depth examination of circRNAs, focusing first on their involvement in non-neoplastic diseases such as respiratory, endocrine, metabolic, musculoskeletal, cardiovascular, and renal disorders. We then explore their roles in tumors, with particular emphasis on exosomal circular RNAs, which are crucial for cancer initiation, progression, and resistance to treatment. By detailing their biogenesis, functions, and impact on disease mechanisms, this review underscores the potential of circRNAs as diagnostic biomarkers and therapeutic targets. The review not only enhances our understanding of circRNAs' roles in specific diseases and tumor types but also highlights their potential as novel diagnostic and therapeutic tools, thereby paving the way for future clinical investigations and potential therapeutic interventions.

Differential Gene Expression in the Penile Cavernosum of Streptozotocin-Induced Diabetic Rats

PURPOSE

Men with diabetes mellitus (DM) often present with severe erectile dysfunction (ED). This ED is less responsive to current pharmacological therapies. If we know the upregulated or downregulated genes of diabetic ED, we can inhibit or enhance the expression of such genes through RNA or gene overexpression.

METHODS

To investigate gene changes associated with ED in type 1 DM, we examined the alterations of gene expression in the cavernosum of streptozotocin-induced diabetic rats. Specifically, we considered 11,636 genes (9,623 upregulated and 2,013 downregulated) to be differentially expressed in the diabetic rat cavernosum group (n=4) compared to the control group (n=4). The analysis of differentially expressed genes using the gene ontology (GO) classification indicated that the following were enriched: downregulated genes such as cell cycle, extracellular matrix, glycosylphosphatidylinositol-anchor biosynthesis and upregulated genes such as calcium signaling, neurotrophin signaling, apoptosis, arginine and proline metabolism, gap junction, transforming growth factor-β signaling, tight junction, vascular smooth muscle contraction, and vascular endothelial growth factor (VEGF) signaling. We examined a more than 2-fold upregulated or downregulated change in expression, using real time polymerase chain reaction. Analysis of differentially expressed genes, using the GO classification, indicated the enrichment.

RESULTS

Of the 41,105 genes initially considered, statistical filtering of the array analysis showed 9,623 upregulated genes and 2,013 downregulated genes with at least 2-fold changes in expression (P<0.05). With Bonferroni correction, SLC2A9 (solute carrier family 2 member 9), LRRC20 (leucine rick repeat containing 20), PLK1 (polo like kinase 1), and AATK (apoptosis-associated tyrosine kinase) were all 2-fold changed genes.

CONCLUSION

This study broadens the scope of candidate genes that may be relevant to the pathophysiology of diabetic ED. In particular, their enhancement or inhibition could represent a novel treatment for diabetic ED.

Multiple novel functions of circular RNAs in diabetes mellitus

Circular RNAs (circRNAs), as an emerging group of non-coding RNAs (ncRNAs), have received the attention given evidence indicating that these novel ncRNAs are implicated in various biological processes. Due to the absence of 5' and 3' ends in circ-RNAs, their two ends are covalently bonded together, and they are synthesised from pre-mRNAs in a process called back-splicing, which makes them more stable than linear RNAs. There is accumulating evidence showing that circRNAs play a critical role in the pathogenesis of diabetes mellitus (DM). Moreover, it has been indicated that dysregulation of circRNAs has made them promising diagnostic biomarkers for the detection of DM. Recently, increasing attention has been paid to investigate the mechanisms underlying the DM process. It has been demonstrated that there is a strong correlation between the expression of circRNAs and DM. Hence, our aim is to discuss the crosstalk between circRNAs and DM and its complications.

Circular RNAs in diabetes and its complications: Current knowledge and future prospects

A novel class of non-coding RNA transcripts called circular RNAs (circRNAs) have been the subject of significant recent studies. Accumulating evidence points that circRNAs play an important role in the cellular processes, inflammatory expression, and immune responses through sponging miRNA, binding, or translating in proteins. Studies have found that circRNAs are involved in the physiologic and pathologic processes of diabetes. There has been an increased focus on the relevance of between abnormal circRNA expression and the development and progression of various types of diabetes and diabetes-related diseases. These circRNAs not only serve as promising diagnostic and prognostic molecular biomarkers, but also have important biological roles in islet cells, diabetes, and its complications. In addition, many circRNA signaling pathways have been found to regulate the occurrence and development of diabetes. Here we comprehensively review and discuss recent advances in our understanding of the physiologic function and regulatory mechanisms of circRNAs on pancreatic islet cells, different subtypes in diabetes, and diabetic complications.

Let7b-5p inhibits insulin secretion and decreases pancreatic β-cell mass in mice

MicroRNAs are crucial regulators for the development, mass and function of pancreatic β-cells. MiRNA dysregulation is associated with β-cell dysfunction and development of diabetes. The members of let7 family are important players in regulating cellular growth and metabolism. In this study we investigated the functional role of let7b-5p in the mouse pancreatic β-cells. We generated pancreatic β-cell-specific let7b-5p transgenic mouse model and analyzed the glucose metabolic phenotype, β-cells mass and insulin secretion in vivo. Luciferase reporter assay, immunofluorescence staining and western blot were carried out to study the target genes of let7b-5p in β-cells. Let7b-5p overexpression impaired the insulin production and secretion of β-cells and resulted impaired glucose tolerance in mice. The overexpressed let7b-5p inhibited pancreatic β-cell proliferation and decreased the expression of cyclin D1 and cyclin D2. Our findings demonstrated that let7b-5p was critical in regulating the proliferation and insulin secretion of pancreatic β-cells.

Exosomal Circular RNA hsa_circ_0046060 of Umbilical Cord Mesenchymal Stromal Cell Ameliorates Glucose Metabolism and Insulin Resistance in Gestational Diabetes Mellitus via the miR-338-3p/G6PC2 Axis

BACKGROUND

Impaired glucose metabolism and insulin sensitivity have been linked to the pathogenesis of gestational diabetes mellitus (GDM). Exosomes secreted by the umbilical cord mesenchymal stromal cells (UMSCs) and circular RNAs (circRNAs) derived from exosomes have been shown to be associated with the progression of GDM-related complications.

METHODS

UMSCs were isolated from umbilical cords and identified through flow cytometry. Exosomes were isolated from UMSCs and were then characterized. The expression levels of RNA of hsa_circ_0046060, mmu_circ_0002819, and miR-338-3p were determined by quantitative real-time polymerase chain reaction (RT-qPCR). The intracellular glucose intake and glycogen content were measured using a High Sensitivity Glucose Assay Kit and Glycogen Assay Kit, respectively. Bioinformatics analysis and luciferase reporter assay were used to validate interactions among hsa_circ_0046060, miR-338-3p, and G6PC2. The expression of insulin receptor substrate-1 (IRS-1) and its phosphorylated form, (p-IRS-1), as well as G6PC2, was determined through western blotting.

RESULTS

UMSCs and exosomes were successfully isolated and identified. The upregulation of hsa_circ_0046060 decreased the intracellular glucose content in L-02 cells (43.45 vs. 16.87 pM/mg, P=0.0002), whereas shRNA-mediated downregulation reversed this effect (16.87 vs. 33.16 pM/mg, P=0.0011). Mmu_circ_0002819 in mice aggravated dysregulated glucose metabolism (49.88 vs. 21.69 pM/mg, P=0.0031) and insulin sensitivity (0.20 vs. 0.11 mg/mL, P=0.03) in GDM mice, which was abrogated by the knockdown of mmu_circ_0002819. The results of luciferase reporter assay revealed that miR-338-3p and G6PC2 were the potential targets of has_circ_0046060. Western blotting results showed that the reduced activation of IRS-1 induced by GDM (1.25 vs. 0.54, P=0.0001) could be rescued by the administration of si-circ-G-UMSC-EXOs (0.54 vs. 1.17, P=0.0001).

CONCLUSION

Taken together, the inhibition of hsa_circ_0046060 expression in exosomes from GDM-derived UMSCs can alleviate GDM by reversing abnormal glucose metabolism and insulin resistance in vivo and in vitro.

Circular RNAs in metabolism and metabolic disorders

Metabolic syndrome (MetS) is a serious health condition triggered by hyperglycemia, dyslipidemia, and abnormal adipose deposition. Recently, circular RNAs (circRNAs) have been proposed as key molecular players in metabolic homeostasis due to their regulatory effects on genes linked to the modulation of multiple aspects of metabolism, including glucose and lipid homeostasis. Dysregulation of circRNAs can lead to metabolic disorders, indicating that circRNAs represent plausible potential targets to alleviate metabolic abnormalities. More recently, a series of circulating circRNAs have been identified to act as both essential regulatory molecules and biomarkers for the progression of metabolism-related disorders, including type 2 diabetes mellitus (T2DM or T2D) and cardiovascular disease (CVD). The findings of this study highlight the function of circRNAs in signaling pathways implicated in metabolic diseases and their potential as future therapeutics and disease biomarkers.

Current treatment options and challenges in patients with Type 1 diabetes: Pharmacological, technical advances and future perspectives

Type 1 diabetes mellitus imposes a significant burden of complications and mortality, despite important advances in treatment: subjects affected by this disease have also a worse quality of life-related to disease management. To overcome these challenges, different new approaches have been proposed, such as new insulin formulations or innovative devices. The introduction of insulin pumps allows a more physiological insulin administration with a reduction of HbA1c level and hypoglycemic risk. New continuous glucose monitoring systems with better accuracy have allowed, not only better glucose control, but also the improvement of the quality of life. Integration of these devices with control algorithms brought to the creation of the first artificial pancreas, able to independently gain metabolic control without the risk of hypo- and hyperglycemic crisis. This approach has revolutionized the management of diabetes both in terms of quality of life and glucose control. However, complete independence from exogenous insulin will be obtained only by biological approaches that foresee the replacement of functional beta cells obtained from stem cells: this will be a major challenge but the biggest hope for the subjects with type 1 diabetes. In this review, we will outline the current scenario of innovative diabetes management both from a technological and biological point of view, and we will also forecast some cutting-edge approaches to reduce the challenges that hamper the definitive cure of diabetes.

Therapeutic Potentials of MicroRNAs for Curing Diabetes Through Pancreatic β-Cell Regeneration or Replacement

MicroRNAs are a type of noncoding RNAs that regulates the expression of target genes at posttranscriptional level. MicroRNAs play essential roles in regulating the expression of different genes involved in pancreatic development, β-cell mass maintenance, and β-cell function. Alteration in the level of miRNAs involved in β-cell function leads to the diabetes. Being an epidemic, diabetes threatens the life of millions of patients posing a pressing demand for its urgent resolve. However, the currently available therapies are not substantial to cure the diabetic epidemic. Thus, researchers are trying to find new ways to replenish the β-cell mass in patients with diabetes. One promising approach is the in vivo regeneration of β-cell mass or increasing the efficiency of β-cell function. Another clinical strategy is the transplantation of in vitro developed β-like cells. Owing to their role in pancreatic β-cell development, maintenance, functioning and their involvement in diabetes, overexpression or attenuation of different miRNAs can cause β-cell regeneration in vivo or can direct the differentiation of various kinds of stem/progenitor cells to β-like cells in vitro. Here, we will summarize different strategies used by researchers to investigate the therapeutic potentials of miRNAs, with focus on miR-375, for curing diabetes through β-cell regeneration or replacement.

Role of miRNAs in the pathogenesis of T2DM, insulin secretion, insulin resistance, and β cell dysfunction: the story so far

Diabetes, the most common endocrine disorder, also known as a silent killer disease, is characterized by uncontrolled hyperglycemia. According to the International Diabetes Federation, there were 451 million people with diabetes mellitus worldwide in 2017. It is a multifactorial syndrome caused by genetic as well as environmental factors. Noncoding RNAs, especially the miRNAs, play a significant role in the development as well as the progression of the disease. This is on account of insulin resistance or defects in β cell function. Various miRNAs including miR-7, miR-9, miR-16, miR-27, miR-24, miR-29, miR-124a, miR-135, miR-130a, miR-144, miR-181a, and miR-375 and many more have been associated with insulin resistance and other pathogenic conditions leading to the development of the disease. These miRNAs play significant roles in various pathways underlying insulin resistance such as PI3K, AKT/GSK, and mTOR. The main target genes of these miRNAs are FOXO1, FOXA2, STAT3, and PTEN. The miRNAs carry out important functions in insulin target tissues like the adipose tissue, liver, and muscle. MiRNAs miR-9, miR-375, and miR-124a, are also associated with the secretion of insulin from pancreatic cells. There is an interplay between the miRNAs and pancreatic cell growth, especially the miRNAs affecting development and proliferation of these cells. Most of the miRNAs target more than one gene which not only justifies their use as biomarkers but also their therapeutic potential. The current review has been compiled with an aim to discuss the role of various miRNAs involved in various pathogenic mechanisms including insulin resistance, insulin secretion, and the β cell dysfunction.

Roles of Noncoding RNAs in Islet Biology

The discovery that most mammalian genome sequences are transcribed to ribonucleic acids (RNA) has revolutionized our understanding of the mechanisms governing key cellular processes and of the causes of human diseases, including diabetes mellitus. Pancreatic islet cells were found to contain thousands of noncoding RNAs (ncRNAs), including micro-RNAs (miRNAs), PIWI-associated RNAs, small nucleolar RNAs, tRNA-derived fragments, long non-coding RNAs, and circular RNAs. While the involvement of miRNAs in islet function and in the etiology of diabetes is now well documented, there is emerging evidence indicating that other classes of ncRNAs are also participating in different aspects of islet physiology. The aim of this article will be to provide a comprehensive and updated view of the studies carried out in human samples and rodent models over the past 15 years on the role of ncRNAs in the control of α- and β-cell development and function and to highlight the recent discoveries in the field. We not only describe the role of ncRNAs in the control of insulin and glucagon secretion but also address the contribution of these regulatory molecules in the proliferation and survival of islet cells under physiological and pathological conditions. It is now well established that most cells release part of their ncRNAs inside small extracellular vesicles, allowing the delivery of genetic material to neighboring or distantly located target cells. The role of these secreted RNAs in cell-to-cell communication between β-cells and other metabolic tissues as well as their potential use as diabetes biomarkers will be discussed. © 2020 American Physiological Society. Compr Physiol 10:893-932, 2020.

lncRNA Gm10451 regulates PTIP to facilitate iPSCs-derived β-like cell differentiation by targeting miR-338-3p as a ceRNA

iPSCs-derived insulin-producing cell transplantation is a promising strategy for diabetes therapy. Although there have been many protocols of mature, glucose-responsive β cells induced in vitro over the past few years, many underlying problems remain to be resolved. As a crucial regulator, long noncoding RNAs (lncRNAs) participate in numerous biological processes, including the maintenance of pluripotency, and stem cell differentiation. In this study, we identified a novel lncRNA Gm10451 as a functional regulator for β-like cell differentiation. Localized to the cytoplasm, Gm10451 regulates histone H3K4 methyltransferase complex PTIP to facilitate Insulin⁺/Nkx6.1⁺ β-like cell differentiation by targeting miR-338-3p as a competing endogenous RNA (ceRNA). miR-338-3p has also been shown to suppress Nkx6.1⁺ early-stage β-like cell differentiation by targeting PTIP. Following transplantation into streptozotocin (STZ)-mice, Gm10451 loss in β-like cells prevented the expression of mature β-cell makers, such as Insulin, Nkx6.1, and Mafa. Accordingly, hyperglycemia in the mice was not resolved. Taken together, this study provides an efficient epigenetic target for generating more mature and functional iPSCs-derived β-like cells. We anticipate that pancreatic organoids, which are generated from human stem cells, biological materials, and epigenetic modifications, can be used in the future as a novel diabetes treatment option.

Circulating miR-338 Cluster activities on osteoblast differentiation: Potential Diagnostic and Therapeutic Targets for Postmenopausal Osteoporosis

MicroRNAs (miRNAs) are the most abundant RNA species found in serum, and recently, several miRNAs have been found to be associated with osteoporosis. However, the development of such associated miRNAs into diagnostic and therapeutic targets remains unaddressed, mostly because of a lack of functional validation. Here, we identified circulating miR-338 associated with postmenopausal osteoporosis, and performed functional validation in vivo and in vitro.

Methods: We collected the serum from postmenopausal osteoporosis patients (N=15) and female volunteers of the same age but with normal bone density (N=15) and examined the enrichment of miR-338 cluster. We also confirmed such enrichment using mice subjected to ovariectomy at different stages. We employed primary bone marrow stromal cells from mice and the MC-3T3 cell line along with CRISPR, RNA-seq and ChIP-qPCR to validate the biological function of secreted miR-338 cluster on osteoblastic differentiation and their upstream regulators. Moreover, we generated miR-338 knockout mice and OVX mice injected with an inhibitor against miR-338 cluster to confirm its biological function in vivo.

Results: We observed a significant enrichment of miR-338 cluster in postmenopausal osteoporosis patients. Such enrichment was also prominent in serum from mice subjected to ovariectomy and was detected much earlier than bone density decreases revealed by micro-CT. We also confirmed the presence of an estrogen-dependent Runx2/Sox4/miR-338 positive feedback loop that modulated osteoblast differentiation, providing a possible explanation for our clinical findings. Moreover, deletion of the miR-338 cluster or direct intravenous injection of an miR-338 cluster inhibitor significantly prevented osteoporosis after ovariectomy.

Conclusion: Circulating miR-338 cluster in the serum could serve as a promising diagnostic and therapeutic target for postmenopausal osteoporosis patients.

Non-Coding RNA in Pancreas and β-Cell Development

In this review, we provide an overview of the current knowledge on the role of different classes of non-coding RNAs for islet and β-cell development, maturation and function. MicroRNAs (miRNAs), a prominent class of small RNAs, have been investigated for more than two decades and patterns of the roles of different miRNAs in pancreatic fetal development, islet and β-cell maturation and function are now emerging. Specific miRNAs are dynamically regulated throughout the period of pancreas development, during islet and β-cell differentiation as well as in the perinatal period, where a burst of β-cell replication takes place. The role of long non-coding RNAs (lncRNA) in islet and β-cells is less investigated than for miRNAs, but knowledge is increasing rapidly. The advent of ultra-deep RNA sequencing has enabled the identification of highly islet- or β-cell-selective lncRNA transcripts expressed at low levels. Their roles in islet cells are currently only characterized for a few of these lncRNAs, and these are often associated with β-cell super-enhancers and regulate neighboring gene activity. Moreover, ncRNAs present in imprinted regions are involved in pancreas development and β-cell function. Altogether, these observations support significant and important actions of ncRNAs in β-cell development and function.

miR‑338‑3p mediates gluconeogenesis via targeting of PP4R1 in hepatocytes

Hyperglycaemia is a characteristic of type 2 diabetes. In hepatocytes, impaired insulin sensitivity leads to increased gluconeogenesis and decreased glycogenesis. MicroRNA (miR)‑338‑3p is associated with tumour necrosis factor (TNF)‑α‑induced suppression of hepatic glycogenesis via regulation of protein phosphatase 4 regulatory subunit 1 (PP4R1). However, the effect of miR‑338‑3p on gluconeogenesis in hepatocytes remains unknown. In a previous study, it was demonstrated that miR‑338‑3p is downregulated in the livers of mice and in mouse HEPA1‑6 hepatocytes following treatment with TNF‑α. In the present study, the effect of miR‑338‑3p on TNF‑α‑induced gluconeogenesis in hepatocytes was investigated. The levels of phosphorylated‑FOXO1/FOXO1, phosphoenolpyruvate carboxykinase (PEPCK), peroxisome proliferator‑activated receptor γ coactivator (PGC‑1α) and glucose‑6‑phosphatase (G6Pase) were measured by western blotting. The mRNA levels of PEPCK, PGC‑1α and G6Pase were determined by quantitative polymerase chain reaction. Pyruvate tolerance testing was used to determine the gluconeogenesis of mouse livers. The results demonstrated that treatment with TNF‑α resulted in increased levels of gluconeogenesis in the livers of mice and decreased miR‑338‑3p expression levels in HEPA1‑6 cells. Overexpression of miR‑338‑3p reversed TNF‑α‑induced glucose production via enhancement of phosphorylated forkhead box O1 levels and downregulation of the expression levels of genes associated with gluconeogenesis, including peroxisome proliferator‑activated receptor γ coactivator‑1α, phosphoenolpyruvate carboxykinase and glucose‑6‑phosphatase. However, inhibition of miR‑338‑3p expression was revealed to enhance gluconeogenesis in the livers of mice and in HEPA1‑6 cells. Furthermore, downregulation of PP4R1 was revealed to attenuate the effect on glucose production following treatment with miR‑338‑3p inhibitors. In conclusion, the results of the present study revealed that miR‑338‑3p may be involved in TNF‑α‑mediated gluconeogenesis via targeting of PP4R1 in hepatocytes.

Circular RNAs as novel regulators of β-cell functions in normal and disease conditions

OBJECTIVE

There is strong evidence for an involvement of different classes of non-coding RNAs, including microRNAs and long non-coding RNAs, in the regulation of β-cell activities and in diabetes development. Circular RNAs were recently discovered to constitute a substantial fraction of the mammalian transcriptome but the contribution of these non-coding RNAs in physiological and disease processes remains largely unknown. The goal of this study was to identify the circular RNAs expressed in pancreatic islets and to elucidate their possible role in the control of β-cells functions.

METHODS

We used a microarray approach to identify circular RNAs expressed in human islets and searched their orthologues in RNA sequencing data from mouse islets. We then measured the level of four selected circular RNAs in the islets of different Type 1 and Type 2 diabetes models and analyzed the role of these circular transcripts in the regulation of insulin secretion, β-cell proliferation, and apoptosis.

RESULTS

We identified thousands of circular RNAs expressed in human pancreatic islets, 497 of which were conserved in mouse islets. The level of two of these circular transcripts, circHIPK3 and ciRS-7/CDR1as, was found to be reduced in the islets of diabetic db/db mice. Mimicking this decrease in the islets of wild type animals resulted in impaired insulin secretion, reduced β-cell proliferation, and survival. ciRS-7/CDR1as has been previously proposed to function by blocking miR-7. Transcriptomic analysis revealed that circHIPK3 acts by sequestering a group of microRNAs, including miR-124-3p and miR-338-3p, and by regulating the expression of key β-cell genes, such as Slc2a2, Akt1, and Mtpn.

CONCLUSIONS

Our findings point to circular RNAs as novel regulators of β-cell activities and suggest an involvement of this novel class of non-coding RNAs in β-cell dysfunction under diabetic conditions.

Noncoding RNAs Carried by Extracellular Vesicles in Endocrine Diseases

RNA molecules are essential and fine regulators of important biological processes. Their role is well documented also in the endocrine system, both in physiological and pathological conditions. Increasing interest is arising about the function and the importance of noncoding RNAs shuttled by extracellular vesicles (EVs). In fact, EV membrane protects nucleic acids from enzyme degradation. Nowadays, the research on EVs and their cargoes, as well as their biological functions, faces the lack of standardization in EV purification. Here, the main techniques for EV isolation are discussed and compared for their advantages and vulnerabilities. Despite the possible discrepancy due to methodological variability, EVs and their RNA content are reported to be key mediators of intercellular communication in pathologies of main endocrine organs, including the pancreas, thyroid, and reproductive system. In particular, the present work describes the role of RNAs contained in EVs in pathogenesis and progression of several metabolic dysfunctions, including obesity and diabetes, and their related manifestations. Their importance in the establishment and progression of thyroid autoimmunity disorders and complicated pregnancy is also discussed. Preliminary studies highlight the attractive possibility to use RNAs contained in EVs as biomarkers suggesting their exploitation for new diagnostic approaches in endocrinology.

MicroRNAs as stress regulators in pancreatic beta cells and diabetes

Background

MicroRNAs have emerged as important regulatory non-coding RNAs that tune cellular responses to physiological perturbations and disease conditions. An increasing body of literature underlines the important roles of miRNA function in pancreatic β-cells in response to metabolic, genetic and inflammatory stress. Lessons from genetic loss- and gain-of-function studies have implicated several highly expressed and evolutionary conserved miRNAs in stress signal modulation, resolution and buffering, thereby forming stabilizing miRNA networks that preserve β-cell differentiation, function, proliferation and cell survival.

Scope of Review

This review will summarize our current knowledge of how biologically relevant miRNAs regulate stress responses in pancreatic β-cells, discuss the challenges and opportunities associated with using secreted miRNAs as biomarkers and forecast how mechanistic knowledge of miRNA function can be exploited in developing miRNA-based therapeutics.

Major Conclusions

miRNAs play important roles in the function, differentiation, proliferation, and survival of pancreatic β-cells. Many miRNA families that are regulated by metabolic, genetic, and inflammatory stressors have been found to coordinate the adaptive responses of β-cells in vivo in conditions such as obesity and diabetes.

AAV-mediated pancreatic overexpression of Igf1 counteracts progression to autoimmune diabetes in mice

Objective

Type 1 diabetes is characterized by autoimmune destruction of β-cells leading to severe insulin deficiency. Although many improvements have been made in recent years, exogenous insulin therapy is still imperfect; new therapeutic approaches, focusing on preserving/expanding β-cell mass and/or blocking the autoimmune process that destroys islets, should be developed. The main objective of this work was to test in non-obese diabetic (NOD) mice, which spontaneously develop autoimmune diabetes, the effects of local expression of Insulin-like growth factor 1 (IGF1), a potent mitogenic and pro-survival factor for β-cells with immunomodulatory properties.

Methods

Transgenic NOD mice overexpressing IGF1 specifically in β-cells (NOD-IGF1) were generated and phenotyped. In addition, miRT-containing, IGF1-encoding adeno-associated viruses (AAV) of serotype 8 (AAV8-IGF1-dmiRT) were produced and administered to 4- or 11-week-old non-transgenic NOD females through intraductal delivery. Several histological, immunological, and metabolic parameters were measured to monitor disease over a period of 28–30 weeks.

Results

In transgenic mice, local IGF1 expression led to long-term suppression of diabetes onset and robust protection of β-cell mass from the autoimmune insult. AAV-mediated pancreatic-specific overexpression of IGF1 in adult animals also dramatically reduced diabetes incidence, both when vectors were delivered before pathology onset or once insulitis was established. Transgenic NOD-IGF1 and AAV8-IGF1-dmiRT-treated NOD animals had much less islet infiltration than controls, preserved β-cell mass, and normal insulinemia. Transgenic and AAV-treated islets showed less expression of antigen-presenting molecules, inflammatory cytokines, and chemokines important for tissue-specific homing of effector T cells, suggesting IGF1 modulated islet autoimmunity in NOD mice.

Conclusions

Local expression of Igf1 by AAV-mediated gene transfer counteracts progression to diabetes in NOD mice. This study suggests a therapeutic strategy for autoimmune diabetes in humans.

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Local pancreatic IGF1 expression prevents spontaneous autoimmune diabetes.

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Protection achieved after one-time local administration of IGF1-encoding AAV vectors.

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Efficacious in animals treated early or once autoimmunity is already established.

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Protection through maintenance of β-cell mass and endogenous insulin secretion.

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Treatment leads to reduced infiltration and expression of immunity genes in islets.

MicroRNAs as regulators of beta-cell function and dysfunction

In the last decade, there has been an explosion in both the number of and knowledge about miRNAs associated with both type 1 and type 2 diabetes. Even though we are presently in the initial stages of understanding how this novel class of posttranscriptional regulators are involved in diabetes, recent studies have demonstrated that miRNAs are important regulators of the islet transcriptome, controlling apoptosis, differentiation and proliferation, as well as regulating unique islet and beta-cell functions and pathways such as insulin expression, processing and secretion. Furthermore, a large number of miRNAs have been linked to diabetogenic processes induced by elevated levels of glucose, free fatty acids and inflammatory cytokines. Thus, miRNAs are novel therapeutic targets with the potential of protecting the beta-cell, and there is proof of principle that miRNA antagonists, so-called antagomirs, are effective in vivo for other disorders. miRNAs are exported out of cells in exosomes, raising the intriguing possibility of cell-to-cell communication between distant tissues via miRNAs and that miRNAs can be used as biomarkers of beta-cell function, mass and survival. The purpose of this review is to provide a status on how miRNAs control beta-cell function and viability in health and disease.

Role of microRNAs in the age-associated decline of pancreatic beta cell function in rat islets

AIMS/HYPOTHESIS

Ageing can lead to reduced insulin sensitivity and loss of pancreatic beta cell function, predisposing individuals to the development of diabetes. The aim of this study was to assess the contribution of microRNAs (miRNAs) to age-associated beta cell dysfunction.

METHODS

The global mRNA and miRNA profiles of 3- and 12-month-old rat islets were collected by microarray. The functional impact of age-associated differences in miRNA expression was investigated by mimicking the observed changes in primary beta cells from young animals.

RESULTS

Beta cells from 12-month-old rats retained normal insulin content and secretion, but failed to proliferate in response to mitotic stimuli. The islets of these animals displayed modifications at the level of several miRNAs, including upregulation of miR-34a, miR-124a and miR-383, and downregulation of miR-130b and miR-181a. Computational analysis of the transcriptomic modifications observed in the islets of 12-month-old rats revealed that the differentially expressed genes were enriched for miR-34a and miR-181a targets. Indeed, the induction of miR-34a and reduction of miR-181a in the islets of young animals mimicked the impaired beta cell proliferation observed in old animals. mRNA coding for alpha-type platelet-derived growth factor receptor, which is critical for compensatory beta cell mass expansion, is directly inhibited by miR34a and is likely to be at least partly responsible for the effects of this miRNA.

CONCLUSIONS/INTERPRETATION

Changes in the level of specific miRNAs that occur during ageing affect the proliferative capacity of beta cells. This might reduce their ability to expand under conditions of increased insulin demand, favouring the development of type 2 diabetes.

Insulin secretion in health and disease: nutrients dictate the pace

Insulin is a key hormone controlling metabolic homeostasis. Loss or dysfunction of pancreatic β-cells lead to the release of insufficient insulin to cover the organism needs, promoting diabetes development. Since dietary nutrients influence the activity of β-cells, their inadequate intake, absorption and/or utilisation can be detrimental. This review will highlight the physiological and pathological effects of nutrients on insulin secretion and discuss the underlying mechanisms. Glucose uptake and metabolism in β-cells trigger insulin secretion. This effect of glucose is potentiated by amino acids and fatty acids, as well as by entero-endocrine hormones and neuropeptides released by the digestive tract in response to nutrients. Glucose controls also basal and compensatory β-cell proliferation and, along with fatty acids, regulates insulin biosynthesis. If in the short-term nutrients promote β-cell activities, chronic exposure to nutrients can be detrimental to β-cells and causes reduced insulin transcription, increased basal secretion and impaired insulin release in response to stimulatory glucose concentrations, with a consequent increase in diabetes risk. Likewise, suboptimal early-life nutrition (e.g. parental high-fat or low-protein diet) causes altered β-cell mass and function in adulthood. The mechanisms mediating nutrient-induced β-cell dysfunction include transcriptional, post-transcriptional and translational modifications of genes involved in insulin biosynthesis and secretion, carbohydrate and lipid metabolism, cell differentiation, proliferation and survival. Altered expression of these genes is partly caused by changes in non-coding RNA transcripts induced by unbalanced nutrient uptake. A better understanding of the mechanisms leading to β-cell dysfunction will be critical to improve treatment and find a cure for diabetes.