MicroRNAs: an adaptive mechanism in the pancreatic β-cell…and beyond?

Recent protocols have been developed to differentiate human stem cells and fibroblasts into insulin-producing cells capable of releasing the hormone in a glucose-stimulated manner. Limitations remain which prevent bringing these protocols to a clinical setting as these models must still undergo complete characterization. Advances in sequencing technologies have driven the identification of several non-coding RNA species including microRNAs (miRNAs). While their diversity and unique expression patterns across different tissues have made deciphering their precise functional role a significant challenge, studies using both cell lines and transgenic mouse models have made substantial progress in understanding their regulatory role on exocytosis and proliferation of the {beta}-cell. These results also indicate miRNAs play an integral role in the fundamental mechanics of how the cell manages the balance between these independent functions. Continued investigation into miRNA function may uncover mechanisms which can be exploited to improve differentiation protocols in producing fully mature {beta}-cells

Insm1 cooperates with Neurod1 and Foxa2 to maintain mature pancreatic β-cell function

Key transcription factors control the gene expression program in mature pancreatic {beta}-cells, but their integration into regulatory networks is little understood. Here, we show that Insm1, Neurod1 and Foxa2 directly interact and together bind regulatory sequences in the genome of mature pancreatic {beta}-cells. We used Insm1 ablation in mature {beta}-cells in mice and found pronounced deficits in insulin secretion and gene expression. Insm1-dependent genes identified previously in developing {beta}-cells markedly differ from the ones identified in the adult. In particular, adult mutant {beta}-cells resemble immature {beta}-cells of newborn mice in gene expression and functional properties. We defined Insm1, Neurod1 and Foxa2 binding sites associated with genes deregulated in Insm1 mutant {beta}-cells. Remarkably, combinatorial binding of Insm1, Neurod1 and Foxa2 but not binding of Insm1 alone explained a significant fraction of gene expression changes. Human genomic sequences corresponding to the murine sites occupied by Insm1/Neurod1/Foxa2 were enriched in single nucleotide polymorphisms associated with glycolytic traits. Thus, our data explain part of the mechanisms by which {beta}-cells maintain maturity: Combinatorial Insm1/Neurod1/Foxa2 binding identifies regulatory sequences that maintain the mature gene expression program in {beta}-cells, and disruption of this network results in functional failure

Argonaute2 regulates the pancreatic β-cell secretome

Argonaute2 (Ago2) is an established component of the microRNA-induced silencing complex. Similar to miR-375 loss-of-function studies, inhibition of Ago2 in the pancreatic beta-cell resulted in enhanced insulin release underlining the relationship between these two genes. Moreover, as the most abundant microRNA in pancreatic endocrine cells, miR-375 was also observed to be enriched in Ago2-associated complexes. Both Ago2 and miR-375 regulate the pancreatic beta-cell secretome and we identified using quantitative mass spectrometry the enhanced release of a set of proteins or secretion signature in response to a glucose stimulus using the murine beta-cell line, MIN6. In addition, loss of Ago2 resulted in the increased expression of miR-375 target genes, including gephyrin and ywhaz. These targets positively contribute to exocytosis indicating they may mediate the functional role of both miR-375 and Ago proteins in the pancreatic beta-cell by influencing the secretory pathway. This study specifically addresses the role of Ago2 in the systemic release of proteins from beta-cells and highlights the contribution of the microRNA pathway to the function of this cell type

SORCS1 and SORCS3 control energy balance and orexigenic peptide production

SORCS1 and SORCS3 are two related sorting receptors expressed in neurons of the arcuate nucleus of the hypothalamus. Using mouse models with individual or dual receptor deficiencies, we document a previously unknown function of these receptors in central control of metabolism. Specifically, SORCS1 and SORCS3 act as intracellular trafficking receptors for tropomyosin-related kinase B to attenuate signaling by brain-derived neurotrophic factor, a potent regulator of energy homeostasis. Loss of the joint action of SORCS1 and SORCS3 in mutant mice results in excessive production of the orexigenic neuropeptide agouti-related peptide and in a state of chronic energy excess characterized by enhanced food intake, decreased locomotor activity, diminished usage of lipids as metabolic fuel, and increased adiposity, albeit at overall reduced body weight. Our findings highlight a novel concept in regulation of the melanocortin system and the role played by trafficking receptors SORCS1 and SORCS3 in this process

La donazione d'organi da neonati anencefalici e il principio di dignità in giurisprudenza

Altered growth and development of the endocrine pancreas is a frequent cause of the hyperglycemia associated with diabetes. Here we show that microRNA-375 (miR-375), which is highly expressed in pancreatic islets, is required for normal glucose homeostasis. Mice lacking miR-375 (375KO) are hyperglycemic, exhibit increased total pancreatic alpha-cell numbers, fasting and fed plasma glucagon levels, and increased gluconeogenesis and hepatic glucose output. Furthermore, pancreatic beta-cell mass is decreased in 375KO mice as a result of impaired proliferation. In contrast, pancreatic islets of obese mice (ob/ob), a model of increased beta-cell mass, exhibit increased expression of miR-375. Genetic deletion of miR-375 from these animals (375/ob) profoundly diminished the proliferative capacity of the endocrine pancreas and resulted in a severely diabetic state. Bioinformatic analysis of transcript data from 375KO islets revealed that miR-375 regulates a cluster of genes controlling cellular growth and proliferation. These data provide evidence that miR-375 is essential for normal glucose homeostasis, alpha- and beta-cell turnover, and adaptive beta-cell expansion in response to increasing insulin demand in insulin resistance

microRNA-184 induces a commitment switch to epidermal differentiation

miR-184 is a highly evolutionary conserved microRNA (miRNA) from fly to human. The importance of miR-184 was underscored by the discovery that point mutations in miR-184 gene led to corneal/lens blinding disease. However, miR-184-related function in vivo remained unclear. Here, we report that the miR-184 knockout mouse model displayed increased p63 expression in line with epidermal hyperplasia, while forced expression of miR-184 by stem/progenitor cells enhanced the Notch pathway and induced epidermal hypoplasia. In line, miR-184 reduced clonogenicity and accelerated differentiation of human epidermal cells. We showed that by directly repressing cytokeratin 15 (K15) and FIH1, miR-184 induces Notch activation and epidermal differentiation. The disease-causing miR-184C57U mutant failed to repress K15 and FIH1 and to induce Notch activation, suggesting a loss-of-function mechanism. Altogether, we propose that, by targeting K15 and FIH1, miR-184 regulates the transition from proliferation to early differentiation, while mis-expression or mutation in miR-184 results in impaired homeostasis

Differential impact of glucose administered intravenously and orally on circulating mir-375 levels in human subjects

Background: To date, numerous nucleic acid species have been detected in the systemic circulation including microRNAs (miRNAs); however their functional role in this compartment remains unclear. Objective: The aim of this study was to determine whether systemic levels of miRNAs abundant in blood, including the neuroendocrine tissue-enriched miR-375, are altered in response to a glucose challenge. Design: Twelve healthy males were recruited for an acute cross-over study which consisted of two tests each following an eight-hour fasting period. An oral glucose tolerance test (OGTT) was performed and blood samples were collected over a 3-hour period. Following a period of at least one week, the same participants were administered an isoglycemic intravenous glucose infusion (IIGI) with the same blood collection protocol. Results: The glucose response curve following the IIGI mimicked that obtained after the OGTT, but as expected systemic insulin levels were lower during the IIGI compared to the OGTT (P<0.05). MiR-375 levels in circulation were increased only in response to an OGTT and not during an IIGI. In addition, the response to the OGTT also coincided with the transient increase of circulating glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2), and glucose-dependent insulinotropic polypeptide (GIP). Conclusions: The present findings show levels of miR-375 increase following administration of an OGTT and in light of its enrichment in cells of the gut, suggest that the gastrointestinal tract may play a significant role to the abundance and function of this microRNA in the blood

The RhoGAP Stard13 controls insulin secretion through F-actin remodeling

OBJECTIVE: Actin cytoskeleton remodeling is necessary for glucose-stimulated insulin secretion in pancreatic {beta}-cells. A mechanistic understanding of actin dynamics in the islet is paramount to a better comprehension of {beta}-cell dysfunction in diabetes. Here, we investigate the Rho GTPase regulator Stard13 and its role in F-actin cytoskeleton organization and islet function in adult mice. METHODS: We used Lifeact-EGFP transgenic animals to visualize actin cytoskeleton organization and dynamics in vivo in the mouse islets. Furthermore, we applied this model to study actin cytoskeleton and insulin secretion in mutant mice deleted for Stard13 selectively in pancreatic cells. We isolated transgenic islets for 3D-imaging and perifusion studies to measure insulin secretion dynamics. In parallel, we performed histological and morphometric analyses of the pancreas and used in vivo approaches to study glucose metabolism in the mouse. RESULTS: In this study, we provide the first genetic evidence that Stard13 regulates insulin secretion in response to glucose. Postnatally, Stard13 expression became restricted to the mouse pancreatic islets. We showed that Stard13 deletion results in a marked increase in actin polymerization in islet cells, which is accompanied by severe reduction of insulin secretion in perifusion experiments. Consistently, Stard13-deleted mice displayed impaired glucose tolerance and reduced glucose-stimulated insulin secretion. CONCLUSIONS: Taken together, our results suggest a previously unappreciated role for the RhoGAP protein Stard13 in the interplay between actin cytoskeletal remodeling and insulin secretion

Neuronal cell adhesion molecule 1 regulates leptin sensitivity and bone mass

The central nervous system is widely known to exert control over our systemic physiology via several mechanisms including the regulation of skeletal metabolism. Neuronal circuits within the hypothalamus have been shown to impact bone mass via leptin-dependent and independent mechanisms; however, the full extent to which the brain controls bone homeostasis is not known. We previously identified cell adhesion molecule1 (Cadm1) as a regulator of body weight and energy homeostasis via its expression in multiple regions of the brain. Here, we show that loss of Cadm1 expression in excitatory neurons results in increased leptin sensitivity in addition to a concomitant reduction in bone mass. Femoral length, bone mineral content, diaphyseal cross-sectional area, and bone strength were all lower in Cadm1-deficient animals. Conversely, inducing expression of Cadm1 in excitatory neurons decreased leptin sensitivity and increased femoral length, bone mineral content, and diaphyseal cross-sectional area. Together, these results illustrate an essential role for this synaptic protein in the neuronal regulation of skeletal bone metabolism