RNA-seq-based identification of Star upregulation by islet amyloid formation

Molecular mapping and functional validation of GLP-1R cholesterol binding sites in pancreatic beta cells

G protein-coupled receptors (GPCRs) are integral membrane proteins which closely interact with their plasma membrane lipid microenvironment. Cholesterol is a lipid enriched at the plasma membrane with pivotal roles in the control of membrane fluidity and maintenance of membrane microarchitecture, directly impacting on GPCR stability, dynamics, and function. Cholesterol extraction from pancreatic beta cells has previously been shown to disrupt the internalisation, clustering, and cAMP responses of the glucagon-like peptide-1 receptor (GLP-1R), a class B1 GPCR with key roles in the control of blood glucose levels via the potentiation of insulin secretion in beta cells and weight reduction via the modulation of brain appetite control centres. Here, we unveil the detrimental effect of a high cholesterol diet on GLP-1R-dependent glucoregulation in vivo, and the improvement in GLP-1R function that a reduction in cholesterol synthesis using simvastatin exerts in pancreatic islets. We next identify and map sites of cholesterol high occupancy and residence time on active vs inactive GLP-1Rs using coarse-grained molecular dynamics (cgMD) simulations, followed by a screen of key residues selected from these sites and detailed analyses of the effects of mutating one of these, Val229, to alanine on GLP-1R-cholesterol interactions, plasma membrane behaviours, clustering, trafficking and signalling in INS-1 832/3 rat pancreatic beta cells and primary mouse islets, unveiling an improved insulin secretion profile for the V229A mutant receptor. This study (1) highlights the role of cholesterol in regulating GLP-1R responses in vivo; (2) provides a detailed map of GLP-1R - cholesterol binding sites in model membranes; (3) validates their functional relevance in beta cells; and (4) highlights their potential as locations for the rational design of novel allosteric modulators with the capacity to fine-tune GLP-1R responses.

Explaining Type 2 Diabetes with Transcriptomic Signatures of Pancreatic β-Cell Dysfunction and Death Induced by Human Islet Amyloid Polypeptide

Amyloid deposits formed by misfolding and aggregation of human islet amyloid polypeptide (hIAPP) are one of the key pathophysiological features of type 2 diabetes mellitus (T2DM) and have been associated with the loss of function and viability of the pancreatic β-cells. The molecular processes by which hIAPP induces cytotoxicity in these cells are not well understood. To the best of our knowledge, this is the first report describing findings from the combined analysis of Affymetrix microarray and high-throughput sequencing (HTS) Gene Expression Omnibus (GEO) datasets of hIAPP-transgenic (Tg) mice islets. In brief, using GEO data, we compared in silico the pancreatic islets obtained from hIAPP-Tg and wild-type mice. Affymetrix microarray datasets (GSE84423, GSE85380, and GSE94672) and HTS datasets (GSE135276 and GSE148809) were chosen. Weighted gene coexpression network analysis was performed using GSE135276 to identify the coexpressed gene networks and establish a correlation pattern between gene modules and hIAPP overexpression under hyperglycemic conditions. Subsequently, we analyzed differential gene expression with the remaining datasets. Network analysis was performed to identify hub genes and the associated pathways using Cytoscape. Key findings from the present study include identification of seven hub genes, namely, Ins2, Agt, Jun, Fos, CD44, Igf1, and Ppar-γ, significantly involved in the process(es) of insulin synthesis and secretion, development of insulin resistance, oxidative stress, inflammation, mitophagy, and apoptosis. In conclusion, we propose that these hub genes can help explain T2DM pathogenesis and can be potentially utilized to develop therapeutic interventions targeting hIAPP for clinical management of T2DM.

Increased Steroidogenic Acute Regulatory Protein Contributes to Cholesterol-induced β-Cell Dysfunction

Hypercholesterolemia is often observed in individuals with type 2 diabetes. Cholesterol accumulation in subcellular compartments within islet β-cells can result in insulin secretory dysfunction, which is a key pathological feature of diabetes. Previously, we demonstrated that expression of the mitochondrial cholesterol transport protein, steroidogenic acute regulatory protein (StAR), is induced in islets under conditions of β-cell dysfunction. However, whether it contributes to mitochondrial cholesterol accumulation in β-cells and cholesterol-induced β-cell dysfunction has not been determined. Thus, we sought to examine the role of StAR in isolated mouse islets under conditions of excess exogenous cholesterol. Cholesterol treatment of islets upregulated StAR expression, which was associated with cholesterol accumulation in mitochondria, decreased mitochondrial membrane potential and impaired mitochondrial oxidative phosphorylation. Impaired insulin secretion and reduced islet insulin content were also observed in cholesterol-laden islets. To determine the impact of StAR overexpression in β-cells per se, a lentivirus was used to increase StAR expression in INS-1 cells. Under these conditions, StAR overexpression was sufficient to increase mitochondrial cholesterol content, impair mitochondrial oxidative phosphorylation, and reduce insulin secretion. These findings suggest that elevated cholesterol in diabetes may contribute to β-cell dysfunction via increases in StAR-mediated mitochondrial cholesterol transport and accumulation.

The islet tissue plasminogen activator/plasmin system is upregulated with human islet amyloid polypeptide aggregation and protects beta cells from aggregation-induced toxicity

AIMS/HYPOTHESIS

Apart from its fibrinolytic activity, the tissue plasminogen activator (tPA)/plasmin system has been reported to cleave the peptide amyloid beta, attenuating brain amyloid deposition in Alzheimer's disease. As aggregation of human islet amyloid polypeptide (hIAPP) is toxic to beta cells, we sought to determine whether activation of the fibrinolytic system can also reduce islet amyloid deposition and its cytotoxic effects, which are both observed in type 2 diabetes.

METHODS

The expression of Plat (encoding tPA) and plasmin activity were measured in isolated islets from amyloid-prone hIAPP transgenic mice or non-transgenic control islets expressing non-amyloidogenic mouse islet amyloid polypeptide cultured in the absence or presence of the amyloid inhibitor Congo Red. Plat expression was also determined in hIAPP-treated primary islet endothelial cells, bone marrow-derived macrophages (BMDM) and INS-1 cells, in order to determine the islet cell type(s) producing tPA in response to hIAPP aggregation. Cell-free thioflavin-T assays and MS were used to respectively monitor hIAPP aggregation kinetics and investigate plasmin cleavage of hIAPP. Cell viability was assessed in INS-1 beta cells treated with hIAPP with or without plasmin. Finally, to confirm the findings in human samples, PLAT expression was measured in freshly isolated islets from donors with and without type 2 diabetes.

RESULTS

In isolated islets from transgenic mice, islet Plat expression and plasmin activity increased significantly with the process of amyloid deposition (p≤0.01, n=5); these effects were not observed in islets from non-transgenic mice and were blocked by Congo Red (p≤0.01, n=4). In response to hIAPP exposure, Plat expression increased in BMDM and INS-1 cells vs vehicle-treated cells (p≤0.05, n=4), but not in islet endothelial cells. Plasmin reduced hIAPP fibril formation in a dose-dependent manner in a cell-free system, and restored hIAPP-induced loss of cell viability in INS-1 beta cells (p≤0.01, n=5). Plasmin cleaved monomeric hIAPP, inducing a rapid decrease in the abundance of full-length hIAPP and the appearance of hIAPP 1-11 and 12-37 fragments. hIAPP 12-37, which contains the critical amyloidogenic region, was not toxic to INS-1 cells. Finally, PLAT expression was significantly increased by 2.4-fold in islets from donors with type 2 diabetes (n=4) vs islets from donors without type 2 diabetes (n=7) (p≤0.05).

CONCLUSIONS/INTERPRETATION

The fibrinolytic system is upregulated in islets with hIAPP aggregation. Plasmin rapidly degrades hIAPP, limiting its aggregation into amyloid and thus protecting beta cells from hIAPP-induced toxicity. Thus, increasing islet plasmin activity might be a strategy to limit beta cell loss in type 2 diabetes.

Moringa oleifera Lam.: a comprehensive review on active components, health benefits and application

Moringa oleifera Lam. is an edible therapeutic plant that is native to India and widely cultivated in tropical countries. In this paper, the current application of M. oleifera was discussed by summarizing its medicinal parts, active components and potential mechanism. The emerging products of various formats such as drug preparation and product application reported in the last years were also clarified. Based on literature reports, the unique components and biological activities of M. oleifera need to be further studied. In the future, a variety of new technologies should be applied to the development of M. oleifera products, to enrich the varieties of dosage forms, improve the bitter taste masking technology, and make it better for use in the fields of food and medicine.

Amylin regulates testosterone levels via steroidogenesis-related enzymes in the central nervous system of male mice

Amylin is a peripheral satiation signal polypeptide co-secreted with insulin by pancreatic β-cells in response to nutrient ingestion. Amylin participates in the eating-inhibitory effect and regulates energy metabolism by acting on the central nervous system (CNS). However, the role of amylin in regulating the biosynthesis of steroid hormones, such as testosterone, through the hypothalamic-pituitary-gonadal axis (HPG) remains unexplored. However, only limited evidence is available on the involvement of amylin in steroid synthesis, we hypothesize that amylin regulates testosterone levels via steroidogenesis-related enzymes in the CNS. In this study, we elucidated the effect of intraperitoneal injection of amylin on the protein expression of steroidogenesis-related enzymes, including 3β-hydroxysteroid dehydrogenase (3β-HSD), cytochrome P450 17A1 (CYP17A1), and steroidogenic acute regulatory protein (StAR), and phospho-extracellular signal-regulated kinase (pERK). Additionally, the effect of amylin on testosterone levels in male mice was examined. Our results suggested that 3β-HSD and CYP17A1 neurons were widely expressed in the CNS of male mice, whereas StAR neurons were mainly expressed in the zona incerta (ZI) and locus coeruleus (LC) regions. Intraperitoneal injection of amylin significantly reduced (p < 0.01) the expression of 3β-HSD, CYP17A1, and StAR in ZI and other areas near the third ventricle (3 V) but increased (p < 0.01) pERK expression, brain testosterone levels, serum FSH, serum LH, and decreased (p < 0.01) serum testosterone levels in mice. In conclusion, amylin regulates testosterone levels via steroidogenesis-related enzymes in the central nervous system of male mice.

IAPP-induced beta cell stress recapitulates the islet transcriptome in type 2 diabetes

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by islet amyloid and toxic oligomers of islet amyloid polypeptide (IAPP). We posed the questions, (1) does IAPP toxicity induce an islet response comparable to that in humans with type 2 diabetes, and if so, (2) what are the key transcriptional drivers of this response?

METHODS

The islet transcriptome was evaluated in five groups of mice: beta cell specific transgenic for (1) human IAPP, (2) rodent IAPP, (3) human calpastatin, (4) human calpastatin and human IAPP, and (5) wild-type mice. RNA sequencing data was analysed by differential expression analysis and gene co-expression network analysis to establish the islet response to adaptation to an increased beta cell workload of soluble rodent IAPP, the islet response to increased expression of oligomeric human IAPP, and the extent to which the latter was rescued by suppression of calpain hyperactivation by calpastatin. Rank-rank hypergeometric overlap analysis was used to compare the transcriptome of islets from human or rodent IAPP transgenic mice vs humans with prediabetes or type 2 diabetes.

RESULTS

The islet transcriptomes in humans with prediabetes and type 2 diabetes are remarkably similar. Beta cell overexpression of soluble rodent or oligomer-prone human IAPP induced changes in islet transcriptome present in prediabetes and type 2 diabetes, including decreased expression of genes that confer beta cell identity. Increased expression of human IAPP, but not rodent IAPP, induced islet inflammation present in prediabetes and type 2 diabetes in humans. Key mediators of the injury responses in islets transgenic for human IAPP or those from individuals with type 2 diabetes include STAT3, NF-κB, ESR1 and CTNNB1 by transcription factor analysis and COL3A1, NID1 and ZNF800 by gene regulatory network analysis.

CONCLUSIONS/INTERPRETATION

Beta cell injury mediated by IAPP is a plausible mechanism to contribute to islet inflammation and dedifferentiation in type 2 diabetes. Inhibition of IAPP toxicity is a potential therapeutic target in type 2 diabetes.

Unpacking the aggregation-oligomerization-fibrillization process of naturally-occurring hIAPP amyloid oligomers isolated directly from sera of children with obesity or diabetes mellitus

The formation of amyloid oligomers and fibrils of the human islet amyloid polypeptide (hIAPP) has been linked with β- cell failure and death which causes the onset, progression, and comorbidities of diabetes. We begin to unpack the aggregation-oligomerization-fibrillization process of these oligomers taken from sera of pediatric patients. The naturally occurring or real hIAPP (not synthetic) amyloid oligomers (RIAO) were successfully isolated, we demonstrated the presence of homo (dodecamers, hexamers, and trimers) and hetero-RIAO, as well as several biophysical characterizations which allow us to learn from the real phenomenon taking place. We found that the aggregation/oligomerization process is active in the sera and showed that it happens very fast. The RIAO can form fibers and react with anti-hIAPP and anti-amyloid oligomers antibodies. Our results opens the epistemic horizon and reveal real differences between the four groups (Controls vs obesity, T1DM or T2DM) accelerating the process of understanding and discovering novel and more efficient prevention, diagnostic, transmission and therapeutic pathways.