Insulin granule biogenesis and exocytosis

The insulin secretory granule is a hotspot for autoantigen formation in type 1 diabetes

In type 1 diabetes, the insulin-producing beta cells of the pancreas are destroyed through the activity of autoreactive T cells. In addition to strong and well-documented HLA class II risk haplotypes, type 1 diabetes is associated with noncoding polymorphisms within the insulin gene locus. Furthermore, autoantibody prevalence data and murine studies implicate insulin as a crucial autoantigen for the disease. Studies identify secretory granules, where proinsulin is processed into mature insulin, stored and released in response to glucose stimulation, as a source of antigenic epitopes and neoepitopes. In this review, we integrate established concepts, including the role that susceptible HLA and thymic selection of the T cell repertoire play in setting the stage for autoimmunity, with emerging insights about beta cell and insulin secretory granule biology. In particular, the acidic, peptide-rich environment of secretory granules combined with its array of enzymes generates a distinct proteome that is unique to functional beta cells. These factors converge to generate non-templated peptide sequences that are recognised by autoreactive T cells. Although unanswered questions remain, formation and presentation of these epitopes and the resulting immune responses appear to be key aspects of disease initiation. In addition, these pathways may represent important opportunities for therapeutic intervention.

Long-term ketogenic diet causes hyperlipidemia, liver dysfunction, and glucose intolerance from impaired insulin trafficking and secretion in mice

A ketogenic diet (KD) is a very low-carbohydrate, very high-fat diet proposed to treat obesity and type 2 diabetes. While KD grows in popularity, its effects on metabolic health are understudied. Here we show that, in male and female mice, while KD protects against weight gain and induces weight loss, over long-term, mice develop hyperlipidemia, hepatic steatosis, and severe glucose intolerance. Unlike high fat diet-fed mice, KD mice are not insulin resistant and have low levels of insulin. Hyperglycemic clamp and ex vivo GSIS revealed cell-autonomous and whole-body impairments in insulin secretion. Major ER/Golgi stress and disrupted ER-Golgi protein trafficking was indicated by transcriptomic profiling of KD islets and confirmed by electron micrographs showing a dilated Golgi network likely responsible for impaired insulin granule trafficking and secretion. Overall, our results suggest long-term KD leads to multiple aberrations of metabolic parameters that caution its systematic use as a health promoting dietary intervention.

RNA-binding protein PCBP2 regulates pancreatic β cell function and adaptation to glucose

Glucose plays a key role in shaping pancreatic β cell function. Thus, deciphering the mechanisms by which this nutrient stimulates β cells holds therapeutic promise for combating β cell failure in type 2 diabetes (T2D). β Cells respond to hyperglycemia in part by rewiring their mRNA metabolism, yet the mechanisms governing these changes remain poorly understood. Here, we identify a requirement for the RNA-binding protein PCBP2 in maintaining β cell function basally and during sustained hyperglycemic challenge. PCBP2 was induced in primary mouse islets incubated with elevated glucose and was required to adapt insulin secretion. Transcriptomic analysis of primary Pcbp2-deficient β cells revealed impacts on basal and glucose-regulated mRNAs encoding core components of the insulin secretory pathway. Accordingly, Pcbp2-deficient β cells exhibited defects in calcium flux, insulin granule ultrastructure and exocytosis, and the amplification pathway of insulin secretion. Further, PCBP2 was induced by glucose in primary human islets, was downregulated in islets from T2D donors, and impacted genes commonly altered in islets from donors with T2D and linked to single-nucleotide polymorphisms associated with T2D. Thus, these findings establish a paradigm for PCBP2 in governing basal and glucose-adaptive gene programs critical for shaping the functional state of β cells.

OSBP-mediated PI(4)P-cholesterol exchange at endoplasmic reticulum-secretory granule contact sites controls insulin secretion

Insulin is packaged into secretory granules that depart the Golgi and undergo a maturation process that involves changes in the protein and lipid composition of the granules. Here, we show that insulin secretory granules form physical contacts with the endoplasmic reticulum and that the lipid exchange protein oxysterol-binding protein (OSBP) is recruited to these sites in a Ca2+-dependent manner. OSBP binding to insulin granules is positively regulated by phosphatidylinositol-4 (PI4)-kinases and negatively regulated by the PI4 phosphate (PI(4)P) phosphatase Sac2. Loss of Sac2 results in excess accumulation of cholesterol on insulin granules that is normalized when OSBP expression is reduced, and both acute inhibition and small interfering RNA (siRNA)-mediated knockdown of OSBP suppress glucose-stimulated insulin secretion without affecting insulin production or intracellular Ca2+ signaling. In conclusion, we show that lipid exchange at endoplasmic reticulum (ER)-granule contact sites is involved in the exocytic process and propose that these contacts act as reaction centers with multimodal functions during insulin granule maturation.

In situ structure of actin remodeling during glucose-stimulated insulin secretion using cryo-electron tomography

Actin mediates insulin secretion in pancreatic β-cells through remodeling. Hampered by limited resolution, previous studies have offered an ambiguous depiction as depolymerization and repolymerization. We report the in situ structure of actin remodeling in INS-1E β-cells during glucose-stimulated insulin secretion at nanoscale resolution. After remodeling, the actin filament network at the cell periphery exhibits three marked differences: 12% of actin filaments reorient quasi-orthogonally to the ventral membrane; the filament network mainly remains as cell-stabilizing bundles but partially reconfigures into a less compact arrangement; actin filaments anchored to the ventral membrane reorganize from a "netlike" to a "blooming" architecture. Furthermore, the density of actin filaments and microtubules around insulin secretory granules decreases, while actin filaments and microtubules become more densely packed. The actin filament network after remodeling potentially precedes the transport and release of insulin secretory granules. These findings advance our understanding of actin remodeling and its role in glucose-stimulated insulin secretion.

Phogrin Regulates High-Fat Diet-Induced Compensatory Pancreatic β-Cell Growth by Switching Binding Partners

The receptor protein tyrosine phosphatase phogrin primarily localizes to hormone secretory granules in neuroendocrine cells. Concurrent with glucose-stimulated insulin secretion, phogrin translocates to pancreatic β-cell plasma membranes, where it interacts with insulin receptors (IRs) to stabilize insulin receptor substrate 2 (IRS2) that, in turn, contributes to glucose-responsive β-cell growth. Pancreatic β-cell development was not altered in β-cell-specific, phogrin-deficient mice, but the thymidine incorporation rate decreased in phogrin-deficient islets with a moderate reduction in IRS2 protein expression. In this study, we analyzed the β-cell response to high-fat diet stress and found that the compensatory expansion in β-cell mass was significantly suppressed in phogrin-deficient mice. Phogrin-IR interactions occurred only in high-fat diet murine islets and proliferating β-cell lines, whereas they were inhibited by the intercellular binding of surface phogrin under confluent cell culture conditions. Thus, phogrin could regulate glucose-stimulated compensatory β-cell growth by changing its binding partner from another β-cell phogrin to IR in the same β-cells.

Rediscovering the intricacies of secretory granule biogenesis

Regulated secretion, an essential cellular process, relies on secretory granules (SGs) for the controlled release of a diverse range of cargo molecules, including proteins, peptides, hormones, enzymes, and neurotransmitters. SG biogenesis encompasses cargo selection, sorting, packaging, and trafficking, with the trans-Golgi Network (TGN) playing a central role. Research in the last three decades has revealed significant components required for SG biogenesis; however, no cargo receptor transferring granule cargo from the TGN to immature SGs (ISGs) has yet been identified. Consequently, recent research has devoted significant attention to studying receptor-independent cargo sorting mechanisms, shedding new light on the complexities of regulated secretion. Understanding the underlying molecular and biophysical mechanisms behind cargo sorting into ISGs holds great promise for advancing our knowledge of cellular communication and disease mechanisms.

Spatiotemporal regulation of GIPR signaling impacts glucose homeostasis as revealed in studies of a common GIPR variant

OBJECTIVE

Glucose-dependent insulinotropic polypeptide (GIP) has a role in controlling postprandial metabolic tone. In humans, a GIP receptor (GIPR) variant (Q354, rs1800437) is associated with a lower body mass index (BMI) and increased risk for Type 2 Diabetes. To better understand the impacts of GIPR-Q354 on metabolism, it is necessary to study it in an isogeneic background to the predominant GIPR isoform, E354. To accomplish this objective, we used CRISPR-CAS9 editing to generate mouse models of GIPR-Q354 and GIPR-E354. Here we characterize the metabolic effects of GIPR-Q354 variant in a mouse model (GIPR-Q350).

METHODS

We generated the GIPR-Q350 mice for in vivo studies of metabolic impact of the variant. We isolated pancreatic islets from GIPR-Q350 mice to study insulin secretion ex vivo. We used a β-cell cell line to understand the impact of the GIPR-Q354 variant on the receptor traffic.

RESULTS

We found that female GIPR-Q350 mice are leaner than littermate controls, and male GIPR-Q350 mice are resistant to diet-induced obesity, in line with the association of the variant with reduced BMI in humans. GIPR-Q350 mice of both sexes are more glucose tolerant and exhibit an increased sensitivity to GIP. Postprandial GIP levels are reduced in GIPR-Q350 mice, revealing feedback regulation that balances the increased sensitivity of GIP target tissues to secretion of GIP from intestinal endocrine cells. The increased GIP sensitivity is recapitulated ex vivo during glucose stimulated insulin secretion assays in islets. Generation of cAMP in islets downstream of GIPR activation is not affected by the Q354 substitution. However, post-activation traffic of GIPR-Q354 variant in β-cells is altered, characterized by enhanced intracellular dwell time and increased localization to the Trans-Golgi Network (TGN).

CONCLUSIONS

Our data link altered intracellular traffic of the GIPR-Q354 variant with GIP control of metabolism. We propose that this change in spatiotemporal signaling underlies the physiologic effects of GIPR-Q350/4 and GIPR-E350/4 in mice and humans. These findings contribute to a more complete understanding of the impact of GIPR-Q354 variant on glucose homeostasis that could perhaps be leveraged to enhance pharmacologic targeting of GIPR for the treatment of metabolic disease.

Update on the ZNT8 epitope and its role in the pathogenesis of type 1 diabetes

Type 1 diabetes (T1D) is an organ-specific chronic autoimmune disease mediated by autoreactive T cells. ZnT8 is a pancreatic islet-specific zinc transporter that is mainly located in β cells. It not only participates in the synthesis, storage and secretion of insulin but also maintains the structural integrity of insulin. ZnT8 is the main autoantigen recognized by autoreactive CD8+ T cells in children and adults with T1D. This article summarizes the latest research results on the T lymphocyte epitope and B lymphocyte epitope of ZnT8 in the current literature. The structure and expression of ZnT8, the role of ZnT8 in insulin synthesis and its role in autoimmunity are reviewed. ZnT8 is the primary autoantigen of T1D and is specifically expressed in pancreatic islets. Thus, it is one of biomarkers for the diagnosis of T1D. It has broad prospects for further research on immunomodulators for the treatment of T1D.

BRSK2 in pancreatic β cells promotes hyperinsulinemia-coupled insulin resistance and its genetic variants are associated with human type 2 diabetes

Brain-specific serine/threonine-protein kinase 2 (BRSK2) plays critical roles in insulin secretion and β-cell biology. However, whether BRSK2 is associated with human type 2 diabetes mellitus (T2DM) has not been determined. Here, we report that BRSK2 genetic variants are closely related to worsening glucose metabolism due to hyperinsulinemia and insulin resistance in the Chinese population. BRSK2 protein levels are significantly elevated in β cells from T2DM patients and high-fat diet (HFD)-fed mice due to enhanced protein stability. Mice with inducible β-cell-specific Brsk2 knockout (βKO) exhibit normal metabolism with a high potential for insulin secretion under chow-diet conditions. Moreover, βKO mice are protected from HFD-induced hyperinsulinemia, obesity, insulin resistance, and glucose intolerance. Conversely, gain-of-function BRSK2 in mature β cells reversibly triggers hyperglycemia due to β-cell hypersecretion-coupled insulin resistance. Mechanistically, BRSK2 senses lipid signals and induces basal insulin secretion in a kinase-dependent manner. The enhanced basal insulin secretion drives insulin resistance and β-cell exhaustion and thus the onset of T2DM in mice fed an HFD or with gain-of-function BRSK2 in β cells. These findings reveal that BRSK2 links hyperinsulinemia to systematic insulin resistance via interplay between β cells and insulin-sensitive tissues in the populations carrying human genetic variants or under nutrient-overload conditions.

Vascular endothelial growth factor B improves impaired glucose tolerance through insulin-mediated inhibition of glucagon secretion

BACKGROUND

Impaired glucose tolerance (IGT) is a homeostatic state between euglycemia and hyperglycemia and is considered an early high-risk state of diabetes. When IGT occurs, insulin sensitivity decreases, causing a reduction in insulin secretion and an increase in glucagon secretion. Recently, vascular endothelial growth factor B (VEGFB) has been demonstrated to play a positive role in improving glucose metabolism and insulin sensitivity. Therefore, we constructed a mouse model of IGT through high-fat diet feeding and speculated that VEGFB can regulate hyperglycemia in IGT by influencing insulin-mediated glucagon secretion, thus contributing to the prevention and cure of prediabetes.

AIM

To explore the potential molecular mechanism and regulatory effects of VEGFB on insulin-mediated glucagon in mice with IGT.

METHODS

We conducted in vivo experiments through systematic VEGFB knockout and pancreatic-specific VEGFB overexpression. Insulin and glucagon secretions were detected via enzyme-linked immunosorbent assay, and the protein expression of phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) was determined using western blot. Further, mRNA expression of forkhead box protein O1, phosphoenolpyruvate carboxykinase, and glucose-6 phosphatase was detected via quantitative polymerase chain reaction, and the correlation between the expression of proteins was analyzed via bioinformatics.

RESULTS

In mice with IGT and VEGFB knockout, glucagon secretion increased, and the protein expression of PI3K/AKT decreased dramatically. Further, in mice with VEGFB overexpression, glucagon levels declined, with the activation of the PI3K/AKT signaling pathway.

CONCLUSION

VEGFB/vascular endothelial growth factor receptor 1 can promote insulin-mediated glucagon secretion by activating the PI3K/AKT signaling pathway to regulate glucose metabolism disorders in mice with IGT.

Insulin C-peptide secretion on-a-chip to measure the dynamics of secretion and metabolism from individual islets

First-phase glucose-stimulated insulin secretion is mechanistically linked to type 2 diabetes, yet the underlying metabolism is difficult to discern due to significant islet-to-islet variability. Here, we miniaturize a fluorescence anisotropy immunoassay onto a microfluidic device to measure C-peptide secretion from individual islets as a surrogate for insulin (InsC-chip). This method measures secretion from up to four islets at a time with ∼7 s resolution while providing an optical window for real-time live-cell imaging. Using the InsC-chip, we reveal two glucose-dependent peaks of insulin secretion (i.e., a double peak) within the classically defined 1st phase (<10 min). By combining real-time secretion and live-cell imaging, we show islets transition from glycolytic to oxidative phosphorylation (OxPhos)-driven metabolism at the nadir of the peaks. Overall, these data validate the InsC-chip to measure glucose-stimulated insulin secretion while revealing new dynamics in secretion defined by a shift in glucose metabolism.

Secretory Conservation in Insulin Producing Cells: Is There a System-Level Law of Mass Action in Biology?

Altered secretion of insulin from pancreatic β-cells can manifest into disorders. For example, a lack of endogenously produced and/or secreted insulin results in Type 1 diabetes (and other associated subtypes). Pancreatic β-cells are the endocrine secretory cells that promote insulin secretion in response to glucose stimulation. Secretion in response to extracellular triggers is an interplay among various signaling pathways, transcription factors, and molecular mechanisms. The Mouse Insulinoma 6 (MIN6) cell line serves as a model system for gaining mechanistic insights into pancreatic β-cell functions. It is obvious that higher glucose consumption and increased insulin secretion are correlated. However, it has been reported that intracellular ATP levels remain ∼ constant beyond the extracellular glucose (EG) concentration of 10 mM. Therefore, any cause-effect relationship between glucose consumption (GC) and enhanced insulin secretion (eIS) remains unclear. We also found that total cellular protein, as well as total protein content in the culture "supernatant," remains constant regardless of varying EG concentrations. This indicated that eIS may be at the cost of (a) intracellular synthesis of other proteins and (b) secretion of other secretory proteins, or both (a) and (b), somehow coupled with GC by cells. To gain insights into the above, we carried out a transcriptome study of MIN6 cells exposed to hypoglycemic (HoG = 2.8 mM EG) and hyperglycemic (HyG = 25 mM EG) conditions. Expression of transcripts was analyzed in terms of Fragments Per Kilobase of transcript per Million mapped reads and Transcripts Per Million (FPKM and TPM) as well as values obtained by normalizing w.r.t. "∑(FPKM)" and "∑(TPM)." We report that HyG extracellular conditions lead to an ∼2-fold increase in insulin secretion compared to HoG measured by the enzyme-linked immunosorbent assay (ELISA) and transcripts of secreted proteins as well as their isoforms decreased in HyG conditions compared to HoG. Our results show for the first time that eIS in HyG conditions is at the cost of reduced transcription of other secreted proteins and is coupled with higher GC. The higher GC at increased extracellular glucose also indicates a yet undiscovered role of glucose molecules enhancing insulin secretion, since ATP levels resulting from glucose metabolism have been reported to be constant above an EG concentration of 10 mM. While extrapolation of our results to clinical implications is ambitious at best, this work reports novel cellular level aspects that seem relevant in some clinical observations pertaining to Type 1 diabetes. In addition, the conservatory nature of cellular secretions in insulin-secreting cells, discovered here, may be a general feature in cell biology.

Vascular endothelial growth factor B regulates insulin secretion in β cells of type 2 diabetes mellitus mice via PLCγ and the IP3R‑evoked Ca2+/CaMK2 signaling pathway

Vascular endothelial growth factor B (VEGFB) plays a crucial role in glucolipid metabolism and is highly associated with type 2 diabetes mellitus (T2DM). The role of VEGFB in the insulin secretion of β cells remains unverified. Thus, the present study aimed to discuss the effect of VEGFB on regulating insulin secretion in T2DM development, and its underlying mechanism. A high‑fat diet and streptozocin (STZ) were used for inducing T2DM in mice model, and VEGFB gene in islet cells of T2DM mice was knocked out by CRISPR Cas9 and overexpressed by adeno‑Associated Virus (AAV) injection. The effect of VEGFB and its underlying mechanism was assessed by light microscopy, electron microscopy and fluorescence confocal microscopy, enzyme‑linked immunosorbent assay, mass spectrometer and western blot analysis. The decrement of insulin secretion in islet β cell of T2DM mice were aggravated and blood glucose remained at a high level after VEGFB knockout (KO). However, glucose tolerance and insulin sensitivity of T2DM mice were improved after the AAV‑VEGFB186 injection. VEGFB KO or overexpression can inhibit or activate PLCγ/IP3R in a VEGFR1‑dependent manner. Then, the change of PLCγ/IP3R caused by VEGFB/VEGFR1 will alter the expression of key factors on the Ca2+/CaMK2 signaling pathway such as PPP3CA. Moreover, VEGFB can cause altered insulin secretion by changing the calcium concentration in β cells of T2DM mice. These findings indicated that VEGFB activated the Ca2+/CaMK2 pathway via VEGFR1‑PLCγ and IP3R pathway to regulate insulin secretion, which provides new insight into the regulatory mechanism of abnormal insulin secretion in T2DM.

Advances in the interaction between endoplasmic reticulum stress and osteoporosis

The endoplasmic reticulum (ER) is the main site for protein synthesis, folding, and secretion, and accumulation of the unfolded/misfolded proteins in the ER may induce ER stress. ER stress is an important participant in various intracellular signaling pathways. Prolonged- or high-intensity ER stress may induce cell apoptosis. Osteoporosis, characterized by imbalanced bone remodeling, is a global disease caused by many factors, such as ER stress. ER stress stimulates osteoblast apoptosis, increases bone loss, and promotes osteoporosis development. Many factors, such as the drug's adverse effects, metabolic disorders, calcium ion imbalance, bad habits, and aging, have been reported to activate ER stress, resulting in the pathological development of osteoporosis. Increasing evidence shows that ER stress regulates osteogenic differentiation, osteoblast activity, and osteoclast formation and function. Various therapeutic agents have been developed to counteract ER stress and thereby suppress osteoporosis development. Thus, inhibition of ER stress has become a potential target for the therapeutic management of osteoporosis. However, the in-depth understanding of ER stress in the pathogenesis of osteoporosis still needs more effort.

Regulation of insulin secretion by the post-translational modifications

Post-translational modification (PTM) has a significant impact on cellular signaling and function regulation. In pancreatic β cells, PTMs are involved in insulin secretion, cell development, and viability. The dysregulation of PTM in β cells is clinically associated with the development of diabetes mellitus. Here, we summarized current findings on major PTMs occurring in β cells and their roles in insulin secretion. Our work provides comprehensive insight into understanding the mechanisms of insulin secretion and potential therapeutic targets for diabetes from the perspective of protein PTMs.

Restoration of PITPNA in Type 2 diabetic human islets reverses pancreatic beta-cell dysfunction

Defects in insulin processing and granule maturation are linked to pancreatic beta-cell failure during type 2 diabetes (T2D). Phosphatidylinositol transfer protein alpha (PITPNA) stimulates activity of phosphatidylinositol (PtdIns) 4-OH kinase to produce sufficient PtdIns-4-phosphate (PtdIns-4-P) in the trans-Golgi network to promote insulin granule maturation. PITPNA in beta-cells of T2D human subjects is markedly reduced suggesting its depletion accompanies beta-cell dysfunction. Conditional deletion of Pitpna in the beta-cells of Ins-Cre, Pitpnaflox/flox mice leads to hyperglycemia resulting from decreasing glucose-stimulated insulin secretion (GSIS) and reducing pancreatic beta-cell mass. Furthermore, PITPNA silencing in human islets confirms its role in PtdIns-4-P synthesis and leads to impaired insulin granule maturation and docking, GSIS, and proinsulin processing with evidence of ER stress. Restoration of PITPNA in islets of T2D human subjects reverses these beta-cell defects and identify PITPNA as a critical target linked to beta-cell failure in T2D.

Genome-wide association analysis identifies ancestry-specific genetic variation associated with acute response to metformin and glipizide in SUGAR-MGH

AIMS/HYPOTHESIS

Characterisation of genetic variation that influences the response to glucose-lowering medications is instrumental to precision medicine for treatment of type 2 diabetes. The Study to Understand the Genetics of the Acute Response to Metformin and Glipizide in Humans (SUGAR-MGH) examined the acute response to metformin and glipizide in order to identify new pharmacogenetic associations for the response to common glucose-lowering medications in individuals at risk of type 2 diabetes.

METHODS

One thousand participants at risk for type 2 diabetes from diverse ancestries underwent sequential glipizide and metformin challenges. A genome-wide association study was performed using the Illumina Multi-Ethnic Genotyping Array. Imputation was performed with the TOPMed reference panel. Multiple linear regression using an additive model tested for association between genetic variants and primary endpoints of drug response. In a more focused analysis, we evaluated the influence of 804 unique type 2 diabetes- and glycaemic trait-associated variants on SUGAR-MGH outcomes and performed colocalisation analyses to identify shared genetic signals.

RESULTS

Five genome-wide significant variants were associated with metformin or glipizide response. The strongest association was between an African ancestry-specific variant (minor allele frequency [MAFAfr]=0.0283) at rs149403252 and lower fasting glucose at Visit 2 following metformin (p=1.9×10-9); carriers were found to have a 0.94 mmol/l larger decrease in fasting glucose. rs111770298, another African ancestry-specific variant (MAFAfr=0.0536), was associated with a reduced response to metformin (p=2.4×10-8), where carriers had a 0.29 mmol/l increase in fasting glucose compared with non-carriers, who experienced a 0.15 mmol/l decrease. This finding was validated in the Diabetes Prevention Program, where rs111770298 was associated with a worse glycaemic response to metformin: heterozygous carriers had an increase in HbA1c of 0.08% and non-carriers had an HbA1c increase of 0.01% after 1 year of treatment (p=3.3×10-3). We also identified associations between type 2 diabetes-associated variants and glycaemic response, including the type 2 diabetes-protective C allele of rs703972 near ZMIZ1 and increased levels of active glucagon-like peptide 1 (GLP-1) (p=1.6×10-5), supporting the role of alterations in incretin levels in type 2 diabetes pathophysiology.

CONCLUSIONS/INTERPRETATION

We present a well-phenotyped, densely genotyped, multi-ancestry resource to study gene-drug interactions, uncover novel variation associated with response to common glucose-lowering medications and provide insight into mechanisms of action of type 2 diabetes-related variation.

DATA AVAILABILITY

The complete summary statistics from this study are available at the Common Metabolic Diseases Knowledge Portal ( https://hugeamp.org ) and the GWAS Catalog ( www.ebi.ac.uk/gwas/ , accession IDs: GCST90269867 to GCST90269899).

Rab26 restricts insulin secretion via sequestering Synaptotagmin-1

Rab26 is known to regulate multiple membrane trafficking events, but its role in insulin secretion in pancreatic β cells remains unclear despite it was first identified in the pancreas. In this study, we generated Rab26-/- mice through CRISPR/Cas9 technique. Surprisingly, insulin levels in the blood of the Rab26-/- mice do not decrease upon glucose stimulation but conversely increase. Deficiency of Rab26 promotes insulin secretion, which was independently verified by Rab26 knockdown in pancreatic insulinoma cells. Conversely, overexpression of Rab26 suppresses insulin secretion in both insulinoma cell lines and isolated mouse islets. Islets overexpressing Rab26, upon transplantation, also failed to restore glucose homeostasis in type 1 diabetic mice. Immunofluorescence microscopy revealed that overexpression of Rab26 results in clustering of insulin granules. GST-pulldown experiments reveal that Rab26 interacts with synaptotagmin-1 (Syt1) through directly binding to its C2A domain, which interfering with the interaction between Syt1 and SNAP25, and consequently inhibiting the exocytosis of newcomer insulin granules revealed by TIRF microscopy. Our results suggest that Rab26 serves as a negative regulator of insulin secretion, via suppressing insulin granule fusion with plasma membrane through sequestering Syt1.

Dysfunctions, molecular mechanisms, and therapeutic strategies of pancreatic β-cells in diabetes

Pancreatic beta-cell death has been established as a critical mediator in the progression of type 1 and type 2 diabetes mellitus. Beta-cell death is associated with exacerbating hyperglycemia and insulin resistance and paves the way for the progression of DM and its complications. Apoptosis has been considered the primary mechanism of beta-cell death in diabetes. However, recent pieces of evidence have implicated the substantial involvement of several other novel modes of cell death, including autophagy, pyroptosis, necroptosis, and ferroptosis. These distinct mechanisms are characterized by their unique biochemical features and often precipitate damage through the induction of cellular stressors, including endoplasmic reticulum stress, oxidative stress, and inflammation. Experimental studies were identified from PubMed literature on different modes of beta cell death during the onset of diabetes mellitus. This review summarizes current knowledge on the crucial pathways implicated in pancreatic beta cell death. The article also focuses on applying natural compounds as potential treatment strategies in inhibiting these cell death pathways.

Promoting β-cells function by the recapitulation of in vivo microenvironmental differentiation signals

The study aims to transdifferentiate rat bone marrow-derived mesenchymal stem cells (BM-MSCs) more efficiently into islet-like cells and encapsulate and transplant them with vital properties like stability, proliferation, and metabolic activity enhanced for the treatment of T1DM. Trans-differentiation of BM-MCs into islet-like cells induced by high glucose concentration combined with Nicotinamide, ꞵ-Mercaptoethanol, ꞵ-Cellulin, and IGF-1. Glucose challenge assays and gene expression profiles were used to determine functionality. Microencapsulation was performed using the vibrating nozzle encapsulator droplet method with a 1% alginate concentration. Encapsulated ꞵ-cells were cultured in a fluidized-bed bioreactor with 1850 μL/min fluid flow rates and a superficial velocity of 1.15 cm/min. The procedure was followed by transplanting transdifferentiated cells into the omentum of streptozotocin (STZ)-induced diabetic Wistar rats. Changes in weight, glucose, insulin, and C-peptide levels were monitored for 2 months after transplantation. PDX1, INS, GCG, NKx2.2, NKx6.1, and GLUT2 expression levels revealed the specificity of generated β-cells with higher viability (about 20%) and glucose sensitivity about twofold more. The encapsulated β-cells decreased the glucose levels in STZ-induced rats significantly (P < 0.05) 1 week after transplantation. Also, the weight and levels of insulin and C-peptide reached the control group. In contrast to the treated, the sham group displayed a consistent decline in weight and died when loss reached > 20% at day ~ 55. The coated cells secrete significantly higher amounts of insulin in response to glucose concentration changes. Enhanced viability and functionality of β-cells can be achieved through differentiation and culturing, a promising approach toward insulin therapy alternatives.

Biomedical importance of the ubiquitin-proteasome system in diabetes and metabolic transdifferentiation of pancreatic duct epithelial cells into β-cells

The ubiquitin-proteasome system (UPS) is a major pathway for cellular protein degradation. The molecular function of the UPS is the removal of damaged proteins, and this function is applied in many biological processes, including inflammation, proliferation, and apoptosis. Accumulating evidence also suggests that the UPS also has a key role in pancreatic β-cell transdifferentiation in diabetes and can be targeted for treatment of diabetic diseases. In this review, we summarized the mechanistic roles of the UPS in the biochemical activities of pancreatic β-cells, including the role of the UPS in insulin synthesis and secretion, as well as β-cell degradation. Also, we discuss how the UPS mediates the transdifferentiation of pancreatic duct epithelial cells into β-cells as the experimental basis for the development of new strategies for the treatment of diabetes in regenerative medicine.

Understanding Insulin in the Age of Precision Medicine and Big Data: Under-Explored Nature of Genomics

Insulin is amongst the human genome's most well-studied genes/proteins due to its connection to metabolic health. Within this article, we review literature and data to build a knowledge base of Insulin (INS) genetics that influence transcription, transcript processing, translation, hormone maturation, secretion, receptor binding, and metabolism while highlighting the future needs of insulin research. The INS gene region has 2076 unique variants from population genetics. Several variants are found near the transcriptional start site, enhancers, and following the INS transcripts that might influence the readthrough fusion transcript INS-IGF2. This INS-IGF2 transcript splice site was confirmed within hundreds of pancreatic RNAseq samples, lacks drift based on human genome sequencing, and has possible elevated expression due to viral regulation within the liver. Moreover, a rare, poorly characterized African population-enriched variant of INS-IGF2 results in a loss of the stop codon. INS transcript UTR variants rs689 and rs3842753, associated with type 1 diabetes, are found in many pancreatic RNAseq datasets with an elevation of the 3'UTR alternatively spliced INS transcript. Finally, by combining literature, evolutionary profiling, and structural biology, we map rare missense variants that influence preproinsulin translation, proinsulin processing, dimer/hexamer secretory storage, receptor activation, and C-peptide detection for quasi-insulin blood measurements.

Modulation of Unfolded Protein Response Restores Survival and Function of β-Cells Exposed to the Endocrine Disruptor Bisphenol A

Diabetes is a metabolic disease that currently affects nearly half a billion people worldwide. β-cells dysfunction is one of the main causes of diabetes. Exposure to endocrine-disrupting chemicals is correlated with increased diabetes incidence. We hypothesized that treatment with bisphenol A (BPA) induces endoplasmic reticulum (ER) stress that activates the unfolded protein response (UPR), leading to impaired function of the β-cells, which over time, can cause diabetes. In this study, we aimed to evaluate UPR pathways activation under BPA treatment in β-cells and possible recovery of ER homeostasis. MIN6 cells (mouse insulinoma cell line) and isolated pancreatic islets from NOR (non-obese diabetes resistant) mice were treated with BPA. We analyzed the impact of BPA on β-cell viability, the architecture of the early secretory pathway, the synthesis and processing of insulin and the activation of UPR sensors and effectors. We found that the addition of the chemical chaperone TUDCA rescues the deleterious effects of BPA, resulting in improved viability, morphology and function of the β-cells. In conclusion, we propose that modulators of UPR can be used as therapeutic interventions targeted towards regaining β-cells homeostasis.

Phosphatidylinositol-4-phosphate signaling regulates dense granule biogenesis and exocytosis in Toxoplasma gondii

Phosphoinositide metabolism defines the foundation of a major signaling pathway that is conserved throughout the eukaryotic kingdom. The 4-OH phosphorylated phosphoinositides such as phosphatidylinositol-4-phosphate (PtdIns4P) and phosphatidylinositol-4,5-bisphosphate are particularly important molecules as these execute intrinsically essential activities required for the viability of all eukaryotic cells studied thus far. Using intracellular tachyzoites of the apicomplexan parasite Toxoplasma gondii as model for assessing primordial roles for PtdIns4P signaling, we demonstrate the presence of PtdIns4P pools in Golgi/trans-Golgi (TGN) system and in post-TGN compartments of the parasite. Moreover, we show that deficits in PtdIns4P signaling result in structural perturbation of compartments that house dense granule cargo with accompanying deficits in dense granule exocytosis. Taken together, the data report a direct role for PtdIns4P in dense granule biogenesis and exocytosis. The data further indicate that the biogenic pathway for secretion-competent dense granule formation in T. gondii is more complex than simple budding of fully matured dense granules from the TGN.

Peptide Inhibitors of Insulin Fibrillation: Current and Future Challenges

Amyloidoses include a large variety of local and systemic diseases that share the common feature of protein unfolding or refolding into amyloid fibrils. The most studied amyloids are those directly involved in neurodegenerative diseases, while others, such as those formed by insulin, are surprisingly far less studied. Insulin is a very important polypeptide that plays a variety of biological roles and, first and foremost, is at the basis of the therapy of diabetic patients. It is well-known that it can form fibrils at the site of injection, leading to inflammation and immune response, in addition to other side effects. In this concise review, we analyze the current knowledge on insulin fibrillation, with a focus on the development of peptide-based inhibitors, which are promising candidates for their biocompatibility but still pose challenges to their effective use in therapy.

A Novel Somatic Mutation Implicates ATP6V0D1 in Proinsulin Processing

Context

Prohormone convertase 1/3 (PC1/3), encoded by protein convertase subtilisin kexin type 1 (PCSK1), converts inactive prohormones into biologically active peptides. Somatic mutations of insulinomas are associated with genetic defects interfering with control of insulin secretion from pancreatic beta cells. However, somatic mutations in proinsulinomas have not been described.

Objective

We report a case of a proinsulinoma, with suppressed insulin and C-peptide levels.

Methods

A 70-year-old woman presented with a 20-year history of "blackouts." During a 72-hour fast, blood glucose level dropped to 1.9 mmol/L with suppressed plasma insulin and C-peptide levels, but proinsulin levels were raised at 37 pmol/L (<10 pmol/L).

Results

Imaging revealed 3 distinct DOTATATE-avid pancreatic lesions. Laparoscopic spleen-preserving distal pancreatomy was performed. In view of discordant insulin, C-peptide, and proinsulin levels, whole exome sequencing analysis was performed on the tumor. In the somatic exome of the tumor, we found mutations in PCSK expression regulators, as well as a novel truncating somatic mutation in ATP6V0D1, a subunit of the ion pump that acidifies the β-cell compartments where the PCSKs act.

Conclusion

Appropriately suppressed insulin levels in the context of hypoglycemia do not always indicate the absence of a neuroendocrine islet cell tumor and proinsulin levels may be indicated to solidify the diagnosis. In the context of elevated proinsulin levels, low insulin and C-peptide levels might be explained by somatic mutations that likely implicate proinsulin processing within the tumor. Furthermore, we propose several mechanistic candidates, including ATP6V0D1. Experimental validation using cellular approaches may in future confirm pathomechanisms involved in this rare condition.

The mechanisms of chromogranin B-regulated Cl- homeostasis

Chloride is the most abundant inorganic anions in almost all cells and in human circulation systems. Its homeostasis is therefore important for systems physiology and normal cellular activities. This topic has been extensively studied with chloride loaders and extruders expressed in both cell surfaces and intracellular membranes. With the newly discovered, large-conductance, highly selective Cl- channel formed by membrane-bound chromogranin B (CHGB), which differs from all other known anion channels of conventional transmembrane topology, and is distributed in plasma membranes, endomembrane systems, endosomal, and endolysosomal compartments in cells expressing it, we will discuss the potential physiological importance of the CHGB channels to Cl- homeostasis, cellular excitability and volume control, and cation uptake or release at the cellular and subcellular levels. These considerations and CHGB's association with human diseases make the CHGB channel a possible druggable target for future molecular therapeutics.

Setting the Stage for Insulin Granule Dysfunction during Type-1-Diabetes: Is ER Stress the Culprit?

Type-1-diabetes (T1D) is a multifactorial disorder with a global incidence of about 8.4 million individuals in 2021. It is primarily classified as an autoimmune disorder, where the pancreatic β-cells are unable to secrete sufficient insulin. This leads to elevated blood glucose levels (hyperglycemia). The development of T1D is an intricate interplay between various risk factors, such as genetic, environmental, and cellular elements. In this review, we focus on the cellular elements, such as ER (endoplasmic reticulum) stress and its consequences for T1D pathogenesis. One of the major repercussions of ER stress is defective protein processing. A well-studied example is that of islet amyloid polypeptide (IAPP), which is known to form cytotoxic amyloid plaques when misfolded. This review discusses the possible association between ER stress, IAPP, and amyloid formation in β-cells and its consequences in T1D. Additionally, ER stress also leads to autoantigen generation. This is driven by the loss of Ca++ ion homeostasis. Imbalanced Ca++ levels lead to abnormal activation of enzymes, causing post-translational modification of β-cell proteins. These modified proteins act as autoantigens and trigger the autoimmune response seen in T1D islets. Several of these autoantigens are also crucial for insulin granule biogenesis, processing, and release. Here, we explore the possible associations between ER stress leading to defects in insulin secretion and ultimately β-cell destruction.

Sunlight-Controllable Biopharmaceutical Production for Remote Emergency Supply of Directly Injectable Therapeutic Proteins

Biopharmaceutical manufacturing requires specialized facilities and a long-range cold supply chain for the delivery of the therapeutics to patients. In order to produce biopharmaceuticals in locations lacking such infrastructure, a production process is designed that utilizes the trigger-inducible release of large quantities of a stored therapeutic protein from engineered endocrine cells within minutes to generate a directly injectable saline solution of the protein. To illustrate the versatility of this approach, it is shown that not only insulin, but also glucagon-like peptide 1 (GLP-1), nanoluciferase (NLuc), and the model biopharmaceutical erythropoietin (EPO) can be trigger-inducibly released, even when using biologically inactive insulin as a carrier. The facilitating beta cells are engineered with a controllable TRPV1-mediated Ca²⁺ influx that induces the fusion of cytoplasmic storage vesicles with the membrane, leading to the release of the stored protein. When required, the growth medium is exchanged for saline solution, and the system is stimulated with the small molecule capsaicin, with a hand-warming pack, or simply by using sunlight. Injection of insulin saline solution obtained in this way into a type-1 diabetes mouse model results in the regulation of blood glucose levels. It is believed that this system will be readily adaptable to deliver various biopharmaceutical proteins at remote locations.

Quantitative, in situ visualization of intracellular insulin vesicles in pancreatic beta cells

Characterizing relationships between Zn²⁺, insulin, and insulin vesicles is of vital importance to the study of pancreatic beta cells. However, the precise content of Zn²⁺ and the specific location of insulin inside insulin vesicles are not clear, which hinders a thorough understanding of the insulin secretion process and diseases caused by blood sugar dysregulation. Here, we demonstrated the colocalization of Zn²⁺ and insulin in both single extracellular insulin vesicles and pancreatic beta cells by using an X-ray scanning coherent diffraction imaging (ptychography) technique. We also analyzed the elemental Zn²⁺ and Ca²⁺ contents of insulin vesicles using electron microscopy and energy dispersive spectroscopy (EDS) mapping. We found that the presence of Zn²⁺ is an important characteristic that can be used to distinguish insulin vesicles from other types of vesicles in pancreatic beta cells and that the content of Zn²⁺ is proportional to the size of insulin vesicles. By using dual-energy contrast X-ray microscopy and scanning transmission X-ray microscopy (STXM) image stacks, we observed that insulin accumulates in the off-center position of extracellular insulin vesicles. Furthermore, the spatial distribution of insulin vesicles and their colocalization with other organelles inside pancreatic beta cells were demonstrated using three-dimensional (3D) imaging by combining X-ray ptychography and an equally sloped tomography (EST) algorithm. This study describes a powerful method to univocally describe the location and quantitative analysis of intracellular insulin, which will be of great significance to the study of diabetes and other blood sugar diseases.

Microtubules in Pancreatic β Cells: Convoluted Roadways Toward Precision

Pancreatic islet β cells regulate glucose homeostasis via glucose-stimulated insulin secretion (GSIS). Cytoskeletal polymers microtubules (MTs) serve as tracks for the transport and positioning of secretory insulin granules. MT network in β cells has unique morphology with several distinct features, which support granule biogenesis (via Golgi-derived MT array), net non-directional transport (via interlocked MT mesh), and control availability of granules at secretion sites (via submembrane MT bundle). The submembrane MT array, which is parallel to the plasma membrane and serves to withdraw excessive granules from the secretion hot spots, is destabilized and fragmented downstream of high glucose stimulation, allowing for regulated secretion. The origin of such an unusual MT network, the features that define its functionality, and metabolic pathways that regulate it are still to a large extent elusive and are a matter of active investigation and debate. Besides the MT network itself, it is important to consider the interplay of molecular motors that drive and fine-tune insulin granule transport. Importantly, activity of kinesin-1, which is the major MT-dependent motor in β cells, transports insulin granules, and has a capacity to remodel MT network, is also regulated by glucose. We discuss yet unknown potential avenues toward understanding how MT network and motor proteins provide control for secretion in coordination with other GSIS-regulating mechanisms.

Glucose-Dependent miR-125b Is a Negative Regulator of β-Cell Function

Impaired pancreatic β-cell function and insulin secretion are hallmarks of type 2 diabetes. miRNAs are short, noncoding RNAs that silence gene expression vital for the development and function of β cells. We have previously shown that β cell-specific deletion of the important energy sensor AMP-activated protein kinase (AMPK) results in increased miR-125b-5p levels. Nevertheless, the function of this miRNA in β cells is unclear. We hypothesized that miR-125b-5p expression is regulated by glucose and that this miRNA mediates some of the deleterious effects of hyperglycemia in β cells. Here, we show that islet miR-125b-5p expression is upregulated by glucose in an AMPK-dependent manner and that short-term miR-125b-5p overexpression impairs glucose-stimulated insulin secretion (GSIS) in the mouse insulinoma MIN6 cells and in human islets. An unbiased, high-throughput screen in MIN6 cells identified multiple miR-125b-5p targets, including the transporter of lysosomal hydrolases M6pr and the mitochondrial fission regulator Mtfp1. Inactivation of miR-125b-5p in the human β-cell line EndoCβ-H1 shortened mitochondria and enhanced GSIS, whereas mice overexpressing miR-125b-5p selectively in β cells (MIR125B-Tg) were hyperglycemic and glucose intolerant. MIR125B-Tg β cells contained enlarged lysosomal structures and had reduced insulin content and secretion. Collectively, we identify miR-125b as a glucose-controlled regulator of organelle dynamics that modulates insulin secretion.

In Vitro Assays to Identify Metabolism-Disrupting Chemicals with Diabetogenic Activity in a Human Pancreatic β-Cell Model

There is a need to develop identification tests for Metabolism Disrupting Chemicals (MDCs) with diabetogenic activity. Here we used the human EndoC-βH1 β-cell line, the rat β-cell line INS-1E and dispersed mouse islet cells to assess the effects of endocrine disruptors on cell viability and glucose-stimulated insulin secretion (GSIS). We tested six chemicals at concentrations within human exposure (from 0.1 pM to 1 µM). Bisphenol-A (BPA) and tributyltin (TBT) were used as controls while four other chemicals, namely perfluorooctanoic acid (PFOA), triphenylphosphate (TPP), triclosan (TCS) and dichlorodiphenyldichloroethylene (DDE), were used as “unknowns”. Regarding cell viability, BPA and TBT increased cell death as previously observed. Their mode of action involved the activation of estrogen receptors and PPARγ, respectively. ROS production was a consistent key event in BPA-and TBT-treated cells. None of the other MDCs tested modified viability or ROS production. Concerning GSIS, TBT increased insulin secretion while BPA produced no effects. PFOA decreased GSIS, suggesting that this chemical could be a “new” diabetogenic agent. Our results indicate that the EndoC-βH1 cell line is a suitable human β-cell model for testing diabetogenic MDCs. Optimization of the test methods proposed here could be incorporated into a set of protocols for the identification of MDCs.

Endoplasmic Reticulum (ER) Stress and Its Role in Pancreatic β-Cell Dysfunction and Senescence in Type 2 Diabetes

An increased life span and accompanying nutritional affluency have led to a rapid increase in diseases associated with aging, such as obesity and type 2 diabetes, imposing a tremendous economic and health burden on society. Pancreatic β-cells are crucial for controlling glucose homeostasis by properly producing and secreting the glucose-lowering hormone insulin, and the dysfunction of β-cells determines the outcomes for both type 1 and type 2 diabetes. As the native structure of insulin is formed within the endoplasmic reticulum (ER), ER homeostasis should be appropriately maintained to allow for the proper metabolic homeostasis and functioning of β-cells. Recent studies have found that cellular senescence is critically linked with cellular stresses, including ER stress, oxidative stress, and mitochondrial stress. These studies implied that β-cell senescence is caused by ER stress and other cellular stresses and contributes to β-cells’ dysfunction and the impairment of glucose homeostasis. This review documents and discusses the current understanding of cellular senescence, β-cell function, ER stress, its associated signaling mechanism (unfolded protein response), and the effect of ER stress on β-cell senescence and dysfunction.

Role of selenoprotein P expression in the function of pancreatic β cells: Prevention of ferroptosis-like cell death and stress-induced nascent granule degradation

Selenoprotein P (SELENOP) is a major selenium (Se)-containing protein (selenoprotein) in human plasma that is mainly synthesized in the liver. SELENOP transports Se to the cells, while SELENOP synthesized in peripheral tissues is incorporated in a paracrine/autocrine manner to maintain the levels of cellular selenoproteins, called the SELENOP cycle. Pancreatic β cells, responsible for the synthesis and secretion of insulin, are known to express SELENOP. Here, using MIN6 cells as a mouse model for pancreatic β cells and Selenop small interfering (si)RNA, we found that Selenop gene knockdown (KD) resulted in decreased cell viability, cellular pro/insulin levels, insulin secretion, and levels of several cellular selenoproteins, including glutathione peroxidase 4 (Gpx4) and selenoprotein K (Selenok). These dysfunctions induced by Selenop siRNA were recovered by the addition of Se. Ferroptosis-like cell death, regulated by Gpx4, was involved in the decrease of cell viability by Selenop KD, while stress-induced nascent granule degradation (SINGD), regulated by Selenok, was responsible for the decrease in proinsulin. SINGD was also observed in the pancreatic β cells of Selenop knockout mice. These findings indicate a significant role of SELENOP expression for the function of pancreatic β cells by maintaining the levels of cellular selenoproteins such as GPX4 and SELENOK.

Lipid Droplets' Role in the Regulation of β-Cell Function and β-Cell Demise in Type 2 Diabetes

During development of type 2 diabetes (T2D), excessive nutritional load is thought to expose pancreatic islets to toxic effects of lipids and reduce β-cell function and mass. However, lipids also play a positive role in cellular metabolism and function. Thus, proper trafficking of lipids is critical for β cells to maximize the beneficial effects of these molecules while preventing their toxic effects. Lipid droplets (LDs) are organelles that play an important role in the storage and trafficking of lipids. In this review, we summarize the discovery of LDs in pancreatic β cells, LD lifecycle, and the effect of LD catabolism on β-cell insulin secretion. We discuss factors affecting LD formation such as age, cell type, species, and nutrient availability. We then outline published studies targeting critical LD regulators, primarily in rat and human β-cell models, to understand the molecular effect of LD formation and degradation on β-cell function and health. Furthermore, based on the abnormal LD accumulation observed in human T2D islets, we discuss the possible role of LDs during the development of β-cell failure in T2D. Current knowledge indicates that proper formation and clearance of LDs are critical to normal insulin secretion, endoplasmic reticulum homeostasis, and mitochondrial integrity in β cells. However, it remains unclear whether LDs positively or negatively affect human β-cell demise in T2D. Thus, we discuss possible research directions to address the knowledge gap regarding the role of LDs in β-cell failure.

The changing view of insulin granule mobility: From conveyor belt to signaling hub

Before the advent of TIRF microscopy the fate of the insulin granule prior to secretion was deduced from biochemical investigations, electron microscopy and electrophysiological measurements. Since Calcium-triggered granule fusion is indisputably necessary to release insulin into the extracellular space, much effort was directed to the measure this event at the single granule level. This has also been the major application of the TIRF microscopy of the pancreatic beta cell when it became available about 20 years ago. To better understand the metabolic modulation of secretion, we were interested to characterize the entirety of the insulin granules which are localized in the vicinity of the plasma membrane to identify the characteristics which predispose to fusion. In this review we concentrate on how the description of granule mobility in the submembrane space has evolved as a result of progress in methodology. The granules are in a state of constant turnover with widely different periods of residence in this space. While granule fusion is associated +with prolonged residence and decreased lateral mobility, these characteristics may not only result from binding to the plasma membrane but also from binding to the cortical actin web, which is present in the immediate submembrane space. While granule age as such affects granule mobility and fusion probability, the preceding functional states of the beta cell leave their mark on these parameters, too. In summary, the submembrane granules form a highly dynamic heterogeneous population and contribute to the metabolic memory of the beta cells.

Targeting the insulin granule for modulation of insulin exocytosis

The pancreatic β-cells control insulin secretion in the body to regulate glucose homeostasis, and β-cell stress and dysfunction is characteristic of Type 2 Diabetes. Pharmacological targeting of the β-cell to increase insulin secretion is typically utilised, however, extended use of common drugs such as sulfonylureas are known to result in secondary failure. Moreover, there is evidence they may induce β-cell failure in the long term. Within β-cells, insulin secretory granules (ISG) serve as compartments to store, process and traffic insulin for exocytosis. There is now growing evidence that ISG exist in multiple populations, distinct in their protein composition, motility, age, and capacity for secretion. In this review, we discuss the implications of a heterogenous ISG population in β-cells and highlight the need for more understanding into how unique ISG populations may be targeted in anti-diabetic therapies.

Chronic binge alcohol and ovariectomy-mediated impaired insulin responsiveness in SIV-infected female rhesus macaques

Aging people living with HIV (PLWH), especially postmenopausal women may be at higher risk of comorbidities associated with HIV, antiretroviral therapy (ART), hypogonadism, and at-risk alcohol use. Our studies in simian immunodeficiency virus (SIV)-infected male macaques demonstrated that chronic binge alcohol (CBA) reduced acute insulin response to glucose (AIRG), and at-risk alcohol use decreased HOMA-β in PLWH. The objective of this study was to examine the impact of ovariectomy (OVX) on glucose-insulin dynamics and integrity of pancreatic endocrine function in CBA/SIV-infected female macaques. Female macaques were administered CBA (12-15 g/kg/wk) or isovolumetric water (VEH) intragastrically. Three months after initiation of CBA/VEH administration, all macaques were infected with SIVmac251, and initiated on antiretroviral therapy (ART) 2.5 mo postinfection. After 1 mo of ART, macaques were randomized to OVX or sham surgeries (n = 7 or 8/group), and euthanized 8 mo post-OVX (study endpoint). Frequently sampled intravenous glucose tolerance tests (FSIVGTT) were performed at selected time points. Pancreatic gene expression and islet morphology were determined at study endpoint. There was a main effect of CBA to decrease AIRG at Pre-SIV and study endpoint. There were no statistically significant OVX effects on AIRG (P = 0.06). CBA and OVX decreased the expression of pancreatic markers of insulin docking and release. OVX increased endoplasmic stress markers. CBA but not OVX impaired glucose-insulin expression dynamics in SIV-infected female macaques. Both CBA and OVX altered integrity of pancreatic endocrine function. These findings suggest increased vulnerability of PLWH to overt metabolic dysfunction that may be exacerbated by alcohol use and ovarian hormone loss.

Changes in granule mobility and age contribute to changes in insulin secretion after desensitization or rest

INTRODUCTION

Functional impairment of the stimulus secretion coupling in pancreatic beta cells is an essential component of type 2 diabetes. It is known that prolonged stimulation desensitizes the secretion of insulin and thus contributes to beta cell dysfunction. Beta cell rest, in contrast, was shown to enhance the secretory response. Here, the underlying mechanisms were investigated.

RESEARCH DESIGN AND METHODS

To characterize the consequences of desensitization or rest for the number and mobility of submembrane granules, insulin-secreting MIN6 cells were desensitized by 18-hour culture with 500 µM tolbutamide or rested by 18-hour culture with 1 µM clonidine. The granules were labeled by hIns-EGFP or hIns-DsRed E5, imaged by TIRF microscopy of the cell footprint area and analyzed with an observer-independent program. Additionally, the insulin content and secretion were measured.

RESULTS

Concurrent with the insulin content, submembrane granules were only slightly reduced after desensitization but markedly increased after rest. Both types of pretreatment diminished arrivals and departures of granules in the submembrane space and increased the proportion of immobile long-term resident granules, but desensitization lowered and rest increased the number of exocytoses, in parallel with the effect on insulin secretion. Labeling with hIns-DsRed E5 ('timer') showed that desensitization did not affect the proportion of aged granules, whereas rest increased it. Aged granules showed a high mobility and made up only a minority of long-term residents. Long-term resident granules were more numerous after rest and had a lower lateral mobility, suggesting a firmer attachment to the membrane.

CONCLUSION

The number, mobility and age of submembrane granules reflect the preceding functional states of insulin-secreting cells. Representing the pool of releasable granules, their quantity and quality may thus form part of the beta cell memory on renewed stimulation.

The β Cell in Diabetes: Integrating Biomarkers With Functional Measures

The pathogenesis of hyperglycemia observed in most forms of diabetes is intimately tied to the islet β cell. Impairments in propeptide processing and secretory function, along with the loss of these vital cells, is demonstrable not only in those in whom the diagnosis is established but typically also in individuals who are at increased risk of developing the disease. Biomarkers are used to inform on the state of a biological process, pathological condition, or response to an intervention and are increasingly being used for predicting, diagnosing, and prognosticating disease. They are also proving to be of use in the different forms of diabetes in both research and clinical settings. This review focuses on the β cell, addressing the potential utility of genetic markers, circulating molecules, immune cell phenotyping, and imaging approaches as biomarkers of cellular function and loss of this critical cell. Further, we consider how these biomarkers complement the more long-established, dynamic, and often complex measurements of β-cell secretory function that themselves could be considered biomarkers.

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

Targeting Small GTPases and Their Prenylation in Diabetes Mellitus

A fundamental role of pancreatic β-cells to maintain proper blood glucose level is controlled by the Ras superfamily of small GTPases that undergo post-translational modifications, including prenylation. This covalent attachment with either a farnesyl or a geranylgeranyl group controls their localization, activity, and protein–protein interactions. Small GTPases are critical in maintaining glucose homeostasis acting in the pancreas and metabolically active tissues such as skeletal muscles, liver, or adipocytes. Hyperglycemia-induced upregulation of small GTPases suggests that inhibition of these pathways deserves to be considered as a potential therapeutic approach in treating T2D. This Perspective presents how inhibition of various points in the mevalonate pathway might affect protein prenylation and functioning of diabetes-affected tissues and contribute to chronic inflammation involved in diabetes mellitus (T2D) development. We also demonstrate the currently available molecular tools to decipher the mechanisms linking the mevalonate pathway’s enzymes and GTPases with diabetes.

Emerging Roles of Small GTPases in Islet β-Cell Function

Several small guanosine triphosphatases (GTPases) from the Ras protein superfamily regulate glucose-stimulated insulin secretion in the pancreatic islet β-cell. The Rho family GTPases Cdc42 and Rac1 are primarily involved in relaying key signals in several cellular functions, including vesicle trafficking, plasma membrane homeostasis, and cytoskeletal dynamics. They orchestrate specific changes at each spatiotemporal region within the β-cell by coordinating with signal transducers, guanine nucleotide exchange factors (GEFs), GTPase-activating factors (GAPs), and their effectors. The Arf family of small GTPases is involved in vesicular trafficking (exocytosis and endocytosis) and actin cytoskeletal dynamics. Rab-GTPases regulate pre-exocytotic and late endocytic membrane trafficking events in β-cells. Several additional functions for small GTPases include regulating transcription factor activity and mitochondrial dynamics. Importantly, defects in several of these GTPases have been found associated with type 2 diabetes (T2D) etiology. The purpose of this review is to systematically denote the identities and molecular mechanistic steps in the glucose-stimulated insulin secretion pathway that leads to the normal release of insulin. We will also note newly identified defects in these GTPases and their corresponding regulatory factors (e.g., GDP dissociation inhibitors (GDIs), GEFs, and GAPs) in the pancreatic β-cells, which contribute to the dysregulation of metabolism and the development of T2D.

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.

Patterns of Insulin Secretion During First-Phase Insulin Secretion in Normal Chinese Adults

BACKGROUND

The increase in diabetes worldwide is alarming. Decreased acute insulin response to intravenous glucose tolerance test (IVGTT) during first-phase insulin secretion (FPIS) is a characteristic of diabetes. However, knowledge of the insulin secretion characteristics identified by different time to glucose peak in subjects with different metabolic state is sparse.

AIMS

This study aimed to find different patterns of FPIS in subjects with normal glucose tolerance (NGT) and analyzed the relationship between insulin secretion patterns and the risk for development of type 2 diabetes mellitus (T2DM).

METHODS

A total of 126 subjects were divided into three groups during a 10-min IVGTT, including NGT with time to glucose peak after 3 min (G1, n = 21), NGT with time to glucose peak at 3 min (G2, n = 95), and prediabetes or diabetes with time to glucose peak at 3 min (G3, n = 10). Glucose, insulin, and C-peptide concentrations at 0, 3, 5, 7, and 10 min during the IVGTT were tested. IVGTT-based indices were calculated to evaluate the insulin secretion and insulin sensitivity.

RESULTS

Age, body mass index (BMI), waist-to-hip ratio, triglyceride (TG), and hemoglobin A1c (HbA1c) of subjects were gradually higher, while high-density lipoprotein cholesterol (HDL-C) was gradually lower from G1 to G3 (p for linear trend <0.05), and the differences between G1 and G2 were also statistically significant (p < 0.05). Glucose peak of most participants in G1 converged at 5 min, and the curves shape of insulin and C-peptide in G2 were the sharpest among three groups. There was no significant difference in all IVGTT-based indices between G1 and G2, but AUC_Ins, AUC_Ins/AUC_Glu, and △Ins₃/△Glu₃ in G2 were the highest, and the p-value for linear trend of those indices among three groups were statistically significant (p < 0.05).

CONCLUSIONS

Two patterns of FPIS were in subjects with NGT, while subjects with later time to glucose peak during FPIS might be less likely to develop T2DM in the future.