SERCA2 regulates proinsulin processing and processing enzyme maturation in pancreatic beta cells

Wolfram syndrome 2 gene (CISD2) deficiency disrupts Ca2+-mediated insulin secretion in β-cells

OBJECTIVE

Diabetes, characterized by childhood-onset, autoantibody-negativity and insulin-deficiency, is a major manifestation of Wolfram syndrome 2 (WFS2), which is caused by recessive mutations of CISD2. Nevertheless, the mechanism underlying β-cell dysfunction in WFS2 remains elusive. Here we delineate the essential role of CISD2 in β-cells.

METHODS

We use β-cell specific Cisd2 knockout (Cisd2KO) mice, a CRISPR-mediated Cisd2KO MIN6 β-cell line and transcriptomic analysis.

RESULTS

Four findings are pinpointed. Firstly, β-cell specific Cisd2KO in mice disrupts systemic glucose homeostasis via impairing β-granules synthesis and insulin secretion; hypertrophy of the β-islets and the presence of a loss of identity that affects certain β-cells. Secondly, Cisd2 deficiency leads to impairment of glucose-induced extracellular Ca2+ influx, which compromises Ca2+-mediated insulin secretory signaling, causing mitochondrial dysfunction and, thereby impairing insulin secretion in the MIN6-Cisd2KO β-cells. Thirdly, transcriptomic analysis of β-islets reveals that Cisd2 modulates proteostasis and ER stress, mitochondrial function, insulin secretion and vesicle transport. Finally, the activated state of two potential upstream regulators, Glis3 and Hnf1a, is significantly suppressed under Cisd2 deficiency; notably, their downstream target genes are deeply involved in β-cell function and identity.

CONCLUSIONS

These findings provide mechanistic insights and form a basis for developing therapeutics for the effective treatment of diabetes in WFS2 patients.

Secretory stimuli distinctly regulate insulin secretory granule maturation through structural remodeling

Insulin secretory granule (ISG) maturation is a crucial aspect of insulin secretion and glucose homeostasis. The regulation of this maturation remains poorly understood, especially how secretory stimuli affect ISG maturity and subcellular localization. In this study, we used soft X-tomography (SXT) to quantitatively map ISG morphology, density, and location in single INS-1E and mouse pancreatic β-cells under the effect of various secretory stimuli. We found that the activation of glucokinase (GK), gastric inhibitory polypeptide receptor (GIPR), glucagon-like peptide-1 receptor (GLP-1R), and G-protein coupled receptor 40 (GPR40) promote ISG maturation. Each stimulus induces unique structural remodeling in ISGs, by altering size and density, depending on the specific signaling cascades activated. These distinct ISG subpopulations mobilize and redistribute in the cell altering overall cellular structural organization. Our results provide insight into how current diabetes and obesity therapies impact ISG maturation and may inform the development of future treatments that target maturation specifically.

New Perspectives in Studying Type 1 Diabetes Susceptibility Biomarkers

Type 1 diabetes (T1D) is generally viewed as an etiologic subtype of diabetes caused by the autoimmune destruction of the insulin-secreting β-cells. It has been known that autoreactive T cells unfortunately destroy healthy β-cells. However, there has been a notion of etiologic heterogeneity around the world implicating a varying incidence of a non-autoimmune subgroup of T1D related to insulin deficiency associated with decreased β cell mass, in which the β-cell is the key contributor to the disease. Beta cell dysfunction, reduced mass, and apoptosis may lead to insufficient insulin secretion and ultimately to the development of T1D. Interestingly, Korean as well as other ethnic genetic results have also suggested that genes related with insulin deficiency, let alone those of immune regulation, were associated with the risk of T1D in the young. Genes related with insulin secretion may influence the phenotype of diabetes differentially and different genes may be working on different steps of T1D development. Although we admit the consensus that islet autoimmunity is an essential component in the pathogenesis of T1D, however, dysfunction might occur not only in the immune system but also in the β-cells, the defect of which may induce further dysfunction of the immune system. These arguments stem from the fact that the β-cell might be the trigger of an autoimmune response. This emergent view has many parallels with the fact that by their nature and function, β-cells are prone to biosynthetic stress with limited measures for self-defense. Beta cell stress may induce an immune attack that has considerable negative effects on the production of a vital hormone, insulin. If then, both β-cell stress and islet autoimmunity can be harnessed as targets for intervention strategies. This also may explain why immunotherapy at best delays the progression of T1D and suggests the use of alternative therapies to expand β-cells, in combination with immune intervention strategies, to reverse the disease. Future research should extend to further investigate β-cell biology, in addition to studies of immunologic areas, to find appropriate biomarkers of T1D susceptibility. This will help to decipher β-cell characteristics and the factors regulating their function to develop novel therapeutic approaches.

Exploring proinsulin proteostasis: insights into beta cell health and diabetes

Proinsulin misfolding is central to diabetes. This review examines the cellular mechanisms regulating proinsulin proteostasis in pancreatic β-cells, encompassing genetic factors such as insulin gene mutations, and exploring the roles of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), ER redox balance, mitochondrial function, and the influence of extrinsic factors. Mutations in the INS gene, particularly those affecting cysteine residues, impair folding and disulfide bond formation, often exhibiting dominant-negative effects on the wild-type proinsulin. The importance of ER quality control mechanisms, including chaperones and oxidoreductases, in facilitating proper folding and degradation of misfolded proinsulin is emphasized. Disruptions in these systems, due to genetic mutations, ER stress, or impaired ER-to-Golgi trafficking, lead to proinsulin accumulation and β-cell dysfunction. The unfolded protein response (UPR), especially the PERK and IRE1α-XBP1 pathways, emerges as a central regulator of protein synthesis and ER stress management. The review also discusses the role of mitochondrial health, ER redox state, and extrinsic factors such as diet and medications in influencing proinsulin proteostasis. Finally, the structural insights from NMR and molecular dynamics simulations are discussedhighlighting the dynamics of misfolding and underscoring the importance of disulfide bonds. These mechanistic insights suggest innovative strategies targeting thiol/disulfide redox systems in cells to mitigate protein misfolding diseases including diabetes.

Fatty Acid Transport Protein-2 (FATP2) Inhibition Enhances Glucose Tolerance through α-Cell-mediated GLP-1 Secretion

Type 2 diabetes affects more than 30 million people in the US, and a major complication is kidney disease. During the analysis of lipotoxicity in diabetic kidney disease, global fatty acid transport protein-2 (FATP2) gene deletion was noted to markedly reduce plasma glucose in db/db mice due to sustained insulin secretion. To identify the mechanism, we observed that islet FATP2 expression was restricted to α-cells, and α-cell FATP2 was functional. Direct evidence of FATP2KO-induced α-cell-mediated GLP-1 secretion included increased GLP-1-positive α-cell mass in FATP2KO db/db mice, small molecule FATP2 inhibitor enhancement of GLP-1 secretion in αTC1-6 cells and human islets, and exendin[9-39]-inhibitable insulin secretion in FATP2 inhibitor-treated human islets. FATP2-dependent enteroendocrine GLP-1 secretion was excluded by demonstration of similar glucose tolerance and plasma GLP-1 concentrations in db/db FATP2KO mice following oral versus intraperitoneal glucose loading, non-overlapping FATP2 and preproglucagon mRNA expression, and lack of FATP2/GLP-1 co-immunolocalization in intestine. We conclude that FATP2 deletion or inhibition exerts glucose-lowering effects through α-cell-mediated GLP-1 secretion and paracrine β-cell insulin release.

Graphical abstract

Disrupted Endoplasmic Reticulum Ca2+ Handling: A Harβinger of β-Cell Failure

The β-cell workload increases in the setting of insulin resistance and reduced β-cell mass, which occurs in type 2 and type 1 diabetes, respectively. The prolonged elevation of insulin production and secretion during the pathogenesis of diabetes results in β-cell ER stress. The depletion of β-cell Ca2+ER during ER stress activates the unfolded protein response, leading to β-cell dysfunction. Ca2+ER is involved in many pathways that are critical to β-cell function, such as protein processing, tuning organelle and cytosolic Ca2+ handling, and modulating lipid homeostasis. Mutations that promote β-cell ER stress and deplete Ca2+ER stores are associated with or cause diabetes (e.g., mutations in ryanodine receptors and insulin). Thus, improving β-cell Ca2+ER handling and reducing ER stress under diabetogenic conditions could preserve β-cell function and delay or prevent the onset of diabetes. This review focuses on how mechanisms that control β-cell Ca2+ER are perturbed during the pathogenesis of diabetes and contribute to β-cell failure.

Genetics and diet shape the relationship between islet function and whole body metabolism

Despite the fact that genes and the environment are known to play a central role in islet function, our knowledge of how these parameters interact to modulate insulin secretory function remains relatively poor. Presently, we performed ex vivo glucose-stimulated insulin secretion and insulin content assays in islets of 213 mice from 13 inbred mouse strains on chow, Western diet (WD), and a high-fat, carbohydrate-free (KETO) diet. Strikingly, among these 13 strains, islets from the commonly used C57BL/6J mouse strain were the least glucose responsive. Using matched metabolic phenotyping data, we performed correlation analyses of isolated islet parameters and found a positive correlation between basal and glucose-stimulated insulin secretion, but no relationship between insulin secretion and insulin content. Using in vivo metabolic measures, we found that glucose tolerance determines the relationship between ex vivo islet insulin secretion and plasma insulin levels. Finally, we showed that islet glucose-stimulated insulin secretion decreased with KETO in almost all strains, concomitant with broader phenotypic changes, such as increased adiposity and glucose intolerance. This is an important finding as it should caution against the application of KETO diet for beta-cell health. Together these data offer key insights into the intersection of diet and genetic background on islet function and whole body glucose metabolism.NEW & NOTEWORTHY Thirteen strains of mice on chow, Western diet, and high-fat, carbohydrate-free (KETO), correlating whole body phenotypes to ex vivo pancreatic islet functional measurements, were used. The study finds a huge spectrum of functional islet responses and insulin phenotypes across all strains and diets, with the ubiquitous C57Bl/6J mouse exhibiting the lowest secretory response of all strains, highlighting the overall importance of considering genetic background when investigating islet function. Ex vivo basal and stimulated insulin secretion are correlated in the islet, and KETO imparts widescale downregulation of islet insulin secretion.

The Ailing β-Cell in Diabetes: Insights From a Trip to the ER: The 2023 Outstanding Scientific Achievement Award Lecture

The synthesis, processing, and secretion of insulin by the pancreatic β-cell is key for the maintenance of systemic metabolic homeostasis, and loss or dysfunction of β-cells underlies the development of both type 1 diabetes (T1D) and type 2 diabetes (T2D). Work in the Evans-Molina laboratory over the past 15 years has pioneered the idea that regulation of calcium dynamics is critical to β-cell biology and diabetes pathophysiology. In this article, I will share three vignettes from the laboratory that demonstrate our bench-to-bedside approach to determining mechanisms of β-cell stress that could improve therapeutic options and outcomes for individuals living with diabetes. The first of these vignettes will illustrate a role for the sarcoendoplasmic reticulum calcium ATPase (SERCA) pump in the regulation of endoplasmic reticulum (ER) calcium, protein trafficking, and proinsulin processing within the β-cell. The second vignette will highlight how alterations in β-cell calcium signaling intersect with T1D pathogenesis. The final vignette will demonstrate how activation of β-cell stress pathways may serve as an anchor to inform biomarker strategies in T1D. Lastly, I will share my vision for the future of diabetes care, where multiple biomarkers of β-cell stress may be combined with additional immune and metabolic biomarkers to better predict disease risk and improve therapies to prevent or delay T1D development.