Pancreatic β-Cell-Specific Deletion of VPS41 Causes Diabetes Due to Defects in Insulin Secretion

Emerging approaches for the development of artificial islets

The islet of Langerhans, functioning as a "mini organ", plays a vital role in regulating endocrine activities due to its intricate structure. Dysfunction in these islets is closely associated with the development of diabetes mellitus (DM). To offer valuable insights for DM research and treatment, various approaches have been proposed to create artificial islets or islet organoids with high similarity to natural islets, under the collaborative effort of biologists, clinical physicians, and biomedical engineers. This review investigates the design and fabrication of artificial islets considering both biological and tissue engineering aspects. It begins by examining the natural structures and functions of native islets and proceeds to analyze the protocols for generating islets from stem cells. The review also outlines various techniques used in crafting artificial islets, with a specific focus on hydrogel-based ones. Additionally, it provides a concise overview of the materials and devices employed in the clinical applications of artificial islets. Throughout, the primary goal is to develop artificial islets, thereby bridging the realms of developmental biology, clinical medicine, and tissue engineering.

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.

Sustained hyperglycemia specifically targets translation of mRNAs for insulin secretion

Pancreatic β cells are specialized for coupling glucose metabolism to insulin peptide production and secretion. Acute glucose exposure robustly and coordinately increases translation of proinsulin and proteins required for secretion of mature insulin peptide. By contrast, chronically elevated glucose levels that occur during diabetes impair β cell insulin secretion and have been shown experimentally to suppress insulin translation. Whether translation of other genes critical for insulin secretion is similarly downregulated by chronic high glucose is unknown. Here, we used high-throughput ribosome profiling and nascent proteomics in MIN6 insulinoma cells to elucidate the genome-wide impact of sustained high glucose on β cell mRNA translation. Before induction of ER stress or suppression of global translation, sustained high glucose suppressed glucose-stimulated insulin secretion and downregulated translation of not only insulin, but also mRNAs related to insulin secretory granule formation, exocytosis, and metabolism-coupled insulin secretion. Translation of these mRNAs was also downregulated in primary rat and human islets following ex vivo incubation with sustained high glucose and in an in vivo model of chronic mild hyperglycemia. Furthermore, translational downregulation decreased cellular abundance of these proteins. Our study uncovered a translational regulatory circuit during β cell glucose toxicity that impairs expression of proteins with critical roles in β cell function.

Sustained hyperglycemia specifically targets translation of mRNAs for insulin secretion

Pancreatic β-cells are specialized for coupling glucose metabolism to insulin peptide production and secretion. Acute glucose exposure robustly and coordinately increases translation of proinsulin and proteins required for secretion of mature insulin peptide. By contrast, chronically elevated glucose levels that occur during diabetes impair β-cell insulin secretion and have been shown experimentally to suppress insulin translation. Whether translation of other genes critical for insulin secretion are similarly downregulated by chronic high glucose is unknown. Here, we used high-throughput ribosome profiling and nascent proteomics in MIN6 insulinoma cells to elucidate the genome-wide impact of sustained high glucose on β-cell mRNA translation. Prior to induction of ER stress or suppression of global translation, sustained high glucose suppressed glucose-stimulated insulin secretion and downregulated translation of not only insulin, but also of mRNAs related to insulin secretory granule formation, exocytosis, and metabolism-coupled insulin secretion. Translation of these mRNAs was also downregulated in primary rat and human islets following ex-vivo incubation with sustained high glucose and in an in vivo model of chronic mild hyperglycemia. Furthermore, translational downregulation decreased cellular abundance of these proteins. Our findings uncover a translational regulatory circuit during β-cell glucose toxicity that impairs expression of proteins with critical roles in β-cell function.

Mutations in CmVPS41 controlling resistance to cucumber mosaic virus display specific subcellular localization

Resistance to cucumber mosaic virus (CMV) in melon (Cucumis melo L.) has been described in several exotic accessions and is controlled by a recessive resistance gene, cmv1, that encodes a vacuolar protein sorting 41 (CmVPS41). cmv1 prevents systemic infection by restricting the virus to the bundle sheath cells, preventing viral phloem entry. CmVPS41 from different resistant accessions carries two causal mutations, either a G85E change, found in Pat-81 and Freeman's cucumber, or L348R, found in PI161375, cultivar Songwhan Charmi (SC). Here, we analyzed the subcellular localization of CmVPS41 in Nicotiana benthamiana and found differential structures in resistant and susceptible accessions. Susceptible accessions showed nuclear and membrane spots and many transvacuolar strands, whereas the resistant accessions showed many intravacuolar invaginations. These specific structures colocalized with late endosomes. Artificial CmVPS41 carrying individual mutations causing resistance in the genetic background of CmVPS41 from the susceptible variety Piel de Sapo (PS) revealed that the structure most correlated with resistance was the absence of transvacuolar strands. Coexpression of CmVPS41 with viral movement proteins, the determinant of virulence, did not change these localizations; however, infiltration of CmVPS41 from either SC or PS accessions in CMV-infected N. benthamiana leaves showed a localization pattern closer to each other, with up to 30% cells showing some membrane spots in the CmVPS41SC and fewer transvacuolar strands (reduced from a mean of 4 to 1-2) with CmVPS41PS. Our results suggest that the distribution of CmVPS41PS in late endosomes includes transvacuolar strands that facilitate CMV infection and that CmVPS41 re-localizes during viral infection.

Endocrine secretory granule production is caused by a lack of REST and intragranular secretory content and accelerated by PROX1

Endocrine secretory granules (ESGs) are morphological characteristics of endocrine/neuroendocrine cells and store peptide hormones/neurotransmitters. ESGs contain prohormones and ESG-related molecules, mainly chromogranin/secretogranin family proteins. However, the precise mechanism of ESG formation has not been elucidated. In this study, we experimentally induced ESGs in the non-neuroendocrine lung cancer cell line H1299. Since repressive element 1 silencing transcription factor (REST) and prospero homeobox 1 (PROX1) are closely associated with the expression of ESG-related molecules, we edited the REST gene and/or transfected PROX1 and then performed molecular biology, immunocytochemistry, and electron and immunoelectron microscopy assays to determine whether ESG-related molecules and ESGs were induced in H1299 cells. Although chromogranin/secretogranin family proteins were induced in H1299 cells by knockout of REST and the induction was accelerated by the PROX1 transgene, the ESGs could not be defined by electron microscopy. However, a small number of ESGs were detected in the H1299 cells lacking REST and expressing pro-opiomelanocortin (POMC) by electron microscopy. Furthermore, many ESGs were produced in the REST-lacking and PROX1- and POMC-expressing H1299 cells. These findings suggest that a lack of REST and the expression of genes related to ESG content are indispensable for ESG production and that PROX1 accelerates ESG production.

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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.

Neurodegenerative VPS41 variants inhibit HOPS function and mTORC1-dependent TFEB/TFE3 regulation

Vacuolar protein sorting 41 (VPS41) is as part of the Homotypic fusion and Protein Sorting (HOPS) complex required for lysosomal fusion events and, independent of HOPS, for regulated secretion. Here, we report three patients with compound heterozygous mutations in VPS41 (VPS41S285P and VPS41R662* ; VPS41c.1423-2A>G and VPS41R662* ) displaying neurodegeneration with ataxia and dystonia. Cellular consequences were investigated in patient fibroblasts and VPS41-depleted HeLa cells. All mutants prevented formation of a functional HOPS complex, causing delayed lysosomal delivery of endocytic and autophagic cargo. By contrast, VPS41S285P enabled regulated secretion. Strikingly, loss of VPS41 function caused a cytosolic redistribution of mTORC1, continuous nuclear localization of Transcription Factor E3 (TFE3), enhanced levels of LC3II, and a reduced autophagic response to nutrient starvation. Phosphorylation of mTORC1 substrates S6K1 and 4EBP1 was not affected. In a C. elegans model of Parkinson's disease, co-expression of VPS41S285P /VPS41R662* abolished the neuroprotective function of VPS41 against α-synuclein aggregates. We conclude that the VPS41 variants specifically abrogate HOPS function, which interferes with the TFEB/TFE3 axis of mTORC1 signaling, and cause a neurodegenerative disease.

Isolation and Proteomics of the Insulin Secretory Granule

Insulin, a vital hormone for glucose homeostasis is produced by pancreatic beta-cells and when secreted, stimulates the uptake and storage of glucose from the blood. In the pancreas, insulin is stored in vesicles termed insulin secretory granules (ISGs). In Type 2 diabetes (T2D), defects in insulin action results in peripheral insulin resistance and beta-cell compensation, ultimately leading to dysfunctional ISG production and secretion. ISGs are functionally dynamic and many proteins present either on the membrane or in the lumen of the ISG may modulate and affect different stages of ISG trafficking and secretion. Previously, studies have identified few ISG proteins and more recently, proteomics analyses of purified ISGs have uncovered potential novel ISG proteins. This review summarizes the proteins identified in the current ISG proteomes from rat insulinoma INS-1 and INS-1E cell lines. Here, we also discuss techniques of ISG isolation and purification, its challenges and potential future directions.