ER Redox Homeostasis Regulates Proinsulin Trafficking and Insulin Granule Formation in the Pancreatic Islet β-Cell

IER3IP1 Mutations Cause Neonatal Diabetes Due to Impaired Proinsulin Trafficking

ARTICLE HIGHLIGHTS

IER3IP1 mutations are linked to the development of microcephaly, epilepsy, and early-onset diabetes syndrome 1. However, the underlying molecular mechanisms of cell dysfunction are unknown. Using targeted genome editing, we generated specific IER3IP1 mutations in human embryonic stem cell lines that were differentiated into pancreatic islet lineages. Loss of IER3IP1 resulted in a threefold reduction in endoplasmic reticulum-to-Golgi trafficking of proinsulin in stem cell-derived β-cells, leading to β-cell dysfunction both in vitro and in vivo. Loss of IER3IP1 also triggered increased markers of endoplasmic reticulum stress, indicating the pivotal role of the endoplasmic reticulum-to-Golgi trafficking pathway for β-cell homeostasis and function.

The role of endoplasmic reticulum stress in type 2 diabetes mellitus mechanisms and impact on islet function

Type 2 diabetes mellitus (T2DM) is a globally prevalent metabolic disorder characterized by insulin resistance and dysfunction of islet cells. Endoplasmic reticulum (ER) stress plays a crucial role in the pathogenesis and progression of T2DM, especially in the function and survival of β-cells. β-cells are particularly sensitive to ER stress because they require substantial insulin synthesis and secretion energy. In the early stages of T2DM, the increased demand for insulin exacerbates β-cell ER stress. Although the unfolded protein response (UPR) can temporarily alleviate this stress, prolonged or excessive stress leads to pancreatic cell dysfunction and apoptosis, resulting in insufficient insulin secretion. This review explores the mechanisms of ER stress in T2DM, particularly its impact on islet cells. We discuss how ER stress activates UPR signaling pathways to regulate protein folding and degradation, but when stress becomes excessive, these pathways may contribute to β-cell death. A deeper understanding of how ER stress impacts islet cells could lead to the development of novel T2DM treatment strategies aimed at improving islet function and slowing disease progression.

Breaking the Feedback Loop of β-Cell Failure: Insight into the Pancreatic β-Cell's ER-Mitochondria Redox Balance

Pancreatic β-cells rely on a delicate balance between the endoplasmic reticulum (ER) and mitochondria to maintain sufficient insulin stores for the regulation of whole animal glucose homeostasis. The ER supports proinsulin maturation through oxidative protein folding, while mitochondria supply the energy and redox buffering that maintain ER proteostasis. In the development of Type 2 diabetes (T2D), the progressive decline of β-cell function is closely linked to disruptions in ER-mitochondrial communication. Mitochondrial dysfunction is a well-established driver of β-cell failure, whereas the downstream consequences for ER redox homeostasis have only recently emerged. This interdependence of ER-mitochondrial functions suggests that an imbalance is both a cause and consequence of metabolic dysfunction. In this review, we discuss the regulatory mechanisms of ER redox control and requirements for mitochondrial function. In addition, we describe how ER redox imbalances may trigger mitochondrial dysfunction in a vicious feed forward cycle that accelerates β-cell dysfunction and T2D onset.

VPS41 deletion triggers progressive loss of insulin stores and downregulation of β-cell identity

Vacuolar protein sorting-associated protein 41 (VPS41) has been established as a requirement for normal insulin secretory function in pancreatic β cells. Genetic deletion of VPS41 in mouse pancreatic β cells results in diabetes, although the mechanisms are not understood. Presently, we show that VPS41 deletion results in rapid mature insulin degradation and downregulation of β-cell identity. This phenotype is observed in vivo, with VPS41KO mice displaying progressive loss of insulin content and β-cell function with age. In acute VPS41 depletion in vitro, the loss of insulin is associated with increased degradative pathway activity, increased Adapter Protein 3 complex colocalization with lysosomes, increased nuclear localization of transcription factor E3, and downregulation of PDX1 and INS mRNA expression. Inhibition of lysosomal degradation rescues the rapidly depleted insulin content. These data evidence a VPS41-dependent mechanism for both insulin content degradation and loss of β-cell identity in β cells.NEW & NOTEWORTHY In this study, we show that acute VPS41 deletion results in rapid degradation of insulin, whereas chronic VPS41 deletion results in downregulation of β-cell identity. In acute VPS41 depletion in vitro, the loss of insulin is associated with increased degradative pathway activity, increased Adapter Protein 3 complex colocalization with lysosomes, increased nuclear localization of transcription factor E3, and downregulation of PDX1 and INS mRNA expression. Inhibition of lysosomal degradation rescues the rapidly depleted insulin content.

Molecular puzzle of insulin: structural assembly pathways and their role in diabetes

Properly folded proteins are essential for virtually all cellular processes including enzyme catalysis, signal transduction, and structural support. The cells have evolved intricate mechanisms of control, such as the assistance of chaperones and proteostasis networks, to ensure that proteins mature and fold correctly and maintain their functional conformations. Here, we review the mechanisms governing the folding of key hormonal regulators or glucose homeostasis. The insulin synthesis in pancreatic β-cells begins with preproinsulin production. During translation, the insulin precursor involves components of the endoplasmic reticulum (ER) translocation machinery, which are essential for proper orientation, translocation, and cleavage of the signal peptide of preproinsulin. These steps are critical to initiate the correct folding of proinsulin. Proinsulin foldability is optimized in the ER, an environment evolved to support the folding process and the formation of disulfide bonds while minimizing misfolding. This environment is intricately linked to ER stress response pathways, which have both beneficial and potentially harmful effects on pancreatic β-cells. Proinsulin misfolding can result from excessive biosynthetic ER load, proinsulin gene mutations, or genetic predispositions affecting the ER folding environment. Misfolded proinsulin leads to deficient insulin production and contributes to diabetes pathogenesis. Understanding the mechanisms of protein folding is critical for addressing diabetes and other protein misfolding-related diseases.

Gene and phenome-based analysis of the shared genetic architecture of eye diseases

While many eye disorders are linked through defects in vascularization and optic nerve degeneration, genetic correlation studies have yielded variable results despite shared features. For example, glaucoma and myopia both share optic neuropathy as a feature, but genetic correlation studies demonstrated minimal overlap. By leveraging electronic health record (EHR) resources that contain genetic variables such as genetically predicted gene expression (GPGE), researchers have the potential to improve the identification of shared genetic drivers of disease by incorporating knowledge of shared features to identify disease-causing mechanisms. In this study, we examined shared genetic architecture across eye diseases. Our gene-based approach used transcriptome-wide association methods to identify shared transcriptomic profiles across eye diseases within BioVU, Vanderbilt University Medical Center's (VUMC's) EHR-linked biobank. Our phenome-based approach leveraged phenome-wide association studies (PheWASs) to identify eye disease comorbidities. Using the beta estimates from the significantly associated comorbidities, we constructed a phenotypic risk score (PheRS) representing a weighted sum of an individual's eye disease comorbidities. This PheRS is predictive of eye disease status and associated with the altered GPGE of significant genes in an independent population. The implementation of both gene- and phenome-based approaches can expand genetic associations and shed greater insight into the underlying mechanisms of shared genetic architecture across eye diseases.

A metabolic redox relay supports ER proinsulin export in pancreatic islet β cells

ER stress and proinsulin misfolding are heralded as contributing factors to β cell dysfunction in type 2 diabetes, yet how ER function becomes compromised is not well understood. Recent data identify altered ER redox homeostasis as a critical mechanism that contributes to insulin granule loss in diabetes. Hyperoxidation of the ER delays proinsulin export and limits the proinsulin supply available for insulin granule formation. In this report, we identified glucose metabolism as a critical determinant in the redox homeostasis of the ER. Using multiple β cell models, we showed that loss of mitochondrial function or inhibition of cellular metabolism elicited ER hyperoxidation and delayed ER proinsulin export. Our data further demonstrated that β cell ER redox homeostasis was supported by the metabolic supply of reductive redox donors. We showed that limiting NADPH and thioredoxin flux delayed ER proinsulin export, whereas thioredoxin-interacting protein suppression restored ER redox and proinsulin trafficking. Taken together, we propose that β cell ER redox homeostasis is buffered by cellular redox donor cycles, which are maintained through active glucose metabolism.

Proinsulin folding and trafficking defects trigger a common pathological disturbance of endoplasmic reticulum homeostasis

Primary defects in folding of mutant proinsulin can cause dominant-negative proinsulin accumulation in the endoplasmic reticulum (ER), impaired anterograde proinsulin trafficking, perturbed ER homeostasis, diminished insulin production, and β-cell dysfunction. Conversely, if primary impairment of ER-to-Golgi trafficking (which also perturbs ER homeostasis) drives misfolding of nonmutant proinsulin-this might suggest bi-directional entry into a common pathological phenotype (proinsulin misfolding, perturbed ER homeostasis, and deficient ER export of proinsulin) that can culminate in diminished insulin storage and diabetes. Here, we've challenged β-cells with conditions that impair ER-to-Golgi trafficking, and devised an accurate means to assess the relative abundance of distinct folded/misfolded forms of proinsulin using a novel nonreducing SDS-PAGE/immunoblotting protocol. We confirm abundant proinsulin misfolding upon introduction of a diabetogenic INS mutation, or in the islets of db/db mice. Whereas blockade of proinsulin trafficking in Golgi/post-Golgi compartments results in intracellular accumulation of properly-folded proinsulin (bearing native disulfide bonds), impairment of ER-to-Golgi trafficking (regardless whether such impairment is achieved by genetic or pharmacologic means) results in decreased native proinsulin with more misfolded proinsulin. Remarkably, reversible ER-to-Golgi transport defects (such as treatment with brefeldin A or cellular energy depletion) upon reversal quickly restore the ER folding environment, resulting in the disappearance of pre-existing misfolded proinsulin while preserving proinsulin bearing native disulfide bonds. Thus, proper homeostatic balance of ER-to-Golgi trafficking is linked to a more favorable proinsulin folding (as well as trafficking) outcome.

Loss of the Golgi-localized v-ATPase subunit does not alter insulin granule formation or pancreatic islet β-cell function

Delayed Golgi export of proinsulin has recently been identified as an underlying mechanism leading to insulin granule loss and β-cell secretory defects in type 2 diabetes (T2D). Because acidification of the Golgi lumen is critical for proinsulin sorting and delivery into the budding secretory granule, we reasoned that dysregulation of Golgi pH may contribute to proinsulin trafficking defects. In this report, we examined pH regulation of the Golgi and identified a partial alkalinization of the Golgi lumen in a diabetes model. To further explore this, we generated a β-cell specific knockout (KO) of the v0a2 subunit of the v-ATPase pump, which anchors the v-ATPase to the Golgi membrane. Although loss of v0a2 partially neutralized Golgi pH and was accompanied by distension of the Golgi cisternae, proinsulin export from the Golgi and insulin granule formation were not affected. Furthermore, β-cell function was well preserved. β-cell v0a2 KO mice exhibited normal glucose tolerance in both sexes, no genotypic difference to diet-induced obesity, and normal insulin secretory responses. Collectively, our data demonstrate the v0a2 subunit contributes to β-cell Golgi pH regulation but suggest that additional disturbances to Golgi structure and function contribute to proinsulin trafficking defects in diabetes.NEW & NOTEWORTHY Delayed proinsulin export from the Golgi in diabetic β-cells contributes to decreased insulin granule formation, but the underlying mechanisms are not clear. Here, we explored if dysregulation of Golgi pH can alter Golgi function using β-cell specific knockout (KO) of the Golgi-localized subunit of the v-ATPase, v0a2. We show that partial alkalinization of the Golgi dilates the cisternae, but does not affect proinsulin export, insulin granule formation, insulin secretion, or glucose homeostasis.

Endoplasmic reticulum stress in the pathogenesis of alcoholic liver disease

The endoplasmic reticulum (ER) plays a pivotal role in protein synthesis, folding, and modification. Under stress conditions such as oxidative stress and inflammation, the ER can become overwhelmed, leading to an accumulation of misfolded proteins and ensuing ER stress. This triggers the unfolded protein response (UPR) designed to restore ER homeostasis. Alcoholic liver disease (ALD), a spectrum disorder resulting from chronic alcohol consumption, encompasses conditions from fatty liver and alcoholic hepatitis to cirrhosis. Metabolites of alcohol can incite oxidative stress and inflammation in hepatic cells, instigating ER stress. Prolonged alcohol exposure further disrupts protein homeostasis, exacerbating ER stress which can lead to irreversible hepatocellular damage and ALD progression. Elucidating the contribution of ER stress to ALD pathogenesis may pave the way for innovative therapeutic interventions. This review delves into ER stress, its basic signaling pathways, and its role in the alcoholic liver injury.

Molecular Mechanisms for the Vicious Cycle between Insulin Resistance and the Inflammatory Response in Obesity

The comprehensive anabolic effects of insulin throughout the body, in addition to the control of glycemia, include ensuring lipid homeostasis and anti-inflammatory modulation, especially in adipose tissue (AT). The prevalence of obesity, defined as a body mass index (BMI) ≥ 30 kg/m², has been increasing worldwide on a pandemic scale with accompanying syndemic health problems, including glucose intolerance, insulin resistance (IR), and diabetes. Impaired tissue sensitivity to insulin or IR paradoxically leads to diseases with an inflammatory component despite hyperinsulinemia. Therefore, an excess of visceral AT in obesity initiates chronic low-grade inflammatory conditions that interfere with insulin signaling via insulin receptors (INSRs). Moreover, in response to IR, hyperglycemia itself stimulates a primarily defensive inflammatory response associated with the subsequent release of numerous inflammatory cytokines and a real threat of organ function deterioration. In this review, all components of this vicious cycle are characterized with particular emphasis on the interplay between insulin signaling and both the innate and adaptive immune responses related to obesity. Increased visceral AT accumulation in obesity should be considered the main environmental factor responsible for the disruption in the epigenetic regulatory mechanisms in the immune system, resulting in autoimmunity and inflammation.

Synchronized proinsulin trafficking reveals delayed Golgi export accompanies β-cell secretory dysfunction in rodent models of hyperglycemia

The pancreatic islet β-cell's preference for release of newly synthesized insulin requires careful coordination of insulin exocytosis with sufficient insulin granule production to ensure that insulin stores exceed peripheral demands for glucose homeostasis. Thus, the cellular mechanisms regulating insulin granule production are critical to maintaining β-cell function. In this report, we utilized the synchronous protein trafficking system, RUSH, in primary β-cells to evaluate proinsulin transit through the secretory pathway leading to insulin granule formation. We demonstrate that the trafficking, processing, and secretion of the proinsulin RUSH reporter, proCpepRUSH, are consistent with current models of insulin maturation and release. Using both a rodent dietary and genetic model of hyperglycemia and β-cell dysfunction, we show that proinsulin trafficking is impeded at the Golgi and coincides with the decreased appearance of nascent insulin granules at the plasma membrane. Ultrastructural analysis of β-cells from diabetic leptin receptor deficient mice revealed gross morphological changes in Golgi structure, including shortened and swollen cisternae, and partial Golgi vesiculation, which are consistent with defects in secretory protein export. Collectively, this work highlights the utility of the proCpepRUSH reporter in studying proinsulin trafficking dynamics and suggests that altered Golgi export function contributes to β-cell secretory defects in the pathogenesis of Type 2 diabetes.