Effects of proinsulin misfolding on β-cell dynamics, differentiation and function in diabetes

SEL1L-HRD1 ER-Associated Degradation Facilitates Prohormone Convertase 2 Maturation and Glucagon Production in Islet α Cells

Proteolytic cleavage of proglucagon by prohormone convertase 2 (PC2) is required for islet α cells to generate glucagon. However, the regulatory mechanisms underlying this process remain largely unclear. Here, we report that SEL1L-HRD1 endoplasmic reticulum (ER)-associated degradation (ERAD), a highly conserved protein quality control system responsible for clearing misfolded proteins from the ER, plays a key role in glucagon production by regulating turnover of the nascent proform of the PC2 enzyme (proPC2). Using a mouse model with SEL1L deletion in proglucagon-expressing cells, we observed a progressive decline in stimulated glucagon secretion and a reduction in pancreatic glucagon content. Mechanistically, we found that endogenous proPC2 is a substrate of SEL1L-HRD1 ERAD, and that degradation of misfolded proPC2 ensures the maturation of activation-competent proPC2 protein. These findings identify ERAD as a novel regulator of PC2 biology and an essential mechanism for maintaining α cell function.

Wolfram Syndrome Type I Case Report and Review-Focus on Early Diagnosis and Genetic Variants

Background and Objectives: Wolfram syndrome type 1 (OMIM# 222300; ORPHAcode 3463) is an extremely rare autosomal recessive syndrome with a 25% recurrence risk in children. It is characterized by the presence of juvenile-onset diabetes mellitus (DM), progressive optic atrophy (OA), diabetes insipidus (DI), and sensorineural deafness (D), often referred to by the acronym DIDMOAD. It is a severe neurodegenerative disease with a life expectancy of 39 years, with death occurring due to cerebral atrophy. For a positive diagnosis, the presence of diabetes mellitus and optic nerve atrophy is sufficient. The disease occurs because of pathogenic variants in the WFS1 gene. The aim of this article is to present a case report of Wolfram Syndrome Type I, alongside a review of genetic variants, clinical manifestations, diagnosis, therapy, and long-term management. Emphasizing the importance of early diagnosis and a multidisciplinary approach, the study aims to enhance understanding and improve outcomes for patients with this complex syndrome. Materials and Methods: A case of a 28-year-old patient diagnosed with DM at the age of 6 and with progressive optic atrophy at 26 years old is presented. Molecular diagnosis revealed the presence of a heterozygous nonsense variant WFS1 c.1943G>A (p.Trp648*), and a heterozygous missense variant WFS1 c.1675G>C (p.Ala559Pro). Results: The molecular diagnosis of the patient confirmed the presence of a heterozygous nonsense variant and a heterozygous missense variant in the WFS1 gene, correlating with the clinical presentation of Wolfram syndrome type 1. Both allelic variants found in our patient have been previously described in other patients, whilst this combination has not been described before. Conclusions: This case report and review underscores the critical role of early recognition and diagnosis in Wolfram syndrome, facilitated by genetic testing. By identifying pathogenic variants in the WFS1 gene, genetic testing not only confirms diagnosis but also guides clinical management and informs genetic counseling for affected families. Timely intervention based on genetic insights can potentially reduce the progressive multisystem manifestations of the syndrome, thereby improving the quality of life and outcomes for patients.

Bax Inhibitor-1 preserves pancreatic β-cell proteostasis by limiting proinsulin misfolding and programmed cell death

The prevalence of diabetes steadily increases worldwide mirroring the prevalence of obesity. Endoplasmic reticulum (ER) stress is activated in diabetes and contributes to β-cell dysfunction and apoptosis through the activation of a terminal unfolded protein response (UPR). Our results uncover a new role for Bax Inhibitor-One (BI-1), a negative regulator of inositol-requiring enzyme 1 (IRE1α) in preserving β-cell health against terminal UPR-induced apoptosis and pyroptosis in the context of supraphysiological loads of insulin production. BI-1-deficient mice experience a decline in endocrine pancreatic function in physiological and pathophysiological conditions, namely obesity induced by high-fat diet (HFD). We observed early-onset diabetes characterized by hyperglycemia, reduced serum insulin levels, β-cell loss, increased pancreatic lipases and pro-inflammatory cytokines, and the progression of metabolic dysfunction. Pancreatic section analysis revealed that BI-1 deletion overburdens unfolded proinsulin in the ER of β-cells, confirmed by ultrastructural signs of ER stress with overwhelmed IRE1α endoribonuclease (RNase) activity in freshly isolated islets. ER stress led to β-cell dysfunction and islet loss, due to an increase in immature proinsulin granules and defects in insulin crystallization with the presence of Rod-like granules. These results correlated with the induction of autophagy, ER phagy, and crinophagy quality control mechanisms, likely to alleviate the atypical accumulation of misfolded proinsulin in the ER. In fine, BI-1 in β-cells limited IRE1α RNase activity from triggering programmed β-cell death through apoptosis and pyroptosis (caspase-1, IL-1β) via NLRP3 inflammasome activation and metabolic dysfunction. Pharmaceutical IRE1α inhibition with STF-083010 reversed β-cell failure and normalized the metabolic phenotype. These results uncover a new protective role for BI-1 in pancreatic β-cell physiology as a stress integrator to modulate the UPR triggered by accumulating unfolded proinsulin in the ER, as well as autophagy and programmed cell death, with consequences on β-cell function and insulin secretion. In pancreatic β-cells, BI-1-/- deficiency perturbs proteostasis with proinsulin misfolding, ER stress, terminal UPR with overwhelmed IRE1α/XBP1s/CHOP activation, inflammation, β-cell programmed cell death, and diabetes.

Beta-cell function in type 2 diabetes (T2DM): Can it be preserved or enhanced?

Type 2 diabetes (T2DM) is a complex metabolic disorder manifested by hyperglycemia, insulin resistance, and deteriorating beta-cell function. A way to prevent progression of the disease might be to enhance beta-cell function and insulin secretion. However, most previous studies examined beta-cell function while patients were using glycemia-lowering agents without an adequate period off medications (washout). In the present review we focus on studies with a washout period. We performed a literature search (2010 to June 2021) using beta-cell function and enhancement. The evidence shows that beta-cell function can be enhanced. Bariatric surgery and very low calorie diets show improvement in beta-cell function in many individuals. In addition, use of glucagon-like peptide-1 receptor agonists for prolonged periods (3 years or more) can also lead to improvement of beta-cell function. Further research is needed to understand the mechanisms leading to improved beta-cell function and identify agents that could enhance beta-cell function in patients with T2DM.

Role and Mechanism of Endoplasmic Reticulum Stress in Mice Pancreatic Islet Dysfunction After Severe Burns

This study attempted to investigate the role and mechanism of endoplasmic reticulum (ER) stress in the islet dysfunction in mice after severe burns. C57BL/6 mice were randomly divided into the sham group, burn group, and burn+4-phenylbutyric acid (4-PBA) group. Mice were burned with full thickness of 30% total surface area (TBSA), and 4-PBA solution was intraperitoneally injected into mice in burn+4-PBA group. Glucose-stimulated insulin secretion (GSIS), Fasting blood glucose (FBG) and glucose tolerance were detected 24 hours post severe burns. The ER stress-related pathway markers immunoglobulin binding protein (BIP), X-box binding protein 1 (XBP1), phosphorylation-PKR-like ER kinase (p-PERK), phosphorylation-eukaryotic translation initiation factor 2α (p-eIF2α), CHOP, activating transcription factor 6 (ATF6), apoptosis-related protein Cleaved-Caspase 3, and islet cell apoptosis were measured. Mice were characterized with elevated FBG, decreased glucose tolerance and GSIS levels post severe burns. The expression of BIP, XBP1, p-PERK, p-eIF2α, CHOP, ATF6, Cleaved-Caspase 3, and islet cell apoptosis were increased significantly after severe burns. 4-PBA treatment contributed to decreased FBG, improved glucose tolerance, increased GSIS, inhibited islet ER stress, and reduced pancreatic islet cell apoptosis in mice post severe burns. ER stress occurs in islets of severely burned mice, which leads to increased apoptosis of islet cells, thus resulting in islet dysfunction.

First-phase insulin secretion: can its evaluation direct therapeutic approaches?

Our work is aimed at unraveling the role of the first-phase insulin secretion in the natural history of type 2 diabetes mellitus (T2DM) and its interrelationship with insulin resistance and with β cell function and mass. Starting from pathophysiology, we investigate the impact of impaired secretion on glucose homeostasis and explore postmeal hyperglycemia as the main clinical feature, underlining its relevance in the management of the disease. We also review dietary and pharmacological approaches aimed at improving early secretory defects and restoring residual β cell function. Furthermore, we discuss possible approaches to detect early secretory defects in clinical practice. By providing a journey through human and animal data, we attempt a unification of the recent evidence in an effort to offer a new outlook on β cell secretion.

Wolfram Syndrome 1: A Pediatrician's and Pediatric Endocrinologist's Perspective

Wolfram syndrome 1 (WS1) is a rare autosomal recessive neurodegenerative disease caused by mutations in WFS1 and WFS2 genes that produce wolframin, a protein involved in endoplasmic reticulum calcium homeostasis and cellular apoptosis. Its main clinical features are diabetes insipidus (DI), early-onset non-autoimmune insulin-dependent diabetes mellitus (DM), gradual loss of vision due to optic atrophy (OA) and deafness (D), hence the acronym DIDMOAD. Several other features from different systems have been reported such as urinary tract, neurological, and psychiatric abnormalities. In addition, endocrine disorders that can appear during childhood and adolescence include primary gonadal atrophy and hypergonadotropic hypogonadism in males and menstrual cycle abnormalities in females. Further, anterior pituitary dysfunction with deficient GH and/or ACTH production have been described. Despite the lack of specific treatment for the disease and its poor life expectancy, early diagnosis and supportive care is important for timely identifying and adequately managing its progressive symptoms. The current narrative review focuses on the pathophysiology and the clinical features of the disease, with a special emphasis on its endocrine abnormalities that appear during childhood and adolescence. Further, therapeutic interventions that have been proven to be effective in the management of WS1 endocrine complications are discussed.

Intra-islet insulin synthesis defects are associated with endoplasmic reticulum stress and loss of beta cell identity in human diabetes

AIMS/HYPOTHESIS

Endoplasmic reticulum (ER) stress and beta cell dedifferentiation both play leading roles in impaired insulin secretion in overt type 2 diabetes. Whether and how these factors are related in the natural history of the disease remains, however, unclear.

METHODS

In this study, we analysed pancreas biopsies from a cohort of metabolically characterised living donors to identify defects in in situ insulin synthesis and intra-islet expression of ER stress and beta cell phenotype markers.

RESULTS

We provide evidence that in situ altered insulin processing is closely connected to in vivo worsening of beta cell function. Further, activation of ER stress genes reflects the alteration of insulin processing in situ. Using a combination of 17 different markers, we characterised individual pancreatic islets from normal glucose tolerant, impaired glucose tolerant and type 2 diabetic participants and reconstructed disease progression.

CONCLUSIONS/INTERPRETATION

Our study suggests that increased beta cell workload is accompanied by a progressive increase in ER stress with defects in insulin synthesis and loss of beta cell identity.

Effects of Initial Combinations of Gemigliptin Plus Metformin Compared with Glimepiride Plus Metformin on Gut Microbiota and Glucose Regulation in Obese Patients with Type 2 Diabetes: The INTESTINE Study

The efficacy and safety of medications can be affected by alterations in gut microbiota in human beings. Among antidiabetic medications, incretin-based therapy such as dipeptidyl peptidase 4 inhibitors might affect gut microbiomes, which are related to glucose metabolism. This was a randomized, controlled, active-competitor study that aimed to compare the effects of combinations of gemigliptin−metformin vs. glimepiride−metformin as initial therapies on gut microbiota and glucose homeostasis in drug-naïve patients with type 2 diabetes. Seventy drug-naïve patients with type 2 diabetes (mean age, 52.2 years) with a glycated hemoglobin (HbA1c) level ≥7.5% were assigned to either gemigliptin−metformin or glimepiride−metformin combination therapies for 24 weeks. Changes in gut microbiota, biomarkers linked to glucose regulation, body composition, and amino acid blood levels were investigated. Although both treatments decreased the HbA1c levels significantly, the gemigliptin−metformin group achieved HbA1c ≤ 7.0% without hypoglycemia or weight gain more effectively than did the glimepiride−metformin group (59% vs. 24%; p < 0.05). At the phylum level, the Firmicutes/Bacteroidetes ratio tended to decrease after gemigliptin−metformin therapy (p = 0.065), with a notable depletion of taxa belonging to Firmicutes, including Lactobacillus, Ruminococcus torques, and Streptococcus (all p < 0.05). However, regardless of the treatment modality, a distinct difference in the overall gut microbiome composition was noted between patients who reached the HbA1c target goal and those who did not (p < 0.001). Treatment with gemigliptin−metformin resulted in a higher achievement of the glycemic target without hypoglycemia or weight gain, better than with glimepiride−metformin; these improvements might be related to beneficial changes in gut microbiota.

Deterioration of Glycemic Control in Youth-Onset Type 2 Diabetes: What Are the Early and Late Predictors?

OBJECTIVE

We examined predictors of early and late loss of glycemic control in individuals with youth-onset type 2 diabetes, as well as predictors of short-term deterioration in youth from the Treatment Options for type 2 Diabetes in Adolescents and Youth (TODAY) study.

METHODS

Demographic, physical, and biochemical measures at baseline and 48 months, and change over time, were examined in 584 participants separated into those with loss of glycemic control (sustained HbA1c ≥ 8%) before 48 months or at 48 months or later, and those who remained in control until the end of the study (median 6.8 years). Univariate and multivariate models, and receiver operating characteristic curve analyses were performed.

RESULTS

Approximately 45% of youth remained in control at 48 months; of these, 30% subsequently lost glycemic control prior to the end of follow-up. Predictors of early loss of glycemic control included baseline HbA1c, C-peptide index, oral disposition index, proinsulin, and proinsulin to insulin ratio. Predictors of late loss included baseline measures of insulin secretion and change in HbA1c and insulin processing at 48 months. A baseline HbA1c cutoff of ≥ 6.2% was optimally predictive of loss of glycemic control at any time, while an absolute rise in HbA1c > 0.5% related to loss of glycemic control within 3 to 6 months.

CONCLUSION

This analysis demonstrates that youth with type 2 diabetes at risk for loss of glycemic control, including impending rapid deterioration, can be identified using available clinical measures, allowing for closer monitoring of at-risk youth, and facilitating the design of research on better therapeutic options.

A multi-omics approach identifies pancreatic cancer cell extracellular vesicles as mediators of the unfolded protein response in normal pancreatic epithelial cells

Although cancer-derived extracellular vesicles (cEVs) are thought to play a pivotal role in promoting cancer progression events, their precise effect on neighbouring normal cells is unknown. In this study, we investigated the impact of pancreatic cancer ductal adenocarcinoma (PDAC) derived EVs on recipient non-tumourigenic pancreatic normal epithelial cells upon internalization. We demonstrate that cEVs are readily internalized and induce endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) in treated normal pancreatic epithelial cells within 24 h. We further show that PDAC cEVs increase cell proliferation, migration, and invasion and that these changes are regulated at least in part, by the UPR mediator DDIT3. Subsequently, these cells release several inflammatory cytokines. Leveraging a layered multi-omics approach, we analysed EV cargo from a panel of six PDAC and two normal pancreas cell lines, using multiple EV isolation methods. We found that cEVs were enriched for an array of biomolecules which can induce or regulate ER stress and the UPR, including palmitic acid, sphingomyelins, metabolic regulators of tRNA charging and proteins which regulate trafficking and degradation. We further show that palmitic acid, at doses relevant to those found in cEVs, is sufficient to induce ER stress in normal pancreas cells. These results suggest that cEV cargo packaging may be designed to disseminate proliferative and invasive characteristics upon internalization by distant recipient normal cells, hitherto unreported. This study is among the first to highlight a major role for PDAC cEVs to induce stress in treated normal pancreas cells that may modulate a systemic response leading to altered phenotypes. These findings highlight the importance of EVs in mediating disease aetiology and open potential areas of investigation toward understanding the role of cEV lipids in promoting cell transformation in the surrounding microenvironment.

Effects of hypothyroidism on the female pancreas involve the regulation of estrogen receptors

This study aimed to investigate the impact of short-time hypothyroidism on the expression of aromatase, estrogen receptors (ERα, β), and GPR30 in the pancreas of female rabbits. The formation of new islets and the expression of insulin, GLUT4, and lactate dehydrogenase (LDH) were also analyzed. This purpose is based on actions that thyroid hormones and estrogens have on β-cells differentiation, acinar cell function, and insulin secretion. Twelve Chinchilla-breed adult virgin female rabbits were divided into control (n = 6) and hypothyroid (n = 6; methimazole 10 mg/kg for 30 days) groups. In the complete pancreas, expressions of aromatase and estrogen receptors, as well as proinsulin, GLUT4, and LDH were determined by western blot. Characteristics of islets were measured in slices of the pancreas with immunohistochemistry for insulin. Islet and acinar cells express aromatase, ERα, ERβ, and GPR30. Hypothyroidism increased the expression of ERα and diminished that for aromatase, ERβ, and GPR30 in the pancreas. It also promoted a high number of extra small islets (new islets) and increased the expression of proinsulin and GLUT4 in the pancreas. Our results show that actions of thyroid hormones and estrogens on β-cells neogenesis, acinar cell function, and synthesis and secretion of insulin are linked. Thus, the effects of hypothyroidism on the pancreas could include summatory actions of thyroid hormones plus estrogens. Our findings indicate the importance of monitoring estrogen levels and actions on the pancreas of hypothyroid women, particularly when serum estrogen concentrations are affected such as menopausal, pregnant, and those with contraceptive use.

Efficacy of GLP-1 Agonist Therapy in Autosomal Dominant WFS1-Related Disorder: A Case Report

BACKGROUND

Wolfram syndrome is a rare neurodegenerative disorder, characterized by the presence of diabetes insipidus, diabetes mellitus, optic atrophy, and sensorineural deafness. The majority of cases are due to autosomal recessive biallelic variants in the WFS1 gene; however, pathogenic autosomal dominant (AD) mutations have also been described. Glucagon-like peptide (GLP-1) agonists have been studied in both animal models and humans with classic Wolfram syndrome.

CASE

We present a 15-year-old female with a personal and family history of congenital strabismus, bilateral cataracts, low-frequency sensorineural hearing loss, and diabetes mellitus. Trio whole exome sequencing revealed a previously unknown maternally inherited heterozygous variant in exon 8 of the WFS1 gene c.2605_2616del12 p.Ser869_His872del, leading to the diagnosis of AD WFS1-related disorder. Treatment with a GLP-1 agonist resulted in marked improvement in glycemic control and discontinuation of insulin therapy. This patient's response to a GLP-1 agonist provides suggestive indirect evidence for a role of WFS1 on β-cell endoplasmic reticulum stress and suggests that treatment with a GLP-1 agonist should be considered in patients with dominant forms of WS.

Pharmacological Inhibition of Inositol-Requiring Enzyme 1α RNase Activity Protects Pancreatic Beta Cell and Improves Diabetic Condition in Insulin Mutation-Induced Diabetes

β-cell ER stress plays an important role in β-cell dysfunction and death during the pathogenesis of diabetes. Proinsulin misfolding is regarded as one of the primary initiating factors of ER stress and unfolded protein response (UPR) activation in β-cells. Here, we found that the ER stress sensor inositol-requiring enzyme 1α (IRE1α) was activated in the Akita mice, a mouse model of mutant insulin gene-induced diabetes of youth (MIDY), a monogenic diabetes. Normalization of IRE1α RNase hyperactivity by pharmacological inhibitors significantly ameliorated the hyperglycemic conditions and increased serum insulin levels in Akita mice. These benefits were accompanied by a concomitant protection of functional β-cell mass, as shown by the suppression of β-cell apoptosis, increase in mature insulin production and reduction of proinsulin level. At the molecular level, we observed that the expression of genes associated with β-cell identity and function was significantly up-regulated and ER stress and its associated inflammation and oxidative stress were suppressed in islets from Akita mice treated with IRE1α RNase inhibitors. This study provides the evidence of the in vivo efficacy of IRE1α RNase inhibitors in Akita mice, pointing to the possibility of targeting IRE1α RNase as a therapeutic direction for the treatment of diabetes.

Limitations of the fasting proinsulin to insulin ratio as a measure of β-cell health in people with and without impaired glucose tolerance

BACKGROUND

The fasting proinsulin to insulin ratio is elevated in people with type 2 diabetes and has been suggested as a marker of β-cell health. However, its utility in discriminating between individuals with varying degrees of β-cell dysfunction is unclear. Proinsulin has a very different half-life to insulin and unlike insulin does not undergo hepatic extraction prior to reaching the systemic circulation. Given these limitations, we sought to examine the relationship between fasting and postprandial concentrations of β-cell polypeptides (proinsulin, insulin and C-peptide) in people with normal and impaired glucose tolerance in differing metabolic environments.

DESIGN

Subjects were studied on two occasions in random order while undergoing an oral challenge. During one study day, free fatty acids were elevated (to induce insulin resistance) by infusion of Intralipid with heparin. Proinsulin to insulin and proinsulin to C-peptide ratios were calculated for the 0-, 30-, 60- and 240-minute time points. Insulin action (Si) and β-cell responsivity (Φ) indices were calculated using the oral minimal model.

RESULTS

The fasting proinsulin to c-peptide or fasting proinsulin to insulin ratios did not differ between groups and did not predict subsequent β-cell responsivity to glucose during the glycerol or Intralipid study days in either group.

CONCLUSIONS

Among nondiabetic individuals, the fasting proinsulin to insulin ratio is not a useful marker of β-cell function.

Crosstalk between endoplasmic reticulum stress and oxidative stress: Focus on protein disulfide isomerase and endoplasmic reticulum oxidase 1

Cellular stress and inflammation, establishing as disease pathology, have reached great heights in the last few decades. Stress conditions such as hyperglycemia, hyperlipidemia and lipoproteins are known to disturb proteostasis resulting in the accumulation of unfolded or misfolded proteins, alteration in calcium homeostasis culminating in unfolded protein response. Protein disulfide isomerase and endoplasmic reticulum oxidase-1 are the key players in protein folding. The protein folding process assisted by endoplasmic reticulum oxidase-1 results in the production of reactive oxygen species in the lumen of the endoplasmic reticulum. Production of reactive oxygen species beyond the quenching capacity of the antioxidant systems perturbs ER homeostasis. Endoplasmic reticulum stress also induces the production of cytokines leading to inflammatory responses. This has been proven to be the major causative factor for various pathophysiological states compared to other cellular triggers in diseases, which further manifests to increased oxidative stress, mitochondrial dysfunction, and altered inflammatory responses, deleterious to cellular physiology and homeostasis. Numerous studies have drawn correlations between the progression of several diseases in association with endoplasmic reticulum stress, redox protein folding, oxidative stress and inflammatory responses. This review aims to provide an insight into the role of protein disulfide isomerase and endoplasmic reticulum oxidase-1 in endoplasmic reticulum stress, unfolded protein response, mitochondrial dysfunction, and inflammatory responses, which exacerbate the progression of various diseases.

The healthy exocrine pancreas contains preproinsulin-specific CD8 T cells that attack islets in type 1 diabetes

Preproinsulin (PPI) is presumably a crucial islet autoantigen found in patients with type 1 diabetes (T1D) but is also recognized by CD8⁺ T cells from healthy individuals. We quantified PPI-specific CD8⁺ T cells within different areas of the human pancreas from nondiabetic controls, autoantibody-positive donors, and donors with T1D to investigate their role in diabetes development. This spatial cellular quantitation revealed unusually high frequencies of autoreactive CD8⁺ T cells supporting the hypothesis that PPI is indeed a key autoantigen. To our surprise, PPI-specific CD8⁺ T cells were already abundantly present in the nondiabetic pancreas, thus questioning the dogma that T1D is caused by defective thymic deletion or systemic immune dysregulation. During T1D development, these cells accumulated in and around islets, indicating that an islet-specific trigger such as up-regulation of major histocompatibility complex class I might be essential to unmask beta cells to the immune system.

Role of Proinsulin Self-Association in Mutant INS Gene-Induced Diabetes of Youth

Abnormal interactions between misfolded mutant and wild-type (WT) proinsulin (PI) in the endoplasmic reticulum (ER) drive the molecular pathogenesis of mutant INS gene-induced diabetes of youth (MIDY). How these abnormal interactions are initiated remains unknown. Normally, PI-WT dimerizes in the ER. Here, we suggest that the normal PI-PI contact surface, involving the B-chain, contributes to dominant-negative effects of misfolded MIDY mutants. Specifically, we find that PI B-chain tyrosine-16 (Tyr-B16), which is a key residue in normal PI dimerization, helps confer dominant-negative behavior of MIDY mutant PI-C(A7)Y. Substitutions of Tyr-B16 with either Ala, Asp, or Pro in PI-C(A7)Y decrease the abnormal interactions between the MIDY mutant and PI-WT, rescuing PI-WT export, limiting ER stress, and increasing insulin production in β-cells and human islets. This study reveals the first evidence indicating that noncovalent PI-PI contact initiates dominant-negative behavior of misfolded PI, pointing to a novel therapeutic target to enhance PI-WT export and increase insulin production.

The Endoplasmic Reticulum and Calcium Homeostasis in Pancreatic Beta Cells

The endoplasmic reticulum (ER) mediates the first steps of protein assembly within the secretory pathway and is the site where protein folding and quality control are initiated. The storage and release of Ca2+ are critical physiological functions of the ER. Disrupted ER homeostasis activates the unfolded protein response (UPR), a pathway which attempts to restore cellular equilibrium in the face of ER stress. Unremitting ER stress, and insufficient compensation for it results in beta-cell apoptosis, a process that has been linked to both type 1 diabetes (T1D) and type 2 diabetes (T2D). Both types are characterized by progressive beta-cell failure and a loss of beta-cell mass, although the underlying causes are different. The reduction of mass occurs secondary to apoptosis in the case of T2D, while beta cells undergo autoimmune destruction in T1D. In this review, we examine recent findings that link the UPR pathway and ER Ca2+ to beta cell dysfunction. We also discuss how UPR activation in beta cells favors cell survival versus apoptosis and death, and how ER protein chaperones are involved in regulating ER Ca2+ levels. Abbreviations: BiP, Binding immunoglobulin Protein ER; endoplasmic reticulum; ERAD, ER-associated protein degradation; IFN, interferon; IL, interleukin; JNK, c-Jun N-terminal kinase; KHE, proton-K+ exchanger; MODY, maturity-onset diabetes of young; PERK, PRKR-like ER kinase; SERCA, Sarco/Endoplasmic Reticulum Ca2+-ATPases; T1D, type 1 diabetes; T2D, type 2 diabetes; TNF, tumor necrosis factor; UPR, unfolded protein response; WRS, Wolcott-Rallison syndrome.

Wolfram syndrome, a rare neurodegenerative disease: from pathogenesis to future treatment perspectives

Background

Wolfram syndrome (WS), a rare genetic disorder, is considered the best prototype of endoplasmic reticulum (ER) diseases. Classical WS features are childhood-onset diabetes mellitus, optic atrophy, deafness, diabetes insipidus, neurological signs, and other abnormalities. Two causative genes (WFS1 and WFS2) have been identified. The transmission of the disease takes place in an autosomal recessive mode but autosomal dominant mutations responsible for WS-related disorders have been described. Prognosis is poor, death occurs at the median age of 39 years with a major cause represented by respiratory failure as a consequence of brain stem atrophy and neurodegeneration. The aim of this narrative review is to focus on etiology, pathogenesis and natural history of WS for an adequate patient management and for the discussion of future therapeutic interventions.

Main body

WS requires a multidisciplinary approach in order to be successfully treated. A prompt diagnosis decreases morbidity and mortality through prevention and treatment of complications. Being a monogenic pathology, WS represents a perfect model to study the mechanisms of ER stress and how this condition leads to cell death, in comparison with other prevalent diseases in which multiple factors interact to produce the disease manifestations. WS is also an important disease prototype to identify drugs and molecules associated with ER homeostasis. Evidence indicates that specific metabolic diseases (type 1 and type 2 diabetes), neurodegenerative diseases, atherosclerosis, inflammatory pathologies and also cancer are closely related to ER dysfunction.

Conclusions

Therapeutic strategies in WS are based on drug repurposing (i.e., investigation of approved drugs for novel therapeutic indications) with the aim to stop the progression of the disease by reducing the endoplasmic reticulum stress. An extensive understanding of WS from pathophysiology to therapy is fundamental and more studies are necessary to better manage this devastating disease and guarantee the patients a better quality of life and longer life expectancy.

Proinsulin misfolding is an early event in the progression to type 2 diabetes

Biosynthesis of insulin – critical to metabolic homeostasis – begins with folding of the proinsulin precursor, including formation of three evolutionarily conserved intramolecular disulfide bonds. Remarkably, normal pancreatic islets contain a subset of proinsulin molecules bearing at least one free cysteine thiol. In human (or rodent) islets with a perturbed endoplasmic reticulum folding environment, non-native proinsulin enters intermolecular disulfide-linked complexes. In genetically obese mice with otherwise wild-type islets, disulfide-linked complexes of proinsulin are more abundant, and leptin receptor-deficient mice, the further increase of such complexes tracks with the onset of islet insulin deficiency and diabetes. Proinsulin-Cys(B19) and Cys(A20) are necessary and sufficient for the formation of proinsulin disulfide-linked complexes; indeed, proinsulin Cys(B19)-Cys(B19) covalent homodimers resist reductive dissociation, highlighting a structural basis for aberrant proinsulin complex formation. We conclude that increased proinsulin misfolding via disulfide-linked complexes is an early event associated with prediabetes that worsens with ß-cell dysfunction in type two diabetes.

Our body fine-tunes the amount of sugar in our blood thanks to specialized ‘beta cells’ in the pancreas, which can release a hormone called insulin. To produce insulin, the beta cells first need to build an early version of the molecule – known as proinsulin – inside a cellular compartment called the endoplasmic reticulum. This process involves the formation of internal staples that keep the molecule of proinsulin folded correctly.

Individuals developing type 2 diabetes have spikes of sugar in their blood, and so their bodies often respond by trying to make large amounts of insulin. After a while, the beta cells can fail to keep up, which brings on the full-blown disease.

However, scientists have discovered that early in type 2 diabetes, the endoplasmic reticulum of beta cells can already show signs of stress; yet, the exact causes of this early damage are still unknown. To investigate this, Arunagiri et al. looked into whether proinsulin folds correctly during the earliest stages of type 2 diabetes. Biochemical experiments showed that even healthy beta cells contained some misfolded proinsulin molecules, where the molecular staples that should fold proinsulin internally were instead abnormally linking proinsulin molecules together. Further work revealed that the misfolded proinsulin was accumulating inside the endoplasmic reticulum. Finally, obese mice that were in the earliest stages of type 2 diabetes had the highest levels of abnormal proinsulin in their beta cells.

Overall, the work by Arunagiri et al. suggests that large amounts of proinsulin molecules stapling themselves to each other in the endoplasmic reticulum of beta cells could be an early hallmark of the disease, and could make it get worse. A separate study by Jang et al. also shows that a protein that limits the misfolding of proinsulin is key to maintain successful insulin production in animals eating a Western-style, high fat diet.

Hundreds of millions of people around the world have type 2 diabetes, and this number is rising quickly. Detecting and then fixing early problems associated with the condition may help to stop the disease in its track.