Kathryn L. Lipson

WFS1 is a novel component of the unfolded protein response and maintains homeostasis of the endoplasmic reticulum in pancreatic beta-cells

In Wolfram syndrome, a rare form of juvenile diabetes, pancreatic β-cell death is not accompanied by an autoimmune response. Although it has been reported that mutations in the WFS1 gene are responsible for the development of this syndrome, the precise molecular mechanisms underlying β-cell death caused by the WFS1 mutations remain unknown. Here we report that WFS1 is a novel component of the unfolded protein response and has an important function in maintaining homeostasis of the endoplasmic reticulum (ER) in pancreatic β-cells. WFS1 encodes a transmembrane glyco-protein in the ER. WFS1 mRNA and protein are induced by ER stress. The expression of WFS1 is regulated by inositol requiring 1 and PKR-like ER kinase, central regulators of the unfolded protein response. WFS1 is normally up-regulated during insulin secretion, whereas inactivation of WFS1 in β-cells causes ER stress and β-cell dysfunction. These results indicate that the pathogenesis of Wolfram syndrome involves chronic ER stress in pancreatic β-cells caused by the loss of function of WFS1.

Wolfram syndrome 1 gene negatively regulates ER stress signaling in rodent and human cells

Wolfram syndrome is an autosomal-recessive disorder characterized by insulin-dependent diabetes mellitus, caused by nonautoimmune loss of beta cells, and neurological dysfunctions. We have previously shown that mutations in the Wolfram syndrome 1 (WFS1) gene cause Wolfram syndrome and that WFS1 has a protective function against ER stress. However, it remained to be determined how WFS1 mitigates ER stress. Here we have shown in rodent and human cell lines that WFS1 negatively regulates a key transcription factor involved in ER stress signaling, activating transcription factor 6alpha (ATF6alpha), through the ubiquitin-proteasome pathway. WFS1 suppressed expression of ATF6alpha target genes and repressed ATF6alpha-mediated activation of the ER stress response element (ERSE) promoter. Moreover, WFS1 stabilized the E3 ubiquitin ligase HRD1, brought ATF6alpha to the proteasome, and enhanced its ubiquitination and proteasome-mediated degradation, leading to suppression of ER stress signaling. Consistent with these data, beta cells from WFS1-deficient mice and lymphocytes from patients with Wolfram syndrome exhibited dysregulated ER stress signaling through upregulation of ATF6alpha and downregulation of HRD1. These results reveal a role for WFS1 in the negative regulation of ER stress signaling and in the pathogenesis of diseases involving chronic, unresolvable ER stress, such as pancreatic beta cell death in diabetes.

Transcriptional Regulation of VEGF-A by the Unfolded Protein Response Pathway

Background Angiogenesis is crucial to many physiological and pathological processes including development and cancer cell survival. Vascular endothelial growth factor-A (VEGFA) is the predominant mediator of angiogenesis in the VEGF family. During development, adverse environmental conditions like nutrient deprivation, hypoxia and increased protein secretion occur. IRE1α, PERK, and ATF6α, master regulators of the unfolded protein response (UPR), are activated under these conditions and are proposed to have a role in mediating angiogenesis. Principal Findings Here we show that IRE1α, PERK, and ATF6α powerfully regulate VEGFA mRNA expression under various stress conditions. In Ire1α−/− and Perk−/− mouse embryonic fibroblasts and ATF6α-knockdown HepG2 cells, induction of VEGFA mRNA by endoplasmic reticulum stress is attenuated as compared to control cells. Embryonic lethality of Ire1α−/− mice is due to the lack of VEGFA induction in labyrinthine trophoblast cells of the developing placenta. Rescue of IRE1α and PERK in Ire1α−/− and Perk−/− cells respectively, prevents VEGFA mRNA attenuation. We further report that the induction of VEGFA by IRE1α, PERK and ATF6 involves activation of transcription factors, spliced-XBP-1, ATF4 and cleaved ATF6 respectively. Conclusions/Significance Our results reveal that the IRE1α-XBP-1, PERK-ATF4, and ATF6α pathways constitute novel upstream regulatory pathways of angiogenesis by modulating VEGF transcription. Activation of these pathways helps the rapidly growing cells to obtain sufficient nutrients and growth factors for their survival under the prevailing hostile environmental conditions. These results establish an important role of the UPR in angiogenesis.

The Role of IRE1α in the Degradation of Insulin mRNA in Pancreatic β-Cells

Background The endoplasmic reticulum (ER) is a cellular compartment for the biosynthesis and folding of newly synthesized secretory proteins such as insulin. Perturbations to ER homeostasis cause ER stress and subsequently activate cell signaling pathways, collectively known as the Unfolded Protein Response (UPR). IRE1α is a central component of the UPR. In pancreatic β-cells, IRE1α also functions in the regulation of insulin biosynthesis. Principal Findings Here we report that hyperactivation of IRE1α caused by chronic high glucose treatment or IRE1α overexpression leads to insulin mRNA degradation in pancreatic β-cells. Inhibition of IRE1α signaling using its dominant negative form prevents insulin mRNA degradation. Islets from mice heterozygous for IRE1α retain expression of more insulin mRNA after chronic high glucose treatment than do their wild-type littermates. Conclusions/Significance These results reveal a role of IRE1α in insulin mRNA expression under ER stress conditions caused by chronic high glucose. The rapid degradation of insulin mRNA could provide immediate relief for the ER and free up the translocation machinery. Thus, this mechanism would preserve ER homeostasis and help ensure that the insulin already inside the ER can be properly folded and secreted. This adaptation may be crucial for the maintenance of β-cell homeostasis and may explain why the β-cells of type 2 diabetic patients with chronic hyperglycemia stop producing insulin in the absence of apoptosis. This mechanism may also be involved in suppression of the autoimmune type 1 diabetes by reducing the amount of misfolded insulin, which could be a source of “neo-autoantigens.”

Endoplasmic Reticulum Stress-Induced Apoptosis and Autoimmunity in Diabetes

Increasing evidence suggests that stress signaling pathways emanating from the endoplasmic reticulum (ER) are important to the pathogenesis of both type 1 and type 2 diabetes. Recent observations indicate that ER stress signaling participates in maintaining the ER homeostasis of pancreatic beta-cells. Either a high level of ER stress or defective ER stress signaling in beta-cells may cause an imbalance in ER homeostasis and lead to beta-cell apoptosis and autoimmune response. In addition, it has been suggested that ER stress attributes to insulin resistance in patients with type 2 diabetes. It is necessary to study the relationship between ER stress and diabetes in order to develop new therapeutic approaches to diabetes based on drugs that block the ER stress-mediated cell-death pathway and insulin resistance.

Dynamic Glucoregulation and Mammalian-Like Responses to Metabolic and Developmental Disruption in Zebrafish

Zebrafish embryos are emerging as models of glucose metabolism. However, patterns of endogenous glucose levels, and the role of the islet in glucoregulation, are unknown. We measured absolute glucose levels in zebrafish and mouse embryos, and demonstrate similar, dynamic glucose fluctuations in both species. Further, we show that chemical and genetic perturbations elicit mammalian-like glycemic responses in zebrafish embryos. We show that glucose is undetectable in early zebrafish and mouse embryos, but increases in parallel with pancreatic islet formation in both species. In zebrafish, increasing glucose is associated with activation of gluconeogenic phosphoenolpyruvate carboxykinase1 (pck1) transcription. Non-hepatic Pck1 protein is expressed in mouse embryos. We show using RNA in situ hybridization, that zebrafish pck1 mRNA is similarly expressed in multiple cell types prior to hepatogenesis. Further, we demonstrate that the Pck1 inhibitor 3-mercaptopicolinic acid suppresses normal glucose accumulation in early zebrafish embryos. This shows that pre- and extra-hepatic pck1 is functional, and provides glucose locally to rapidly developing tissues. To determine if the primary islet is glucoregulatory in early fish embryos, we injected pdx1-specific morpholinos into transgenic embryos expressing GFP in beta cells. Most morphant islets were hypomorphic, not a genetic, but embryos still exhibited persistent hyperglycemia. We conclude from these data that the early zebrafish islet is functional, and regulates endogenous glucose. In summary, we identify mechanisms of glucoregulation in zebrafish embryos that are conserved with embryonic and adult mammals. These observations justify use of this model in mechanistic studies of human metabolic disease.

CHOP Mediates Endoplasmic Reticulum Stress-Induced Apoptosis in Gimap5-Deficient T Cells

Gimap5 (GTPase of the immunity-associated protein 5) has been linked to the regulation of T cell survival, and polymorphisms in the human GIMAP5 gene associate with autoimmune disorders. The BioBreeding diabetes-prone (BBDP) rat has a mutation in the Gimap5 gene that leads to spontaneous apoptosis of peripheral T cells by an unknown mechanism. Because Gimap5 localizes to the endoplasmic reticulum (ER), we hypothesized that absence of functional Gimap5 protein initiates T cell death through disruptions in ER homeostasis. We observed increases in ER stress-associated chaperones in T cells but not thymocytes or B cells from Gimap5−/− BBDP rats. We then discovered that ER stress-induced apoptotic signaling through C/EBP-homologous protein (CHOP) occurs in Gimap5−/− T cells. Knockdown of CHOP by siRNA protected Gimap5−/− T cells from ER stress-induced apoptosis, thereby identifying a role for this cellular pathway in the T cell lymphopenia of the BBDP rat. These findings indicate a direct relationship between Gimap5 and the maintenance of ER homeostasis in the survival of T cells.

Leptin treatment confers clinical benefit at multiple stages of virally induced type 1 diabetes in BB rats

The adipokine, leptin, regulates blood glucose and the insulin secretory function of beta cells, while also modulating immune cell function. We hypothesized that the dual effects of leptin may prevent or suppress the autoreactive destruction of beta cells in a virally induced rodent model of type 1 diabetes. Nearly 100% of weanling BBDR rats treated with the combination of an innate immune system activator, polyinosinic:polycytidylic acid (pIC), and Kilham rat virus (KRV) become diabetic within a predictable time frame. We utilized this model to test the efficacy of leptin in preventing diabetes onset, remitting new onset disease, and preventing autoimmune recurrence in diabetic rats transplanted with syngeneic islet grafts. High doses of leptin delivered via an adenovirus vector (AdLeptin) or alzet pump prevented diabetes in>90% of rats treated with pIC+KRV. The serum hyperleptinemia generated by this treatment was associated with decreased body weight, decreased non-fasting serum insulin levels, and lack of islet insulitis in leptin-treated rats. In new onset diabetics, hyperleptinemia prevented rapid weight loss and diabetic ketoacidosis, and temporarily restored euglycemia. Leptin treatment also prolonged the survival of syngeneic islets transplanted into diabetic BBDR rats. In diverse therapeutic settings, we found leptin treatment to have significant beneficial effects in modulating virally induced diabetes. These findings merit further evaluation of leptin as a potential adjunct therapeutic agent for treatment of human type 1 diabetes.

A Novel Role for the Centrosomal Protein, Pericentrin, in Regulation of Insulin Secretory Vesicle Docking in Mouse Pancreatic β-cells

The centrosome is important for microtubule organization and cell cycle progression in animal cells. Recently, mutations in the centrosomal protein, pericentrin, have been linked to human microcephalic osteodysplastic primordial dwarfism (MOPD II), a rare genetic disease characterized by severe growth retardation and early onset of type 2 diabetes among other clinical manifestations. While the link between centrosomal and cell cycle defects may account for growth deficiencies, the mechanism linking pericentrin mutations with dysregulated glucose homeostasis and pre-pubertal onset of diabetes is unknown. In this report we observed abundant expression of pericentrin in quiescent pancreatic β-cells of normal animals which led us to hypothesize that pericentrin may have a critical function in β-cells distinct from its known role in regulating cell cycle progression. In addition to the typical centrosome localization, pericentrin was also enriched with secretory vesicles in the cytoplasm. Pericentrin overexpression in β-cells resulted in aggregation of insulin-containing secretory vesicles with cytoplasmic, but not centrosomal, pericentriolar material and an increase in total levels of intracellular insulin. RNAi- mediated silencing of pericentrin in secretory β-cells caused dysregulated secretory vesicle hypersecretion of insulin into the media. Together, these data suggest that pericentrin may regulate the intracellular distribution and secretion of insulin. Mice transplanted with pericentrin-depleted islets exhibited abnormal fasting hypoglycemia and inability to regulate blood glucose normally during a glucose challenge, which is consistent with our in vitro data. This previously unrecognized function for a centrosomal protein to mediate vesicle docking in secretory endocrine cells emphasizes the adaptability of these scaffolding proteins to regulate diverse cellular processes and identifies a novel target for modulating regulated protein secretion in disorders such as diabetes.

Insulin regulates carboxypeptidase E by modulating translation initiation scaffolding protein eIF4G1 in pancreatic β cells

Significance Elevated circulating proinsulin and a poor biological response to insulin are observed early in individuals with type 2 diabetes. Genome-wide association studies have recently identified genes associated with proinsulin processing, and clinical observations suggest that elevated proinsulin and its intermediates are markers of dysfunctional insulin-secreting β cells. Here, we propose a previously unidentified mechanism in the regulation of an enzyme that is involved in proinsulin processing called carboxypeptidase E (CPE). Disruption of insulin signaling in β cells reduces expression of a scaffolding protein, eukaryotic translation initiation factor 4 gamma 1, that is required for the initiation of translation and occurs via regulation of two transcription factors, namely, pancreatic and duodenal homeobox 1 and sterol regulatory element-binding protein 1. Together, these effects lead to reduced levels of CPE protein and poor proinsulin processing in β cells. Insulin resistance, hyperinsulinemia, and hyperproinsulinemia occur early in the pathogenesis of type 2 diabetes (T2D). Elevated levels of proinsulin and proinsulin intermediates are markers of β-cell dysfunction and are strongly associated with development of T2D in humans. However, the mechanism(s) underlying β-cell dysfunction leading to hyperproinsulinemia is poorly understood. Here, we show that disruption of insulin receptor (IR) expression in β cells has a direct impact on the expression of the convertase enzyme carboxypeptidase E (CPE) by inhibition of the eukaryotic translation initiation factor 4 gamma 1 translation initiation complex scaffolding protein that is mediated by the key transcription factors pancreatic and duodenal homeobox 1 and sterol regulatory element-binding protein 1, together leading to poor proinsulin processing. Reexpression of IR or restoring CPE expression each independently reverses the phenotype. Our results reveal the identity of key players that establish a previously unknown link between insulin signaling, translation initiation, and proinsulin processing, and provide previously unidentified mechanistic insight into the development of hyperproinsulinemia in insulin-resistant states.