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.
Previous SectionNext SectionIn 1938, Wolfram and Wagener (1) analyzed four siblings with the combination of juvenile diabetes and optic atrophy, thus providing the first report of Wolfram syndrome. Because a significant portion of patients with Wolfram syndrome develop diabetes insipidus and auditory nerve deafness, this syndrome is also referred to as the diabetes insipidus, diabetes mellitus, optic atrophy, and deafness syndrome (2, 3). Its pathogenesis is still unknown.
Although patients with Wolfram syndrome are not generally obese and do not have insulitis, the β-cells in their pancreatic islets are selectively destroyed (4). Families that exhibit Wolfram syndrome share mutations in a gene encoding the WFS1 protein, a transmembrane protein in the endoplasmic reticulum (ER)3 (5, 6). WFS1 serves as an ER calcium channel (7), suggesting that this molecule may have a function in ER homeostasis. Therefore, inactivation of WFS1 may cause imbalances in ER homeostasis.
The ER is an important cellular compartment for the folding of newly synthesized secretory proteins such as proinsulin. Imbalance in ER homeostasis elicits stress in this organelle. ER stress is defined as an imbalance between the actual folding capacity of the ER and the demand placed on this organelle. The unfolded protein response (UPR), an adaptive response that counteracts ER stress, has three components as follows: gene expression, translational attenuation, and ER-associated protein degradation system (8-10). Accumulating evidence suggests that a high level of ER stress or defective ER stress signaling (i.e. the UPR) causes β-cell death during the development of diabetes (9, 11-13).
Inositol requiring 1 (IRE1), a sensor for unfolded and misfolded proteins in the ER, is a central regulator of the UPR. IRE1α, which is expressed ubiquitously, has a high level of expression in the pancreas and placenta (14, 15); IRE1β is expressed only in epithelial cells of the gastrointestinal tract (16, 17). The presence of unfolded proteins in the ER causes dimerization, trans-autophosphorylation, and consequent activation of IRE1. Activated IRE1 splices XBP-1 (X-box binding protein-1) mRNA, leading to synthesis of the active transcription factor XBP-1 and up-regulation of UPR genes (18, 19). In contrast, prolonged ER stress activates the cell-death pathway through IRE1. Under chronic ER stress, IRE1 recruits tumor necrosis factor receptor-associated factor 2 (20), which activates apoptosis-signaling kinase 1 (ASK1) (21, 22). Activated ASK1 leads to the activation of c-Jun N-terminal protein kinase and, in the presence of extreme ER stress, induces apoptosis (23). It has been suggested that this pathway is important for insulin resistance in patients with type 2 diabetes (24). Obesity causes ER stress in the liver and leads to hyperactivation of c-Jun N-terminal protein kinase signaling. This causes serine phosphorylation of insulin receptor substrate-1 and inhibits insulin action in liver cells. In addition, tumor necrosis factor receptor-associated factor 2 recruitment by IRE1 causes clustering and activation of caspase-12 at the ER membrane (25). Activated caspase-12 induces apoptosis under pathological ER stress conditions (26).
Two more upstream components in the UPR, PKR-like ER kinase (PERK) and activating transcription factor 6 (ATF6) (27-29), are also sensors of unfolded or misfolded proteins and are activated by the accumulation of such proteins in the ER. PERK is highly expressed in pancreatic islets (29, 30). Activated PERK phosphorylates the α subunit of eukaryotic translation initiation factor 2 (eIF2α), which leads to the attenuation of general protein translation. This reduces the ER workload and protects cells from apoptosis resulting from ER stress (31). In Wolcott-Rallison syndrome, a rare form of juvenile diabetes, mutations in the EIF2AK3 gene encoding PERK have been reported (32). PERK knock-out mice also develop diabetes because of the high level of ER stress in the pancreas (33, 34), strongly suggesting that β-cell death in patients with Wolcott-Rallison syndrome is caused by ER stress. ATF6 is a bZIP-containing transcription factor in the ER. Under ER stress, ATF6 is cleaved and released from the ER. The bZIP domain of ATF6 then translocates into the nucleus and up-regulates the UPR-specific downstream genes. The physiological role of ATF6 in pancreatic β-cells is not yet known.
Increasing evidence suggests that a high level of ER stress and defective ER stress signaling are important in the pathogenesis of diabetes. It is highly likely that downstream components of ER stress signaling maintain ER homeostasis in pancreatic β-cells. Therefore, defective ER stress signaling could cause a high level of ER stress in pancreatic β-cells and lead to β-cell dysfunction and diabetes. Pancreatic β-cells are specialized in proinsulin folding and insulin secretion. It is possible that β-cells have a unique downstream component of ER stress signaling. In this study we investigated whether Wolfram syndrome gene 1 (WFS1) is a component of ER stress signaling and has a function in maintaining ER homeostasis in β-cells.