Molecular Pathophysiology of Wolfram Syndrome and Endoplasmic Reticulum Stress-associated Pancreatic β-cell Failure.

Wolfram综合征是由WFS 1基因突变引起的，其特征是幼年型糖尿病和进行性视神经萎缩。在患者的胰岛中，β细胞被选择性地丢失。WFS 1基因编码一种内质网（ER）膜包埋蛋白（Wolframin）。我们推测，钨蛋白参与ER应激反应。用ER应激诱导的化学物质如毒胡萝卜素或衣霉素处理MIN 6细胞，增加wfs 1 mRNA水平。当用WFS 1基因启动子-荧光素酶报告系统检测时，通过用这些试剂处理细胞激活了人WFS 1启动子。此外，我们发现在源自秋田小鼠的小鼠胰岛素瘤细胞中，错误折叠的突变胰岛素表达诱导了内质网应激，WFS 1 mRNA和蛋白表达增加。WFS 1启动子在这些秋田小鼠来源的β细胞中更活跃。总的来说，我们的数据表明，Wfs 1的表达是由ER应激诱导的。在Wfs 1-/-小鼠中，胰腺β细胞的丢失在很大程度上取决于它们的遗传背景。为了验证Wfs 1缺乏的胰腺β细胞更容易受到ER应激的假设，我们将C57 BL/6 J背景下的Wfs 1-/-小鼠与轻度肥胖/胰岛素抵抗的遗传模型agglutinallethalyellow（A/Ay）小鼠杂交。ER伴侣Bip/GRP 78在A/Ay小鼠的胰岛中表达增加，表明由于胰岛素生物合成和分泌的需求增加而存在ER应激。尽管C57 BL/6 J背景下的Wfs 1-/-小鼠或A/Ay小鼠均未发生明显的糖尿病，但所有Wfs 1-/- A/Ay小鼠均发生了明显的糖尿病。Wfs 1-/- A/Ay小鼠胰腺β细胞凋亡明显减少。这些数据表明，缺乏Wfs 1的胰腺β细胞更容易受到ER应激诱导的凋亡。

Wolfram syndrome, caused by the mutations in the WFS1 gene, is characterized by juvenile onset diabetes mellitus and progressive optic atrophy. In the patient's islets, β-cells are selectively lost. The WFS1 gene encodes an endoplasmic reticulum (ER) membrane-embedded protein (Wolframin). We speculated that wolframin is involved in the ER stress responses. Treatment of MIN6 cells with ER stress-inducible chemical such as thapsigargin or tunicamycin, increased wfs1 mRNA levels. Human WFS1 promoter was activated by treatment of the cells with these reagents when it was examined by the WFS1 gene promoter-luciferase reporter system. In addition, we found increased WFS1 mRNA and protein expression in the mouse insulinoma cells derived from Akita mouse, in which ER stress is induced by the misfolded mutant insulin expression. WFS1 promoter was more active in these Akita mouse-derived β-cells. Collectively, our data suggest that WFS1 expression is induced by ER stress.In Wfs1-/- mice, pancreatic β-cell loss largely depends on their genetic backgrounds. To test the hypothesis that the Wfs1-lacking pancreatic β-cells are more susceptible to ER stress, we crossed Wfs1-/- mice on the C57BL/6J background with agouti lethal yellow (A/Ay) mice, a genetic model of mild obesity/insulin resistance. ER chaperon Bip/GRP78 expression increased in the pancreatic islets of A/Ay mice, suggesting the presence of ER stress due to increased demands of insulin biosynthesis and secretion. Although neither the Wfs1-/- mice or the A/Ay mice on the C57BL/6J background develop overt diabetes, all Wfs1-/- A/Ay mice developed overt diabetes. Pancreatic β-cells markedly reduced in the Wfs1-/- A/Ay mice due to apoptosis. These data suggest that the pancreatic β-cells lacking Wfs1 to be more susceptible to ER stress-induced apoptosis.