Division of Endocrinology, Department of Bio-Signal Analysis, Yamaguchi University Graduate School of Medicine, Ube, Yamaguchi, Japan.
Hum Mol Genet. 2011 Apr 1;20(7):1274-84. doi: 10.1093/hmg/ddq568. Epub 2011 Jan 3.
Wolfram syndrome is an autosomal recessive disorder characterized by juvenile-onset insulin-dependent diabetes mellitus and optic atrophy. The gene responsible for the syndrome (WFS1) encodes an endoplasmic reticulum (ER) resident transmembrane protein. The Wfs1-null mouse exhibits progressive insulin deficiency causing diabetes. Previous work suggested that the function of the WFS1 protein is connected to unfolded protein response and to intracellular Ca(2+) homeostasis. However, its precise molecular function in pancreatic β-cells remains elusive. In our present study, immunofluorescent and electron-microscopic analyses revealed that WFS1 localizes not only to ER but also to secretory granules in pancreatic β-cells. Intragranular acidification was assessed by measuring intracellular fluorescence intensity raised by the acidotrophic agent, 3-[2,4-dinitroanilino]-3'-amino-N-methyldipropyramine. Compared with wild-type β-cells, there was a 32% reduction in the intensity in WFS1-deficient β-cells, indicating the impairment of granular acidification. This phenotype may, at least partly, account for the evidence that Wfs1-null islets have impaired proinsulin processing, resulting in an increased circulating proinsulin level. Morphometric analysis using electron microscopy evidenced that the density of secretory granules attached to the plasma membrane was significantly reduced in Wfs1-null β-cells relative to that in wild-type β-cells. This may be relevant to the recent finding that granular acidification is required for the priming of secretory granules preceding exocytosis and may partly explain the fact that glucose-induced insulin secretion is profoundly impaired in young prediabetic Wfs1-null mice. These results thus provide new insights into the molecular mechanisms of β-cell dysfunction in patients with Wolfram syndrome.
Wolfram 综合征是一种常染色体隐性遗传病，其特征为青少年起病的胰岛素依赖型糖尿病和视神经萎缩。引起该综合征的基因（WFS1）编码内质网（ER）驻留的跨膜蛋白。Wfs1 基因敲除小鼠表现为进行性胰岛素缺乏导致糖尿病。先前的研究表明，WFS1 蛋白的功能与未折叠蛋白反应和细胞内 Ca(2+) 稳态有关。然而，其在胰岛β细胞中的确切分子功能仍不清楚。在本研究中，免疫荧光和电子显微镜分析显示，WFS1 不仅定位于 ER，也定位于胰岛β细胞的分泌颗粒中。通过测量酸化剂 3-[2,4-二硝基苯氨基]-3'-氨基-N-甲基二丙基哌嗪引起的细胞内荧光强度来评估颗粒内酸化。与野生型β细胞相比，WFS1 缺陷型β细胞的荧光强度降低了 32%，表明颗粒酸化受损。这种表型可能至少部分解释了 Wfs1 基因敲除胰岛中胰岛素原加工受损的证据，导致循环胰岛素原水平增加。电子显微镜的形态计量学分析表明，与野生型β细胞相比，Wfs1 基因敲除型β细胞附着在质膜上的分泌颗粒的密度显著降低。这可能与最近的发现有关，即颗粒酸化是在胞吐作用之前分泌颗粒的启动所必需的，并且部分解释了年轻的糖尿病前期 Wfs1 基因敲除小鼠葡萄糖诱导的胰岛素分泌严重受损的事实。这些结果为 Wolfram 综合征患者β细胞功能障碍的分子机制提供了新的见解。
