Department of Physiology, University of Toronto, Toronto, Ontario, Canada.
University of Kansas Medical Center, Kansas City, Kansas, United States of America.
PLoS One. 2015 Mar 25;10(3):e0119136. doi: 10.1371/journal.pone.0119136. eCollection 2015.
Zinc has an important role in normal pancreatic beta cell physiology as it regulates gene transcription, insulin crystallization and secretion, and cell survival. Nevertheless, little is known about how zinc is transported through the plasma membrane of beta cells and which of the class of zinc influx transporters (Zip) is involved. Zip4 was previously shown to be expressed in human and mouse beta cells; however, its function there is still unknown. Therefore, the aim of this study was to define the zinc transport role of Zip4 in beta cells. To investigate this, Zip4 was over-expressed in MIN6 beta cells using a pCMV6-Zip4GFP plasmid. Organelle staining combined with confocal microscopy showed that Zip4 exhibits a widespread localization in MIN6 cells. Time-lapse zinc imaging experiments showed that Zip4 increases cytoplasmic zinc levels. This resulted in increased granular zinc content and glucose-stimulated insulin secretion. Interestingly, it is unlikely that the increased glucose stimulated insulin secretion was triggered by a modulation of mitochondrial function, as mitochondrial membrane potential remained unchanged. To define the role of Zip4 in-vivo, we generated a beta cell-specific knockout mouse model (Zip4BKO). Deletion of the Zip4 gene was confirmed in Zip4BKO islets by PCR, RT-PCR, and immuno-histochemistry. Zip4BKO mice showed slightly improved glucose homeostasis but no change in insulin secretion during an oral glucose tolerance test. While Zip4 was not found to be essential for proper glucose homeostasis and insulin secretion in vivo in mice, this study also found that Zip4 mediates increases in cytoplasmic and granular zinc pools and stimulates glucose dependant insulin secretion in-vitro.
锌在正常胰腺β细胞生理中具有重要作用，因为它调节基因转录、胰岛素结晶和分泌以及细胞存活。然而，人们对锌如何穿过β细胞的质膜运输以及哪种锌内流转运体（Zip）参与其中知之甚少。Zip4 先前已被证明在人和鼠的β细胞中表达；然而，其在那里的功能仍然未知。因此，本研究的目的是确定 Zip4 在β细胞中的锌转运作用。为了研究这一点，使用 pCMV6-Zip4GFP 质粒在 MIN6 β细胞中过表达 Zip4。细胞器染色结合共聚焦显微镜显示，Zip4 在 MIN6 细胞中广泛定位。时间 lapse 锌成像实验表明，Zip4 增加细胞质锌水平。这导致颗粒内锌含量增加和葡萄糖刺激的胰岛素分泌增加。有趣的是，增加的葡萄糖刺激胰岛素分泌不太可能是通过调节线粒体功能触发的，因为线粒体膜电位保持不变。为了确定 Zip4 在体内的作用，我们生成了一种β细胞特异性敲除小鼠模型（Zip4BKO）。通过 PCR、RT-PCR 和免疫组织化学证实了 Zip4BKO 胰岛中 Zip4 基因的缺失。Zip4BKO 小鼠的葡萄糖稳态略有改善，但口服葡萄糖耐量试验中胰岛素分泌没有变化。虽然在体内没有发现 Zip4 对小鼠正常葡萄糖稳态和胰岛素分泌是必需的，但这项研究还发现，Zip4 介导细胞质和颗粒锌池的增加，并刺激体外葡萄糖依赖性胰岛素分泌。
