Molecular functions of human zinc transporter-8 in pancreatic beta cells

我们研究的长期目标是了解锌转运蛋白-8 （ZnT8）的分子和细胞功能如何调节人类胰腺β细胞对炎症应激和自身免疫攻击的病理生理反应。ZnT8是一种组织特异性锌转运蛋白，在产生胰岛素的β细胞中具有极高的表达水平，在葡萄糖刺激下将约50%的生物合成能力用于胰岛素的产生和分泌。ZnT8的主要功能是维持胰岛素分泌颗粒中适当折叠和结晶包装所需的高锌浓度，但越来越多的证据表明，ZnT8是一种动态蛋白，在细胞表面和内质网（ER）上具有额外的功能作用，其中ZnT8是未折叠蛋白反应的主要客户蛋白，导致ZnT8泛素化，免疫蛋白酶体降解和抗原呈递。此外，在胰岛自身免疫进展为显性1型糖尿病的早期阶段，ZnT8是自身反应性B细胞靶向的主要细胞表面自身抗原。ZnT8的多效性作用塑造了β细胞的独特生物学特性，并调节了它们对疾病驱动的炎症应激的易感性。因此，人类ZnT8是急性胰岛炎症期间β细胞自身免疫破坏的主要自身抗原，也是慢性低度炎症下β细胞功能衰竭的主要内质网应激负担。目前尚不清楚人类ZnT8的病理生理反应如何增加细胞对炎症应激和自身免疫攻击的易感性。本研究将阐明细胞因子诱导ZnT8泛素化和降解（Aim-1）的分子细节，阐明ZnT8自身抗原在人β细胞（Aim-2）上的加工和提呈。揭示驱动ZnT8靶向、降解和抗原呈递的分子机制将揭示内质网应激和ZnT8免疫原性如何通过炎症应激调节，从而增加1型和2型糖尿病的风险。

The long-term goal of our research is to understand how the molecular and cellular functions of zinc transporter-8 (ZnT8) modulate pathophysiologic responses of human pancreatic beta cells to inflammatory stress and autoimmune attack. ZnT8 is a tissue-specific zinc transporter with an exceedingly high expression level in the insulin-producing beta cells that dedicate ~50% of biosynthetic capacity to insulin production and secretion upon glucose stimulation. The primary function of ZnT8 is to maintain a high zinc concentration required for proper insulin folding and crystalline packing in the insulin secretory granules, but growing evidence suggests that ZnT8 is a dynamic protein with additional functional roles on the cell surface and at the endoplasmic reticulum (ER) where ZnT8 is a major client protein of unfolded protein response leading to ZnT8 ubiquitination, immunoproteasome degradation and antigenic presentation. In addition, ZnT8 is a major cell-surface autoantigen targeted by autoreactive B cell in the earlier phase of islet autoimmunity progression to overt type-1 diabetes. The pleiotropic roles of ZnT8 shape the unique biology of beta cells and modulate their susceptibility to disease-driving inflammatory stress. Accordingly, human ZnT8 is a major self-antigen targeted for autoimmune destruction of beta cells during acute islet inflammation, and also a major ER stress burden contributing to functional failure of beta cells under chronic, low-grade inflammation. At present, it is unclear how pathophysiologic responses of human ZnT8 may increase the cell vulnerability to inflammatory stress and autoimmune attack. The proposed research will elucidate the molecular details of cytokine-induced ZnT8 ubiquitination and degradation (Aim-1), and elucidate the processing and presentation of ZnT8 autoantigen on human beta cells (Aim-2). Uncovering the molecular mechanisms driving ZnT8 targeting, degradation, and antigenic presentation will inform how ER stress and ZnT8 immunogenicity may be regulated by inflammatory stress to increase the risk of both type-1 and type-2 diabetes.