Coupling of Insulin Secretion and Display of a Granule-resident Zinc Transporter ZnT8 on the Surface of Pancreatic Beta Cells

Qiong Huang,Chengfeng Merriman,Hao Zhang,Dax Fu

doi:10.1074/jbc.m116.772152

Abstract

The islet-specific zinc transporter ZnT8 mediates zinc enrichment in the insulin secretory granules of the pancreatic beta cell. This granular zinc transporter is also a major self-antigen found in type 1 diabetes patients. It is not clear whether ZnT8 can be displayed on the cell surface and how insulin secretion may regulate the level of ZnT8 exposure to extracellular immune surveillance. Here we report specific antibody binding to the extracellular surface of rat insulinoma INS-1E cells that stably expressed a tagged human zinc transporter ZnT8. Flow cytometry analysis after fluorescent antibody labeling revealed strong correlations among the levels of ZnT8 expression, its display on the cell surface, and glucose-stimulated insulin secretion (GSIS). Glucose stimulation increased the surface display of endogenous ZnT8 from a basal level to 32.5% of the housekeeping Na+/K+ ATPase on the cell surface, thereby providing direct evidence for a GSIS-dependent surface exposure of the ZnT8 self-antigen. Moreover, the variation in tagged-ZnT8 expression and surface labeling enabled sorting of heterogeneous beta cells to subpopulations that exhibited marked differences in GSIS with parallel changes in endogenous ZnT8 expression. The abundant surface display of endogenous ZnT8 and its coupling to GSIS demonstrated the potential of ZnT8 as a surface biomarker for tracking and isolating functional beta cells in mixed cell populations.

Highlights

The pancreatic beta cell is ranked among the most specialized cells in the human body, functioning as a sole source for providing insulin in response to the blood glucose level [1]
Whether the granule-resident ZnT8 is linked to the insulin secretory pathway and how glucose stimulation may modulate the level of ZnT8 surface display
Specific antibody labeling of tagged-ZnT8 on the cell surface enabled flow cytometry sorting of heterogeneous INS-1E cells to subpopulations with varied glucose-stimulated insulin secretion (GSIS) phenotypes, suggesting that ZnT8 may be used as a surface biomarker for tracking and isolating functional beta cells in mixed cell populations

Summary

Introduction

The pancreatic beta cell is ranked among the most specialized cells in the human body, functioning as a sole source for providing insulin in response to the blood glucose level [1]. The low ZnT8-GFP and ZnT8-FLAG/GFP signals observed in stably transfected INS-1E cells (left panel, Fig. 1C) suggested that the expression levels of both tagged-ZnT8 variants were negligible relative to that of endogenous ZnT8.

Results

Conclusion