Loss of tetraspanin-7 expression reduces pancreatic beta-cell exocytosis Ca2+ sensitivity but has limited effect on systemic metabolism [mouse]

Background: Tetraspanin-7 (Tspan7) is an islet autoantigen involved in autoimmune type 1 diabetes and known to regulate beta-cell L-type Ca2+ channel activity. However, the role of Tspan7 in pancreatic beta-cell function is not yet fully understood. Methods: Histological analyses were conducted using immunostaining. Whole-body metabolism was tested using glucose tolerance test. Islet hormone secretion was quantified using static batch incubation or dynamic perifusion. Beta-cell transmembrane currents, electrical activity and exocytosis were measured using whole-cell patch-clamping and capacitance measurements. Gene expression was studied using mRNA-sequencing and quantitative PCR. Results: Tspan7 is expressed in insulin-containing granules of pancreatic beta-cells. Tspan7-knockout mice (Tspan7 y/- mouse) exhibit reduced body weight and ad libitum plasma glucose but normal glucose tolerance. Tspan7y/- islets have normal insulin content and glucose- or tolbutamide-stimulated insulin secretion. Depolarisation-triggered Ca2+ current was enhanced in Tspan7y/- beta-cells, but beta-cell electrical activity and depolarisation-evoked exocytosis were unchanged suggesting that exocytosis was less sensitive to Ca2+. TSPAN7 knockdown (KD) in human pseudo-islets led to a significant reduction in high K+-stimulated insulin secretion. Transcriptomic analyses show that TSPAN7 KD in human pseudo-islets correlated with changes in genes involved in hormone secretion, apoptosis and ER stress. Consistent with rodent beta-cells, exocytotic Ca2+ sensitivity was reduced in a human beta cell line (EndoC-bH1) following Tspan7 KD. Conclusion: Tspan7 is involved in the regulation of Ca2+-dependent exocytosis in beta-cells. Its function is more significant in human beta-cells than their rodent counterparts Overall design: Comparative gene expression profiling analysis of RNA-seq data for pancreatic islet from WT and TSPAN7-KD mouse.