Tetraspanin‐7 regulation of L‐type voltage‐dependent calcium channels controls pancreatic β‐cell insulin secretion

Abstract

Key points Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca 2+ ) handling. TSPAN‐7 is enriched in pancreatic islet cells; however, the function of islet TSPAN‐7 has not been identified. Here, we characterize how β‐cell TSPAN‐7 regulates Ca 2+ handling and hormone secretion. We find that TSPAN‐7 reduces β‐cell glucose‐stimulated Ca 2+ entry, slows Ca 2+ oscillation frequency and decreases glucose‐stimulated insulin secretion. TSPAN‐7 controls β‐cell function through a direct interaction with L‐type voltage‐dependent Ca 2+ channels (Ca V 1.2 and Ca V 1.3), which reduces channel Ca 2+ conductance. TSPAN‐7 slows activation of Ca V 1.2 and accelerates recovery from voltage‐dependent inactivation; TSPAN‐7 also slows Ca V 1.3 inactivation kinetics. These findings strongly implicate TSPAN‐7 as a key regulator in determining the set‐point of glucose‐stimulated Ca 2+ influx and insulin secretion. Abstract Glucose‐stimulated insulin secretion (GSIS) is regulated by calcium (Ca 2+ ) entry into pancreatic β‐cells through voltage‐dependent Ca 2+ (Ca V ) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca 2+ handling, and thus they may also modulate GSIS. TSPAN‐7 is the most abundant islet TSPAN and immunostaining of mouse and human pancreatic slices shows that TSPAN‐7 is highly expressed in β‐ and α‐cells; however, the function of islet TSPAN‐7 has not been determined. Here, we show that TSPAN‐7 knockdown (KD) increases glucose‐stimulated Ca 2+ influx into mouse and human β‐cells. Additionally, mouse β‐cell Ca 2+ oscillation frequency was accelerated by TSPAN‐7 KD. Because TSPAN‐7 KD also enhanced Ca 2+ entry when membrane potential was clamped with depolarization, the effect of TSPAN‐7 on Ca V channel activity was examined. TSPAN‐7 KD enhanced L‐type Ca V currents in mouse and human β‐cells. Conversely, heterologous expression of TSPAN‐7 with Ca V 1.2 and Ca V 1.3 L‐type Ca V channels decreased Ca V currents and reduced Ca 2+ influx through both channels. This was presumably the result of a direct interaction of TSPAN‐7 and L‐type Ca V channels because TSPAN‐7 coimmunoprecipitated with both Ca V 1.2 and Ca V 1.3 from primary human β‐cells and from a heterologous expression system. Finally, TSPAN‐7 KD in human β‐cells increased basal (5.6 m M glucose) and stimulated (45 m M KCl + 14 m M glucose) insulin secretion. These findings strongly suggest that TSPAN‐7 modulation of β‐cell L‐type Ca V channels is a key determinant of β‐cell glucose‐stimulated Ca 2+ entry and thus the set‐point of GSIS.

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