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

KEY POINTS: Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca ) 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 handling and hormone secretion. We find that TSPAN-7 reduces β-cell glucose-stimulated Ca entry, slows Ca oscillation frequency and decreases glucose-stimulated insulin secretion. TSPAN-7 controls β-cell function through a direct interaction with L-type voltage-dependent Ca channels (Ca 1.2 and Ca 1.3), which reduces channel Ca conductance. TSPAN-7 slows activation of Ca 1.2 and accelerates recovery from voltage-dependent inactivation; TSPAN-7 also slows Ca 1.3 inactivation kinetics. These findings strongly implicate TSPAN-7 as a key regulator in determining the set-point of glucose-stimulated Ca influx and insulin secretion.

ABSTRACT: Glucose-stimulated insulin secretion (GSIS) is regulated by calcium (Ca ) entry into pancreatic β-cells through voltage-dependent Ca (Ca ) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca 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 influx into mouse and human β-cells. Additionally, mouse β-cell Ca oscillation frequency was accelerated by TSPAN-7 KD. Because TSPAN-7 KD also enhanced Ca entry when membrane potential was clamped with depolarization, the effect of TSPAN-7 on Ca channel activity was examined. TSPAN-7 KD enhanced L-type Ca currents in mouse and human β-cells. Conversely, heterologous expression of TSPAN-7 with Ca 1.2 and Ca 1.3 L-type Ca channels decreased Ca currents and reduced Ca influx through both channels. This was presumably the result of a direct interaction of TSPAN-7 and L-type Ca channels because TSPAN-7 coimmunoprecipitated with both Ca 1.2 and Ca 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 mM glucose) and stimulated (45 mM KCl + 14 mM glucose) insulin secretion. These findings strongly suggest that TSPAN-7 modulation of β-cell L-type Ca channels is a key determinant of β-cell glucose-stimulated Ca entry and thus the set-point of GSIS.