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

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Tetraspanin (TSPAN) proteins regulate many biological processes, including intracellular calcium (Ca²⁺ ) 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 Ca²⁺ handling and hormone secretion. We find that TSPAN-7 reduces β-cell glucose-stimulated Ca²⁺ entry, slows Ca²⁺ oscillation frequency and decreases glucose-stimulated insulin secretion. TSPAN-7 controls β-cell function through a direct interaction with L-type voltage-dependent Ca²⁺ channels (Ca_V 1.2 and Ca_V 1.3), which reduces channel Ca²⁺ 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 Ca²⁺ influx and insulin secretion.

Abstract

Glucose-stimulated insulin secretion (GSIS) is regulated by calcium (Ca²⁺ ) entry into pancreatic β-cells through voltage-dependent Ca²⁺ (Ca_V ) channels. Tetraspanin (TSPAN) transmembrane proteins control Ca²⁺ 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 Ca²⁺ influx into mouse and human β-cells. Additionally, mouse β-cell Ca²⁺ oscillation frequency was accelerated by TSPAN-7 KD. Because TSPAN-7 KD also enhanced Ca²⁺ 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 Ca²⁺ 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 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_V channels is a key determinant of β-cell glucose-stimulated Ca²⁺ entry and thus the set-point of GSIS.

SUBMITTER: Dickerson MT

PROVIDER: S-EPMC8095317 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundProper glycemic control is an important goal of critical care medicine, including perioperative patient care that can influence patients' prognosis. Insulin secretion from pancreatic β-cells is generally assumed to play a critical role in glycemic control in response to an elevated blood glucose concentration. Many animal and human studies have demonstrated that perioperative drugs, including volatile anesthetics, have an impact on glucose-stimulated insulin secretion (GSIS). However, the effects of the intravenous anesthetic propofol on glucose metabolism and insulin sensitivity are largely unknown at present.MethodsThe effect of propofol on insulin secretion under low glucose or high glucose was examined in mouse MIN6 cells, rat INS-1 cells, and mouse pancreatic β-cells/islets. Cellular oxygen or energy metabolism was measured by Extracellular Flux Analyzer. Expression of glucose transporter 2 (GLUT2), potassium channels, and insulin mRNA was assessed by qRT-PCR. Protein expression of voltage-dependent potassium channels (Kv2) was also assessed by immunoblot. Propofol's effects on potassium channels including stromatoxin-1-sensitive Kv channels and cellular oxygen and energy metabolisms were also examined.ResultsWe showed that propofol, at clinically relevant doses, facilitates insulin secretion under low glucose conditions and GSIS in MIN6, INS-1 cells, and pancreatic β-cells/islets. Propofol did not affect intracellular ATP or ADP concentrations and cellular oxygen or energy metabolism. The mRNA expression of GLUT2 and channels including the voltage-dependent calcium channels Cav1.2, Kir6.2, and SUR1 subunit of KATP, and Kv2 were not affected by glucose or propofol. Finally, we demonstrated that propofol specifically blocks Kv currents in β-cells, resulting in insulin secretion in the presence of glucose.ConclusionsOur data support the hypothesis that glucose induces membrane depolarization at the distal site, leading to KATP channel closure, and that the closure of Kv channels by propofol depolarization in β-cells enhances Ca2+ entry, leading to insulin secretion. Because its activity is dependent on GSIS, propofol and its derivatives are potential compounds that enhance and initiate β-cell electrical activity.

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

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