GLP-1 stimulates insulin secretion by PKC-dependent TRPM4 and TRPM5 activation
Makoto Shigeto, … , Graham Ladds, Patrik Rorsman
Makoto Shigeto, … , Graham Ladds, Patrik Rorsman
Published November 16, 2015
Citation Information: J Clin Invest. 2015;125(12):4714-4728. https://doi.org/10.1172/JCI81975.
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Research Article Cell biology

GLP-1 stimulates insulin secretion by PKC-dependent TRPM4 and TRPM5 activation

Abstract

Strategies aimed at mimicking or enhancing the action of the incretin hormone glucagon-like peptide 1 (GLP-1) therapeutically improve glucose-stimulated insulin secretion (GSIS); however, it is not clear whether GLP-1 directly drives insulin secretion in pancreatic islets. Here, we examined the mechanisms by which GLP-1 stimulates insulin secretion in mouse and human islets. We found that GLP-1 enhances GSIS at a half-maximal effective concentration of 0.4 pM. Moreover, we determined that GLP-1 activates PLC, which increases submembrane diacylglycerol and thereby activates PKC, resulting in membrane depolarization and increased action potential firing and subsequent stimulation of insulin secretion. The depolarizing effect of GLP-1 on electrical activity was mimicked by the PKC activator PMA, occurred without activation of PKA, and persisted in the presence of PKA inhibitors, the KATP channel blocker tolbutamide, and the L-type Ca2+ channel blocker isradipine; however, depolarization was abolished by lowering extracellular Na+. The PKC-dependent effect of GLP-1 on membrane potential and electrical activity was mediated by activation of Na+-permeable TRPM4 and TRPM5 channels by mobilization of intracellular Ca2+ from thapsigargin-sensitive Ca2+ stores. Concordantly, GLP-1 effects were negligible in Trpm4 or Trpm5 KO islets. These data provide important insight into the therapeutic action of GLP-1 and suggest that circulating levels of this hormone directly stimulate insulin secretion by β cells.

Authors

Makoto Shigeto, Reshma Ramracheya, Andrei I. Tarasov, Chae Young Cha, Margarita V. Chibalina, Benoit Hastoy, Koenraad Philippaert, Thomas Reinbothe, Nils Rorsman, Albert Salehi, William R. Sones, Elisa Vergari, Cathryn Weston, Julia Gorelik, Masashi Katsura, Viacheslav O. Nikolaev, Rudi Vennekens, Manuela Zaccolo, Antony Galione, Paul R.V. Johnson, Kohei Kaku, Graham Ladds, Patrik Rorsman

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Figure 1

Stimulatory effects of picomolar concentrations of GLP-1 on insulin secretion, electrical activity, and [Ca2+]i.

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Stimulatory effects of picomolar concentrations of GLP-1 on insulin secr...
(A) Insulin secretion measured at 6 mM glucose (black circle) and increasing concentrations of GLP-1 (n = 4–8 experiments). The white circle shows insulin secretion measured at 1 mM glucose in the absence of GLP-1. †P<0.05 vs. 1 mM glucose, *P < 0.05 vs. 6 mM glucose (1-way ANOVA with Dunnett’s post-hoc test). (B) Insulin secretion at 1, 4, 6, and 10 mM glucose in the absence and presence of 1 pM GLP-1 (n = 9 experiments). *P < 0.01 vs. no GLP-1 (Student’s t test). (C) GLP-1 (1 pM) stimulated insulin secretion from perfused mouse pancreas (n = 4). (D) Membrane potential recording from a β cell exposed sequentially to 1 pM and 10 nM GLP-1 (representative of 24 of 25 cells). (E) Spontaneous [Ca2+]i oscillations in a β cell within an intact islet exposed to 1 pM GLP-1 (representative of 23 of 35 cells in 3 islets from 3 mice). (F) Insulin secretion in mouse islets at 6 mM glucose in the absence and presence of 1 pM GLP-1 and/or 100 nM exendin (9-39) (Ex9-39). *P < 0.05 vs. 1 mM glucose, †P < 0.05 vs. 6 mM glucose (n = 3–5; 1-way ANOVA with Dunnett’s post-hoc test). (G) As in F but using human islets (n = 8–12 with islets from 2–4 donors). *P < 0.05 vs. 1 mM glucose, †P < <0.05 vs. 6 mM glucose (n = 3–5; 1-way ANOVA with Dunnett’s post-hoc test).