GLP-1 stimulates insulin secretion by PKC-dependent TRPM4 and TRPM5 activation
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
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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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 2

Stimulatory effects of low concentrations of GLP-1 depend on L-type Ca2+ channels.

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Stimulatory effects of low concentrations of GLP-1 depend on L-type Ca2+...
(A) GLP-1 (1 pM) enhances mouse β cell Ca2+ currents (ICa) evoked by 20-ms depolarization from –70 mV to 0 mV (n = 7 cells from 5 mice). (B) Current-voltage (I-V) relationships of whole-cell Ca2+ currents in the absence (black) and presence (red) of GLP-1. Current amplitudes have been normalized to cell capacitance. Ca2+ current activation curves are shown in the inset in the absence and presence of GLP-1. *P < 0.05 vs. control by paired Student’s t test (n = 5 cells from 3 mice). (C) GLP-1 inhibits ICa when added in the presence of isradipine (added 5 minutes prior to the addition of GLP-1) (n = 5 cells from 3 mice). (D) Effects of GLP-1 on insulin secretion in mouse islets in the absence and presence of GLP-1 and/or isradipine. Isradipine was included during the 30-minute preincubation. *P < 0.05 vs. 1 mM glucose; †P < 0.05 vs. 6 mM glucose; ‡P < 0.01 vs. 6 mM glucose plus GLP-1 (n = 7–8; 1-way ANOVA with Dunnett’s post-hoc test). (E and F) As in A and C but using human β cells (n = 13 cells from 6 donors in E; n = 6 cells from 3 donors in F). Rectangles indicate the period used for measuring charge entry (QCa) to avoid contribution of voltage-gated Na+ current (initial spiky component). (G) Exocytosis measured as an increase in membrane capacitance evoked by 500-ms depolarizations from –70 mV to 0 mV under control conditions (6 mM glucose, black) and 10 minutes after addition of GLP-1 (red). A control experiment showing that when GLP-1 was not added, the exocytotic responses remained small and stable for 10 minutes (blue; n = 5), is also shown. (H) Exocytosis elicited by a train of three 500-ms depolarizations from –70 mV to 0 mV in a human β cell before (black) and 10 minutes after (red) the addition of GLP-1 (representative of 5 cells from 3 donors).