TRPM4 controls insulin secretion in pancreatic β-cells

Abstract

TRPM4 is a calcium-activated non-selective cation channel that is widely expressed and proposed to be involved in cell depolarization. In excitable cells, TRPM4 may regulate calcium influx by causing the depolarization that drives the activation of voltage-dependent calcium channels. We here report that insulin-secreting cells of the rat pancreatic beta-cell line INS-1 natively express TRPM4 proteins and generate large depolarizing membrane currents in response to increased intracellular calcium. These currents exhibit the characteristics of TRPM4 and can be suppressed by expressing a dominant negative TRPM4 construct, resulting in significantly decreased insulin secretion in response to a glucose stimulus. Reduced insulin secretion was also observed with arginine vasopressin stimulation, a Gq-coupled receptor agonist in beta-cells. Moreover, the recruitment of TRPM4 currents was biphasic in both INS-1 cells as well as HEK-293 cells overexpressing TRPM4. The first phase is due to activation of TRPM4 channels localized within the plasma membrane followed by a slower secondary phase, which is caused by the recruitment of TRPM4-containing vesicles to the plasma membrane during exocytosis. The secondary phase can be observed during perfusion of cells with increasing [Ca(2+)](i), replicated with agonist stimulation, and coincides with an increase in cell capacitance, loss of FM1-43 dye, and vesicle fusion. Our data suggest that TRPM4 may play a key role in the control of membrane potential and electrical activity of electrically excitable secretory cells and the dynamic translocation of TRPM4 from a vesicular pool to the plasma membrane via Ca(2+)-dependent exocytosis may represent a key short- and midterm regulatory mechanism by which cells regulate electrical activity.

Increased IgE-dependent mast cell activation and anaphylactic responses in mice lacking the calcium-activated nonselective cation channel TRPM4

“…In electrically nonexcitable cells, such as mast cells, in which Ca 2+ entry occurs through store-or receptor-operated channels, depolarization of the membrane potential would decrease the rate of Ca 2+ influx 12,19 . That hypothesis is supported by data from studies of cell lines in which TRPM4 was overexpressed 16 or TRPM4 function and expression was 'knocked down' 20,21 .…”

“…Adult Trpm4 +/+ and Trpm4 -/-mice were of similar size and weight (at 6-10 weeks of age: Trpm4 +/+ , 30.7 ± 0.5 g (n ¼ 14 mice); Trpm4 -/-, 30.5 ± 0.7 g (n ¼ 14 mice); P 4 0.05). Contrary to what could be anticipated from a published study of a pancreatic b-cell line 20 , glucose clearance was identical in Trpm4 +/+ and Trpm4 -/-mice challenged with intraperitoneal glucose, and glucose-induced insulin release from pancreatic islets remained unchanged after deletion of Trpm4 ( Supplementary Fig. 2 online).…”

“…A function for TRPM4 as a regulator of membrane potential in cell lines has been suggested by RNA interference and antisense 'geneknockdown' strategies 20,21,41 . It has been proposed that suppression of TRPM4 expression converts oscillatory changes in [Ca 2+ ] cyt into longlasting, sustained increases in [Ca 2+ ] cyt in a Jurkat T cell line 21 .…”

Structures of the calcium-activated, non-selective cation channel TRPM4

“…Thus, its function has been implicated in many important physiological processes including the Bayliss effect in cerebral arteries in response to stretch 17,18 , the breath pacemaking in brainstem neurons 19,20 , the insulin secretion of pancreatic β cells in response to glucose uptake 21 , and the Ca 2+ -dependent immune response in T lymphocytes and dendritic cells 22 . In cardiac cells, several mutations in TRPM4 were found to be associated with human heart conduction dysfunction 23–27 .…”

Purinergic P2Y1 receptors take centre stage in autocrine stimulation of human beta cells

“…Further studies are needed to clarify the molecular identity of this conductance. Members of the transient receptor potential family of proteins constitute promising candidates [21,22].…”