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

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 Ca(2+) 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 Ca(2+) from thapsigargin-sensitive Ca(2+) 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.Peer reviewe

Potentiation of TRPM5 ion channel activity reveals a novel pathway for the development of antidiabetic medication

In this thesis we explored the therapeutic potential of modulating TRPM5 ion channel activity in the context of high fat diet-induced diabetes in mice. TRPM5 is a monovalent selective cation channel that is activated by increases in intracellular calcium. The channel is functionally expressed in the type II taste receptor cells in the taste buds, where it is essential for the transduction of sweet, bitter and umami taste. TRPM5 expression was also shown in pancreatic beta-cells. Trpm5-/- mice have impaired insulin secretion and a prediabetic glucose metabolism. Therefore potentiation of TRPM5 might lead to increased taste perception and increased insulin secretion. At the start of this project no compounds were available that could accomplish this. In a first part of this study, we developed a thallium-based fluorescence screening method to detect TRPM5 modulating molecules in a high throughput. We found that ketoconazole and robustic acid (amongst others) are potent inhibitors of TRPM5 and confirmed their inhibiting effect with patch clamp experiments. We identified and describe the first known TRPM5 potentiators chloro-acetoxy-quinoline and exemestane. More interesting was the observation that glimepiride, an oral antidiabetic, was a potentiator of TRPM5 activity. This shows that TRPM5 is already a target in the treatment of diabetes, albeit unknown. A second part of the study focused on the interaction between steviol glycosides and TRPM5. We identified steviol glycosides, the sweet molecules in the stevia rebaudiana plant, as selective TRPM5 potentiators. We analyzed in detail the effects of stevioside and steviol on TRPM5. We showed that the overwhelming sweet taste of steviol glycosides can be explained by a potentiation of TRPM5 in the taste receptor cells. Furthermore, steviol on its own does not taste sweet, but it increases taste perception for sweet, bitter and umami compounds in a TRPM5-dependent way. We measured potentiation of TRPM5 currents in an overexpression system and in pancreatic beta-cells. We observed increased calcium oscillations upon application of stevioside in WT but not on Trpm5-/- mice. We finally analyzed the effect of stevioside administration in a high-fat diet induced diabetic mouse model. We found that stevioside increased the insulin secretion due to potentiation of TRPM5 in the beta-cells of the pancreas. This led to the prevention of severe hyperglycemia in these mice and effectively delayed the onset of diabetes. The effects of stevioside are mediated through TRPM5, as no e ects were observed in knockout animals. Taken together, i) our data con rm the essential role of TRPM5 in taste perception and insulin secretion, which was previously suggested from studies with knockout mice. ii) We show that TRPM5 is essential for the biological action of steviol and steviol glycosides iii) We show that potentiation of TRPM5 channel activity is an efficient strategy to prevent the development of type 2 diabetes, at least in mice and possibly also in humans.status: publishe

Transient Receptor Potential (TRP) Cation Channels in Diabetes

Transient receptor potential (TRP) proteins constitute a family of cation channels with diverse permeation and gating properties. Consequently, they have a complex role in mammalian physiology, ranging from temperature sensation in the skin through blood pressure regulation to Ca2 + reabsorption in kidneys and intestines. Increasing evidence has implicated TRP channels in the pathology of diabetes, both on the level of insulin release from the pancreatic ß cells and in secondary conditions such as diabetic neuropathy, nephropathy, and vasculopathy. In this review, we summarize these recent findings.status: publishe

Stevia Sweetener Enhances Pancreatic Beta-Cell Function by Potentiating TRPM5 Channel Activity

Steviol glycosides as stevioside and rebaudioside A are natural, non-caloric sweet-tasting organic molecules, present in the extracts of the scrub plant Stevia rebaudiana, which are widely used as sweeteners in consumer foods and beverages. TRPM5 is a Ca2+-activated cation channel expressed in pancreatic β-cells. We show potentiation of TRPM5 with stevioside, rebaudioside A and their aglycon steviol. In this way, there is an enhanced glucose-induced insulin secretion in a Trpm5-dependent manner. The daily consumption of stevioside prevents the development of high-fat-diet-induced diabetic hyperglycaemia in wild-type mice, but not in Trpm5−/− mice. In isolated human pancreatic islets, there is a stevioside-induced potentiation of the intracellular calcium dynamics. This is indicative for increased insulin secretion, as is observed with mice. These results elucidate a molecular mechanism of action of steviol glycosides and identify TRPM5 as a potential target to prevent and treat type 2 diabetes.status: publishe

A Thallium-Based Screening Procedure to Identify Molecules That Modulate the Activity of Ca2+-Activated Monovalent Cation-Selective Channels

TRPM5 functions as a calcium-activated monovalent cation-selective ion channel and is expressed in a variety of cell types. Dysfunction of this type of channel has been recently implied in cardiac arrhythmias, diabetes, and other pathologies. Therefore, a growing interest has emerged to develop the pharmacology of these ion channels. We optimized a screening assay based on the thallium flux through the TRPM5 channel and a fluorescent thallium dye as a probe for channel activity. We show that this assay is capable of identifying molecules that inhibit or potentiate calcium-activated monovalent cation-selective ion channels.status: publishe