ATP-regulated potassium channels and voltage-gated calcium channels in pancreatic alpha and beta cells: Similar functions but reciprocal effects on secretion

Closure of ATP-regulated K+ channels (KATP channels) plays a central role in glucose-stimulated insulin secretion in beta cells. KATP channels are also highly expressed in glucagon-producing alpha cells, where their function remains unresolved. Under hypoglycaemic conditions, KATP channels are open in alpha cells but their activity is low and only ~1% of that in beta cells. Like beta cells, alpha cells respond to hyperglycaemia with KATP channel closure, membrane depolarisation and stimulation of action potential firing. Yet, hyperglycaemia reciprocally regulates glucagon (inhibition) and insulin secretion (stimulation). Here we discuss how this conundrum can be resolved and how reduced KATP channel activity, via membrane depolarisation, paradoxically reduces alpha cell Ca2+ entry and glucagon exocytosis. Finally, we consider whether the glucagon secretory defects associated with diabetes can be attributed to impaired KATP channel regulation and discuss the potential for remedial pharmacological intervention using sulfonylureas. © 2014 Springer-Verlag Berlin Heidelberg

Defining the ionic mechanisms of optogenetic control of vascular tone by channelrhodopsin-2

Background and Purpose Optogenetic control of electromechanical coupling in vascular smooth muscle cells (VSMCs) is emerging as a powerful research tool with potential applications in drug discovery and therapeutics. However, the precise ionic mechanisms involved in this control remain unclear. Experimental Approach Cell imaging, patch-clamp electrophysiology and muscle tension recordings were used to define these mechanisms over a wide range of light stimulations. Key Results Transgenic mice expressing a channelrhodopsin-2 variant [ChR2(H134R)] selectively in VSMCs were generated. Isolated VSMCs obtained from these mice demonstrated blue light-induced depolarizing whole-cell currents. Fine control of artery tone was attained by varying the intensity of the light stimulus. This arterial response was sufficient to overcome the endogenous, melanopsin-mediated, light-evoked, arterial relaxation observed in the presence of contractile agonists. Ca2+ entry through voltage-gated Ca2+ channels, and opening of plasmalemmal depolarizing channels (TMEM16A and TRPM) and intracellular IP3 receptors were involved in the ChR2(H134R)-dependent arterial response to blue light at intensities lower than ~0.1 mW·mm^-2. Light stimuli of greater intensity evoked a significant Ca2+ influx directly through ChR2(H134R) and produced marked intracellular alkalinization of VSMCs. Conclusions and Implications We identified the range of light intensity allowing optical control of arterial tone, primarily by means of endogenous channels and without substantial alteration to intracellular pH. Within this range, mice expressing ChR2(H134R) in VSMCs are a powerful experimental model for achieving accurate and tuneable optical voltage-clamp of VSMCs and finely graded control of arterial tone, offering new approaches to the discovery of vasorelaxant drugs.</p

CPT1a-dependent long-chain fatty acid oxidation contributes to maintaining glucagon secretion from pancreatic islets

Glucagon, the principal hyperglycemic hormone, is secreted from pancreatic islet α cells as part of the counter-regulatory response to hypoglycemia. Hence, secretory output from α cells is under high demand in conditions of low glucose supply. Many tissues oxidize fat as an alternate energy substrate. Here, we show that glucagon secretion in low glucose conditions is maintained by fatty acid metabolism in both mouse and human islets, and that inhibiting this metabolic pathway profoundly decreases glucagon output by depolarizing α cell membrane potential and decreasing action potential amplitude. We demonstrate, by using experimental and computational approaches, that this is not mediated by the KATP channel, but instead due to reduced operation of the Na+-K+ pump. These data suggest that counter-regulatory secretion of glucagon is driven by fatty acid metabolism, and that the Na+-K+ pump is an important ATP-dependent regulator of α cell function

Fumarate Hydratase Deletion in Pancreatic beta Cells Leads to Progressive Diabetes

We thank the Wellcome Trust for core infrastructure support to the Wellcome Trust Centre for Human Genetics, Oxford (090532/Z/ 09/Z), a Wellcome Trust Senior Investigator Award (095531/Z/11/Z) to P.R., and grants to F.M.A. (884655 and 089795). T.S. is supported by a Grant-inAid for Scientific Research on Innovative Areas, Japan (22134007) and the Yamagata Prefectural Government and City of Tsuruoka; H.M., P.S., and P.R. are supported by the Swedish Research Council; P.R. is supported by the Knut and Alice Wallenbergs Stiftelse (Wallenberg Scholars); and E.R. is supported by FAPESP. R.M.d.R. and J.T.-R. were partially supported by a grant from the Spanish Ministry of Science and Technology: SAF2009/1267. F.M.A. holds a Royal Society/Wolfson merit award and an ERC Advanced Investigatorship (322620). A.L. is a Barts and The London Charity post-doctoral fellow. R.R. and Q.Z. are Diabetes UK RD Lawrence fellows, and A.H. holds a PhD studentship funded by Diabetes UK

The structural basis of lipid scrambling and inactivation in the endoplasmic reticulum scramblase TMEM16K

Membranes in cells have defined distributions of lipids in each leaflet, controlled by lipid scramblases and flip/floppases. However, for some intracellular membranes such as the endoplasmic reticulum (ER) the scramblases have not been identified. Members of the TMEM16 family have either lipid scramblase or chloride channel activity. Although TMEM16K is widely distributed and associated with the neurological disorder autosomal recessive spinocerebellar ataxia type 10 (SCAR10), its location in cells, function and structure are largely uncharacterised. Here we show that TMEM16K is an ER-resident lipid scramblase with a requirement for short chain lipids and calcium for robust activity. Crystal structures of TMEM16K show a scramblase fold, with an open lipid transporting groove. Additional cryo-EM structures reveal extensive conformational changes from the cytoplasmic to the ER side of the membrane, giving a state with a closed lipid permeation pathway. Molecular dynamics simulations showed that the open-groove conformation is necessary for scramblase activity