Ca2+ channels and excitation-contraction coupling

Metabolic fate of glucose and candidate signaling and excess-fuel detoxification pathways in pancreatic β-cells

Glucose metabolism promotes insulin secretion in β-cells via metabolic coupling factors that are incompletely defined. Moreover, chronically elevated glucose causes β-cell dysfunction, but little is known about how cells handle excess fuels to avoid toxicity. Here we sought to determine which among the candidate pathways and coupling factors best correlates with glucose-stimulated insulin secretion (GSIS), define the fate of glucose in the β-cell, and identify pathways possibly involved in excess-fuel detoxification. We exposed isolated rat islets for 1 h to increasing glucose concentrations and measured various pathways and metabolites. Glucose oxidation, oxygen consumption, and ATP production correlated well with GSIS and saturated at 16 mm glucose. However, glucose utilization, glycerol release, triglyceride and glycogen contents, free fatty acid (FFA) content and release, and cholesterol and cholesterol esters increased linearly up to 25 mm glucose. Besides being oxidized, glucose was mainly metabolized via glycerol production and release and lipid synthesis (particularly FFA, triglycerides, and cholesterol), whereas glycogen production was comparatively low. Using targeted metabolomics in INS-1(832/13) cells, we found that several metabolites correlated well with GSIS, in particular some Krebs cycle intermediates, malonyl-CoA, and lower ADP levels. Glucose dose-dependently increased the dihydroxyacetone phosphate/glycerol 3-phosphate ratio in INS-1(832/13) cells, indicating a more oxidized state of NAD in the cytosol upon glucose stimulation. Overall, the data support a role for accelerated oxidative mitochondrial metabolism, anaplerosis, and malonyl-CoA/lipid signaling in β-cell metabolic signaling and suggest that a decrease in ADP levels is important in GSIS. The results also suggest that excess-fuel detoxification pathways in β-cells possibly comprise glycerol and FFA formation and release extracellularly and the diversion of glucose carbons to triglycerides and cholesterol esters.

A role for cytosolic isocitrate dehydrogenase as a negative regulator of glucose signaling for insulin secretion in pancreatic ß-cells

Cytosolic NADPH may act as one of the signals that couple glucose metabolism to insulin secretion in the pancreatic ß-cell. NADPH levels in the cytoplasm are largely controlled by the cytosolic isoforms of malic enzyme and isocitrate dehydrogenase (IDHc). Some studies have provided evidence for a role of malic enzyme in glucose-induced insulin secretion (GIIS) via pyruvate cycling, but the role of IDHc in ß-cell signaling is unsettled. IDHc is an established component of the isocitrate/α-ketoglutarate shuttle that transfers reducing equivalents (NADPH) from the mitochondrion to the cytosol. This shuttle is energy consuming since it is coupled to nicotinamide nucleotide transhydrogenase that uses the mitochondrial proton gradient to produce mitochondrial NADPH and NAD(+) from NADP(+) and NADH. To determine whether flux through IDHc is positively or negatively linked to GIIS, we performed RNAi knockdown experiments in ß-cells. Reduced IDHc expression in INS 832/13 cells and isolated rat islet ß-cells resulted in enhanced GIIS. This effect was mediated at least in part via the KATP-independent amplification arm of GIIS. IDHc knockdown in INS 832/13 cells did not alter glucose oxidation but it reduced fatty acid oxidation and increased lipogenesis from glucose. Metabolome profiling in INS 832/13 cells showed that IDHc knockdown increased isocitrate and NADP(+) levels. It also increased the cellular contents of several metabolites linked to GIIS, in particular some Krebs cycle intermediates, acetyl-CoA, glutamate, cAMP and ATP. The results identify IDHc as a component of the emerging pathways that negatively regulate GIIS.

Identification of Glucose-Dependent Insulin Secretion Targets in Pancreatic β Cells by Combining Defined-Mechanism Compound Library Screening and siRNA Gene Silencing

Identification and validation of novel drug targets continues to be a major bottleneck in drug development, particularly for polygenic complex diseases such as type 2 diabetes. Here, the authors describe an approach that allows researchers to rapidly identify and validate potential drug targets by combining chemical tools and RNA interference technology. As a proof-of-concept study, the known mechanism Sigma LOPAC library was used to screen for glucose-dependent insulin secretion (GDIS) in INS-1 832/13 cells. In addition to several mechanisms that are known to regulate GDIS (such as cyclic adenosine monophosphate—specific phosphodiesterases, adrenoceptors, and Ca2+ channels), the authors find that several of the dopamine receptor ( DRD) antagonists significantly enhance GDIS, whereas DRD agonists profoundly inhibit GDIS. Subsequent siRNA studies in the same cell line indicate that knockdown of DRD2 enhanced GDIS. Furthermore, selective DRD2 antagonists and agonists also enhance or suppress, respectively, GDIS in isolated rat islets. The data support that the approach described here offers a rapid and effective way for target identification and validation. ( Journal of Biomolecular Screening 2008;128-134)

A role for ATP-citrate lyase, malic enzyme, and pyruvate/citrate cycling in glucose-induced insulin secretion

In pancreatic beta-cells, metabolic coupling factors generated during glucose metabolism and pyruvate cycling through anaplerosis/cataplerosis processes contribute to the regulation of insulin secretion. Pyruvate/citrate cycling across the mitochondrial membrane leads to the production of malonyl-CoA and NADPH, two candidate coupling factors. To examine the implication of pyruvate/citrate cycling in glucose-induced insulin secretion (GIIS), different steps of the cycle were inhibited in INS 832/13 cells by pharmacological inhibitors and/or RNA interference (RNAi) technology: mitochondrial citrate export, ATP-citrate lyase (ACL), and cytosolic malic enzyme (ME1). The inhibitors of the di- and tri-carboxylate carriers, n-butylmalonate and 1,2,3-benzenetricarboxylate, respectively, reduced GIIS, indicating the importance of transmitochondrial transport of tri- and dicarboxylates in the action of glucose. To directly test the role of ACL and ME1 in GIIS, small hairpin RNA (shRNA) were used to selectively decrease ACL or ME1 expression in transfected INS 832/13 cells. shRNA-ACL reduced ACL protein levels by 67%, and this was accompanied by a reduction in GIIS. The amplification/K(ATP)-independent pathway of GIIS was affected by RNAi knockdown of ACL. The ACL inhibitor radicicol also curtailed GIIS. shRNA-ME1 reduced ME1 activity by 62% and decreased GIIS. RNAi suppression of either ACL or ME1 did not affect glucose oxidation. However, because ACL is required for malonyl-CoA formation, inhibition of ACL expression by shRNA-ACL decreased glucose incorporation into palmitate and increased fatty acid oxidation in INS 832/13 cells. Taken together, the results underscore the importance of pyruvate/citrate cycling in pancreatic beta-cell metabolic signaling and the regulation of GIIS.

A pyruvate cycling pathway involving cytosolic NADP-dependent isocitrate dehydrogenase regulates glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) from pancreatic islet beta-cells is central to control of mammalian fuel homeostasis. Glucose metabolism mediates GSIS in part via ATP-regulated K+ (KATP) channels, but multiple lines of evidence suggest participation of other signals. Here we investigated the role of cytosolic NADP-dependent isocitrate dehydrogenase (ICDc) in control of GSIS in beta-cells. Delivery of small interfering RNAs specific for ICDc caused impairment of GSIS in two independent robustly glucose-responsive rat insulinoma (INS-1-derived) cell lines and in primary rat islets. Suppression of ICDc also attenuated the glucose-induced increments in pyruvate cycling activity and in NADPH levels, a predicted by-product of pyruvate cycling pathways, as well as the total cellular NADP(H) content. Metabolic profiling of eight organic acids in cell extracts revealed that suppression of ICDc caused increases in lactate production in both INS-1-derived cell lines and primary islets, consistent with the attenuation of pyruvate cycling, with no significant changes in other intermediates. Based on these studies, we propose that a pyruvate cycling pathway involving ICDc plays an important role in control of GSIS.

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.

L-Type Ca2+ channels mediate the slow Ca2+-dependent afterhyperpolarization current in rat CA3 pyramidal cells in vitro

Single-electrode voltage-clamp recordings were obtained from CA3 pyramidal cells in rat hippocampal organotypic slice cultures, and the slow Ca2+-dependent K+ current or afterhyperpolarization current (IAHP) was elicited with brief depolarizing voltage jumps. The slow IAHP was suppressed by the selective L-type Ca2+ channel antagonists isradipine (2 microM) or nifedipine (10 microM). In contrast, neither omega-conotoxin MVIIA (1 microM) nor omega-agatoxin IVA (200 nM), N-type and P/Q-type Ca2+ channel antagonists, respectively, attenuated this slow outward current. The slow IAHP was significantly reduced by thapsigargin (10 microM), a Ca2+ ATPase inhibitor that depletes intracellular Ca2+ stores, and by ryanodine (10-100 microM), which blocks Ca2+-induced Ca2+ release from intracellular compartments. At this concentration thapsigargin did not modify high-threshold Ca2+ current, which was, however, blocked by isradipine. Thus, in hippocampal CA3 pyramidal cells, Ca2+ influx through L-type Ca2+ channels is necessary to trigger the slow IAHP. Furthermore, intracellular Ca2+-activated Ca2+ stores represent a critical component in the transduction pathway leading to the generation of the slow IAHP.

The lifetime of inositol 1,4,5-trisphosphate in single cells

In many eukaryotic cell types, receptor activation leads to the formation of inositol 1,4,5-trisphosphate (IP3) which causes calcium ions (Ca) to be released from internal stores. Ca release was observed in response to the muscarinic agonist carbachol by fura-2 imaging of N1E-115 neuroblastoma cells. Ca release followed receptor activation after a latency of 0.4 to 20 s. Latency was not caused by Ca feedback on IP3 receptors, but rather by IP3 accumulation to a threshold for release. The dependence of latency on carbachol dose was fitted to a model in which IP3 synthesis and degradation compete, resulting in gradual accumulation to a threshold level at which Ca release becomes regenerative. This analysis gave degradation rate constants of IP3 in single cells ranging from 0 to 0.284 s-1 (0.058 +/- 0.067 s-1 SD, 53 cells) and a mean IP3 lifetime of 9.2 +/- 2.2 s. IP3 degradation was also measured directly with biochemical methods. This gave a half life of 9 +/- 2 s. The rate of IP3 degradation sets the time frame over which IP3 accumulations are integrated as input signals. IP3 levels are also filtered over time, and on average, large-amplitude oscillations in IP3 in these cells cannot occur with period < 10 s.