T-type calcium channels facilitate insulin secretion by enhancing general excitability in the insulin-secreting beta-cell line, INS-1

The T-type calcium channel CaV3.2 regulates insulin secretion in the pancreatic β-cell

Voltage-gated Ca²⁺ (Ca_V) channel dysfunction leads to impaired glucose-stimulated insulin secretion in pancreatic β-cells and contributes to the development of type-2 diabetes (T2D). The role of the low-voltage gated T-type Ca_V channels in β-cells remains obscure. Here we have measured the global expression of T-type Ca_V3.2 channels in human islets and found that gene expression of CACNA1H, encoding Ca_V3.2, is negatively correlated with HbA1c in human donors, and positively correlated with islet insulin gene expression as well as secretion capacity in isolated human islets. Silencing or pharmacological blockade of Ca_V3.2 attenuates glucose-stimulated cytosolic Ca²⁺ signaling, membrane potential, and insulin release. Moreover, the endoplasmic reticulum (ER) Ca²⁺ store depletion is also impaired in Ca_V3.2-silenced β-cells. The linkage between T-type (Ca_V3.2) and L-type Ca_V channels is further identified by the finding that the intracellular Ca²⁺ signaling conducted by Ca_V3.2 is highly dependent on the activation of L-type Ca_V channels. In addition, CACNA1H expression is significantly associated with the islet predominant L-type CACNA1C (Ca_V1.2) and CACNA1D (Ca_V1.3) genes in human pancreatic islets. In conclusion, our data suggest the essential functions of the T-type Ca_V3.2 subunit as a mediator of β-cell Ca²⁺ signaling and membrane potential needed for insulin secretion, and in connection with L-type Ca_V channels.

Beta-Cell Ion Channels and Their Role in Regulating Insulin Secretion

Beta cells of the pancreatic islet express many different types of ion channels. These channels reside in the β-cell plasma membrane as well as subcellular organelles and their coordinated activity and sensitivity to metabolism regulate glucose-dependent insulin secretion. Here, we review the molecular nature, expression patterns, and functional roles of many β-cell channels, with an eye toward explaining the ionic basis of glucose-induced insulin secretion. Our primary focus is on K_ATP and voltage-gated Ca²⁺ channels as these primarily regulate insulin secretion; other channels in our view primarily help to sculpt the electrical patterns generated by activated β-cells or indirectly regulate metabolism. Lastly, we discuss why understanding the physiological roles played by ion channels is important for understanding the secretory defects that occur in type 2 diabetes. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Seeking the exclusive binding region of phenylalkylamine derivatives on human T-type calcium channels via homology modeling and molecular dynamics simulation approach

Pharmaceutical features of phenylalkylamine derivatives (PAAs) binding to calcium channels have been studied extensively in the past decades. Only a few PAAs have the binding specificity on calcium channels, for example, NNC 55‐0396. Here, we created the homology models of human Ca_v3.2, Ca_v3.3 and use them as a receptor on the rigid docking tests. The nonspecific calcium channel blocker mibefradil showed inconsistent docking preference across four domains; however, NNC 55‐0396 had a unique binding pattern on domain II specifically. The subsequent molecular dynamics (MD) simulations identified that Ca_v3.1, Ca_v3.2, and Ca_v3.3 share domain II when Ca²⁺ appearing in the neighbor region of selective filters (SFs). Moreover, free‐energy perturbation analysis suggests single mutation of lysine at P‐loop domain III, or threonine at the P‐loop domain II largely reduced the total amount of hydration‐free energy in the system. All these findings suggest that P‐loop and segment six domain II in the T‐type calcium channels (TCCs) are crucial for attracting the PAAs with specificity as the antagonist.

The Transcription Factor Shox2 Shapes Neuron Firing Properties and Suppresses Seizures by Regulation of Key Ion Channels in Thalamocortical Neurons

Thalamocortical neurons (TCNs) play a critical role in the maintenance of thalamocortical oscillations, dysregulation of which can result in certain types of seizures. Precise control over firing rates of TCNs is foundational to these oscillations, yet the transcriptional mechanisms that constrain these firing rates remain elusive. We hypothesized that Shox2 is a transcriptional regulator of ion channels important for TCN function and that loss of Shox2 alters firing frequency and activity, ultimately perturbing thalamocortical oscillations into an epilepsy-prone state. In this study, we used RNA sequencing and quantitative PCR of control and Shox2 knockout mice to determine Shox2-affected genes and revealed a network of ion channel genes important for neuronal firing properties. Protein regulation was confirmed by Western blotting, and electrophysiological recordings showed that Shox2 KO impacted the firing properties of a subpopulation of TCNs. Computational modeling showed that disruption of these conductances in a manner similar to Shox2's effects modulated frequency of oscillations and could convert sleep spindles to near spike and wave activity, which are a hallmark for absence epilepsy. Finally, Shox2 KO mice were more susceptible to pilocarpine-induced seizures. Overall, these results reveal Shox2 as a transcription factor important for TCN function in adult mouse thalamus.

Divalent cation influx and calcium homeostasis in germinal vesicle mouse oocytes

Prior to maturation, mouse oocytes are arrested at the germinal vesicle (GV) stage during which they experience constitutive calcium (Ca²⁺) influx and spontaneous Ca²⁺ oscillations. The oscillations cease during maturation but Ca²⁺ influx continues, as the oocytes' internal stores attain maximal content at the culmination of maturation, the metaphase II stage. The identity of the channel(s) that underlie this Ca²⁺ influx has not been completely determined. GV and matured oocytes are known to express three Ca²⁺ channels, Ca_V3.2, TRPV3 and TRPM7, but females null for each of these channels are fertile and their oocytes display minor modifications in Ca²⁺ homeostasis, suggesting a complex regulation of Ca²⁺ influx. To define the contribution of these channels at the GV stage, we used different divalent cations, pharmacological inhibitors and genetic models. We found that the three channels are active at this stage. Ca_V3.2 and TRPM7 channels contributed the majority of Ca²⁺ influx, as inhibitors and oocytes from homologous knockout (KO) lines showed severely reduced Ca²⁺ entry. Sr²⁺ influx was promoted by Ca_V3.2 channels, as Sr²⁺ oscillations were negligible in Ca_V3.2-KO oocytes but robust in control and Trpv3-KO GV oocytes. Mn²⁺ entry relied on expression of Ca_V3.2 and TRPM7 channels, but Ni²⁺ entry depended on the latter. Ca_V3.2 and TRPV3 channels combined to fill the Ca²⁺ stores, although Ca_V3.2 was the most impactful. Studies with pharmacological inhibitors effectively blocked the influx of divalent cations, but displayed off-target effects, and occasionally agonist-like properties. In conclusion, GV oocytes express channels mediating Ca²⁺ and other divalent cation influx that are pivotal for fertilization and early development. These channels may serve as targets for intervention to improve the success of assisted reproductive technologies.

Enhanced expression of β cell CaV3.1 channels impairs insulin release and glucose homeostasis

We reveal that increased expression of Ca_V3.1 channels in rat islets selectively impairs first-phase glucose-stimulated insulin secretion. This deterioration is recapitulated in human islets. Its causal role in diabetes development is clearly manifested in an in vivo diabetic model. Mechanistically, this is due to reduction of phosphorylated FoxO1 in the cytoplasm, elevated FoxO1 nuclear retention, and decreased syntaxin 1A, SNAP-25, and synaptotagmin III in a Ca_V3.1 channel- and calcineurin-dependent manner. Our findings suggest that elevated expression of Ca_V3.1 channels in pancreatic islets drives FoxO1-mediated down-regulation of exocytotic proteins resulting in the diabetic phenotypes of impaired insulin secretion and aberrant glucose homeostasis. This causal connection pinpoints β cell Ca_V3.1 channels as a potential druggable target for antidiabetes therapy.

Voltage-gated calcium 3.1 (Ca_V3.1) channels are absent in healthy mouse β cells and mediate minor T-type Ca²⁺ currents in healthy rat and human β cells but become evident under diabetic conditions. Whether more active Ca_V3.1 channels affect insulin secretion and glucose homeostasis remains enigmatic. We addressed this question by enhancing de novo expression of β cell Ca_V3.1 channels and exploring the consequent impacts on dynamic insulin secretion and glucose homeostasis as well as underlying molecular mechanisms with a series of in vitro and in vivo approaches. We now demonstrate that a recombinant adenovirus encoding enhanced green fluorescent protein–Ca_V3.1 subunit (Ad-EGFP-Ca_V3.1) efficiently transduced rat and human islets as well as dispersed islet cells. The resulting Ca_V3.1 channels conducted typical T-type Ca²⁺ currents, leading to an enhanced basal cytosolic-free Ca²⁺ concentration ([Ca²⁺]_i). Ad-EGFP-Ca_V3.1-transduced islets released significantly less insulin under both the basal and first phases following glucose stimulation and could no longer normalize hyperglycemia in recipient rats rendered diabetic by streptozotocin treatment. Furthermore, Ad-EGFP-Ca_V3.1 transduction reduced phosphorylated FoxO1 in the cytoplasm of INS-1E cells, elevated FoxO1 nuclear retention, and decreased syntaxin 1A, SNAP-25, and synaptotagmin III. These effects were prevented by inhibiting Ca_V3.1 channels or the Ca²⁺-dependent phosphatase calcineurin. Enhanced expression of β cell Ca_V3.1 channels therefore impairs insulin release and glucose homeostasis by means of initial excessive Ca²⁺ influx, subsequent activation of calcineurin, consequent dephosphorylation and nuclear retention of FoxO1, and eventual FoxO1-mediated down-regulation of β cell exocytotic proteins. The present work thus suggests an elevated expression of Ca_V3.1 channels plays a significant role in diabetes pathogenesis.

Oxidative Stress Induces Disruption of the Axon Initial Segment

The axon initial segment (AIS), the domain responsible for action potential initiation and maintenance of neuronal polarity, is targeted for disruption in a variety of central nervous system pathological insults. Previous work in our laboratory implicates oxidative stress as a potential mediator of structural AIS alterations in two separate mouse models of central nervous system inflammation, as these effects were attenuated following reactive oxygen species scavenging and NADPH oxidase-2 ablation. While these studies suggest a role for oxidative stress in modulation of the AIS, the direct effects of reactive oxygen and nitrogen species (ROS/RNS) on the stability of this domain remain unclear. Here, we demonstrate that oxidative stress, as induced through treatment with 3-morpholinosydnonimine (SIN-1), a spontaneous ROS/RNS generator, drives a reversible loss of AIS protein clustering in primary cortical neurons in vitro. Pharmacological inhibition of both voltage-dependent and intracellular calcium (Ca²⁺) channels suggests that this mechanism of AIS disruption involves Ca²⁺ entry specifically through L-type voltage-dependent Ca²⁺ channels and its release from IP₃-gated intracellular stores. Furthermore, ROS/RNS-induced AIS disruption is dependent upon activation of calpain, a Ca²⁺-activated protease previously shown to drive AIS modulation. Overall, we demonstrate for the first time that oxidative stress, as induced through exogenously applied ROS/RNS, is capable of driving structural alterations in the AIS complex.

Calcium Channels in Postnatal Development of Rat Pancreatic Beta Cells and Their Role in Insulin Secretion

Pancreatic beta cells during the first month of development acquire functional maturity, allowing them to respond to variations in extracellular glucose concentration by secreting insulin. Changes in ionic channel activity are important for this maturation. Within the voltage-gated calcium channels (VGCC), the most studied channels are high-voltage-activated (HVA), principally L-type; while low-voltage-activated (LVA) channels have been poorly studied in native beta cells. We analyzed the changes in the expression and activity of VGCC during the postnatal development in rat beta cells. We observed that the percentage of detection of T-type current increased with the stage of development. T-type calcium current density in adult cells was higher than in neonatal and P20 beta cells. Mean HVA current density also increased with age. Calcium current behavior in P20 beta cells was heterogeneous; almost half of the cells had HVA current densities higher than the adult cells, and this was independent of the presence of T-type current. We detected the presence of α1G, α1H, and α1I subunits of LVA channels at all ages. The Cav 3.1 subunit (α1G) was the most expressed. T-type channel blockers mibefradil and TTA-A₂ significantly inhibited insulin secretion at 5.6 mM glucose, which suggests a physiological role for T-type channels at basal glucose conditions. Both, nifedipine and TTA-A₂, drastically decreased the beta-cell subpopulation that secretes more insulin, in both basal and stimulating glucose conditions. We conclude that changes in expression and activity of VGCC during the development play an important role in physiological maturation of beta cells.

Calcium modulation of exocytosis-linked plasma membrane potential oscillations in INS-1 832/13 cells

Oscillations in plasma membrane potential initiated by substrate-dependent blockade of ATP-sensitive K+ channels in insulin-secreting INS-1 832/13 are differentially linked to distinct voltage-activated Ca2+ channels and drive exocytosis. Ca2+ feeds back to control oscillation frequency, amplitude and prevalence.

Ca2+ influx-mediated dilation of the endoplasmic reticulum and c-FLIPL downregulation trigger CDDO-Me-induced apoptosis in breast cancer cells

The synthetic triterpenoid 2-cyano-3, 12-dioxooleana-1, 9(11)-dien-C28-methyl ester (CDDO-Me) is considered a promising anti-tumorigenic compound. In this study, we show that treatment with CDDO-Me induces progressive endoplasmic reticulum (ER)-derived vacuolation in various breast cancer cells and ultimately kills these cells by inducing apoptosis. We found that CDDO-Me-induced increases in intracellular Ca2+ levels, reflecting influx from the extracellular milieu, make a critical contribution to ER-derived vacuolation and subsequent cell death. In parallel with increasing Ca2+ levels, CDDO-Me markedly increased the generation of reactive oxygen species (ROS). Interestingly, there exists a reciprocal positive-regulatory loop between Ca2+ influx and ROS generation that triggers ER stress and ER dilation in response to CDDO-Me. In addition, CDDO-Me rapidly reduced the protein levels of c-FLIPL (cellular FLICE-inhibitory protein) and overexpression of c-FLIPL blocked CDDO-Me-induced cell death, but not vacuolation. These results suggest that c-FLIPL downregulation is a key contributor to CDDO-Me-induced apoptotic cell death, independent of ER-derived vacuolation. Taken together, our results show that ER-derived vacuolation via Ca2+ influx and ROS generation as well as caspase activation via c-FLIPL downregulation are responsible for the potent anticancer effects of CDDO-Me on breast cancer cells.

Lipopolysaccharides upregulate calcium concentration in mouse uterine smooth muscle cells through the T-type calcium channels

Infection is a significant cause of preterm birth. Abnormal changes in intracellular calcium signals are the ultimate triggers of early uterine contractions that result in preterm birth. T‑type calcium channels play an important role in the pathogenesis of cancer, as well as endocrine and cardiovascular diseases. However, there are limited studies on their role in uterine contractions and parturition. In the present study, mouse uterine smooth muscle cells were isolated and treated with lipopolysaccharides (LPS) to mimic the microenvironment of uterine infection in vitro to investigate the role of T‑type calcium channels in the process of infection‑induced preterm birth. The results from quantitative polymerase chain reaction and western blot analysis showed that LPS significantly induced the expression of the Cav3.1 and Cav3.2 subtypes of T‑type calcium channels. Measurements of intracellular calcium concentration showed a significant increase in response to LPS. However, these effects can be reversed by T‑type calcium channel blockers. Western blot analysis further indicated that LPS induced the activation of the nuclear factor (NF)‑κB signaling pathway, and endothelin‑1 (ET‑1) was significantly upregulated, whereas NF‑κB inhibitors significantly inhibited the LPS‑induced upregulation of Cav3.1, Cav3.2 and ET‑1 expression. In addition, ET‑1 directly induced Cav3.1 and Cav3.2 expression, whereas ET‑1 antagonists inhibited the LPS‑induced upregulation of Cav3.1 and Cav3.2 expression. In conclusion, the present study demonstrates that infection triggers the upregulation of T‑type calcium channels and promotes calcium influx. This process relies on the activation of the NF‑κB/ET‑1 signaling pathway. The T‑type calcium channel is expected to become an effective target for the prevention of infection‑induced preterm birth.

Ionic mechanisms in pancreatic β cell signaling

The function and survival of pancreatic β cells critically rely on complex electrical signaling systems composed of a series of ionic events, namely fluxes of K(+), Na(+), Ca(2+) and Cl(-) across the β cell membranes. These electrical signaling systems not only sense events occurring in the extracellular space and intracellular milieu of pancreatic islet cells, but also control different β cell activities, most notably glucose-stimulated insulin secretion. Three major ion fluxes including K(+) efflux through ATP-sensitive K(+) (KATP) channels, the voltage-gated Ca(2+) (CaV) channel-mediated Ca(2+) influx and K(+) efflux through voltage-gated K(+) (KV) channels operate in the β cell. These ion fluxes set the resting membrane potential and the shape, rate and pattern of firing of action potentials under different metabolic conditions. The KATP channel-mediated K(+) efflux determines the resting membrane potential and keeps the excitability of the β cell at low levels. Ca(2+) influx through CaV1 channels, a major type of β cell CaV channels, causes the upstroke or depolarization phase of the action potential and regulates a wide range of β cell functions including the most elementary β cell function, insulin secretion. K(+) efflux mediated by KV2.1 delayed rectifier K(+) channels, a predominant form of β cell KV channels, brings about the downstroke or repolarization phase of the action potential, which acts as a brake for insulin secretion owing to shutting down the CaV channel-mediated Ca(2+) entry. These three ion channel-mediated ion fluxes are the most important ionic events in β cell signaling. This review concisely discusses various ionic mechanisms in β cell signaling and highlights KATP channel-, CaV1 channel- and KV2.1 channel-mediated ion fluxes.

Angiotensin type 2 receptor in pancreatic islets of adult rats: a novel insulinotropic mediator

In the present study, we evaluated the relative abundance of angiotensin type 2 receptor (AT2R) protein in various tissues of adult rats. We found that pancreatic islets expressed the highest AT2R protein compared with all other tissues. Accordingly, we then determined the functional significance of AT2R in the endocrine pancreas in in vivo and in vitro experiments by using angiotensin II (ANG II) alone, losartan (Los; AT1R antagonist), compound 21 (C21; AT2R agonist), and PD-123319 (PD; AT2R antagonist). Experiments carried out in rats indicated that, 1) ANG II treatment significantly increased plasma insulin concentration (1.51 ± 0.20 vs. 0.82 ± 0.14 ng/ml, n = 7, P < 0.05) in the fed state. This insulinotropic effect was further augmented by combined treatment with ANG II + Los (2.31 ± 0.25 ng/ml, n = 7, P < 0.01). C21 also elevated insulin levels (2.13 ± 0.20 ng/ml, n = 7, P < 0.01), which was completely abolished by PD. 2) ANG II impaired glucose tolerance, whereas ANG II + Los or C21 improved this function. 3) All treated rats displayed an enhanced insulin secretory response to a glucose challenge. 4) All treated rats displayed upregulated proinsulin 2 mRNA and insulin protein expression in the pancreas. In in vitro experiments using INS-1E cells and isolated rat islets, we found that AT2R activation significantly improved insulin biosynthesis and secretion. These results suggest that the AT2R functions as an insulinotropic mediator. AT2R and its downstream signaling pathways may be potential therapeutic targets for diabetes.

Hydrogen sulfide-induced inhibition of L-type Ca2+ channels and insulin secretion in mouse pancreatic beta cells

AIMS/HYPOTHESIS

L-type voltage-dependent Ca(2+) channels (VDCCs) in pancreatic beta cells play a critical role in regulating insulin secretion. The gasotransmitter H(2)S is mostly generated from L-cysteine in pancreatic beta cells by cystathionine γ-lyase (CSE) and has been reported to inhibit insulin release by opening ATP-sensitive K(+) channels. However, whether and how H(2)S affects VDCCs in beta cells is unknown.

METHODS

The whole-cell patch-clamp technique was used to record VDCCs in beta cells from Cse (also known as Cth)-knockout (KO) and wild-type (WT) mice. Insulin secretion from pancreatic islets and endogenous H(2)S production in pancreas were measured.

RESULTS

The H(2)S donor NaHS reversibly decreased L-type VDCC current density in a concentration-dependent fashion in WT pancreatic beta cells, and the current density was further inhibited by nifedipine. Furthermore, NaHS inhibited the channel recovery from depolarisation-induced inactivation, but did not shift the current-voltage (I-V) relationship. ACS67, another H(2)S donor, also inhibited L-type VDCCs in beta cells. Inhibiting CSE activity with DL-propargylglycine increased the basal L-channel activity of beta cells from WT mice, but not that of beta cells from Cse-KO mice. Beta cells from Cse-KO mice displayed higher L-type VDCC density than those from WT mice. Insulin secretion from pancreatic islets was elevated in Cse-KO mice compared with WT mice. NaHS dose-dependently inhibited glucose-stimulated insulin secretion, which was further inhibited by nifedipine. Bay K-8644 increased glucose-stimulated insulin secretion, but this was counteracted by NaHS and nifedipine.

CONCLUSIONS/INTERPRETATION

Exogenous and endogenous H(2)S inhibit L-type VDCC activity and pancreatic insulin secretion, constituting a novel mechanism for the regulation of insulin secretion by the CSE/H(2)S system.

The antimicrobial peptide LL-37 alters human osteoblast Ca2+ handling and induces Ca2+-independent apoptosis

The human antimicrobial peptide cathelicidin LL-37 has, besides its antimicrobial properties, also been shown to regulate apoptosis in a cell type-specific manner. Mechanisms involved in LL-37-regulated apoptotic signaling are not identified. Here, we show that LL-37 reduces the human osteoblast-like MG63 cell number and cell viability in the micromolar concentration range with an IC50 value of about 5 µM. Treatment with 4 µM LL-37 increased the number of annexin V-positive cells and stimulated activation of caspase 3 showing that LL-37 promotes apoptosis. Treatment with 4 µM LL-37 caused an acute and sustained rise in intracellular Ca(2+) concentration assessed by laser-scanning confocal microscopy of Fluo-4-AM-loaded MG63 cells. LL-37 increased Ca(2+) also in the presence of the respective L- and T-type voltage-sensitive Ca(2+) channel blockers nifedipine and NiCl2. LL-37 had no effect on Ca(2+) in cells incubated with Ca(2+)-free solution. LL-37 (4 and 8 µM) reduced the MG63 cell number both in the presence and absence of Ca(2+) in the medium. In conclusion, LL-37 reduces the osteoblast cell number by promoting apoptosis, and furthermore, LL-37 stimulates Ca(2+) inflow via a mechanism independent of voltage-sensitive Ca(2+) channels. Interestingly, LL-37-induced lowering of the cell number seems to be mediated via a mechanism independent of Ca(2+).

Ion channels and regulation of insulin secretion in human β-cells: a computational systems analysis

In mammals an increase in glucose leads to block of ATP dependent potassium channels in pancreatic β cells leading to membrane depolarization. This leads to the repetitive firing of action potentials that increases calcium influx and triggers insulin granule exocytosis. Several important differences between species in this process suggest that a dedicated human-oriented approach is advantageous as extrapolating from rodent data may be misleading in several respects. We examined depolarization-induced spike activity in pancreatic human islet-attached β-cells employing whole-cell patch-clamp methods. We also reviewed the literature concerning regulation of insulin secretion by channel activity and constructed a data-based computer model of human β cell function. The model couples the Hodgkin-Huxley-type ionic equations to the equations describing intracellular Ca²⁺ homeostasis and insulin release. On the basis of this model we employed computational simulations to better understand the behavior of action potentials, calcium handling and insulin secretion in human β cells under a wide range of experimental conditions. This computational system approach provides a framework to analyze the mechanisms of human β cell insulin secretion.

Thiopental-induced insulin secretion via activation of IP3-sensitive calcium stores in rat pancreatic β-cells

While glucose-stimulated insulin secretion depends on Ca(2+) influx through voltage-gated Ca(2+) channels in the cell membrane of the pancreatic β-cell, there is also ample evidence for an important role of intracellular Ca(2+) stores in insulin secretion, particularly in relation to drug stimuli. We report here that thiopental, a common anesthetic agent, triggers insulin secretion from the intact pancreas and primary cultured rat pancreatic β-cells. We investigated the underlying mechanisms by measurements of whole cell K(+) and Ca(2+) currents, membrane potential, cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), and membrane capacitance. Thiopental-induced insulin secretion was first detected by enzyme-linked immunoassay, then further assessed by membrane capacitance measurement, which revealed kinetics distinct from glucose-induced insulin secretion. The thiopental-induced secretion was independent of cell membrane depolarization and closure of ATP-sensitive potassium (K(ATP)) channels. However, accompanied by the insulin secretion stimulated by thiopental, we recorded a significant intracellular [Ca(2+)] increase that was not from Ca(2+) influx across the cell membrane, but from intracellular Ca(2+) stores. The thiopental-induced [Ca(2+)](i) rise in β-cells was sensitive to thapsigargin, a blocker of the endoplasmic reticulum Ca(2+) pump, as well as to heparin (0.1 mg/ml) and 2-aminoethoxydiphenyl borate (2-APB; 100 μM), drugs that inhibit inositol 1,4,5-trisphosphate (IP(3)) binding to the IP(3) receptor, and to U-73122, a phospholipase C inhibitor, but insensitive to ryanodine. Thapsigargin also diminished thiopental-induced insulin secretion. Thus, we conclude that thiopental-induced insulin secretion is mediated by activation of the intracellular IP(3)-sensitive Ca(2+) store.

Intracellular serotonin modulates insulin secretion from pancreatic beta-cells by protein serotonylation

Non-neuronal, peripheral serotonin deficiency causes diabetes mellitus and identifies an intracellular role for serotonin in the regulation of insulin secretion.

While serotonin (5-HT) co-localization with insulin in granules of pancreatic β-cells was demonstrated more than three decades ago, its physiological role in the etiology of diabetes is still unclear. We combined biochemical and electrophysiological analyses of mice selectively deficient in peripheral tryptophan hydroxylase (Tph1−/−) and 5-HT to show that intracellular 5-HT regulates insulin secretion. We found that these mice are diabetic and have an impaired insulin secretion due to the lack of 5-HT in the pancreas. The pharmacological restoration of peripheral 5-HT levels rescued the impaired insulin secretion in vivo. These findings were further evidenced by patch clamp experiments with isolated Tph1−/− β-cells, which clearly showed that the secretory defect is downstream of Ca²⁺-signaling and can be rescued by direct intracellular application of 5-HT via the clamp pipette. In elucidating the underlying mechanism further, we demonstrate the covalent coupling of 5-HT by transglutaminases during insulin exocytosis to two key players in insulin secretion, the small GTPases Rab3a and Rab27a. This renders them constitutively active in a receptor-independent signaling mechanism we have recently termed serotonylation. Concordantly, an inhibition of such activating serotonylation in β-cells abates insulin secretion. We also observed inactivation of serotonylated Rab3a by enhanced proteasomal degradation, which is in line with the inactivation of other serotonylated GTPases. Our results demonstrate that 5-HT regulates insulin secretion by serotonylation of GTPases within pancreatic β-cells and suggest that intracellular 5-HT functions in various microenvironments via this mechanism in concert with the known receptor-mediated signaling.

Diabetes is the most prevalent metabolic disease and one that affects individuals of every social and economic status. The disease can arise as a result of reduced secretion of insulin from pancreatic β-cells or reduced action of insulin on its target organs. Therefore, understanding how to prevent and treat diabetes requires an extensive knowledge of the regulation of insulin secretion. In this study, we identify the hormone serotonin as a new regulator of insulin secretion and thereby attribute a function to the co-localization of serotonin and insulin in pancreatic β-cells that was first observed 30 years ago but until now not understood. We first demonstrate that a lack of serotonin in β-cells of transgenic mice leads to reduced insulin secretion and diabetes mellitus and that pharmacological replenishment of serotonin rescues insulin secretion in these mice. Interestingly, serotonin mainly acts not as an intercellular signaling molecule via its traditional surface receptors but intracellularly via regulation of the activity of target proteins through covalent coupling of serotonin to them. This coupling, called serotonylation, activates specific small GTPases, which in turn promote glucose-mediated insulin secretion. Adding this receptor-independent signaling mechanism to the multifarious regulatory functions of serotonin, we hypothesize that protein serotonylation modulates physiological secretion processes in all serotonin-containing tissues.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces calcium influx through T-type calcium channel and enhances lysosomal exocytosis and insulin secretion in INS-1 cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been associated with diabetes in several epidemiological studies. However, the diabetogenic action of TCDD on pancreatic cells is unclear. Here, we investigated the direct toxic effects of TCDD on a rat insulin-secreting beta cell line. We found that TCDD enhances exocytosis of MTT formazan and lysosomal proteins such as beta-hexosaminindase and Lamp-1. This TCDD-induced exocytosis was abrogated by T-type calcium channel blockers (mibefradil, flunarizine) but not by an aryl hydrocarbon receptor antagonist (alpha-naphtoflavone). Indeed, cytosolic calcium levels were increased by TCDD. Furthermore, TCDD stimulated insulin secretion, which was inhibited by flunarizine. Taken together, our results suggest that TCDD-induced calcium influx via T-type channels regulates vesicular trafficking, such as lysosomal and secretory granule exocytosis, and that TCDD might exert adverse effects on beta cells by continuous insulin release followed by beta cell exhaustion. This could contribute to the link between TCDD exposure and the risk of developing diabetes.

Channel regulation of glucose sensing in the pancreatic beta-cell

Mammalian beta-cells are acutely and chronically regulated by sensing surrounding glucose levels that determine the rate at which insulin is secreted, to maintain euglycemia. Experimental research in vitro and in vivo has shown that, when these cells are exposed to adverse conditions like long periods of hypoglycemia or hyperglycemia, their capability to sense glucose is decreased. Understanding the normal physiology and identifying the main players along this route becomes paramount. In this review, we have taken on the task of looking at the role that ion channels play in the regulation of this process, delineating the different families, and describing the signaling that parallels the glucose sensing process that results in insulin release.

Characterization of voltage operated R-type Ca2+ channels in modulating somatostatin receptor subtype 2- and 3-dependent inhibition of insulin secretion from INS-1 cells

Somatostatin (SST) inhibits Ca(2+) entry into pancreatic B-cells via voltage-operated Ca(2+) channels (VOCCs) of L-type, leading to the suppression of insulin secretion. Activation of R-type channels increases insulin secretion. However, the role of R-type Ca(2+) channels (Ca(V)2.3) in mediating the effects of SST on insulin secretion has not been so far investigated. Here, we identify the SST-receptor subtypes (SSTR) expressed on insulin-producing INS-1 cells by RT-PCR and by functional assays. The role of R-type channels in regulating [Ca(2+)](i) in response to SST-treatment was detected by cell fluorescence imaging and patch-clamp technique. INS-1 expressed SSTR2 and SSTR3 and agonists (ag.) selective for these receptors reduced 10 nM exendin-4/20 mM glucose-stimulated insulin secretion. Surprisingly, SST and SST2-ag. transiently increased [Ca(2+)](i). Subsequently, these agonists led to a decrease in [Ca(2+)](i) below the basal levels. In contrast, SST3-ag. failed to induce a transient peak of [Ca(2+)](i). Instead, a persistent minor suppression of [Ca(2+)](i) was detected from 25 min. R-type channel blocker SNX-482 altered [Ca(2+)](i) in SST- and SST2-ag.-treated cells. Notably, the inhibition of insulin secretion by SST and SST2-ag., but not SST3-ag. was attenuated by SNX-482. Taken together, SST and SSTR2 regulate [Ca(2+)](i) and insulin secretion in INS-1 cells via R-type channels. In contrast, the R-type calcium channel does not mediate the effects of SST3-ag. on insulin secretion. We conclude that R-type channels play a major role in the inhibition of insulin secretion by somatostatin in INS-1 cells.

Physiological development of insulin secretion, calcium channels, and GLUT2 expression of pancreatic rat beta-cells

Insulin secretion in mature beta-cells increases vigorously when extracellular glucose concentration rises. Glucose-stimulated insulin secretion depends on Ca(2+) influx through voltage-gated Ca(2+) channels. During fetal development, this structured response is not well established, and it is after birth that beta-cells acquire glucose sensitivity and a robust secretion. We compared some elements of glucose-induced insulin secretion coupling in beta-cells obtained from neonatal and adult rats and found that neonatal cells are functionally immature compared with adult cells. We observed that neonatal cells secrete less insulin and cannot sense changes in extracellular glucose concentrations. This could be partially explained because in neonates Ca(2+) current density and synthesis of mRNA alpha1 subunit Ca(2+) channel are lower than in adult cells. Interestingly, immunostaining for alpha1B, alpha1C, and alpha1D subunits in neonatal cells is similar in cytoplasm and plasma membrane, whereas it occurs predominantly in the plasma membrane in adult cells. We also observed that GLUT2 expression in adult beta-cells is mostly located in the membrane, whereas in neonatal cells glucose transporters are predominantly in the cytoplasm. This could explain, in part, the insensitivity to extracellular glucose in neonatal beta-cells. Understanding neonatal beta-cell physiology and maturation contributes toward a better comprehension of type 2 diabetes physiopathology, where alterations in beta-cells include diminished L-type Ca(2+) channels and GLUT2 expression that results in an insufficient insulin secretion.

The role of voltage-gated calcium channels in pancreatic beta-cell physiology and pathophysiology

Voltage-gated calcium (CaV) channels are ubiquitously expressed in various cell types throughout the body. In principle, the molecular identity, biophysical profile, and pharmacological property of CaV channels are independent of the cell type where they reside, whereas these channels execute unique functions in different cell types, such as muscle contraction, neurotransmitter release, and hormone secretion. At least six CaValpha1 subunits, including CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1, have been identified in pancreatic beta-cells. These pore-forming subunits complex with certain auxiliary subunits to conduct L-, P/Q-, N-, R-, and T-type CaV currents, respectively. beta-Cell CaV channels take center stage in insulin secretion and play an important role in beta-cell physiology and pathophysiology. CaV3 channels become expressed in diabetes-prone mouse beta-cells. Point mutation in the human CaV1.2 gene results in excessive insulin secretion. Trinucleotide expansion in the human CaV1.3 and CaV2.1 gene is revealed in a subgroup of patients with type 2 diabetes. beta-Cell CaV channels are regulated by a wide range of mechanisms, either shared by other cell types or specific to beta-cells, to always guarantee a satisfactory concentration of Ca2+. Inappropriate regulation of beta-cell CaV channels causes beta-cell dysfunction and even death manifested in both type 1 and type 2 diabetes. This review summarizes current knowledge of CaV channels in beta-cell physiology and pathophysiology.

Low-threshold exocytosis induced by cAMP-recruited CaV3.2 (alpha1H) channels in rat chromaffin cells

We have studied the functional role of CaV3 channels in triggering fast exocytosis in rat chromaffin cells (RCCs). CaV3 T-type channels were selectively recruited by chronic exposures to cAMP (3 days) via an exchange protein directly activated by cAMP (Epac)-mediated pathway. Here we show that cAMP-treated cells had increased secretory responses, which could be evoked even at very low depolarizations (-50, -40 mV). Potentiation of exocytosis in cAMP-treated cells did not occur in the presence of 50 microM Ni2+, which selectively blocks T-type currents in RCCs. This suggests that the "low-threshold exocytosis" induced by cAMP is due to increased Ca2+ influx through cAMP-recruited T-type channels, rather than to an enhanced secretion downstream of Ca2+ entry, as previously reported for short-term cAMP treatments (20 min). Newly recruited T-type channels increase the fast secretory response at low voltages without altering the size of the immediately releasable pool. They also preserve the Ca2+ dependence of exocytosis, the initial speed of vesicle depletion, and the mean quantal size of single secretory events. All this indicates that cAMP-recruited CaV3 channels enhance the secretory activity of RCCs at low voltages by coupling to the secretory apparatus with a Ca2+ efficacy similar to that of already existing high-threshold Ca2+ channels. Finally, using RT-PCRs we found that the fast inactivating low-threshold Ca2+ current component recruited by cAMP is selectively associated to the alpha1H (CaV3.2) channel isoform.

Towards selective antagonists of T-type calcium channels: design, characterization and potential applications of NNC 55-0396

NNC 55-0396 is a structural analog of mibefradil (Ro 40-5967) that inhibits both T-type and high-voltage-activated (HVA) Ca2+ channels with a higher selectivity for T-type Ca2+ channels. The inhibitory effect of mibefradil on HVA Ca2+ channels can be attributed to a hydrolyzed metabolite of the drug: the methoxy acetate side chain of mibefradil is removed by intracellular enzymes, thus it forms (1S,2S)-2-(2-(N-[(3-benzoimidazol-2-yl)propyl]-N-methylamino)ethyl)-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-naphtyl hydroxy dihydrochloride (dm-mibefradil), which causes potent inhibition of HVA Ca2+ currents. By replacing the methoxy acetate chain of mibefradil with cyclopropanecarboxylate, a more stable analog was developed (NNC 55-0396). The acute IC50 of NNC 55-0396 to block recombinant Cav3.1 T-type channels expressed in HEK293 cells is approximately 7 muM, whereas 100 microM NNC 55-0396 has no detectable effect on high voltage-activated currents in INS-1 cells. Block of T-type Ca2+ current was partially reduced by membrane hyperpolarization and was enhanced at high stimulus frequency. Washing NNC 55-0396 out of the recording chamber did not reverse the T-type Ca2+ current activity, suggesting that the compound dissolves in or passes through the plasma membrane to exert its effect; however, intracellular perfusion of the compound did not block T-type Ca2+ currents, arguing against a cytoplasmic route of action. We conclude that NNC 55-0396, by virtue of its modified structure, does not produce the metabolite that causes inhibition of L-type Ca2+ channel channels, thus rendering it more selective to T-type Ca2+ channels.

Role of high-voltage-activated calcium channels in glucose-regulated beta-cell calcium homeostasis and insulin release

High-voltage-activated (HVA) calcium channels are known to be the primary source of calcium for glucose-stimulated insulin secretion. However, few studies have investigated how these channels can be regulated by chronically elevated levels of glucose. In the present study, we determined the level of expression of the four major HVA calcium channels (N-type, P/Q-type, L(C)-type, and L(D)-type) in rat pancreatic beta-cells. Using quantitative real-time PCR (QRT-PCR), we found the expression of all four HVA genes in rat insulinoma cells (INS-1) and in primary isolated rat islet cells. We then determined the role of each channel in insulin secretion by using channel-selective antagonists. Insulin secretion analysis revealed that N- and L-type channels are both involved in immediate glucose-induced insulin secretion. However, L-type was preferentially coupled to secretion at later time points. P/Q-type channels were not found to play a role in insulin secretion at any stage. It was also found that long-term exposure to elevated glucose increases basal calcium in these cells. Interestingly, chronically elevated glucose decreased the mRNA expression of the channels involved with insulin secretion and diminished the level of stimulated calcium influx in these cells. Using whole cell patch clamp, we found that N- and L-type channel currents increase gradually subsequent to lower intracellular calcium perfusion, suggesting that these channels may be regulated by glucose-induced changes in calcium.

Beta-cell CaV channel regulation in physiology and pathophysiology

The beta-cell is equipped with at least six voltage-gated Ca2+ (CaV) channel alpha1-subunits designated CaV1.2, CaV1.3, CaV2.1, CaV2.2, CaV2.3, and CaV3.1. These principal subunits, together with certain auxiliary subunits, assemble into different types of CaV channels conducting L-, P/Q-, N-, R-, and T-type Ca2+ currents, respectively. The beta-cell shares customary mechanisms of CaV channel regulation with other excitable cells, such as protein phosphorylation, Ca2+-dependent inactivation, and G protein modulation. However, the beta-cell displays some characteristic features to bring these mechanisms into play. In islet beta-cells, CaV channels can be highly phosphorylated under basal conditions and thus marginally respond to further phosphorylation. In beta-cell lines, CaV channels can be surrounded by tonically activated protein phosphatases dominating over protein kinases; thus their activity is dramatically enhanced by inhibition of protein phosphatases. During the last 10 years, we have revealed some novel mechanisms of beta-cell CaV channel regulation under physiological and pathophysiological conditions, including the involvement of exocytotic proteins, inositol hexakisphosphate, and type 1 diabetic serum. This minireview highlights characteristic features of customary mechanisms of CaV channel regulation in beta-cells and also reviews our studies on newly identified mechanisms of beta-cell CaV channel regulation.

Pathophysiological Significance of T-type Ca2+ Channels: T-type Ca2+ Channels and Drug Development

T-type Ca(2+) channels are present in cardiovascular, neuronal, and endocrine systems; and they are now receiving attention as novel therapeutic targets. Many drugs and compounds non-specificaly block T-type Ca(2+) channels. Certain dihydropyridine compounds, such as efonidipine, have blocking activity on both L-type and T-type Ca(2+) channels which possibly underlies their excellent clinical profiles such as minimum reflex tachycardia and renal protection. Selective inhibitors of T-type Ca(2+) channels, such as non-hydrolyzable mibefradil and R(-)-efonidipine, are powerful pharmacological tools for further studies and may lead to the development of novel therapeutic strategies.

Cav3.1 (alpha1G) T-type Ca2+ channels mediate vaso-occlusion of sickled erythrocytes in lung microcirculation

In the present study, we demonstrate that lung microvascular endothelial cells express a Cav3.1 (alpha1G) T-type voltage-gated Ca2+ channel, whereas lung macrovascular endothelial cells do not express voltage-gated Ca2+ channels. Voltage-dependent activation indicates that the Cav3.1 T-type Ca2+ current is shifted to a positive potential, at which maximum current activation is -10 mV; voltage-dependent conductance and inactivation properties suggest a "window current" in the range of -60 to -30 mV. Thrombin-induced transitions in membrane potential activate the Cav3.1 channel, resulting in a physiologically relevant rise in cytosolic Ca2+. Furthermore, activation of the Cav3.1 channel induces a procoagulant endothelial phenotype; eg, channel inhibition attenuates increased retention of sickled erythrocytes in the inflamed pulmonary circulation. We conclude that activation of the Cav3.1 channels selectively induces phenotypic changes in microvascular endothelial cells that mediate vaso-occlusion by sickled erythrocytes in the inflamed lung microcirculation.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Functional coupling between 'R-type' Ca2+ channels and insulin secretion in the insulinoma cell line INS-1

Among voltage-gated Ca2+ channels the non-dihydropyridine-sensitive alpha1E subunit is functionally less well characterized than the structurally related alpha1A (omega-agatoxin-IVA sensitive, P- /Q-type) and alpha1B (omega-conotoxin-GVIA sensitive, N-type) subunits. In the rat insulinoma cell line, INS-1, a tissue-specific splice variant of alpha1E (alpha1Ee) has been characterized at the mRNA and protein levels, suggesting that INS-1 cells are a suitable model for investigating the function of alpha1Ee. In alpha1E-transfected human embryonic kidney (HEK-293) cells the alpha1E-selective peptide antagonist SNX-482 (100 nM) reduces alpha1Ed- and alpha1Ee-induced Ba2+ inward currents in the absence and presence of the auxiliary subunits beta3 and alpha2delta-2 by more than 80%. The inhibition is fast and only partially reversible. No effect of SNX-482 was detected on the recombinant T-type Ca2+ channel subunits alpha1G, alpha1H, and alpha1I showing that the toxin from the venom of Hysterocrates gigas is useful as an alpha1E-selective antagonist. After blocking known components of Ca2+ channel inward current in INS-1 cells by 2 microM (+/-)-isradipine plus 0.5 microM omega-conotoxin-MVIIC, the remaining current is reduced by 100 nM SNX-482 from -12.4 +/- 1.2 pA/pF to -7.6 +/- 0.5 pA/pF (n = 9). Furthermore, in INS-1 cells, glucose- and KCl-induced insulin release are reduced by SNX-482 in a dose-dependent manner leading to the conclusion that alpha1E, in addition to L-type and non-L-type (alpha1A-mediated) Ca2+ currents, is involved in Ca2+ dependent insulin secretion of INS-1 cells.

Localized calcium influx in pancreatic beta-cells: its significance for Ca2+-dependent insulin secretion from the islets of Langerhans

Ca2+ influx through voltage-dependent Ca2+ channels plays a crucial role in stimulus-secretion coupling in pancreatic islet beta-cells. Molecular and physiologic studies have identified multiple Ca2+ channel subtypes in rodent islets and insulin-secreting cell lines. The differential targeting of Ca2+ channel subtypes to the vicinity of the insulin secretory apparatus is likely to account for their selective coupling to glucose-dependent insulin secretion. In this article, I review these studies. In addition, I discuss temporal and spatial aspects of Ca2+ signaling in beta-cells, the former involving the oscillatory activation of Ca2+ channels during glucose-induced electrical bursting, and the latter involving [Ca2+]i elevation in restricted microscopic "domains," as well as direct interactions between Ca2+ channels and secretory SNARE proteins. Finally, I review the evidence supporting a possible role for Ca2+ release from the endoplasmic reticulum in glucose-dependent insulin secretion, and evidence to support the existence of novel Ca2+ entry pathways. I also show that the beta-cell has an elaborate and complex set of [Ca2+]i signaling mechanisms that are capable of generating diverse and extremely precise [Ca2+]i patterns. These signals, in turn, are exquisitely coupled in space and time to the beta-cell secretory machinery to produce the precise minute-to-minute control of insulin secretion necessary for body energy homeostasis.

T-type calcium currents in rat cardiomyocytes during postnatal development: contribution to hormone secretion

T-type Ca(2+) channels have been suggested to play a role in cardiac automaticity, cell growth, and cardiovascular remodeling. Although three genes encoding for a T-type Ca(2+) channel have been identified, the nature of the isoform(s) supporting the cardiac T-type Ca(2+) current (I(Ca,T)) has not yet been determined. We describe the postnatal evolution of I(Ca,T) density in freshly dissociated rat atrial and ventricular myocytes and its functional properties at peak current density in young atrial myocytes. I(Ca,T) displays a classical low activation threshold, rapid inactivation kinetics, negative steady-state inactivation, slow deactivation, and the presence of a window current. Interestingly, I(Ca,T) is poorly sensitive to Ni(2+) and insensitive to R-type current toxin SNX-482. RT-PCR experiments and comparison of functional properties with recombinant Ca(2+) channel subtypes suggest that neonatal I(Ca,T) is related to the alpha(1G)-subunit. Atrial natriuretic factor (ANF) secretion was measured using peptide radioimmunoassays in atrial tissue. Pharmacological dissection of ANF secretion indicates an important contribution of I(Ca,T) to Ca(2+) signaling during the neonatal period.

Increased T-type Ca2+ channel activity as a determinant of cellular toxicity in neuronal cell lines expressing polyglutamine-expanded human androgen receptors

We have analyzed Ca2+ currents in two neuroblastoma-motor neuron hybrid cell lines that expressed normal or glutamine-expanded human androgen receptors (polyGln-expanded AR) either transiently or stably. The cell lines express a unique, low-threshold, transient type of Ca2+ current that is not affected by L-type Ca2+ channel blocker (PN 200-110), N-type Ca2+ channel blocker (omega-conotoxin GVIA) or P-type Ca2+ channel blocker (Agatoxin IVA) but is blocked by either Cd2+ or Ni2+. This pharmacological profile most closely resembles that of T-type Ca2+ channels [1-3]. Exposure to androgen had no effect on control cell lines or cells transfected with normal AR but significantly changed the steady-state activation in cells transfected with expanded AR. The observed negative shift in steady-state activation results in a large increase in the T-type Ca2+ channel window current. We suggest that Ca2+ overload due to abnormal voltage-dependence of transient Ca2+ channel activation may contribute to motor neuron toxicity in spinobulbar muscular atrophy (SBMA). This hypothesis is supported by the additional finding that, at concentrations that selectively block T-type Ca2+ channel currents, Ni2+ significantly reduced cell death in cell lines transfected with polyGln-expanded AR.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

A low voltage-activated Ca2+ current mediates cytokine-induced pancreatic beta-cell death

Insulin-dependent diabetes mellitus is characterized by the selective destruction of pancreatic beta-cells. Chronic treatment with cytokines induced a low voltage-activated (LVA) Ca2+ current in mouse beta-cells. The concomitant increase in the basal cytoplasmic free Ca2+ concentration ([Ca2+]i) was associated with DNA fragmentation and cell death. Antagonists of LVA Ca2+ channels prevented this elevation of basal [Ca2+]i and DNA fragmentation and reduced the percentage of cell death. Exposure to cytokines did not affect the profile of Ca2+ currents or basal [Ca2+]i in glucagon-secreting alpha-cells. An increased Ca2+ signal through LVA Ca2+ channels may thus be a key feature in cytokine-induced beta-cell destruction.