Purified skeletal muscle 1,4-dihydropyridine receptor forms phosphorylation-dependent oligomeric calcium channels in planar bilayers

Mechanisms and Physiological Implications of Cooperative Gating of Ion Channels Clusters

Ion channels play a central role in the regulation of nearly every cellular process. Dating back to the classic 1952 Hodgkin-Huxley model of the generation of the action potential, ion channels have always been thought of as independent agents. A myriad of recent experimental findings exploiting advances in electrophysiology, structural biology, and imaging techniques, however, have posed a serious challenge to this long-held axiom as several classes of ion channels appear to open and close in a coordinated, cooperative manner. Ion channel cooperativity ranges from variable-sized oligomeric cooperative gating in voltage-gated, dihydropyridine-sensitive Ca_v1.2 and Ca_v1.3 channels to obligatory dimeric assembly and gating of voltage-gated Na_v1.5 channels. Potassium channels, transient receptor potential channels, hyperpolarization cyclic nucleotide-activated channels, ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP₃Rs) have also been shown to gate cooperatively. The implications of cooperative gating of these ion channels range from fine tuning excitation-contraction coupling in muscle cells to regulating cardiac function and vascular tone, to modulation of action potential and conduction velocity in neurons and cardiac cells, and to control of pace-making activity in the heart. In this review, we discuss the mechanisms leading to cooperative gating of ion channels, their physiological consequences and how alterations in cooperative gating of ion channels may induce a range of clinically significant pathologies.

Mechanisms and Regulation of Cardiac CaV1.2 Trafficking

During cardiac excitation contraction coupling, the arrival of an action potential at the ventricular myocardium triggers voltage-dependent L-type Ca²⁺ (Ca_V1.2) channels in individual myocytes to open briefly. The level of this Ca²⁺ influx tunes the amplitude of Ca²⁺-induced Ca²⁺ release from ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum and thus the magnitude of the elevation in intracellular Ca²⁺ concentration and ultimately the downstream contraction. The number and activity of functional Ca_V1.2 channels at the t-tubule dyads dictates the amplitude of the Ca²⁺ influx. Trafficking of these channels and their auxiliary subunits to the cell surface is thus tightly controlled and regulated to ensure adequate sarcolemmal expression to sustain this critical process. To that end, recent discoveries have revealed the existence of internal reservoirs of preformed Ca_V1.2 channels that can be rapidly mobilized to enhance sarcolemmal expression in times of acute stress when hemodynamic and metabolic demand increases. In this review, we provide an overview of the current thinking on Ca_V1.2 channel trafficking dynamics in the heart. We highlight the numerous points of control including the biosynthetic pathway, the endosomal recycling pathway, ubiquitination, and lysosomal and proteasomal degradation pathways, and discuss the effects of β-adrenergic and angiotensin receptor signaling cascades on this process.

Local regulation of L-type Ca²⁺ channel sparklets in arterial smooth muscle

This review addresses the latest advances in our understanding of the regulation of a novel Ca²⁺ signal called L-type Ca²⁺ channel sparklets in arterial smooth muscle. L-type Ca²⁺ channel sparklets are elementary Ca²⁺ influx events produced by the opening of a single or a small cluster of L-type Ca²⁺ channels. These Ca²⁺ signals were first visualized in the vasculature in arterial smooth muscle cells. In these cells, L-type Ca²⁺ channel sparklets display two functionally distinct gating modalities that regulate local and global [Ca²⁺](i). The activity of L-type Ca²⁺ channel sparklets varies regionally within a cell depending on the dynamic activity of a cohort of protein kinases and phosphatases recruited to L-type Ca²⁺ channels in the arterial smooth muscle sarcolemma in a complex coordinated by the scaffolding molecule AKAP150. We also described a mechanism whereby clusters of L-type Ca²⁺ channels gate cooperatively to amplify intracellular Ca²⁺ signals with likely pathological consequences.

Expression-dependent pharmacology of transient receptor potential vanilloid subtype 1 channels in Xenopus laevis oocytes

Transient receptor potential vanilloid subfamily member 1 channels are polymodal sensors of noxious stimuli and integral players in thermosensation, inflammation and pain signaling. It has been shown previously that under prolonged stimulation, these channels show dynamic pore dilation, providing a pathway for large and otherwise relatively impermeant molecules. Further, we have shown recently that these nonselective cation channels, when activated by capsaicin, are potently and reversibly blocked by external application of quaternary ammonium compounds and local anesthetics. Here we describe a novel phenomenon in transient receptor potential channel pharmacology whereby their expression levels in Xenopus laevis oocytes, as assessed by the magnitude of macroscopic currents, are negatively correlated with extracellular blocker affinity: small current densities give rise to nanomolar blockade by quaternary ammoniums and this affinity decreases linearly as current density increases. Possible mechanisms to explain these data are discussed in light of similar observations in other channels and receptors.

L-type Ca2+ channel function during Timothy syndrome

Voltage-gated, dihydropyridine-sensitive L-type Ca(2+) channels are multimeric proteins composed of a pore-forming transmembrane α(1) subunit (Ca(v)1.2) and accessory β, α(2)δ, and γ subunits. Ca(2+) entry via Ca(v)1.2 channels shapes the action potential (AP) of cardiac myocytes and is required for excitation-contraction coupling. Two de novo point mutations of Ca(v)1.2 glycine residues, G406R and G402S, cause a rare multisystem disorder called Timothy syndrome (TS). Here, we discuss recent work on the mechanisms by which Ca(v)1.2 channels bearing TS mutations display slowed inactivation that leads to increased Ca(2+) influx, prolonging the cardiac AP and promoting lethal arrhythmias. Based on these studies, we propose a model in which the scaffolding protein AKAP79/150 stabilizes the open conformation of Ca(v)1.2-TS channels and facilitates physical interactions among adjacent channels via their C-tails, increasing the activity of adjoining channels and amplifying Ca(2+) influx.

CaV1.2 sparklets in heart and vascular smooth muscle

CaV1.2 sparklets are local elevations in intracellular Ca(2+) ([Ca(2+)]i) resulting from the opening of a single or small cluster of voltage-gated, dihydropyridine-sensitive CaV1.2 channels. Activation of CaV1.2 sparklets is an early event in the signaling cascade that couples membrane depolarization to contraction (i.e., excitation-contraction coupling) in cardiac and arterial smooth muscle. Here, we review recent work on the molecular and biophysical mechanisms that regulate CaV1.2 sparklet activity in these cells. CaV1.2 sparklet activity is tightly regulated by a cohort of protein kinases and phosphatases that are targeted to specific regions of the sarcolemma by the anchoring protein AKAP150. We discuss a model for the local control of Ca(2+) influx via CaV1.2 channels in which a signaling complex formed by AKAP79/150, protein kinase C, protein kinase A, and calcineurin regulates the activity of individual CaV1.2 channels and also facilitates the coordinated activation of small clusters of these channels. This results in amplification of Ca(2+) influx, which strengthens excitation-contraction coupling in cardiac and vascular smooth muscle.

Ca2+ signaling amplification by oligomerization of L-type Cav1.2 channels

Ca(2+) influx via L-type Ca(v)1.2 channels is essential for multiple physiological processes, including gene expression, excitability, and contraction. Amplification of the Ca(2+) signals produced by the opening of these channels is a hallmark of many intracellular signaling cascades, including excitation-contraction coupling in heart. Using optogenetic approaches, we discovered that Ca(v)1.2 channels form clusters of varied sizes in ventricular myocytes. Physical interaction between these channels via their C-tails renders them capable of coordinating their gating, thereby amplifying Ca(2+) influx and excitation-contraction coupling. Light-induced fusion of WT Ca(v)1.2 channels with Ca(v)1.2 channels carrying a gain-of-function mutation that causes arrhythmias and autism in humans with Timothy syndrome (Ca(v)1.2-TS) increased Ca(2+) currents, diastolic and systolic Ca(2+) levels, contractility and the frequency of arrhythmogenic Ca(2+) fluctuations in ventricular myocytes. Our data indicate that these changes in Ca(2+) signaling resulted from Ca(v)1.2-TS increasing the activity of adjoining WT Ca(v)1.2 channels. Collectively, these data support the concept that oligomerization of Ca(v)1.2 channels via their C termini can result in the amplification of Ca(2+) influx into excitable cells.

1alpha,25(OH)2 vitamin D3 actions on ion channels in osteoblasts

Membrane-initiated cellular responses to steroids include modulation of ion channel activities via signal transduction pathways. However, the molecular mechanisms involved in nongenomic actions remain only partially understood. Our research has focused on the rapid effects of 1alpha,25(OH)(2) Vitamin D(3) [1,25D] on L-type Ca(2+) [L-Ca] and DIDS-sensitive Cl(-) channels in osteoblasts. Physiological nanomolar concentrations of hormonally active 1,25D promote rapid (1-5 min) potentiation of outward Cl(-) currents in osteosarcoma ROS 17/2.8 cells and mouse primary osteoblasts. In addition, 1,25D increases inward barium currents through L-Ca channels at low depolarizing potentials within seconds in a fashion similar to the 1,4-dihydropyridine [DHP] agonist Bay K8644. We found that second messenger cAMP is involved in 1,25D potentiation of Cl(-) and Ca(2+) channels. Nongenomic 1,25D effects on ion channel activities in osteoblasts appear to involve different mechanisms that include a possible direct interaction with the L-Ca channel molecule, on one hand, and signaling through the cAMP pathway, on the other. Rapid 1,25D actions on Cl(-) and Ca(2+) currents seem to couple to secretory activities in osteoblasts, thus contributing to bone mass formation.

Interactions between electron and proton currents in excised patches from human eosinophils

The NADPH–oxidase is a plasma membrane enzyme complex that enables phagocytes to generate superoxide in order to kill invading pathogens, a critical step in the host defense against infections. The oxidase transfers electrons from cytosolic NADPH to extracellular oxygen, a process that requires concomitant H⁺ extrusion through depolarization-activated H⁺ channels. Whether H⁺ fluxes are mediated by the oxidase itself is controversial, but there is a general agreement that the oxidase and H⁺ channel are intimately connected. Oxidase activation evokes profound changes in whole-cell H⁺ current (I_H), causing an approximately −40-mV shift in the activation threshold that leads to the appearance of inward I_H. To further explore the relationship between the oxidase and proton channel, we performed voltage-clamp experiments on inside-out patches from both resting and phorbol-12-myristate-13-acetate (PMA)-activated human eosinophils. Proton currents from resting cells displayed slow voltage-dependent activation, long-term stability, and were blocked by micromolar internal [Zn²⁺]. I_H from PMA-treated cells activated faster and at lower voltages, enabling sustained H⁺ influx, but ran down within minutes, regaining the current properties of nonactivated cells. Bath application of NADPH to patches excised from PMA-treated cells evoked electron currents (I_e), which also ran down within minutes and were blocked by diphenylene iodonium (DPI). Run-down of both I_H and I_e was delayed, and sometimes prevented, by cytosolic ATP and GTP-γ-S. A good correlation was observed between the amplitude of I_e and both inward and outward I_H when a stable driving force for e⁻ was imposed. Combined application of NADPH and DPI reduced the inward I_H amplitude, even in the absence of concomitant oxidase activity. The strict correlation between I_e and I_H amplitudes and the sensitivity of I_H to oxidase-specific agents suggest that the proton channel is either part of the oxidase complex or linked by a membrane-limited mediator.

L-type voltage-gated calcium channels: understanding function through structure

L-type voltage-gated calcium channels (VGCCs) are multisubunit membrane proteins that regulate calcium influx into excitable cells. Within the last two years there have been four separate reports describing the structure of the skeletal muscle VGCC determined by electron microscopy and single particle analysis methods. There are some discrepancies between the structures, as well as reports for both monomeric and dimeric forms of the channel. This article considers each of the VGCC structures in terms of similarities and differences with an emphasis upon translation of data into a biological context.

The Three-dimensional Structure of the Cardiac L-type Voltage-gated Calcium Channel

We describe here the first three-dimensional structure of the cardiac L-type voltage-gated calcium channel (VGCC) purified from bovine heart. The structure was determined by electron microscopy and single particle analysis of negatively stained complexes, using the angular reconstitution method. The cardiac VGCC can be isolated as a stable dimer, as reported previously for the skeletal muscle VGCC, with a central aqueous chamber formed by the two halves of the complex. Moreover, we demonstrate that the dimeric cardiac VGCC binds the dihydropyridine [3H]azidopine with a Kd approximately 310 pM. We have compared the cardiac VGCC structure with the skeletal muscle form, determined using the same reconstructive methodology, allowing us to identify common and distinct features of the complexes. By using antibody and lectin-gold labeling, we have localized the intracellular beta polypeptides and the extracellular glycosylation sites of the skeletal muscle VGCC, which can be correlated to the cardiac three-dimensional structure. From the data presented here the assignment of the orientation of the VGCC complexes with respect to the lipid bilayer is now possible. A difference between the cardiac and skeletal muscle ion channels is apparent in the putative transmembrane region, which would be consistent with the absence of the gamma subunit in the cardiac VGCC assembly.

G Protein Modulation of Voltage-Gated Calcium Channels

Calcium influx into any cell requires fine tuning to guarantee the correct balance between activation of calcium-dependent processes, such as muscle contraction and neurotransmitter release, and calcium-induced cell damage. G protein-coupled receptors play a critical role in negative feedback to modulate the activity of the CaV2 subfamily of the voltage-dependent calcium channels, which are largely situated on neuronal and neuro-endocrine cells. The basis for the specificity of the relationships among membrane receptors, G proteins, and effector calcium channels will be discussed, as well as the mechanism by which G protein-mediated inhibition is thought to occur. The inhibition requires free G beta gamma dimers, and the cytoplasmic linker between domains I and II of the CaV2 alpha 1 subunits binds G beta gamma dimers, whereas the intracellular N terminus of CaV2 alpha 1 subunits provides essential determinants for G protein modulation. Evidence suggests a key role for the beta subunits of calcium channels in the process of G protein modulation, and the role of a class of proteins termed "regulators of G protein signaling" will also be described.

Multiple molecular mechanisms of 1 alpha,25(OH)2-vitamin D3 rapid modulation of three ion channel activities in osteoblasts

Rapid nongenomic responses to steroids include modulation of ion channel activities on the cell membrane of target cells, but little is known about the molecular mechanisms involved. In this paper we investigate the mechanisms underlying the combined action of the secosteroid hormone 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)(2)D3] on three different ion channel types in rat osteoblasts, which include a voltage-gated L-type Ca(2+) channel, a mechanosensitive Cl(-) channel, and a stretch-activated cation (SA-Cat) channel. We found that physiological nanomolar concentrations of 1alpha,25(OH)(2)D3 rapidly modify the overall electrical activity of the membrane in ROS 17/2.8 cells. 1alpha,25(OH)(2)D3 increases the osteoblast L-type Ca(2+) channel activity at low depolarizing voltages in a fashion similar to the 1,4-dihydropyridine (DHP) agonist Bay K8644. At highly depolarizing potentials 1alpha,25(OH)(2)D3 potentiates volume-sensitive Cl(-) currents through mechanisms that may involve a putative membrane receptor. We show for the first time that 1alpha,25(OH)(2)D3 also increases inward currents through SA-Cat channels at positive membrane voltages in a dose-dependent manner. Contrary to our expectations, the stereoisomer 1beta,25(OH)(2)D3, which suppresses 1alpha,25(OH)(2)D3 activation of osteoblast Cl(-) currents, mimicked 1alpha,25(OH)(2)D3 agonist effects on Ca(2+) and SA-Cat channel activities. Cyclic AMP is involved in 1alpha,25(OH)(2)D3 effects on both Ca(2+) and SA-Cat channels, but not in Cl(-) channels. We conclude that 1alpha,25(OH)(2)D3 rapid effects on ion channel activities in ROS 17/2.8 cells occur through multiple mechanisms that, on the one hand, involve a possible direct interaction with the L-type Ca(2+) channel molecule and, on the other hand, molecular pathways that may include a putative membrane receptor.

Tandem gramicidin channels cross-linked by streptavidin

The interaction of biotin-binding proteins with biotinylated gramicidin (gA5XB) was studied by monitoring single-channel activity and sensitized photoinactivation kinetics. It was discovered that the addition of streptavidin or avidin to the bathing solutions of a bilayer lipid membrane (BLM) with incorporated gA5XB induced the opening of a channel characterized by approximately doubled single-channel conductance and extremely long open-state duration. We believe that the deceleration of the photoinactivation kinetics observed here with streptavidin and previously (Rokitskaya, T.I., Y.N. Antonenko, E.A. Kotova, A. Anastasiadis, and F. Separovic. 2000. Biochemistry. 39:13053-13058) with avidin reflects the formation of long-lived channels of this type. Both opening and closing of the double-conductance channels occurred via a transient sub-state of the conductance coinciding with that of the usual single-channel transition. The appearance of the double-conductance channels after the addition of streptavidin was preceded by bursts of fast fluctuations of the current with the open state duration of the individual events of 60 ms. The streptavidin-induced double-conductance channels appeared to be inherent only to the gramicidin analogue with a biotin group linked to the COOH terminus through a long linker arm. Including biotinylated phosphatidylethanolamine into the BLM prevented the formation of the double-conductance channels even with the excess streptavidin. In view of the results obtained here, it is suggested that the double-conductance channel represents a tandem of two neighboring gA5XB channels with their COOH termini being cross-linked by the bound streptavidin at both sides of the BLM. The finding that streptavidin induces the formation of the tandem gramicidin channel comprising two channels functioning in concert is considered to be relevant to the physiologically important phenomenon of ligand-induced receptor oligomerization.

Structure and regulation of voltage-gated Ca2+ channels

Voltage-gated Ca(2+) channels mediate Ca(2+) entry into cells in response to membrane depolarization. Electrophysiological studies reveal different Ca(2+) currents designated L-, N-, P-, Q-, R-, and T-type. The high-voltage-activated Ca(2+) channels that have been characterized biochemically are complexes of a pore-forming alpha1 subunit of approximately 190-250 kDa; a transmembrane, disulfide-linked complex of alpha2 and delta subunits; an intracellular beta subunit; and in some cases a transmembrane gamma subunit. Ten alpha1 subunits, four alpha2delta complexes, four beta subunits, and two gamma subunits are known. The Cav1 family of alpha1 subunits conduct L-type Ca(2+) currents, which initiate muscle contraction, endocrine secretion, and gene transcription, and are regulated primarily by second messenger-activated protein phosphorylation pathways. The Cav2 family of alpha1 subunits conduct N-type, P/Q-type, and R-type Ca(2+) currents, which initiate rapid synaptic transmission and are regulated primarily by direct interaction with G proteins and SNARE proteins and secondarily by protein phosphorylation. The Cav3 family of alpha1 subunits conduct T-type Ca(2+) currents, which are activated and inactivated more rapidly and at more negative membrane potentials than other Ca(2+) current types. The distinct structures and patterns of regulation of these three families of Ca(2+) channels provide a flexible array of Ca(2+) entry pathways in response to changes in membrane potential and a range of possibilities for regulation of Ca(2+) entry by second messenger pathways and interacting proteins.

cAMP-dependent phosphorylation sites and macroscopic activity of recombinant cardiac L-type calcium channels

The involvement of cAMP-dependent phosphorylation sites in establishing the basal activity of cardiac L-type Ca2+ channels was studied in HEK 293 cells transiently cotransfected with mutants of the human cardiac alpha1 and accessory subunits. Systematic individual or combined elimination of high consensus protein kinase A (PKA) sites, by serine to alanine substitutions at the amino and carboxyl termini of the alpha1 subunit, resulted in Ca2+ channel currents indistinguishable from those of wild type channels. Dihydropyridine (DHP)-binding characteristics were also unaltered. To explore the possible involvement of nonconsensus sites, deletion mutants were used. Carboxyl-terminal truncations of the alpha1 subunit distal to residue 1597 resulted in increased channel expression and current amplitudes. Modulation of PKA activity in cells transfected with the wild type channel or any of the mutants did not alter Ca2+ channel functions suggesting that cardiac Ca2+ channels expressed in these cells behave, in terms of lack of PKA control, like Ca2+ channels of smooth muscle cells.

Known calcium channel alpha1 subunits can form low threshold small conductance channels with similarities to native T-type channels

Native T-type voltage-dependent calcium channels are low voltage-activated and have a small single channel conductance of 5-8 pS, which distinguishes them from any known cloned calcium channels whose conductances are 12-25 pS. Here, we show that when alpha1B, alpha1E, or alpha1C are expressed in COS7 cells, which contain no endogenous calcium channel subunits or calcium channels, they each exhibit a 4-7 pS channel as well as a large conductance channel. At low depolarizations, or when the alpha1 subunit is expressed in the absence of auxiliary alpha2-delta or beta subunits, the small conductance channels are seen alone, and their biophysical properties, including voltage dependence and kinetics of activation and inactivation, are very similar to native T-type calcium channels.

Regulation of mouse skeletal muscle L-type Ca2+ channel by activation of the insulin-like growth factor-1 receptor

We investigated the modulation of the skeletal muscle L-type Ca2+ channel/dihydropyridine receptor in response to insulin-like growth factor-1 receptor (IGF-1R) activation in single extensor digitorum longus muscle fibers from adult C57BL/6 mice. The L-type Ca2+ channel activity in its dual role as a voltage sensor and a selective Ca2+-conducting pore was recorded in voltage-clamp conditions. Peak Ca2+ current amplitude consistently increased after exposure to 20 ng/ml IGF-1 (EC50 = 5.6 +/- 1.8 nM). Peak IGF-1 effect on current amplitude at -20 mV was 210 +/- 18% of the control. Ca2+ current potentiation resulted from a shift in 13 mV of the Ca2+ current-voltage relationship toward more negative potentials. The IGF-1-induced facilitation of the Ca2+ current was not associated with an effect on charge movement amplitude and/or voltage distribution. These phenomena suggest that the L-type Ca2+ channel structures involved in voltage sensing are not involved in the response to the growth factor. The modulatory effect of IGF-1 on L-type Ca2+ channel was blocked by tyrosine kinase and PKC inhibitors, but not by a cAMP-dependent protein kinase inhibitor. IGF-1-dependent phosphorylation of the L-type Ca2+ channel alpha1 subunit was demonstrated by incorporation of [gamma-32P]ATP to monolayers of adult fast-twitch skeletal muscles. IGF-1 induced phosphorylation of a protein at the 165 kDa band, corresponding to the L-type Ca2+ channel alpha1 subunit. These results show that the activation of the IGF-1R facilitates skeletal muscle L-type Ca2+ channel activity via a PKC-dependent phosphorylation mechanism.

Regulation of mouse skeletal muscle L-type Ca2+ channel by activation of the insulin-like growth factor-1 receptor

We investigated the modulation of the skeletal muscle L-type Ca2+ channel/dihydropyridine receptor in response to insulin-like growth factor-1 receptor (IGF-1R) activation in single extensor digitorum longus muscle fibers from adult C57BL/6 mice. The L-type Ca2+ channel activity in its dual role as a voltage sensor and a selective Ca2+-conducting pore was recorded in voltage-clamp conditions. Peak Ca2+ current amplitude consistently increased after exposure to 20 ng/ml IGF-1 (EC50 = 5.6 +/- 1.8 nM). Peak IGF-1 effect on current amplitude at -20 mV was 210 +/- 18% of the control. Ca2+ current potentiation resulted from a shift in 13 mV of the Ca2+ current-voltage relationship toward more negative potentials. The IGF-1-induced facilitation of the Ca2+ current was not associated with an effect on charge movement amplitude and/or voltage distribution. These phenomena suggest that the L-type Ca2+ channel structures involved in voltage sensing are not involved in the response to the growth factor. The modulatory effect of IGF-1 on L-type Ca2+ channel was blocked by tyrosine kinase and PKC inhibitors, but not by a cAMP-dependent protein kinase inhibitor. IGF-1-dependent phosphorylation of the L-type Ca2+ channel alpha1 subunit was demonstrated by incorporation of [gamma-32P]ATP to monolayers of adult fast-twitch skeletal muscles. IGF-1 induced phosphorylation of a protein at the 165 kDa band, corresponding to the L-type Ca2+ channel alpha1 subunit. These results show that the activation of the IGF-1R facilitates skeletal muscle L-type Ca2+ channel activity via a PKC-dependent phosphorylation mechanism.

Structural and functional diversity of voltage-activated calcium channels

Data gathered from the expression of cDNAs that encode the subunits of voltage-dependent Ca2+ channels have demonstrated important structural and functional similarities among these channels. Despite these convergences, there are also significant differences in the nature and functional importance of subunit-subunit and protein-Ca2+ channel interactions. There is evidence demonstrating that the functional differences between Ca2+ channel subtypes is due to several factors, including the expression of distinct alpha 1 subunit proteins, the selective association of structural subunits and modulatory proteins, and differences in posttranslational processing and cell regulation. We summarize several avenues of research that should provide significant clues about the structural features involved in the biophysical and functional diversity of voltage-dependent Ca2+ channels.

Facilitation of Ca2+ current in excitable cells

Voltage-dependent Ca2+ channels are one of the main routes for the entry of Ca2+ into excitable cells. These channels are unique in cell-signalling terms in that they can transduce an electrical signal (membrane depolarization) via Ca2+ entry into a chemical signal, by virtue of the diverse range of intracellular Ca(2+)-dependent enzymes and processes. In a variety of cell types, currents through voltage-dependent Ca2+ channels can be increased in amplitude by a number of means. Although the term facilitation was originally defined as an increase of Ca2+ current resulting from one or a train of prepulses to depolarizing voltages, there is a great deal of overlap between facilitation by this means and enhancement by other routes, such as phosphorylation.

Multiple genes, tissue specificity, and expression-dependent modulationcontribute to the functional diversity of potassium channels in Arabidopsis thaliana

K+ channels play diverse roles in mediating K+ transport and in modulating the membrane potential in higher plant cells during growth and development. Some of the diversity in K+ channel functions may arise from the regulated expression of multiple genes encoding different K+ channel polypeptides. Here we report the isolation of a novel Arabidopsis thaliana cDNA (AKT2) that is highly homologous to the two previously identified K+ channel genes, KAT1 and AKT1. This cDNA mapped to the center of chromosome 4 by restriction fragment length polymorphism analysis and was highly expressed in leaves, whereas AKT1 was mainly expressed in roots. In addition, we show that diversity in K+ channel function may be attributable to differences in expression levels. Increasing KAT1 expression in Xenopus oocytes by polyadenylation of the KAT1 mRNA increased the current amplitude and led to higher levels of KAT1 protein, as assayed in western blots. The increase in KAT1 expression in oocytes produced shifts in the threshold potential for activation to more positive membrane potentials and decreased half-activation times. These results suggest that different levels of expression and tissue-specific expression of different K+ channel isoforms can contribute to the functional diversity of plant K+ channels. The identification of a highly expressed, leaf-specific K+ channel homolog in plants should allow further molecular characterization of K+ channel functions for physiological K+ transport processes in leaves.

Effects of phosphorylation on ion channel function

A fundamental property of ion channels is their ability to be modulated by intracellular second messenger systems acting via covalent modifications of the channel protein itself. One such important biochemical reaction is phosphorylation on serine, threonine, and tyrosine residues. Ion channels in the kidney are no exception. Moreover, many ion channels, including many amiloride-sensitive epithelial Na+ channels, are subject to modulation by a multiplicity of inputs. For example, renal Na+ channels are not gated by voltage in their unphosphorylated state. However, upon phosphorylation by PKA plus ATP, these channels become voltage-dependent as well as having their open probability increased. Phosphorylation by PKC inhibits channel activity regardless of whether the channel was previously phosphorylated by PKA. Likewise, Na+ channel ADP-ribosylation by PTX overrides the actions of cAMP-dependent phosphorylation. Consistent with this idea is the fact that the phosphorylation sites for PKA and PKC and the ADP-ribosylation sites occur on different polypeptides comprising the channel complex. Epithelial Na+ channel activity is also regulated by methylation, arachidonic acid metabolites, and by interactions with cytoskeletal components. An exciting new age in understanding renal Na+ channel function has begun. Canessa and collaborators [103, 104] and Lingueglia et al [105] have, for the first time, identified by expression cloning an amiloride-sensitive Na+ channel from rat distal colon. The messenger RNA encoding the subunits comprising this channel are expressed in the distal tubule and cortical collecting tubule of the kidney (Rossier, unpublished observations). In addition, our laboratory has successfully cloned a mammalian homologue of this same channel from bovine renal papillary collecting ducts [106].(ABSTRACT TRUNCATED AT 250 WORDS)

Sites of selective cAMP-dependent phosphorylation of the L-type calcium channel alpha 1 subunit from intact rabbit skeletal muscle myotubes

The principal (alpha 1) subunit of purified skeletal muscle dihydropyridine-sensitive (L-type) calcium channels is present in full-length (212 kDa) and COOH-terminal truncated (190 kDa) forms, which are both phosphorylated by cAMP-dependent protein kinase (cA-PK) in vitro. Immunoprecipitation of the calcium channel from rabbit muscle myotubes in primary cell culture followed by phosphorylation with cA-PK, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and two-dimensional phosphopeptide mapping revealed comparable phosphorylation of three COOH-terminal phosphopeptides found in the purified full-length alpha 1 subunit. Stimulation of muscle myotubes with a permeant cAMP analogue, 8-(4-chlorophenylthio) adenosine 3',5'-cyclic monophosphate, prior to immunoprecipitation of alpha 1 results in a 60-80% reduction of cA-PK catalyzed "back" phosphorylation of each of these sites in vitro in calcium channels purified from the cells, indicating that these sites are phosphorylated in vivo in response to increased intracellular cAMP. Serine 687, the most rapidly phosphorylated site in the truncated 190-kDa alpha 1 subunit, was observed as a minor phosphopeptide whose level of phosphorylation was not significantly affected by stimulation of endogenous cA-PK in the myotubes. The COOH-terminal sites, designated tryptic phosphopeptides 4, 5, and 6, were identified as serine 1757 (phosphopeptides 4 and 6) and 1854 (phosphopeptide 5) by a combination of protease cleavage, phosphorylation of synthetic peptides and fusion proteins, specific immunoprecipitation, and phosphopeptide mapping. Phosphorylation of serines 1757 and 1854 in the COOH-terminal region of the 212-kDa alpha 1 subunit in intact skeletal muscle cells may play a pivotal role in the regulation of calcium channel function by cA-PK.

Single calcium channel behavior in native skeletal muscle

The purpose of this study was to use whole-cell and cell-attached patches of cultured skeletal muscle myotubes to study the macroscopic and unitary behavior of voltage-dependent calcium channels under similar conditions. With 110 mM BaCl2 as the charge carrier, two types of calcium channels with markedly different single-channel and macroscopic properties were found. One class was DHP-insensitive, had a single-channel conductance of approximately 9 pS, yielded ensembles that displayed an activation threshold near -40 mV, and activated and inactivated rapidly in a voltage-dependent manner (T current). The second class could only be well resolved in the presence of the DHP agonist Bay K 8644 (5 microM) and had a single-channel conductance of approximately 14 pS (L current). The 14-pS channel produced ensembles exhibiting a threshold of approximately -10 mV that activated slowly (tau act approximately 20 ms) and displayed little inactivation. Moreover, the DHP antagonist, (+)-PN 200-110 (10 microM), greatly increased the percentage of null sweeps seen with the 14-pS channel. The open probability versus voltage relationship of the 14-pS channel was fitted by a Boltzmann distribution with a VP0.5 = 6.2 mV and kp = 5.3 mV. L current recorded from whole-cell experiments in the presence of 110 mM BaCl2 + 5 microM Bay K 8644 displayed similar time- and voltage-dependent properties as ensembles of the 14-pS channel. Thus, these data are the first comparison under similar conditions of the single-channel and macroscopic properties of T current and L current in native skeletal muscle, and identify the 9- and 14-pS channels as the single-channel correlates of T current and L current, respectively.

Voltage-dependent potentiation of L-type Ca2+ channels in skeletal muscle cells requires anchored cAMP-dependent protein kinase

Skeletal muscle L-type Ca2+ channels respond to trains of brief depolarizations with a strong shift of the voltage dependence of channel activation toward more negative membrane potentials and slowing of channel deactivation. Increased Ca2+ entry resulting from this potentiation of channel activity may increase contractile force in response to tetanic stimuli. This voltage-dependent Ca2+ channel potentiation requires phosphorylation by cAMP-dependent protein kinase (PKA) at a rate that suggests that kinase and channel may be maintained in close proximity through kinase anchoring. A peptide derived from the conserved kinase-binding domain of a PKA-anchoring protein (AKAP) prevents potentiation by endogenous PKA as effectively as inhibition of PKA by a specific peptide inhibitor or by omission of ATP from the intracellular solution. In contrast, a proline-substituted mutant of AKAP peptide has no effect. Potentiation in the presence of 2 microM exogenous catalytic subunit of PKA is unaffected, indicating that kinase anchoring is specifically blocked by the AKAP peptide. No effects of these agents were observed on the level or voltage dependence of basal Ca2+ channel activity before potentiation, suggesting that close physical proximity between the skeletal muscle Ca2+ channel and PKA is critical for voltage-dependent potentiation of Ca2+ channel activity but not for basal activity.

Nimodipine decreases calcium action potentials in rabbit hippocampal CA1 neurons in an age-dependent and concentration-dependent manner

Intracellular recordings were made from rabbit hippocampal CA1 neurons in vitro using slices from aging and young adult rabbits. Calcium action potentials were studied in the presence of 4 microns tetrodotoxin using electrodes filled with 2M CsCl. Increasing concentrations of the dihydropyridine L-type calcium channel antagonist nimodipine were tested on the amplitude and time course of calcium action potentials. The calcium action potential (AP) consisted of two components: an initial fast phase followed by a slower plateau phase. No difference in the peak amplitude of the initial fast phase was observed between age groups. The amplitude and duration of the slower plateau phase of the calcium AP was significantly larger in aging neurons. Switching to a zero Ca2+ medium in the presence of 200 microns CdCl2 completely blocked the calcium AP. Nimodipine decreased the plateau phase of the calcium AP at concentrations as low as 100 nm in aging neurons and 10 microns in young neurons. Switching to higher concentrations of nimodipine did not reveal any substantially increased block of the calcium AP plateau phase. These data suggest that enhanced calcium influx through L-type calcium channels is largely responsible for the enhanced calcium action potentials observed in aging CA1 neurons. The action of nimodipine in reducing the plateau phase of the calcium action potential may underlie the drug's notable ability to improve learning in hippocampally dependent tasks in aging animals.

Selective dephosphorylation of the subunits of skeletal muscle calcium channels by purified phosphoprotein phosphatases

Multiple sites on the alpha 1 and beta subunits of purified skeletal muscle calcium channels are phosphorylated by cyclic AMP-dependent protein kinase, resulting in three different tryptic phosphopeptides derived from each subunit. Phosphoprotein phosphatases dephosphorylated these sites selectively. Phosphoprotein phosphatase 1 (PP1) and phosphoprotein phosphatase 2A (PP2A) dephosphorylated both alpha 1 and beta subunits at similar rates, whereas calcineurin dephosphorylated beta subunits preferentially. PP1 dephosphorylated phosphopeptides 1 and 2 of the alpha 1 subunit more rapidly than phosphopeptide 3. In contrast, PP2A dephosphorylated phosphopeptide 3 of the alpha 1 subunit preferentially. All three phosphoprotein phosphatases preferentially dephosphorylated phosphopeptide 1 of the beta subunit and dephosphorylated phosphopeptides 2 and 3 more slowly. Mn2+ increased the rate and extent of dephosphorylation of all sites by calcineurin so that > 80% dephosphorylation of both alpha 1 and beta subunits was obtained. The results demonstrate selective dephosphorylation of different phosphorylation sites on the alpha 1 and beta subunits of skeletal muscle calcium channels by the three principal serine/threonine phosphoprotein phosphatases.

Negative cooperativity may explain flat concentration-response curves of ATP-sensitive potassium channels

Blockage of ATP-sensitive K+ channels by various drugs has been reported to exhibit a weak concentration dependence with Hill coefficients below unity. This phenomenon is interpreted by a negative cooperativity between K+ channels whereby drug binding to one channel lowers the drug affinities of neighbouring channels. Results are presented for a dimeric and a tetrameric channel model and compared with published experimental data.

Multiple levels of native cardiac Na+ channels at elevated temperature measured with high-bandwidth/low-noise patch clamp

Currents through single Na+ channels were studied in cell-attached patches of enzymatically dispersed heart cells of the mouse with a low-noise patch-clamp technique that allows evaluation of current levels at temperatures of up to 35 degrees C with bandwidths of up to 13 kHz. Noise arising from the pipette and the holder was reduced by the use of short (total length 8 mm) patch pipettes, which were sealed at their end with oil and inserted for only 1.5 mm into an appropriately tipped holder. At 9 degrees C (filter 5 kHz), channel openings were regularly dominated by one open level, and amplitude histograms could be fitted with high accuracy with a sum of Gaussian curves. Above 24 degrees C (filter 10 or 13 kHz), however, channel-open levels were heterogeneous with maximum levels of up to 4.5 pA at -50 mV. Amplitude histograms with improved resolution, based on variance calculation with window widths of 75 microseconds or 195 microseconds, confirmed the observed heterogeneity of open levels. Regular level patterns were not found. The frequency of the largest levels strongly varied from patch to patch and intermediate levels were always the most frequent. A corresponding dissociation of amplitudes was also observed at 35 degrees C. Averaged currents, formed from trace ensembles including only levels below arbitrarily set borders, obeyed equal kinetics. It is concluded that at low temperature the conductance of single Na+ channel currents is much more homogeneous than at 24 degrees C and above, where the same channels have multiple open states with different conductance.

Nerve growth factor-stimulated calcium uptake into PC12 cells: uniqueness of the channel and evidence for phosphorylation

Nerve growth factor stimulates the uptake of radioactive calcium into PC12 cells. This stimulation is inhibited by low concentrations of dideoxyforskolin or staurosporine, and by high concentrations of nifedipine or cadmium. On the other hand, neither dideoxyforskolin nor staurosporine inhibited the stimulation of calcium uptake caused by BK-8644 or adenosine triphosphate (ATP). Nickel inhibited only the effect of ATP on calcium uptake, and actually stimulated the effects of either BK-8644 or nerve growth factor. Down-regulation of L-calcium channels by BK-8644 blocked the subsequent stimulation of calcium uptake by this agent, but not the stimulation by nerve growth factor. Conversely, pre-treatment of the cells with nerve growth factor inhibited the subsequent stimulation of calcium uptake by nerve growth factor, but not the stimulation by BK-8644. The effects of BK-8644 and nerve growth factor on calcium uptake were additive, as were the effects of nerve growth factor and ATP. Phosphatase 2A inhibited the effect of nerve growth factor on calcium uptake, but did not influence the action of BK-8644. On the other hand, calcineurin inhibited the effect of BK-8644 on calcium uptake, but potentiated the action of nerve growth factor. Calmidazolium or fluphenazine also inhibited the effect of nerve growth factor on calcium uptake, but okadaic acid stimulated it. A comparison of the effects of these inhibitors on the actions of various calcium channel agonists shows that the channels on which the action of nerve growth factor is exerted are different than either the L-type calcium channels or the ATP-activated calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ and voltage inactivate Ca2+ channels in guinea-pig ventricular myocytes through independent mechanisms

1. L-type Ca2+ currents and Ca2+ channel gating currents were studied in isolated guinea-pig ventricular heart cells using the whole-cell patch-clamp technique, in order to investigate the mechanism of Ca(2+)-dependent inactivation. The effect of altering the intracellular Ca2+ concentration ([Ca2+]i) on these currents was studied through photorelease of intracellular Ca2+ ions using the photolabile Ca2+ chelators DM-nitrophen and nitr-5. 2. We found that step increases in [Ca2+]i produced by photorelease could either increase or decrease the L-type Ca2+ current. Specifically, Ca2+ photorelease from DM-nitrophen almost exclusively caused inactivation of the Ca2+ current. In contrast, Ca2+ photorelease from nitr-5 had a biphasic effect: a small, rapid inactivation of the Ca2+ current was followed by a slow potentiation. These two Ca(2+)-dependent processes seemed to differ in their Ca2+ dependence, as small Ca2+ photoreleases elicited potentiation without a preceding inactivation, whereas larger photoreleases elicited both inactivation and potentiation. 3. The mechanism of the Ca(2+)-dependent inactivation of Ca2+ channels was explored by comparing the effects of voltage and photoreleased Ca2+ on the Ca2+ current and the Ca2+ channel gating current. Voltage was found to reduce both the Ca2+ current and the gating current proportionally. However, Ca2+ photorelease from intracellular DM-nitrophen inactivated the Ca2+ current without having any effect on the gating current. 4. The dephosphorylation hypothesis for Ca(2+)-dependent inactivation was tested by applying isoprenaline to the cells before eliciting a maximal rise of [Ca2+]i (maximal flash intensity, zero external [Na+]i). Isoprenaline could completely prevent Ca(2+)-dependent inactivation under these conditions, even when [Ca2+]i rose so high as to cause an irreversible contracture of the cell. 5. We concluded from these experiments that voltage and Ca2+ ions inactivate the L-type Ca2+ channel through separate, independent mechanisms. In addition, we found that Ca(2+)-dependent inactivation does not result in the immobilization of gating charge, and apparently closes the Ca2+ permeation pathway through a mechanism that does not involve the voltage-sensing region of the channel. Furthermore, we found that Ca(2+)-dependent inactivation is entirely sensitive to beta-adrenergic stimulation. These facts suggest that either Ca(2+)-dependent inactivation results from Ca(2+)-dependent dephosphorylation of the Ca2+ channel, or that Ca(2+)-dependent inactivation is modulated by protein kinase A.

Characterization of the two size forms of the alpha 1 subunit of skeletal muscle L-type calcium channels

The molecular properties of two size forms of the alpha 1 subunit of purified skeletal muscle calcium channels were analyzed. The minor, full-length, form, alpha 1(212), was found to have an apparent molecular mass of 214 kDa by Ferguson plot analysis, while the major, truncated, form, now designated alpha 1(190), had an apparent molecular mass of 193 kDa. Antibody mapping of the C-terminal region of alpha 1(190) with 10 anti-peptide antibodies placed the C terminus between residues 1685 and 1699. Three consensus sites for cAMP-dependent protein phosphorylation are present in the C-terminal region of alpha 1(212) but not in alpha 1(190), and they may be important for the regulation of the ion conductance activity of the calcium channel.

Calcium channels from Cyprinus carpio skeletal muscle

The complete amino acid sequence of the L-type calcium channel alpha 1 subunit from the carp (Cyprinus carpio) white skeletal muscle was deduced by cDNA cloning and sequence analysis. The open reading frame encodes 1852 amino acids (Mr 210,060). A 155-amino acid COOH-terminal sequence (after the fourth internal repeat) is evolutionarily preserved (90% homology) and may represent an important functional domain of L-type calcium channels. The photolabeled, membrane-bound, and purified carp alpha 1 subunits have masses of 211 and 190 kDa. The purified channel could not be phosphorylated by cAMP-dependent protein kinase. Two glycoproteins (alpha 2 subunits) are associated with the alpha 1 subunit and change their apparent masses from 235 and 220 kDa to 159 kDa upon reduction of disulfide bonds. Nucleic acid hybridization with alpha 2 cDNA revealed an 8.0-kilobase transcript in carp skeletal muscle. Evidence for a copurification of subunits similar in size to mammalian beta or gamma subunits was not obtained.

Ca2+ channel currents in rat sensory neurones: interaction between guanine nucleotides, cyclic AMP and Ca2+ channel ligands

1. The characteristics have been examined of the high threshold calcium channel current in cultured rat dorsal root ganglion (DRG) neurones recorded in the presence of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S; 200 microM in the patch pipette). This current, termed IBa, GTP gamma S, was slowly activating and showed little inactivation over 100 ms. 2. External application of forskolin (10 microM) to elevate internal cyclic AMP levels increased the amplitude of IBa, GTP gamma S whereas it had no effect on the control IBa. This cyclic AMP-dependent protein kinase (PKI; 25 microM). 3. The cyclic AMP-dependent phosphorylation induced enhancement of IBa, GTP gamma S was voltage dependent and either did not occur or was observed only transiently at a holding potential (VH) of -30 mV. The forskolin-stimulated enhancement seen at VH -80 mV was lost with a t1/2 of about 1 min when VH was depolarized to -30 mV. Cholera toxin pre-treatment also increased the amplitude of IBa, GTP gamma S at VH -80 mV but not at VH -30 mV. 4. The calcium channel antagonist (-)-202-791 (5 microM) increased the amplitude of IBa, GTP gamma S when applied at VH -80 mV, but either not, or only transiently, at VH -30 mV, as previously observed. This 'agonist' effect of (-)-202-791 was prevented by PKI and was occluded by prior enhancement of IBa, GTP gamma S with forskolin. (-)-202-791 did not increase cyclic AMP levels in DRG neurones. 5. The 'agonist' response of IBa, GTP gamma S to D600 (10 microM) was also occluded by application of forskolin (10 microM) in the patch pipette. Forskolin alone, applied in this manner, increased IBa, GTP gamma S to a similar extent to D600 applied alone. 6. The agonist effect of (+)-202-791 (5 microM) on IBa, GTP gamma S was not prevented by prior enhancement with forskolin, nor was it prevented by PKI. 7. In conclusion, internal GTP gamma S activates G proteins which may interact directly with calcium channels to influence the kinetics of activation and to reduce steady-state inactivation of the channels. There is also an indirect effect on the generation of second messengers such as cyclic AMP. It is likely that forskolin enhances IBa, GTP gamma S by increasing activated Gs coupling to adenylyl cyclase and increasing cyclic AMP generation. The mechanism of action of (-)-202-791 to enhance IBa, GTP gamma S also involves cyclic AMP-dependent phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)