Molecular localization of ion selectivity sites within the pore of a human L-type cardiac calcium channel

A lysine residue from an extracellular turret switches the ion preference in a Cav3 T-Type channel from calcium to sodium ions

Cav3 T-type calcium channels from great pond snail Lymnaea stagnalis have a selectivity-filter ring of five acidic residues, EE(D)DD. Splice variants with exons 12b or 12a spanning the extracellular loop between the outer helix IIS5 and membrane-descending pore helix IIP1 (IIS5-P1) in Domain II of the pore module possess calcium selectivity or dominant sodium permeability, respectively. Here, we use AlphaFold2 neural network software to predict that a lysine residue in exon 12a is salt-bridged to the aspartate residue immediately C terminal to the second-domain glutamate in the selectivity filter. Exon 12b has a similar folding but with an alanine residue in place of lysine in exon 12a. We express LCav3 channels with mutated exons Ala-12b-Lys and Lys-12a-Ala and demonstrate that they switch the ion preference to high sodium permeability and calcium selectivity, respectively. We propose that in the calcium-selective variants, a calcium ion chelated between Domain II selectivity-filter glutamate and aspartate is knocked-out by the incoming calcium ion in the process of calcium permeation, whereas sodium ions are repelled. The aspartate is neutralized by the lysine residue in the sodium-permeant variants, allowing for sodium permeation through the selectivity-filter ring of four negatively charged residues akin to the prokaryotic sodium channels with four glutamates in the selectivity filter. The evolutionary adaptation in invertebrate LCav3 channels highlight the involvement of a key, ubiquitous aspartate, "a calcium beacon" of sorts in the outer pore of Domain II, as determinative for the calcium ion preference over sodium ions through eukaryotic Cav1, Cav2, and Cav3 channels.

Cav2.3 channel function and Zn2+-induced modulation: potential mechanisms and (patho)physiological relevance

Voltage-gated calcium channels (VGCCs) are critical for Ca²⁺ influx into all types of excitable cells, but their exact function is still poorly understood. Recent reconstruction of homology models for all human VGCCs at atomic resolution provides the opportunity for a structure-based discussion of VGCC function and novel insights into the mechanisms underlying Ca²⁺ selective flux through these channels. In the present review, we use these data as a basis to examine the structure, function, and Zn²⁺-induced modulation of Ca_v2.3 VGCCs, which mediate native R-type currents and belong to the most enigmatic members of the family. Their unique sensitivity to Zn²⁺ and the existence of multiple mechanisms of Zn²⁺ action strongly argue for a role of these channels in the modulatory action of endogenous loosely bound Zn²⁺, pools of which have been detected in a number of neuronal, endocrine, and reproductive tissues. Following a description of the different mechanisms by which Zn²⁺ has been shown or is thought to alter the function of these channels, we discuss their potential (patho)physiological relevance, taking into account what is known about the magnitude and function of extracellular Zn²⁺ signals in different tissues. While still far from complete, the picture that emerges is one where Ca_v2.3 channel expression parallels the occurrence of loosely bound Zn²⁺ pools in different tissues and where these channels may serve to translate physiological Zn²⁺ signals into changes of electrical activity and/or intracellular Ca²⁺ levels.

A pore-localizing CACNA1C-E1115K missense mutation, identified in a patient with idiopathic QT prolongation, bradycardia, and autism spectrum disorder, converts the L-type calcium channel into a hybrid nonselective monovalent cation channel

BACKGROUND

Gain-of-function variants in the CACNA1C-encoded L-type calcium channel (LTCC, Ca_v1.2) cause type 8 long QT syndrome (LQT8). The pore region contains highly conserved glutamic acid (E) residues that collectively form the LTCC's selectivity filter. Here, we identified and characterized a pore-localizing missense variant, E1115K, that yielded a novel perturbation in the LTCC.

OBJECTIVE

The purpose of this study was to determine whether CACNA1C-E1115K alters the LTCC's selectivity and is the substrate for the patient's LQTS.

METHODS

The proband was a 14-year-old male with idiopathic QT prolongation and bradycardia. Genetic testing revealed a missense variant, CACNA1C-E1115K. The whole-cell patch clamp technique was used to measure CACNA1C-WT and -E1115K currents when heterologously expressed in TSA201 cells.

RESULTS

The CACNA1C-E1115K channel exhibited no inward calcium current. Instead, robust cardiac transient outward potassium current (I_to)-like outward currents that were blocked significantly by nifedipine were measured when 2 mM/0.1 mM extracellular/intracellular CaCl2 or 4 mM/141 mM extracellular/intracellular KCl was applied. Furthermore, when 140 mM extracellular NaCl was applied, the CACNA1C-E1115K channel revealed both robust inward persistent Na⁺ currents with slower inactivation and outward currents, which were also nifedipine sensitive. In contrast, CACNA1C-WT revealed only a small inward persistent Na⁺ current without a robust outward current.

CONCLUSION

This CACNA1C-E1115K variant destroyed the LTCC's calcium selectivity and instead converted the mutant channel into a channel with a marked increase in sodium-mediated inward currents and potassium-mediated outward currents. Despite the anticipated 50% reduction in LTCC, the creation of a new population of channels with accentuated inward and outward currents represents the likely pathogenic substrates for the patient's LQTS and arrhythmia phenotype.

Intramembranal disulfide cross-linking elucidates the super-quaternary structure of mammalian CatSpers

CatSper is a voltage-dependent calcium channel located in the plasma membrane of the sperm flagellum and is responsible for triggering hyperactive motility. A homology model for the transmembrane region was built in which the arrangement of the subunits around the pseudo-four-fold symmetry axis was deduced by the pairing of conserved transmembranal cysteines across mammals. Directly emergent of the predicted quaternary structure is an architecture in which tetramers polymerize through additional, highly conserved cysteines, creating one or more double-rows channels extending the length of the principal piece of the mammalian sperm tail. The few species that are missing these cysteines are eusocial or otherwise monogamous, suggesting that sperm competition is selective for a disulfide-crosslinked macromolecular architecture. The model suggests testable hypotheses for how CatSper channel opening might behave in response to pH, 2-arachidonoylglycerol, and mechanical force. A flippase function is hypothesized, and a source of the concomitant disulfide isomerase activity is found in CatSper-associated proteins β, δ and ε.

CaV1.3 L-type Ca2+ channel contributes to the heartbeat by generating a dihydropyridine-sensitive persistent Na+ current

The spontaneous activity of sinoatrial node (SAN) pacemaker cells is generated by a functional interplay between the activity of ionic currents of the plasma membrane and intracellular Ca²⁺ dynamics. The molecular correlate of a dihydropyridine (DHP)-sensitive sustained inward Na⁺ current (I_st), a key player in SAN automaticity, is still unknown. Here we show that I_st and the L-type Ca²⁺ current (I_Ca,L) share Ca_V1.3 as a common molecular determinant. Patch-clamp recordings of mouse SAN cells showed that I_st is activated in the diastolic depolarization range, and displays Na⁺ permeability and minimal inactivation and sensitivity to I_Ca,L activators and blockers. Both Ca_V1.3-mediated I_Ca,L and I_st were abolished in Ca_V1.3-deficient (Ca_V1.3^−/−) SAN cells but the Ca_V1.2-mediated I_Ca,L current component was preserved. In SAN cells isolated from mice expressing DHP-insensitive Ca_V1.2 channels (Ca_V1.2^DHP−/−), I_st and Ca_V1.3-mediated I_Ca,L displayed overlapping sensitivity and concentration–response relationships to the DHP blocker nifedipine. Consistent with the hypothesis that Ca_V1.3 rather than Ca_V1.2 underlies I_st, a considerable fraction of I_Ca,L was resistant to nifedipine inhibition in Ca_V1.2^DHP−/− SAN cells. These findings identify Ca_V1.3 channels as essential molecular components of the voltage-dependent, DHP-sensitive I_st Na⁺ current in the SAN.

A voltage-gated calcium channel regulates lysosomal fusion with endosomes and autophagosomes and is required for neuronal homeostasis

Autophagy helps deliver sequestered intracellular cargo to lysosomes for proteolytic degradation and thereby maintains cellular homeostasis by preventing accumulation of toxic substances in cells. In a forward mosaic screen in Drosophila designed to identify genes required for neuronal function and maintenance, we identified multiple cacophony (cac) mutant alleles. They exhibit an age-dependent accumulation of autophagic vacuoles (AVs) in photoreceptor terminals and eventually a degeneration of the terminals and surrounding glia. cac encodes an α1 subunit of a Drosophila voltage-gated calcium channel (VGCC) that is required for synaptic vesicle fusion with the plasma membrane and neurotransmitter release. Here, we show that cac mutant photoreceptor terminals accumulate AV-lysosomal fusion intermediates, suggesting that Cac is necessary for the fusion of AVs with lysosomes, a poorly defined process. Loss of another subunit of the VGCC, α2δ or straightjacket (stj), causes phenotypes very similar to those caused by the loss of cac, indicating that the VGCC is required for AV-lysosomal fusion. The role of VGCC in AV-lysosomal fusion is evolutionarily conserved, as the loss of the mouse homologues, Cacna1a and Cacna2d2, also leads to autophagic defects in mice. Moreover, we find that CACNA1A is localized to the lysosomes and that loss of lysosomal Cacna1a in cerebellar cultured neurons leads to a failure of lysosomes to fuse with endosomes and autophagosomes. Finally, we show that the lysosomal CACNA1A but not the plasma-membrane resident CACNA1A is required for lysosomal fusion. In summary, we present a model in which the VGCC plays a role in autophagy by regulating the fusion of AVs with lysosomes through its calcium channel activity and hence functions in maintaining neuronal homeostasis.

A voltage-gated calcium channel required for neurotransmitter release also regulates the fusion of neuronal lysosomes with endosomes and autophagosomes, thereby helping to maintain cellular homeostasis.

Autophagy is a cellular process used by cells to prevent the accumulation of toxic substances. It delivers misfolded proteins and damaged organelles by fusing autophagosomes—organelles formed by a double membrane that surrounds the “debris” to be eliminated—with lysosomes. How this fusion process is regulated during autophagy, however, remains to be established. Here, we analyze this process in flies and mice, and find that loss of different subunits of a specific type of Voltage Gated Calcium Channel (VGCC) leads to defects in lysosomal fusion with autophagosomes in neurons. It was already known that VGCCs control calcium entry at synaptic terminals to promote the fusion of synaptic vesicles with the plasma membrane, and that mutations in the subunits of VGCCs in humans cause neurological diseases. Our data indicate that defects in autophagy and lysosomal fusion are independent of defects in synaptic vesicle fusion and neurotransmitter release, and we show that a specific VGCC is present on lysosomal membranes where it is required for lysosomal fusion with endosomes and autophagosomes. These observations suggest that the fusion events required in autophagy rely on mechanisms similar to those that trigger the fusion of synaptic vesicles with the presynaptic membrane.

Voltage-gated calcium channels: Determinants of channel function and modulation by inorganic cations

Voltage-gated calcium channels (VGCCs) represent a key link between electrical signals and non-electrical processes, such as contraction, secretion and transcription. Evolved to achieve high rates of Ca(2+)-selective flux, they possess an elaborate mechanism for selection of Ca(2+) over foreign ions. It has been convincingly linked to competitive binding in the pore, but the fundamental question of how this is reconcilable with high rates of Ca(2+) transfer remains unanswered. By virtue of their similarity to Ca(2+), polyvalent cations can interfere with the function of VGCCs and have proven instrumental in probing the mechanisms underlying selective permeation. Recent emergence of crystallographic data on a set of Ca(2+)-selective model channels provides a structural framework for permeation in VGCCs, and warrants a reconsideration of their diverse modulation by polyvalent cations, which can be roughly separated into three general mechanisms: (I) long-range interactions with charged regions on the surface, affecting the local potential sensed by the channel or influencing voltage-sensor movement by repulsive forces (electrostatic effects), (II) short-range interactions with sites in the ion-conducting pathway, leading to physical obstruction of the channel (pore block), and in some cases (III) short-range interactions with extracellular binding sites, leading to non-electrostatic modifications of channel gating (allosteric effects). These effects, together with the underlying molecular modifications, provide valuable insights into the function of VGCCs, and have important physiological and pathophysiological implications. Allosteric suppression of some of the pore-forming Cavα1-subunits (Cav2.3, Cav3.2) by Zn(2+) and Cu(2+) may play a major role for the regulation of excitability by endogenous transition metal ions. The fact that these ions can often traverse VGCCs can contribute to the detrimental intracellular accumulation of metal ions following excessive release of endogenous Cu(2+) and Zn(2+) or exposure to non-physiological toxic metal ions.

Evolution of voltage-gated ion channels at the emergence of Metazoa

Voltage-gated ion channels are large transmembrane proteins that enable the passage of ions through their pore across the cell membrane. These channels belong to one superfamily and carry pivotal roles such as the propagation of neuronal and muscular action potentials and the promotion of neurotransmitter secretion in synapses. In this review, we describe in detail the current state of knowledge regarding the evolution of these channels with a special emphasis on the metazoan lineage. We highlight the contribution of the genomic revolution to the understanding of ion channel evolution and for revealing that these channels appeared long before the appearance of the first animal. We also explain how the elucidation of channel selectivity properties and function in non-bilaterian animals such as cnidarians (sea anemones, corals, jellyfish and hydroids) can contribute to the study of channel evolution. Finally, we point to open questions and future directions in this field of research.

Ion selectivity strategies of sodium channel selectivity filters

CONSPECTUS: Sodium ion channels selectively transport Na(+) cations across the cell membrane. These integral parts of the cell machinery are implicated in regulating the cardiac, skeletal and smooth muscle contraction, nerve impulses, salt and water homeostasis, as well as pain and taste perception. Their malfunction often results in various channelopathies of the heart, brain, skeletal muscles, and lung; thus, sodium channels are key drug targets for various disorders including cardiac arrhythmias, heart attack, stroke, migraine, epilepsy, pain, cancer, and autoimmune disorders. The ability of sodium channels to discriminate the native Na(+) among other competing ions in the surrounding fluids is crucial for proper cellular functions. The selectivity filter (SF), the narrowest part of the channel's open pore, lined with amino acid residues that specifically interact with the permeating ion, plays a major role in determining Na(+) selectivity. Different sodium channels have different SFs, which vary in the symmetry, number, charge, arrangement, and chemical type of the metal-ligating groups and pore size: epithelial/degenerin/acid-sensing ion channels have generally trimeric SFs lined with three conserved neutral serines and/or backbone carbonyls; eukaryotic sodium channels have EKEE, EEKE, DKEA, and DEKA SFs with an invariant positively charged lysine from the second or third domain; and bacterial voltage-gated sodium (Nav) channels exhibit symmetrical EEEE SFs, reminiscent of eukaryotic voltage-gated calcium channels. How do these different sodium channel SFs achieve high selectivity for Na(+) over its key rivals, K(+) and Ca(2+)? What factors govern the metal competition in these SFs and which of these factors are exploited to achieve Na(+) selectivity in the different sodium channel SFs? The free energies for replacing K(+) or Ca(2+) bound inside different model SFs with Na(+), evaluated by a combination of density functional theory and continuum dielectric calculations, have shed light on these questions. The SFs of epithelial and eukaryotic Nav channels select Na(+) by providing an optimal number and ligating strength of metal ligands as well as a rigid pore whose size fits the cognate Na(+) ideally. On the other hand, the SFs of bacterial Nav channels select Na(+), as the protein matrix attenuates ion-protein interactions relative to ion-solvent interactions by enlarging the pore and allowing water to enter, so the ion interacts indirectly with the conserved glutamates via bridging water molecules. This shows how these various SFs have adapted to the specific physicochemical properties of the native ion, using different strategies to select Na(+) among its contenders.

Structural and stoichiometric determinants of Ca2+ release-activated Ca2+ (CRAC) channel Ca2+-dependent inactivation

Depletion of intracellular Ca(2+) stores in mammalian cells results in Ca(2+) entry across the plasma membrane mediated primarily by Ca(2+) release-activated Ca(2+) (CRAC) channels. Ca(2+) influx through these channels is required for the maintenance of homeostasis and Ca(2+) signaling in most cell types. One of the main features of native CRAC channels is fast Ca(2+)-dependent inactivation (FCDI), where Ca(2+) entering through the channel binds to a site near its intracellular mouth and causes a conformational change, closing the channel and limiting further Ca(2+) entry. Early studies suggested that FCDI of CRAC channels was mediated by calmodulin. However, since the discovery of STIM1 and Orai1 proteins as the basic molecular components of the CRAC channel, it has become apparent that FCDI is a more complex phenomenon. Data obtained using heterologous overexpression of STIM1 and Orai1 suggest that, in addition to calmodulin, several cytoplasmic domains of STIM1 and Orai1 and the selectivity filter within the channel pore are required for FCDI. The stoichiometry of STIM1 binding to Orai1 also has emerged as an important determinant of FCDI. Consequently, STIM1 protein expression levels have the potential to be an endogenous regulator of CRAC channel Ca(2+) influx. This review discusses the current understanding of the molecular mechanisms governing the FCDI of CRAC channels, including an evaluation of further experiments that may delineate whether STIM1 and/or Orai1 protein expression is endogenously regulated to modulate CRAC channel function, or may be dysregulated in some pathophysiological states.

Sodium channel selectivity and conduction: prokaryotes have devised their own molecular strategy

The molecular strategy for alkali cation selectivity by a bacterial sodium channel resembles those of eukaryotic calcium and potassium channels, rather than those of eukaryotic sodium channels.

Striking structural differences between voltage-gated sodium (Nav) channels from prokaryotes (homotetramers) and eukaryotes (asymmetric, four-domain proteins) suggest the likelihood of different molecular mechanisms for common functions. For these two channel families, our data show similar selectivity sequences among alkali cations (relative permeability, Pion/PNa) and asymmetric, bi-ionic reversal potentials when the Na/K gradient is reversed. We performed coordinated experimental and computational studies, respectively, on the prokaryotic Nav channels NaChBac and NavAb. NaChBac shows an “anomalous,” nonmonotonic mole-fraction dependence in the presence of certain sodium–potassium mixtures; to our knowledge, no comparable observation has been reported for eukaryotic Nav channels. NaChBac’s preferential selectivity for sodium is reduced either by partial titration of its highly charged selectivity filter, when extracellular pH is lowered from 7.4 to 5.8, or by perturbation—likely steric—associated with a nominally electro-neutral substitution in the selectivity filter (E191D). Although no single molecular feature or energetic parameter appears to dominate, our atomistic simulations, based on the published NavAb crystal structure, revealed factors that may contribute to the normally observed selectivity for Na over K. These include: (a) a thermodynamic penalty to exchange one K⁺ for one Na⁺ in the wild-type (WT) channel, increasing the relative likelihood of Na⁺ occupying the binding site; (b) a small tendency toward weaker ion binding to the selectivity filter in Na–K mixtures, consistent with the higher conductance observed with both sodium and potassium present; and (c) integrated 1-D potentials of mean force for sodium or potassium movement that show less separation for the less selective E/D mutant than for WT. Overall, tight binding of a single favored ion to the selectivity filter, together with crucial inter-ion interactions within the pore, suggests that prokaryotic Nav channels use a selective strategy more akin to those of eukaryotic calcium and potassium channels than that of eukaryotic Nav channels.

Bacterial sodium channels: models for eukaryotic sodium and calcium channels

Eukaryotic sodium and calcium channels are made up of four linked homologous but different transmembrane domains. Bacteria express sodium channels comprised of four identical subunits, each being analogous to a single homologous domain of their eukaryotic counterparts. Key elements of primary structure are conserved between bacterial and eukaryotic sodium and calcium channels. The simple protein structure of the bacterial channels has allowed extensive structure-function probes of key regions as well as allowing determination of several X-ray crystallographic structures of these channels. The structures have revealed novel features of sodium and calcium channel pores and elucidated the structural importance of many of the conserved features of primary sequence. The structural information has also formed the basis for computational studies probing the basis for sodium and calcium selectivity and gating.

Energetics of discrete selectivity bands and mutation-induced transitions in the calcium-sodium ion channels family

We use Brownian dynamics (BD) simulations to study the ionic conduction and valence selectivity of a generic electrostatic model of a biological ion channel as functions of the fixed charge Q(f) at its selectivity filter. We are thus able to reconcile the discrete calcium conduction bands recently revealed in our BD simulations, M0 (Q(f)=1e), M1 (3e), M2 (5e), with a set of sodium conduction bands L0 (0.5e), L1 (1.5e), thereby obtaining a completed pattern of conduction and selectivity bands vs Q(f) for the sodium-calcium channels family. An increase of Q(f) leads to an increase of calcium selectivity: L0 (sodium-selective, nonblocking channel) → M0 (nonselective channel) → L1 (sodium-selective channel with divalent block) → M1 (calcium-selective channel exhibiting the anomalous mole fraction effect). We create a consistent identification scheme where the L0 band is putatively identified with the eukaryotic sodium channel The scheme created is able to account for the experimentally observed mutation-induced transformations between nonselective channels, sodium-selective channels, and calcium-selective channels, which we interpret as transitions between different rows of the identification table. By considering the potential energy changes during permeation, we show explicitly that the multi-ion conduction bands of calcium and sodium channels arise as the result of resonant barrierless conduction. The pattern of periodic conduction bands is explained on the basis of sequential neutralization taking account of self-energy, as Q(f)(z,i)=ze(1/2+i), where i is the order of the band and z is the valence of the ion. Our results confirm the crucial influence of electrostatic interactions on conduction and on the Ca(2+)/Na(+) valence selectivity of calcium and sodium ion channels. The model and results could be also applicable to biomimetic nanopores with charged walls.

Structure of a prokaryotic sodium channel pore reveals essential gating elements and an outer ion binding site common to eukaryotic channels

Voltage-gated sodium channels (NaVs) are central elements of cellular excitation. Notwithstanding advances from recent bacterial NaV (BacNaV) structures, key questions about gating and ion selectivity remain. Here, we present a closed conformation of NaVAe1p, a pore-only BacNaV derived from NaVAe1, a BacNaV from the arsenite oxidizer Alkalilimnicola ehrlichei found in Mono Lake, California, that provides insight into both fundamental properties. The structure reveals a pore domain in which the pore-lining S6 helix connects to a helical cytoplasmic tail. Electrophysiological studies of full-length BacNaVs show that two elements defined by the NaVAe1p structure, an S6 activation gate position and the cytoplasmic tail "neck", are central to BacNaV gating. The structure also reveals the selectivity filter ion entry site, termed the "outer ion" site. Comparison with mammalian voltage-gated calcium channel (CaV) selectivity filters, together with functional studies, shows that this site forms a previously unknown determinant of CaV high-affinity calcium binding. Our findings underscore commonalities between BacNaVs and eukaryotic voltage-gated channels and provide a framework for understanding gating and ion permeation in this superfamily.

The mitochondrial calcium uniporter is a multimer that can include a dominant-negative pore-forming subunit

Mitochondrial calcium uniporter (MCU) channel is responsible for Ruthenium Red-sensitive mitochondrial calcium uptake. Here, we demonstrate MCU oligomerization by immunoprecipitation and Förster resonance energy transfer (FRET) and characterize a novel protein (MCUb) with two predicted transmembrane domains, 50% sequence similarity and a different expression profile from MCU. Based on computational modelling, MCUb includes critical amino-acid substitutions in the pore region and indeed MCUb does not form a calcium-permeable channel in planar lipid bilayers. In HeLa cells, MCUb is inserted into the oligomer and exerts a dominant-negative effect, reducing the [Ca(2+)]mt increases evoked by agonist stimulation. Accordingly, in vitro co-expression of MCUb with MCU drastically reduces the probability of observing channel activity in planar lipid bilayer experiments. These data unveil the structural complexity of MCU and demonstrate a novel regulatory mechanism, based on the inclusion of dominant-negative subunits in a multimeric channel, that underlies the fine control of the physiologically and pathologically relevant process of mitochondrial calcium homeostasis.

Characterization of inward currents and channels underlying burst activity in motoneurons of crab cardiac ganglion

Large cell motoneurons in the Cancer borealis cardiac ganglion generate rhythmic bursts of action potentials responsible for cardiac contractions. While it is well known that these burst potentials are dependent on coordinated interactions among depolarizing and hyperpolarizing conductances, the depolarizing currents present in these cells, and their biophysical characteristics, have not been thoroughly described. In this study we used a combined molecular biology and electrophysiology approach to look at channel identity, expression, localization, and biophysical properties for two distinct high-voltage-activated calcium currents present in these cells: a slow calcium current ( ICaS) and a transient calcium current ( ICaT). Our data indicate that CbCaV1 is a putative voltage-gated calcium channel subunit in part responsible for an L-type current, while CbCaV2 (formerly cacophony) is a subunit in part responsible for a P/Q-type current. These channels appear to be localized primarily to the somata of the motoneurons. A third calcium channel gene (CbCaV3) was identified that encodes a putative T-type calcium channel subunit and is expressed in these cells, but electrophysiological studies failed to detect this current in motoneuron somata. In addition, we identify and characterize for the first time in these cells a calcium-activated nonselective cationic current ( ICAN), as well as a largely noninactivating TTX-sensitive current reminiscent of a persistent sodium current. The identification and further characterization of these currents allow both biological and modeling studies to move forward with more attention to the complexity of interactions among these distinct components underlying generation of bursting output in motoneurons.

How "Pharmacoresistant" is Cav2.3, the Major Component of Voltage-Gated R-type Ca2+ Channels?

Membrane-bound voltage-gated Ca²⁺ channels (VGCCs) are targets for specific signaling complexes, which regulate important processes like gene expression, neurotransmitter release and neuronal excitability. It is becoming increasingly evident that the so called “resistant” (R-type) VGCC Ca_v2.3 is critical in several physiologic and pathophysiologic processes in the central nervous system, vascular system and in endocrine systems. However its eponymous attribute of pharmacologic inertness initially made in depth investigation of the channel difficult. Although the identification of SNX-482 as a fairly specific inhibitor of Ca_v2.3 in the nanomolar range has enabled insights into the channels properties, availability of other pharmacologic modulators of Ca_v2.3 with different chemical, physical and biological properties are of great importance for future investigations. Therefore the literature was screened systematically for molecules that modulate Ca_v2.3 VGCCs.

Voltage-driven Ca(2+) binding at the L-type Ca(2+) channel triggers cardiac excitation-contraction coupling prior to Ca(2+) influx

The activation of the ryanodine Ca(2+) release channels (RyR2) by the entry of Ca(2+) through the L-type Ca(2+) channels (Cav1.2) is believed to be the primary mechanism of excitation-contraction (EC) coupling in cardiac cells. This proposed mechanism of Ca(2+)-induced Ca(2+) release (CICR) cannot fully account for the lack of a termination signal for this positive feedback process. Using Cav1.2 channel mutants, we demonstrate that the Ca(2+)-impermeable α(1)1.2/L775P/T1066Y mutant introduced through lentiviral infection into neonate cardiomyocytes triggers Ca(2+) transients in a manner independent of Ca(2+) influx. In contrast, the α(1)1.2/L775P/T1066Y/4A mutant, in which the Ca(2+)-binding site of the channel was destroyed, supports neither the spontaneous nor the electrically evoked contractions. Ca(2+) bound at the channel selectivity filter appears to initiate a signal that is conveyed directly from the channel pore to RyR2, triggering contraction of cardiomyocytes prior to Ca(2+) influx. Thus, RyR2 is activated in response to a conformational change in the L-type channel during membrane depolarization and not through interaction with Ca(2+) ions diffusing in the junctional gap space. Accordingly, termination of the RyR2 activity is achieved when the signal stops upon the return of the L-channel to the resting state. We propose a new model in which the physical link between Cav1.2 and RyR2 allows propagation of a conformational change induced at the open pore of the channel to directly activate RyR2. These results highlight Cav1.2 as a signaling protein and provide a mechanism for terminating the release of Ca(2+) from RyR2 through protein-protein interactions. In this model, the L-type channel is a master regulator of both initiation and termination of EC coupling in neonate cardiomyocytes.

Competition among Ca2+, Mg2+, and Na+ for model ion channel selectivity filters: determinants of ion selectivity

Because voltage-gated ion channels play critical biological roles, understanding how they can discriminate the native metal ion from rival cations in the milieu is of great interest. Although Ca(2+), Mg(2+), and Na(+) are present in comparable concentrations outside the cell, the factors governing the competition among these cations for the selectivity filter of voltage-gated Ca(2+) ion channel remain unclear. Using density functional theory combined with continuum dielectric methods, we evaluate the effect of (1) the number, chemical type, and charge of the ligands lining the pore, (2) the pore's rigidity, size, symmetry, and solvent accessibility, and (3) the Ca(2+) hydration number outside the selectivity filter on the competition among Ca(2+), Mg(2+), and Na(+) in model selectivity filters. The calculations show how the outcome of this competition depends on the interplay between electronic and solvation effects. Selectivity for monovalent Na(+) over divalent Ca(2+)/Mg(2+) is achieved when solvation effects outweigh electrostatic effects; thus filters comprising a few weak charge-donating groups such as Ser/Thr side chains, where electrostatic effects are relatively weak and are easily overcome by solvation effects, are Na(+)-selective. In contrast, selectivity for divalent Ca(2+)/Mg(2+) over monovalent Na(+) is achieved when metal-ligand electrostatic effects outweigh solvation effects. The key differences in selectivity between Mg(2+) and Ca(2+) lie in the pore size, oligomericity, and solvent accessibility. The results, which are consistent with available experimental data, reveal how the structure and composition of the ion channel selectivity pore had adapted to the specific physicochemical properties of the native metal ion to enhance the competitiveness of the native metal toward rival cations.

Sodium leak channels in neuronal excitability and rhythmic behaviors

Extracellular K⁺, Na⁺, and Ca²⁺ ions all influence the resting membrane potential of the neuron. However, the mechanisms by which extracellular Na⁺ and Ca²⁺ regulate basal neuronal excitability are not well understood. Recent findings suggest that NALCN, in association with UNC79 and UNC80, contributes a basal Na⁺ leak conductance in neurons. Mutations in Nalcn, Unc79, or Unc80 lead to severe phenotypes that include neonatal lethality and disruption in rhythmic behaviors. This review discusses the properties of the NALCN complex, its regulation, and its contribution to neuronal function and animal behavior.

Two sets of amino acids of the domain I of Cav2.3 Ca(2+) channels contribute to their high sensitivity to extracellular protons

Extracellular acidification decreases Ca(2+) current amplitude and produces a depolarizing shift in the activation potential (Va) of voltage-gated Ca(2+) channels (VGCC). These effects are common to all VGCC, but differences exist between Ca(2+) channel types and the underlying molecular mechanisms remain largely unknown. We report here that the changes in current amplitude induced by extracellular acidification or alkalinisation are more important for Cav2.3 R type than for Cav2.1 P/Q-type Ca(2+) channels. This difference results from a higher shift of Va combined with a modification of channel conductance. Although involved in the sensitivity of channel conductance to extracellular protons, neither the EEEE locus nor the divalent cation selectivity locus could explain the specificity of the pH effects. We show that this specificity involves two separate sets of amino acids within domain I of the Cavα subunit. Residues of the voltage sensor domain and residues in the pore domain mediate the effects of extracellular protons on Va and on channel conductance, respectively. These new insights are important for elucidating the molecular mechanisms that control VGCC gating and conductance and for understanding the role of extracellular protons in other channels or membrane-tethered enzymes with similar pore and/or voltage sensor domains.

Structural studies of ion permeation and Ca2+ blockage of a bacterial channel mimicking the cyclic nucleotide-gated channel pore

Cyclic nucleotide-gated (CNG) channels play an essential role in the visual and olfactory sensory systems and are ubiquitous in eukaryotes. Details of their underlying ion selectivity properties are still not fully understood and are a matter of debate in the absence of high-resolution structures. To reveal the structural mechanism of ion selectivity in CNG channels, particularly their Ca(2+) blockage property, we engineered a set of mimics of CNG channel pores for both structural and functional analysis. The mimics faithfully represent the CNG channels they are modeled after, permeate Na(+) and K(+) equally well, and exhibit the same Ca(2+) blockage and permeation properties. Their high-resolution structures reveal a hitherto unseen selectivity filter architecture comprising three contiguous ion binding sites in which Na(+) and K(+) bind with different ion-ligand geometries. Our structural analysis reveals that the conserved acidic residue in the filter is essential for Ca(2+) binding but not through direct ion chelation as in the currently accepted view. Furthermore, structural insight from our CNG mimics allows us to pinpoint equivalent interactions in CNG channels through structure-based mutagenesis that have previously not been predicted using NaK or K(+) channel models.

Possible roles of exceptionally conserved residues around the selectivity filters of sodium and calcium channels

In the absence of x-ray structures of sodium and calcium channels their homology models are used to rationalize experimental data and design new experiments. A challenge is to model the outer-pore region that folds differently from potassium channels. Here we report a new model of the outer-pore region of the NaV1.4 channel, which suggests roles of highly conserved residues around the selectivity filter. The model takes from our previous study (Tikhonov, D. B., and Zhorov, B. S. (2005) Biophys. J. 88, 184-197) the general disposition of the P-helices, selectivity filter residues, and the outer carboxylates, but proposes new intra- and inter-domain contacts that support structural stability of the outer pore. Glycine residues downstream from the selectivity filter are proposed to participate in knob-into-hole contacts with the P-helices and S6s. These contacts explain the adapted tetrodotoxin resistance of snakes that feed on toxic prey through valine substitution of isoleucine in the P-helix of repeat IV. Polar residues five positions upstream from the selectivity filter residues form H-bonds with the ascending-limb backbones. Exceptionally conserved tryptophans are engaged in inter-repeat H-bonds to form a ring whose π-electrons would facilitate passage of ions from the outer carboxylates to the selectivity filter. The outer-pore model of CaV1.2 derived from the NaV1.4 model is also stabilized by the ring of exceptionally conservative tryptophans and H-bonds between the P-helices and ascending limbs. In this model, the exceptionally conserved aspartate downstream from the selectivity-filter glutamate in repeat II facilitates passage of calcium ions to the selectivity-filter ring through the tryptophan ring. Available experimental data are discussed in view of the models.

Structural modeling of calcium binding in the selectivity filter of the L-type calcium channel

Calcium channels play crucial physiological roles. In the absence of high-resolution structures of the channels, the mechanism of ion permeation is unknown. Here we used a method proposed in an accompanying paper (Cheng and Zhorov in Eur Biophys J, 2009) to predict possible chelation patterns of calcium ions in a structural model of the L-type calcium channel. We compared three models in which two or three calcium ions interact with the four selectivity filter glutamates and a conserved aspartate adjacent to the glutamate in repeat II. Monte Carlo energy minimizations yielded many complexes with calcium ions bound to at least two selectivity filter carboxylates. In these complexes calcium-carboxylate attractions are counterbalanced by calcium-calcium and carboxylate-carboxylate repulsions. Superposition of the complexes suggests a high degree of mobility of calcium ions and carboxylate groups of the glutamates. We used the predicted complexes to propose a permeation mechanism that involves single-file movement of calcium ions. The key feature of this mechanism is the presence of bridging glutamates that coordinate two calcium ions and enable their transitions between different chelating patterns involving four to six oxygen atoms from the channel protein. The conserved aspartate is proposed to coordinate a calcium ion incoming to the selectivity filter from the extracellular side. Glutamates in repeats III and IV, which are most distant from the repeat II aspartate, are proposed to coordinate the calcium ion that leaves the selectivity filter to the inner pore. Published experimental data and earlier proposed permeation models are discussed in view of our model.

Chlamydomonas CAV2 encodes a voltage- dependent calcium channel required for the flagellar waveform conversion

Cilia and flagella can alter their beating patterns through changes in membrane excitation mediated by Ca(2+) influx. The ion channel that generates this Ca(2+) influx and its cellular distribution have not been identified. In this study, we analyzed the Chlamydomonas ppr2 mutant, which is deficient in the production of a flagellar Ca(2+) current and consequently has a defective photophobic response and mechanoshock response. ppr2 had a mutation in CAV2, which encodes a homolog of the alpha(1) subunit of voltage-dependent calcium channels (VDCCs). CAV2 has four domains, each with six transmembrane segments and EEEE loci in the ion-selective filter, which are typical of VDCCs in vertebrates. Interestingly, we found that CAV2 primarily localized toward the distal part of flagella. We provide evidence that CAV2 is transported toward the flagellar tip via intraflagellar transport (IFT) because CAV2 accumulated near the flagellar base when IFT was blocked. The results of this study suggest that the Ca(2+) influx of Chlamydomonas flagella is mediated by the VDCC, CAV2, whose distribution is biased to the distal region of the flagellum.

Molecular determinant for specific Ca/Ba selectivity profiles of low and high threshold Ca2+ channels

Voltage-gated Ca²⁺ channels (VGCC) play a key role in many physiological functions by their high selectivity for Ca²⁺ over other divalent and monovalent cations in physiological situations. Divalent/monovalent selection is shared by all VGCC and is satisfactorily explained by the existence, within the pore, of a set of four conserved glutamate/aspartate residues (EEEE locus) coordinating Ca²⁺ ions. This locus however does not explain either the choice of Ca²⁺ among other divalent cations or the specific conductances encountered in the different VGCC. Our systematic analysis of high- and low-threshold VGCC currents in the presence of Ca²⁺ and Ba²⁺ reveals highly specific selectivity profiles. Sequence analysis, molecular modeling, and mutational studies identify a set of nonconserved charged residues responsible for these profiles. In HVA (high voltage activated) channels, mutations of this set modify divalent cation selectivity and channel conductance without change in divalent/monovalent selection, activation, inactivation, and kinetics properties. The Ca_V2.1 selectivity profile is transferred to Ca_V2.3 when exchanging their residues at this location. Numerical simulations suggest modification in an external Ca²⁺ binding site in the channel pore directly involved in the choice of Ca²⁺, among other divalent physiological cations, as the main permeant cation for VGCC. In LVA (low voltage activated) channels, this locus (called DCS for divalent cation selectivity) also influences divalent cation selection, but our results suggest the existence of additional determinants to fully recapitulate all the differences encountered among LVA channels. These data therefore attribute to the DCS a unique role in the specific shaping of the Ca²⁺ influx between the different HVA channels.

Structural insight into Ca2+ specificity in tetrameric cation channels

Apparent blockage of monovalent cation currents by the permeating blocker Ca(2+) is a physiologically essential phenomenon relevant to cyclic nucleotide-gated (CNG) channels. The recently determined crystal structure of a bacterial homolog of CNG channel pores, the NaK channel, revealed a Ca(2+) binding site at the extracellular entrance to the selectivity filter. This site is not formed by the side-chain carboxylate groups from the conserved acidic residue, Asp-66 in NaK, conventionally thought to directly chelate Ca(2+) in CNG channels, but rather by the backbone carbonyl groups of residue Gly-67. Here we present a detailed structural analysis of the NaK channel with a focus on Ca(2+) permeability and blockage. Our results confirm that the Asp-66 residue, although not involved in direct chelation of Ca(2+), plays an essential role in external Ca(2+) binding. Furthermore, we give evidence for the presence of a second Ca(2+) binding site within the NaK selectivity filter where monovalent cations also bind, providing a structural basis for Ca(2+) permeation through the NaK pore. Compared with other Ca(2+)-binding proteins, both sites in NaK present a novel mode of Ca(2+) chelation, using only backbone carbonyl oxygen atoms from residues in the selectivity filter. The external site is under indirect control by an acidic residue (Asp-66), making it Ca(2+)-specific. These findings give us a glimpse of the possible underlying mechanisms allowing Ca(2+) to act both as a permeating ion and blocker of CNG channels and raise the possibility of a similar chemistry governing Ca(2+) chelation in Ca(2+) channels.

The Neuronal Channel NALCN Contributes Resting Sodium Permeability and Is Required for Normal Respiratory Rhythm

Sodium plays a key role in determining the basal excitability of the nervous systems through the resting "leak" Na(+) permeabilities, but the molecular identities of the TTX- and Cs(+)-resistant Na(+) leak conductance are totally unknown. Here we show that this conductance is formed by the protein NALCN, a substantially uncharacterized member of the sodium/calcium channel family. Unlike any of the other 20 family members, NALCN forms a voltage-independent, nonselective cation channel. NALCN mutant mice have a severely disrupted respiratory rhythm and die within 24 hours of birth. Brain stem-spinal cord recordings reveal reduced neuronal firing. The TTX- and Cs(+)-resistant background Na(+) leak current is absent in the mutant hippocampal neurons. The resting membrane potentials of the mutant neurons are relatively insensitive to changes in extracellular Na(+) concentration. Thus, NALCN, a nonselective cation channel, forms the background Na(+) leak conductance and controls neuronal excitability.

Conserved motifs in voltage-sensing and pore-forming modules of voltage-gated ion channel proteins

Voltage-gated ion channels (VGCs) mediate selective diffusion of ions across cell membranes to enable many vital cellular processes. Three-dimensional structure data are lacking for VGC proteins; hence, to better understand their function, there is a need to identify the conserved motifs using sequence analysis methods. In this study, we have used a profile-to-profile alignment method to identify several new conserved motifs specific to each transmembrane segment (TMS) of the voltage-sensing and the pore-forming modules of Ca2+, Na+, and K+ channel subfamilies. For Ca2+ and Na+, the functional theme of motif conservation is similar in all segments while they differ with those of the K+ channel proteins. Nevertheless, the conservation is strikingly similar in the S4 segment of the voltage-sensing module across all subfamilies. In each subfamily and for each TMS, we have identified conserved motifs/residues and correlated their functional significance and disease associations in human, using mutational data from the literature.

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.

CRACM1 multimers form the ion-selective pore of the CRAC channel

Receptor-mediated Ca(2+) release from the endoplasmic reticulum (ER) is often followed by Ca(2+) entry through Ca(2+)-release-activated Ca(2+) (CRAC) channels in the plasma membrane . RNAi screens have identified STIM1 as the putative ER Ca(2+) sensor and CRACM1 (Orai1; ) as the putative store-operated Ca(2+) channel. Overexpression of both proteins is required to reconstitute CRAC currents (I(CRAC); ). We show here that CRACM1 forms multimeric assemblies that bind STIM1 and that acidic residues in the transmembrane (TM) and extracellular domains of CRACM1 contribute to the ionic selectivity of the CRAC-channel pore. Replacement of the conserved glutamate in position 106 of the first TM domain of CRACM1 with glutamine (E106Q) acts as a dominant-negative protein, and substitution with aspartate (E106D) enhances Na(+), Ba(2+), and Sr(2+) permeation relative to Ca(2+). Mutating E190Q in TM3 also affects channel selectivity, suggesting that glutamate residues in both TM1 and TM3 face the lumen of the pore. Furthermore, mutating a putative Ca(2+) binding site in the first extracellular loop of CRACM1 (D110/112A) enhances monovalent cation permeation, suggesting that these residues too contribute to the coordination of Ca(2+) ions to the pore. Our data provide unequivocal evidence that CRACM1 multimers form the Ca(2+)-selective CRAC-channel pore.

The ionic dependence of voltage-activated inward currents in the pharyngeal muscle of Caenorhabditis elegans

The pharynx of Caenorhabditis elegans consists of a syncytium of radially orientated muscle cells that contract synchronously and rhythmically to ingest and crush bacteria and pump them into the intestine of the animal. The action potentials that support this activity are superficially similar to vertebrate cardiac action potentials in appearance with a long, calcium-dependent plateau phase. Although the pharyngeal muscle can generate action potentials in the absence of external calcium ions, action potentials are absent when sodium is removed from the extracellullar solution (Franks et al. 2002). Here we have used whole cell patch clamp recordings from the pharynx and show low voltage-activated inward currents that are present in zero external calcium and reduced in zero external sodium ions. Whilst the lack of effect of zero calcium when sodium ions are present is not surprising in view of the known permeability of voltage-gated calcium channels to sodium ions, the reduction in current in zero sodium when calcium ions are present is harder to explain in terms of a conventional voltage-gated calcium channel. Inward currents were also recorded from egl-19 (n582) which has a loss of function mutation in the pharyngeal L-type calcium channel and these were also markedly reduced in zero external sodium. Despite this apparent dependence on external sodium ions, the current was partially blocked by the divalent cations, cadmium, barium and nickel. Using single-channel recordings we identified a cation channel for which the open-time duration was increased by depolarisation. In inside-out patches, the single-channel conductance was highest in symmetrical sodium solution. Further studies are required to determine the contribution of these channels to the pharyngeal action potential.

The Ion Channel Narrow Abdomen Is Critical for Neural Output of the Drosophila Circadian Pacemaker

Circadian clocks consist of transcriptional feedback loops housed in interdependent pacemaker neurons. Yet little is known about the neuronal output components essential for rhythmic behavior. Drosophila mutants of a putative ion channel, narrow abdomen (na), exhibit poor circadian rhythms and suppressed daylight activity. We find that NA is expressed in pacemaker neurons and induced expression within circadian neurons is sufficient to rescue these mutant phenotypes. Selective na rescue in distinct pacemaker neurons influences rhythmicity and timing of behavior. Oscillations of the clock protein PERIOD are intact in na mutants, indicating an output role. Pore residues are required for robust rescue consistent with NA action as an ion channel. In na mutants, expression of potassium currents and the key neuropeptide PDF are elevated, the latter consistent with reduced release. These data implicate NA and the pacemaker neural network in controlling phase and rhythmicity.

Functional architecture of the inner pore of a voltage-gated Ca2+ channel

The inner pore of voltage-gated Ca²⁺ channels (VGCCs) is functionally important, but little is known about the architecture of this region. In K⁺ channels, this part of the pore is formed by the S6/M2 transmembrane segments from four symmetrically arranged subunits. The Ca²⁺ channel pore, however, is formed by four asymmetric domains of the same (α₁) subunit. Here we investigated the architecture of the inner pore of P/Q-type Ca²⁺ channels using the substituted-cysteine accessibility method. Many positions in the S6 segments of all four repeats of the α₁ subunit (Ca_v2.1) were modified by internal methanethiosulfonate ethyltrimethylammonium (MTSET). However, the pattern of modification does not fit any known sequence alignment with K⁺ channels. In IIS6, five consecutive positions showed clear modification, suggesting a likely aqueous crevice and a loose packing between S6 and S5 segments, a notion further supported by the observation that some S5 positions were also accessible to internal MTSET. These results indicate that the inner pore of VGCCs is indeed formed by the S6 segments but is different from that of K⁺ channels. Interestingly some residues in IIIS6 and IVS6 whose mutations in L-type Ca²⁺ channels affect the binding of dihydropyridines and phenylalkylamines and are thought to face the pore appeared not to react with internal MTSET. Probing with qBBr, a rigid thiol-reactive agent with a dimension of 12 Å × 10 Å × 6 Å suggests that the inner pore can open to >10 Å. This work provides an impetus for future studies on ion permeation, gating, and drug binding of VGCCs.

Contrasting Effects of Cd2+ and Co2+ on the Blocking/Unblocking of Human Cav3 Channels

Inorganic ions have been used widely to investigate biophysical properties of high voltage-activated calcium channels (HVA: Ca(v)1 and Ca(v)2 families). In contrast, such information regarding low voltage-activated calcium channels (LVA: Ca(v)3 family) is less documented. We have studied the blocking effect of Cd2+, Co2+ and Ni2+ on T-currents expressed by human Ca(v)3 channels: Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3. With the use of the whole-cell configuration of the patch-clamp technique, we have recorded Ca2+ (2 mM: ) currents from HEK-293 cells stably expressing recombinant T-type channels. Cd2+ and Co2+ block was 2- to 3-fold more potent for Ca(v)3.2 channels (EC50 = 65 and 122 microM, respectively) than for the other two LVA channel family members. Current-voltage relationships indicate that Co2+ and Ni2+ shift the voltage dependence of Ca(v)3.1 and Ca(v)3.3 channels activation to more positive potentials. Interestingly, block of those two Ca(v)3 channels by Co2+ and Ni2+ was drastically increased at extreme negative voltages; in contrast, block due to Cd2+ was significantly decreased. This unblocking effect was slightly voltage-dependent. Tail-current analysis reveals a differential effect of Cd2+ on Ca(v)3.3 channels, which can not close while the pore is occupied with this metal cation. The results suggest that metal cations affect differentially T-type channel activity by a mechanism involving the ionic radii of inorganic ions and structural characteristics of the channels pore.

Amino acid substitutions in the pore of the Ca(V)1.2 calcium channel reduce barium currents without affecting calcium currents

Ba(2+) currents through Ca(V)1.2 Ca(2+) channels are typically twice as large as Ca(2+) currents. Replacing Phe-1144 in the pore-loop of domain III with glycine and lysine, and Tyr-1152 with lysine, reduces whole-cell G(Ba)/G(Ca) from 2.2 (wild-type) to 0.95, 1.21, and 0.90, respectively. Whole-cell and single-channel measurements indicate that reductions in G(Ba)/G(Ca) result specifically from a decrease in Ba(2+) conductance and not changes in V(h) or P(O). Half-maximal block of I(Li) is increased by 3.2-, 3.8-, and 1.6-fold in Ca(2+), and 3.8-, 4.2-, and 1.8-fold in Ba(2+) for F1144G, Y1152K, and F1144K, respectively. High affinity interactions of individual divalent cations to the pore are not important for determining G(Ba)/G(Ca), because the fold increases in IC(50) values for Ba(2+) and Ca(2+) are similar. On the contrary, conductance-concentration curves indicate that G(Ba)/G(Ca) is reduced because the interactions of multiple Ba(2+) ions in the mutant pores are altered. The complexity of these interactions is exemplified by the anomalous mole fraction effect, which is flattened for F1144G and FY/GK but accentuated for F1144K. In summary, the physicochemical properties of the amino acid residues at positions 1144 and 1152 are crucial to the pore's ability to distinguish between multiple Ba(2+) ions and Ca(2+) ions.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Key roles of Phe1112 and Ser1115 in the pore-forming IIIS5-S6 linker of L-type Ca2+ channel alpha1C subunit (CaV 1.2) in binding of dihydropyridines and action of Ca2+ channel agonists

Voltage-dependent L-type Ca2+ channels are modulated by the binding of Ca2+ channel antagonists and agonists to the pore-forming alpha1c subunit (CaV 1.2). We recently identified Ser1115 in IIIS5-S6 linker of alpha1C subunit as a critical determinant of the action of 1,4-dihydropyridine agonists. In this study, we applied alanine-scanning mutational analysis in IIIS5-S6 linker of rat brain alpha1C subunit (rbCII) to illustrate the role of pore-forming IIIS5-S6 linker in the action of Ca2+ channel modulators. Ca2+ channel currents through wild-type (rbCII) or mutated alpha1C subunits, transiently expressed in BHK6 cells with beta1a and alpha2/delta subunits, were analyzed. The replacement of Phe1112 by Ala (F1112A) significantly impaired the sensitivity to Ca2+ channel agonists (S)-(-)-Bay k 8644 and FPL-64176, and modestly to 1,4-dihydropyridine (DHP) antagonists. The low sensitivity of F1112A and S1115A to DHP antagonists was consistent with the reduced binding affinity for [3H](+)PN200-110. The replacement of Phe1112 by Tyr, but not by Ala, restored the long openings produced by FPL-64176, thus indicating the critical role of aromatic ring of Phe1112 in the Ca2+ channel agonist action. Interestingly, double-mutant Ca2+ channel (F1112A/S1115A) failed to discriminate between Ca2+ channel agonist (S)-(-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl] phenyl)-3-pyridine carboxylic acid methyl ester (Bay k 8644) and antagonist (R)-(+)-Bay k 8644 and was blocked by the two enantiomers in an identical manner. These results indicate that both Phe1112 and Ser1115 in linker IIIS5-S6 are required for the action of Ca2+ channel agonists. A model of the DHP receptor is proposed to visualize possible interactions of Phe1112, Ser1115, and other DHP-sensing residues with a typical DHP ligand nifedipine.

Permeation and selectivity in calcium channels

Recent advances-both experimental and theoretical-provide a tentative image of the structures in Ca channels that make them exceptionally selective. The image is very different from K channels, which obtain high selectivity with a rigid pore that tightly fits K(+) ions and is lined by carbonyl oxygens of the polypeptide backbone. Ca channels rely on four glutamate residues (the EEEE locus), whose carboxyl side chains likely reach into the pore lumen to interact with passing Ca(2+) ions. The structure is thought to be flexible, tightly binding a single Ca(2+) ion in order to block Na(+) flux but rearranging to interact with multiple Ca(2+) ions to allow Ca(2+) flux. The four glutamates are not equivalent, a fact that seems important for Ca(2+) permeation. This review describes the experimental evidence that leads to these conclusions and the attempts by theorists to explain the combination of high selectivity and high flux that characterizes Ca channels.