Eric A. Ertel

Nomenclature of voltage-gated sodium channels

Voltage-gated Ca2+ channels mediate calcium influx in response to membrane depolarization and regulate intracellular processes such as contraction, secretion, neurotransmission, and gene expression. They are members of a gene superfamily of transmembrane ion channel proteins that includes voltage-gated K+ and Na+ channels. The Ca2+ channels that have been characterized biochemically are complex proteins composed of four or five distinct subunits, which are encoded by multiple genes. The α1 subunit of 190–250 kDa is the largest subunit, and it incorporates the conduction pore, the voltage sensor and gating apparatus, and the known sites of channel regulation by second messengers, drugs, and toxins. An intracellular β subunit and a transmembrane, disulfide-linked α2δ subunit complex are components of most types of Ca2+ channels. A γ subunit has also been found in skeletal muscle Ca2+ channels, and related subunits are expressed in heart and brain. Although these auxiliary subunits modulate the properties of the channel complex, the pharmacological and electrophysiological diversity of Ca2+ channels arises primarily from the existence of multiple forms of α1 subunits. Mammalian α1 subunits are encoded by at least ten distinct genes. Historically, various names have been given to the corresponding gene products, giving rise to distinct and sometimes confusing nomenclatures. In 1994, some of us proposed a unified nomenclature based on the most widely accepted system at the time: α1 subunits were referred to as α1S for the original skeletal muscle isoform and α1A through α1E for those discovered subsequently (Birnbaumer et al. 1994xBirnbaumer, L., Campbell, K.P., Catterall, W.A., Harpold, M.M., Hofmann, F., Horne, W.A., Mori, Y., Schwartz, A., Snutch, T.P., Tanabe, T. et al. Neuron. 1994; 13: 505–506Abstract | Full Text PDF | PubMed | Scopus (264)See all ReferencesBirnbaumer et al. 1994). Since then, four new α1 subunits have been identified, which were named α1F through α1I.Ca2+ currents recorded in different cell types have diverse physiological and pharmacological properties, and an alphabetical nomenclature has also evolved for the distinct classes of Ca2+ currents. L-type Ca2+ currents require a strong depolarization for activation, are long lasting, and are blocked by the organic L-type Ca2+ channel antagonists, including dihydropyridines, phenylalkylamines, and benzothiazepines. They are the main Ca2+ currents recorded in muscle and endocrine cells, where they initiate contraction and secretion. N-type, P/Q-type, and R-type Ca2+ currents also require strong depolarization for activation. They are unaffected by L-type Ca2+ antagonist drugs but are blocked by specific polypeptide toxins from snail and spider venoms. They are expressed primarily in neurons, where they initiate neurotransmission at most fast synapses. T-type Ca2+ currents are activated by weak depolarizations and are transient. They are resistant to both organic antagonists and to the snake and spider toxins used to define the N- and P/Q-type Ca2+ currents. They are expressed in a wide variety of cell types, where they are involved in shaping the action potential and controlling patterns of repetitive firing.As new Ca2+ channel genes are cloned, it is apparent that these two alphabetical nomenclatures will overlap at α1L, which may not mediate an L-type Ca2+ current and therefore may create confusion. Moreover, the present alphabetical nomenclature does not reveal the structural relationships among the α1 subunits, which can be grouped into three families: (1) α1S, α1C, α1D, and α1F; (2) α1A, α1B, and α1E; and (3) α1G, α1H, and α1I. The complete amino acid sequences of these α1 subunits are more than 70% identical within a family but less than 40% identical among families. These family relationships are illustrated for the more conserved transmembrane and pore domains in Figure 1Figure 1. Division of calcium channels into these three families is phylogenetically ancient, as representatives of each are found in the C. elegans genome. Ideally, a nomenclature for Ca2+ channel α1 subunits should provide a systematic organization based on their structural relationships and should be coordinated with nomenclatures for the other families of voltage-gated ion channels of different ionic selectivities (ie., K+ and Na+).Figure 1Phylogeny of Voltage-Gated Ca2+ Channel α1 SubunitsOnly the membrane-spanning segments and the pore loops (∼350 amino acids) are compared. First, all sequence pairs were compared, which clearly defines three families with intrafamily sequence identities above 80% (CaV1.m, CaV2.m, CaV3.m). Then, a consensus sequence was defined for each family, and these three sequences were compared to one another, with interfamily sequence identities of ∼52% (CaV1.m versus CaV2.m) and 28% (CaV3.m versus CaV1.m or CaV2.m).View Large Image | View Hi-Res Image | Download PowerPoint SlideFor these reasons, we wish to propose a new nomenclature of voltage-gated Ca2+ channels (Table 1Table 1), which is more systematic and mimics the well-defined K+ channel nomenclature (Chandy et al., 1991xChandy, K.G. Nature. 1991; 352: 26Crossref | PubMedSee all ReferencesChandy et al., 1991). This nomenclature uses a numerical system (KV1.1, KV2.1, KV3.1, etc.) to define families and subfamilies of K+ channels based on similarities in amino acid sequences. In a similar manner, we propose that Ca2+ channels should be renamed using the chemical symbol of the principal permeating ion (Ca) with the principal physiological regulator (voltage) indicated as a subscript (CaV). The numerical identifier would correspond to the CaV channel α1 subunit gene family (1 through 3 at present) and the order of discovery of the α1 subunit within that family (1 through m). According to this nomenclature, the CaV1 family (CaV1.1 through CaV1.4) includes channels containing α1S, α1C, α1D, and α1F, which mediate L-type Ca2+ currents (Table 1Table 1). The CaV2 family (CaV2.1 through CaV2.3) includes channels containing α1A, α1B, and α1E, which mediate P/Q-type, N-type, and R-type Ca2+ currents, respectively (Table 1Table 1). The CaV3 family (CaV3.1 through CaV3.3) includes channels containing α1G, α1H, and α1I, which mediate T-type Ca2+ currents (Table 1Table 1). When specific reference to the α1 subunit within the Ca2+ channel complex is intended, the designation α11.m, α12.m, or α13.m may be used, where the numeral m represents the individual gene/protein within the family. Where applicable, lowercase letters are used to distinguish alternatively spliced variants (e.g., CaV1.2a corresponds to channels containing the cardiac variant of the former α1C). Such a systematic nomenclature has proved successful for the KV channel proteins. Its strength resides in the rational basis derived from the structural relationships among the channel proteins and the ease and precision with which new channels can be added.Table 1Proposed Nomenclature for Cloned Voltage-Gated Ca2+ Channel α1 SubunitsNameFormer NamesAccession NumberGene Name and Human ChromosomeSplice TypesFormer NamesPrimary TissuesCav1.1 α11.1α1S, α1Skm, CaCh1X05921CACNA1S; 1q31-32skeletal muscleCav1.2α1C, rbC, CaCh2CaCh2, X15539CACNA1C; 12p13.3Cav1.2aα1C-aheartα11.2Cav1.2bα1C-bsmooth musclerbC-I, M67516; rbC-II, M67515Cav1.2cα1C-bbrain, heart, pituitary, adrenalCav1.3 α11.3α1D, rbD, CaCh3M76558CACNA1D; 3p14.3brain, pancreas, kidney, ovary, cochleaCav1.4α1FAJ224874CACNA1F; Xp11.23retinaα11.4Cav2.1α1A, rbA, CaCh4, BIrbA, M64373; BI-1, X57476CACNA1A; 19p13Cav2.1aBI1brain, cochlea, pituitaryα12.1BI-2, X57477Cav2.1bBI2brain, cochlea, pituitaryCav2.2α1B, rbB, CaCh5, BIIIrbB, M92905; BIII, D14157;CACNA1B; 9q34Cav2.2aα1B-1brain, nervous systemα12.2human α1B, M94172Cav2.2bα1B-2brain, nervous systemCav2.3α1E, rbE, CaCh6, BIIrbE, L15453, BII-1, X67855;CACNA1E; 1q25-31Cav2.3aBIIbrain, cochlea, retina, heart,α12.3human α1E, L29384pituitaryCav2.3bBII2brain, cochlea, retinaCav3.1α1GAF027984; AF029228CACNA1G; 17q22Cav3.1abrain, nervous systemα13.1Cav3.2α1HAF051946; AF073931CACNA1H; 16p13.3Cav3.2abrain, heart, kidney, liverα13.2Cav3.3α1IAF086827CACNA1I; 22q12.3-13-2Cav3.3abrainα13.3The cloned voltage-gated Ca2+ channels and most widely studied alternate splice forms are presented together with the proposed nomenclature and previous nomenclatures.The nomenclature of the auxiliary subunits is not modified, since it already includes numbers for the gene family and lowercase letters for the splice variants. Thus, the subunit compositions of the voltage-dependent Ca2+ channels CaVn.mx may be described as α1n.mx/βm′x′/γm′′x′′/α2δm′′′x′′′ complexes, where the number n defines a main family, the numbers m, m′, m′′, and m′′′ refer to the individual genes/proteins within the families, and the letters x, x′, x′′, and x′′′ identify the splice variants. Standard prefixes can be placed in front of the channel name to identify the species of origin. In this notation, the skeletal muscle calcium channel would be written α11.1a/β1a/γ1a/α2δ1a. With this new nomenclature, the CaV designation may also be used to identify calcium channel auxiliary subunits such as CaVβ or CaVγ independent of their presence in a calcium channel complex.We hope that this new nomenclature for α1 subunits will be a stimulus to further research on voltage-gated Ca2+ channels by providing a common, easily accessible standard of reference for scientists working in this field. A full-length review article** is planned to present a more detailed proposal for nomenclature of the many alternate splice forms of the α1 subunits and the auxiliary subunits of Ca2+ channels that have been described in cDNA cloning experiments.*This nomenclature has been approved by the Nomenclature Committee of the International Union of Pharmacology, and a review article giving more details of the nomenclature for calcium channel subunits and splice variants is planned for Pharmacological Reviews.

Letter to the EditorNomenclature of Voltage-Gated Calcium Channels

As new Ca 2ϩ channel genes are cloned, it is apparent that these two alphabetical nomenclatures will overlap at ␣ 1L , which may not mediate an L-type Ca 2ϩ current and Voltage-gated Ca 2ϩ channels mediate calcium influx in therefore may create confusion. Moreover, the present response to membrane depolarization and regulate in-alphabetical nomenclature does not reveal the structural tracellular processes such as contraction, secretion, relationships among the ␣ 1 subunits, which can be neurotransmission, and gene expression. They are mem-grouped into three families: (1) ␣ 1S , ␣ 1C , ␣ 1D , and ␣ 1F ; (2) bers of a gene superfamily of transmembrane ion chan-The complete nel proteins that includes voltage-gated K ϩ and Na ϩ amino acid sequences of these ␣ 1 subunits are more channels. The Ca 2ϩ channels that have been character-than 70% identical within a family but less than 40% ized biochemically are complex proteins composed of identical among families. These family relationships are four or five distinct subunits, which are encoded by illustrated for the more conserved transmembrane and multiple genes. The ␣ 1 subunit of 190–250 kDa is the pore domains in Figure 1. Division of calcium channels largest subunit, and it incorporates the conduction pore, into these three families is phylogenetically ancient, as the voltage sensor and gating apparatus, and the known representatives of each are found in the C. elegans ge-sites of channel regulation by second messengers, nome. Ideally, a nomenclature for Ca 2ϩ channel ␣ 1 sub-drugs, and toxins. An intracellular ␤ subunit and a trans-units should provide a systematic organization based on membrane, disulfide-linked ␣ 2 ␦ subunit complex are their structural relationships and should be coordinated components of most types of Ca 2ϩ channels. A ␥ subunit with nomenclatures for the other families of voltage-has also been found in skeletal muscle Ca 2ϩ channels, gated ion channels of different ionic selectivities (ie., K ϩ and related subunits are expressed in heart and brain. and Na ϩ). Although these auxiliary subunits modulate the proper-For these reasons, we wish to propose a new nomen-ties of the channel complex, the pharmacological and clature of voltage-gated Ca 2ϩ channels (Table 1), which electrophysiological diversity of Ca 2ϩ channels arises is more systematic and mimics the well-defined K ϩ primarily from the existence of multiple forms of ␣ 1 sub-channel nomenclature (Chandy et al., 1991). This no-units. Mammalian ␣ 1 …

Discovery and main pharmacological properties of mibefradil (Ro 40-5967), the first selective T-type calcium channel blocker

Properties of mibefradil Mibefradil is a novel calcium channel antagonist with structural and pharmacological characteristics clearly distinct from those of classical calcium antagonists. It is a potent vasodilator with a high selectivity for the coronary vasculature over the peripheral vasculature and the myocardium. Most importantly, this compound can relax vascular muscle and slow the heart rate without reducing cardiac contractility. In addition, it does not stimulate neurohormonal reflexes and it exhibits a good pharmacological profile characterized by a long duration of action. Mechanism of action The mechanism of action of mibefradil is characterized by the selective blockade of transient, low- voltage-activated (T-type) calcium channels over long-lasting, high-voltage-activated (L-type) calcium channels, which is probably responsible for many of its unique properties. Clinical use of mibefradil Although calcium antagonists are mainly used for the treatment of hypertension and angina pectoris, there is strong preclinical evidence that mibefradil may also be beneficial in the treatment of congestive heart failure.

T-Type Ca2+ Channels and Pharmacological Blockade: Potential Pathophysiological Relevance

Low-voltage–activated T-type Ca2+ channelsare present in most excitable tissues including the heart (mainly pacemakercells), smooth muscle, central and peripheral nervous systems, and endocrinetissues, but also in non-excitable cells, such as osteoblasts, fibroblasts,glial cells, etc. Although they comprise a slightly heterogeneouspopulation, these channels share many defining characteristics: smallconductance (<10 pS), similar Ca2+ andBa2+ permeabilities, slow deactivation, and avoltage-dependent inactivation rate. In addition, activation at lowvoltages, rapid inactivation, and blockade by Ni2+ areclassical properties of T-type Ca2+ channels, which areless specific. T-type Ca2+ channels are weakly blocked bystandard Ca2+ antagonists. Pharmacological blockers arescarce and often lack specificity and/or potency. The physiologicalmodulation of T-type Ca2+ currents is complex: they areenhanced by endothelin-1, angiotensin II (AT1-receptor), ATP,and isoproterenol (cAMP-independent), but are reduced by angiotensin II(AT2-receptor), somatostatin and atrial natriuretic peptide.Norepinephrine enhances these currents in some cells but decreases them inothers. T-type Ca2+ currents have many known or suggestedphysiological and pathophysiological roles in growth (protein synthesis,cell differentiation, and proliferation), neuronal firing regulation, someaspects of genetic hypertension, cardiac hypertrophy, cardiac fibrosis,cardiac rhythm (normal and abnormal), and atherosclerosis. Mibefradil is anew Ca2+ antagonist that is effective in hypertension andangina pectoris. Its favorable pharmacological profile and limited sideeffects appear to be related to selective block of T-typeCa2+ channels: mibefradil reduces vascular resistance andheart rate without negative inotropy or neurohormonal stimulation, and italso has significant antiproliferative actions.

Low-voltage-activated T-type Ca2+ channels

Although progress in our understanding of T channels and their physiological role has been slower than with other Ca2+ channels, it was clear during this two-day workshop that interest and research in the field remain very intense. Advances have been hampered by many factors: small current amplitude, lack of pharmacological tools, apparent heterogeneity, and lack of a cloned channel. Nevertheless, many interesting roles for T channels have been described, which point to a generally subtle modulatory action. Furthermore, recent results suggest that the above barriers might soon be abolished: new pharmacological tools (mibefradil and newer generation compounds) with T-channel selectivity are being developed and many groups claim to be close to cloning a T channel.

Pharmacology of Cav3 (T-Type) Channels

Voltage-operated Ca2+ channels are complex membrane proteins that are responsible for mediating Ca2+ influx in response to membrane depolarization (Hille 1984). Ca2+ entry through these channels serves to regulate intracellular processes such as contraction, secretion, neurotransmission, and gene expression, in response to action potentials and other membrane potential changes. Although all voltage-operated Ca2+ channels are activated by a conformational modification of protein structure induced by a change in membrane potential, these channels can vary considerably in their electrophysiological properties such as voltage-dependences, activation and inactivation kinetics, sensitivity to extracellular and intracellular ligands, ion selectivity, and single channel properties. Historically, the characterization of voltage-operated Ca2+ channels has progressed on two separate fronts: the eleetrophysiological and pharmacological characterization of voltage-operated Ca2+ channel currents and the biochemical and molecular characterization of voltage-operated Ca2+ channel proteins. This has led to the emergence of two separate nomenclatures. The correspondence between these two nomenclatures has been difficult to define because many of the features used to associate cloned and native Ca2+ channels depend on many factors, such as cellular environment, co-expression of accessory subunits, and modulation by second messengers and G-proteins.

Calcium channel blockers and calcium channels

In 1883, Ringer showed that to get isolated hearts to contract, it was necessary to have Ca2+ ions in the perfusion medium [421]. This was the first demonstration of the critical role of calcium in cellular activity. Remarkably, a hundred years passed before the importance of calcium was recognized in processes other than muscle contraction, and almost as long before the cellular mechanisms responsible for calcium regulation started to be understood. Nowadays, it is acknowledged that calcium is the most ubiquitous intracellular signaling molecule and that the concentration of Ca2+ ions is under very tight and dynamic control in all major cell compartments (cytoplasm, nucleus, endoplasmic reticulum, mitochondria). In each compartment, this control is achieved through the interplay of transmembrane entry and extrusion systems (channels, exchangers, and transporters) and of buffering systems (Ca2+-binding proteins). Some of these buffers also participate in the further processing of the Ca2+ concentration signals. An excellent general review by Brini and Carafoli [61] provides detailed information on intracellular calcium signaling and more specific papers are available on Ca2+ stores in the endoplasmic reticulum [463], in the nucleus [50], and in mitochondria [137, 422].