Lutz Birnbaumer

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.

Receptor-effector coupling by G proteins.

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

The TRP Channels, a Remarkably Functional Family

TRP cation channels display an extraordinary assortment of selectivities and activation mechanisms, some of which represent previously unrecognized modes for regulating ion channels. Moreover, the biological roles of TRP channels appear to be equally diverse and range from roles in pain perception to male aggression.

trp, a Novel Mammalian Gene Family Essential for Agonist-Activated Capacitative Ca2+ Entry

SUMMARY Capacitative calcium entry (CCE) describes CA2+ influx into cells that replenishes CA2+ stores emptied through the action of IP3 and other agents. It is an essential component of cellular responses to many hormones and growth factors. The molecular basis of this form of Ca2+ entry is complex and may involve more than one type of channel. Studies on visual signal transduction in Drosophila led to the hypothesis that a protein encoded in trp may be a component of CCE channels. We reported the existence of six trp-related genes in the mouse genome. Expression in L cells of small portions of these genes in antisense orientation suppressed CCE. Expression in COS cells of two full-length cDNAs encoding human trp homologs, Htrp1 and Htrp3, increased CCE. This identifies mammalian gene products that participate in CCE. We propose that trp homologs are subunits of CCE channels, not unlike those of classical voltage-gated ion channels.

Functional interaction between InsP3 receptors and store-operated Htrp3 channels

Calcium ions are released from intracellular stores in response to agonist-stimulated production of inositol 1,4,5-trisphosphate (InsP3), a second messenger generated at the cell membrane. Depletion of Ca2+ from internal stores triggers a capacitative influx of extracellular Ca2+ across the plasma membrane,. The influx of Ca2+ can be recorded as store-operated channels (SOC) in the plasma membrane or as a current known as the Ca2+-release-activated current (Icrac). A critical question in cell signalling is how SOC and Icrac sense and respond to Ca2+-store depletion: in one model, a messenger molecule is generated that activates Ca2+ entry in response to store depletion,; in an alternative model, InsP3 receptors in the stores are coupled to SOC and Icrac. The mammalian Htrp3 protein forms a well defined store-operated channel, and so provides a suitable system for studying the effect of Ca2+-store depletion on SOC and Icrac. We show here that Htrp3 channels stably expressed in HEK293 cells are in a tight functional interaction with the InsP3 receptors. Htrp3 channels present in the same plasma membrane patch can be activated by Ca2+ mobilization in intact cells and by InsP3 in excised patches. This activation of Htrp3 by InsP3 is lost on extensive washing of excised patches but is restored by addition of native or recombinant InsP3-bound InsP3 receptors. Our results provide evidence for the coupling hypothesis, in which InsP3 receptors activated by InsP3 interact with SOC and regulate Icrac.

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 …

A Unified Nomenclature for the Superfamily of TRP Cation Channels

The TRP superfamily includes a diversity of non-voltage-gated cation channels that vary significantly in their selectivity and mode of activation. Nevertheless, members of the TRP superfamily share significant sequence homology and predicted structural similarities. Currently, most of the genes and proteins that comprise the TRP superfamily have multiple names and, in at least one instance, two distinct genes belonging to separate subfamilies have the same name. Moreover, there are many cases in which highly related proteins that belong to the same subfamily have unrelated names. Therefore, to minimize confusion, we propose a unified nomenclature for the TRP superfamily.The current effort to unify the TRP nomenclature focuses on three subfamilies (TRPC, TRPV, and TRPM) that bear significant similarities to the founding member of this superfamily, Drosophila TRP, and which include highly related members in worms, flies, mice, and humans (Table 1)(Table 1). Members of the three subfamilies contain six transmembrane segments, a pore loop separating the final two transmembrane segments, and similarity in the lengths of the cytoplasmic and extracellular loops. In addition, the charged residues in the S4 segment that appear to contribute to the voltage sensor in voltage-gated ion channels are not conserved. The TRP-Canonical (TRPC) subfamily (formerly short-TRPs or STRPs) is comprised of those proteins that are the most highly related to Drosophila TRP. The TRPV subfamily (formerly OTRPC), is so named based on the original designation, Vanilloid Receptor 1 (VR1), for the first mammalian member of this subfamily (now TRPV1). The name for the TRPM subfamily (formerly long-TRPs or LTRPs) is derived from the first letter of Melastatin, the former name (now TRPM1) of the founding member of this third subfamily of TRP-related proteins. Based on amino acid homologies, the mammalian members of these three subfamilies can be subdivided into several groups each (Table 2Table 2 and Figure 1Figure 1) .Table 1Number of TRP Genes in Worms (C. elegans), Flies (Drosophila melanogaster), Mice, and HumansSubfamilyWormsFliesMiceHumansTRPC3376aaTRPV5255TRPM4188aTRPC2 is a pseudogene and is not counted.Table 2Nomenclature of the Mammalian TRP SuperfamilyNameGroupFormer NamesAccession NumbersTRPC11TRP1CAA61447, AAA93252TRPC1TRPC22TRP2X89067, AAD17195, AAD17196, AAG29950, AAG29951, AAD31453,TRPC2CAA06964TRPC33TRP3AAC51653TRPC3TRPC44TRP4CAA68125, BAA23599TRPC4TRPC54TRP5AAC13550, CAA06911, CAA06912TRPC5TRPC63TRP6NP_038866TRPC6TRPC73TRP7AAD42069, NP_065122TRPC7TRPV11VR1AAC53398OTRPC1TRPV21VRL-1AAD26363, AAD26364, BAA78478OTRPC2GRCTRPV3 (not assigned)TRPV42OTRPC4AAG17543, AAG16127, AAG28027, AAG28028, AAG28029,VR-OACCAC20703TRP12VRL-2TRPV53ECaC1CAB40138CaT2TRPV63CaT1AAD47636ECaC2CAC20416CaT-LCAC20417TRPM11MelastatinAAC13683, AAC80000TRPM22TRPC7BAA34700LTRPC2TRPM31KIAA1616AA038185LTRPC3TRPM43TRPM4H18835LTRPC4TRPM53MTR1AAF26288LTRPC5TRPM64Chak2AF350881TRPM74TRP-PLIKAAF73131Chak1LTRPC7TRPM82TRP-p8AC005538Indicated are the suggested gene and protein names, the groups within each subfamily, the former names, and accession numbers.Figure 1Phylogenetic Tree of the TRP SuperfamilyThe tree, which was adapted from Clapham et al., 2001 (Nat. Rev. Neurosci. 2, 387–396), was calculated using the neighbor-joining method and human, rat, and mouse sequences.View Large Image | View Hi-Res Image | Download PowerPoint SlideThe numbering system for the mammalian TRPC, TRPV, and TRPM proteins takes into account the order of their discovery and, in as many cases as possible, the number that has already been assigned to the genes and proteins (Table 2)(Table 2). In the case of the TRPV proteins, the numbering system is also based in part on the groupings of the TRPV proteins. New members of each subfamily will maintain the same root name and, with the exception of TRPV3, will be assigned the next number in the sequence. Currently, TRPV3 is unassigned to maintain the TRPV1/ TRPV2 and TRPV5/TRPV6 groupings and so that the former OTRPC4 could be renamed TRPV4. The next TRPV protein will be designated TRPV3.We hope this new nomenclature will add clarity to the field and simplify the naming of new members of the TRP superfamily. We recommend that accession numbers be used whenever it is necessary to unambiguously specify a given variant resulting from alternative mRNA splicing. Finally, this nomenclature has been approved by the HUGO Gene Nomenclature Committee and we recommend that this system be used in all future publications concerning TRPC, TRPV, and TRPM subfamily members.

Ulcerative colitis and adenocarcinoma of the colon in G alpha i2-deficient mice.

G proteins are involved in cellular signalling and regulate a variety of biological processes including differentiation and development. We have generated mice deficient for the G protein subunit αi2 (Gαi2) by homologous recombination in embryonic stem cells. Gαi2–deficient mice display growth retardation and develop a lethal diffuse colitis with clinical and histopathological features closely resembling ulcerative colitis in humans, including the development of adenocarcinoma of the colon. Prior to clinical symptoms, the mice show profound alterations in thymocyte maturation and function. The study of these animals should provide important insights into the pathogenesis of ulcerative colitis as well as carcinogenesis.

Increased Vascular Smooth Muscle Contractility in TRPC6−/− Mice

ABSTRACT Among the TRPC subfamily of TRP (classical transient receptor potential) channels, TRPC3, -6, and -7 are gated by signal transduction pathways that activate C-type phospholipases as well as by direct exposure to diacylglycerols. Since TRPC6 is highly expressed in pulmonary and vascular smooth muscle cells, it represents a likely molecular candidate for receptor-operated cation entry. To define the physiological role of TRPC6, we have developed a TRPC6-deficient mouse model. These mice showed an elevated blood pressure and enhanced agonist-induced contractility of isolated aortic rings as well as cerebral arteries. Smooth muscle cells of TRPC6-deficient mice have higher basal cation entry, increased TRPC-carried cation currents, and more depolarized membrane potentials. This higher basal cation entry, however, was completely abolished by the expression of a TRPC3-specific small interference RNA in primary TRPC6 − / − smooth muscle cells. Along these lines, the expression of TRPC3 in wild-type cells resulted in increased basal activity, while TRPC6 expression in TRPC6 −/− smooth muscle cells reduced basal cation influx. These findings imply that constitutively active TRPC3-type channels, which are up-regulated in TRPC6-deficient smooth muscle cells, are not able to functionally replace TRPC6. Thus, TRPC6 has distinct nonredundant roles in the control of vascular smooth muscle tone.

CLONING AND EXPRESSION OF A NOVEL MAMMALIAN HOMOLOG OF DROSOPHILA TRANSIENT RECEPTOR POTENTIAL (TRP) INVOLVED IN CALCIUM ENTRY SECONDARY TO ACTIVATION OF RECEPTORS COUPLED BY THE GQ CLASS OF G PROTEIN

Hormonal stimulation of Gq-protein coupled receptors triggers Ca2+ mobilization from internal stores. This is followed by a Ca2+ entry through the plasma membrane.Drosophila Trp and Trpl proteins have been implicated in Ca2+ entry and three mammalian homologues ofDrosophila Trp/Trpl, hTrp1, hTrp3 and bTrp4 (also bCCE) have been cloned and expressed. Using mouse brain RNA as template, we report here the polymerase chain reaction-based cloning and functional expression of a novel Trp, mTrp6. The cDNA encodes a protein of 930 amino acids, the sequence of which is 36.8, 36.3, 43.1, 38.6, and 74.1% identical to Drosophila Trp and Trpl, bovine Trp4, and human Trp1 and Trp3, respectively. Transient expression of mTrp6 in COS.M6 cells by transfection of the full-length mTrp6 cDNA increases Ca2+ entry induced by stimulation of co-transfected M5 muscarinic acetylcholine receptor with carbachol (CCh), as seen by dual wavelength fura 2 fluorescence ratio measurements. The mTrp6-mediated increase in Ca2+ entry activity was blocked by SKF-96365 and La3+. Ca2+ entry activity induced by thapsigargin was similar in COS cells transfected with or without the mTrp6 cDNA. The thapsigargin-stimulated Ca2+ entry could not be further stimulated by CCh in control cells but was markedly increased in mTrp6-transfected cells. Records of whole cell transmembrane currents developed in response to voltage ramps from −80 to +40 mV in control HEK cells and HEK cells stably expressing mTrp6 revealed the presence of a muscarinic receptor responsive non-selective cation conductance in Trp6 cells that was absent in control cells. Our data support the hypothesis that mTrp6 encodes an ion channel subunit that mediates Ca2+ entry stimulated by a G-protein coupled receptor, but not Ca2+ entry stimulated by intracellular Ca2+store depletion.