calcium channelopathies

REVIEW ARTICLE

Calcium Channelopathies

Ricardo Felix

Department of Cell Biology, Center for Research and Advanced Studies, National Polytechnic Institute(Cinvestav-IPN), Mexico City, Mexico

Received December 11, 2005; Accepted January 20, 2006

Abstract

Intracellular calcium ([Ca2+]i) is highly regulated in eukaryotic cells. The free [Ca2+]i is approx-imately four orders of magnitude less than that in the extracellular environment. It is, there-fore, an electrochemical gradient favoring Ca2+ entry, and transient cellular activation increasingCa2+ permeability will lead to a transient increase in [Ca2+]i. These transient rises of [Ca2+]i trig-ger or regulate diverse intracellular events, including metabolic processes, muscle contraction,secretion of hormones and neurotransmitters, cell differentiation, and gene expression. Hence,changes in [Ca2+]i act as a second messenger system coordinating modifications in the externalenvironment with intracellular processes. Notably, information on the molecular genetics ofthe membrane channels responsible for the influx of Ca2+ ions has led to the discovery thatmutations in these proteins are linked to human disease. Ca2+ channel dysfunction is now knownto be the basis for several neurological and muscle disorders such as migraine, ataxia, and peri-odic paralysis. In contrast to other types of genetic diseases, Ca2+ channelopathies can be stud-ied with precision by electrophysiological methods, and in some cases, the results have beenhighly rewarding with a biophysical phenotype that correlates with the ultimate clinical phe-notype. This review outlines recent advances in genetic, molecular, and pathophysiologicalaspects of human Ca2+ channelopathies.

doi: 10.1385/NMM:8:3:307

Index Entries: Absence epilepsy; Ca2+ channels; cerebellar ataxia; CSNB2; EA2; FHM1;HypoPP; MHS; SCA6.

NeuroMolecular MedicineCopyright © 2006 Humana Press Inc.All rights of any nature whatsoever reserved.ISSN0895-8696/06/08:307–318/$30.00 (Online) 1559-1174doi: 10.1385/NMM:8:3:307

NeuroMolecular Medicine 307 Volume 8, 2006

*Author to whom all correspondence and reprint requests should be addressed. E-mail: [email protected]

Introduction

Voltage-gated Ca2+ (CaV) channels are membersof a family of transmembrane proteins that act astransducers of electrical signals into other forms of

activity within a cell. In response to membranedepolarization, CaV channels conduct Ca2+ ions intothe cell in a selective manner. Once within the cell,Ca2+ acts as an intracellular messenger capableof initiating diverse physiological events such as

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contraction, secretion, neurotransmission, and geneexpression (Hille, 2001).

Functional Diversity of Voltage-Gated Ca2+

Channels

CaV channels are functionally diverse and havebeen well characterized in terms of biophysicaland pharmacological properties. Five differenttypes of CaV channels have thus far been definedand named T, L, N, P/Q, and R (Hille, 2001; Cat-terall et al., 2005). These channel types can also beseparated into two classes based on threshold ofvoltage-activation: low voltage- and high voltage-activated channels (LVA and HVA, respectively).T-type channels are the only identified member ofthe LVA class. These channels are activated at neg-ative potentials and then rapidly inactivate aftersmall changes in the transmembrane voltage. Theyexhibit small conductance and slow rate of deac-tivation. HVA class, on the other hand, groups theL-, N-, P/Q-, and R-type Ca2+ channels. In general,these channels are activated at relatively depo-larized membrane potentials, inactivate slowlyduring depolarization (except for the R-type thatinactivates rapidly) and show faster rates of deac-tivation and larger conductance values than LVAchannels. HVACa2+ channels can be further dividedinto two main categories based on drug sensitiv-ity. The first category includes the dihydropyri-dine (DHP) sensitive L-type Ca2+ channels. Thesechannels present a wide distribution in brain,skeletal muscle, heart, and other tissues. Experi-mental evidence favors a major role for L-typeCa2+ channels excitation–contraction (EC) cou-pling of skeletal and cardiac muscle (Hille, 2001;Dirksen, 2002). The second category comprises theDHP insensitive Ca2+ channels (non-L-type chan-nels), which are preferentially expressed in thenervous system. These channels are responsiblefor the N- and P/Q-type Ca2+ currents that are selectively blocked by specific small polypep-tide toxins from snail and spider venoms, as wellas for the R-type currents, which are unaffected bymost organic antagonists of Ca2+ channels (Doer-ing and Zamponi, 2003). N- and P/Q-type Ca2+

channels are localized in presynaptic nerve termi-nals and are crucial elements in neuronal excita-tion–secretion (ES) coupling (Catterall, 2000; Hille,2001; Catterall et al., 2005).

Structural Diversity of CaV Channels

According to molecular data, HVA channels aremade up of at least four elements: one pore-formingα1-subunit (Fig. 1A) and three regulatory (α2δ, β, andγ-subunits (Catterall, 2000; Arikkath and Campbell,2003; Lacinova, 2005) (Fig. 1B). Functional differ-ences between CaV channel types are attributed toseveral factors including the expression of distinctα1-subunits and the selective association of regula-tory subunits. Expression of cloned α1-genes hasrevealed many functional roles for this subunitincluding sensing transmembrane voltage, channelactivity, and binding of pharmacological agents(Varadi et al., 1995; Catterall, 2000; Lacinova, 2005).Likewise, the α2δ-subunit is a transmembrane com-plex derived from a common precursor protein thatis proteolytically processed to yield two separatepeptides (α2 and δ) that remain linked by disulfidebonds. This regulatory subunit can associate directlywith the α1-pore-forming subunit and alter itsbiophysical and binding properties (Felix, 1999;Klugbauer et al., 2003). On the other hand, theβ-subunit does not cross the plasma membrane, butit can also directly interact with the α1-subunit andappears to be important for trafficking and expres-sion of the kinetic properties of the channel (Walkerand De Waard, 1998; Dolphin, 2003). Lastly, theγ-subunit is a membrane-spanning protein that mightdownregulate Ca2+ channel activity in myocytes andneurons by decreasing the number of functionalchannels at the plasma membrane and causing ahyperpolarizing shift in the inactivation curve ofthe channels (Black, 2003; Kang and Campbell, 2003).

A Functional-Phylogenetic Nomenclature of Voltage-Gated Ca2+ Channels

Mammalian genomes contain 10 genes codingfor the α1-subunits of CaV channels. Initially, a prac-tical although arbitrary nomenclature was pro-posed in which the α1-subunits were referred to asα1S for the original skeletal muscle isoform and α1Athrough α1E for those discovered subsequently(Birnbaumer et al., 1994). More recently, a unifiednomenclature has been adopted in which the CaV1subfamily (CaV1.1–CaV1.4) includes channels con-taining α1S, α1C, α1D, and α1F, which mediate DHP-sensitive L-type Ca2+ currents. The CaV2 subfamily(CaV2.1–CaV2.3) includes channels containing α1A,

Calcium Channelopathies 309


α1B, and α1E, which originate P/Q-, N-, and R-typeCa2+ currents, respectively. Lastly, the CaV3 sub-family (CaV3.1–CaV3.3) includes channels contain-ing α1G, α1H, and α1I, which mediate T-type Ca2+

currents (Ertel et al., 2000; Catterall et al., 2005).

Inherited CaV Channel Diseases

Diverse genetic studies have linked CaVchannelsto human neurological disorders and animal seizure

models (Felix, 2000, 2002; Lorenzon and Beam, 2000;Striessnig et al., 2004; Pietrobon, 2005a) (Fig. 1B).The first human hereditary diseases recognized asbeing caused by spontaneous mutations affectingCaV channels, familial hemiplegic migraine type-1(FHM1) and episodic ataxia type-2 (EA2) were iden-tified as mutations in the human CaV2.1 α1-subunitCACNA1A gene (formerly CACNL1A4) (Ophoffetal., 1996). The central role of CaV2.1channels in neu-ronal excitability, integration, and Ca2+-dependentneurotransmitter release (Jun et al., 1999; Hille, 2001;

Fig. 1. Molecular structure of CaV channels and mutations associated diseases. (A) The pore-forming α1-subunitof CaV channels is made up of four repeated domains (I–IV) with each domain containing six transmembraneα-helices (segments S1–S6). Segments S5 and S6 of each domain are connected through an external loop, which re-inserts into the membrane, and together provide the outer vestibule and ion-conducting pore lining of the channel.The intracellular loops connecting repeated domains are important sites for channel modulation. The S4 segment ofeach homologous domain contains positively charged lysines or arginines (+) which renders this segment sensitiveto changes in voltage.(B) HVA CaV channels are oligomeric complexes of a pore-forming α1-subunit, an intracellu-lar β subunit, and transmembrane α2δ and γ-subunits. CaVα1-subunit (approx 200 kDa) is made up of four homolo-gous membrane-spanning internal domains surrounding a central pore (panel A). The CaVα2δ-complex (approx 170kDa) derives from a common precursor protein that is proteolytically processed to yield separate CaVα2 and CaVδproteins that remain linked by disulfide bonds. CaVβ is a cytoplasmic protein (approx 70 kDa), whereas CaVγ is approx36 kDa protein that is predicted to contain four transmembrane domains. Indicated are the mutations associated withCaV channel diseases in humans (left) and mice (right). (References for these mutations are given in the text.)

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Catterall et al., 2005) suggests that the human CaV2.1channelopathies might primarily be synaptic dis-eases. The different disorders probably arise fromdisruption of the finely tuned balance between exci-tation and inhibition in neuronal circuits of specificbrain regions (Pietrobon, 2005a).

Episodic Ataxia Type-2 and FamilialHemiplegic Migraine Type-1

EA2 is an autosomal-dominant paroxysmal cere-bral disorder, characterized by ataxia and migraine-like symptoms, interictal nystagmus, and cerebellaratrophy. FHM1 is a rare autosomal-dominant sub-type of migraine with aura, associated with ictal hemi-paresis and, in some cases, progressive cerebellaratrophy. In EA2, two mutations disrupting the openreading frame of the gene were originally found:1 bp deletion (C4073) resulting in a truncated Ca2+

channel α1-subunit and a glycine–alanine transitionof the first nucleotide of intron 24. Four different mis-sense mutations were identified in FHM1 patients:a transition from glycine–alanine at codon 192, lead-ing to substitution of an arginine–glutamine (R192Q)within the fourth segment of the first membrane span-ning domain (IS4; Fig. 1A); a missense mutation atthe second repeat, replacing a threonine–methionine(T666M); and two mutations located in the sixthtransmembrane spanning segment of repeats II andIV (V714Aand I1811L, respectively). Functional stud-ies provided evidence that three of these mutationsaffect the kinetic properties and the voltage depen-dence of activation of the CaV2.1 channel (Kraus etal.,1998). With these findings, the study of CaV channelsentered to an exciting phase in which evidence formissense mutations, truncated and expanded ver-sions of the CaV2.1 α1-subunit has been linked to sev-eral diseases of mice and humans. To date, 22mutations in CACNA1A have been associated to theEA2 phenotype and 13 have been reported for FMH1(Baloh and Jen, 2002; van den Maagdenberg et al.,2002). Most of them are nonsense mutations (Friendet al., 1999; Jen et al., 1999), although a few might becritical missense mutations (Guida et al., 2001; Wapplet al., 2002; van den Maagdenberg et al., 2002). There-fore, CaV2.1 mutations associated with EA2 andFMH1 does not lead to a complete loss of mutantchannel function in all cases (Wappl et al., 2002).

When expressed in heterologous systems, typicalCaV2.1 EA2 mutations produce either nonfunctional

or channels with altered biophysical properties(Guida et al., 2001; Jen et al., 2001; Wappl et al., 2002;Spacey et al., 2004). Given that Purkinje cells in thecerebellum are enriched in CaV2.1 channels, alter-ations in the firing patterns of these neurons prob-ably underlie the motor dysfunctions in ataxichumans with CaV2.1 EA2 mutations. Therefore, ithas been speculated that in some patients the patho-physiological mechanisms of disease might includean impairment of the release of neurotransmitters(Jen et al., 2001; Maselli et al., 2003).

Spinocerebellar Ataxia Type 6

A locus for spinocerebellar ataxia type 6 (SCA6),another autosomal-dominant paroxysmal cerebraldisorder, was mapped to chromosome 19p13.1 inthe same interval as the EA2 and FHM1 locus(Ishikawa et al., 1997; Jodice et al., 1997), indicatingthat EA2, FHM1, and SCA6 are allelic CaV channeldisorders. SCA6 is characterized by late-onsetslow-progressive cerebellar ataxia and Purkinje cell-predominant degeneration in the cerebellum (Sasakiet al., 1998) and appears to be more frequent in Japan-ese populations (Takano et al., 1998). Interestinglyenough, a polymorphic CAG repeat was identifiedin the human CaV2.1 α1-subunit (Matsuyama et al.,1997; Zhuchenko et al., 1997), providing the mole-cular basis for this disorder. Expansion of CAGrepeats encoding polyglutamine (polyQ) tracts hasbeen linked to several neurodegenerative diseases,including Huntington’s disease and other forms ofspinocerebellar ataxia. However, among the gluta-mine repeat disorders, SCA6 is unmatched in thatthe functional properties of the affected gene prod-uct, the CaV2.1 channel, has been extensively char-acterized, and that even subtle changes in itsproperties can be precisely detected, whereas func-tions of the proteins affected in other glutaminerepeat disorders are mostly unknown. Although thepathophysiological mechanisms of SCA6 remainelusive, experimental evidence shows thatexpanded polyQ can alter the functional propertiesof the CaV2.1 channel, causing shifts in the voltagedependence of activation and altering the inactiva-tion properties of the channels (Matsuyama et al.,1999; Restituito et al., 2000; Toru et al., 2000). It hasbeen also reported that the mutation might causean increase in the protein expression at the cell sur-face (Piedras-Renteria et al., 2001). Likewise, it has



been suggested that expanded polyQ form aggre-gates in the nucleus and exert a toxiceffect (Christo-pher, 1997). In SCA6, it seems like the length ofglutamine repeats is not long enough to form thisaggregate (Matsuyama et al., 1999). Instead, cyto-plasmic aggregations of the CaV2.1 channel proteincan be found in the cytoplasm of Purkinje neurons,which might be associated with apoptotic cell death(Ishikawa et al., 1999).

Recent genetic studies indicate that expansion ofthe CAG trinucleotide repeat in the CACNA1Ageneencoding the CaV2.1 subunit might be an age of dis-ease onset (AO) modifier in another type of spino-cerebellar ataxia, SCA2 (Pulst et al., 2005). SCA2 isa polyQ disease of ataxin-2, a cytoplasmic proteinof unknown function with abundant expression inembryonic and adult tissues (Pulst et al., 1996;Nechiporuk et al., 1998). The phenotype of affectedindividuals includes cerebellar ataxia, spasticity,neuropathy, dementia, and parkinsonian features(Geschwind et al., 1997). SCA2 shares with otherpolyQ diseases an inverse correlation betweenrepeat length and AO, although there is a signifi-cant amount of AO variance that is not determinedby the length of the mutant CAG repeat in the ataxin-2 gene (Pulst et al., 2005). Interestingly, by using astrategy of allelic association in individuals highlydiscordant in AO, Pulst and colleagues (2005) iden-tified alleles of the CACNA1A gene as modifiers ofAO in SCA2 patients. Variation in the polyQdomain of CACNA1A might influence AO in SCA2by either directly interacting with ataxin-2 or byaltering CaV2.1 channel function (Pulst et al., 2005).

Incomplete X-Linked Congenital StationaryNight Blindness

Incomplete X-linked congenital stationary nightblindness (CSNB2) is a recessive nonprogressiveretinal disorder characterized by night blindness,decreased visual acuity, myopia, nystagmus, andstrabismus. Electrophysiological data suggested adefect in retinal neurotransmission, whereas mole-cular studies localized the CSNB2 locus to chro-mosome Xp11.23 (Bech-Hansen et al., 1998a,b).Interestingly, a Ca2+ channel α1-subunit gene(CACNA1F), which shares high homology to theDHP-sensitive L-type channel, was identified in thisregion (Bech-Hansen et al., 1998b; Strom et al., 1998).The protein product of this gene (CaV1.4) is localized

to photoreceptor cell bodies and the synaptic ter-minals of rodphotoreceptors in the retina (Morganset al., 2001). Extensive genetic analysis of CACNA1Fin CSNB2 patients revealed more than 40 differentmutations (Striessnig et al., 2004), including non-sense and frameshift mutations that would be pre-dicted to cause premature protein truncation,suggesting that aberrations in the CaV1.4 channelcause this disorder (Bech-Hansen et al., 1998a,b;Strom et al., 1998; Boycott et al., 2001; Jacobi et al.,2003). Based on the electroretinographic phenotypeof CSNB2 patients, CaV1.4 channels has been local-ized to the axon terminal of photoreceptor cells andthe mutation seems to result in a loss of signal trans-mission from photoreceptors to bipolar cells.

Most CaV1.4 CSNB2 mutations are predicted tocause severe structural changes, unlikely to supportsignificant channel activity. However, after het-erologous expression, some single missense muta-tions have not lead to a complete loss-of-channelfunction. Actually, some mutations might evenenhance activity (Striessnig et al., 2004). These datathus suggest that clinical CSNB2 symptoms mightresult not only from complete loss-of-function butalso from channel-gating changes (including gain-of-function) and/or decreased expression of mutantchannels with unchanged gating behavior (Striessniget al., 2004).

Hypokalaemic Periodic Paralysis

Hypokalaemic periodic paralysis (HypoPP) is anautosomal-dominant muscle disease manifested byepisodic weakness associated with low serum K+.It is thought to arise also from the abnormal func-tion of CaV channels. The HypoPP locus has beenlocalized to chromosome 1q31-32 and the geneencoding the skeletal muscle CaV1.1 channelCACNA1S (formerly CACNL1A3) maps to thesame region (Fontaine et al., 1994). The CaV1.1 chan-nel serve a dual role as a Ca2+-conducting pore andas the voltage sensor coupling T-tubule depolar-ization to Ca2+ release from the sarcoplasmic retic-ulum (SR; Rios and Brum, 1987). Interestingly, twomutations (R1239G and R1239H) in the CACNA1Sgene have been associated to HypoPP. These muta-tions occur at one of two adjacent nucleotides withinthe same codon, causing a substitution of a highlyconserved arginine in the S4 segment (the voltagesensor; Fig. 1A) of domain IV with either histidine

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or glycine (Ptacek et al., 1994). Heterologous expres-sion of these mutant subunits, as well as a thirdmutation in the S4 segment of domain II (R528H),showed reduced L-type Ca2+ current amplitude andaltered activation properties of the recombinantchannels (Lapie et al., 1996; Morrill and Cannon,1999). Electrophysiological studies in humanmyotubes cultured from patients with the R528Hand the R1239H mutations have shown to cause amild reduction in current density in HypoPP cells(Sipos et al., 1995; Morrill et al., 1998). In addition,the mutation in the putative voltage sensor, IIS4(R528H), apparently slows the kinetics of activation(Morrill et al., 1998) and shifts the voltage depen-dence of inactivation to more negative potentials(Sipos et al., 1995). Although HypoPP is considereda Ca2+ channelopathy, experimental evidence sug-gests that the mutations within the CaV channelsmight exert their effect by causing an adverse inter-action with another ion channels. Analysis of humanmuscle from patients with HypoPP showed that theactivity of the ATP-sensitive K+ (KATP) channels isabnormally reduced (Tricarico et al., 1999). Thediminished conductance of these channels mightlead to an accumulation of K+ intracellularly, caus-ing depolarization of the muscle cell membrane andhypokalaemia extracellularly. The involvement ofthese channels are supported by observations thatthe K+ channel opener cromakalim can reverse someof the pathophysiological manifestations in this dis-order (Grafe et al., 1990).

Malignant Hyperthermia Susceptibility

Malignant hyperthermia susceptibility (MHS) isa potentially fatal autosomal-dominant disorder ofskeletal muscle that has been linked to a mutationin an intracellular Ca2+-releasing channel, the ryan-odine receptor type 1 (RYR1). MHS is characterizedby the susceptibility of otherwise healthy individ-uals to severe adverse reactions to volatile anes-thetics or depolarizing muscle relaxants (Jurkat-Rottet al., 2002; Striessnig et al., 2004).

Interestingly, in some MHS families, linkage tomarkers flanking the CACNA2D1 (formerlyCACNL2A) locus on chromosome 7 (Iles et al., 1994)has also been found. This gene encodes the α2δ-auxiliary subunit of the CaV channels, which areintimately associated at the skeletal muscle triadicjunctions with the RYR1. However, although

detailed analysis of the sequence and genomic struc-ture of the CACNA2D1 gene have been performed,mutations within the coding region have not yetbeen identified (Schleithoff et al., 1999). Given thatthe promoter region remains to be analyzed, andthe eventuality of an intronic mutation is still pos-sible, CACNA2D1 remains as a candidate gene forMHS in individuals that show no mutation in theRYR1. Likewise, two further loci for MHS have beenpublished, one locates to chromosome 1q, the siteof a candidate gene, CACNA1S, encoding the CaV1.1channel α1-subunit, and the second on chromosome5p to which no known candidate has been mappedyet (Monnier et al., 1997; Robinson et al., 1997).Sequence analysis shows that an arginine–histidinesubstitution at residue 1086, resulting from the tran-sition of A for G3333, segregates with the MHS phe-notype (Monnier et al., 1997; Stewart et al., 2001).As CaV1.1 mainly serves as the voltage sensor ofRyR1 rather than a Ca2+ channel, these mutationsmight alter functional interaction between these twoCa2+ channels (Weiss et al., 2004). Finally, althoughin some families the phenotype and the presence ofthis mutations are discordant (Brooks et al., 2002),it has been reported that several independent genesmight influence MHS in an individual family (Mon-nier et al., 2002; Robinson et al., 2003).

Mouse Models of Ca2+ Channelopathies

Eight spontaneous mutations (tottering, leaner,rolling Nagoya, rocker, lethargic, ducky, entla, andstargazer) in genes encoding the four subunits thatform the multimeric neuronal CaVcomplex have beendirectly related with cerebellar ataxia and general-ized absence epilepsy in mice (Fig. 1B). The discov-ery and systematic analysis of these animal modelsis helping to clarify how different mutations affectchannel function and how altered channel functionproduces disease (Felix, 2002; Pietrobon, 2005a).Likewise, muscular dysgenesis (mdg) is a loss-of-function mutation transmitted as a single autosomal-recessive gene. Affected individuals are unable tobreathe and die perinatally (Adams and Beam, 1990).Functional and molecular studies have shown thatthe probable gene defect that is the critical locus forthe development of the dysgenic phenotype islocated in or in close proximity to the CACNA1Sgene that encodes the α1-subunit of the CaV1.1 chan-nel, which is essential for EC coupling (Adams and



Beam, 1990; Dirksen, 2002). In addition, studies onmice in which CaV channel subunits have beendeleted, have provided important information aboutthe roles carried out by these molecules in cell func-tion and disease. In particular, a mouse with a dele-tion of the β2-subunit of CaV channels in the centralnervous system (CNS) was recently shown to havea similar phenotype as CSNB2 patients (Ball andGregg, 2002; Ball et al., 2002) the visual disorder asso-ciated with mutations in the retina-specific CaV1.4channels. The use of the powerful mouse geneticshas also allowed the creation of the first knockin (KI)mutant mouse model carrying a human disease-causing Ca2+ channel mutation, the CaV2.1 R192QFHM1 mutation (van den Maagdenberg et al., 2004).TheFHM1 KI mouse shows multiple gain-of-functioneffects, which include increased CaV2.1 current den-sity in cerebellar granule cells and cortical pyramidalneurons, enhanced glutamate release from corticalneurons, decreased trigger threshold for cortical-spreading depression (CSD), and enhanced neuro-transmissionat the neuromuscular junction (van denMaagdenberg et al., 2004; Kaja et al., 2005). Inter-estingly, the enhanced release of glutamate as a con-sequence of gain-of-function of CaV2.1 channelsmight underlie the susceptibility to CSD, the mech-anism underlying the migraine aura in FHM1(Pietrobon, 2005b). In addition, a decreased G protein-mediated presynaptic inhibition of glutamatergicsynapses might contribute to trigger some FHM1attacks (Pietrobon, 2005a).

Pharmacology of Ca2+

Channelopathies

EA2 attacks are remarkably sensitive to acetazo-lamide, a carbonic anhydrase inhibitor (Griggs et al.,1978). It has been postulated that this drug mightwork by increasing the extracellular concentrationof free protons in the cerebellum (Bain et al., 1992).Although protons can block CaV1 channels by inter-acting with a site in the pore that is critical for ionpermeation (Prod’hom et al., 1987; Chen et al., 1996),further studies need to be done to establish howacidification might stabilize the transient dysfunc-tion of the mutated CaV2.1 channel. From the clin-ical point of view, the acetazolamide treatmenttypically begins with a low dose of 125 mg/d andthen can be increased to an average effective dose

of approx 500–750 mg/d. Most patients experienceasthenia and paresthesias after taking the drug, butthese symptoms usually decrease over time (Balohand Jen, 2002). The main long-term side effect seemsto be the development of nephrolithiasis, which canbe decreased if the patient regularly drinks citrusjuices (Baloh and Jen, 2002).

Although FHM1 is a difficult-to-treat disorder, ithas been reported that two Ca2+ antagonists flu-narizine and verapamil might prevent hemiplegicmigraine (Silver and Andermann, 1993; Yu andHorowitz, 2003). However, it is unlikely that thetherapeutic effect of flunarizine (a Ca2+ channelblocker with a complex pharmacological profile)and verapamil (a CaV1 channel blocker), mightinvolve a direct interaction with the mutated CaV2.1channel. Instead, these agents seem to be acting byreversing the vasospasm during the hemiplegicmigrainous process (Ng et al., 2000). It is worth clar-ifying, however, that it is unknown whether mutatedCaV2.1 channels are sensitive to verapamil or flu-narizine and whether hemiplegic migraine is anexclusive CaV2.1 channel dysfunction. In line withthis, CaV1 channels are upregulated in the Purkinjecells in CACNA1A knockout mice (Jun et al., 1999),suggesting a role for these channels in disorderswith CaV2.1 channel dysfunction.

In HypoPP, during an acute attack, oral potas-sium chloride is the treatment of choice, and for pre-venting attacks acetazolamide has been shown tobe of benefit (Davies and Hanna, 2001). Dichlor-phenamide, another inhibitor of carbonic anhy-drase, is also effective in the prevention of episodicweakness in HypoPP (Tawil et al., 2000).

As discussed earlier, MHS is a life-threateningcondition triggered in genetically susceptible indi-viduals when certain potent inhalation anestheticsor succinylcholine are administered. Death canresult from cardiac arrest, brain damage, internalhemorrhaging, or failure of other body systems. Inorder to prevent crisis in patients with MHS, non-triggering anesthetic agents and nondepolarizingmuscle relaxants are recommended. Drugs com-monly used include barbiturates, benzodiazepines,and propofol. Appropriate muscle relaxants includeatracurium or vecuronium (Davies and Hanna,2001). The moment that any signs of an attack ofMHS occur during an anaesthetic, intravenousdantrolene therapy (2.5 mg/kg) should be imme-diately given (Chartrand, 2003). Dantrolene is

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thought to decrease the SR Ca2+ release by blockingthe RyR1 (MacLennan et al., 1996).

Concluding Remarks

Ca2+ channels are present in the plasma mem-brane of eukaryotic cells, in which they coordinatesuch diverse functions as neurotransmission, con-traction, and secretion. Over the past decade, it hasbecome increasingly clear that mutations in thegenes that encode the proteins of Ca2+ channels canresult in pathological states (Fig. 1B). Whereas thepathophysiology of certain Ca2+ channelopathies isknown with some detail, becauseCa2+ channels andtheir mutants can be studied by highly sensitiveandelaborate electrophysiological techniques (even atthe level of single molecules), there are some otherCa2+ channel diseases whose complex pathophysi-ology remains incompletely understood severalyears after identification of the underlying gene.

However, the identification of the molecular basisof Ca2+ channel diseases will benefit undoubtedlythe individual patients. Insight into the structureand function of channel proteins coupled with theknowledge of genetic and disease-induced regula-tion of Ca2+ channels could improve diagnosis andoffer specific candidate genes for the developmentof appropriate pharmacotherapy. On the assump-tion that defined Ca2+ channel mutations are linkedto specific diseases, it might be feasible to conductmolecular diagnosis. Likewise, knowledge of spe-cific mutations might alsolead to validation of moresuitable animal models of disease to help assess-ment of novel compounds. In the coming years,modulating Ca2+ channel gene expression via genedelivery approaches could present an additionalavenue to treat Ca2+ channelopathies.

Acknowledgments

I acknowledge support from National Councilfor Science and Technology (Conacyt, Mexico) andgratefully thank A. Sandoval for artwork.

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