Gyula Varadi

Molecular determinants of Ca2+ channel function and drug action

Molecular cloning has revealed the existence of six high-voltage activated Ca2+ channel types. Expression studies have shown that basic high-voltage activated channel function, which is typical for the L-(skeletal muscle, cardiac muscle and neuroendocrine tissue), N-, P-, Q- and R-type channels is carried by the corresponding alpha 1 subunits. Auxiliary subunits, such as alpha 2/delta and beta, modulate the kinetics of activation, inactivation, current density and drug binding, thereby creating considerable potential for multiple Ca2+ channel functions. Glutamic acid residues in the pore (P) loops are molecular components that impart high selectivity for Ca+. Binding or pharmacologically active sites for Ca2+ channel drugs have been localized on various segments of the alpha 1 subunit in close proximity to the pore lining. In this article, Gyula Varadi and colleagues review the roles of the different subunits in Ca2+ channel function and suggest that Ca2+ channel drugs act by blocking or, in some cases, activating channel function via binding directly or indirectly to the pore structure of the channel.

Multiple Modulation Pathways of Calcium Channel Activity by a β Subunit DIRECT EVIDENCE OF β SUBUNIT PARTICIPATION IN MEMBRANE TRAFFICKING OF THE α1C SUBUNIT

In order to study the precise mechanisms of α1 subunit modulation by an auxiliary β subunit of voltage-dependent calcium channels, a recombinant β3 subunit fusion protein was produced and introduced into oocytes that express the human α1C subunit. Injection of the β3 subunit protein rapidly modulated the current kinetics and voltage dependence of activation, whereas massive augmentation of peak current amplitude occurred over a longer time scale. Consistent with the latter, a severalfold increase in the amount of the α1C subunit in the plasma membrane was detected by quantitative confocal laser-scanning microscopy after β3subunit injection. Pretreatment of oocytes with bafilomycin A1, a vacuolar type H+-ATPase inhibitor, abolished the increase of the α1C subunit in the plasma membrane, attenuated current increase, but did not affect the modulation of current kinetics and voltage dependence by the β3 subunit. These results provide clear evidence that the β subunit modifies the calcium channel complex in a binary fashion; one is an allosteric modulation of the α1 subunit function and the other is a chaperoning of the α1 subunit to the plasma membrane.

Evidence for the existence of a cardiac specific isoform of the α1 subunit of the voltage dependent calcium channel

Biochemical, pharmacological and electrophysiological evidence implies the existence of tissue specific isoforms of the L‐type VDCC. The α1 and α2 subunits of the skeletal muscle calcium channel have been previously cloned and their amino acid sequence deduced. Here we report the isolation and sequencing of a partial cDNA that encodes a heart specific isoform of the α1 subunit. The amino acid sequence deduced from this part cDNA clone shows 64.7% similarity with the skeletal muscle α1 subunit. Northern analysis reveals 2 hybridizing bands, 8.5 and 13 kb, in contrast to one 6.5 kb band in the skeletal muscle. Selective inhibition of mRNA expression in Xenopus oocytes by complementary oligodeoxynucleotides derived from the heart clone provides further evidence that the cDNA corresponds to an essential component of the VDCC. These data further support the existence of tissue‐specific isoforms of the L‐type VDCC.

Molecular Elements of Ion Permeation and Selectivity within Calcium Channels

Voltage-dependent calcium channels are located in the plasma membrane and form a highly selective conduit by which Ca2+ ions enter all excitable cells and some nonexcitable cells. Extensive characterization studies have revealed the existence of one low (T) and five high-voltage-activated calcium channel types (L, N, P, Q, and R). The high voltage-activated calcium channels have been found to exist as heteromultimers, consisting of an alpha1, beta, alpha2/delta, and gamma subunit. Molecular cloning has revealed the existence of 10 channel transcripts, and expression of these cloned calcium channel genes has shown that basic voltage-activated calcium channel function is strictly carried by the corresponding alpha1 subunits. In turn, the auxiliary subunits serve to modulate calcium channel function by altering the voltage dependence of channel gating, kinetics, and current amplitude, thereby creating a likelihood for calcium channels with multiple properties. Although for calcium channels to be effective, Ca2+ ions must enter selectively through the pore of the alpha1-subunit, bypassing competition with other extracellular ions. The structural determinants of this highly selective Ca2+ filter reside within the four glutamic acid residues located at homologous positions within each of the four pore-forming segments. Together, these residues form a single or multiple Ca2+ affinity site(s) that entrap calcium ions, which are then electrostatically repulsed through the intracellular opening of the pore. This mechanism of high-selectivity calcium filtration, the spatial arrangement of pore glutamic acid residues, and the coordination chemistry of calcium binding are discussed in this review.

Developmental regulation of expression of the α1 and α2 subunits mRNAs of the voltage-dependent calcium channel in a differentiating myogenic cell line

The voltage‐dependent calcium channel (VDCC) in skeletal muscle probably plays a key role in transducing membrane charge movement to the calcium release channel. We report here that the expression of VDCC α1 and α2 mRNAs is developmentally regulated in differentiating C2Cl2 myogenic cells. The α1 mRNA is not detectable in the myoblast form of C2Cl2 cells while its expression is induced 20‐fold in differentiated myotubes. In contrast, the α2 mRNA is weakly expressed in myoblasts but is also induced upon myogenic differentiation.

A region in IVS5 of the human cardiac L-type calcium channel is required for the use-dependent block by phenylalkylamines and benzothiazepines.

Mutations in motif IVS5 and IVS6 of the human cardiac calcium channel were made using homologous residues from the rat brain sodium channel 2a. [3H]PN200-110 and allosteric binding assays revealed that the dihydropyridine and benzothiazepine receptor sites maintained normal coupling in the chimeric mutant channels. Whole cell voltage clamp recording fromXenopus oocytes showed a dramatically slowed inactivation and a complete loss of use-dependent block for mutations in the cytoplasmic connecting link to IVS5 (HHT-5371) and in IVS5 transmembrane segment (HHT-5411) with both diltiazem and verapamil. However, the use-dependent block by isradipine was retained by these two mutants. For mutants HHT-5411 and HHT-5371, the residual current appeared associated with a loss of voltage dependence in the rate of inactivation indicating a destabilization of the inactivated state. Furthermore, both HHT-5371 and -5411 recovered from inactivation significantly faster after drug block than that of the wild type channel. Our data demonstrate that accelerated recovery of HHT-5371 and HHT-5411 decreased accumulation of these channels in inactivation during pulse trains and suggest a close link between inactivation gating of the channel and use-dependent block by phenylalkylamines and benzothiazepines and provide evidence of a role for the transmembrane and cytoplasmic regions of IVS5 in the use-dependent block by diltiazem and verapamil.

Architecture of Ca2+ Channel Pore-lining Segments Revealed by Covalent Modification of Substituted Cysteines

The cysteine accessibility method was used to explore calcium channel pore topology. Cysteine mutations were introduced into the SS1-SS2 segments of Motifs I-IV of the human cardiac L-type calcium channel, expressed in Xenopusoocytes and the current block by methanethiosulfonate compounds was measured. Our studies revealed that several consecutive mutants of motifs II and III are accessible to methanethiosulfonates, suggesting that these segments exist as random coils. Motif I cysteine mutants exhibited an intermittent sensitivity to these compounds, providing evidence for a β-sheet secondary structure. Motif IV showed a periodic sensitivity, suggesting the presence of an α-helix. These studies reveal that the SS1-SS2 segment repeat in each motif have non-uniform secondary structures. Thus, the channel architecture evolves as a highly distorted 4-fold pore symmetry.

Molecular Studies on the Voltage Dependence of Dihydropyridine Action on L-type Ca2+ Channels CRITICAL INVOLVEMENT OF TYROSINE RESIDUES IN MOTIF IIIS6 AND IVS6

The interaction site(s) of dihydropyridine (DHP) antagonists and agonists have been identified by site-directed mutagenesis and localized on motifs IIIS5, IIIS6, and IVS6 of L-type voltage-gated calcium channels. In this study, we investigated the voltage-dependent action of DHPs with mutants of the IIIS6 and IVS6 segments of a cardiac calcium channel. Tyrosine residues in both motifs (Tyr1178 and Tyr1489) strongly contributed to the action of DHP agonists and antagonists. When these two sites were mutated, the communication between the voltage sensor and the DHP interaction site(s) was substantially impaired. In contrast, mutants of a nearby Ile (Ile1182) had much less influence on DHP agonist and antagonist interaction, and the voltage dependence of DHP antagonists was very similar to that of the wild type. The effect of a mutating of Ile1182, on agonist or antagonist action, however, depended strongly on the type of amino acid change. When Ile1182 was substituted with alanine, small changes were noted for DHP agonist and antagonist action. Changing this site into phenylalanine, however, significantly decreased the action of the DHP antagonist. These data show that Ile1182 can preferentially interact with DHP antagonists, but has a lesser contribution in agonist interaction. Thus, even though the agonist and antagonist interaction sites for DHPs with L-type calcium channels may overlap, some amino acids in this site may exhibit a preference for either DHP enantiomers.

Characterization of auto-regulation of the human cardiac α1 subunit of the L-type calcium channel: Importance of the C-terminus

The carboxyl terminal of the L-type calcium channel α1C subunit comprises approximately one third of the primary structure of the α1 subunit (> 700 amino acids residues). This region is sensitive to limited posttranslational processing. In heart and brain the α1C subunits are found to be truncated but the C-terminal domain remains functionally present. Based on our previous data we hypothesized that the distal C-terminus (approximately residues 1650–1950) harbors an important, predominantly inhibitory domain. We generated C-terminal-truncated α1C mutants, and after expressing them in combination with a β3 subunit in HEK-293 cells, electrophysiological experiments were carried out. In order to dissect the important inhibitory part of the C-terminus, trypsin was dialyzed into the cells. The data provide evidence that there are multiple residues within the inhibitory domain that are crucial to the inhibitory process as well as to the enhancement of expressed current by intracellular application of proteases. In addition, the expression of the chimeric mutant α1CΔ1673-DRK1 demonstrated that the C-terminal is specific for the heart channel.

The Role of Region IVS5 of the Human Cardiac Calcium Channel in Establishing Inactivated Channel Conformation USE-DEPENDENT BLOCK BY BENZOTHIAZEPINES

The role of inactivated channel conformation and use dependence for diltiazem, a specific benzothiazepine calcium channel inhibitor, was studied in chimeric constructs and point mutants created in the IVS5 transmembrane segment of the L-type cardiac calcium channel. All mutations, chimeric or point mutations, were restricted to IVS5, while the YAI-containing segment in IVS6, i.e. the primary interaction site with benzothiazepines, remained intact. Slowed inactivation rate and incomplete steady state inactivation, a behavior of some mutants, were accompanied by a reduced or by a complete loss of use-dependent block by diltiazem. Single channel properties of mutants that lost use dependence toward diltiazem were characterized by drastically elongated mean open times and distinctly slower time constants of open time distribution. Mutation of individual residues of the IVMLF segment in IVS5 did not mimic the complete loss of use dependence as observed for the replacement of the whole stretch. These results establish evidence that amino acids that govern inactivation and the drug-binding site and other amino acids that are located distal from the putative drug-binding site contribute significantly to the function of the benzothiazepine receptor region. The data are consistent with a complex “pocket” conformation that is responsive to a specific class of L-type calcium channel inhibitors. The data allow for a concept that multiple sites within regions of the α1 subunit contribute to auto-regulation of the L-type Ca2+ channel.