Insights into the molecular mechanism of hERG1 channel activation and blockade by drugs.

Blockade of the human ether-a-go-go related gene 1 (hERG1) channel has been associated with an increased duration of ventricular repolarization, causing prolongation of the time interval between Q and T waves (long QT syndrome, or LQTS). LQTS may result in serious cardiovascular disorders such as tachyarrhythmia and sudden cardiac death. Diverse types of organic compounds bind to the wide intracellular cavity in the pore domain of hERG channels, leading to a full or partial blockade of ion current through the pore. The drug-induced blockade of the hERG-related component of the potassium current is thought to be a major reason for drug-induced arrhythmias in humans. Identification of specific interactions governing the high-affinity blockade of cardiac potassium (K-) channels is crucial both for the prevention of unintended ion channel block and for the design of ion channel modulators. A plethora of ligand- and receptor-based models of K-channels have been created to address these challenges. In this paper, we review the current state of knowledge regarding the structure-function relationship of hERG and discuss progress in the use of molecular modeling for developing both blockers and activators of hERG.

Decrease in density of INa is in the common final pathway to heart block in murine hearts overexpressing calcineurin.

Overexpression of calcineurin in transgenic mouse heart results in massive cardiac hypertrophy followed by sudden death. Sudden deaths are caused by abrupt transitions from sinus rhythm to heart block (asystole) in calcineurin-overexpressing (CN) mice. Preliminary studies showed decreased maximum change in potential over time (dV/dt(max)) of phase 0 of the action potential. Accordingly, the hypothesis was tested that decreased activity of the sodium channel contributes to heart block. Profound decreases in activity of sodium currents (I(Na)) paralleled the changes in action potential characteristics. Progressive age-dependent decreases were observed such that at 42-50 days of life little sodium channel function existed. However, this was not paralleled by decreased protein expression as assessed by immunocytochemistry or by Western blot. Since calcineurin can interact with the ryanodine receptor, we assessed whether chronic in vitro treatment with BAPTA-AM, thapsigargin, and ryanodine could rescue the decrease of I(Na). All of these treatments rescued I(Na) to levels indistinguishable from wild type. The nonspecific PKC inhibitor bisindolylmaleimide I also rescued the decrease of I(Na). To assess whether decreased sodium channel activity contributes to sudden death in vivo, the response to encainide (20 mg/kg) was assessed: 6 of 10 young CN mice died because of asystole, whereas 0 of 10 wild-type mice died (P < 0.01). Moreover, encainide produced exaggerated prolongation of the QRS width in sinus beats before the heart block. Catecholamine tone appears necessary to support life in older CN mice because propranolol (1 mg/kg) triggered asystolic death in five of six CN mice. We conclude that decrease in sodium channel activity is in the common final pathway to asystole in CN mice.

Overexpression of calcineurin in mouse causes sudden cardiac death associated with decreased density of K+ channels.

BACKGROUND: Overexpression of calcineurin in transgenic (TG) mice results in cardiac hypertrophy and unexpected deaths. METHODS AND RESULTS: None of the TG survived beyond 24 weeks (n=38) whereas all of the wildtype (WT, n=47) survived. Prolongation of repolarization preceded the development of sustained pleomorphic ventricular tachycardia and high degree atrioventricular block, which occurred during spontaneous sudden deaths. Since depolarization-activated K(+) channels contribute dominantly to repolarization in mice, we hypothesized that the TG would decrease these K(+) currents and that the in vivo administration of cyclosporin A (CsA), a calcineurin inhibitor, would reduce this effect. CsA reversed cardiac hypertrophy: capacitance measurements of WT left ventricular myocytes (127+/-7 pF; n=45) and CsA-treated TG (129+/-14 pF; n=17) were significantly lower than in placebo-treated TG (220+/-11 pF; n=41; P<0.001 by ANOVA). Independent of whether the data fit a bi- or a tri-exponential model, the density of I(tof) was significantly reduced in TG versus WT and CsA reversed this effect. While I(tos) and I(Kslow) were also reduced in TG, CsA does not reverse this change because long-term in vivo CsA treatment of WT also reduces I(tos) and I(Kslow.) To assess whether the decreased 'repolarization reserve' contributed to arrhythmogenesis, the residual I(Kr) was blocked by dofetilide precipitating pleomorphic ventricular tachycardias. CONCLUSION: Since the downregulation of I(tof) was observed with overexpression of calcineurin and was also reversed by the calcineurin inhibitor CsA, we conclude that downregulation of I(tof) is a consequence of calcineurin overexpression.

Novel gain-of-function mechanism in K(+) channel-related long-QT syndrome: altered gating and selectivity in the HERG1 N629D mutant.

The N629D mutation, adjacent to the GFG signature sequence of the HERG1 A K(+) channel, causes long-QT syndrome (LQTS). Expression of N629D in Xenopus oocytes produces a rapidly activating, noninactivating current. N629D is nonselective among monovalent cations; permeation of K(+) was similar to that of Na(+) or Cs(+). During repolarization to potentials between -30 and -70 mV, N629D manifested an inward tail current, which was abolished by replacement of extracellular Na(+) (Na(+)(e)) with extracellular N-methyl-D-glucamine (NMG(e)). Because LQTS occurs in heterozygous patients, we coexpressed N629D and wild type (WT) at equimolar concentrations. Heteromultimer formation was demonstrated by analyzing the response to 0 [K(+)](e). The outward time-dependent current was nearly eliminated for WT at 0 [K(+)](e), whereas no reduction was observed for homomultimeric N629D or for the equimolar coexpressed current. To assess physiological significance, dofetilide-sensitive currents were recorded during application of simulated action potential clamps. During phase 3 repolarization, WT manifested outward currents, whereas homomultimeric N629D manifested inward depolarizing currents. During coexpression studies, variable phenotypes were observed ranging from a reduction in outward repolarizing current to net inward depolarizing current during phase 3. In summary, N629D replaces the WT outward repolarizing tail current with an inward depolarizing sodium current, which is expected to delay later stages of repolarization and contribute to arrhythmogenesis. Thus, the consequences of N629D resemble the pathophysiology seen in LQT3 Na(+) channel mutations and may be considered the first LQTS K(+) channel mutation that exhibits gain of function.

Molecular determinant of high-affinity dofetilide binding to HERG1 expressed in Xenopus oocytes: involvement of S6 sites.

This study reports that the affinity of HERG1 A for dofetilide is decreased from 0.125 +/- 0.003 microM for wild-type (WT) channels to 15 +/- 3 microM for F656V, a mutation in the COOH-terminal half of the S6. Similarly, the IC(50) for quinidine was increased from 8 +/- 4 microM for WT to 219 +/- 65 microM for the F656V mutation, whereas affinity for external tetraethylammonium was similar for WT (51 +/- 10 mM) and F656V (36 +/- 10 mM, NS). Kinetics of onset of inactivation of F656V was similar to WT but kinetics of deactivation, activation, and recovery from inactivation differed from WT. However, mutations in nearby amino acids in the S6 more strikingly altered deactivation, activation, and recovery from inactivation but had little effect on affinity for dofetilide. To assess the effects of disruption of inactivation, the S631A mutation was made. The S631A mutation altered the IC(50) for dofetilide to 20 +/- 3 microM, but the IC(50) for quinidine was unchanged at 8 +/- 4 microM for WT and 10 +/- 1 microM for S631A. To address whether the F656V mutation alters the IC(50) for dofetilide in a channel that does not inactivate, the double mutation S631A/F656V was made. The IC(50) for dofetilide of the double mutation was 32 +/- 3 microM, which is not substantially different than that of S631A. These data support the notion that allosteric changes occurring during the process of inactivation are necessary for high-affinity dofetilide binding. In conclusion, the Phe-656 residue of HERG is a molecular determinant of high-affinity dofetilide binding.

Electrophysiological characterization of an alternatively processed ERG K+ channel in mouse and human hearts.

Mutants of HERG, the human form of ERG (the ether-a-go-go-related K+ channel gene), are responsible for some forms of the long-QT syndrome, an abnormality of cardiac repolarization. HERG was cloned from brain and has properties similar but not identical to the rapidly activating component of the native cardiac K+ channel current (Ikr). We identified in the mouse an alternatively processed form of ERG (MERG B) that is expressed abundantly in heart but only in trace amounts in brain. MERG B has a unique 36-amino acid NH2-terminal domain that is strongly basic and considerably shorter than the 376-amino acid NH2-terminal domain of HERG. When expressed in Xenopus oocytes, the kinetics of activation and deactivation of the MERG B current were best fit by a biexponential function, with the fast components dominant over the slow components. The fast component of activation had a mean tau value of 163 +/- 16 ms at -20 mV and 8 +/- 4 ms at +20 mV (n = 4). The fast component of deactivation had a mean tau value of 145 +/- 29 ms at -20 mV and 12 +/- 4 ms at -90 mV (n = 4). The MERG B current was blocked by the selective IKr blocker, dofetilide, with an IC50 of 54 nmol/L. In addition, we isolated HERG B, the human homologue of MERG B, which has electrophysiological characteristics qualitatively similar to those of MERG B. We have identified ERG B, an alternatively processed isoform of the ERG gene, expressed selectively in heart and with electrophysiological characteristics similar to those of native cardiac IKr.

[3H]dofetilide binding to cardiac myocytes: modulation by extracellular potassium.

UNLABELLED: The radioligand [3H]dofetilide binds specifically to the delayed rectifier potassium channel and provides a biochemical approach to study interactions of Class III drugs with this channel. However, previous studies have examined the binding of [3H]dofetilide to cardiac myocytes only at extracellular potassium of 135 mM. Because previous electrophysiological studies have shown that hyperkalemia could alter the pharmacological responses to I(Kr) channel blockers, the hypothesis tested in this study was that changing ionic conditions would alter characteristics of [3H]dofetilide binding. RESULTS: under physiological conditions (Na+ 135 mM, K+ 5 mM), [3H]dofetilide bound to two sites on guinea-pig ventricular myocytes (a high-affinity site, K(d) 26+/-8 nM, B(max) 81+/-12 fmol/10(6) cells: and a low-affinity site, K(d) 1.6+/-0.8 microM, B(max) 1003+/-173 fmol/10(6) cells, n=11). Binding properties were not altered by changes in osmolarity or extracellular sodium. However, when extracellular K+ was increased to 20 mM, a single binding site was observed with an affinity K(d) of 120+12 nM and a B(max) of 303+/-57 fmol/10(6) cells (P<0.05; n=6). To establish whether this effect was mediated at the high-affinity site we assessed the effects of elevated extracellular potassium on a biological model, neonatal mouse myocytes, that expressed solely the high-affinity binding sites. The K(d) values for binding to fetal mouse cardiac myocytes at an extracellular K+ of 5 mM and 20 mM were also significantly different, 29+/-10 and 230+/-46 nM, respectively. In conclusion, [3H]dofetilide binding to its high-affinity site is modulated by extracellular potassium.

Brain-specific tropomyosins TMBr-1 and TMBr-3 have distinct patterns of expression during development and in adult brain.

In this study we report on the developmental and regional expression of two brain-specific isoforms of tropomyosin, TMBr-1 and TMBr-3, that are generated from the rat alpha-tropomyosin gene via the use of alternative promoters and alternative RNA splicing. Western blot analysis using an exon-specific peptide polyclonal antibody revealed that the two isoforms are differentially expressed in development with TMBr-3 appearing in the embryonic brain at 16 days of gestation, followed by the expression of TMBr-1 at 20 days after birth. TMBr-3 was detected in all brain regions examined, whereas TMBr-1 was detected predominantly in brain areas that derived from the prosencephalon. Immunocytochemical studies on mixed primary cultures made from rat embryonic midbrain indicate that expression of the brain-specific epitope is restricted to neurons. The developmental pattern and neuronal localization of these forms of tropomyosin suggest that these isoforms have a specialized role in the development and plasticity of the nervous system.

A vertebrate actin-related protein is a component of a multisubunit complex involved in microtubule-based vesicle motility.

Actin is a cytoskeletal protein which is highly conserved across eukaryotic phyla. Actin filaments, in association with a family of myosin motor proteins, are required for cellular motile processes as diverse as vesicle transport, cell locomotion and cytokinesis. Many organisms have several closely related actin isoforms. In addition to conventional actins, yeasts contain actin-related proteins that are essential for viability. We show here that vertebrates also contain an actin-related protein (actin-RPV). Actin-RPV is a major component of the dynactin complex, an activator of dynein-driven vesicle movement, indicating that unlike conventional actins which work in conjunction with myosin motors, actin-RPV may be involved in cytoplasmic movements via a microtubule-based system.

Identification of act2, an essential gene in the fission yeast Schizosaccharomyces pombe that encodes a protein related to actin.

Actins are a family of highly conserved proteins that are ubiquitously found among eukaryotic organisms. All actins that have previously been identified, including those of animals, plants, fungi, and protozoa, are 374-376 amino acids long and exhibit at least 70% amino acid sequence identity when compared with one another. We have cloned a gene from the fission yeast Schizosaccharomyces pombe that encodes a distantly related member of the actin protein family, herein referred to as act2. In contrast to all other actins, the derived amino acid sequence reveals that act2 is 427 residues long and exhibits only 35-40% identity to actins, including act1 from Sch. pombe. Comparison to the known x-ray crystallographic structure of rabbit skeletal muscle actin indicates that the ATP and divalent metal ion binding sites are largely conserved in act2, while regions involved in actin-actin and actin-myosin interactions are relatively divergent. Disruption of the act2 gene demonstrated that this gene encodes a function essential for germination of haploid spores. These findings indicate that while act2 and act1 are related proteins, they appear to have distinct functions. In addition, they demonstrate that the actin protein family is more diverse than was previously thought.

The molecular basis for tropomyosin isoform diversity.

The tropomyosins are a family of actin filament binding proteins. In multicellular animals, they exhibit extensive cell type specific isoform diversity. In this essay we discuss the genetic mechanisms by which this diversity is generated and its possible significance to cellular function.

Four fibroblast tropomyosin isoforms are expressed from the rat alpha-tropomyosin gene via alternative RNA splicing and the use of two promoters.

cDNA clones encoding four rat tropomyosin isoforms, termed TM-2, TM-3, TM-5a, and TM-5b, were isolated and characterized. All are derived from the alpha-tropomyosin gene via alternative RNA processing and the use of two alternate promoters. The cDNA sequences predict that TM-2 and TM-3 both contain 284 amino acids and differ from each other only at an internal region of the protein from amino acids 189 through 213, due to alternative splicing of exons 6a and 6b. TM-5a and TM-5b both contain 248 amino acids and differ from each other only at an internal exon encoding amino acids 153 through 177, also due to alternative splicing of exons 6a and 6b. The differences in the amino acid sequence encoded by these alternate exons affects the theoretical actin-binding pattern of the tropomyosins, such that TM-5b is expected to bind actin with greater affinity than TM-5a. TM-2 and TM-3 are transcribed from the upstream promoter, and TM-5a and TM-5b are transcribed from an internal promoter. In addition, all four isoforms contain the identical COOH-terminal coding region. RNA protection analyses revealed that the mRNA for each isoform is expressed in a number of different tissues and cell types, although the expression of some isoforms is restricted to particular cell types. Furthermore, the expression of mRNA encoding these isoforms was found to be altered in a number of different virally transformed cell lines. The changes in the expression of tropomyosin mRNAs in transformed cells reflect changes in the relative use of the two promoters, as well as the relative use of alternatively spliced exons 6a and 6b.

Structure and complete nucleotide sequence of the gene encoding rat fibroblast tropomyosin 4.

We have isolated and determined the complete nucleotide sequence of the gene that encodes the 248 amino acid residue fibroblast tropomyosin, TM-4. The TM-4 sequence is encoded by eight exons, which span approximately 16,000 bases. The position of the intron-exon splice junctions relative to the final transcript are identical to those present in other vertebrate tropomyosin genes and the Drosophila melanogaster TMII gene. We have found no evidence that the rat TM-4 gene is alternatively spliced, unlike all the other tropomyosin genes from multicellular organisms that have been described. Typical vertebrate tropomyosin genes contain some, or all, of alternatively spliced exons 1a and 1b, 2a and 2b, 6a and 6b, and 9a, 9b, 9c and 9d in addition to common exons 3, 4, 5, 7 and 8. The rat fibroblast TM-4 mRNA is encoded by sequences most similar to exons 1b, 3, 4, 5, 6b, 7, 8 and 9d. Two exon-like sequences that are highly similar to alternatively spliced exons 2b and 9a of the rat beta-tropomyosin gene and the human TMnm gene have been located in the appropriate region of the gene encoding rat fibroblast TM-4. However, several mutations in these sequences render them non-functional as tropomyosin coding exons. We have termed these exon-like sequences, vestigial exons. The evolutionary relationship of the rat TM-4 gene relative to other vertebrate tropomyosin genes is discussed.

Three novel brain tropomyosin isoforms are expressed from the rat alpha-tropomyosin gene through the use of alternative promoters and alternative RNA processing.

cDNA clones encoding three novel tropomyosins, termed TMBr-1, TMBr-2, and TMBr-3, were isolated and characterized from a rat brain cDNA library. All are derived from a single gene, which was previously found to express striated muscle alpha-tropomyosin and a number of other tropomyosin isoforms via an alternative splicing mechanism (N. Ruiz-Opazo and B. Nadal-Ginard, J. Biol. Chem. 262:4755-4765, 1987; D. F. Wieczorek, C. W. J. Smith, and B. Nadal-Ginard, Mol. Cell. Biol. 8:679-694, 1988). The derived amino acid sequences revealed that TMBr-1 contains 281 amino acids, TMBr-2 contains 251 amino acids, and TMBr-3 contains 245 amino acids. All three proteins contain a region that is identical to amino acids 81 through 258 of skeletal muscle alpha-tropomyosin. TMBr-1 is identical to striated muscle alpha-tropomyosin from amino acids 1 through 258 but contains a novel COOH-terminal region from amino acids 259 through 281. TMBr-2 and TMBr-3 both contain identical NH2-terminal sequences from amino acids 1 through 44 which were found to be expressed from a novel promoter. TMBr-3 contains the same COOH-terminal region as TMBr-1, whereas TMBr-2 contains a second novel COOH-terminal region. The genomic organization of the exons encoding TMBr-1, TMBr-2, and TMBr-3 were determined. These studies revealed a previously uncharacterized promoter located in the internal region of the alpha-TM gene as well as two novel COOH-terminal coding exons. The alpha-TM gene is a complex transcription unit containing 15 exons including two alternative promoters, two internal mutually exclusive exon cassettes, and four alternatively spliced 3' exons that encode four different COOH-terminal coding regions. A total of nine distinct mRNAs are known to be expressed from the alpha-TM gene in a cell type-specific manner in tissues such as striated muscle, smooth muscle, kidney, liver, brain, and fibroblasts. The mRNAs encoding TMBr-1, TMBr-2, and TMBr-3 were found to be expressed only in brain tissue, with TMBr-3 being expressed at much greater levels than TMBr-1 and TMBr-2. The individual structural characteristics of each brain alpha-tropomyosin isoform and their possible functions are discussed.

An abundant and novel protein of 22 kDa (SM22) is widely distributed in smooth muscles. Purification from bovine aorta.

Using a rabbit polyclonal-antibody preparation directed against the chicken gizzard protein, we demonstrated by immunoblotting the presence of the 22 kDa protein (SM22) in a variety of chicken smooth-muscle-containing organs, including uterus, intestine, gizzard, oesophagus and aorta. Protein SM22 was present in only trace amounts in brain, liver and heart, and could not be detected in chicken breast muscle. The antibody preparation did not cross-react with extracts of bovine aorta. However, the presence of SM22 as a major component in bovine aorta and pig carotid was demonstrated by its co-migration with the purified chicken gizzard protein on one- and two-dimensional polyacrylamide electrophoretic gels. Its molar abundance relative to actin was estimated to be 0.9:6.0 and 1.4:6.0 for bovine aorta and pig carotid respectively. Like the chicken gizzard protein, it separates on pH-gradient electrophoresis into at least three variants, alpha, beta and gamma, with similar apparent Mr. Purification of the aorta SM22 showed it to have a similar amino acid composition to the chicken gizzard protein. We conclude that SM22 is widely distributed and an abundant and unique protein component of smooth-muscle tissues of birds and mammals.

Amino acid sequence of chicken gizzard smooth muscle SM22 alpha.

The complete amino acid sequence of SM22 alpha, a novel and abundant 22-kDa protein from chicken gizzard smooth muscle, was determined by a combination of automated and manual Edman degradation methods on fragments produced by suitable chemical and proteolytic cleavages. The protein consists of a single polypeptide chain of 197 residues, has a Mr of 21, 978, and a net charge of +4.5 at neutral pH. The pattern of alternating hydrophilic and hydrophobic regions throughout the length of SM23 alpha is typical of a globular protein. The overall secondary structural analysis, using several algorithms based on the sequence, predicts approximately 31% alpha-helix, 24% beta-sheet, 18% beta-turn, and 27% random coil. A search against the National Biomedical Research Foundation Protein Sequence Databank (Washington) and GenBank (Los Alamos) failed to demonstrate significant similarity with any other protein of known sequence.

Isolation and characterization of an abundant and novel 22-kDa protein (SM22) from chicken gizzard smooth muscle.

Chicken gizzard smooth muscle contains a highly abundant protein (SM22) with an apparent Mr on sodium dodecyl sulfate-polyacrylamide electrophoretic gels of 23,000. The ratio of actin:SM22:tropomyosin in this tissue is estimated to be 6.5(+/- 0.8):2.0(+/- 0.2):1.0. At least three isoelectric isoforms are present in ratios of alpha:beta:gamma of 14:5:1 with alpha the most basic and gamma the most acidic. A method for the purification of SM22 and partial separation of its isoforms is described. Amino acid analyses of purified alpha and beta demonstrate the presence of 1 and 2 half-cystines, respectively, and a lower content of basic amino acids in beta. A value of 22,000 for the Mr of alpha estimated by sedimentation equilibrium indicated its presence as a monomer at physiological ionic strengths. Estimates of the translational frictional coefficient (f/fmin) of alpha calculated from its Stokes radius (25.5 A) and Mr were consistent with its existence as a moderately asymmetric globular protein. Calculations based on its far-ultraviolet CD spectrum provided values of 37% alpha-helix, 31% beta-sheet, 5% beta-turn, and 27% random coil. SM22 was shown not to share functional properties with several proteins of similar Mr and isoelectric point such as myokinase, brain 23-kDa protein, and troponin I. We conclude that it is a novel protein not previously isolated or characterized from any tissue.

Drugs that block calmoduLin activity inhibit cell-to-cell coupling in the epidermis of Tenebrio molitor.

In many cell systems, the permeability of membrane junctions is modulated by the cytoplasmic level of free Ca++. To examine whether the calcium-dependent regulatory protein calmodulin is involved in this process, the ability of anticalmodulin drugs to influence the cell-to-cell passage of injected current and an organic tracer was tested using standard intracellular glass microelectrode techniques. Several antipsychotics and local anesthetics were found to block junctional communication in the epidermis of the beetle Tenebrio molitor. Treatment of the epidermis with chlorpromazine (0.25 mM) raised intercellular resistance two- to threefold within 20 to 25 min; cell-to-cell passage of electrical current was abolished within 41 +/- 5 min. Loss of electrotonic coupling was accompanied by a block in the cell-to-cell movement of the organic tracer carboxyfluorescein. The reaction is fully reversible, with normal electrotonic coupling being restored within 2 to 4 hr. Other antipsychotics and local anesthetics had similar effects on cell coupling. The order of potency found was: trifluoperazine greater than thioridazine greater than D-butaclamol greater than chlorprothixine = chlorpromazine greater than L-butaclamol greater than dibucaine greater than tetracaine. The relative uncoupling potencies of these drugs correlate well with their known ability to inhibit calmodulin-dependent phosphodiesterase activity. Other anesthetic compounds, procaine and pentobarbital, did not block cell-to-cell communication. Altering the extracellular Ca++ concentration did not affect the rate of uncoupling by antipsychotics, while chelation of extracellular Ca++ with EGTA raised electrotonic coupling. The effect of three metabolic inhibitors on coupling was also examined. Iodoacetate uncoupled the epidermal cells while DNP and cyanide did not. These results are discussed in terms of possible mechanisms by which calmodulin may control junctional communication in this tissue.