Regulation of KChIP2 potassium channel beta subunit gene expression underlies the gradient of transient outward current in canine and human ventricle

Molecular and functional characterization of Kv4.2 and KChIP2 expressed in the porcine left ventricle

Recent studies showed that the Ca(2+)-independent transient outward current (I (to)) is very small or even not detectable in the porcine left ventricle. We investigated whether an altered molecular expression or function of voltage-dependent potassium channels belonging to the Kv4 sub-family and their ancillary Ca(2+)-binding beta sub-unit KChIP2, which contribute to the major fraction of I (to )in other species, may underlie this lack of a significant I (to) in the porcine left ventricle. RT-PCR analysis with degenerate primers showed that both Kv4 mRNA and KChIP2 mRNA are expressed in porcine left ventricular tissue and in isolated ventricular myocytes. PCR cloning and sequence analysis predicted proteins with >98% identity to rat and human Kv4.2 and >99% identity to rat and human KChIP2. Heterologous expression of porcine Kv4.2 in Xenopus laevis oocytes gave rise to currents with characteristic properties of rat and human Kv4.2, and co-expression with its KChIP2 sub-unit increased current density (tenfold), slowed inactivation (twofold) and accelerated recovery from inactivation (tenfold). Kv4.2 immunohistochemistry in porcine left ventricular tissue revealed a predominant membrane-bound signal. Relative quantification of gene expression indicated that Kv4.2 and KChIP2 mRNA and protein are expressed at comparable ratios in porcine and rat left ventricular tissues, which displays a large I (to). Collectively, these data demonstrate that the lack of a significant I (to) in the porcine left ventricle does not result from dysfunctional or insufficiently expressed Kv4.2 and KChIP2 sub-units.

Spatial distributions of Kv4 channels and KChip2 isoforms in the murine heart based on laser capture microdissection

OBJECTIVE

Regional differences in repolarizing K(+) current densities and expression levels of their molecular components are important for coordinating the pattern of electrical excitation and repolarization of the heart. The small size of hearts from mice may obscure these interventricular and/or transmural expression differences of K(+) channels. We have examined this possibility in adult mouse ventricle using a technology that provides very high spatial resolution of tissue collection.

METHODS

Conventional manual dissection and laser capture microdissection (LCM) were utilized to dissect tissue from distinct ventricular regions. RNA was isolated from epicardial, mid-myocardial and endocardial layers of both the right and left ventricles. Real-time RT-PCR was used to quantify the transcript expression in these different regions.

RESULTS

LCM revealed significant interventricular and transmural gradients for both Kv4.2 and the alpha-subunit of KChIP2. The expression profile of a second K(+) channel transcript, Kir2.1, which is responsible for the inwardly rectifying K(+) current I(k1), showed no interventricular or transmural gradients and therefore served as a negative control.

CONCLUSIONS

Our findings are in contrast to previous reports of a relatively uniform left ventricular transmural pattern of expression of Kv4.2, Kv4.3 and KChIP2 in adult mouse heart, which appear to be different than that in larger mammals. Specifically, our results demonstrate significant epi- to endocardial differences in the patterns of expression of both Kv4.2 and KChIP2.

Potassium channel remodeling in cardiac hypertrophy

Cardiac hypertrophy is an adaptive process against increased work loads; however, hypertrophy also presents substrates for lethal ventricular arrhythmias, resulting in sudden arrhythmic deaths that account for about one third of deaths in cardiac hypertrophy. To maintain physiological cardiac function in the face of increased work loads, hypertrophied cardiomyocytes undergo K(+) channel remodeling that provides a prolongation in action potential duration and an increase in Ca(2+) entry. Increased Ca(2+) entry, in turn, activates signaling mechanisms including a calcineruin/NFAT pathway to permit remodeling of the K(+) channels. This results in a positive feedback loop between the K(+) channel remodeling and altered Ca(2+) handling; this loop may represent a potential therapeutic target against sudden arrhythmic deaths in cardiac hypertrophy. The purposes of this review are to: (1) discuss types of K(+) channels and their mRNA that undergo remodeling in cardiac hypertrophy; (2) report on recent research on molecular mechanisms of K(+) channel remodeling; and (3) address physiological events underlying new therapeutic modalities to ameliorate arrhythmias and sudden death in cardiac hypertrophy.

Three-dimensional structure of the KChIP1-Kv4.3 T1 complex reveals a cross-shaped octamer

Brain I(A) and cardiac I(to) currents arise from complexes containing Kv4 voltage-gated potassium channels and cytoplasmic calcium-sensor proteins (KChIPs). Here, we present X-ray crystallographic and small-angle X-ray scattering data that show that the KChIP1-Kv4.3 N-terminal cytoplasmic domain complex is a cross-shaped octamer bearing two principal interaction sites. Site 1 comprises interactions between a unique Kv4 channel N-terminal hydrophobic segment and a hydrophobic pocket formed by displacement of the KChIP H10 helix. Site 2 comprises interactions between a T1 assembly domain loop and the KChIP H2 helix. Functional and biochemical studies indicate that site 1 influences channel trafficking, whereas site 2 affects channel gating, and that calcium binding is intimately linked to KChIP folding and complex formation. Together, the data resolve how Kv4 channels and KChIPs interact and provide a framework for understanding how KChIPs modulate Kv4 function.

Calcineurin increases cardiac transient outward K+ currents via transcriptional up-regulation of Kv4.2 channel subunits

Fast transient outward potassium currents (I(to,f)) are critical determinants of regional heterogeneity of cardiomyocyte repolarization as well as cardiomyocyte contractility. Additionally, I(to,f) densities are markedly down-regulated in cardiac hypertrophy and heart disease, conditions associated with activation of the serine/threonine phosphatase calcineurin (Cn). In this study, we investigated the regulation of I(to,f) expression by Cn in cultured neonatal rat ventricular myocytes (NRVMs) with and without alpha(1)-adrenoreceptor stimulation with phenylephrine (PE). Overexpression of constitutively active Cn in NRVMs induced hypertrophy and caused profound increases in I(to,f) density as well as Kv4.2 mRNA and protein expression and promoter activity, without affecting Kv4.3 or KChIP2 levels. The effects of Cn on hypertrophy, I(to,f), and Kv4.2 transcription were associated with NFAT activation and were abrogated by NFAT inhibition. Despite activating Cn and inducing hypertrophy in NRVMs, PE resulted in profound down-regulation of I(to,f) densities as well as Kv4.2, Kv4.3, and KChIP2 expression. Although hypertrophy and NFAT activation were inhibited by the Cn inhibitory peptide CAIN, I(to,f) and Kv4.2 expression were further reduced by CAIN, whereas Cn overexpression eliminated PE-induced reductions in I(to,f) and Kv4.2 expression without affecting Kv4.3 or KChIP2 levels. We conclude that Cn increases cardiac I(to,f) densities by positively regulating Kv4.2 gene transcription. Consistent with this conclusion, we found that I(to,f) was increased in myocytes isolated from young mice overexpressing Cn prior to the development of heart disease. This positive regulation of Kv4.2 transcription by Cn activation is expected to minimize the reductions in I(to,f) and Kv4.2 expression observed in hypertrophic cardiomyocytes.

Species and tissue differences in the expression of DPPY splicing variants

The non-functional dipeptidyl peptidase, DPPY (DPP10), regulates the expression and gating of K+ channels in Kv4 family by tightly binding to these pore-forming subunits. Neural tissue-specific expression of this and the related DPPX (DPP6) is thought to confer rapid inactivation and other unique properties of neuronal Kv4 channels. Here we report that DPPY mRNA is abundant in human adrenal gland, but very low in the corresponding rat tissue. Furthermore, multiple DPPY splicing variants with alternative first exons are significant in the brain, whereas the expression of DPPY gene in the adrenal gland and pancreas is predominantly initiated at the two latter sites. These splicing variants, as well as an N-terminal peptide-deleted DPPY, produce similar changes in Kv4.3 gating. Thus, transcription of DPPY gene is species- and tissue-specifically controlled.

C-terminal Domain of Kv4.2 and Associated KChIP2 Interactions Regulate Functional Expression and Gating of Kv4.2

The Kv4.2 transient voltage-dependent potassium current contributes to the morphology of the cardiac action potential as well as to neuronal excitability and firing frequency. Here we report profound effects of the Kv4.2 C terminus on the surface expression and activation gating properties of Kv4.2 that are modulated by the direct interaction between KChIP2, an auxiliary regulatory subunit, and the C terminus of Kv4.2. We show that increasingly large truncations of the C terminus of rat Kv4.2 (wild type) cause a progressive decrease of Kv4.2 current along with a shift in voltage-dependent activation that is closely correlated with negative charge deletion. Co-expression of more limited Kv4.2 C-terminal truncation mutants (T588 and T528) with KChIP2 results in a doubling of Kv4.2 protein expression and up to an 8-fold increase in Kv4.2 current amplitude. Pulsechase experiments show that co-expression with KChIP2 slows Kv4.2 wild type degradation 8-fold. Co-expression of KChIP2 with an intermediate-length C-terminal truncation mutant (T474) shifts Kv4.2 activation voltage dependence and enhances expression of Kv4.2 current. The largest truncation mutants (T417 and DeltaC) show an intracellular localization with no measurable currents and no response to KChIP2 co-expression. Co-immunoprecipitation and competitive glutathione S-transferase-binding assays indicate a direct interaction between KChIP2 and the Kv4.2 C terminus with a relative binding affinity comparable with that of the N terminus. Overall, these results suggest that the C-terminal domain of Kv4.2 plays a critical role in voltage-dependent activation and functional expression that is mediated by direct interaction between the Kv4.2 C terminus and KChIP2.

Gene expression profiling of human cardiac potassium and sodium channels

BACKGROUND

The native cardiac ion currents and the action potential itself are the results of the concerted action of several different ion channels. The electrophysiological properties of cardiac cells are determined by the composition of ion channels and by their absolute abundance and proportional ratio.

METHODS

Our aim in this study was to compare the gene expression level of a representative panel of cardiac ion channels with each other and to compare the same channels in the atrium and ventricle of the human heart using quantitative real-time PCR analysis.

RESULTS

We obtained a significant difference in the gene expression levels in 21 of 35 channels between atrium and ventricle of healthy human hearts. Further, we found that the expression levels of Kv1.5 and Kv2.1 transcripts in the ventricle were very high, and that mRNAs for Kv1.7 and Kv3.4 are highly abundant in both the atrium and ventricle, which might indicate a functional role of these ion channel subunits in the formation of action potential in the human ventricle and both in the atrium and ventricle, respectively.

CONCLUSIONS

This is the first report on the expression of several ion channel subunits, such as Kv1.7, Kv3.3 or Kv3.4 in human cardiomyocytes. The expression levels of these genes are comparable with that of well known ion channel subunits. Therefore, it is reasonable to assume, that these ion channel subunits may contribute to native currents in the human myocardium.

Voltage-gated potassium channels: regulation by accessory subunits

Voltage-gated potassium channels regulate cell membrane potential and excitability in neurons and other cell types. A precise control of neuronal action potential patterns underlies the basic functioning of the central and peripheral nervous system. This control relies on the adaptability of potassium channel activities. The functional diversity of potassium currents, however, far exceeds the considerable molecular diversity of this class of genes. Potassium current diversity contributes to the specificity of neuronal firing patterns and may be achieved by regulated transcription, RNA splicing, and posttranslational modifications. Another mechanism for regulation of potassium channel activity is through association with interacting proteins and accessory subunits. Here the authors highlight recent work that addresses this growing area of exploration and discuss areas of future investigation.

Contributions of sustained INa and IKv43 to transmural heterogeneity of early repolarization and arrhythmogenesis in canine left ventricular myocytes

The roles of sustained components of I(Na) and I(Kv43) in shaping the action potentials (AP) of myocytes isolated from the canine left ventricle (LV) have not been studied in detail. Here we investigate the hypothesis that these two currents can contribute substantially to heterogeneity of early repolarization and arrhythmic risk. Quantitative data from voltage-clamp and expression profiling experiments were used to complete meaningful modifications to an existing "local control" model of canine midmyocardial myocyte excitation-contraction coupling for epicardial and endocardial cells. We include 1) heterogeneous I(Kv43), I(Ks), and I(SERCA) density; 2) modulation of I(Kv43) by Kv channel interacting protein type 2 (KChIP2) channel subunits; 3) a possible Ca(2+)-dependent open-state inactivation of I(Kv43); and 4) a sustained component of the inward Na(+) current, I(NaL). The resulting simulations illustrate ways in which KChIP2- and Ca(2+)-dependent control of I(Kv43) can result in a sustained outward current that can neutralize I(NaL) in a rate- and myocyte subtype-dependent manner. Both these currents appear to play significant roles in modulating AP duration and rate dependence in midmyocardial myocytes. Furthermore, an increased ratio of I(Kv43) to I(NaL) is capable of protecting epicardial myocytes from the early afterdepolarizations resulting from the SCN5A-I1768V mutation-induced increase in I(NaL). Experimentally observed transmural differences in Ca(2+) handling, including greater sarcoplasmic reticulum Ca(2+) content and faster Ca(2+) transient decay rates on the epicardium, were recapitulated in our simulations. By design, these models allow upward integration into organ models or may be used as a basis for further investigations into cellular heterogeneities.

Differential Calcineurin/NFATc3 Activity Contributes to the I to Transmural Gradient in the Mouse Heart

Kv4 channels are differentially expressed across the mouse left ventricular free wall. Accordingly, the transient outward K + current ( I to ), which is produced by Kv4 channels, is greater in left ventricular epicardial (EPI) than in endocardial (ENDO) cells. However, the mechanisms underlying heterogeneous Kv4 expression in the heart are unclear. Here, we tested the hypothesis that differential [Ca 2+ ] i and calcineurin/NFATc3 signaling in EPI and ENDO cells contributes to the gradient of I to function in the mouse left ventricle. In support of this hypothesis, we found that [Ca 2+ ] i , calcineurin, and NFAT activity were greater in ENDO than in EPI myocytes. However, the amplitude of I to was the same in ENDO and EPI cells when [Ca 2+ ] i , calcineurin, and NFAT activity were equalized. Consistent with this, we observed complete loss of I to and Kv4 heterogeneity in NFATc3-null mice. Interestingly, Kv4.3, Kv4.2, and KChIP2 genes had different apparent thresholds for NFATc3-dependent suppression and were ordered as Kv4.3≈KChIP2>Kv4.2. Based on these data, we conclude that calcineurin and NFATc3 constitute a Ca 2+ -driven signaling module that contributes to the nonuniform distribution of Kv4 expression, and hence I to function, in the mouse left ventricle.

A potassium channel (Kv4) cloned from the heart of the tunicate Ciona intestinalis and its modulation by a KChIP subunit

Voltage-gated ion channels of the Kv4 subfamily produce A-type currents whose properties are tuned by accessory subunits termed KChIPs, which are a family of Ca2+ sensor proteins. By modifying expression levels and the intrinsic biophysical properties of Kv4 channels, KChIPs modulate the excitability properties of neurons and myocytes. We studied how a Kv4 channel from a tunicate, the first branching clade of the chordates, is modulated by endogenous KChIP subunits. BLAST searches in the genome of Ciona intestinalis identified a single Kv4 gene and a single KChIP gene, implying that the diversification of both genes occurred during early vertebrate evolution, since the corresponding mammalian gene families are formed by several paralogues. In this study we describe the cloning and characterization of a tunicate Kv4 channel, CionaKv4, and a tunicate KChIP subunit, CionaKChIP. We demonstrate that CionaKChIP strongly modulates CionaKv4 by producing larger currents that inactivate more slowly than in the absence of the KChIP subunit. Furthermore, CionaKChIP shifted the midpoints of activation and inactivation and slowed deactivation and recovery from inactivation of CionaKv4. Modulation by CionaKChIP requires the presence of the intact N terminus of CionaKv4 because, except for a minor effect on inactivation, CionaKChIP did not modulate CionaKv4 channels that lacked amino acids 2-32. In summary, our results suggest that modulation of Kv4 channels by KChIP subunits is an ancient mechanism for modulating electrical excitability.

Transient outward potassium current and Ca2+ homeostasis in the heart: beyond the action potential

The present review deals with Ca2+-independent, K+-carried transient outward current (Ito), an important determinant of the early repolarization phase of the myocardial action potential. The density of total Ito and of its fast and slow components (I(to,f) and I(to,s), respectively), as well as the expression of their molecular correlates (pore-forming protein isoforms Kv4.3/4.2 and Kv1.4, respectively), vary during postnatal development and aging across species and regions of the heart. Changes in Ito may also occur in disease conditions, which may affect the profile of cardiac repolarization and vulnerability to arrhythmias, and also influence excitation-contraction coupling. Decreased Ito density, observed in immature and aging myocardium, as well as during several types of cardiomyopathy and heart failure, may be associated with action potential prolongation, which favors Ca2+ influx during membrane depolarization and limits voltage-dependent Ca2+ efflux via the Na+/Ca2+ exchanger. Both effects contribute to increasing sarcoplasmic reticulum (SR) Ca2+ content (the main source of contraction-activating Ca2+ in mammalian myocardium), which, in addition to the increased Ca2+ influx, should enhance the amount of Ca2+ released by the SR during systole. This change usually takes place under conditions in which SR function is depressed, and may be adaptive since it provides partial compensation for SR deficiency, although possibly at the cost of asynchronous SR Ca2+ release and greater propensity to triggered arrhythmias. Thus, Ito modulation appears to be an additional mechanism by which excitation-contraction coupling in myocardial cells is indirectly regulated.

KChIP2b modulates the affinity and use-dependent block of Kv4.3 by nifedipine

Rapidly activating Kv4 voltage-gated ion channels are found in heart, brain, and diverse other tissues including colon and uterus. Kv4.3 can co-assemble with KChIP ancillary subunits, which modify kinetic behavior. We examined the affinity and use dependence of nifedipine block on Kv4.3 and its modulation by KChIP2b. Nifedipine (150 microM) reduced peak Kv4.3 current approximately 50%, but Kv4.3/KChIP2b current only approximately 27%. Nifedipine produced a very rapid component of open channel block in both Kv4.3 and Kv4.3/KChIP2b. However, recovery from the blocked/inactivated state was strongly sensitive to KChIP2b. Kv4.3 Thalf,recovery was slowed significantly by nifedipine (120.0+/-12.4 ms vs. 213.1+/-18.2 ms), whereas KChIP2b eliminated nifedipine's effect on recovery: Kv4.3/KChIP2b Thalf,recovery was 45.3+/-7.2 ms (control) and 47.8+/-8.2 ms (nifedipine). Consequently, Kv4.3 exhibited use-dependent nifedipine block in response to a series of depolarizing pulses which was abolished by KChIP2b. KChIPs alter drug affinity and use dependence of Kv4.3.

Pharmacogenetics and cardiac ion channels

Ion channels control electrical excitability in living cells. In mammalian heart, the opposing actions of Na(+) and Ca(2+) ion influx, and K(+) ion efflux, through cardiac ion channels determine the morphology and duration of action potentials in cardiac myocytes, thus controlling the heartbeat. The last decade has seen a leap in our understanding of the molecular genetic origins of inherited cardiac arrhythmia, largely through identification of mutations in cardiac ion channels and the proteins that regulate them. Further, recent advances have shown that 'acquired arrhythmias', which occur more commonly than inherited arrhythmias, arise due to a variety of environmental factors including side effects of therapeutic drugs and often have a significant genetic component. Here, we review the pharmacogenetics of cardiac ion channels-the interplay between genetic and pharmacological factors that underlie human cardiac arrhythmias.

Transmural gradients in Na/K pump activity and [Na+]I in canine ventricle

There are well-documented differences in ion channel activity and action potential shape between epicardial (EPI), midmyocardial (MID), and endocardial (ENDO) ventricular myocytes. The purpose of this study was to determine if differences exist in Na/K pump activity. The whole cell patch-clamp was used to measure Na/K pump current (I(P)) and inward background Na(+)-current (I(inb)) in cells isolated from canine left ventricle. All currents were normalized to membrane capacitance. I(P) was measured as the current blocked by a saturating concentration of dihydro-ouabain. [Na(+)](i) was measured using SBFI-AM. I(P)(ENDO) (0.34 +/- 0.04 pA/pF, n = 17) was smaller than I(P)(EPI) (0.68 +/- 0.09 pA/pF, n = 38); the ratio was 0.50 with I(P)(MID) being intermediate (0.53 +/- 0.13 pA/pF, n = 19). The dependence of I(P) on [Na(+)](i) or voltage was essentially identical in EPI and ENDO (half-maximal activation at 9-10 mM [Na(+)](i) or approximately -90 mV). Increasing [K(+)](o) from 5.4 to 15 mM caused both I(P)(ENDO) and I(P)(EPI) to increase, but the ratio remained approximately 0.5. I(inb) in EPI and ENDO were nearly identical ( approximately 0.6 pA/pF). Physiological [Na(+)](i) was lower in EPI (7 +/- 2 mM, n = 31) than ENDO (12 +/- 3 mM, n = 29), with MID being intermediate (9 +/- 3 mM, n = 22). When cells were paced at 2 Hz, [Na(+)](i) increased but the differences persisted (ENDO 14 +/- 3 mM, n = 10; EPI 9 +/- 2 mM, n = 10; and MID intermediate, 11 +/- 2 mM, n = 9). Based on these results, the larger I(P) in EPI appears to reflect a higher maximum turnover rate, which implies either a larger number of active pumps or a higher turnover rate per pump protein. The transmural gradient in [Na(+)](i) means physiological I(P) is approximately uniform across the ventricular wall, whereas transporters that utilize the transmembrane electrochemical gradient for Na(+), such as Na/Ca exchange, have a larger driving force in EPI than ENDO.

Molecular/genetic determinants of repolarization and their modification by environmental stress

Although a variety of factors, inherited or environmental, can influence expression of ion channel proteins to impact on repolarization, that environment can affect genetic determinants of repolarization for intervals of varying duration is a concept that is not as generally appreciated as it should be. In the following pages we review the molecular/genetic determinants of cardiac repolarization and summarize how pathologic events and environmental intrusions can affect these determinants. Understanding the chains of events involved should yield insights into both the causes and potential avenues of treatment for abnormalities of repolarization.

Mitogen-activated protein kinases control cardiac KChIP2 gene expression

Hypertrophied myocardium is associated with reductions in the transient outward K(+) current (Ito) and expression of pore-forming Kv4.2/4.3 and auxiliary KChIP2 subunits. Here we show that KChIP2 mRNA and protein levels are dramatically decreased to 10% to 30% of control levels in the left ventricle of aorta-constricted rats in vivo and phenylephrine (PE)-treated myocytes in vitro. PE also markedly decreases Ito density. Inhibition of protein kinase Cs (PKCs) does not affect the PE-induced reduction in KChIP2 mRNA level, whereas activation of PKC with phorbol ester (phorbol myristate [PMA]) causes a marked reduction in KChIP2 mRNA level. Pharmacological inhibition of MEKs or overexpression of a dominant-negative MEK1 increases the basal KChIP2 mRNA expression and blocks the PMA-induced decrease in auxiliary subunit mRNA level. In addition, a constitutively active MEK1 decreases the basal KChIP2 mRNA level, and PMA causes no further reduction in auxiliary subunit mRNA level in active MEK1-expressing cells. Furthermore, pharmacological inhibition of JNKs or overexpression of a dominant-negative JNK1 prevents the PE-induced, but not PMA-induced, reduction in KChIP2 mRNA expression. These results suggest that downregulation of KChIP2 expression significantly contributes to the hypertrophy-associated reduction in Ito density. They also indicate that the expression of KChIP2 mRNA is controlled by the 2 branches of mitogen-activated protein kinase pathways: JNKs play a predominant role in mediating the PE-induced reduction, whereas the MEK-ERK pathway influences the basal expression and mediates the PKC-mediated downregulation.

Targeted deletion of Kv4.2 eliminates I(to,f) and results in electrical and molecular remodeling, with no evidence of ventricular hypertrophy or myocardial dysfunction

Previous studies have demonstrated a role for voltage-gated K+ (Kv) channel alpha subunits of the Kv4 subfamily in the generation of rapidly inactivating/recovering cardiac transient outward K+ current, I(to,f), channels. Biochemical studies suggest that mouse ventricular I(to,f) channels reflect the heteromeric assembly of Kv4.2 and Kv4.3 with the accessory subunits, KChIP2 and Kvbeta1, and that Kv4.2 is the primary determinant of regional differences in (mouse ventricular) I(to,f) densities. Interestingly, the phenotypic consequences of manipulating I(to,f) expression in different mouse models are distinct. In the experiments here, the effects of the targeted deletion of Kv4.2 (Kv4.2(-/-)) were examined. Unexpectedly, voltage-clamp recordings from Kv4.2(-/-) ventricular myocytes revealed that I(to,f) is eliminated. In addition, the slow transient outward K+ current, I(to,s), and the Kv1.4 protein (which encodes I(to,s)) are upregulated in Kv4.2(-/-) ventricles. Although Kv4.3 mRNA/protein expression is not measurably affected, KChIP2 expression is markedly reduced in Kv4.2(-/-) ventricles. Similar to Kv4.3, expression of Kvbeta1, as well as Kv1.5 and Kv2.1, is similar in wild-type and Kv4.2(-/-) ventricles. In addition, and in marked contrast to previous findings in mice expressing a truncated Kv4.2 transgene, the elimination I(to,f) in Kv4.2(-/-) mice does not result in ventricular hypertrophy. Taken together, these findings demonstrate not only an essential role for Kv4.2 in the generation of mouse ventricular I(to,f) channels but also that the loss of I(to,f) per se does not have overt pathophysiological consequences.

Transient outward potassium current, 'Ito', phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms

At least two functionally distinct transient outward K(+) current (I(to)) phenotypes can exist across the free wall of the left ventricle (LV). Based upon their voltage-dependent kinetics of recovery from inactivation, these two phenotypes are designated 'I(to,fast)' (recovery time constants on the order of tens of milliseconds) and 'I(to,slow)' (recovery time constants on the order of thousands of milliseconds). Depending upon species, either I(to,fast), I(to,slow) or both current phenotypes may be expressed in the LV free wall. The expression gradients of these two I(to) phenotypes across the LV free wall are typically heterogeneous and, depending upon species, may consist of functional phenotypic gradients of both I(to,fast) and I(to,slow) and/or density gradients of either phenotype. We review the present evidence (molecular, biophysical, electrophysiological and pharmacological) for Kv4.2/4.3 alpha subunits underlying LV I(to,fast) and Kv1.4 alpha subunits underlying LV I(to,slow) and speculate upon the potential roles of each of these currents in determining frequency-dependent action potential characteristics of LV subepicardial versus subendocardial myocytes in different species. We also review the possible functional implications of (i) ancillary subunits that regulate Kv1.4 and Kv4.2/4.3 (Kvbeta subunits, DPPs), (ii) KChIP2 isoforms, (iii) spider toxin-mediated block of Kv4.2/4.3 (Heteropoda toxins, phrixotoxins), and (iv) potential mechanisms of modulation of I(to,fast) and I(to,slow) by cellular redox state, [Ca(2)(+)](i) and kinase-mediated phosphorylation. I(to) phenotypic activation and state-dependent gating models and molecular structure-function relationships are also discussed.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Identification and characterization of small molecule modulators of KChIP/Kv4 function

Potassium channels and their associated subunits are important contributors to electrical excitability in many cell types. In this study, a yeast two-hybrid assay was used to identify inhibitors such as a diaryl-urea compound (CL-888) that binds to and modulates the formation of the Kv4/KChIP complex. CL-888 altered the apparent affinity of KChIP1 to Kv4.3-N in a Biacore assay, but did not dissociate the two proteins in size-exclusion chromatography experiments. Kv4.2/KChIP1 current amplitude and kinetics were altered with compound exposure, supporting the hypothesis of a compound-induced conformational change in the protein complex. Fluorescence spectroscopy of a unique tryptophan residue in KChIP1 was consistent with compound binding to the protein. Molecular modeling using the KChIP1 crystal structure indicates that compound binding may occur in a small tryptophan-containing binding pocket located on the hydrophilic side of the protein.

Recent advances in understanding sex differences in cardiac repolarization

A number of gender differences exist in the human electrocardiogram (ECG): the P-wave and P-R intervals are slightly longer in men than in women, whilst women have higher resting heart rates than do men, but a longer rate-corrected QT (QT(C)) interval. Women with the LQT1 and LQT2 variants of congenital long-QT syndrome (LQTS) are at greater risk of adverse cardiac events. Similarly, many drugs associated with acquired LQTS have a greater risk of inducing torsades de pointes (TdP) arrhythmia in women than in men. There are also male:female differences in Brugada syndrome, early repolarisation syndrome and sudden cardiac death. The differences in the ECG between men and women, and in particular those relating to the QT interval, have been explored experimentally and provide evidence of differences in the processes underlying ventricular repolarization. The data available from rabbit, canine, rat, mouse and guinea pig models are reviewed and highlight involvement of male:female differences in Ca and K currents, although the possible involvement of rapid and persistent Na current and Na-Ca exchange currents cannot yet be excluded. The mechanisms underlying observed differences remain to be elucidated fully, but are likely to involve the influence of gonadal steroids. With respect to the QT interval and risk of TdP, a range of evidence implicates a protective role of testosterone in male hearts, possibly by both genomic and non-genomic pathways. Evidence regarding oestrogen and progesterone is less unequivocal, although the interplay between these two hormones may influence both repolarization and pro-arrhythmic risk.

Asymmetrical distribution of ion channels in canine and human left-ventricular wall: epicardium versus midmyocardium

The aim of the present study was to compare the distribution of ion currents and the major underlying ion channel proteins in canine and human subepicardial (EPI) and midmyocardial (MID) left-ventricular muscle. Ion currents and action potentials were recorded from canine cardiomyocytes derived from the very superficial EPI and central MID regions of the left ventricle. Amplitude, duration and the maximum velocity of depolarization of the action potential were significantly greater in MID than EPI myocytes, whereas phase-1 repolarization was more pronounced in the EPI cells. Amplitudes of the transient outwards K+ current (29.5+/-1.5 vs. 19.0+/-2.3 pA/pF at +50 mV) and the slow component of the delayed rectifier K+ current (10.3+/-2.3 vs. 6.5+/-1.0 pA/pF at +50 mV) were significantly larger in EPI than in MID myocytes under whole-cell voltage-clamp conditions. The densities of the inwards rectifier K+ current, rapid delayed rectifier K+ current and L-type Ca2+ current were similar in both cell types. Expression of channel proteins in both canine and human ventricular myocardium was determined by Western blotting. In the canine heart, the expression of Kv4.3, Kv1.4, KChIP2 and KvLQT1 was significantly higher, and that of Nav1.5 and MinK much lower, in EPI than in MID. No significant EPI-MID differences were observed in the expression of the other channel proteins studied (Kir2.1, alpha1C, HERG and MiRP1). Similar results were obtained in human hearts, although the HERG was more abundant in the EPI than in the MID layer. In the canine heart, the EPI-MID differences in ion current densities were proportional to differences in channel protein expression. Except for the density of HERG, the pattern of EPI-MID distribution of ion-channel proteins was identical in canine and human ventricles.

Molecular mechanisms underlying K+ current downregulation in canine tachycardia-induced heart failure

Heart failure (HF) is characterized by marked prolongation of action potential duration and reduction in cellular repolarization reserve. These changes are caused in large part by HF-induced K+ current downregulation. Molecular mechanisms underlying these changes remain unclear. We determined whether downregulation of K+ currents in a canine model of tachycardia-induced HF is caused by altered expression of underlying K+ channel α- and β-subunits encoding these currents. K+ channel subunit expression was quantified in normal and failing dogs at the mRNA and protein levels in epicardial (Epi), midmyocardial (Mid), and endocardial (Endo) layers of left ventricle. Analysis of mRNA and protein levels of candidate genes encoding the transient outward K+ current ( Ito) revealed marked reductions in canine cKv4.3 expression in HF in Epi (44% mRNA, 39% protein), Mid (52% mRNA, 34% protein), and Endo (49% mRNA, 73% protein) layers and a paradoxical enhancement (41% Epi, 97% Mid, 113% Endo) in cKv1.4 protein levels, without significant changes in Kv channel-interacting protein cKChIP2 expression. Expression of cKir2.1, the gene underlying inward rectifier K+ current ( IK1), was unaffected by HF at mRNA and protein levels despite significant reduction in IK1, whereas canine ether-à-go-go-related gene (cERG), which encodes the rapidly activating component of the delayed rectifier current ( IK), exhibited increased protein expression. HF was not accompanied by significant changes in cKvLQT1 or cMinK mRNA and protein levels. These data indicate that 1) downregulation of Ito in HF is associated with decreased cKv4.3 and not cKv1.4 or cKChIP2, and 2) alterations in both the rapidly activating and slowly activating components of IK as well as IK1 in nonischemic dilated cardiomyopathy are not caused by changes in either transcript or immunoreactive protein levels of relevant channel subunits, which suggests posttranslational modification of these currents by HF.

Molecular correlates of altered expression of potassium currents in failing rabbit myocardium

Action potential (AP) prolongation is a hallmark of failing myocardium. Functional downregulation of K currents is a prominent feature of cells isolated from failing ventricles. The detailed changes in K current expression differ depending on the species, the region of the heart, and the mechanism of induction of heart failure. We used complementary approaches to study K current downregulation in pacing tachycardia-induced heart failure in the rabbit. The AP duration (APD) at 90% repolarization was significantly longer in cells isolated from failing hearts compared with controls (539 +/- 162 failing vs. 394 +/- 114 control, P < 0.05). The major K currents in the rabbit heart, inward rectifier potassium current (I(K1)), transient outward (I(to)), and delayed rectifier current (I(K)) were functionally downregulated in cells isolated from failing ventricles. The mRNA levels of Kv4.2, Kv1.4, KChIP2, and Kir2.1 were significantly downregulated, whereas the Kv4.3, Erg, KvLQT1, and minK were unaltered in the failing ventricles compared with the control left ventricles. Significant downregulation in the long splice variant of Kv4.3, but not in the total Kv4.3, Kv4.2, and KChIP2 immunoreactive protein, was observed in cells isolated from the failing ventricle with no change in Kv1.4, KvLQT1, and in Kir2.1 immunoreactive protein levels. Multiple cellular and molecular mechanisms underlie the downregulation of K currents in the failing rabbit ventricle.

Mutations in the genes KCND2 and KCND3 encoding the ion channels Kv4.2 and Kv4.3, conducting the cardiac fast transient outward current (ITO,f), are not a frequent cause of long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is a hereditary cardiac arrhythmogenic disorder characterized by prolongation of the QT interval in the electrocardiogram, torsades de pointes arrhythmia, and syncopes and sudden death. LQTS is caused by mutations in ion channel genes. However, only in half of the families is it possible to identify mutations in one of the seven known LQTS genes, why further genetic heterogeneity is expected. The genes KCND2 and KCND3, encoding the alpha-subunits of the voltage-gated potassium channels Kv4.2 and Kv4.3 conducting the fast transient outward current (I(TO,f)) of the cardiac action potential (AP) in the myocardium, have been associated with prolongation of AP duration and QT prolongation in murine models.

METHODS

KCND2 and KCND3 were examined for mutations using single-strand conformation polymorphism (SSCP) analysis in 43 unrelated LQTS patients, where mutations in the coding regions of known LQTS genes had been excluded.

RESULTS

Seven single nucleotide polymorphismsm (SNPs) were found, two exonic SNPs in KCND2 and three exonic and two intronic in KCND3. None of the five exonic SNPs had coding effect. All seven SNPs are considered normal variants.

CONCLUSION

The data suggest that mutations in KCND2 and KCND3 are not a frequent cause of long QT syndrome.

Nature, significance, and mechanisms of electrical heterogeneities in ventricle

Previously, dispersion of repolarization (DOR) has been extensively linked to the development of arrhythmias and sudden cardiac death. The electrical heterogeneities that cause DOR between transmural myocyte layers have been reported in a wide variety of animals and humans. The underlying causes of transmural electrical heterogeneities are in part due to heterogeneous functional expression of proteins responsible for ion handling. Recently, we found that electrophysiologic heterogeneities between subepicardial and midmyocardial cells can form a substrate for reentrant ventricular arrhythmias. However, cell-to-cell coupling through gap junctions is expected to attenuate transmural heterogeneities between cell types spanning the ventricular wall. In this article we review a hypothesis that regional uncoupling resulting from expression patterns of gap junctions across the ventricular wall underlies DOR, and DOR can be amplified under disease conditions which remodel gap junctions. We find the principle gap junction protein, connexin43 (Cx43), is selectively reduced in the subepicardium (by 24%) compared to deeper layers of normal canine left ventricle. Additionally, the greatest DOR occurs within the subepicardial-midmyocardial interface, precisely where Cx43 expression is reduced. The present data suggests that ion channel and gap junction heterogeneities act in conjunction to form and maintain transmural DOR. Importantly, both ion channel and gap junction remodeling occurs during many disease states such as heart failure. Importantly, in the absence of ion channel remodeling, pharmacological uncoupling increases transmural DOR, particularly within the epicardial-midmyocardial interface, to values observed in heart failure. Therefore, these data suggest that heterogeneous Cx43 expression produces functionally significant electrophysiologic heterogeneities across the ventricular wall and may be a mechanism for promoting DOR which underlie arrhythmias in heart failure.

Ion channels as novel therapeutic targets in heart failure

Electrophysiological remodeling in heart failure (HF) is characterized by major changes in ion channel function and expression that alter the electrical phenotype and predispose to the development of lethal ventricular tachyarrhythmias. In this article, we provide a review of our current understanding of HF-induced ion channel dysfunction by highlighting changes in potassium and sodium currents, pumps, and exchangers as well as calcium handling proteins. We further relate these changes in ion channel function to abnormalities in impulse generation, conduction, and repolarization with a view towards identifying potentially novel targets for anti-arrhythmic therapy for this public health epidemic.

Regulation of cardiac shal-related potassium channel Kv 4.3 by serum- and glucocorticoid-inducible kinase isoforms in Xenopus oocytes

The human cardiac transient outward potassium current I(to) is formed by co-assembly of voltage-dependent K(+) channel (Kv 4.3) pore-forming alpha-subunits with differently spliced K channel interacting protein (KChIP) accessory proteins. I(to) is of considerable importance for the normal course of the cardiac ventricular action potential. The present study was performed to determine whether isoforms of the serum- and glucocorticoid-inducible kinase (SGK) family influence Kv 4.3/KChIP2b channel activity in the Xenopus laevis heterologous expression system. Co-expression of SGK1, but not of SGK2 or SGK3, increased Kv 4.3/KChIP2b channel currents. The up-regulation of the current was not due to changes in the activation curve or changes of channel inactivation. The currents in oocytes expressing Kv 4.3 alone were smaller than those in Kv 4.3/KChIP2b expressing oocytes, but were still stimulated by SGK1. The effect of wild-type SGK1 was mimicked by constitutively active SGK1 (SGK1 S422D) but not by an inactive mutant (SGK1 K127N). The current amplitude increase mediated by SGK1 was not dependent on NEDD4.2 or RAB5, nor did it reflect increased cell surface expression. In conclusion, SGK1 stimulates Kv 4.3 potassium channels expressed in Xenopus oocytes by a novel mechanism distinct from the known NEDD4.2-dependent pathway.

Transmural distribution of connexins in rodent hearts

INTRODUCTION

Electrophysiologic heterogeneity across the ventricular wall is a result of differential transmural expression of various ion channel proteins that underlie the different action potential waveforms observed in epicardial, midmyocardial, and endocardial regions. Cardiac connexins mediate cell-to-cell communication, are critical for normal impulse propagation, and play a role in electrophysiologic remodeling in disease states. However, little is known about the transmural distribution of cardiac gap junction proteins.

METHODS AND RESULTS

Connexin expression in epicardium, midmyocardium, and endocardium was assessed immunohistochemically in mouse and rat hearts. The total connexin protein content within different ventricular regions was measured by immunoblotting. Connexin43 is twice as abundant in midmyocardium and endocardium compared with epicardium in the mouse but not in the rat. Connexin45 is expressed equally across the left ventricular wall.

CONCLUSION

Epicardial myocytes express significantly less Cx43 and therefore may be less well coupled than midmyocardial and endocardial myocytes. A transmural gradient of connexin43 expression across the left ventricular free wall likely results in differences in the stoichiometry of connexins expressed in different regions of the heart.

Angiotensin Receptor Type 1 Forms a Complex with the Transient Outward Potassium Channel Kv4.3 and Regulates Its Gating Properties and Intracellular Localization

We report a novel signal transduction complex of the angiotensin receptor type 1. In this complex the angiotensin receptor type 1 associates with the potassium channel alpha-subunit Kv4.3 and regulates its intracellular distribution and gating properties. Co-localization of Kv4.3 with angiotensin receptor type 1 and fluorescent resonance energy transfer between those two proteins labeled with cyan and yellow-green variants of green fluorescent protein revealed that Kv4.3 and angiotensin receptor type I are located in close proximity to each other in the cell. The angiotensin receptor type 1 also co-immunoprecipitates with Kv4.3 from canine ventricle or when co-expressed with Kv4.3 and its beta-subunit KChIP2 in human embryonic kidney 293 cells. Treatment of the cells with angiotensin II results in the internalization of Kv4.3 in a complex with the angiotensin receptor type 1. When stimulated with angiotensin II, angiotensin receptors type 1 modulate gating properties of the remaining Kv4.3 channels on the cell surface by shifting their activation voltage threshold to more positive values. We hypothesize that the angiotensin receptor type 1 provides its internalization molecular scaffold to Kv4.3 and in this way regulates the cell surface representation of the ion channel.

Transmural expression of transient outward potassium current subunits in normal and failing canine and human hearts

The transient outward current (I(to)), an important contributor to transmural electrophysiological heterogeneity, is significantly remodelled in congestive heart failure (CHF). The molecular bases of transmural I(to) gradients and CHF-dependent ionic remodelling are incompletely understood. To elucidate these issues, we studied mRNA and protein expression of Kv4.3 and KChIP2, the principal alpha and beta subunits believed to form I(to), in epicardial and endocardial tissues and in isolated cardiomyocytes from control dogs and dogs with CHF induced by 240 beats min(-1) ventricular tachypacing. CHF decreased I(to) density in both epicardium and endocardium (by 73 and 55% at +60 mV, respectively), without a significant change in relative current density (endocardium/epicardium 0.11 control, 0.17 CHF). There were transmural gradients in mRNA expression of both Kv4.3 (endocardium/epicardium ratio 0.3 under control conditions) and KChIP2 (endocardium/epicardium ratio 0.2 control), which remained in the presence of CHF (Kv4.3 endocardium/epicardium ratio 0.4; KChIP2 0.4). There were qualitatively similar protein expression gradients in human and canine cardiac tissues and isolated canine cardiomyocytes; however, the KChIP2 gradient was only detectable with a highly selective monoclonal antibody and closely approximated the I(to) density gradient. Kv4.3 mRNA expression was reduced by CHF, but KChIP2 mRNA was not significantly changed. CHF decreased Kv4.3 protein expression in canine cardiac tissues and cardiomyocytes, as well as in terminally failing human heart tissue samples, but KChIP2 protein was not down-regulated in any of the corresponding sample sets. We conclude that both Kv4.3 and KChIP2 may contribute to epicardial-endocardial gradients in I(to), and that I(to) down-regulation in human and canine CHF appears due primarily to changes in Kv4.3.

alpha1-Adrenoceptors stimulate a Galphas protein and reduce the transient outward K+ current via a cAMP/PKA-mediated pathway in the rat heart

alpha(1)-Adrenoceptor stimulation prolongs the duration of the cardiac action potentials and leads to positive inotropic effects by inhibiting the transient outward K(+) current (I(to)). In the present study, we have examined the role of several protein kinases and the G protein involved in I(to) inhibition in response to alpha(1)-adrenoceptor stimulation in isolated adult rat ventricular myocytes. Our findings exclude the classic alpha(1)-adrenergic pathway: activation of the G protein G(alphaq), phospholipase C (PLC), and protein kinase C (PKC), because neither PLC, nor PKC, nor G(alphaq) blockade prevents the alpha(1)-induced I(to) reduction. To the contrary, the alpha(1)-adrenoceptor does not inhibit I(to) in the presence of protein kinase A (PKA), adenylyl cyclase, or G(alphas) inhibitors. In addition, PKA and adenylyl cyclase activation inhibit I(to) to the same extent as phenylephrine. Finally, we have shown a functional coupling between the alpha(1)-adrenoceptor and G(alphas) in a physiological system. Moreover, this coupling seems to be compartmentalized, because the alpha(1)-adrenoceptor increases cAMP levels only in intact cells, but not in isolated membranes, and the effect on I(to) disappears when the cytoskeleton is disrupted. We conclude that alpha(1)-adrenoceptor stimulation reduces the amplitude of the I(to) by activating a G(alphas) protein and the cAMP/PKA signaling cascade, which in turn leads to I(to) channel phosphorylation.

Cardiac Memory Evolves With Age in Association With Development of the Transient Outward Current

BACKGROUND

Calcium-insensitive transient outward current (I(to)) is important to the development of cardiac memory (CM), which itself reflects the capacity of the heart to remodel electrophysiologically. We used cardiac pacing to test the hypothesis that CM evolution can be explained by developmental maturation of I(to).

METHODS AND RESULTS

Acutely anesthetized dogs from 1 day old to adult were paced from the left ventricle (VP, n=29) or left atrial appendage (AP, n=12) to induce CM. T-wave vector displacement (TVD) obtained during VP was greater than with AP (adults, 0.39+/-0.06 mV; neonates, 0.04+/-0.01 mV; P<0.05). TVD began to increase at approximately 40 days of age, reaching adult levels by approximately 200 days. Microelectrode studies performed in 18 dogs (ages 3 to 94 days) after completing the CM protocol and 20 additional dogs (1 day old to adult) revealed that the epicardial action potential notch was absent in neonates, became apparent in the young, and was deepest in adults. The relationship between TVD and epicardial notch was such that as notch magnitude increased, TVD increased (r=-0.65, P<0.05). KChIP2 and Kv4.3 mRNA (measured via reverse transcription-polymerase chain reaction) also increased with age.

CONCLUSIONS

The inducibility of CM gradually increases with age in association with evolution of the epicardial action potential notch and mRNA expression for KChIP2 and Kv4.3. This suggests that the capacity of the heart to remodel electrophysiologically and to manifest memory during development depends in part on evolution of the determinants of I(to).

Structure and function of Kv4-family transient potassium channels

Shal-type (Kv4.x) K(+) channels are expressed in a variety of tissue, with particularly high levels in the brain and heart. These channels are the primary subunits that contribute to transient, voltage-dependent K(+) currents in the nervous system (A currents) and the heart (transient outward current). Recent studies have revealed an enormous degree of complexity in the regulation of these channels. In this review, we describe the surprisingly large number of ancillary subunits and scaffolding proteins that can interact with the primary subunits, resulting in alterations in channel trafficking and kinetic properties. Furthermore, we discuss posttranslational modification of Kv4.x channel function with an emphasis on the role of kinase modulation of these channels in regulating membrane properties. This concept is especially intriguing as Kv4.2 channels may integrate a variety of intracellular signaling cascades into a coordinated output that dynamically modulates membrane excitability. Finally, the pathophysiology that may arise from dysregulation of these channels is also reviewed.

Protein–protein interactions of KChIP proteins and Kv4.2

To prove heteromeric assembly of KChIP proteins, the present study is carried out. The results of chemical crosslinking and pull down assay revealed that KChIP1, KChIP2.1, and KChIP2.2 could form homo- as well as hetero-oligomer, and this oligomerization exhibited a Ca(2+)-dependent manner. Moreover, homomeric and heteromeric assembly of KChIPs did not perturb their interaction with Kv4.2 K(+) channel, indicating that the region associated with oligomerization of KChIPs was distinct from that for binding with Kv4.2. Together with previous findings that the net effects of KChIP proteins on the molecular properties and trafficking of Kv channel were different, these observations open a fascinating possibility that the electrophysiological properties of Kv channel may be differently regulated by homomeric and heteromeric assembly of KChIPs.

Sino-atrial nodal cells of mammalian hearts: ionic currents and gene expression of pacemaker ionic channels

The cardiac pacemaker is a sino-atrial (SA) nodal cell. The signal induced by this pacemaker is distributed over the heart surface by a specialised conduction system and is clinically recorded as the ECG. The SA nodal cells are highly resistant to cardiac failure and ischemia. Under calcium overload conditions, some dysrythmias of SA nodal cells occur easily. Morphological analysis under these conditions shows swelling of the cisternae of the Golgi apparatus, with little or no other histological change or damage being observed. The rate of sinus rhythm is quite different between various species. The investigations of SA nodal cells have so far clarified the pacemaker mechanisms involved. A number of ionic channel currents or pacemaker currents, contribute to the depolarization of the pacemaker potential (phase 4). This will not occur with a single current. Recent experiments have identified several novel pacemaker currents and have also revealed several differences in the pacemaker currents between species. The marked hyperpolarization-activated inward current (I(f)) appears in SA nodal cells of most species, while the inwardly rectifying K+ current (I(K1)) with masked I(f) current is found in those of the rat and monkey. In addition, the rapidly activated current (I(Kr)) and slowly activated current (I(Ks)) of the delayed rectifier K+ current (I(K)) contribute to the pacemaker potential in guinea pig SA nodal cells, with only the I(Ks) current in porcine SA nodal cells and only the I(Kr) current in the rat and rabbit. These differences in ionic channels presumably result from differences in gene expression. Some smooth muscle cells also possess the capacity to beat spontaneously. Uterine smooth muscle cells also exhibit an I(f) current. The basal mechanism for spontaneous activity in both SA nodal cells and smooth muscle cells is almost the same, but some differences in the ionic channels and their genetic expression may contribute to their respective pacemaker currents.

Action potential duration determines sarcoplasmic reticulum Ca2+ reloading in mammalian ventricular myocytes

After sarcoplasmic reticulum (SR) Ca2+ depletion in intact ventricular myocytes, electrical activity promotes SR Ca2+ reloading and recovery of twitch amplitude. In ferret, recovery of twitch and caffeine-induced contracture required fewer twitches than in rabbit or rat. In rat, there was no difference in action potential duration at 90% repolarization (APD90) at steady state (SS) versus at the first post-depletion (PD) twitch. The SS APD90 was similar in ferret and rabbit (but longer than in rat). However, compared to SS, the PD APD90 was lengthened in ferret, but shortened in rabbit. When rabbit myocytes were subjected to AP-clamp patterns during SR Ca2+ reloading (ferret- or rabbit-type APs), reloading was much faster using the ferret AP templates. We conclude that the faster SR Ca2+ refilling in ferret is due to the increased Ca2+ influx during the longer PD AP. The PD versus SS APD90 difference was suppressed by thapsigargin in ferret (indicating Ca2+ dependence). In rabbit, the PD AP shortening depended on the preceding diastolic interval (rather than Ca2+), because rest produced the same AP shortening, and SS APD90 increased as a function of frequency (in contrast to ferret). Transient outward current (Ito) was larger and recovered from inactivation much faster in ferret than in rabbit. Moreover, slow Ito recovery (tau approximately 3 s) in rabbit was a much larger fraction of Ito. Our data and a computational model (including two Ito components) suggest that in rabbit the slowly recovering Ito is responsible for short post-rest and PD APs, for the unusual frequency dependence of APD90, and ultimately for the slower post-depletion SR Ca2+ reloading.

Propagation of normal beats and re-entry in a computational model of ventricular cardiac tissue with regional differences in action potential shape and duration

There is substantial experimental evidence from studies using both intact tissue and isolated single cells to support the existence of different cell types within the ventricular wall of the heart, each possessing different electrical properties. However other studies have failed to find these differences, and instead support the idea that electrical coupling in vivo between regions with different cell types smoothes out differences in action potential shape and duration. In this study we have used a computational model of electrical activation in heterogenous 2D and 3D cardiac tissue to investigate the propagation of both normal beats and arrhythmias. We used the Luo-Rudy dynamic model for guinea pig ventricular cells, with simplified Ca2+ handling and transmural heterogeneity in IKs and Ito. With normal cell-to-cell coupling, a layer of M cells was not necessary for the formation of an upright T wave in the simulated electrocardiogram, and the amplitude and configuration of the T wave was not greatly affected by the thickness and configuration of the M cell layer. Transmural gradients in repolarisation pushed re-entrant waves with an intramural filament towards either the base or the apex of the ventricles, and caused transient break up of re-entry with a transmural filament.

Novel KChIP2 isoforms increase functional diversity of transient outward potassium currents

Kv4.3 channels conduct transient outward K(+) currents in the human heart and brain where they mediate the early phase of action potential repolarization. KChIP2 proteins are members of a new class of calcium sensors that modulate the surface expression and biophysical properties of Kv4 K(+) channels. Here we describe three novel isoforms of KChIP2 with an alternatively spliced C-terminus (KChIP2e, KChIP2f) or N-terminus (KChIP2g). KChIP2e and KChIP2f are expressed in the human atrium, whereas KChIP2g is predominantly expressed in the brain. The KChIP2 isoforms were coexpressed with Kv4.3 channels in Xenopus oocytes and currents recorded with two-microelectrode voltage-clamp techniques. KChIP2e caused a reduction in current amplitude, an acceleration of inactivation and a slowing of the recovery from inactivation of Kv4.3 currents. KChIP2f increased the current amplitude and slowed the rate of inactivation, but did not alter the recovery from inactivation or the voltage of half-maximal inactivation of Kv4.3 channels. KChIP2g increased current amplitudes, slowed the rate of inactivation and shifted the voltage of half-maximal inactivation to more negative potentials. The biophysical changes induced by these alternatively spliced KChIP2 proteins differ markedly from previously described KChIP2 proteins and would be expected to increase the diversity of native transient outward K(+) currents.

Regulation of Ion Channel Expression

A potentially important mechanism controlling ion channel expression is homeostatic regulation, which can act to maintain a stable electrophysiological phenotype in cardiac myocytes as well as to provide plasticity in response to genetic, pathological, or pharmacological insults. The capabilities and limitations of the homeostatic regulatory mechanisms that contribute to the control of cardiac ion channel expression are the primary topic of this review.

Three-dimensional structure of I(to); Kv4.2-KChIP2 ion channels by electron microscopy at 21 Angstrom resolution

Regulatory KChIP2 subunits assemble with pore-forming Kv4.2 subunits in 4:4 complexes to produce native voltage-gated potassium (Kv) channels like cardiac I(to) and neuronal I(A) subtypes. Here, negative stain electron microscopy (EM) and single particle averaging reveal KChIP2 to create a novel approximately 35 x 115 x 115 Angstrom, intracellular fenestrated rotunda: four peripheral columns that extend down from the membrane-embedded portion of the channel to enclose the Kv4.2 "hanging gondola" (a platform held beneath the transmembrane conduction pore by four internal columns). To reach the pore from the cytosol, ions traverse one of four external fenestrae to enter the rotundal vestibule and then cross one of four internal windows in the gondola.

An LQT mutant minK alters KvLQT1 trafficking

Cardiac I(Ks), the slowly activated delayed-rectifier K(+) current, is produced by the protein complex composed of alpha- and beta-subunits: KvLQT1 and minK. Mutations of genes encoding KvLQT1 and minK are responsible for the hereditary long QT syndrome (loci LQT1 and LQT5, respectively). MinK-L51H fails to traffic to the cell surface, thereby failing to produce effective I(Ks). We examined the effects that minK-L51H and an endoplasmic reticulum (ER)-targeted minK (minK-ER) exerted over the electrophysiology and biosynthesis of coexpressed KvLQT1. Both minK-L51H and minK-ER were sequestered primarily in the ER as confirmed by lack of plasma membrane expression. Glycosylation and immunofluorescence patterns of minK-L51H were qualitatively different for minK-ER, suggesting differences in trafficking. Cotransfection with the minK mutants resulted in reduced surface expression of KvLQT1 as assayed by whole cell voltage clamp and immunofluorescence. MinK-L51H reduced current amplitude by 91% compared with wild-type (WT) minK/KvLQT1, and the residual current was identical to KvLQT1 without minK. The phenotype of minK-L51H on I(Ks) was not dominant because coexpressed WT minK rescued the current and surface expression. Collectively, our data suggest that ER quality control prevents minK-L51H/KvLQT1 complexes from trafficking to the plasma membrane, resulting in decreased I(Ks). This is the first demonstration that a minK LQT mutation is capable of conferring trafficking defects onto its associated alpha-subunit.

Regulation of Kv4.3 voltage-dependent gating kinetics by KChIP2 isoforms

We conducted a kinetic analysis of the voltage dependence of macroscopic inactivation (tau(fast), tau(slow)), closed-state inactivation (tau(closed,inact)), recovery (tau(rec)), activation (tau(act)), and deactivation (tau(deact)) of Kv4.3 channels expressed alone in Xenopus oocytes and in the presence of the calcium-binding ancillary subunits KChIP2b and KChIP2d. We demonstrate that for all expression conditions, tau(rec), tau(closed,inact) and tau(fast) are components of closed-state inactivation transitions. The values of tau(closed,inact) and tau(fast) monotonically merge from -30 to -20 mV while the values of tau(closed,inact) and tau(rec) approach each other from -60 to -50 mV. These data generate classic bell-shaped time-constant-potential curves. With the KChIPs, these curves are distinct from that of Kv4.3 expressed alone due to acceleration of tau(rec) and slowing of tau(closed,inact) and tau(fast). Only at depolarized potentials where channels open is tau(slow) detectable suggesting that it represents an open-state inactivation mechanism. With increasing depolarization, KChIPs favour this open-state inactivation mechanism, supported by the observation of larger transient reopening currents upon membrane hyperpolarization compared to Kv4.3 expressed alone. We propose a Kv4.3 gating model wherein KChIP2 isoforms accelerate recovery, slow closed-state inactivation, and promote open-state inactivation. This model supports the observations that with KChIPs, closed-state inactivation transitions are [Ca(2+)](i)-independent, while open-state inactivation is [Ca(2+)](i)-dependent. The selective KChIP- and Ca(2+)-dependent modulation of Kv4.3 inactivation mechanisms predicted by this model provides a basis for dynamic modulation of the native cardiac transient outward current by intracellular Ca(2+) fluxes during the action potential.

Ito channels are octomeric complexes with four subunits of each Kv4.2 and K+ channel-interacting protein 2

Mammalian voltage-gated K+ channels are assemblies of pore-forming alpha-subunits and modulating beta-subunits. To operate correctly, Kv4 alpha-subunits in the heart and central nervous system require recently identified beta-subunits of the neuronal calcium sensing protein family called K+ channel-interacting proteins (KChIPs). Here, Kv4.2.KChIP2 channels are purified, integrity of isolated complexes confirmed, molar ratio of the subunits determined, and subunit valence established. A complex has 4 subunits of each type, a stoichiometry expected for other channels employing neuronal calcium sensing beta-subunits.

Altered Abeta formation and long-term potentiation in a calsenilin knock-out

Calsenilin has been identified as a presenilin-binding protein, a transcription factor regulating dynorphin expression, and a beta-subunit of Kv4 channels and could, thus, be a multifunctional protein. To study these functions of calsenilin in vivo and to determine the neuroanatomical expression pattern of calsenilin, we generated mice with a disruption of the calsenilin gene by the targeted insertion of the beta-galactosidase gene. We found that calsenilin expression (as represented by beta-galactosidase activity) is very restricted but overlaps better with that of presenilins and Kv4 channels than with dynorphin, suggesting that calsenilin may regulate presenilin and Kv4 channels in brain. Abeta peptide levels are reduced in calsenilin knock-out mice, demonstrating that calsenilin affects presenilin-dependent gamma-cleavage in vivo. Furthermore, long-term potentiation (LTP) in dentate gyrus of hippocampus, in which calsenilin is strongly and selectively expressed, is enhanced in calsenilin knock-out mice. This enhancement of LTP coincides with a downregulation of the Kv4 channel-dependent A-type current and can be mimicked in wild-type animals by a Kv4 channel blocker. The data presented here show that lack of calsenilin affects both Abeta formation and the A-type current. We suggest that these effects are separate events, caused by a common mechanism possibly involving protein transport.

Effective association of Kv channel-interacting proteins with Kv4 channel is mediated with their unique core peptide

Kv channel-interacting proteins (KChIPs) and neuronal calcium sensor-1 (NCS-1) have been shown to interact with Kv4 channel alpha-subunits to regulate the expression and/or gating of these channels. Here we examine the specificity and sites of these proteins for interaction with Kv channel proteins. Immunoprecipitation and green fluorescent protein imaging show that KChIPs (but not NCS-1) effectively bind to Kv4.3 protein and localize at the plasma membrane when channel proteins are coexpressed. Analysis with chimeric proteins between KChIP2 and NCS-1 reveals that the three regions of KChIP2 (the linker between the first and second EF hands, the one between the third and fourth EF hands, and the C-terminal peptide after the fourth EF hand) are necessary and sufficient for its effective binding to Kv4.3 protein. The chimera with these three KChIP2 portions slowed inactivation and facilitated recovery from inactivation of Kv4.3 current. These results indicate that the sequence difference in these three regions between KChIPs and NCS-1 determines the specificity and affinity for interaction with Kv4 protein. Because the three identified regions surround the large hydrophobic crevice based on the NCS-1 crystal structure, this crevice may be the association site of KChIPs for the channel protein.