Michihiko Tada

Direct association of the gap junction protein connexin-43 with ZO-1 in cardiac myocytes.

The gap junction protein connexin-43 is normally located at the intercalated discs of cardiac myocytes, and it plays a critical role in the synchronization of their contraction. The mechanism by which connexin-43 is localized within cardiac myocytes is unknown. However, localization of connexin-43 likely involves an interaction with the cytoskeleton; immunofluorescence microscopy showed that in cardiac myocytes, connexin-43 specifically colocalizes with the cytoskeletal proteins ZO-1 and α-spectrin. In transfected HEK293 cells, immunoprecipitation experiments using coexpressed epitope-tagged connexin-43 and ZO-1 indicated that ZO-1 links connexin-43 with α-spectrin. The domains responsible for the protein-protein interaction between connexin-43 and ZO-1 were identified using affinity binding assays with deleted ZO-1 and connexin-43 fusion proteins. Immunoblot analysis of associated proteins showed that the C-terminal domain of connexin-43 binds to the N-terminal domain of ZO-1. The role of this linkage in gap junction formation was examined by a dominant-negative assay using the N-terminal domain of ZO-1. Overexpression of the N-terminal domain of ZO-1 in connexin-43-expressing cells resulted in redistribution of connexin-43 from cell-cell interfaces to cytoplasmic structures; this intracellular redistribution of connexin-43 coincided with a loss of electrical coupling. We therefore conclude that the linkage between connexin-43 and α-spectrin, via ZO-1, may serve to localize connexin-43 at the intercalated discs, thereby generating functional gap junctions in cardiac myocytes.

Phospholamban inhibitory function is activated by depolymerization

Phospholamban (PLN), a homopentameric, integral membrane protein, reversibly inhibits cardiac sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) activity through intramembrane interactions. Here, alanine-scanning mutagenesis of the PLN transmembrane sequence was used to identify two functional domains on opposite faces of the transmembrane helix. Mutations in one face diminish inhibitory interactions with transmembrane sequences of SERCA2a, but have relatively little effect on the pentameric state, while mutations in the other face activate inhibitory interactions and enhance monomer formation. Double mutants are monomeric, but loss of inhibitory function is dominant over activation of inhibitory function. These observations support the proposal that the SERCA2a interaction site lies on the helical face which is not involved in pentamer formation. Four highly inhibitory mutants are effectively devoid of pentamer, suggesting that pentameric PLN represents a less active or inactive reservoir that dissociates to provide inhibitory monomeric PLN subunits. A model is presented in which the degree of PLN inhibition of SERCA2a activity is ultimately determined by the concentration of the inhibited PLN monomer·SERCA2a heterodimeric complex. The concentration of this inhibited complex is determined by the dissociation constant for the PLN pentamer (which is mutation-sensitive) and by the dissociation constant for the PLN/SERCA2a heterodimer (which is likely to be mutation-sensitive).

Transmembrane Helix M6 in Sarco(endo)plasmic Reticulum Ca2+-ATPase Forms a Functional Interaction Site with Phospholamban EVIDENCE FOR PHYSICAL INTERACTIONS AT OTHER SITES

In an earlier study (Kimura, Y., Kurzydlowski, K., Tada, M., and MacLennan, D. H. (1997)J. Biol. Chem. 272, 15061–15064), mutation of amino acids on one face of the phospholamban (PLN) transmembrane helix led to loss of PLN inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) molecules. This helical face was proposed to form a site of PLN interaction with a transmembrane helix in SERCA molecules. To determine whether predicted transmembrane helices M4, M5, M6, or M8 in SERCA1a interact with PLN, SERCA1a mutants were co-expressed with wild-type PLN and effects on Ca2+dependence of Ca2+ transport were measured. Wild-type inhibitory interactions shifted apparent Ca2+ affinity of SERCA1a by an average of −0.34 pCa units, but four of the seven mutations in M4 led to a more inhibitory shift in apparent Ca2+ affinity, averaging −0.53 pCa units. Seven mutations in M5 led to an average shift of −0.32 pCa units and seven mutations in M8 led to an average shift of −0.30pCa units. Among 11 mutations in M6, 1, Q791A, increased the inhibitory shift (−0.59 pCa units) and 5, V795A (−0.11), L802A (−0.07), L802V (−0.04), T805A (−0.11), and F809A (−0.12), reduced the inhibitory shift, consistent with the view that Val795, Leu802, Thr805, and Phe809, located on one face of a predicted M6 helix, form a site in SERCA1a for interaction with PLN. Those mutations in M4, M6, or M8 of SERCA1a that enhanced PLN inhibitory function did not enhance PLN physical association with SERCA1a, but mutants V795A and L802A in M6, which decreased PLN inhibitory function, decreased physical association, as measured by co-immunoprecipitation. In related studies, those PLN mutants that gained inhibitory function also increased levels of co-immunoprecipitation of wild-type SERCA1a and those that lost inhibitory function also reduced association, correlating functional interaction sites with physical interaction sites. Thus, both functional and physical data confirm that PLN interacts with M6 SERCA1a.

Phospholamban Domain Ib Mutations Influence Functional Interactions with the Ca2+-ATPase Isoform of Cardiac Sarcoplasmic Reticulum

Alanine-scanning mutagenesis of amino acids 21–30, forming cytoplasmic domain Ib in phospholamban (PLN), revealed that mutation to Ala of Asn27, Gln29, and Asn30 results in gain of inhibitory function. In an earlier study (Kimura, Y., Kurzydlowski, K., Tada, M., and MacLennan, D. H. (1997) J. Biol. Chem.272, 15061–15064), gain of function in PLN transmembrane domain II mutants was correlated with pentamer destabilization, leading to proposals that the PLN monomer is the active inhibitory species, that dissociation of the PLN pentamer is one determinant of PLN inhibitory function and that dissociation of the PLN·cardiac sarco(endo)plasmic Ca2+-ATPase isoform (SERCA2a) complex is a second determinant. Because each of the new domain Ib mutants contained a normal ratio of pentamer to monomer in SDS-polyacrylamide gel electrophoresis, gain of function must have resulted from mechanisms other than destabilization of pentameric structure. Evidence that domain Ib and domain II mutants act through different sites and different mechanisms was provided by a monomeric double mutant, N30A/I40A, in which the enhanced inhibitory function of each single mutant was additive. Evidence for an alteration in stability of the PLN/SERCA2a heterodimer was obtained in a study of double mutant N27A/N34A in which inhibitory function was regained by combining a gain of function, domain Ib mutation with a loss of function domain II mutation. These results support the proposal that PLN inhibition of SERCA2a involves, first, depolymerization of PLN and, second, the formation of inhibitory interactions between monomeric PLN and SERCA2a.

Intercellular Calcium Signaling via Gap Junction in Connexin-43-transfected Cells

In excitable cells, intracellular Ca2+ is released via the ryanodine receptor from the intracellular Ca2+ storing structure, the sarcoplasmic reticulum. To determine whether this released Ca2+propagates through gap junctions to neighboring cells and thereby constitutes a long range signaling network, we developed a cell system in which cells expressing both connexin-43 and ryanodine receptor are surrounded by cells expressing only connexin-43. When the ryanodine receptor in cells was activated by caffeine, propagation of Ca2+ from these caffeine-responsive cells to neighboring cells was observed with a Ca2+ imaging system using fura-2/AM. Inhibitors of gap junctional communication rapidly and reversibly abolished this propagation of Ca2+. Together with the electrophysiological analysis of transfected cells, the observed intercellular Ca2+ wave was revealed to be due to the reconstituted gap junction of transfected cells. We next evaluated the functional roles of cysteine residues in the extracellular loops of connexin-43 in gap junctional communication. Mutations of Cys54, Cys187, Cys192, and Cys198 to Ser showed the failure of Ca2+propagation to neighboring cells in accordance with the electrical uncoupling between transfected cells, whereas mutations of Cys61 and Cys68 to Ser showed the same pattern as the wild type. [14C]Iodoacetamide labeling of free thiols of cysteine residues in mutant connexin-43s showed that two pairs of intramolecular disulfide bonds are formed between Cys54 and Cys192 and between Cys187and Cys198. These results suggest that intercellular Ca2+ signaling takes place in cultured cells expressing connexin-43, leading to their own synchronization and that the extracellular disulfide bonds of connexin-43 are crucial for this process.

Molecular regulation of phospholamban function and expression.

Intracellular levels of cAMP regulated by the beta-adrenergic actions of catecholamines play a key in the metabolic, electrical, and mechanical performance of the cardiac muscles. Among a number of biological actions of cAMP, the excitation-contraction coupling process in cardiac myocytes is markedly affected by cAMP through its stimulatory effect on cAMP-dependent protein kinase. Phospholamban, which is expressed in the sarcoplasmic reticulum of cardiac, slow-twitch skeletal, and smooth muscles, is one of the substrates for cAMP-dependent protein kinase. Phospholamban regulates the activity of Ca ATPase in the sarcoplasmic reticulum membranes in a manner dependent on the phosphorylation state of cAMP-dependent protein kinase, thereby changing the mechanical performance of the cardiac muscles. This Ca regulatory mechanism of phospholamban-Ca ATPase system is mediated by a direct protein-protein interaction between two proteins. This review focuses on recent advances in understanding the role of phospholamban molecule in the regulation of Ca transport by cardiac muscle sarcoplasmic reticulum.

Inducible nitric oxide synthase augments injury elicited by oxidative stress in rat cardiac myocytes

The effects of nitric oxide (NO) produced by cardiac inducible NO synthase (iNOS) on myocardial injury after oxidative stress were examined. Interleukin-1β induced cultured rat neonatal cardiac myocytes to express iNOS. After induction of iNOS,l-arginine enhanced NO production in a concentration-dependent manner. Glutathione peroxidase (GPX) activity in myocytes was attenuated by elevated iNOS activity and by an NO donor, S-nitroso- N-acetyl-penicillamine (SNAP). Although NO production by iNOS did not induce myocardial injury, NO augmented release of lactate dehydrogenase from myocyte cultures after addition of H2O2(0.1 mM, 1 h). Inhibition of iNOS with Nω-nitro-l-arginine methyl ester ameliorated the effects of NO-enhancing treatments on myocardial injury and GPX activity. SNAP augmented the myocardial injury induced by H2O2. Inhibition of GPX activity with antisense oligodeoxyribonucleotide for GPX mRNA increased myocardial injury by H2O2. Results suggest that the induction of cardiac iNOS promotes myocardial injury due to oxidative stress via inactivation of the intrinsic antioxidant enzyme, GPX.The effects of nitric oxide (NO) produced by cardiac inducible NO synthase (iNOS) on myocardial injury after oxidative stress were examined: Interleukin-1 beta induced cultured rat neonatal cardiac myocytes to express iNOS. After induction of iNOS, L-arginine enhanced NO production in a concentration-dependent manner. Glutathione peroxidase (GPX) activity in myocytes was attenuated by elevated iNOS activity and by an NO donor, S-nitroso-N-acetyl-penicillamine (SNAP). Although NO production by iNOS did not induce myocardial injury, NO augmented release of lactate dehydrogenase from myocyte cultures after addition of H2O2 (0.1 mM, 1 h). Inhibition of iNOS with N omega-nitro-L-arginine methyl ester ameliorated the effects of NO-enhancing treatments on myocardial injury and GPX activity. SNAP augmented the myocardial injury induced by H2O2. Inhibition of GPX activity with antisense oligodeoxyribonucleotide for GPX mRNA increased myocardial injury by H2O2. Results suggest that the induction of cardiac iNOS promotes myocardial injury due to oxidative stress via inactivation of the intrinsic antioxidant enzyme, GPX.

PHOSPHOLAMBAN DOMAIN I/CYTOCHROME B5 TRANSMEMBRANE SEQUENCE CHIMERAS DO NOT INHIBIT SERCA2A

A series of chimeras between the transmembrane domains of phospholamban (PLN) and cytochrome b 5 were coexpressed with the Ca2+‐ATPase of cardiac sarcoplasmic reticulum (SERCA2a). The chimeric molecules were not inhibitory, in line with our view that inhibitory PLN/SERCA2a interactions occur in transmembrane sequences, while cytoplasmic interactions regulate the inhibitory interactions in a four‐base circuit.

Direct cardiotoxic effects of cocaine and cocaethylene on isolated cardiomyocytes

We investigated the cardiotoxic effects of cocaine and cocaethylene on the Ca2+ flux responsible for excitation-contraction coupling in isolated ventricular rat myocytes. We simultaneously measured intracellular Ca2+ transients and cell length in isolated cardiac myocytes loaded with a fluorescent Ca2+ indicator, indo-1, during electrical field stimulation at 1 Hz. The cell length was estimated by video dimension analysis. We also measured the activities of Ca2+ ATPase and Ca2+ release channels of cardiac sarcoplasmic reticulum membrane vesicles. Both cocaine and cocaethylene produced significant decreases in both peak intracellular Ca2+ and the cell-contraction rate in a dose-dependent manner. The K0.5 for the reduction of peak intracellular Ca2+ was 157.5 microM for cocaine, but 90.0 microM for cocaethylene. Both cocaethylene and cocaine inhibited neither Ca2+ ATPase nor Ca2+ release channel activity. These results demonstrate that cocaethylene has a more potent direct negative inotropic action on cardiomyocytes, without preventing Ca2+ flux through the cardiac sarcoplasmic reticulum membrane.