Istaroxime treatment ameliorates calcium dysregulation in a zebrafish model of phospholamban R14del cardiomyopathy

The heterozygous Phospholamban p.Arg14del mutation is found in patients with dilated or arrhythmogenic cardiomyopathy. This mutation triggers cardiac contractile dysfunction and arrhythmogenesis by affecting intracellular Ca2+ dynamics. Little is known about the physiological processes preceding induced cardiomyopathy, which is characterized by sub-epicardial accumulation of fibrofatty tissue, and a specific drug treatment is currently lacking. Here, we address these issues using a knock-in Phospholamban p.Arg14del zebrafish model. Hearts from adult zebrafish with this mutation display age-related remodeling with sub-epicardial inflammation and fibrosis. Echocardiography reveals contractile variations before overt structural changes occur, which correlates at the cellular level with action potential duration alternans. These functional alterations are preceded by diminished Ca2+ transient amplitudes in embryonic hearts as well as an increase in diastolic Ca2+ level, slower Ca2+ transient decay and longer Ca2+ transients in cells of adult hearts. We find that istaroxime treatment ameliorates the in vivo Ca2+ dysregulation, rescues the cellular action potential duration alternans, while it improves cardiac relaxation. Thus, we present insight into the pathophysiology of Phospholamban p.Arg14del cardiomyopathy.

Decreased inward rectifier current in adult rabbit ventricular myocytes maintained in primary culture: a single-channel study

OBJECTIVE

Regulation of ion channel function in heart has been shown to be affected by changes in the cellular environment. Recently it was shown that rabbit ventricular myocytes kept in primary culture, show a strong reduction in inward rectifier current (IK1). The aim of the present study was to elucidate the mechanism underlying this decrease in IK1, using single-channel measurements. In addition, we studied the effects of primary culture on the ATP-regulated K+ (K.ATP) channel, also a member of the inwardly rectifying K+ channel family.

METHODS

Adult rabbit ventricular myocytes were cultured for up to 3 days in Ham's F-10 medium complemented with 1% rabbit serum and 5% glutamine. IK1 and K.ATP channel activity was studied in the inside-out patch configuration of the patch-clamp technique with equimolar K+ concentrations (140 mM K+) on the intra- and extracellular side. Single channel characteristics were determined at various times during culture and compared to those present in freshly isolated myocytes.

RESULTS

IK1 channels in freshly isolated myocytes (day 0) had a single-channel conductance of 56.1 +/- 2.5 pS (mean +/- SEM) and an open probability of 0.64 +/- 0.05 (mean +/- SEM). Neither the single-channel conductance nor the open probability (Po) underwent significant changes during culture. The mean number of channels per patch, however, was drastically reduced from 1.2 +/- 0.3 (mean +/- SEM) at day 0 to 0.17 +/- 0.06 at day three. K.ATP channel density and open probability, on the other hand, were both increased with an optimum at day two. Po increased from 0.27 +/- 0.06 at day 0 to 0.63 +/- 0.06 at day three. The mean number of channels per patch was 2.29 +/- 0.57 and 3.25 +/- 0.48 at days 0 and 3 respectively. The unitary current amplitude at -50 mV remained unchanged, suggesting no change in the K.ATP single-channel conductance.

CONCLUSIONS

The decrease in IK1 in rabbit ventricular myocytes as has been observed during primary culture is the result of a reduction in the number of active channels and not of altered kinetic or conductive channel properties. The increase in K.ATP channel activity under the same conditions suggests that gene expression of both channel types is differently regulated.

Lysis of Lactococcus lactis subsp. cremoris SK110 and its nisin-immune transconjugant in relation to flavor development in cheese

To develop a nisin-producing cheese starter, Lactococcus lactis subsp. cremoris SK110 was conjugated with transposon Tn5276-NI, which codes for nisin immunity but not for nisin production. Cheese made with transconjugant SK110::Tn5276-NI as the starter was bitter. The muropeptide of the transconjugant contained a significantly greater amount of tetrapeptides than the muropeptide of strain SK110, which could have decreased the susceptibility of the cells to lysis and thereby the release of intracellular debittering enzymes.

Spatial distribution of connexin43, the major cardiac gap junction protein, visualizes the cellular network for impulse propagation from sinoatrial node to atrium

Myocytes are electrically coupled by gap junctions, which are composed of low-resistance intercellular channels. The major cardiac gap junction protein is connexin43 (Cx43). The distribution of Cx43 has been studied by immunofluorescence to visualize the electrical coupling between atrial tissue and sinoatrial node. From modeling studies, this coupling was inferred to be gradual in order to shield the sinoatrial node from the atrial hyperpolarizing influence. The actual Cx43 labeling pattern did not show the expected gradient but instead a rather black and white staining in a striking pattern of strands of cells. We used an immunohistochemical marker (anti-alpha-smooth muscle actin [alpha SMA]) that specifically cross-reacts with guinea pig sinoatrial node cells together with Cx43 antibody to stain previously electrophysiologically mapped sinoatrial nodes. We found that in the guinea pig sinoatrial node the impulse originates in an alpha SMA-positive, virtually Cx43-negative, region (primary pacemaker region). The impulse then travels obliquely upward to the crista terminalis through a region where layers of alpha SMA-positive cells alternate with layers of Cx43-positive SMA-negative cells. The layers of Cx43-positive cells appear to become broader and thicker in the direction of the crista terminalis, whereas the layers of alpha SMA-positive cells become thinner and narrower. Lateral contacts between Cx43- and alpha SMA-positive cells were very sparse and only detected where the Cx43-positive strands ended (the region where alpha SMA-positive cells fill the whole space between endocardium and epicardium, ie, the putative primary pacemaker region). From these results, we conclude that the primary pacemaker is shielded from the hyperpolarizing influence of the atrium by a gradient in coupling brought about by tissue geometric factors rather than by a gradient of gap junction density.

The peptidoglycan composition of a Staphylococcus aureus mutant selected for reduced methicillin resistance

The peptidoglycan of a Tn551 mutant of Staphylococcus aureus (RUSA208) selected for reduced methicillin resistance was analyzed by reversed-phase high pressure liquid chromatography and mass spectrometry. RUSA208 is a member of a cluster of Tn551 mutants located on fragment A of SmaI digests but is distinct from the femA and femB class of mutants. The peptidoglycan of RUSA208 contained normal parental muropeptides but in diminished amounts only. The major muropeptides of RUSA208 were new components eluting with somewhat longer retention times from the column. Amino acid analysis of these new muropeptides showed identical compositions to the corresponding peaks in the parental strain, but mass spectrometry revealed increased molecular weights by the following mass units: 1 (in monomers), 1 or 2 (in dimers), and 2 or 3 (in trimers). These observations suggest that in RUSA208 the mutational block may be in the amidation of the stem peptide glutamate residues, resulting in the replacement of isoglutamine with free glutamic acid in one or more of the cell wall stem peptides.

Differences in gap junction channels between cardiac myocytes, fibroblasts, and heterologous pairs

Cultures of neonatal rat heart cells contain predominantly myocytes and fibroblastic cells. Most abundant are groups of synchronously contracting myocytes, which are electrically well coupled through large gap junctions. Cardiac fibroblasts may be electrically coupled to each other and to adjacent myocytes, be it with low intercellular conductances. Nevertheless, synchronously beating myocytes interconnected via a fibroblast were present, demonstrating that nonexcitable cardiac cells are capable of passive impulse conduction. In fibroblast pairs as well as in myocyte-fibroblast cell pairs, no sensitivity to junctional voltage could be detected when transjunctional conductance was > 1-2 nS. However, in pairs coupled by a conductance of < 1 nS, complex voltage-dependent gating was evident; gap junction channel open probability decreased with increasing junctional voltage but a nongated residual conductance remained at all voltages tested. Single gap junction channel conductance between fibroblasts was approximately 21 pS, very similar to an approximately 18-pS channel conductance that was found between myocytes next to the major conductance of 43 pS. Single-channel conductance in heterologous myocyte-fibroblast gap junctions was approximately 32 pS, which matches the theoretical value of 29 pS for gap junction channels composed of a fibroblast connexon and the major myocyte connexon. A site-directed antibody against rat heart gap junction protein connexin43 recognized gap junctions between neonatal cardiomyocytes, as demonstrated by immunocytochemical labeling. In contrast, junctions between fibroblasts showed no labeling, while in myocyte-fibroblast junctions labeling occasionally was present. Our results suggest the existence of two gap junction proteins between neonatal rat cardiocytes, connexin43 and another yet unidentified connexin. An alternative explanation (cell-specific regulation of the conductance of connexin43 channels) is discussed.

Cross-linkage and cross-linking of peptidoglycan in Escherichia coli: definition, determination, and implications

The glycan chains in peptidoglycan or murein are cross-linked by transpeptidation of the peptide side chains. To assess the fraction of side chains involved in cross-bridges, distinction has been made between cross-linkage and cross-linking. The first expression refers to the situation in unlabeled (or fully labeled) peptidoglycan, and the second refers to pulse-labeled peptidoglycan. It is argued that for the determination of the cross-linking value, the mode of insertion as denoted by the so-called acceptor/donor radioactivity ratio should be taken into account.

Structure and function of the simian sinoatrial node (Macaca fascicularis)

In isolated preparations of the simian (Macaca fascicularis) heart, we studied the activation pattern within the sinoatrial node, using the conventional microelectrode technique. After electrophysiological experiments we subjected three preparations to a correlative light microscopical and two to an electron microscopical investigation. The sinoatrial node of the Macaca fascicularis is characterized by unifocal impulse generation. The impulse is propagated preferentially in an oblique direction towards the inferior vena cava, which is a unique direction compared to all other mammals studied so far. Possible consequences for A-V nodal input are discussed. Conduction block was seen in an oblique superior direction towards the atrial septum. In this small zone of blocked conduction double component action potentials could be recorded. The morphology of the sinoatrial node of the Macaca fascicularis is essentially the same as found in other mammals.

Gap junction formation and functional interaction between neonatal rat cardiocytes in culture: a correlative physiological and ultrastructural study

The time course of gap junction formation and growth, following contraction synchronization of cardiac myocytes in culture, has been studied in a combined (electro) physiological and ultrastructural study. In cultures of collagenase-dissociated neonatal rat cardiocytes, pairs of spontaneously beating myocytes synchronized their contractions within one beat interval within 2-20 min after they apparently had grown into contact. 45 sec after the first synchronized beat an appreciable junctional region containing several small gap junctions was already present. In the following 30 min, neither the area of individual gap junctions nor their total area increased. 75 min after synchronization both the area of individual gap junctions and their total area had increased by a factor of 10-15 with respect to what was found in the first half hour. In the period between 75 and 300 min again no further increase in gap junctional area was found. In double voltage-clamp experiments, gap junctions between well-coupled cells behaved like ohmic conductors. In poorly coupled cells, in which the number of functional gap-junctional channels was greatly reduced, the remaining channels showed voltage-dependent gating. Their single-channel conductance was 40-50 pS. The electrophysiologically measured junctional conductance agreed well with the conductance calculated from the morphometrically determined gap-junctional area. It is concluded that a rapid initial gap junction formation occurs during the 2-20 min period prior to synchronization by assembly of functional channels from existing channel precursors already present in the cell membranes. It then takes at least another 30 min before the gap-junctional area increases possibly by de novo synthesis or by recruitment from intracellular stores or from nonjunctional membranes, a process completed in the next 45 min.

Single channel currents of homo- and heterologous gap junctions between cardiac fibroblasts and myocytes

Recently, the use of the double whole-cell patch-clamp technique enabled conductance measurements of single gap junctional channels. Different values have been measured in pairs of rat lacrimal cells (6), murine acinar cells and chinese hamster ovary cells (9), embryonic chick heart- (10) and neonatal rat heart myocytes (7). We here present evidence that the conductance of gap junction channels between two different cell types originating from the same tissue, neonatal rat heart, is different. In mixed cultures of cardiac fibroblasts and myocytes, gap junction channels between fibroblasts have a single channel conductance of only 22 pS, while those between myocytes have a conductance of 43 pS. Fibroblasts can be electrically coupled to myocytes through channels having an intermediate conductance of 29 pS, a value which matches very well with the theoretically expected conductance of a gap junction channel composed of a fibroblast- and a myoblast connexon (hemichannel). These data provide direct evidence on the single channel level that in heterologous gap junction channels the composing connexons retain their cell-specific properties.

Functional morphology of the pig sinoatrial node

The porcine sinoatrial node in an isolated right atrium preparation is characterized by unifocal impulse generation. It has a rather elongated shape and the larger part of its volume is taken up by collagen and fibroblasts. The impulse appears to emerge from a site where the percentage of myofilaments is relatively low. The impulse is propagated faster towards the crista terminalis than to the interatrial septum with preference for the oblique-upward direction. A very large zone of cells with low excitability is located at the interatrial septal side of the node.

Functional morphology of the mammalian sinuatrial node

The primary pacemaker area is located at the site with lowest percentage of myofilaments and the highest rate of diastolic depolarization in rabbit, guinea-pig, cat and pig. All investigated sinuatrial nodes contained large amounts, 45% or more, of collagen. There was, however, substantially more collagen in the sinuatrial nodes of the cat and the pig than in the rabbit and the guinea-pig. This had, however, no consequences for the sinuatrial conduction time and the regularity of the beat-to-beat cycle length in the different species, because the rabbit and cat had comparable sinuatrial conduction times, although their nodal collagen content was very different and the beat-to-beat cycle length showed a comparable variability in the different species. We conclude that extensive proliferation of collagen and fibroblasts does not necessarily impair intercellular coupling in the sinuatrial node.

Functional and morphological organization of the cat sinoatrial node

The feline sinoatrial node has a unifocal impulse generation as previously described for rodents. Its main component is collagen. The primary pacemaker consists of at most 2000 cells, but appears to function normally with less than 500 cells. Primary pacemaker cells are found in the area where empty cells are predominant. A negative correlation between myofilament density and diastolic depolarization rate, known to exist in the rabbit and guinea-pig, is absent in the cat. Gap junctions are seen in the center and in the periphery of the nodal region, but they are extremely rare. The electrophysiological characteristics of the primary pacemaker of the cat are quite similar to those of the rabbit, although the nodal morphology is very different. Abrupt transitions from one cell type into another are observed in the feline sinoatrial node. From this morphological point of view the feline sinoatrial node resembles the canine and human sinoatrial nodes more than the lapine sinoatrial node.

Functional and morphological organization of the guinea-pig sinoatrial node compared with the rabbit sinoatrial node

The primary pacemaker, i.e. the group of pacemaker cells discharging the sinoatrial node comprises less than 1000 cells in the guinea-pig and about 5000 cells in the rabbit. These primary pacemaker cells are described as 'central nodal' cells in light microscopy and as 'typical nodal' cells in electron microscopy. The action potential of the leading cells has a higher upstroke velocity in the guinea-pig than in the rabbit (6.2 v. 1.9 V/s). Gap junctions have been observed even in the very center of the node in both species. A zone of double-component action potentials at the septal margin of the node was observed in the rabbit, but not in the guinea-pig. Evidence is presented for abrupt transitions in electrophysiological as well as in ultrastructural characteristics in the guinea-pig sinoatrial node. The differences in intrinsic cycle length between both species but also between individuals of the same species are discussed.

Cycle length dependence of the chronotropic effects of adrenaline, acetylcholine, Ca2+ and Mg2+ in the Guinea-pig sinoatrial node

Ca (1.1-5.5 mM) has a positive chronotropic action on isolated right atria of the guinea-pig. The magnitude of the response depends on the cycle length. Magnitude and cycle length dependence of the Ca response are independent of beta-blockade by propranolol. Mg (0.6-6.0 mM) has a negative chronotropic action. At 6.0 mM it interferes with responses to adrenaline and acetylcholine by preventing pacemaker shifts. Adrenaline has a positive chronotropic action in a cycle length dependent manner. A shift of pacemaker dominance under the influence of adrenaline to an identical site in all preparations (as in the rabbit) was not observed. However, pacemaker shifts in the presence of adrenaline do occur and they are always directed towards the inferior part of the node. Acetylcholine has a negative chronotropic action, independent of cycle length. Acetylcholine also induces pacemaker shifts. Contrary to the pacemaker shifts caused by adrenaline, the new, acetylcholine-induced pacemaker center, has an identical site in all preparations. This was previously observed in the rabbit too. The acetylcholine-induced center is located about 1 mm inferior from the primary center. During exposure to acetylcholine different action potentials may be recorded at the epi- and endocardial side of the preparation, but only close to the Ach-induced center. The acetylcholine-induced center is located at the epicardial side. The response to acetylcholine predominates over the response to adrenaline. All results are discussed in comparison with our previous findings in the rabbit.