Early presence of phospholamban in developing a chick heart

Regulation of the sarcoplasmic reticulum calcium pump by divergent phospholamban isoforms in zebrafish

The sarcoplasmic reticulum calcium pump (SERCA) is regulated by the small integral membrane proteins phospholamban (PLN) and sarcolipin (SLN). These regulators have homologous transmembrane regions, yet they differ in their cytoplasmic and luminal domains. Although the sequences of PLN and SLN are practically invariant among mammals, they vary in fish. Zebrafish (zf) appear to harbor multiple PLN isoforms, one of which contains 18 sequence variations and a unique luminal extension. Characterization of this isoform (zfPLN) revealed that SERCA inhibition and reversal by phosphorylation were comparable with human PLN. To understand the sequence variations in zfPLN, chimeras were created by transferring the N terminus, linker, and C terminus of zfPLN onto human PLN. A chimera containing the N-terminal domain resulted in a mild loss of function, whereas a chimera containing the linker domain resulted in a gain of function. This latter effect was due to changes in basic residues in the linker region of PLN. Removing the unique luminal domain of zfPLN ((53)SFHGM) resulted in loss of function, whereas adding this domain to human PLN had a minimal effect on SERCA inhibition. We conclude that the luminal extension contributes to SERCA inhibition but only in the context of zfPLN. Although this domain is distinct from the SLN luminal tail, zfPLN appears to use a hybrid PLN-SLN inhibitory mechanism. Importantly, the different zebrafish PLN isoforms raise the interesting possibility that sarcoplasmic reticulum calcium handling and cardiac contractility may be regulated by the differential expression of PLN functional variants.

Phospholamban and cardiac function: a comparative perspective in vertebrates

Phospholamban (PLN) is a small phosphoprotein closely associated with the cardiac sarcoplasmic reticulum (SR). Dephosphorylated PLN tonically inhibits the SR Ca-ATPase (SERCA2a), while phosphorylation at Ser16 by PKA and Thr17 by Ca(2+) /calmodulin-dependent protein kinase (CaMKII) relieves the inhibition, and this increases SR Ca(2+) uptake. For this reason, PLN is one of the major determinants of cardiac contractility and relaxation. In this review, we attempted to highlight the functional significance of PLN in vertebrate cardiac physiology. We will refer to the huge literature on mammals in order to describe the molecular characteristics of this protein, its interaction with SERCA2a and its role in the regulation of the mechanic and the electric performance of the heart under basal conditions, in the presence of chemical and physical stresses, such as β-adrenergic stimulation, response to stretch, force-frequency relationship and intracellular acidosis. Our aim is to provide the basis to discuss the role of PLN also on the cardiac function of nonmammalian vertebrates, because so far this aspect has been almost neglected. Accordingly, when possible, the literature on PLN will be analysed taking into account the nonuniform cardiac structural and functional characteristics encountered in ectothermic vertebrates, such as the peculiar and variable organization of the SR, the large spectrum of response to stresses and the disaptive absence of crucial proteins (i.e. haemoglobinless and myoglobinless species).

Presence of functional sarcoplasmic reticulum in the developing heart and its confinement to chamber myocardium

During development fast-contracting atrial and ventricular chambers develop from a peristaltic-contracting heart tube. This study addresses the question of whether chamber formation is paralleled by a matching expression of the sarcoplasmic reticulum (SR) Ca(2+) pump. We studied indo-1 Ca(2+) transients elicited by field stimulation of linear heart tube stages and of explants from atria and outflow tracts of the prototypical preseptational E13 rat heart. Ca(2+) transients of H/H 11+ chicken hearts, which constitute the prototypic linear heart tube stage, were sensitive to verapamil only, indicating a minor contribution of Ca(2+)-triggered SR Ca(2+) release. Outflow tract transients displayed sensitivity to the inhibitors similar to that of the linear heart tube stages. Atrial Ca(2+) transients disappeared upon addition of ryanodine, tetracaine, or verapamil, indicating the presence of Ca(2+)-triggered SR Ca(2+) release. Quantitative radioactive in situ hybridization on sections of E13 rat hearts showed approximately 10-fold higher SERCA2a mRNA levels in the atria compared to nonmyocardial tissue and approximately 5-fold higher expression in compact ventricular myocardium. The myocardium of atrioventricular canal, outflow tract, inner curvature, and ventricular trabecules displayed weak expression. Immunohistochemistry on sections of rat and human embryos showed a similar pattern. The significance of these findings is threefold. (i) A functional SR is present long before birth. (ii) SR development is concomitant with cardiac chamber development, explaining regional differences in cardiac function. (iii) The pattern of SERCA2a expression underscores a manner of chamber development by differentiation at the outer curvature, rather than by segmentation of the linear heart tube.

Effects of verapamil and ryanodine on activity of the embryonic chick heart during anoxia and reoxygenation

Perturbations of the trans-sarcolemmal and sarcoplasmic Ca2+ transport contribute to the abnormal myocardial activity provoked by anoxia and reoxygenation. Whether Ca2+ pools of the extracellular compartment and sarcoplasmic reticulum (SR) are involved to the same extent in the dysfunction of the anoxic-reoxygenated immature heart has not been investigated. Spontaneously contracting hearts isolated from 4-day-old chick embryos were submitted to repeated anoxia (1 min) followed by reoxygenation (5 min). Heart rate, atrioventricular propagation velocity, ventricular shortening, velocities of contraction and relaxation, and incidence of arrhythmias were studied, recorded continuously. Addition of verapamil (10 nM), which blocks selectively sarcolemmal L-type Ca2+ channels, was expected to protect against excessive entry of extracellular Ca2+, whereas addition of ryanodine (10 nM), which opens the SR Ca2+ release channel, was expected to increase cytosolic Ca2+ concentration. Verapamil (a) had no dromotropic effect by contrast to adult heart, (b) attenuated ventricular contracture induced by repeated anoxia, (c) shortened cardioplegia induced by reoxygenation, and (d) had remarkable antiarrhythmic properties during reoxygenation specially. On the other hand, ryanodine potentiated markedly arrhythmias both during anoxia and at reoxygenation. Thus despite its immaturity, the SR seems to be functional early in the developing chick heart and involved in the reversible dysfunction induced by anoxia-reoxygenation. Moreover, Ca2+ entry through L-type channels appears to worsen arrhythmias especially during reoxygenation. These findings show that the Ca2+-handling systems involved in irregular activity in immature heart, such as the embryonic chick heart, may differ from those in the adult.

Excitation-contraction coupling in the day 15 embryonic chick heart with persistent truncus arteriosus

Ca2+ transients were examined in embryonic chick hearts with an experimentally induced cardiac neural crest-related outflow tract defect known as persistent truncus arteriosus (PTA). In all of the animal models of neural crest-related heart defects, prenatal mortality is too high to be attributed to structural defects of the heart alone, suggesting that there is altered development of the myocardium. Earlier reports indicating reduced L-type Ca2+ current in hearts with PTA suggest that poor viability may be related to impairment of cardiac excitation-contraction coupling. To test this hypothesis, direct measurements of the systolic Ca2+ transient in fura-2-loaded myocytes from normal hearts and hearts with PTA were carried out. We found that Ca2+ transients were severely depressed in hearts with PTA and difficult to measure above background noise unless signal averaged or treated with isoproterenol (ISO). We confirmed that the reduced Ca2+ transients were due, at least partly, to a reduction in L-type Ca2+ current. In addition we found that although ISO could raise the L-type current in hearts with PTA to the level found in normal hearts in the absence of ISO, it could not fully restore the Ca2+ transient. Furthermore, caffeine-stimulated Ca2+ transients were diminished in size and the time-to-peak and the decaying phase were significantly slowed. Interestingly, these observations were not accompanied by a reduction in the number of Ca2+ release channels. These results indicated an impairment of SR function in addition to the reduction in L-type Ca2+ current. These results strongly support our hypothesis that the poor viability of embryos with PTA is due to impaired cardiac excitation-contraction coupling.

Patterns of expression of sarcoplasmic reticulum Ca(2+)-ATPase and phospholamban mRNAs during rat heart development

This study reports the clonal analysis and sequence of rat phospholamban (PLB) cDNA clones and the temporal appearance and patterns of distribution of the mRNAs encoding sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA2) and PLB in the developing rat heart determined by in situ hybridization. Both proteins play a critical role in the contraction-relaxation cycle of the heart. SERCA2 mRNA is already abundantly present in the first stage studied, in the cardiogenic plate of the 9-day-old presomite embryo, before the occurrence of the first contractions. This very early expression makes it an excellent marker for the study of early heart development. Subsequently, SERCA2 mRNA becomes expressed in a craniocaudal gradient, being highest at the venous pole and decreasing in concentration toward the arterial pole of the heart. PLB mRNA can be detected in hearts from 12 days of development onward in a virtually opposite gradient. In essence, these patterns do not change during further development. PLB mRNA levels remain highest in the ventricle and outflow tract, whereas SERCA2 mRNA prevails in the inflow tract and atrium, although the difference between atrium and ventricle becomes less pronounced. These observations are compatible with a model in which the upstream part of the heart (inflow tract and atrium) would have a greater capacity to clear calcium and hence would have a longer duration of the diastole than the downstream compartments (atrioventricular canal, ventricle, and outflow tract), similar to the observed pattern of contraction of the embryonic heart. The sinoatrial and atrioventricular nodes do not reveal an expression pattern of SERCA2 and PLB mRNA that allows one to distinguish them from the surrounding atrial working myocardium. However, the ventricular part of the conduction system, comprising atrioventricular bundle and bundle branches, are almost devoid of SERCA2 mRNA.

Calcium-containing vacuolated mitochondria during early heart development in chick embryos as demonstrated by cytochemistry and X-ray microanalysis

The ultrastructure of mitochondria in the developing heart was examined in chick embryos from 2 to 7 days after fertilization. Vacuolated mitochondria were observed in the heart muscle cells of embryos at all stages examined. The number of vacuolated mitochondria as a percentage of total mitochondria in muscle cells was high in embryos at 3 and 4 days and was much higher in the ventricular cells than in the atrial cells. Examination by cytochemistry and X-ray microanalysis revealed the accumulation of calcium in the vacuoles of mitochondria. These results suggest important roles for vacuolated mitochondria in the regulation of the intracellular concentration of calcium during the early development of the chick heart.

Expression of phospholamban mRNA during early avian muscle morphogenesis is distinct from that of alpha-actin

We have studied the expression of phospholamban during the early development of chick embryos by in situ hybridization and have compared it to that of alpha-cardiac and alpha-skeletal actin. In adult cross-striated muscles there is only one phospholamban gene and it is expressed exclusively in the heart and slow muscles. In the heart phospholamban transcripts were first detected at stage 14 in the region of presumptive ventricle and at stage 20 in the atrium. In the myotomal portion of the somites phospholamban mRNA was first detected at stage 20, which lagged behind the appearance of the alpha-actins. In the limb rudiments all three mRNAs were barely detectable through stage 24, but increased by stage 28+. However, quantitative analysis of signal intensity at stage 28+ indicated that less phospholamban mRNA is present in the limb bud than in the myotome since for phospholamban the ratio of the signal density in the myotome to that in the limb rudiments was about twice the value of the ratio determined for the alpha-actins. Northern blot analysis of embryonic day 11 chick fast pectoralis muscle showed that phospholamban mRNA was not detected in vivo while alpha-cardiac actin mRNA was. Moreover, no phospholamban mRNA was detected in primary cultures derived from pectoralis muscle of the same age. In concert with previous observations that phospholamban is not detectable at stage 30-32 in wing or thigh muscle, these results suggest that phospholamban mRNA is expressed independently of the alpha-actins in the limb buds during early myogenesis.

Mouse phospholamban gene expression during development in vivo and in vitro

To establish a murine model that may allow for definition of the precise role of phospholamban in myocardial contractility through selective perturbations in the phospholamban gene, we initiated studies on the role of phospholamban in the murine heart. Intact beating hearts were perfused in the absence or presence of isoproterenol, and quantitative measurements of cardiac performance were obtained. Isoproterenol stimulation was associated with increases in the affinity of the sarcoplasmic reticulum Ca2+ pump for Ca2+ that were due to phospholamban phosphorylation. To assess the regulation of phospholamban gene expression during murine development, Northern blot and polymerase chain reaction analyses were used. Phospholamban mRNA was first detected in murine embryos on the ninth day of development (the time when the cardiac tube begins to contract). In murine embryoid bodies, which have been shown to recapitulate several aspects of cardiogenesis, phospholamban mRNA was detected on the seventh day (the time when spontaneous contractions are first observed). Only those embryoid bodies that exhibited contractions expressed phospholamban transcripts, and these were accompanied by expression of the protein, as revealed by immunofluorescence microscopy. Sequence analysis of the cDNA encoding phospholamban in embryoid bodies indicated complete homology to that in adult hearts. The deduced amino acid sequence of murine phospholamban was identical to rabbit cardiac phospholamban but different from dog cardiac and human cardiac phospholamban by one amino acid. These data suggest that phospholamban, the regulator of the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum, is present very early in murine cardiogenesis in utero and in vitro, and this may constitute an important determinant for proper development of myocardial contractility.

Comparative analysis of phospholamban phosphorylation in crude membranes of vertebrate hearts

Phospholamban, a sarcoplasmic reticulum phosphoprotein, is present in the hearts of mammalian, avian, amphibian, and fish species. Phylogenetic changes are indicated by marked differences among species in cardiac phospholamban content and by the absence of Ca2+/calmodulin-dependent phospholamban phosphorylation at an early developmental stage.

Temporal appearance and distribution of the Ca2+ + Mg2+ ATPase of the sarcoplasmic reticulum in developing chick myocardium as determined by immunofluorescence labeling

The temporal appearance and distribution of the Ca2+ + Mg2+ ATPase of the sarcoplasmic reticulum were determined in the developing chick heart (stage 9 to stage 16) by indirect immunofluorescence labeling. The results obtained showed that the Ca2+ + Mg2+ ATPase was first observed in the bulbus ventricular region of the single tubular heart at stage 9 to 10 of development, when these myocardial cells first contract. As the atrial and later the sinus venosus tissues became incorporated into the single tubular heart the Ca2+ + Mg2+ ATPase was also observed in these regions, however, the highest density of Ca2+ + Mg2+ ATPase labeling was generally observed in the region of the heart most recently incorporated. These results suggest that the sarcoplasmic reticulum is present and perhaps functional in the regulation of the cytoplasmic Ca2+ concentration and thereby the contraction-relaxation cycle in myocardial cells when the first contraction occurs, as well as throughout all subsequent stages of development. Furthermore comparison between the relative density and intensity of the Ca2+ + Mg2+ ATPase labeling and the intrinsic rate of contraction of the myocardial cells in the various regions of the heart (A. Barry, 1942, J. Exp. Zool. 91, 119-130) supports the possibility that a positive correlation exists between these two characteristics of the myocardial cells.

Sarcolemmal phospholamban is phosphorylated in isolated rat hearts perfused with isoprenaline

Phosphorylation of phospholamban in cardiac sarcolemma is implicated in the increased influx of Ca2+ through the slow calcium channel induced by catecholamines. A method is described for the preparation of highly purified sarcolemmal vesicles from rat heart, and this has been used to examine the phosphorylation of phospholamban in 32Pi-perfused rat hearts. Phospholamban phosphorylation is increased 3-fold after 30 s of perfusion with 0.1 microM isoprenaline. The time course of this increase precedes the inotropic response by 5-10 s.