Acquisition of multiple nuclei and the activity of DNA polymerase alpha and reinitiation of DNA replication in terminally differentiated adult cardiac muscle cells in culture

Cardiac myocyte cell cycle control in development, disease, and regeneration

Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle soon after birth in mammals. Although the extent to which adult cardiac myocytes are capable of cell cycle reentry is controversial and species-specific differences may exist, it appears that for the vast majority of adult cardiac myocytes the predominant form of growth postnatally is an increase in cell size (hypertrophy) not number. Unfortunately, this limits the ability of the heart to restore function after any significant injury. Interest in novel regenerative therapies has led to the accumulation of much information on the mechanisms that regulate the rapid proliferation of cardiac myocytes in utero, their cell cycle exit in the perinatal period, and the permanent arrest (terminal differentiation) in adult myocytes. The recent identification of cardiac progenitor cells capable of giving rise to cardiac myocyte-like cells has challenged the dogma that the heart is a terminally differentiated organ and opened new prospects for cardiac regeneration. In this review, we summarize the current understanding of cardiomyocyte cell cycle control in normal development and disease. In addition, we also discuss the potential usefulness of cardiomyocyte self-renewal as well as feasibility of therapeutic manipulation of the cardiac myocyte cell cycle for cardiac regeneration.

A precise pharmacodynamic study showing the advantage of a marked reduction in cardiotoxicity in continuous infusion of doxorubicin

We have already shown that the antileukemic activity of daunorubicin that had been reported to be dependent on the area under the concentration - time curve (AUC) was actually peak concentration (Cmax) dependent. The antitumor activity of doxorubicin (DXR) has also been reported to be dependent on AUC, whereas its cumulative cardiotoxicity has been reported to be Cmax dependent. In this study, we evaluated whether the antileukemic and cardiotoxic effects of DXR were AUC or Cmax dependent, and compared their cytotoxic effects, utilizing the computer-controlled in vitro pharmacokinetic simulation system or a conventional culture system for a leukemic cell line and measuring the intracellular ATP amount or the proportion of beating cells for the cardiotoxicity. In leukemic cells, the cytotoxic rate decreased as the simulated infusion time or exposure time increased with the same AUC value in the simulation and conventional culture system (P < 0.05 and <0.01, respectively). The intracellular ATP and proportion of beating cells also increased with prolonged DXR exposure time with the same constant concentration - time product value (P < 0.05 and <0.0001, respectively) in heart cells. These results indicated that both the antileukemic effects and the cardiotoxicity were Cmax dependent. However, a comparison of the two showed that cardiotoxicity was more Cmax dependent than the antileukemic effect. These results suggested that the continuous infusion treatment schedule of DXR may have the clinical advantage of reducing cardiotoxicity more markedly than the antileukemic effect.

p38 MAP kinase inhibition enables proliferation of adult mammalian cardiomyocytes

Adult mammalian cardiomyocytes are considered terminally differentiated and incapable of proliferation. Consequently, acutely injured mammalian hearts do not regenerate, they scar. Here, we show that adult mammalian cardiomyocytes can divide. One important mechanism used by mammalian cardiomyocytes to control cell cycle is p38 MAP kinase activity. p38 regulates expression of genes required for mitosis in cardiomyocytes, including cyclin A and cyclin B. p38 activity is inversely correlated with cardiac growth during development, and its overexpression blocks fetal cardiomyocyte proliferation. Activation of p38 in vivo by MKK3bE reduces BrdU incorporation in fetal cardiomyocytes by 17.6%. In contrast, cardiac-specific p38alpha knockout mice show a 92.3% increase in neonatal cardiomyocyte mitoses. Furthermore, inhibition of p38 in adult cardiomyocytes promotes cytokinesis. Finally, mitosis in adult cardiomyocytes is associated with transient dedifferentiation of the contractile apparatus. Our findings establish p38 as a key negative regulator of cardiomyocyte proliferation and indicate that adult cardiomyocytes can divide.

Adult-derived liver stem cells acquire a cardiomyocyte structural and functional phenotype ex vivo

We examined the differentiation potential of an adult liver stem cell line (WB F344) in a cardiac microenvironment, ex vivo. WB F344 cells were established from a single cloned nonparenchymal epithelial cell isolated from a normal male adult rat liver. Genetically modified, WB F344 cells that express beta-galactosidase and green fluorescent protein or only beta-galactosidase were co-cultured with dissociated rat or mouse neonatal cardiac cells. After 4 to 14 days, WB F344-derived cardiomyocytes expressed cardiac-specific proteins and exhibited myofibrils, sarcomeres, and a nascent sarcoplasmic reticulum. Further, rhythmically beating WB F344-derived cardiomyocytes displayed calcium transients. Fluorescent recovery after photobleaching demonstrated that WB F344-derived cardiomyocytes were coupled with adjacent neonatal cardiomyocytes and other WB F344-derived cardiomyocytes. Fluorescence in situ hybridization experiments suggested that fusion between WB F344 cells and neonatal mouse cardiomyocytes did not take place. Collectively, these results support the conclusion that these adult-derived liver stem cells respond to signals generated in a cardiac microenvironment ex vivo acquiring a cardiomyocyte phenotype and function. The identification ex vivo of microenvironmental signals that appear to cross germ layer and species specificities should prove valuable in understanding the molecular basis of adult stem cell differentiation and phenotypic plasticity.

Heterogeneous proliferative potential in regenerative adult newt cardiomyocytes

Adult newt cardiomyocytes, in contrast to their mammalian counterparts, can proliferate after injury and contribute to the functional regeneration of the heart. In order to understand the mechanisms underlying this plasticity we performed longitudinal studies on single cardiomyocytes in culture. We find that the majority of cardiomyocytes can enter S phase, a process that occurs in response to serum-activated pathways and is dependent on the phosphorylation of the retinoblastoma protein. However, more than half of these cells stably arrest at either entry to mitosis or during cytokinesis, thus resembling the behaviour observed in mammalian cardiomyocytes. Approximately a third of the cells progress through mitosis and may enter successive cell divisions. When cardiomyocytes divided more than once, the proliferative behaviour of sister cells was significantly correlated, in terms of whether they underwent a subsequent cell cycle, and if so, the duration of that cycle. These observations suggest a mechanism whereby newt heart regeneration depends on the retention of proliferative potential in a subset of cardiomyocytes. The regulation of the remaining newt cardiomyocytes is similar to that described for their mammalian counterparts, as they arrest during mitosis or cytokinesis. Understanding the nature of this block and why it arises in some but not other newt cardiomyocytes may lead to an augmentation of the regenerative potential in the mammalian heart.

Cardiac-specific overexpression of cyclin-dependent kinase 2 increases smaller mononuclear cardiomyocytes

Cyclin-dependent kinase 2 (cdk2) plays a critical role in the G1- to S-phase checkpoint of the cell cycle. Adult cardiomyocytes are believed to withdraw from the cell cycle. To determine whether forced overexpression of cdk2 results in altered cell-cycle regulation in the adult heart, we generated transgenic mice specifically overexpressing cdk2 in hearts. Transgenic hearts expressed high levels of both cdk2 mRNA and catalytically active cdk2 proteins. Cdk2 overexpression significantly increased the levels of cdk4 and cyclins A, D3, and E. There was an increase in both DNA synthesis and proliferating cell nuclear antigen levels in the adult transgenic hearts. The ratio of heart weight to body weight in cdk2 transgenic mice was significantly increased in neonatal day 2 but not in adults compared with that of wild-type mice. Analysis of dispersed individual adult cardiomyocytes showed a 5.6-fold increase in the proportion of smaller mononuclear cardiomyocytes in the transgenic mice. Echocardiography revealed that transgenic heart was functionally normal. However, adult transgenic ventricles expressed beta-myosin heavy chain and atrial natriuretic factor. Surgically induced pressure overload caused an exaggerated maladaptive hypertrophic response in transgenic mice but did not change the proportion of mononuclear cardiomyocytes. The data suggest that overexpression of cdk2 promotes smaller, less-differentiated mononuclear cardiomyocytes in adult hearts that respond in an exaggerated manner to pressure overload.

Modulation of L-type calcium channel expression during retinoic acid-induced differentiation of H9C2 cardiac cells

The molecular mechanisms underlying the developmental regulation of L-type voltage-dependent Ca(2+) channels (VDCCs) are still unknown. In this study, we have characterized the expression patterns of skeletal (alpha(1S)) and cardiac (alpha(1C)) L-type VDCCs during cardiogenic differentiation in H9C2 cells that derived from embryonic rat heart. We report that chronic treatment of H9C2 cells with 10 nM all-trans-retinoic acid (all-trans-RA) enhanced cardiac Ca(2+) channel expression, as demonstrated by reverse transcription-polymerase chain reaction, immunoblotting, and indirect immunofluorescence studies, as well as patch-clamp experiments. In addition, RA treatment prevented expression of functional skeletal L-type VDCCs, which were restricted to myotubes that spontaneously appear in control H9C2 cultures undergoing myogenic transdifferentiation. The use of specific skeletal and cardiac markers indicated that RA, by preventing myogenic transdifferentiation, preserves cardiac differentiation of this cell line. Altogether, we provide evidence that cardiac and skeletal subtype-specific L-type Ca(2+) channels are relevant functional markers of differentiated cardiac and skeletal myocytes, respectively. In conclusion, our data demonstrate that in vitro RA stimulates cardiac (alpha(1C)) L-type Ca(2+) channel expression, therefore supporting the hypothesis that the RA pathway might be involved in the tissue specific expression of Ca(2+) channels in mature cardiac cells.

In vitro reestablishment of cell-cell contacts in adult rat cardiomyocytes. Functional role of transmembrane components in the formation of new intercalated disk-like cell contacts

Primary adult rat cardiomyocytes (ARC)in culture are shown to be a model system for cardiac cell hypertrophy in vitro. ARC undergo a process of morphological transformation and grow only by increase in cell size, however, without loss of the cardiac phenotype. The isolated cells spread and establish new cell-cell contacts, eventually forming a two-dimensional heart tissue-like synchronously beating cell sheet. The reformation of specific cell contacts (intercalated disks) is shown also between ventricular and atrial cardiomyocytes by using antibodies against the gap junction protein connexin-43 and after microinjection into ARC of N-cadherin cDNA fused to reporter green fluorescent protein (GFP) cDNA. The expressed fusion protein allowed the study of live cell cultures and of the dynamics of the adherens junction protein N-cadherin during the formation of new cell-cell contacts. The possible use of the formed ARC cell-sheet cells under microgravity conditions as a test system for the reformation of the cytoskeleton of heart muscle cells is proposed.

Unusual myocardial (myostromal) "repair" in cardiac transplant patient

A 57-year-old man received a cardiac allograft for severe ischemic heart disease. His endomyocardial biopsy at eight weeks postoperatively showed a focus of unusual myocardial morphology characterized by small diameter myocytes associated with loose, myxoid appearing stroma and a myocytic mitotic figure. We feel this may represent a unique type of myocardial repair.

Comparison of mitosis in binucleated and mononucleated newt cardiac myocytes

BACKGROUND

The cultured adult newt ventricular myocyte has been shown to undergo mitosis and cytokinesis in a fully differentiated state. Insight into its proliferation and cellular changes during the repair process involves obtaining a better understanding of the nuclear pattern (mononucleated, binucleated, or multinucleated) resulting from mitotic events. Mitosis is easily observable in cultured newt cardiac myocytes using phase-contrast microscopy.

METHODS

From days 8-19 in culture, the process of mitosis in mononucleated and binucleated newt ventricular myocytes was recorded and timed by using time-lapse video microscopy. Cultured cardiac myocytes were double-stained for myosin and F-actin by using fluorescein isothiocyanate (FITC)-labeled MF20 and rhodamine phalloidin.

RESULTS

Mitotic, mononucleated myocytes produced mononucleated daughter cells in 80% of the cases, whereas 20% were single, binucleated myocytes, In binucleated myocytes, only 32% underwent complete cytokinesis to produce two binucleated daughter cells, whereas 68% resulted in variably nucleated myocytes. Mononucleated and binucleated myocytes undergoing mitosis had similar time intervals for the period from nuclear breakdown (prometaphase) to the start of anaphase (108.7 minutes and 94.5 minutes, respectively), but the period between anaphase and midbody formation was significantly shorter in binucleated than in mononucleated myocytes (43.5 minutes and 69.3 minutes, respectively). The myofibrillae were not as well organized in binucleated myocytes as those observed in mononucleated myocytes.

CONCLUSIONS

Mitosis in vitro appears to proceed more rapidly in binucleated newt cardiac myocytes, which have more poorly organized myofibrillae than mononucleated myocytes. Mitosis of cultured binucleated myocytes commonly results in variably nucleated daughter cells, whereas mononucleated myocytes produce predominantly mononucleated daughter cells.

DNA synthesis in adult feline ventricular myocytes. Comparison of hypoxic and normoxic states

Adult mammalian ventricular myocytes are terminally differentiated cells, and the prevailing perception has been that DNA synthesis and repair are not active. We tested the hypothesis that there is potential for DNA synthesis and repair by studying the ability of whole-cell extracts from adult myocytes to incorporate [alpha-32P]dCTP into damaged plasmids. Left ventricular myocytes were isolated from adult cat hearts by collagenase dissociation. Cells were maintained in room air (control extract, CE) or made ischemic (IE) with N2 displacement of O2 and extracted for total protein. The nicked form of the plasmid was produced by exposure to an Fe3+/ascorbic acid free radical generating system. Both IE and CE degraded the supercoiled form of the plasmid and incorporated [alpha-32P]dCTP into the nicked (32P/DNA mass; CE = 2.2, IE = 3.0) and linear forms (32P/DNA mass; CE = 28.7, IE = 25.2). Exposure of plasmids to UV light did not inhibit incorporation of label. Inhibition studies with the cell extracts suggested a participation of polymerase delta in myocyte DNA repair/synthesis. Myocyte extract was as active as extract from rapidly growing COS cells at incorporating labeled nucleotides into plasmid DNA. The ability of intact myocytes to incorporate [alpha-32P]dCTP into endogenous DNA was measured in isolated cells made permeable with saponin. Studies were done in room air or N2. Permeable cells incorporated [alpha-32P]dCTP into nuclear DNA, but maximal specific activity of DNA was observed at 15 minutes with ischemia and at 60 minutes with room air control cells (ischemia, 1.34 +/- 0.5, 0.86 +/- 0.33, 0.60 +/- 0.04; air, 1.0, 1.28 +/- 0.20, 1.87 +/- 0.38, at 15, 30, and 60 minutes, respectively). These data indicate that mammalian adult ventricular myocytes can actively repair and/or synthesize both exogenous and endogenous DNA. A DNA synthetic response to cellular damage may have important pathological and clinical implications.

Myoblast transplantation in the porcine model: a potential technique for myocardial repair

The use of transgenic cells transplanted in syngeneic rodents has shown modest success, but allogeneic and xenogeneic transplants have not been uniformly successful. To assess the feasibility of xenogeneic and allogeneic myoblast transplantation, we subjected seven adult swine to transplantation of murine atrial tumor cells (xenogeneic), neonatal porcine myocytes (allogeneic), and human fetal cardiomyocytes into the left ventricular wall. After general anesthesia, isolated cells were injected along the anterior and posterior walls of the porcine left ventricle. All the animals were immuno-suppressed and observed for 1 month after injection, at which time they were killed and analyzed. This report will present results primarily concerned with the success of human cell transfers. In all injected sites examined, the transplanted cells thrived within the host myocardium with no significant rejection. Transplant cells formed close associations with host myocytes that resembled nascent intercalated disks on electron microscopy. These cells also contained myofibrils and other cell architecture resembling the transplanted cell lines. Additionally, these cells appeared to produce an angiogenic influence resulting in the proliferation of the surrounding microvasculature. We believe that these findings indicate successful xenogeneic and allogeneic myoblast cell transplantation in a large animal model. These experiments set the stage for future studies to assess the ability of these cells to form a syncytium, contract, and potentially repair failed myocardium.

Transformation of adult ventricular myocytes with the temperature sensitive A58 (tsA58) mutant of the SV40 large T antigen

Freshly isolated ventricular myocytes have been used extensively as an adult cardiac model system. Due to their inability to undergo cytokinesis in vitro and their dedifferentiated properties in long-term culture, they can not be used for extended studies. Recent reports tell of the establishment of fetal and neonatal cardiac cell lines and the development of adult cardiomyocytes from transgenic animals. A recent report by Kirshenbaum [1], is the first to demonstrate insertion of genes in to adult ventricular myocytes using viral infection. This paper discusses the infection of primary adult differentiated cardiomyocytes with the SV40 large T antigen and subsequent proliferation under temperature sensitive control. Upon further characterization, the cells could be used as a model to study muscle differentiation and repair as well as adult cardiac cell physiology.

Factors altering DNA synthesis in the cardiac myocyte of the adult newt, Notophthalmus viridescens

To better understand the mechanisms governing the proliferation of cardiac myocytes it is important to identify the factors controlling this phenomenon, and to characterize their actions. DNA synthesis was quantified in vitro in ventricular myocytes from the adult red-spotted newt, Notophthalmus viridescens. Ventricles were enzymatically separated and plated onto laminin. Myocytes were fed modified L-15 medium with 10% fetal bovine serum, and were variously treated with transforming growth factor-beta, transforming growth factor-beta combined with platelet-derived growth factor, acidic fibroblast growth factor, basic fibroblast growth factor, 12-0-tetradecanoylphorbol-13-acetate, heparin, or conditioned medium from ventricular myocytes or non-myocytes (primarily endothelial cells). With their final feeding the cells were given 1 mu Ci/ml of tritiated thymidine, and 24 hours later were fixed and stained. Dishes were coated with photographic emulsion, exposed, and developed. The percent of cells with labeled nuclei was determined. Experimental media that significantly increased DNA synthesis included those containing acidic fibroblast growth factor (121% of control), basic fibroblast growth factor (119% of control), 12-0-tetradecanoylphorbol-13-acetate (233% of control) and conditioned medium from ventricular myocytes (230% of control) or non-myocytes (128% of control). Media significantly inhibiting DNA synthesis were those containing heparin (31% of control), transforming growth factor-beta (38% of control), non-myocyte conditioned medium and heparin (75% of control), or transforming growth factor-beta and platelet-derived growth factor (63% of control).

Acidic and basic fibroblast growth factors in adult rat heart myocytes. Localization, regulation in culture, and effects on DNA synthesis

Basic fibroblast growth factor (bFGF) and acidic fibroblast growth factor (aFGF) are involved in the induction of embryonic mesoderm, angiogenesis, neuronal differentiation, and proliferation and survival of many cell types. In cardiac myocytes their roles are not well understood. Effects of fibroblast growth factors on reexpression of fetal actin genes have been reported. In freshly isolated adult rat cardiac myocytes, bFGF mRNA was not detectable by in situ hybridization, although the cells contained significant amounts of bFGF and aFGF as quantified by radioimmunoassays, mitogen assays with immunoneutralization, and Western blotting. After culturing, bFGF mRNA was detected (aFGF mRNA was not studied), and the cells contained 2.5-fold more bFGF and 60% more aFGF than freshly isolated cells. The FGFs were not found in conditioned medium. They were localized, especially in cultured cells, to the nucleus. Cultured myocytes bound fourfold more 125I-FGF than freshly isolated cells and expressed the fibroblast growth factor R-1 (flg) gene. The addition of bFGF or aFGF in serum-free medium to pure populations of myocytes (after 10 days in culture, at which time they are spread, beating, and multinucleated) led to increased thymidine incorporation. Expression of fibroblast growth factors and fibroblast growth factor receptors by adult cardiac myocytes that survive the shock and "dedifferentiation" of culturing may contribute to DNA synthesis and, by analogy, to other cell types, to regulation of ribosomal and actin genes, and to cell survival. These possibilities and their in vivo relevance will require further study.

Proliferating cell nuclear antigen in developing and adult rat cardiac muscle cells

During early development, rat cardiac muscle cells actively proliferate. Shortly after birth, division of cardiac muscle cells ceases, whereas DNA synthesis continues for approximately 2 weeks at a progressively diminishing rate. Little DNA synthesis or cell division occurs in adult cardiocytes. Thus, developing cardiac muscle cells are an ideal system in which to examine the expression of cell cycle-regulated genes during development. We chose to examine proliferating cell nuclear antigen (PCNA), a gene expressed at the G1/S phase boundary of the cell cycle. Northern blots of RNA from cardiac muscle cells from 18-day-old rat fetuses and from day 0, 5, and 14 neonatal as well as adult rat hearts revealed that the PCNA mRNA was found in cardiac muscle cells from all ages. However, because it was possible that this was a result of fibroblast PCNA gene expression, we used reverse transcription followed by polymerase chain reaction to see if it was possible to detect the message for PCNA in cardiac muscle cells from all ages. Because of the great sensitivity of this technique, RNA was recovered from 25 isolated adult cardiac muscle cells. Polymerase chain reaction amplification products for PCNA produced from the RNA isolated from these cells conclusively demonstrated that mRNA for this gene, which normally is associated with proliferating cells, is expressed in adult cardiac muscle cells that no longer divide. Furthermore, Western blot analysis demonstrated that the PCNA protein was found only in embryonic and neonatal cells and not in adult rat cardiac muscle cells. Therefore, it might be inferred from these data that PCNA might be regulated at the posttranscriptional level in adult cardiac muscle cells.

Localization of bFGF-like proteins as punctate inclusions in the preseptation myocardium of the chicken embryo

Immunocytochemistry has been employed to map the appearance of bFGF-like proteins in precardiac and preseptation myocardial cells between stages 6 and 15 of chicken embryogenesis. Stage 6 embryos exhibited no staining, with the exception of a subtle signal in endoderm cells. At subsequent stages, staining was observed only in cells of the developing myocardium, first appearing at the time of heart tube fusion (stage 9+) as punctate cytoplasmic aggregates. While the expression of bFGF-like antigen was temporally similar to that of myosin heavy chain, their staining patterns differed in that bFGF-like proteins were nonsarcomeric and did not extend into the inflow or outflow tracts. Western blotting of heparin agarose affinity-isolated proteins from stage 15 hearts revealed an antigen migrating at approximately 19 kDa. In contrast with the unique localization of bFGF-like proteins in myocardial cells, FGF receptor (FGFR) staining was widely distributed in the embryo; however, concentrated deposits of FGFR were detected in endothelial and myocardial cells, which diminished in the myocardium but not in the endothelium by stage 15. These results suggest that FGF-like proteins may have autocrine and/or paracrine functions during early cardiac morphogenesis.

Modulators of cyclic AMP metabolism induce syncytiotrophoblast formation in vitro

During placental development cytotrophoblast stem cells fuse to form the syncytiotrophoblast, a multinucleate cytoplasm with a brush border in contact with the maternal blood. Biochemical differentiation including the expression of placental-specific proteins and hormones accompanies this maturation. However, the biochemical mechanisms responsible for these events are unknown. We have defined a system in which single cytotrophoblast-like cells of the human choriocarcinoma (BeWo) cell line undergo fusion and extensive morphological differentiation following their treatment with effectors of cyclic AMP metabolism. Forskolin incubation caused a dose-dependent increase in intracellular and secreted cyclic AMP and a coordinate fusion of cells which yielded syncytia containing hundreds of nuclei per cytoplasm and a mature dense "placental-like" brush border. These fused cells also synthesized and secreted large amounts of both subunits of chorionic gonadotropin. However, they continued to synthesize several other placenta-specific proteins--placental-like alkaline phosphatase, placental lactogen, and SP1--at rates similar to those in control cells. Other reported effectors of cyclic AMP metabolism also induced cell fusion, although theophylline, an inhibitor of phosphodiesterase, induced fusion by a cyclic AMP-independent mechanism. Additionally, unlike the case with forskolin, treatment of BeWo cells with theophylline did not induce other morphological features of mature syncytiotrophoblasts. Thus, this system will allow one to examine the biochemical mechanism of placental cell fusion in the absence of other variables of cell differentiation.

Culture and characterization of fetal human atrial and ventricular cardiac muscle cells

Atrial and ventricular cardiac muscle cells isolated from 14- to 18-wk old fetal human hearts were grown in culture and characterized. Once established in culture the flattened cells contracted spontaneously and possessed differentiated ultrastructural characteristics including organized sarcomeres, intercalated discs, and transverse tubules with couplings. Atrial granules were present in the cultured atrial cells. Some cultured ventricular myocytes also contained electron-dense granules associated with Golgi cisternae, which were similar in size and appearance to atrial granules. The cultured ventricular myocytes divided and expressed the genes for thymidine kinase, histone H4, myosin heavy chain, muscle-specific creatine kinase, atrial natriuretic factor, and insulin-like growth factor II. These results establish that differentiated fetal human heart muscle cells can be cultured in sufficient quantities for biochemical, molecular, and morphological analyses.

Basic fibroblast growth factor in atria and ventricles of the vertebrate heart

Extracts from atrial and ventricular heart tissue of several species (chicken, rat, sheep, and cow) are strongly mitogenic for chicken skeletal myoblasts, with the highest apparent concentration of biological activity in the atrial extracts. Using several approaches (biological activity assay and biochemical and immunological analyses), we have established that (a) all cardiac extracts contain an 18,000-D peptide which is identified as basic fibroblast growth factor (bFGF) since it elutes from heparin-Sepharose columns at salt concentrations greater than 1.4 M and is recognized by bFGF-specific affinity-purified antibodies; (b) bFGF is more abundant in the atrial extracts in all species so examined; (c) avian cardiac tissue extracts contain the highest concentration of immunoreactive bFGF; and (d) avian ventricles contain a higher relative molecular mass (23,000-D) bFGF-like peptide which is absent from atrial extracts. Examination of frozen bovine cardiac tissue sections by indirect immunofluorescence using anti-bFGF antibodies shows bFGF-like reactivity associated with nuclei and intercalated discs of muscle fibers. There is substantial accumulation of bFGF around atrial but not ventricular myofibers, resulting most likely from more extensive endomysium in the atria. Blood vessels and single, nonmuscle, connective tissue cells react strongly with the anti-bFGF antibodies. Higher bFGF content and pericellular distribution in atrial muscles suggest a correlation with increased regenerative potential in this tissue. Distribution within the myofibers is intriguing, raising the possibility for an intimate and continuous involvement of bFGF-like components with normal myocardial function.

Polygons and adhesion plaques and the disassembly and assembly of myofibrils in cardiac myocytes

Successive stages in the disassembly of myofibrils and the subsequent assembly of new myofibrils have been studied in cultures of dissociated chick cardiac myocytes. The myofibrils in trypsinized and dispersed myocytes are sequentially disassembled during the first 3 d of culture. They split longitudinally and then assemble into transitory polygons. Multiples of single sarcomeres, the cardiac polygons, are analogous to the transitory polygonal configurations assumed by stress fibers in spreading fibroblasts. They differ from their counterparts in fibroblasts in that they consist of muscle alpha-actinin vertices and muscle myosin heavy chain struts, rather than of the nonmuscle contractile protein isoforms of stress fiber polygons. EM sections reveal the vertices and struts in cardiac polygons to be typical Z and A bands. Most cardiac polygons are eliminated by day 5 of culture. Concurrent with the disassembly and elimination of the original myofibrils new myofibrils are rapidly assembled elsewhere in the same myocyte. Without exception both distal tips of each nascent myofibril terminate in adhesion plaques. The morphology and composition of the adhesion plaques capping each end of each myofibril are similar to those of the termini of stress fibers in fibroblasts. However, whereas the adhesion complexes involving stress fibers in fibroblasts consist of vinculin/nonmuscle alpha-actinin/beta- and gamma-actins, the analogous structures in myocytes involving myofibrils consist of vinculin/muscle alpha-actinin/alpha-actin. The addition of 1.7-2.0 microns sarcomeres to the distal tips of an elongating myofibril, irrespective of whether the myofibril consists of 1, 10, or several hundred tandem sarcomeres, occurs while the myofibril appears to remain linked to its respective adhesion plaques. The adhesion plaques in vitro are the equivalent of the in vivo intercalated discs, both in terms of their molecular composition and with respect to their functioning as initiating sites for the assembly of new sarcomeres. How 1.7-2.0 microns nascent sarcomeres can be added distally during elongation while the tips of the myofibrils remain inserted into submembranous adhesion plaques is unknown.

Primary cell culture and morphological characterization of ventricular myocytes from the adult newt, Notophthalmus viridescens

Previous work has demonstrated that adult newt cardiac myocytes possess a proliferative ability in response to an experimentally induced injury, in vivo. This study describes an in vitro model in which the proliferative events of the adult cardiac myocyte may be studied. Ventricles were minced and then enzymatically dissociated in a Ca++- and MG++-free salt solution containing 0.5% trypsin and 625 U/ml of CLS II collagenase for 8 to 10 hours at 25 degrees C. Enzyme digests were preplated and then cultured on bovine corneal endothelial-derived basement membrane "carpets" in either serum-free or serum-supplemented modified Leibovitz's medium for up to 30 days. Light and transmission electron microscopic characterization demonstrated that a majority of the myocytes underwent an initial period of disorganization characterized by a "rounding up" of the cell and a loss of myofibrillar organization. Once the myocytes had attached to the culture substratum they began to spread out, underwent a reassembly of their contractile elements, resumed spontaneous contractions, and demonstrated ultrastructural evidence of protein synthesis. Mitosis was observed in several myocytes 8 to 15 days following isolation. In 15-day serum-supplemented and serum-free cultures, 6.5% +/- 0.9% and 8.1% +/- 1.4% of the myocytes were binucleated, respectively. These results demonstrate that adult newt ventricular myocytes can be successfully placed into primary culture and are capable of undergoing mitosis. This work may be considered as a foundation for future investigations which will focus on the mechanisms which control cardiac myocyte proliferation.

Differentiation of adult rat cardiac myocytes in cell culture

Cardiac myocytes isolated from adult rat hearts were grown on laminin coated culture dishes for more than a month. During this time, the cells underwent a morphological transformation which has also been referred to by others as cell remodeling (Guo J-X, Jacobson SL, Brown DL: Cell Mot Cytoskeleton 1986;6:291-304). This results in a change in myocyte morphology from its typical in vivo cylindrical shape to one which is more pleiomorphic. Despite the long-term change in morphology, myocytes expressed for differing lengths of time several aspects of the adult phenotype as evidenced by the following: 1) maintenance of cylindrical shape and/or evident cross-striations for the first 24-48 hours in culture, 2) reappearance of cross-striations during the second week in culture, 3) little or no spontaneous contractility for the first 4 days in culture, 4) expression of only the V1 isoform of myosin for at least 7 days, and 5) altered myosin isoform expression in response to changes in environmental conditions. These factors taken together suggest that in culture the adult cardiac myocyte remains a highly differentiated cell (as opposed to possible dedifferentiation) and maintains many of its previous in vivo characteristics. Such highly differentiated adult cells should be suitable as an in vitro system for studying the direct cellular effects of factors which regulate growth and differentiation of the in vivo heart.

Analysis of DNA synthesis in cell cultures of the adult newt cardiac myocyte

Cell division in the adult cardiac myocyte has been examined in a number of different species in vivo and in vitro. The newt cardiac myocyte responds to trauma in vivo with proliferation. It has recently been successfully placed into cell culture. The purpose of the present study was to analyze the process of DNA synthesis in these cultures. The myocytes were cultured in modified Leibovitz L-15 medium on a bovine corneal endothelial cell membrane carpet and were incubated with tritiated thymidine (1 microCi/ml) for 24 hr prior to fixation at 10, 15, 20 and 30 days. Labeling indices were determined to be 10.5 +/- 2.5, 16.5 +/- 2.8, 10.5 +/- 2.2, and 2.9 +/- 0.6, respectively. When myocytes were exposed to 1 microCi/ml tritiated thymidine continuously from the fifth to the thirtieth day in culture, the labeling index was 34.5 +/- 6.8. Comparison of DNA synthesis in the in vivo and in vitro systems indicated comparable patterns, although there was an earlier onset of activity in culture. Between 8 and 15 days in culture, myocyte mitoses were regularly observed. Myocytes in metaphase contained well-organized myofibrillae, suggesting that mitosis may occur with highly differentiated morphology in vitro. It appears that this system will be useful in the definition of mechanisms involved in both initiating and stopping proliferative events in the cardiac myocyte.

Polyploidization of transplanted cardiac myocytes

Pieces of cardiac ventricular tissue of late embryonic or 1-day postnatal rats, implanted beneath the kidney capsule of adult syngeneic hosts, formed viable, beating transplants. These transplants were investigated over a 40-day postoperative course. In the transplants, cellular binucleation and nuclear polyploidization occurred according to the same schedule as in the heart in situ. The composition of the classes of myocytes was identical both in the hearts in situ and in transplants, but the number of non-diploid myocytes in the intact heart reached 90%, whereas in transplants it varied from 30 to 60%. In contrast to the heart in situ, myocytes in transplants grew feebly after the phase of polyploidization. From these data one can conclude that under conditions of transplantation the temporal sequence of cellular binucleation and nuclear polyploidization follows the normal course, but that a greater number of myocytes remain in a diploid state than is the case in the normal heart. The growth of cardiac myocytes seems to be related to their level of function.

Nuclear characteristics of cardiac myocytes following the proliferative response to mincing of the myocardium in the adult newt, Notophthalmus viridescens

Amphibian cardiac myocytes are predominantly mononucleated and have been demonstrated to respond to injury with DNA synthesis and mitosis. The nature of this response with regard to nuclear number and ploidy is unclear. In this study, the apex of the newt ventricle was minced and replaced, increasing the reactive area of the wound. At 45 days after mincing following multiple injections of tritiated thymidine (2.5 microCi/animal, 20 Ci/mM) 15 to 20 days after mincing, three ventricular zones were isolated and fixed: Zone 1, the minced area; Zone 2, extending approximately 500 micron proximally from the amputation plane; and Zone 3, the portion proximal to Zone 2. Myocytes separated in 50% KOH were examined for DNA synthesis by autoradiography and for nuclear number and DNA content using a scanning microdensitometer on Feulgen-Naphthol yellow S-stained cells. No labeled myocyte nuclei were found in control hearts and 98.3% of the myocytes were 2C. At 45 days, 46.78% of myocyte nuclei within Zone 1 were labeled, while 13% were non-diploid. In Zone 2, 9.25% were labeled with 4.8% non-diploid. In Zone 3, 1.1% were labeled, with 2.8% non-diploid. The newt ventricle's response to injury apparently may involve complete mitosis and cytokinesis, resulting in mononucleated diploid cells.

Growth factors and TPA stimulate DNA synthesis and alter the morphology of cultured terminally differentiated adult rat cardiac muscle cells

Previous studies have established that the terminally differentiated ventricular cardiac muscle cell of the adult rat reinitiates semiconservative DNA replication when grown in culture (W. C. Claycomb and H. D. Bradshaw, Jr., 1983, Dev. Biol. 90, 331-337). Work reported here shows that several growth factors and chemicals will stimulate this DNA synthetic activity in a concentration-dependent manner. Autoradiographic experiments establish that this stimulated DNA synthesis is due to cells not previously synthesizing DNA being induced to enter the S phase of the cell cycle. By far the greatest stimulation (250%) is observed with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Fifty ng/ml is the optimal concentration, and the maximal effect is observed 5 days after adding TPA. TPA also substantially increases the protein content of the cultured myocytes. Diacylyglcerols (DAG) induce these same changes, indicating that the effect of TPA is mediated by protein kinase C. The morphology of the cultured cardiac muscle cells is profoundly altered by TPA and DAG. TPA- and DAG-treated myocytes spread more thinly on the surface of the culture flask, acquire multiple nuclei, and undergo nucleolar fragmentation. The myofibrillar ultrastructure of the treated cells becomes almost totally disorganized, and intermediate filaments and rough endoplasmic reticulum accumulate in the cytoplasm. These TPA results suggest a possible relationship between the degree of ultrastructural differentiation of the ventricular cardiac muscle cell and DNA synthetic activity. This easily altered cellular plasticity should be very useful for studies of the regulation of cardiac muscle cell proliferation and cell differentiation.

Proto-oncogene expression in proliferating and differentiating cardiac and skeletal muscle

We have examined the expression of 13 proto-oncogenes in proliferating and terminally differentiated cardiac and skeletal muscle. Total RNA was prepared from intact ventricular cardiac-muscle tissue and from purified ventricular cardiac-muscle cells of neonatal and adult rats and from cultured proliferating and terminally differentiated L6A1 rat skeletal-muscle cells. cDNA probes for histone H4, thymidine kinase, myosin heavy chain and M-creatine kinase were used to assess cellular proliferation and differentiation. Oncogenes c-myc, c-raf, c-erb-A, c-ras-H, c-ski, and c-sis were expressed in both proliferating and differentiated cardiac muscle tissue and cells, whereas c-myb expression was not observed in either. c-src was expressed only in neonatal cardiac muscle tissue and cells. c-fms, c-abl, and c-ras-K were expressed in tissue from both neonatal and adult animals but only in purified cells from neonatal animals. c-fes/fps was expressed only in neonatal cardiac muscles cells. c-fos expression was not observed in cardiac-muscle tissue from either neonatal or adult rats, but surprisingly was abundantly expressed in freshly isolated cardiac-muscle cells from animals of both ages. These results emphasize that biochemical analysis using intact cardiac-muscle tissue may not necessarily reflect muscle-specific cell processes. They also show that the expression of c-fos can be activated by the cell isolation procedure. c-myc, c-ski, c-ras-H, c-ras-K, c-abl, c-raf and c-erb-A were expressed in both proliferating and terminally differentiated skeletal-muscle cells, whereas c-myb, c-fos, c-src and c-fms transcripts were observed only in proliferating cells. c-fes/fps and c-sis were not expressed in dividing or fused skeletal-muscle cells. These results demonstrate unique tissue and cell-specific patterns of proto-oncogene expression and suggest that these genes may be involved with the regulation of cellular proliferation and terminal differentiation in striated muscle.

Changes in binucleation and cellular dimensions of rat left atrial myocytes after induced left ventricular infarction

The left atrium of young rats has previously been demonstrated to respond with DNA synthesis and binucleation 11 days after left ventricular infarction. This investigation was designed to examine the hypertrophic response of the left atrial myocyte of the rat at 20 and 60 days after ventricular infarction. Male Sprague-Dawley rats were subjected to left coronary artery ligation (CAL) or sham operation. Following enzymatic separation, left atrial myocytes were examined at 20 and 60 days postoperation for number of nuclei and cellular dimensions (cell length, width and area, and nuclear area). Results demonstrated that the level of binucleation at 20 days (77.3%) and 60 days (71.3%) was nearly twice that observed in sham-operated animals, which were 33.1% binucleated at 20 days and 43.5% binucleated at 60 days. In both mononucleated and binucleated myocytes, the mean lengths, widths, and cell areas from CAL hearts were significantly greater than those of corresponding sham-operated animals. In all cases, these values were larger in binucleated myocytes than in mononucleated cells. The mean area of CAL cells was approximately twice that of sham-operated myocytes. With regard to mean lengths and widths, although both were greater in the CAL animals, there was a decrease in length and increase in width between 20 and 60 days in the CAL group. Mean nuclear areas were significantly greater in CAL myocytes than in those from the sham-operated group. These increases in nuclear number and cellular dimensions of the atrial myocyte are prominent features of the response to the stress imposed by left ventricular infarction.

Biochemical characterization and development of DNA polymerases alpha and delta in the neonatal rat heart

The ontogeny of DNA polymerase activity in the neonatal rat heart was studied. The DNA polymerase activities in rat heart extracts were identified as DNA polymerase alpha and DNA polymerase delta activities by their purification and characterization, by the use of a specific inhibitor (BuAdATP), and by a specific monoclonal antibody against DNA polymerase alpha. Using these inhibitors, it was shown that the two activities declined in parallel during the maturation and terminal differentiation of the heart.

DNA synthesis of adult mammalian cardiac muscle cells in long-term culture

Adult rat cardiac ventricular muscle cells were isolated and cultured in monolayer for 30-45 days. Most of the cardiac muscle cells undergo external and internal structural alterations, resembling embryonic/neonatal cardiac muscle cells in culture (Nag and Cheng, 1981; Nag et al., 1983). These cultured cells underwent DNA synthesis and mitosis as revealed by autoradiography studies that involved the exposure of the cells to [3H]-thymidine for 24 hr prior to the termination of the culture at selected intervals. During the first week of culture, cardiac muscle cells showed less than 5% labeled cells. The labeling index of myocytes attained a peak in the second week of culture, exhibiting approximately 23% labeled cells. The labeling indices of cardiac muscle cells declined over the period of 30 days of culture. During the end of the incubation period, approximately 4% of the myocytes were labeled. When the extent of the total cell population involved in DNA synthesis was examined by exposing the cells to [3H]-thymidine continuously for long periods of time, it was observed that approximately 26% of the cardiac muscle cells regained the capacity for DNA synthesis during 1-10 days of culture. From day 1 to day 14, approximately 29% of the total muscle cell population was labeled. When the cells were exposed to the radioactive isotope continuously for 30 days, approximately 31% of the cells incorporated radioactive isotope, showing their capacity for DNA synthesis. Approximately 90% of the cardiac muscle cells in long-term culture contained more than one nucleus. The nuclei were often observed in multiples of two. Labeled mitotic apparatus was observed in cardiac myocytes, indicating the replication of DNA, followed by karyokinesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Culture of atrial and ventricular cardiac muscle cells from the adult squirrel monkey Saimiri sciureus

Atrial and ventricular cardiac muscle cells isolated from the adult squirrel monkey Saimiri sciureus were cultured and characterized by light and transmission electron microscopy (TEM). Freshly isolated cells were striated and cylindrical in shape and, when placed in culture, rounded up and lost their highly organized morphology. With prolonged time in culture, they spread out on the surface of the culture flask and reacquired many of the internal ultrastructural characteristics of their in vivo atrial and ventricular cardiac muscle cell counterparts. Autoradiographic experiments indicated that both atrial and ventricular myocytes synthesized DNA when grown in culture. In some binucleated atrial cells only one nucleus became labelled. These studies show that it is now possible to culture cardiac muscle cells isolated from an adult primate.

Genome multiplication in cardiomyocytes of fast- and slow-growing mice

The number of myocytes and the percentage of cells with a high degree of ploidy increased in the heart ventricles of fast-growing mice compared with slow-growing ones. The mean incidence of octa- and hexadecaploid (by summary DNA content) myocytes was 7% in the slow-growing and 23% in the fast-growing, weaned mice. In these groups, the total myocyte number varied by 20%. There were 43% more myocyte genomes in the heart ventricles of the fast-growing mice than in those of the slow-growing mice. The same differences in cell number and ploidy persist in 90-day-old mice in spite of feeding ad libitum after weaning.

The relationship between stress fiber-like structures and nascent myofibrils in cultured cardiac myocytes

The topographical relationship between stress fiber-like structures (SFLS) and nascent myofibrils was examined in cultured chick cardiac myocytes by immunofluorescence microscopy. Antibodies against muscle-specific light meromyosin (anti-LMM) and desmin were used to distinguish cardiac myocytes from fibroblastic cells. By various combinations of staining with rhodamine-labeled phalloidin, anti-LMM, and antibodies against chick brain myosin and smooth muscle alpha-actinin, we observed the following relationships between transitory SFLS and nascent and mature myofibrils: (a) more SFLS were present in immature than mature myocytes; (b) in immature myocytes a single fluorescent fiber would stain as a SFLS distally and as a striated myofibril proximally, towards the center of the cell; (c) in regions of a myocyte not yet penetrated by the elongating myofibrils, SFLS were abundant; and (d) in regions of a myocyte with numerous mature myofibrils, SFLS had totally disappeared. Spontaneously contracting striated myofibrils with definitive Z-band regions were present long before anti-desmin localized in the I-Z-band region and long before morphologically recognizable structures periodically link Z-bands to the sarcolemma. These results suggest a transient one-on-one relationship between individual SFLS and newly emerging individual nascent myofibrils. Based on these and other relevant data, a complex, multistage molecular model is presented for myofibrillar assembly and maturation. Lastly, it is of considerable theoretical interest to note that mature cardiac myocytes, like mature skeletal myotubes, lack readily detectable stress fibers.

Isolation and culture of the terminally differentiated adult mammalian ventricular cardiac muscle cell

We have carried out systematic studies to optimize and standardize methodology to isolate and culture the adult rat ventricular cardiac muscle cell. Four hearts were perfused simultaneously with a calcium-free medium containing collagenase. The ventricular tissue was then minced and further digested to liberate individual cells. Approximately 16 million rod-shaped muscle cells were obtained. The plating efficiency has been greatly improved by culturing the cells in a conditioned medium prepared from a rabbit corneal cell line. This medium also contained added fetal bovine serum, essential and nonessential amino acids, vitamins, insulin, transferrin, and 25 trace minerals. The culture flasks were precoated with rat-tail collagen. Fibroblast contamination was virtually eliminated by including cytosine arabinoside in the medium during the first 7 d of culture. After this time the cells could be cultured in the absence of serum in a chemically defined medium composed of MEM, vitamins, nonessential amino acids, and trace minerals. They continued to contract spontaneously and do well in this medium for at least 3 d thereafter. This improved methodology resulted in a reproducible culture system with improved plating efficiency. It provided a new and unique system to study the structure and function of the adult mammalian ventricular cardiac muscle cell.

Long-term primary cultures of adult human and rat cardiomyocytes

An improved protocol for isolating and culturing adult mammalian cardiomyocytes is presented. Problems of establishing and maintaining cultured adult human and rat cardiomyocytes, and some of their properties, are discussed.

Morphological dedifferentiation of adult cardiac myocytes in coculture with hepatocytes

When adult heart cells are plated on a dish covered with a monolayer of hepatocytes gradual morphological changes are observed. While during the first day the myofibrils are still organized in rod-like shape, later the cells become flat and spread on top of underlying hepatocytes. After two days most cells have a flat, polygonal appearance with spread myofibrillar bundles. At this stage they start spontaneous rhythmic contractions which are characteristic for embryonic myocytes, but not for isolated adult ventricular cells. In this culture myocytes form specific contact structures to adjacent myocytes as well as to hepatocytes. These results demonstrate the phenotypical plasticity of adult heart muscle cells which are believed to be terminally differentiated.