MC29-immortalized clonal avian heart cell lines can partially differentiate in vitro

Establishment methods and research progress of livestock and poultry immortalized cell lines: A review

An infinite cell line is one of the most favored experimental tools and plays an irreplaceable role in cell-based biological research. Primary cells from normal animal tissues undergo a limited number of divisions and subcultures in vitro before they enter senescence and die. On the contrary, an infinite cell line is a population of non-senescent cells that could proliferate indefinitely in vitro under the stimulation of external factors such as physicochemical stimulation, virus infection, or transfer of immortality genes. Cell immortalization is the basis for establishing an infinite cell line, and previous studies have found that methods to obtain immortalized cells mainly included physical and chemical stimulations, heterologous expression of viral oncogenes, increased telomerase activity, and spontaneous formation. However, some immortalized cells do not necessarily proliferate permanently even though they can extend their lifespan compared with primary cells. An infinite cell line not only avoids the complicated process of collecting primary cell, it also provides a convenient and reliable tool for studying scientific problems in biology. At present, how to establish a stable infinite cell line to maximize the proliferation of cells while maintaining the normal function of cells is a hot issue in the biological community. This review briefly introduces the methods of cell immortalization, discusses the related progress of establishing immortalized cell lines in livestock and poultry, and compares the characteristics of several methods, hoping to provide some ideas for generating new immortalized cell lines.

Human Induced Pluripotent Stem Cell as a Disease Modeling and Drug Development Platform-A Cardiac Perspective

A comprehensive understanding of the pathophysiology and cellular responses to drugs in human heart disease is limited by species differences between humans and experimental animals. In addition, isolation of human cardiomyocytes (CMs) is complicated because cells obtained by biopsy do not proliferate to provide sufficient numbers of cells for preclinical studies in vitro. Interestingly, the discovery of human-induced pluripotent stem cell (hiPSC) has opened up the possibility of generating and studying heart disease in a culture dish. The combination of reprogramming and genome editing technologies to generate a broad spectrum of human heart diseases in vitro offers a great opportunity to elucidate gene function and mechanisms. However, to exploit the potential applications of hiPSC-derived-CMs for drug testing and studying adult-onset cardiac disease, a full functional characterization of maturation and metabolic traits is required. In this review, we focus on methods to reprogram somatic cells into hiPSC and the solutions for overcome immaturity of the hiPSC-derived-CMs to mimic the structure and physiological properties of the adult human CMs to accurately model disease and test drug safety. Finally, we discuss how to improve the culture, differentiation, and purification of CMs to obtain sufficient numbers of desired types of hiPSC-derived-CMs for disease modeling and drug development platform.

Experimental models of cardiac physiology and pathology

Experimental models of cardiac disease play a key role in understanding the pathophysiology of the disease and developing new therapies. The features of the experimental models should reflect the clinical phenotype, which can have a wide spectrum of underlying mechanisms. We review characteristics of commonly used experimental models of cardiac physiology and pathophysiology in all translational steps including in vitro, small animal, and large animal models. Understanding their characteristics and relevance to clinical disease is the key for successful translation to effective therapies.

Deacetylation of Histone H4 Accompanying Cardiomyogenesis is Weakened in HDAC1-Depleted ES Cells

Cell differentiation into cardiomyocytes requires activation of differentiation-specific genes and epigenetic factors that contribute to these physiological processes. This study is focused on the in vitro differentiation of mouse embryonic stem cells (mESCs) induced into cardiomyocytes. The effects of clinically promising inhibitors of histone deacetylases (HDACi) on mESC cardiomyogenesis and on explanted embryonic hearts were also analyzed. HDAC1 depletion caused early beating of cardiomyocytes compared with those of the wild-type (wt) counterpart. Moreover, the adherence of embryonic bodies (EBs) was reduced in HDAC1 double knockout (dn) mESCs. The most important finding was differentiation-specific H4 deacetylation observed during cardiomyocyte differentiation of wt mESCs, while H4 deacetylation was weakened in HDAC1-depleted cells induced to the cardiac pathway. Analysis of the effect of HDACi showed that Trichostatin A (TSA) is a strong hyperacetylating agent, especially in wt mESCs, but only SAHA reduced the size of the beating areas in EBs that originated from HDAC1 dn mESCs. Additionally, explanted embryonic hearts (e15) responded to treatment with HDACi: all of the tested HDACi (TSA, SAHA, VPA) increased the levels of H3K9ac, H4ac, H4K20ac, and pan-acetylated lysines in embryonic hearts. This observation shows that explanted tissue can be maintained in a hyperacetylation state several hours after excision, which appears to be useful information from the view of transplantation strategy and the maintenance of gene upregulation via acetylation in tissue intended for transplantation.

Experimental Models of Cardiovascular Diseases: Overview

Cardiovascular disease is one of the most common causes of deaths in clinics. Experimental models of cardiovascular diseases are essential to understand disease mechanism, to provide accurate diagnoses, and to develop new therapies. Large numbers of experimental models have been proposed and replicated by many laboratories in the past. Models with significant advantages are chosen and became more popular. Particularly, feasibility, reproducibility, and human disease resemblance are the common key factors for frequently used cardiovascular disease models. In this chapter, we provide a brief overview of these experimental models used for in vitro, in vivo, and in silico studies of cardiovascular diseases.

In vitro cellular models for cardiac development and pharmacotoxicology

Permanent cultures of cardiac cells described so far have limited value for studying cell biology and pharmacology of the developing heart because of the loss of proliferative capacity and cardiac-specific properties of cardiomyocytes during long-term cultivation. Pluripotent embryonic carcinoma (EC) and embryonic stem (ES) cells cultivated as permanent lines offer a new approach for studying cardiogenic differentiation in vitro. We describe cardiogenesis in vitro by differentiating EC and ES cells by way of embryo-like aggregates (embryoid bodies) into spontaneously beating cardiomyocytes. During cardiomyocyte differentiation three distinct developmental stages were defined by expression of specific action potentials and ionic currents measured by the whole-cell patch-clamp technique. Whereas early differentiated cardiomyocytes are characterized by action potentials and ionic currents typical for early pacemaker cells, terminally differentiated cardiomyocytes show action potentials and ionic currents inherent to ventricular-, atrial- or sinus nodal-like cells. These functional characteristics are in accordance with the expression of alpha- and beta-cardiac myosin heavy chain at early differentiation stages and the additional expression of ventricular-specific MLC-2V and atrial-specific ANF genes at terminal stages demonstrated by reverse transcription polymerase chain reaction (RT-PCR) analysis. Pharmacological studies performed by measuring chronotropic responses and by analysing the Ca(2+) channel activity correspond to data obtained with cardiac cells from living organisms. For testing the influence of exogenous compounds on cardiac differentiation the teratogenic compound retinoic acid (RA) was applied during distinct stages of embryoid body development. A temporally controlled influence of RA on cardiac differentiation and expression of cardiac-specific genes was found. We conclude that ES cell-derived cardiomyocytes provide an excellent cellular model to study early cardiac development and to perform pharmacological and embryotoxicological investigations.

Cell Lines as In Vitro Models for Drug Screening and Toxicity Studies

Cell culture is highly desirable, as it provides systems for ready, direct access and evaluation of tissues. The use of tissue culture is a valuable tool to study problems of clinical relevance, especially those related to diseases, screening, and studies of cell toxicity mechanisms. Ready access to the cells provides the possibility for easy studies of cellular mechanisms that may suggest new potential drug targets and, in the case of pathological-derived tissue, it has an interesting application in the evaluation of therapeutic agents that potentially may treat the dysfunction. However, special considerations must be addressed to establish stable in vitro function. In primary culture, these factors are primarily linked to greater demands of tissue to adequately survive and develop differentiated conditions in vitro. Additional requirements include the use of special substrates (collagen, laminin, extracellular matrix preparations, etc.), growth factors and soluble media supplements, some of which can be quite complex in their composition. These demands, along with difficulties in obtaining adequate tissue amounts, have prompted interest in developing immortalized cell lines which can provide unlimited tissue amounts. However, cell lines tend to exhibit problems in stability and/or viability, though they serve as a feasible alternative, especially regarding new potential applications in cell transplant therapy. In this regard, stem cells may also be a source for the generation of various cell types in vitro. This review will address aspects of cell culture system application, with focus on immortalized cell lines, in studying cell function and dysfunction with the primary aim being to identify cell targets for drug screening.

A role for Tbx5 in proepicardial cell migration during cardiogenesis

Transcriptional regulatory cascades during epicardial and coronary vascular development from proepicardial progenitor cells remain to be defined. We have used immunohistochemistry of human embryonic tissues to demonstrate that the TBX5 transcription factor is expressed not only in the myocardium, but also throughout the embryonic epicardium and coronary vasculature. TBX5 is not expressed in other human fetal vascular beds. Furthermore, immunohistochemical analyses of human embryonic tissues reveals that unlike their epicardial counterparts, delaminating epicardial-derived cells do not express TBX5 as they migrate through the subepicardium before undergoing epithelial-mesenchymal transformation required for coronary vasculogenesis. In the chick, Tbx5 is expressed in the embryonic proepicardial organ (PEO), which is composed of the epicardial and coronary vascular progenitor cells. Retrovirus-mediated overexpression of human TBX5 inhibits cell incorporation of infected proepicardial cells into the nascent chick epicardium and coronary vasculature. TBX5 overexpression as well as antisense-mediated knockdown of chick Tbx5 produce a cell-autonomous defect in the PEO that prevents proepicardial cell migration. Thus, both increasing and decreasing Tbx5 dosage impairs development of the proepicardium. Culture of explanted PEOs demonstrates that untreated chick proepicardial cells downregulate Tbx5 expression during cell migration. Therefore, we propose that Tbx5 participates in regulation of proepicardial cell migration, a critical event in the establishment of the epicardium and coronary vasculature.

DSCAM: an endogenous promoter drives expression in the developing CNS and neural crest

The development of central nervous system (CNS) neuronal networks involves processes including neuroblast migration, axonal pathfinding, and synaptogenesis. To evaluate the role of the axonal guidance molecule DSCAM in CNS connectivity, we generated a lacZ reporter construct, Pr1.8-betagal, containing a 1.8kb fragment of the human DSCAM promoter region, and analyzed its expression in four E12.5 transgenic mouse embryos. We found that Pr1.8-betagal drives lacZ expression in the choroid plexus and roof of the fourth ventricle, the floor plate of the fourth ventricle, pons and medulla oblongata, and the eye, limb buds, and dorsal root ganglion. This recapitulates a subset of DSCAM expression as demonstrated by in situ hybridization, supporting this 1.8kb fragment as a component of the endogenous DSCAM promoter. The Pr1.8-betagal expression pattern supports a role for DSCAM in CNS development, providing an endogenous promoter to investigate the contribution of DSCAM to Down syndrome neural defects.

TBX5 transcription factor regulates cell proliferation during cardiogenesis

Mutations in human TBX5, a member of the T-box transcription factor gene family, cause congenital cardiac septation defects and isomerism in autosomal dominant Holt-Oram syndrome. To determine the cellular function of TBX5 in cardiogenesis, we overexpressed wild-type and mutant human TBX5 isoforms in vitro and in vivo. TBX5 inhibited cell proliferation of D17 canine osteosarcoma cells and MEQC quail cardiomyocyte-like cells in vitro. Mutagenesis of the 5' end of the T-box but not the 3' end of the T-box abolished this effect. Overexpression of TBX5 in embryonic chick hearts showed that TBX5 inhibits myocardial growth and trabeculation. TBX5 effects in vivo were abolished by Gly80Arg missense mutation of the 5' end of the T-box. PCNA analysis in transgenic chick hearts revealed that TBX5 overexpression does suppress embryonic cardiomyocyte proliferation in vivo. Inhibitory effects of TBX5 on cardiomyocyte proliferation include a noncell autonomous process in vitro and in vivo. TBX5 inhibited proliferation of both nontransgenic cells cocultured with transgenic cells in vitro and nontransgenic cardiomyocytes in transgenic chick hearts with mosaic expression of TBX5 in vivo. Immunohistochemical studies of human embryonic tissues, including hearts, also demonstrated that TBX5 expression is inversely related to cellular proliferation. We propose that TBX5 can act as a cellular arrest signal during vertebrate cardiogenesis and thereby participate in modulation of cardiac growth and development.

Optimal lipofection reagent varies with the molecular modifications of the DNA

Cationic lipid reagents differ in their cytofection efficacy with different cell types. No evidence has addressed whether the same lipid reagent is best for different DNAs in a single cell line. Immortalized avian embryonic cardiomyocytes cultured in vitro were tested with 15 cationic lipid reagents using (A) a beta-gal expression plasmid, (B) a fluorescein-tagged, phosphorothioate-modified ODN B, (C) a fluorescein-tagged, ethoxy-modified ODN C with the same nucleotide sequence as ODN B, and (D) a fluorescein-tagged, phosphorothioate-modified ODN D with a different nucleotide sequence from ODNs B and C. Cytofection was scored as percent of cells expressing beta-gal activity or showing diffuse cellular fluorescence. The best lipid reagents for the phosphorothioate-modified ODNs were ODN-specific and markedly different from the best lipid reagents for the expression plasmid or for the ethoxy-modified ODN. These results suggest that the best cationic lipid reagent for a particular cell type varies with the physical and chemical form of the DNA being transfected into the cells.

HL-1 cells: a cardiac muscle cell line that contracts and retains phenotypic characteristics of the adult cardiomyocyte

We have derived a cardiac muscle cell line, designated HL-1, from the AT-1 mouse atrial cardiomyocyte tumor lineage. HL-1 cells can be serially passaged, yet they maintain the ability to contract and retain differentiated cardiac morphological, biochemical, and electrophysiological properties. Ultrastructural characteristics typical of embryonic atrial cardiac muscle cells were found consistently in the cultured HL-1 cells. Reverse transcriptase-PCR-based analyses confirmed a pattern of gene expression similar to that of adult atrial myocytes, including expression of alpha-cardiac myosin heavy chain, alpha-cardiac actin, and connexin43. They also express the gene for atrial natriuretic factor. Immunohistochemical staining of the HL-1 cells indicated that the distribution of the cardiac-specific markers desmin, sarcomeric myosin, and atrial natriuretic factor was similar to that of cultured atrial cardiomyocytes. A delayed rectifier potassium current (IKr) was the most prominent outward current in HL-1 cells. The activating currents displayed inward rectification and deactivating current tails were voltage-dependent, saturated at >>+20 mV, and were highly sensitive to dofetilide (IC50 of 46.9 nM). Specific binding of [3H]dofetilide was saturable and fit a one-site binding isotherm with a Kd of 140 +/- 60 nM and a Bmax of 118 fmol per 10(5) cells. HL-1 cells represent a cardiac myocyte cell line that can be repeatedly passaged and yet maintain a cardiac-specific phenotype.

Cloning and sequencing of a developmentally regulated avian mRNA containing the LEA motif found in plant seed proteins

We report the cloning of a bromodeoxyuridine (BrdU)-sensitive transcript of 918 bp from an immortalized quail heart cell line containing an open reading frame (ORF) of 215 amino acids (aa) (approximately 23 kDa). Analysis of the secondary structure predicts two amphipathic alpha-helices with oppositely oriented amphipathic surfaces at the C-terminus of the protein. Each of the helices contains an LEA (late embryogenesis abundant) consensus sequence (A/TAEKAK/RETKD) which has been previously described only in a group of plant seed-specific proteins. Temporal and spatial distribution patterns of the transcript during chick embryo development were examined by whole-mount in situ hybridization and Northern blot analysis. At H&H (Hamburger and Hamilton, 1951) stages 11-14, the message was expressed strongly in blood islands in the area opaca. At day 5, strong signals were found in the liver primordia, mesonephrons, and nephric duct. Frozen sections of whole mount-stained embryonic liver demonstrated that the message was restricted to developing blood cells. The expression pattern of this transcript suggests that its protein product may be involved in hematopoiesis during avian development.

Expression of avian glypican is developmentally regulated

An avian cDNA homologue of human and rat glypicans has been cloned from a stage 17 chicken heart cDNA library and used to analyze the distribution of this proteoglycan during development by Northern analysis and whole mount in situ hybridization. At stages 7-12, strong signals were detected in the cephalic region of the neural folds, rostral portion of paraxial mesoderm, and newly formed epithelial somites. At stages 20-25, strong expression was observed in the mantle zone of the telencephalon, the apical epidermal ridge and proximal region of developing limb. Transcripts also were found in the truncus arteriosus and arteriovenous-canal region of the heart, but not in the myocardium. This distribution pattern suggests that the avian glypican may be involved in the morphogenesis of limb, somite, heart, and brain. The expression of glypican also overlaps FGFs in limb bud, FGF receptors in heart and somite, and NGF receptors in forebrain. The affinity of heparan sulfate proteoglycans for growth factors and the distribution of the avian glypican are consistent with a role for this molecule in growth factor-mediated signals.

Establishment of the mesodermal cell line QCE-6. A model system for cardiac cell differentiation

The QCE-6 cell line was derived from precardiac mesoderm of the Japanese quail. As previously reported, these cells are able to differentiate into two distinct cardiac cell types with myocardial or endocardial endothelial cell properties. This present communication describes in detail the derivation of this cell line and further characterizes the nontreated and induced myocardial and endothelial phenotypes of these cells. The QCE-6 cells exhibit an epithelial morphology, as well as the pattern of protein expression, that is characteristic of precardiac mesoderm. Treatment with retinoic acid, basic fibroblast growth factor (bFGF), transforming growth factor (TGF)-beta 2, and TGF-beta 3 induces these cells to differentiate and produce mixed cultures of epithelial and mesenchymal cells. The epithelial cells express myosin, desmin, and cardiac troponin I in a punctate pattern throughout the cytoplasm. These sarcomeric proteins become organized in a premyofibrillar pattern when TGF-beta 1, platelet-derived growth factor (PDGF)-BB, and insulin-like growth factor (IGF) II are added in combination along with retinoic acid, bFGF, TGF-beta 2, and TGF-beta 3. Also, these treatments induce Na+,K(+)-ATPase expression. When the QCE-6 cells are cultured on collagen type I, the mesenchymal cells that are promoted by retinoic acid, bFGF, TGF-beta 2, and TGF-beta 3 will invade the gel. These mesenchymal cells are positive for QH1 and JB3, which are both markers for presumptive endocardial cells within the early cardiogenic mesoderm. The addition of both PDGF-BB and IGF II to QCE-6 cell cultures will inhibit the ability of retinoic acid, bFGF, TGF-beta 2, and TGF-beta 3 to induce both the mesenchymal morphology and QH1 and JB3 expression. Collectively, these results suggest that the proces of cardiac cell differentiation is regulated by multiple signals and that early cardiogenic mesoderm contains a bipotential stem cell that can give rise to both the myocardial and endocardial lineages. More important, since the QCE-6 cells are representative of early cardiogenic cells, this cell line offers a unique model system to study cardiac cell differentiation.

Differential localization of cytoplasmic myosin II isoforms A and B in avian interphase and dividing embryonic and immortalized cardiomyocytes and other cell types in vitro

Two principal isoforms of cytoplasmic myosin II, A and B (CMIIA and CMIIB), are present in different proportions in different tissues. Isoform-specific monoclonal and polyclonal antibodies to avian CMIIA and CMIIB reveal the cellular distributions of these isoforms in interphase and dividing embryonic avian cardiac, intestinal epithelial, spleen, and dorsal root ganglia cells in primary cell culture. Embryonic cardiomyocytes react with antibodies to CMIIB but not to CMIIA, localize CMIIB in stress-fiber-like-structures during interphase, and markedly concentrate CMIIB in networks in the cleavage furrow during cytokinesis. In contrast, cardiac fibroblasts localize both CMIIA and CMIIB in stress fibers and networks during interphase, and demonstrate slight and independently regulated concentration of CMIIA and CMIIB in networks in their cleavage furrows. V-myc-immortalized cardiomyocytes, an established cell line, have regained the ability to express CMIIA, as well as CMIIB, and localize both CMIIA and CMIIB in stress fibers and networks in interphase cells and in cleavage furrows in dividing cells. Conversely, some intestinal epithelial, spleen, and dorsal root ganglia interphase cells express only CMIIA, organized primarily in networks. Of these, intestinal epithelial cells express both CMIIA and CMIIB when they divide, whereas some dividing cells from both spleen and dorsal root ganglia express only CMIIA and concentrate it in their cleavage furrows. These results suggest that within a given tissue, different cell types express different isoforms of CMII, and that cells expressing either CMIIA or CMIIB alone, or simultaneously, can form a cleavage furrow and divide.

Infection with the avian polyomavirus, BFDV, selectively affects myofibril structure in embryonic chick ventricle cardiomyocytes

Embryonic cardiomyocytes can both beat and divide. They assemble cardiac muscle-specific proteins into sarcomeric myofibrils and contract. In addition, they periodically synthesize DNA, complete mitosis, disassemble sarcomeric myofibrils in the area of the mitotic spindle, assemble cytoplasmic isoform-specific proteins into a cleavage furrow contractile ring, undergo cytokinesis, and then reform sarcomeric myofibrils in daughter cells. Little is known about how embryonic cardiomyocytes disassemble their myofibrils as they traverse the cell cycle and divide. In the present study, beating embryonic avian ventricular cardiomyocytes in primary culture were stimulated to initiate DNA synthesis without subsequent mitosis or cytokinesis by infection with the lytic avian polyomavirus, Budgerigar Fledgling Disease Virus (BFDV). Within 48 hours, infected, adherent cardiomyocytes disassemble most of their sarcomeric myofibrils, retaining cardiac myosin only in thin myofibrils with disrupted sarcomeric periodicity and in amorphous nonfibrillar pools. By 72 hours, infected cardiomyocytes contain no myofibrils and no longer react with antibodies to cardiac myosin. In contrast, infected cardiomyocytes continue to display cytoplasmic myosin localized in stress-fiber-like-structures in adherent cells, or in disrupted fibers and dispersed pools in detaching cells. Infected cardiomyocytes also continue to display interphase-like arrays of polymerized microtubules, even when rounded-up just prior to lysis. These results suggest that polyomavirus infection may provide a useful model system for further study of the regulation of myofibrils disassembly in embryonic cardiomyocytes.

Patterns of integration and expression of retroviral, non-replicative vectors in avian embryos: embryo developmental stage and virus subgroup envelope modulate tissue-tropism

We previously demonstrated that Avian Leukemia Viruses (ALV) carrying the v-myc gene specifically induce two types of tumors, cardiomyocytic tumors when the virus is injected before embryonic day 3 (E3), skin tumors when the virus is injected at E3 or E5. Aiming to elucidate the mechanisms which determine this time-dependent change in target, we infected chick and quail embryos at E3 and E5 with replication-deficient, lacZ gene-carrying, ALV-based viruses produced by a packaging cell line. Three constructs driven by 3 different Long Terminal Repeats (LTRs) were tested and yielded similar results. When the constructs were inoculated at E3 and the lacZ gene product revealed 5 days later, around 70% of the embryos carried lacZ+ clones in the heart, around 50% had positive clones in the skin anywhere on the body, while a few embryos displayed clones in internal organs (liver, stomach, lungs). Immunocytological identification of the heart cell type(s) expressing the virus revealed that the only cells infected were cardiomyocytes. When the constructs were inoculated at E5, no lacZ+ clones appeared in the heart but all were located in the cephalic skin. In order to examine the relationship between viral integration and expression, DNA of different organs or tissues from lacZ stained embryos was analyzed by PCR. A tight correlation between integration and expression in the heart and in the skin was revealed in most cases. In contrast, a significant PCR signal was often detected in the liver or the stomach despite weak or absent expression as revealed by lacZ+ clones. We then investigated the influence of envelope glycoprotein subgroups on the tropism of these constructs. The lacZ vector driven by RAV-2 LTRs was packaged as subgroups A, B or E viral particles. The A subgroup, used in the part of the study described above, infects both chick and quail while the B and E subgroups are specific for chick or quail respectively. These B and E subgroups induced lacZ+ clones in the heart (after E3 injection) while no clones or only a few were detected in the skin either after E3 or E5 injection. The following conclusions can be drawn: 1) cardiomyocytes are at E3 the major target for integration and expression of ALV-derived viruses in vivo; 2) targets change rapidly with embryonic age; and 3) tissue-specific infections depend on the envelope subgroup, thus presumably on the presence of the cognate receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac growth factors

The role of polypeptide growth factors in cardiovascular ontogeny, function, and pathologic states is poorly understood. Recent investigations demonstrate that the myocardium produces both known and novel growth factors, which are highly regulated during development and disease, and have suggested that peptide growth factors may direct cardiac organogenesis and adaptation. Aspects of growth factor production, transduction, and action in myocardium are distinct to the cardiac muscle lineage and were not foreseen from results in simpler systems. Transforming growth factor beta 1 and fibroblast growth factors (FGFs) selectively up-regulate an ensemble of tissue-specific genes associated with the fetal myocardium. One of these, encoding the skeletal muscle isoform of alpha-actin, is activated by basic FGF yet is inhibited by acidic FGF. A serum response element of this gene is selectively induced, in cardiac myocytes, by basic FGF but not acidic FGF. Thus, cardiac muscle is an especially intriguing model for the analysis of growth factor signalling pathways that control differentiated gene transcription.