Co-regulation of the atrial natriuretic factor and cardiac myosin light chain-2 genes during alpha-adrenergic stimulation of neonatal rat ventricular cells. Identification of cis sequences within an embryonic and a constitutive contractile protein gene which mediate inducible expression

Potential role of helix-loop-helix proteins in cardiac gene expression

Because helix-loop-helix (HLH) transcription factors appear to play an important role in mesodermal development, we have investigated the potential role of these factors in cardiac gene expression. HLH proteins interact with DNA at consensus "E-box" sites and may be tissue specific or more widely expressed. We have examined cardiac cells for expression and regulation of widely expressed factors Pan1/Pan2 and the inhibitor of differentiation (Id) by RNase protection analysis. The effect of MyoD, Id, and Pan1/Pan2 expression on skeletal and cardiac promoters in cardiac cells was examined by transient cotransfection studies. Our results indicate that neonatal ventricular cells are a functional HLH environment, because MyoD can activate a skeletal muscle-specific promoter in these cells. MyoD, however, has no effect on the expression of several genes that are expressed in cardiac cells. In addition, Id may be an early response gene for signal transduction in cardiac cells, because increases in Id mRNA occurred within 30 minutes of stimulation with serum or phenylephrine. Activities of three cardiac promoter elements in primary ventricular myocytes were not downregulated by Id. Surprisingly, expression of Pan1 and Pan2 exhibited a strong negative effect on cardiac expression of the myosin light chain-2 promoter.

Effect of difluoromethylornithine on atrial natriuretic peptide in rat atria

Polyamines are aliphatic cations known to have a role in cellular growth and differentiation. In this study, difluoromethylornithine (DFMO), an inhibitor of polyamine synthesis, was used to investigate its effect on atrial natriuretic peptide (ANP) in atria of rat. The rats were given DFMO (2%) in drinking water for 10 days and three intraperitoneal administrations (200 mg/kg), 24, 18 and 1 h prior to experiment. Radioimmunoassay of ANP in atrial extracts indicated that DFMO treatment increased ANP contents of atria. ANP from atrial extracts was immunoprecipitated using ANP antibody and the immunoprecipitate was resolved and detected by SDS-PAGE and Western blotting. The predominant ANP peptide in both control and DFMO group migrated at 17 kDa. The synthesis of ANP was studied following the intravenous administration of [35S]methionine. ANP in atrial extracts was immunoprecipitated by ANP antibody. The incorporation of ANP into control and DFMO group peaked at 30 min and returned to a basal level by 60 min. DFMO decreased the incorporation of [35S]methionine into ANP. At 30 min following the administration of [35S]methionine, putrescine restored the synthesis of tricholoroprecipitable atrial proteins, but had no effect on the synthesis of ANP. At 60 min following, the amount of labeled ANP in DFMO + putrescine-treated group was significantly lower than that in DFMO group. These results indicate that polyamines influence both the synthesis and secretion of ANP.

Role of epicardial mesothelial cells in the modification of phenotype and function of adult rat ventricular myocytes in primary coculture

Adult rat ventricular myocytes undergo a well-documented sequence of phenotypic changes during adaptation to primary culture. However, we observed that coculture of myocytes with a specific subset of nonmyocyte cardiac cells could slow and even reverse the process of adaptation. These nonmyocyte cells were isolated and identified by immunohistochemical and ultrastructural criteria as being of epicardial mesothelial origin. When added to long-term primary cultures of adult ventricular myocytes, epicardial mesothelial cells appeared to induce myofibrillar arrays that were more organized than those seen in noncocultured myocytes; these changes that occurred were concurrent with the appearance of large amplitude contractions and multicellular synchronous beating that was facilitated by gap junctions between myocytes and epicardial mesothelial cells. The changes in morphology and function were accompanied by a marked increase in beta-myosin heavy chain isoform transcription in cocultured myocytes, a return to the ratio of cardiac to skeletal alpha-actin expected in adult rat myocardium, and a much reduced expression of smooth muscle alpha-actin. These changes in myocyte phenotype and function appeared to require epicardial cell-myocyte contact, or close apposition, because media conditioned by epicardial mesothelial cells alone or in coculture had no effect. Thus, these rapid and reversible changes in myocyte ultrastructure, function, and gene expression may provide a useful in vitro model with which to study the mechanism responsible for regulating the plasticity of ventricular myocyte phenotype and the role of specific cell-cell interactions.

Microinjection of antibodies and expression vectors into living myocardial cells. Development of a novel approach to identify candidate genes that regulate cardiac growth and hypertrophy

BACKGROUND

Microinjection approaches in the cardiac cell context have allowed delivery of various calcium dyes and monitoring of short-term physiological responses. However, unlike other cell types, it has proved difficult to microinject myocardial cells without the concomitant loss of long-term cell viability.

METHODS AND RESULTS

An analysis of experimental variables was conducted to adapt microinjection techniques to the neonatal rat ventricular cell context. Among the variables optimized were the selection of culture dishes, plating substrate, microinjection parameters, and a variety of maneuvers to inhibit myocyte hypercontracture, injury, and consequent death after micropuncture. With the modified technique, the percentage of injected cells that maintained long-term viability (48 hours) increased from less than 1% to 30%. Similarly, an increased efficiency of gene transfer and expression (measured as the percentage of injected cells that express the delivered gene) was obtained after either cytoplasmic or nuclear injection of a beta-galactosidase expression vector into cardiac myocytes. Microinjection of marker immunoglobulin G does not interfere with the induction of the hypertrophic response or the expression of a coinjected atrial natriuretic factor promoter-luciferase reporter fusion gene construct.

CONCLUSIONS

To the best of our knowledge, this study provides the first description of the efficient microinjection of neonatal cardiac muscle cells with maintenance of long-term cell viability. The microinjection technique is now a viable approach to examine cause-and-effect relations between specific gene products and any defined feature or response of cardiac myocytes that can be assayed at a single-cell level.

Signaling mechanisms for the activation of an embryonic gene program during the hypertrophy of cardiac ventricular muscle

To study the signaling mechanisms which mediate ventricular hypertrophy, we utilized the induction of the ANF gene as a marker of the hypertrophic response. The induction of the atrial natriuretic factor gene (ANF) is one of the most conserved features of ventricular hypertrophy, occurring in multiple species (mouse, rat, hamster, canine, and human) in response to diverse stimuli (hormonal, mechanical, pressure/volume overload, genetic, IHSS, hypertension, etc.). The ANF gene is expressed in both the atrial and ventricular compartments during embryonic development, but shortly after birth ANF expression is down-regulated to negligible levels in the adult myocardium. Since the reactivation of ANF gene expression in the hypertrophied ventricle is a hallmark of the activation of an embryonic gene program, it has also become of interest to determine if similar mechanisms activate ANF expression during hypertrophy and the initial stages of cardiogenesis. A combination of cotransfection, microinjection, and transgenic approaches has been coupled to well characterized cultured cell systems and in vivo murine models employing normal and transgenic mice. The microinjection of oncogenic RAS proteins into living myocardial cells does not lead to the activation of cell proliferation, but activates ANF gene expression, as assessed by immunofluorescence. Co-transfection of mutant and wild-type RAS expression vectors with a ANF-luciferase fusion gene supports a direct effect of activated RAS on ANF gene transcription. Co-transfection of a dominant negative RAS vector effectively inhibits the induction of the ANF gene during alpha adrenergic mediated hypertrophy of ventricular muscle cells, thereby establishing that a RAS-mediated pathway is required for ANF induction.(ABSTRACT TRUNCATED AT 250 WORDS)