Changes in gene expression during cardiac growth

Disease severity-related alterations of cardiac microRNAs in experimental pulmonary hypertension

Right ventricular (RV) failure is the primary cause of death in pulmonary arterial hypertension (PAH). We hypothesized that heart‐relevant microRNAs, that is myomiRs (miR‐1, miR‐133a, miR‐208, miR‐499) and miR‐214, can have a role in the right ventricle in the development of PAH. To mimic PAH, male Wistar rats were injected with monocrotaline (MCT, 60 mg/kg, s.c.); control group received vehicle. MCT rats were divided into two groups, based on the clinical presentation: MCT group terminated 4 weeks after MCT administration and prematurely terminated group (ptMCT) displaying signs of terminal disease. Myocardial damage genes and candidate microRNAs expressions were determined by RT‐qPCR. Reduced blood oxygen saturation, breathing disturbances, RV enlargement as well as elevated levels of markers of myocardial damage confirmed PH in MCT animals and were more pronounced in ptMCT. MyomiRs (miR‐1/miR‐133a/miR‐208a/miR‐499) were decreased and the expression of miR‐214 was increased only in ptMCT group (P < 0.05). The myomiRs negatively correlated with Fulton index as a measure of RV hypertrophy in MCT group (P < 0.05), whereas miR‐214 showed a positive correlation (P < 0.05). We conclude that the expression of determined microRNAs mirrored the disease severity and targeting their pathways might represent potential future therapeutic approach in PAH.

Gender Modulates the Expression of Calcium-Regulating Proteins in Pediatric Atrial Myocardium

A differential expression of sarcoplasmic reticulum calcium-ATPase (SERCA2a) and phospholamban (PLB) characterizes the remodeling process in heart failure and atrial arrhythmias in adult patients. Gender is known to modulate the course and Prognosis of different forms of heart disease. We hypothesized that gender plays a role in molecular changes of myocardial calcium regulating components already in childhood. Moreover, we studied the influence of volume overloaded (VO) on SERCA2a and PLB in pediatric patients. Quantitative reverse transcription-polymerase chain reaction was used to measure mRNA expression of SERCA2a and PLB in atrial myocardium from 30 pediatric patients (12 girls, 18 boys). Eighteen patients had VO right atria, and 12 patients had not-overloaded atria (NO). Protein expression was studied by Western blot. In the entire population, SERCA2a and PLB expression was not different between girls and boys. If hemodynamic overload was taken into account, SERCA2a mRNA expression was significantly reduced in the VO group compared with the NO group (P = 0.021). The VO versus NO difference was restricted to toys, which corresponds to a highly significant interaction of gender versus VO status (P = 0.002). The PLB to SERCA2a Protein ratio was significantly lower in girls (P = 0.028). The decrease in SERCA2a mRNA expression in VO atrial myocardium and the PLB to SERCA2a ratio of protein expression was modulated by gender in this pediatric population. To our knowledge, this study is the first to show the impact of gender on the differential expression of calcium-regulating components in Pediatric cardiac patients.

Fibroblast growth factor receptor 1 signaling in adult cardiomyocytes increases contractility and results in a hypertrophic cardiomyopathy

Fibroblast growth factors (FGFs) and their receptors are highly conserved signaling molecules that have been implicated in postnatal cardiac remodeling. However, it is not known whether cardiomyocyte-expressed FGF receptors are necessary or sufficient for ventricular remodeling in the adult heart. To determine whether cardiomyocytes were competent to respond to an activated FGF receptor, and to determine if this signal would result in the development of hypertrophy, we engineered a doxycycline (DOX)-inducible, cardiomyocyte-specific, constitutively active FGF receptor mouse model (αMHC-rtTA, TRE-caFgfr1-myc). Echocardiographic and hemodynamic analysis indicated that acute expression of caFGFR1 rapidly and directly increased cardiac contractility, while chronic expression resulted in significant hypertrophy with preservation of systolic function. Subsequent histologic analysis showed increased cardiomyocyte cross-sectional area and regions of myocyte disarray and fibrosis, classic features of hypertrophic cardiomyopathy (HCM). Analysis of downstream pathways revealed a lack of clear activation of classical FGF-mediated signaling pathways, but did demonstrate a reduction in Serca2 expression and troponin I phosphorylation. Isolated ventricular myocytes showed enhanced contractility and reduced relaxation, an effect that was partially reversed by inhibition of actin-myosin interactions. We conclude that adult cardiomyocytes are competent to transduce FGF signaling and that FGF signaling is sufficient to promote increased cardiomyocyte contractility in vitro and in vivo through enhanced intrinsic actin-myosin interactions. Long-term, FGFR overexpression results in HCM with a dynamic outflow tract obstruction, and may serve as a unique model of HCM.

Altered in vivo left ventricular torsion and principal strains in hypothyroid rats

The twisting and untwisting motions of the left ventricle (LV) lead to efficient ejection of blood during systole and filling of the ventricle during diastole. Global LV mechanical performance is dependent on the contractile properties of cardiac myocytes; however, it is not known how changes in contractile protein expression affect the pattern and timing of LV rotation. At the myofilament level, contractile performance is largely dependent on the isoforms of myosin heavy chain (MHC) that are expressed. Therefore, in this study, we used MRI to examine the in vivo mechanical consequences of altered MHC isoform expression by comparing the contractile properties of hypothyroid rats, which expressed only the slow β-MHC isoform, and euthyroid rats, which predominantly expressed the fast α-MHC isoform. Unloaded shortening velocity (V(o)) and apparent rate constants of force development (k(tr)) were measured in the skinned ventricular myocardium isolated from euthyroid and hypothyroid hearts. Increased expression of β-MHC reduced LV torsion and fiber strain and delayed the development of peak torsion and strain during systole. Depressed in vivo mechanical performance in hypothyroid rats was related to slowed cross-bridge performance, as indicated by significantly slower V(o) and k(tr), compared with euthyroid rats. Dobutamine infusion in hypothyroid hearts produced smaller increases in torsion and strain and aberrant transmural torsion patterns, suggesting that the myocardial response to β-adrenergic stress is compromised. Thus, increased expression of β-MHC alters the pattern and decreases the magnitude of LV rotation, contributing to reduced mechanical performance during systole, especially in conditions of increased workload.

A family of microRNAs encoded by myosin genes governs myosin expression and muscle performance

Myosin is the primary regulator of muscle strength and contractility. Here we show that three myosin genes, Myh6, Myh7, and Myh7b, encode related intronic microRNAs (miRNAs), which, in turn, control muscle myosin content, myofiber identity, and muscle performance. Within the adult heart, the Myh6 gene, encoding a fast myosin, coexpresses miR-208a, which regulates the expression of two slow myosins and their intronic miRNAs, Myh7/miR-208b and Myh7b/miR-499, respectively. miR-208b and miR-499 play redundant roles in the specification of muscle fiber identity by activating slow and repressing fast myofiber gene programs. The actions of these miRNAs are mediated in part by a collection of transcriptional repressors of slow myofiber genes. These findings reveal that myosin genes not only encode the major contractile proteins of muscle, but act more broadly to influence muscle function by encoding a network of intronic miRNAs that control muscle gene expression and performance.

Determination of rate constants for turnover of myosin isoforms in rat myocardium: implications for in vivo contractile kinetics

The ventricles of small mammals express mostly alpha-myosin heavy chain (alpha-MHC), a fast isoform, whereas the ventricles of large mammals, including humans, express approximately 10% alpha-MHC on a predominately beta-MHC (slow isoform) background. In failing human ventricles, the amount of alpha-MHC is dramatically reduced, leading to the hypothesis that even small amounts of alpha-MHC on a predominately beta-MHC background confer significantly higher rates of force development in healthy ventricles. To test this hypothesis, it is necessary to determine the fundamental rate constants of cross-bridge attachment (f(app)) and detachment (g(app)) for myosins composed of 100% alpha-MHC or beta-MHC, which can then be used to calculate twitch time courses for muscles expressing variable ratios of MHC isoforms. In the present study, rat skinned trabeculae expressing either 100% alpha-MHC or 100% beta-MHC were used to measure ATPase activity, isometric force, and the rate constant of force redevelopment (k(tr)) in solutions of varying Ca(2+) concentrations. The rate of ATP utilization was approximately 2.5-fold higher in preparations expressing 100% alpha-MHC compared with those expressing only beta-MHC, whereas k(tr) was 2-fold faster in the alpha-MHC myocardium. From these variables, we calculated f(app) to be approximately threefold higher for alpha-MHC than beta-MHC and g(app) to be twofold higher in alpha-MHC. Mathematical modeling of isometric twitches predicted that small increases in alpha-MHC significantly increased the rate of force development. These results suggest that low-level expression of alpha-MHC has significant effects on contraction kinetics.

Pivotal role of cardiac lineage protein-1 (CLP-1) in transcriptional elongation factor P-TEFb complex formation in cardiac hypertrophy

OBJECTIVE

Our aim was to determine if the expression pattern of CLP-1 in developing heart is consistent with its role in controlling RNA transcript elongation by transcriptional elongation factor b (P-TEFb) and if the inhibitory control exerted over P-TEFb by CLP-1 is released under hypertrophic conditions.

METHODS

We performed immunoblot and immunofluorescence analysis of CLP-1 and the P-TEFb components cdk9 and cyclin T in fetal mouse heart and 2 day post-natal mouse cardiomyocytes to determine if they are co-localized. We induced hypertrophy in rat cardiomyocytes either by mechanical stretch or treatment with hypertrophic agents such as endothelin-1 and phenylephrine to determine if CLP-1 is released from P-TEFb in response to hypertrophic stimuli. The involvement of the Jak/STAT signal transduction pathway in this process was studied by blocking this pathway with the Jak2 kinase inhibitor, AG490, and assessing the association of CLP-1 with P-TEFb complexes.

RESULTS

We found that CLP-1 is expressed along with P-TEFb components in developing heart during the period in which knockout mice lacking the CLP-1 gene develop cardiac hypertrophy and die. Under conditions of hypertrophy induced by mechanical stretch or agonist treatment, CLP-1 dissociates from the P-TEFb complex, a finding consistent with the de-repression of P-TEFb kinase activity seen in hypertrophic cardiomyocytes. Blockage of Jak/STAT signaling by AG490 prevented release of CLP-1 from P-TEFb despite the ongoing presence of hypertrophic stimulation by mechanical stretch.

CONCLUSIONS

CLP-1 expression in developing heart and isolated post-natal cardiomyocytes colocalizes with P-TEFb expression and therefore has the potential to regulate RNA transcript elongation by controlling P-TEFb cdk9 kinase activity in heart. We further conclude that the dissociation of CLP-1 from P-TEFb is responsive to hypertrophic stimuli transduced by cellular signal transduction pathways. This process may be part of the genomic stress response resulting in increased RNA transcript synthesis in hypertrophic cardiomyocytes.

Role of myosin heavy chain composition in the stretch activation response of rat myocardium

The speed and force of myocardial contraction during systolic ejection is largely dependent on the intrinsic contractile properties of cardiac myocytes. As the myosin heavy chain (MHC) isoform of cardiac muscle is an important determinant of the contractile properties of individual myocytes, we studied the effects of altered MHC isoform expression in rat myocardium on the mechanical properties of skinned ventricular preparations. Skinned myocardium from thyroidectomized rats expressing only the beta MHC isoform displayed rates of force redevelopment that were about 2.5-fold slower than in myocardium from hyperthyroid rats expressing only the alpha MHC isoform, but the amount of force generated at a given level of Ca2+ activation was not different. Because recent studies suggest that the stretch activation response in myocardium has an important role in systolic function, we also examined the effect of MHC isoform expression on the stretch activation response by applying a rapid stretch (1% of muscle length) to an otherwise isometrically contracting muscle fibre. Sudden stretch of myocardium resulted in a concomitant increase in force that quickly decayed to a minimum and was followed by a delayed redevelopment of force (i.e. stretch activation) to levels greater than prestretch force. beta MHC expression dramatically slowed the overall rate of the stretch activation response compared to expression of alpha MHC isoform; specifically, the rate of force decay was approximately 2-fold slower and the rate of delayed force development was approximately 2.5-fold slower. In contrast, MHC isoform had no effect on the amplitude of the stretch activation response. Collectively, these data show that expression of beta MHC in myocardium dramatically slows rates of cross-bridge recruitment and detachment which would be expected to decrease power output and contribute to depressed systolic function in end-stage heart failure.

Developmental regulation of cardiac MAP4 protein expression

It has been shown that the level of expression of microtubule-associated protein 4 (MAP4) mRNAs changes throughout neonatal heart development [Chapin SJ, et al. 1995. Biochemistry 34:2289]. In the present study, both immunofluorescence and western blotting methods were used to monitor MAP4 protein expression levels in the developing heart. By both methods, it was shown that the levels of total MAP4 protein were maximal during the first postnatal week, and then declined progressively to adulthood. In addition, four major electrophoretic species that reacted with MAP4-specific antibodies (called bands 1-4) were observed in all heart tissue samples. Three of the four bands decreased in abundance throughout postnatal development, but at different rates. The fourth band remained relatively constant in abundance with increasing postnatal age. To determine if phosphorylation events might contribute to this heterogeneity, western blotting experiments using phospho-specific antibodies and phosphatase digestion of extract samples were performed. No phosphorylation-specific antibody staining was observed and no significant changes were demonstrated in the bands after phosphatase treatment, implying that the observed complexity was due mainly to alternative start site or differential isoform expression. Finally, it was discovered that cardiomyocyte MAP4 associated with drug- and cold-stable microtubules in early neonatal myocytes. Thus, the complex regulation of MAP4 protein expression may play a key role in the functional differentiation of myocyte microtubules during heart development.

Phenotypic plasticity of adult myocardium: molecular mechanisms

Cardiac phenotypic plasticity (so-called cardiac remodelling, CR) is characterized by changes in myocardial structure that happen in response to either mechanical overload or a loss of substance such as that occurring after myocardial infarction.

Mechanosensation is a widespread biological process and is inextricably mixed with other transduction systems from hormones and vasopeptides, which ultimately produce post-translational modifications of transcription factors. The expression of the four main transcription factors during cardiogenesis is also enhanced as a link to foetal reprogramming.

CR results from re-expression of the foetal programme, which is mostly adaptive, but also from several other phenotypic modifications that are not usually adaptive, such as fibrosis. (i) The initial determinant is mechanical,and re-expression of the foetal programme includes a global increase in genetic expression with cardiac hypertrophy, re-expression of genes that are normally not expressed in the adult ventricles, repression of genes not expressed during the foetal life, and activation of pre-exisiting stem cells. Microarray technology has revealed a coordinated change in expression of genes pertaining to signal transduction, metabolic function, structure and motility,and cell organism defence. The physiological consequence is a better adapted muscle. (ii) During clinical conditions, the effects of mechanics are modified by several interfering determinants that modify CR, including senescence,obesity, diabetes, ischemia and the neurohormonal reaction. Each of these factors can alter myocardial gene expression and modify molecular remodelling of mechanical origin.

Finally, as compared to evolutionary phenotypic plasticity described in plants and insects in response to variations in environmental conditions, in CR, the environmental factor is internal, plasticity is primarily adaptive,and it involves coordinated changes in over 1400 genes. Study of reaction norms showed that the genotypes from different animal species are similarly plastic, but there are transgenic models in which adaptation to mechanics is not caused by hypertrophy but by qualitative changes in gene expression.

Ablation of the CLP-1 gene leads to down-regulation of the HAND1 gene and abnormality of the left ventricle of the heart and fetal death

We have recently reported that cardiac lineage protein-1 (CLP-1), a nuclear protein with an acidic region that constitutes a potential protein-protein interaction domain, regulates transcription of the cardiac myosin light chain-2v (MLC-2v) gene promoter in a manner consistent with its being a transcriptional co-activator or regulator. To test the postulate that CLP-1 is a regulator of cardiac genes we ablated the CLP-1 gene in mice. Past embryonic day (E)16.5, CLP-1 null alleles did not show Mendelian inheritance suggesting that absence of CLP-1 was lethal in late fetal stages. CLP-1 (-/-) fetal hearts exhibited a reduced left ventricular chamber with thickened myocardial walls, features suggestive of cardiac hypertrophy. Electron microscopic analysis of E16.5 CLP-1 (-/-) ventricular myocardium showed a marked decline in cell density and altered nuclear and myofibril morphologies similar to that seen in animal models of hypertrophic heart. Analysis of contractile and non-contractile protein genes known to be re-expressed during cardiac hypertrophy showed them to have higher expression levels in CLP-1 (-/-) hearts thereby confirming the hypertrophic phenotype at the molecular level. Analysis of cardiac development genes showed that expression of the HAND1 transcription factor, a gene involved in patterning of the heart tube and down-regulated in hypertrophic hearts, was also significantly reduced in CLP-1 (-/-) fetal hearts. CLP-1 and HAND1 have similar expression patterns in the developing heart ventricles. These data suggest that CLP-1 and the HAND transcription factors may be part of a genetic program critical to proper heart development, perturbation of which can lead to cardiomyopathy.

Transforming growth factor beta in cardiovascular development and function

Transforming growth factor betas (TGFbetas) are pleiotropic cytokines involved in many biological processes. Genetic engineering and tissue explanation studies have revealed specific non-overlapping roles for TGFbeta ligands and their signaling molecules in development and in normal function of the cardiovascular system in the adult. In the embryo, TGFbetas appear to be involved in epithelial-mesenchymal transformations (EMT) during endocardial cushion formation, and in epicardial epithelial-mesenchymal transformations essential for coronary vasculature, ventricular myocardial development and compaction. In the adult, TGFbetas are involved in cardiac hypertrophy, vascular remodeling and regulation of the renal renin-angiotensin system. The evidence for TGFbeta activities during cardiovascular development and physiologic function will be given and areas which need further investigation will be discussed.

Adenosine A1 and A2A receptor cross-talk during ageing in the rat myocardium

Adenosine (Ado), a naturally occurring autacoid, exerts cardioprotective effects against myocardial ischemia and reperfusion injury, through activation of its receptors type 1 (A1) and 2A (A2A). Since ageing involves a complex change in these effects, we evaluated A1 and A2A gene expression in left (LV) and right ventricle (RV) from 2-, 5-, 12-, and 21-month-old Sprague-Dawley rats. LV end-diastolic (EDD) and end-systolic (ESD) internal dimensions (mm) and LV fractional shortening (FS, %) were measured by M-mode echocardiography. Senescence was associated with a reduction in FS (42+/-1, 38+/-2, 39+/-2 and 35+/-2, in 2-, 5-, 12- and 21-month-old rats; p<0.02) and increases in EDD (7.5+/-0.2, 8.1+/-0.2, 8.5+/-0.2 and 8.8+/-0.2; p<0.001) and ESD (4.2+/-0.1, 4.4+/-0.2, 4.7+/-0.2 and 5.1+/-0.2; p=0.002). Ado A1 mRNA levels were highest in 12 and 21-month-old animals in both ventricles (LV: p<0.001; RV: p=0.001). By contrast, Ado A2A gene expression was lower in the aged LV (p<0.001), but higher in the aged RV (p<0.001). These modifications of Ado receptor gene expression and especially the increase in A1 receptor mRNA may partially explain the stronger antiadrenergic effects of Ado in the senescent heart.

Expression of protein phosphatases during postnatal development of rabbit heart

Protein phosphatases play a major role in the regulation of L-type calcium current (I(Ca)) in heart cells. We previously showed developmental differences in the effects of inhibitors of protein phosphatases (PP's) on the modulation of I(Ca), with greater stimulatory effects on I(Ca) observed in newborn than in adult ventricular cells. We hypothesized that this developmental difference might be due to greater expression and levels of PP 1 and PP 2A in newborn than in adult ventricular cells. We thus determined the mRNA expression of alpha and beta subunits of PP 1 and the a subunit of PP 2A in adult and newborn rabbit ventricles and levels of PP 1 and PP 2A in total homogenates, particulate membranes, and in soluble fraction prepared from isolated ventricular myocytes from adult and newborn rabbits. RT-PCR analysis demonstrated the presence of mRNA of these subunits of PP's in both newborn and adult ventricles. Northern blot analysis using 32P labeled cDNA probes specific for PP 1alpha, PP 1beta and PP 2Aalpha showed that the expression of steady state mRNA levels for PP 1alpha, PP 1beta and PP 2Aalpha were much higher in newborn compared to adult rabbit ventricles. mRNA for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and for sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) in rabbit ventricles were measured as controls. GAPDH did not show significant developmental changes while mRNA for SERCA was higher in adult compared to newborns. Western blot analysis showed that PP 1 and PP 2A protein levels were also much higher in newborn compared to adult rabbit ventricular cells. Immunoblot analysis in particulate membranes and soluble fraction showed that PP1 was mainly membrane bound while PP 2 was present only in soluble fraction. These findings suggest that the two major protein phosphatases (PP 1 and PP 2A) in heart are expressed at much higher levels in newborn and decline to lower levels in adult ventricular myocytes. The presence of high levels of PP's and particularly PP 1 in newborn cells may be responsible for the greater dependence of newborn cells on the inhibition of PP as a mechanism of action of beta-agonist isoproterenol on I(Ca).

TGF-beta1 mediates the hypertrophic cardiomyocyte growth induced by angiotensin II

Angiotensin II (Ang II), a potent hypertrophic stimulus, causes significant increases in TGFb1 gene expression. However, it is not known whether there is a causal relationship between increased levels of TGF-beta1 and cardiac hypertrophy. Echocardiographic analysis revealed that TGF-beta1-deficient mice subjected to chronic subpressor doses of Ang II had no significant change in left ventricular (LV) mass and percent fractional shortening during Ang II treatment. In contrast, Ang II-treated wild-type mice showed a >20% increase in LV mass and impaired cardiac function. Cardiomyocyte cross-sectional area was also markedly increased in Ang II-treated wild-type mice but unchanged in Ang II-treated TGF-beta1-deficient mice. No significant levels of fibrosis, mitotic growth, or cytokine infiltration were detected in Ang II-treated mice. Atrial natriuretic factor expression was approximately 6-fold elevated in Ang II-treated wild-type, but not TGF-beta1-deficient mice. However, the alpha- to beta-myosin heavy chain switch did not occur in Ang II-treated mice, indicating that isoform switching is not obligatorily coupled with hypertrophy or TGF-beta1. The Ang II effect on hypertrophy was shown not to result from stimulation of the endogenous renin-angiotensis system. These results indicate that TGF-beta1 is an important mediator of the hypertrophic growth response of the heart to Ang II.

Application of cDNA microarrays in determining molecular phenotype in cardiac growth, development, and response to injury

BACKGROUND

Normal myocardial development and the tissue response to cardiac stress are accompanied by marked changes in gene expression; however, the extent of these changes and their significance remain to be fully explored. We used cDNA microarrays for gene expression profiling in rat cardiac tissue samples to study developmental transitions and the response to myocardial infarction (MI).

METHODS AND RESULTS

Microarrays with rat cDNAs for 86 known genes and 989 anonymous cDNAs obtained by molecular subtraction (representational difference analysis) of mRNA from sham-operated and 6-week post-MI samples were used in 2-color hybridization experiments. Twelve known genes previously associated with myocardial development were identified together with 10 uncharacterized expressed sequence tags and 36 genes not previously associated with cardiac development. After MI, genes associated with myocardial stress and wound healing exhibited differences in magnitude and expression kinetics, and 14 genes not previously associated with MI were identified. In situ hybridization revealed mRNA localization characteristic of wound healing and vascular and cardiomyocyte reactivity.

CONCLUSIONS

Tissue analysis of gene expression with cDNA microarrays provides a measure of transcriptional or posttranscriptional regulation and cellular recruitment. Our results demonstrate the complexity of gene regulation in the developing myocardium and show that cDNA microarrays can be used to monitor the evolution of the cardiac stress-inducible phenotype.

Fibroblast growth factor-2 mediates pressure-induced hypertrophic response

In vitro, fibroblast growth factor-2 (FGF2) has been implicated in cardiomyocyte growth and reexpression of fetal contractile genes, both markers of hypertrophy. However, its in vivo role in cardiac hypertrophy during pressure overload is not well characterized. Mice with or without FGF2 (Fgf2(+/+) and Fgf2(-/-), respectively) were subjected to transverse aortic coarctation (AC). Left ventricular (LV) mass and wall thickness were assessed by echocardiography preoperatively and once a week postoperatively for 10 weeks. In vivo LV function during dobutamine stimulation, cardiomyocyte cross-sectional area, and recapitulation of fetal cardiac genes were also measured. AC Fgf2(-/-) mice develop significantly less hypertrophy (4-24% increase) compared with AC Fgf2(+/+) mice (41-52% increase). Cardiomyocyte cross-sectional area is significantly reduced in AC Fgf2(-/-) mice. Noncoarcted (NC) and AC Fgf2(-/-) mice have similar beta-adrenergic responses, but those of AC Fgf2(+/+) mice are blunted. A lack of mitotic growth in both AC Fgf2(+/+) and Fgf2(-/-) hearts indicates a hypertrophic response of cardiomyocytes. Consequently, FGF2 plays a major role in cardiac hypertrophy. Comparison of alpha- and beta-cardiac myosin heavy chain mRNA and protein levels in NC and AC Fgf2(+/+) and Fgf2(-/-) mice indicates that myosin heavy chain composition depends on hemodynamic stress rather than on FGF2 or hypertrophy, and that isoform switching is transcriptionally, not posttranscriptionally, regulated.

Interaction between the renin-angiotensin system and insulin-like growth factor I in aorto-caval fistula-induced cardiac hypertrophy in rats

The effects of angiotensin converting enzyme inhibition and angiotensin II receptor blockade on the development of cardiac hypertrophy and myocardial insulin-like growth factor I (IGF-I) in volume overload were studied in male Wistar rats with aorto-caval fistulas (ACF). Rats were treated with ramipril (RAM, 3 mg kg(-1) day(-1)) for 4-20 days or losartan (LOS, 10 mg kg(-1) day(-1)) for 2-7 days. Myocardial IGF-I and IGF-I receptor (IGF-I-R) mRNA were determined by solution hybridization. ACF caused hypertrophy of left (LV) and right ventricles (RV). Hypertrophy appeared on day 2 and reached maximal values of +60% in LV and +75% in RV at day 12. Systolic blood pressure was initially reduced 15% but recovered by day 12. RAM abolished the recovery of blood pressure. Furthermore, RAM attenuated RV hypertrophy by 17% on day 7 and on day 20, RV weights were close to values found in controls. Beginning on day 9, RAM reduced LV weight back to control levels in parallel to blood pressure. In contrast, LOS affected neither RV nor LV hypertrophy. RV IGF-I mRNA increased 60-100% on day 7 alone in RV in ACF. RAM potentiated the increase in RV IGF-I to +400% and induced an increase in RV IGF-I-R mRNA on day 7 (+90%) in ACF. LOS did not affect RV IGF-I. Development of cardiac hypertrophy in ACF seemed independent of angiotensin II. RV hypertrophy was associated with activation of IGF-I independent of the renin-angiotensin system. IGF-I was further potentiated when development of hypertrophy was attenuated, possibly indicative of a greater urge for compensational growth in a relatively thinner and more volume-distended chamber.

Molecular mechanisms of myocardial remodeling

"Remodeling" implies changes that result in rearrangement of normally existing structures. This review focuses only on permanent modifications in relation to clinical dysfunction in cardiac remodeling (CR) secondary to myocardial infarction (MI) and/or arterial hypertension and includes a special section on the senescent heart, since CR is mainly a disease of the elderly. From a biological point of view, CR is determined by 1 ) the general process of adaptation which allows both the myocyte and the collagen network to adapt to new working conditions; 2) ventricular fibrosis, i.e., increased collagen concentration, which is multifactorial and caused by senescence, ischemia, various hormones, and/or inflammatory processes; 3) cell death, a parameter linked to fibrosis, which is usually due to necrosis and apoptosis and occurs in nearly all models of CR. The process of adaptation is associated with various changes in genetic expression, including a general activation that causes hypertrophy, isogenic shifts which result in the appearance of a slow isomyosin, and a new Na+-K+-ATPase with a low affinity for sodium, reactivation of genes encoding for atrial natriuretic factor and the renin-angiotensin system, and a diminished concentration of sarcoplasmic reticulum Ca2+-ATPase, beta-adrenergic receptors, and the potassium channel responsible for transient outward current. From a clinical point of view, fibrosis is for the moment a major marker for cardiac failure and a crucial determinant of myocardial heterogeneity, increasing diastolic stiffness, and the propensity for reentry arrhythmias. In addition, systolic dysfunction is facilitated by slowing of the calcium transient and the downregulation of the entire adrenergic system. Modifications of intracellular calcium movements are the main determinants of the triggered activity and automaticity that cause arrhythmias and alterations in relaxation.

Cellular distribution of Ca2+ pumps and Ca2+ release channels in rat cardiac hypertrophy induced by aortic stenosis

BACKGROUND

The response of ventricular myocytes to pressure overload is heterogeneous and not spatially coordinated. We investigated whether or not the alterations in SERCA and RyR gene expression are homogeneous within the myocardium.

METHODS AND RESULTS

The cellular distribution of mRNAs and proteins encoding the 2 sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) isoforms (SERCA 2a and 2b) and 2 Ca2+ release channels (the ryanodine receptor, RyR, and the IP3 receptor, IP3R) were analyzed by in situ hybridization and immunofluorescence, respectively. Analyses were performed during early (1 and 5 days) and late (1 month) stages of cardiac hypertrophy induced in rat by thoracic aortic stenosis (AS). The results indicated that 1 and 5 days after AS, the cellular distribution of SERCA 2a and RyR2 mRNAs in right ventricle and atrium was similar to controls but the mRNA levels appeared to decrease in some areas of the left ventricle (LV). One month after AS, the distribution of SERCA 2a mRNA and protein became heterogeneous throughout the LV, whereas RyR2 mRNA and protein levels were decreased in a homogeneous manner. SERCA 2b, poorly expressed in both cardiomyocytes and vessels of controls, was increased 4-fold 1 month after AS in coronary arteries only. In both sham (Sh) and AS, SERCA 3 and IP3R mRNAs were mainly found in the vessels.

CONCLUSIONS

In severe hypertrophy, decreased accumulation of SERCA 2a was heterogeneous and not compensated by an induction of SERCA 2b in the cardiomyocytes. Decrease in RyR2 expression was more homogeneous and not compensated by an increased IP3R expression.

Translocation of 60S ribosomal subunit in spreading cardiac myocytes

Cardiac myocytes in culture undergo considerable structural reorganization. The remodeling of the myofibrils and the nonmyofibrillar cytoskeleton that occurs in the spreading cardiac myocytes resembles the cellular features observed in the hypertrophying heart. In this study we examined the distribution of the large 60S ribosomal subunit in freshly isolated cardiac myocytes and during the course of attachment and spreading in culture. Initially, anti-60S immunolabeling was scattered widely throughout the sarcoplasm of the dissociated cardiac myocytes. After attachment to the substrate, the 60S ribosomal subunit attained wide sarcoplasmic localization before a sarcomere-related staining pattern appeared in the spreading cell. Double labeling experiments with alpha-actinin confirmed co-localization of the 60S ribosomal subunit with nascent and mature myofibrils. These findings demonstrate that translocation of the 60S ribosomal subunit coincides with the cytoskeletal reorganization taking place in these cells. Moreover, the close association between the myofibrils indicates a particular role for the ribosomes in maintenance and growth of the contractile apparatus. (J Histochem Cytochem 46:963-969, 1998)

Regional activation of the immediate-early response gene c-fos in infarcted rat hearts

Regional infarction of the left ventricle is followed by hypertrophy of the viable myocardium. This compensatory growth of cardiac myocytes requires induction of gene transcription and synthesis of proteins. In this study, we examined the expression of the immediate-early response gene c-fos following ligation of the left coronary artery in rat hearts. RNase protection assay demonstrated a rapid increase in the c-fos mRNA level in the ventricular myocardium. After two days of infarction, the c-fos expression was attenuated and was comparable to that observed in sham-operated control hearts. In situ tissue distribution of Fos protein-like immunoreactivity revealed the appearance of positively stained cells adjacent to the lateral border of the ischaemic myocardium, in the left ventricular subendocardial areas, in the papillary muscles of the left ventricle, in the proximity of great transmural vessels, and focally in the normo-perfused subepicardial myocardium. Double staining using antibodies recognizing the Fos protein and alpha-actinin, confirmed that the accumulation of nuclear Fos protein-like immunoreactivity was mainly seen in the cardiac myocytes. However, double staining of the Fos protein and Hoechst DNA labelling showed that detectable immunoreactivity occurred only in a limited proportion of the total nuclei present in these myocardial regions. Moreover, the regions showing c-fos activation correspond to the areas in which the appearance of subsequent growth responses are most pronounced following myocardial infarction. The present results therefore indicate that an early and regional c-fos activation is taking place in viable cardiac myocytes following left coronary artery ligation, and that c-fos is a possible regulating factor of sequential events leading to altered pattern of gene expression and protein synthesis in the hypertrophying heart.

Cardiac phosphocreatine deficiency induced by GPA during postnatal development in rat

The effect of chronic administration of beta-guanidinopropionic acid (GPA) on the protein profiling, energy metabolism and right ventricular (RV) function was studied in the rat heart during the weaning and adolescence period. GPA was given in tap water (1-1.5%) using pair drink controls. The feeding of animals with GPA solution for a six week period resulted in elevation of heart to body weight ratio due to body growth retardation. GPA accumulated in the myocardium up to 67.37 +/- 5.3 mumoles.g dry weight and the tissue content of total creatine, phosphocreatine and ATP was significantly decreased to 15%, 9% and 65% of control values respectively. Total activity of creatine kinase (CK) was not changed, but the proportion of mitochondrial (Mi) CK isoenzyme was decreased; the percentage of MB isoenzyme of CK was significantly higher. GPA treatment resulted in an elevation of the content of cardiac collagenous proteins and decrease of non-collagenous proteins in the heart; in parallel, a decrease of the collagen I to collagen III ratio was detected. The function of the RV was assessed using an isolated perfused heart with RV performing pressure-volume work. As compared to pair-drink controls, RV function was significantly impaired the GPA group: at any given right atrial filling pressure, the RV systolic pressure and the rate of pressure development were decreased by almost a factor of two. Elevation of the RV diastolic pressure with increasing pulmonary artery diastolic pressure was also significantly steeper in the GPA group which also showed decrease of cardiac output, especially at high outflow resistance. It may be assumed that chronic administration of GPA deeply influenced metabolic parameters, protein profiles and contractile function of the developing heart. On the other hand, concentrations of glucose, total lipids and triglycerides in blood plasma were not affected. All these data confirm the concept that the CK system is of central importance both for heart function and for the regulation of normal growth of cardiac myocytes.

Localization of proliferating cell nuclear antigen in the developing and mature rat heart cell

BACKGROUND

The cardiac muscle cell ceases to divide shortly after birth; this cessation is followed by a limited period when DNA synthesis and karyokinesis occur without cytokinesis. The regulation of this process is not known. The purpose of this study is to explore the possible events that could lead to the cessation of cardiac muscle cell division. One protein requisite for DNA synthesis is proliferating cell nuclear antigen (PCNA). This protein is the auxiliary protein of DNA polymerase delta.

METHODS

Rats of fetal age day 18 or days 0, 4, 8, 12, and 16 after birth were obtained. In addition, adult hearts were used for this study. Hearts from the fetal day-18 rats and the day-0 neonatal rats were digested. Cardiac myocytes were isolated and placed in culture for an analysis of DNA synthesis by using tridiated thymidine. Ventricular muscle tissue was isolated from hearts of all ages and frozen in liquid nitrogen for Northern and Western blot analyses.

RESULTS

Tridiated thymidine analysis revealed that, although serum stimulation significantly increased the number of labeled fetal cardiac muscle cells, it did not have that effect on neonatal cardiac muscle cells in culture. Northern blot analysis revealed that the steady state levels of mRNA for PCNA remained constant from fetal day 18 through day 4 after birth. Steady state levels declined during the second postnatal week and then reached basal levels by day 16. PCNA message was still present in adult heart tissue. By using indirect immunofluorescence and Western blotting, PCNA protein could be located in the nucleus of cardiac muscle cells during the first 2 weeks after birth. At 16 days after birth, the protein was found in the cytoplasm in very low amounts but was not found in the nucleus. The protein was barely detectable by Western blotting in the cytoplasmic fraction from the adult myocardium.

CONCLUSIONS

The results of this study suggest that the PCNA message and protein product declined after birth, but both were present at low levels in the adult myocardium. However, the PCNA protein was not translocated to the nucleus in adult myocardial cells. The events involving PCNA correlated closely with the time period when cell division and then DNA synthesis ceased in these cells.

Time-dependent decreases of atrial natriuretic peptide release from the isolated rat atrium: evidence for a readily releasable pool

In vitro, atrial distension causes a rapid increase in atrial natriuretic peptide (ANP) release. This stretch-induced release, however, declines to baseline levels within minutes without significant depletion of the total hormone stores. It has been observed that the basal rate of ANP release from isolated atria also declines over time despite evidence that the tissue retains its viability. We examined this time-dependency of ANP release from isolated rat atria and some parameters that may explain the diminishing release. Mean ANP secretion was 60 pg/min for both spontaneously beating and electrically paced preparations. Although ANP secretion steadily declined over time, there was no time-dependent effect on the amplitude of intracellularly recorded action potentials. The total ANP content in atria obtained after dissection was 133 +/- 28.9 micrograms/g (n = 3) which was not significantly different from atria that were perfused for 3 h (137 +/- 21.2 micrograms/g; n = 3). Only the 28 amino acid circulating form of ANP was released. The ANP mRNA appeared to be partially degraded in atria after 30 min equilibration or after perfusion for 3 h. These results demonstrated that ANP release from isolated atrial preparations declines steadily despite the maintenance of normal electrophysiological activity. This decline was not due to significant depletion of the ANP stores suggesting that a readily releasable pool of ANP exists and represents only a small fraction of the total hormone stores. Finally, degradation of ANP mRNA implies a reduction of de novo synthesis in our preparation which suggests that the observed depletion of the releasable pool was related to a decline in newly synthesized ANP.

Decrease in beta 1-adrenergic and M2-muscarinic receptor mRNA levels and unchanged accumulation of mRNAs coding for G alpha i-2 and G alpha s proteins in rat cardiac hypertrophy

During compensatory cardiac hypertrophy in the rat, hemodynamic overload induces a parallel decrease in the densities of both beta 1-adrenergic (beta 1-AR) and M2-muscarinic (M2-MR) receptors in the left ventricle, but the total number of receptors remains unchanged. It is not known whether this reduction is transcriptionally or translationally regulated, or if the functionally closely linked alpha-subunits of G protein (G alpha s and G alpha i-2) partake in this regulation. In order to resolve these questions, the absolute concentrations of mRNAs for both receptors and for G alpha s and G alpha i-2 were quantified by slot blot analysis of the left ventricles of adult rats 5 weeks after aortic banding. The results showed a significant decrease of both receptor mRNA levels in hypertrophied left ventricle (beta 1-AR: -48%; M2-MR: -42%) that paralleled the reduction in receptor protein densities and was negatively correlated with the left ventricular weight/body weight ratio (LVW/BW). By contrast, the relative levels of G alpha s and G alpha i-2 mRNAs remained unchanged, and both accumulated proportionally to the increase in LVW/BW. These results show that the beta 1-AR and the M2-MR were pretranslationally regulated. This suggests the hypothesis that the corresponding genes do not follow the general increase in transcriptional activity. By contrast, the genes coding for G alpha s and G alpha i-2 may follow the general pattern of activation during hypertrophy. Receptors and coupling proteins belong to two different groups of genes that are controlled by distinct mechanisms of regulation.

The standard electrocardiogram as a screening test for hypertrophic cardiomyopathy

Phenotypic heterogeneity in hypertrophic cardiomyopathy (HC) makes definitive diagnosis difficult, particularly during family screening. We studied the electrocardiogram (ECG) as a potential initial screening test in patients with HC. Using accepted diagnostic criteria, we examined the ECGs and echocardiograms of 159 patients with a confirmed clinical or genetic diagnosis of HC. An abnormal ECG was found in 154 patients (97%) while only 146 (92%) showed an abnormal echocardiogram. Of the former, 9 patients (6%) had normal echocardiograms and had been diagnosed on the basis of identification of a mutation in the beta myosin heavy chain gene (n = 8) or obligate carrier status (n = 1). Only 1 of these 9 patients was under age 20, the time at which hypertrophy is normally expressed on the echocardiogram. The remaining 5 patients (3%) without ECG abnormality consisted of 1 patient with an echocardiogram clearly diagnostic of HC and 4 clinically normal patients (aged 13, 24, 29, and 33 years) with normal echocardiograms who had been diagnosed by mutation identification (n = 3) or obligate carrier status (n = 1). Thus only these latter 4 patients (3%) would not have been diagnosed as having HC based on an abnormal ECG and/or abnormal echocardiogram. Screening relatives for HC by ECG criteria alone detects all those whom an echocardiogram will diagnose. While echocardiography aids in the specificity of HC diagnosis, the ECG, within the context of a family with a proven case of HC, is a more sensitive marker of the disease. It is therefore both a cost-effective and useful tool for screening those to proceed to echocardiography.

Adaptational response in transcription factors during development of myocardial hypertrophy

Cardiac hypertrophy is characterized, among others, by the molecular events which selectively activate the expression of genes for contractile proteins within individual myocardial cells. As such, myosin light chain 2 (MLC-2), which is upregulated in the hypertrophic state in both rat and human, serves as a marker for hypertrophy. In an attempt to investigate the gene regulatory mechanisms of this phenomenon, we tested the hypothesis that certain transcription factors are directly involved in the development of cardiac hypertrophy by demonstrating the presence of cardiac tissue-specific regulatory elements in the 5'-flanking region of the MLC-2 promoter and testing them in the gel mobility shift assay for their binding activity to nuclear proteins from hypertrophied and normal cardiac tissue. In nuclear extracts from the ventricular tissues of the spontaneously hypertensive rat (SHR), distinctive changes in two families of activator proteins, the A/T-rich DNA-binding transcription factors, myocyte enhancer factor (MEF-2) and CArG-binding factor, manifested in a developmentally dictated manner paralleling the evolution of cardiac hypertrophy in these animals. Extracts isolated from brains and skeletal muscle tissues from the same animals did not exhibit the changes in binding activity. Also, the changes were not apparent when a distal negative regulatory element (CSS), which confers cardiac-specific expression, was tested in gel mobility shift assays. The ubiquitous TATA-binding proteins remained unchanged in comparing SHR with the control strain WKY in the same assay. These data support the notion that the expression of specific transcription factors is modulated in response to hypertrophy related signals which execute changes at the gene level effecting the enrichment of certain contractile proteins in an effort discrete and estranged from the basal transcription machinery.

Type V, but not type VI, adenylyl cyclase mRNA accumulates in the rat heart during ontogenic development. Correlation with increased global adenylyl cyclase activity

Type V and VI adenylyl cyclase mRNAs are the two main cyclase isoforms expressed in the mammalian heart. A recent report has shown that their expression is differentially regulated during ontogenic development, but the accumulation of the two mRNA species and their concentration ratio have not been determined. We thus determined the accumulation and the relative amounts of type V and VI adenylyl cyclase mRNA in fetal, neonatal and adult rat hearts, using a sensitive ribonuclease protection assay. In 18-day-old fetuses, the two adenylyl cyclase mRNA isoforms were weakly expressed in approximately equal amounts (type V mRNA/type VI mRNA = 0.93 +/- 0.09). Further development was characterized by a sharp increase in type V adenylyl cyclase mRNA (x 1.9 in neonates v fetuses, P < 0.01; x 2.4 and x 4.5 in adults v neonates and fetuses, respectively, P < 0.01 for both comparisons) and a slight, non-significant fall in type VI mRNA (P = 0.16). As a result, the type V mRNA/type VI mRNA ratio was 2.86 +/- 0.57 and 9.09 +/- 1.21 in neonatal hearts and adult ventricles, respectively (P < 0.01 v ratio in fetal hearts for both comparisons; P < 0.01 for ratio in adult ventricles v ratio in neonatal hearts), and the overall amount of the two mRNA isoforms was 2.3 times greater in adult than in fetal hearts (P < 0.01). This increase was paralleled by an increase in basal and isoproterenol- and forskolin-stimulated adenylyl cyclase activities in adult hearts compared to fetal and neonatal hearts (P < 0.01 for the three comparisons). Our results demonstrate that type V adenylyl cyclase mRNA accumulates in the rat heart after birth to become the highly predominant isoform in the adult heart. They further suggest that the increase in cardiac adenylyl cyclase activity observed during rat development is due to this accumulation.

Fibronectin expression during physiological and pathological cardiac growth

Fibronectin (FN) is a dimeric glycoprotein found in the extracellular matrix of most tissues that serves as a bridge between cells and the interstitial collagen meshwork and influences diverse processes including cell growth, adhesion, migration, and wound repair. Multiple FN forms arise by the alternative splicing of a primary transcript originating from a single gene. The spatial and temporal alterations in FN expression in the myocardium has been studied in models of cardiac growth in vivo such as fetal development, and hypertrophy secondary to pressure overload. This review focuses on the differential expression of FN isoforms that are observed in different models of cardiac growth. Using a combination of qualitative and quantitative analyses it is shown that in the rat myocardium: (1) the FN phenotype is developmentally regulated, (2) the re-expression of the fetal FN isoforms is observed in different models of cardiac hypertrophy secondary to a sudden or progressive hypertension and (3) the changes in cardiac FN expression affect mostly the coronary artery smooth muscle cells.

Is the senescent heart overloaded and already failing?

Heart failure mainly occurs during the last decades of life, and it is important to know if the senescent heart is not an already failing heart. During aging, both contraction and relaxation of papillary muscle are impaired. Such an impairment is compensated in vivo and the cardiac output remains normal. In spite of a loss in myocytes, the heart weight/body weight ratio is unchanged, but the myocytes are bigger. Arrhythmias are permanent and are accompanied by a loss of the normal heart rate variability. Changes in specific mRNAs include: a shift in myosin heavy chain (MHC) isogene expression leading to an increased beta MHC content; decreased densities of Ca2+ ATPase of the sarcoplasmic reticulum, beta 1-adrenergic receptor, and muscarinic receptors; and attenuation of the Na+/Ca2+ exchange activity. Most of these changes, but not all, resemble those observed during cardiac overload and are accompanied by an increased duration of both the action potential and the intracellular calcium transient. However, the senescent heart is still able to further modify its phenotype in response to mechanical overload. The senescent heart is a diseased heart, and the origin of the "disease" is multifactorial and includes the general process of senescence, hormonal changes, and the myocardial consequences of senescence of the vessels.

Compartmentation of creatine kinases during perinatal development of mammalian heart

Maturation of the cardiac cell is characterized by increasing diversity of isozymic expression of creatine kinases. Expression of the M-CK isozyme always precedes that of mitochondrial isozyme (mi-CK), however the expression of an isoform does not inform about its localization or cellular function. The functional role of isozymes binding to sites of energy utilization and production characteristic of the adult myocardium can be evidenced by the functional coupling of M-CK to myofibrillar ATPase and mito-CK to translocase in Triton X-100 and saponin skinned fibers. Functional activity of M-CK and mito-CK were investigated during perinatal development. Both functional activities appear during late fetal life in species mature at birth like guinea pig, and in the first postnatal weeks in immature species like rat or rabbit. Thus, the functional activity of bound CK isozymes is not associated with birth per se but with the general process of cell maturation. Localization of CK in the cytosol appears optimal for the transfer of glycolytic production of ATP to sites of utilization in an immature heart. During cell maturation, the increasing contribution of oxidative phosphorylation to ATP production, the apparition and binding of mi-CK to mitochondria, the binding of M-CK to myofibrils, turn the cell in a compartmentalized system of energy production. This provides the cellular basis for energy transfer by the PCr-Cr-CK system between sites of ATP production and utilization. Compartmentation of both Ca handling and energy turnover leads to a highly structured cell organization and could be essential for the efficiency of heart function.

Activation of myoD gene transcription by 3,5,3'-triiodo-L-thyronine: a direct role for the thyroid hormone and retinoid X receptors

Thyroid hormones are major determinants of skeletal muscle differentiation in vivo. Triiodo-L-thyronine treatment promotes terminal muscle differentiation and results in increased MyoD gene transcription in myogenic cell lines; furthermore myoD and fast myosin heavy chain gene expression are activated in rodent slow twitch muscle fibers (Molecular Endocrinology 6: 1185-1194, 1992; Development 118: 1137-1147, 1993). We have identified a T3 response element (TRE) in the mouse MyoD promoter between nucleotide positions -337 and -309 (5' CTGAGGTCAGTACAGGCTGGAGGAGTAGA 3'). This sequence conferred an appropriate T3 response to an enhancerless SV40 promoter. In vitro binding studies showed that the thyroid hormone receptor alpha (TR alpha) formed a heterodimeric complex, with either the retinoid X receptor alpha or gamma 1 isoforms (RXR alpha, RXR gamm), on the MyoD TRE that was specifically competed by other well characterised TREs and not by other response elements. Analyses of this heterodimer with a battery of steroid hormone response elements indicated that the complex was efficiently competed by a direct repeat of the AGGTCA motif separated by 4 nucleotides as predicted by the 3-4-5 rule. EMSA experiments demonstrated that the nuclear factor(s) present in muscle cells that bound to the myoD TRE were constitutively expressed during myogenesis; this complex was competed by the myosin heavy chain, DR-4 and PAL-0 TREs in a sequence specific fashion. Western blot analysis indicated that TR alpha 1 was constitutively expressed during C2C12 differentiation. Mutagenesis of the myoD TRE indicated that the sequence of the direct repeats (AGGTCA) and the 4 nucleotide gap were necessary for efficient binding to the TR alpha/RXR alpha heterodimeric complex. In conclusion our data suggest that the TRE in the helix loop helix gene, myoD, is a target for the direct heterodimeric binding of TR alpha and RXR alpha/gamma. These results provide a molecular mechanism/model for the effects of triiodo-L-thyronine on in vitro myogenesis; the activation of myoD gene expression in the slow twitch fibres and the cascade of myogenic events regulated by thyroid hormone.

Cardiac and skeletal myopathy in beta myosin heavy-chain simian virus 40 tsA58 transgenic mice

The mechanisms regulating cardiac muscle differentiation and development are incompletely understood. To examine the relationships between cardiocyte proliferation and differentiation, we tested the ability of a fragment from the rat beta myosin heavy-chain (MHC beta) gene to correctly target expression of a thermolabile simian virus 40 large tumor antigen allele (tsA58) in the developing mouse. Transgene expression in the heart was observed as early as 10 days postconception and was developmentally regulated in parallel with the endogenous MHC beta gene. Expression was also detected in developing skeletal muscle, although at low levels. Despite the temperature sensitivity of the mutant large tumor antigen protein, a subset of transgenic mice in several lineages developed marked cardiac and skeletal myopathies.

Regional appearance of atrial natriuretic peptide in the ventricles of infarcted rat hearts

The appearance of atrial natriuretic peptide (ANP) in the ventricular myocardium was investigated in rat hearts subjected to severe left ventricular infarction. The left coronary artery was ligated for 1, 2, 3, 4 and 6 days and for 3 weeks, and the tissue was prepared for microscopic examination of immunoreactive ANP and for electron microscopy. In the normal and sham-operated hearts, and in hearts subjected to 1 day of coronary ligation, ANP immunoreactivity was restricted to a few ventricular myocytes of the conduction system. Following 2-3 days of coronary ligation, ANP immunoreactivity was detected in the viable myocardium of the lateral border of the infarct and in a few layers of viable cardiac myocytes located in the subendocardial areas of the ischemic left free ventricular wall. Further, during the following days and after 3 weeks of coronary ligation, a gradient of specific labeling was commonly seen across the lateral border area of the infarct. Thus, the strongest immunoreactivities were present in the cardiac myocytes located adjacent to the non-contracting myocardium. Electron microscopic examination of the immunoreactive cardiac myocytes confirmed the presence of electron-dense specific granules within these cells. The present findings suggest that the increased regional production of ANP within the ventricular myocardium is induced by increased mechanical stretch of the cardiac myocytes, and that this might contribute to the increased release of ANP in myocardial infarction.

Calponin and SM 22 as differentiation markers of smooth muscle: spatiotemporal distribution during avian embryonic development

Calponin and SM 22 are two proteins related in sequence that are particularly abundant in smooth muscle cells. Here, the distribution patterns of calponin and SM 22 were compared with that of other smooth muscle contractile and cytoskeletal components in the avian embryo using immunofluorescence microscopy and immunoblotting. Like myosin-light-chain kinase and heavy caldesmon, both calponin and SM 22 were more or less exclusively found in smooth muscle cells, during embryonic development and in the adult. Labelling of other cell types including striated muscle was not observed. In contrast, tropomyosin, smooth muscle alpha-actin, filamin and desmin could also be detected in many other cell types in addition to smooth muscles, at least during part of embryonic life. Calponin and SM 22 appeared almost synchronously during the differentiation of all smooth muscle cell populations, though with a slight time difference in the case of the aorta. The appearance of calponin, SM 22 and heavy caldesmon was generally delayed in relation to desmin, tropomyosin, smooth muscle alpha-actin, myosin-light-chain kinase and filamin and a marked increase in abundance of these proteins was observed in the late embryo and in the adult. From these observations we can conclude that both calponin and SM 22 belong to a group of late differentiation determinants in smooth muscle and may constitute convenient and reliable markers to follow the differentiation of most, if not all, smooth muscle cell populations.

Arterial smooth muscle cell phenotype in stroke-prone spontaneously hypertensive rats

The aim of this study was to determine the phenotype of smooth muscle cells in the arteries of chronically hypertensive animals and to analyze the effects of treatments known to increase the survival of the animal without a clear effect on its hypertensive state. Stroke-prone spontaneously hypertensive rats (SHRSP) kept on a 1% sodium drinking solution were untreated or treated with one of two diuretics, indapamide (3 mg/kg per day) or hydrochlorothiazide (20 mg/kg per day), from 6 to 13 weeks of age. Phenotype was characterized by the immunolabeling of arteries with antibodies raised against a cellular form (EIIIA) of fibronectin, alpha-smooth muscle actin, and nonmuscle myosin. We demonstrated that phenotypes of smooth muscle cells of the SHRSP differ from those found in Wistar-Kyoto rats. The difference in phenotype is specific for the vessel type: ie, an increased expression of nonmuscle myosin in the aorta and of both EIIIA fibronectin and nonmuscle myosin in the coronary arteries. The two diuretics (1) had no effect on blood pressure, (2) prevented or did not prevent the increase in medial thickness, and (3) prevented changes in both smooth muscle cell phenotype and ischemic tissular lesions. Taken together, the results suggest that in SHRSP the changes in the phenotype of smooth muscle cells and the thickness of arteries are unrelated events. We propose that the maintenance of the contractile phenotype of the arterial smooth muscle cells could be an essential parameter involved in the prevention of the deleterious consequences characteristic of a severe hypertensive state.

Decreased accumulation of beta 1-adrenergic receptor, G alpha s and total myosin heavy chain messenger RNAs in the left ventricle of senescent rat heart

The expression of genes coding for the beta 1-adrenergic receptor (beta 1-AR), the alpha subunit of Gs and total myosin heavy chain (MHC) was compared between left ventricles (LV's) from young (6-7 weeks old) and old (22 months old) rats. The mRNA levels were quantitated by Northern or Slot blots analyses using specific DNA probes. Ageing was found to be associated with a reduction in beta 1-AR (77%), G alpha s (33%) and, total MHC (51%) mRNA levels with no concomitant change in 18S RNA and poly(A+) mRNA levels. These results indicate that transcriptional and/or post-transcriptional mechanisms participate in the control of beta-adrenergic receptor density during ageing. As in the senescent LV, beta 1-AR mRNA level is reduced in the hypertrophied LV, whereas the level of G alpha s mRNA is reduced in the senescent but not in the hypertrophied LV. From our data we conclude (1) that a dual mechanism may operate during ageing, mechanical factors indirectly regulating beta 1-AR mRNA level, while changes in G alpha s mRNA level do not depend on hemodynamic load and (2) that the re-expression of beta-MHC mRNA does not compensate for the decreased accumulation of alpha-MHC mRNA which results in a large decrease in the level of total MHC mRNA in the senescent LV.

The molecular biology of heart failure

In the mammalian heart, the development of cardiac hypertrophy is a common feature that normally precedes all forms of heart failure. This adaptive process involves molecular changes in the myocardium, including the altered expression of several genes encoding proteins for contraction and relaxation. The expression of myosin heavy chain (MHC) and sarcomeric alpha-actin messenger ribonucleic acid (mRNA) changes qualitatively during cardiac hypertrophy; however, their accumulations are not coordinated. Skeletal alpha-actin transcripts accumulate throughout the ventricles and earlier than beta-MHC transcripts, which accumulate primarily around large coronary vessels. Skeletal alpha-actin transcripts also "hyperaccumulate" relative to cardiac alpha-actin mRNA, whose expression does not change. Expression of MHC isomRNA shows an inverse relation; as beta-MHC accumulates, alpha-MHC decreases in abundance. From nuclear run-on assays, we present evidence that the accumulation of these gene products is at least under partial transcriptional control with developmental growth, suggesting that those changes that occur with hypertrophy and heart failure may be primarily transcriptionally regulated. The expression of the mRNA for the calcium-adenosine triphosphate (Ca(2+)-ATPase) of the sarcoplasmic reticulum changes quantitatively with cardiac hypertrophy without the reexpression of a different isoform. The relative mRNA and protein concentrations for this protein diminish with both cardiac hypertrophy and heart failure, a change that may partially explain the delayed relaxation rates seen in hypertrophied and failing hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning and developmental expression of human cardiac troponin T

We have isolated a full-size cDNA coding for cardiac troponin T (cTnT) from a human adult heart library, using a slow skeletal TnT probe. This cDNA detected a 1.2 kb mRNA in fetal and post-natal human heart, the amount of which increased during ontogenic development. Interestingly, a similar transcript was coexpressed in fetal skeletal muscle, together with the 0.9 kb slow skeletal muscle mRNA, and its expression was down-regulated during further development.

Coexpression of alpha and beta myosin heavy-chain isoforms in atria of neonates and infants with congenital heart disease

The relative amounts of cardiac myosin heavy-chain isoforms (MyHC) in right atrial tissue (RA) of 16 neonates and children suffering from congenital heart disease have been investigated. Quantification of MyHC was based on one-dimensional gel electrophoresis and on histometrical evaluation of cyro-sections stained with monoclonal antibodies against alpha- and beta-MyHC. The mean right atrial pressures ranged from 2 to 14 Hg. The RA load was normal in eight patients (5.1 +/- 1.3 mm Hg) and overloaded in eight cases (10 +/- 2.5 mm Hg). The arterial oxygen saturation was normal in 12 and ranged between 85% and 89% in four cases. In all patients a large proportion of atrial myocytes coexpressed alpha- and beta-MyHC. However, in the cases with pressure overloaded RA the amount of beta-MyHC was found to be 1.6 times higher than in the cases with normal pressure. This indicates an adaptational response to overload, as was previously described for the adult human heart. In light of this finding, it seems important to conserve as much as possible of the trained right atrial wall during a Fontan type of operation.

Alpha-, beta-MHC mRNA quantification in adult cardiomyocytes by in situhybridization: effect of thyroid hormone

Cardiac myocytes isolated from adult rats and cultured for up to 5 days in a defined serum- and 3,5,3'-triiodothyronine-(T3) free medium were processed for in situ hybridization using [35S]cRNA probes specific for alpha- or beta-myosin heavy chain (MHC) mRNAs. A computer-assisted image analysis system was used to quantitate the hybridization signals within individual myocytes (100 cells/experimental point). The method was validated by comparison with dot-blot quantitation. The mean alpha-MHC mRNA density per cell decreased by 50% (P < 0.01) after 2 days in culture and remained stable thereafter, whereas the relative amount of beta-MHC mRNA did not increase until day 5. Addition of 10(-12) M T3 to the culture medium for 2 or 3 days was sufficient to maintain alpha-MHC mRNA levels similar to the day 0 values, whereas 10(-9) M T3 was necessary to completely inhibit beta-MHC mRNA expression. The independent analysis of myocytes exhibiting different morphological phenotypes with time in culture demonstrated that rounded myocytes contain relatively more alpha-MHC mRNA and were as sensitive to T3 as their rod-shaped counterparts. Their beta-MHC RNA content was similar to that found in rod-shaped cells and was still depressed by T3. In conclusion, we show that 1) physiological doses of T3 are sufficient to maintain in vitro a MHC phenotype close to that observed in vivo in adult, 2) the dose responsiveness of adult myocytes to T3 differs from that reported in neonatal myocytes, and 3) the alpha-MHC mRNA content and the T3 sensitivity of spheroidal myocytes imply that there is no alteration in their state of maturation.