The embryological basis of subclinical hypertrophic cardiomyopathy.

Hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomeric proteins, the commonest being MYBPC3 encoding myosin-binding protein C. It is characterised by left ventricular hypertrophy but there is an important pre-hypertrophic phenotype with features including crypts, abnormal mitral leaflets and trabeculae. We investigated these during mouse cardiac development using high-resolution episcopic microscopy. In embryonic hearts from wildtype, homozygous (HO) and heterozygous (HET) Mybpc3-targeted knock-out (KO) mice we show that crypts (one or two) are a normal part of wildtype development but they almost all resolve by birth. By contrast, HO and HET embryos had increased crypt presence, abnormal mitral valve formation and alterations in the compaction process. In scarce normal human embryos, crypts were sometimes present. This study shows that features of the human pre-hypertrophic HCM phenotype occur in the mouse. In an animal model we demonstrate that there is an embryological HCM phenotype. Crypts are a normal part of cardiac development but, along with the mitral valve and trabeculae, their developmental trajectory is altered by the presence of HCM truncating Mybpc3 gene mutation.

Effects of chronic hypoxia on cardiac function measured by pressure-volume catheter in fetal chickens.

Hypoxia is a common component of many developmental insults and has been studied in early-stage chicken development. However, its impact on cardiac function and arterial-ventricular coupling in late-stage chickens is relatively unknown. To test the hypothesis that hypoxic incubation would reduce baseline cardiac function but protect the heart during acute hypoxia in late-stage chickens, white Leghorn eggs were incubated at 21% O2 or 15% O2. At 90% of incubation (19 days), hypoxic incubation caused growth restriction (-20%) and increased the LV-to-body ratio (+41%). Left ventricular (LV) pressure-volume loops were measured in anesthetized chickens in normoxia and acute hypoxia (10% O2). Hypoxic incubation lowered the maximal rate of pressure generation (ΔP/ΔtMax; -22%) and output (-57%), whereas increasing end-systolic elastance (ELV; +31%) and arterial elastance (EA; +122%) at similar heart rates to normoxic incubation. Both hypoxic incubation and acute hypoxia lengthened the half-time of relaxation (τ; +24%). Acute hypoxia reduced heart rate (-8%) and increased end-diastolic pressure (+35%). Hearts were collected for mRNA analysis. Hypoxic incubation was marked by decreased mRNA expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase 2, Na(+)/Ca(2+) exchanger 1, phospholamban, and ryanodine receptor. In summary, hypoxic incubation reduces LV function in the late-stage chicken by slowing pressure generation and relaxation, which may be driven by altered intracellular excitation-contraction coupling. Cardiac efficiency is greatly reduced after hypoxic incubation. In both incubation groups acute hypoxia reduced diastolic function.

Cardiomyopathy in offspring of diabetic rats is associated with activation of the MAPK and apoptotic pathways.

BACKGROUND: Maternal diabetes affects the developing fetal cardiovascular system. Newborn offspring of diabetic mothers can have a transient cardiomyopathy. We hypothesized that cardiomyopathic remodeling is associated with activation of the mitogen activated protein kinase (MAPK) signaling and apoptotic pathways. METHODS: To evaluate the effects of moderate and severe maternal hyperglycemia, pregnant rats were made diabetic with an injection of 50 mg/kg of streptozotocin. Moderately well controlled maternal diabetes was achieved with twice daily glucose checks and insulin injections. No insulin was given to severely diabetic dams. Offspring of moderate and severe diabetic mothers (OMDM and MSDM, respectively) were studied on postnatal days 1 (NB1) and 21 (NB21). Echocardiograms were performed to evaluate left ventricular (LV) dimensions and function. Myocardial MAPK and apoptotic protein levels were measured by Western blot. RESULTS: OMDM had increased cardiac mass at NB1 compared to controls that normalized at NB21. OSDM demonstrated microsomia with relative sparing of cardiac mass and a dilated cardiomyopathy at NB1. In both models, there was a persistent increase in the HW:BW and significant activation of MAPK and apoptotic pathways at NB21. CONCLUSION: The degree of maternal hyperglycemia determines the type of cardiomyopathy seen in the offspring, while resolution of both the hypertrophic and dilated cardiomyopathies is associated with activation of MAPK signaling and apoptotic pathways.

Revisiting animal models of aortic stenosis in the early gestation fetus.

BACKGROUND: Mechanisms leading to left ventricular hypoplasia and endocardial fibroelastosis in the fetus remain unknown. Prevailing theory is that obstruction to blood flow through the left ventricle leads to elevated end-diastolic pressures, compromised myocardial perfusion, and endocardial ischemia. Fetal interventions are now being performed, based on the presumption that they would prevent such pathogenic mechanisms. METHODS: Forty first-trimester fetal sheep (mean gestational age, 53 days) were studied. Severe fetal left ventricular outflow obstruction was created by banding the ascending aorta in 25 fetuses; 15 control fetuses underwent "sham" surgery with thoracotomy. Serial fetal echocardiography was used to assess left ventricular growth and fetal hemodynamics. Findings were correlated to morphologic and histopathologic changes, and intracardiac pressure measurements obtained from fetal cardiac catheterization. RESULTS: Surviving banded fetuses (n = 13) had one of two phenotypes: compensatory left ventricular hypertrophy (n = 7) or noncompensatory left ventricular dilatation (n = 6) with hydrops and severe left ventricular dysfunction. All fetuses had elevated left ventricular end-diastolic pressures (mean, 21 mm Hg; range, 14 to 28 mm Hg), which correlated to the gradient across the ascending aorta (mean, 41 mm Hg; range, 28 to 73 mm Hg). In vivo echocardiography findings were incongruous with those at autopsy, and demonstrated preservation of left ventricular growth indices in all fetuses. Endocardial fibroelastosis and myocardial fibrosis were not observed in any banded fetus. CONCLUSIONS: While early gestational obstruction to flow can compromise left ventricular function in the fetus, it does not retard normal growth. Similarly, an elevated left ventricular end-diastolic pressure is not sufficient to cause myocardial fibrosis or endocardial fibroelastosis in the fetus.

Mitogen-activated protein kinase activation and regulation in the pressure-loaded fetal ovine heart.

The mitogen-activated protein (MAP) kinase signaling pathways help to mediate the hypertrophic response of the pressure-loaded adult heart, although their importance in fetal myocardium is less known. The goal of this study was to determine the role the MAP kinase signaling pathways play in regulating the response of the fetal heart to a pressure load. Aortic (Ao) and pulmonary artery (PA) bands were placed in 132-day fetal sheep for 7 days. Protein levels of the total and active (phosphorylated) terminal MAP kinases extracellular signal-regulated kinase (ERK/P-ERK), c-Jun NH(2)-terminal kinase (JNK/P-JNK), and p38/P-p38 and the MAP kinase phosphatases MKP-1, MKP-2, and MKP-3 were made in the right and left ventricular (RV and LV) free walls. In both Ao- and PA-banded animals, total heart weight normalized to body weight was significantly increased, largely due to an increase in RV free wall mass in the Ao-banded animals and an increase in septal mass in the PA-banded fetuses. Total protein levels of the three terminal kinases and of P-ERK and P-JNK remained stable in both groups of banded animals. However, P-p38 was significantly increased in RV and LV of Ao- and PA-banded fetuses. Whereas MKP-1 and MKP-2 protein levels were unchanged following Ao- and PA-banding, MKP-3 protein levels were significantly increased in the RV of the PA-banded animals. These findings indicate that the MAP kinase signaling pathways are active in the fetal heart and help to modulate the response of prenatal myocardium to a pressure load.

Heart block in mice overexpressing calcineurin but not NF-AT3.

OBJECTIVE: Overexpression of calcineurin causes cardiac hypertrophy and arrhythmic deaths. During disease development, sinus bradycardia followed by high degree atrioventricular (AV) block finally culminating in ventricular asystole has been observed over time in calcineurin hearts. AV block is associated with the development of pleomorphic ventricular tachycardia in mice and downregulation of potassium currents in ventricular myocytes. We tested the hypothesis that the abnormalities of AV block and propensity to ventricular tachycardia relate to overexpression of calcineurin independent of the development of hypertrophy. METHODS: Cardiac electrophysiologic properties were compared in isolated perfused hearts with ventricular hypertrophy due to overexpression of calcineurin or NF-AT3 and in their corresponding wild types at 15 or 30 days of age. RESULTS: Compared to wild-type hearts, significant prolongation of sinus node recovery times was noted in both NF-AT3 and calcineurin hearts. Compared to wild-type hearts, Wenckebach cycle length (WCL) and the left ventricular effective refractory period (LVERP) were significantly prolonged in the calcineurin hearts (p<0.05) but not NF-AT3 hearts. In calcineurin mice, left ventricular effective refractory period impinged on Wenckebach cycle length resulting in a significant correlation between left ventricular effective refractory period and Wenckebach cycle length (r(2)=0.56). No such correlation was observed for wild type or NF-AT3 hearts. At 30 days of development, ventricular tachycardia developed in 70% of calcineurin hearts compared to 0% wild-type hearts (p=0.003), whereas ventricular tachycardia was observed in 33% of NF-AT3 hearts and 10% of corresponding wild-type hearts (p=NS). CONCLUSIONS: The prolonged ventricular refractoriness, seen only in calcineurin hearts, impinges on Wenckebach cycle length resulting in heart block and is associated with propensity to ventricular tachycardia.

Left ventricular alterations in a model of fetal left ventricular overload.

Congenital aortic coarctation is well tolerated by the fetus because the foramen ovale and ductus arteriosus equalize intracardiac and great arteries pressures and shunts. The pathologic consequences only emerge after birth with closure of the foramen ovale and ductus arteriosus. There is, however, no documentation of myocardial effects in utero of the left ventricular (LV) pressure overload induced by aortic banding. We investigated whether prenatal aortic banding could be detrimental at the structural and/or functional level. The goal of the present study was to investigate the cardiac effects of LV pressure overload in a fetal lamb model. Nine fetal lambs underwent preductal banding of the aortic arch in utero at midgestation (CoA group), whereas their twins underwent sham surgery. All fetuses were studied between 27 and 37 d after surgery for LV pressure, anatomic and histologic anomalies, and steady state sarcoendoplasmic reticulum calcium ATPase (SERCA 2a) mRNA and protein levels and pump activity. Surgery resulted in severe aortic coarctation in all the animals in the CoA group and was associated with a 65% increase in the LV weight to body weight ratio relative to the sham-operated group (p < 0.001). Hemodynamic and histologic studies showed an evolutionary pattern depending on duration of the experimental coarctation with a shift occurring at 30 d of coarctation. The initial response of cardiomyocytes to ventricular overload was hypertrophy of the myocytes, followed by myocyte hyperplasia. Compared with sham, there was an apparent decrease in the percentage of binucleated cells in the CoA group after 30 d of coarctation. The earliest response to LV pressure overload appears to occur at the molecular level. Indeed, sarcoendoplasmic reticulum calcium ATPase (SERCA 2a) mRNA levels fell significantly to only 28.6% of the sham group value (p = 0.023), independently of the duration of coarctation. In the fetal lamb, the pressure overload-induced hypertrophy resulting from progressive aortic coarctation leads to hemodynamic and lesional abnormalities and slows ontogenic maturation.