Echocardiographic evaluation of ventricular function in mice.

Ventricular dysfunction remains a hallmark of most cardiac disease. The mouse has become an essential model system for cardiovascular biology, and echocardiography an established tool in the study of normal and genetically altered mice. This review describes the measurement of ventricular function, most often left ventricular function, by echocardiographic methods in mice. Technical limitations related to the small size and rapid heart rate in the mouse initially argued for the performance of echocardiography under anesthesia. More recently, higher frame rates and smaller probes operating at higher frequencies have facilitated imaging of conscious mice in some, but not all, experimental protocols and conditions. Ventricular function may be qualitatively and quantitatively evaluated under both conditions. Particular detail is provided for measurement under conscious conditions, and measurement under conscious and sedated or anesthestized conditions are contrasted. Normal values for echocardiographic indices for the common C57BL/6 strain are provided. Diastolic dysfunction is a critical pathophysiologic component of many disease states, and progress in the echocardiographic evaluation of diastolic function is discussed. Finally, echocardiography exists among several competing imaging technologies, and these alternatives are compared.

Death, cardiac dysfunction, and arrhythmias are increased by calmodulin kinase II in calcineurin cardiomyopathy.

BACKGROUND: Activation of cellular Ca2+ signaling molecules appears to be a fundamental step in the progression of cardiomyopathy and arrhythmias. Myocardial overexpression of the constitutively active Ca2+-dependent phosphatase calcineurin (CAN) causes severe cardiomyopathy marked by left ventricular (LV) dysfunction, arrhythmias, and increased mortality rate, but CAN antagonist drugs primarily reduce hypertrophy without improving LV function or risk of death. METHODS AND RESULTS: We found that activity and expression of a second Ca2+-activated signaling molecule, calmodulin kinase II (CaMKII), were increased in hearts from CAN transgenic mice and that CaMKII-inhibitory drugs improved LV function and suppressed arrhythmias. We devised a genetic approach to "clamp" CaMKII activity in CAN mice to control levels by interbreeding CAN transgenic mice with mice expressing a specific CaMKII inhibitor in cardiomyocytes. We developed transgenic control mice by interbreeding CAN transgenic mice with mice expressing an inactive version of the CaMKII-inhibitory peptide. CAN mice with CaMKII inhibition had reduced risk of death and increased LV and ventricular myocyte function and were less susceptible to arrhythmias. CaMKII inhibition did not reduce transgenic overexpression of CAN or expression of endogenous CaMKII protein or significantly reduce most measures of cardiac hypertrophy. CONCLUSIONS: CaMKII is a downstream signal in CAN cardiomyopathy, and increased CaMKII activity contributes to cardiac dysfunction, arrhythmia susceptibility, and longevity during CAN overexpression.

Calmodulin kinase II inhibition protects against myocardial cell apoptosis in vivo.

Inhibition of the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) or depletion of sarcoplasmic reticulum (SR) Ca(2+) stores protects against apoptosis from excessive isoproterenol (Iso) stimulation in cultured ventricular myocytes, suggesting that CaMKII inhibition could be a novel approach to reducing cell death in conditions of increased adrenergic tone, such as myocardial infarction (MI), in vivo. We used mice with genetic myocardial CaMKII inhibition due to transgenic expression of a highly specific CaMKII inhibitory peptide (AC3-I) to test whether CaMKII was important for apoptosis in vivo. A second line of mice expressed a scrambled, inactive form of AC3-I (AC3-C). AC3-C and wild-type (WT) littermates were used as controls. AC3-I mice have reduced SR Ca(2+) content and are resistant to Iso- and MI-induced apoptosis compared with AC3-C and WT mice. Phospholamban (PLN) is a target for modulation of SR Ca(2+) content by CaMKII. PLN(-/-) mice have increased susceptibility to Iso-induced apoptosis. Verapamil pretreatment prevented Iso-induced apoptosis in PLN(-/-) mice, indicating the involvement of a Ca(2+)-dependent pathway. AC3-I and AC3-C mice were bred into a PLN(-/-) background. Loss of PLN increased and equalized SR Ca(2+) content in AC3-I, AC3-C, and WT mice and abolished the resistance to apoptosis in AC3-I mice after MI. There was a trend (P = 0.07) for increased Iso-induced apoptosis in AC3-I mice lacking PLN compared with AC3-I mice with PLN. These findings indicate CaMKII is proapoptotic in vivo and suggest that regulation of SR Ca(2+) content by PLN contributes to the antiapoptotic mechanism of CaMKII inhibition.

Abnormal mitochondrial bioenergetics and heart rate dysfunction in mice lacking very-long-chain acyl-CoA dehydrogenase.

Mitochondrial very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is associated with severe hypoglycemia, cardiac dysfunction, and sudden death in neonates and children. Sudden death is common, but the underlying mechanisms are not fully understood. We report on a mouse model of VLCAD deficiency with a phenotype induced by the stresses of fasting and cold, which includes hypoglycemia, hypothermia, and severe bradycardia. The administration of glucose did not rescue the mice under stress conditions, but rewarming alone consistently led to heart rate recovery. Brown adipose tissue (BAT) from the VLCAD-/- mice showed elevated levels of the uncoupling protein isoforms and peroxisome proliferator-activated receptor-alpha. Biochemical assessment of the VLCAD(/- mice BAT showed increased oxygen consumption, attributed to uncoupled respiration in the absence of stress. ADP-stimulated respiration was 23.05 (SD 4.17) and 68.24 (SD 6.3) nmol O2.min(-1).mg mitochondrial protein(-1) for VLCAD+/+ and VLCAD-/- mice, respectively (P < 0.001), and carbonyl cyanide p-trifluoromethoxyphenylhydrazone-stimulated respiration was 35.9 (SD 3.6) and 49.3 (SD 9) nmol O2.min(-1).mg mitochondrial protein(-1) for VLCAD+/+ and VLCAD-/- mice, respectively (P < 0.20), but these rates were insufficient to protect them in the cold. We conclude that disturbed mitochondrial bioenergetics in BAT is a critical contributing factor for the cold sensitivity in VLCAD deficiency. Our observations provide insights into the possible mechanisms of stress-induced death in human newborns with abnormal fat metabolism and elucidate targeting of specific substrates for particular metabolic needs.

Calmodulin kinase II inhibition protects against structural heart disease.

Beta-adrenergic receptor (betaAR) stimulation increases cytosolic Ca(2+) to physiologically augment cardiac contraction, whereas excessive betaAR activation causes adverse cardiac remodeling, including myocardial hypertrophy, dilation and dysfunction, in individuals with myocardial infarction. The Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) is a recently identified downstream element of the betaAR-initiated signaling cascade that is linked to pathological myocardial remodeling and to regulation of key proteins involved in cardiac excitation-contraction coupling. We developed a genetic mouse model of cardiac CaMKII inhibition to test the role of CaMKII in betaAR signaling in vivo. Here we show CaMKII inhibition substantially prevented maladaptive remodeling from excessive betaAR stimulation and myocardial infarction, and induced balanced changes in excitation-contraction coupling that preserved baseline and betaAR-stimulated physiological increases in cardiac function. These findings mark CaMKII as a determinant of clinically important heart disease phenotypes, and suggest CaMKII inhibition can be a highly selective approach for targeting adverse myocardial remodeling linked to betaAR signaling.

Temporal changes in ventricular function assessed echocardiographically in conscious and anesthetized mice.

The mouse is an important model system for cardiovascular biology, with echocardiography a critical tool for noninvasive measurement of cardiac morphology and function. The feasibility and short-term temporal consistency of repeated echocardiographic measurements in conscious mice has not been previously evaluated. We performed serial 2-dimensional guided M-mode transthoracic echocardiographic measurements at 5- to 10-minute intervals over 60 minutes in conscious mice and in mice treated with 1 of 3 anesthetic regimens: ketamine and acepromazine (n = 14); pentobarbital (n = 14); and ketamine and xylazine (n = 13). Unanesthetized mice received intraperitoneal saline (n = 6) or no injection (n = 7). In sequentially repeated measurements over 1 hour in conscious mice, none of the measured or derived echocardiographic parameters differed from baseline, whereas all 3 anesthetic regimens produced significant, prolonged, and temporally variable decreases in heart rate and fractional shortening. The relationship between heart rate and fractional shortening was not altered by anesthetic choice. Serial echocardiographic assessments of cardiac function, dimension, and mass can be performed with high reproducibility in conscious mice.

Very-long-chain acyl-coenzyme a dehydrogenase deficiency in mice.

Fatty acid oxidation (FAO) defects are inborn errors of metabolism clinically associated with cardiomyopathy and sudden infant death syndrome (SIDS). FAO disorders often present in infancy with myocardial dysfunction and arrhythmias after exposure to stresses such as fasting, exercise, or intercurrent viral illness. It is uncertain whether the heart, in the absence of stress, is normal. We generated very-long-chain acyl-coenzyme A dehydrogenase (VLCAD)-deficient mice by homologous recombination to define the onset and molecular mechanism of myocardial disease. We found that VLCAD-deficient hearts have microvesicular lipid accumulation, marked mitochondrial proliferation, and demonstrated facilitated induction of polymorphic ventricular tachycardia, without antecedent stress. The expression of acyl-CoA synthase (ACS1), adipophilin, activator protein 2, cytochrome c, and the peroxisome proliferator activated receptor gamma coactivator-1 were increased immediately after birth, preceding overt histological lipidosis, whereas ACS1 expression was markedly downregulated in the adult heart. We conclude that mice with VLCAD deficiency have altered expression of a variety of genes in the fatty acid metabolic pathway from birth, reflecting metabolic feedback circuits, with progression to ultrastructural and physiological correlates of the associated human disease in the absence of stress.