Functional rescue of elastin insufficiency in mice by the human elastin gene: implications for mouse models of human disease.

Diseases linked to the elastin gene arise from loss-of-function mutations leading to protein insufficiency (supravalvular aortic stenosis) or from missense mutations that alter the properties of the elastin protein (dominant cutis laxa). Modeling these diseases in mice is problematic because of structural differences between the human and mouse genes. To address this problem, we developed a humanized elastin mouse with elastin production being controlled by the human elastin gene in a bacterial artificial chromosome. The temporal and spatial expression pattern of the human transgene mirrors the endogenous murine gene, and the human gene accurately recapitulates the alternative-splicing pattern found in humans. Human elastin protein interacts with mouse elastin to form functional elastic fibers and when expressed in the elastin haploinsufficient background reverses the hypertension and cardiovascular changes associated with that phenotype. Elastin from the human transgene also rescues the perinatal lethality associated with the null phenotype. The results of this study confirm that reestablishing normal elastin levels is a logical objective for treating diseases of elastin insufficiency such as supravalvular aortic stenosis. This study also illustrates how differences in gene structure and alternative splicing present unique problems for modeling human diseases in mice.

Neurally-mediated increase in calcineurin activity regulates cardiac contractile function in absence of hypertrophy.

OBJECTIVE: The calcineurin pathway has been involved in the development of cardiac hypertrophy, yet it remains unknown whether calcineurin activity can be regulated in myocardium independently from hypertrophy and cardiac load. METHODS: To test that hypothesis, we measured calcineurin activity in a rat model of infrarenal aortic constriction (IR), which affects neurohormonal pathways without increasing cardiac afterload. RESULTS: In this model, there was no change in arterial pressure over the 4-week experimental period, and the left ventricle/body weight ratio did not increase. At 2 weeks after IR, calcineurin activity was increased 1.8-fold (P<0.05) and remained elevated at 4 weeks (1.7-fold, P<0.05). Similarly, the cardiac activity of calcium calmodulin kinase II (CaMKII) was increased significantly after IR, which confirms a regulation of Ca(2+)-dependent enzymes in this model. In cardiac myocytes, the increased activity of calcineurin was accompanied by a significant decrease in L-type Ca(2+) channel activity (I(Ca)) and contraction velocity (-dL/dt). Cardiac denervation prevented the activation of calcineurin after IR, which demonstrates that a neurohormonal mechanism is responsible for the changes in enzymatic activity. In addition, cardiac denervation suppressed the effects of IR on I(Ca) and -dL/dt, which shows that calcineurin activation is related to altered contractility. However, action potential duration, the densities of inward rectifier K(+) currents (I(K1)), and outward K(+) currents (I(to) and I(K)) were not altered in IR myocytes. CONCLUSIONS: Calcineurin can be activated in the heart through a neural stimulus, which induces alterations in Ca(2+) currents and contractility. These effects occur in the absence of myocyte hypertrophy, electrophysiological changes in action potential, and K(+) channel currents.

Differential responses of canine myosin ATPase activity and tissue gases in the pressure-overloaded ventricle dependent upon degree of obstruction: mild versus severe pulmonic and aortic stenosis.

Mild pulmonic stenosis, induced in dogs by banding the pulmonary artery, elevated right ventricular peak systolic pressure to 60% above the control and elevated right ventricular K+- and Ca2+- activated myosin ATPase activities. In contrast, severe pulmonic stenosis, which elevated right ventricular peak systolic pressure to 300% above the control, did not produce an increase in myosin enzymatic ATPase Vmax values but caused a decrease in myosin activity. Mild aortic stenosis, induced by banding the ascending aorta, forcing a transaortic pressure gradient of 25 mm Hg, caused an elevation in left ventricular muosin ATPase, whereas severe aortic banding, brought about by creating a transaortic pressure gradient of 55 mm Hg, never caused an elevation in left ventricular myosin enzymatic Vmax values, but, like severe pulmonic banding, caused a decrease in K+- and Ca2+- activated myosin activities. Normal left ventricular myosin Vmax values in mumol of PO4/mg-min at 37 degrees C were: K+ = 2.84 +/- 0.22, and Ca2+ = 0.97 +/- 0.14. For right ventricular myosin they were: K+ = 2.15 +/- 0.16, and Ca2+ =0.74 +/- 0.10. Analyses of tissue gases, based on mass spectrometry data, showed that the hypertrophied ventricles had an elevated tissue pCO2 and an elevation in the cGMP/cAMP ratio.