Phosphorylation status of regulatory proteins and functional characteristics in myocardium of dilated cardiomyopathy of Syrian hamsters

Role of β-adrenergic signaling in masseter muscle

In skeletal muscle, the major isoform of β-adrenergic receptor (β-AR) is β2-AR and the minor isoform is β1-AR, which is opposite to the situation in cardiac muscle. Despite extensive studies in cardiac muscle, the physiological roles of the β-AR subtypes in skeletal muscle are not fully understood. Therefore, in this work, we compared the effects of chronic β1- or β2-AR activation with a specific β1-AR agonist, dobutamine (DOB), or a specific β2-AR agonist, clenbuterol (CB), on masseter and cardiac muscles in mice. In cardiac muscle, chronic β1-AR stimulation induced cardiac hypertrophy, fibrosis and myocyte apoptosis, whereas chronic β2-AR stimulation induced cardiac hypertrophy without histological abnormalities. In masseter muscle, however, chronic β1-AR stimulation did not induce muscle hypertrophy, but did induce fibrosis and apoptosis concomitantly with increased levels of p44/42 MAPK (ERK1/2) (Thr-202/Tyr-204), calmodulin kinase II (Thr-286) and mammalian target of rapamycin (mTOR) (Ser-2481) phosphorylation. On the other hand, chronic β2-AR stimulation in masseter muscle induced muscle hypertrophy without histological abnormalities, as in the case of cardiac muscle, concomitantly with phosphorylation of Akt (Ser-473) and mTOR (Ser-2448) and increased expression of microtubule-associated protein light chain 3-II, an autophagosome marker. These results suggest that the β1-AR pathway is deleterious and the β2-AR is protective in masseter muscle. These data should be helpful in developing pharmacological approaches for the treatment of skeletal muscle wasting and weakness.

Oscillation of cAMP and Ca(2+) in cardiac myocytes: a systems biology approach

Cyclic adenosine monophosphate (cAMP) and Ca(2+) levels may oscillate in harmony within excitable cells; a mathematical oscillation loop model, the Cooper model, of these oscillations was developed two decades ago. However, in that model all adenylyl cyclase (AC) isoforms were assumed to be inhibited by Ca(2+), and it is now known that the heart expresses multiple AC isoforms, among which the type 5/6 isoforms are Ca(2+)-inhibitable whereas the other five (AC2, 3, 4, 7, and 9) are not. We used a computational systems biology approach with CellDesigner simulation software to develop a comprehensive graphical map and oscillation loop model for cAMP and Ca(2+). This model indicated that Ca(2+)-mediated inhibition of AC is essential to create oscillations of Ca(2+) and cAMP, and the oscillations were not altered by incorporation of phosphodiesterase-mediated cAMP hydrolysis or PKA-mediated inhibition of AC into the model. More importantly, they were created but faded out immediately in the co-presence of Ca(2+)-noninhibitable AC isoforms. Because the subcellular locations of AC isoforms are different, spontaneous cAMP and Ca(2+) oscillations may occur within microdomains containing only Ca(2+)-inhibitable isoforms in cardiac myocytes, which might be necessary for fine tuning of excitation-contraction coupling.

Effects of protein kinase a on the phosphorylation status and transverse stiffness of cardiac myofibrils

Stimulation of β-adrenergic receptors in cardiac myocytes activates cyclic AMP-dependent protein kinase A (PKA). PKA-mediated phosphorylation of myofibrils decreases their longitudinal stiffness, but its effect on transverse stiffness is not fully understood. We thus examined the effects of PKA treatment on the transverse stiffness of cardiac myofibrils by atomic force microscopy and determined the phosphorylation levels of myofibril components by SDS-PAGE. Transverse stiffness was significantly decreased by PKA treatment concomitantly with increased phosphorylation of troponin I, myosin-binding protein C, and titin (also called connectin). Subsequent treatment with protein phosphatase 1 abrogated these PKA-mediated effects.

Implications of protease activation in cardiac dysfunction and development of genetic cardiomyopathy in hamsters

It has become evident that protein degradation by proteolytic enzymes, known as proteases, is partly responsible for cardiovascular dysfunction in various types of heart disease. Both extracellular and intracellular alterations in proteolytic activities are invariably seen in heart failure associated with hypertrophic cardiomyopathy, dilated cardiomyopathy, hypertensive cardiomyopathy, diabetic cardiomyopathy, and ischemic cardiomyopathy. Genetic cardiomyopathy displayed in different strains of hamsters provides a useful model for studying heart failure due to either cardiac hypertrophy or cardiac dilation. Alterations in the function of several myocardial organelles such as sarcolemma, sarcoplasmic reticulum, myofibrils, mitochondria, as well as extracellular matrix have been shown to be due to subcellular remodeling as a consequence of changes in gene expression and protein content in failing hearts from cardiomyopathic hamsters. In view of the increased activities of various proteases, including calpains and matrix metalloproteinases in the hearts of genetically determined hamsters, it is proposed that the activation of different proteases may also represent an important determinant of subcellular remodeling and cardiac dysfunction associated with genetic cardiomyopathy.