| Signaling pathways of the cardiac myocyte are important mediators of cardiac muscle function. One of the signaling pathways that may be involved in normal and abnormal cardiac function is the Rac/PAK signaling pathway 1. Rac is a monomeric GTPase belonging to the Ras superfamily of small GTPases and p21-activated kinase (PAK) is a key effector of Rac. These signaling intermediates have been studied mostly in the context of non-muscle cells where they are involved in regulation of cytoskeletal organization, apoptosis, the oxidative response and transcription. Importantly, both Rae and PAK have been implicated in cardiac ischemia/reperfusion injury, hypertrophy and failure. This thesis rests on the general hypothesis that the Rac/PAK signaling pathway is involved in normal and abnormal heart function. More specifically, the predominant function of this pathway in the cardiac myocyte is postulated to be dynamically altered in accordance with changing intrinsic and extrinsic cellular conditions.; In this work, both reductionist and proteomic approaches were applied to the study of the role of the Rac/PAK signaling pathway in normal and abnormal cardiac function. PAK was found to increase the calcium sensitivity of Triton-skinned cardiac muscle fiber bundles (ΔpCa50 ≅ 0.25 units) via a mechanism involving phosphorylation of desmin, troponin T, troponin I and an unidentified 70 kDa protein.; A proteomic approach was applied to the study of a transgenic mouse model of dilated cardiomyopathy in which Rac1 was constitutively expressed specifically in the myocardium. Importantly, we found a 2.5-fold (p < 0.05) increase in the protein amounts of both tubulin α and β chains in the myocardium of Rac1 compared to non-transgenic (NTG) mice. In normal heart function, the Rac/PAK signaling pathway may be involved establishing the calcium-sensitivity of the myofilament in accordance with environmental demands. Under conditions of cardiac failure, this signaling cascade may be involved not only in increasing the calcium-sensitivity of the myofilament, but also, in transcription activation and cytoskeletal rearrangement, functions which may serve to promote or diminish the aberrant phenotype depending on the stage of disease progression.; 1In this thesis, “Rac/PAK signaling pathway” is used to refer to either Rac or PAK acting independently, or Rac and PAK acting together, as a signaling module. |
