Cardiac Thin Filament Structural Modulation by Sarcomere Length

Abstract

Myofilament Length Dependent Activation (LDA) forms the cellular basis of the Frank-Starling law observed on the heart. LDA has been studied intensively and appears to be modulated through various mechanisms, such as contractile proteins composition and their phosphorylation status. However, the cellular molecular mechanisms that underlie this phenomenon are still not well characterized.The aim of our study is to determine whether LDA is regulated through cTnC structural changes upon stretch. Accordingly, we used a single attached skinned cardiac myocyte in combination with confocal fluorescent measurement. Using this technique, we found that stretch of a relaxed cell, in the absence of Ca2+, resulted in marked alterations of cTnC structure as reported by cTnC-T53C-IAF confocal fluorescence. Moreover, titin mutant cells show a drastic alteration of both passive tension and myofilament sensitivity to calcium (pCa50) upon stretch. Consistent with this finding, by employing time-resolved x-ray diffraction of intact, electrically stimulated rat myocardium, we found marked changes in troponin and myosin structure upon stretch in the diastolic phase (i.e. when cross-bridges are not active). Moreover, we repeated these x-ray experiments using rat myocardium that expresses an unusually long titin molecule and found that, when compared to WT, diastolic stretch in these muscles did not induce the structural changes in troponin and myosin observed in WT. These results strongly suggest a direct effect of sarcomere length on thin and thick filament and implicate titin to be the molecule underling the length signal transduction for LDA.