*4.1. Phosphorylation of the PEVK Element of Titin Increases Cardiomyocyte Passive Tension*

In addition to genetic and transcriptional modifications, post-translational modifications of titin also play an important role in cardiac physiology and changes associated with heart failure. The most well described post-translational modification of titin is phosphorylation and dephosphorylation [76]. Based on proteomic analysis, there are hundreds of predicted phosphorylation sites on titin [77–79]. Only a few of these potential phosphosites have been studied and demonstrated to impart functional

changes on titin with a majority of the studied sites located in the spring-like I-band domain [17]. Phosphorylation within different regions of the I-band have different effects on the functional properties of titin. The PEVK element has a predominantly negative charge, therefore phosphorylation of this element with a positively charged phosphate group increases electrostatic attraction making the extensible region more rigid [76]. Treatment of human cardiomyocytes with the protein kinase C alpha (PKCα) has been shown to cause phosphorylation of the PEVK element and increase passive tension [80]. In a study of cardiomyocytes isolated by biopsy from patients with mixed systolic and diastolic heart failure, there was greater activation of PKCα within the cardiomyocytes compared to control samples. This increase in PKCα activity also correlated with increased passive tension studied on skinned myocardial fibers [81]. PKCα is activated by excess catecholamines or hypertrophic signaling cascades and may represent a terminal pathway in heart failure that leads to increased myofilament stiffness. Activation of PKCα and the resultant increase in passive tension of cardiomyocytes may be a compensatory change to increase passive stiffness in patients with DCM in order to improve cardiac function. PKCα mediated increased passive stiffness may also represent a dysregulated pathway that leads to pathologic passive tension and may result in diastolic dysfunction [82,83]. Additional studies comparing PKCα activation and passive tension between cardiomyocytes isolated from patients with DCM and patients with diastolic dysfunction may be helpful to better understand the molecular pathways involving PKCα and heart failure. It should be considered that modification of this pathway represents a viable therapeutic target for treatment of heart failure when it is more clearly understood.
