Excision of titin's cardiac PEVK spring element abolishes PKCα-induced increases in myocardial stiffness

Abstract

Protein kinase C-alpha (PKCα) was recently reported to increase myocardial stiffness, an effect that was proposed to be due to phosphorylation of two highly conserved sites (S11878 and S12022) within the proline-glutamic acid-valine-lysine (PEVK) rich spring element of titin. To test this proposal we investigated the effect of PKCα on phosphorylation and passive stiffness in a mouse model lacking the titin exons that contain these two phosphorylation sites, the PEVK knockout (KO). We used skinned, gelsolin-extracted, left ventricular myocardium from wildtype and PEVK KO mice. Consistent with previous work we found that PKCα increased passive stiffness in the WT myocardium by 27±6%. Importantly, this effect was completely abolished in KO myocardium. In addition, increases in the elastic and viscous moduli at a wide range of frequencies (properties important in diastolic filling) following PKCα incubation (27±3% and 20±4%, respectively) were also ablated in the KO. Back phosphorylation assays showed that titin phosphorylation following incubation with PKCα was significantly reduced by 36±12% in skinned PEVK KO myocardial tissues. The remaining phosphorylation in the KO suggests that PKCα sites exist in the titin molecule outside the PEVK region; these sites are not involved in increasing passive stiffness. Our results firmly support that the PEVK region of cardiac titin is phosphorylated by PKCα and that this increases passive tension. Thus, the PEVK spring element is the critical site of PKCα's involvement in passive myocardial stiffness.