CONICET | Buscador de Institutos y Recursos Humanos

Titin-based passive stiffness is post-translationally regulated by several kinases that phosphorylate specific spring elements located within titinLs elastic I-band region. Whether titin is phosphorylated by calcium/calmodulin dependent protein kinase II (CaMKII), an important regulator of cardiac function and disease, has not been addressed. The aim of this work was to determine whether CaMKIIÂ, the predominant CaMKII isoform in the heart, phosphorylates titin, and to use phosphorylation assays and mass spectrometry to study which of titinLs spring elements might be targeted by CaMKIIÂ. It was found that CaMKIIÂ phosphorylates titin in mouse LV skinned fibers, that the CaMKIIÂ sites can be dephosphorylated by protein phosphatase 1 (PP1), and that under baseline conditions, in both intact isolated hearts and skinned myocardium, about half of the CaMKIIÂ sites are phosphorylated. Mass spectrometry revealed that both the N2B and PEVK segments are targeted by CaMKIIÂ at several conserved serine residues. Whether phosphorylation of titin by CaMKIIÂ occurs in vivo, was tested in several conditions using back phosphorylation assays and phospho-specific antibodies to CaMKIIÂ sites. Reperfusion following global ischemia increased the phosphorylation level of CaMKIIÂ sites on titin and this effect was abolished by the CaMKII inhibitor KN-93. No changes in the phosphorylation level of the PEVK element were found suggesting that the increased phosphorylation level of titin in IR (ischemia reperfusion) might be due to phosphorylation of the N2B element. The findings of these studies show for the first time that titin can be phosphoryalated by CaMKIIÂ, both in vitro and in vivo, and that titinLs molecular spring region that determines diastolic stiffness is a target of CaMKIIÂ.