CaMKII-dependent phospholamban phosphorylation as a mechanism to limit Ca2+ overload

MUNDIÑA-WEILENMANN C; VITTONE L; SAID M,; KRANIAS EG; MATTIAZZI A

Brisbane, Australia

Congreso; XVIII World Congress International Society for Heart Research; 2004

International Society for Heart Research

Phosphorylation of phospholamban (PLN) relieves its inhibitory effects on SERCA2a activity resulting in : a) increased sarcoplasmic reticulum (SR) Ca2 + uptake rate, which accelerates myocardial relaxation ; and b) increased SR Ca2 + load and release, which modulates myocardial contractility. The physiological significance of the phosphorylation of PLN at Thr17 by CaMKII remains unclear. While b-adrenergic agonists consistently increase Ser16 phosphorylation (PKA site), an increase in Thr17 phosphorylation is not always found by this intervention.We have shown that Thr17 phosphorylation, independently of that of Ser16, requires the increase in intracellular Ca2 + together with the inhibition of phosphatases, two conditions met by the b-adrenergic stimulation and experimentally, by the perfusion of high Ca2 + with phosphatase inhibitors or acidic pH. We studied PLN phosphorylation in the perfused heart under two pathological situations in which increased intracellular Ca2 + co-exits with acidic pH: reperfusion after ischemia and recovery from hypercapnic acidosis. PLN was phosphorylated at Thr17, independently of Ser16, at the onset of reflow and immediately after hypercapnia was settled. The mechanical recovery after both, the ischemic and acidic insults, was impaired when the increase in Thr17 phosphorylation was abolished by CaMKII inhibition. Studies in transgenic animals with Thr17 mutated to Ala confirmed these results: mice with mutated PLN have lower contractile and delayed relaxation recoveries after ischemia than mice with non-mutated PLN. Although phosphorylation of PLN at Thr17 may not be sufficient to prevent myocardial injury, it may provide a mechanism to limit the intracellular Ca2 + overload in pathological situations.