Cardiac ryanodine receptor phosphorylation by CaM Kinase II: keeping the balance right

Phosphorylation of the cardiac ryanodine receptor (RyR(2)) is a key mechanism regulating sarcoplasmic reticulum (SR) Ca2+ release. Differences in opinion have arisen over the importance assigned to specific phosphorylation sites on RyR(2), over the kinase (s) suggested to directly phosphorylate RyR(2) and surrounding the possibility that altered phosphorylation of RyR(2) is associated with contractile dysfunction observed in heart failure. Ca2+/calmodulin dependent protein kinase II (CaMKII) can phosphorylate RyR(2) and modulate its activity. This phosphorylation positively modulates cardiac inotropic function but in extreme situations such as heart failure, elevated CaMKII activity can adversely increase Ca2+ release from the SR and lead to arrhythmogenesis. Although other kinases can phosphorylate RyR(2), most notably cAMP-dependent protein kinase (PKA), evidence for a key role of CaMKII in mediating RyR(2)-dependent Ca2+ release is emerging. Future challenges include (i) fully identifying mechanisms of CaMKII interaction with the RyR(2) complex and (ii) given the ubiquitous expression of CaMKII, developing selective strategies to modulate RyR(2)-targeted CaMKII activity and allow improved understanding of its role in normal and diseased heart.