β-Adrenergic receptor signaling in the heart: Role of CaMKII

The multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII) targets a number of Ca2+ homeostatic proteins and regulates gene transcription. Many of the substrates phosphorylated by CaMKII are also substrates for protein kinase A (PKA), the best known downstream effector of β-adrenergic receptor (β-AR) signaling. While PKA and CaMKII are conventionally considered to transduce signals through separate pathways, there is a body of evidence suggesting that CaMKII is activated in response to β-AR stimulation and that some of the downstream effects of β-AR stimulation are actually mediated by CaMKII. The signaling pathway through which β-AR stimulation activates CaMKII, in parallel with or downstream of PKA, is not well-defined. This review considers the evidence for and mechanisms by which CaMKII is activated in response to β-AR stimulation. In addition the potential role of CaMKII in β-AR regulation of cardiac function is considered. Notably, although many CaMKII targets (e.g., phospholamban or the ryanodine receptor) are central to the regulation of Ca2+ handling, and effects of CaMKII on Ca2+ handling are detectable, inhibition or gene deletion of CaMKII has relatively little effect on the acute physiological contractile response to β-AR. On the other hand CaMKII expression and activity are increased in heart failure, a pathophysiological condition characterized by chronic stimulation of cardiac β-ARs. Blockade of β-ARs is an accepted therapy for treatment of chronic heart failure although the rationale for its beneficial effects in cardiomyocytes is uncertain. There is growing evidence that inhibition or gene deletion of CaMKII also has a significant beneficial impact on the development of heart failure. The possibility that excessive β-AR stimulation is detrimental because of its effects on CaMKII mediated Ca2+ handling disturbances (e.g., ryanodine receptor phosphorylation and diastolic SR Ca2+ leak) is an intriguing hypothesis that merits future consideration.