Calmodulin-dependent protein kinase II: linking heart failure and arrhythmias

Abstract

Understanding relationships between heart failure and arrhythmias, important causes of suffering and sudden death, remains an unmet goal for biomedical researchers and physicians. Evidence assembled over the past decade supports a view that activation of the multifunctional Ca(2+) and calmodulin-dependent protein kinase II (CaMKII) favors myocardial dysfunction and cell membrane electrical instability. CaMKII activation follows increases in intracellular Ca(2+) or oxidation, upstream signals with the capacity to transition CaMKII into a Ca(2+) and calmodulin-independent constitutively active enzyme. Constitutively active CaMKII appears poised to participate in disease pathways by catalyzing the phosphorylation of classes of protein targets important for excitation-contraction coupling and cell survival, including ion channels and Ca(2+) homeostatic proteins, and transcription factors that drive hypertrophic and inflammatory gene expression. This rich diversity of downstream targets helps to explain the potential for CaMKII to simultaneously affect mechanical and electrical properties of heart muscle cells. Proof-of-concept studies from a growing number of investigators show that CaMKII inhibition is beneficial for improving myocardial performance and for reducing arrhythmias. We review the molecular physiology of CaMKII and discuss CaMKII actions at key cellular targets and results of animal models of myocardial hypertrophy, dysfunction, and arrhythmias that suggest CaMKII inhibition may benefit myocardial function while reducing arrhythmias.

Figure 1. CaMKII becomes constitutively active by autophosphorylation and/or oxidation and constitutively active CaMKII promotes core events important for heart failure and arrhythmias

The CaMKII holoenzyme consists of two hexameric stacked rings. Under resting conditions the catalytic subunit is conformationally constrained so that CaMKII is inactive (top). CaMKII is initially activated when Ca²⁺ bound calmodulin (Ca²⁺/CaM, dumbbell shapes) bind to the regulatory domain (blue segments) causing a more extended conformation where the catalytic domain (green) becomes accessible to substrate proteins and ATP. Sustained increases in Ca²⁺ lead to autophosphorylation at Thr 287, enhancing the avidity of CaM binding but also inducing a Ca²⁺/CaM-independent form of CaMKII after CaM unbinding. Oxidation of Met 281/282 induces a Ca²⁺/CaM-independent form of CaMKII without CaM trapping. Increased levels of autophosphorylated or oxidized CaMKII favor heart failure and arrhythmias, at least in part, by activating hypertrophic and inflammatory gene programs, inducing proarrhythmic electrical remodeling, increasing myocyte death and fibrosis and defective intracellular Ca²⁺ homeostasis.

Figure 2. The domain structure of a CaMKII monomer

Each CaMKII monomer has a C terminus association domain and an N terminus catalytic domain. The internal regulatory domain consists of a C terminal side Ca²⁺/CaM binding region (CaM-B) and an N terminal side autoinhibitory region (AI). Oxidation at Met 281/282 or autophosphorylation at Thr 287 prevent reassociation of the catalytic domain and the AI sequence, leading to constitutive, Ca²⁺/CaM-independent CaMKII activity.

Figure 3. Myocardial cell ultrastructure creates close associations between CaMKII targets important for membrane excitability and intracellular Ca²⁺ homeostasis

Activated CaMKII enhances cellular Ca²⁺ entry by catalyzing phosphorylation of voltage-gated Ca²⁺ channels (mostly Ca_V1.2 in ventricle) and ryanodine receptor (RyR2) Ca²⁺ release channels. Ca_V1.2 and RyR2 are brought into close association (~10 nm) by Ca_V1.2 enrichment on T-tubular sarcolemmal invaginations. CaMKII phosphorylation of voltage-gated Na⁺ channels (mostly Na_V1.5 in ventricle) increases subsarcolemmal Na⁺ concentration that disadvantages the electrochemical driving force for Ca²⁺ extrusion from the cell by the Na⁺/Ca²⁺ exchanger (NCX), leading to increased intracellular Ca²⁺ concentration.

Figure 4. CaMKII anchoring at Ca_V1.2 and Na_V1.5

CaMKII associates with two important voltage-gated ion channels by catalytic domain binding to a recognition sequence that resembles the autoinhibitory region of the CaMKII regulatory domain. A sequence conserved across validated CaMKII targets is present on the β subunit of CaV1.2 (bottom panel) and on β_IV spectrin, a cytoskeletal protein associated with Ankyrin G and the Na_V1.5 (top panel). These sequences are required for CaMKII to regulate Ca_V1.2 and Na_V1.5 currents.

Figure 5. Oxidized CaMKII leads to SA node cell death and SA node dysfunction

Under physiological conditions (top) the SA node initiates each heart beat by inducing an inward current of sufficient magnitude to depolarize surrounding atrial myocardium with high fidelity. Activation of NADPH oxidase by angiotensin II leads to CaMKII activation by oxidation and excessive oxidized CaMKII leads to SA node cell apoptosis. SA node dysfunction occurs when SA node cell loss beyond a critical threshold prevents high fidelity capture of surrounding myocardium (bottom).