Figure 3. Simplified representation of the excitation-contraction coupling in cardiomyocytes and main gene therapy targets. The cardiac action potential is initiated in the myocardial cell membrane, and the depolarization results in inward Ca2+ through L-type Ca2+ channels from extracellular to intracellular fluid during plateau. Entry of Ca2+ into the myocardial cell triggers the release of more Ca2+ from stores in the sarcoplasmic reticulum (SR) through ryanodine receptors. Ca2+ binds to troponin C, tropomyosin is moved, and the actin and myosin can bind and tension is produced. Relaxation occurs when Ca2+ reaccumulated in the SR by the action of the Ca2+ ATPase and Ca2+–Na+ exchange in the sarcolemmal membrane. After the onset of HF, most promising targets are: 1. SERCA2a (Lilac Star). Abnormal Ca2+ leak from SR; depressed SR Ca2+ re-uptake caused by low expression of cardiac sarcoplasmic reticulum Ca2+ ATPase (SERCA2a) pump and an increased activity of endogenous inhibitor of SERCA2a, phospholamban. SERCA2a has a critical role in Ca2+ regulation. The overexpression of SERCA2a is one of the targets for the treatment of HF. 2. βARKct (Lilac Star). Molecular abnormalities associated with HF include the uncoupling of the β-adrenergic receptor system, enhanced expression and activity of the G protein-coupled receptor kinase and loss of βAR inotropic reserve. βARKct gene delivery approach has the potential to resolve βAR downregulation and desensitization. 3. Phospholamban (Lilac Star) is an endogenous inhibitor of the SR Ca2+-ATPase. Phosphorylation of phospholamban by cyclic AMP- dependent or calmodulin-dependent protein kinases (PKA or CaMKII) relieves this inhibition, allowing faster twitchrelaxation and decline of intracellular Ca2+. Because the SR Ca2+-ATPase competes better with Na+/Ca2+ exchange, phosphorylation of phospholamban also enhances Ca2+ content in the SR. 4. S100A1 (Lilac Star) plays a role in increasing SERCA2a activity, diminishing diastolic SR Ca2+ leak, and augmenting systolic open probability of the ryanodine receptors, causing an overall gain in SR Ca2+ cycling. Also, S100A1 regulates SERCA2A-phospholamban function, resulting in a balanced enhancement of SR Ca2+ release and uptake. 5. In failing hearts, the downregulation of adrenergic receptor and cAMP-dependent protein kinase signaling leads to the inactivation of inhibitor-1 which, in turn, results in increased activity of Protein Phosphatase 1 (Lilac Star). This activation leads to the dephosphorylation of phospholamban thus reducing calcium uptake by SERCA-2a. PKA, protein kinase A; RyR, ryanodine receptors; FKB12, calstabin 2; AngII, angiotensin II; ET-1, endothelin 1; NE, norepinephrine; ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; AC, adenylyl cyclase; PDE, phosphodiesterase; Gq, class of guanine nucleotide-binding proteins; PLCβ, phospholipase C beta; DAG, diacylglycerol; IP3, inositol trisphosphate; Gs/Gi, stimulatory/inhibitory G protein; L-type, long lasting dihydropyridine receptors. Lilac stars indicate the main gene therapy targets. Reproduced with modifications .
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