Tale of two kinases: Protein kinase A and Ca2+/calmodulin-dependent protein kinase II in pre-diabetic cardiomyopathy

Figure 1 β-Adrenergic stimulation in the normal heart and pre-diabetic cardiomyopathy. Left panel: β-adrenergic stimulation in the normal heart. In physiological excitation-contraction coupling, membrane depolarization activates L-type voltage-dependent Ca²⁺ channels, inducing a small Ca²⁺ influx (I_Ca) that triggers the activation of cardiac ryanodine receptors (RyR2, PDB accession code: 6WOV). This triggers the release of sufficient Ca²⁺ from the lumen of the sarcoplasmic reticulum to the cytoplasm to elicit contraction. During relaxation, Ca²⁺ is primarily removed from the cytoplasm by the sarcoplasmic reticulum Ca²⁺ ATPase (PDB accession code: 6HXB), which resequesters Ca²⁺ into the sarcoplasmic reticulum lumen. Ca²⁺ is also extruded by the Na⁺/Ca²⁺ exchanger (PDB accession code: 3US9), while a small amount of Ca²⁺ is taken up by the mitochondrial calcium uniporter (PDB accession code: 6WDN). Noradrenaline activates β₁-adrenoceptors (β₁-ARs, PDB accession code: 6H70) located at the sarcolemma of cardiomyocytes; agonist-bound β₁-ARs stimulate Ga_s proteins and therefore one or more isoforms of adenylyl cyclase (PDB accession code: 6R3Q), leading to cAMP formation and the activation of the cAMP-dependent protein kinase (PDB accession code: 3FHI). Protein kinase A phosphorylates several Ca²⁺ handling proteins, including RyR2 at Ser²⁸⁰⁸ and phospholamban (PDB accession code: 2LPF) at Ser¹⁶; the latter increases sarcoplasmic reticulum Ca²⁺ ATPase pump activity. Ca²⁺ binds to calmodulin, and the complex Ca/calmodulin binds to and activates Ca²⁺/calmodulin-dependent protein kinase II (PDB accession code: 3SOA), which phosphorylates RyR at Ser²⁸¹⁴ and phospholamban at Thr¹⁷. The exchange protein directly activated by cAMP (Epac) is also involved in Ca²⁺/calmodulin-dependent protein kinase II activation; however, its role in pre-diabetic cardiomyopathy has not yet been addressed; thus, it is not depicted in the figure. Right panel: β-adrenergic stimulation in pre-diabetic cardiomyopathy. In the presence of obesity, increased triglyceride levels, decreased high-density lipoprotein cholesterol, hypertension, and/or insulin resistance (all Metabolic Syndrome components), and abnormal β₁-AR activation (associated with either chronic sympathetic tone or changes in β-AR expression) dysregulates excitation-contraction coupling in cardiac cells. Pre-diabetic cardiomyopathy is characterized by abnormal diastolic Ca²⁺ leak (diastolic dysfunction) due to augmented RyR2 phosphorylation at Ser²⁸⁰⁸ and Ser²⁸¹⁴ in the absence of adrenergic stimulation, generating spontaneous Ca²⁺ waves that may induce pro-arrhythmogenic events through altered Na⁺/Ca²⁺ exchanger activity. In addition, phosphorylated phospholamban (at Ser¹⁶ and Thr¹⁷) detaches from sarcoplasmic reticulum Ca²⁺ ATPase 2a, augmenting its activity; finally, Ca²⁺ transient amplitude decreases and leads to impaired cell contraction. NA: Noradrenaline; AR: Adrenoceptors; NCX: Na⁺/Ca²⁺ exchanger; AC: Adenylyl cyclase; PKA: Protein kinase A; CaMKII: Ca²⁺/calmodulin-dependent protein kinase II; CaM: Calmodulin; RyR: Ryanodine receptor; LTCC: L-type voltage-dependent Ca²⁺ channels; PLN: Phospholamban; HDL-C: High-density lipoprotein cholesterol; TG: Triglycerides; SERCA: Sarcoplasmic reticulum Ca²⁺ ATPase.