Ca2+/calmodulin-dependent kinase IIδC-induced chronic heart failure does not depend on sarcoplasmic reticulum Ca2+...

Hyperactivity of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) has emerged as a central cause of pathologic remodelling in heart failure. It has been suggested that CaMKII-induced hyperphosphorylation of the ryanodine receptor 2 (RyR2) and consequently increased diastolic Ca²⁺ leak from the sarcoplasmic reticulum (SR) is a crucial mechanism by which increased CaMKII activity leads to contractile dysfunction. We aim to evaluate the relevance of CaMKII-dependent RyR2 phosphorylation for CaMKII-induced heart failure development in vivo.

We crossbred CaMKIIδC overexpressing [transgenic (TG)] mice with RyR2-S2814A knock-in mice that are resistant to CaMKII-dependent RyR2 phosphorylation. Ca²⁺-spark measurements on isolated ventricular myocytes confirmed the severe diastolic SR Ca²⁺ leak previously reported in CaMKIIδC TG [4.65 ± 0.73 mF/F₀ vs. 1.88 ± 0.30 mF/F₀ in wild type (WT)]. Crossing in the S2814A mutation completely prevented SR Ca²⁺-leak induction in the CaMKIIδC TG, both regarding Ca²⁺-spark size and frequency, demonstrating that the CaMKIIδC-induced SR Ca²⁺ leak entirely depends on the CaMKII-specific RyR2-S2814 phosphorylation. Yet, the RyR2-S2814A mutation did not affect the massive contractile dysfunction (ejection fraction = 12.17 ± 2.05% vs. 45.15 ± 3.46% in WT), cardiac hypertrophy (heart weight/tibia length = 24.84 ± 3.00 vs. 9.81 ± 0.50 mg/mm in WT), or severe premature mortality (median survival of 12 weeks) associated with cardiac CaMKIIδC overexpression. In the face of a prevented SR Ca²⁺ leak, the phosphorylation status of other critical CaMKII downstream targets that can drive heart failure, including transcriptional regulator histone deacetylase 4, as well as markers of pathological gene expression including Xirp2, Il6, and Col1a1, was equally increased in hearts from CaMKIIδC TG on a RyR WT and S2814A background.

S2814 phosphoresistance of RyR2 prevents the CaMKII-dependent SR Ca²⁺ leak induction but does not prevent the cardiomyopathic phenotype caused by enhanced CaMKIIδC activity. Our data indicate that additional mechanisms-independent of SR Ca²⁺ leak-are critical for the maladaptive effects of chronically increased CaMKIIδC activity with respect to heart failure.