Abstract 15993: Ablation of Phosphatase-1-Inhibitor-1 is Detrimental in CaMKII Overexpression Despite Reduction of SR Ca2+ Leak
Background: CaMKII is a central regulator of cardiac myocyte Ca2+ homeostasis. It is hyperactive in human heart failure (HF) and transgenic CaMKII overexpression leads to severe HF. Detrimental CaMKII effects have been strongly associated with increased SR Ca2+ leak due to CaMKII dependent hyperphosphorylation of RyR2. Ablation of phosphatase-1-inhibitor-1 (I-1-KO) results in reduced RyR2 phosphorylation exclusively at the CaMKII site associated with protection against catecholamine induced HF progression and lethal arrhythmias.
Hypothesis: Ablation of I-1 might be beneficial in HF specifically by targeting CaMKII mediated SR Ca2+ leak.
Methods/results: I-1-KO mice were crossed with CaMKIIδc transgenic mice (TG), resulting in wildtype (WT), KO, TG and TG/KO mice. As expected, echocardiography showed already at 6 weeks of age a severe CaMKII-associated HF phenotype. Surprisingly, cardiac function was even further impaired in TG/KO compared to TG not only with respect to contractile dysfunction (FAS (%): WT 46±3, KO 49±4, TG 20±3, TG/KO 12±2; n≥9), but also to exaggerated cardiac hypertrophy (LVM/BW (mg/g): WT 3.8±0.3, KO 4.0±0.2, TG 5.2±0.3, TG/KO 6.0±0.4; n≥9). Even more surprising, this is despite a reduction of SR Ca2+ leak due to I-1 ablation as assessed by Ca2+ sparks in isolated myocytes (CaSpF (1/s*100µm): WT 0.64±0.08, KO 0.44±0.07, TG 1.05±0.11, TG/KO 0.79±0.07; n≥349), and also despite reduced arrhythmic non-stimulated-events during 20s of rest in these cells (WT 31%, KO 17%, TG 54%, TG/KO 45%). Despite reduced SR Ca2+ leak, Ca2+ transient amplitude in myocytes was not significantly altered or even diminished upon I-1-KO (Fura-2 ratio at 4Hz: WT 0.28±0.02, KO 0.25±0.02, TG 0.23±0.02, TG/KO 0.19±0.01).
Conclusion: Contrasting our initial hypothesis, ablation of I-1 does not protect from CaMKII-induced HF, but even worsens cardiac function in this context. This is despite the expected reduction of SR Ca2+ leak upon I-1 ablation. We hypothesize that other as yet unknown SR-independent I-1 effects/downstream targets or unexpected effects of I-1 on SR Ca2+ reuptake are responsible. In conclusion, caution appears to be warranted with respect to inhibiting I-1 in context of a strong inductor of HF such as CaMKII overexpression.
- © 2011 by American Heart Association, Inc.