Elevated local [Ca2+] and CaMKII promote spontaneous Ca2+ release in ankyrin-B-deficient hearts

Aims

Loss-of-function mutations in the cytoskeletal protein ankyrin-B (AnkB) cause ventricular tachyarrhythmias in humans. Previously, we found that a larger fraction of the sarcoplasmic reticulum (SR) Ca²⁺ leak occurs through Ca²⁺ sparks in AnkB-deficient (AnkB^+/–) mice, which may contribute to arrhythmogenicity via Ca²⁺ waves. Here, we investigated the mechanisms responsible for increased Ca²⁺ spark frequency in AnkB^+/– hearts.

Methods and results

Using immunoblots and phospho-specific antibodies, we found that phosphorylation of ryanodine receptors (RyRs) by CaMKII is enhanced in AnkB^+/– hearts. In contrast, the PKA-mediated RyR phosphorylation was comparable in AnkB^+/– and wild-type (WT) mice. CaMKII inhibition greatly reduced Ca²⁺ spark frequency in myocytes from AnkB^+/– mice but had little effect in the WT. Global activities of the major phosphatases PP1 and PP2A were similar in AnkB^+/– and WT hearts, while CaMKII autophosphorylation, a marker of CaMKII activation, was increased in AnkB^+/– hearts. Thus, CaMKII-dependent RyR hyperphosphorylation in AnkB^+/– hearts is caused by augmented CaMKII activity. Intriguingly, CaMKII activation is limited to the sarcolemma–SR junctions since non-junctional CaMKII targets (phospholamban, HDAC4) are not hyperphosphorylated in AnkB^+/– myocytes. This local CaMKII activation may be the consequence of elevated [Ca²⁺] in the junctional cleft caused by reduced Na⁺/Ca²⁺ exchange activity. Indeed, using the RyR-targeted Ca²⁺ sensor GCaMP2.2-FBKP12.6, we found that local junctional [Ca²⁺] is significantly elevated in AnkB^+/– myocytes.

Conclusions

The increased incidence of pro-arrhythmogenic Ca²⁺ sparks and waves in AnkB^+/– hearts is due to enhanced CaMKII-mediated RyR phosphorylation, which is caused by higher junctional [Ca²⁺] and consequent local CaMKII activation.

from #AlexandrosSfakianakis via Alexandros G.Sfakianakis on Inoreader http://ift.tt/2alH7WX
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