title: Elevated local [Ca2+] and CaMKII promote spontaneous Ca2+ release in ankyrin-B-deficient hearts.
abstract: 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) Ca(2+) leak occurs through Ca(2+) sparks in AnkB-deficient (AnkB(+/-)) mice, which may contribute to arrhythmogenicity via Ca(2+) waves. Here, we investigated the mechanisms responsible for increased Ca(2+) spark frequency in AnkB(+/-) hearts.
  
  
  
  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 Ca(2+) 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 [Ca(2+)] in the junctional cleft caused by reduced Na(+)/Ca(2+) exchange activity. Indeed, using the RyR-targeted Ca(2+) sensor GCaMP2.2-FBKP12.6, we found that local junctional [Ca(2+)] is significantly elevated in AnkB(+/-) myocytes.
  
  
  
  The increased incidence of pro-arrhythmogenic Ca(2+) sparks and waves in AnkB(+/-) hearts is due to enhanced CaMKII-mediated RyR phosphorylation, which is caused by higher junctional [Ca(2+)] and consequent local CaMKII activation.
authors.affiliation: Department of Pharmacology and Nutritional Sciences, University of Kentucky, 900 S Limestone, Lexington, KY 40536, USA.
authors.forename: Iuliana
authors.initials: I
authors.lastname: Popescu
authors.affiliation: Department of Pharmacology, University of California Davis, Davis, CA 95616, USA.
authors.forename: Samuel
authors.initials: S
authors.lastname: Galice
authors.affiliation: The Dorothy M. Davis Heart and Lung Research Institute, The Ohio State University Wexner Medical Center, Columbus, OH 43210, USA Department of Physiology and Cell Biology, The Ohio State University Wexner Medical Center, Columbus, OH 43210, USA Department of Internal Medicine, The Ohio State University Wexner Medical Center, Columbus, OH 43210, USA.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Mohler
authors.affiliation: Department of Pharmacology and Nutritional Sciences, University of Kentucky, 900 S Limestone, Lexington, KY 40536, USA s.despa@uky.edu.
authors.forename: Sanda
authors.initials: S
authors.lastname: Despa
mesh_terms: D000818:Animals; D017487:Ankyrins / Q000172:deficiency* / Q000235:genetics; D001145:Arrhythmias, Cardiac / Q000201:enzymology / Q000235:genetics / Q000503:physiopathology; D015374:Biosensing Techniques; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2 / Q000378:metabolism*; D004789:Enzyme Activation; D005838:Genotype; D007365:Intercellular Junctions / Q000201:enzymology; D008564:Membrane Potentials; D018345:Mice, Knockout; D032383:Myocytes, Cardiac / Q000201:enzymology*; D010641:Phenotype; D010766:Phosphorylation; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000201:enzymology; D013997:Time Factors; D015854:Up-Regulation
chemical_list: C498995:Ank2 protein, mouse; D017487:Ankyrins; D019837:Ryanodine Receptor Calcium Release Channel; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D002118:Calcium
keywords: Ankyrin-B; Ca2+ sparks; CaMKII; Junctions; Local Ca2+ concentration