Ryanodine receptor cluster fragmentation and redistribution in persistent atrial fibrillation enhance calcium release.
OBJECTIVE
In atrial fibrillation (AF), abnormalities in Ca(2+) release contribute to arrhythmia generation and contractile dysfunction. We explore whether ryanodine receptor (RyR) cluster ultrastructure is altered and is associated with functional abnormalities in AF.


RESULTS
Using high-resolution confocal microscopy (STED), we examined RyR cluster morphology in fixed atrial myocytes from sheep with persistent AF (N = 6) and control (Ctrl; N = 6) animals. RyR clusters on average contained 15 contiguous RyRs; this did not differ between AF and Ctrl. However, the distance between clusters was significantly reduced in AF (288 ± 12 vs. 376 ± 17 nm). When RyR clusters were grouped into Ca(2+) release units (CRUs), i.e. clusters separated by <150 nm, CRUs in AF had more clusters (3.43 ± 0.10 vs. 2.95 ± 0.02 in Ctrl), which were more dispersed. Furthermore, in AF cells, more RyR clusters were found between Z lines. In parallel experiments, Ca(2+) sparks were monitored in live permeabilized myocytes. In AF, myocytes had >50% higher spark frequency with increased spark time to peak (TTP) and duration, and a higher incidence of macrosparks. A computational model of the CRU was used to simulate the morphological alterations observed in AF cells. Increasing cluster fragmentation to the level observed in AF cells caused the observed changes, i.e. higher spark frequency, increased TTP and duration; RyR clusters dispersed between Z-lines increased the occurrence of macrosparks.


CONCLUSIONS
In persistent AF, ultrastructural reorganization of RyR clusters within CRUs is associated with overactive Ca(2+) release, increasing the likelihood of propagating Ca(2+) release.
Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium Department of Molecular Neuroscience, George Mason University, Fairfax, VA, USA Institute of Cardiovascular Sciences, University of Glasgow, Glasgow, UK.
Niall
N
Macquaide
School of Systems Biology, George Mason University, Manassas, VA, USA.
Hoang-Trong Minh
HT
Tuan
Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, KU Leuven, Leuven, Belgium.
Jun-Ichi
J
Hotta
Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, KU Leuven, Leuven, Belgium.
Wouter
W
Sempels
Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium.
Ilse
I
Lenaerts
Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium.
Patricia
P
Holemans
Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, KU Leuven, Leuven, Belgium.
Johan
J
Hofkens
Department of Cardiology, University Hospitals Leuven, Leuven, Belgium.
M Saleet
MS
Jafri
Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium Department of Molecular Neuroscience, George Mason University, Fairfax, VA, USA Institute of Cardiovascular Sciences, University of Glasgow, Glasgow, UK.
Rik
R
Willems
Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium Karin.Sipido@med.kuleuven.be Niall.Macquaide@glasgow.ac.uk.
Karin R
KR
Sipido
D000818:Animals; D001281:Atrial Fibrillation / Q000378:metabolism* / Q000503:physiopathology; D020013:Calcium Signaling; D003198:Computer Simulation; D004195:Disease Models, Animal; D006325:Heart Atria / Q000378:metabolism / Q000503:physiopathology / Q000648:ultrastructure; D007700:Kinetics; D018613:Microscopy, Confocal; D008955:Models, Cardiovascular; D008958:Models, Molecular; D032383:Myocytes, Cardiac / Q000378:metabolism* / Q000648:ultrastructure; D011487:Protein Conformation; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism* / Q000648:ultrastructure; D012756:Sheep; D013329:Structure-Activity Relationship
D019837:Ryanodine Receptor Calcium Release Channel
Atrial fibrillation; Atrial myocytes; Ryanodine receptor; Sarcoplasmic reticulum; Super-resolution microscopy