Modest reductions of cardiac calsequestrin increase sarcoplasmic reticulum Ca2+ leak independent of luminal Ca2+ and trigger ventricular arrhythmias in mice.
Cardiac calsequestrin-null mice (Casq2-/-) display catecholaminergic ventricular tachycardia akin to humans with CASQ2 mutations. However, the specific contribution of Casq2 deficiency to the arrhythmia phenotype is difficult to assess because Casq2-/- mice also show significant reductions in the sarcoplasmic reticulum (SR) proteins junctin and triadin-1 and increased SR volume. Furthermore, it remains unknown whether Casq2 regulates SR Ca2+ release directly or indirectly by buffering SR luminal Ca2+. To address both questions, we examined heterozygous (Casq2+/-) mice, which have a 25% reduction in Casq2 but no significant decrease in other SR proteins. Casq2+/- mice (n=35) challenged with isoproterenol displayed 3-fold higher rates of ventricular ectopy than Casq2+/+ mice (n=31; P<0.05). Programmed stimulation induced significantly more ventricular tachycardia in Casq2+/- mice than in Casq2+/+ mice. Field-stimulated Ca2+ transients, cell shortening, L-type Ca2+ current, and SR volume were not significantly different in Casq2+/- and Casq2+/+ myocytes. However, in the presence of isoproterenol, SR Ca2+ leak was significantly increased in Casq2+/- myocytes (Casq2+/- 0.18+/-0.02 F(ratio) versus Casq2+/+ 0.11+/-0.01 F(ratio), n=57, 60; P<0.01), resulting in a significantly higher rate of spontaneous SR Ca2+ releases and triggered beats. SR luminal Ca2+ measured using Mag-Fura-2 was not altered by Casq2 reduction. As a result, the relationship between SR Ca2+ leak and SR luminal Ca2+ was significantly different between Casq2+/- and Casq2+/+ myocytes (P<0.01). Thus, even modest reductions in Casq2 increase SR Ca2+ leak and cause ventricular tachycardia susceptibility under stress. The underlying mechanism is likely the direct regulation of SR Ca2+ release channels by Casq2 rather than altered luminal Ca2+.
Oates Institute for Experimental Therapeutics, and Division of Clinical Pharmacology, Vanderbilt University Medical Center, 1265 Medical Research Building IV, Nashville, TN 37232-0575, USA.
Nagesh
N
Chopra
Prince J
PJ
Kannankeril
Tao
T
Yang
Thinn
T
Hlaing
Izabela
I
Holinstat
Kristen
K
Ettensohn
Karl
K
Pfeifer
Brandy
B
Akin
Larry R
LR
Jones
Clara
C
Franzini-Armstrong
Björn C
BC
Knollmann
D000818:Animals; D064032:Calbindin 2; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism*; D020013:Calcium Signaling; D002155:Calsequestrin / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D002304:Cardiac Pacing, Artificial; D003971:Diastole; D004195:Disease Models, Animal; D006339:Heart Rate; D015640:Ion Channel Gating; D007545:Isoproterenol; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac / Q000378:metabolism* / Q000648:ultrastructure; D064030:S100 Calcium Binding Protein G / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure; D017180:Tachycardia, Ventricular / Q000139:chemically induced / Q000235:genetics / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology; D013997:Time Factors; D016276:Ventricular Function; D018879:Ventricular Premature Complexes / Q000139:chemically induced / Q000235:genetics / Q000378:metabolism* / Q000473:pathology / Q000503:physiopathology
D064032:Calbindin 2; D015220:Calcium Channels; D002155:Calsequestrin; D064030:S100 Calcium Binding Protein G; C514388:casq2 protein, mouse; D007545:Isoproterenol; D002118:Calcium