Background:
Calsequestrins (Casqs), comprising the Casq1 and Casq2 isoforms, buffer Ca
2+
and regulate its release in the sarcoplasmic reticulum (SR) of skeletal and cardiac muscle, respectively. Human inherited diseases associated with mutations in
CASQ1
or
CASQ2
include malignant hyperthermia/environmental heat stroke (MH/EHS) and catecholaminergic polymorphic ventricular tachycardia. However, patients with an MH/EHS event often suffer from arrhythmia for which the underlying mechanism remains unknown.
Methods:
Working hearts from conventional (
Casq1
-KO) and cardiac-specific (
Casq1
-CKO)
Casq1
knockout mice were monitored
in vivo
and
ex vivo
by electrocardiogram and electrical mapping, respectively. MH was induced by 2% isoflurane and treated intraperitoneally with dantrolene. Time-lapse imaging was used to monitor intracellular Ca
2+
activity in isolated mouse cardiomyocytes or neonatal rat ventricular myocytes (NRVMs) with knockdown, over-expression or truncation of the
Casq1
gene. Conformational change in both Casqs was determined by crosslinking Western blot analysis.
Results:
Like MH/EHS patients,
Casq1
-KO and
Casq1
-CKO mice had faster basal heart rate, and ventricular tachycardia upon exposure to 2% isoflurane, which could be relieved by dantrolene. Basal sinus tachycardia and ventricular ectopic electrical triggering also occurred in
Casq1
-KO hearts
ex vivo
. Accordingly, the ventricular cardiomyocytes from
Casq1
-CKO mice displayed dantrolene-sensitive increased Ca
2+
waves and diastole premature Ca
2+
transients/oscillations upon isoflurane. NRVMs with Casq1-knockdown had enhanced spontaneous Ca2+ sparks/transients upon isoflurane, while cells over-expressing Casq1 exhibited decreased Ca2+ sparks/transients that were absent in cells with truncation of 9 amino acids at the C-terminus of Casq1. Structural evaluation showed that most of the Casq1 protein was present as a polymer and physically interacted with RyR2 in the ventricular SR. The Casq1 isoform was also expressed in human myocardium. Mechanistically, exposure to 2% isoflurane or heating at 41ºC induced Casq1 oligomerization in mouse ventricular and skeletal muscle tissues, leading to a reduced Casq1/RyR2 interaction and increased RyR2 activity in the ventricle.
Conclusions:
Casq1 is expressed in the heart, where it regulates SR Ca
2+
release and heart rate. Casq1 deficiency independently causes MH/EHS-like ventricular arrhythmia by trigger-induced Casq1 oligomerization and a relief of its inhibitory effect on RyR2-mediated Ca
2+
release, thus revealing a new inherited arrhythmia and a novel mechanism for MH/EHS arrhythmogenesis.
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