Abstract
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a
familial arrhythmogenic disorder associated with mutations in the
cardiac ryanodine receptor (RyR2) and cardiac calsequestrin (CASQ2)
genes. Previous in vitro studies suggested that RyR2 and CASQ2 interact
as parts of a multimolecular Ca$^2+$-signaling complex; however,
direct evidence for such interactions and their potential significance
to myocardial function remain to be determined. We identified a novel
CASQ2 mutation in a young female with a structurally normal heart
and unexplained syncopal episodes. This mutation results in the nonconservative
substitution of glutamine for arginine at amino acid 33 of CASQ2
(R33Q). Adenoviral-mediated expression of CASQ2(R33Q) in adult rat
myocytes led to an increase in excitation-contraction coupling gain
and to more frequent occurrences of spontaneous propagating (Ca$^2+$
waves) and local Ca$^2+$ signals (sparks) with respect to control
cells expressing wild-type CASQ2 (CASQ2WT). As revealed by a Ca$^2+$
indicator entrapped inside the sarcoplasmic reticulum (SR) of permeabilized
myocytes, the increased occurrence of spontaneous Ca$^2+$ sparks
and waves was associated with a dramatic decrease in intra-SR Ca$^2+$.
Recombinant CASQ2WT and CASQ2R33Q exhibited similar Ca$^2+$-binding
capacities in vitro; however, the mutant protein lacked the ability
of its WT counterpart to inhibit RyR2 activity at low luminal Ca$^2+$
in planar lipid bilayers. We conclude that the R33Q mutation disrupts
interactions of CASQ2 with the RyR2 channel complex and impairs regulation
of RyR2 by luminal Ca$^2+$. These results show that intracellular
Ca$^2+$ cycling in normal heart relies on an intricate interplay
of CASQ2 with the proteins of the RyR2 channel complex and that disruption
of these interactions can lead to cardiac arrhythmia.
- acid
- amino
- animals;
- arginine;
- artificial,
- binding,
- calcium
- calcium,
- calsequestrin,
- cardiac
- cardiac,
- catecholamines,
- channel,
- competitive;
- death,
- etiology;
- exercise;
- female;
- genetics/metabolism/physiopathology
- genetics/metabolism;
- genetics;
- glutamine;
- humans;
- intracellular
- membranes,
- metabolism;
- methods;
- mutation;
- myocytes,
- pacing,
- patch-clamp
- proteins,
- rats;
- receptor
- recombinant
- release
- reticulum,
- ryanodine
- sarcoplasmic
- substitution;
- sudden,
- syncope,
- tachycardia,
- techniques;
- ventricular,
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