Proteins within the intracellular calcium store determine cardiac RyR channel activity and cardiac output

Abstract

The contractile function of the heart requires the release of Ca(2+) from intracellular Ca(2+) stores in the sarcoplasmic reticulum (SR) of cardiac muscle cells. The efficacy of Ca(2+) release depends on the amount of Ca(2+) loaded into the Ca(2+) store and the way in which this 'Ca(2+) load' influences the activity of the cardiac ryanodine receptor Ca(2+) release channel (RyR2). The effects of the Ca(2+) load on Ca(2+) release through RyR2 are facilitated by: (i) the sensitivity of RyR2 itself to luminal Ca(2+) concentrations; and (ii) interactions between the cardiac Ca(2+) -binding protein calsequestrin (CSQ) 2 and RyR2, transmitted through the 'anchoring' proteins junctin and/or triadin. Mutations in RyR2 are linked to catecholaminergic polymorphic ventricular tachycardia (CPVT) and sudden cardiac death. The tachycardia is associated with changes in the sensitivity of RyR2 to luminal Ca(2+) . Triadin-, junctin- or CSQ-null animals survive, but their longevity and ability to tolerate stress is compromised. These studies reveal the importance of the proteins in normal muscle function, but do not reveal the molecular nature of their functional interactions, which must be defined before changes in the proteins leading to CPVT and heart disease can be understood. Herein, we discuss known interactions between the RyR, triadin, junctin and CSQ with emphasis on the cardiac isoforms of the proteins. Where there is little known about the cardiac isoforms, we discuss evidence from skeletal isoforms.

Calsequestrin interacts directly with the cardiac ryanodine receptor luminal domain

“…3F) is also consistent with the increase in Ca 2+ -binding sites within the store provided by CSQ2, indicating that more time is required to reach a high-enough level of free luminal Ca 2+ to trigger the next Ca 2+ -release event (Jiang et al, 2002). In addition, it has been reported that although RyR2 activity decreases when CSQ2 is dissociated (Wei et al, 2009), the relative open probability increases more steeply with luminal Ca 2+ after CSQ2 dissociation (Dulhunty et al, 2012), in agreement with our findings that the rate of spontaneous Ca 2+ release is increased in the absence of CSQ2 (Fig. 3C).…”

Calcium Signaling and Cardiac Arrhythmias

“…95 CASQ2 is part of the RyR2 macromolecular complex which also involves the SR-proteins TRDN and JCTN. 96, 97 . The levels of these proteins are often dramatically altered when CASQ2 is genetically ablated or mutated.…”

A novel cytoplasmic interaction between junctin and ryanodine receptor calcium release channels

“…Indeed junctin-knockout and knockout or mutation of CSQ both increase channel leak and can lead to arrhythmia (Altschafl et al, 2011; Faggioni and Knollmann, 2012). In bilayer studies, experimental removal of CSQ2 from RyR2 increases the sensitivity of RyR2 to changes in luminal Ca 2+ , in a manner that could lead to arrhythmia (Dulhunty et al, 2012). Similar changes in sensitivity to luminal [Ca 2+ ] are seen with junctin knockout (Altschafl et al, 2011) and are likely due to removal of the link to CSQ2.…”

“…Inherited or acquired changes in Ca 2+ release lead to skeletal and cardio-myopathies and cardiac death (Györke and Carnes, 2008). The efficacy of Ca 2+ release depends an influence of the ‘Ca 2+ load’ in the SR on the activity of ryanodine receptor (RyR) Ca 2+ release channels, which is facilitated by a Ca 2+ -dependent interaction between CSQ proteins (also known as CASQ) and the RyR through the ‘anchoring’ protein junctin, which is a non-catalytic splice variant encoded by the aspartate-β-hydroxylase ( Asph ) gene (Dulhunty et al, 2009; Dulhunty et al, 2012; Wei et al, 2009a). A second ‘anchoring’ protein, triadin also binds to CSQ and the RyR, but does not transmit signals from CSQ to the RyR in skeletal muscle in vitro (Wei et al, 2009a), although it does influence Ca 2+ release during excitation–contraction coupling in intact myotubes (Goonasekera et al, 2007) and anchoring of CSQ to the junctional SR membrane (Boncompagni et al, 2012).…”