Flux regulation of cardiac ryanodine receptor channels

Zhong

et al. 2014

Biochemical Journal

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Cardiac calsequestrin (CASQ2) is commonly believed to serve as the sarcoplasmic reticulum (SR) luminal Ca2+ sensor. Ablation of CASQ2 promotes spontaneous Ca2+ waves (SCWs) and catecholaminergic polymorphic ventricular tachycardia (CPVT) upon stress but not at rest. How SCWs and CPVT are triggered by stress in the absence of the CASQ2-based luminal Ca2+ sensor is an important unresolved question. Here we assessed the role of the newly identified RyR2-resident luminal Ca2+ sensor in determining SCW propensity, CPVT susceptibility, and cardiac hypertrophy in Casq2 knockout (KO) mice. We crossbred Casq2 KO mice with RyR2 mutant (E4872Q+/−) mice, which lack RyR2-resident SR luminal Ca2+ sensing, to generate animals with both deficiencies. Casq2+/− and Casq2−/− mice showed stress-induced ventricular tachyarrhythmias (VTs), whereas Casq2+/−/E4872Q+/− and Casq2−/−/E4872Q+/− mice displayed little or no stress-induced VTs. Confocal Ca2+ imaging revealed that Casq2−/− hearts frequently exhibited SCWs after extracellular Ca2+ elevation or adrenergic stimulation, while Casq2−/−/E4872Q+/− hearts had few or no SCWs under the same conditions. Cardiac hypertrophy developed and CPVT susceptibility increased with age in Casq2−/− mice, but not in Casq2−/−/E4872Q+/− mice. However, the amplitudes and dynamics of voltage-induced Ca2+ transients in Casq2−/− and Casq2−/−/E4872Q+/− hearts were not significantly different. Our results indicate that SCWs, CPVT and hypertrophy in Casq2 null cardiac muscle are governed by the RyR2-resident luminal Ca2+ sensor. This implies that defects in CASQ2-based luminal Ca2+ sensing can be overridden by the RyR2-resident luminal Ca2+ sensor. This makes this RyR2-resident sensor a promising molecular target for the treatment of Ca2+-mediated arrhythmias.

Section: Discussionmentioning

confidence: 99%

“…How elevating SR Ca 2+ load opens the RyR2 channel and triggers SCWs is unclear. The RyR2 channel is thought to be regulated by an intra-SR Ca 2+ sensing mechanism(s) that promotes RyR2 opening as SR Ca 2+ content rises [28-32]. The molecular nature of the intra-SR Ca 2+ sensor is debated.…”

Section: Introductionmentioning

confidence: 99%

The cardiac ryanodine receptor luminal Ca2+ sensor governs Ca2+ waves, ventricular tachyarrhythmias and cardiac hypertrophy in calsequestrin-null mice

Zhang

Zhong

et al. 2014

Biochemical Journal

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“…Studies of chimerical channels of mammalian Slo1 and dSlo1 revealed that the S0 TM segment is important for the functional effects of β subunits (Wallner et al, 1996; Morrow et al, 2006; Lee et al, 2010). By engineering disulfide linkages, Marx and colleagues model S0 outside of the VSD adjacent to the S3 and S4 helices, while the two TM segments of the β subunits (TM1 and TM2) are packed close to each other at the mouth of the cleft between VSDs of two adjacent Slo1 subunits (Liu et al, 2008a, 2010; Figure 1B). Within this cleft, TM1 is close to S1 of one VSD and TM2 close to S0 of the adjacent VSD (Liu et al, 2008b, 2010; Wu et al, 2009; Zakharov et al, 2009).…”

Section: Bk Channel β Subunitsmentioning

confidence: 99%

Regulation of Voltage-Activated K+ Channel Gating by Transmembrane β Subunits

Sun¹,

Zaydman²,

Cui³

2012

Front. Pharmacol.

Voltage-activated K+ (KV) channels are important for shaping action potentials and maintaining resting membrane potential in excitable cells. KV channels contain a central pore-gate domain (PGD) surrounded by four voltage-sensing domains (VSDs). The VSDs will change conformation in response to alterations of the membrane potential thereby inducing the opening of the PGD. Many KV channels are heteromeric protein complexes containing auxiliary β subunits. These β subunits modulate channel expression and activity to increase functional diversity and render tissue specific phenotypes. This review focuses on the KV β subunits that contain transmembrane (TM) segments including the KCNE family and the β subunits of large conductance, Ca2+- and voltage-activated K+ (BK) channels. These TM β subunits affect the voltage-dependent activation of KV α subunits. Experimental and computational studies have described the structural location of these β subunits in the channel complexes and the biophysical effects on VSD activation, PGD opening, and VSD–PGD coupling. These results reveal some common characteristics and mechanistic insights into KV channel modulation by TM β subunits.

“…One proposed mechanism, the “feed-through” hypothesis, suggests that luminal Ca 2+ passes through an open RyR2 and acts on its own cytosolic Ca 2+ activation site 14, 36 . However, there is an accumulating body of evidence indicating that luminal Ca 2+ activation of single RyR2 is mediated by some luminal Ca 2+ sensing mechanism(s) that is (or are) structurally distinct from the RyR’s cytosolic Ca 2+ activation site 13, 19, 37–40 . The molecular nature of the luminal Ca 2+ sensing mechanism(s) is poorly understood.…”

Section: Introductionmentioning

confidence: 99%

The ryanodine receptor store-sensing gate controls Ca2+ waves and Ca2+-triggered arrhythmias

Wang

et al. 2014

Nat Med

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180

244

Spontaneous Ca2+ release from intracellular stores is important for various physiological and pathological processes. In cardiac muscle cells, spontaneous store overload-induced Ca2+ release (SOICR) can result in Ca2+ waves, a major cause of ventricular tachyarrhythmias (VTs) and sudden death. The molecular mechanism underlying SOICR has been a mystery for decades. Here, we show that a point mutation E4872A in the helix bundle crossing (the proposed gate) of the cardiac ryanodine receptor (RyR2) completely abolishes luminal, but not cytosolic, RyR2 Ca2+ activation. Introducing metal-binding histidines at this site converts RyR2 into a luminal Ni2+ gated channel. Mouse hearts harboring an RyR2 mutation at this site (E4872Q+/−) are resistant to store overload-induced Ca2+ waves and completely protected against Ca2+-triggered VTs. These data show that the RyR2 gate directly senses store Ca2+, explaining RyR2 store Ca2+ regulation, Ca2+ wave initiation, and Ca2+-triggered arrhythmias. This novel store-sensing gate structure is conserved in all RyRs and inositol 1,4,5-trisphosphate receptors.