Ryanodine receptor sensitivity governs the stability and synchrony of local calcium release during cardiac excitation-contraction coupling

Abstract

Calcium-induced calcium release is the principal mechanism that triggers the cell-wide [Ca2+]i transient that activates muscle contraction during cardiac excitation-contraction coupling (ECC). Here, we characterize this process in mouse cardiac myocytes with a novel mathematical action potential (AP) model that incorporates realistic stochastic gating of voltage-dependent L-type calcium (Ca2+) channels (LCCs) and sarcoplasmic reticulum (SR) Ca2+ release channels (the ryanodine receptors, RyR2s). Depolarization of the sarcolemma during an AP stochastically activates the LCCs elevating subspace [Ca2+] within each of the cell’s 20,000 independent calcium release units (CRUs) to trigger local RyR2 opening and initiate Ca2+ sparks, the fundamental unit of triggered Ca2+ release. Synchronization of Ca2+ sparks during systole depends on the nearly uniform cellular activation of LCCs and the likelihood of local LCC openings triggering local Ca2+ sparks (ECC fidelity). The detailed design and true SR Ca2+ pump/leak balance displayed by our model permits investigation of ECC fidelity and Ca2+ spark fi-delity, the balance between visible (Ca2+ spark) and invisible (Ca2+ quark/sub-spark) SR Ca2+ release events. Excess SR Ca2+ leak is examined as a disease mechanism in the context of “catecholaminergic polymorphic ventricular tachycardia (CPVT)”, a Ca2+-dependent arrhythmia. We find that that RyR2s (and therefore Ca2+ sparks) are relatively insensitive to LCC openings across a wide range of membrane potentials; and that key differences exist between Ca2+ sparks evoked during quiescence, diastole, and systole. The enhanced RyR2 [Ca2+]i sensitivity during CPVT leads to increased Ca2+ spark fidelity resulting in asynchronous systolic Ca2+ spark activity. It also produces increased diastolic SR Ca2+ leak with some prolonged Ca2+ sparks that at times become “metastable” and fail to efficiently terminate. There is a huge margin of safety for stable Ca2+ handling within the cell and this novel mechanistic model provides insight into the molecular signaling characteristics that help maintain overall Ca2+ stability even under the conditions of high SR Ca2+ leak during CPVT. Finally, this model should provide tools for investigators to examine normal and pathological Ca2+ signaling characteristics in the heart.

Quantitative tests reveal that microtubules tune the healthy heart but underlie arrhythmias in pathology

“…; Wescott et al . ). The compartmental Monte Carlo model used here consists of ordinary differential equations (ODEs) representing the time‐evolution of various intracellular ion concentrations (i.e.…”

“…See Wescott et al . () for more details. For the purposes of this study, the cell's sarcolemmal membrane was clamped to −80 mV to simulate a quiescent cardiomyocyte.…”

“…Each RyR2's gating is determined by the following two‐state, continuous‐time Markov chain represented by the following transition‐state diagram, where k 12 is the basal RyR2 opening rate in μM −η s −1 , Φ is a luminal regulation function (see Wescott et al . ), c ds is the subspace [Ca 2+ ], η is the RyR2 Ca 2+ ‐binding cooperativity constant and k 21 is the intrinsic RyR2 closing rate in s −1 .…”

“…Cardiac excitation-contraction coupling (ECC) is governed by voltage-dependent calcium (Ca 2+ ) influx that rapidly activates Ca 2+ sparks, the elementary Ca 2+ release events arising from clusters of type 2 ryanodine receptor (RyR2) Ca 2+ channels in the sarcoplasmic reticulum (SR) membrane (Cheng et al 1993;Cheng & Lederer, 2008). Informed by experimental measures, computational models of Ca 2+ sparks and cardiac SR Ca 2+ release have granted further insights into ECC by generating predictions that can inform future experimentation, and provided quantitative characterizations of events that are currently difficult (or impossible) to investigate experimentally (Smith et al 1998;Sobie et al 2002Sobie et al , 2006Sobie et al , 2017Ramay et al 2011;Williams et al 2011;Wagner et al 2012;Greiser et al 2014;Walker et al 2014;Brandenburg et al 2016;Wescott et al 2016).…”

Axial tubule junctions control rapid calcium signaling in atria

“…Quantitative RyR2 cluster modeling supports local and global AT coupling. To integrate the observed local and global Ca 2+ signals mechanistically, we used an existing mathematical ECC model that incorporates the latest biophysical information regarding local RyR2 cluster dynamics (35). This model was modified to account for our data on AT-associated Ca 2+ -signaling functions and discrete localizations of highphos RyR2 clusters at AT and S sites ( Figure 4C).…”