In silico simulations reveal that RYR distribution affects the dynamics of calcium release in cardiac myocytes

Abstract

The dyads of cardiac myocytes contain ryanodine receptors (RYRs) that generate calcium sparks upon activation. To test how geometric factors of RYR distribution contribute to the formation of calcium sparks, which cannot be addressed experimentally, we performed in silico simulations on a large set of models of calcium release sites (CRSs). Our models covered the observed range of RYR number, density, and spatial arrangement. The calcium release function of CRSs was modeled by RYR openings, with an open probability dependent on concentrations of free Ca2+ and Mg2+ ions, in a rapidly buffered system, with a constant open RYR calcium current. We found that simulations of spontaneous sparks by repeatedly opening one of the RYRs in a CRS produced three different types of calcium release events (CREs) in any of the models. Transformation of simulated CREs into fluorescence signals yielded calcium sparks with characteristics close to the observed ones. CRE occurrence varied broadly with the spatial distribution of RYRs in the CRS but did not consistently correlate with RYR number, surface density, or calcium current. However, it correlated with RYR coupling strength, defined as the weighted product of RYR vicinity and calcium current, so that CRE characteristics of all models followed the same state-response function. This finding revealed the synergy between structure and function of CRSs in shaping dyad function. Lastly, rearrangements of RYRs simulating hypothetical experiments on splitting and compaction of a dyad revealed an increased propensity to generate spontaneous sparks and an overall increase in calcium release in smaller and more compact dyads, thus underlying the importance and physiological role of RYR arrangement in cardiac myocytes.

Magnesium Ions Moderate Calcium-Induced Calcium Release in Cardiac Calcium Release Sites by Binding to Ryanodine Receptor Activation and Inhibition Sites

“…The intensity of fluorescence is directly proportional to the concentration of calcium-bound Fluo-4 [Ca 2+ -Fluo4], thus the change of fluorescence was reported as Δ F / F 0 . The calculated spatial profile of [Ca 2+ -Fluo4] was convolved with a Gaussian point spread function with FWHM = 400, 400, and 800 nm for the X, Y, and Z coordinate, respectively, to approximate the image observed by fluorescent microscopy ( Smith et al, 1998 ; Iaparov et al, 2021 ).…”

“…For quantitative characterization of the geometrical arrangement of RyRs, we used the concept of vicinity ( Iaparov et al, 2021 ), an extension of the concept of adjacency ( Walker et al, 2015 ) to arbitrary RyR-RyR distances. The RyR vicinities v i and the group vicinity v were determined according to Iaparov et al (2021) as:…”

“…where N RyR is the number of RyRs in the CRS, and C ij is the reciprocal value of the distance between the i th and the j th RyR ( Iaparov et al, 2021 ). Out of 100 generated release sites, the one that showed the highest range of RyR vicinities was selected for simulations ( Figure 1 ).…”

“…Postulating binding of Ca 2+ ions to the RyR activation site as instantaneous and unbinding of Mg 2+ from the calcium activation site as slow predicted 2-3 simultaneously open RyR channels at the peak of a spark ( Zahradnikova and Zahradnik, 2012 ). Considering the equilibration between Ca 2+ and Mg 2+ on the RyR activation site as instantaneous predicted three types of calcium release events differing in the number of open RyRs ( Iaparov et al, 2021 ): CREs consisting of only a single RyR opening that did not activate nearby RyRs in a CRS (quarks); CREs that involved activation of a small subset of RyRs and simultaneous opening of 2-3 RyRs in a CRS (blip); and CREs that involved sequential activation of all RyRs in the CRS and simultaneous opening of about half of the RyRs in the CRS (sparks). This in silico study indicated the role of dynamic Ca 2+ and Mg 2+ interaction with RyRs of a calcium release site that can be solved only by a RyR model explicitly incorporating the respective rate constants.…”

Computationally efficient model of myocardial electromechanics for multiscale simulations

“…The major source of Ca 2 in the cytosol is calcium-induced calcium release (CICR) from SR. CICR is a complex process activated by increased Ca 2+ concentration in the narrow subspace between the cell membrane with the L-type Ca 2+ -channels (dihydropyridine receptors; DHPR) and the SR membrane with ryanodine receptors (RyR) as well as by high Ca 2+ concentration in the SR lumen. Several sophisticated models partially reviewed by Hinch et al [39] as well as a non-Markovian one [40] and a model of interacting clusters of DHPR and RyR [41] were suggested for simulation of different aspects of CICR. Here we neglected all details that are not necessary for describing major feature of CIRC in normally contracting cardiac muscle cell and set the CICR flux in a rather simple form [3]:…”