Nanoscale organization of ryanodine receptor distribution and phosphorylation pattern determines the dynamics of calcium sparks

Abstract

Super-resolution imaging techniques have provided a better understanding of the relationship between the nanoscale organization and function of ryanodine receptors (RyRs) in cardiomyocytes. Recent data have indicated that this relationship is disrupted in heart failure (HF), as RyRs are dispersed into smaller and more numerous clusters. However, RyRs are also hyperphosphorylated in this condition, and this is reported to occur preferentially within the cluster centre. Thus, the combined impact of RyR relocalization and sensitization on Ca2+ spark generation in failing cardiomyocytes is likely complex and these observations suggest that both the nanoscale organization of RyRs and the pattern of phosphorylated RyRs within clusters could be critical determinants of Ca2+ spark dynamics. To test this hypothesis, we used computational modeling to quantify the relationships between RyR cluster geometry, phosphorylation patterns, and sarcoplasmic reticulum (SR) Ca2+ release. We found that RyR cluster disruption results in a decrease in spark fidelity and longer sparks with a lower amplitude. Phosphorylation of some RyRs within the cluster can play a compensatory role, recovering healthy spark dynamics. Interestingly, our model predicts that such compensation is critically dependent on the phosphorylation pattern, as phosphorylation localized within the cluster center resulted in longer Ca2+ sparks and higher spark fidelity compared to a uniformly distributed phosphorylation pattern. Our results strongly suggest that both the phosphorylation pattern and nanoscale RyR reorganization are critical determinants of Ca2+ dynamics in HF.

Spine apparatus modulates Ca2+in spines through spatial localization of sources and sinks

“…In all cases, the total number of RyRs was held constant (60 RyRs) (Section 4.3.1) and we conducted | N | = 250 simulations for each scenario. As we and others have shown before ( 32 , 34 ), due to the stochastic nature of the model, RyRs open only in a subset of those simulations n ∈ N . Thus the remaining set N \ n is the set of simulations where RyRs did not open.…”

“…To investigate the crosstalk between the geometric features listed above and Ca 2+ dynamics, we developed a stochastic computational model of the main molecular components in spines with a focus on RyR-mediated CICR. In our model, we considered the role of RyR clustering inspired by work in cardiomyocytes ( 32 – 34 ) and the role of RyR localization based on experimental observations ( 27 ). Since the ER and the spine have unique shapes ( 10 , 16 , 17 ), we also investigated how the geometries of both the spine head and the ER (non-laminar and SA) could impact the RyR-driven Ca 2+ dynamics using idealized and realistic geometries.…”

Luminal transport through intact endoplasmic reticulum limits the magnitude of localized Ca2+signals

“…Modeling studies of Ca 2+ release have highlighted the importance of several key physical parameters, including the distribution and opening probability of channels on the ER membrane (38)(39)(40)(41), the buffering capacity of luminal Ca 2+binding proteins (38), and the relative Ca 2+ diffusivity in the lumen and cytoplasm (42). Model results have indicated that the mobility of luminal buffer plays an important role in supporting Ca 2+ release currents when free Ca 2+ diffusivity in the lumen is much less than the cytoplasm (38), and that the luminal buffering capacity largely determines the total Ca 2+ levels stored in the lumen (39).…”

Correlative super-resolution analysis of cardiac calcium sparks and their molecular origins in health and disease

“…They also remain powerful tools for interrogating this structure-function relationship in the context of biochemical modification or multi-molecular partnering that can shift the underpinning determinants of Ca 2+ sparks (e.g. RyR2 open probability and luminal [Ca 2+ ] of the ER or, in muscle, sarcoplasmic reticulum; hereafter SR) [ 31 , 37 , 38 ]. A significant gap that has remained however is the experimental evidence directly correlating the spark properties observed in situ with the true distribution of RyR2s.…”