Structural Integrity of RyR2 Clusters Controls Cardiac Calcium Leak

Figures

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  Figure 1.. Architecture of RyR2 clusters. (A) Adjoining configuration. RyR2 tetramers are positioned in a side-by-side array, with neighboring channels aligned in rows and columns. In this arrangement, adjacent channels make contact along their lateral surfaces, consistent with the “side-by-side” geometry observed in cardiac cells. (B) Oblique configuration. RyR2 tetramers are connected in an alternating “checkerboard” pattern. This arrangement corresponds to the oblique interaction identified in structural studies, where subunits from diagonally offset channels (1–3 and 2–4) form the interface.

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  Figure 2.. Stochastic activation of a 10 × 10 RyR2 cluster connected in the oblique configuration. The cytosolic Ca concentration is fixed at co=6.0μM. Three independent simulation runs are shown corresponding to the red, black, and blue lines. The activation time T (for the simulation shown in black) is the time when the channels are all closed, to when no exceeds nc=5. The parameters used in this simulation are δ=0.5,σ=0.5,g=25μM,kfo=0.005(μM⋅ms)−1, and kb=0.3(ms)−1.

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  Figure 3.. Dependence of spontaneous spark timing on diastolic Ca co. (A) Mean waiting time T to a spontaneous Ca spark as a function of diastolic Ca concentration co for the oblique cluster configuration. T is computed by averaging over 50 independent simulation runs. Error bars shown are computed as the standard deviation of 5 independent sets of 10 simulation runs each. (B) Same simulation as in A but with RyR2 channels arranged in the adjoining configuration. (C) Same simulations as in A but with cluster sizes of 5 × 5, 10 × 10, and 20 × 20. (D) Same simulations as in B with the indicated cluster sizes. For all simulations in C and D we have fixed σ=0.5.

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  Figure 4.. Dependence of spontaneous Ca spark timing on inter-subunit coupling strength. (A) Dependence of T on σ at co=5.0μM for a 10 × 10 cluster. Red (black) line corresponds to the adjoining (oblique) configuration. (B) Dependence of T on σ at high Ca (co=100μM). The waiting time increases only modestly, indicating weak coupling effects at elevated Ca. Points plotted are averaged over 50 samples and the error bar error bars shown are estimated by computing the standard deviation of 5 independent sets of 10 runs each.

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  Figure 5.. Dependence of spontaneous spark timing on cluster connectivity. Mean waiting time T to a spontaneous Ca spark as function of the connection probability p. In this simulation the connectivity strength is set to σ=1.0 and co=2.0μM. Red line (black line) denotes the adjoining (oblique) cluster configurations of size 10 × 10. Here, T is computed by averaging over 50 independent simulations, and the error bars are computed as the standard deviation 5 sets of 10 simulation runs each.

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  Figure 6.. Spark timing in heterogeneous clusters. (A) Representative cluster morphologies generated by varying the clustering parameter α in the preferential attachment algorithm. Low values yield dispersed clusters with irregular boundaries, intermediate values produce partially cohesive structures, and high values generate compact, dense aggregates. In this simulation we place 50 RyR2 channels on a grid of size 20 × 20. Neighboring channels are connected according to the oblique geometry. (B) Dependence of the mean waiting time T for spontaneous Ca spark initiation on the clustering parameter. Red and black line corresponds to σ=1 and σ=0.5 respectively. The waiting time T is computed by averaging over 50 independent simulation runs. For each simulation run a different cluster arrangement is generated using the preferential growth algorithm. Here, we set co=2.5μM, and error bars are computed using 5 sets of 10 simulation runs.