RYR2 phosphomutants show that the tetramers and the junctional sarcoplasmic reticulum form a structural syncytium: Calcium Signaling and Excitation–Contraction in Cardiac, Skeletal and Smooth Muscle

We previously showed that RYR2 tetramers are distributed nonuniformly within ventricular dyads, and that physiological and pathological factors can alter their relative positions. Agents that decreased Ca²⁺ spark frequency, high Mg²⁺, and saturating concentrations of the immunophilins FKBP12 and FKBP12.6 drew the receptors together, minimizing their nearest-neighbor distance and reducing the size of the clusters. Activating kinases with a phosphorylation cocktail did the opposite. The purpose of this study is to test the hypothesis that phosphorylation of RYR2 is required for the structural changes we have observed. We measured junctional sarcoplasmic reticulum (jSR) lengths using 2-D transmission electron microscopy (TEM) and directly visualized RYR2 distribution using dual-tilt electron tomography in phosphomutant mice S2808A, S2814A, S2814D, and S2030A. Mouse hearts were hung on a Langendorff and treated with either saline or 300 nmol/liter isoproterenol (ISO) for 2 min before being fixed and sectioned for analysis. We found that (1) RYR2 distribution in mouse ventricles is comparable to that reported for rats and humans, (2) the response to ISO applied to an intact, beating heart is identical to a phosphorylation cocktail applied to isolated permeabilized myocytes, and (3) all of the mutations produced significant changes in the tetramer arrangements and/or NND relative to wild-type (WT) mice. Our 2-D TEM measurements showed that (1) in WT mice, ISO significantly increased the length of the jSR, (2) ISO significantly increased the jSR lengths of WT, S2814A, and S2808A mice, but not the S2030A mouse, and (3) the jSR length of the S2814D mouse was significantly greater than WT, but not WT + ISO or S2814D + ISO, indicating that a mutation of the RYR2 alone caused a significant change in the jSR length. These results indicate that the tetramers and the jSR form a structural syncytium.