Dysregulated ryanodine receptors mediate cellular toxicity. Restoration of normal phenotype by FKBP12.6

Ca²⁺ homeostasis is a vital cellular control mechanism in which Ca²⁺ release from intracellular stores plays a central role. Ryanodine receptor (RyR)-mediated Ca²⁺ release is a key modulator of Ca²⁺ homeostasis, and the defective regulation of RyR is pathogenic. However, the molecular events underlying RyR-mediated pathology remain undefined. Cells stably expressing recombinant human RyR2 (Chinese hamster ovary cells, CHO^hRyR2) had similar resting cytoplasmic Ca²⁺ levels ([Ca²⁺]_c) to wild-type CHO cells (CHO^WT) but exhibited increased cytoplasmic Ca²⁺ flux associated with decreased cell viability and proliferation. Intracellular Ca²⁺ flux increased with human RyR2 (hRyR2) expression levels and determined the extent of phenotypic modulation. Co-expression of FKBP12.6, but not FKBP12, or incubation of cells with ryanodine suppressed intracellular Ca²⁺ flux and restored normal cell viability and proliferation. Restoration of normal phenotype was independent of the status of resting [Ca²⁺]_c or ER Ca²⁺ load. Heparin inhibition of endogenous inositol trisphosphate receptors (IP₃R) had little effect on intracellular Ca²⁺ handling or viability. However, purinergic stimulation of endogenous IP₃R resulted in apoptotic cell death mediated by hRyR2 suggesting functional interaction occurred between IP₃R and hRyR2 Ca²⁺ release channels. These data demonstrate that defective regulation of RyR causes altered cellular phenotype via profound perturbations in intracellular Ca²⁺ signaling and highlight a key modulatory role of FKBP12.6 in hRyR2 Ca²⁺ channel function.