Overexpression of junctin causes adaptive changes in cardiac myocyte Ca²⁺ signaling

Abstract

In cardiac muscle, junctin forms a quaternary protein complex with the ryanodine receptor (Ry R), calsequestrin, and triadin 1 at the luminal face of the junctional sarcoplasmic reticulum (j SR). By binding directly the Ry R and calsequestrin, junctin may mediate the Ca²⁺-dependent regulatory interactions between both proteins. To gain more insight into the underlying mechanisms of impaired contractile relaxation in transgenic mice with cardiac-specific overexpression of junctin (TG), we studied cellular Ca²⁺ handling in these mice. We found that the SR Ca²⁺ load was reduced by 22 percent in cardiomyocytes from TG mice. Consistent with this, the frequency of Ca²⁺ sparks was diminished by 32 percent. The decay of spontaneous Ca2+ sparks was prolonged by 117 percent in TG. This finding was associated with a lower Na⁺-Ca²⁺ exchanger (NCX) protein expression (by 67 percent) and a higher basal Ry R phosphorylation at Ser²⁸⁰⁹ (by 64 percent) in TG. The shortening- and Δ(Ca)_i-frequency relationships (0.5–4 Hz) were flat in TG compared to wild-type (WT) which exhibited a positive staircase for both parameters. Furthermore, increasing stimulation frequencies hastened the time of relaxation and the decay of (Ca)_i by a higher percentage in TG. We conclude that the impaired relaxation in TG may result from a reduced NCX expression and/or a higher SR Ca²⁺ leak. The altered shortening-frequency relationship in TG seems to be a consequence of an impaired excitation–contraction coupling with depressed SR Ca²⁺release at higher rates of stimulation. Our data suggest that the more prominent frequency-dependent hastening of relaxation in TG results from a stimulation of SR Ca²⁺ transport reflected by corresponding changes of (Ca)_i.