Impaired relaxation in transgenic mice overexpressing junctin

Objective: Junctin is a major transmembrane protein in cardiac junctional sarcoplasmic reticulum, which forms a quaternary complex with the ryanodine receptor (Ca²⁺ release channel), triadin, and calsequestrin. Methods: To better understand the role of junctin in excitation-contraction coupling in the heart, we generated transgenic mice with targeted overexpression of junctin to mouse heart, using the α-MHC promoter to drive protein expression. Results: The protein was overexpressed 10-fold in mouse ventricles and overexpression was accompanied by cardiac hypertrophy (19%). The levels of two other junctional SR-proteins, the ryanodine receptor and triadin, were reduced by 32% and 23%, respectively. However, [³H]ryanodine binding and the expression levels of calsequestrin, phospholamban and SERCA2a remained unchanged. Cardiomyocytes from junctin-overexpressing mice exhibited impaired relaxation: Ca²⁺ transients decayed at a slower rate and cell relengthening was prolonged. Isolated electrically stimulated papillary muscles from junctin-overexpressing hearts exhibited prolonged mechanical relaxation, and echocardiographic parameters of relaxation were prolonged in the living transgenic mice. The amplitude of caffeine-induced Ca²⁺ transients was lower in cardiomyocytes from junctin-overexpressing mice. The inactivation kinetics of L-type Ca²⁺ channel were prolonged in junctin-overexpressing cardiomyocytes using Ca²⁺ or Ba²⁺ as charge carriers. Conclusion: Our data provide evidence that cardiac-specific overexpression of junctin is accompanied by impaired myocardial relaxation with prolonged Ca²⁺ transient kinetics on the cardiomyocyte level.