Exacerbation of dystrophic cardiomyopathy by phospholamban deficiency-mediated chronically increased cardiac Ca 2+ cycling in vivo

Cardiomyopathy is a significant contributor to morbidity and mortality in Duchenne Muscular Dystrophy (DMD). Membrane instability, leading to intracellular calcium (Ca2+) mishandling and overload, causes myocyte death and subsequent fibrosis in DMD cardiomyopathy. On a cellular level, cardiac myocytes from mdx mice have dysregulated Ca2+ handling, including increased resting Ca2+ and slow Ca2+ decay, especially evident under stress conditions. Sarco(endo)plasmic reticulum Ca2+ ATPase and its regulatory protein phospholamban (PLN) are potential therapeutic targets for DMD cardiomyopathy owing to their key role in regulating intracellular Ca2+ cycling. We tested the hypothesis that enhanced cardiac Ca2+ cycling would remediate cardiomyopathy caused by dystrophin deficiency. We used a genetic complementation model approach by crossing dystrophin-deficient mdx mice with PLN knockout mice (termed DKO). As expected, adult cardiac myocytes isolated from DKO mice exhibited increased contractility and faster relaxation associated with increased Ca2+ transient peak height and faster Ca2+ decay rate compared to controls. However, in comparison to wild-type, mdx and PLNKO animals, DKO mice unexpectedly had reduced in vivo systolic and diastolic function measured by echocardiography. Further, Evan's blue dye uptake was increased in DKO hearts compared to control, mdx and PLNKO hearts, demonstrating increased membrane damage, which subsequently led to increased fibrosis in the DKO myocardium in vivo. In conclusion, despite enhanced intracellular Ca2+ handling at the myocyte level, DMD cardiomyopathy was exacerbated owing to unregulated chronic increases in Ca2+ cycling in DKO mice in vivo. These findings have potential important implications for ongoing therapeutic strategies for the dystrophic heart.