Sarcolipin Deletion in mdx Mice Worsens Dystrophic Pathology by Impairing Calcineurin Signaling and Reducing Utrophin Expression

Abstract

Duchenne's muscular dystrophy (DMD) is the most severe form of muscular dystrophy affecting 1 in 3500 live male births. There is no cure for DMD and males typically succumb to early mortality induced by respiratory and cardiac insufficiency. Thus, finding viable therapeutic targets is of utmost importance. Sarcolipin (SLN), a sarco(endo)plasmic reticulum Ca2+‐ATPase (SERCA) pump inhibitor, is highly upregulated in dystrophic muscles from mdx mice, a mouse model for DMD. To date, the role of SLN in muscular dystrophy remains unknown, and therefore, in this study we crossed SlnKO mice with mdx mice to generate the mdx/SlnKO double knockout mutant mouse. We hypothesized that Sln deletion would worsen dystrophic myopathy based on unpublished results from our laboratory and previous studies showing that SLN can stimulate calcineurin – a Ca2+‐dependent serine/threonine phosphatase shown to counteract muscular dystrophy. Our findings show that mdx/SlnKO myofibers from soleus (P = 0.0001) and diaphragm (P < 0.0001) were significantly smaller than those from mdx. Furthermore, mdx/SlnKO soleus and diaphragm muscles displayed a slow‐to‐fast fiber type shift, which was associated with a significant reduction in utrophin (P = 0.004). Utrophin, a dystrophin homolog that confers compensatory membrane stability, is largely controlled by calcineurin, suggesting that SLN in mdx mice stimulates calcineurin signaling. In addition, the significant reductions in utrophin expression in the mdx/SlnKO mice can account for their worsened dystrophic pathology revealed by greater serum creatine kinase (P = 0.01) and greater variability in myofiber size (minimal Feret's diameter, P = 0.01) compared with mdx. Finally, lower central nuclei counts in muscles from mdx/SlnKO mice compared with mdx (P = 0.04) suggests that genetic deletion of Sln also potentially impairs the muscles’ regenerative capacity. Taken together, these results, for the first time clearly demonstrate SLN's vital role in countering muscular dystrophy.Support or Funding InformationThis work was supported by the Canadian Institutes of Health Research (CIHR; MOP 86618 and MOP 47296 to A.R.T).