Isolation of Primary Myoblasts and Characterization of the Endoplasmic Reticulum Stress Response in Wild-type and mdx Mice

Duchenne Muscular Dystrophy (DMD) is a devastating muscle disease caused by a recessive X- linked mutation in the dystrophin gene. This mutation leads to a loss of the dystrophin- glycoprotein complex (DGC) and destabilization of the sarcolemmal membrane. In the past, it has been assumed that mutant dystrophin protein is rapidly degraded in DMD muscle due to the lack of dystrophin detection in DMD muscle lysates. A recent study in the Crosbie-Watson group showed the accumulation of truncated dystrophin in the ER/Golgi of the mdx mice, leading our lab to examine the unfolded protein response (UPR) in these mice. The UPR is a cascade of molecular events that function to stabilize the cell as either mis-folded or unfolded proteins overload the ER by attenuating transcription and up-regulating chaperones to assist with the accumulation of proteins. We hypothesize that loss of dystrophin and accumulation of compensatory proteins in the ER lead to the UPR in DMD muscle. We performed a cell viability assay to examine isolated myoblasts' ability to tolerate stress when treated with tunicamycin, a known ER stressor, and found mdx myoblasts displayed a lower percentage of viable cells when compared to wild-type. Many UPR proteins, such as BiP, PERK, and cleaved ATF6 also display an increased abundance in immunoblots from mdx myotubes as well as total skeletal muscle when compared to wild-type myotubes. Curcumin treatment decreased protein expression of BiP and ATF6, but up-regulated the PERK pathway. When analyzing the UPR before and after the onset of pathology at 3 and 10 weeks of age in mdx mice, we found 3-week old mdx mice had increased abundance of UPR proteins compared to wild-type, but this difference dissipated in 10- week old mice. ATF6 also increased in abundance in 10-week old mice. Interestingly, we also found an increase in the accumulation of alpha dystroglycan (DG), a dystrophin binding protein, in the ER after the onset of pathology in mdx mice compared to the wild-type controls. The effects of the accumulation of alpha-DG and the physiological effects of UPR activation in dystrophin- deficient muscle remains to be understood and ER function needs to be further analyzed in future studies