Abstract
Duchenne Muscular Dystrophy (DMD) is a prevalent variant of muscular dystrophy affecting approximately 1 in 3500 male infants. This disorder stems from mutations in the DMD gene which encodes for dystrophin, a protein responsible for the structural integrity of skeletal muscle fibres through the formation of dystrophin-associated glycoprotein complexes (DGCs). Due to the lack of dystrophin, dystrophic muscle fibres become susceptible to damage during muscular contractions. With age, the muscle undergoes degeneration and increased fibrosis as a result of mechanical stress and impaired Ca2+ homeostasis. Key Ca2+ handling proteins that regulate Ca2+ homeostasis within skeletal muscle include the Dihydropyridine receptor (DHPR), Ryanodine receptor 1 (RyR1), Calsequestrin 1 (CSQ1), Sarco/endoplasmic reticulum Ca2+ ATPase 1 & 2 (SERCA1 & SERCA2), and Stromal interacting molecule 1 (STIM1). In DMD, changes in expression and activity of these Ca2+ handling proteins contribute to elevated intracellular Ca2+ levels, leading to the activation of downstream apoptotic and necrotic signalling pathways. DMD is a progressive disorder initially affecting the fast-twitch fibres of the lower limb musculature. One such muscle affected is the tibialis anterior (TA), a mixed fibre type muscle predominantly composed of fast-twitch fibres. A common animal model used to study DMD is the C57BL/10ScSn-dmdmdx mice. The mdx mice display similar phenotypes to DMD patients, however, they experience an intense phase of degeneration/regeneration of muscle fibres around the age of 3-4 weeks and then pathology appears to plateau until at least 12 months of age.
Expression of DHPR, RyR1, SERCA1, SERCA2, CSQ1, and STIM1 in 21-28 day, 6 month, and 1 year + old mdx mice TA were examined using Western Blotting. We found no significant difference between all mdx mice age groups of both DHPR and RyR1 expression. SERCA1 and CSQ1 expressions were significantly increased between the 21-28 day and 1yr+ mdx mice and between the 6 month and 1yr+ mdx mice, while SERCA2 expression was only significantly increased between the 21-28 day and 1yr+ mdx mice groups.
Although DHPR and RyR1 had no significant change in expression in the TA across the different age groups of mdx mice, RyR1 trended towards significance between the 21-28 day and 1yr+ groups. Overall, the results indicated that there is a relationship between age and expression of SERCA1, SERCA2, and CSQ1 in the TA of mdx mice. These findings align with the notion of impaired Ca2+ handling as a mechanism involved in pathological increases in intracellular Ca2+, thus enhancing the progression of DMD.