Ultrastructural immunodetection of the regulatory factors PAX7 and MyoD in myonuclei of sedentary and trained old mice

The plasticity of skeletal muscle and its capability of regeneration drastically decrease during ageing, resulting in a loss of motor units, called sarcopenia. Sarcopenia may depend on a reduced efficiency of muscle tissue regeneration likely due to a decrease in the amount of satellite cells (SC) and a decline in SC myogenic potential. Previous studies showed that ageing is characterized by a reduction in two important myogenic regulatory factors, PAX7 and MyoD, which are specific markers of SC functional state. Since some studies suggest that an adapted aerobic physical exercise may be an efficient non-pharmacological approach to mitigate the effect of aging on skeletal muscle, we focused our attention on the possible effects that training may induce on the ultrastructural distribution and relative amount of PAX7 and MyoD in myonuclei of old mice. Since MyoD and PAX7 are expressed in both SCs and myonuclei, ultrastructural immunocytochemistry is a suitable and unique approach for detecting, locating and quantifying these protein factors in the myonuclei of sarcopenic mice before and after mild physical exercise, thus providing basic information to understand their functional modulation in relation to ageing and training. Our results suggest that, in response to the mild physical exercise, the increase of MyoD in myonuclei is involved in their reactivation; such activation is not mediated by PAX7, which did not increase and seemed to find in the nucleolus its storage site in aged myonuclei.