Transcriptional profiling of skeletal muscle reveals factors that are necessary to maintain satellite cell integrity during ageing.

Skeletal muscle ageing is characterized by faulty degenerative/regenerative processes that promote the decline of its mass, strength, and endurance. In this study, we used a transcriptional profiling method to better understand the molecular pathways and factors that contribute to these processes. To more appropriately contrast the differences in regenerative capacity of old muscle, we compared it with young muscle, where robust growth and efficient myogenic differentiation is ongoing. Notably, in old mice, we found a severe deficit in satellite cells activation. We performed expression analyses on RNA from the gastrocnemius muscle of young (3-week-old) and old (24-month-old) mice. The differential expression highlighted genes that are involved in the efficient functioning of satellite cells. Indeed, the greatest number of up-regulated genes in young mice encoded components of the extracellular matrix required for the maintenance of the satellite cell niche. Moreover, other genes included Wnt inhibitors (Wif1 and Sfrp2) and Notch activator (Dner), which are putatively involved in the interconnected signalling networks that control satellite cell function. The widespread expression differences for inhibitors of TGFbeta signalling further emphasize the shortcomings in satellite cell performance. Therefore, we draw attention to the breakdown of features required to maintain satellite cell integrity during the ageing process.

A novel hindlimb immobilization procedure for studying skeletal muscle atrophy and recovery in mouse.

Skeletal muscle atrophy is a serious concern for patients afflicted by limb restriction due to surgery (e.g., arthrodesis), several articular pathologies (e.g., arthralgia), or simply following cast immobilization. To study the molecular events involved in this immobilization-induced debilitating condition, a convenient mouse model for atrophy is lacking. Here we provide a new immobilization procedure exploiting the normal flexion of the mouse hindlimb using a surgical staple to fix the ventral part of the foot to the distal part of the calf. Histological analysis revealed that our approach induced significant skeletal muscle atrophy by reducing the myofiber size of the tibialis anterior (TA) muscle by 36% compared with the untreated contralateral TA within a few days postimmobilization. Two molecular markers for atrophy, atrogin-1/muscle atrophy F-box (atrogin-1/MAFbx) and muscle ring finger 1 (MuRF-1) mRNAs, were significantly upregulated by 1.9- and 5.9-fold, respectively. Interestingly, our model also revealed the presence of an early inflammatory process during atrophy, characterized by the mRNA upregulation of TNF-alpha, IL-1, and IL-6 (1.9-, 2.4-, and 3.4-fold, respectively) simultaneously with the upregulation of the common leukocyte marker CD45 (6.1-fold). Moreover, muscle rapidly recovered on remobilization, an event associated with significantly increased levels of uncoupling protein-3 and peroxisome proliferator-activated receptor gamma coactivator-1alpha mRNA, key components of prooxidative muscle metabolism. This model offers unexpected new insights into the molecular events involved in immobilization atrophy.