Selected Contribution: Identification of differentially expressed genes between young and old rat soleus muscle during recovery from immobilization-induced atrophy.

After cessation of hindlimb immobilization, which resulted in a 27-37% loss in soleus mass, the atrophied soleus muscle of young but not old rats regrows to its mass before treatment. We hypothesized that during remobilization the mRNA levels of growth potentiating factor(s) would be present in the soleus muscle of young (3- to 4-mo-old) but absent in old (30- to 31-mo-old) Fischer 344 x Brown Norway rats or that mRNAs for growth inhibitory factor(s) would be absent in young but present in old. Gene expression levels of >24,000 transcripts were determined by using Affymetrix RGU34A-C high-density oligonucleotide microarrays in soleus muscles at 3, 6, 10, and 30 days of remobilization after cessation of a 10-day period of hindlimb immobilization. Each muscle sample was applied to an independent set of arrays. Recovery-related differences were determined by using a three-factor ANOVA with a false discovery rate-adjustment of P = 0.01, which yielded 64 significantly different probe sets. Elfin, amphiregulin, and clusterin mRNAs were selected for further confirmation by real-time PCR. Elfin mRNA levels were less in old than in young rats at 6, 10, and 30 days of remobilization. Amphiregulin expression exhibited a unique spike on the 10th day of successful regrowth in young rats but remained unchanged old. Clusterin mRNA was unchanged in young muscles but was elevated on the 3rd, 6th, and 10th days of recovery in old soleus muscles. The mRNAs identified as differentially expressed between young and old recovery may modulate muscle growth that could highlight new candidate mechanisms to explain the failure of old soleus muscle to recover lost muscle mass.