Effects of 48h Lower Limb Unloading in Human Skeletal Muscle

Although short-term disuse does not result in measurable muscle atrophy, studies suggest that molecular changes associated with protein degradation may be initiated within days of the onset of a disuse stimulus. We examined the global gene expression patterns in sedentary men (n = 7, mean age S.D = 22.1 3.7 yr) following 48h unloading (UL) via unilateral lower limb suspension and 24h reloading (RL). Biopsy samples of the left vastus lateralis muscle were collected at baseline, 48h UL, and 24h RL. Expression changes were measured by microarray and gene clustering; identification of enriched functions and canonical pathways were performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathway Analysis (IPA). Four genes were validated with qRT-PCR, and protein levels were measured with Western blot. Of the upregulated genes after UL, the most enriched functional group and highest ranked canonical pathway were related to protein ubiquitination. The oxidative stress response pathway was the second highest ranked canonical pathway. Of the downregulated genes, functions related to mitochondrial metabolism were the mostly highly enriched. In general, gene expression patterns following UL persisted following RL. qRT-PCR confirmed increases in mRNA for UPP-related E3 ligase Atrogin1 (but not accompanying increases in protein products) and stress response gene heme oxygenase-1 (HMOX, which showed a trend towards increases in protein products at 48h UL) as well as extracellular matrix (ECM) component COL4.

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Reich KA, Chen YW, Thompson PD, Hoffman EP, Clarkson PM