Initial events in the muscle atrophy program
The aim of this dissertation was to explore alterations in human skeletal muscle during a controlled period of immobilization and following spinal cord injury (SCI). Study I examined the effects of an imposed period of immobilization of the adductor pollicis (AP) muscle on muscle function and volume in young (18-25 years) and aged (60-75 years) men. Muscle strength and volume of the AP was assessed before and after immobilization. Results from Study I show that although the older and younger adults had disparate losses in muscle volume they experienced similar losses in muscle strength. These results confirm that even short periods of inactivity promote more rapid losses in muscle volume in older adults, while the ability to generate force is maintained.
Study II and Study III of this dissertation explored the molecular alterations in human skeletal muscle in response to immobilization or SCI. Muscle biopsies were analyzed at both the transcriptional (microarray analysis, quantitative Real-Time Polymerase Chain Reaction (qRT-PCR)) and translational (Western blotting, Immunohistochemistry (IHC)) level. In Study II, the knee joint of five young men (aged 18-25 years) was immobilized, and muscle biopsies were performed before and after 48h of leg immobilization. Immobilization resulted in increased expression of genes involved in the ubiquitin proteasome pathway and metallothionein function, but no change in respective protein products. However, results of Study II also showed a decrease in gene expression and protein products for the collagens, which are involved in extracellular matrix (ECM) integrity, indicating that disruption to the ECM is an initial step in the muscle atrophy program following immobilization.
In Study III, analysis of muscle biopsies taken two and five days post-SCI, compared to healthy controls, showed increased gene expression for genes that encode components of the ubiquitin proteasome pathway, metallothioneins, and secretory leukocyte protease inhibitors. Western blotting and IHC showed that protein products for components of the proteolytic core and the metallothioneins increased by five days post-SCI, and protein products were localized to the ECM. These results indicate that components of the ECM are initial targets of proteolytic activity within the first days following SCI.