Journal
JOURNAL OF APPLIED PHYSIOLOGY
Volume 95, Issue 2, Pages 791-802Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/japplphysiol.01113.2002
Keywords
premessenger ribonucleic acid; protein translation markers; protein degradation enzymes; insulin-like growth factor I; myogenic regulatory factors
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Funding
- NIAMS NIH HHS [AR-30346] Funding Source: Medline
- NINDS NIH HHS [NS-16333] Funding Source: Medline
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We examined the expression of several molecular markers of protein balance in response to skeletal muscle atrophy induced by spinal cord isolation ( SI; i.e., a complete transection of the spinal cord at both a midthoracic and a high sacral level plus complete deafferentation between the two transection sites). This treatment nearly eliminates neuromuscular activity ( activation and loading) of the hindlimb muscles while maintaining neuromuscular connectivity. SI was associated with a reduced transcriptional activity (via pre-mRNA analyses) of myosin heavy chain (MHC) and actin. In addition, there was an increased gene expression of enzyme systems impacting protein degradation (calpain-1; plus enzymes associated with polyubquitination processes) that could further contribute to the protein deficits in the SI muscles via degradative pathways. IGF-I receptor and binding protein-5 mRNA expression was induced throughout the 15-day period of SI, whereas IGF-I mRNA was induced at 8 and 15 days. These responses occurred in the absence of an upregulation of translational regulatory proteins (p70 S6 kinase; eukaryotic 4E binding protein 1) to compensate for the decreased protein translational capacity. These data collectively demonstrate that 1) the molecular changes accompanying SI-induced muscle atrophy are not necessarily the reverse of those occurring during muscle hypertrophy, and 2) the rapid and marked atrophy that defines this model of muscle inactivity is likely the result of multifactorial processes affecting transcription, translation, and protein degradation.
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