Identification of a molecular signature of sarcopenia.

Investigating the molecular mechanisms underlying sarcopenia in humans with the use of microarrays has been complicated by low sample size and the variability inherent in human gene expression profiles. We have conducted a study using Affymetrix GeneChips to identify a molecular signature of aged skeletal muscle. The molecular signature was defined as the set of expressed genes that best distinguished the vastus lateralis muscle of young (n = 10) and older (n = 12) male subjects, when a k-nearest neighbor supervised classification method was used in conjunction with a signal-to-noise ratio gene selection method and a holdout cross-validation procedure. The age-specific expression signature was comprised of 45 genes; 27 were upregulated and 18 were downregulated. This signature also correctly classified 75% of the muscle samples from young and older subjects published by an independent laboratory, based on their expression profiles. The signature revealed increased expression of several genes involved in mediating cellular responses to inflammation and apoptosis, including complement component C1QA, Galectin-1, C/EBP-beta, and FOXO3A, among others. The increased expressions of genes that regulate pre-mRNA splicing, localization, and modification of RNA comprise markers of the aging signature. Downregulated genes in the signature were the glutamine transporter SLC38A1, a TRAF-6 inhibitory zinc finger protein, and membrane-bound transcription factor protease S2P, among others. The sarcopenia signature developed here will be useful as a molecular model to judge the effectiveness of exercise and other therapeutic treatments aimed at ameliorating the effects of muscle loss associated with aging.

Transcriptional profile of a myotube starvation model of atrophy.

Skeletal muscle wasting is a pervasive phenomenon that can result from a wide range of pathological conditions as well as from habitual muscular inactivity. The present work describes a cell-culture condition that induces significant atrophy in skeletal muscle C2C12 myotubes. The failure to replenish differentiation media in mature myotubes leads to rapid atrophy (53% in diameter), which is referred to here as starvation. Affymetrix microarrays were used to develop a transcriptional profile of control (fed) vs. atrophied (nonfed) myotubes. Myotube starvation was characterized by an upregulation of genes involved in translational inhibition, amino acid biosynthesis and transport, and cell cycle arrest/apoptosis, among others. Downregulated genes included several structural and regulatory elements of the extracellular matrix as well as several elements of Wnt/frizzled and TGF-beta signaling pathways. Interestingly, the characteristic transcriptional upregulation of the ubiquitin-proteasome system, calpains, and cathepsins known to occur in multiple in vivo models of atrophy were not seen during myotube starvation. With the exception of the downregulation of extracellular matrix genes, serine protease inhibitor genes, and the upregulation of the translation initiation factor PHAS-I, this model of atrophy in cell culture has a transcriptional profile quite distinct from any study published to date with atrophy in whole muscle. These data show that, although the gross morphology of atrophied muscle fibers may be similar in whole muscle vs. myotube culture, the processes by which this phenotype is achieved differ markedly.

Global analysis of gene expression patterns during disuse atrophy in rat skeletal muscle.

Muscular inactivity leads to atrophy, weakness, and decreased fatigue resistance. In order to provide a window into the dynamic processes that underlie muscle atrophy, we performed global gene expression analysis of rat soleus muscles using Affymetrix GeneChips at 1, 4, 7 and 14 days of hindlimb unloading. Expression of 309 known genes was significantly changed by at least 2-fold (212 upregulated, 97 downregulated). K-means clustering was used to divide these genes into co-regulated clusters based on the similarity of temporal expression patterns. This allowed the development of a timeline of the atrophy process with respect to the behaviour of genes in a broad array of functional categories. Regulatory genes were often upregulated early, in either a transient or sustained manner, but they also populated clusters with later patterns of activation, suggesting different phases of molecular adaptations. Other early events were the activation of ubiquitination genes and downregulation of protein chaperones. In comparison, clusters representing slightly delayed activation patterns included genes involved in fast contraction, glycolysis, translational inhibition, oxidative stress, protein degradation, and amino acid catabolism. Downregulated genes exhibited fewer unique expression patterns and included structural and regulatory genes of the extracellular matrix and cytoskeleton, and genes that define a slow-oxidative phenotype. Other novel findings include the tight co-activation of proteasome subunit and ubiquitination genes, differential regulation of serine proteases and serine protease inhibitors, and the identification of transcriptional, signalling, growth and cell cycle genes that probably play a role in the atrophy process. The present work has uncovered temporal patterns of gene expression that highlight the molecular processes that underlie muscle atrophy and provide new avenues for study.

Molecular events in skeletal muscle during disuse atrophy.

This review summarizes the current knowledge of the molecular processes underlying skeletal muscle atrophy due to disuse. Because the processes involved with muscle wasting due to illness are similar to disuse, this literature is used for comparison. Areas that are ripe for further study and that will advance our understanding of muscle atrophy are suggested.