Skeletal muscle atrophy caused by unloading is characterized by both decreased responsiveness to myogenicThe impairment of growth factor signaling is a near-universal feature of skeletal myopathies induced by unloading (6, 13). Clinical trials have established that during unloading, muscle tissue fails to respond to IGF-1, a dominant myotrophic hormone (7,19,34). Under normal conditions and in response to hypertrophic stimuli, IGF-1 promotes muscle growth and suppresses muscle loss largely through the Akt-dependent phosphorylation and cytosolic sequestration of FOXO transcription factors in skeletal myocytes, which leads to the inhibition of FOXO-dependent gene expression (38, 41). In contrast, IGF-1-dependent Akt signaling is impaired during muscle atrophy, which decreases the phosphorylation and increases the transactivation of FOXO target genes. In particular, FOXO regulates the expression of atrophy-related genes (atrogenes) that encode atrogin-1/MAFbx and MuRF-1, which are RING-type ubiquitin ligases that are critical mediators of atrophic myopathies in vivo (3,14). Atrogin-1 and MuRF-1 regulate the degradation of key proteins involved in striated muscle growth and differentiation, including MyoD, calcineurin, and troponin-I (24, 27, 47). Although diminished growth factor responsiveness and enhanced proteolysis both are major atrophyrelated processes, the mechanisms by which skeletal muscle becomes refractory to the trophic actions of muscle growth factors during unloading are not well defined.In a previous study designed to evaluate changes in skeletal muscle gene expression in rats exposed to a 16-day spaceflight (30), we identified novel potential atrogenes (37) using microarray analysis. The response of skeletal muscle to mechanical stress is accompanied by marked alterations in atrogene expression, and we showed that microgravity induces Siah-1A, MuRF-1 (30), and atrogin-1 (see Table S1 in the supplemental material). Microgravity also resulted in the increased expression of Cbl-b (greater than eightfold). Cbl-b is another RING-type ubiquitin ligase previously established as a negative regulator of receptor tyrosine kinase signaling in a variety of cells (23,45). These results complement our recent finding that Cbl-b downregulates bone formation

Although muscle atrophy is a serious problem during spaceflight, little is known about the sequence of molecular events leading to atrophy in response to microgravity. We carried out a spaceflight experiment using Caenorhabditis elegans onboard the Japanese Experiment Module of the International Space Station. Worms were synchronously cultured in liquid media with bacterial food for 4 days under microgravity or on a 1-G centrifuge. Worms were visually observed for health and movement and then frozen. Upon return, we analyzed global gene and protein expression using DNA microarrays and mass spectrometry. Body length and fat accumulation were also analyzed. We found that in worms grown from the L1 larval stage to adulthood under microgravity, both gene and protein expression levels for muscular thick filaments, cytoskeletal elements, and mitochondrial metabolic enzymes decreased relative to parallel cultures on the 1-G centrifuge (95% confidence interval (P⩽0.05)). In addition, altered movement and decreased body length and fat accumulation were observed in the microgravity-cultured worms relative to the 1-G cultured worms. These results suggest protein expression changes that may account for the progressive muscular atrophy observed in astronauts.

How microgravitational space environments affect aging is not well understood. We observed that, in Caenorhabditis elegans, spaceflight suppressed the formation of transgenically expressed polyglutamine aggregates, which normally accumulate with increasing age. Moreover, the inactivation of each of seven genes that were down-regulated in space extended lifespan on the ground. These genes encode proteins that are likely related to neuronal or endocrine signaling: acetylcholine receptor, acetylcholine transporter, choline acetyltransferase, rhodopsin-like receptor, glutamate-gated chloride channel, shaker family of potassium channel, and insulin-like peptide. Most of them mediated lifespan control through the key longevity-regulating transcription factors DAF-16 or SKN-1 or through dietary-restriction signaling, singly or in combination. These results suggest that aging in C. elegans is slowed through neuronal and endocrine response to space environmental cues.