Severe disruptions in the use of skeletal muscle, caused by trauma, such as peripheral nerve damage, inevitably lead to a decrease in myofiber size, or muscle atrophy. Moreover, even modest decreases in the use of skeletal myofibers, caused by immobilization, prolonged bed-rest or aging, leads to muscle atrophy. Atrophic myofibers have a reduced capacity to generate force but retain most of the structural features that are characteristic of normal muscle. Thus, even in the absence of innervation, denervated myofibers do not undergo degeneration and severe wasting. Indeed, after short periods of inactivity, atrophy is reversible, and myofibers regain their normal size. Even after prolonged periods of disuse, degeneration is uncommon, and atrophy can be partially reversed.
The pathways that regulate atrophy are poorly understood, but signals that stimulate phosphatidylinositol-3-kinase (PI3K) and Akt inhibit atrophy, and constitutively active Akt can promote muscle hypertrophy. Activated Akt phosphorylates FoxO transcription factors, which inhibits their nuclear import and prevents activation of FoxO-target genes, some of which encode for E3 ubiquitin ligases that participate in proteasome-mediated protein degradation. Nutritional starvation, catabolic steroids and cancer cachexia also lead to a decrease in myofiber size, either by activating a similar FoxO-atrophy-program and/or by inducing autophagy.
Previously, we used a subtractive-hybridization and cloning strategy to identify genes that are expressed in skeletal muscle and regulated by innervation. We found that Runx1 (AML1), a DNA-binding protein that is homologous to Drosophila Runt and has critical roles in hematopoiesis and leukemogenesis, is poorly expressed in innervated muscle but strongly induced in muscle shortly after denervation (Zhu et al., 1994). To study the role of Runx1 in skeletal myofibers, we inactivated the runx1 gene selectively in skeletal muscle (Wang et al., 2005). We found that induction of Runx1 is required to sustain muscle by preventing denervated myofibers from undergoing severe muscle wasting, accompanied by myofibrillar disorganization and the accumulation of autophagic vacuoles, structural defects found in a variety of congenital myopathies and in critical illness myopathy. These data indicate that muscle disuse, including denervation, places demands on the muscle that are met, in part, by induction of Runx1. In the absence of Runx1 induction, disused myofibers cannot meet these demands and undergo severe wasting. As Runx1 induction is critical to sustain muscle during periods of reduced neural/muscle activity, we are currently studying how the runx1 gene is regulated by innervation.

We used a microarray screen to identify targets of Runx1. We found that only twenty-nine genes are mis-regulated (≥three-fold) in denervated muscle lacking Runx1, suggesting that only a few genes are responsible for the dramatic muscle wasting observed in runx1 mutant mice (Wang et al., 2005). The twenty-nine mis-regulated genes encode ion channels (five genes), signaling molecules (fourteen genes) and structural proteins (four genes) but not transcription factors, indicating that the identified genes are good candidates for direct targets of Runx1. Sixteen genes are not appropriately up-regulated or maintained in runx1 mutant denervated muscle, suggesting that Runx1 activates their expression. Thirteen genes are expressed at unusually high levels in denervated muscle lacking Runx1, suggesting that Runx1 represses their expression. The genes that are mis-regulated in runx1 mutant muscle provide clues to the causes for the profound structural changes. Current experiments are designed to determine which of the Runx1-target genes are responsible for the structural changes and wasting of denervated runx1 mutant muscle.