Tissue remodeling is an important physiological process that allows skeletal muscle to respond to environmental demands, promoting adaptive changes in cytoarchitecture and protein composition in response to a variety of stimuli. A host of extracellular agonists, receptors, protein kinases, intermediate molecules, and transcription factors participate in signal transduction pathways, promoting specific cellular responses to environmental cues. Perturbations in these pathways can result in chronic protein degradation, one of the most devastating consequences of defects in muscle survival mechanisms.
Disruption of skeletal muscle homeostasis can appear spontaneously at all ages, resulting in weakness, muscle atrophy, and degeneration. The genetic defects responsible for muscle dysfunction in several inherited pathologies have been well characterized, as in muscular dystrophies where the disease results from mutations in the giant gene encoding dystrophin, a structural protein (1
). However, muscle wasting is often a secondary consequence of other pathological states. For example, cardiac cachexia leads to progressive muscle atrophy, which has been increasingly appreciated as a powerful negative predictive factor in heart failure (2
). Moreover, most cancer patients suffer from muscle degeneration (4
), which also occurs in patients with dysfunction of the immune system (5
Despite its widespread occurrence, the molecular underpinnings of muscle atrophy have remained elusive. Subtle alterations in signaling pathways have been identified as leading to significant defects in muscle metabolism, yet the field has been stalled in devising successful therapeutic strategies for treatment of this debilitating and often fatal group of human ailments. The complexity of muscle types, the intimate relationship between structural integrity and mechanical function, and the sensitivity of skeletal muscle to metabolic perturbations have impeded rapid progress in successful clinical intervention. The increasingly limited regenerative properties of aging skeletal muscle further compound the devastating effects of wasting.
Pathological loss of muscle mass occurs primarily through enhanced protein breakdown mediated by ubiquitin, a small, highly conserved protein that becomes covalently linked to lysine residues of intracellular proteins destined for degradation (6
). The critical regulators of protein ubiquitination are tissue-specific E3 ligases (7
) that catalyze the transfer of activated ubiquitin, selectively targeting specific proteins for rapid degradation through the 26S proteasome (6
). The crucial role of the ubiquitin-proteasome pathway has been demonstrated in several models of muscular atrophy that include starvation (7
), uremia (9
), diabetes (10
), and sepsis (11
). The characterization of molecular pathways that regulate muscle catabolism through control of ubiquitin-proteasome–mediated degradation is of critical importance in designing candidate gene targets for the attenuation of muscle wasting.
Maintenance of muscle mass is also profoundly influenced by secondary factors, such as inflammation. Indeed, systemic inflammation rather than disuse has been cited as the primary cause of muscle wasting during aging and chronic disorders (12
). Among the pathways controlling inflammation, those activating transcription factor NF-κB play a major pleiotropic role in the modulation of immune, inflammatory, cell survival, and proliferative responses (13
). In unstimulated cells, NF-κB is kept inactive via association with inhibitor of NF-κB (IκB) proteins. Induced phosphorylation of IκB is mediated by IκB kinase (IKK) complex, which contains 2 catalytic subunits (IKK1/α and IKK2/β) (14
) and a regulatory subunit termed NF-κB essential modulator/IKKγ (NEMO/IKKγ) (15
). Upon encountering a variety of stimuli, including TNF-α, IL-1, and other growth factors, IκBα is phosphorylated and degraded through the ubiquitination pathway, rendering NF-κB free to accumulate in the nucleus and bind to its target genes (16
). In muscle tissue cultures, NF-κB influences myogenic growth and differentiation (17
), downregulates MyoD expression (19
), and induces muscle atrophy in mice (21
). However, less is known about the in vivo contribution of NF-κB to skeletal muscle development and regeneration.
To address the specific physiological role of proinflammatory signaling in skeletal muscle homeostasis and to bypass the embryonic lethality caused by systemic NF-κB depletion (22
), we targeted the IKK2 subunit of the IKK complex, using the Cre/loxP recombination system, to restrict inactivation of the gene to muscle cells. We also took advantage of recent studies on the role of IGF-1 in the promotion of muscle regeneration and resolution of the inflammatory response (24
). Here we report that mouse muscles depleted of NF-κB signaling exhibit an increase in muscle strength, maintain normal muscle physiology, block protein degradation under atrophy conditions, and display enhanced muscle regeneration in response to injury. Combined with supplemental IGF-1 in the form of a muscle-specific transgene expressing a local Igf-1
), abrogation of NF-κB signaling provides an even greater protection against muscle atrophy. These results place the control of inflammatory pathways at center stage for clinical intervention in muscle atrophy and degeneration and underscore the potential power of combination therapies to counter the devastating consequences of muscle wasting.