Muscle Wasting in Chronic Obstructive Pulmonary Disease: Not Enough Autophagy?

Chronic obstructive pulmonary disease (COPD) is an inflammatory lung disease characterized by airway obstruction and inflammation. Pulmonary emphysema, a form of COPD, represents a major public health problem worldwide (1). Patients with COPD frequently experience exercise intolerance, which negatively affects their quality of life and worsens morbidity andmortality (2). Growing evidence indicates that patients with emphysema develop locomotor skeletal muscle wasting and weakness, which contributes to adverse clinical outcomes, independent of lung function impairment (2, 3). The etiology of skeletal muscle dysfunction in patients with COPD is multifactorial and includes systemic inflammation, oxidative stress, malnutrition, and enhanced proteolysis. Recent studies have reported that dysregulation of autophagy may be involved in the pathogenesis of skeletal muscle dysfunction in patients with COPD (4, 5). Autophagy is the catabolic process by which cytosolic proteins or damaged organelles, such as mitochondria and peroxisomes, are enclosed by a double membrane autophagosome and subsequently delivered to lysosomes for degradation. It plays a key role in numerous physiological and pathological processes, including maintenance of muscle mass, contractile function, and myofiber integrity (6, 7). In this issue of the Journal, Balnis and colleagues (pp. 623–637) report on the role of autophagy in the impairment of the myogenesis program of muscle satellite cells (SCs) using a murine model of emphysema in which IL13 (T-helper cell type 2 cytokine IL-13) is overexpressed in pulmonary cells (8). They conclude that inhibiting autophagy impairs myogenic and replicative capacities of muscle SCs in emphysematous mice, leading eventually to muscle atrophy and contractile dysfunction. The IL13model was chosen because it replicates most features of COPD in patients, including pulmonary emphysema, decreased oxygen saturation, and subsequent development of limbmuscle atrophy and weakness. The authors demonstrate that the development of emphysema triggers significant fiber atrophy and leads to decreases in the maximum isometric force of the extensor digitorum longus (EDL) muscle (see Reference 8 Figure 1). Specific maximum isometric EDL force (force normalized per surface area) did not vary in comparison with control mice (IL13), suggesting that muscle atrophy is the primary cause of limbmuscle weakness in this model. This conclusion is supported by previous observations by the same group that the muscle-specific ubiquitin E3 ligases atrogin-1 andMuRF1, which play important roles in muscle atrophy in various catabolic conditions, are significantly upregulated in EDLmuscles from IL13 mice (9). The first question the authors attempt to answer is whether autophagy is impaired in the skeletal muscles of emphysematous mice. In line with previous reports in patients with COPD (4, 5), they found that several autophagy-related genes (Ulk1, Beclin1, Lc3, Sqstm1, Atg7, andAtg3) were upregulated in the limbmuscles of these mice. Of note, though, when quantifying autophagy, measuring autophagy-related gene expression alone is not an adequate method, given the fact that impaired autophagosome–lysosome fusion or lysosome dysfunction may develop with no accompanying changes in autophagy gene expression (10). Thus, one of the shortcomings of the study is that it lacks in vivomuscle autophagic flux measurements. Future studies should be designed to address this issue. Skeletal muscle autophagic flux can be measured in experimental emphysema models by using autophagy inhibitors such as leupeptin or colchicine, as previously reported by our group (11). The second question the authors attempt to answer is whether the regenerative capacity of an SC is dysregulated in emphysematous mice. They demonstrate that differentiation of primary SCs derived from IL13mice significantly decreases relative to those derived from IL13mice, suggesting that the in vitromyogenic capacity of SCs is indeed impaired by emphysema. Furthermore, using a barium chloride injury model to elicit fiber necrosis in the tibialis anterior (TA) muscle, they showed that the fiber size of regenerating TA muscles from IL13mice was significantly smaller that that from wild-type mice, suggesting impairment of the in vivomyogenesis program. Interestingly, in the regenerating muscles of both the IL13 and IL13mice, the expressions of desmin and embryonic myosin heavy chain (markers of regeneration) were similar, indicating the degree of impairment in the emphysematous mice was relatively mild. The third question the authors attempt to address is whether the replicative capacity of an SC is disrupted in the muscles of emphysematous mice. Ethynyl-deoxyuridine incorporation assays resulted in significant decreases in the proliferation of IL13 SCs relative to IL13 SCs. To establish that altered regeneration and replication are intrinsic defects of IL13 SCs rather than related to the COPD environment, they conducted transplantation experiments in which red fluorescent protein–labeled SCs were injected into barium chloride–injured muscles. The injured muscles were examined 2 weeks later, and lineage tracing confirmed that the defects in IL13 SCs were indeed intrinsic. To identify the degree to which autophagy contributed to these defects, the authors measured Beclin1 protein concentrations and counted the number of LC3 punctae in these cells. Both appeared to be significantly higher in IL13 SCs relative to IL13 SCs, but direct measurements of in vivo autophagic flux using bafilomycin (inhibitor of lysosomal degradation) suggested that the rate of autophagosome formation in IL13 SCs was actually significantly lower than in IL13 SCs. So, they then administered spermidine (inducer of autophagy) to the drinking water of IL13 mice to confirm that decreased autophagy was, in fact, the mechanism underlying the observed impairments to the myogenic