Sarcopenia, the age-related loss of skeletal muscle mass, is characterized by a deterioration of muscle quantity and quality leading to a gradual slowing of movement, a decline in strength and power, increased risk of fall-related injury, and often, frailty [43]. von Haehling et al. [44] have estimated its prevalence at 513% for elderly people aged 60–70 years and 11–50% for those aged 80 years or above. Lean muscle mass generally contributes up to ∼50% of total body weight in young adults, but declines with aging to 25% at 75–80 years old [45].

A decline in autophagy during normal aging has been described for invertebrates and higher organisms [46]. Inefficient autophagy has been attributed a major role in the age-related accumulation of damaged cellular components, such as undergradable lysosome-bound lipofuscin, protein aggregates, and damaged mitochondria [46]. Demontis and Perrimon [47] showed that the function of autophagy/lysosome system of protein degradation declined during aging in the skeletal muscle of Drosophila. This results in the progressive accumulation of polyubiquitin protein aggregates in senescent Drosophila muscle. Intriguingly, overexpresssion of the FOXO increases the expression of many autophagy genes, preserves the function of the autophagy pathway, and prevents the accumulation of polyybiquitin protein aggregates in sarcopenic Drosophila muscle [48]. Several investigators reported the autophagic changes in aged mammalian skeletal muscle [17, 26, 49–51]. Compared with those in young male Fischer 344 rats, amounts of Beclin-1 were significantly increased in the plantaris muscles of senescent rats [51]. Using Western blot of fractionated homogenates and immunofluorescence microscopy, we recently demonstrated the selective induction of p62/SQSTM1 and Beclin-1 but not LC3 in the cytosol of sarcopenic muscle fibers in mice [17]. In addition, we observed a significant smaller p62/SQSTM1-positive muscle fibers in aged muscle compared to the surrounding p62/SQSTM1-negative fibers [17], (Fig. 4.1). In contrast, aging did not influence the amounts of Atg7 and Atg9 proteins in rat plantaris muscle [51]. Western blot analysis by Wohlgemuth et al. [51] clearly showed a marked increase in the amount of LC3 in muscle during aging. However, they could not demonstrate an aging-related increase of the ratio of LC3-II to LC3-I, a better biochemical marker to assess ongoing autophagy. In addition, we failed to detect marked increase in LC3-I and LC3-II (active form) proteins in aged quadriceps muscle [17]. In contrast, Wenz et al. [26] recognized a significant increase in the ratio of LC3-II to LC3-I during aging (3 vs. 22 months) in the biceps femoris muscle of wild-type mice. None of the studies determining the transcript level of autophagy-linked molecules found a significant increase with age [17, 49, 51]. Not all contributors to autophagy signaling seem to change similarly at both mRNA and protein levels in senescent skeletal muscle. Therefore, sarcopenia may include a partial defect of autophagy signaling, although more exhaustive investigation is needed in this field. Intriguingly, more recent study [16] using biopsy samples of young and aged human volunteers clearl showed tha age-dependent autophagic defect such as the decrease in the mount of Atg7 protein and in the ration of LC3-II/LC3-I protein.

Life-long caloric restriction alone, or combined with voluntary exercise, resulted in mild reduction of LC3 expression and lipidation coupled with increased lysosome-associated membrane protein 2 expression, suggesting a potential increase in autophagy flux. No significant age-related increase in autophagy-linked molecules was observed in MCK-PGC-1α mice. PGC-1α may also enhance autophagic flux. More recently, glycogen synthase kinase (GSK)-3α was proposed as a critical regulator of aging in various organs (skeletal muscle, heart, liver, bone, etc.) via modulating mTORC1 and autophagy [52]. Intriguingly, mice with null mutation of GSK-3α showed premature death and acceleration of age-related pathologies such as vacuolar degeneration, large tubular aggregates, sarcomere disruption, and striking sarcopenia in cardiac and skeletal muscle [52]. These GSK-3α knockout mice exhibited marked activation of mTORC1 and associated suppression of several autophagy molecules. Indeed, unrestrained activation of mTORC1 leads to profound inhibition of autophagy [53]. Therefore, it is expected that pharmacological inhibition (everolimus) of mTORC1 rescued the muscular disorder resembling sarcopenia in GSK-3α knockout mice [52]. Enhancement of autophagy flux (exercise, caloric restriction, etc.) would be a potential strategy attenuating sarcopenia as well as various type of muscular dystrophy with autophagy defect [36, 54]. Figure 4.2 summarizes a possible adaptation of autophagy-linked molecules (LC3 and p62/SQSTM1) in sarcopenic muscle.

Cachexia is a complex metabolic syndrome characterized by a severe and involunrtary loss of muscle mass. Cachexia is associated not only with chronic diseases, most commonly cancer, but also with other inflammatory conditions such as chronic obstructive pulmonary disease (COPD), heart failure, chronic kidney disease, AIDS and sepsis [56]. The overall prevalence of cachexia is approximately 1% of the global patient population, which can increase to 50–80% in cancer patients [56,57]. Indeed, almost 80% of cancer patients suffereing cachexia will be dead within 1 year of diagnosis.

As for cancer cachexia, earlier results obtained on muscles isolated from cachectic animals led us to rule out a substantial role for lysosomes in overall protein degradation [58]. In contrast, an elevation of total lysosome protease activity has been observed in the skeletal muscle and liver of tumor-bearing rats [59]. In addition, increased levels of cathepsin L. mRNA have been reported in the skeletal muscle of septic or tumor-bearing rats, whereas cathepsin B gene expression has been shown to be enhanced in muscle biopsy samples obtained from patients with lung cancer [60, 61]. Furthermore, a few general observations suggested that autophagy can be activated in the muscle of animals bearing Lewis lung carcinoma (LLC) or colon 26 (C26) tumor [62, 63]. More recently, Penna et al. [64] investigated whether autophagy signaling was elevated in muscle using three different models of cancer cachexia. They observed marked increases in the levels of Beclin-1, p62, and LC3-II (the lipidated form; a reliable marker of autophagosome formation) in muscle in C26-bearing mice. In addition, Penna et al. [64] evaluated autophagic markers in the gastrocnemius muscle of rats bearing Yoshida AH-130 hepatoma or of mice transplanted with LLC. Several autophagic markers were upregulated in the muscle of these two cancer cachexia rodent models, although there was some difference in the adaptive manner. Furthermore, OP den Kamp et al. [65] indicated that the levels of both LC3-I and -II proteins but not LC3B mRNA were significantly increased in the vastus lateralis muscle of patients with lung cancer cachexia. Esophageal cancer patients also appear to exhibit higher LC3-II/I ratios and levels of cathepsin B and L. expression in muscle [66]. Since they did not detect a significant change of proteasome, calpain, or caspase 3 activity in the muscle of these patients, they consider that the autophagic-lysosomal pathway is the main proteolytic system in the muscle in esophageal cancer cachexia. Table4.2 summarizes the adaptive changes in several molecules in utophagy in cancer cachexia.

The functional importance of autophagy in the pathogenesis of lung disease in COPD patients has been demonstrated by Chen et al. [69] who described significant increases of autophagy in clinical lung samples taken from COPD patients. LC3B, Beclin-1, Atg7, and Atg5 were all upregulated, and autophagosome formation was visualized using electron microscopy.

In addition, Ryter et al. [70] have also described increased autophagy in clinical specimens of the lung from patients with COPD. They showed the increased expression and activation of autophagic regulator proteins (i.e., LC3B, Beclin-1, Atg5, Atg7) in lung. Similar evidence of increased autophagy was observed in mice subjected to chronic inhalation of cigarette smoke [69] and in lung epithelial cells exposed to aqueous cigarette smoke extracts [71]. Taking these findings together, autophagy seems to be activated in lung as a stress response. To date, little research has been completed on the contribution of the autophagy system to protein degradation and loss of skeletal muscle mass in COPD patients. Using muscle biopsy samples obtained from severe COPD patients with marked atrophy [forced expiratory volume in 1 s value of 35 ± 2% of predicted], Plant et al. [72] demonstrated that there was no difference in the levels of Beclin-1 and LC3 transcripts in the quadriceps muscle of patients with COPD compared with those in control individuals. On the basis of these results, Plant et al. [72] concluded that autophagy is not activated in muscles of COPD patients. However, they assessed the degree of autophagy by measuring mRNA levels only. More recently, Guo et al. [73] performed a pilot experiment using Western blot and real-time PCR mRNA measurements to evaluate autophagy-related gene expression of muscle biopsies obtained from cases of severe COPD. These experiments revealed significant increases in the intensity of LC3-II protein in muscle of COPD patients compared with that in control subjects. In addition, they also observed significant increases in Beclin-1 and p62 protein levels in muscle biopsies of COPD patients indicating the activation of autophagy. More complete elucidation of the functional role of autophagy in muscle of COPD patients remains to be determined, but some research in this field has been undertaken. It is probable that the activation of autophagy in the muscle of COPD patients is modulated by several factors, such as oxidative stress, inflammation, malnutrition, and therapeutic medication, as proposed in an excellent systematic review by Hussain and Sandri [74].

One original study investigated the relationship between chronic heart failure (CHF) and autophagy signaling in skeletal muscle [75]. It was suggested that there is a difference in the manner of autophagic adaptation between soleus (slow-type) and plantaris (fast-type) muscles by using rats with myocardial infarction. In fact, the transcription levels of GABARAPL1 and Atg7 were increased in the plantaris but not the soleus muscle. However, the expression levels of other autophagic markers (Beclin-1 and Atg12) did not change significantly. In addition, an autophagy-activating marker (LC3-II/I) also did not increase in both muscles. However, there have been no studies examining the autophagy in muscle in cases of CHF. It remains to be elucidated whether CHF includes autophagic activation in skeletal muscle similar to muscle in cancer cachexia and COPD.

A finely tuned system for protein degradation and organelle removal is required for the proper function and contractility of skeletal muscle [14]. Inhibition/alteration of autophagy contributes to myofiber degeneration leading to accumulation of abnormal (dysfunctional) organelles and of unfolded and aggregation-prone proteins [24, 76], which are typical features of several myopathies [36, 77]. Generation of Atg5 and Atg7 muscle-specific knockout mice confirmed the physiological importance of the autophagy system in muscle mass maintenance [24, 78]. The muscle-specific Atg7 knockout mice are characterized by the presence of abnormal mitochondria, oxidative stress, accumulation of polyubiquitinated proteins, and consequent sarcomere disorganization [24]. In addition, the central role of the autophagy-lysosome system in muscle homeostasis is highlighted by lysosomal storage diseases (Pompe disease, Danon disease, and X-linked myopathy), a group of debilitating muscle disorders characterized by alterations in lysosomal proteins and autophagosome buildup [79]. Intriguingly, all of these myopathies exhibit the accumulation of autophagic vacuoles inside myofibers due to defects in their clearance.