Cachexia with muscle wasting is prevalent and closely associated with mortality and morbidity in patients with CKD.1 The pathophysiology of muscle wasting in CKD is complex. Inadequate nutritional intake, physical inactivity from muscle weakness, systemic inflammation and aberrant signaling of neuropeptides have been implicated.2 To date, there is no effective therapy. Exercise training has well documented health benefits, including maintenance of muscle mass as well as increased physical performance resulting from changes in muscle fiber phenotype leading to increased mitochondrial biogenesis. The molecular signaling mechanism underlying adaptations to increased physical activity in CKD-associated muscle wasting is not well understood.
Wang et al.3 investigated microRNA-23a (miR-23a) and miR-27a, located in an NFAT-regulated gene cluster, as potential candidates for exercise mimetics in mice with CKD cachexia. miR-23a expression is decreased in mice with CKD. Resistance overload exercise increased muscle miR-23a and miR-27a expression in CKD. In vivo transfection of miR-23a/miR-27a cluster precursor in CKD mice attenuated muscle wasting, improved grip strength, and corrected the aberrant signaling of ubiquitin ligases, muscle ring finger 1, and atrogin 1, which are the signature of muscle wasting in CKD. In silico analysis identified PTEN and caspase-7 as targets for miR-23a and Fox-O1 as target of miR-27a in muscle. The authors concluded that the miR-23a/miR-27a cluster might play an important role in the molecular signaling of exercise-induced adaptations in CKD muscle and suggested that this pathway may provide the basis of pharmaceutical exercise mimetics for CKD cachexia and wasting.
miRs have long been associated with myogenesis. miR-1 knockout flies show premature death from failure of skeletal muscle development.4 Knockout of miRs in mice, however, produces little in terms of pathologic phenotype, such as would be predicted from in vitro studies.5 Overlap and redundancy of various miR members could be the reason. Double knockout of miR-133a-1 and miR-133a-2 resulted in skeletal muscle myopathy, which was not shown in the single knockouts.6 miRs regulate satellite cell proliferation. In vitro miR-27a is known to promote myoblast proliferation by inhibiting myostatin.7 However, miR-23a affects myoblast differentiation via regulation of myosin heavy-chain gene transcription.8 Exercise has known effects on miR expression. Endurance exercise increases miR-1 and miR-133 in the short term9 but decreases the resting levels of miR-1 and miR-133 in the long term.10
In this study, Wang et al.3 first determined the effect of resistance exercise–prevented muscle wasting in both transverse abdominal (TA) and soleus muscles. Muscle overloading specifically improved miR-23a/miR-27a expression, whereas miR-24 and miR-29a expressions did not change in CKD mice. Then, they showed that overexpression of miR-23a/27a/24–2 by transfection of vectors into the TA muscle improved TA muscle mass as well as grip strength in CKD mice. The TA muscle provides thoracic and pelvic stability, which may improve efficiency of muscle recruitment in the extremities. In this study, only grip strength, which is mostly a test of upper limb muscle function, was tested. Rotarod activity and treadmill running tests, which include and are more reflective of lower limb function, were not studied. The demonstration of improved grip strength is important, because the quantitation of lean body mass/muscle mass is fraught with methodologic confounders, such as hydration and quality of muscle tissue with respect to the amount of fibrosis and fat, in CKD. Indeed, the demonstration of simultaneous improvements in muscle mass and muscle function is significant, because the direct correlation of the quantity and function of muscles in chronic diseases presenting with cachexia, such as CKD, cannot be assumed. This is clearly shown in a recent phase 3 trial of anamorelin in patients with cachexia from nonsmall cell lung cancer, which showed that increase in lean body mass was not accompanied by improvements in handgrip strength.11 Demonstration of improved muscle function has obvious translational implication in the therapeutic approach for muscle wasting in CKD. Significantly, there is recent evidence of the prognostic value of grip strength in predicting all-cause and cardiovascular mortality in a large longitudinal population study of >140,000 healthy participants in 17 countries of varying incomes and sociocultural settings.12
Wang et al.3 explored the molecular mechanism of the effect of the miR-23a/miR-27a cluster on CKD-associated muscle wasting by focusing on the atrophy-related signaling. They showed improvements in Akt/FoxO signaling, caspase-3–mediated proteolysis, muscle regeneration pathway, and myostatin signaling as well as inflammatory cytokines. Furthermore, they performed in silico exploration and identified PTEN and caspase-7 as potential molecular targets for miR-23a as well as FoxO and myostatin for miR-27a. In addition to maintaining muscle mass or preventing muscle wasting, exercise training is known to trigger in skeletal muscle remodeling that progressively enhances substrate utilization; this acts to reduce muscle fatigue, leading to improved muscle strength and performance. Furthermore, exercise training can cause muscle fiber type remodeling from less to more oxidative fibers, leading to high maximal oxidative capacity and improvement in exercise endurance. Wang et al.3 did not investigate whether miR-23a/miR-27a has an effect on substrate utilization or muscle fiber–type remodeling. A summary of miR effects on skeletal muscle biology in CKD with respect to pathways related to exercise mimetics is summarized in Figure 1.
The concept of exercise mimetics (that genetic manipulation and drug treatment may result in similar molecular adaptations elicited by exercise) has stimulated intense investigations. This approach makes the assumption that artificial reproduction of the exercise-induced molecular adaptations in disease conditions leading to physical inactivity may have the same salutary benefits of exercise training. The results of this line of investigation are, however, mixed. Wang et al.13 showed that reproducing molecular signatures of an oxidative phenotype of skeletal muscle by overexpressing PPARδ or treatment of PPARδ agonist may protect against development of obesity and insulin resistance in rodents fed a high-fat diet. PPARβ/δ agonist treatment did not increase endurance in sedentary mice, despite its molecular effects. However, PPARβ/δ agonist treatment and exercise training did synergistically improve physical endurance as well as oxidative muscle phenotype. Impressively, the pharmacologic AMPK activation by AICAR increases running endurance by as much as 44% in untrained mice.14
Furthermore, exercise training leads to multiorgan adaptations in addition to those shown in skeletal muscle. Thus, it would be simplistic to attribute the multiple health benefits of increased physical performance from exercise training to merely adaptations in skeletal muscle phenotype and genotype. Physical inactivity has been recognized as a major health hazard responsible for modern epidemics of lifestyle disorders, such as cardiovascular disease, obesity, and type 2 diabetes.15 Although not established as yet in CKD, the same pathophysiology may be similar. Thus, exercise mimetic for treating muscle wasting is an interesting and novel approach, but its validity in improving cachexia and its associated mortality and morbidity effects needs to be established in more robust rodent and human studies before it can be established.
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