10. Aging-related Elevation in Phd2 is Responsible for the Loss of Muscle Adaptation to Exercise

BACKGROUND: Exercise is necessary for the maintenance of skeletal muscle with aging. However, muscle adaptation to exercise becomes limited with aging, possibly due to a pathologic increase in prolyl hydroxylase domain (PHD)2, which inhibits downstream hypoxia signaling. We hypothesize age-related increase of PHD2 and loss of hypoxia pathway signaling limits muscle adaptation to exercise. METHOD: Young (12-14 weeks old) and old (24-26 months old) mice were subjected to either 40 minutes of a progressive running training protocol on a treadmill, designed to exercise the animals at approximately 70% of VO2max, 5 times per week for 8 weeks total, or no additional training. Following the exercise regimen, the whole gastrocnemius (G) was subjected to RNA Seq transcriptomic analysis. Next, mice with muscle specific PHD2 overexpression were created to further assess whether loss of hypoxia signaling impairs muscle adaptation to exercise and underwent a similar training regimen. Finally, to determine whether augmentation of hypoxia signaling could restore exercise adaptation in old mice, ML228, a hypoxia activator, was administered to old mice, which subsequently underwent exercise training. RESULTS: Young, trained mice exhibited a significant increase in maximum distance running (250%, p<0.001), maximal running speed (23%, p=0.03), and lean muscle mass (15%, p<0.001) in comparison to age matched groups which did not undergo regimented training. In contrast, old mice did not benefit from regimented aerobic training in these measures. Oxidative fiber types, Type I and IIa, which are responsible for increased endurance capacity, increased by 250% (p=0.02) and 30% (p=0.03), respectively, in young mice but did not increase in old. Transcriptome analysis of muscles from young mice demonstrated differential regulation of 120 genes with exercise. None of these genes were similarly regulated in the old group. Genes most upregulated following exercise in young mice were direct targets of the hypoxia signaling pathway. Immunoblotting demonstrated that PHD2 decreased 2-fold (p<0.01) and downstream hypoxia targets increased with exercise in young mice, but this decrease was absent in old mice following exercise. To determine if this increase in PHD2 could be causative for loss of exercise adaptation, we next generated mice with muscle specific knock in of PHD2 (mPHD2) and compared these to littermate controls, with and without exercise. mPHD2 mice expressed 4-fold higher levels of PHD2 in muscle and, similarly to old mice, did not exhibit improvements in maximal running distance, maximal running speed, lean muscle mass, or fiber type adaptation. ML228, a non-specific hypoxia signaling activator, restored loss of exercise adaptation in old mice. CONCLUSION: Age-related elevation of skeletal muscle PHD2 hinders physiologic and fiber type adaptation in muscle in response to exercise. Pharmacological activation of hypoxia signaling offers a promising method for improving exercise-based therapies for muscle loss in aging.