Muscle‐specific changes in protein synthesis with aging and reloading after disuse atrophy

Abstract

BackgroundSuccessful strategies to halt or reverse sarcopenia require a basic understanding of the factors that cause muscle loss with age. Acute periods of muscle loss in older individuals have an incomplete recovery of muscle mass and strength, thus accelerating sarcopenic progression. The purpose of the current study was to further understand the mechanisms underlying the failure of old animals to completely recover muscle mass and function after a period of hindlimb unloading.MethodsHindlimb unloading was used to induce muscle atrophy in Fischer 344–Brown Norway (F344BN F1) rats at 24, 28, and 30 months of age. Rats were hindlimb unloaded for 14 days and then reloaded at 24 months (Reloaded 24), 28 months (Reloaded 28), and 24 and 28 months (Reloaded 24/28) of age. Isometric torque was determined at 24 months of age (24 months), at 28 months of age (28 months), immediately after 14 days of reloading, and at 30 months of age (30 months). During control or reloaded conditions, rats were labelled with deuterium oxide (D2O) to determine rates of muscle protein synthesis and RNA synthesis.ResultsAfter 14 days of reloading, in vivo isometric torque returned to baseline in Reloaded 24, but not Reloaded 28 and Reloaded 24/28. Despite the failure of Reloaded 28 and Reloaded 24/28 to regain peak force, all groups were equally depressed in peak force generation at 30 months. Increased age did not decrease muscle protein synthesis rates, and in fact, increased resting rates of protein synthesis were measured in the myofibrillar fraction (Fractional synthesis rate (FSR): %/day) of the plantaris (24 months: 2.53 ± 0.17; 30 months: 3.29 ± 0.17), and in the myofibrillar (24 months: 2.29 ± 0.07; 30 months: 3.34 ± 0.11), collagen (24 months: 1.11 ± 0.07; 30 months: 1.55 ± 0.14), and mitochondrial (24 months: 2.38 ± 0.16; 30 months: 3.20 ± 0.10) fractions of the tibialis anterior (TA). All muscles increased myofibrillar protein synthesis (%/day) in Reloaded 24 (soleus: 3.36 ± 0.11, 5.23 ± 0.19; plantaris: 2.53 ± 0.17, 3.66 ± 0.07; TA: 2.29 ± 0.14, 3.15 ± 0.12); however, in Reloaded 28, only the soleus had myofibrillar protein synthesis rates (%/day) >28 months (28 months: 3.80 ± 0.10; Reloaded 28: 4.86 ± 0.19). Across the muscles, rates of protein synthesis were correlated with RNA synthesis (all muscles combined, R 2 = 0.807, P < 0.0001).ConclusionsThese data add to the growing body of literature that indicate that changes with age, including following disuse atrophy, differ by muscle. In addition, our findings lead to additional questions of the underlying mechanisms by which some muscles are maintained with age while others are not.

Endurance-Type Exercise Increases Bulk and Individual Mitochondrial Protein Synthesis Rates in Rats

“…We chose to assess protein synthetic responses in the soleus muscle primarily due to the high type I muscle fiber content and aerobic nature of wheel running exercise. As soleus muscle may possess a greater sensitivity to induce transcriptional and/or protein synthetic responses to physical (in)activity when compared to other muscles (e.g., plantaris or tibialis anterior) (Miller et al, 2019), we should be careful when translating these findings to other muscle groups.…”

Massage as a mechanotherapy promotes skeletal muscle protein and ribosomal turnover but does not mitigate muscle atrophy during disuse in adult rats

“…After incubation, the mixture was centrifuged at 12 000 g for 10 minutes at 4°C to pellet RNA. The RNA pellet was isolated, rinsed with 1 mL of 75% ethanol and resuspended in 50 µL of molecular biology grade H 2 O. RNA integrity was assessed using the Agilent Bioanalyzer (Agilent Technologies) and RNA concentration was determined using a Nano Drop (Thermo Fisher Scientific) to then calculate total RNA per mg muscle …”

“…A separate RNA isolation of ~15 to 25 mg of frozen muscle was performed for ribosomal turnover analysis according to our previously published procedures . We utilized Trizol for RNA isolation exactly the same as described above for Total RNA isolation.…”

“…The fractional molar isotope abundances at m ⁄z 212 (M0) and 213 (M1) of the pentafluorobenzyl triacetyl derivative of purine ribose were quantified using MassHunter software. All analyses were corrected for abundance with an unenriched pentafluorobenzyl triacetyl purine ribose derivative standard …”

Identification and Characterization of Fbxl22, a novel skeletal muscle atrophy-promoting E3 ubiquitin ligase

“…The manifestation of the degenerative phenotype in Fbxl22 transfected muscles appears to differ between muscles and is dependent on which Fbxl22 isoform is overexpressed, this being reflected in the subtle differences observed in our biochemical analyses. We have previously observed muscle-specific changes in levels of atrophy upon implementation of a hindlimb unloading model (52)(53)(54). While differences in muscle fiber type, neural input, and/or loading history may all contribute to our previous findings, it is important to highlight that the changes in E3 ubiquitin ligase expression (MuRF1 and MAFbx) varied between the rat soleus, medial gastrocnemius and TA muscles during the unloading period (53).…”