Morphological and molecular aspects of immobilization-induced muscle atrophy in rats at different stages of postnatal development: the role of autophagy

Effects of Acupotomy on Immobilization-Induced Gastrocnemius Contracture and Fibrosis in Rats via Wnt/β-Catenin Signaling

OBJECTIVE

To determine whether acupotomy ameliorates immobilization-induced muscle contracture and fibrosis via Wnt/β-catenin signaling pathway.

METHODS

Thirty Wistar rats were randomly divided into 5 groups (n=6) by a random number table, including control, immobilization, passive stretching, acupotomy, and acupotomy 3 weeks (3-w) groups. The rat model of gastrocnemius contracture was established by immobilizing the right hind limb in plantar flexion for 4 weeks. Rats in the passive stretching group received passive stretching at gastrocnemius, a daily series of 10 repetitions for 30 s each at 30-s intervals for 10 consecutive days. Rats in the acupotomy and acupotomy 3-w groups received acupotomy once and combined with passive stretching at gastrocnemius a daily series of 10 repetitions for 30 s each at 30-s intervals for 10 consecutive days. Additionally, rats in the acupotomy 3-w group were allowed to walk freely for 3 weeks after 10-day therapy. After treatment, range of motion (ROM), gait analysis [i.e., paw area, stance/swing and maximum ratio of paw area to paw area duration (Max dA/dT)], gastrocnemius wet weight and the ratio of muscle wet weight to body weight (MWW/BW) were tested. Gastrocnemius morphometric and muscle fiber cross-sectional area (CSA) were assessed by hematoxylin-eosin staining. Fibrosis-related mRNA expressions (i.e., Wnt 1, β-catenin, axin-2, α-smooth muscle actin, fibronectin, and types I and III collagen) were measured using real-time quantitative polymerase chain reactions. Wnt 1, β-catenin and fibronectin concentrations were measured by enzyme-linked immunosorbent assay. Types I and III collagen in the perimysium and endomysium were analyzed using immunofluorescence.

RESULTS

Compared with the control group, ROM, gait function, muscle weight, MWW/BW and CSA were significantly decreased in the immobilization group (all P<0.01), while protein levels of types I and III collagen, Wnt 1, β-catenin, fibronectin and mRNA levels of fibrosis-related genes were obviously increased (all P<0.01). Treatment with passive stretching or acupotomy restored ROM and gait function and increased muscle wet weight, MWW/BW and CSA (all P<0.05), while protein expression levels of Wnt 1, β-catenin, fibronectin, types I and III collagen and mRNA levels of fibrosis-related genes were remarkably declined compared with the immobilization group (all P<0.05). Compared with passive stretching group, ROM, gait function, MWW was remarkably restored (all P<0.05), and mRNA levels of fibrosis-related genes as well as protein expression levels of Wnt 1, β-catenin, fibronectin, types I and III collagen in the acupotomy group were obviously decreased (all P<0.05). Compared with the acupotomy group, ROM, paw area, Max dA/dT, and MWW were restored (all P<0.05), and mRNA levels of fibrosis-related genes along with protein levels of Wnt 1, β-catenin, fibronectin, types I and III collagen in the acupotomy 3-w group were decreased (P<0.05).

CONCLUSION

Improvements in motor function, muscle contractures, and muscle fibrosis induced by acupotomy correlates with the inhibition of Wnt/β-catenin signaling pathway.

AKT controls protein synthesis and oxidative metabolism via combined mTORC1 and FOXO1 signalling to govern muscle physiology

BACKGROUND

Skeletomuscular diseases result in significant muscle loss and decreased performance, paralleled by a loss in mitochondrial and oxidative capacity. Insulin and insulin-like growth factor-1 (IGF-1) are two potent anabolic hormones that activate a host of signalling intermediates including the serine/threonine kinase AKT to influence skeletal muscle physiology. Defective AKT signalling is associated with muscle pathology, including cachexia, sarcopenia, and disuse; however, the mechanistic underpinnings remain unresolved.

METHODS

To elucidate the role of AKT signalling in muscle mass and physiology, we generated both congenital and inducible mouse models of skeletal muscle-specific AKT deficiency. To understand the downstream mechanisms mediating AKT's effects on muscle biology, we generated mice lacking AKT1/2 and FOXO1 (M-AKTFOXO1TKO and M-indAKTFOXO1TKO) to inhibit downstream FOXO1 signalling, AKT1/2 and TSC1 (M-AKTTSCTKO and M-indAKTTSCTKO) to activate mTORC1, and AKT1/2, FOXO1, and TSC1 (M-QKO and M-indQKO) to simultaneously activate mTORC1 and inhibit FOXO1 in AKT-deficient skeletal muscle. Muscle proteostasis and physiology were assessed using multiple assays including metabolic labelling, mitochondrial function, fibre typing, ex vivo physiology, and exercise performance.

RESULTS

Here, we show that genetic ablation of skeletal muscle AKT signalling resulted in decreased muscle mass and a loss of oxidative metabolism and muscle performance. Specifically, deletion of muscle AKT activity during development or in adult mice resulted in a significant reduction in muscle growth by 30-40% (P  < 0.0001; n = 12-20) and 15% (P < 0.01 and P < 0.0001; n = 20-30), respectively. Interestingly, this reduction in muscle mass was primarily due to an ~40% reduction in protein synthesis in both M-AKTDKO and M-indAKTDKO muscles (P < 0.05 and P < 0.01; n = 12-20) without significant changes in proteolysis or autophagy. Moreover, a significant reduction in oxidative capacity was observed in both M-AKTDKO (P < 0.05, P < 0.01 and P < 0.001; n = 5-12) and M-indAKTDKO (P < 0.05 and P < 0.01; n = 4). Mechanistically, activation and inhibition of mTORC1/FOXO1, respectively, but neither alone, were sufficient to restore protein synthesis, muscle oxidative capacity, and muscle function in the absence of AKT in vivo. In a mouse model of disuse-induced muscle loss, simultaneous activation of mTORC1 and inhibition of FOXO1 preserved muscle mass following immobilization (~5-10% reduction in casted M-indFOXO1TSCDKO muscles vs. ~30-40% casted M-indControl muscles, P < 0.05 and P < 0.0001; n = 8-16).

CONCLUSIONS

Collectively, this study provides novel insights into the AKT-dependent mechanisms that underlie muscle protein homeostasis, function, and metabolism in both normal physiology and disuse-induced muscle wasting.

Redox Control of Proteolysis During Inactivity-Induced Skeletal Muscle Atrophy

Significance: Skeletal muscles play essential roles in key body functions including breathing, locomotion, and glucose homeostasis; therefore, maintaining healthy skeletal muscles is important. Prolonged periods of muscle inactivity (e.g., bed rest, mechanical ventilation, or limb immobilization) result in skeletal muscle atrophy and weakness. Recent Advances: Disuse skeletal muscle atrophy occurs due to both accelerated proteolysis and decreased protein synthesis with proteolysis playing a leading role in some types of inactivity-induced atrophy. Although all major proteolytic systems are involved in inactivity-induced proteolysis in skeletal muscles, growing evidence indicates that both calpain and autophagy play an important role. Regulation of proteolysis in skeletal muscle is under complex control, but it is established that activation of both calpain and autophagy is directly linked to oxidative stress. Critical Issues: In this review, we highlight the experimental evidence that supports a cause and effect link between reactive oxygen species (ROS) and activation of both calpain and autophagy in skeletal muscle fibers during prolonged inactivity. We also review the sources of oxidant production in muscle fibers during inactivity-induced atrophy, and provide a detailed discussion on how ROS activates both calpain and autophagy during disuse muscle wasting. Future Directions: Future studies are required to delineate the specific mechanisms by which ROS activates both calpain and autophagy in skeletal muscles during prolonged periods of contractile inactivity. This knowledge is essential to develop the most effective strategies to protect against disuse muscle atrophy. Antioxid. Redox Signal. 33, 559-569.

Remodeling of the skeletal muscle and postsynaptic component after short-term joint immobilization and aquatic training

Joint immobilization is commonly used as a conservative treatment for osteoarticular and musculotendinous traumas. However, joint immobilization might elicit degenerative effects on the neuromuscular system and muscle atrophy. For this reason, the choice of strategies that mitigate these effects is essential in the post-immobilization period. Therefore, this study aimed to investigate the impact of aquatic training on the morphology of muscle fibers and motor endplates of the gastrocnemius muscle in the post-immobilization period. Male Wistar rats (90 days old) were divided into groups: Sedentary: no procedure; Immobilization: joint immobilization protocol (10 days); Immobilization/non-training: joint immobilization protocol (10 days) followed by four weeks without exercise intervention; Immobilization/training: joint immobilization protocol (10 days) and post-immobilization aquatic training (4 weeks). After the procedures, we quantified the cross-sectional area (CSA), volume and numerical density of different myofibers types, and total and stained area and perimeter of the motor endplate. We demonstrate the following main results: (a) short-term joint immobilization resulted in myofibers atrophy; however, we verified a small change in the postsynaptic component; (b) the period of inactivity after immobilization caused severe changes in the motor endplate (lower stained area, stained perimeter, total area, and total perimeter) and maintenance of muscle atrophy due to immobilization; (c) the prescription of post-immobilization exercise proved to be effective in restoring muscle morphology and inducing plasticity in the motor endplate. We conclude that short-term joint immobilization (10 days) results in atrophy type I and II myofibers, in addition to a decline in the total perimeter of the motor endplate. Besides, the post-immobilization period appears to be decisive in muscle and postsynaptic remodeling. Thus, aquatic training is effective in stimulating adjustments associated with muscle hypertrophy and plasticity of the motor endplate during the post-immobilization period.

Muscle-specific activation of calpain system in hindlimb unloading rats and hibernating Daurian ground squirrels: a comparison between artificial and natural disuse

To determine whether the regulation of calpain system is involved in non-hibernators and hibernators in disused condition, the soleus (SOL) and extensor digitorum longus (EDL) muscles were used for investigating the muscle mass, the ratio of muscle wet weight/body weight (MWW/BW), fiber-type distribution, fiber cross-sectional area (CSA), and the protein expression of MuRF1, calpain-1, calpain-2, calpastatin, desmin, troponin T, and troponin C in hindlimb unloading rats and hibernating Daurian ground squirrels. The muscle mass, MWW/BW, and fiber CSA were found significantly decreased in SOL and EDL of hindlimb unloading rats, but unchanged in hibernating ground squirrels. The MuRF1 expression was increased in both SOL and EDL of unloading rats, while it was only increased in SOL, but maintained in EDL of hibernating ground squirrels. The expression levels of calpain-1 and calpain-2 were increased in different degrees in unloaded SOL and EDL in rats, while they were maintained in EDL and even reduced in SOL of hibernating ground squirrels. Besides, the expression of calpastatin was decreased in unloaded rats, but increased in hibernating ground squirrels. The desmin expression was decreased in unloaded rats, but maintained in hibernating squirrels. Interestingly, the levels of troponin T and troponin C were decreased in both SOL and EDL of unloaded rats, but increased in hibernating ground squirrels with muscle-type specificity. In conclusion, differential calpain activation and substrate-selective degradation in slow and fast muscles are involved in the mechanisms of muscle atrophy of unloaded rats and remarkable ability of muscle maintenance of hibernating ground squirrels.

Macrophage polarization contributes to local inflammation and structural change in the multifidus muscle after intervertebral disc injury

PURPOSE

Intervertebral disk (IVD) lesion and its subsequent degeneration have a profound effect on the multifidus muscle. The subacute/early chronic phase of multifidus remodeling after IVD lesion has been proposed to be regulated by inflammatory processes. The balance between pro-inflammatory (M1) and anti-inflammatory (M2) macrophages plays an important role in maintaining tissue integrity after injury. The localization, polarization of macrophage subtypes and their mediation of the pro-inflammatory cytokine tumor necrosis factor (TNF) are unknown in paraspinal muscles during IVD degeneration. A sheep model of IVD degeneration was used to investigate the role of macrophages and TNF in the structural alterations that occur within the multifidus muscle.

METHODS

Anterolateral lesions were induced at L3-4 IVD in sheep. Multifidus muscle tissue at L4 was harvested 3 and 6 months after lesion and used for immunofluorescence assays to examine total macrophage number, macrophage polarization between M1 and M2, and to assess the localization of TNF expression in muscle, adipose and connective tissues from injured and naïve control animals.

RESULTS

A greater proportion of M1 macrophages is present in muscle at both 3 and 6 months after IVD lesion, and adipose tissue at 6 months. Total number of macrophages is unchanged. At 6 months, expression of TNF is increased in adipose and connective tissue and the proportion of TNF expressed by M1 macrophages is increased.

CONCLUSIONS

These data support the proposal that macrophages and TNF (pro-inflammatory cytokine) play an active role in the subacute/early chronic phase of remodeling in muscle, adipose and connective tissues of the multifidus during IVD degeneration. This presents a novel target for treatment. These slides can be retrieved under Electronic Supplementary Material.

Reduced skeletal muscle fiber size following caloric restriction is associated with calpain-mediated proteolysis and attenuation of IGF-1 signaling

Caloric restriction decreases skeletal muscle mass in mammals, principally due to a reduction in fiber size. The effect of suboptimal nutrient intake on skeletal muscle metabolic properties in neonatal calves was examined. The longissimus muscle (LM) was collected after a control (CON) or caloric restricted (CR) diet was cosnumed for 8 wk and muscle fiber size, gene expression, and metabolic signal transduction activity were measured. Results revealed that CR animals had smaller (P < 0.05) LM fiber cross-sectional area than CON, as expected. Western blot analysis detected equivalent amounts of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) but reduced (P < 0.05) amounts of the splice-variant, PGC1α-4 in CR LM. Expression of IGF-1, a PGC1α-4 target gene, was 40% less (P < 0.05) in CR than CON. Downstream mediators of autocrine IGF-1 signaling also are attenuated in CR by comparison with CON. The amount of phosphorylated AKT1 was less (P < 0.05) in CR than CON. The ratio of p4EBP1^T37/46 to total 4EBP1, a downstream mediator of AKT1, did not differ between CON and CR. By contrast, protein lysates from CR LM contained less (P < 0.05) total glycogen synthase kinase-3β (GSK3β) and phosphorylated GSK3β than CON LM, suggesting blunted protein synthesis. Smaller CR LM fiber size associates with increased (P < 0.05) calpain 1 (CAPN1) activity coupled with lower (P < 0.05) expression of calpastatin, the endogenous inhibitor of CAPN1. Atrogin-1 and MuRF expression and autophagy components were unaffected by CR. Thus CR suppresses the hypertrophic PGC1α-4/IGF-1/AKT1 pathway while promoting activation of the calpain system.