Leucine supplementation improves skeletal muscle regeneration after cryolesion in rats

Simulated altitude is medicine: intermittent exposure to hypobaric hypoxia and cold accelerates injured skeletal muscle recovery

Muscle injuries are the leading cause of sports casualties. Because of its high plasticity, skeletal muscle can respond to different stimuli to maintain and improve functionality. Intermittent hypobaric hypoxia (IHH) improves muscle oxygen delivery and utilization. Hypobaria coexists with cold in the biosphere, opening the possibility to consider the combined use of both environmental factors to achieve beneficial physiological adjustments. We studied the effects of IHH and cold exposure, separately and simultaneously, on muscle regeneration. Adult male rats were surgically injured in one gastrocnemius and randomly assigned to the following groups: (1) CTRL: passive recovery; (2) COLD: intermittently exposed to cold (4°C); (3) HYPO: submitted to IHH (4500 m); (4) COHY: exposed to intermittent simultaneous cold and hypoxia. Animals were subjected to these interventions for 4 h/day for 9 or 21 days. COLD and COHY rats showed faster muscle regeneration than CTRL, evidenced after 9 days at histological (dMHC-positive and centrally nucleated fibre reduction) and functional levels after 21 days. HYPO rats showed a full recovery from injury (at histological and functional levels) after 9 days, while COLD and COHY needed more time to induce a total functional recovery. IHH can be postulated as an anti-fibrotic treatment since it reduces collagen I deposition. The increase in the pSer473Akt/total Akt ratio observed after 9 days in COLD, HYPO and COHY, together with the increase in the pThr172AMPKα/total AMPKα ratio observed in the gastrocnemius of HYPO, provides clues to the molecular mechanisms involved in the improved muscle regeneration. KEY POINTS: Only intermittent hypobaric exposure accelerated muscle recovery as early as 9 days following injury at histological and functional levels. Injured muscles from animals treated with intermittent (4 h/day) cold, hypobaric hypoxia or a simultaneous combination of both stimuli regenerated histological structure and recovered muscle function 21 days after injury. The combination of cold and hypoxia showed a blunting effect as compared to hypoxia alone in the time course of the muscle recovery. The increased expression of the phosphorylated forms of Akt observed in all experimental groups could participate in the molecular cascade of events leading to a faster regeneration. The elevated levels of phosphorylated AMPKα in the HYPO group could play a key role in the modulation of the inflammatory response during the first steps of the muscle regeneration process.

The Effect of Downhill Running on Quadriceps Muscle in Growth-Restricted Mice

INTRODUCTION

Growth restriction (GR) reduces ribosome abundance and skeletal muscle mass in mice. A reduction in skeletal muscle mass increases the risk of frailty and is associated with high morbidity and mortality rates. As eccentric type exercise increases muscle mass, this investigation aimed to determine if eccentric loading of skeletal muscle via downhill running (DHR) increased muscle mass in GR mice.

METHODS

Mice were growth-restricted either gestational undernutrition (GUN, n = 8 litters), postnatal undernutrition (PUN, n = 8 litters), or were not restricted (CON, n = 8 litters) via a validated cross-fostering nutritive model. On postnatal day (PN) 21, all mice were weaned to a healthy diet, isolating the period of GR to early life as seen in humans. At PN45, mice were assigned to either a DHR (CON, n = 4 litters; GUN, n = 4 litters; PUN, n = 4 litters) or sedentary (SED: CON, n = 4 litters; GUN, n = 4 litters; PUN, n = 4 litters) group. Downhill running (16% decline: 18 m·min -1 ) was performed in 30-min bouts, three times per week, for 12 wk on a rodent treadmill. At PN129, the quadriceps femoris was dissected and evaluated for mass, myofiber size and type, and molecular markers of growth.

RESULTS

Following training, CON-DHR mice having larger cells than CON-SED, GUN-SED, PUN-SED, and PUN-DHR mice ( P < 0.05). The PUN group (as compared with CON) had reduced body mass ( P < 0.001), upstream binding factor abundance ( P = 0.012), phosphor-mTOR ( P < 0.001), and quadriceps mass ( P = 0.02). The GUN and PUN groups had increased MuRF1 abundance ( P < 0.001) compared with CON ( P < 0.001).

CONCLUSIONS

The blunted response to training suggests GR mice may have anabolic resistance when exposed to eccentric type exercise.

Evaluation and management of body composition changes in cancer patients

Wasting in cancer patients has long been recognized as a condition that adversely affects cancer patients' quality of life, treatment tolerance, and oncological outcomes. Historically, this condition was mainly evaluated by changes in body weight. However, this approach is not quite accurate because body weight is the overall change of all body compartments. Conditions such as edema and ascites can mask the severity of muscle and adipose tissue depletion. Changes in body composition assessment in cancer patients have historically been underappreciated because of the limited availability of measurement tools. As more evidence highlighting the importance of body composition has emerged, it is imperative to apply a more precise evaluation of nutritional status and a more targeted approach to provide nutritional support for cancer patients. In this review, we will discuss the modalities for evaluating body composition and how to manage body composition changes in cancer patients.

Evolutionarily divergent mTOR remodels translatome for tissue regeneration

An outstanding mystery in biology is why some species, such as the axolotl, can regenerate tissues whereas mammals cannot1. Here, we demonstrate that rapid activation of protein synthesis is a unique feature of the injury response critical for limb regeneration in the axolotl (Ambystoma mexicanum). By applying polysome sequencing, we identify hundreds of transcripts, including antioxidants and ribosome components that are selectively activated at the level of translation from pre-existing messenger RNAs in response to injury. By contrast, protein synthesis is not activated in response to non-regenerative digit amputation in the mouse. We identify the mTORC1 pathway as a key upstream signal that mediates tissue regeneration and translational control in the axolotl. We discover unique expansions in mTOR protein sequence among urodele amphibians. By engineering an axolotl mTOR (axmTOR) in human cells, we show that these changes create a hypersensitive kinase that allows axolotls to maintain this pathway in a highly labile state primed for rapid activation. This change renders axolotl mTOR more sensitive to nutrient sensing, and inhibition of amino acid transport is sufficient to inhibit tissue regeneration. Together, these findings highlight the unanticipated impact of the translatome on orchestrating the early steps of wound healing in a highly regenerative species and provide a missing link in our understanding of vertebrate regenerative potential.

Exploring skeletal muscle tolerance and whole-body metabolic effects of FDA-approved drugs in a volumetric muscle loss model

Volumetric muscle loss (VML) is associated with persistent functional impairment due to a lack of de novo muscle regeneration. As mechanisms driving the lack of regeneration continue to be established, adjunctive pharmaceuticals to address the pathophysiology of the remaining muscle may offer partial remediation. Studies were designed to evaluate the tolerance and efficacy of two FDA-approved pharmaceutical modalities to address the pathophysiology of the remaining muscle tissue after VML injury: (1) nintedanib (an anti-fibrotic) and (2) combined formoterol and leucine (myogenic promoters). Tolerance was first established by testing low- and high-dosage effects on uninjured skeletal muscle mass and myofiber cross-sectional area in adult male C57BL/6J mice. Next, tolerated doses of the two pharmaceutical modalities were tested in VML-injured adult male C57BL/6J mice after an 8-week treatment period for their ability to modulate muscle strength and whole-body metabolism. The most salient findings indicate that formoterol plus leucine mitigated the loss in muscle mass, myofiber number, whole-body lipid oxidation, and muscle strength, and resulted in a higher whole-body metabolic rate (p ≤ 0.016); nintedanib did not exacerbate or correct aspects of the muscle pathophysiology after VML. This supports ongoing optimization efforts, including scale-up evaluations of formoterol treatment in large animal models of VML.

Maternal vitamin D deficiency affects the morphology and function of glycolytic muscle in adult offspring rats

BACKGROUND

Fetal stage is a critical developmental window for the skeletal muscle, but little information is available about the impact of maternal vitamin D (Vit. D) deficiency (VDD) on offspring lean mass development in the adult life of male and female animals.

METHODS

Female rats (Wistar Hannover) were fed either a control (1000 IU Vit. D3/kg) or a VDD diet (0 IU Vit. D3/kg) for 6 weeks and during gestation and lactation. At weaning, male and female offspring were randomly separated and received a standard diet up to 180 days old.

RESULTS

Vitamin D deficiency induced muscle atrophy in the male (M-VDD) offspring at the end of weaning, an effect that was reverted along the time. Following 180 days, fast-twitch skeletal muscles [extensor digitorum longus (EDL)] from the M-VDD showed a decrease (20%; P < 0.05) in the number of total fibres but an increase in the cross-sectional area of IIB (17%; P < 0.05), IIA (19%; P < 0.05) and IIAX (21%; P < 0.05) fibres. The fibre hypertrophy was associated with the higher protein levels of MyoD (73%; P < 0.05) and myogenin (55% %; P < 0.05) and in the number of satellite cells (128.8 ± 14 vs. 91 ± 7.6 nuclei Pax7 + in the M-CTRL; P < 0.05). M-VDD increased time to fatigue during ex vivo contractions of EDL muscles and showed an increase in the phosphorylation levels of IGF-1/insulin receptor and their downstream targets related to anabolic processes and myogenic activation, including Ser ⁴⁷³ Akt and Ser ^21/9 GSK-3β. In such muscles, maternal VDD induced a compensatory increase in the content of calcitriol (two-fold; P < 0.05) and CYP27B1 (58%; P < 0.05), a metabolizing enzyme that converts calcidiol to calcitriol. Interestingly, most morphological and biochemical changes found in EDL were not observed in slow-twitch skeletal muscles (soleus) from the M-VDD group as well as in both EDL and soleus muscles from the female offspring.

CONCLUSIONS

These data show that maternal VDD selectively affects the development of type-II muscle fibres in male offspring rats but not in female offspring rats and suggest that the enhancement of their size and fatigue resistance in fast-twitch skeletal muscle (EDL) is probably due to a compensatory increase in the muscle content of Vit. D in the adult age.

Molecular and biochemical regulation of skeletal muscle metabolism

Skeletal muscle hypertrophy is a culmination of catabolic and anabolic processes that are interwoven into major metabolic pathways, and as such modulation of skeletal muscle metabolism may have implications on animal growth efficiency. Muscle is composed of a heterogeneous population of muscle fibers that can be classified by metabolism (oxidative or glycolytic) and contractile speed (slow or fast). Although slow fibers (type I) rely heavily on oxidative metabolism, presumably to fuel long or continuous bouts of work, fast fibers (type IIa, IIx, and IIb) vary in their metabolic capability and can range from having a high oxidative capacity to a high glycolytic capacity. The plasticity of muscle permits continuous adaptations to changing intrinsic and extrinsic stimuli that can shift the classification of muscle fibers, which has implications on fiber size, nutrient utilization, and protein turnover rate. The purpose of this paper is to summarize the major metabolic pathways in skeletal muscle and the associated regulatory pathways.

The Effect of Leucine Supplementation on Sarcopenia-Related Measures in Older Adults: A Systematic Review and Meta-Analysis of 17 Randomized Controlled Trials

Background

The role of leucine in sarcopenia prevention remains unclear. We aimed to summarize the published data from randomized controlled trials (RCTs) to estimate the effect of leucine supplementation on sarcopenia-related measures in older adults.

Methods

A systematic literature search was performed using the electronic databases PubMed, Embase, and Web of Science with restriction to randomized controlled trials design from January 1, 2009 to March 19, 2022. Sarcopenia-related measures included handgrip strength, total lean mass, gait speed, leg press, 6-min walk test, short-physical performance battery, timed up-and-go test and 30-s chair-stand test. Fixed- and random-effects meta-analysis models were used to generate pooled weighted mean differences (WMDs) and 95% CIs. Heterogeneity was examined in subgroup and sensitivity analyses. Publication bias assessments were performed.

Results

A total of 17 RCTs enrolling 1418 subjects were identified. Leucine-isolated supplementation showed no effect on total lean mass (WMD = 0.03 kg, 95% CI: –0.51, 0.57, P = 0.917), handgrip strength (WMD = 1.23 kg, 95% CI: –0.58, 3.03, P = 0.183) and leg press (WMD = –1.35 kg, 95% CI: –7.46, 4.77, P = 0.666). However, leucine-combined supplementation including vitamin D showed a significant improvement in handgrip strength (WMD = 2.17 kg, 95% CI: 0.24, 4.10, P = 0.027) and gait speed (WMD = 0.03 m/s, 95% CI: 0.01, 0.05, P = 0.008).

Conclusion

Leucine-isolated supplementation did not improve muscle mass and strength in elderly. However, leucine-combined supplementation including vitamin D exhibited a significant benefit for muscle strength and performance including handgrip strength and gait speed in older adults. A combination of nutritional supplements would be a viable option for improving sarcopenia.

Histomorphological and functional contralateral symmetry in the gastrocnemius muscles of the laboratory rat

It is usual in anatomical and physiological research to assess the effects of some intervention on extremities (e.g., training programmes or injury recovery protocols) using one muscle for the intervention and its contralateral as control. However, the existence of laterality (left‐handedness or right‐handedness) in athletes of different specialities is widely recognized. In rats, gastrocnemius is one of the muscles most widely used because of its importance in locomotion and high relative limb mass. Since we have not found studies reporting laterality assessment on the morphology and function in rat gastrocnemius, our study aimed to evaluate the fibre histochemical, morphometrical and muscle force contractile properties between right and left gastrocnemius of the laboratory rat. Fibre‐type proportion, fibre morphometrical measurements, muscle capillarization and muscle force properties were analysed in the right and left gastrocnemius of six male rats. No statistically significant differences (p = 0.265) were found in gastrocnemius to body weight ratio (‰) between right (6.55 ± 0.40) and left (6.49 ± 0.40) muscles. The muscles analysed showed a great degree of heterogeneity in fibre type distribution, having three clearly distinguished regions named red, mixed and white. In the three regions, there were no statistical differences in fibre type proportions between right and left gastrocnemius, as is indicated by the p‐values (from 0.203 to 0.941) obtained after running t‐Student paired tests for each fibre type. When analysing fibre cross‐sectional area, individual fibre capillarization and fibre circularity, no significant differences between right and left gastrocnemius in any of these morphometrical parameters were found in any muscle region or fibre type. Most of the p‐values (70%) resulting from running t‐Student paired tests were higher than 0.400, and the lowest p‐value was 0.115. Seemingly, global capillary and fibre densities were not statistically different between right and left sides in all muscle regions with p‐values ranging from 0.337 to 0.812. Force parameters normalized to gastrocnemius mass (mN g⁻¹) did not show any significant difference between right (PF = 74.0 ± 13.4, TF = 219.4 ± 13.0) and left (PF = 70.9 ± 10.7, TF = 213.0 ± 18.0) muscles with p = 0.623 (PF) and p = 0.514 (TF). Twitch time parameters (ms) also lacked significant differences between the two sides (CT: 43.4 ± 8.6 vs. 45.0 ± 14.3, p = 0.639; HRT: 77.6 ± 15.0 vs. 82.3 ± 25.3, p = 0.475). Finally, both muscles also showed similar (p = 0.718) fatigue properties. We did find an absence of laterality at the morphological and functional levels, which raises the possibility of using right and left gastrocnemius muscles interchangeably for experimental designs where one muscle is used to analyse data after a physiological intervention and its contralateral muscle plays the control role, thus allowing unbiased paired comparisons to derive accurate conclusions.

Glutamine supplementation improves contractile function of regenerating soleus muscles from rats

This study evaluated the effects of glutamine supplementation immediately after freezing injury on morphological and contractile function of regenerating soleus muscles from rats. Young male Wistar rats were subjected to cryolesion of soleus muscles, and immediately after received a daily supplementation of glutamine (1 g/kg/day). The muscles were evaluated on post-injury days 3 and 10. Glutamine-supplemented injured muscles had a lower number of CD11b positive immune cells and higher mRNA levels of IL-4 compared to those from the cryolesioned muscles analyzed on post-injury day 3. The mRNA and protein expression levels of the myogenic transcription factor MyoD were also higher in glutamine-supplemented injured muscles than in injured muscles examined on post-cryolesion day 3. In addition, glutamine-supplemented injured muscles had a higher size of their regenerating myofibers, attenuated decline in maximum tetanic strength and improved fatigue resistance compared to those from injured muscles evaluated on post-cryolesion day 10. No effect was observed in uninjured muscles supplemented with glutamine. Our results suggest that glutamine supplementation improves the resolution of inflammation, as well as the size and functional recovery of regenerating myofibers from soleus muscles by accelerating the up-regulation of IL-4 and MyoD expression. Future non-pharmacological rehabilitation studies are warranted to investigate the effect of glutamine supplementation on the outcome of injured skeletal muscles.

Beta-hydroxy-beta-methylbutyrate associated with low-intensity exercise training improves skeletal muscle regeneration through the IGF-Akt pathway

The effect of beta-hydroxy-beta-methylbutyrate (HMB) supplementation associated with exercise training at different intensities and frequencies on skeletal muscle regeneration of muscle-injured rats was investigated. Male Wistar rats were divided into sedentary and trained groups. The sedentary groups were subdivided into non-injured (SED-Ct), non-injured supplemented with HMB (SED-Ct-HMB), injured (SED), and injured with HMB (SED-HMB), and the trained groups were injured, supplemented with HMB, and then divided into training three times a week without load (HT3) or with load (HT3L) and training five times a week without load (HT5) and with load (HT5L). The rats received a daily dose of HMB associated with 60 min of swimming with or without 5% body mass load for 14 days. On the 15th day, cryoinjury was performed in the right tibialis anterior muscle (TA), and 48 h later, supplementation and training continued for 15 days. After the last session, the TA was dissected and a cross-sectional area (CSA) of muscle fibers was used to determine the percentage of CSA fibers and connective tissue (%CT), as well as the total and phosphorylated protein contents. SED-HMB showed increased CSA and decreased %CT and TGF-β when compared to SED. HT3 showed increased CSA and reduced %CT accompanied by increased IGF-1/Akt, myogenin, and MuRF1, and decreased TGF-β. The CSA of HT5L also increased, but at the cost of a higher %CT compared to the other groups. Our results demonstrated that HMB associated with training without load and with lower frequency per week may be a valuable strategy for skeletal muscle regeneration.

Aerobic Exercise Training and In Vivo Akt Activation Counteract Cancer Cachexia by Inducing a Hypertrophic Profile through eIF-2α Modulation

Simple Summary

Chronic disease-related muscle atrophy is a serious public health problem since it reduces mobility and contributes to increases in hospitalization costs. Unfortunately, there is no approved treatment for muscle wasting at present. Thus, an understanding of the mechanisms underlying the control of muscle mass and function under chronic diseases can pave the way for the discovery of innovative therapeutic strategies to counteract muscle wasting. Since numerous types of cancer induce cachexia, which has no cure nor an effective treatment, the main proposal here was to study the effects of AET in cancer cachexia, and to investigate, through in vivo manipulation of the Akt/mTORC1 pathway, whether the cachectic muscle still presents conditions to respond adaptively to hypertrophic stimuli. Our results could provide a basis for innovative research lines to better understand muscle plasticity and to investigate potential therapeutic approaches necessary to prevent muscle loss.

Abstract

Cancer cachexia is a multifactorial and devastating syndrome characterized by severe skeletal muscle mass loss and dysfunction. As cachexia still has neither a cure nor an effective treatment, better understanding of skeletal muscle plasticity in the context of cancer is of great importance. Although aerobic exercise training (AET) has been shown as an important complementary therapy for chronic diseases and associated comorbidities, the impact of AET on skeletal muscle mass maintenance during cancer progression has not been well documented yet. Here, we show that previous AET induced a protective mechanism against tumor-induced muscle wasting by modulating the Akt/mTORC1 signaling and eukaryotic initiation factors, specifically eIF2-α. Thereafter, it was determined whether the in vivo Akt activation would induce a hypertrophic profile in cachectic muscles. As observed for the first time, Akt-induced hypertrophy was able and sufficient to either prevent or revert cancer cachexia by modulating both Akt/mTORC1 pathway and the eIF-2α activation, and induced a better muscle functionality. These findings provide evidence that skeletal muscle tissue still preserves hypertrophic potential to be stimulated by either AET or gene therapy to counteract cancer cachexia.

Pyruvate Might Bridge Gut Microbiota and Muscle Health in Aging Mice After Chronic High Dose of Leucine Supplementation

Background: The previous studies demonstrated that there might be complex and close relationships among leucine supplementation, gut microbiota, and muscle health, which still needs further investigation.

Aims: This study aimed to explore the associations of gut microbiota with muscle health after leucine intake.

Methods: In this study, 19-month-old male C57BL/6j mice (n = 12/group) were supplemented with ultrapure water, low dose of leucine (500 mg/kg·d), and high dose of leucine (1,250 mg/kg·d) for 12 weeks by oral gavage. The mice fecal samples in each group before and after supplementation were collected for baseline and endpoint gut microbiota analysis by using 16S rDNA amplicon sequencing. Meanwhile, ultrasound measurement, H&E staining, myofiber cross-sectional area (CSA) measurement, and western blotting were performed in the quadriceps subsequently. The pyruvate levels were detected in feces.

Results: Improvement in muscle of histology and ultrasonography were observed after both low and high dose of leucine supplementation. High dose of leucine supplementation could promote skeletal muscle health in aging mice via regulating AMPKα/SIRT1/PGC-1α. The richness and diversities of microbiota as well as enriched metabolic pathways were altered after leucine supplementation. Firmicutes-Bacteroidetes ratio was significantly decreased in high-leucine group. Moreover, pyruvate fermentation to propanoate I were negatively associated with differential species and the pyruvate levels were significantly increased in feces after high dose of leucine supplementation.

Conclusions: Chronic high dose of leucine supplementation changed gut microbiota composition and increased pyruvate levels in the feces, which possibly provides a novel direction for promoting muscle health in aging mice.

Resistance Exercise and Whey Protein Supplementation Reduce Mechanical Allodynia and Spinal Microglia Activation After Acute Muscle Trauma in Rats

Muscle injury caused by direct trauma to the skeletal muscle is among the main musculoskeletal disorders. Non-pharmacological treatments have been effective in controlling muscle injury–induced pain; however, there are just a few studies in the literature investigating this response. Thus, the present study aimed to evaluate the effect of a resistance exercise training protocol combined or not with whey protein supplementation on mechanical allodynia induced by muscle injury. In addition, we also investigated the involvement of spinal glial cells in this process. For this purpose, male Wistar rats underwent a muscle injury model induced by direct trauma to the gastrocnemius muscle. Mechanical allodynia was measured by a digital von Frey algesimeter test. To evaluate the effect of exercise and/or supplementation on mechanical allodynia, the animals practiced exercises three times a week for 14 days and received supplementation daily for 14 days, respectively. Moreover, the effect of both the participation of spinal glial cells in the muscle injury and the resistance exercise training and/or whey protein supplementation on these cells was also investigated by the Western blot assay. The results demonstrated that resistance exercise training and whey protein supplementation, combined or alone, reduced mechanical allodynia. These treatments also reduced the number of interstitial cells and pro-inflammatory cytokine IL-6 levels in the injured muscle. It was also found that spinal microglia and astrocytes are involved in muscle injury, and that resistance exercise training combined with whey protein supplementation inhibits spinal microglia activation. The results suggest that both resistance exercise training and whey protein supplementation may be effective non-pharmacological treatments to control pain in the muscle after injury induced by acute trauma.

Leucine-Rich Diet Improved Muscle Function in Cachectic Walker 256 Tumour-Bearing Wistar Rats

Skeletal muscle atrophy occurs in several pathological conditions, such as cancer, especially during cancer-induced cachexia. This condition is associated with increased morbidity and poor treatment response, decreased quality of life, and increased mortality in cancer patients. A leucine-rich diet could be used as a coadjutant therapy to prevent muscle atrophy in patients suffering from cancer cachexia. Besides muscle atrophy, muscle function loss is even more important to patient quality of life. Therefore, this study aimed to investigate the potential beneficial effects of leucine supplementation on whole-body functional/movement properties, as well as some markers of muscle breakdown and inflammatory status. Adult Wistar rats were randomly distributed into four experimental groups. Two groups were fed with a control diet (18% protein): Control (C) and Walker 256 tumour-bearing (W), and two other groups were fed with a leucine-rich diet (18% protein + 3% leucine): Leucine Control (L) and Leucine Walker 256 tumour-bearing (LW). A functional analysis (walking, behaviour, and strength tests) was performed before and after tumour inoculation. Cachexia parameters such as body weight loss, muscle and fat mass, pro-inflammatory cytokine profile, and molecular and morphological aspects of skeletal muscle were also determined. As expected, Walker 256 tumour growth led to muscle function decline, cachexia manifestation symptoms, muscle fibre cross-section area reduction, and classical muscle protein degradation pathway activation, with upregulation of FoxO1, MuRF-1, and 20S proteins. On the other hand, despite having no effect on the walking test, inflammation status or muscle oxidative capacity, the leucine-rich diet improved muscle strength and behaviour performance, maintained body weight, fat and muscle mass and decreased some protein degradation markers in Walker 256 tumour-bearing rats. Indeed, a leucine-rich diet alone could not completely revert cachexia but could potentially diminish muscle protein degradation, leading to better muscle functional performance in cancer cachexia.

Metformin and leucine increase satellite cells and collagen remodeling during disuse and recovery in aged muscle

Loss of muscle mass and strength after disuse followed by impaired muscle recovery commonly occurs with aging. Metformin (MET) and leucine (LEU) individually have shown positive effects in skeletal muscle during atrophy conditions but have not been evaluated in combination nor tested as a remedy to enhance muscle recovery following disuse atrophy in aging. The purpose of this study was to determine if a dual treatment of metformin and leucine (MET + LEU) would prevent disuse-induced atrophy and/or promote muscle recovery in aged mice and if these muscle responses correspond to changes in satellite cells and collagen remodeling. Aged mice (22-24 months) underwent 14 days of hindlimb unloading (HU) followed by 7 or 14 days of reloading (7 or 14 days RL). MET, LEU, or MET + LEU was administered via drinking water and were compared to Vehicle (standard drinking water) and ambulatory baseline. We observed that during HU, MET + LEU resolved whole body grip strength and soleus muscle specific force decrements caused by HU. Gastrocnemius satellite cell abundance was increased with MET + LEU treatment but did not alter muscle size during disuse or recovery conditions. Moreover, MET + LEU treatment alleviated gastrocnemius collagen accumulation caused by HU and increased collagen turnover during 7 and 14 days RL driven by a decrease in collagen IV content. Transcriptional pathway analysis revealed that MET + LEU altered muscle hallmark pathways related to inflammation and myogenesis during HU. Together, the dual treatment of MET and LEU was able to increase muscle function, satellite cell content, and reduce collagen accumulation, thus improving muscle quality during disuse and recovery in aging.

Skeletal Muscle and Childhood Cancer: Where are we now and where we go from here

Skeletal muscle (muscle) is essential for physical health and for metabolic integrity, with sarcopenia (progressive muscle mass loss and weakness), a pre-curser of aging and chronic disease. Loss of lean mass and muscle quality (force generation per unit of muscle) in the general population are associated with fatigue, weakness, and slowed walking speed, eventually interfering with the ability to maintain physical independence, and impacting participation in social roles and quality of life. Muscle mass and strength impairments are also documented during childhood cancer treatment, which often persist into adult survivorship, and contribute to an aging phenotype in this vulnerable population. Although several treatment exposures appear to confer increased risk for loss of mass and strength that persists after therapy, the pathophysiology responsible for poor muscle quantity and quality is not well understood in the childhood cancer survivor population. This is partly due to limited access to both pediatric and adult survivor muscle tissue samples, and to difficulties surrounding non-invasive investigative approaches for muscle assessment. Because muscle accounts for just under half of the body's mass, and is essential for movement, metabolism and metabolic health, understanding mechanisms of injury responsible for both initial and persistent dysfunction is important, and will provide a foundation for intervention. The purpose of this review is to provide an overview of the available evidence describing associations between childhood cancer, its treatment, and muscle outcomes, identifying gaps in current knowledge.

Firearms-related skeletal muscle trauma: pathophysiology and novel approaches for regeneration

One major cause of traumatic injury is firearm-related wounds (i.e., ballistic trauma), common in both civilian and military populations, which is increasing in prevalence and has serious long-term health and socioeconomic consequences worldwide. Common primary injuries of ballistic trauma include soft-tissue damage and loss, haemorrhage, bone fracture, and pain. The majority of injuries are of musculoskeletal origin and located in the extremities, such that skeletal muscle offers a major therapeutic target to aid recovery and return to normal daily activities. However, the underlying pathophysiology of skeletal muscle ballistic trauma remains poorly understood, with limited evidence-based treatment options. As such, this review will address the topic of firearm-related skeletal muscle injury and regeneration. We first introduce trauma ballistics and the immediate injury of skeletal muscle, followed by detailed coverage of the underlying biological mechanisms involved in regulating skeletal muscle dysfunction following injury, with a specific focus on the processes of muscle regeneration, muscle wasting and vascular impairments. Finally, we evaluate novel approaches for minimising muscle damage and enhancing muscle regeneration after ballistic trauma, which may have important relevance for primary care in victims of violence.

cAMP-dependent protein kinase inhibits FoxO activity and regulates skeletal muscle plasticity in mice

Although we have shown that catecholamines suppress the activity of the Ubiquitin-Proteasome System (UPS) and atrophy-related genes expression through a cAMP-dependent manner in skeletal muscle from rodents, the underlying mechanisms remain unclear. Here, we report that a single injection of norepinephrine (NE; 1 mg kg^-1 ; s.c) attenuated the fasting-induced up-regulation of FoxO-target genes in tibialis anterior (TA) muscles by the stimulation of PKA/CREB and Akt/FoxO1 signaling pathways. In addition, muscle-specific activation of PKA by the overexpression of PKA catalytic subunit (PKAcat) suppressed FoxO reporter activity induced by (1) a wild-type; (2) a non-phosphorylatable; (3) a non-phosphorylatable and non-acetylatable forms of FoxO1 and FoxO3; (4) downregulation of FoxO protein content, and probably by (5) PGC-1α up-regulation. Consistently, the overexpression of the PKAcat inhibitor (PKI) up-regulated FoxO activity and the content of Atrogin-1 and MuRF1, as well as induced muscle fiber atrophy, the latter effect being prevented by the overexpression of a dominant negative (d. n.) form of FoxO (d.n.FoxO). The sustained overexpression of PKAcat induced fiber-type transition toward a smaller, slower, and more oxidative phenotype and improved muscle resistance to fatigue. Taken together, our data provide the first evidence that endogenous PKA activity is required to restrain the basal activity of FoxO and physiologically important to maintain skeletal muscle mass.

Emerging Strategies Targeting Catabolic Muscle Stress Relief

Skeletal muscle wasting represents a common trait in many conditions, including aging, cancer, heart failure, immobilization, and critical illness. Loss of muscle mass leads to impaired functional mobility and severely impedes the quality of life. At present, exercise training remains the only proven treatment for muscle atrophy, yet many patients are too ill, frail, bedridden, or neurologically impaired to perform physical exertion. The development of novel therapeutic strategies that can be applied to an in vivo context and attenuate secondary myopathies represents an unmet medical need. This review discusses recent progress in understanding the molecular pathways involved in regulating skeletal muscle wasting with a focus on pro-catabolic factors, in particular, the ubiquitin-proteasome system and its activating muscle-specific E3 ligase RING-finger protein 1 (MuRF1). Mechanistic progress has provided the opportunity to design experimental therapeutic concepts that may affect the ubiquitin-proteasome system and prevent subsequent muscle wasting, with novel advances made in regards to nutritional supplements, nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) inhibitors, myostatin antibodies, β₂ adrenergic agonists, and small-molecules interfering with MuRF1, which all emerge as a novel in vivo treatment strategies for muscle wasting.

Roles of mTOR Signaling in Tissue Regeneration

The mammalian target of rapamycin (mTOR), is a serine/threonine protein kinase and belongs to the phosphatidylinositol 3-kinase (PI3K)-related kinase (PIKK) family. mTOR interacts with other subunits to form two distinct complexes, mTORC1 and mTORC2. mTORC1 coordinates cell growth and metabolism in response to environmental input, including growth factors, amino acid, energy and stress. mTORC2 mainly controls cell survival and migration through phosphorylating glucocorticoid-regulated kinase (SGK), protein kinase B (Akt), and protein kinase C (PKC) kinase families. The dysregulation of mTOR is involved in human diseases including cancer, cardiovascular diseases, neurodegenerative diseases, and epilepsy. Tissue damage caused by trauma, diseases or aging disrupt the tissue functions. Tissue regeneration after injuries is of significance for recovering the tissue homeostasis and functions. Mammals have very limited regenerative capacity in multiple tissues and organs, such as the heart and central nervous system (CNS). Thereby, understanding the mechanisms underlying tissue regeneration is crucial for tissue repair and regenerative medicine. mTOR is activated in multiple tissue injuries. In this review, we summarize the roles of mTOR signaling in tissue regeneration such as neurons, muscles, the liver and the intestine.

miR-29c improves skeletal muscle mass and function throughout myocyte proliferation and differentiation and by repressing atrophy-related genes

AIM

To identify microRNAs (miRs) involved in the regulation of skeletal muscle mass. For that purpose, we have initially utilized an in silico analysis, resulting in the identification of miR-29c as a positive regulator of muscle mass.

METHODS

miR-29c was electrotransferred to the tibialis anterior to address its morphometric and functional properties and to determine the level of satellite cell proliferation and differentiation. qPCR was used to investigate the effect of miR-29c overexpression on trophicity-related genes. C2C12 cells were used to determine the impact of miR-29c on myogenesis and a luciferase reporter assay was used to evaluate the ability of miR-29c to bind to the MuRF1 3'UTR.

RESULTS

The overexpression of miR-29c in the tibialis anterior increased muscle mass by 40%, with a corresponding increase in fibre cross-sectional area and force and a 30% increase in length. In addition, satellite cell proliferation and differentiation were increased. In C2C12 cells, miR-29c oligonucleotides caused increased levels of differentiation, as evidenced by an increase in eMHC immunostaining and the myotube fusion index. Accordingly, the mRNA levels of myogenic markers were also increased. Mechanistically, the overexpression of miR-29c inhibited the expression of the muscle atrophic factors MuRF1, Atrogin-1 and HDAC4. For the key atrogene MuRF1, we found that miR-29c can bind to its 3'UTR to mediate repression.

CONCLUSIONS

The results herein suggest that miR-29c can improve skeletal muscle size and function by stimulating satellite cell proliferation and repressing atrophy-related genes. Taken together, our results indicate that miR-29c might be useful as a future therapeutic device in diseases involving decreased skeletal muscle mass.

Leucine attenuates muscle atrophy and autophagosome formation by activating PI3K/AKT/mTOR signaling pathway in rotator cuff tears

Rotator cuff tears (RCTs), the most common tendon injury, are always accompanied by muscle atrophy, which is characterized by excessive protein degradation. Autophagy-lysosome systems are the crucial proteolytic pathways and are activated in atrophying muscle. Thus, inhibition of the autophagy-lysosome pathway might be an alternative way to minimize skeletal muscle atrophy. In this present study, combined with a tendon transection-induced rat model of massive rotator cuff tears, HE staining and transmission electron microscopy methods, we found leucine supplementation effectively prevented muscle atrophy, muscle injury and autophagosome formation. Utilizing immunoblotting, we discovered that leucine supplementation is able to inhibit the rise in autophagy-related protein expression (including LC3, Atrogin-1, MuRF1, Bnip3 and FoxO3) driven by tendon transection. The PI3K/AKT/mTOR pathway that was essential in autophagosome formation and autophagy was blocked in atrophying muscle and reactivated in the presence of leucine. Once administrated with LY294002 (PI3K inhibitor) and Rapamycin (mTOR inhibitor), leucine mediated by the anti-atrophic effects was nearly blunted. These results suggest that leucine potentially attenuates tendon transection-induced muscle atrophy through autophagy inhibition via activating the PI3K/AKT/mTOR pathway.

BGP-15 improves contractile function of regenerating soleus muscle

This study investigated the effect of the heat shock protein inducer O-[3-piperidino-2-hydroxy-1-propyl]-nicotinic amidoxime (BGP-15) on the morphology and contractile function of regenerating soleus muscles from mice. Cryolesioned soleus muscles from young mice treated daily with BGP-15 (15 mg/Kg) were evaluated on post-cryolesion day 10. At this time point, there was a significant decrease in the cross-sectional area of regenerating myofibers, maximal force, specific tetanic force, and fatigue resistance of regenerating soleus muscles. BGP-15 did not reverse the decrease in myofiber cross-sectional area but effectively prevented the reduction in tetanic force and fatigue resistance of regenerating muscles. In addition, BGP-15 treatment increased the expression of embryonic myosin heavy chain (e-MyHC), MyHC-II and MyHC-I in regenerating muscles. Although BGP-15 did not alter voltage dependent anion-selective channel 2 (VDAC2) expression in cryolesioned muscles, it was able to increase inducible 70-kDa heat shock protein (HSP70) expression. Our results suggest that BGP-15 improves strength recovery in regenerating soleus muscles by accelerating the re-expression of adult MyHC-II and MyHC-I isoforms and HSP70 induction. The beneficial effects of BGP-15 on the contractile function of regenerating muscles reinforce the potential of this molecule to be used as a therapeutic agent.

Effect of 8-week leucine supplementation and resistance exercise training on muscle hypertrophy and satellite cell activation in rats

We investigated the effects of regular leucine intake and/or resistance exercise training on skeletal muscle hypertrophy and satellite cell activity after the administration of different doses of leucine. Ten-week-old Sprague-Dawley rats were assigned to six groups (n = 7 per group): a control group (Con), two groups receiving either 10% (0.135 g/kg.wt) (Leu10) or 50% (0.675 g/kg.wt) (Leu50) leucine supplementation, and three exercise groups receiving 0% (Ex), 10% (Leu10Ex), and 50% (Leu50Ex) leucine supplementation. The rats performed ladder climbing exercises thrice per week for 8 weeks, and received leucine supplements at the same time daily. Muscle phenotypes were assessed by immunohistochemistry. MyoD, myogenin, and IGF1 protein levels were determined by western blot. The Leu50Ex group displayed significantly higher numbers of positive embryonic myosin fibers (0.35 ± 0.08, 250%) and myonuclei (3.29 ± 0.3, 118.7%) than all other groups. And exercise training groups increased the cross-sectional area, the number of satellite cells and protein expression of MyoD, myogenin, and IGF1alpha relative to the Control group (P < 0.05). However, Only leucine supplementation group did not increase skeletal muscle hypertrophy and satellite cell activity, regardless of the dose (P > 0.05). Leucine intake accompanied by regular exercise training may increase satellite cell activation in skeletal muscles, and improve muscle quality more effectively than continuous leucine ingestion alone.

The Physiological Mechanisms of Effect of Vitamins and Amino Acids on Tendon and Muscle Healing: A Systematic Review

Aims/Objectives: To evaluate the current literature via systematic review to ascertain whether amino acids/vitamins provide any influence on musculotendinous healing and if so, by which physiological mechanisms. Methods: EBSCO, PubMed, ScienceDirect, Embase Classic/Embase, and MEDLINE were searched using terms including “vitamins,” “amino acids,” “healing,” “muscle,” and “tendon.” The primary search had 479 citations, of which 466 were excluded predominantly due to nonrandomized design. Randomized human and animal studies investigating all supplement types/forms of administration were included. Critical appraisal of internal validity was assessed using the Cochrane risk of Bias Tool or the Systematic Review Centre for Laboratory Animal Experimentation Risk of Bias Tool for human and animal studies, respectively. Two reviewers performed duel data extraction. Results: Twelve studies met criteria for inclusion: eight examined tendon healing and four examined muscle healing. All studies used animal models, except two human trials using a combined integrator. Narrative synthesis was performed via content analysis of demonstrated statistically significant effects and thematic analysis of proposed physiological mechanisms of intervention. Vitamin C/taurine demonstrated indirect effects on tendon healing through antioxidant activity. Vitamin A/glycine showed direct effects on extracellular matrix tissue synthesis. Vitamin E shows an antiproliferative influence on collagen deposition. Leucine directly influences signaling pathways to promote muscle protein synthesis. Discussion: Preliminary evidence exists, demonstrating that vitamins and amino acids may facilitate multilevel changes in musculotendinous healing; however, recommendations on clinical utility should be made with caution. All animal studies and one human study showed high risk of bias with moderate interobserver agreement (k = 0.46). Currently, there is limited evidence to support the use of vitamins and amino acids for musculotendinous injury. Both high-quality animal experimentation of the proposed mechanisms confirming the physiological influence of supplementation and human studies evaluating effects on tissue morphology and biochemistry are required before practical application.

EFFECTS OF BETA-HYDROXY-BETA-METHYL BUTYRATE IN MUSCLE REGENERATION OF RATS

ABSTRACT Introduction: Studies have shown that beta-hydroxy-beta-methyl butyrate (HMB) supplementation increases muscle strength and mass. Objective: To evaluate the effect of HMB supplementation on the muscle regeneration process in young and sedentary rats. Methods: Twenty-four male Wistar rats two months old were divided into two groups: lesion (LE) and supplemented (S), and evaluated in two moments - seven days (LE7; S7, n=6) and 21 days (LE21; S21, n=6). The right tibialis anterior muscle was subjected to cryolesion in all animals. After the injury, the LE group remained in the vivarium without any intervention. Group S received HMB calcium supplementation diluted in water by gavage (320 mg/kg/weight per day). The injury tibialis anterior (ITA), the tibialis anterior (TA), and the left soleus (SOL) muscles were removed, weighted and divided transversally into two parts, one for the analysis of the cross-sectional area (CSA) and the area of inflammation/regeneration and the other part to measure the muscular glycogen concentration. Data were evaluated using the SAS program considering mean and standard deviation. For analysis of variance the ANOVA test was used followed by the Tukey-HSD test (p≤0.05). Results: The ITA muscle weight was higher in S21 compared to S7 (p<0.05). The groups LE21 and S21 presented greater CSA of muscle fibers area and smaller ITA regeneration/inflammation area (p<0.05) when compared with the LE7 and S7 groups. There was an increase in muscle glycogen levels in S7 group compared to LE7 and S21 groups for TA and SOL (p<0.01), as well as in S21 group compared to LE21 for SOL (p<0.05). Conclusion: HMB did not influence the muscle regeneration process and did not favor anabolic activity in the different muscular fibers of young sedentary rats. Level of Evidence II; Therapeutic studies - Investigation of treatment results.

Protein ingestion preserves proteasome activity during intense aseptic inflammation and facilitates skeletal muscle recovery in humans

The ubiquitin–proteasome system (UPS) is the main cellular proteolytic system responsible for the degradation of normal and abnormal (e.g. oxidised) proteins. Under catabolic conditions characterised by chronic inflammation, the UPS is activated resulting in proteolysis, muscle wasting and impaired muscle function. Milk proteins provide sulphur-containing amino acid and have been proposed to affect muscle inflammation. However, the response of the UPS to aseptic inflammation and protein supplementation is largely unknown. The aim of this study was to investigate how milk protein supplementation affects UPS activity and skeletal muscle function under conditions of aseptic injury induced by intense, eccentric exercise. In a double-blind, cross-over, repeated measures design, eleven men received either placebo (PLA) or milk protein concentrate (PRO, 4×20 g on exercise day and 20 g/d for the following 8 days), following an acute bout of eccentric exercise (twenty sets of fifteen eccentric contractions at 30°/s) on an isokinetic dynamometer. In each trial, muscle biopsies were obtained from the vastus lateralis muscle at baseline, as well as at 2 and 8 d post exercise, whereas blood samples were collected before exercise and at 6 h, 1 d, 2 d and 8 d post exercise. Muscle strength and soreness were assessed before exercise, 6 h post exercise and then daily for 8 consecutive days. PRO preserved chymotrypsin-like activity and attenuated the decrease of strength, facilitating its recovery. PRO also prevented the increase of NF-κB phosphorylation and HSP70 expression throughout recovery. We conclude that milk PRO supplementation following exercise-induced muscle trauma preserves proteasome activity and attenuates strength decline during the pro-inflammatory phase.

From Nutrient to MicroRNA: a Novel Insight into Cell Signaling Involved in Skeletal Muscle Development and Disease

Skeletal muscle is a remarkably complicated organ comprising many different cell types, and it plays an important role in lifelong metabolic health. Nutrients, as an external regulator, potently regulate skeletal muscle development through various internal regulatory factors, such as mammalian target of rapamycin (mTOR) and microRNAs (miRNAs). As a nutrient sensor, mTOR, integrates nutrient availability to regulate myogenesis and directly or indirectly influences microRNA expression. MiRNAs, a class of small non-coding RNAs mediating gene silencing, are implicated in myogenesis and muscle-related diseases. Meanwhile, growing evidence has emerged supporting the notion that the expression of myogenic miRNAs could be regulated by nutrients in an epigenetic mechanism. Therefore, this review presents a novel insight into the cell signaling network underlying nutrient-mTOR-miRNA pathway regulation of skeletal myogenesis and summarizes the epigenetic modifications in myogenic differentiation, which will provide valuable information for potential therapeutic intervention.

Contractile function recovery in severely injured gastrocnemius muscle of rats treated with either oleic or linoleic acid

NEW FINDINGS

What is the central question of this study? Oleic and linoleic acids modulate fibroblast proliferation and myogenic differentiation in vitro. However, their in vivo effects on muscle regeneration have not yet been examined. We investigated the effects of either oleic or linoleic acid on a well-established model of muscle regeneration after severe laceration. What is the main finding and its importance? We found that linoleic acid increases fibrous tissue deposition and impairs muscle regeneration and recovery of contractile function, whereas oleic acid has the opposite effects in severely injured gastrocnemius muscle, suggesting that linoleic acid has a harmful effect and oleic acid a potential therapeutic effect on muscle regeneration. Oleic and linoleic acids control fibroblast proliferation and myogenic differentiation in vitro; however, there was no study in skeletal muscle in vivo. The aim of this study was to evaluate the effects of either oleic or linoleic acid on the fibrous tissue content (collagen deposition) of muscle and recovery of contractile function in rat gastrocnemius muscle after being severely injured by laceration. Rats were supplemented with either oleic or linoleic acid for 4 weeks after laceration [0.44 g (kg body weight)^-1 day^-1 ]. Muscle injury led to an increase in oleic-to-stearic acid and palmitoleic-to-palmitic acid ratios, suggesting an increase in Δ⁹ desaturase activity. Increased fibrous tissue deposition and reduced isotonic and tetanic specific forces and resistance to fatigue were observed in the injured muscle. Supplementation with linoleic acid increased the content of eicosadienoic (20:2, n-6) and arachidonic (20:4, n-6) acids, reduced muscle mass and fibre cross-sectional areas, increased fibrous tissue deposition and further reduced the isotonic and tetanic specific forces and resistance to fatigue induced by laceration. Supplementation with oleic acid increased the content of docosahexaenoic acid (22:6, n-3) and abolished the increase in fibrous tissue area and the decrease in isotonic and tetanic specific forces and resistance to fatigue induced by muscle injury. We concluded that supplementation with linoleic acid impairs muscle regeneration and increases fibrous tissue deposition, resulting in impaired recovery of contractile function. Oleic acid supplementation reduced fibrous tissue deposition and improved recovery of contractile function, attenuating the tissue damage caused by muscle injury.

Differential effects of leucine supplementation in young and aged mice at the onset of skeletal muscle regeneration

Aging decreases the ability of skeletal muscle to respond to injury. Leucine has been demonstrated to target protein synthetic pathways in skeletal muscle thereby enhancing this response. However, the effect of aging on leucine-induced alterations in protein synthesis at the onset of skeletal muscle regeneration has not been fully elucidated. The purpose of this study was to determine if aging alters skeletal muscle regeneration and leucine-induced alterations in markers of protein synthesis. The tibialis anterior of young (3 months) and aged (24 months) female C57BL/6J mice were injected with either bupivacaine or PBS, and the mice were given ad libitum access to leucine-supplemented or normal drinking water. Protein and gene expression of markers of protein synthesis and degradation, respectively, were analyzed at three days post-injection. Following injury in young mice, leucine supplementation was observed to elevate only p-p70S6K. In aged mice, leucine was shown to elicit higher p-mTOR content with and without injury, and p-4EBP-1 content post-injury. Additionally in aged mice, leucine was shown to elicit higher content of relative p70S6K post-injury. Our study shows that leucine supplementation affects markers of protein synthesis at the onset of skeletal muscle regeneration differentially in young and aged mice.

Leucine-enriched essential amino acids attenuate inflammation in rat muscle and enhance muscle repair after eccentric contraction

Eccentric exercise results in prolonged muscle damage that may lead to muscle dysfunction. Although inflammation is essential to recover from muscle damage, excessive inflammation may also induce secondary damage, and should thus be suppressed. In this study, we investigated the effect of leucine-enriched essential amino acids on muscle inflammation and recovery after eccentric contraction. These amino acids are known to stimulate muscle protein synthesis via mammalian target of rapamycin (mTOR), which, is also considered to alleviate inflammation. Five sets of 10 eccentric contractions were induced by electrical stimulation in the tibialis anterior muscle of male SpragueDawley rats (8–9 weeks old) under anesthesia. Animals received a 1 g/kg dose of a mixture containing 40 % leucine and 60 % other essential amino acids or distilled water once a day throughout the experiment. Muscle dysfunction was assessed based on isometric dorsiflexion torque, while inflammation was evaluated by histochemistry. Gene expression of inflammatory cytokines and myogenic regulatory factors was also measured. We found that leucine-enriched essential amino acids restored full muscle function within 14 days, at which point rats treated with distilled water had not fully recovered. Indeed, muscle function was stronger 3 days after eccentric contraction in rats treated with amino acids than in those treated with distilled water. The amino acid mix also alleviated expression of interleukin-6 and impeded infiltration of inflammatory cells into muscle, but did not suppress expression of myogenic regulatory factors. These results suggest that leucine-enriched amino acids accelerate recovery from muscle damage by preventing excessive inflammation.

Skeletal muscle regeneration and impact of aging and nutrition

After skeletal muscle injury a regeneration process takes place to repair muscle. Skeletal muscle recovery is a highly coordinated process involving cross-talk between immune and muscle cells. It is well known that the physiological activities of both immune cells and muscle stem cells decline with advancing age, thereby blunting the capacity of skeletal muscle to regenerate. The age-related reduction in muscle repair efficiency contributes to the development of sarcopenia, one of the most important factors of disability in elderly people. Preserving muscle regeneration capacity may slow the development of this syndrome. In this context, nutrition has drawn much attention: studies have demonstrated that nutrients such as amino acids, n-3 polyunsaturated fatty acids, polyphenols and vitamin D can improve skeletal muscle regeneration by targeting key functions of immune cells, muscle cells or both. Here we review the process of skeletal muscle regeneration with a special focus on the cross-talk between immune and muscle cells. We address the effect of aging on immune and skeletal muscle cells involved in muscle regeneration. Finally, the mechanisms of nutrient action on muscle regeneration are described, showing that quality of nutrition may help to preserve the capacity for skeletal muscle regeneration with age.

Cryotherapy Reduces Inflammatory Response Without Altering Muscle Regeneration Process and Extracellular Matrix Remodeling of Rat Muscle

The application of cryotherapy is widely used in sports medicine today. Cooling could minimize secondary hypoxic injury through the reduction of cellular metabolism and injury area. Conflicting results have also suggested cryotherapy could delay and impair the regeneration process. There are no definitive findings about the effects of cryotherapy on the process of muscle regeneration. The aim of the present study was to evaluate the effects of a clinical-like cryotherapy on inflammation, regeneration and extracellular matrix (ECM) remodeling on the Tibialis anterior (TA) muscle of rats 3, 7 and 14 days post-injury. It was observed that the intermittent application of cryotherapy (three 30-minute sessions, every 2 h) in the first 48 h post-injury decreased inflammatory processes (mRNA levels of TNF-α, NF-κB, TGF-β and MMP-9 and macrophage percentage). Cryotherapy did not alter regeneration markers such as injury area, desmin and Myod expression. Despite regulating Collagen I and III and their growth factors, cryotherapy did not alter collagen deposition. In summary, clinical-like cryotherapy reduces the inflammatory process through the decrease of macrophage infiltration and the accumulation of the inflammatory key markers without influencing muscle injury area and ECM remodeling.

Reviewing the Effects of L-Leucine Supplementation in the Regulation of Food Intake, Energy Balance, and Glucose Homeostasis

Leucine is a well-known activator of the mammalian target of rapamycin (mTOR). Because mTOR signaling regulates several aspects of metabolism, the potential of leucine as a dietary supplement for treating obesity and diabetes mellitus has been investigated. The objective of the present review was to summarize and discuss the available evidence regarding the mechanisms and the effects of leucine supplementation on the regulation of food intake, energy balance, and glucose homeostasis. Based on the available evidence, we conclude that although central leucine injection decreases food intake, this effect is not well reproduced when leucine is provided as a dietary supplement. Consequently, no robust evidence indicates that oral leucine supplementation significantly affects food intake, although several studies have shown that leucine supplementation may help to decrease body adiposity in specific conditions. However, more studies are necessary to assess the effects of leucine supplementation in already-obese subjects. Finally, although several studies have found that leucine supplementation improves glucose homeostasis, the underlying mechanisms involved in these potential beneficial effects remain unknown and may be partially dependent on weight loss.

mTOR is necessary for proper satellite cell activity and skeletal muscle regeneration

The serine/threonine kinase mammalian target of rapamycin (mTOR) is a key regulator of protein synthesis, cell proliferation and energy metabolism. As constitutive deletion of Mtor gene results in embryonic lethality, the function of mTOR in muscle stem cells (satellite cells) and skeletal muscle regeneration remains to be determined. In this study, we established a satellite cell specific Mtor conditional knockout (cKO) mouse model by crossing Pax7(CreER) and Mtor(flox/flox) mice. Skeletal muscle regeneration after injury was severely compromised in the absence of Mtor, indicated by increased number of necrotic myofibers infiltrated by Evans blue dye, and reduced number and size of regenerated myofibers in the Mtor cKO mice compared to wild type (WT) littermates. To dissect the cellular mechanism, we analyzed satellite cell-derived primary myoblasts grown on single myofibers or adhered to culture plates. The Mtor cKO myoblasts exhibited defective proliferation and differentiation kinetics when compared to myoblasts derived from WT littermates. At the mRNA and protein levels, the Mtor cKO myoblasts expressed lower levels of key myogenic determinant genes Pax7, Myf5, Myod, Myog than did the WT myoblasts. These results suggest that mTOR is essential for satellite cell function and skeletal muscle regeneration through controlling the expression of myogenic genes.

Effects of free leucine supplementation and resistance training on muscle strength and functional status in older adults: a randomized controlled trial

OBJECTIVE

To assess the effect of free leucine supplementation combined with resistance training versus resistance training only on muscle strength and functional status in older adults.

METHODS

This was a randomized, double-blind, placebo-controlled, parallel study with two intervention groups. Thirty older adults were randomly assigned to receive either 10 g leucine/day (leucine group [LG], n=15) or a placebo (control group [CG], n=15), plus resistance training over a 12-week period. Maximal overcoming isometric leg strength, functional status, nutritional status, body composition, health-related quality of life, depression, and dietary intake were assessed at 4 and 12 weeks. Missing data at 12 weeks were handled using mixed models for repeated measurements for data imputation.

RESULTS

Twenty-four subjects completed the 4-week assessment and eleven completed the 12-week intervention. Clinically significant gains were found in isometric leg strength at both assessment time points. Analysis of the effect size also showed how participants in LG outperformed those in CG for chair stands and the timed up and go test. No significant changes were observed for the rest of the outcomes.

CONCLUSION

Our combined analysis showed moderate changes in isometric leg muscle strength and certain components of functional status. The magnitude of changes found on these outcomes should be qualified as a positive effect of the concomitant intervention.

Leucine-enriched essential amino acids attenuate muscle soreness and improve muscle protein synthesis after eccentric contractions in rats

Eccentric exercise results in prolonged muscle weakness and muscle soreness, which are typical symptoms of muscle damage. Recovery from muscle damage is related to mammalian target of rapamycin (mTOR) activity. Leucine-enriched essential amino acids (LEAAs) stimulate muscle protein synthesis via activation of the mTOR pathway. Therefore, we investigated the effect of LEAAs on muscle protein synthesis and muscle soreness after eccentric contractions (EC). Male Sprague–Dawley rats (9–11 weeks old) were administered an LEAA solution (AminoL40; containing 40 % leucine and 60 % other essential amino acids) at 1 g/kg body weight or distilled water (control) 30 min before and 10 min after EC. Tibialis anterior (TA) muscle was exposed to 500 EC by electrical stimulation under anesthesia. The fractional synthesis rate (FSR; %/h) in the TA muscle was measured by incorporating l-[ring-²H₅] phenylalanine into skeletal muscle protein. Muscle soreness was evaluated by the paw withdrawal threshold using the Randal–Selitto test with some modifications from 1 to 3 days after EC. The FSR in the EC-control group (0.147 ± 0.016 %/h) was significantly lower than in the sedentary group (0.188 ± 0.016 %/h, p < 0.05). AminoL40 administration significantly mitigated the EC-induced impairment of the FSR (0.172 ± 0.018 %/h). EC decreased the paw withdrawal threshold at 1 and 2 days after EC, which indicated that EC induced muscle soreness. Furthermore, AminoL40 administration alleviated the decreased paw withdrawal threshold. These findings suggest that LEAA supplementation improves the rate of muscle protein synthesis and ameliorates muscle soreness after eccentric exercise.

The combination of ursolic acid and leucine potentiates the differentiation of C2C12 murine myoblasts through the mTOR signaling pathway

Aging causes phenotypic changes in skeletal muscle progenitor cells that lead to the progressive loss of myogenic differentiation and thus a decrease in muscle mass. The naturally occurring triterpene, ursolic acid, has been reported to be an effective agent for the prevention of muscle loss by suppressing degenerative muscular dystrophy. Leucine, a branched-chain amino acid, and its metabolite, β-hydroxy-β-methylbutyric acid, have been reported to enhance protein synthesis in skeletal muscle. Therefore, the aim of the present study was to investigate whether the combination of ursolic acid and leucine promotes greater myogenic differentiation compared to either agent alone in C2C12 murine myoblasts. Morphological changes were observed and creatine kinase (CK) activity analysis was performed to determine the conditions through which the combination of ursolic acid and leucine would exert the most prominent effects on muscle cell differentiation. The effect of the combination of ursolic acid and leucine on the expression of myogenic differentiation marker genes was examined by RT-PCR and western blot analysis. The combination of ursolic acid (0.5 µM) and leucine (10 µM) proved to be the most effective in promoting myogenic differentiation. The combination of ursolic acid and leucine significantly increased CK activity than treatment with either agent alone. The level of myosin heavy chain, a myogenic differentiation marker protein, was also enhanced by the combination of ursolic acid and leucine. The combination of ursolic acid and leucine significantly induced the expression of myogenic differentiation marker genes, such as myogenic differentiation 1 (MyoD) and myogenin, at both the mRNA and protein level. In addition, the number of myotubes and the fusion index were increased. These findings indicate that the combination of ursolic acid and leucine promotes muscle cell differentiation, thus suggesting that this combination of agents may prove to be beneficial in increasing muscle mass.

Regenerated soleus muscle shows reduced creatine kinase efflux after contractile activity in vitro

Regenerated skeletal muscles show less muscle damage after strenuous muscle exercise. The aim of the studies was to investigate if the regeneration is associated with reduced muscle creatine kinase (CK) efflux immediately after the exercise. Cryolesion was applied to the soleus muscle of 3-month-old C57BL/6J male mice. Then total CK efflux was assessed in vitro in the regenerated muscles without exercise or after 100 eccentric contractions. The same measurements were performed in the control muscles, which were not exposed to cryolesion. Regenerated muscles generated weaker (P < 0.05) twitches, but stronger (P < 0.05) 150-Hz and 300-Hz tetani with prolonged (P < 0.01) contraction times compared with the control muscles. There was no difference between regenerated and control muscles in the total CK efflux without exercise, but only control muscles showed an increase (P < 0.001) in the CK efflux after the exercise. Our results suggest that muscle regeneration is associated with modulation of contractile properties and improvement in muscle resistance to damage after eccentric exercise.

Leucine supplementation accelerates connective tissue repair of injured tibialis anterior muscle

This study investigated the effect of leucine supplementation on the skeletal muscle regenerative process, focusing on the remodeling of connective tissue of the fast twitch muscle tibialis anterior (TA). Young male Wistar rats were supplemented with leucine (1.35 g/kg per day); then, TA muscles from the left hind limb were cryolesioned and examined after 10 days. Although leucine supplementation induced increased protein synthesis, it was not sufficient to promote an increase in the cross-sectional area (CSA) of regenerating myofibers (p > 0.05) from TA muscles. However, leucine supplementation reduced the amount of collagen and the activation of phosphorylated transforming growth factor-β receptor type I (TβR-I) and Smad2/3 in regenerating muscles (p < 0.05). Leucine also reduced neonatal myosin heavy chain (MyHC-n) (p < 0.05), increased adult MyHC-II expression (p < 0.05) and prevented the decrease in maximum tetanic strength in regenerating TA muscles (p < 0.05). Our results suggest that leucine supplementation accelerates connective tissue repair and consequent function of regenerating TA through the attenuation of TβR-I and Smad2/3 activation. Therefore, future studies are warranted to investigate leucine supplementation as a nutritional strategy to prevent or attenuate muscle fibrosis in patients with several muscle diseases.

Impaired structural and functional regeneration of skeletal muscles from β2-adrenoceptor knockout mice

Aims

β2-adrenergic stimulation causes beneficial effects on structure and function of regenerating muscles; thus, the β2-adrenoceptor may play an important role in the muscle regenerative process. Here, we investigated the role of the β₂-adrenoceptor in skeletal muscle regeneration.

Methods

Tibialis anterior (TA) muscles from β2-adrenoceptor knockout (β₂KO) mice were cryolesioned and analysed after 1, 3, 10 and 21 days. The role of β2-adrenoceptor on regenerating muscles was assessed through the analysis of morphological and contractile aspects, M1 and M2 macrophage profile, cAMP content, and activation of TGF-β signalling elements.

Results

Regenerating muscles from β₂KO mice showed decreased calibre of regenerating myofibres and reduced muscle contractile function at 10 days when compared with those from wild type. The increase in cAMP content in muscles at 10 days post-cryolesion was attenuated in the absence of the β₂-adrenoceptor. Furthermore, there was an increase in inflammation and in the number of macrophages in regenerating muscles lacking the β2-adrenoceptor at 3 and 10 days, a predominance of M1 macrophage phenotype, a decrease in TβR-I/Smad2/3 activation, and in the Smad4 expression at 3 days, while akirin1 expression increased at 10 days in muscles from β₂KO mice when compared to those from wild type.

Conclusions

Our results suggest that the β2-adrenoceptor contributes to the regulation of the initial phases of muscle regeneration, especially in the control of macrophage recruitment in regenerating muscle through activation of TβR-I/Smad2/3 and reduction in akirin1 expression. These findings have implications for the future development of better therapeutic approaches to prevent or treat muscle injuries.

The neural mobilization technique modulates the expression of endogenous opioids in the periaqueductal gray and improves muscle strength and mobility in rats with neuropathic pain

BACKGROUND

The neural mobilization (NM) technique is a noninvasive method that has been proven to be clinically effective in reducing pain; however, the molecular mechanisms involved remain poorly understood. The aim of this study was to analyze whether NM alters the expression of the mu-opioid receptor (MOR), the delta-opioid receptor (DOR) and the Kappa-opioid receptor (KOR) in the periaqueductal gray (PAG) and improves locomotion and muscle force after chronic constriction injury (CCI) in rats.

METHODS

The CCI was imposed on adult male rats followed by 10 sessions of NM every other day, starting 14 days after the CCI injury. At the end of the sessions, the PAG was analyzed using Western blot assays for opioid receptors. Locomotion was analyzed by the Sciatic functional index (SFI), and muscle force was analyzed by the BIOPAC system.

RESULTS

An improvement in locomotion was observed in animals treated with NM compared with injured animals. Animals treated with NM showed an increase in maximal tetanic force of the tibialis anterior muscle of 172% (p < 0.001) compared with the CCI group. We also observed a decrease of 53% (p < 0.001) and 23% (p < 0.05) in DOR and KOR levels, respectively, after CCI injury compared to those from naive animals and an increase of 17% (p < 0.05) in KOR expression only after NM treatment compared to naive animals. There were no significant changes in MOR expression in the PAG.

CONCLUSION

These data provide evidence that a non-pharmacological NM technique facilitates pain relief by endogenous analgesic modulation.