Lysosomal proteolysis in skeletal muscle

The mechanosensitive gene arrestin domain containing 2 regulates myotube diameter with direct implications for disuse atrophy with aging

Arrestin domain containing 2 and 3 (Arrdc2/3) are genes whose mRNA contents are decreased in young skeletal muscle following mechanical overload. Arrdc3 is linked to the regulation of signaling pathways in nonmuscle cells that could influence skeletal muscle size. Despite a similar amino acid sequence, Arrdc2 function remains undefined. The purpose of this study was to further explore the relationship of Arrdc2/Arrdc3 expression with changes in mechanical load in young and aged muscle and define the effect of Arrdc2/3 expression on C2C12 myotube diameter. In young and aged mice, mechanical load was decreased using hindlimb suspension whereas mechanical load was increased by reloading previously unloaded muscle or inducing high-force contractions. Arrdc2 and Arrdc3 mRNAs were overexpressed in C2C12 myotubes using adenoviruses. Myotube diameter was determined 48-h posttransfection, and RNA sequencing was performed on those samples. Arrdc2 and Arrdc3 mRNA content was higher in the unloaded muscle within 1 day of disuse and remained higher up through 10 days. The induction of Arrdc2 mRNA was more pronounced in aged muscle than young muscle in response to unloading. Reloading previously unloaded muscle of young and aged mice restored Arrdc2 and Arrdc3 levels to ambulatory levels. Increasing mechanical load beyond normal ambulatory levels lowered Arrdc2 mRNA, but not Arrdc3 mRNA, in young and aged muscle. Arrdc2 overexpression only was sufficient to lower myotube diameter in C2C12 cells in part by altering the transcriptome favoring muscle atrophy. These data are consistent with Arrdc2 contributing to disuse atrophy, particularly in aged muscle.NEW & NOTEWORTHY We establish Arrdc2 as a novel mechanosensitive gene highly induced in response to mechanical unloading, particularly in aged muscle. Arrdc2 induction in C2C12 myotubes is sufficient to produce thinner myotubes and a transcriptional landscape consistent with muscle atrophy and disuse.

Spatial transcriptomics reveals alterations in perivascular macrophage lipid metabolism in the onset of Wooden Breast myopathy in broiler chickens

This study aims to use spatial transcriptomics to characterize the cell-type-specific expression profile associated with the microscopic features observed in Wooden Breast myopathy. 1 cm3 muscle sample was dissected from the cranial part of the right pectoralis major muscle from three randomly sampled broiler chickens at 23 days post-hatch and processed with Visium Spatial Gene Expression kits (10X Genomics), followed by high-resolution imaging and sequencing on the Illumina Nextseq 2000 system. WB classification was based on histopathologic features identified. Sequence reads were aligned to the chicken reference genome (Galgal6) and mapped to histological images. Unsupervised K-means clustering and Seurat integrative analysis differentiated histologic features and their specific gene expression pattern, including lipid laden macrophages (LLM), unaffected myofibers, myositis and vasculature. In particular, LLM exhibited reprogramming of lipid metabolism with up-regulated lipid transporters and genes in peroxisome proliferator-activated receptors pathway, possibly through P. Moreover, overexpression of fatty acid binding protein 5 could enhance fatty acid uptake in adjacent veins. In myositis regions, increased expression of cathepsins may play a role in muscle homeostasis and repair by mediating lysosomal activity and apoptosis. A better knowledge of different cell-type interactions at early stages of WB is essential in developing a comprehensive understanding.

Effects of pre-slaughter fasting on antemortem skeletal muscle protein degradation levels and postmortem muscle free amino acid concentrations in broiler chickens

This study investigated the effects of pre-slaughter fasting time on the relationship between skeletal muscle protein degradation levels at slaughter and chicken meat quality after 48 h of postmortem aging. Twenty-four broiler chicks at 0 d of age were used in this study until 28 d of age. At 27 d of age, the chickens were assigned to 4 treatment groups: 0 h of fasting (0H), 8 h of fasting (8H), 16 h of fasting (16H), or 24 h of fasting (24H). They were slaughtered at 28 d of age. Blood samples were collected before fasting and immediately before slaughter. Plasma Nτ-methylhistidine concentration, an index of skeletal muscle protein degradation level, and muscle free amino acid concentration were analyzed. Antemortem changes in individual plasma Nτ-methylhistidine concentrations were significantly increased in 8H, 16H, and 24H compared to that in 0H (P < 0.05). After 48 h of postmortem storage, the glutamic acid content in the pectoralis major muscles increased with fasting time (P < 0.05), and the umami taste of chicken soup in the fasting groups (8H, 16H, 24H) was higher than that in the 0H group (P < 0.05). The antemortem changes in plasma Nτ-methylhistidine concentrations were correlated with glutamic acid content in the pectoralis major muscles (r = 0.57, P < 0.05) and umami taste (r = 0.66, P < 0.05). These results suggest that skeletal muscle protein degradation levels at slaughter are related to postmortem chicken meat quality, especially glutamic acid content and umami taste.

Protein Turnover in Skeletal Muscle: Looking at Molecular Regulation towards an Active Lifestyle

Skeletal muscle is a highly plastic tissue, able to change its mass and functional properties in response to several stimuli. Skeletal muscle mass is influenced by the balance between protein synthesis and breakdown, which is regulated by several signaling pathways. The relative contribution of Akt/mTOR signaling, ubiquitin-proteasome pathway, autophagy among other signaling pathways to protein turnover and, therefore, to skeletal muscle mass, differs depending on the wasting or loading condition and muscle type. By modulating mitochondria biogenesis, PGC-1α has a major role in the cell's bioenergetic status and, thus, on protein turnover. In fact, rates of protein turnover regulate differently the levels of distinct protein classes in response to atrophic or hypertrophic stimuli. Mitochondrial protein turnover rates may be enhanced in wasting conditions, whereas the increased turnover of myofibrillar proteins triggers muscle mass gain. The present review aims to update the knowledge on the molecular pathways implicated in the regulation of protein turnover in skeletal muscle, focusing on how distinct muscle proteins may be modulated by lifestyle interventions with emphasis on exercise training. The comprehensive analysis of the anabolic effects of exercise programs will pave the way to the tailored management of muscle wasting conditions.

Cathepsins in oral diseases: mechanisms and therapeutic implications

Cathepsins are a type of lysosomal globulin hydrolase and are crucial for many physiological processes, including the resorption of bone matrix, innate immunity, apoptosis, proliferation, metastasis, autophagy, and angiogenesis. Findings regarding their functions in human physiological processes and disorders have drawn extensive attention. In this review, we will focus on the relationship between cathepsins and oral diseases. We highlight the structural and functional properties of cathepsins related to oral diseases, as well as the regulatory mechanisms in tissue and cells and their therapeutic uses. Elucidating the associated mechanism between cathepsins and oral diseases is thought to be a promising strategy for the treatment of oral diseases and may be a starting point for further studies at the molecular level.

Molecular mechanisms of post-burn muscle wasting and the therapeutic potential of physical exercise

After a severe burn injury, a systemic stress response activates metabolic and inflammatory derangements that, among other, leads to muscle mass loss (muscle wasting). These negative effects on skeletal muscle continue for several months or years and are aggravated by short-term and long-term disuse. The dynamic balance between muscle protein synthesis and muscle protein breakdown (proteolysis) is regulated by complex signalling pathways that leads to an overall negative protein balance in skeletal muscle after a burn injury. Research concerning these molecular mechanisms is still scarce and inconclusive, understanding of which, if any, molecular mechanisms contribute to muscle wasting is of fundamental importance in designing of therapeutic interventions for burn patients as well. This review not only summarizes our present knowledge of the molecular mechanisms that underpin muscle protein balance but also summarizes the effects of exercise on muscle wasting post-burn as promising strategy to counteract the detrimental effects on skeletal muscle. Future research focusing on the pathways causing post-burn muscle wasting and the different effects of exercise on them is needed to confirm this hypothesis and to lay the foundation of therapeutic strategies.

Invited review: Muscle protein breakdown and its assessment in periparturient dairy cows

Mobilization of body reserves including fat, protein, and glycogen is necessary to overcome phases of negative nutrient balance typical for high-yielding dairy cows during the periparturient period. Skeletal muscle, the largest internal organ in mammals, plays a crucial role in maintaining metabolic homeostasis. However, unlike in liver and adipose tissue, the metabolic and regulatory role of skeletal muscle in the adaptation of dairy cows to the physiological needs of pregnancy and lactation has not been studied extensively. The functional integrity and quality of skeletal muscle are maintained through a constant turnover of protein, resulting from both protein breakdown and protein synthesis. Thus, muscle protein breakdown (MPB) and synthesis are intimately connected and tightly controlled to ensure proper protein homeostasis. Understanding the regulation of MPB, the catabolic component of muscle turnover, and its assessment are therefore important considerations to provide information about the timing and extent of tissue mobilization in periparturient dairy cows. Based on animal models and human studies, it is now evident that MPB occurs via the integration of 3 main systems: autophagy-lysosomal, calpain Ca²⁺-dependent cysteine proteases, and the ubiquitin-proteasome system. These 3 main systems are interconnected and do not work separately, and the regulation is complex. The ubiquitin-proteasomal system is the most well-known cellular proteolytic system and plays a fundamental role in muscle physiology. Complete degradation of a protein often requires a combination of the systems, depending on the physiological situation. Determination of MPB in dairy cows is technically challenging, resulting in a relative dearth of information. The methods for assessing MPB can be divided into either direct or indirect measurements, both having their strengths and limitations. Available information on the direct measures of MPB primarily comes from stable isotopic tracer methods and those of indirect measurements from assessing expression and activity measures of the components of the 3 MPB systems in muscle biopsy samples. Other indirect approaches (i.e., potential indicators of MPB), including ultrasound imaging and measuring metabolites from muscle degradation (i.e., 3-methylhistidine and creatinine), seem to be applicable methods and can provide useful information about the extent and timing of MPB. This review presents our current understanding, including methodological considerations, of the process of MPB in periparturient dairy cows.

Using Milk Thistle (Silybum marianum) Extract to Improve the Welfare, Growth Performance and Meat Quality of Broiler Chicken

Simple Summary

The latest trends in livestock husbandry and breeding are directed towards expanding production, which includes the use of natural products, such as herbs, in animal nutrition. The aim of the study was to determine the effects of an aqueous solution of milk thistle administered at chosen concentrations on the welfare, production results, selected physicochemical properties of the breast muscles, the activity of selected enzymes, and the antioxidant potential of the blood serum and breast muscles of broiler chickens. On the basis of the obtained results, it was found that both concentration levels of the milk thistle extract showed a significant effect on some of the growth performance, antioxidant capacity in the blood serum, and in pectoral muscle and welfare.

Abstract

Modern poultry production looks for feed and water additives that improve the welfare of chickens. The utilization of herbs as additives in feed or drinking water is becoming increasingly well known. The aim of this study was to determine the effect of milk thistle extract (Silybum marianum), administered in drinking water, on chickens’ welfare, production results, selected physicochemical properties of the breast muscles, the activity of selected enzymes, and the antioxidant potential of blood serum and breast muscles. A total of 102 one-day-old male ROSS 308 chicks were allocated into three treatment groups. The material was an infusion of dried milk thistle seeds in two doses, administered in drinking water for chickens (group: C = 0; E1 = 0.24; E2 = 0.36 g/day/animal) under standard rearing conditions. As a result of adding the milk thistle to the drinking water, there was an improvement in the welfare of the chickens and in the production results, enzymatic capacity of selected enzymes, and antioxidant capacity in the blood serum and in pectoral muscle (p ≤ 0.05). The addition of a milk thistle seed infusion for chickens can be used in poultry production to improve the rearing performance antioxidant capacity and welfare of chickens.

Effect of Lysosomal Cathepsin L on Proteolysis of Beef Myofibrillar Proteins In Vivo and In Vitro

This study investigated the effects of cathepsin L on proteolysis of beef myofibrillar proteins in vivo and in vitro. Results indicated that cathepsin L affected the degradation of desmin and troponin-T during postmortem aging, and the extent of degradation increased from 1 d to 14 d postmortem. No detectable degradation of titin, nebulin, and α-actinin in the presence of cathepsin L inhibitor was observed during postmortem aging. In vitro, cathepsin L affected the degradation of titin, nebulin, and troponin-T, and the extent of degradation increased with increasing incubation time. Nevertheless, cathepsin L did not cause the degradation of α-actinin and desmin, regardless of incubation temperature. The different results between in vitro and in vivo experiments might mainly depend on different treatment temperatures. Overall, these results indicated that cathepsin L participated in the degradation of myofibrillar proteins and meat tenderization.

Magnesium Homeostasis in Myogenic Differentiation-A Focus on the Regulation of TRPM7, MagT1 and SLC41A1 Transporters

Magnesium (Mg) is essential for skeletal muscle health, but little is known about the modulation of Mg and its transporters in myogenic differentiation. Here, we show in C2C12 murine myoblasts that Mg concentration fluctuates during their differentiation to myotubes, declining early in the process and reverting to basal levels once the cells are differentiated. The level of the Mg transporter MagT1 decreases at early time points and is restored at the end of the process, suggesting a possible role in the regulation of intracellular Mg concentration. In contrast, TRPM7 is rapidly downregulated and remains undetectable in myotubes. The reduced amounts of TRPM7 and MagT1 are due to autophagy, one of the proteolytic systems activated during myogenesis and essential for the membrane fusion process. Moreover, we investigated the levels of SLC41A1, which increase once cells are differentiated, mainly through transcriptional regulation. In conclusion, myogenesis is associated with alterations of Mg homeostasis finely tuned through the modulation of MagT1, TRPM7 and SLC41A1.

The Influence of the Type of Dry-Cured Italian PDO Ham on Cathepsin B Activity Trend during Processing

Cathepsin B activity was measured during processing in hams originating from the main Italian prosciutto PDOs: Parma, San Daniele and Toscano. Sixty-five heavy pig thighs, from sixty-five Italian large white x Italian Landrace pigs bred and slaughtered in the same conditions were considered. Five thighs represented the post-mortem control time. The other 60 were distributed one plant per PDO, following a balanced plan. The thighs were sampled at the biceps femoris in groups of four per plant in the following ripening phases: salting, resting, drying, greasing, end of curing. The activity of the Cathepsin B (U/g protein) was determined by means of fluorescence measurements. The Cathepsin B ripening trend of the various PDOs was significantly different, particularly during the initial and mid-curing stage. This activity correlates with the proteolysis index through a PDO dependent pattern, indicating that different processing conditions can influence the quality of prosciutto, since they determine its biochemical development.

Autophagy-lysosomal signaling responses to heat stress in tenotomy-induced rat skeletal muscle atrophy

AIMS

The autophagy-lysosomal system plays a crucial role in maintaining muscle proteostasis. Excessive stimulation of the autophagic machinery is a major contributor to muscle atrophy induced by tendon transection. Hyperthermia is known to attenuate muscle protein loss during disuse conditions; however, little is known regarding the response of the autophagy pathway to heat stress following tenotomy-induced muscle atrophy. The purpose of this study was to evaluate whether heat stress would have a beneficial impact on the activation of autophagy in tenotomized soleus and plantaris muscles.

MAIN METHODS

Male Wistar rats were divided into control, control plus heat stress, tenotomy, and tenotomy plus heat stress groups. The effects of tenotomy were evaluated at 8 and 14 days with heat treatment applied using thermal blankets (30 min. day^-1, at 40.5-41.5 °C, for 7 days).

KEY FINDINGS

Heat stress could normalize tenotomy-induced muscle loss and over-activation of autophagy-lysosomal signaling; this effect was evidently observed in soleus muscle tenotomized for 14 days. The autophagy-related proteins LC3B-II and LC3B-II/I tended to decrease, and lysosomal cathepsin L protein expression was significantly suppressed. While p62/SQSTM1 was not altered in response to intermittent heat exposure in tenotomized soleus muscle at day 14. Phosphorylation of the 4E-BP1 protein was significantly increased in tenotomized plantaris muscle; whereas heat stress had no impact on phosphorylation of Akt and FoxO3a proteins in both tenotomized muscles examined.

SIGNIFICANCE

Our results provide evidence that heat stress associated attenuation of tenotomy-induced muscle atrophy is mediated through limiting over-activation of the autophagy-lysosomal pathway in oxidative and glycolytic muscles.

Skeletal Muscle Wasting and Function Impairment in Intensive Care Patients With Severe COVID-19

Background: Intensive care patients commonly develop muscle wasting and functional impairment. However, the role of severe COVID-19 in the magnitude of muscle wasting and functionality in the acute critical disease is unknown.

Objective: To perform a prospective characterization to evaluate the skeletal muscle mass and functional performance in intensive care patients with severe COVID-19.

Methods: Thirty-two critically ill patients (93.8% male; age: 64.1 ± 12.6 years) with the diagnosis of the severe COVID-19 were prospectively recruited within 24 to 72 h following intensive care unit (ICU) admission, from April 2020 to October 2020, at Hospital Sírio-Libanês in Brazil. Patients were recruited if older than 18 years old, diagnosis of severe COVID-19 confirmed by RT-PCR, ICU stay and absence of limb amputation. Muscle wasting was determined through an ultrasound measurement of the rectus femoris cross-sectional area, the thickness of the anterior compartment of the quadriceps muscle (rectus femoris and vastus intermedius), and echogenicity. The peripheral muscle strength was assessed with a handgrip test. The functionality parameter was determined through the ICU mobility scale (IMS) and the International Classification of Functioning, Disability and Health (ICF). All evaluations were performed on days 1 and 10.

Results: There were significant reductions in the rectus femoris cross-section area (−30.1% [95% IC, −26.0% to −34.1%]; P < 0.05), thickness of the anterior compartment of the quadriceps muscle (−18.6% [95% IC, −14.6% to 22.5%]; P < 0.05) and handgrip strength (−22.3% [95% IC, 4.7% to 39.9%]; P < 0.05) from days 1 to 10. Patients showed increased mobility (0 [0–5] vs 4.5 [0–8]; P < 0.05), improvement in respiratory function (3 [3–3] vs 2 [1–3]; P < 0.05) and structure respiratory system (3 [3–3] vs 2 [1–3]; P < 0.05), but none of the patients returned to normal levels.

Conclusion: In intensive care patients with severe COVID-19, muscle wasting and decreased muscle strength occurred early and rapidly during 10 days of ICU stay with improved mobility and respiratory functions, although they remained below normal levels. These findings may provide insights into skeletal muscle wasting and function in patients with severe COVID-19.

Histopathological changes and oxidative damage in type I and type II muscle fibers in rats undergoing paradoxical sleep deprivation

BACKGROUND

previous studies have shown that muscle atrophy is observed after sleep deprivation (SD) protocols; however, the mechanisms responsible are not fully understood. Muscle trophism can be modulated by several factors, including energy balance (positive or negative), nutritional status, oxidative stress, the level of physical activity, and disuse. The metabolic differences that exist in different types of muscle fiber may also be the result of different adaptive responses. To better understand these mechanisms, we evaluated markers of oxidative damage and histopathological changes in different types of muscle fibers in sleep-deprived rats.

METHODS

Twenty male Wistar EPM-1 rats were randomly allocated in two groups: a control group (CTL group; n = 10) and a sleep deprived group (SD group; n = 10). The SD group was submitted to continuous paradoxical SD for 96  h; the soleus (type I fibers) and plantar (type II fiber) muscles were analyzed for histopathological changes, trophism, lysosomal activity, and oxidative damage. Oxidative damage was assessed by lipid peroxidation and nuclear labeling of 8-OHdG.

RESULTS

The data demonstrated that SD increased the nuclear labeling of 8-OHdG and induced histopathological changes in both muscles, being more evident in the soleus muscle. In the type I fibers there was signs of tissue degeneration, inflammatory infiltrate and tissue edema. Muscle atrophy was observed in both muscles. The concentration of malondialdehyde, and cathepsin L activity only increased in type I fibers after SD.

CONCLUSION

These data indicate that the histopathological changes observed after 96 h of SD in the skeletal muscle occur by different processes, according to the type of muscle fiber, with muscles predominantly composed of type I fibers undergoing greater oxidative damage and catabolic activity, as evidenced by a larger increase in 8-OHdG labeling, lipid peroxidation, and lysosomal activity.

Sarcopenia - Molecular mechanisms and open questions

Sarcopenia represents a muscle-wasting syndrome characterized by progressive and generalized degenerative loss of skeletal muscle mass, quality, and strength occurring during normal aging. Sarcopenia patients are mainly suffering from the loss in muscle strength and are faced with mobility disorders reducing their quality of life and are, therefore, at higher risk for morbidity (falls, bone fracture, metabolic diseases) and mortality. Several molecular mechanisms have been described as causes for sarcopenia that refer to very different levels of muscle physiology. These mechanisms cover e. g. function of hormones (e. g. IGF-1 and Insulin), muscle fiber composition and neuromuscular drive, myo-satellite cell potential to differentiate and proliferate, inflammatory pathways as well as intracellular mechanisms in the processes of proteostasis and mitochondrial function. In this review, we describe sarcopenia as a muscle-wasting syndrome distinct from other atrophic diseases and summarize the current view on molecular causes of sarcopenia development as well as open questions provoking further research efforts for establishing efficient lifestyle and therapeutic interventions.

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.

Zinc at the crossroads of exercise and proteostasis

Zinc is an essential element for all forms of life, and one in every ten human proteins is a zinc protein. Zinc has catalytic, structural and signalling functions and its correct homeostasis affects many cellular processes. Zinc deficiency leads to detrimental consequences, especially in tissues with high demand such as skeletal muscle. Zinc cellular homeostasis is tightly regulated by different transport and buffer protein systems. Specifically, in skeletal muscle, zinc has been found to affect myogenesis and muscle regeneration due to its effects on muscle cell activation, proliferation and differentiation. In relation to skeletal muscle, exercise has been shown to modulate zinc serum and urinary levels and could directly affect cellular zinc transport. The oxidative stress induced by exercise may provide the basis for the mild zinc deficiency observed in athletes and could have severe consequences on health and sport performance. Proteostasis is induced during exercise and zinc plays an essential role in several of the associated pathways.

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Zinc deficiency could be a crucial issue in sport performance for athletes.

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Exercise could modulate zinc serum and cellular homeostasis.

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Zinc is part of proteostatic systems critical during exercise.

Effects of dietary lysine on productivity, reproductive performance, protein and lipid metabolism-related gene expression in laying duck breeders

This study investigated whether dietary lysine (Lys) affects productive performance and expression of genes related to protein and lipid metabolism in laying duck breeders. Longyan duck breeders (n = 540, 19 wk of age) were randomly assigned to 6 groups with 6 replicates of 15 birds each. Breeders were fed diets with 6 total Lys levels (6.4, 7.2, 8.0, 8.8, 9.6, and 10.4 g/kg) for 26 wk duration. Egg production, egg weight, egg mass, feed conversion ratio, hatchability, hatchling weight, albumen weight, eggshell weight, yolk weight, and yolk proportion increased with dietary Lys levels (P < 0.05). Dietary Lys level had a linear (P < 0.05) and quadratic (P < 0.05) effects on maternal hepatic expression of mechanistic target of rapamycin, eukaryotic translation initiation factor 4E binding protein 1, ubiquitin conjugating enzyme E2K (UBE2K), cathepsin B (CTSB), and quadratically (P < 0.05) increased the concentrations of plasma Lys, leucine, threonine, and tryptophan in duck breeders. In contrast, maternal dietary Lys suppressed expression of proteasome 26S subunit, UBE2K, and CTSB in the liver of hatchlings. Moreover, relative expression of peroxisome proliferator-activated receptors alpha, carnitine palmitoyltransferase 1A, and very low density apolipoprotein-II increased linearly (P < 0.05) and quadratically (P < 0.05), and that of VLDL receptor (VLDLR) decreased quadratically (P < 0.05) in the liver of duck breeders with increasing dietary Lys levels; hepatic triglyceride and cholesterol contents were reduced. Maternal dietary Lys suppressed hepatic expression of VLDLR in the hatchlings. A diet containing 8.6 g Lys/kg promoted protein turnover and lipid metabolism in laying duck breeders, which positively reflected in the productivity and reproductive performance.

Muscle alterations in the development and progression of cancer-induced muscle atrophy: a review

Cancer cachexia-cancer-associated body weight and muscle loss-is a significant predictor of mortality and morbidity in cancer patients across a variety of cancer types. However, despite the negative prognosis associated with cachexia onset, there are no clinical therapies approved to treat or prevent cachexia. This lack of treatment may be partially due to the relative dearth of literature on mechanisms occurring within the muscle before the onset of muscle wasting. Therefore, the purpose of this review is to compile the current scientific literature on mechanisms contributing to the development and progression of cancer cachexia, including protein turnover, inflammatory signaling, and mitochondrial dysfunction. We define "development" as changes in cell function occurring before the onset of cachexia and "progression" as alterations to cell function that coincide with the exacerbation of muscle wasting. Overall, the current literature suggests that multiple aspects of cellular function, such as protein turnover, inflammatory signaling, and mitochondrial quality, are altered before the onset of muscle loss during cancer cachexia and clearly highlights the need to study more thoroughly the developmental stages of cachexia. The studying of these early aberrations will allow for the development of effective therapeutics to prevent the onset of cachexia and improve health outcomes in cancer patients.

miR-125b-5p targeting TRAF6 relieves skeletal muscle atrophy induced by fasting or denervation

Background

Skeletal muscle atrophy, characterized by accelerated protein degradation, occurs in such conditions as unloading, immobilization, fasting, and denervation. Effective treatments for skeletal muscle atrophy are not yet available. Considering that microRNAs (miRs) may play an important role in the regulation of muscle atrophy, in the present study, we aimed to examine the effect of miR-125b-5p-based therapeutic strategies on skeletal muscle atrophy, and to explore the underlying mechanisms.

Methods

Fasting-induced atrophic mouse C2C12 myotubes and denervated rat tibialis anterior (TA) muscles were used as in vitro and in vivo models of skeletal muscle atrophy, respectively. The morphological parameters of skeletal muscle were measured by immunostaining-based quantification. The interaction between miR-125b-5p and TRAF6 3'-UTR was detected by luciferase reporter analysis. The mRNA and protein expressions were determined by real-time qPCR and Western blot analysis respectively. The miR mimics/agomir and miR inhibitor/antagomir were transfected into C2C12 myotubes and TA muscles respectively to alter the expression of miR-125b-5p.

Results

The expression of miR-125b-5p was down-regulated in both atrophic C2C12 myotubes and denervated TA muscles. The interaction between miR-125b-5p and TRAF6 3'-UTR was identified. Overexpression of miR-125b-5p protected skeletal muscle samples from atrophy in vitro and in vivo by targeting TRAF6 through inactivation of several ubiquitin-proteasome system (UPS)- and autophagy-lysosome system (ALS)-related proteins.

Conclusions

Overexpression of miR-125b-5p may provide a promising therapeutic approach to treat muscle atrophy.

Skeletal muscle atrogenes: From rodent models to human pathologies

Skeletal muscle atrophy is a common side effect of most human diseases. Muscle loss is not only detrimental for the quality of life but it also dramatically impairs physiological processes of the organism and decreases the efficiency of medical treatments. While hypothesized for years, the existence of an atrophying programme common to all pathologies is still incompletely solved despite the discovery of several actors and key regulators of muscle atrophy. More than a decade ago, the discovery of a set of genes, whose expression at the mRNA levels were similarly altered in different catabolic situations, opened the way of a new concept: the presence of atrogenes, i.e. atrophy-related genes. Importantly, the atrogenes are referred as such on the basis of their mRNA content in atrophying muscles, the regulation at the protein level being sometimes more complicate to elucidate. It should be noticed that the atrogenes are markers of atrophy and that their implication as active inducers of atrophy is still an open question for most of them. While the atrogene family has grown over the years, it has mostly been incremented based on data coming from rodent models. Whether the rodent atrogenes are valid for humans still remain to be established. An "atrogene" was originally defined as a gene systematically up- or down-regulated in several catabolic situations. Even if recent works often restrict this notion to the up-regulation of a limited number of proteolytic enzymes, it is important to keep in mind the big picture view. In this review, we provide an update of the validated and potential rodent atrogenes and the metabolic pathways they belong, and based on recent work, their relevance in human physio-pathological situations. We also propose a more precise definition of the atrogenes that integrates rapid recovery when catabolic stimuli are stopped or replaced by anabolic ones.

Pyropia yezoensis Protein Prevents Dexamethasone-Induced Myotube Atrophy in C2C12 Myotubes

Glucocorticoids (GCs), which are endocrine hormones released under stress conditions, can cause skeletal muscle atrophy. This study investigated whether Pyropia yezoensis crude protein (PYCP) inhibits synthetic GCs dexamethasone (DEX)-induced myotube atrophy associated with proteolytic systems. Mouse skeletal muscle C2C12 myotubes were treated with DEX in the presence or absence of PYCP. DEX exposure (100 μM) for 24 h significantly decreased myotube diameter and myogenin expression, which were all increased by treatment with 20 and 40 μg/mL PYCP. Additionally, PYCP significantly reduced the nuclear expression of the forkhead box transcription factors, FoxO1 and FoxO3a, and ubiquitin-proteasome pathway activation. Further mechanistic research revealed that PYCP inhibited the autophagy-lysosome pathway in DEX-induced C2C12 myotubes. These findings indicate that PYCP prevents DEX-induced myotube atrophy through the regulation of FoxO transcription factors, followed by the inhibition of the ubiquitin-proteasome and autophagy-lysosome pathways. Therefore, we suggest that inhibiting these two proteolytic processes with FoxO transcription factors is a promising strategy for preventing DEX-related myotube atrophy.

p38β MAPK mediates ULK1-dependent induction of autophagy in skeletal muscle of tumor-bearing mice

Muscle wasting is the key manifestation of cancer-associated cachexia, a lethal metabolic disorder seen in over 50% of cancer patients. Autophagy is activated in cachectic muscle of cancer hosts along with the ubiquitin-proteasome pathway (UPP), contributing to accelerated protein degradation and muscle wasting. However, established signaling mechanism that activates autophagy in response to fasting or denervation does not seem to mediate cancer-provoked autophagy in skeletal myocytes. Here, we show that p38β MAPK mediates autophagy activation in cachectic muscle of tumor-bearing mice via novel mechanisms. Complementary genetic and pharmacological manipulations reveal that activation of p38β MAPK, but not p38α MAPK, is necessary and sufficient for Lewis lung carcinoma (LLC)-induced autophagy activation in skeletal muscle cells. Particularly, muscle-specific knockout of p38β MAPK abrogates LLC tumor-induced activation of autophagy and UPP, sparing tumor-bearing mice from muscle wasting. Mechanistically, p38β MAPK-mediated activation of transcription factor C/EBPβ is required for LLC-induced autophagy activation, and upregulation of autophagy-related genes LC3b and Gabarapl1. Surprisingly, ULK1 activation (phosphorylation at S555) by cancer requires p38β MAPK, rather than AMPK. Activated ULK1 forms a complex with p38β MAPK in myocytes, which is markedly increased by a tumor burden. Overexpression of a constitutively active p38Tbeta; MAPK in HEK293 cells increases phosphorylation at S555 and other amino acid residues of ULK1, but not several of AMPK-mediated sites. Finally, ULK1 activation is abrogated in tumor-bearing mice with muscle-specific knockout of p38β MAPK. Thus, p38β MAPK appears a key mediator of cancer-provoked autophagy activation, and a therapeutic target of cancer-induced muscle wasting.

Dietary supplementation with alkylresorcinols prevents muscle atrophy through a shift of energy supply

It has been reported that phytoextracts that contain alkylresorcinols (ARs) protect against severe myofibrillar degeneration found in isoproterenol-induced myocardial infarction. In this study, we examined the effect of dietary ARs derived from wheat bran extracts on muscle atrophy in denervated mice. The mice were divided into the following four groups: (1) sham-operated (control) mice fed with normal diet (S-ND), (2) denervated mice fed with normal diet (D-ND), (3) control mice fed with ARs-supplemented diet (S-AR) and (4) denervated mice fed with ARs-supplemented diet (D-AR). The intake of ARs prevented the denervation-induced reduction of the weight of the hind limb muscles and the myofiber size. However, the expression of ubiquitin ligases and autophagy-related genes, which is associated with muscle proteolysis, was slightly higher in D-AR than in D-ND. Moreover, the abundance of the autophagy marker p62 was significantly higher in D-AR than in D-ND. Muscle atrophy has been known to be associated with a disturbed energy metabolism. The expression of pyruvate dehydrogenase kinase 4 (PDK4), which is related to fatty acid metabolism, was decreased in D-ND as compared with that in S-ND. In contrast, dietary supplementation with ARs inhibited the decrease of PDK4 expression caused by denervation. Furthermore, the abnormal expression pattern of genes related to the abundance of lipid droplets-coated proteins that was induced by denervation was improved by ARs. These results raise the possibility that dietary supplementation with ARs modifies the disruption of fatty acid metabolism induced by lipid autophagy, resulting in the prevention of muscle atrophy.

Low-temperature long-time cooking of meat: Eating quality and underlying mechanisms

Heat treatment of meat at temperatures between 50 and 65 °C, for extended periods of time, is known as low-temperature long-time (LTLT) cooking. This cooking method produces meat that has increased tenderness and better appearance than when cooked at higher temperatures. Public concerns regarding this method have focused on the ability to design heat treatments that can reach microbiological safety. The heat treatment induces modification of the meat structure and its constituents, which can explain the desirable eating quality traits obtained. Denaturation, aggregation, and degradation of myofibrillar, sarcoplasmic and connective tissue proteins occur depending on the combination of time and temperature during the heat treatment. The protein changes, especially in relation to collagen denaturation, along with proteolytic activity, have often been regarded to be the main contributors to the increased meat tenderness. The mechanisms involved and the possible contribution of other factors are reviewed and discussed.

A review of telomere length in sarcopenia and frailty

Sarcopenia and frailty are associated with several important health-related adverse events, including disability, loss of independence, institutionalization and mortality. Sarcopenia can be considered a biological substrate of frailty, and the prevalence of both these conditions progressively increases with age. Telomeres are nucleoprotein structures located at the end of linear chromosomes and implicated in cellular ageing, shorten with age, and are associated with various age-related diseases. In addition, telomere length (TL) is widely considered a molecular/cellular hallmark of the ageing process. This narrative review summarizes the knowledge about telomeres and analyzes for the first time a possible association of TL with sarcopenia and frailty. The overview provided by the present review suggests that leukocyte TL as single measurement, calculated by quantitative real-time polymerase chain reaction (qRT-PCR), cannot be considered a meaningful biological marker for complex, multidimensional age-related conditions, such as sarcopenia and frailty. Panels of biomarkers, including TL, may provide more accurate assessment and prediction of outcomes in these geriatric syndromes in elderly people.

Investigating the Life Expectancy and Proteolytic Degradation of Engineered Skeletal Muscle Biological Machines

A combination of techniques from 3D printing, tissue engineering and biomaterials has yielded a new class of engineered biological robots that could be reliably controlled via applied signals. These machines are powered by a muscle strip composed of differentiated skeletal myofibers in a matrix of natural proteins, including fibrin, that provide physical support and cues to the cells as an engineered basement membrane. However, maintaining consistent results becomes challenging when sustaining a living system in vitro. Skeletal muscle must be preserved in a differentiated state and the system is subject to degradation by proteolytic enzymes that can break down its mechanical integrity. Here we examine the life expectancy, breakdown, and device failure of engineered skeletal muscle bio-bots as a result of degradation by three classes of proteases: plasmin, cathepsin L, and matrix metalloproteinases (MMP-2 and MMP-9). We also demonstrate the use of gelatin zymography to determine the effects of differentiation and inhibitor concentration on protease expression. With this knowledge, we are poised to design the next generation of complex biological machines with controllable function, specific life expectancy and greater consistency. These results could also prove useful for the study of disease-specific models, treatments of myopathies, and other tissue engineering applications.

GROWTH AND DEVELOPMENT SYMPOSIUM: Adenosine monophosphate-activated protein kinase and mitochondria in Rendement Napole pig growth

The Rendement Napole mutation (RN-), which is well known to influence pork quality, also has a profound impact on metabolic characteristics of muscle. Pigs with RN- possess a SNP in the γ3 subunit of adenosine monophosphate (AMP)-activated protein kinase (AMPK); AMPK, a key energy sensor in skeletal muscle, modulates energy producing and energy consuming pathways to maintain cellular homeostasis. Importantly, AMPK regulates not only acute response to energy stress but also facilitates long-term adaptation via changes in gene and protein expression. The RN- allele increases AMPK activity, which alters the metabolic phenotype of skeletal muscle by increasing mitochondrial content and oxidative capacity. Fibers with greater oxidative capacity typically exhibit increased protein turnover and smaller fiber size, which indicates that RN- pigs may exhibit decreased efficiency and growth potential. However, whole body and muscle growth of RN- pigs appear similar to that of wild-type pigs and despite increased oxidative capacity, fibers maintain the capacity for hypertrophic growth. This indicates that compensatory mechanisms may allow RN- pigs to achieve rates of muscle growth similar to those of wild-type pigs. Intriguingly, lipid oxidation and mitochondria function are enhanced in RN- pig muscle. Thus far, characteristics of RN- muscle are largely based on animals near market weight. To better understand interaction between energy signaling and protein accretion in muscle, further work is needed to define age-dependent relationships between AMPK signaling, metabolism, and muscle growth.

The Role of Exercise and TFAM in Preventing Skeletal Muscle Atrophy

Skeletal muscle atrophy is the consequence of protein degradation exceeding protein synthesis. This arises for a multitude of reasons including the unloading of muscle during microgravity, post-surgery bedrest, immobilization of a limb after injury, and overall disuse of the musculature. The development of therapies prior to skeletal muscle atrophy settings to diminish protein degradation is scarce. Mitochondrial dysfunction is associated with skeletal muscle atrophy and contributes to the induction of protein degradation and cell apoptosis through increased levels of ROS observed with the loss of organelle function. ROS binds mtDNA, leading to its degradation and decreasing functionality. Mitochondrial transcription factor A (TFAM) will bind and coat mtDNA, protecting it from ROS and degradation while increasing mitochondrial function. Exercise stimulates cell signaling pathways that converge on and increase PGC-1α, a well-known activator of the transcription of TFAM and mitochondrial biogenesis. Therefore, in the present review we are proposing, separately, exercise and TFAM treatments prior to atrophic settings (muscle unloading or disuse) alleviate skeletal muscle atrophy through enhanced mitochondrial adaptations and function. Additionally, we hypothesize the combination of exercise and TFAM leads to a synergistic effect in targeting mitochondrial function to prevent skeletal muscle atrophy. J. Cell. Physiol. 232: 2348-2358, 2017. © 2016 The Authors. Journal of Cellular Physiology Published by © 2016 Wiley Periodicals, Inc.

Activin A induces skeletal muscle catabolism via p38β mitogen-activated protein kinase

BACKGROUND

Activation of type IIB activin receptor (ActRIIB) in skeletal muscle leads to muscle atrophy because of increased muscle protein degradation. However, the intracellular signalling mechanism that mediates ActRIIB-activated muscle catabolism is poorly defined.

METHODS

We investigated the role of p38β mitogen-activated protein kinases (MAPK) in mediating ActRIIB ligand activin A-activated muscle catabolic pathways in C2C12 myotubes and in mice with perturbation of this kinase pharmacologically and genetically.

RESULTS

Treatment of C2C12 myotubes with activin A or myostatin rapidly activated p38 MAPK and its effector C/EBPβ within 1 h. Paradoxically, Akt was activated at the same time through a p38 MAPK-independent mechanism. These events were followed by up-regulation of ubiquitin ligases atrogin1 (MAFbx) and UBR2 (E3α-II), as well as increase in LC3-II, a marker of autophagosome formation, leading to myofibrillar protein loss and myotube atrophy. The catabolic effects of activin A were abolished by p38α/β MAPK inhibitor SB202190. Using small interfering RNA-mediated gene knockdown, we found that the catabolic activity of activin A was dependent on p38β MAPK specifically. Importantly, systemic administration of activin A to mice similarly activated the catabolic pathways in vivo, and this effect was blocked by SB202190. Further, activin A failed to activate the catabolic pathways in mice with muscle-specific knockout of p38β MAPK. Interestingly, activin A up-regulated MuRF1 in a p38 MAPK-independent manner, and MuRF1 did not appear responsible for activin A-induced myosin heavy chain loss and muscle atrophy.

CONCLUSIONS

ActRIIB-mediated activation of muscle catabolism is dependent on p38β MAPK-activated signalling.

Metabolic Changes During Cancer Cachexia Pathogenesis

Wasting of adipose tissue and skeletal muscle is a hallmark of metastatic cancer and a major cause of death. Like patients with cachexia caused by other chronic infections or inflammatory diseases, the cancer subject manifests both malnutrition and metabolic stress. Both carbohydrate utilization and amino acid incorporation are decreased in the muscles of cancer cachexia patients. Cancer cells affect host metabolism in two ways: (a) their own metabolism of nutrients into other metabolites and (b) circulating factors they secrete or induce the host to secrete. Accelerated glycolysis and lactate production, i.e., the Warburg effect and the resultant increase in Cori cycle activity, are the most widely discussed metabolic effects. Meanwhile, although a large number of pro-cachexia circulating factors have been found, such as TNFa, IL-6, myostatin, and PTHrp, none have been shown to be a dominant factor that can be targeted singly to treat cancer cachexia in humans. It is possible that given the complex multifactorial nature of the cachexia secretome, and the personalized differences between cancer patients, targeting any single circulating factor would always be insufficient to treat cachexia for all patients. Here we review the metabolic changes that occur in response to tumor growth and tumor-secreted factors during cachexia.

Effects of high EPA and high DHA fish oils on changes in signaling associated with protein metabolism induced by hindlimb suspension in rats

The effects of either eicosapentaenoic (EPA)- or docosahexaenoic (DHA)-rich fish oils on hindlimb suspension (HS)-induced muscle disuse atrophy were compared. Daily oral supplementations (0.3 mL/100 g b.w.) with mineral oil (MO) or high EPA or high DHA fish oils were performed in adult rats. After 2 weeks, the animals were subjected to HS for further 2 weeks. The treatments were maintained alongside HS At the end of 4 weeks, we evaluated: body weight gain, muscle mass and fat depots, composition of fatty acids, cross-sectional areas (CSA) of the soleus muscle and soleus muscle fibers, activities of cathepsin L and 26S proteasome, and content of carbonylated proteins in the soleus muscle. Signaling pathway activities associated with protein synthesis (Akt, p70S6K, S6, 4EBP1, and GSK3-beta) and protein degradation (atrogin-1/MAFbx, and MuRF1) were evaluated. HS decreased muscle mass, CSA of soleus muscle and soleus muscle fibers, and altered signaling associated with protein synthesis (decreased) and protein degradation (increased). The treatment with either fish oil decreased the ratio of omega-6/omega-3 fatty acids and changed protein synthesis-associated signaling. EPA-rich fish oil attenuated the changes induced by HS on 26S proteasome activity, CSA of soleus muscle fibers, and levels of p-Akt, total p70S6K, p-p70S6K/total p70S6K, p-4EBP1, p-GSK3-beta, p-ERK2, and total ERK 1/2 proteins. DHA-rich fish oil attenuated the changes induced by HS on p-4EBP1 and total ERK1 levels. The effects of EPA-rich fish oil on protein synthesis signaling were more pronounced. Both EPA- and DHA-rich fish oils did not impact skeletal muscle mass loss induced by non-inflammatory HS.

The role of E3 ubiquitin-ligases MuRF-1 and MAFbx in loss of skeletal muscle mass

The ubiquitin-proteasome system (UPS) is the main regulatory mechanism of protein degradation in skeletal muscle. The ubiquitin-ligase enzymes (E3s) have a central role in determining the selectivity and specificity of the UPS. Since their identification in 2001, the muscle specific E3s, muscle RING finger-1 (MuRF-1) and muscle atrophy F-box (MAFbx), have been shown to be implicated in the regulation of skeletal muscle atrophy in various pathological and physiological conditions. This review aims to explore the involvement of MuRF-1 and MAFbx in catabolism of skeletal muscle during various pathologies, such as cancer cachexia, sarcopenia of aging, chronic kidney disease (CKD), diabetes, and chronic obstructive pulmonary disease (COPD). In addition, the effects of various lifestyle and modifiable factors (e.g. nutrition, exercise, cigarette smoking, and alcohol) on MuRF-1 and MAFbx regulation will be discussed. Finally, evidence of potential strategies to protect against skeletal muscle wasting through inhibition of MuRF-1 and MAFbx expression will be explored.

Redox control of skeletal muscle atrophy

Skeletal muscles comprise the largest organ system in the body and play an essential role in body movement, breathing, and glucose homeostasis. Skeletal muscle is also an important endocrine organ that contributes to the health of numerous body organs. Therefore, maintaining healthy skeletal muscles is important to support overall health of the body. Prolonged periods of muscle inactivity (e.g., bed rest or limb immobilization) or chronic inflammatory diseases (i.e., cancer, kidney failure, etc.) result in skeletal muscle atrophy. An excessive loss of muscle mass is associated with a poor prognosis in several diseases and significant muscle weakness impairs the quality of life. The skeletal muscle atrophy that occurs in response to inflammatory diseases or prolonged inactivity is often associated with both oxidative and nitrosative stress. In this report, we critically review the experimental evidence that provides support for a causative link between oxidants and muscle atrophy. More specifically, this review will debate the sources of oxidant production in skeletal muscle undergoing atrophy as well as provide a detailed discussion on how reactive oxygen species and reactive nitrogen species modulate the signaling pathways that regulate both protein synthesis and protein breakdown.

Connexin hemichannels explain the ionic imbalance and lead to atrophy in denervated skeletal muscles

Denervated fast skeletal muscles undergo atrophy, which is associated with an increase in sarcolemma permeability and protein imbalance. However, the mechanisms responsible for these alterations remain largely unknown. Recently, a close association between de novo expression of hemichannels formed by connexins 43 and 45 and increase in sarcolemma permeability of denervated fast skeletal myofibers was demonstrated. However, it remains unknown whether these connexins cause the ionic imbalance of denervates fast myofibers. To elucidate the latter and the role of hemichannels formed by connexins (Cx HCs) in denervation-induced atrophy, skeletal myofibers deficient in Cx43 and Cx45 expression (Cx43^fl/flCx45^fl/fl:Myo-Cre mice) and control (Cx43^fl/flCx45^fl/fl mice) were denervated and several muscle features were systematically analyzed at different post-denervation (PD) times (1, 3, 5, 7 and 14days). The following sequence of events was found in denervated myofibers of Cx43^fl/flCx45^fl/fl mice: 1) from day 3 PD, increase in sarcolemmal permeability, 2) from day 5 PD, increases of intracellular Ca²⁺ and Na⁺ signals as well as a significant increase in protein synthesis and degradation, yielding a negative protein balance and 3) from day 7 PD, a fall in myofibers cross-section area. All the above alterations were either absent or drastically reduced in denervated myofibers of Cx43^fl/flCx45^fl/fl:Myo-Cre mice. Thus, the denervation-induced Cx HCs expression is an early event that precedes the electrochemical gradient dysregulation across the sarcolemma and critically contributes to the progression of skeletal muscle atrophy. Consequently, Cx HCs could be a therapeutic target to drastically prevent the denervation-induced atrophy of fast skeletal muscles.

Differential role of endogenous cathepsin and microorganism in texture softening of ice-stored grass carp (Ctenopharyngodon idella) fillets

BACKGROUND

Texture deterioration often negatively affects sensory attributes and commercial values of ice-stored fish fillets. The mechanism of softening of fish fillets during chilling storage is not fully resolved. Grass carp is a predominant freshwater fish species in China. The objective of the present study was to investigate the differential role of endogenous cathepsin and microorganisms in texture softening of ice-stored grass carp fillets.

RESULTS

The fillets were immersed in either NaN3 solution to reduce microbial activity or in iodoacetic acid solution to exclude cathepsin activity before ice storage. Treatment with NaN3 reduced microbial load of fillets below 2 log CFU g(-1) muscle during the entire storage period, and had no significant influence on the cathepsin activity and proteolysis. But the shear force of fillets treated with NaN3 decreased by 66% after 21 days of storage. Meanwhile, treatment with iodoacetic acid inactivated cathepsin B and B + L but did not significantly affect the microbial growth of fillets. Compared to NaN3 treatment, iodoacetic acid effectively alleviated softening and inhibited the increase in TCA-soluble peptides during storage.

CONCLUSION

This study demonstrated that proteolysis induced by endogenous cathepsins, rather than microorganisms, plays an important role in texture softening of ice-stored grass carp fillets. © 2015 Society of Chemical Industry.

Skeletal muscle wasting and renewal: a pivotal role of myokine IL-6

Adult skeletal tissue is composed of heterogeneous population of cells that constantly self-renew by means of a controlled process of activation and proliferation of tissue-resident stem cells named satellite cells. Many growth factors, cytokines and myokines produced by skeletal muscle cells play critical roles in local regulation of the inflammatory process and skeletal muscle regeneration during different pathological conditions. IL-6 is a pleiotropic cytokine released in large amount during infection, autoimmunity and cancer. Low levels of IL-6 can promote activation of satellite cells and myotube regeneration while chronically elevated production promote skeletal muscle wasting. These distinct effects may be explained by a crosstalk of the IL-6/IL-6 receptor and gp130 trans-signaling pathway that oppose to regenerative and anti-inflammatory of the classical IL-6 receptor signaling pathway. Here we discuss on potential therapeutic strategies using monoclonal antibodies to IL-6R for the treatment of skeletal muscle wasting and cachexia. We also highlight on the IL-6/JAK/STAT and FGF/p38αβ MAPK signaling pathways in satellite cell activation and the use of protein kinase inhibitors for tailoring and optimizing satellite cell proliferation during the skeletal muscle renewal. Future investigations on the roles of the IL-6 classical and trans-signaling pathways in both immune and non-immune cells in skeletal muscle tissue will provide new basis for therapeutic approaches to reverse atrophy and degeneration of skeletal muscles in cancer and inflammatory diseases.

Lessons from mammalian hibernators: molecular insights into striated muscle plasticity and remodeling

Striated muscle shows an amazing ability to adapt its structural apparatus based on contractile activity, loading conditions, fuel supply, or environmental factors. Studies with mammalian hibernators have identified a variety of molecular pathways which are strategically regulated and allow animals to endure multiple stresses associated with the hibernating season. Of particular interest is the observation that hibernators show little skeletal muscle atrophy despite the profound metabolic rate depression and mechanical unloading that they experience during long weeks of torpor. Additionally, the cardiac muscle of hibernators must adjust to low temperature and reduced perfusion, while the strength of contraction increases in order to pump cold, viscous blood. Consequently, hibernators hold a wealth of knowledge as it pertains to understanding the natural capacity of myocytes to alter structural, contractile and metabolic properties in response to environmental stimuli. The present review outlines the molecular and biochemical mechanisms which play a role in muscular atrophy, hypertrophy, and remodeling. In this capacity, four main networks are highlighted: (1) antioxidant defenses, (2) the regulation of structural, contractile and metabolic proteins, (3) ubiquitin proteosomal machinery, and (4) macroautophagy pathways. Subsequently, we discuss the role of transcription factors nuclear factor (erythroid-derived 2)-like 2 (Nrf2), Myocyte enhancer factor 2 (MEF2), and Forkhead box (FOXO) and their associated posttranslational modifications as it pertains to regulating each of these networks. Finally, we propose that comparing and contrasting these concepts to data collected from model organisms able to withstand dramatic changes in muscular function without injury will allow researchers to delineate physiological versus pathological responses.

Molecular pathways leading to loss of skeletal muscle mass in cancer cachexia--can findings from animal models be translated to humans?

Background

Cachexia is a multi-factorial, systemic syndrome that especially affects patients with cancer of the gastrointestinal tract, and leads to reduced treatment response, survival and quality of life. The most important clinical feature of cachexia is the excessive wasting of skeletal muscle mass. Currently, an effective treatment is still lacking and the search for therapeutic targets continues. Even though a substantial number of animal studies have contributed to a better understanding of the underlying mechanisms of the loss of skeletal muscle mass, subsequent clinical trials of potential new drugs have not yet yielded any effective treatment for cancer cachexia. Therefore, we questioned to which degree findings from animal studies can be translated to humans in clinical practice and research.

Discussion

A substantial amount of animal studies on the molecular mechanisms of muscle wasting in cancer cachexia has been conducted in recent years. This extensive review of the literature showed that most of their observations could not be consistently reproduced in studies on human skeletal muscle samples. However, studies on human material are scarce and limited in patient numbers and homogeneity. Therefore, their results have to be interpreted critically.

Summary

More research is needed on human tissue samples to clarify the signaling pathways that lead to skeletal muscle loss, and to confirm pre-selected drug targets from animal models in clinical trials. In addition, improved diagnostic tools and standardized clinical criteria for cancer cachexia are needed to conduct standardized, randomized controlled trials of potential drug candidates in the future.

Acquired Muscle Weakness in the Surgical Intensive Care Unit

Muscle weakness is common in the surgical intensive care unit (ICU). Low muscle mass at ICU admission is a significant predictor of adverse outcomes. The consequences of ICU-acquired muscle weakness depend on the underlying mechanism. Temporary drug-induced weakness when properly managed may not affect outcome. Severe perioperative acquired weakness that is associated with adverse outcomes (prolonged mechanical ventilation, increases in ICU length of stay, and mortality) occurs with persistent (time frame: days) activation of protein degradation pathways, decreases in the drive to the skeletal muscle, and impaired muscular homeostasis. ICU-acquired muscle weakness can be prevented by early treatment of the underlying disease, goal-directed therapy, restrictive use of immobilizing medications, optimal nutrition, activating ventilatory modes, early rehabilitation, and preventive drug therapy. In this article, the authors review the nosology, epidemiology, diagnosis, and prevention of ICU-acquired weakness in surgical ICU patients.

Alteration of cathepsin-D expression in atrophied muscles and apoptotic myofibers by hindlimb unloading in a low-temperature environment

[Purpose] The purpose of this study was to elucidate the cathepsin-D involvement in signaling pathways for the survival and apoptosis of myofibers in rats with hindlimb-unloading in a low-temperature environment. [Subjects and Methods] Wistar rats were divided into two groups: a control group and a group that underwent hindlimb unloading in a low-temperature environment to induce muscle apoptosis. Cathepsin-D localization in the soleus and extensor digitorum longus muscles, along with the expression of cathepsin-D in apoptotic myofibers, was examined. Expression of the active and inactive forms of cathepsin-D was also analyzed. [Results] Cathepsin-D was mainly expressed in type I myofibers and was observed to have punctate patterns in the control group. In the hindlimb unloading in a low-temperature environment group, the type I myofiber composition ratio decreased, and caspase-3 activation and TUNEL-positive apoptotic myofibers were observed. In caspase-3-activated myofibers, cathepsin-D overexpression and leakage of it into the cytoplasm were observed. In the hindlimb unloading in a low-temperature environment group, the amount of inactive cathepsin-D decreased, whereas that of the active form increased. [Conclusion] Cathepsin-D was deduced to be indicative of a myofiber-type classification and a factor related to myofiber type maintenance. In addition, cathepsin-D leakage into the cytoplasm was appeared to be involved in caspase-3 activation in the hindlimb unloading in a low-temperature environment group.