L-Lysine suppresses myofibrillar protein degradation and autophagy in skeletal muscles of senescence-accelerated mouse prone 8

Autophagy: Are Amino Acid Signals Dependent on the mTORC1 Pathway or Independent?

Autophagy is a kind of "self-eating" phenomenon that is ubiquitous in eukaryotic cells. It mainly manifests in the damaged proteins or organelles in the cell being wrapped and transported by the autophagosome to the lysosome for degradation. Many factors cause autophagy in cells, and the mechanism of nutrient-deficiency-induced autophagy has been a research focus. It has been reported that amino-acid-deficiency-induced cellular autophagy is mainly mediated through the mammalian rapamycin target protein complex 1 (mTORC1) signaling pathway. In addition, some researchers also found that non-mTORC1 signaling pathways also regulate autophagy, and the mechanism of autophagy occurrence induced by the deficiency of different amino acids is not precisely the same. Therefore, this review aims to summarize the process of various amino acids regulating cell autophagy and provide a narrative review on the molecular mechanism of amino acids regulating autophagy.

"Diet for the prevention and management of sarcopenia"

Sarcopenia is a geriatric condition characterized by a progressive loss of skeletal muscle mass and strength, with an increased risk of adverse health outcomes (e.g., falls, disability, institutionalization, reduced quality of life, mortality). Pharmacological remedies are currently unavailable for preventing the development of sarcopenia, halting its progression, or impeding its negative health outcomes. The most effective strategies to contrast sarcopenia rely on the adoption of healthier lifestyle behaviors, including adherence to high-quality diets and regular physical activity. In this review, the role of nutrition in the prevention and management of sarcopenia is summarized. Special attention is given to current "blockbuster" dietary regimes and agents used to counteract age-related muscle wasting, together with their putative mechanisms of action. Issues related to the design and implementation of effective nutritional strategies are discussed, with a focus on unanswered questions on the most appropriate timing of nutritional interventions to preserve muscle health and function into old age. A brief description is also provided on new technologies that can facilitate the development and implementation of personalized nutrition plans to contrast sarcopenia.

mTORC1 Mediates the Processes of Lysine Regulating Satellite Cells Proliferation, Apoptosis, and Autophagy

Lysine (Lys) is essential for skeletal muscle growth and protein synthesis in mammals. However, the regulatory network underlying Lys-regulated skeletal muscle development is unknown. To determine whether any cross-talk occurs among mammalian targets of rapamycin complex 1 (mTORC1) and Lys in the regulation of muscle satellite cells (SCs) proliferation, we applied the treatment rapamycin (a mTORC1 inhibitor) and MHY1485 (a mTORC1 activator) on Lys-added or -deficient SCs. The results show Lys deprivation significantly decreases SCs viability, protein synthesis, and cell cycling, increases autophagy and apoptosis, and inhibits the mTORC1 signaling pathway. Restoration of Lys content significantly attenuates this effect. mTORC1 signaling pathway activation during Lys deprivation or mTORC1 signaling pathway inhibition during Lys addition attenuates the effect of Lys deprivation or addition on SCs viability, protein synthesis, cell cycling, autophagy, and apoptosis. In conclusion, Lys could improve SCs proliferation, and inhibit SCs apoptosis and autophagy, via the mTORC1 signaling pathway.

Sarcopenia, frailty and type 2 diabetes mellitus (Review)

Skeletal muscle is the largest and most energy‑consuming organ in the human body, which plays an important role in energy metabolism and glucose uptake. There is a notable decrease in glucose uptake in the skeletal muscle of patients with type 2 diabetes mellitus (DM). Endurance exercise can reduce hyperglycemia and improve insulin resistance in patients with type 2 DM. Insulin exerts a variety of effects, many of which are mediated by Akt, including increasing glucose uptake, promoting glycogen synthesis and inhibiting glycogen degradation, increasing free fatty acid uptake, increasing protein synthesis, promoting muscle hypertrophy and inhibiting protein degradation. Skeletal muscle mass progressively declines with aging, resulting in loss of muscle strength and physical function. Sarcopenia is a syndrome characterized by loss of skeletal muscle mass and muscle weakness or loss of physical function, and frailty is another syndrome that has received great interest in recent years. Decreased organ function results in vulnerability to external stress. Frailty is associated with falls, fractures and hospitalization; however, there is the reversibility of returning to a healthy state with appropriate interventions. Frailty is classified into three subgroups: Physical frailty, social frailty and cognitive frailty, whereby sarcopenia is the main component of physical frailty. The present review discusses the associations between sarcopenia, frailty and type 2 DM based on current evidence.

Common Muscle Metabolic Signatures Highlight Arginine and Lysine Metabolism as Potential Therapeutic Targets to Combat Unhealthy Aging

Biological aging research is expected to reveal modifiable molecular mechanisms that can be harnessed to slow or possibly reverse unhealthy trajectories. However, there is first an urgent need to define consensus molecular markers of healthy and unhealthy aging. Established aging hallmarks are all linked to metabolism, and a ‘rewired’ metabolic circuitry has been shown to accelerate or delay biological aging. To identify metabolic signatures distinguishing healthy from unhealthy aging trajectories, we performed nontargeted metabolomics on skeletal muscles from 2-month-old and 21-month-old mice, and after dietary and lifestyle interventions known to impact biological aging. We hypothesized that common metabolic signatures would highlight specific pathways and processes promoting healthy aging, while revealing the molecular underpinnings of unhealthy aging. Here, we report 50 metabolites that commonly distinguished aging trajectories in all cohorts, including 18 commonly reduced under unhealthy aging and 32 increased. We stratified these metabolites according to known relationships with various aging hallmarks and found the greatest associations with oxidative stress and nutrient sensing. Collectively, our data suggest interventions aimed at maintaining skeletal muscle arginine and lysine may be useful therapeutic strategies to minimize biological aging and maintain skeletal muscle health, function, and regenerative capacity in old age.

β-Cryptoxanthin Improves p62 Accumulation and Muscle Atrophy in the Soleus Muscle of Senescence-Accelerated Mouse-Prone 1 Mice

We investigated the effects of β-cryptoxanthin on skeletal muscle atrophy in senescence-accelerated mouse-prone 1 (SAMP1) mice. For 15 weeks, SAMP1 mice were intragastrically administered vehicle or β-cryptoxanthin. At 35 weeks of age, the skeletal muscle mass in SAMP1 mice was reduced compared with that in control senescence-accelerated mouse-resistant 1 (SAMR1) mice. β-cryptoxanthin increased muscle mass with an increase in the size of muscle fibers in the soleus muscle of SAMP1 mice. The expressions of autophagy-related factors such as beclin-1, p62, LC3-I, and LC3-II were increased in the soleus muscle of SAMP1 mice; however, β-cryptoxanthin administration inhibited this increase. Unlike in SAMR1 mice, p62 was punctately distributed throughout the cytosol in the soleus muscle fibers of SAMP1 mice; however, β-cryptoxanthin inhibited this punctate distribution. The cross-sectional area of p62-positive fiber was smaller than that of p62-negative fiber, and the ratio of p62-positive fibers to p62-negative fibers was increased in SAMP1 mice. β-cryptoxanthin decreased this ratio in SAMP1 mice. Furthermore, β-cryptoxanthin decreased the autophagy-related factor expression in murine C2C12 myotube. The autophagy inhibitor bafilomycin A1, but not the proteasome inhibitor MG132, inhibited the β-cryptoxanthin-induced decrease in p62 and LC3-II expressions. These results indicate that β-cryptoxanthin inhibits the p62 accumulation in fibers and improves muscle atrophy in the soleus muscle of SAMP1 mice.

Lactobacillus paracasei PS23 decelerated age-related muscle loss by ensuring mitochondrial function in SAMP8 mice

Sarcopenia is a common impairment in the elderly population responsible for poor outcomes later in life; it can be caused by age-related alternations. Only a few strategies have been reported to reduce sarcopenia. Lactobacillus paracasei PS23 (LPPS23) has been reported to delay some age-related disorders. Therefore, here we investigated whether LPPS23 decelerates age-related muscle loss and its underlying mechanism. Female senescence-accelerated mouse prone-8 (SAMP8) mice were divided into three groups (n=6 each): non-aging (16-week-old), control (28-week-old), and PS23 (28-week-old) groups. The control and PS23 groups were given saline and LPPS23, respectively. We evaluated the effects of LPPS23 by analyzing body weight and composition, muscle strength, protein uptake, mitochondrial function, reactive oxygen species (ROS), antioxidant enzymes, and inflammation-related cytokines. LPPS23 significantly attenuated age-related decreases of muscle mass and strength. Compared to the control group, the non-aging and PS23 groups exhibited higher mitochondrial function, IL10, antioxidant enzymes, and protein uptake. Moreover, inflammatory cytokines and ROS were lower in the non-aging and PS23 groups than the control group. Taken together, LPPS23 extenuated sarcopenia progression during aging; this effect might have been enacted by preserving the mitochondrial function via reducing age-related inflammation and ROS and by retaining protein uptake in the SAMP8 mice.

Regulation of Skeletal Muscle Function by Amino Acids

Amino acids are components of proteins that also exist free-form in the body; their functions can be divided into (1) nutritional, (2) sensory, and (3) biological regulatory roles. The skeletal muscle, which is the largest organ in the human body, representing ~40% of the total body weight, plays important roles in exercise, energy expenditure, and glucose/amino acid usage—processes that are modulated by various amino acids and their metabolites. In this review, we address the metabolism and function of amino acids in the skeletal muscle. The expression of PGC1α, a transcriptional coactivator, is increased in the skeletal muscle during exercise. PGC1α activates branched-chain amino acid (BCAA) metabolism and is used for energy in the tricarboxylic acid (TCA) cycle. Leucine, a BCAA, and its metabolite, β-hydroxy-β-methylbutyrate (HMB), both activate mammalian target of rapamycin complex 1 (mTORC1) and increase protein synthesis, but the mechanisms of activation appear to be different. The metabolite of valine (another BCAA), β-aminoisobutyric acid (BAIBA), is increased by exercise, is secreted by the skeletal muscle, and acts on other tissues, such as white adipose tissue, to increase energy expenditure. In addition, several amino acid-related molecules reportedly activate skeletal muscle function. Oral 5-aminolevulinic acid (ALA) supplementation can protect against mild hyperglycemia and help prevent type 2 diabetes. β-alanine levels are decreased in the skeletal muscles of aged mice. β-alanine supplementation increased the physical performance and improved the executive function induced by endurance exercise in middle-aged individuals. Further studies focusing on the effects of amino acids and their metabolites on skeletal muscle function will provide data essential for the production of food supplements for older adults, athletes, and individuals with metabolic diseases.

Submandibular gland-specific inflammaging-induced hyposalivation in the male senescence-accelerated mouse prone -1 line (SAM-P1)

Aging has pronounced effects on mammalian tissues and cells, but the impacts of aging on salivary gland function are relatively unknown. This study aims to evaluate the effects of aging on submandibular gland (SMG) and parotid gland (PG) functions in the male senescence-accelerated mouse. In vivo analysis at the systemic level revealed that salivary secretion induced by pilocarpine, a muscarinic agonist, from the SMG was significantly decreased in aged mice, whereas salivary secretion from the PG was not affected. To evaluate organ-level function, the SMG was perfused with the muscarinic agonists carbachol and calcium ionophore A23187 ex vivo to induce salivary secretion, and decreased saliva production was also observed in the aged SMG. Histological analysis revealed the presence of CD4-positive lymphocytes infiltrating the aged SMG. Furthermore, real-time PCR revealed that the aged SMG exhibited accelerated cell aging, increased levels of the inflammatory cytokine interleukin-6, and decreased mRNA levels of the water channel protein aquaporin-5 (AQP5). In summary, these results demonstrate that SMG function in aged mice was diminished, and that cell senescence, chronic inflammation, and the decreased gene expression of AQP5 are the likely causes of hyposalivation in the SMG of aged mice.

Essential amino acids exhibit variable effects on protein degradation in rainbow trout (Oncorhynchus mykiss) primary myocytes

The functional role of amino acids as regulators of protein degradation was investigated using primary myogenic precursor cell culture as in vitro model of rainbow trout white muscle. Seven-day old myocytes were starved of amino acids for two hours then exposed to media that contained amino acid treatments, during which protein degradation rates were analyzed over five hours by measuring cellular release of ³H-tyrosine. Increasing concentrations of essential amino acids (EAA) reduced protein degradation rates; this effect was dose-dependent within the physiological range found in plasma. Addition of leucine or phenylalanine at 5 mM and 2.5 mM, respectively, decreased rates of protein degradation compared to media without amino acid supplementation, suggesting that these amino acids directly regulate muscle proteolysis. Protein degradation rates were similar in cells exposed to media without EAA and media lacking only leucine, further supporting a role for leucine as a central regulator of protein turnover. Addition of 5 mM lysine or valine to media without amino acids increased protein degradation; this response was attenuated as EAA were added back into media, supporting that a lysine or valine imbalance is costly for muscle protein retention. In summary, there is evidence for amino acids as both positive and negative regulators of protein turnover in rainbow trout muscle. These findings suggest that there may be an optimal plasma amino acid profile that minimizes protein turnover and that this could be achieved through diet formulation.

L-Lysine Attenuates Hepatic Steatosis in Senescence-Accelerated Mouse Prone 8 Mice

Non-alcoholic hepatic steatosis is a phenotype of metabolic syndrome, and aging is a risk factor for this condition. Senescence-accelerated mouse prone 8 (SAMP8) is a murine model for studying aging-associated disorders. We here investigated the effect of dietary supplementation with L-lysine (Lys) on non-alcoholic hepatic steatosis in SAMP8 mice. Triglyceride (TG) and cholesterol (Chol) accumulated in the livers of SAMP8 mice fed a standard diet at 36 wk of age. However, intake of a Lys-rich diet for 2 mo prevented the accumulation of TG and Chol in the liver. Plasma alanine aminotransferase activity, an index of liver injury, was decreased by Lys. The mRNA expression levels of peroxisome proliferator-activated receptor gamma coactivator 1-α and carnitine palmitoyltransferase 1a, which regulate β-oxidation, were increased in the livers of SAMP8 mice fed the Lys-rich diet. Taken together, our study suggests dietary intake of Lys prevents hepatic steatosis by stimulating β-oxidation in SAMP8 mice.

Metabolic signature of the aging eye in mice

Aging is a major risk factor for age-related ocular diseases including age-related macular degeneration in the retina and retinal pigment epithelium (RPE), cataracts in the lens, glaucoma in the optic nerve, and dry eye syndrome in the cornea. We used targeted metabolomics to analyze metabolites from young (6 weeks) and old (73 weeks) eyes in C57 BL6/J mice. Old mice had diminished electroretinogram responses and decreased number of photoreceptors in their retinas. Among the 297 detected metabolites, 45-114 metabolites are significantly altered in aged eye tissues, mostly in the neuronal tissues (retina and optic nerve) and less in cornea, RPE/choroid, and lens. We noted that changes of metabolites in mitochondrial metabolism and glucose metabolism are common features in the aged retina, RPE/choroid, and optic nerve. The aging retina, cornea, and optic nerve also share similar changes in Nicotinamide adenine dinucleotide (NAD), 1-methylnicotinamides, 3-methylhistidine, and other methylated metabolites. Metabolites in taurine metabolism are strikingly influenced by aging in the cornea and lens. In conclusion, the aging eye has both common and tissue-specific metabolic signatures. These changes may be attributed to dysregulated mitochondrial metabolism, reprogrammed glucose metabolism and impaired methylation in the aging eye. Our findings provide biochemical insights into the mechanisms of age-related ocular changes.