Voluntary wheel running is beneficial to the amino acid profile of lysine-deficient rats

Lysine or Threonine Deficiency Decreases Body Weight Gain in Growing Rats despite an Increase in Food Intake without Increasing Energy Expenditure in Response to FGF21

The objective of this study is to evaluate the effects of a strictly essential amino acid (lysine or threonine; EAA) deficiency on energy metabolism in growing rats. Rats were fed for three weeks severely (15% and 25% of recommendation), moderately (40% and 60%), and adequate (75% and 100%) lysine or threonine-deficient diets. Food intake and body weight were measured daily and indirect calorimetry was performed the week three. At the end of the experimentation, body composition, gene expression, and biochemical analysis were performed. Lysine and threonine deficiency induced a lower body weight gain and an increase in relative food intake. Lysine or threonine deficiency induced liver FGF21 synthesis and plasma release. However, no changes in energy expenditure were observed for lysine deficiency, unlike threonine deficiency, which leads to a decrease in total and resting energy expenditure. Interestingly, threonine severe deficiency, but not lysine deficiency, increase orexigenic and decreases anorexigenic hypothalamic neuropeptides expression, which could explain the higher food intake. Our results show that the deficiency in one EAA, induces a decrease in body weight gain, despite an increased relative food intake, without any increase in energy expenditure despite an induction of FGF21.

Lysine suppresses myofibrillar protein degradation by regulating the autophagic-lysosomal system through phosphorylation of Akt in C2C12 cells

The prevention of muscle wasting is important for maintaining quality of life, since loss of muscle mass can lead to a bedridden state and decreased resistance to diseases. The prevention of muscle wasting requires an increase in protein synthesis and a decrease in protein degradation in skeletal muscle. We previously showed that lysine (Lys) markedly suppressed myofibrillar protein degradation by inhibiting the autophagic-lysosomal system via the mammalian target of rapamycin (mTOR) and other signal molecules in C2C12 cells. In this study, we investigated the involvement of Akt and adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK), two regulators of autophagy, on the suppressive effects of Lys on myofibrillar protein degradation in C2C12 cells. Lys induced the phosphorylation of Akt, but the suppressive effects of Lys on myofibrillar protein degradation and autophagy were completely abolished in the presence of Akt1/2 kinase inhibitor (Akti). Lys suppressed the phosphorylation of AMPK, but this effect was also abolished by Akti. On the other hand, AMPK activation by 5-aminoimidazole-4-carboxamide-1-β-D-ribonucleoside (AICAR) did not affect either Akt activity or the autophagic-lysosomal system in C2C12 cells treated with Lys. These results indicate that regulation of AMPK activity is not essential for the regulation of autophagy by Lys. Taken together, our results show that Lys suppresses myofibrillar protein degradation by the autophagic-lysosomal system through the phosphorylation of Akt in C2C12 cells.

Regulation of skeletal muscle protein degradation and synthesis by oral administration of lysine in rats

Several catabolic diseases and unloading induce muscle mass wasting, which causes severe pathological progression in various diseases and aging. Leucine is known to attenuate muscle loss via stimulation of protein synthesis and suppression of protein degradation in skeletal muscle. The aim of this study was to investigate the effects of lysine intake on protein degradation and synthesis in skeletal muscle. Fasted rats were administered 22.8-570 mg Lys/100 g body weight and the rates of myofibrillar protein degradation were assessed for 0-6 h after Lys administration. The rates of myofibrillar protein degradation evaluated by MeHis release from the isolated muscles were markedly suppressed after administration of 114 mg Lys/100 g body weight and of 570 mg Lys/100 g body weight. LC3-II, a marker of the autophagic-lysosomal pathway, tended to decrease (p=0.05, 0.08) after Lys intake (114 mg/100 g body weight). However, expression of ubiquitin ligase E3 atrogin-1 mRNA and levels of ubiquitinated proteins were not suppressed by Lys intake. Phosphorylation levels of mTOR, S6K1 and 4E-BP1 in the gastrocnemius muscle were not altered after Lys intake. These results suggest that Lys is able to suppress myofibrillar protein degradation at least partially through the autophagic-lysosomal pathway, not the ubiquitin-proteasomal pathway, whereas Lys might be unable to stimulate protein synthesis within this time frame.

Lysine suppresses protein degradation through autophagic-lysosomal system in C2C12 myotubes

Muscle mass is determined between protein synthesis and protein degradation. Reduction of muscle mass leads to bedridden condition and attenuation of resistance to diseases. Moreover, bedridden condition leads to additional muscle loss due to disuse muscle atrophy. In our previous study (Sato et al. 2013), we showed that administered lysine (Lys), one of essential amino acid, suppressed protein degradation in skeletal muscle. In this study, we investigated that the mechanism of the suppressive effects of Lys on skeletal muscle proteolysis in C2C12 cell line. C2C12 myotubes were incubated in the serum-free medium containing 10 mM Lys or 20 mM Lys, and myofibrillar protein degradation was determined by the rates of 3-methylhistidine (MeHis) release from the cells. The mammalian target of rapamycin (mTOR) activity from the phosphorylation levels of p70-ribosormal protein S6 kinase 1 and eIF4E-binding protein 1 and the autophagic-lysosomal system activity from the ratio of LC3-II/I in C2C12 myotubes stimulated by 10 mM Lys for 0-3 h were measured. The rates of MeHis release were markedly reduced by addition of Lys. The autophagic-lysosomal system activity was inhibited upon 30 min of Lys supplementation. The activity of mTOR was significantly increased upon 30 min of Lys supplementation. The suppressive effect of Lys on the proteolysis by the autophagic-lysosomal system was maintained partially when mTOR activity was inhibited by 100 nM rapamycin, suggesting that some regulator other than mTOR signaling, for example, Akt, might also suppress the autophagic-lysosomal system. From these results, we suggested that Lys suppressed the activity of the autophagic-lysosomal system in part through activation of mTOR and reduced myofibrillar protein degradation in C2C12 myotubes.

Enhanced voluntary wheel running in GPRC6A receptor knockout mice

GPRC6A is an amino acid-sensing receptor highly expressed in the brain and in skeletal muscle. Although recent evidence suggests that genetically engineered GPRC6A receptor knockout (KO) mice are susceptible to develop subtle endocrine and metabolic disturbances, the underlying disruptions in energy metabolism are largely unexplored. Based on GPRC6A's expression pattern and ligand preferences, we hypothesize that the receptor may impact energy metabolism via regulating physical activity levels. Thus, in the present study, we exposed GPRC6A receptor KO mice and their wild-type (WT) littermates to voluntary wheel running and forced treadmill exercise. Moreover, we assessed energy expenditure in the basal state, and evaluated the effects of wheel running on food intake, body composition, and a range of exercise-induced central and peripheral biomarkers. We found that adaptation to voluntary wheel running is affected by GPRC6A, as ablation of the receptor significantly enhances wheel running in KO relative to WT mice. Both genotypes responded to voluntary exercise by increasing food intake and improving body composition to a similar degree. In conclusion, these data demonstrate that the GPRC6A receptor is involved in regulating exercise behaviour. Future studies are highly warranted to delineate the underlying molecular details and to assess if these findings hold any translational value.

Severe dietary lysine restriction affects growth and body composition and hepatic gene expression for nitrogen metabolism in growing rats

Dietary lysine restriction may differentially affect body growth and lipid and nitrogen metabolism, depending on the degree of lysine restriction. This study was conducted to examine the effect of dietary lysine restriction on growth and lipid and nitrogen metabolism with two different degree of lysine restriction. Isocaloric amino acid-defined diets containing 1.4% lysine (adequate), 0.70% lysine (50% moderate lysine restriction) and 0.35% lysine (75% severe lysine restriction) were fed from the age of 52 to 77 days for 25 days in male Sprague-Dawley rats. The 75% severe lysine restriction increased (p < 0.05) food intake, but retarded (p < 0.05) growth, increased (p < 0.05) liver and muscle lipid contents and abdominal fat accumulation, increased (p < 0.05) blood urea nitrogen levels and mRNA levels of the serine-synthesizing 3-phosphoglycerate dehydrogenase gene, but decreased (p < 0.05) urea cycle arginase gene mRNA levels. In contrast, the 50% lysine restriction did not significantly (p > 0.05) affect body growth and lipid and nitrogen metabolism. Our results demonstrate that severe 75% lysine restriction has detrimental effects on body growth and deregulate lipid and nitrogen metabolism.

The brain's response to an essential amino acid-deficient diet and the circuitous route to a better meal

The essential (indispensable) amino acids (IAA) are neither synthesized nor stored in metazoans, yet they are the building blocks of protein. Survival depends on availability of these protein precursors, which must be obtained in the diet; it follows that food selection is critical for IAA homeostasis. If even one of the IAA is depleted, its tRNA becomes quickly deacylated and the levels of charged tRNA fall, leading to disruption of global protein synthesis. As they have priority in the diet, second only to energy, the missing IAA must be restored promptly or protein catabolism ensues. Animals detect and reject an IAA-deficient meal in 20 min, but how? Here, we review the molecular basis for sensing IAA depletion and repletion in the brain's IAA chemosensor, the anterior piriform cortex (APC). As animals stop eating an IAA-deficient meal, they display foraging and altered choice behaviors, to improve their chances of encountering a better food. Within 2 h, sensory cues are associated with IAA depletion or repletion, leading to learned aversions and preferences that support better food selection. We show neural projections from the APC to appetitive and consummatory motor control centers, and to hedonic, motivational brain areas that reinforce these adaptive behaviors.

Somatostatin is involved in anorexia in mice fed a valine-deficient diet

The ingestion of a valine (Val)-deficient diet results in a significant reduction of food intake and body weight within 24 h, and this phenomenon continues throughout the period over which such a diet is supplied. Both microarray and real-time PCR analyses revealed that the expression of somatostatin mRNA was increased in the hypothalamus in anorectic mice that received a Val-deficient diet. On the other hand, when somatostatin was administered intracerebroventricularly to intact animals that were fed a control diet, their 24-h food intake decreased significantly. In addition, Val-deficient but not pair-fed mice or those fasted for 24 h showed a less than 0.5-fold decrease in the hypothalamic mRNA expression levels of Crym, Foxg1, Itpka and two unknown EST clone genes and a more than twofold increase in those of Slc6a3, Bdh1, Ptgr2 and one unknown EST clone gene. These results suggest that hypothalamic somatostatin and genes responsive to Val deficiency may be involved in the central mechanism of anorexia induced by a Val-deficient diet.

The seasonal fluctuation of plasma amino acids in aquarium-maintained bottlenose dolphins (Tursiops truncatus)

Although there has been extensive research on plasma amino acid profiles of mammals, there is currently a lack of information on seasonal differences in the concentrations of plasma amino acids specifically in cetaceans. The present study examined the response of the plasma amino acids to seasonal changes in the culture environment after controlling for the effect of sex and age. Significant seasonal changes in plasma carnosine (P=0.012), cystine (P=0.0014), isoleucine (P=0.0042), methionine (P=0.002), ornithine (P=0.0096), and taurine (P=0.032) were observed. These amino acids were mainly related to capacity for exercise, ammonia detoxification, thermoregulation, and osmoregulation. We proposed that optimizing plasma amino acids levels by supplementation of amino acids should be of considerable benefit for aquarium-maintained bottlenose dolphins. This study constitutes a first step towards improving our understanding of the metabolism of aquarium-maintained bottlenose dolphins. We also revealed that the ratio of tryptophan to large neutral amino acids significantly declined (P=0.0076), suggesting reduction in serotonin synthesis in winter and autumn. Although further studies are needed, this finding implied that bottlenose dolphins could produce behavioral changes seasonally by the alteration of serotonin activity. To better understand the metabolic machinery for amino acids that facilitate the adaptation of marine mammals to their environments, it is essential to continue monitoring of and further investigations into relationships between plasma amino acids and specific environmental factors.

Dietary deficiency of essential amino acids rapidly induces cessation of the rat estrous cycle

Reproductive functions are regulated by the sophisticated coordination between the neuronal and endocrine systems and are sustained by a proper nutritional environment. Female reproductive function is vulnerable to effects from dietary restrictions, suggesting a transient adaptation that prioritizes individual survival over reproduction until a possible future opportunity for satiation. This adaptation could also partially explain the existence of amenorrhea in women with anorexia nervosa. Because amino acid nutritional conditions other than caloric restriction uniquely alters amino acid metabolism and affect the hormonal levels of organisms, we hypothesized that the supply of essential amino acids in the diet plays a pivotal role in the maintenance of the female reproductive system. To test this hypothesis, we examined ovulatory cyclicity in female rats under diets that were deficient in threonine, lysine, tryptophan, methionine or valine. Ovulatory cyclicity was monitored by daily cytological evaluations of vaginal smears. After continuous feeding of the deficient diet, a persistent diestrus or anovulatory state was induced most quickly by the valine-deficient diet and most slowly by the lysine-deficient diet. A decline in the systemic insulin-like growth factor 1 level was associated with a dietary amino acid deficiency. Furthermore, a paired group of rats that were fed an isocaloric diet with balanced amino acids maintained normal estrous cyclicity. These disturbances of the estrous cycle by amino acid deficiency were quickly reversed by the consumption of a normal diet. The continuous anovulatory state in this study is not attributable to a decrease in caloric intake but to an imbalance in the dietary amino acid composition. With a shortage of well-balanced amino acid sources, reproduction becomes risky for both the mother and the fetus. It could be viewed as an adaptation to the diet, diverting resources away from reproduction and reallocating them to survival until well-balanced amino acid sources are found.

Characteristic metabolism of free amino acids in cetacean plasma: cluster analysis and comparison with mice

From an evolutionary perspective, the ancestors of cetaceans first lived in terrestrial environments prior to adapting to aquatic environments. Whereas anatomical and morphological adaptations to aquatic environments have been well studied, few studies have focused on physiological changes. We focused on plasma amino acid concentrations (aminograms) since they show distinct patterns under various physiological conditions. Plasma and urine aminograms were obtained from bottlenose dolphins, pacific white-sided dolphins, Risso's dolphins, false-killer whales and C57BL/6J and ICR mice. Hierarchical cluster analyses were employed to uncover a multitude of amino acid relationships among different species, which can help us understand the complex interrelations comprising metabolic adaptations. The cetacean aminograms formed a cluster that was markedly distinguishable from the mouse cluster, indicating that cetaceans and terrestrial mammals have quite different metabolic machinery for amino acids. Levels of carnosine and 3-methylhistidine, both of which are antioxidants, were substantially higher in cetaceans. Urea was markedly elevated in cetaceans, whereas the level of urea cycle-related amino acids was lower. Because diving mammals must cope with high rates of reactive oxygen species generation due to alterations in apnea/reoxygenation and ischemia-reperfusion processes, high concentrations of antioxidative amino acids are advantageous. Moreover, shifting the set point of urea cycle may be an adaption used for body water conservation in the hyperosmotic sea water environment, because urea functions as a major blood osmolyte. Furthermore, since dolphins are kept in many aquariums for observation, the evaluation of these aminograms may provide useful diagnostic indices for the assessment of cetacean health in artificial environments in the future.