Triennial Growth Symposium: leucine acts as a nutrient signal to stimulate protein synthesis in neonatal pigs

Subcellular protein turnover in human neural progenitor cells revealed by correlative electron microscopy and nanoscale secondary ion mass spectrometry imaging

Protein turnover is a critical process for accurate cellular function, in which damaged proteins in the cells are gradually replaced with newly synthesized ones. Many previous studies on cellular protein turnover have used stable isotopic labelling by amino acids in cell culture (SILAC), followed by proteomic bulk analysis. However, this approach does not take into account the heterogeneity observed at the single-cell and subcellular levels. To address this, we investigated the protein turnover of neural progenitor cells at the subcellular resolution, using correlative TEM and NanoSIMS imaging, relying on a pulse-chase analysis of isotopically-labelled protein precusors. Cellular protein turnover was found significantly heterogenous across individual organelles, which indicates a possible relation between protein turnover and subcellular activity. In addition, different isotopically-labelled amino acids provided different turnover patterns, in spite of all being protein precursors, suggesting that they undergo distinct protein synthesis and metabolic pathways at the subcellular level.

Changes in Skeletal Muscle Protein Metabolism Signaling Induced by Glutamine Supplementation and Exercise

AIM

To evaluate the effects of resistance exercise training (RET) and/or glutamine supplementation (GS) on signaling protein synthesis in adult rat skeletal muscles.

METHODS

The following groups were studied: (1) control, no exercise (C); (2) exercise, hypertrophy resistance exercise training protocol (T); (3) no exercise, supplemented with glutamine (G); and (4) exercise and supplemented with glutamine (GT). The rats performed hypertrophic training, climbing a vertical ladder with a height of 1.1 m at an 80° incline relative to the horizontal with extra weights tied to their tails. The RET was performed three days a week for five weeks. Each training session consisted of six ladder climbs. The extra weight load was progressively increased for each animal during each training session. The G groups received daily L-glutamine by gavage (one g per kilogram of body weight per day) for five weeks. The C group received the same volume of water during the same period. The rats were euthanized, and the extensor digitorum longus (EDL) muscles from both hind limbs were removed and immediately weighed. Glutamine and glutamate concentrations were measured, and histological, signaling protein contents, and mRNA expression analyses were performed.

RESULTS

Supplementation with free L-glutamine increased the glutamine concentration in the EDL muscle in the C group. The glutamate concentration was augmented in the EDL muscles from T rats. The EDL muscle mass did not change, but a significant rise was reported in the cross-sectional area (CSA) of the fibers in the three experimental groups. The levels of the phosphorylated proteins (pAkt/Akt, pp70S6K/p70S6K, p4E-BP1/4E-BP1, and pS6/S6 ratios) were significantly increased in EDL muscles of G rats, and the activation of p4E-BP1 was present in T rats. The fiber CSAs of the EDL muscles in T, G, and GT rats were increased compared to the C group. These changes were accompanied by a reduction in the 26 proteasome activity of EDL muscles from T rats.

CONCLUSION

Five weeks of GS and/or RET induced muscle hypertrophy, as indicated by the increased CSAs of the EDL muscle fibers. The increase in CSA was mediated via the upregulated phosphorylation of Akt, 4E-BP1, p70S6k, and S6 in G animals and 4E-BP1 in T animals. In the EDL muscles from T animals, a decrease in proteasome activity, favoring a further increase in the CSA of the muscle fibers, was reported.

The effect of supplementation of essential amino acid combinations in a low crude protein diet on growth performance in weanling pigs

The present study investigated the impact of providing different supplemental essential amino acids (EAA) in a low crude protein (CP) diet on growth performance in weanling pigs. A total of 324 mixed-sex 24-d weaned piglets (initial BW 6.9 ± 0.34 kg) were used in a 27-d growth trial with six dietary treatments immediately post-weaning. The first two treatments were a control standard CP (19%) diet (positive control; PC) and a negative control (NC) diet with low CP (16%) and reduced Ile, Leu, and histidine levels. The rest of the treatments had low CP with varied EAA types and levels; T1 had similar Ile, Leu, and His levels as PC but with low CP (16%), while T2 had low CP and 10% higher His, Thr, Trp, and Met+Cys compared to PC. The T3 was a low CP diet with 10% supplemental Leu, Ile, and Val compared to PC, while T4 was a low CP diet with 10% supplementation with all the EAA except Lys compared to PC. The initial body weight (BW) was not statistically different (P > 0.05) among the treatments. Also, on d 6, no statistical differences in BW were observed among the treatments. The average BW recorded on d 13, 20, and 27 showed significant treatment differences where the PC had consistently higher BW than all the other treatments (P < 0.05). The average daily gain (ADG) of the PC was higher than the rest of the treatments. Between d 13 and 20, the average daily feed intake (ADFI) for PC was not different from NC and T1 (P > 0.05), but compared to T2, T3, and T4, the PC treatment showed a high ADFI (P < 0.05). Overall (d 0-27), the ADFI for PC was not different from T1 and was significantly higher than all other treatments. Overall, results showed that the gain to feed (G:F) ratio was higher (P < 0.05) for PC compared to other dietary treatments. In summary, although the treatments (T1-T4) consisted of varying levels of EAA above the recommended requirement levels for optimal performance, we did not see a significant impact on growth performance improvement, which may indicate that the targeted EAA (His, Val, Thr, lle, Leu, Trp, and Met) may not have been limiting in these diets. On the other hand, the phenylalanine (Phe) requirement may be limited in the current formulations, or perhaps the EAA: total N ratio in T1, T2, T3, and T4 may have been too high, resulting in the inefficiency of EAA utilization for growth.

Role of Branched-chain Amino Acid Metabolism in Tumor Development and Progression

Branched-chain amino acids (BCAAs), isoleucine, leucine and valine, are essential amino acids with vital roles in protein synthesis and energy production. We reviewed the fundamentals of BCAA metabolism in advanced cancer patients. BCAAs and various catabolic products act as signalling molecules, which activate mechanisms ranging from protein synthesis to insulin secretion. Recently, BCAA metabolism has been suggested to contribute to cancer progression. Of particular interest is the modulation of the mTOR activity by BCAAs. There are likely multiple pathways involved in BCAA metabolism implicated in carcinogenesis. Understanding the mechanism(s) underlying altered BCAAs metabolism will significantly advance the current understanding of nutrient involvement in carcinogenesis and direct future studies to unravel the significance of BCCA metabolites in tumor development and progression.

Plant Proteins for Future Foods: A Roadmap

Protein calories consumed by people all over the world approximate 15-20% of their energy intake. This makes protein a major nutritional imperative. Today, we are facing an unprecedented challenge to produce and distribute adequate protein to feed over nine billion people by 2050, in an environmentally sustainable and affordable way. Plant-based proteins present a promising solution to our nutritional needs due to their long history of crop use and cultivation, lower cost of production, and easy access in many parts of the world. However, plant proteins have comparatively poor functionality, defined as poor solubility, foaming, emulsifying, and gelling properties, limiting their use in food products. Relative to animal proteins, including dairy products, plant protein technology is still in its infancy. To bridge this gap, advances in plant protein ingredient development and the knowledge to construct plant-based foods are sorely needed. This review focuses on some salient features in the science and technology of plant proteins, providing the current state of the art and highlighting new research directions. It focuses on how manipulating plant protein structures during protein extraction, fractionation, and modification can considerably enhance protein functionality. To create novel plant-based foods, important considerations such as protein-polysaccharide interactions, the inclusion of plant protein-generated flavors, and some novel techniques to structure plant proteins are discussed. Finally, the attention to nutrition as a compass to navigate the plant protein roadmap is also considered.

The efficacy of essential amino acid supplementation for augmenting dietary protein intake in older adults: implications for skeletal muscle mass, strength and function

The primary aim of this review is to evaluate the efficacy of essential amino acid (EAA) supplementation as a strategy to increase dietary protein intake and improve muscle mass, strength and function in older adults. A sufficient daily protein intake is widely recognised to be fundamental for the successful management of sarcopenia in older undernourished adults. In practice, optimising protein intakes in older adults is complex, requiring consideration of the dose and amino acid composition (i.e. a complete EAA profile and abundant leucine content) of ingested protein on a per meal basis, alongside the age-related decline in appetite and the satiating properties of protein. Recent studies in older adults demonstrate that EAA-based supplements are non-satiating and can be administered alongside food to enhance the anabolic properties of a meal containing a suboptimal dose of protein; an effect magnified when combined with resistance exercise training. These findings support the notion that EAA supplementation could serve as an effective strategy to improve musculoskeletal health in older adults suffering from non-communicable diseases such as sarcopenia. Compliance is critical for the long-term success of complex interventions. Hence, aspects of palatability and desire to eat are important considerations regarding EAA supplementation. In conclusion, EAA-based supplements enriched with l-leucine offer an alternative strategy to whole protein sources to assist older adults in meeting protein recommendations. In practice, EAA supplements could be administered alongside meals of suboptimal protein content, or alternatively between meals on occasions when older adults achieve their per meal protein intake recommendations.

Acute Effect of a Protein Supplement on Targeted Plasma Amino Acid Profile among Healthy Asian Indians: A Randomized Controlled Trial

Background

Indians have a poor protein intake in terms of quantity as well as quality because of their predominantly cereal-based diet. However, there is limited information on circulatory amino acid levels in healthy Indians. Herein, we evaluated the acute effect of a protein supplement on the plasma levels of essential amino acids (EAAs) in healthy Indian adults, using targeted EAA analysis.

Methods

In this double-blind, randomized, crossover study, 20 healthy Indian adults were randomized to receive the test protein supplement (treatment arm, n = 10) or placebo (control arm, n = 10) with milk, after overnight fasting. After 7 days, the participants returned for the crossover treatment. Blood samples were collected at baseline and at 60 and 120 min after protein/placebo consumption. Plasma EAA levels were estimated using liquid chromatography-tandem mass spectrometry. Repeated measures ANOVA was performed to assess the effect of treatment on EAA levels. P values < 0.05 were considered statistically significant.

Results

At baseline, mean plasma levels did not differ significantly between the two arms for any of the EAAs. In the treatment arm, the mean levels of all EAAs increased significantly from baseline to 60 min (P < 0.01), with no significant change from 60 to 120 min. There was no significant change in amino acid levels in the control arm. The magnitude as well as percentage of increase from baseline to 60 min was significantly greater in the treatment arm than in the control arm for all EAAs.

Conclusion

Compared to placebo, protein supplement increased circulatory amino acid levels in healthy Indians. The observed increase in EAA levels and its role in conjunction with exercise in both healthy and diseased states need to be further evaluated. This is the first dataset exploring targeted EAA profiles and the effect of a protein supplement among healthy Indians. The clinical trial is registered with CTRI/2018/12/016777.

The Role of the Anabolic Properties of Plant- versus Animal-Based Protein Sources in Supporting Muscle Mass Maintenance: A Critical Review

Plant-sourced proteins offer environmental and health benefits, and research increasingly includes them in study formulas. However, plant-based proteins have less of an anabolic effect than animal proteins due to their lower digestibility, lower essential amino acid content (especially leucine), and deficiency in other essential amino acids, such as sulfur amino acids or lysine. Thus, plant amino acids are directed toward oxidation rather than used for muscle protein synthesis. In this review, we evaluate the ability of plant- versus animal-based proteins to help maintain skeletal muscle mass in healthy and especially older people and examine different nutritional strategies for improving the anabolic properties of plant-based proteins. Among these strategies, increasing protein intake has led to a positive acute postprandial muscle protein synthesis response and even positive long-term improvement in lean mass. Increasing the quality of protein intake by improving amino acid composition could also compensate for the lower anabolic potential of plant-based proteins. We evaluated and discussed four nutritional strategies for improving the amino acid composition of plant-based proteins: fortifying plant-based proteins with specific essential amino acids, selective breeding, blending several plant protein sources, and blending plant with animal-based protein sources. These nutritional approaches need to be profoundly examined in older individuals in order to optimize protein intake for this population who require a high-quality food protein intake to mitigate age-related muscle loss.

Meta-regression analysis to predict the influence of branched-chain and large neutral amino acids on growth performance of pigs1

A meta-analysis was conducted to evaluate the effects of branched-chain amino acids (BCAA), their interactions, and interactions with large neutral amino acids (LNAA) to develop prediction equations for growth performance of pigs. Data from 25 papers, published from 1995 to 2018, for a total of 44 trials and 210 observations were recorded in a database. Diets were reformulated using the NRC (2012) loading values to estimate nutrient concentrations. The response variables were average daily gain (ADG), average daily feed intake (ADFI), and gain-to-feed ratio (G:F). The predictor variables tested included average body weight (BW), crude protein, neutral detergent fiber, Ile:Lys, Leu:Lys, Val:Lys, BCAA:Lys, Ile:Leu, Val:Leu, Ile:Val, (Ile+Val):Leu, Trp:Lys, Leu:Trp, Ile:Trp, Val:Trp, BCAA:Trp, Met:Lys, Leu:Met, Ile:Met, Val:Met, BCAA:Met, His:Lys, Leu:His, Ile:His, Val:His, BCAA:His, Thr:Lys, Leu:Thr, Ile:Thr, Val:Thr, BCAA:Thr, (Phe+Tyr):Lys, Leu:(Phe+Tyr), Ile:(Phe+Tyr), Val:(Phe+Tyr), BCAA:(Phe+Tyr), LNAA:Lys, Leu:LNAA, Ile:LNAA, Val:LNAA, and BCAA:LNAA. Amino acids were expressed on standardized ileal digestible basis. The MIXED procedure of SAS (SAS Institute Inc., Cary, NC) was used to develop the equations. The inverse of squared SEM was used to account for heterogeneous errors using the WEIGHT statement. Models were selected with a step-wise manual forward selection. In order to be included in the final model, predictor variables had to be statistically significant (P < 0.05) and provide an improvement of at least 2 points in Bayesian information criterion. The optimum equations were: ADG, g = - 985.94 + (15.2499 × average BW (kg)) - (0.08885 × average BW × average BW) + (1.063 × Leu:Lys) + (20.2659 × Ile:Lys) - (0.1479 × Ile:Lys × Ile:Lys) + (9.2243 × (Ile+Val):Leu) - (0.03321 × (Ile+Val):Leu × (Ile+Val):Leu) - (0.4413 × Ile:Trp); G:F, g/kg = 648.3 - (6.2974 × average BW (kg)) + (0.02051 × average BW × average BW) + (0.5396 × Ile:Lys) + (1.7284 × Val:Lys) - (0.00795 × Val:Lys × Val:Lys) - (1.7594 × Met:Lys); and ADFI, kg = predicted ADG/predicted G:F. Overall, the prediction equations suggest that increasing Leu:Lys negatively impacts ADG due to a reduction in G:F and ADFI caused by insufficient levels of other BCAA and LNAA relative to Leu. According to the model, the addition of Val, Ile, and Trp, alone or in combination, has the potential to counteract the negative effects of high dietary Leu concentrations on growth performance.

Leucine Promotes the Growth of Fetal Pigs by Increasing Protein Synthesis through the mTOR Signaling Pathway in Longissimus Dorsi Muscle at Late Gestation

Leucine (Leu) plays an important role in protein synthesis and metabolism. The present study tested whether Leu supplementation in the diet for sows during late pregnancy could improve piglet birth weight, and it also investigated the possible underlying mechanism. Two hundred sows at day 70 of pregnancy were selected and assigned to four groups fed with following four diets until farrowing, respectively: corn and soybean meal-based diet group (CON), CON + 0.40% Leu, CON + 0.80% Leu, and CON + 1.20% Leu. We found that supplementing with 0.80% Leu significantly increased mean piglet birth weight ( P < 0.05). Supplementation with 0.40, 0.80, and 1.20% Leu increased the plasma concentration of Leu, while decreasing the plasma concentrations of valine (Val) and isoleucine (Ile) in both farrowing sows and newborn piglets ( P < 0.05). The protein expressions of amino acid transporters (including LAT1, SNAT1, SNAT2, 4F2hc, and rBAT) in duodenum, jejunum, ileum, longissimus dorsi muscle of newborn piglets, and placenta of sows showed a difference among the CON group and Leu supplemented groups. Expressions of p-mTOR, p-4E-BP1, and p-S6K1 in longissimus dorsi muscle were also enhanced in each of the supplemental Leu groups compared to CON ( P < 0.05). Collectively, these results indicated that 0.40-0.80% Leu supplementation during late gestation enhanced birth weight of fetal pigs by increasing protein synthesis through modulation of the plasma amino acids profile, amino acid transporters expression, and mTOR signaling pathway.

Leucine modulates the IPEC-J2 cell proteome associated with cell proliferation, metabolism and phagocytosis

Leucine can affect intestinal protein expressions, and improve mucosal immune function. However, little study has been conducted to determine the change of protein component by leucine treatment in intestine epithelial cells. The present study was to cover the key proteins and cell pathways that could be regulated by leucine treatment in porcine intestinal epithelial cell line (IPEC-J2) cells with the approach of proteome analysis. A total number of 3,211 proteins were identified in our approach by searching the database of Uniprot sus scrofa. Among identified proteins, there were 101 proteins expressed differently between control group and leucine group. Compared with the control group, there were 50 up-regulated proteins and 51 down-regulated proteins in leucine group. In these proteins, leucine treatment decreased the expression of some proteins including pyruvate kinase, glyceraldehyde-3-phosphate dehydrogenase, E3 ubiquitin ligase, cathepsin D, caspase 3 and caspase 6, and increased the levels of some proteins, such as some eukaryotic translation initiation factors, ubiquitin carboxyl-terminal hydrolase, DNA-related RNA polymerase II, urokinase plasminogen activator, cyclin-dependent kinase inhibitor 2b, MutL homolog 1, 5-methylcytosine binding domain 4, polymerase δ, α-tubulin, syntaxin 18, Ras homolog D, actin related protein 2/3 complex and cofilin. Via the analysis of Gene Ontology and pathways, these proteins in IPEC-J2 cells were related with some physiological functions, such as protein metabolism, glycolysis, cell proliferation, apoptosis and phagocytosis. Thus, these results suggest that leucine affects gut barrier function possibly via regulating cell proliferation and apoptosis, metabolism and phagocytosis.

Leucine Protects Against Skeletal Muscle Atrophy in Lipopolysaccharide-Challenged Rats

Skeletal muscle atrophy is a decrease in muscle mass that occurs when protein degradation exceeds protein synthesis. Leucine (Leu), an essential branched-chain amino acid in animal nutrition, regulates skeletal muscle protein metabolism. Two experiments were conducted to evaluate whether Leu could alleviate lipopolysaccharide (LPS)-induced skeletal muscle wasting by modulating skeletal muscle protein synthesis and degradation. A total of 24 rats were randomly allocated into three groups (n = 8): (1) non-challenged control; (2) LPS-challenged control; and (3) LPS +3.0% Leu. Rats were fed with control or Leu-supplemented (part of the casein was replaced with 3.0% Leu) diets throughout the trial and were injected intraperitoneally with sterile saline or LPS at days 6, 11, 16, and 21. On the morning of day 22, serum samples were collected and rats were then sacrificed for liver and muscle analysis. In vitro protein degradation, nuclear factor-κB (NF-κB) activity, and proteolytic enzyme activities of the muscles from immune-challenged rats were also measured. Our results showed that the LPS challenge resulted in not only enhanced serum interleukin-1 and liver C-reactive protein (CRP) concentrations but also decreased the average daily body weight gain and muscle fiber diameter. However, dietary Leu inclusion attenuated the increase in CRP level and the decrease in muscle fiber diameter. Importantly, the LPS challenge caused a significant elevation in the muscle proteolysis rate, but dietary Leu supplementation significantly blocked the muscle proteolysis. The mRNA expression of NF-κB, muscle atrophy F-box (MAFbx), and muscle ring finger 1 (MuRF1) was upregulated by the LPS challenge in gastrocnemius muscles, but was downregulated by Leu supplementation. Interestingly, when muscles from the LPS-challenged rats were incubated with Leu in vitro, proteasome-, calpain-, and cathepsin-L-dependent muscle proteolysis and NF-κB activity were decreased. Collectively, the data suggest that Leu supplementation could inhibit excessive skeletal muscle degradation, as well as enhance protein synthesis and, thus, attenuate the negative effects caused by the LPS-induced immune challenge.

Enteral β-hydroxy-β-methylbutyrate supplementation increases protein synthesis in skeletal muscle of neonatal pigs

Many low-birth weight infants are at risk for poor growth due to an inability to achieve adequate protein intake. Administration of the amino acid leucine stimulates protein synthesis in skeletal muscle of neonates. To determine the effects of enteral supplementation of the leucine metabolite β-hydroxy-β-methylbutyrate (HMB) on protein synthesis and the regulation of translation initiation and degradation pathways, overnight-fasted neonatal pigs were studied immediately (F) or fed one of five diets for 24 h: low-protein (LP), high-protein (HP), or LP diet supplemented with 4 (HMB4), 40 (HMB40), or 80 (HMB80) μmol HMB·kg body wt(-1)·day(-1) Cell replication was assessed from nuclear incorporation of BrdU in the longissimus dorsi (LD) muscle and jejunum crypt cells. Protein synthesis rates in LD, gastrocnemius, rhomboideus, and diaphragm muscles, lung, and brain were greater in HMB80 and HP and in brain were greater in HMB40 compared with LP and F groups. Formation of the eIF4E·eIF4G complex and S6K1 and 4E-BP1 phosphorylation in LD, gastrocnemius, and rhomboideus muscles were greater in HMB80 and HP than in LP and F groups. Phosphorylation of eIF2α and eEF2 and expression of SNAT2, LAT1, MuRF1, atrogin-1, and LC3-II were unchanged. Numbers of BrdU-positive myonuclei in the LD were greater in HMB80 and HP than in the LP and F groups; there were no differences in jejunum. The results suggest that enteral supplementation with HMB increases skeletal muscle protein anabolism in neonates by stimulation of protein synthesis and satellite cell proliferation.

Pulsatile delivery of a leucine supplement during long-term continuous enteral feeding enhances lean growth in term neonatal pigs

Neonatal pigs are used as a model to study and optimize the clinical treatment of infants who are unable to maintain oral feeding. Using this model, we have shown previously that pulsatile administration of leucine during continuous feeding over 24 h via orogastric tube enhanced protein synthesis in skeletal muscle compared with continuous feeding alone. To determine the long-term effects of leucine pulses, neonatal piglets (n = 11-12/group) were continuously fed formula via orogastric tube for 21 days, with an additional parenteral infusion of either leucine (CON + LEU; 800 μmol·kg^-1·h^-1) or alanine (CON + ALA) for 1 h every 4 h. The results show that body and muscle weights and lean gain were ∼25% greater, and fat gain was 48% lower in CON + LEU than CON + ALA; weights of other tissues were unaffected by treatment. Fractional protein synthesis rates in longissimus dorsi, gastrocnemius, and soleus muscles were ∼30% higher in CON + LEU compared with CON + ALA and were associated with decreased Deptor abundance and increased mTORC1, mTORC2, 4E-BP1, and S6K1 phosphorylation, SNAT2 abundance, and association of eIF4E with eIF4G and RagC with mTOR. There were no treatment effects on PKB, eIF2α, eEF2, or PRAS40 phosphorylation, Rheb, SLC38A9, v-ATPase, LAMTOR1, LAMTOR2, RagA, RagC, and LAT1 abundance, the proportion of polysomes to nonpolysomes, or the proportion of mRNAs encoding rpS4 or rpS8 associated with polysomes. Our results demonstrate that pulsatile delivery of a leucine supplement during 21 days of continuous enteral feeding enhances lean growth by stimulating the mTORC1-dependent translation initiation pathway, leading to protein synthesis in skeletal muscle of neonates.

Effects of different doses of leucine ingestion following eight weeks of resistance exercise on protein synthesis and hypertrophy of skeletal muscle in rats

[Purpose]

This study was designed to determine the appropriate Leucine intake volume to obtain the effects of restoring damaged muscle through the synthesis of muscle proteins to increase skeletal muscle and improve exercise performance, and to achieve enhanced muscle hypertrophy.

[Methods]

To clarify the effects of leucine on skeletal muscle hypertrophy of SD rats, following eight weeks of resistance exercise (climbing ladder), the mass of the FHL (Flexor hallucis longus) was measured after extraction, after which change in the activity of muscle signaling proteins (PKB/Akt, mTOR, p70S6K, 4EBP1) was analyzed.

[Results]

The expressions of PKB/Akt, mTOR and p70S6K were increased in L5 (Leucine 50% administration group) compared with the control group (CON) and exercise group (Ex, exercise training group); EL1 (exercise + 10% leucine administration group) and EL5 (exercise + 50% Leucine administration) also exhibited increased expressions of PKB/Akt, mTOR, and p70S6K, while no difference between EL1 and EL5 were observed. No significant differences in 4EBP1 were found among any of the groups. In addition, there were no differences in FHL mass, while relative mass (FHL/body mass) was increased in the exercise group (Ex, EL1, EL5) compared with the control group. No differences were observed among the exercise groups.

[Conclusion]

The present study demonstrated that the relative body mass was increased in the EX group compared with the CON group, while no significant differences in muscle mass could be found among the groups. Even though some signaling proteins were increased, or some differences existed among groups, there were no differences in muscle mass between the leucine administration and exercise training combined with leucine administration groups in the present study.

Bolus vs. continuous feeding to optimize anabolism in neonates

PURPOSE OF REVIEW

Neonates with feeding difficulties can be fed by orogastric tube, using either continuous or bolus delivery. This review reports on recent findings that bolus is advantageous compared to continuous feeding in supporting optimal protein anabolism.

RECENT FINDINGS

Whether bolus or continuous feeding is more beneficial has been controversial, largely due to limitations inherent in clinical studies, such as the presence of confounding variables and the inability to use invasive approaches. Recent studies using the piglet as a model of the human neonate showed that, compared to continuous feeding, bolus feeding enhances protein synthesis and promotes greater protein deposition. The increase in protein synthesis occurs in muscles of varying fiber type and in visceral tissues whereas muscle protein degradation is largely insensitive to feeding pattern. This higher protein synthesis rate is enabled by the rapid and profound increases in circulating amino acids and insulin that occur following a bolus feed, which activate the intracellular signaling pathways leading to mRNA translation.

SUMMARY

Recent findings indicate that bolus feeding enhances protein synthesis more than continuous feeding and promotes greater protein anabolism. The difference in response is attributable to the pulsatile pattern of amino acid-induced and insulin-induced translation initiation induced only by bolus feeding.

Leucine supplementation improves skeletal muscle regeneration after cryolesion in rats

This study was undertaken in order to provide further insight into the role of leucine supplementation in the skeletal muscle regeneration process, focusing on myofiber size and strength recovery. Young (2-month-old) rats were subjected or not to leucine supplementation (1.35 g/kg per day) started 3 days prior to cryolesion. Then, soleus muscles were cryolesioned and continued receiving leucine supplementation until 1, 3 and 10 days later. Soleus muscles from leucine-supplemented animals displayed an increase in myofiber size and a reduction in collagen type III expression on post-cryolesion day 10. Leucine was also effective in reducing FOXO3a activation and ubiquitinated protein accumulation in muscles at post-cryolesion days 3 and 10. In addition, leucine supplementation minimized the cryolesion-induced decrease in tetanic strength and increase in fatigue in regenerating muscles at post-cryolesion day 10. These beneficial effects of leucine were not accompanied by activation of any elements of the phosphoinositide 3-kinase/Akt/mechanistic target of rapamycin signalling pathway in the regenerating muscles. Our results show that leucine improves myofiber size gain and strength recovery in regenerating soleus muscles through attenuation of protein ubiquitination. In addition, leucine might have therapeutic effects for muscle recovery following injury and in some muscle diseases.

The suckling piglet as an agrimedical model for the study of pediatric nutrition and metabolism

The neonatal pig ranks among the most prominent research models for the study of pediatric nutrition and metabolism. Its precocial development at birth affords ready adaptation to artificial rearing systems, and research using this model spans a wide array of nutrients. Sophisticated in vitro and in vivo methodologies supporting both invasive, reduction-science research as well as whole-animal preclinical investigations have been developed. Potential applications may dually benefit both agricultural and medical sciences (e.g., "agrimedical research"). The broad scope of this review is to outline the fundamental elements of the piglet model and to highlight key aspects of relevance to various macronutrients, including lipids, carbohydrates, proteins/amino acids, and calcium/phosphorus. The review examines similarities between piglets and infants and also piglet idiosyncrasies, concluding that, overall, the piglet represents an adaptable and robust model for pediatric nutrition and metabolism research.

Muscle free amino acid profiles are related to differences in skeletal muscle growth between single and twin ovine fetuses near term

Twin sheep fetuses have reduced skeletal muscle weight near birth relative to singles as a result of restricted muscle hypertrophy. Intracellular free amino acids (FAA) are reported to regulate metabolic pathways which control muscle protein accretion, whereby reduced intracellular content of specific FAA may reduce their activation and therefore, muscle hypertrophy. The aim of this study was to determine whether differences in muscle weight between singleton and twin fetuses, under different maternal conditions is associated with reduced concentration of specific FAA. The FAA content in the semitendinosus muscle (ST) in singleton and twin fetuses (rank) at 140 days of gestation from heavy (H) or light (L) ewes fed ad libitum (A) or maintenance (M) level of nutrition was measured. Muscle weight was reduced in twin fetuses compared to singletons in all groups. Reduced concentrations of leucine, threonine and valine, but higher concentrations of methionine, ornithine, lysine and serine were found in twin fetuses compared to singletons. Maternal size and nutrition interaction with rank resulted in reduced glutamine in twins from HM-ewes (H-ewes under M nutrition) compared to their singleton counterparts. Maternal weight interaction with pregnancy rank reduced the concentration of arginine in twins, with a larger effect on H-ewes compared with L-ewes. Maternal size interaction with pregnancy rank resulted in twins from M-ewes to have lower alanine, while twins from A-ewes had lower aspartic acid concentration compared to singletons. The ST muscle weight was positively correlated only with arginine concentration after taking into account rank, size and nutrition. The present results indicate that reduced concentrations of specific intracellular FAA, such as arginine, leucine, valine, glutamine, which are known to play a role in muscle growth, could be acting as limiting factors for muscle hypertrophy in twin fetuses during late gestation. Ewe size and nutrition can influence the concentration of specific FAA in muscle and should be considered in any intervention plan to improve twin fetal muscle growth.

Leucine pulses enhance skeletal muscle protein synthesis during continuous feeding in neonatal pigs

Infants unable to maintain oral feeding can be nourished by orogastric tube. We have shown that orogastric continuous feeding restricts muscle protein synthesis compared with intermittent bolus feeding in neonatal pigs. To determine whether leucine infusion can be used to enhance protein synthesis during continuous feeding, neonatal piglets received the same amount of formula enterally by orogastric tube for 25.25 h continuously (CON) with or without LEU or intermittently by bolus every 4 h (BOL). For the CON+LEU group, leucine pulses were administered parenterally (800 μmol·kg(-1)·h(-1)) every 4 h. Insulin and glucose concentrations increased after the BOL meal and were unchanged in groups fed continuously. LEU infusion during CON feeding increased plasma leucine after the leucine pulse and decreased essential amino acids compared with CON feeding. Protein synthesis in longissimus dorsi (LD), gastrocnemius, and soleus muscles, but not liver or heart, were greater in CON+LEU and BOL than in the CON group. BOL feeding increased protein synthesis in the small intestine. Muscle S6K1 and 4E-BP1 phosphorylation and active eIF4E·eIF4G complex formation were higher in CON+LEU and BOL than in CON but AMPKα, eIF2α, and eEF2 phosphorylation were unchanged. LC3-II-to-total LC3 ratio was lower in CON+LEU and BOL than in CON, but there were no differences in atrogin-1 and MuRF-1 abundance and FoxO3 phosphorylation. In conclusion, administration of leucine pulses during continuous orogastric feeding in neonates increases muscle protein synthesis by stimulating translation initiation and may reduce protein degradation via the autophagy-lysosome, but not the ubiquitin-proteasome pathway.

Inhibition of mammalian target of rapamycin signaling by CCI-779 (temsirolimus) induces growth inhibition and cell cycle arrest in Cashmere goat fetal fibroblasts (Capra hircus)

The mammalian target of rapamycin (mTOR) is a Ser/Thr kinase. It plays an evolutionarily conserved role in regulating cell growth, proliferation, survival, and metabolism via different cellular processes. The purpose of this study was to explore the inhibitory effects of CCI-779 (temsirolimus), a specific mTOR inhibitor, on mTOR signaling, and examine the mechanism of cell growth suppression by CCI-779 in Cashmere goat fetal fibroblasts (GFb cells). GFb cells were sensitive to CCI-779 and the survival rate of cells treated with >3.0 μM of CCI-779 was significantly reduced compared with the control (p<0.01). CCI-779 inhibited the phosphorylation of mTOR (at Ser2448) and S6 (at Ser240/244), and the expression of mTOR, p70S6K, and S6. Thus, CCI-779 was toxic to GFb cells, and it induced a dose-dependent decrease in cell proliferation and caused G1/S cell cycle arrest. Taken together, these data show that CCI-779 can inhibit mTOR signaling and proliferation in GFb cells in vitro. Therefore, mTOR is an important regulator for GFb cell growth and proliferation.

Molecular characterization and functional analysis of Cashmere goat mammalian target of rapamycin

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved protein kinase that belongs to the phosphatidylinositol kinase-related kinase family. We describe our molecular characterization of mTOR and its function (GenBank accession HM114224) in Cashmere goat (Capra hircus). The goat mTOR complementary DNA is 8617 bp, comprising an open reading frame of 7650 bp--corresponding to a polypeptide of 2549 amino acids--and a 909 bp 3' untranslated region with a polyA tract and a polyadenylation signal at nucleotides 8575-8580. In a bioinformatics analysis, goat mTOR has typical sites of activity and domains. mTOR mRNA was measured in brain, heart, testis, liver, spleen, kidney, and lung by real-time polymerase chain reaction, and the expression of mTOR in fetal fibroblasts was detected by western blot. The viability of fetal fibroblasts was inhibited on treatment with CCI-779, a specific inhibitor of mTOR. Our data supplied evidence that the transcription of mTOR was detected in the seven tissues in Cashmere goat, and mTOR protein was translated in fetal fibroblasts. The proliferation of fetal fibroblasts decreases on inhibition of mTOR.