Activation by insulin and amino acids of signaling components leading to translation initiation in skeletal muscle of neonatal pigs is developmentally regulated

Can milk replacer allowance affect animal performance, body development, metabolism, and skeletal muscle hypertrophy in pre-weaned dairy kids?

We aimed to evaluate performance, body development, metabolism, and expression of genes related to skeletal muscle hypertrophy in non-castrated male dairy kids fed with different levels of MR during the pre-weaning period. Sixty newborn male kids, not castrated, from Saanen and Swiss Alpine breeds, with an average body weight (BW) of 3.834 ± 0.612 kg, were distributed in a randomized block design. Breeds were the block factor in the model (random effect). Kids were allocated into 2 nutrition plans (n = 30 kids per treatment) categorized as follows: low nutritional plan (LNP; 1L MR/kid/day) or high nutritional plan (HNP; 2L MR/kid/day). All kids were harvested at 45 d of life. The majority of nitrogen balance variables were affected by the nutritional plan (P < 0.050). Morphometric measures and body condition score (2.99 - LNP vs. 3.28 - HNP) were affected by nutritional plan (P < 0.050), except hip height, thoracic depth and hip width. The nutritional plan affected the body components (P < 0.050), except esophagus and trachea. Animal performance and carcass traits were influenced by nutritional plan (P < 0.050), except carcass dressing (48.56% on average). Nutritional plan affected (P < 0.050) some blood profile variables as the total cholesterol (141.35 vs. 113.25 mg/dL), triglycerides (60.53 vs. 89.05 mg/dL), LDL (79.76 vs. 33.66 g/mL) and IGF-1 (17.77 vs. 38.55 ng/mL) for LNP and HNP respectively. Hypertrophy was greater in HNP than LNP animals (P < 0.050), being represented by the proportion of sarcoplasm (39.76 vs. 31.99%). LNP had a greater mTOR abundance than HNP (P = 0.045), but AMPK was not affected by the nutritional plan. Our findings show that a higher milk replacer allowance enhances animal performance, body development, metabolic parameters, and cellular hypertrophy in pre-weaned dairy kids.

Early postnatal nutrition and renal consequences in preterm infants

Perinatal nutritional factors may lead to decreased nephron endowment, decreased kidney function, and long-term development of chronic kidney disease and non-communicable diseases. At the same time, optimal postnatal nutrition and catch-up growth are associated with better neurodevelopmental outcomes in preterm infants. Therefore, nutritional management of preterm infants is a major challenge for neonatologists. In this context, the Section of Nutrition, Gastroenterology and Metabolism reviewed the current knowledge on nutritional issues related to kidney function. This narrative review discusses the clinical impact of early postnatal nutrition on long-term kidney function. In preterm infants, data are largely lacking to determine the extent to which early nutrition contributes to nephrogenesis and nephron endowment. However, some nutritional principles may help clinicians better protect the developing kidney in preterm infants. IMPACT: Clinical data show that preterm infants are an emerging population at high risk for chronic kidney disease. Both undernutrition and overnutrition can alter long-term kidney function. In preterm infants, data are largely lacking to determine the extent to which early postnatal nutrition contributes to nephrogenesis, nephron endowment and increased risk for chronic kidney disease. Some nutritional principles may help clinicians better protect the developing kidney in preterm infants: avoiding extrauterine growth restriction; providing adequate protein and caloric intakes; limiting exposure to high and prolonged hyperglycaemia; avoiding micronutrient deficiencies and maintaining acid-base and electrolyte balance.

Chronic Fetal Leucine Infusion Increases Rate of Leucine Oxidation but Not of Protein Synthesis in Late Gestation Fetal Sheep

BACKGROUND

Leucine increases protein synthesis rates in postnatal animals and adults. Whether supplemental leucine has similar effects in the fetus has not been determined.

OBJECTIVE

To determine the effect of a chronic leucine infusion on whole-body leucine oxidation and protein metabolic rates, muscle mass, and regulators of muscle protein synthesis in late gestation fetal sheep.

METHODS

Catheterized fetal sheep at ∼126 d of gestation (term = 147 d) received infusions of saline (CON, n = 11) or leucine (LEU; n = 9) adjusted to increase fetal plasma leucine concentrations by 50%-100% for 9 d. Umbilical substrate net uptake rates and protein metabolic rates were determined using a 1-13C leucine tracer. Myofiber myosin heavy chain (MHC) type and area, expression of amino acid transporters, and abundance of protein synthesis regulators were measured in fetal skeletal muscle. Groups were compared using unpaired t tests.

RESULTS

Plasma leucine concentrations were 75% higher in LEU fetuses compared with CON by the end of the infusion period (P < 0.0001). Umbilical blood flow and uptake rates of most amino acids, lactate, and oxygen were similar between groups. Fetal whole-body leucine oxidation was 90% higher in LEU (P < 0.0005) but protein synthesis and breakdown rates were similar. Fetal and muscle weights and myofiber areas were similar between groups, however, there were fewer MHC type IIa fibers (P < 0.05), greater mRNA expression levels of amino acid transporters (P < 0.01), and a higher abundance of signaling proteins that regulate protein synthesis (P < 0.05) in muscle from LEU fetuses.

CONCLUSIONS

A direct leucine infusion for 9 d in late gestation fetal sheep does not increase protein synthesis rates but results in higher leucine oxidation rates and fewer glycolytic myofibers. Increasing leucine concentrations in the fetus stimulates its own oxidation but also increases amino acid transporter expression and primes protein synthetic pathways in skeletal muscle.

Prematurity blunts protein synthesis in skeletal muscle independently of body weight in neonatal pigs

BACKGROUND

Postnatal growth failure in premature infants is associated with reduced lean mass accretion. Prematurity impairs the feeding-induced stimulation of translation initiation and protein synthesis in the skeletal muscle of neonatal pigs. The objective was to determine whether body weight independently contributes to the blunted postprandial protein synthesis.

METHODS

Preterm and term pigs that were either fasted or fed were stratified into quartiles according to birth weight to yield preterm and term groups of similar body weight; first and second quartiles of preterm pigs and third and fourth quartiles of term pigs were compared (preterm-fasted, n = 23; preterm-fed, n = 25; term-fasted, n = 21; term-fed, n = 21). Protein synthesis rates and mechanistic target of rapamycin complex 1 (mTORC1) activation in skeletal muscle were determined.

RESULTS

Relative body weight gain was lower in preterm compared to term pigs. Prematurity attenuated the feeding-induced increase in mTORC1 activation in longissimus dorsi and gastrocnemius muscles (P < 0.05). Protein synthesis in gastrocnemius (P < 0.01), but not in longissimus dorsi muscle, was blunted by preterm birth.

CONCLUSION

A lower capacity of skeletal muscle to respond adequately to feeding may contribute to reduced body weight gain and lean mass accretion in preterm infants.

IMPACT

This study has shown that the feeding-induced increase in protein synthesis of skeletal and cardiac muscle is blunted in neonatal pigs born preterm compared to pigs born at term independently of birth weight. These findings support the notion that preterm birth, and not low birth weight, impairs the capacity of skeletal and cardiac muscle to upregulate mechanistic target of rapamycin-dependent anabolic signaling pathways and protein synthesis in response to the postprandial increase in insulin and amino acids. These observations suggest that a blunted anabolic response to feeding contributes to reduced lean mass accretion and altered body composition in preterm infants.

Preterm birth alters the feeding-induced activation of Akt signaling in the muscle of neonatal piglets

BACKGROUND

Postnatal lean mass accretion is commonly reduced in preterm infants. This study investigated mechanisms involved in the blunted feeding-induced activation of Akt in the skeletal muscle of preterm pigs that contributes to lower protein synthesis rates.

METHODS

On day 3 following cesarean section, preterm and term piglets were fasted or fed an enteral meal. Activation of Akt signaling pathways in skeletal muscle was determined.

RESULTS

Akt1 and Akt2, but not Akt3, phosphorylation were lower in the skeletal muscle of preterm than in term pigs (P < 0.05). Activation of Akt-positive regulators, PDK1 and mTORC2, but not FAK, were lower in preterm than in term (P < 0.05). The formation of Akt complexes with GAPDH and Hsp90 and the abundance of Ubl4A were lower in preterm than in term (P < 0.05). The abundance of Akt inhibitors, PHLPP and SHIP2, but not PTEN and IP6K1, were higher in preterm than in term pigs (P < 0.05). PP2A activation was inhibited by feeding in term but not in preterm pigs (P < 0.05).

CONCLUSIONS

Our results suggest that preterm birth impairs regulatory components involved in Akt activation, thereby limiting the anabolic response to feeding. This anabolic resistance likely contributes to the reduced lean accretion following preterm birth.

IMPACT

The Akt-mTORC1 pathway plays an important role in the regulation of skeletal muscle protein synthesis in neonates. This is the first evidence to demonstrate that, following preterm birth, the postprandial activation of positive regulators of Akt in the skeletal muscle is reduced, whereas the activation of negative regulators of Akt is enhanced. This anabolic resistance of Akt signaling in response to feeding likely contributes to the reduced accretion of lean mass in premature infants. These results may provide potential novel molecular targets for intervention to enhance lean growth in preterm neonates.

Intermittent Bolus Compared With Continuous Feeding Enhances Insulin and Amino Acid Signaling to Translation Initiation in Skeletal Muscle of Neonatal Pigs

BACKGROUND

Nutrition administered as intermittent bolus feeds rather than continuously promotes greater protein synthesis rates in skeletal muscle and enhances lean growth in a neonatal piglet model. The molecular mechanisms responsible remain unclear.

OBJECTIVES

We aimed to identify the insulin- and/or amino acid-signaling components involved in the enhanced stimulation of skeletal muscle by intermittent bolus compared to continuous feeding in neonatal pigs born at term.

METHODS

Term piglets (2-3 days old) were fed equal amounts of sow milk replacer [12.8 g protein and 155 kcal/(kg body weight · d)] by orogastric tube as intermittent bolus meals every 4 hours (INT) or by continuous infusion (CTS). After 21 days, gastrocnemius muscle samples were collected from CTS, INT-0 (before a meal), and INT-60 (60 minutes after a meal) groups (n = 6/group). Insulin- and amino acid-signaling components relevant to mechanistic target of rapamycin complex (mTORC) 1 activation and protein translation were measured.

RESULTS

Phosphorylation of the insulin receptor, IRS-1, PDK1, mTORC2, pan-Akt, Akt1, Akt2, and TSC2 was 106% to 273% higher in the skeletal muscle of INT-60 piglets than in INT-0 and CTS piglets (P  < 0.05), but phosphorylation of PTEN, PP2A, Akt3, ERK1/2, and AMPK did not differ among groups, nor did abundances of PHLPP, SHIP2, and Ubl4A. The association of GATOR2 with Sestrin1/2, but not CASTOR1, was 51% to 52% lower in INT-60 piglets than in INT-0 and CTS piglets (P  < 0.05), but the abundances of SLC7A5/LAT1, SLC38A2/SNAT2, SLC38A9, Lamtor1/2, and V-ATPase did not differ. Associations of mTOR with RagA, RagC, and Rheb and phosphorylation of S6K1 and 4EBP1, but not eIF2α and eEF2, were 101% to 176% higher in INT-60 piglets than in INT-0 and CTS piglets (P < 0.05).

CONCLUSIONS

The enhanced rates of muscle protein synthesis and growth with intermittent bolus compared to continuous feeding in a neonatal piglet model can be explained by enhanced activation of both the insulin- and amino acid-signaling pathways that regulate translation initiation.

Prematurity blunts the insulin- and amino acid-induced stimulation of translation initiation and protein synthesis in skeletal muscle of neonatal pigs

Extrauterine growth restriction in premature infants is largely attributed to reduced lean mass accretion and is associated with long-term morbidities. Previously, we demonstrated that prematurity blunts the feeding-induced stimulation of translation initiation signaling and protein synthesis in skeletal muscle of neonatal pigs. The objective of the current study was to determine whether the blunted feeding response is mediated by reduced responsiveness to insulin, amino acids, or both. Pigs delivered by cesarean section preterm (PT; 103 days, n = 25) or at term (T; 112 days, n = 26) were subject to euinsulinemic-euaminoacidemic-euglycemic (FAST), hyperinsulinemic-euaminoacidemic-euglycemic (INS), or euinsulinemic-hyperaminoacidemic-euglycemic (AA) clamps four days after delivery. Indices of mechanistic target of rapamycin complex 1 (mTORC1) signaling and fractional protein synthesis rates were measured after 2 h. Although longissimus dorsi (LD) muscle protein synthesis increased in response to both INS and AA, the increase was 28% lower in PT than in T. Upstream of mTORC1, Akt phosphorylation, an index of insulin signaling, was increased with INS but was 40% less in PT than in T. The abundances of mTOR·RagA and mTOR·RagC, indices of amino acid signaling, increased with AA but were 25% less in PT than in T. Downstream of mTORC1, eIF4E·eIF4G abundance was increased by both INS and AA but attenuated by prematurity. These results suggest that preterm birth blunts both insulin- and amino acid-induced activation of mTORC1 and protein synthesis in skeletal muscle, thereby limiting the anabolic response to feeding. This anabolic resistance likely contributes to the high prevalence of extrauterine growth restriction in prematurity.NEW & NOTEWORTHY Extrauterine growth faltering is a major complication of premature birth, but the underlying cause is poorly understood. Our results demonstrate that preterm birth blunts both the insulin-and amino acid-induced activation of mTORC1-dependent translation initiation and protein synthesis in skeletal muscle, thereby limiting the anabolic response to feeding. This anabolic resistance likely contributes to the reduced accretion of lean mass and extrauterine growth restriction of premature infants.

Intermittent Bolus Feeding Enhances Organ Growth More Than Continuous Feeding in a Neonatal Piglet Model

BACKGROUND

Orogastric tube feeding is frequently prescribed for neonates who cannot ingest food normally. In a piglet model of the neonate, greater skeletal muscle growth is sustained by upregulation of translation initiation signaling when nutrition is delivered by intermittent bolus meals, rather than continuously.

OBJECTIVES

The objective of this study was to determine the long-term effects of feeding frequency on organ growth and the mechanism by which feeding frequency modulates protein anabolism in these organs.

METHODS

Eighteen neonatal pigs were fed by gastrostomy tube the same amount of a sow milk replacer either by continuous infusion (CON) or on an intermittent bolus schedule (INT). After 21 d of feeding, the pigs were killed without interruption of feeding (CON; n = 6) or immediately before (INT-0; n = 6) or 60 min after (INT-60; n = 6) a meal, and fractional protein synthesis rates and activation indexes of signaling pathways that regulate translation initiation were measured in the heart, jejunum, ileum, kidneys, and liver.

RESULTS

Compared with continuous feeding, intermittent feeding stimulated the growth of the liver (+64%), jejunum (+48%), ileum (+40%), heart (+64%), and kidney (+56%). The increases in heart, kidney, jejunum, and ileum masses were proportional to whole body lean weight gain, but liver weight gain was greater in the INT-60 than the CON, and intermediate for the INT-0 group. For the liver and ileum, but not the heart, kidney, and jejunum, INT-60 compared with CON pigs had greater fractional protein synthesis rates (22% and 48%, respectively) and was accompanied by an increase in ribosomal protein S6 kinase 1 and eukaryotic initiation factor 4E binding protein 1 phosphorylation.

CONCLUSIONS

These results suggest that intermittent bolus compared with continuous orogastric feeding enhances organ growth and that in the ileum and liver, intermittent feeding enhances protein synthesis by stimulating translation initiation.

Insulin Stimulation of Protein Synthesis and mTOR Signaling in Chick Myotube Cultures

Insulin stimulates protein synthesis in skeletal muscles. Protein synthesis is controlled by the mechanistic target of rapamycin (mTOR) signaling in skeletal muscles. This study was conducted to investigate the effect of insulin on protein synthesis and mTOR signaling in chick myotube cultures. Chick myotubes were incubated with insulin (1 µg/ml) for 1 h. Protein synthesis, measured using the surface sensing of translation method, was significantly increased by insulin. The phosphorylation of AKT (Thr308 and Ser473), p70 ribosomal S6 kinase 1 (S6K1, Thr389), S6 ribosomal protein (Ser235/236), and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1, Thr37/46) was also significantly increased by insulin. These results suggest that insulin stimulates protein synthesis via mTOR signaling (phosphorylation of AKT, S6K1, S6 ribosomal protein, and 4E-BP1) in chick myotube cultures.

Differential regulation of mTORC1 activation by leucine and β-hydroxy-β-methylbutyrate in skeletal muscle of neonatal pigs

Leucine (Leu) and its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1)-dependent protein synthesis in the skeletal muscle of neonatal pigs. This study aimed to determine whether HMB and Leu utilize common nutrient-sensing mechanisms to activate mTORC1. In study 1, neonatal pigs were fed one of five diets for 24 h: low protein (LP), high protein (HP), or LP supplemented with 4 (LP+HMB4), 40 (LP+HMB40), or 80 (LP+HMB80) μmol HMB·kg body wt^-1·day^-1. In study 2, neonatal pigs were fed for 24 h: LP, LP supplemented with Leu (LP+Leu), or HP diets delivering 9, 18, and 18 mmol Leu·kg body wt^-1·day^-1, respectively. The upstream signaling molecules that regulate mTORC1 activity were analyzed. mTOR phosphorylation on Ser2448 and Ser2481 was greater in LP+HMB40, LP+HMB80, and LP+Leu than in LP and greater in HP than in HMB-supplemented groups (P < 0.05), whereas HP and LP+Leu were similar. Rheb-mTOR complex formation was lower in LP than in HP (P < 0.05), with no enhancement by HMB or Leu supplementation. The Sestrin2-GATOR2 complex was more abundant in LP than in HP and was reduced by Leu (P < 0.05) but not HMB supplementation. RagA-mTOR and RagC-mTOR complexes were higher in LP+Leu and HP than in LP and HMB groups (P < 0.05). There were no treatment differences in RagB-SH3BP4, Vps34-LRS, and RagD-LRS complex abundances. Phosphorylation of Erk1/2 and TSC2, but not AMPK, was lower in LP than HP (P < 0.05) and unaffected by HMB or Leu supplementation. Our results demonstrate that HMB stimulates mTORC1 activation in neonatal muscle independent of the leucine-sensing pathway mediated by Sestrin2 and the Rag proteins.NEW & NOTEWORTHY Dietary supplementation with either leucine or its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulates protein synthesis in skeletal muscle of the neonatal pig. Our results demonstrate that both leucine and HMB stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) phosphorylation in neonatal muscle. This leucine-stimulated process involves dissociation of the Sestrin2-GATOR2 complex and increased binding of Rag A/C to mTOR. However, HMB's activation of mTORC1 is independent of this leucine-sensing pathway.

Prematurity blunts the feeding-induced stimulation of translation initiation signaling and protein synthesis in muscle of neonatal piglets

Postnatal growth of lean mass is commonly blunted in preterm infants and may contribute to short- and long-term morbidities. To determine whether preterm birth alters the protein anabolic response to feeding, piglets were delivered at term or preterm, and fractional protein synthesis rates (K_s) were measured at 3 days of age while fasted or after an enteral meal. Activation of signaling pathways that regulate protein synthesis and degradation were determined. Relative body weight gain was lower in preterm than in term. Gestational age at birth (GAB) did not alter fasting plasma glucose or insulin, but when fed, plasma insulin and glucose rose more slowly, and reached peak value later, in preterm than in term. Feeding increased K_s in longissimus dorsi (LD) and gastrocnemius muscles, heart, pancreas, and kidney in both GAB groups, but the response was blunted in preterm. In diaphragm, lung, jejunum, and brain, feeding increased K_s regardless of GAB. Liver K_s was greater in preterm than term and increased with feeding regardless of GAB. In all tissues, changes in 4EBP1, S6K1, and PKB phosphorylation paralleled changes in K_s. In LD, eIF4E·eIF4G complex formation, phosphorylation of TSC2, mTOR, and rpS6, and association of mammalian target of rapamycin (mTOR1) complex with RagA, RagC, and Rheb were increased by feeding and blunted by prematurity. There were no differences among groups in LD protein degradation markers. Our results demonstrate that preterm birth reduces weight gain and the protein synthetic response to feeding in muscle, pancreas, and kidney, and this is associated with blunted insulin- and/or amino acid-induced translation initiation signaling.

Peptidomic analysis reveals multiple protection of human breast milk on infants during different stages

It has been shown that human breast milk (HBM) is an important nutrient for the growth and development of newborns. Currently, peptide drugs provide promising regimes in neonatal disease treatment, especially peptides from HBM that exhibit multiple functions within cells. To explore the potential biological function peptides among the colostrum, transition and mature milk from mother of extremely low birth weight children (the samples were collected from Women's Hospital of Nanjing Medical University from December 2016 to February 2017). A total of 3,182 nonredundant peptides were identified and compared among colostrum, transitional and mature milk using liquid chromatography/mass spectrometry technology, and the numbers and fragments of peptides were various. The isoelectric point and molecular weight analysis of the differentially expressed peptides basically accord with the range of mass spectrometry identification (<3 kDa). Gene Ontology analysis and Pathway analysis, restriction sites analysis, as well as bioinformatics analysis showed that these differentially expressed peptides enriched a variety of biological processes. We identified several putative peptides that might have bioactive effects in diseases and development of newborns, which will inform further functional investigations. Our preliminary research provided a better understanding of the function of peptides during the newborn periods. Furthermore, it laid a foundation for discovering new peptide drugs in neonatal disease treatment.

Regulation of Muscle Growth in Early Postnatal Life in a Swine Model

Skeletal muscle growth during the early postnatal period is rapid in the pig and dependent on the capacity of muscle to respond to anabolic and catabolic stimuli. Muscle mass is driven by the balance between protein synthesis and degradation. Among these processes, muscle protein synthesis in the piglet is exceptionally sensitive to the feeding-induced postprandial changes in insulin and amino acids, whereas muscle protein degradation is affected only during specific catabolic states. The developmental decline in the response of muscle to feeding is associated with changes in the signaling pathways located upstream and downstream of the mechanistic target of rapamycin protein complex. Additionally, muscle growth is supported by an accretion of nuclei derived from satellite cells. Activated satellite cells undergo proliferation, differentiation, and fusion with adjacent growing muscle fibers. Enhancing early muscle growth through modifying protein synthesis, degradation, and satellite cell activity is key to maximizing performance, productivity, and lifelong pig health.

Amino Acid- and Insulin-Induced Activation of mTORC1 in Neonatal Piglet Skeletal Muscle Involves Sestin2-GATOR2, Rag A/C-mTOR, and RHEB-mTOR Complex Formation

Background

Feeding stimulates protein synthesis in skeletal muscle of neonates and this response is regulated through activation of mechanistic target of rapamycin complex 1 (mTORC1). The identity of signaling components that regulate mTORC1 activation in neonatal muscle has not been fully elucidated.

Objective

We investigated the independent effects of the rise in amino acids (AAs) and insulin after a meal on the abundance and activation of potential regulators of mTORC1 in muscle and whether the responses are modified by development.

Methods

Overnight-fasted 6- and 26-d-old pigs were infused for 2 h with saline (control group) or with a balanced AA mixture (AA group) or insulin (INS group) to achieve fed levels while insulin or AAs, respectively, and glucose were maintained at fasting levels. Muscles were analyzed for potential mTORC1 regulatory mechanisms and results were analyzed by 2-factor ANOVA followed by Tukey's post hoc test.

Results

The abundances of DEP domain-containing mTOR-interacting protein (DEPTOR), growth factor receptor bound protein 10 (GRB10), and regulated in development and DNA damage response 2 (REDD2) were lower (65%, 73%, and 53%, respectively; P < 0.05) and late endosomal/lysosomal adaptor, MAPK and mTOR activator 1/2 (LAMTOR1/2), vacuolar H+-ATPase (V-ATPase), and Sestrin2 were higher (94%, 141%, 145%, and 127%, respectively; P < 0.05) in 6- than in 26-d-old pigs. Both AA and INS groups increased phosphorylation of GRB10 (P < 0.05) compared with control in 26- but not in 6-d-old pigs. Formation of Ras-related GTP-binding protein A (RagA)-mTOR, RagC-mTOR, and Ras homolog enriched in brain (RHEB)-mTOR complexes was increased (P < 0.05) and Sestrin2-GTPase activating protein activity towards Rags 2 (GATOR2) complex was decreased (P < 0.05) by both AA and INS groups and these responses were greater (P < 0.05) in 6- than in 26-d-old pigs.

Conclusion

The results suggest that formation of RagA-mTOR, RagC-mTOR, RHEB-mTOR, and Sestrin2-GATOR2 complexes may be involved in the AA- and INS-induced activation of mTORC1 in skeletal muscle of neonates after a meal and that enhanced activation of the mTORC1 signaling pathway in neonatal muscle is in part due to regulation by DEPTOR, GRB10, REDD2, LAMTOR1/2, V-ATPase, and Sestrin2.

In ovo feeding of l-arginine regulates intestinal barrier functions of posthatch broilers by activating the mTOR signaling pathway

BACKGROUND

During the last phase of incubation, dramatic physiological and metabolic changes occur in chick embryos, and supplies of nutrients and energy are always insufficient. This study investigated the effects of in ovo feeding (IOF) of l-arginine (Arg) on the hatchability, growth performance, intestinal development and functions of posthatch broilers.

RESULTS

The IOF of Arg increased (P < 0.05) the feed intake and body weight gain during 1-21 days and 1-42 days, and the intestinal weight of 7- and 21-day-old broilers, compared with non-injected control and diluent-injected groups. The IOF of Arg increased (P < 0.05) villus height (VH), ratio of VH to crypt depth (CD) and density of goblet cells, and decreased (P < 0.05) the CD in jejunum of 1-, 7- and 21-day-old broilers. The IOF of Arg also increased (P < 0.05) the percentage of proliferating cell nuclear antigen positive cells of villus, and the mRNA expressions of mucin-2, claudin-1, zonula occludens-1 and -2 in jejunal mucosa of 21-day-old broilers. Meanwhile, IOF of Arg increased (P < 0.05) the protein abundance of phosphorylated mechanistic target of rapamycin (mTOR), ribosomal protein S6 kinase 1 and eukaryotic initiation factor 4E binding protein 1 in jejunal mucosa.

CONCLUSION

The IOF of Arg improved the development and barrier functions of small intestine, which might be associated with activating the mTOR pathway. In addition, the improved intestinal development might explain the improvement in feed intake and consequently the growth performance of broilers. Therefore, IOF of Arg solution could be an effective technology for regulating early nutrition supply and subsequent growth development in the poultry industry. © 2017 Society of Chemical Industry.

Digestion and nitrogen balance using swine diets containing increasing proportions of coproduct ingredients and formulated using the net energy system

Rising feed expenditures demand that our industry pursues strategies to lower the cost of production. One option is the adoption of the NE system, although many producers are hesitant to proceed without proof that NE estimates are reliable. The objective of this experiment was to compare the apparent total tract digestibility (ATTD) of energy and nutrients and the N retention (NR) of diets formulated using the NE system with increasing quantities of coproduct ingredients. The 5 dietary treatments included a control corn-soybean meal diet (CTL); the CTL plus 6% each of corn distiller's dried grains with solubles (DDGS), corn germ meal, and wheat middlings and NE equal to the CTL by adding soybean oil (CONS-18); the CONS-18 diet, without oil added, with NE content lower than the CTL (DECL-18); the CTL plus 12% each of corn DDGS, corn germ meal, and wheat middlings and NE equal to the CTL by adding soybean oil (CONS-36); and the CONS-36 diet, without oil added, with NE content lower than the CTL (DECL-36). Diets were formulated for both the growing period (GP; 40 to 70 kg) and the finishing period (FP; 70 to 110 kg). Forty gilts (PIC 337 × C22 or C29; 38.5 ± 0.4 kg initial BW) were randomly assigned to treatment and received feed and water ad libitum (8 pigs per treatment). For the last 13 d of the GP and FP, pigs were transferred to metabolism crates, where 2 total urine and fecal collections (d 4 to 6 and d 11 to 13) were performed. The GP fed diets with coproduct ingredients had lower ATTD of DM, N, and GE than those fed the CTL ( < 0.050). The ATTD of N and GE progressively decreased as coproduct inclusion increased from 0 to 18 to 36% in the FP ( < 0.010). In the GP and FP, there were no differences in ATTD of DM, N, or GE between CONS-18 and DECL-18 or between CONS-36 and DECL-36 ( > 0.050). The NR declined on all coproduct diets in the GP ( = 0.010) and tended to decline in the FP ( = 0.079). There were no differences in NR between CONS-18 and DECL-18 or between CONS-36 and DECL-36 ( > 0.050). In conclusion, digestion of diets containing up to 36% coproducts and formulated using NE resulted in expected DE and ME values; NR of diets with coproducts was lower than that of the simple CTL, which is not related to the accuracy of the energy estimations but rather to other factors such as imbalances in the AA concentrations or to postabsorptive energy metabolism, factors not accounted for by the current energy systems approach.

Myoblast replication is reduced in the IUGR fetus despite maintained proliferative capacity in vitro

Adults who were affected by intrauterine growth restriction (IUGR) suffer from reductions in muscle mass and insulin resistance, suggesting muscle growth may be restricted by molecular events that occur during fetal development. To explore the basis of restricted fetal muscle growth, we used a sheep model of progressive placental insufficiency-induced IUGR to assess myoblast proliferation within intact skeletal muscle in vivo and isolated myoblasts stimulated with insulin in vitro Gastrocnemius and soleus muscle weights were reduced by 25% in IUGR fetuses compared to those in controls (CON). The ratio of PAX7+ nuclei (a marker of myoblasts) to total nuclei was maintained in IUGR muscle compared to CON, but the fraction of PAX7+ myoblasts that also expressed Ki-67 (a marker of cellular proliferation) was reduced by 23%. Despite reduced proliferation in vivo, fetal myoblasts isolated from IUGR biceps femoris and cultured in enriched media in vitro responded robustly to insulin in a dose- and time-dependent manner to increase proliferation. Similarly, insulin stimulation of IUGR myoblasts upregulated key cell cycle genes and DNA replication. There were no differences in the expression of myogenic regulatory transcription factors that drive commitment to muscle differentiation between CON and IUGR groups. These results demonstrate that the molecular machinery necessary for transcriptional control of proliferation remains intact in IUGR fetal myoblasts, indicating that in vivo factors such as reduced insulin and IGF1, hypoxia and/or elevated counter-regulatory hormones may be inhibiting muscle growth in IUGR fetuses.

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.

Impact of prolonged leucine supplementation on protein synthesis and lean growth in neonatal pigs

Most low-birth weight infants experience extrauterine growth failure due to reduced nutrient intake as a result of feeding intolerance. The objective of this study was to determine whether prolonged enteral leucine supplementation improves lean growth in neonatal pigs fed a restricted protein diet. Neonatal pigs (n = 14-16/diet, 5 days old, 1.8 ± 0.3 kg) were fed by gastric catheter a whey-based milk replacement diet with either a high protein (HP) or restricted protein (RP) content or RP supplemented with leucine to the same level as in the HP diet (RPL). Pigs were fed 40 ml·kg body wt(-1)·meal(-1) every 4 h for 21 days. Feeding the HP diet resulted in greater total body weight and lean body mass compared with RP-fed pigs (P < 0.05). Masses of the longissimus dorsi muscle, heart, and kidneys were greater in the HP- than RP-fed pigs (P < 0.05). Body weight, lean body mass, and masses of the longissimus dorsi, heart, and kidneys in pigs fed the RPL diet were intermediate to RP- and HP-fed pigs. Protein synthesis and mTOR signaling were increased in all muscles with feeding (P < 0.05); leucine supplementation increased mTOR signaling and protein synthesis rate in the longissimus dorsi (P < 0.05). There was no effect of diet on indices of protein degradation signaling in any tissue (P > 0.05). Thus, when protein intake is chronically restricted, the capacity for leucine supplementation to enhance muscle protein accretion in neonatal pigs that are meal-fed milk protein-based diets is limited.

Leucine is a major regulator of muscle protein synthesis in neonates

Approximately 10% of infants born in the United States are of low birth weight. Growth failure during the neonatal period is a common occurrence in low birth weight infants due to their inability to tolerate full feeds, concerns about advancing protein supply, and high nutrient requirements for growth. An improved understanding of the nutritional regulation of growth during this critical period of postnatal growth is vital for the development of strategies to improve lean gain. Past studies with animal models have demonstrated that muscle protein synthesis is increased substantially following a meal and that this increase is due to the postprandial rise in amino acids as well as insulin. Both amino acids and insulin act independently to stimulate protein synthesis in a mammalian target of rapamycin-dependent manner. Further studies have elucidated that leucine, in particular, and its metabolites, α-ketoisocaproic acid and β-hydroxy-β-methylbutyrate, have unique anabolic properties. Supplementation with leucine, provided either parenterally or enterally, has been shown to enhance muscle protein synthesis in neonatal pigs, making it an ideal candidate for stimulating growth of low birth weight infants.

Regulation of skeletal muscle energy/nutrient-sensing pathways during metabolic adaptation to fasting in healthy humans

During fasting, rapid metabolic adaptations are required to maintain energy homeostasis. This occurs by a coordinated regulation of energy/nutrient-sensing pathways leading to transcriptional activation and repression of specific sets of genes. The aim of the study was to investigate how short-term fasting affects whole body energy homeostasis and skeletal muscle energy/nutrient-sensing pathways and transcriptome in humans. For this purpose, 12 young healthy men were studied during a 24-h fast. Whole body glucose/lipid oxidation rates were determined by indirect calorimetry, and blood and skeletal muscle biopsies were collected and analyzed at baseline and after 10 and 24 h of fasting. As expected, fasting induced a time-dependent decrease in plasma insulin and leptin levels, whereas levels of ketone bodies and free fatty acids increased. This was associated with a metabolic shift from glucose toward lipid oxidation. At the molecular level, activation of the protein kinase B (PKB/Akt) and mammalian target of rapamycin pathways was time-dependently reduced in skeletal muscle during fasting, whereas the AMP-activated protein kinase activity remained unaffected. Furthermore, we report some changes in the phosphorylation and/or content of forkhead protein 1, sirtuin 1, and class IIa histone deacetylase 4, suggesting that these pathways might be involved in the transcriptional adaptation to fasting. Finally, transcriptome profiling identified genes that were significantly regulated by fasting in skeletal muscle at both early and late time points. Collectively, our study provides a comprehensive map of the main energy/nutrient-sensing pathways and transcriptomic changes during short-term adaptation to fasting in human skeletal muscle.

Regulation of protein degradation pathways by amino acids and insulin in skeletal muscle of neonatal pigs

BACKGROUND

The rapid gain in lean mass in neonates requires greater rates of protein synthesis than degradation. We previously delineated the molecular mechanisms by which insulin and amino acids, especially leucine, modulate skeletal muscle protein synthesis and how this changes with development. In the current study, we identified mechanisms involved in protein degradation regulation. In experiment 1, 6- and 26-d-old pigs were studied during 1) euinsulinemic-euglycemic-euaminoacidemic, 2) euinsulinemic-euglycemic-hyperaminoacidemic, and 3) hyperinsulinemic-euglycemic-euaminoacidemic clamps for 2 h. In experiment 2, 5-d-old pigs were studied during 1) euinsulinemic-euglycemic-euaminoacidemic-euleucinemic, 2) euinsulinemic-euglycemic-hypoaminoacidemic-hyperleucinemic, and 3) euinsulinemic-euglycemic-euaminoacidemic-hyperleucinemic clamps for 24 h. We determined in muscle indices of ubiquitin-proteasome, i.e., atrogin-1 (MAFbx) and muscle RING-finger protein-1 (MuRF1) and autophagy-lysosome systems, i.e., unc51-like kinase 1 (UKL1), microtubule-associated protein light chain 3 (LC3), and lysosomal-associated membrane protein 2 (Lamp-2). For comparison, we measured ribosomal protein S6 (rpS6) and eukaryotic initiation factor 4E (eIF4E) activation, components of translation initiation.

RESULTS

Abundance of atrogin-1, but not MuRF1, was greater in 26- than 6-d-old pigs and was not affected by insulin, amino acids, or leucine. Abundance of ULK1 and LC3 was higher in younger pigs and not affected by treatment. The LC3-II/LC3-I ratio was reduced and ULK1 phosphorylation increased by insulin, amino acids, and leucine. These responses were more profound in younger pigs. Abundance of Lamp-2 was not affected by treatment or development. Abundance of eIF4E, but not rpS6, was higher in 6- than 26-d-old-pigs but unaffected by treatment. Phosphorylation of eIF4E was not affected by treatment, however, insulin, amino acids, and leucine stimulated rpS6 phosphorylation, and the responses decreased with development.

CONCLUSIONS

The rapid growth of neonatal muscle is in part due to the positive balance between the activation of protein synthesis and degradation signaling. Insulin, amino acids, and, particularly, leucine, act as signals to modulate muscle protein synthesis and degradation in neonates.

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.

Viscera and muscle protein synthesis in neonatal pigs is increased more by intermittent bolus than by continuous feeding

BACKGROUND

Continuous and intermittent bolus orogastric feedings are strategies used in infants unable to tolerate normal feeds.

METHODS

To determine the effects of feeding modality on protein synthesis in different tissues, neonatal pigs received a balanced formula by orogastric tube as an intermittent bolus feed every 4 h or as a continuous infusion, or were fasted overnight.

RESULTS

As compared with fasting, protein synthesis in gastrocnemius, masseter, and soleus muscles; left ventricle; liver; pancreas; jejunum; and kidney increased in bolus- and continuously fed pigs, but the greatest increase occurred after a bolus meal. Tuberous sclerosis complex (TSC2), the proline-rich AKT substrate of 40 kDa (PRAS40), eukaryotic initiation factor (eIF) 4E binding protein (4EBP1), and ribosomal protein S6 kinase 1 (S6K1) phosphorylation in all tissues, and the proportion of ribosomal protein S4 in liver polysomes were enhanced 90 min following the bolus meal but not immediately before the meal or during continuous feeding. Eukaryotic elongation factor 2 (eEF2) and eIF2α phosphorylation were unaffected by feeding.

CONCLUSION

These results suggest that intermittent bolus feeding increases protein synthesis in muscles of different fiber types and visceral tissues to a greater extent than continuous feeding by stimulating translation initiation.

Impact of fasting on the rhythmic expression of myogenic and metabolic factors in skeletal muscle of adult mice

Circadian rhythms and metabolism are tightly integrated, and rhythmic expression of metabolic factors is common in homeostatic processes. We measured the temporal changes in the expression of myogenic regulatory factors and expression and activity level of molecules involved in protein metabolism in skeletal muscles and livers in mice and examined the impact of fasting. Tissues were collected over 24 h (at zeitgeber times ZT1, ZT5, ZT9, ZT13, ZT17, ZT21, and ZT1 the following day) from adult male C57Bl/6J mice that had been either freely fed or fasted for 24 h. In skeletal muscle, there was a robust rise in the mRNA expression of the myogenic regulatory factors MyoD and myogenin during dark hours which was strongly suppressed by fasting. Circadian pattern was observed for mRNA of MuRF1, Akt1, and ribosomal protein S6 in muscles in fed and fasted mice and for Fbxo32 in fed mice. Activity (phosphorylation) levels of Akt(Ser473) displayed temporal regulation in fasted (but not fed) mice and were high at ZT9. Fasting caused significant reductions in phosphorylation for both Akt and S6 in muscles, indicative of inactivation. Hepatic phosphorylated Akt(Ser473) and S6(Ser235/236) proteins did not exhibit daily rhythms. Fasting significantly reduced hepatic Akt(473) phosphorylation compared with fed levels, although (unlike in muscle) it did not affect S6(Ser235/236) phosphorylation. This in vivo circadian study addresses for the first time the signaling activities of key molecules related to protein turnover and their possible cross-regulation of expression of genes related to protein degradation.

Short-term feed deprivation rapidly induces the protein degradation pathway in skeletal muscles of young mice

Analysis of protein kinase B (AKT) and S6 kinase1 (p70S6K) activity is widely used to assess the efficacy of interventions designed to increase or maintain skeletal muscle mass; these studies are often performed on feed-deprived mice. One problem associated with feed deprivation is that it promotes catabolism, and young or metabolically compromised mice may have less tolerance. The aim of our study was to determine the effect of various times of feed deprivation on the activity of AKT and p70S6K signaling and markers of protein catabolism in young, growing mice compared with adult mice. Young 23-d-old and adult 3-mo-old mice were feed deprived for 8, 10, and 12 h starting at 0700 h. In addition, adult mice were feed deprived for 24 h. AKT(Ser473) phosphorylation decreased by 50 and 76% from fed amounts by 10 and 12 h of feed deprivation, respectively, in young but not adult muscles. In adult muscles, feed deprivation for 24 h reduced AKT(Ser473) phosphorylation by 70%. Significant de-phosphorylation of p70S6K(Thr389) occurred in all feed-deprived young and adult mice. There was an increase in muscle RING-finger protein-1 (Murf1; 133-1245%) and muscle atrophy F-box protein or Atrogin-1 (Fbxo32; 210-2420%) mRNA in all young but not adult groups deprived of feed for 8-12 h, and there was a trend (P = 0.08) toward increased MURF1 associated with the contractile protein-enriched fraction isolated from young muscles of mice feed deprived for 12 h. This study demonstrates that skeletal muscles of young mice respond rapidly to feed deprivation by decreasing AKT activity and upregulating the protein degradation program.

Could muscle deformity in children with spastic cerebral palsy be related to an impairment of muscle growth and altered adaptation?

Skeletal muscle deformity is common in children with spastic cerebral palsy (CP), but the underlying mechanisms are unclear. This review explores some possible factors which may influence the development of muscle deformity in CP. Normal muscle function and growth appear to depend on the interaction of neuronal, endocrinal, nutritional, and mechanical factors, and also on the development of an appropriate balance between muscle protein synthesis and degradation, and between the development of contractile and non-contractile components. In this context, the changes seen in muscle in children with CP are reviewed and discussed. It is suggested that the development of muscle deformity in children with CP may be related to a multifactorial impairment of muscle growth, on which adaptation of the extracellular matrix due to altered loading may be imposed.

Abundance of amino acid transporters involved in mTORC1 activation in skeletal muscle of neonatal pigs is developmentally regulated

Previously we demonstrated that the insulin- and amino acid-induced activation of the mammalian target of rapamycin complex 1 (mTORC1) is developmentally regulated in neonatal pigs. Recent studies have indicated that members of the System A transporter (SNAT2), the System N transporter (SNAT3), the System L transporters (LAT1 and LAT2), and the proton-assisted amino acid transporters (PAT1 and PAT2) have crucial roles in the activation of mTORC1 and that the abundance of amino acid transporters is positively correlated with their activation. This study aimed to determine the effect of the post-prandial rise in insulin and amino acids on the abundance or activation of SNAT2, SNAT3, LAT1, LAT2, PAT1, and PAT2 and whether the response is modified by development. Overnight fasted 6- and 26-day-old pigs were infused for 2 h with saline (Control) or with insulin or amino acids to achieve fed levels while amino acids or insulin, respectively, as well as glucose were maintained at fasting levels. The abundance of SNAT2, SNAT3, LAT1, LAT2, PAT1, and PAT2 was higher in muscle of 6- compared with 26-day-old pigs. The abundance of the PAT2-mTOR complex was greater in 6- than in 26-day-old pigs, consistent with the higher activation of mTORC1. Neither insulin nor amino acids altered amino acid transporter or PAT2-mTOR complex abundance. In conclusion, the amino acid transporters, SNAT 2/3, LAT 1/2, and PAT1/2, likely have important roles in the enhanced amino acid-induced activation of mTORC1 in skeletal muscle of the neonate.

Anabolic signaling and protein deposition are enhanced by intermittent compared with continuous feeding in skeletal muscle of neonates

Orogastric tube feeding is indicated for neonates with impaired ability to ingest and can be administered by intermittent bolus or continuous schedule. Our aim was to determine whether feeding modalities affect muscle protein deposition and to identify mechanisms involved. Neonatal pigs were overnight fasted (FAS) or fed the same amount of food continuously (CON) or intermittently (INT; 7 × 4 h meals) for 29 h. For 8 h, between hours 20 and 28, pigs were infused with [(2)H(5)]phenylalanine and [(2)H(2)]tyrosine, and amino acid (AA) net balances were measured across the hindquarters. Insulin, branched-chain AA, phenylalanine, and tyrosine arterial concentrations and whole body phenylalanine and tyrosine fluxes were greater for INT after the meal than for CON or FAS. The activation of signaling proteins leading to initiation of mRNA translation, including eukaryotic initiation factor (eIF)4E·eIF4G complex formation in muscle, was enhanced by INT compared with CON feeding or FAS. Signaling proteins of protein degradation were not affected by feeding modalities except for microtubule-associated protein light chain 3-II, which was highest in the FAS. Across the hindquarters, AA net removal increased for INT but not for CON or FAS, with protein deposition greater for INT. This was because protein synthesis increased following feeding for INT but remained unchanged for CON and FAS, whereas there was no change in protein degradation across any dietary treatment. These results suggest that muscle protein accretion in neonates is enhanced with intermittent bolus to a greater extent than continuous feeding, mainly by increased protein synthesis.

Intermittent bolus feeding has a greater stimulatory effect on protein synthesis in skeletal muscle than continuous feeding in neonatal pigs

Orogastric tube feeding, using either continuous or intermittent bolus delivery, is common in infants for whom normal feeding is contraindicated. To compare the impact of different feeding strategies on muscle protein synthesis, after withholding food overnight, neonatal pigs received a complete formula orally as a bolus feed every 4 h or were continuously fed. Protein synthesis rate and translational mechanisms in skeletal muscle were examined after 0, 24, and 25.5 h. Plasma amino acid and insulin concentrations increased minimally and remained constant in continuously fed compared to feed-deprived pigs; however, the pulsatile meal feeding pattern was mimicked in bolus-fed pigs. Muscle protein synthesis was stimulated by feeding and the greatest response occurred after a bolus meal. Bolus but not continuous feeds increased polysome aggregation, the phosphorylation of protein kinase B, tuberous sclerosis complex 2, proline-rich Akt substrate of 40 kDa, eukaryotic initiation factor (eIF) 4E binding protein (4EBP1), and rp S6 kinase and enhanced dissociation of the 4EBP1 ·eIF4E complex and formation of the eIF4E ·eIF4G complex compared to feed deprivation (P < 0.05). Activation of insulin receptor substrate-1, regulatory associated protein of mammalian target of rapamycin, AMP-activated protein kinase, eukaryotic elongation factor 2, and eIF2α phosphorylation were unaffected by either feeding modality. These results suggest that in neonates, intermittent bolus feeding enhances muscle protein synthesis to a greater extent than continuous feeding by eliciting a pulsatile pattern of amino acid- and insulin-induced translation initiation.

Differential regulation of protein synthesis and mTOR signaling in skeletal muscle and visceral tissues of neonatal pigs after a meal

Protein synthesis (PS) increases after a meal in neonates, but the time course of the changes in PS in different tissues after a meal is unknown. We aimed to evaluate the changes in tissue PS, mammalian target of rapamycin complex 1 (mTORC1) activation, and proportion of ribosomal protein (rp) mRNAs in polysomes over 4 h after a bolus meal in neonatal pigs (n = 6/group; 5- to 7-d-old). The results show a more sustained increase in PS in glycolytic compared with mixed fiber type muscles and no changes in oxidative muscles. PS increased in liver, jejunum, and pancreas but not in kidney and heart. Feeding did not affect AMP-activated protein kinase or RAS-related GTP binding B activation. Phosphorylation of tuberous sclerosis complex 2, proline-rich Akt substrate of 40 kD, mTOR, eukaryotic initiation factor 4E binding protein, and rp S6 kinase 1 increased in all tissues after feeding. The proportion of mRNAs encoding rp S4 and S8 in liver polysomes increased within 30 min postfeeding. These results suggest that feeding stimulates mTORC1 signaling in muscle and viscera, but mTORC1 activation alone is not sufficient to stimulate PS in all tissues.

Essential amino acids regulate both initiation and elongation of mRNA translation independent of insulin in MAC-T cells and bovine mammary tissue slices

Current nutrient requirement models assume fixed efficiencies of absorbed amino acid (AA) conversion to milk protein. Regulation of mammary protein synthesis (PS) potentially violates this assumption by changing the relationship between AA supply and milk protein output. The objective of this study was to investigate the effects of essential AA (EAA) and insulin on cellular signaling and PS rates in bovine mammary cells. MAC-T cells were subjected to 0 or 100% of normal EAA concentrations in DMEM/F12 and 0 or 100 μg insulin/L in a 2 × 2 factorial arrangement of treatments. Lactogenic bovine mammary tissue slices (MTS) were subjected to the same treatments, except low-EAA was 5% of normal DMEM/F12 concentrations. In MAC-T cells, EAA increased phosphorylation of mammalian target of rapamycin (mTOR; Ser2448), ribosomal protein S6 kinase 1 (S6K1; Thr389), eIF4E binding protein 1 (4EBP1; Thr37/46), and insulin receptor substrate 1 (IRS1; Ser1101), and reduced phosphorylation of eukaryotic elongation factor 2 (eEF2; Thr56) and eukaryotic initiation factor (eIF) 2-α (Ser51). In the presence of insulin, phosphorylation of Akt (Ser473), mTOR, S6K1, 4EBP1, and IRS1 increased in MAC-T cells. In MTS, EAA had similar effects on phosphorylation of signaling proteins and increased mammary PS rates. Insulin did not affect MTS signaling, perhaps due to inadequate levels. Effects of EAA and insulin were independent and additive for mTOR signaling in MAC-T cells. EAA did not inhibit insulin stimulation of Akt phosphorylation. PS rates were strongly associated with phosphorylation of 4EBP1 and eEF2 in MTS. EAA availability affected translation initiation and elongation control points to more strongly regulate PS than insulin.

Review: Ammonia emissions from dairy farms and beef feedlots

Hristov, A. N., Hanigan, M., Cole, A., Todd, R., McAllister T. A., Ndegwa, P. and Rotz, A. 2011. Review: Ammonia emissions from dairy farms and beef feedlots. Can. J. Anim. Sci. 91: 1–35. Ammonia emitted from animal feeding operations is an environmental and human health hazard, contributing to eutrophication of surface waters and nitrate contamination of ground waters, soil acidity, and fine particulate matter formation. It may also contribute to global warming through nitrous oxide formation. Along with these societal concerns, ammonia emission is a net loss of manure fertilizer value to the producer. A significant portion of cattle manure nitrogen, primarily from urinary urea, is converted to ammonium and eventually lost to the atmosphere as ammonia. Determining ammonia emissions from cattle operations is complicated by the multifaceted nature of the factors regulating ammonia volatilization, such as manure management, ambient temperature, wind speed, and manure composition and pH. Approaches to quantify ammonia emissions include micrometeorological methods, mass balance accounting and enclosures. Each method has its advantages, disadvantages and appropriate application. It is also of interest to determine the ammonia emitting potential of manure (AEP) independent of environmental factors. The ratio of nitrogen to non-volatile minerals (phosphorus, potassium, ash) or nitrogen isotopes ratio in manure has been suggested as a useful indicator of AEP. Existing data on ammonia emission factors and flux rates are extremely variable. For dairy farms, emission factors from 0.82 to 250 g ammonia per cow per day have been reported, with an average of 59 g per cow per day (n=31). Ammonia flux rates for dairy farms averaged 1.03 g m−2 h−1 (n=24). Ammonia losses are significantly greater from beef feedlots, where emission factors average 119 g per animal per day (n=9) with values as high as 280 g per animal per day. Ammonia flux rate for beef feedlots averaged 0.174 g m−2 h−1 (n=12). Using nitrogen mass balance approaches, daily ammonia nitrogen losses of 25 to 50% of the nitrogen excreted in manure have been estimated for dairy cows and feedlot cattle. Practices to mitigate ammonia emissions include reducing excreted N (particularly urinary N), acidifying ammonia sources, or binding ammonium to a substrate. Reducing crude protein concentration in cattle diets and ruminal protein degradability are powerful tools for reducing N excretion, AEP, and whole-farm ammonia emissions. Reducing dietary protein can also benefit the producer by reducing feed cost. These interventions, however, have to be balanced with the risk of lost production. Manure treatment techniques that reduce volatile N species (e.g., urease inhibition, pH reduction, nitrification-denitrification) are also effective for mitigating ammonia emissions. Another option for reducing ammonia emissions is capture and treatment of released ammonia. Examples in the latter category include biofilters, permeable and impermeable covers, and manure incorporation into the soil for crop or pasture production. Process-level simulation of ammonia formation and emission provides a useful tool for estimating emissions over a wide range of production practices and evaluating the potential benefits of mitigation strategies. Reducing ammonia emissions from dairy and beef cattle operations is critical to achieving environmentally sustainable animal production that will benefit producers and society at large.

Leucine supplementation of a low-protein meal increases skeletal muscle and visceral tissue protein synthesis in neonatal pigs by stimulating mTOR-dependent translation initiation

Protein synthesis and eukaryotic initiation factor (eIF) activation are increased in skeletal muscle of neonatal pigs parenterally infused with amino acids. Leucine appears to be the most effective single amino acid to trigger these effects. To examine the response to enteral leucine supplementation, overnight food-deprived 5-d-old pigs were gavage fed at 0 and 60 min a: 1) low-protein diet (LP); 2) LP supplemented with leucine (LP+L) to equal leucine in the high-protein diet (HP); or 3) HP diet. Diets were isocaloric and equal in lactose. Fractional protein synthesis rates and translation initiation control mechanisms were examined in skeletal muscles and visceral tissues 90 min after feeding. Protein synthesis rates in longissimus dorsi, gastrocnemius, and masseter muscles, heart, jejunum, kidney, and pancreas, but not liver, were greater in the LP+L group compared with the LP group and did not differ from the HP group. Feeding LP+L and HP diets compared with the LP diet increased phosphorylation of mammalian target of rapamycin (mTOR), 4E-binding protein 1, ribosomal protein S6 kinase-1, and eIF4G and formation of the active eIF4E·eIF4G complex in longissimus dorsi muscle. In all tissues except liver, activation of mTOR effectors increased in pigs fed LP+L and HP vs. LP diets. Our results suggest that leucine supplementation of a low-protein meal stimulates protein synthesis in muscle and most visceral tissues to a rate similar to that achieved by feeding a high-protein meal and this stimulation involves activation of mTOR downstream effectors.

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

The postprandial increases in AA and insulin independently stimulate protein synthesis in skeletal muscle of piglets. Leucine is an important mediator of the response to AA. We have shown that the postprandial increase in leucine, but not isoleucine or valine, acutely stimulates muscle protein synthesis in piglets. Leucine increases muscle protein synthesis by modulating the activation of mammalian target of rapamycin (mTOR) complex 1 and signaling components of translation initiation. Leucine increases the phosphorylation of mTOR, 70-kDa ribosomal protein S6 kinase-1, eukaryotic initiation factor (eIF) 4E-binding protein-1, and eIF4G; decreases eIF2α phosphorylation; and increases the association of eIF4E with eIF4G. However, leucine does not affect the upstream activators of mTOR, that is, protein kinase B, adenosine monophosphate-activated protein kinase, and tuberous sclerosis complex 1/2, or the activation of translation elongation regulator, eukaryotic elongation factor 2. The action of leucine can be replicated by α-ketoisocaproate but not by norleucine. Interference by rapamycin with the raptor-mTOR interaction blocks leucine-induced muscle protein synthesis. The acute leucine-induced stimulation of muscle protein synthesis is not maintained for prolonged periods, despite continued activation of mTOR signaling, because circulating AA fall as they are utilized for protein synthesis. However, when circulating AA concentrations are maintained, the leucine-induced stimulation of muscle protein synthesis is maintained for prolonged periods. Thus, leucine acts as a nutrient signal to stimulate translation initiation, but whether this translates into a prolonged increase in protein synthesis depends on the sustained availability of all AA.

The abundance and activation of mTORC1 regulators in skeletal muscle of neonatal pigs are modulated by insulin, amino acids, and age

Mammalian target of rapamycin complex 1 (mTORC1) signaling is crucial for the regulation of protein synthesis. Most of known mTORC1 regulators have been isolated and characterized using cell culture systems, and the physiological roles of these regulators have not been fully tested in vivo. Previously we demonstrated that the insulin (INS) and amino acid (AA)-induced activation of mTORC1 is developmentally regulated in skeletal muscle (Suryawan A et al. Am J Physiol Endocrinol Metab 293: E1597-E1605, 2007). The present study aimed to characterize in more detail the effects of the postprandial rise in INS and AA on the activation and abundance of mTORC1 regulators in muscle and how this is modified by development. Overnight fasted 6- and 26-day-old pigs were studied during 1) euinsulinemic-euglycemic-euaminoacidemic conditions (control), 2) euinsulinemic-euglycemic-hyperaminoacidemic clamps (AA), and 3) hyperinsulinemic-euglycemic-euaminoacidemic clamps (INS). INS, but not AA, enhanced the PRAS40 phosphorylation, and this effect was greater in 6- than in 26-day old pigs. Phospholipase D1 (PLD1) abundance and phosphorylation, and the association of PLD1 with Ras homolog enriched in brain (Rheb), were greater in the younger pigs. Neither INS, AA, nor age altered the abundance of Rheb, vacuolar protein sorting 34 (Vps34), or FK506-binding protein 38 (FKBP38). Although INS and AA had no effect, the abundance of ras-related GTP binding B (RagB) and the association of RagB with Raptor were greater in 6- than in 26-day-old pigs. Neither INS, AA, nor age altered AMPK-induced phosphorylation of Raptor. Our results suggest that the enhanced activation of mTORC1 in muscle of neonatal pigs is in part due to regulation by PRAS40, PLD1, and the Rag GTPases.

A growth stimulus is needed for IGF-1 to induce skeletal muscle hypertrophy in vivo

Here, we characterise new strains of normal and dystrophic (mdx) mice that overexpress Class 2 IGF-1 Ea in skeletal myofibres. We show that transgenic mice have increased muscle levels of IGF-1 (~13-26 fold) and show striking muscle hypertrophy (~24-56% increase in mass). Adult normal muscles were resistant to elevated IGF-1; they reached adult steady state and maintained the same mass from 3 to 12 months. By contrast, dystrophic muscles from mdx/IGF-1(C2:Ea) mice continued to increase in mass during adulthood. IGF-1 signalling was evident only in muscles that were growing as a result of normal postnatal development (23-day-old mice) or regenerating in response to endogenous necrosis (adult mdx mice). Increased phosphorylation of Akt at Ser473 was not evident in fasted normal adult transgenic muscles, but was 1.9-fold higher in fasted normal young transgenic muscles compared with age-matched wild-type controls and fourfold higher in fasted adult mdx/IGF-1(C2:Ea) compared with mdx muscles. Muscles of adult mdx/IGF-1(C2:Ea) mice showed higher p70S6K(Thr421/Ser424) phosphorylation and both young transgenic and adult mdx/IGF-1(C2:Ea) mice had higher phosphorylation of rpS6(Ser235/236). The level of mRNA encoding myogenin was increased in normal young (but not adult) transgenic muscles, indicating enhanced myogenic differentiation. These data demonstrate that elevated IGF-1 has a hypertrophic effect on skeletal muscle only in growth situations.

Stimulation of muscle protein synthesis by prolonged parenteral infusion of leucine is dependent on amino acid availability in neonatal pigs

The postprandial rise in amino acids, particularly leucine, stimulates muscle protein synthesis in neonates. Previously, we showed that a 1-h infusion of leucine increased protein synthesis, but this response was not sustained for 2 h unless the leucine-induced decrease in amino acids was prevented. To determine whether a parenteral leucine infusion can stimulate protein synthesis for a more prolonged, clinically relevant period if baseline amino acid concentrations are maintained, overnight food-deprived neonatal pigs were infused for 24 h with saline, leucine (400 mumol.kg(-1). h(-1)), or leucine with replacement amino acids. Amino acid replacement prevented the leucine-induced decrease in amino acids. Muscle protein synthesis was increased by leucine but only when other amino acids were supplied to maintain euaminoacidemia. Leucine did not affect activators of mammalian target of rapamycin (mTOR), i.e. protein kinase B, AMP-activated protein kinase, tuberous sclerosis complex 2, or eukaryotic elongation factor 2. There was no effect of treatment on the association of mTOR with regulatory associated protein of mammalian target of rapamycin (raptor), G-protein beta subunit-like protein, or rictor or the phosphorylation of raptor or proline-rich Akt substrate of 40 kDa. Phosphorylation of mTOR and its downstream targets, eukaryotic initiation factor (eIF) 4E binding protein and ribosomal protein S6 kinase, and the eIF4E . eIF4G association were increased and eIF2alpha phosphorylation was reduced by leucine and was not further altered by correcting for the leucine-induced hypoaminoacidemia. Thus, prolonged parenteral infusion of leucine activates mTOR and its downstream targets in neonatal skeletal muscle, but the stimulation of protein synthesis also is dependent upon amino acid availability.

Amino acids and insulin are regulators of muscle protein synthesis in neonatal pigs

The stage of development between birth and weaning in mammals is a period of very rapid growth that is crucial for the long-term well-being of the animal. The rate of protein deposition in neonatal animals is very high because dietary protein is efficiently utilized to increase body protein mass. Our studies in neonatal pigs have shown that this high efficiency of protein deposition is largely due to the marked increase in protein synthesis after feeding, and this response is particularly profound in the skeletal muscle. The enhanced stimulation of muscle protein synthesis in neonates after feeding is independently mediated by the rise in insulin and amino acids and this response declines with age. Intracellular signaling components that respond to the postprandial rise in amino acids and insulin have been identified and their activation has been shown to be elevated in skeletal muscle of neonatal pigs after a meal and to decrease with development. The enhanced activation of these components in the amino acid and insulin signaling pathways in neonatal muscle contributes to the high rate of muscle protein synthesis and rapid gain in skeletal muscle mass in newborn pigs, which are essential determinants of efficient growth during development.

Differences in the glucose-induced insulin response and the peripheral insulin responsiveness between neonatal calves of the Belgian Blue, Holstein-Friesian, and East Flemish breeds

Decreased insulin sensitivity (IS) in dairy cows supports milk yield but increases the risk for metabolic and reproductive disorders. Although several inducers of decreased IS are known, it is unclear to what extent it is congenitally determined. The main aim was to investigate differences in IS between neonatal calves of the Belgian Blue (BB) breed, reared for beef production, and the Holstein-Friesian (HF) breed, reared for milk yield. Additionally, a small number of East Flemish (EF) calves, a local dual-purpose breed, were compared with the 2 other breeds. Ten BB, 12 HF, and 4 EF calves with similar age, ration, and housing were selected. In the intravenous glucose tolerance test, blood samples were taken at regular intervals after an intravenous glucose bolus of 150 mg/kg. Area under the curve (AUC), peak concentration, and elimination rate of insulin and glucose were computed. The quantitative insulin sensitivity check index (QUICKI) and revised QUICKI were computed using basal glucose, insulin, and nonesterified fatty acid concentrations. In the intravenous insulin tolerance test, blood samples were obtained from 4 calves of each breed at regular times after an intravenous insulin challenge of 0.05 IU/kg. Based on the decline in glucose concentrations relative to basal levels, the insulin-stimulated blood glucose response was computed. Basal insulin concentrations were higher in HF (1.58 +/- 0.40 microU/mL) than in BB calves (0.35 +/- 0.09 mmol/L). Compared with BB calves, HF and EF calves had higher basal glucose concentrations (4.40 +/- 0.16 vs. 5.70 +/- 0.35 and 5.81 +/- 0.13 mmol/L, respectively), insulin peak concentrations (4.62 +/- 1.09 vs. 9.70 +/- 1.45 and 16.44 +/- 5.58 microU/mL, respectively), insulin AUC (86.71 +/- 18.81 vs. 222.65 +/- 45.00 and 293.69 +/- 109.22 microU/mL.min, respectively), and glucose AUC (256.22 +/- 17.53 vs. 335.66 +/- 18.74 and 321.03 +/- 10.05 mmol/L min, respectively). Glucose elimination rates were lower in HF (1.37 +/- 0.22%/min) than in BB calves (2.35 +/- 0.25%/min). The QUICKI was lower in HF and EF than in BB calves (0.52 +/- 0.039 and 0.57 +/- 0.068 vs. 0.76 +/- 0.038, respectively), and the revised QUICKI was lower in HF (0.86 +/- 0.11) than in BB calves (1.59 +/- 0.17). The insulin-stimulated blood glucose response did not differ between breeds. Because management differences were negligible, our results suggest breed-specific differences in glucose partitioning and IS. These findings may reflect different rearing purposes of the breeds, although extrapolation of the data to adult animals should be done cautiously.

Feeding rapidly stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing translation initiation

Food consumption increases protein synthesis in most tissues by promoting translation initiation, and in the neonate, this increase is greatest in skeletal muscle. In this study, we aimed to identify the currently unknown time course of changes in the rate of protein synthesis and the activation of factors involved in translation in neonatal muscle after a meal. After overnight food deprivation, 36 5- to 7-d-old piglets were administered a nutritionally complete bolus i.g. meal and were killed immediately before or 30, 60, 90, 120, or 240 min later. The increase in skeletal muscle protein synthesis peaked 30 min after the meal and this was sustained through 120 min, returning to baseline thereafter. The relative proportion of polysomes to nonpolysomes was higher only after 30 min. Protein kinase B phosphorylation peaked 30 min after feeding and returned to baseline by 90 min. The phosphorylation of mammalian target of rapamycin, eukaryotic initiation factor (eIF) 4E binding protein (4E-BP1), ribosomal protein S6, and eIF4G was increased within 30 min of feeding and persisted through 120 min, but all had returned to baseline by 240 min. The association of 4E-BP1.eIF4E was reduced and eIF4E.eIF4G increased 30 min after receiving a meal, remaining so for 120 min, before returning to baseline at 240 min. Thus, in neonates, food consumption rapidly increased skeletal muscle protein synthesis by enhancing translation initiation and this increase was sustained for at least 120 min after the meal but returned to baseline by 240 min after the feeding.

The effect of the protein level in a pre-starter diet on the post-hatch performance and activation of ribosomal protein S6 kinase in muscle of neonatal broilers

The cytoplasmic serine/threonine ribosomal protein S6 kinase (S6K1) plays a critical role in controlling protein translation. There is evidence that amino acids regulate S6K1 and protein synthesis in avian species, but the effect of dietary protein level on the activation of S6K1 in neonatal chicks is unknown. Therefore, the aim of the present experiment was to investigate the effect of different protein levels, supplied during the first 5 d post-hatch, on body growth, breast muscle development and on the activation of S6K1 and its downstream target, the S6, in neonatal chicks. Chicks were fed a pre-starter diet during the first 5 d post-hatch containing low (19·6 % crude protein (CP); LP), medium (23·1 % CP; MP) or high (26·7 % CP) levels (HP) of protein. Weight gain of chicks fed the HP diet was higher (P < 0·05) compared with those fed the LP diet during day (d)3–d5 and the numerical advantage of this group was maintained from d2 to d7. On d2 and d3, greater levels of S6K1 and S6 phosphorylation and/or activity were observed in chicks receiving the HP diet compared with LP and MP diets, without differences between results of the latter two dietary treatments. In conclusion, the present results suggest that early protein nutrition impacts the development of broiler chicks.

Regulation of muscle growth in neonates

PURPOSE OF REVIEW

This review reports recent findings on the multiple factors that regulate skeletal muscle growth in neonates.

RECENT FINDINGS

Skeletal muscle is the fastest growing protein mass in neonates. The high rate of neonatal muscle growth is due to accelerated rates of protein synthesis accompanied by the rapid accumulation of muscle nuclei. Feeding profoundly stimulates muscle protein synthesis in neonates and the response decreases with age. The feeding-induced stimulation of muscle protein synthesis is modulated by enhanced sensitivity to the postprandial rise in insulin and amino acids. Insulin and amino acid signaling components have been identified that are involved in the feeding-induced stimulation of protein synthesis in neonatal muscle. The enhanced activation of these signaling components in skeletal muscle of the neonate contributes to the high rate of muscle protein synthesis and rapid gain in muscle protein mass in neonates.

SUMMARY

Recent findings suggest that the immature muscle has a heightened capacity to activate signaling cascades that promote translation initiation in response to the postprandial rise in insulin and amino acids thereby enabling their efficient utilization for muscle growth. This capacity is further supported by enhanced satellite cell proliferation, but how these two processes are linked remains to be established.