Mechanisms involved in the nutritional regulation of mRNA translation: features of the avian model

Early Growth and Protein-Energy Metabolism in Chicken Lines Divergently Selected on Ultimate pH

In chickens, a divergent selection on the Pectoralis major pHu allowed the creation of the pHu+ and pHu− lines, which represent a unique model for studying the biological control of carbohydrate storage in muscle. The present study aimed to describe the early mechanisms involved in the establishment of pHu+ and pHu− phenotypes. At hatching, pHu+ chicks were slightly heavier but exhibited lower plasma glucose and triglyceride and higher uric acid. After 5 days, pHu+ chicks exhibited higher breast meat yield compared to pHu− while their body weight was no different. At both ages, in vivo muscle glycogen content was lower in pHu+ than in pHu− muscles. The lower ability of pHu+ chicks to store carbohydrate in their muscle was associated with the increased expression of SLC2A1 and SLC2A3 genes coding glucose transporters 1 and 3, and of CS and LDHα coding key enzymes of oxidative and glycolytic pathways, respectively. Reduced muscle glycogen content at hatching of the pHu+ was concomitant with higher activation by phosphorylation of S6 kinase 1/ribosomal protein S6 pathway, known to activate protein synthesis in chicken muscle. In conclusion, differences observed in muscle at slaughter age in the pHu+ and pHu− lines are already present at hatching. They are associated with several changes related to both carbohydrate and protein metabolism, which are likely to affect their ability to use eggs or exogenous nutrients for muscle growth or energy storage.

Endogenous CSE/Hydrogen Sulfide System Regulates the Effects of Glucocorticoids and Insulin on Muscle Protein Synthesis

Aims

Insulin and glucocorticoids play crucial roles in skeletal muscle protein turnover. Fast-twitch glycolytic fibres are more susceptible to atrophy than slow-twitch oxidative fibres. Based on accumulating evidence, hydrogen sulfide (H₂S) is a physiological mediator of this process. The regulatory effect of H₂S on protein synthesis in fast-twitch fibres was evaluated.

Results

A NaHS (sodium hydrosulfide) injection simultaneously increased the diameter of M. pectoralis major (i.e., fast-twitch glycolytic fibres) and activated the mammalian target of the rapamycin (mTOR)/p70S6 kinase (p70S6K) pathway. Dexamethasone (DEX) inhibited protein synthesis, downregulated mTOR and p70S6K phosphorylation, and suppressed the expression of the cystathionine γ-lyase (CSE) protein in myoblasts. The precursor of H₂S, L-cysteine, completely abolished the inhibitory effects of DEX. The CSE inhibitor DL-propargylglycine (PAG) completely abrogated the effects of RU486 on blocking the suppressive effects of DEX. The H₂S donor NaHS increased the H₂S concentrations and abrogated the inhibitory effects of DEX on protein synthesis. Insulin increased protein synthesis and upregulated CSE expression. However, PAG abrogated the stimulatory effects of insulin on protein synthesis and the activity of the mTOR/p70S6K pathway.

Innovation

These results demonstrated that CSE/H₂S regulated protein synthesis in fast-twitch muscle fibres, and glucocorticoids and insulin regulated protein synthesis in an endogenous CSE/H₂S system-dependent manner.

Conclusions

The results from the present study suggest that the endogenous CSE/H₂S system regulates fast-twitch glycolytic muscle degeneration and regeneration.

Constant heat stress reduces skeletal muscle protein deposition in broilers

BACKGROUND

This experiment was conducted to evaluate the effects of constant heat stress on growth performance and protein metabolism in skeletal muscle of Arbor Acres broilers.

RESULTS

Two hundred and seventy 21-day-old Arbor Acres broilers with similar body weight (1298 ± 28 g) were selected for a 3-week trial (29-49 days of age). The broilers were randomly assigned to three groups including the control group, constant heat stress group and pair-fed group. Up-regulation of the rectal temperature and the mRNA expression of heat shock protein 70 in liver indicate that the model for constant heat stress was success. The average daily gain, feed conversion ratio, breast and thigh muscle weight, percentage of breast muscle, crude protein content in breast and thigh muscle in constant heat stress group were significantly lower than in control group and pair-fed group. Serum uric acid content and the glutamic-oxaloacetic transaminase activity were significantly higher, while protein content and glutamic-pyruvate transaminase activity were significantly lower in liver of heat stress group than of the control and pair-fed groups. The expression of insulin-like growth factor 1, phosphatidylinositol 3-kinase and p70S6 kinase associated with protein synthesis were lower in breast muscle but higher in thigh muscle in heat stress group compared to the control or fed-pair groups. In thigh muscles, the expression of muscle ring-finger protein-1 and MAFbx associated with protein degradation were higher in the heat stress group than in the control and pair-fed groups.

CONCLUSION

Poor performance of the birds under heat stress may be due to lower synthesis and increased degradation of proteins.

Modulation of the insulin anabolic signalling cascade in growing chickens byn-3 PUFA

n-3 PUFA are crucial for health and development. Their effects as regulators of lipid and glucose metabolism are well documented. They also appear to affect protein metabolism, especially by acting on insulin sensitivity. The aim of the present study was to investigate the role ofn-3 PUFA, i.e. the precursor α-linolenic acid (ALA) 18 : 3n-3 or long-chain PUFA (LC-PUFA), in chickens, by focusing on their potential function as co-regulators of the insulin anabolic signalling cascade. Ross male broilers were divided into six dietary treatment groups. Diets were isoproteic (22 % crude protein) and isoenergetic (12·54 MJ metabolisable energy/kg) and contained similar lipid levels (6 %) provided by different proportions of various lipid sources: oleic sunflower oil rich in 18 : 1n-9 as control; fish oil rich in LC-PUFA; rapeseed and linseed oils providing ALA. The provision of diets enriched withn-3 PUFA, i.e. rich in LC-PUFA or in the precursor ALA, for 3 weeks improved the growth performance of chickens, whereas that of only the ALA diet enhanced the development of the pectoralis major muscle. At 23 d of age, we studied the insulin sensitivity of the pectoralis major muscle and liver of chickens after an intravenous injection of insulin or saline. The present results indicate that the activation patterns ofn-3 PUFA are different in the liver and muscles. An ALA-enriched diet may improve insulin sensitivity in muscles, with greater activation of the insulin-induced 70 kDa ribosomal protein S6 kinase/ribosomal protein S6 pathway involved in the translation of mRNA into proteins, thereby potentially increasing muscle protein synthesis and growth. Our findings provide a basis on which to optimise dietary fatty acid provision in growing animals.

The effect of albumen removal before incubation (embryonic protein under-nutrition) on the post-hatch performance, regulators of protein translation activation and proteolysis in neonatal broilers

Albumen was removed from broiler eggs before the start of incubation to induce prenatal protein under-nutrition in chicken embryos. With this method, the direct effect of protein deficiency was investigated, differing from mammalian models manipulating the maternal diet where indirect, hormonal effects can interfere. Based on the estimated albumen/egg weight ratio, 10 % of albumen was removed with an 18G needle, after making a hole at the sharp end of the egg with another 18G needle. Eggs were taped thereafter. The sham group underwent the same procedure, except that no albumen was removed. Control eggs did not receive any treatment. The removal of albumen decreased both embryonic and post-hatch body weight up to day 7 compared with the control group. On embryonic day 18, embryos from the albumen-deprived group had higher plasma uric acid levels compared with the sham (P= 0·016) and control (P= 0·009) groups. Moreover, a lower plasma amino acid concentration was observed at hatch compared with the sham (P= 0·038) and control (P= 0·152) groups. These findings indicate an altered protein metabolism. At hatch, a higher mRNA expression of muscle ring finger-1 (MuRF1), a gene related to proteolysis, was observed in albumen-deprived chicks compared with the control and sham chicks, together with an up-regulated expression of atrogin-1 (another atrogene) at this time point in the male protein-deficient chicks. These findings suggest that muscle proteolysis is transiently increased by the removal of albumen before the start of incubation. No evidence was found for altered protein synthesis capacity and translational efficiency in albumen-deprived chicks.

Responses to nutrients in farm animals: implications for production and quality

It is well known that any quantitative (energy and protein levels) and qualitative (nature of the diet, nutrient dynamic) changes in the feeding of animals affect metabolism. Energy expenditure and feed efficiency at the whole-body level, nutrient partitioning between and within tissues and organs and, ultimately, tissue and organ characteristics are the major regulated traits with consequences on the quality of the meat and milk produced. Recent progress in biology has brought to light important biological mechanisms which explain these observations: for instance, regulation by the nutrients of gene expression or of key metabolic enzyme activity, interaction and sometimes cross-regulation or competition between nutrients to provide free energy (ATP) to living cells, indirect action of nutrients through a complex hormonal action, and, particularly in herbivores, interactions between trans-fatty acids produced in the rumen and tissue metabolism. One of the main targets of this nutritional regulation is a modification of tissue insulin sensitivity and hence of insulin action. In addition, the nutritional control of mitochondrial activity (and hence of nutrient catabolism) is another major mechanism by which nutrients may affect body composition and tissue characteristics. These regulations are of great importance in the most metabolically active tissues (the digestive tract and the liver) and may have undesirable (i.e. diabetes and obesity in humans) or desirable consequences (such as the production of fatty liver by ducks and geese, and the production of fatty and hence tasty meat or milk with an adapted fatty acid profile).

Regulators of protein metabolism are affected by cyclical nutritional treatments with diets varying in protein and energy content

There is evidence that the E3 ubiquitin ligases muscle ring finger-1 (MuRF1) and atrogin-1, which mediate the ubiquitination of certain proteins and thereby their proteolysis, are regulated by cyclical nutritional treatments varying in lysine content. In order to explore further the regulatory mechanisms involved in metabolic adaptation to dietary changes, we investigated the effects of daily variations in energy [2800 (E-) followed by 3200 kcal/kg (E+)], protein [230 (P+) followed by 150g/kg (P-)] or both [E-P+ followed by E+P-] on muscle protein metabolism in 2-week-old male broiler chickens. Growth performance was similar for all treatments. Expression of atrogin-1 and MuRF1 was changed by alternation of diets varying in protein (higher expression with P- vs. P+) and energy content (higher expression with E- vs. E+). The expression of atrogin-1 was regulated with mixed diets (increase in E+P- vs. E-P+) but not that of MuRF1. Such regulation may involve the mammalian target of rapamycin (mTOR), which was more phosphorylated with P+ than with P-. Eukaryotic initiation factor 4E binding protein, p70S6 kinase and ribosomal protein S6, which are mTOR targets known to control protein synthesis, were highly activated by increased protein content (P+ vs. P-). The mechanisms coordinating the protein synthesis/proteolysis balance remain to be characterized.

Methionine deprivation regulates the S6K1 pathway and protein synthesis in avian QM7 myoblasts without activating the GCN2/eIF2 alpha cascade

Amino acids modulate mRNA translation through the 70 kDa ribosomal protein S6 kinase (S6K1) and the general control nondepressible 2 protein kinase (GCN2)/eukaryotic initiation factor 2 alpha eIF2 alpha pathways. The aim of the present study was therefore to explore the signaling cascades potentially modulated by methionine availability in quail muscle QM7 myoblasts using media providing all other amino acids. Methionine deprivation caused a lower S6K1 phosphorylation compared with control (Ctl) cells. Supplying the methionine-deprived media with L- and DL-methionine isomers restored S6K1 phosphorylation to the levels observed in Ctl cells. Methionine also regulated downstream S6K1 targets (i.e. ribosomal protein S6 and eukaryotic elongation factor 2), modulated translation preinitiation complex (PIC) assembly, and stimulated protein synthesis. Replacing the lacking methionine with D-methionine or its hydroxyanalog [2-hydroxy-(4-methylthio) butanoic acid] did not restore S6K1 activation or protein synthesis. Conversely, the S6K1 pathway was activated by a methionine precursor, the ketoanalog of methionine. Methionine availability regulated the GCN2/eIF2 alpha pathway. However, our results indicate that methionine deprivation led to lower protein synthesis without activating eIF2 alpha phosphorylation, a process known to limit the formation of the 43S PIC. Using the amino acid alcohol methioninol did not decrease S6K1 phosphorylation or activity and did not alter the regulation of protein synthesis by methionine. These findings suggest that methionine exerts an effect on S6K1 signaling and protein synthesis in avian QM7 myoblasts through a mechanism partly independent of the global regulation via tRNA charging.

Effects of heat exposure on Akt/S6K1 signaling and expression of genes related to protein and energy metabolism in chicken (Gallus gallus) pectoralis major muscle

In order to improve understanding of the heat-induced changes in muscle growth, we determined the expression of genes related to protein and energy metabolism in the pectoralis major muscle of chickens. We also explored the protein kinase B (PKB also called Akt)/p70 S6 kinase (S6K1)/S6 pathway that mediates anabolic signals thereby regulating metabolism and hypertrophic/atrophic balance. Four-week-old chickens were exposed to 32 or 22 degrees C for 1 week. Chickens from both groups were then fasted for 16 h or left fed, and submitted to an oral administration of glucose-arginine to induce an anabolic response (30-min treatment) or left untreated. High ambient temperature and the associated decrease in feed intake modified the expression of certain energy-related genes (e.g. -40% for PGC-1alpha) and protein metabolism (e.g. about +80% for atrogin-1), but the expression of several muscle metabolism-related genes considered here was unchanged. The capacity for muscle protein synthesis, i.e. RNA/protein ratio, was reduced in warm conditions (approximately -20%). Slightly lower activation of S6 induced by glucose-arginine treatment was found at 32 degrees C compared to 22 degrees C, which might indicate somewhat lower efficiency of mRNA translation. Analysis of glucose/insulin balance suggested changes in glucose metabolism under heat exposure. However, this remains to be characterized.

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.

Tissue-specific regulation of S6K1 by insulin in chickens divergently selected for growth

In chickens, insulin injection leads to the activation of the early steps of insulin receptor signaling in liver but not in muscles. Paradoxically, muscle p70 S6 kinase (S6K1), a kinase controlling protein synthesis and growth, was markedly activated in response to insulin. The aim of this study was to further investigate S6K1 regulation and activation using chickens divergently selected for growth, i.e. fast- (FGL) and slow- (SGL) growing lines. In the Pectoralis major muscle, insulin stimulated S6K1 phosphorylation on T389 in FGL and SGL chickens, whereas S6K1 phosphorylation on T421/S424 was increased by insulin only in FGL chickens. Moreover, insulin-related increase in muscle S6K1 activity was greater in FGL chickens than in SGL chickens. Surprisingly, liver S6K1 was insulin insensitive in the two genotypes. Such difference of regulation between tissues and between genotypes was not observed for the protein kinase B, which is involved in insulin signaling upstream of S6K1, or for eukaryotic initiation factor 4E-binding protein. Interestingly, insulin-activated a S6K1 downstream target, the ribosomal protein S6, irrespective of tissue, suggesting that a pathway different of the S6K1 cascade may be involved in S6 phosphorylation in chicken liver. In conclusion, the regulation of S6K1 differs between the liver and muscle and between chickens divergently selected for growth. Our results suggest a potential involvement of S6K1 in the control of muscle growth and an open issue concerning S6K1 function in chicken liver.

Involvement of the ERK1/2 MAPK pathway in insulin-induced S6K1 activation in avian cells

In mammals, insulin regulates S6K1, a key enzyme involved in the control of protein synthesis, via the well-documented phosphoinositide-3'kinase (PI3K) pathway. Conversely, S6K1 is activated by insulin in avian muscle despite the relative insulin insensitivity of the PI3K pathway in this tissue. Mitogen-activated protein kinase (MAPK) cascade is another insulin sensitive pathway. The aim of this study was to explore the potential involvement of the ERK1/2 MAPK pathway in the control of p70 S6 kinase (S6K1) in avian species. Firstly, we characterized ERK1/2 MAPK in various chicken tissues. ERK2 was the only isoform detected in avian species whatever the tissue studied. We also showed that ERK2 is activated in vivo by insulin in chicken muscle. The regulation and the role of ERK2 in insulin signaling were next investigated in chicken hepatoma cells (LMH) and primary myoblasts. Insulin stimulation led to ERK2 and S6K1 phosphorylation, and concomitantly increased kinase activity. U0126, an inhibitor of the ERK MAPK pathway, completely abolished insulin-induced S6K1 phosphorylation and activity in chicken myoblasts, whereas its effect was only partial in LMH cells. In conclusion, these results show that ERK1/2 MAPK is involved in the control of S6K1 by insulin in chicken cells, particularly myoblasts.

Mechanisms Regulating Feed Intake, Energy Expenditure, and Body Weight in Poultry

To achieve energy balance and maintain a constant BW, changes in feed intake and energy expenditure must be coordinated and tightly regulated. This may not hold true for some poultry species intensively selected for such economically important traits as growth and meat production. For example, the modern commercial broiler breeder does not adequately control voluntary feed intake to meet its energy requirements and maintain energy balance. As a consequence, feeding must be limited in these birds to avoid overconsumption and excessive fattening during production. It is important to determine a genetic basis to help explain this situation and to offer potential strategies for producing more efficient poultry. This review summarizes what is currently known about the control of feed intake and energy expenditure at the gene level in birds. Highly integrated regulatory systems have been identified that link the control of feeding with the sensing of energy status. How such systems function in poultry is currently being explored. One example recently identified in chickens is the adenosine monophosphate-activated protein kinase pathway that links energy sensing with modulation of metabolic activity to maintain energy homeostasis at the cellular level. In the hypothalamus, this same pathway may also play an important role in regulating feed intake and energy expenditure commensurate with perceived whole body energy needs. Genes encoding key regulatory factors such as hormones, neuropeptides, receptors, enzymes, and transcription factors produce the molecular components that make up intricate and interconnected neural, endocrine, and metabolic pathway networks linking peripheral tissues with the central nervous system. Moreover, coordinate expression of specific gene groups can establish functional pathways that respond to and are regulated by such factors as hormones, nutrients, and metabolites. Thus, with a better understanding of the genetic and molecular basis for regulating feed intake and energy expenditure in birds important progress can be made in developing, evaluating, and managing more efficient commercial poultry lines.

Insulin and amino acid availability regulate atrogin-1 in avian QT6 cells

New evidence has demonstrated that the expression of major genes, termed atrogenes, controls the ubiquitin-proteasome proteolytic pathway. The present work aimed to study the impact of insulin and amino acids on the expression of one of these atrogenes, the E3 ubiquitin ligase Muscle Atrophy F box (MAFbx, also called atrogin-1), in quail muscle (QT6) fibroblasts. First, we characterized atrogin-1 in QT6 cells and demonstrated the insulin sensitivity of these cells. Second, we showed that insulin reduced atrogin-1 mRNA via the phosphatidylinositol-3'kinase (PI3K)/protein kinase B (PKB or AKT)/target of rapamycin (TOR) pathway. Atrogin-1 expression also depended on the availability of an individual amino acid, i.e., methionine. Moreover, the amino acid-induced reduction of atrogin-1 was inhibited by rapamycin, indicating the involvement of the TOR pathway in such regulation. In conclusion, expression of the ubiquitin ligase atrogin-1 is regulated by both insulin and amino acids through the TOR pathway.