Modelling Mammary Metabolism in the Dairy Cow to Predict Milk Constituent Yield, with Emphasis on Amino Acid Metabolism and Milk Protein Production: Model Evaluation

Valine and nonessential amino acids affect bidirectional transport rates of leucine and isoleucine in bovine mammary epithelial cells

A more complete understanding of the mechanisms controlling AA transport in mammary glands of dairy cattle will help identify solutions to increase nitrogen feeding efficiency on farms. It was hypothesized that Ala, Gln, and Gly (NEAAG), which are actively transported into cells and exchanged for all branched-chain AA (BCAA), may stimulate transport of BCAA, and that Val may antagonize transport of the other BCAA due to transporter competition. Thus, we evaluated the effects of varying concentrations of NEAAG and Val on transport and metabolism of the BCAA Ala, Met, Phe, and Thr by bovine mammary epithelial cells. Primary cultures of bovine mammary epithelial cells were assigned to treatments of low (70% of mean in vivo plasma concentrations of lactating dairy cows) and high (200%) concentrations of Val and NEAAG (LVal and LNEAAG, HVal and HNEAAG, respectively) in a 2 × 2 factorial design. Cells were preloaded with treatment media containing [15N]-labeled AA for 24 h. The [15N]-labeled media were replaced with treatment media containing [13C]-labeled AA. Media and cells were harvested from plates at 0, 0.5, 1, 5, 15, 30, 60, and 240 min after application of the [13C]-labeled AA and assessed for [15N]- and [13C]-AA label concentrations. The data were used to derive transport, transamination, irreversible loss, and protein-synthesis fluxes. All Val fluxes, except synthesis of rapidly exchanging tissue protein, increased with the HVal treatment. Interestingly, the rapidly exchanging tissue protein, transamination, and irreversible-loss rate constants decreased with HVal, indicating that the significant flux increases were primarily driven by mass action with the cells resisting the flux increases by downregulating activity. However, the decreases could also reflect saturation of processes that would drive down the mass-action rate constants. This is supported by decreases in the same rate constants for Ile and Leu with HVal. This could be due to either competition for shared transamination and oxidation reactions or a reduction in enzymatic activity. Also, NEAAG did not affect Val fluxes, but influx and efflux rate constants increased for both Val and Leu with HNEAAG, indicating an activating substrate effect. Overall, AA transport rates generally responded concordantly with extracellular concentrations, indicating the transporters are not substrate-saturated within the in vivo range. However, BCAA transamination and oxidation enzymes may be approaching saturation within in vivo ranges. In addition, System L transport activity appeared to be stimulated by as much as 75% with high intracellular concentrations of Ala, Gln, and Gly. High concentrations of Val antagonized transport activity of Ile and Leu by 68% and 15%, respectively, indicating competitive inhibition, but this was only observable at HNEAAG concentrations. The exchange transporters of System L transport 8 of the essential AA that make up approximately 40% of milk protein, so better understanding this transporter is an important step for increased efficiency.

A meta-analysis of the relationship between milk protein production and absorbed amino acids and digested energy in dairy cattle

Milk protein production is the largest draw on AA supplies for lactating dairy cattle. Prior NRC predictions of milk protein production have been absorbed protein (MP)-based and utilized a first-limiting nutrient concept to integrate the effects of energy and protein, which yielded poor accuracy and precision (root mean squared error (RMSE) > 21%). Using a meta-data set gathered, various alternative equation forms considering MP, absorbed total essential AA (EAA), absorbed individual EAA, and digested energy (DE) supplies as additive drivers of production were evaluated, and all were found to be superior in statistical performance to the first limitation approach (RMSE = 14-15%). Inclusion of DE intake and a quadratic term for MP or absorbed EAA supplies were found to be necessary to achieve intercept estimates (non-productive protein use) that were similar to the factorial estimates of NASEM. The partial linear slope for MP was found to be 0.409, which is consistent with the observed slope bias of -0.34g/g when a slope of 0.67 was used for MP efficiency in a first-limiting nutrient system. Replacement of MP with the supplies of individual absorbed EAA expressed in g/d and a common quadratic across the EAA resulted in unbiased predictions with improved statistical performance as compared with MP-based models. Based on Akaike's Information Criterion (AIC) and biological consistency, the best equations included absorbed His, Ile, Lys, Met, Thr, the non-essential AA, and individual DE intakes from fatty acids, neutral detergent fiber, residual organic matter, and starch. Several also contained a term for absorbed Leu. These equations generally had RMSE of 14.3% and a concordance correlations (CCC) of 0.76. Based on the common quadratic and individual linear terms, milk protein response plateaus were predicted at approximately 320 g/d of absorbed His, Ile, and Lys; 395 g/d of absorbed Thr; 550 g/d of absorbed Met; and 70 g/d of absorbed Leu. Therefore, responses to each except Leu are almost linear throughout the normal in vivo range. De-aggregation of the quadratic term and parsing to individual absorbed EAA resulted in non-biological estimates for several EAA indicating over-parameterization. Expression of the EAA as g/100 g of total absorbed EAA or as ratios of DE intake and using linear and quadratic terms for each EAA resulted in similar statistical performance, but the solutions had identifiability problems and several non-biological parameter estimates. The use of ratios also introduced nonlinearity in the independent variables which violates linear regression assumptions. Further screening of the global model using absorbed EAA expressed as g/d with a common quadratic using an all-models approach, and exhaustive cross-evaluation indicated the parameter estimates for body weight, all 4 DE terms, His, Ile, Lys, Met, and the common quadratic term were stable, while estimates for Leu and Thr were known with less certainty. Use of independent and additive terms and a quadratic expression in the equation results in variable efficiencies of conversion. The additivity also provides partial substitution among the nutrients. Both of these prevent establishment of fixed nutrient requirements in support of milk protein production.

Valine promotes milk synthesis by regulating PKM2 nuclear accumulation and histone H3 acetylation through the TAS1R1-mTOR-DDX39B signaling pathway

Valine, a branched-chain amino acid found in dairy cows, has been recognized for its critical role in milk synthesis. However, the precise effect of valine on lactation in dairy cows remains an area of investigation. In our study, bovine mammary epithelial cells (BMECs) were isolated to explore the mechanism through which valine enhances milk synthesis. The results showed that 100 μM valine significantly boosted the milk synthesis via TAS1R1-mTOR-DDX39B signaling pathway in BMECs. Subsequent investigations revealed that DDX39B governs the accumulation of PKM2 in the nuclei of BMECs. This nuclear buildup of PKM2 weakened the interaction between HDAC3 and histone H3, leading to an increase in the acetylation levels of histone H3. In an vivo context, the 0.25 % valine-enriched drinking water notably elevated in the expression of milk protein and fat in these mice. Further examination showed that 0.25 % valine drinking water considerably augmented the protein expression levels of DDX39B, PKM2, and p-mTOR in the mice mammary glands. In summary, our results suggest that valine, by modulating the TAS1R1-mTOR-DDX39B signaling pathway, directs the accumulation of PKM2 in the nucleus. This, in turn, escalates the acetylation levels of histone H3, promoting the synthesis of both milk protein and fat.

Estimation of Individual Glucose Reserves in High-Yielding Dairy Cows

Glucose plays a central role in numerous physiological processes in dairy cows related to immune defence and milk production. A lack of glucose impairs both objectives, although to different degrees. A method for the estimation of glucose balance (GB) in dairy cows was developed to assess glucose reserves in the intermediary metabolism. Digestive fluxes of glucogenic carbon were individually estimated via the Systool Web application based on data on body weight (BW), dry matter intake (DMI), and chemical analyses of feedstuffs. Fluxes of endogenous precursors glycerol, alanine and L-lactate and the glucose demand imposed by major glucose-consuming organs were deduced from BW, lactose yield and lactation stage. GB was calculated for 201 lactations (1 to 105 DIM) of 157 cows fed isoenergetic rations. Individual DMI, BW and milk yield were assessed on a daily basis. The results showed that the GB varied greatly between cows and lactation stages. In the first week of lactation, average daily GB reached levels close to zero (3.2 ± 13.5 mol C) and increased as lactation progressed. Most cows risk substantial shortages of glucose for maintenance during the first weeks of lactation. In face of the specific role of glucose for the functional capability of the immune function, the assessment of glucose reserves is a promising measure for the identification of cows at risk of impaired immunocompetence.

Compositional and functional properties of milk and dairy products derived from cows fed pasture or concentrate-based diets

Worldwide milk production is predominantly founded on indoor, high-concentrate feeding systems, whereas pasture-based feeding systems are most common in New Zealand and Ireland but have received greater attention recently in countries utilizing conventional systems. Consumer interest in 'pasture-fed' dairy products has also increased, arising from environmental, ethical, and nutritional concerns. A substantial body of research exists describing the effect of different feeding strategies on the composition of milk, with several recent studies focusing on the comparison of pasture- and concentrate-based feeding regimes. Significant variation is typically observed in the gross composition of milk produced from different supplemental feeds, but various changes in the discrete composition of macromolecular components in milk have also been associated with dietary influence, particularly in relation to the fatty acid profile. Changes in milk composition have also been shown to have implications for milk and dairy product processability, functionality and sensory properties. Methods to determine the traceability of dairy products or verify marketing claims such as 'pasture-fed' have also been established, based on compositional variation due to diet. This review explores the effects of feed types on milk composition and quality, along with the ultimate effect of diet-induced changes on milk and dairy product functionality, with particular emphasis placed on pasture- and concentrate-based feeding systems.

Assessing amino acid uptake and metabolism in mammary glands of lactating dairy cows intravenously infused with methionine, lysine, and histidine or with leucine and isoleucine

The objective of this study was to evaluate the effect of jugular infusions of 2 groups of AA on essential AA (EAA) transport and metabolism by mammary glands. Four Holstein cows in second lactation (66 ± 10 d in milk) were used in 4 × 4 Latin square design with a 2 × 2 factorial arrangement of treatments. Treatments were jugular infusions of saline; Met, Lys, and His (MKH); Ile and Leu (IL); or MKH plus IL (MKH+IL). Each period consisted of 8 d of no infusion followed by 8 d of jugular vein infusion of the treatment solutions. Amino acids were infused at rates of 21 g of Met, 38 g of Lys, 20 g of His, 50 g of Leu, and 22 g of Ile per day. Cows were fed a basal diet consisting of 15.2% crude protein with adequate rumen degradable protein but 15% deficient in MP based on estimates by Cornell Net Carbohydrate and Protein System (v6.5). On the last day of each period, ¹³C-AA derived from algae was infused into the jugular vein over 6 h, and blood and milk samples were collected before, during, and after infusion. Plasma and milk samples were analyzed for AA isotopic enrichment, and a mammary compartmental model was fitted to the data to derive bidirectional transport and metabolism rates for individual EAA. Influx of Leu increased with IL, whereas influx of other EAA was not different among treatments. Cellular efflux of Met and Lys to venous plasma represented 12 to 34% of influx, whereas cellular efflux of Phe and BCAA represented 29 to 59% of influx. Increased efflux/influx ratios of Ile and Leu with IL but not Met and Lys with MKH demonstrated that increased Ile and Leu influx was mostly returned to plasma resulting in no change in net uptake or efficiency. The isotope results showed that mammary net uptake of Lys and Ile increased during MKH infusion. Net uptake of Met increased with MKH but only in the absence of IL. Catabolism of Lys and Met only increased with MKH alone, resulting in decreased efficiency for milk protein, which demonstrated that Ile and Leu infusion can spare Lys and Met for milk protein synthesis. Total AA uptake to milk output was not different from 1, implying the catabolized Met and Lys contributed nitrogen to nonessential AA. Overall, EAA uptake and metabolism in mammary glands of dairy cows varied across individual EAA and responded differently to respective AA supplements. In addition, uptake, retention, and end use of AA by mammary tissue is variable and dependent on the mix of AA provided. This variability, depending on the mix of AA absorbed, will change the efficiency of utilization of individual AA at the mammary gland level and consequently the whole-body level. Thus, it is inaccurate to use a fixed, constant efficiency within and across AA to represent tissue activity.

Identification of octopus peptide and its promotion of β-casein synthesis in a mouse mammary epithelial cell line

Octopus protein hydrolysate has been reported to increase milk yield and milk protein production. In this paper, the utilization and underlying mechanisms of bioactive peptide fractions from octopus protein hydrolysate on β-casein expression in mouse mammary epithelial cells (HC11) were investigated. Fraction OPH3-1 significantly stimulated cell proliferation and β-casein synthesis in HC11 cells, which was purified by ultra-filtration and gel-filtration chromatography. The MWs of the peptides from OPH3-1 ranged from 525-2,578 Da and consisted of 7-26 amino acid residues. Most of the peptides demonstrated the typical characteristics of milk protein synthesis promotion, especially MGLAGPR, MGDVLNF, EAPLMHV, and TEAPLMHV. Additionally, the mRNA abundances of mTOR, S6K1, 4EBP1, JAK2, and STAT5 were significantly enhanced by OPH3-1, which was consistent with the increased β-casein expression. These results suggest that the OPH3-1 peptides can promote the proliferation of mammary epithelial cells and increase β-casein synthesis. PRACTICAL APPLICATIONS: Breastfeeding mothers are generally recommended to take octopus soup as a daily diet to promote lactation. The peptides fraction OPH3-1 from the enzymatic hydrolysate of Octopus vulgaris which was revealed to significantly stimulate mammary epithelial cell proliferation and β-casein synthesis was obtained. This study suggests that octopus peptides can be used as nutritional supplements to increase the quantity and quality of milk production.

Examination of methionine stimulation of gene expression in dairy cow mammary epithelial cells using RNA-sequencing

In this research communication, a cell model with elevated β-CASEIN synthesis was established by stimulating bovine mammary epithelial cells with 0.6 mM methionine, and the genome-wide gene expression profiles of methionine-stimulated cells and untreated cells were investigated by RNA sequencing. A total of 458 differentially expressed genes (DEGs; 219 upregulated and 239 downregulated) were identified between the two groups. Gene Ontology (GO) analysis showed that the two highest-ranked GO terms in 'molecular function' category were 'binding' and 'catalytic activity', suggesting that milk protein synthesis in methionine-stimulated cells requires induction of gene expression to increase metabolic activity. Kyoto Encyclopedia of Genes and Genomes analysis revealed that within the 'environmental information processing' category, the subcategory that is most highly enriched for DEGs was 'signal transduction'. cGMP-PKG, Rap1, calcium, cAMP, PI3K-AKT, MAPK, and JAK-STAT are the pathways with the highest number of DEGs, suggesting that these signaling pathways have potential roles in mediating methionine-induced milk protein synthesis in bovine mammary epithelial cells. This study provides valuable insights into the physiological and metabolic adaptations in cells stimulated with methionine. Understanding the regulation of this transition is essential for effective intervention in the lactation process.

Use of Mechanistic Nutrition Models to Identify Sustainable Food Animal Production

To feed people in the coming decades, an increase in sustainable animal food production is required. The efficiency of the global food production system is dependent on the knowledge and improvement of its submodels, such as food animal production. Scientists use statistical models to interpret their data, but models are also used to understand systems and to integrate their components. However, empirical models cannot explain systems. Mechanistic models yield insight into the mechanism and provide guidance regarding the exploration of the system. This review offers an overview of models, from simple empirical to more mechanistic models. We demonstrate their applications to amino acid transport, mass balance, whole-tissue metabolism, digestion and absorption, growth curves, lactation, and nutrient excretion. These mechanistic models need to be integrated into a full model using big data from sensors, which represents a new challenge. Soon, training in quantitative and computer science skills will be required to develop, test, and maintain advanced food system models.

Supply of methionine and arginine alters phosphorylation of mechanistic target of rapamycin (mTOR), circadian clock proteins, and α-s1-casein abundance in bovine mammary epithelial cells

Methionine (Met) and arginine (Arg) regulate casein protein abundance through alterations in activity of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway.

The "Grass-Fed" Milk Story: Understanding the Impact of Pasture Feeding on the Composition and Quality of Bovine Milk

Milk is a highly nutritious food that contains an array of macro and micro components, scientifically proven to be beneficial to human health. While the composition of milk is influenced by a variety of factors, such as genetics, health, lactation stage etc., the animal's diet remains a key mechanism by which its nutrition and processing characteristics can be altered. Pasture feeding has been demonstrated to have a positive impact on the nutrient profile of milk, increasing the content of some beneficial nutrients such as Omega-3 polyunsaturated fatty acids, vaccenic acid, and conjugated linoleic acid (CLA), while reducing the levels of Omega-6 fatty acids and palmitic acid. These resultant alterations to the nutritional profile of "Grass-Fed" milk resonate with consumers that desire healthy, "natural", and sustainable dairy products. This review provides a comprehensive comparison of the impact that pasture and non-pasture feeding systems have on bovine milk composition from a nutritional and functional (processability) perspective, highlighting factors that will be of interest to dairy farmers, processors, and consumers.

The effect of nanoemulsified methionine and cysteine on the in vitro expression of casein in bovine mammary epithelial cells

Objective

Dairy cattle nutrient requirement systems acknowledge amino acid (AAs) requirements in aggregate as metabolizable protein (MP) and assume fixed efficiencies of MP used for milk protein. Regulation of mammary protein synthesis may be associated with AA input and milk protein output. The aim of this study was to evaluate the effect of nanoemulsified methionine and cysteine on the in-vitro expression of milk protein (casein) in bovine mammary epithelial cells (MAC-T cells).

Methods

Methionine and cysteine were nonionized using Lipoid S 75 by high-speed homogenizer. The nanoemulsified AA particle size and polydispersity index were determined by dynamic light scattering correlation spectroscopy using a high-performance particle sizer instrument. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay was performed to determine the cytotoxicity effect of AAs with and without nanoionization at various concentrations (100 to 500 μg/mL) in mammary epithelial cells. MAC-T cells were subjected to 100% of free AA and nanoemulsified AA concentration in Dulbecco’s modified Eagle medium/nutrient mixture F-12 (DMEM/F12) for the analysis of milk protein (casein) expression by the quantitative reverse transcription polymerase chain reaction method.

Results

The AA-treated cells showed that cell viability tended to decrease (80%) in proportion to the concentration before nanogenesis, but cell viability increased as much as 90% after nanogenesis. The analysis of the expression of genetic markers related to milk protein indicated that; α_s2-casein increased 2-fold, κ-casein increased 5-fold, and the amount of unchanged β-casein expression was nearly doubled in the nanoemulsified methionine-treated group when compared with the free-nanoemulsified methionine-supplemented group. On the contrary, the non-emulsified cysteine-administered group showed higher expression of genetic markers related to milk protein α_s2-casein, κ-casein, and β-casein, but all the genetic markers related to milk protein decreased significantly after nanoemulsification.

Conclusion

Detailed knowledge of factors, such nanogenesis of methionine, associated with increasing cysteine and decreasing production of genetic markers related to milk protein (casein) will help guide future recommendations to producers for maximizing milk yield with a high level of milk protein casein.

Arteriovenous blood metabolomics: An efficient method to determine the key metabolic pathway for milk synthesis in the intra-mammary gland

The present study aimed to identify metabolic signature changes of the arteriovenous metabolome and the new metabolites that involved in mammary biological process during milk synthesis. GC/MS-based metabolomics profiling of arteriovenous plasma from 30 lactating dairy cows fed three diets identified a total of 144 metabolites. Phenylalanine and tyrosine, involved in aminoacyl-tRNA biosynthesis and phenylalanine metabolism, were shown higher expression in the artery than in the vein based on both GC/MS and targeted analysis for cows fed both alfalfa hay diet and rice straw diet. Mammary uptake or clearance of citric acid, stearic acid, oleic acid, fructose, β-mannosylglycerate, 4-hydroxybutyrate, and D-talose were significantly correlated with milk performance or feed intake, indicating that these metabolites might be newly identified precursors or indicators of milk synthesis. This comprehensive assessment of metabolic changes in the arteriovenous metabolome will provide a fundamental understanding of the key metabolites involved in milk synthesis and shows implications of how metabolites from arteriovenous plasma across MG are involved in biological processes or physiological functions for milk synthesis. The newly identified metabolites from the present study provide potential new targeted insights into the study of physiological process for milk synthesis in the MG.

60S ribosomal protein L35 regulates β-casein translational elongation and secretion in bovine mammary epithelial cells

60S ribosomal protein L35 (RPL35) is an important component of the 60S ribosomal subunit and has a role in protein translation and endoplasmic reticulum (ER) docking. However, few studies have investigated RPL35 in eukaryotes and much remains to be learned. Here, we analyzed the function of RPL35 in β-casein (CSN2) synthesis and secretion in bovine mammary epithelial cells (BMECs). We found that methionine (Met) could promote the expressions of CSN2 and RPL35. Analysis of overexpression and inhibition of RPL35 confirmed that it could mediate the Met signal and regulate CSN2 expression. The mechanism of CSN2 regulation by RPL35 was analyzed by coimmunoprecipitation (Co-IP), colocalization, fluorescence resonance energy transfer (FRET) and gene mutation. We found that RPL35 could control ribosome translational elongation during synthesis of CSN2 by interacting with eukaryotic translational elongation factor 2 (eEF2), and that eEF2 was the signaling molecule downstream of RPL35 controlling this process. RPL35 could also control the secretion of CSN2 by locating it to the ER. Taken together, these results revealed that, RPL35 was an important positive regulatory factor involving in the Met-mediated regulation of CSN2 translational elongation and secretion.

Administration of Exogenous Growth Hormone Is Associated with Changes in Plasma and Intracellular Mammary Amino Acid Profiles and Abundance of the Mammary Gland Amino Acid Transporter SLC3A2 in Mid-Lactation Dairy Cows

The objectives of this study were to (1) identify changes in plasma and mammary intracellular amino acid (AA) profiles in dairy cows treated with growth hormone (GH), and (2) evaluate the expression of mammary gland genes involved in the transport of AA identified in (1). Eight non-pregnant (n = 4 per group) lactating dairy cows were treated with a single subcutaneous injection of either a slow-release formulation of commercially available GH (Lactotropin 500 mg) or physiological saline solution. Six days after treatment, cows were milked and blood collected from the jugular vein for the analysis of free AA in the plasma. Cows were euthanized and mammary tissue harvested. Treatment with GH increased milk, protein, fat and lactose yields, with no effect on dry matter intake. Plasma concentrations of lysine and group I AA decreased significantly, and arginine, methionine, tyrosine and arginine-family AA tended to decrease in GH-treated cows. Concentrations of intracellular glycine, serine and glutamate increased significantly, with a trend for decreased arginine observed in the mammary gland of GH-treated cows. A trend for increased concentrations of intracellular total AA, NEAA and arginine-family AA were observed in the mammary gland of GH-treated cows. Variance in the concentration of plasma methionine, tyrosine, valine, alanine, ornithine, BCAA, EAA was significantly different between treatments. Variance in the concentration of intracellular lysine, valine, glutamine, EAA and group II was significantly different between treatments. AA changes were associated with increased mRNA abundance of the mammary gland AA transporter SLC3A2. We propose that these changes occur to support increased milk protein and fatty acid production in the mammary gland of GH-treated cows via potential mTOR pathway signaling.

Effects of Arginine concentration on the in vitro expression of Casein and mTOR pathway related genes in mammary epithelial cells from dairy cattle

Arginine (Arg) is a conditionally-essential amino acid that is taken up by bovine mammary gland in excess of its output in milk. In this study we evaluated the effects of Arg concentration on the expression of casein and signaling pathway-related genes in mammary epithelial cells. The treatments (applied for 24 h) were designed to be devoid of Arg 0X (control; 0.00 mg/L), resemble the profile of Arg in casein (Arg 1X; 278.00 mg/L), be deficient [Arg 0.25X (69.50 mg/L) and Arg 0.5X (139.00 mg/L)], or be in excess of the amount in casein [Arg 2X (556.00 mg/L), Arg 4X (1,112 mg/L), and Arg 8X (2,224 mg/L)]. The expression of CSN1S, CSN3 and mTOR in the experimental groups was higher than those of the control group (P<0.05). Except for Arg 0.25X and Arg 8X (P>0.05), the expression of CSN1S2, CSN2 and JAK2 in other experimental groups was higher (P<0.05) than those in the control group. Except for Arg 8X (P>0.05), the expression of STAT5 in the other experimental groups was higher than those of the control (P<0.05). It also was observed that except for Arg 0.5X, the S6K expression was higher in other experimental groups than the control (P<0.05). In contrast, except for Arg 0.25X the other experimental groups resulted in lower 4EBP1 expression than the control (P<0.05). Among groups, the expression of CSN1S1, CSN1S2, CSN2, CSN3, JAK2, STAT5, mTOR and S6K gene was highest with Arg 2X (P<0.05); the reverse was true for 4EBP1 gene, with the lowest expression in this group (P<0.05). Taken together, Arg appears to play an important role in the transcriptional regulation of casein genes and mTOR-related genes in bovine mammary epithelial cells.

Ratio of lysine to methionine alters expression of genes involved in milk protein transcription and translation and mTOR phosphorylation in bovine mammary cells

This study was conducted to determine the optimum ratio of lysine and methionine (Lys:Met) to enhance milk protein concentration in vitro, focusing on the regulation of genes related to the JAK2-STAT5 and the mammalian target of rapamycin (mTOR) signaling pathways. A preliminary dose response study revealed that casein concentration peaked (2.5-2.7 ppm) at a supplemental Lys concentration of 1.2 mM and Met at 0.5 mM. At the peak casein concentration cell proliferation rate also was higher. Furthermore, the expression of CSN1S1, CSN1S2, CSN2, CSN3, LALBA, JAK2, STAT5, and MTOR was upregulated with both Lys and Met compared with the control. A subsequent experiment was conducted as a 5 × 3 factorial design with supplemental Lys plus Met at different ratios. When the supplemental concentration of Lys was 1.2 mM and Met was 0.4 mM (∼3:1), the concentration of casein peaked. Therefore, we measured gene expression, mTOR protein expression, and phosphorylated mTOR (p-mTOR) in cultures incubated with 3:1 Lys:Met (Lys&Met). Expression of CSN1S1 and LALBA were the most highly expressed genes (P < 0.01). The upregulation of CSN2, CSN3, CSN1S2 isoforms (P < 0.01) and JAK2, ELF5, mTOR (P < 0.05) was also observed. Total mTOR protein expression was greater (P < 0.05) with Lys alone and also Lys&Met. However, Lys&Met resulted in the greatest (P < 0.05) p-mTOR. Results suggest that peak concentration of casein at a supplemental 3:1 Lys:Met is driven in part via upregulation of the mRNA expression of components of the JAK-STAT and mTOR pathways.

Exploring metabolism flexibility in complex organisms through quantitative study of precursor sets for system outputs

Background

When studying metabolism at the organ level, a major challenge is to understand the matter exchanges between the input and output components of the system. For example, in nutrition, biochemical models have been developed to study the metabolism of the mammary gland in relation to the synthesis of milk components. These models were designed to account for the quantitative constraints observed on inputs and outputs of the system. In these models, a compatible flux distribution is first selected. Alternatively, an infinite family of compatible set of flux rates may have to be studied when the constraints raised by observations are insufficient to identify a single flux distribution. The precursors of output nutrients are traced back with analyses similar to the computation of yield rates. However, the computation of the quantitative contributions of precursors may lack precision, mainly because some precursors are involved in the composition of several nutrients and because some metabolites are cycled in loops.

Results

We formally modeled the quantitative allocation of input nutrients among the branches of the metabolic network (AIO). It corresponds to yield information which, if standardized across all the outputs of the system, allows a precise quantitative understanding of their precursors. By solving nonlinear optimization problems, we introduced a method to study the variability of AIO coefficients when parsing the space of flux distributions that are compatible with both model stoichiometry and experimental data. Applied to a model of the metabolism of the mammary gland, our method made it possible to distinguish the effects of different nutritional treatments, although it cannot be proved that the mammary gland optimizes a specific linear combination of flux variables, including those based on energy. Altogether, our study indicated that the mammary gland possesses considerable metabolic flexibility.

Conclusion

Our method enables to study the variability of a metabolic network with respect to efficiency (i.e. yield rates). It allows a quantitative comparison of the respective contributions of precursors to the production of a set of nutrients by a metabolic network, regardless of the choice of the flux distribution within the different branches of the network.

Gene networks driving bovine mammary protein synthesis during the lactation cycle

A crucial role for both insulin and mTOR in the regulation of milk protein synthesis is emerging. Bovine mammary biopsies harvested during late-pregnancy through end of subsequent lactation were used to evaluate via quantitative PCR the expression of 44 genes involved in pathways of insulin, mTOR, AMPK, and Jak2-Stat5 signalling and also glucose and amino acid (AA) transporters. We observed an increased expression during lactation of ELF5, AA and glucose transporters, insulin signaling pathway components, MAPK14, FRAP1, EIF4EBP2, GSK3A and TSC1 among mTOR signaling-related genes. Among ribosomal components RPL22 was down-regulated. The overall data support a central role of AA and glucose transporters and insulin signaling through mTOR for the regulation of protein synthesis in bovine mammary gland. Furthermore, the existence of translational competition favoring the translation of milk protein transcripts was inferred from the combined dataset.

On the use of milk composition measures to predict the energy balance of dairy cows

Milk composition varies with energy status and was proposed for measuring energy balance on-farm, but the accuracy of prediction using monthly samples is not high. With automated sampling and inline milk analysis, a much higher measurement frequency is possible, and thus improved accuracy of energy balance determination may be expected. Energy balance was evaluated using data in which milk composition was measured at each milking. Three breeds (Danish Holstein, Danish Red, and Jerseys) of cows (623 lactations from 299 cows) in parities 1, 2, and 3+ were used. Data were smoothed using a rolling local regression. Energy balance (EBal) was calculated from changes in body reserves (body weight and body condition score). The relationship between EBal and milk measures was quantified by partial least squares regression (PLS) using group means data. For each day in lactation, the within-breed and parity mean EBal and mean milk measures were used. Further PLS was done using the individual cow data. The initial PLS models included 25 combinations of milk measures allowing a range of nonlinear effects. These combinations were as follows: days in milk (DIM); DIM raised to the powers 2, 3, and 4; milk yield; fat content; protein content; lactose content; fat yield; protein yield; lactose yield; fat:protein ratio; fat:lactose ratio; protein:lactose ratio; and milk yield:lactose ratio, together with 10 "diff()" variables. These variables are the current minus the previous value of the milk measure in question. Using group means data, a very high proportion (96%) of the variability in EBal was explained by the PLS model. A reduced model with only 6 variables explained 94% of the variation in EBal. This model had a prediction error of 3.82 MJ/d; the 25-variable model had a prediction error of 3.11 MJ/d. When using individual rather than group means data, the PLS prediction error was 17.3 MJ/d. In conclusion, the mean Ebal of different parities of Holstein, Danish Red, and Jersey cows can be predicted throughout lactation using 1 common equation based on DIM, milk yield, milk fat, and milk protein measures.

A Model of Net Amino Acid Absorption and Utilization by the Portal-Drained Viscera of the Lactating Dairy Cow

A more complete understanding of amino acid (AA) metabolism by the various tissues of the body is required to improve upon current systems for predicting the use of absorbed AA. The objective of this work was to construct and parameterize a model of net removal of AA by the portal-drained viscera (PDV). Six cows were prepared with arterial, portal, and hepatic catheters and infused abomasally with 0, 200, 400, or 600 g of casein daily. Casein infusion increased milk yield quadratically and tended to increase milk protein yield quadratically. Arterial concentrations of a number of essential AA increased linearly with respect to infusion amount. When infused casein was assumed to have a true digestion coefficient of 0.95, the minimum likely true digestion coefficient for noninfused duodenal protein was found to be 0.80. Net PDV use of AA appeared to be linearly related to total supply (arterial plus absorption), and extraction percentages ranged from 0.5 to 7.25% for essential AA. Prediction errors for portal vein AA concentrations ranged from 4 to 9% of the observed mean concentrations. Removal of AA by PDV represented approximately 33% of total postabsorptive catabolic use, including use during absorption but excluding use for milk protein synthesis, and was apparently adequate to support endogenous N losses in feces of 18.4 g/d. As 69% of this use was from arterial blood, increased PDV catabolism of AA in part represents increased absorption of AA in excess of amounts required by other body tissues. Based on the present model, increased anabolic use of AA in the mammary and other tissues would reduce the catabolic use of AA by the PDV.