Alpha(S1)-casein is required for the efficient transport of beta- and kappa-casein from the endoplasmic reticulum to the Golgi apparatus of mammary epithelial cells

A Comparative Analysis of the Gene Expression Profiles in the Mammary Glands of Lactating and Nonlactating Mares at the Second Month of Gestation

To investigate molecular regulation involved in lactation during pregnancy, this study focused on the transcriptomic profiles of mammary tissue from lactating and non-lactating Mongolian mares at the second month of gestation. A total of 4197 differentially expressed genes were identified by comparing mammary tissues from pregnant mares at two different states, including 1974 differentially expressed genes such as the milk protein-related genes a-s1-casein (CSN1S1), k-casein (CSN3), lactalbumin (LALBA), and lactoferrin (LTF), which were highly expressed in the lactating mares group, and overall, these differentially expressed genes were mainly associated with biological processes such as endoplasmic reticulum protein processing, the Toll-like receptor signaling pathway, steroid biosynthesis, cytokine-cytokine receptor interactions, and amino sugar and nucleotide glycolysis. These findings serve as a foundation for investigating the molecular underpinnings of lactation in pregnant equids.

Site specific insertion of a transgene into the murine α-casein (CSN1S1) gene results in the predictable expression of a recombinant protein in milk

Gene loci of highly expressed genes provide ideal sites for transgene expression. Casein genes are highly expressed in mammals leading to the synthesis of substantial amounts of casein proteins in milk. The α-casein (CSN1S1) gene has assessed as a site of transgene expression in transgenic mice and a mammary gland cell line. A transgene encoding an antibody light chain gene (A1L) was inserted into the α-casein gene using sequential homologous and site-specific recombination. Expression of the inserted transgene is directed by the α-casein promoter, is responsive to lactogenic hormone activation, leads to the synthesis of a chimeric α-casein/A1L transgene mRNA, and secretion of the recombinant A1L protein into milk. Transgene expression is highly consistent in all transgenic lines, but lower than that of the α-casein gene (4%). Recombinant A1L protein accounted for 0.5% and 1.6% of total milk protein in heterozygous and homozygous transgenic mice, respectively. The absence of the α-casein protein in homozygous A1L transgenic mice leads to a reduction of total milk protein and delayed growth of the pups nursed by these mice. Overall, the data demonstrate that the insertion of a transgene into a highly expressed endogenous gene is insufficient to guarantee its abundant expression.

Review: Genetic and protein variants of milk caseins in goats

The milk casein genes in goats, are highly polymorphic genes with numerous synonymous and non-synonymous mutations. So far, 20 protein variants have been reported in goats for alpha-S1-casein, eight for beta-casein, 14 for alpha-S2-casein, and 24 for kappa-casein. This review provides a comprehensive overview on identified milk casein protein variants in goat and non-coding DNA sequence variants with some affecting the expression of the casein genes. The high frequency of some casein protein variants in different goat breeds and geographical regions might reflect specific breeding goals with respect to milk processing characteristics, properties for human nutrition and health, or adaptation to the environment. Because protein names, alongside the discovery of protein variants, go through a historical process, we linked old protein names with new ones that reveal more genetic variability. The haplotypes across the cluster of the four genetically linked casein genes are recommended as a valuable genetic tool for discrimination between breeds, managing genetic diversity within and between goat populations, and breeding strategies. The enormous variation in the casein proteins and genes is crucial for producing milk and dairy products with different properties for human health and nutrition, and for genetic improvement depending on local breeding goals.

α-casein micelles-membranes interaction: Flower-like lipid protein coaggregates formation

BACKGROUND

Environmental conditions regulate the association/aggregation states of proteins and their action in cellular compartments. Analysing protein behaviour in presence of lipid membranes is fundamental for the comprehension of many functional and dysfunctional processes. Here, we present an experimental study on the interaction between model membranes and α-casein. α-casein is the major component of milk proteins and it is recognised to play a key role in performing biological functions. The conformational properties of this protein and its capability to form supramolecular structures, like micelles or irreversible aggregates, are key effectors in functional and pathological effects.

METHODS

By means of quantitative fluorescence imaging and complementary spectroscopic methods, we were able to characterise α-casein association state and the course of events induced by pH changes, which regulate the interaction of this molecule with membranes.

RESULTS

The study of these complex dynamic events revealed that the initial conformation of the protein critically regulates the fate of α-casein, size and structure of the newly formed aggregates and their effect on membrane structures. Disassembly of micelles due to modification in electrostatic interactions results in increased membrane structure rigidity which accompanies the formation of protein lipid flower-like co-aggregates with protein molecules localised in the external part.

GENERAL SIGNIFICANCE

These results may contribute to the comprehension of how the initial state of a protein establishes the course of events that occur upon changes in the molecular environment. These events which may occur in cells may be essential to functional, pathological or therapeutical properties specifically associated to casein proteins.

The Effect of Alpha s1 Genotype on Some Physiological and Chemical Milk Characteristics in Garganica Goat

In the present study individual milk samples were collected from 80 Garganica goats and the population was genotyped for CSN1S1. Seven different alleles were observed with the A allele found as predominant, followed by the F and B allele, and 14 different genotypes were observed. A proteomic approach was used to classify the individual milk on the basis of protein abundance: four levels of αs1-casein (CN) expression were identified as high (HAS-1), intermediate (IAS-1), low (LAS-1), and null (NAS-1). The present paper aimed to investigate the influence of the level of expression of αs1-casein fraction in milk on proteolytic activities, chemical composition, and renneting properties of individual Garganica goat milk. Results from the present study evidenced that the level of expression of αs1-casein fraction in Garganica goat milk affected chemical composition, curd firmness, and indigenous proteolytic enzymes. In particular, fat, protein, and casein content were higher in HAS-1 and IAS-1 compared to LAS-1 and NAS-1. The SCC decreased passing from the HAS-1 to the NAS-1 and the curd firmness displayed the highest value in HAS-1. The activity of plasmin (PL) was higher in HAS-1 and IAS-1 groups compared to the LAS-1 and NAS-1; whereas, PL plasminogen-derived (PG) activity was the highest in NAS-1 group. On the contrary, the activity of elastase was the highest in LAS-1, intermediate in the IAS-1 group, and the lowest in HAS-1 and NAS-1 groups. Present data demonstrated that the level of synthesis of αs1-CN reflects the efficiency of the mammary gland in the activation of the cellular mechanisms and influences the proteolytic activity in milk, especially the PL-PG system.

Dynamic miRNA Landscape Links Mammary Gland Development to the Regulation of Milk Protein Expression in Mice

Simple Summary

Milk synthesis is vital for maintaining the normal growth of newborn animals. Abnormal mammary gland development leads to a decrease in female productivity and the overall productivity of animal husbandry. This study characterized the dynamic miRNA expression profile during the process of mammary gland development, and identified a novel miRNA regulating expression of β-casein—an important milk protein. The results are valuable for studying mammary gland development, increasing milk production, improving the survival rate of pups, and promoting the development of animal husbandry.

Abstract

Mammary gland morphology varies considerably between pregnancy and lactation status, e.g., virgin to pregnant and lactation to weaning. Throughout these critical developmental phases, the mammary glands undergo remodeling to accommodate changes in milk production capacity, which is positively correlated with milk protein expression. The purpose of this study was to investigate the microRNA (miRNA) expression profiles in female ICR mice’s mammary glands at the virgin stage (V), day 16 of pregnancy (P16d), day 12 of lactation (L12d), day 1 of forced weaning (FW1d), and day 3 of forced weaning (FW3d), and to identify the miRNAs regulating milk protein gene expression. During the five stages of testing, 852 known miRNAs and 179 novel miRNAs were identified in the mammary glands. Based on their expression patterns, the identified miRNAs were grouped into 12 clusters. The expression pattern of cluster 1 miRNAs was opposite to that of milk protein genes in mammary glands in all five different stages. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses revealed that the predicted target genes of cluster 1 miRNAs were related to murine mammary gland development and lactation. Furthermore, fluorescence in situ hybridization (FISH) analysis revealed that the novel-mmu-miR424-5p, which belongs to the cluster 1 miRNAs, was expressed in murine mammary epithelial cells. The dual-luciferase reporter assay revealed that an important milk protein gene—β-casein (CSN2)—was regarded as one of the likely targets for the novel-mmu-miR424-5p. This study analyzed the expression patterns of miRNAs in murine mammary glands throughout five critical developmental stages, and discovered a novel miRNA involved in regulating the expression of CSN2. These findings contribute to an enhanced understanding of the developmental biology of mammary glands, providing guidelines for increasing lactation efficiency and milk quality.

Organelles coordinate milk production and secretion during lactation: Insights into mammary pathologies

The mammary gland undergoes a spectacular series of changes during its development and maintains a remarkable capacity to remodel and regenerate during progression through the lactation cycle. This flexibility of the mammary gland requires coordination of multiple processes including cell proliferation, differentiation, regeneration, stress response, immune activity, and metabolic changes under the control of diverse cellular and hormonal signaling pathways. The lactating mammary epithelium orchestrates synthesis and apical secretion of macromolecules including milk lipids, milk proteins, and lactose as well as other minor nutrients that constitute milk. Knowledge about the subcellular compartmentalization of these metabolic and signaling events, as they relate to milk production and secretion during lactation, is expanding. Here we review how major organelles (endoplasmic reticulum, Golgi apparatus, mitochondrion, lysosome, and exosome) within mammary epithelial cells collaborate to initiate, mediate, and maintain lactation, and how study of these organelles provides insight into options to maintain mammary/breast health.

Verification of cheeses authenticity by mass spectrometry

Cheeses are a group of fermented dairy products that are produced all over the world in various forms and flavours. Milk, especially sheep or goat milk, is still regarded as an expensive raw material in the world, which makes milk and milk products highly attractive as a fraud target. Most often, such fraud includes partial or complete substitution with cheaper sorts of milk (e.g. bovine milk). The aim of this work was to verify the authenticity of 27 cheeses commonly emerging on the Czech food market. The cheeses were distinguished on the basis of milk animal species origin. For this purpose, two mass spectrometry techniques were used: matrix-assisted laser desorption/ionization with time of flight mass spectrometry together with principal component analysis method and liquid chromatography coupled with electrospray ionization and quadrupole time-of-flight mass spectrometry. The results were a partial success, because the cheeses could only be partially distinguished with the first mass spectrometry technique probably because of the influence of some protein additive materials in cheeses. The second technique allowed for collecting higher quality results and thus appears to be highly suitable for the research task.

Human Milk Fat Globule Membrane Contains Hundreds of Abundantly Expressed and Nutritionally Beneficial Proteins That Are Generally Lacking in Caprine Milk

BACKGROUND

Bioactive proteins from milk fat globule membrane (MFGM) play extensive roles in cellular processes and defense mechanisms in infants. The aims of this study were to identify differences in protein compositions in human and caprine MFGM using proteomics and evaluate possible nutritional benefits of caprine milk toward an infant's growth, as an alternative when breastfeeding or human milk administration is not possible or inadequate.

MATERIALS AND METHODS

Human and caprine MFGM proteins were isolated and analyzed, initially by polyacrylamide gel electrophoresis, and subsequently by quadrupole time-of-flight liquid chromatography-mass spectrometry. This was then followed by database search and gene ontology analysis. In general, this method selectively analyzed the abundantly expressed proteins in milk MFGM.

RESULTS

Human MFGM contains relatively more abundant bioactive proteins compared with caprine. While a total of 128 abundant proteins were detected in the human MFGM, only 42 were found in that of the caprine. Seven of the bioactive proteins were apparently found to coexist in both human and caprine MFGM.

RESULTS/DISCUSSION

Among the commonly detected MFGM proteins, lactotransferrin, beta-casein, lipoprotein lipase, fatty acid synthase, and butyrophilin subfamily 1 member A1 were highly expressed in human MFGM. On the other hand, alpha-S1-casein and EGF factor 8 protein, which are also nutritionally beneficial, were found in abundance in caprine MFGM. The large number of human MFGM abundant proteins that were generally lacking in caprine appeared to mainly support human metabolic and developmental processes.

CONCLUSION

Our data demonstrated superiority of human MFGM by having more than one hundred nutritionally beneficial and abundantly expressed proteins, which are clearly lacking in caprine MFGM. The minor similarity in the abundantly expressed bioactive proteins in caprine MFGM, which was detected further, suggests that it is still nutritionally beneficial, and therefore should be included when caprine milk-based formula is used as an alternative.

Technical note: Quantification of caseins from a crude extract of mammary epithelial cells

En masse secretion of milk proteins, notably the caseins in the form of casein micelles, is a unique feature of the milk-secreting mammary epithelial cell. Caseins are therefore specific markers of these cells and constitute an ideal tool to monitor their differentiation, as well as functional status, during the development of the gland. To use them as such, a reliable method for quantitative analysis of the caseins from mammary cells or tissue is needed. Here we show that the caseins are heat-stable, a feature that leads to their complete extraction from a complex cellular extract by boiling. This allows for high enrichment and direct analysis of the caseins, even when they are poorly expressed in the starting material.

Fatty acid profile and coagulating ability of milk from Jersey and Friesian cows fed whole flaxseed

The experiment was carried out to evaluate the effects of a moderate level of flaxseed administration on milk coagulation properties and fatty acid profile of milk from two different breeds. The experiment was performed on 20 Italian Friesian cows and 20 Jersey cows divided into 2 groups of 10 animals each. The experimental diets were (1) a traditional diet (CON) administrated as unifeed and no supplemental fat and (2) a diet supplemented with 0·5 kg/d of whole flaxseed (FS). Cows were milked twice daily and milk yield was recorded. Milk samples were analysed at 1, 15, and 30 d of the experiment for composition, pH, and milk coagulation properties. To verify the effects of flaxseed administration on the coagulation properties of milk from Friesian and Jersey cows, an electrophoresis study on casein fractions was performed. Milk fatty acid profile can be improved by administrating a moderate level of flaxseed in the diet, however, milk fatty acid profile from Friesian and Jersey cows showed different contents of C18 : 1 trans-11, SFA and MUFA. The results demonstrated that milk coagulating ability can be increased by flaxseed administration in both breeds as a result of different aggregation of casein micelles.

Distinct roles of prolactin, epidermal growth factor, and glucocorticoids in β-casein secretion pathway in lactating mammary epithelial cells

Beta-casein is a secretory protein contained in milk. Mammary epithelial cells (MECs) synthesize and secrete β-casein during lactation. However, it remains unclear how the β-casein secretion pathway is developed after parturition. In this study, we focused on prolactin (PRL), epidermal growth factor (EGF), and glucocorticoids, which increase in blood plasma and milk around parturition. MECs cultured with PRL, EGF and dexamethasone (DEX: glucocorticoid analog) developed the β-casein secretion pathway. In the absence of PRL, MECs hardly expressed β-casein. EGF enhanced the expression and secretion of β-casein in the presence of PRL and DEX. DEX treatment rapidly increased secreted β-casein concurrent with enhancing β-casein expression. DEX also up-regulated the expression of SNARE proteins, such as SNAP-23, VAMP-8 and Syntaxin-12. Furthermore, PRL and DEX regulated the expression ratio of α_s1-, β- and κ-casein. These results indicate that PRL, EGF and glucocorticoids have distinct roles in the establishment of β-casein secretion pathway.

Lactogenic hormones regulate mammary protein synthesis in bovine mammary epithelial cells via the mTOR and JAK–STAT signal pathways

The expression of CSN3, hormone receptor, the expression of genes regulating the mTOR, JAK–STAT signal pathways, and the relative content of к-casein as well as total casein were determined in the present study to explore the mechanism of the effect of lactogenic hormones on milk-protein synthesis in bovine mammary epithelial cells. The results showed that apoptosis of the cells was increased by inhibitor LY294002, while the expressions of genes encoding PKB, Rheb, PRAS40 and S6K1 in the mTOR signal pathway, JAK2, STAT5A in the JAK–STAT signal pathway, and genes encoding INSR, PRLR, NR3C1 and CSN3 were all downregulated, and the relative contents of κ-casein and total casein were decreased in the mammary epithelial cells compared with those in the control group. Comparatively, the inhibitory effects of AG-490 were more profound than those of LY294002, and the double block using both inhibitors had a greater effect than the single block. The CSN3 gene expression was downregulated and the content of milk casein was decreased by the inhibitors. In addition, the expression of the hormone receptor genes was downregulated. Our results suggest that lactogenic hormones, via their receptors in the membrane, regulated the JAK–STAT and m-TOR signal pathways, and affected cell proliferation and apoptosis, leading to changes in milk-protein synthesis.

The membrane-associated form of α(s1)-casein interacts with cholesterol-rich detergent-resistant microdomains

Caseins, the main milk proteins, interact with colloidal calcium phosphate to form the casein micelle. The mesostructure of this supramolecular assembly markedly influences its nutritional and technological functionalities. However, its detailed molecular organization and the cellular mechanisms involved in its biogenesis have been only partially established. There is a growing body of evidence to support the concept that α(s1)-casein takes center stage in casein micelle building and transport in the secretory pathway of mammary epithelial cells. Here we have investigated the membrane-associated form of α(s1)-casein in rat mammary epithelial cells. Using metabolic labelling we show that α(s1)-casein becomes associated with membranes at the level of the endoplasmic reticulum, with no subsequent increase at the level of the Golgi apparatus. From morphological and biochemical data, it appears that caseins are in a tight relationship with membranes throughout the secretory pathway. On the other hand, we have observed that the membrane-associated form of α(s1)-casein co-purified with detergent-resistant membranes. It was poorly solubilised by Tween 20, partially insoluble in Lubrol WX, and substantially insoluble in Triton X-100. Finally, we found that cholesterol depletion results in the release of the membrane-associated form of α(s1)-casein. These experiments reveal that the insolubility of α(s1)-casein reflects its partial association with a cholesterol-rich detergent-resistant microdomain. We propose that the membrane-associated form of α(s1)-casein interacts with the lipid microdomain, or lipid raft, that forms within the membranes of the endoplasmic reticulum, for efficient forward transport and sorting in the secretory pathway of mammary epithelial cells.

The Application of Genomic Technologies to Investigate the Inheritance of Economically Important Traits in Goats

Goat genomics has evolved at a low pace because of a lack of molecular tools and sufficient investment. Whilst thousands and hundreds of quantitative trait loci (QTL) have been identified in cattle and sheep, respectively, about nine genome scans have been performed in goats dealing with traits as conformation, growth, fiber quality, resistance to nematodes, and milk yield and composition. In contrast, a great effort has been devoted to the characterization of candidate genes and their association with milk, meat, and reproduction phenotypes. In this regard, causal mutations have been identified in the αS1-casein gene that has a strong effect on milk composition and the PIS locus that is linked to intersexuality and polledness. In recent times, the development of massive parallel sequencing technologies has allowed to build a reference genome for goats as well as to monitor the expression of mRNAs and microRNAs in a broad array of tissues and experimental conditions. Besides, the recent design of a 52K SNP chip is expected to have a broad impact in the analysis of the genetic architecture of traits of economic interest as well as in the study of the population structure of goats at a worldwide scale.

A novel highly divergent protein family identified from a viviparous insect by RNA-seq analysis: a potential target for tsetse fly-specific abortifacients

In tsetse flies, nutrients for intrauterine larval development are synthesized by the modified accessory gland (milk gland) and provided in mother's milk during lactation. Interference with at least two milk proteins has been shown to extend larval development and reduce fecundity. The goal of this study was to perform a comprehensive characterization of tsetse milk proteins using lactation-specific transcriptome/milk proteome analyses and to define functional role(s) for the milk proteins during lactation. Differential analysis of RNA-seq data from lactating and dry (non-lactating) females revealed enrichment of transcripts coding for protein synthesis machinery, lipid metabolism and secretory proteins during lactation. Among the genes induced during lactation were those encoding the previously identified milk proteins (milk gland proteins 1-3, transferrin and acid sphingomyelinase 1) and seven new genes (mgp4-10). The genes encoding mgp2-10 are organized on a 40 kb syntenic block in the tsetse genome, have similar exon-intron arrangements, and share regions of amino acid sequence similarity. Expression of mgp2-10 is female-specific and high during milk secretion. While knockdown of a single mgp failed to reduce fecundity, simultaneous knockdown of multiple variants reduced milk protein levels and lowered fecundity. The genomic localization, gene structure similarities, and functional redundancy of MGP2-10 suggest that they constitute a novel highly divergent protein family. Our data indicates that MGP2-10 function both as the primary amino acid resource for the developing larva and in the maintenance of milk homeostasis, similar to the function of the mammalian casein family of milk proteins. This study underscores the dynamic nature of the lactation cycle and identifies a novel family of lactation-specific proteins, unique to Glossina sp., that are essential to larval development. The specificity of MGP2-10 to tsetse and their critical role during lactation suggests that these proteins may be an excellent target for tsetse-specific population control approaches.

Comparison of milk oligosaccharides between goats with and without the genetic ability to synthesize αs1-casein

Milk oligosaccharides (OS)-free complex carbohydrates-confer unique health benefits to the nursing neonate. Though human digestive enzymes cannot degrade these sugars, they provide nourishment to specific commensal microbes and act as decoys to prevent the adhesion of pathogenic micro-organisms to gastrointestinal cells. At present, the limited quantities of human milk oligosaccharides (HMO) impede research on these molecules and their potential applications in functional food formulations. Considerable progress has been made in the study of OS structures; however, the synthetic pathways leading to their synthesis in the mammary gland are poorly understood. Recent studies show that complex OS with fucose and N-acetyl neuraminic acid (key structural elements of HMO bioactivity) exist in goat milk. Polymorphisms in the CSN1S1 locus, which is responsible for synthesis of α_s1-casein, affect lipid and casein micelle structure in goat milk. The present study sought to determine whether CSN1S1 polymorphisms also influence goat milk oligosaccharide (GMO) production and secretion. The GMO compositions of thirty-two goat milk samples, half of which were from genotype A/A (α_s1-casein producers) and half from genotype O/O (α_s1-casein non-producers), were determined with nanoflow liquid chromatography high-accuracy mass spectrometry. This study represents the most exhaustive characterization of GMO to date. A systematic and comprehensive GMO library was created, consolidating information available in the literature with the new findings. Nearly 30 GMO, 11 of which were novel, were confirmed via tandem mass spectrometric analyses. Six fucosylated OS were identified; 4 of these matched HMO compositions and three were identified for the first time in goat milk. Importantly, multivariate statistical analysis demonstrated that the OS profiles of the A/A and O/O genotype milks could be discriminated by the fucosylated OS. Quantitative analysis revealed that the goat milk samples contained 1.17 g/L of OS; however, their concentration in milks from A/A and O/O genotypes was not different. This study provides evidence of a genetic influence on specific OS biosynthesis but not total OS production. The presence of fucosylated GMO suggests that goat milk represents a potential source of bioactive milk OS suitable as a functional food ingredient.

Identification of an intronic regulatory mutation at the buffalo αS1-casein gene that triggers the skipping of exon 6

The characterization of casein polymorphism is an essential step in order to understand the genetic basis of milk quality in dairy ruminants. In this work, we report the identification of a regulatory mutation at the buffalo αs1-casein (CSN1S1) gene that alters the normal processing of the primary transcript. Sequencing of CSN1S1 cDNA from individuals harbouring this new variant revealed that its most distinctive feature is the loss of exon 6 that encodes eight amino acids between positions 35-42 of mature protein. In an effort to map the causal mutation, we sequenced a genomic region spanning exons 5-7 of the buffalo CSN1S1 gene. This experiment allowed us to establish that exon 6-skipping is produced by a G to C substitution at the first position of intron 6 that inactivates the donor splice site. This mutation can be typed by PCR-RFLP by using either TaaI or Bpu10I diagnostic restriction enzymes, and it has a frequency of 0.18 in Romanian buffaloes. This exon skipping phenomenon is the first one described in buffalo CSN1S1 locus.

Goat α(s1)-casein genotype affects milk fat globule physicochemical properties and the composition of the milk fat globule membrane

Milk fat secretion is a complex process that initiates in the endoplasmic reticulum of the mammary epithelial cell by the budding of lipid droplets. Lipid droplets are finally released as fat globules in milk enveloped by the apical plasma membrane of the mammary epithelial cell. The milk fat globule membrane (MFGM) thus comprises membrane-specific proteins and polar lipids (glycerophospholipids and sphingolipids) surrounding a core of neutral lipids (mainly triacylglycerols and cholesterol esters). We have recently described major proteins of the MFGM in the goat and we have highlighted prominent differences between goats and bovine species, especially regarding lactadherin, a major MFGM protein. Here, we show that, in the goat species, the well-documented genetic polymorphism at the α(s1)-casein (CSN1S1) locus affects both structure and composition of milk fat globules. We first evidenced that both milk fat globule size and ζ-potential are related to the α(s1)-casein genotype. At midlactation, goats displaying strong genotypes for α(s1)-casein (A/A goats) produce larger fat globules than goats with a null genotype at the CSN1S1 locus (O/O goats). A linear relationship (R(2)=0.75) between fat content (g/kg) in the milk and diameter of fat globules (μm) was established. Moreover, we found significant differences with regard to MFGM composition (including both polar lipids and MFGM proteins) from goats with extreme genotype at the CSN1S1 locus. At midlactation, the amount of polar lipids is significantly higher in the MFGM from goats with null genotypes for α(s1)-casein (O/O goats; 5.97±0.11mg/g of fat; mean ± standard deviation) than in the MFGM from goats with strong genotypes for α(s1)-casein (A/A goats; 3.96±0.12mg/g of fat; mean ± standard deviation). Two MFGM-associated proteins, namely lactadherin and stomatin, are also significantly upregulated in the MFGM from goats with null genotype for α(s1)-casein at early lactation. Our findings are discussed with regard to techno-functional properties and nutritional value of goat milk. In addition, the genetic polymorphism in the goat species appears to be a tool to provide clues to the lipid secretion pathways in the mammary epithelial cell.

Milk lacking α-casein leads to permanent reduction in body size in mice

The major physiological function of milk is the transport of amino acids, carbohydrates, lipids and minerals to mammalian offspring. Caseins, the major milk proteins, are secreted in the form of a micelle consisting of protein and calcium-phosphate.

We have analysed the role of the milk protein α-casein by inactivating the corresponding gene in mice. Absence of α-casein protein significantly curtails secretion of other milk proteins and calcium-phosphate, suggesting a role for α-casein in the establishment of casein micelles. In contrast, secretion of albumin, which is not synthesized in the mammary epithelium, into milk is not reduced. The absence of α-casein also significantly inhibits transcription of the other casein genes. α-Casein deficiency severely delays pup growth during lactation and results in a life-long body size reduction compared to control animals, but has only transient effects on physical and behavioural development of the pups. The data support a critical role for α-casein in casein micelle assembly. The results also confirm lactation as a critical window of metabolic programming and suggest milk protein concentration as a decisive factor in determining adult body weight.

AlphaS1-casein, which is essential for efficient ER-to-Golgi casein transport, is also present in a tightly membrane-associated form

BACKGROUND

Caseins, the main milk proteins, aggregate in the secretory pathway of mammary epithelial cells into large supramolecular structures, casein micelles. The role of individual caseins in this process and the mesostructure of the casein micelle are poorly known.

RESULTS

In this study, we investigate primary steps of casein micelle formation in rough endoplasmic reticulum-derived vesicles prepared from rat or goat mammary tissues. The majority of both alphaS1- and beta-casein which are cysteine-containing casein was dimeric in the endoplasmic reticulum. Saponin permeabilisation of microsomal membranes in physico-chemical conditions believed to conserve casein interactions demonstrated that rat immature beta-casein is weakly aggregated in the endoplasmic reticulum. In striking contrast, a large proportion of immature alphaS1-casein was recovered in permeabilised microsomes when incubated in conservative conditions. Furthermore, a substantial amount of alphaS1-casein remained associated with microsomal or post-ER membranes after saponin permeabilisation in non-conservative conditions or carbonate extraction at pH11, all in the presence of DTT. Finally, we show that protein dimerisation via disulfide bond is involved in the interaction of alphaS1-casein with membranes.

CONCLUSIONS

These experiments reveal for the first time the existence of a membrane-associated form of alphaS1-casein in the endoplasmic reticulum and in more distal compartments of the secretory pathway of mammary epithelial cells. Our data suggest that alphaS1-casein, which is required for efficient export of the other caseins from the endoplasmic reticulum, plays a key role in early steps of casein micelle biogenesis and casein transport in the secretory pathway.

Probing the interaction between recombinant human myelin basic protein and caseins using surface plasmon resonance and diffusing wave spectroscopy

An intrinsically unstructured human myelin basic protein (hMBP) was expressed in the milk of transgenic cows (TGmilk) and found exclusively associated with the casein micellar phase. The interaction between the recombinant protein and milk caseins was investigated using surface plasmon resonance (SPR). An anti-human myelin basic protein antibody was covalently immobilized to the surface of the sensor chip. Subsequently the interaction between the recombinant protein (captured by this antibody) and caseins was studied in comparison to that noted with its human counterpart. Results showed a calcium-mediated interaction between the recombinant protein and caseins. The order of magnitude of this interaction was in agreement with the number of phosphorylated residues carried by each type of casein (alpha(s)- > beta- > kappa-casein). This selective interaction was not noted between the human protein and milk caseins indicating that the recombinant protein was phosphorylated to a higher extent than the human protein. The obtained results indicated that the co-expression of the recombinant protein and caseins by the mammary gland along with the recombinant protein's ability to form calcium bridges played a key role in the association of the recombinant human myelin basic protein (rhMBP) with the casein micelles of milk. Despite this association between the recombinant protein and milk caseins, light scattering investigations using diffusing wave spectroscopy (DWS) showed no significant differences between the milks of the transgenic and the non-transgenic control cows, with respect to both the average micelle size and surface charges. This was attributed to the low expression levels of the recombinant protein in milk.

Oleate and linoleate stimulate degradation of β-casein in prolactin-treated HC11 mouse mammary epithelial cells

Although virtually all cells store neutral lipids as cytoplasmic lipid droplets, mammary epithelial cells have developed a specialized function to secrete them as milk fat globules. We have used the mammary epithelial cell line HC11 to evaluate the potential connections between the lipid and protein synthetic pathways. We show that unsaturated fatty acids induce a pronounced proliferation of cytoplasmic lipid droplets and stimulate the synthesis of adipose differentiation-related protein. Unexpectedly, the cellular level of beta-casein, accumulated under lactogenic hormone treatment, decreases following treatment of the cells with unsaturated fatty acids. In contrast, saturated fatty acids have no significant effect on either cytoplasmic lipid droplet proliferation or cellular beta-casein levels. We demonstrate that the action of unsaturated fatty acids on the level of beta-casein is post-translational and requires protein synthesis. We have also observed that proteasome inhibitors potentiate beta-casein degradation, indicating that proteasomal activity can destroy some cytosolic protein(s) involved in the process that negatively controls beta-casein levels. Finally, lysosome inhibitors block the effect of unsaturated fatty acids on the cellular level of beta-casein. Our data thus suggest that the degradation of beta-casein occurs via the microautophagic pathway.

Terminal differentiation of goat mammary tissue during pregnancy requires the expression of genes involved in immune functions

Terminal differentiation of mammary tissue into a functional epithelium that synthesizes and secretes milk occurs during pregnancy. The molecular mechanisms underlying this complex process are poorly understood, especially in ruminants. To obtain an overview of the ruminant mammary gland's final differentiation process, we conducted time-course gene expression analysis of five physiological stages: four during pregnancy (P46, P70, P90, and P110) and one after 40 days of lactation (L40). An appropriate loop experimental design was used to follow gene expression profiles. Using three nulliparous (pregnancy) or primiparous (lactation) goats per stage, we performed a comparison starting from nine dye-swaps and using a 22K bovine oligoarray. Statistical analysis revealed that the expression of 1,696 genes varied significantly at least once in the study. These genes fell into 19 clusters based on their expression profiles. Identification of biological functions with Ingenuity Pathway Analysis software revealed several similarities, in keeping with physiological stages described in mice. As in mice, expression of milk protein genes began at midpregnancy, and genes regulating lipid biosynthesis were induced at the onset of lactation. During the first half of pregnancy, the molecular signature of goat mammary tissue was characterized by the expression of genes associated with tissue remodeling and differentiation, while the second half was mainly characterized by the presence of messengers encoding genes involved in cell proliferation. A large number of immune-related genes were also induced, supporting recent speculation that mammary tissue has an original immune function, and the recruitment of migrating hematopoietic cells possibly involved in the branching morphogenesis of the mammary gland. These data hint that the induction of differentiation occurs early in pregnancy, very likely before P46. This period is therefore crucial for obtaining a healthy and productive mammary gland.

Dephosphorylation of alpha(s)- and beta-caseins and its effect on chaperone activity: a structural and functional investigation

Milk casein proteins can act as molecular chaperones: under conditions of stress, such as elevated temperature, molecular chaperones stabilize proteins from unfolding, aggregating, and precipitating. In this study, alpha(s)- and beta-caseins were dephosphorylated using alkaline phosphatase. A structural and functional investigation was undertaken to determine the effect of dephosphorylation on the chaperone activity of alpha(s)- and beta-caseins against two types of protein misfolding, i.e., amorphous aggregation and amyloid fibril assembly. The dephosphorylation of alpha(s)- and beta-caseins resulted in a decrease in the chaperone efficiency against both heat- and reduction-induced amorphously aggregating target proteins. In contrast, dephosphorylation had no effect on the chaperone activity of alpha(s)- and beta-caseins against the amyloid-forming target protein kappa-casein. Circular dichroism and fluorescence spectroscopic data indicated that the loss of negative charge associated with dephosphorylation led to an increase in ordered structure of alpha(s)- and beta-caseins. It is concluded that the flexible, dynamic, and relatively unstructured and amphiphatic nature of alpha(s)- and beta-caseins is important in their chaperone action.

Efeito dos genótipos para alphaS1-caseína sobre as frações proteicas e lipídicas do leite de cabra

O alto polimorfismo encontrado no lócus do gene da αS1-caseína em caprinos, classificado em quatro níveis de expressão - alto, médio, baixo e nulo -, está associado à produção de 3,6; 1,6; 0,6 e 0g/L/alelo, respectivamente. O estudo foi realizado para investigar possíveis variações na produção de leite e seus constituintes, no perfil de caseínas e na lipólise da gordura. Quarenta e quatro cabras foram distribuídas em cinco genótipos: dois homozigotos, um para alta (AA) e outro para produção intermediária (EE), e três heterozigotos chamados AE, AF e EF, para αs1-caseína. Para a lipólise, o leite foi subamostrado em quatro alíquotas que sofreram tratamento térmico no momento da ordenha e após 24h de resfriamento. Diferenças entre genótipos foram observadas para a produção de caseína e de suas frações. As demais variáveis não diferiram entre genótipos. O genótipo AA apresentou os maiores conteúdos de caseína (28,6g/L) e de αS1-cn (22,3%). Os demais genótipos apresentaram média de 20,4g/L. Os grupos AE e AF apresentaram média de 12,1, EE-10,1 e EF-9,1% de αS1-cn. O resfriamento do leite por 24 horas aumentou a taxa de lipólise no leite. A genotipagem das cabras para αS1-cn pode ser usada como ferramenta de seleção com objetivo de obter produtos lácteos com distintos perfis de proteínas.

Fast screening and quantitative evaluation of internally deleted goat alphas1-casein variants by mass spectrometric detection of the signature peptides

Currently, the internally deleted caprine alphas1-casein (alphas1-CN) variants F and G, associated with low casein expression, are detected by means of ordinary descriptive techniques. No relevant procedure is available to detect internally deleted goat alphas1-CN in bulk milks. The availability of full-length and alphas1-CN F and G variants allowed us to further investigate this issue. Using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) and high-performance liquid chromatography (HPLC)/electrospray ionization (ESI)-MS and ESI-MS/MS, tryptic signature peptides alphas1-CN F f59-63/f43-63, alphas1-CN G f4-20/f4-21, alphas1-CN B2 f4-22 Pro16 and alphas1-CN A f4-22 Leu16 were identified. This also helped to solve the interesting question of how the casein variants contribute to the composition of goat's bulk milk. Synthetic peptide analogues with ionization efficiency equivalent to that of tryptic junction peptides were used as internal standards to evaluate alphas1-CN variants, either individually or globally, using bulk milk from a single goat breed as a model system. Here, alphas1-CN F accounted for 0.15+/-0.08% and the alphas1-CN G variant was missing or below the 0.10% detection limit. The analysis of six samples confirmed that alphas1-CN G was missing and that alphas1-CN F occurred at a low frequency in hybrid and local breed bulk milks from Mediterranean areas. In conclusion, a quantitative MS-based application of the signature peptides for full-length and internally deleted variants in goat's casein is provided. The strategy is also suggested for the determination of splice variants in any biological sample.

Characterisation of monotreme caseins reveals lineage-specific expansion of an ancestral casein locus in mammals

Using a milk-cell cDNA sequencing approach we characterised milk-protein sequences from two monotreme species, platypus (Ornithorhynchus anatinus) and echidna (Tachyglossus aculeatus) and found a full set of caseins and casein variants. The genomic organisation of the platypus casein locus is compared with other mammalian genomes, including the marsupial opossum and several eutherians. Physical linkage of casein genes has been seen in the casein loci of all mammalian genomes examined and we confirm that this is also observed in platypus. However, we show that a recent duplication of β-casein occurred in the monotreme lineage, as opposed to more ancient duplications of α-casein in the eutherian lineage, while marsupials possess only single copies of α- and β-caseins. Despite this variability, the close proximity of the main α- and β-casein genes in an inverted tail–tail orientation and the relative orientation of the more distant kappa-casein genes are similar in all mammalian genome sequences so far available. Overall, the conservation of the genomic organisation of the caseins indicates the early, pre-monotreme development of the fundamental role of caseins during lactation. In contrast, the lineage-specific gene duplications that have occurred within the casein locus of monotremes and eutherians but not marsupials, which may have lost part of the ancestral casein locus, emphasises the independent selection on milk provision strategies to the young, most likely linked to different developmental strategies. The monotremes therefore provide insight into the ancestral drivers for lactation and how these have adapted in different lineages.

Effect of alphas1-casein ( CSN1S1) genotype on milk CSN1S1 content in Malagueña and Murciano-Granadina goats

There is substantial evidence showing that the polymorphism of the goat alphas1-casein (CSN1S1) gene has a major effect on milk protein, casein and fat content as well as on cheese yield. However, its influence on the synthesis rate of CSN1S1 has been less studied, with measurements only available in French breeds. In this article, we have measured milk CSN1S1 content in 89 Malagueña and 138 Murciano-Granadina goats with 305 and 460 phenotypic registers, respectively. In the Malagueña breed, average values of CSN1S1 content estimated for BB, BF, EE and FF genotypes were 6.94+/-0.38, 5.36+/-0.22, 4.58+/-0.13 and 3.98+/-0.27 g/l, respectively, being all significantly different (P<0.05). Conversely, in the Murciano-Granadina breed only the BB genotype (8.50+/-0.60 g/l) was significantly associated with increased levels of CSN1S1 (P<0.05), whereas BF (6.56+/-0.82 g/l), EE (6.39+/-0.60 g/l) and EF (6.91+/-0.76 g/l) genotypes displayed non-significant differences when compared with each other. Our results highlight the existence of breed-specific genetic and/or environmental factors modulating the impact of the CSN1S1 gene polymorphism on the synthesis rate of the corresponding protein.