Multispecies comparison of the casein gene loci and evolution of casein gene family

Human casein alpha s1 (CSN1S1) skews in vitro differentiation of monocytes towards macrophages

Background

The milk-derived protein human Casein alpha s1 (CSN1S1) has recently been detected in blood cells and was shown to possess proinflammatory properties. In the present study, we investigated the effect of CSN1S1 on the differentiation of monocytes.

Methods

Primary human monocytes were stimulated with recombinant CSN1S1 and compared to cells stimulated with GM-CSF/IL-4 or M-CSF/IFNγ. Morphological changes were assessed by microscopy and quantification of surface markers of differentiation by FACS analysis. Phagocytic activity of CSN1S1 stimulated cells was measured by quantification of zymosan labeled particle uptake. The role of mitogen activated protein kinases for CSN1S1-induced differentiation of monocytes and proinflammatory cytokine expression was assessed by supplementation of specific inhibitors.

Results

CSN1S1 at a concentration of 10 μg/ml resulted in morphological changes (irregular shape, pseudopodia) and aggregation of cells, comparable to changes observed in M-CSF/IFNγ differentiated macrophages. Surface marker expression was altered after 24 h with an upregulation of CD14 (mean 2.5 fold) and CD64 (1.9 fold) in CSN1S1 stimulated cells. CSN1S1 treated cells showed a characteristic surface marker pattern for macrophages after 120 h of incubation (CD14^high, CD64^high, CD83^low, CD1a^low) comparable to changes observed in M-CSF/IFNγ treated monocytes. Furthermore, phagocytic activity was increased 1.4 and 1.9 fold following stimulation with 10 μg/ml CSN1S1 after 24 and 48 h, respectively. Early GM-CSF, but not GM-CSF/IL-4 induced differentiation of monocytes towards dendritic cells (DC) was inhibited by addition of CSN1S1. Finally, CSN1S1 induced upregulation of CD14 was impeded by inhibition of ERK1/2, while inhibition of the mitogen activated protein kinases JNK and p38 did not influence cellular differentiation. However, JNK and p38 inhibitors impeded CSN1S1 induced secretion of the proinflammatory cytokines IL-1b or IL-6.

Conclusions

CSN1S1 skews in vitro differentiation of monocytes towards a macrophage-like phenotype. Data is accumulating that functions of CSN1S1 are beyond nutritional properties and include immunomodulatory effects.

Sex determination of bovine preimplantation embryos by oligonucleotide microarray

The aim has been to set up a rapid and accurate microarray assay using sandwich mode for sex determination of bovine preimplantation embryos. Twelve sequence-specific oligonucleotide capture probes used to discriminate 12 samples were spotted onto the aldehyde-modified glass slides by Arrayer. The 2 recognition probes used to identify coding regions of the sex-determining region of the Y chromosome gene (SRY) and β-casein (CSN2) reference gene were coupled with biotin. The assay was optimized by using genomic DNA extracted from blood samples of known sex individuals. Polymerase chain reaction (PCR) was used to amplify the fragments in the HMG box region of SRY gene and CSN2 gene with sequence-specific primers. The sex of samples was identified by detecting both the SRY and CSN2 genes simultaneously in 2 reaction cells of microarrays, with the male having SRY and CSN2 signals and the female only CSN2. The sex of 20 bovine preimplantation embryos was determined by oligonucleotide microarray. The protocol was run with a blind test that showed a 100% (82/82) specificity and accuracy in sexing of leukocytes. The bovine embryos were transferred into 20 bovine recipients, with a pregnant rate of 40% (8/20). Three calves were born at term, and 5 fetuses were miscarried. Their sexes were fully in accordance with the embryonic sex predetermination predicted by oligonucleotide microarray. This suggests that the oligonucleotide microarray method of SRY gene analysis can be used in early sex prediction of bovine embryos in breeding programs.

Epigenetic modifications unlock the milk protein gene loci during mouse mammary gland development and differentiation

BACKGROUND

Unlike other tissues, development and differentiation of the mammary gland occur mostly after birth. The roles of systemic hormones and local growth factors important for this development and functional differentiation are well-studied. In other tissues, it has been shown that chromatin organization plays a key role in transcriptional regulation and underlies epigenetic regulation during development and differentiation. However, the role of chromatin organization in mammary gland development and differentiation is less well-defined. Here, we have studied the changes in chromatin organization at the milk protein gene loci (casein, whey acidic protein, and others) in the mouse mammary gland before and after functional differentiation.

METHODOLOGY/PRINCIPAL FINDINGS

Distal regulatory elements within the casein gene cluster and whey acidic protein gene region have an open chromatin organization after pubertal development, while proximal promoters only gain open-chromatin marks during pregnancy in conjunction with the major induction of their expression. In contrast, other milk protein genes, such as alpha-lactalbumin, already have an open chromatin organization in the mature virgin gland. Changes in chromatin organization in the casein gene cluster region that are present after puberty persisted after lactation has ceased, while the changes which occurred during pregnancy at the gene promoters were not maintained. In general, mammary gland expressed genes and their regulatory elements exhibit developmental stage- and tissue-specific chromatin organization.

CONCLUSIONS/SIGNIFICANCE

A progressive gain of epigenetic marks indicative of open/active chromatin on genes marking functional differentiation accompanies the development of the mammary gland. These results support a model in which a chromatin organization is established during pubertal development that is then poised to respond to the systemic hormonal signals of pregnancy and lactation to achieve the full functional capacity of the mammary gland.

Genetic polymorphism at the CSN1S1 gene in Girgentana dairy goat breed

The aim of this work was to evaluate the variability of the αs1-casein locus in the endangered Girgentana dairy goat breed in order to define genetic improvement and a conservation program for this breed. The study was performed on 200 dairy goats by means of different PCR protocols. The most frequent alleles were A (0.590) and F (0.290) followed by B (0.065) and N (0.047). CSN1S1 E allele was identified with a very low frequency (0.008). The most common genotype was AF (0.365) followed by AA (0.340). The high frequency of the strong genotypes is associated with the production of milk with high fat and protein content and with optimal technological properties. In Girgentana goat breed, the CSN1S1 genotype information could be utilised in selection strategies for milk protein content and milk yield, in order to select genetic lines for the production of ‘drinking milk’ using weak and null genotypes, and for niche products using strong genotypes.

Molecular characterization and genetic variability at κ-casein gene (CSN3) in camels

κ-casein is a glycosilated protein belonging to a family of phosphoproteins (αs1, β, αs2, κ) that represent the major protein component in mammalian milk. κ-casein plays an essential role in the casein micelle stabilization, determining the size and the specific function. In the present paper, we report for the first time the characterization of the nucleotide sequence of the whole κ-casein-encoding gene (CSN3) plus 1045 nucleotides at the 5' flanking region in Camelus dromedarius. The promoter region and the complete cDNA were also provided for the first time in Camelus bactrianus. The gene is spread over 9.3kb and consists of 5 exons varying in length from 33bp (exon 3) to 494bp (exon 4), and 4 introns from 1200bp (intron 3) to 2928bp (intron 2). Highly conserved sequences, located in the 5' flanking region, have been found. The regulatory regions of camels seems to be more related to equids than to other compared species. 17 polymorphic sites have been detected, one of these (g.1029T>C) is responsible for the creation of a new putative consensus sequence for the transcription factor HNF-1. In general, these SNPs are the first reported in camels for casein loci. Finally, seven interspersed repeated elements were also identified at intronic level.

G-NEST: a gene neighborhood scoring tool to identify co-conserved, co-expressed genes

BACKGROUND

In previous studies, gene neighborhoods-spatial clusters of co-expressed genes in the genome-have been defined using arbitrary rules such as requiring adjacency, a minimum number of genes, a fixed window size, or a minimum expression level. In the current study, we developed a Gene Neighborhood Scoring Tool (G-NEST) which combines genomic location, gene expression, and evolutionary sequence conservation data to score putative gene neighborhoods across all possible window sizes simultaneously.

RESULTS

Using G-NEST on atlases of mouse and human tissue expression data, we found that large neighborhoods of ten or more genes are extremely rare in mammalian genomes. When they do occur, neighborhoods are typically composed of families of related genes. Both the highest scoring and the largest neighborhoods in mammalian genomes are formed by tandem gene duplication. Mammalian gene neighborhoods contain highly and variably expressed genes. Co-localized noisy gene pairs exhibit lower evolutionary conservation of their adjacent genome locations, suggesting that their shared transcriptional background may be disadvantageous. Genes that are essential to mammalian survival and reproduction are less likely to occur in neighborhoods, although neighborhoods are enriched with genes that function in mitosis. We also found that gene orientation and protein-protein interactions are partially responsible for maintenance of gene neighborhoods.

CONCLUSIONS

Our experiments using G-NEST confirm that tandem gene duplication is the primary driver of non-random gene order in mammalian genomes. Non-essentiality, co-functionality, gene orientation, and protein-protein interactions are additional forces that maintain gene neighborhoods, especially those formed by tandem duplicates. We expect G-NEST to be useful for other applications such as the identification of core regulatory modules, common transcriptional backgrounds, and chromatin domains. The software is available at http://docpollard.org/software.html.

The evolution of milk secretion and its ancient origins

Lactation represents an important element of the life history strategies of all mammals, whether monotreme, marsupial, or eutherian. Milk originated as a glandular skin secretion in synapsids (the lineage ancestral to mammals), perhaps as early as the Pennsylvanian period, that is, approximately 310 million years ago (mya). Early synapsids laid eggs with parchment-like shells intolerant of desiccation and apparently dependent on glandular skin secretions for moisture. Mammary glands probably evolved from apocrine-like glands that combined multiple modes of secretion and developed in association with hair follicles. Comparative analyses of the evolutionary origin of milk constituents support a scenario in which these secretions evolved into a nutrient-rich milk long before mammals arose. A variety of antimicrobial and secretory constituents were co-opted into novel roles related to nutrition of the young. Secretory calcium-binding phosphoproteins may originally have had a role in calcium delivery to eggs; however, by evolving into large, complex casein micelles, they took on an important role in transport of amino acids, calcium and phosphorus. Several proteins involved in immunity, including an ancestral butyrophilin and xanthine oxidoreductase, were incorporated into a novel membrane-bound lipid droplet (the milk fat globule) that became a primary mode of energy transfer. An ancestral c-lysozyme lost its lytic functions in favor of a role as α-lactalbumin, which modifies a galactosyltransferase to recognize glucose as an acceptor, leading to the synthesis of novel milk sugars, of which free oligosaccharides may have predated free lactose. An ancestral lipocalin and an ancestral whey acidic protein four-disulphide core protein apparently lost their original transport and antimicrobial functions when they became the whey proteins β-lactoglobulin and whey acidic protein, which with α-lactalbumin provide limiting sulfur amino acids to the young. By the late Triassic period (ca 210 mya), mammaliaforms (mammalian ancestors) were endothermic (requiring fluid to replace incubatory water losses of eggs), very small in size (making large eggs impossible), and had rapid growth and limited tooth replacement (indicating delayed onset of feeding and reliance on milk). Thus, milk had already supplanted egg yolk as the primary nutrient source, and by the Jurassic period (ca 170 mya) vitellogenin genes were being lost. All primary milk constituents evolved before the appearance of mammals, and some constituents may have origins that predate the split of the synapsids from sauropsids (the lineage leading to 'reptiles' and birds). Thus, the modern dairy industry is built upon a very old foundation, the cornerstones of which were laid even before dinosaurs ruled the earth in the Jurassic and Cretaceous periods.

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.

Progesterone receptor directly inhibits β-casein gene transcription in mammary epithelial cells through promoting promoter and enhancer repressive chromatin modifications

Differentiated HC-11 cells ectopically expressing progesterone receptor (PR) were used to explore the molecular mechanisms by which progesterone suppresses β-casein gene transcription induced by prolactin (PRL) and glucocorticoids in the mammary gland. As detected by chromatin immunoprecipitation assays, treatment of cells with the progestin agonist R5020 induced a rapid recruitment (5 min) of PR to the proximal promoter (-235 bp) and distal enhancer (-6 kb upstream of transcription start site) of β-casein. PR remained bound for 4 h and was dissociated by 24 h after treatment. Despite efficient binding, the hormone agonist-occupied PR did not stimulate transcription of the β-casein gene. Recruitment of signal transducer and activator of transcription 5a, glucocorticoid receptor, and the CCAAT enhancer binding protein β to the enhancer and proximal promoter of β-casein induced by PRL and glucocorticoids was blocked by progestin cotreatment, whereas PR binding was induced under these conditions. PRL/glucocorticoid-induced histone acetylation and the recruitment of the coactivator p300 and RNA polymerase II required for gene activation were also inhibited by progestin. In addition, progestin prevented dissociation of the corepressors Yin and Yang 1 and histone deacetylase 3 from the promoter, and demethylation of lysine 9 of histone 3 induced by PRL and glucocorticoids. These studies are consistent with the conclusion that progesterone interferes with PRL/glucocorticoid induction of β-casein transcription by a physical interaction of PR with the promoter and enhancer that blocks assembly of a transcriptional activation complex and dissociation of corepressors and promotes repressive chromatin modifications. These studies define a novel mechanism of steroid receptor-mediated transcriptional repression of a physiologically important gene in mammary gland development and differentiation.

Evolution of major milk proteins in Mus musculus and Mus spretus mouse species: a genoproteomic analysis

Background

Due to their high level of genotypic and phenotypic variability, Mus spretus strains were introduced in laboratories to investigate the genetic determinism of complex phenotypes including quantitative trait loci. Mus spretus diverged from Mus musculus around 2.5 million years ago and exhibits on average a single nucleotide polymorphism (SNP) in every 100 base pairs when compared with any of the classical laboratory strains. A genoproteomic approach was used to assess polymorphism of the major milk proteins between SEG/Pas and C57BL/6J, two inbred strains of mice representative of Mus spretus and Mus musculus species, respectively.

Results

The milk protein concentration was dramatically reduced in the SEG/Pas strain by comparison with the C57BL/6J strain (34 ± 9 g/L vs. 125 ± 12 g/L, respectively). Nine major proteins were identified in both milks using RP-HPLC, bi-dimensional electrophoresis and MALDI-Tof mass spectrometry. Two caseins (β and α_s1) and the whey acidic protein (WAP), showed distinct chromatographic and electrophoresis behaviours. These differences were partly explained by the occurrence of amino acid substitutions and splicing variants revealed by cDNA sequencing. A total of 34 SNPs were identified in the coding and 3'untranslated regions of the SEG/Pas Csn1s1 (11), Csn2 (7) and Wap (8) genes. In addition, a 3 nucleotide deletion leading to the loss of a serine residue at position 93 was found in the SEG/Pas Wap gene.

Conclusion

SNP frequencies found in three milk protein-encoding genes between Mus spretus and Mus musculus is twice the values previously reported at the whole genome level. However, the protein structure and post-translational modifications seem not to be affected by SNPs characterized in our study. Splicing mechanisms (cryptic splice site usage, exon skipping, error-prone junction sequence), already identified in casein genes from other species, likely explain the existence of multiple α_s1-casein isoforms both in SEG/Pas and C57BL/6J strains. Finally, we propose a possible mechanism by which the hallmark tandem duplication of a 18-nt exon (14 copies) may have occurred in the mouse genome.

Casein α s1 is expressed by human monocytes and upregulates the production of GM-CSF via p38 MAPK

Caseins are major constituents of mammalian milks that are thought to be exclusively expressed in mammary glands and to function primarily as a protein source, as well as to ameliorate intestinal calcium uptake. In addition, proinflammatory and immunomodulatory properties have been reported for bovine caseins. Our aim was to investigate whether human casein α s1 (CSN1S1) is expressed outside the mammary gland and possesses immunomodulatory functions in humans as well. For this purpose, CSN1S1 mRNA was detected in primary human monocytes and CD4(+) and CD8(+) T cells, but not in CD19(+) B cells. CSN1S1 protein was traceable in supernatants of cultured primary human CD14(+) monocytes by ELISA. Similarly, CSN1S1 mRNA and protein were detected in the human monocytic cell lines HL60, U937, and THP1 but not in Mono Mac 6 cells. Moreover, permeabilized human monocytes and HL60 cells could be stained by immunofluorescence, indicating intracellular expression. Recombinant human CSN1S1 was bound to the surface of Mono Mac 6 cells and upregulated the expression of GM-CSF mRNA in primary human monocytes and Mono Mac 6 cells in a time- and concentration-dependent manner. A similar increase in GM-CSF protein was found in the culture supernatants. CSN1S1-dependent upregulation of GM-CSF was specifically blocked by the addition of the p38 MAPK inhibitor ML3403. Our results indicated that human CSN1S1 may possess an immunomodulatory role beyond its nutritional function in milk. It is expressed in human monocytes and stimulates the expression of the proinflammatory cytokine GM-CSF.

Evolution of lactation: ancient origin and extreme adaptations of the lactation system

Lactation, an important characteristic of mammalian reproduction, has evolved by exploiting a diversity of strategies across mammals. Comparative genomics and transcriptomics experiments have now allowed a more in-depth analysis of the molecular evolution of lactation. Milk cell and mammary gland genomic studies have started to reveal conserved milk proteins and other components of the lactation system of monotreme, marsupial, and eutherian lineages. These analyses confirm the ancient origin of the lactation system and provide useful insight into the function of specific milk proteins in the control of lactation. These studies also illuminate the role of milk in the regulation of growth and development of the young beyond simple nutritive aspects.

Short communication: genetic variability in the predicted microRNA target sites of caprine casein genes

The main goal of the current work was to identify single nucleotide polymorphisms (SNP) that might create or disrupt microRNA (miRNA) target sites in the caprine casein genes. The 3' untranslated regions of the goat alpha(S1)-, alpha(S2)-, beta-, and kappa-casein genes (CSN1S1, CSN1S2, CSN2, and CSN3, respectively) were resequenced in 25 individuals of the Murciano-Granadina, Cashmere, Canarian, Saanen, and Sahelian breeds. Five SNP were identified through this strategy: c.175C>T at CSN1S1; c.109T>C, c.139G>C, and c.160T>C at CSN1S2; and c.216C>T at CSN2. Analysis with the Patrocles Finder tool predicted that all of these SNP are located within regions complementary to the seed of diverse miRNA sequences. These in silico results suggest that polymorphism at miRNA target sites might have some effect on casein expression. We explored this issue by genotyping the c.175C>T SNP (CSN1S1) in 85 Murciano-Granadina goats with records for milk CSN1S1 concentrations. This substitution destroys a putative target site for miR-101, a miRNA known to be expressed in the bovine mammary gland. Although TT goats had higher levels (6.25 g/L) of CSN1S1 than their CT (6.05 g/L) and CC (6.04 g/L) counterparts, these differences were not significant. Experimental confirmation of the miRNA target sites predicted in the current work and performance of additional association analyses in other goat populations will be an essential step to find out if polymorphic miRNA target sites constitute an important source of variation in casein expression.

Short communication: molecular genetic characterization of ovine alpha(S1)-casein allele H caused by alternative splicing

Sequencing of ovine CSN1S1*H cDNA showed an absence of exon 8 in comparison with GenBank sequences; the absence was confirmed by protein sequencing. We demonstrated that this allelic aberration is the result of a deletion of 4 nucleotides, the last 3 of exon 8 and the first 1 of intron 8, which are replaced by an insertion of 13 nucleotides in the DNA sequence. The insertion is a precise duplication of a part of the adjacent intronic sequence of CSN1S1*C''. These sequence differences result in an inactivation of the splice donor sequence distal to exon 8, leading to upstream exon skipping during the serial splice reactions of the ovine CSN1S1*H pre-mRNA, and may affect the specific casein expression as well as protein characteristics.

Invited review: milk protein polymorphisms in cattle: effect on animal breeding and human nutrition

The 6 main milk proteins in cattle are encoded by highly polymorphic genes characterized by several nonsynonymous and synonymous mutations, with up to 47 protein variants identified. Such an extensive variation was used for linkage analysis with the description of the casein cluster more than 30 yr ago and has been applied to animal breeding for several years. Casein haplotype effects on productive traits have been investigated considering information on the whole casein complex. Moreover, mutations within the noncoding sequences have been shown to affect the specific protein expression and, as a consequence, milk composition and cheesemaking. Milk protein variants are also a useful tool for breed characterization, diversity, and phylogenetic studies. In addition, they are involved in various aspects of human nutrition. First, the occurrence of alleles associated with a reduced content of different caseins might be exploited for the production of milk with particular nutritional qualities; that is, hypoallergenic milk. On the other hand, the frequency of these alleles can be decreased by selection of sires using simple DNA tests, thereby increasing the casein content in milk used for cheesemaking. Furthermore, the biological activity of peptides released from milk protein digestion can be affected by amino acid exchanges or deletions resulting from gene mutations. Finally, the gene-culture coevolution between cattle milk protein genes and human lactase genes, which has been recently highlighted, is impressive proof of the nonrandom occurrence of milk protein genetic variation over the centuries.

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.

The bovine lactation genome: insights into the evolution of mammalian milk

Comparison of milk protein and mammary genes in the bovine genome with those from other mammals gives insights into the evolution of lactation.

Background

The newly assembled Bos taurus genome sequence enables the linkage of bovine milk and lactation data with other mammalian genomes.

Results

Using publicly available milk proteome data and mammary expressed sequence tags, 197 milk protein genes and over 6,000 mammary genes were identified in the bovine genome. Intersection of these genes with 238 milk production quantitative trait loci curated from the literature decreased the search space for milk trait effectors by more than an order of magnitude. Genome location analysis revealed a tendency for milk protein genes to be clustered with other mammary genes. Using the genomes of a monotreme (platypus), a marsupial (opossum), and five placental mammals (bovine, human, dog, mice, rat), gene loss and duplication, phylogeny, sequence conservation, and evolution were examined. Compared with other genes in the bovine genome, milk and mammary genes are: more likely to be present in all mammals; more likely to be duplicated in therians; more highly conserved across Mammalia; and evolving more slowly along the bovine lineage. The most divergent proteins in milk were associated with nutritional and immunological components of milk, whereas highly conserved proteins were associated with secretory processes.

Conclusions

Although both copy number and sequence variation contribute to the diversity of milk protein composition across species, our results suggest that this diversity is primarily due to other mechanisms. Our findings support the essentiality of milk to the survival of mammalian neonates and the establishment of milk secretory mechanisms more than 160 million years ago.

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.

Analysis of mammary specific gene locus regulation in differentiated cells derived by somatic cell fusion

The transcriptional regulation of a gene is best analysed in the context of its normal chromatin surroundings. However, most somatic cells, in contrast to embryonic stem cells, are refractory to accurate modification by homologous recombination. We show here that it is possible to introduce precise genomic modifications in ES cells and to analyse the phenotypic consequences in differentiated cells by using a combination of gene targeting, site-specific recombination and somatic cell fusion. To provide a proof of principle, we have analysed the regulation of the casein gene locus in mammary gland cells derived from modified murine ES cells by somatic cell fusion. A beta-galactosidase reporter gene was inserted in place of the beta-casein gene and the modified ES cells, which do not express the reporter gene, were fused with the mouse mammary gland cell line HC11. The resulting cell clones expressed the beta-galactosidase gene to a similar extent and with similar hormone responsiveness as the endogenous gene. However, a reporter gene under the control of a minimal beta-casein promoter (encompassing the two consensus STAT5 binding sites which mediate the hormone response of the casein genes) was unable to replicate expression levels or hormone responsiveness of the endogenous gene when inserted into the same site of the casein locus. As expected, these results implicate sequences other than the STAT5 sites in the regulation of the beta-casein gene.

A lactational study of the composition and integrity of casein micelles from the milk of the tammar wallaby (Macropus eugenii)

The amount of casein found in the milk of the tammar wallaby increases as lactation progresses. The increase is due to increasing amounts of beta-casein; the alpha-casein remains largely constant. The alpha-casein is the more highly phosphorylated; the most abundant form is the 10-P, throughout lactation. The level of phosphorylation of beta-casein shifts to lower average values in late lactation, possibly indicating the enzymatic reaction is overloaded by the increasing amounts of beta-casein. Unlike bovine casein micelles, the wallaby micelles are not completely disrupted at pH 7.0 by sequestration of their calcium content with ethylene diamine tetraacetic acid (EDTA). Complete disruption only follows the addition of sodium dodecyl sulphate, indicating considerably greater importance for hydrophobic bonds in maintaining their integrity. This micellar behaviour indicates that, despite the evolutionary divergence of marsupials millennia ago, the caseins of wallaby milk assemble into micelles in much the same fashion as in bovine milk.

Polymorphism of bovine beta-casein and its potential effect on human health

Proteins in bovine milk are a common source of bioactive peptides. The peptides are released by the digestion of caseins and whey proteins. In vitro the bioactive peptide beta-casomorphin 7 (BCM-7) is yielded by the successive gastrointestinal proteolytic digestion of bovine beta-casein variants A1 and B, but this was not seen in variant A2. In hydrolysed milk with variant A1 of beta-casein, BCM-7 level is 4-fold higher than in A2 milk. Variants A1 and A2 of beta-casein are common among many dairy cattle breeds. A1 is the most frequent in Holstein-Friesian (0.310-0.660), Ayrshire (0.432-0.720) and Red (0.710) cattle. In contrast, a high frequency of A2 is observed in Guernsey (0.880-0.970) and Jersey (0.490-0.721) cattle. BCM-7 may play a role in the aetiology of human diseases. Epidemiological evidence from New Zealand claims that consumption of beta-casein A1 is associated with higher national mortality rates from ischaemic heart disease. It seems that the populations that consume milk containing high levels of beta-casein A2 have a lower incidence of cardiovascular disease and type 1 diabetes. BCM-7 has also been suggested as a possible cause of sudden infant death syndrome. In addition, neurological disorders, such as autism and schizophrenia, seem to be associated with milk consumption and a higher level of BCM-7. Therefore, careful attention should be paid to that protein polymorphism, and deeper research is needed to verify the range and nature of its interactions with the human gastrointestinal tract and whole organism.

Effect of κ-Casein Polymorphism on Milk Composition in the Orobica Goat

The aim of this work was to study the effects of isoelectrofocusing (IEF) milk protein variants on milk composition in the Italian Orobica goat breed, which is characterized by a rather high frequency of the kappa-casein (CSN3) B(IEF) allele. Significant associations were found between the IEF phenotype and protein and casein percentages. A favorable effect of the CSN3 B(IEF) variant was found for both protein and casein percentages, with a codominance trend for the 3 phenotypes: BB > AB > AA. Depending on the selection purpose, emphasis could be given to different kappa-casein variants in breeding. The high frequency of B(IEF) could be exploited in breeding strategies to improve the protein and casein percentages when cheese making is a selection objective.

The primary structure of a low-Mr multiphosphorylated variant ofβ-casein in equine milk

Highly phosphorylated casein with a low molecular mass was isolated from Haflinger mare's milk by RP-HPLC. It accounts for 4.0% of the casein content. Its mass was determined by LC-ESI-MS before and after treatment by alkaline phosphatase. The molecular mass found for the apo-form (10,591 +/- 2 Da) is in agreement with its primary structure, which was established by ESI-MS/MS from tryptic peptides. It appeared that this short protein (94 amino acid residues) is an internally truncated form of the full-length equine beta-casein (226 residues). This low-Mr variant of equine beta-casein displays a large deletion (residues 50-181), due to a cryptic splice site usage occurring within exon 7 during the course of primary transcripts processing. The phosphorylation pattern of this equine beta-casein variant was investigated by LC-ESI-MS and 2-DE. Seven phosphorylation forms were identified with one to seven phosphate groups with pIs ranging between 4.67 and 4.01. The major isoforms carry five and six phosphate groups.

kappa-casein-deficient mice fail to lactate

Acquisition of milk production capabilities by an ancestor of mammals is at the root of mammalian evolution. Milk casein micelles are a primary source of amino acids and calcium phosphate to neonates. To understand the role of kappa-casein in lactation, we have created and characterized a null mouse strain (Csnk-/-) lacking this gene. The mutant kappa-casein allele did not affect the expression of other milk proteins in Csnk-/- females. However, these females did not suckle their pups and failed to lactate because of destabilization of the micelles in the lumina of the mammary gland. Thus, kappa-casein is essential for lactation and, consequently, for the successful completion of the process of reproduction in mammals. In view of the extreme structural conservation of the casein locus, as well as the phenotype of Csnk-/- females, we propose that the organization of a functional kappa-casein gene would have been one of the critical events in the evolution of mammals. Further, kappa-casein variants are known to affect the industrial properties of milk in dairy animals. Given the expenses and the time scale of such experiments in livestock species, it is desirable to model the intended genetic modifications in mice first. The mouse strain that we have created would be a useful model to study the effect of kappa-casein variants on the properties of milk and/or milk products.

A review of protein structure and gene organisation for proteins associated with mineralised tissue and calcium phosphate stabilisation encoded on human chromosome 4

Several proteins associated with mineralised tissue (teeth and bone) or involved in calcium phosphate stabilisation in the body fluids, milk and saliva have been mapped to the q arm of human chromosome 4. These include the dentine/bone proteins dentine sialophosphoprotein (DSPP), dentine matrix protein 1 (DMP1), bone sialoprotein (BSP), matrix extracellular phosphoglycoprotein, osteopontin (OPN), enamelin, ameloblastin, milk caseins, salivary statherin, and proline-rich proteins. The proposed function of those that are multiphosphorylated is: (i) the stabilisation of calcium phosphate in solution (e.g. casein, statherin) preventing spontaneous precipitation and seeded-crystal growth or (ii) promoting biomineralisation (e.g. the phosphophoryn domain of DSPP), where the protein described as a template macromolecule, is proposed to act as a nucleator/promoter of crystal growth. The genes of these proteins have been subjected to conserved chromosomal synteny during mammalian evolution. The multiphosphorylated proteins statherin, caseins, phosphophoryn, BSP and OPN have been characterised as intrinsically disordered. The codon usage patterns for the amino acid serine reveal a bias for AGC and AGT codons within the human genes dspp, dmp1 and bsp, mouse dspp and dmp1 but not significantly for statherin or caseins. This pattern was also observed in the gene encoding hen phosvitin that also contains stretches of multiphosphorylated serines and in the dmp1 gene sequences of mammalian, reptilian and avian classes. In conclusion, these intrinsically disordered multiphosphorylated proteins are the translation products of genes displaying examples of codon usage bias, internal repeats and conserved chromosomal synteny within the mammalian class.

Polymorphic AP-1 Binding Site in Bovine CSN1S1 Shows Quantitative Differences in Protein Binding Associated with Milk Protein Expression

Polymorphisms in 5'-flanking regions of milk protein encoding genes can influence the binding activity of the affected response elements and thus have an impact on the expression of the gene products. However, precise quantitative data concerning the binding properties of such variable response elements have so far not been described. In this study we present the results of a quantitative fluorescent electromobility shift assay comparing the allelic variants of a polymorphic activator protein-1 binding site in the promoter region of the bovine alphas1-casein encoding gene (CSN1S1), which is affected by an A-->G exchange at -175 bp (CSN1S1(-175bp)). A supershift assay using a commercial c-jun antibody was carried out to verify the specificity of protein binding. The gel shift analysis revealed specific and significantly reduced protein binding of oligonucleotides containing the G variant of the CSN1S1(-175bp) binding site. Further investigations comprised genotyping of the variable CSN1S1(-175bp) activator protein-1 element by an NmuCl restriction fragment length polymorphism in 62 cows of the breed Simmental and 80 cows of the breed German Holstein. Single milk proteins from at least 4 milk samples per cow were quantified by alkaline urea polyacrylamide gel electrophoresis. Homozygotes for CSN1S1(-175bp)*G were not observed, and the allele frequencies were 0.19 in Simmental and 0.05 in German Holstein. Carriers of CSN1S1(-175bp)*G showed higher content (%) as well as quantity (g/d) of alphas1-casein than CSN1S1(-175bp)*A homozygotes, independent of breed. We assume that the positive association of the CSN1S1(-175bp)*G variant with CSN1S1 expression is likely to be caused by a reduced affinity of the affected response element to a c-jun-containing CSN1S1 dimer with repressor properties.

The goat alphas1-casein gene: gene structure and promoter analysis

The level of alphas1-casein in goat milk shows strong variations determined by at least 15 alleles associated with four different efficiencies of protein synthesis. The nucleotide sequence of the whole goat alphas1-casein-encoding gene (CSN1S1) plus 1973 nucleotides at the 5' flanking region and 610 nucleotides at the 3' flanking region was determined and aligned with its bovine counterpart. The gene is spread over 16.7 kb and consists of 19 exons varying in length from 24 bp (exons 5, 6, 7, 8, 10, 13 and 16) to 385 bp (exon 19) and 18 introns from 90 bp of intron 10 to 1685 bp of intron 2. Furthermore, highly conserved sequences, mainly located in the 5' flanking region, were found between this gene and other casein-encoding genes. Finally, seven interspersed repeated elements (10 in the bovine CSN1S1 gene) were also identified at four different locations of the sequenced region: 5' untranscribed region and introns 2, 8 and 11.

The first intron of the murine beta-casein gene contains a functional promoter

Caseins are the major milk proteins in most mammals. Together with calcium and phosphate they form the casein micelle. The corresponding casein genes are clustered in mammalian genomes and their expression is coordinately regulated with regard to developmental and tissue specificity. Casein gene promoters are responsive to lactogenic hormones, cell-matrix, and cell-cell interactions. Transcriptional enhancer elements are found in the 5(') upstream regions of casein genes but have also been detected in the first intron of the bovine beta-casein gene. We show here that the first intron of the murine beta-casein gene has three discernible functions. First, transcriptional enhancer elements present in the intron increase the basal activity of the beta-casein promoter. In addition, these intronic enhancer elements augment the induction of the beta-casein promoter by lactogenic hormones. Finally, we demonstrate that the first intron of the murine beta-casein gene contains a functional promoter.