The rat casein multigene family. Fine structure and evolution of the beta-casein gene

Comparative Genomics, Evolutionary and Gene Regulatory Regions Analysis of Casein Gene Family in Bubalus bubalis

Buffalo is a luxurious genetic resource with multiple utilities (as a dairy, draft, and meat animal) and economic significance in the tropical and subtropical regions of the globe. The excellent potential to survive and perform on marginal resources makes buffalo an important source for nutritious products, particularly milk and meat. This study was aimed to investigate the evolutionary relationship, physiochemical properties, and comparative genomic analysis of the casein gene family (CSN1S1, CSN2, CSN1S2, and CSN3) in river and swamp buffalo. Phylogenetic, gene structure, motif, and conserved domain analysis revealed the evolutionarily conserved nature of the casein genes in buffalo and other closely related species. Results indicated that casein proteins were unstable, hydrophilic, and thermostable, although αs1-CN, β-CN, and κ-CN exhibited acidic properties except for αs2-CN, which behaved slightly basic. Comparative analysis of amino acid sequences revealed greater variation in the river buffalo breeds than the swamp buffalo indicating the possible role of these variations in the regulation of milk traits in buffalo. Furthermore, we identified lower transcription activators STATs and higher repressor site YY1 distribution in swamp buffalo, revealing its association with lower expression of casein genes that might subsequently affect milk production. The role of the main motifs in controlling the expression of casein genes necessitates the need for functional studies to evaluate the effect of these elements on the regulation of casein gene function in buffalo.

Casein Gene Cluster in Camelids: Comparative Genome Analysis and New Findings on Haplotype Variability and Physical Mapping

The structure of casein genes has been fully understood in llamas, whereas in other camelids, this information is still incomplete. In fact, structure and polymorphisms have been identified in three (CSN1S1, αs1-CN; CSN2, β-CN; CSN3, κ-CN) out of four casein genes, whereas controversial information is available for the CSN1S2 (αs2-CN) in terms of structure and genetic diversity. Data from the genome analysis, whose assembly is available for feral camel, Bactrian, dromedary, and alpaca, can contribute to a better knowledge. However, a majority of the scaffolds available in GenBank are still unplaced, and the comparative annotation is often inaccurate or lacking.Therefore, the aims of this study are 1) to perform a comparative genome analysis and synthesize the literature data on camelids casein cluster; 2) to analyze the casein variability in two dromedary populations (Sudanese and Nigerian) using polymorphisms at CSN1S1 (c.150G > T), CSN2 (g.2126A > G), and CSN3 (g.1029T > C); and 3) to physically map the casein cluster in alpaca. Exon structures, gene and intergenic distances, large insertion/deletion events, SNPs, and microsatellites were annotated. In all camelids, the CSN1S2 consists of 17 exons, confirming the structure of llama CSN1S2 gene. The comparative analysis of the complete casein cluster (∼190kb) shows 12,818 polymorphisms. The most polymorphic gene is the CSN1S1 (99 SNPs in Bactrian vs. 248 in dromedary vs. 626 in alpaca). The less polymorphic is the CSN3 in the Bactrian (22 SNPs) and alpaca (301 SNPs), whereas it is the CSN1S2 in dromedary (79 SNPs). In the two investigated dromedary populations, the allele frequencies for the three markers are slightly different: the allele C at CSN1S1 is very rare in Nigerian (0.054) and Sudanese dromedaries (0.094), whereas the frequency of the allele G at CSN2 is almost inverted. Haplotype analysis evidenced GAC as the most frequent (0.288) and TGC as the rarest (0.005). The analysis of R-banding metaphases hybridized with specific probes mapped the casein genes on chromosome 2q21 in alpaca. These data deepen the information on the structure of the casein cluster in camelids and add knowledge on the cytogenetic map and haplotype variability.

Perinatal exposure to bisphenol A modifies the transcriptional regulation of the β-Casein gene during secretory activation of the rat mammary gland

With the aim to analyze whether bisphenol A (BPA) modifies β-Casein (β-Cas) synthesis and transcriptional regulation in perinatally exposed animals, here, pregnant F0 rats were orally exposed to 0, 0.6 or 52 μg BPA/kg/day from gestation day 9 until weaning. Then, F1 females were bred and mammary glands were obtained on lactation day 2. Perinatal BPA exposure decreased β-Cas expression without modifying the activation of prolactin receptor. It also decreased the expression of glucocorticoid receptor in BPA52-exposed dams and β1 and α6 integrins as well as dystroglycan in both BPA groups. In addition, BPA exposure altered the expression of histone-modifying enzymes and induced histone modifications and DNA methylation in the promoter, enhancer and exon VII of the β-Cas gene. An impaired crosstalk between the extracellular matrix and lactogenic hormone signaling pathways and epigenetic modifications of the β-Cas gene could be the molecular mechanisms by which BPA decreased β-Cas expression.

Molecular Characterization of the Llamas (Lama glama) Casein Cluster Genes Transcripts (CSN1S1, CSN2, CSN1S2, CSN3) and Regulatory Regions

In the present paper, we report for the first time the characterization of llama (Lama glama) caseins at transcriptomic and genetic level. A total of 288 casein clones transcripts were analysed from two lactating llamas. The most represented mRNA populations were those correctly assembled (85.07%) and they encoded for mature proteins of 215, 217, 187 and 162 amino acids respectively for the CSN1S1, CSN2, CSN1S2 and CSN3 genes. The exonic subdivision evidenced a structure made of 21, 9, 17 and 6 exons for the αs1-, β-, αs2- and κ-casein genes respectively. Exon skipping and duplication events were evidenced. Two variants A and B were identified in the αs1-casein gene as result of the alternative out-splicing of the exon 18. An additional exon coding for a novel esapeptide was found to be cryptic in the κ-casein gene, whereas one extra exon was found in the αs2-casein gene by the comparison with the Camelus dromedaries sequence. A total of 28 putative phosphorylated motifs highlighted a complex heterogeneity and a potential variable degree of post-translational modifications. Ninety-six polymorphic sites were found through the comparison of the lama casein cDNAs with the homologous camel sequences, whereas the first description and characterization of the 5'- and 3'-regulatory regions allowed to identify the main putative consensus sequences involved in the casein genes expression, thus opening the way to new investigations -so far- never achieved in this species.

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.

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.

The liver-enriched inhibitory protein isoform of CCAAT/enhancer-binding protein beta, but not nuclear factor-kappaB, mediates the transcriptional inhibition of beta-casein by tumor necrosis factor-alpha

TNF-alpha is a physiological regulator of mammary gland development that stimulates the growth of both normal and malignant mammary epithelial cells in primary culture and inhibits functional differentiation. To understand how TNF exerts its effects, the current study examined the mechanism by which TNF down-regulates expression of the beta-casein and whey acidic protein (WAP) genes. TNF treatment markedly decreased activity of the beta-casein and WAP promoters in transiently transfected HC11 mammary epithelial cells. Overexpression of the nuclear factor-kappaB (NFkappaB) p50 and/or p65 proteins increased the transcriptional activity of the beta-casein and WAP promoters in HC11 cells, suggesting that the inhibitory effect of TNF on transcription of these genes is not mediated by NFkappaB. This was further confirmed in experiments in which an NFkappaB super-repressor was overexpressed, and by deletion of an NFkappaB binding site in the beta-casein promoter. In contrast, we found that TNF induced both nuclear expression and the DNA-binding activity of liver-enriched inhibitory protein (LIP) isoform of CCAAT/enhancer-binding protein beta. Moreover, cotransfection of LIP and beta-casein expression vectors showed that LIP suppressed the transcriptional activity of the beta-casein promoter. Together, these results suggest that LIP plays a critical role in mediating TNF-induced down-regulation of the beta-casein gene.

In vivo regulation of growth hormone-stimulated gene transcription by STAT5b

The long-term effects of growth hormone (GH) are mediated through coordinated changes in gene expression that are the outcome of interactions between hormone-activated signal transduction pathways and specific feedback loops. Recent studies in mice have implicated the transcription factor STAT5b as part of the GH-regulated somatic growth pathway, because mice lacking this protein showed diminished growth rates. To assess the role of Stat5b in GH-stimulated gene expression, we have delivered modified versions of the protein to the liver of pituitary-deficient male rats by quantitative adenovirus-mediated gene transfer. In pilot studies in cell culture, both constitutive-active and dominant-negative STAT5b showed appropriate binding properties toward a specific DNA response element. After in vivo expression, neither protein prevented nuclear accumulation of STATs 1 and 3 in the liver. Dominant-negative STAT5b completely inhibited GH-stimulated transcription of genes encoding the growth-promoting proteins IGF-I, IGF-binding protein-3 (IGFBP-3), and acid-labile subunit (ALS), which comprise the major circulating IGF-I complex, and blocked expression of the GH inhibitors SOCS-1, SOCS-2, and CIS, but had little effect on induction of SOCS-3. Constitutive-active STAT5b stimulated robust transcription of IGF-I, ALS, and IGFBP-3 in the absence of hormone but did little to modify GH-mediated activation of SOCS family genes. An adenovirus encoding EGFP was without effect. These results, in addition to establishing STAT5b as one of the key agents of GH-stimulated gene transcription, demonstrate the feasibility of using in vivo gene transfer to target and dissect the functions of distinct components of complex hormone-activated signal transduction pathways.

Acute control of insulin-like growth factor-I gene transcription by growth hormone through Stat5b

Many of the effects of growth hormone (GH) are mediated by insulin-like growth factor-I (IGF-I), a secreted peptide whose gene transcription is induced by GH by unknown mechanisms. Recent studies in mice have implicated Stat5b as part of a GH-regulated somatic growth pathway, because mice lacking this transcription factor show diminished growth rates and a decline in serum IGF-I levels. To test the role of Stat5b in GH-stimulated IGF-I gene expression, we have delivered modified versions of the protein to pituitary-deficient male rats by quantitative adenovirus-mediated gene transfer. In pilot studies in cell culture, both constitutively active and dominant-negative Stat5b appropriately regulated transcription from a GH-responsive Stat5-dependent reporter gene. After in vivo expression, neither protein impaired GH-induced activation of cytoplasmic signaling pathways or blocked nuclear accumulation of Stats 1 and 3 in the liver, the major site of IGF-I production. Dominant-negative Stat5b completely prevented GH-stimulated IGF-I gene transcription, whereas constitutively active Stat5b led to robust IGF-I gene expression in the absence of hormone. An adenovirus encoding enhanced green fluorescent protein was without effect. Similar results were seen with the GH-responsive Stat5b-dependent Spi 2.1 gene, whereas GH-stimulated c-fos transcription was minimally altered. These results establish Stat5b as a key component of GH-stimulated IGF-I gene transcription, and they demonstrate the feasibility of using in vivo gene transfer to target distinct components of hormone-activated signaling pathways.

Mineralized tissue and vertebrate evolution: the secretory calcium-binding phosphoprotein gene cluster

Gene duplication creates evolutionary novelties by using older tools in new ways. We have identified evidence that the genes for enamel matrix proteins (EMPs), milk caseins, and salivary proteins comprise a family descended from a common ancestor by tandem gene duplication. These genes remain linked, except for one EMP gene, amelogenin. These genes show common structural features and are expressed in ontogenetically similar tissues. Many of these genes encode secretory Ca-binding phosphoproteins, which regulate the Ca-phosphate concentration of the extracellular environment. By exploiting this fundamental property, these genes have subsequently diversified to serve specialized adaptive functions. Casein makes milk supersaturated with Ca-phosphate, which was critical to the successive mammalian divergence. The innovation of enamel led to mineralized feeding apparatus, which enabled active predation of early vertebrates. The EMP genes comprise a subfamily not identified previously. A set of genes for dentine and bone extracellular matrix proteins constitutes an additional cluster distal to the EMP gene cluster, with similar structural features to EMP genes. The duplication and diversification of the primordial genes for enameldentinebone extracellular matrix may have been important in core vertebrate feeding adaptations, the mineralized skeleton, the evolution of saliva, and, eventually, lactation. The order of duplication events may help delineate early events in mineralized skeletal formation, which is a major characteristic of vertebrates.

Purification of porcine beta-casein, N-terminal sequence, quantification in mastitic milk

The objectives of the study were to purify porcine beta-casein from sow's milk, to determine N-terminal amino acid sequence, to develop specific antisera against porcine beta-casein, and to use that antisera to evaluate milk samples from a mastitis study. Milk was collected by hand milking a Yorkshire by Duroc crossbred sow following oxytocin administration on d 27 of lactation. A casein-enriched fraction was then prepared by iso-electric precipitation. Porcine beta-casein was then purified by liquid chromatography on a Mono Q anion-exchange column, and checked for purity with SDS-PAGE. An apparent molecular weight of 29,000 Da was estimated from SDS-PAGE. N-Terminal amino acid sequence was determined by Edman degradation to be RAKEELNASGETVE. Rabbits (n = 2) were immunized with beta-casein mixed with Freund's complete (primary) or incomplete (boosters) adjuvant at 4-wk intervals. Antiserum collected from one rabbit 112 d after primary immunization detected 30 to 100 ng beta-casein by Western blot procedure when used at a dilution of 1:2 x 10(6). The antiserum was specific for porcine beta-casein, but showed some cross-reactivity with equine casein. It was determined by Western blot procedure that mammary inflammation induced by lipopolysaccharide infusion resulted in a 41% decrease in the beta-casein concentration of sow milk.

YY1 Represses Vitamin D Receptor-Mediated 25-Hydroxyvitamin D3 24-Hydroxylase Transcription: Relief of Repression by CREB-Binding Protein

Ying Yang transcription factor (YY1) can repress or activate transcription. 25-Hydroxyvitamin D(3)-24-hydroxylase [24(OH)ase], an enzyme involved in the catabolism of 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)], is up-regulated at the transcriptional level by 1,25-(OH)(2)D(3) to self-induce its deactivation. Here we report that YY1 can repress 1,25-(OH)(2)D(3)-induced 24(OH)ase transcription in CV1 cells transfected with vitamin D receptor (VDR) expression vector or in LLCPK(1) cells that contain VDR endogenously. With increasing amounts of YY1 DNA transfected (500 ng to 2 microg), ligand-dependent VDR activation of 24(OH)ase transcription was steadily repressed (maximum repression was 10-fold). Thus, YY1 may be a key modulator preventing activation at times that do not require the enzyme to be expressed. Relief of YY1 repression was observed in the presence of TFIIB or CBP (CREB binding protein) suggesting that YY1 may exert repression, in part, by sequestering TFIIB/CBP. Glutathione-S-transferase (GST) pull-down assays identified regions in the N and C termini of CBP that can bind YY1. In addition, the N-terminal region of CBP that interacts with YY1 can inhibit YY1 from binding to TFIIB. Thus, CBP may alleviate YY1-mediated repression, in part, by preventing YY1 from binding to TFIIB, which is required for VDR-mediated transcription. In summary, our results suggest that YY1 represses 24(OH)ase transcription, at least in part, by sequestering activator proteins involved in VDR-mediated transcription. In addition, our findings demonstrate a role for CBP in relief of repression of VDR-mediated transcription.

Characterization and expression of the mouse pregnant specific uterus protein gene and its rat homologue in the intestine and uterus

The pregnant specific uterus protein gene (Psup) is a novel mouse gene expressed in pregnant uterus. This paper describes the identification and expression of the rat homologue of Psup. The gene is highly expressed in the duodenum. Expression decreases in a proximal-distal gradient in the small intestine and was not detected in the cecum and colon. The pattern of expression in the mouse was similar. Expression of Psup in the mouse was localized to the epithelial cells in the intestine and pregnant uterus by in situ hybridization. The data show tissue-specific expression of Psup.

Comparative aspects of milk caseins

The caseins comprise the major protein component of milk of most mammals and are secreted as micelles that also carry high concentrations of calcium. They are phosphoproteins that represent the products of four genes, equivalent to those that encode the bovine alpha s1, alpha s2, beta, and kappa-caseins. There is considerable variation in the relative proportions of the particular caseins across species. The primary sequences of the alpha s1, alpha s2, and beta-caseins also show considerable species variation consistent with rapidly evolving genes that are proposed to have a common precursor. In contrast, the kappa-caseins exhibit features that demonstrate a separate origin and function where they are proposed to stabilise the micelle structure. This review focuses on comparative aspects of the caseins across a number of species for which information is now available.

A distal enhancer region in the human beta-casein gene mediates the response to prolactin and glucocorticoid hormones

The 5' flanking region of the human beta-casein gene was investigated for the presence of regulatory sequences mediating the action of the lactogenic hormones prolactin and dexamethasone. DNA encompassing 9389 base pairs of the flanking region was isolated and a sequence comparison performed with regulatory regions previously identified in the beta-casein gene of rodents and ruminants. The analysis revealed the presence of a distal region between -4700 and -4550 with a high percentage of identity to the bovine beta-casein enhancer region, and a proximal region between -1 and -200 similar to the proximal promoter regions found in rodents and ruminants. Reporter gene constructs under the control of the distal or the proximal region of the human beta-casein gene were tested for their responsiveness to prolactin and dexamethasone. In transfection experiments, the distal region functioned as a lactogenic hormone inducible enhancer, whereas the proximal region exhibited low activity. In electromobility shift assays, multiple binding sites for Stat5, CCAAT/enhancer-binding proteins, and Ets domain proteins were identified in the distal human enhancer. These transcription factors have already been demonstrated as important regulators of the transcription of milk protein genes in rodents. Thus, a common set of transcription factors appears to be required for the expression of the human beta-casein gene and of milk protein genes in other species.

A novel complementary deoxyribonucleic acid is abundantly and specifically expressed in the uterus during pregnancy

OBJECTIVE

Our purpose was to identify novel genes expressed by the uterus during late pregnancy.

STUDY DESIGN

A complementary deoxyribonucleic acid library constructed from late pregnancy mouse uterus was screened by differential hybridization with complementary deoxyribonucleic acid probes constructed from late pregnancy mouse uterus and nonpregnant mouse uterus. Radiolabeled complementary deoxyribonucleic acid probes derived from one of the complementary deoxyribonucleic acids isolated were used in northern hybridizations against ribonucleic acid collected from pregnant and nonpregnant uterus and a variety of other mouse tissues.

RESULTS

A total of 40 positive clones were isolated; half were identified as cytotoxic T-lymphocyte antigen-2 alpha (a putative inhibitor of the protease cathepsin L) and the other half represented a novel complementary deoxyribonucleic acid. Conceptual translation of the complementary deoxyribonucleic acid predicted a novel protein of 154 amino acids that is proline rich and acidic (pregnancy-specific uterine protein). Northern hybridizations demonstrated that message is abundant in the uterus during late pregnancy. After birth expression rapidly decreased and message was no longer found in the uterus by the third day. A minimal amount of message is present in placental ribonucleic acid, but expression is otherwise not detected in a variety of adult and fetal tissues surveyed, suggesting that expression of this gene is limited to the pregnant uterus.

CONCLUSIONS

The abundance of message and expression apparently limited to the pregnant uterus suggests that the protein represented by this complementary deoxyribonucleic acid may play an important role in pregnancy.

Hormonally regulated double- and single-stranded DNA-binding complexes involved in mouse beta-casein gene transcription

Transcription of the 252-base pair-long mouse beta-casein gene promoter is induced by the synergistic action of insulin, prolactin, and glucocorticoid in a primary mammary epithelial cell culture. The promoter contains a region termed block C having a highly conserved sequence and position among many casein genes. Mutation of block C reduced the response of the promoter to lactogenic hormones 84%. Nuclear extracts from lactating mouse mammary glands contained both a double-stranded and a single-stranded DNA binding protein complex (DS1 and SS), which specifically bind to the sequences AAATTAGCATGT and CCACAA of block C, respectively. The DS1 and the SS protein complexes were approximately 400 and 280 kDa, respectively. Each complex contained a DNA-binding component(s) having a molecular mass of approximately 120 kDa for DS1 and 80 and 65 kDa for SS. Deoxycholate, which interferes with the protein-protein interactions, inhibited the binding activities of DS1 and SS. The maximal increase in the binding activity of DS1 and SS in the mammary gland occurred during pregnancy and during lactation, respectively. In organ culture, the DS1 activity is increased by epidermal growth factor or prolactin in combination with insulin, whereas the SS activity is enhanced by insulin, prolactin, and glucocorticoid. These results suggest that multiprotein complexes binding to the double- and single-stranded DNA of block C mediate hormonal induction of beta-casein gene transcription.

Mammary gland expression of transgenes and the potential for altering the properties of milk

Transgenic animals are a useful in vivo experimental model for assessing the ability and impact of foreign gene expression in a biological system. Transgenic mice are most commonly used, while transgenic sheep, goats, pigs and cows have also been developed for specific, "applied" purposes. Most of the work directed at targeting expression of transgenes to the mammary gland of an animal, by using a milk gene promoter, has been with the intent of either studying promoter function or recovering the desired protein from the milk. Transgenic technology can also be used to alter the functional and physical properties of milk resulting in novel manufacturing properties. The properties of milk have been altered by adding a new protein with the aim of improving the milk, not of recovering the protein for other uses.

EP-GP and the lipocalin VEGh, two different human salivary 20-kDa proteins

Two salivary 20-kDa proteins [the human lipocalin Von Ebner's gland protein (VEGh) and extraparotid glycoprotein (EP-GP)] show several remarkable similarities and differences. The latter is identical to secretory actin-binding protein (SABP), gross cystic disease fluid protein-15 (GCDFP-15), prolactin-induced protein (PIP), and 17-kDA CD4-binding glycoprotein (gp17). Much is known about the distribution, localization, biochemical characteristics, and molecular biology of these two proteins, yet there are only few clues about their functions.

Synthesis and secretion of caseins by the mouse mammary gland: production and characterization of new polyclonal antibodies

Polyclonal antibodies to mouse alpha- and beta/gamma-caseins were raised in rabbits. These antibodies display tissue- and species specificity as shown by immunoblotting. Immunohistochemical analyses demonstrate that both alpha- and beta/gamma-caseins were synthesized and secreted from virtually all lactating mammary epithelial cells, in a pattern very similar to that of the mouse alpha-lactalbumin. Residual amounts of caseins were located also in the apical surface of epithelial cells surrounding the ducal lumen of virgin mammary gland sections. In contrast to the significant level of gamma-casein in the milk, the amount of this protein compared to alpha- or beta-caseins was extremely low in medium conditioned for 24 h by mammary explants of mid-pregnant mice immediately after explantation or after 4 days.

Disulphide arrangement in bovine caseins: localization of intrachain disulphide bridges in monomers of kappa- and alpha s2-casein from bovine milk

Naturally occurring monomeric kappa-casein and alpha s2-casein in bovine milk were purified by ion-exchange chromatography in order to localize potential intrachain disulphide bridges. Enzymic cleavage of the proteins followed by mass spectrometry and amino acid sequence analysis of cystine-containing peptides revealed the presence of an intrachain disulphide bond in both proteins.

Variants within the 5'-flanking regions of bovine milk protein genes: I. κ-casein-encoding gene

In order to identify DNA variants within the 5'-flanking region of the bovine κ-casein (κCn)-encoding gene, this area of the gene from 13 cows belonging to seven breeds (Holstein Friesian, Brown Swiss, German Simmental, Jersey, Galloway, Scottish Highland and Ceylon Dwarf Zebu) was analysed. For each individual, about 1 kb of the 5'-flanking region including exon I was amplified by polymerase chain reaction (PCR). The biotinylated PCR product was immobilized on magnetic beads followed by direct bidirectional sequencing using an automated DNA sequencer. Fifteen DNA variants were identified, some of which are located within potential regulatory sites and possibly involved in the expression of the κ-casein encoding gene.

Production of pharmaceutical proteins from transgenic animals

Different systems are being studied and used to prepare recombinant proteins for pharmaceutical use. The blood, and still more the milk, from transgenic animals appear a very attractive source of pharmaceuticals. The cells from these animals are expected to produce well-matured proteins in potentially huge amounts. Several problems remain before this process becomes used in a large scale. Gene transfer remains a difficult and costly task for farm animals. The vectors carrying the genes coding for the proteins of interest are of unpredictable efficiency. Improvement of these vectors includes the choice of efficient promoters, introns and transcription terminators, the addition of matrix attached regions (MAR) and specialized chromatin sequences (SCS) to enhance the expression of the transgenes and to insulate them from the chromatin environment. Mice are routinely used to evaluate the gene constructs to be transferred into larger animals. Mice can also be utilized to prepare amounts as high as a few hundred mg of recombinant proteins from their milk. Rabbit appears adequate for amounts not higher than 1 kg per year. For larger quantities, goat, sheep, pig and cow are required. No recombinant proteins extracted from the blood or milk of transgenic animals are yet on the market. The relatively slow but real progress to improving the efficiency of this process inclines to be reasonably optimistic. Predictive reports suggest that 10% of the recombinant proteins, corresponding to a 100 million dollars annual market, will be prepared from the milk of transgenic animals by the end of the century.

Hormonal regulation of transcription factor activity in mammary epithelial cells

The multihormonal control of milk protein gene transcription in mammary epithelial cells has been investigated. Although the hormones regulating milk protein gene expression are known, the interaction of the signal transduction pathways of steroid (glucocorticoids) and peptide (insulin and prolactin) hormones remains undefined in molecular terms. These signals converge on the level of nuclear factors binding to regulatory elements in the beta-casein gene promoter. The promoter has a modular architecture and is composed of positive and negative response elements. Nuclear transcription factors which bind to these elements have been identified. The mammary gland factor, MGF, is an essential mediator of lactogenic hormone action and is itself positively regulated in its DNA binding activity. It binds to the promoter region between positions -80 to -100. MGF counteracts a repressor element, constituted by two components, which is located adjacent to the MGF binding site at positions -100 to -150. The transcription factor YY1 binds to the proximal half of the repressor element which overlaps with the MGF binding site. Specific single-stranded DNA binding proteins contribute to the negative regulation of the promoter by interacting with sequence elements between -160 and -190. DNA binding of these proteins is negatively regulated by the lactogenic hormones.

Structure of the human beta-casein encoding gene

The entire human beta-casein-encoding gene, Bca, was cloned and sequenced. The gene consists of eight exons ranging from 21 to 531 nucleotides (nt) in length and extending over 10,466 nt. Exon-2 contains the translational start, the entire signal sequence and the codons for the two first amino acids of the mature protein. This corresponds to the organization found in other species. The translational stop is localized to exon-7. Exon/intron boundaries are in accordance with the AG/GT rule and conform to suggested consensus sequences. Splice junctions are located between coding triplets. In all other species analyzed, Bca has been found to consist of nine exons; however, within intron-2 of the human gene, a sequence omitted from human mRNA, but corresponding to exon-3 of other known Bca genes, was revealed.

Structure and function of milk protein genes

Interspecies comparisons of cDNA and mosaic milk protein genes have confirmed their high rate of evolution, but the overall gene organization has been conserved. The three Ca-sensitive casein genes, which share common motifs in the promoter region and contain similar sequences that encode signal peptide and multiple phosphorylation sites, probably derived from a common ancestor. alpha s1- and alpha s2-casein genes, divided into many small exons, undergo complex splicing, and the deleted caseins arise from exon skipping. The four bovine casein genes are clustered on 200 kb of chromosome 6. alpha-Lactalbumin and beta-lactoglobulin pseudogenes occur in ruminants. Study of the expression of native and modified milk protein genes in mammary cell lines and transgenic animals and DNA footprinting have shown the occurrence of important regulatory motifs in the proximal 5' flanking region, including one recognized by a specific mammary nuclear factor. Good stage- and tissue-specific expression has been obtained in transgenic animals with milk protein genes having less than a 3-kb 5' flanking region. Better knowledge of both the structure and function of milk protein genes, which has already allowed the use of powerful techniques for the rapid identification of alleles, offers the potential for the genetic modification of milk composition.

Bovine alpha s2-casein D is generated by exon VIII skipping

Bovine alpha s2-casein D (CasD) differs from the common type A by the deletion of a stretch of 9 amino acids (aa) starting at a position not precisely known, either at aa 50, 51, or 52. The sequence of cloned PCR-amplified genomic DNA from three homozygous cows, two unrelated females carrying the CasD allele and one carrying the CasA allele, did not reveal any deletion and showed two identical nucleotide (nt) substitutions in the 1.7-kb region of both CasD alleles encompassing codons 43-75 in the cDNA encoding alpha s2-CasA. This strongly suggests that the deleted bovine alpha s2-CasD arises from skipping the 27-nt exon, now identified as exon VIII, which encodes aa 51-59 of alpha s2-CasA. The G-->T transversion (allele A-->D) affecting the last nt of exon VIII, i.e., the 5' consensus splicing site, might be responsible for the altered splicing of the primary transcript of alpha s2-CasD.

[Expression of recombinant proteins in the milk of transgenic animals]

The bulky production of recombinant proteins can be achieved by procaryotes or eucaryotes cells. Cells from higher eucaryotes may be required when proteins have to be modified post-transcriptionally (glycosylation phosphorylation, cleavage, folding...). Cells from higher vertebrates in culture are used to prepare proteins like human factor VIII and erythropoietin. The use of transgenic organism has been suggested to reach the same goal. Indeed a whole living organism allows a very potent amplification, the number of cells involved in the biosynthesis of the recombinant proteins being very numerous and in the best metabolic conditions. Biological fluids (blood, milk, insect hemolymph, egg white...) and possibly organs from transgenic animals are a priori the best sources of recombinant proteins. Blood is abundant and it is a by-product of slaughter house. Its composition is relatively complex and the circulating recombinant proteins may heavily alter health of animals. Milk is very abundant, its composition is relatively simple, it is poor in proteolytic enzymes and it can be collected easily. Hemolymph from insects is relatively scarce. Egg white will be a possible source of recombinant proteins, when transgenesis has become more accessible in birds. Organs from transgenic animals should be solicited only when a particular cell type is required for the biosynthesis of the recombinant proteins. Milk appears therefore, presently, as the best source of recombinant proteins from transgenic animals. About 15 public and private laboratories try to use these techniques. They consist in preparing vectors containing regulatory regions of one of the milk proteins genes and the coding part (cDNA or gene) of the corresponding proteins to be produced. The transfer of these gene constructs to mouse, rabbit, sheep, goat, pig, shows that these techniques are indeed very promising. A single protein, human alpha 1-antitrypsin produced in milk of transgenic sheep, has presently reached the preparation at an industrial scale. This method has two theoretical limitations: 1) some of the proteins secreted in milk may be not matured as their native counterparts. Experiments carried out so far (about 20 proteins has been produced at an experimental scale) indicate that the mammary cell is able to achieve glycosylation in a correct way; 2) a significant proportion of the recombinant proteins migrate from the alveolar compartment of the mammary gland to blood circulation and they can alter health of lactating animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning of the goat beta-casein-encoding gene and expression in transgenic mice

The goat beta-casein-encoding gene (CSN2), which encodes the most abundant protein of goat milk, has been cloned and sequenced. The intron/exon organization of the 9.0-kb goat CSN2 gene is similar to that of other CSN2 genes. Expression of the goat gene was principally restricted to the mammary gland of lactating transgenic animals. A low level of expression was also observed in skeletal muscle and skin. In contrast to a rat CSN2 transgene [Lee et al., Nucleic Acids Res. 16 (1988) 1027-1041], the goat gene was expressed to a high degree in the lactating mammary gland. Differences in the content or context of regulatory elements may account for the enhanced performance of the goat relative to the rat CSN2 gene in transgenic mice.

Isolation and characterization of the mouse alpha-lactalbumin-encoding gene: interspecies comparison, tissue- and stage-specific expression

The murine alpha-lactalbumin-encoding gene (m alpha La) was isolated and completely sequenced. The 2.3-kb transcription unit shared a similar organization with that of its counterparts from other species. Sequence comparison for the proximal 5'-flanking region indicated the presence of a consensus motif that occurs in all milk-protein-encoding genes, except the kappa-casein-encoding gene. This may correspond to the binding site for the recently identified mammary-gland-specific factor. The m alpha La gene occurs in a single copy per haploid genome and is specifically expressed in the mammary gland where it is induced during late pregnancy.

Differential effects of the simian virus 40 early genes on mammary epithelial cell growth, morphology, and gene expression

To study the effect of SV40 T-antigen in mammary epithelial cells, a rat beta-casein promoter-driven SV40 early-region construct was stably introduced into the clonal mouse mammary epithelial cell line HC11. With the expression of the viral T-antigens under the control of a hormone-inducible promoter, it was possible to dissociate the effects of different levels of T-antigen expression on cell growth, morphology, and gene expression. Following hormonal induction, a rapid but transient induction of T-antigen was observed, followed by a delayed induction of H4 histone mRNA. In T-antigen-positive HC11 cells cultured in the absence of EGF, the expression of basal levels of T-antigen (in the absence of hormonal induction) led to a decreased doubling time and an increased cell density. In the presence of EGF, T-antigen expression resulted additionally in an altered cell morphology. Despite the effects of T-antigen on cell growth and gene expression, the cells were unable to form colonies in soft agar and were nontumorigenic when transplanted into cleared mammary fat pads. They were, however, weakly tumorigenic in nude mice. Relatively high levels of p53 protein synthesis were observed in both the transfected HC11 cells and the parental COMMA-D cells, as compared to 3T3E fibroblasts and another mammary epithelial cell line. The HC11 and COMMA-D cells synthesized approximately equal levels of wild-type and mutated p53 proteins as defined by their reactivities with monoclonal antibodies PAb246 and PAb240, respectively. Interactions between excess p53 and T-antigen may, in part, explain the failure of these cells to display a completely transformed phenotype.

The prospects for domesticating milk protein genes

It is possible to convert milk glands of transgenic animals into bioreactors producing heterologous proteins such as scarce human pharmaceuticals. To predictably and successfully engineer the milk gland, we will need a thorough understanding of its physiology. Expression studies in transgenic animals have located mammary specific and hormone inducible transcription elements in the promoter/upstream regions of milk protein genes, and transfection studies in cell lines or primary cells have identified constitutive and hormone inducible elements. Most importantly, it appears that in addition to individual promoter based transcription elements structural features of milk protein chromosomal loci may contribute to the tight developmental and hormonal regulation. I will discuss milk protein gene regulation with emphasis on regulatory differences between genes and species, and the possibility that transcription elements function only properly within genetically defined chromatin domains. Novel strategies to build mammary expression vectors and to test their functionality without pursuing the standard transgenic route will be presented. Finally, I will discuss homologous recombination with the goal to target milk protein genes. Only through the domestication of milk protein genes will we be able to use their full potential in the mammary bioreactor.

High expression of the caprine beta-casein gene in transgenic mice

An 18-kb caprine genomic DNA fragment, comprising the beta-casein transcription unit with about 3-kb 5' and 6-kb 3' flanking regions, was microinjected into fertilized one-cell murine eggs. All nine lines of transgenic mice obtained expressed the transgene in their mammary glands, as demonstrated by Northern blot analysis of mRNA in miscellaneous tissues, and qualitative and quantitative analysis of caprine beta-casein in milk, using SDS/PAGE, Western blotting and rocket immunoelectrophoresis. Two lines produced milk containing up to 21-24 mg of the exogenous protein/ml, a yield which is roughly twice that found in goat milk. The yield reached at least 40 mg/ml in some progeny of crossbred G1 transgenic mice. Thus, the investigated gene appears to be a good candidate for making hybrid constructs that might promote an efficient production of valuable foreign proteins in milk.

Structure of the gene encoding rabbit alpha s1-casein

The complete nucleotide sequence of the entire rabbit alpha s1-casein-encoding gene Aslca and its flanking regions was determined. These data represent the first complete primary sequence of an Aslca gene. The gene consists of 19 exons spread over 16 kb. Highly conserved sequences were found between this gene and other casein-encoding genes mainly upstream from the gene from position -180 to -10. Several repeated interspersed elements of unknown function were also identified within introns.

Exon-skipping is responsible for the 9 amino acid residue deletion occurring near the N-terminal of human beta-casein

Interspecies comparison and alignment of the beta-casein N-terminal sequence, taking into account its exon modular splitting derived from the known structural organization of the relevant genes, has revealed that a 9 amino acid residue sequence, corresponding to that encoded by the third exon of the other species genes, is lacking in human beta-casein. Using the polymerase chain reaction technique, we have amplified a human genomic 1-kb fragment, spanning from exon 2 to exon 4, which was subsequently cloned and sequenced. One hundred base pairs (bp) upstream from exon 4 and 737 bp downstream of exon 2, a 27-bp virtual exon 3 sequence, probably skipped during the course of pre-mRNA splicing, was identified. We discuss the possibility that this out-splicing event might be due to the weak strength of the 3' acceptor site and/or to the secondary structure sequestering of the branch site sequence.

Exon skipping in human beta-casein

Earlier amino acid alignments of mature beta-caseins showed that the human protein was shifted in alignment relative to other species, with amino acid deletions in the N-terminal region and others inserted in the C-terminal region. Our alignment, based on cDNA sequences and their translation products, has shown that the amino acid deletions correspond exactly to exon 3 in the other species. Cloning and sequencing of a segment of the human beta-casein gene between exons 2 and 4 revealed the presence of an intact exon 3 sequence in the gene. An interruption of the polypyrimidine tract adjacent to the 5' end of exon 3 sequence may account for the omission of the exon from human beta-casein mRNA.

The fatty acid synthase (FAS) gene and its promoter in Rattus norvegicus

Screening of rat liver genomic libraries yielded 5 overlapping clones for rat fatty acid synthase (FAS). From these clones we determined the 18,170 bp sequence of the rat FAS together with 5,028 bp of the 5'-flanking region and 515 bp of the 3'-adjacent genomic sequence. The two FAS transcripts which differ only in the positions of their polyadenylation/termination sites consist of one untranslated and 42 translated exons. Surprisingly, the substrate binding site for enoyl reductase, one of the FAS component functions, is interrupted by an intron. The sizes and the boundaries of the individual domains could be mapped in relation to the exon/intron structure of the gene. These eight partial functions coincide with discrete units of exons. The acyl carrier protein with its prosthetic 4'-phosphopantetheine group is located within a single exon supporting the idea that rat FAS has evolved by gene fusion. Using primer extension the main transcription start site of the FAS mRNA in both hepatic and mammary gland tissues was located at 5,028 bp in the sequence determined. As expected of a gene which is pretranslationally regulated the 5'-flanking region contains, in addition to TATA and CAAT boxes, consensus sequences for several DNA binding proteins.

Multiple mRNA species code for two non-allelic forms of ovine alpha s2-casein

The two non-allelic forms of alpha s2-casein, occurring in ovine milk, differ by an internal deletion of nine amino acid residues, including both cysteine residues at positions 34 and 42 in the mature chain. Sequencing of several alpha s2-casein cDNA, isolated from the mammary cDNA library of a single lactating ewe, showed three new types which differed from that previously studied. In addition to the expected deletion of codons +34 to +42 affecting 30-40% of mRNA, another structural difference involving an internal stretch of 44 nucleotides in the 5' untranslated region, was found. S1-nuclease protection assays confirmed the existence of several types of the relevant mRNA and sequencing of in-vitro-amplified genomic DNA demonstrated the presence of the 44-nucleotide stretch in the alpha s2-casein transcriptional unit, thus ruling out the possibility of a cloning artefact. The different alpha s2-casein mRNA, four in terms of deletion and two in terms of nucleotide substitutions for a given ewe, can be readily explained by partial exon skipping and allelic differences, respectively. This assumption is well supported by the following observations: 5' and 3' ends of both deleted DNA fragments are similar to those of exons; sequences neighbouring the 44-nucleotide stretch of the genomic DNA perfectly match consensus sequences described for 3' and 5' ends of introns; the rather simple patterns observed on Southern blots of different enzymatic digests of genomic DNA strongly suggest the occurrence of only 1 copy alpha s2-casein gene/haploid genome. During the course of evolution, the alpha s2-casein-encoding gene has undergone many mutations and some of them might have occurred in regions corresponding to consensus splicing regions of the pre-mRNA. Thus, complete skipping of some exons might be responsible for the shorter sizes of rat and mouse alpha s2-casein mRNA. If so, the overall organization of the alpha s2-casein gene in the different species might be more similar than expected from structural comparisons of the cognate mRNA or caseins.