Expression analysis of ruminant α-lactalbumin in transgenic mice: Developmental regulation and general location of importantcis-regulatory elements

A Comparative Review of the Extrinsic and Intrinsic Factors Regulating Lactose Synthesis

Milk is critical for the survival of all mammalian offspring, where its production by a mammary gland is also positively associated with its lactose concentration. A clearer understanding of the factors that regulate lactose synthesis stands to direct strategies for improving neonatal health while also highlighting opportunities to manipulate and improve milk production and composition. In this review we draw a cross-species comparison of the extra- and intramammary factors that regulate lactose synthesis, with a special focus on humans, dairy animals, and rodents. We outline the various factors known to influence lactose synthesis including diet, hormones, and substrate supply, as well as the intracellular molecular and genetic mechanisms. We also discuss the strengths and limitations of various in vivo and in vitro systems for the study of lactose synthesis, which remains an important research gap.

Recombinant Derp5 allergen with αS1-casein signal peptide secreted in murine milk protects against dust mite allergen-induced airway inflammation

Recent advances in recombinant technology make transgenic animals that produce pharmaceutical proteins in their milk more feasible. The group 5 allergen isolated from Dermatophagoides pteronyssinus (Derp5) is one of the most important dust mite allergens in humans. The aims of this study were to develop transgenic mice that could secrete recombinant Derp5-containing milk and to demonstrate that ingesting recombinant milk protects against allergic airway inflammation. Two transgenes were constructed separately. The α-LA-Derp5f transgene consisted of the bovine α-lactalbumin (α-LA) promoter and full-length Derp5 cDNA. The α-LA-CN-Derp5t transgene included the α-LA promoter, a leader sequence of αS1-casein (CN), and signal peptide-truncated Derp5 cDNA. Both species of transgenic mice were confirmed to have successful transgene integration and stable germline transmission. Western blot analysis of the milk obtained from the offspring of transgenic mice demonstrated that recombinant Derp5 was secreted successfully in the milk of αLA-CN-Derp5t transgenic mice but not in that of αLA-Derp5f transgenic mice. This study provides new evidence that transgenic mice can secrete recombinant Derp5 efficiently in milk by adding a signal peptide of αS1-casein. The antigenic activity of recombinant Derp5 milk was demonstrated to have a protective effect against allergic airway inflammation in a murine model in which the ingestion of recombinant Derp5-containing milk was used as pretreatment.

Cloning and characterization of 5'-untranslated region of porcine beta casein gene (CSN2)

beta-Casein (CSN2) is a major milk protein in most mammals. The CSN2 gene is generally induced by lactogenic hormones bound to its promoter. The expression of this gene can be enhanced by signal transducers and activators of transcription (STAT) and glucocorticoid receptor (GR). Here, we analyzed the promoter and intron 1 regions of the porcine CSN2 gene. The porcine CSN2 promoter and intron 1 regions (-3098bp to +2446bp) were cloned into the pGL3-Basic vector containing the luciferase reporter gene (pCSN2-PEI). Lactogenic signals induced the transcription of porcine CSN2. By using AG490, a Janus kinase (JAK) inhibitor, we demonstrated that STAT5 positively regulates the transcription of porcine CSN2. Further, seven STAT mutants were generated by site-directed mutagenesis. By performing electrophoretic mobility shift assays (EMSAs), we located a critical element for pCSN2-PEI transcription bound to STAT5 in the -102bp to -84bp region. The construct containing only the promoter region (pCSN2-P), however, did not exert any promotive effects on transcription in two cell types-a mouse mammary epithelial cell line (HC11) and porcine mammary gland epithelial cells (PMECs). Thus, the construct containing intron 1 of porcine CSN2 exerts an elevating effect on transcription. We suggest that the transcription of porcine CSN2 is regulated by lactogenic signals via the STAT5 site (-102bp to -84bp) and intron 1.

In vitro and ex vivo green fluorescent protein expression in alveolar mammary epithelial cells and mammary glands driven by the distal 5'-regulative sequence and intron 1 of the goat beta-casein gene

The 5'-regulative sequence and intron 1 of the goat beta-casein gene from -4044 to +2123 bp was cloned and fused with the reporter gene of green fluorescent protein (GFP) to create a plasmid termed pGB562/GFP. To detect GFP expression, pGB562/GFP was transfected in vitro via liposomes into the mammary epithelial cell line NMuMG. Cells could not express GFP unless the transfected NMuMG cells lined up to create functional alveoli. These functional cells were cultured with lactogenic hormones, including insulin, dexamethasone and prolactin, and were grown on a layer of the extracellular matrix Matrigel. Green fluorescent protein expression levels in NMuMG cells were 25-, 55- and 42-fold those in the control group at 24, 48, and 72 h after pGB562/GFP transfection respectively. In addition, pGB562/GFP was transfected ex vivo by electroporation into mammary gland fragments and cells were then cultured in vitro with a supplement of lactogenic hormones. Strong GFP expression localized in fragments of the mammary gland was observed 24 h after gene transfer. The novel strategy of ex vivo gene transfer into mammary tissue using GFP as a reporter gene to detect the function of a tissue-specific promoter is efficient and convenient. The data obtained herein reveal that the 5'-regulative sequence and intron 1 of the 6.2 kb goat beta-casein gene can enhance the efficiency of transgene expression. Thus, the GB562 sequence may act as a good promoter and effectively elevate the production of exogenous protein in mammary glands.

An expression profile of human alpha-lactalbumin in the milk of transgenic mouse

Five female transgenic mice were produced by microinjection using a construct made up of a 7.3-kb-5' flanking region and a 2.0-kb coding region of human alpha-lactalbumin, as well as a 227-bp 3'-flanking region from bovine growth hormone gene. A founder female expressed human alpha-lactalbumin as much as 0.3 g per liter of its milk, approximately a 3-fold increase in the total alpha-lactalbumin concentration of the transgenic mouse milk. Compared with the normal mice, the expression profile of the halpha-Lac transgene in the transgenics is different during the lactation, showing low level in the first 3 days and becoming increased from day 4, then gradually reaching and stabilizing at the highest level from day 13. In addition, the milk yielding volume in the transgenics tended to be higher than in normal mice, suggesting higher concentrations of alpha-lactalbumin might boost more milk output.

Analysis of the flanking regions of the human alpha-lactalbumin gene responsible for position-effect independent expression

Transgenic rats with the 130 kb bacterial artificial chromosome construct bLA, including the alpha-lactalbumin gene, had position-independent and copy number-dependent expression, which confirmed previous experiments using the 210 kb yeast artificial construct, yLALBA. To identify elements that confer a position effect, we compared the yLALBA and bLA sequences. yLALBA was chimeric. A common 32 kb region was identified and the total nucleotide sequence was determined. We previously analyzed transgenic rats using polymerase chain reaction to compare the integrity and expression of the transgenes. The -6 to +9 kb region is considered to be necessary for position-independent expression. Transgenic rats lacking the -3.4 to -0.85 kb region had a severe position effect. This 2.5 kb region contains two DNaseI hypersensitive sites at -1.0 and -2.8 kb. The 2.5 kb region is proposed to be a locus control region of the human alpha-lactalbumin gene.

Temporal and spatial expression of biologically active human factor VIII in the milk of transgenic mice driven by mammary-specific bovine alpha-lactalbumin regulation sequences

Hemophilia A is one of the major inherited bleeding disorders caused by a deficiency or abnormality in coagulation factor VIII (FVIII). Hemophiliacs have been treated with whole plasma or purified FVIII concentrates. The risk of transmitting blood-borne viruses and the cost of highly purified FVIII are the major factors that restrict prophylaxis in hemophilia therapy. One of the challenges created by the biotechnology revolution is the development of methods for the economical production of highly purified proteins in large scales. Recent developments indicate that manipulating milk composition using transgenesis has focused mainly on the mammary gland as a bioreactor to produce pharmaceuticals. In the present study, a hybrid gene containing bovine alpha-lactalbumin and human FVIII cDNA was constructed for microinjection into the pronuclei of newly fertilized mouse eggs. The alphaLA-hFVIII hybrid gene was confirmed to be successfully integrated and stably germ-line transmitted in 12 (seven females/five males) lines. Western-blot analysis of milk samples obtained from eight of the transgenic founders and F1 offspring indicated that the recombinant hFVIII was secreted into the milk of the transgenic mice. The concentrations of rFVIII ranged from 7.0 to 50.2 microg/ml, over 35-200-fold higher than that in normal human plasma. Up to 13.4 U/ml of rFVIII was detected in an assay in which rFVIII restored normal clotting activity to FVIII-deficient human plasma.

Enhanced branching morphogenesis in mammary glands of mice lacking cell surface beta1,4-galactosyltransferase

Development of the mammary gland is influenced both by the systemic hormonal environment and locally through cell-cell and cell-extracellular matrix (ECM) interactions. We have previously demonstrated aberrant mammary gland morphogenesis in transgenic mice with elevated levels of the long isoform of beta1,4-galactosyltransferase 1 (GalT), a proportion of which is targeted to the plasma membrane, where it plays a role in cell-ECM interactions. Here, we show that mammary glands of mice lacking the long GalT isoform exhibit a complementary phenotype. Cell-surface GalT activity was reduced by over 60%, but because the short GalT isoform is intact, total GalT activity was reduced only slightly relative to wild type. Mammary glands from long GalT-null mice were characterized by excess branching, and this phenotype was accompanied by altered expression of laminin chains. Laminin alpha1 and alpha3 were reduced 2.4- and 3.0-fold, respectively, while expression of laminin gamma2 was elevated 2.3-fold. The expression and cleavage of laminin gamma2 have been correlated with branching and cell migration, and Western blotting revealed an altered pattern in gamma2 cleavage products in long GalT-null mammary glands. We then examined the expression of metalloproteases that cleave laminins or that have been shown to play a role in mammary gland morphogenesis. Expression of MT1-MMP, a membrane-bound protease that can cleave laminin gamma2, was elevated 5.5-fold in the long GalT-nulls. MMP 7 was also elevated 5.1-fold. Our results suggest that expression of surface GalT is important for the proper regulation of matrix expression and deposition, which in turn regulates the proper branching morphogenesis of the mammary epithelial ductal system.

Transgenic animals and nutrition research

Transgenic animals are useful tools for the study of biological functions of proteins and secondary gene products synthesized by the action of protein catalysts. Research in nutrition and allied fields is benefiting from their use as models to contrast normal and altered metabolism. Although food, nutritional products, and ingredients from transgenic animals have not yet reached consumers, the technologies for their production are maturing and yielding exciting results in experimental and farm animals. Regulatory governmental bodies are already issuing guidelines and legislation in anticipation of the advent of these products and ingredients. This review summarizes available technology for the production of transgenic animals, discusses their scientific and commercial potential, and examines ancillary issues relevant to the field of nutrition.

Analysis of control elements for position-independent expression of human alpha-lactalbumin YAC

A major problem in the production of transgenic animal bioreactors using microinjections is the low production rate of high-expressing transgenic animals due to the position effect. We previously reported that transgenic rats carrying the 210 kb yeast artificial chromosome (YAC) including the human alpha-lactalbumin gene express the transgene in a position-independent manner. The 210 kb YAC was thought to have all the elements necessary for position-independent expression. In this paper, we constructed fragmented YAC clones and a cosmid clone, and produced transgenic rats to analyze these elements. Transgenic rats with both the 50 kb upstream and downstream regions of the alpha-lactalbumin gene had position-independent expression. Transgenic rats with the 20 kb upstream and downstream regions, however, had position-dependent expression. Therefore, all the elements necessary for position-independent expression are thought to be located in the 50 kb upstream to 50 kb downstream region of the alpha-lactalbumin gene. Furthermore, we replaced the human alpha-lactalbumin promoter with the bovine alphaS1-casein promoter in the 210 kb YAC and produced transgenic rats. Position-dependent expression was observed. The elements required for position-independent expression of the bovine alphaS1-casein gene are different from those required for the human alpha-lactalbumin gene, despite the fact that the two genes have the same tissue and developmental specificity.

Use of doxycycline-controlled gene expression to reversibly alter milk-protein composition in transgenic mice

A reverse tetracycline transactivator-encoding cDNA under the control of the mammary specific beta-lactoglobulin promoter was linked to a bovine alpha-lactalbumin transcription unit driven by a reverse tetracycline-controlled transactivator/doxycycline-inducible human cytomegalovirus promoter. The construct was microinjected into eggs from alpha-lactalbumin-deficient mice. These mice produce a highly viscous lactose-free milk and have a shortened lactation period. Mice from three out of the nine transgenic lines investigated expressed reverse tetracycline-controlled transactivator mRNA in their lactating mammary glands at levels detectable by Northern analysis. Following doxycycline addition to the drinking water, lactation was fully restored in animals from the three lines. Doxycycline removal resulted in a reversal of phenotype. The observed mammary-specific and high expression of the doxycycline inducible reporter gene (up to 5.2 mg of recombinant alpha-lactalbumin.mL-1 of milk, i.e. up to 13-fold induction) opens up exciting prospects to use the tetracycline system to study the development and functioning of the mammary gland, and to control the production level of active pharmaceutical proteins in the milk of transgenic animals.

Introduction of a proximal Stat5 site in the murine alpha-lactalbumin promoter induces prolactin dependency in vitro and improves expression frequency in vivo

In order to establish a possible correlation between in vitro prolactin induction and the transcriptional activity of mammary gene promoters in transgenic mice, a functional Stat5-binding site was created by means of site-directed mutagenesis at position -70 on a 560 bp murine alpha-lactalbumin promotor linked to a CAT reporter gene. Surprisingly, the wild-type promoter was constitutively active in vitro and could not be induced by prolactin. Introducing the proximal Stat5 site abolished this constitutive activity and resulted in prolactin dependence in both CHO-K1- and HC11-transfected cells. In transgenic mice, both the frequency of lines expressing the transgene and the prevalence of mid to late pregnancy expression were increased.

Characterization and partial purification of bovine alpha-lactalbumin and beta-casein produced in milk of transgenic mice

Bovine alpha-lactalbumin (alpha-LA) and bovine beta-casein (beta-CN), from milk from transgenic mice were characterized and partially purified using electrophoretic, immunoblotting, and chromatographic methods. The transgenically expressed bovine milk proteins were identified using PAGE or by a combination of preparative isoelectrofocusing followed by Western immunoblotting. The heterologous bovine alpha-IA and bovine beta-CN had molecular masses that were identical to those of those of the native proteins. The estimated expression of the proteins was 1.0 mg/ml of milk for alpha-LA and 3.0 mg/ml for beta-CN. The calcium binding of bovine alpha-LA suggested that the protein produced in murine milk has the same electrophoretic shift as native bovine alpha-LA after the removal of calcium. Nitrogen-linked glycosylation of native and murine synthesized bovine alpha-LA was identified by peptide-N-glycosidase F treatment, and the N-terminal amino acid sequence of HPLC-purified bovine alpha-LA from mouse milk was confirmed to be identical to native bovine alpha-LA. In addition, the phosphorylation of the bovine beta-CN expressed in the milk of transgenic mice was the same as that of native bovine beta-CN, as determined by phosphatase digestion.

Transgenic dairy cattle: genetic engineering on a large scale

Amid the explosion of fundamental knowledge generated from transgenic animal models, a small group of scientists has been producing transgenic livestock with goals of improving animal production efficiency and generating new products. The ability to modify mammary-specific genes provides an opportunity to pursue several distinctly different avenues of research. The objective of the emerging gene "pharming" industry is to produce pharmaceuticals for treating human diseases. It is argued that mammary glands are an ideal site for producing complex bioactive proteins that can be cost effectively harvested and purified. Consequently, during the past decade, approximately a dozen companies have been created to capture the US market for pharmaceuticals produced from transgenic bioreactors estimated at $3 billion annually. Several products produced in this way are now in human clinical trials. Another research direction, which has been widely discussed but has received less attention in the laboratory, is genetic engineering of the bovine mammary gland to alter the composition of milk destined for human consumption. Proposals include increasing or altering endogenous proteins, decreasing fat, and altering milk composition to resemble that of human milk. Initial studies using transgenic mice to investigate the feasibility of enhancing manufacturing properties of milk have been encouraging. The potential profitability of gene "pharming" seems clear, as do the benefits of transgenic cows producing milk that has been optimized for food products. To take full advantage of enhanced milk, it may be desirable to restructure the method by which dairy producers are compensated. However, the cost of producing functional transgenic cattle will remain a severe limitation to realizing the potential of transgenic cattle until inefficiencies of transgenic technology are overcome. These inefficiencies include low rates of gene integration, poor embryo survival, and unpredictable transgene behavior.

Position-independent and high-level expression of human alpha-lactalbumin in the milk of transgenic rats carrying a 210-kb YAC DNA

The level of expression of transgenes in transgenic animals varies among lines, and is often much lower than that of endogenous genes (position effects). In order to surmount position effects and establish a more efficient production system of transgenic animals producing pharmaceutical proteins in their milk, transgenic rats carrying 210-kb YAC DNA containing the human alpha-lactalbumin gene were produced. Three transgenic lines transmitted the transgene to the next generation. They had one copy of the alpha-lactalbumin gene and secreted human alpha-lactalbumin in their milk at concentrations of 2.0-4.3 mg/ml. No position effect was seen. The transgene was expressed specifically in the mammary gland of the transgenic rats. The 210-kb region is thought to contain all the DNA elements required for proper expression of the human alpha-lactalbumin gene. The YAC carrying the human alpha-lactalbumin gene is a potential vector for the expression of foreign genes in the mammary gland.

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.

DNA sequence of the porcine alpha-lactalbumin 5' flanking region and single-base polymorphisms within this region

The 5' flanking region of the alpha-lactalbumin (alpha-LA) gene was sequenced for the Duroc, Yorkshire and Meishan breeds of swine to identify potential sequence variants within this regulatory region of the porcine alpha-LA gene. The sequenced region of the gene encompasses 391bp5' of the translation start site to 11bp3' of the translation start site. Within this sequence of the porcine alpha-LA gene two single-base pair differences were detected. One variant occurs at position -178 and the other at position -235 from the translation start site. Each of the variations can be detected by a restriction fragment length polymorphism within a polymerase chain reaction amplified product. The polymorphisms at the -178 and -235 positions appear to be genetically linked in the animals that have been analysed.

Lactation is disrupted by alpha-lactalbumin deficiency and can be restored by human alpha-lactalbumin gene replacement in mice

Mice carrying either a deletion of the murine alpha-lactalbumin (alpha-lac) gene (null allele) or its replacement by the human alpha-lac gene (humanized allele) have been generated by gene targeting. Homozygous null females are alpha-lac-deficient, produce reduced amounts of thickened milk containing little or no lactose, and cannot sustain their offspring. This provides definitive evidence that alpha-lac is required for lactose synthesis and that lactose is important for milk production. Females homozygous for the humanized allele lactate normally, indicating that human alpha-lac can replace murine alpha-lac. Mouse and human alpha-lac expression was compared in mice heterozygous for the humanized allele. The human gene expressed approximately 15-fold greater mRNA and approximately 14-fold greater protein than the mouse, indicating that the major determinants of human alpha-lac expression are close to, or within, the human gene and that the mouse locus does not exert a negative influence on alpha-lac expression. Variations in alpha-lac expression levels in nondeficient mice did not affect milk lactose concentration, but the volume of milk increased slightly in mice homozygous for the humanized allele. These variations demonstrated that alpha-lac expression in mice is gene dosage dependent.

Specific combinations of human serum albumin introns direct high level expression of albumin in transfected COS cells and in the milk of transgenic mice

A new series of expression vectors, each comprised of the beta-lactoglobulin (BLG) promoter driving one of a variety of human serum albumin (HSA) minigenes or the entire gene, were evaluated for their ability to direct expression of HSA in vitro in COS tissue culture cells and into the milk of transgenic mice. Vectors directed a hierarchy of expression levels in vitro, dependent upon the specific complement of HSA introns included. HSA introns acted in a synergistic manner. In addition, minigenes comprised of specific subsets of introns were more efficacious than the entire HSA gene with all of its introns. Transgenic mice expressed as much as 10 mg ml-1 of HSA in their milk. Vectors comprised of specific intron subsets directed levels at 1 mg ml-1 or greater in the milk of 20% of generated transgenics. A statistical correlation between the expression level trend in vitro with the trend of expression in vivo (% which express) at detectable levels (p = 0.0015) and at the level of greater than 0.1 mg ml-1 (p = 0.0156) was demonstrated. A weak correlation existed (p = 0.0526) at in vivo levels of 1 mg ml-1 or greater. These new vectors are expected to direct the production of high levels of HSA in the milk of a large percentage of generated transgenic dairy animals.

Creation and phenotypic analysis of alpha-lactalbumin-deficient mice

alpha-Lactalbumin is an abundant milk-specific calcium metalloprotein which has an evolutionary relationship to lysozyme. It modifies the substrate specificity of a Golgi galactosyltransferase by forming the lactose synthetase binary complex. Lactose, together with other sugars and diffusible ions, is responsible for the osmotic pressure of milk. To assess the involvement of alpha-lactalbumin in lactogenesis, alpha-lactalbumin-deficient mice were created by disrupting the gene by homologous recombination in embryonic stem cells. Homozygous mutant mice are viable and fertile but females cannot feed their offspring. They produce a highly viscous milk that pups appear to be unable to remove from the mammary gland. This milk is rich in fat and protein and is devoid of alpha-lactalbumin and lactose. The phenotype of heterozygous mice was found to be intermediate, with a 40% decrease in alpha-lactalbumin but only a 10-20% decrease in the lactose content of their milk compared with wild-type animals. These results emphasize the key function of alpha-lactalbumin in lactogenesis and open new opportunities to manipulate milk composition.

Variation in expression of a bovine alpha-lactalbumin transgene in milk of transgenic mice

Transgenic mice were produced to study the production of bovine alpha-LA in their milk. A 7.6-kb fragment containing a bovine alpha-LA gene was purified and microinjected into pronuclear stage mouse embryos. This fragment contained 2.0 kb of 5' flanking region, the 1.7-kb coding region, and 2.7 kb of 3' flanking region. Out of 121 potential transgenic founder mice, 3 were identified as being transgenic by the polymerase chain reaction. Multiple mice from the second, third, and fourth generation from each line were milked, and the milk was analyzed using an ELISA assay and Western blots to determine the presence of bovine alpha-LA. Bovine alpha-LA was present at concentrations up to 1.5 mg of protein/ml of mouse milk. The high degree of expression variation between mice within each of the transgenic lines was a characteristic that has not been reported in other studies of transgene expression in milk. Production of bovine alpha-LA in the milk of these transgenic mice showed a high degree of variation both within a lactation and between mice within a line. The bovine alpha-LA concentration in a single line of transgenic mice exhibited as much as a 10-fold variation between mice. Variations as high as 3-fold were detected within a single lactation in the same mouse. These differences in expression appeared to be correlated with mouse milk production; bovine alpha-LA was higher on d 10 and 15 of lactation than on d 5. Transgenic mice that show variation in expression of a bovine gene might offer a unique system for studying quantitative traits in a laboratory model.

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.

Functions of milk protein gene 5' flanking regions on human growth hormone gene

Fragments containing 5' flanking regions of four bovine milk protein genes--alpha lactalbumin (b alpha LA), alpha S1 casein (b alpha S1CN), beta casein (b beta CN), kappa casein (b kappa CN)--and mouse whey acidic protein (mWAP) gene were prepared by PCR and ligated to human growth hormone (hGH) gene. These recombinant DNAs were microinjected into rat embryos to produce transgenic rats, and the functions of the 5' regions to direct secretion of hGH in the milk were tested. Although milk was obtained only in 5 of 19 mWAP/hGH rat lines, more than two-thirds of the rats carrying the other four DNAs produced milk. More than 80% of the lactated rats carrying b alpha LA/, b beta CN/, and mWAP/hGH, and 33% of the lactated b alpha S1CN/hGH rats secreted detectable amounts of hGH (> 0.05 microgram/ml) in the milk. In some rats, the hGH concentrations in the milk were comparable to or more than that of the corresponding milk protein in bovine milk. The ranges of hGH concentrations in the milk of b alpha LA/, b beta CN/, b alpha S1CN/, and mWAP/hGH rats were 1.13-4,360 micrograms/ml, 0.11-10,900 micrograms/ml, 86.8-6,480 micrograms/ml, and 6.87-151 micrograms/ml, respectively. HGH was also detected in the sera of these rats, and some abnormalities of growth and reproduction were observed. All but one virgin mWAP/hGH rat secreted up to 0.0722 microgram/ml of hGH in the serum, and more than half of them showed abnormal fat accumulations at their abdomen.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

[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)

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.