Expression of human lysozyme mRNA in the mammary gland of transgenic mice

Expression of a recombinant anti-programed cell death 1 antibody in the mammary gland of transgenic mice

Nivolumab, a fully human IgG4 anti-programed cell death 1（PD-1）antibody, is recently one of the most popular and successful therapeutic monoclonal antibodies in clinical use. With the increasing demands for Nivolumab and other therapeutic monoclonal antibodies, the mammary gland bioreactor has been regarded as another choice for the production of recombinant monoclonal antibodies besides mammalian cell culture. Here, we expressed a recombinant human anti-PD-1 antibody in the mammary glands of transgenic mice. Two expression vectors were constructed bearing the heavy and light chains of anti-PD-1 antibody respectively under the control of bovine α_s1-casein promoter. Transgenic mice were then generated by co-microinjection of the two expression cassettes. Three F0 founders with both heavy chain and light chain positive were obtained. Transgenes of both chains were detected to be stably transmitted to the offspring. The recombinant antibody was detected in the milk of transgenic mice with the highest expression level up to 80.52 ± 0.82 mg/L and could specifically binds to the human PD-1 antigen. Therefore, our results suggest the feasibility of anti-PD-1 antibody production in the milk of transgenic animals.

STAT5 plays a critical role in regulating the 5'-flanking region of the porcine whey acidic protein gene in transgenic mice

The mammary gland serves as a valuable bioreactor system for the production of recombinant proteins in lactating animals. Pharmaceutical-grade recombinant protein can be harvested from the milk of transgenic animals that carry a protein of interest under the control of promoter regions genes encoding milk proteins. Whey acidic protein (WAP), for example, is predominantly expressed in the mammary gland and is regulated by lactating hormones during pregnancy. We cloned the 5'-flanking region of the porcine WAP gene (pWAP) to confirm the sequence elements in its promoter that are required for gene-expression activity. In the present study, we investigated how lactogenic hormones--including prolactin, hydrocortisone, and insulin--contribute to the transcriptional activation of the pWAP promoter region in mammalian cells, finding that these hormones activate STAT5 signaling, which in turn induce gene expression via STAT5 binding sites in its 5'-flanking region. To confirm the expression and hormonal regulation of the 5'-flanking region of pWAP in vivo, we generated transgenic mice expressing human recombinant granulocyte colony stimulating factor (hCSF2) in the mammary gland under the control of the pWAP promoter. These mice secreted hCSF2 protein in their milk at levels ranging from 242 to 1,274.8 ng/ml. Collectively, our findings show that the pWAP promoter may be useful for confining the expression of foreign proteins to the mammary gland, where they can be secreted along with milk.

Construction and in vivo evaluation of a mammary gland-specific expression vector for human lysozyme

A mammary gland-specific expression vector p205C3 was constructed with the 5'- and 3'-flanking regions of β-lactoglobulin gene and the first intron of β-casein gene of Chinese dairy goat as regulatory sequences. Human lysozyme (hLYZ) cDNA from mammary gland was cloned into p205C3 and the recombinant vector was used to generate transgenic mice by microinjection. Based on the lysoplate assay, four female offspring of one male founder were detected expressing recombinant hLYZ in their milk at the levels of 5-200 mg/l, and the expressed protein had the same molecular weight as that of normal hLYZ. Besides mammary glands, ectopic expressions were also found in the spleens and the small intestines of the transgenic mice. Among the offspring, the female transgenic mice maintained and expressed the transgene stably with a highest expression level of 750 mg/l. Therefore, p205C3 could be used to develop animal mammary gland bioreactors expressing hLYZ.

Low expression of the antibacterial factor L-amino acid oxidase in bovine mammary gland

In the mouse, L-amino acid oxidase (LAO) produces hydrogen peroxide by utilizing free amino acids and is a proven antibacterial factor in mammary glands. Mastitis, a bacterial infection of the mammary gland, is the most frequent disease in dairy cattle. Here, we investigate whether LAO is expressed in the mammary gland of dairy cattle and is antibacterial. In dairy cattle, the expression level of LAO mRNA in the mammary gland was considerably lower than that in mice, and LAO activity was not observed in cattle milk that produced hydrogen peroxide. The expression of LAO mRNA was also low in Japanese Black cattle, the same as in Holstein cattle. A higher LAO mRNA expression was observed in the mastitis glands than in the lactating glands. Furthermore, spleen and lymph nodes expressed high levels of LAO mRNA in dairy cattle. We conclude that mammary glands in dairy cattle have lower ability to express the LAO gene compared to that in mice, which may result in a high incidence of mastitis.

Analysis of Tissue-Specific Expression of Human Type II Collagen cDNA Driven by Different Sizes of the Upstream Region of the β-Casein Promoter

To investigate the ability of 1.8 kb or 3.1 kb bovine beta-casein promoter sequences for the expression regulation of transgene in vivo, transgenic mice were produced with human type II collagen gene fused to 1.8 kb and 3.1 kb of bovine beta-casein promoter by DNA microinjection. Five and three transgenic founder mice were produced using transgene constructs with 1.8 kb and 3.1 kb of bovine beta-casein promoters respectively. Founder mice were outbred with the wild type to produce F1 and F2 progenies. Total RNAs were extracted from four tissues (mammary gland, liver, kidney, and muscle) of female F1 transgenic mice of each transgenic line following parturition. RT-PCR and Northern blot analysis revealed that the expression level of transgene was variable among the transgenic lines, but transgenic mice containing 1.8 kb of promoter sequences exhibited more leaky expression of transgene in other tissues compared to those with 3.1 kb promoter. Moreover, Western blot analysis of transgenic mouse milk showed that human type II collagen proteins secreted into the milk of lactating transgenic mice contained 1.8 kb and 3.1 kb of bovine beta-casein promoter. These results suggest that promoter sequences of 3.1 kb bovine beta-casein gene can be used for induction of mammary gland-specific expression of transgenes in transgenic animals.

Generation of mastitis resistance in cows by targeting human lysozyme gene to β-casein locus using zinc-finger nucleases

Mastitis costs the dairy industry billions of dollars annually and is the most consequential disease of dairy cattle. Transgenic cows secreting an antimicrobial peptide demonstrated resistance to mastitis. The combination of somatic cell gene targeting and nuclear transfer provides a powerful method to produce transgenic animals. Recent studies found that a precisely placed double-strand break induced by engineered zinc-finger nucleases (ZFNs) stimulated the integration of exogenous DNA stretches into a pre-determined genomic location, resulting in high-efficiency site-specific gene addition. Here, we used ZFNs to target human lysozyme (hLYZ) gene to bovine β-casein locus, resulting in hLYZ knock-in of approximately 1% of ZFN-treated bovine fetal fibroblasts (BFFs). Gene-targeted fibroblast cell clones were screened by junction PCR amplification and Southern blot analysis. Gene-targeted BFFs were used in somatic cell nuclear transfer. In vitro assays demonstrated that the milk secreted by transgenic cows had the ability to kill Staphylococcus aureus. We report the production of cloned cows carrying human lysozyme gene knock-in β-casein locus using ZFNs. Our findings open a unique avenue for the creation of transgenic cows from genetic engineering by providing a viable tool for enhancing resistance to disease and improving the health and welfare of livestock.

Zinc-finger nickase-mediated insertion of the lysostaphin gene into the beta-casein locus in cloned cows

Zinc-finger nickases (ZFNickases) are a type of programmable nuclease that can be engineered from zinc-finger nucleases to induce site-specific single-strand breaks or nicks in genomic DNA, which result in homology-directed repair. Although zinc-finger nuclease-mediated gene disruption has been demonstrated in pigs and cattle, they have not been used to target gene addition into an endogenous gene locus in any large domestic species. Here we show in bovine fetal fibroblasts that targeting ZFNickases to the endogenous β-casein (CSN2) locus stimulates lysostaphin gene addition by homology-directed repair. We find that ZFNickase-treated cells can be successfully used in somatic cell nuclear transfer, resulting in live-born gene-targeted cows. Furthermore, the gene-targeted cows secrete lysostaphin in their milk and in vitro assays demonstrate the milk's ability to kill Staphylococcus aureus. Our success with this strategy will facilitate new transgenic technologies beneficial to both agriculture and biomedicine.

The absence of detectable fetal microchimerism in nontransgenic goats (Capra aegagrus hircus) bearing transgenic offspring

Regulations for the disposal of genetically engineered animals are strict due to concern for their inappropriate introduction into the food chain, and of the possible public health and environmental impacts of these organisms. Nontransgenic animals that give birth to transgenic offspring are treated as if they are transgenic due to concern of fetal cells crossing the placental barrier and residing in the mother (fetal-maternal microchimerism). Determining whether or not fetal-fetal or fetal-maternal transfer of DNA or cells occurs during caprine gestation is critical to effectively protect the public without culling animals that pose no risk. Additionally, fetal-maternal transfer, should it exist in the goat, could contraindicate the rebreeding of nontransgenic dams due to the possible transfer of fetal cells from 1 pregnancy to the fetus of subsequent pregnancies. Fetal-maternal transfer in Capra hircus has not been reported in the literature, although it has been reported in another ruminant, Bos taurus. We examined blood from nontransgenic dams that carried transgenic offspring using a PCR method sensitive enough to detect the presence of a spider silk transgene to a 1:100,000 dilution. At this sensitivity, we did not detect the occurrence of fetal-maternal transfer in 5 nontransgenic dams. Likewise, fetal-fetal transfer was not observed from a transgenic to a nontransgenic twin in utero. To test tissue-specific expression of the silk transgene, proteins purified from standard necropsy tissue from a lactating transgenic dam were examined by Western blot analysis. Silk protein expression was only observed in mammary tissue consistent with the tissue specificity of the β-casein promoter used in the transgenic construct. We report evidence collected from a limited caprine breeding pool against transfer of transgenes in utero from fetus to dam and fetus to fetus. In addition, we show evidence that the β-casein promoter in our expression construct is not expressed ectopically as previously suggested. These results suggest that transgene transfer in utero does not occur, but further study is warranted with a larger sample group to confirm these results.

High-level expression of bioactive recombinant human lysozyme in the milk of transgenic mice using a modified human lactoferrin BAC

Transgenesis has been used for expressing human lysozyme (hLZ) in the milk of livestock to improve their disease resistance. Here we describe a human lactoferrin (hLF) BAC as a candidate vector for high-level expression of hLZ in the milk of transgenic mice. Using recombineering, hLF genomic DNA in the hLF BAC was replaced by the hLZ gene (from the ATG start codon to the TAA stop codon), and flanking regions of the hLF gene (a 90-kb 5' and a 30-kb 3') were used as transcriptional control elements for hLZ expression. When this construct was used to generate transgenic mice, rhLZ was highly expressed in the milk of four transgenic mouse lines (1.20-1.76 g/L), was expressed at a lower level in one additional line (0.21 g/L). rhLZ from the milk of these transgenic mice exhibited the same antibacterial activity as native hLZ. Our results suggest a potential approach for producing large amounts of hLZ in the milk of livestock.

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.

Human lysozyme expressed in the mammary gland of transgenic dairy goats can inhibit the growth of bacteria that cause mastitis and the cold-spoilage of milk

The addition of human milk components with intrinsic antimicrobial activity to livestock milk by genetic engineering has the potential to benefit milk safety and production as well as the health of the lactating animal. As a model for the dairy cow, we generated transgenic goats that expressed human lysozyme in their milk at 68% of the levels found in human milk. Milk from these transgenic animals had a bacteriostatic effect on both in vitro and in vivo growth of several microorganisms important to the dairy industry. In vitro, milk from transgenic animals was capable of slowing the growth of mastitis-causing strains of Escherichia coli (P < 0.02) and Staphylococcus aureus (P < 0.05) as well as the cold-spoilage organism Pseudomonas fragi (P < 0.02). The growth of an organism involved in cheese-making, Lactococcus lactis, was not affected by the presence of lysozyme in milk. The supplementation of control milk with purified lysozyme did not achieve the same inhibitory effect as milk from transgenic animals. In vivo, milk from transgenic animals supported less bacterial growth than control milk. This transgenic model demonstrates the possibilities offered by genetic engineering to enhance the antimicrobial nature of milk and the udder.

Designer milk

Dairy biotechnology is fast gaining ground in the area of altering milk composition for processing and/or animal and human health by employing nutritional and genetic approaches. Modification of the primary structure of casein, alteration in the lipid profile, increased protein recovery, milk containing nutraceuticals, and replacement for infant formula offer several advantages in the area of processing. Less fat in milk, altered fatty acid profiles to include more healthy fatty acids such as CLA and ω‐fats, improved amino acid profiles, more protein, less lactose, and absence of β‐lactoglobulin (β‐LG) are some opportunities of “designing” milk for human health benefits. Transgenic technology has also produced farm animals that secrete in their milk, human lactoferrin, lysozyme, and lipase so as to simulate human milk in terms of quality and quantity of these elements that are protective to infants. Cow milk allergenicity in children could be reduced by eliminating the β‐LG gene from bovines. Animals that produce milk containing therapeutic agents such as insulin, plasma proteins, drugs, and vaccines for human health have been genetically engineered. In order to cater to animal health, transgenic animals that express in their mammary glands, various components that work against mastitis have been generated. The ultimate acceptability of the “designer” products will depend on ethical issues such as animal welfare and safety, besides better health benefits and increased profitability of products manufactured by the novel techniques.

Expression and bioactivity of recombinant human lysozyme in the milk of transgenic mice

Human milk lysozyme is an important protein for innate immunity, but human breast milk is a fairly poor source for commercial production of this enzyme. Research on the expression of recombinant human lysozyme (rHlys) is therefore potentially valuable to the dairy industry. In this study, 2 different kinds of transgenic mice, PBC-hLY and PBC-sighLY, were generated and used as system models to express rHlys. Six lines of PBC-hLY transgenic mice with human lysozyme genomic DNA-based constructs were generated, and a maximum expression level of rHlys approaching 0.154 mg/mL was achieved. Antibacterial activity of the whey from PBC-hLY female transgenic mice was determined by a turbidimetric assay. Results showed that antibacterial activity of the whey was strongly enhanced, and confirmed that rHlys retained full activity. For rHlys to be secreted efficiently into the milk of transgenic mice, 5 lines of mice were also generated, in which the signal peptide DNA of bovine beta-casein was substituted for that of lysozyme in PBC-hLY transgenic mice. Compared with PBC-hLY transgenic mice, both the expression levels of rHlys and the antibacterial activity of the whey were much higher in the PBC-sighLY transgenic mice. The concentration of rHlys in one of these mice amounted to 1.405 mg/mL-3 times higher than the level in human whey. The antibacterial activity of the whey was also 3 times higher than that of human whey. The rHlys from both PBC-hLY and PBC-sighLY transgenic mice had the same antibacterial activity as human milk lysozyme. The effect of the signal peptide and copy numbers of the transgene on expression of rHlys was also evaluated. This work will certainly permit a better understanding of how mammary gland bioreactor systems can be applied to produce rHlys in other mammals, such as cattle.

The effect of coating single- and double-stranded DNA with the recombinase A protein of Escherichia coli on transgene integration in mice

Embryo survival and transgene integration rates are two major factors that influence the efficiency of transgenic animal production by pronuclear microinjection. Recombinase A protein-coated transgenes were compared for transgene integration and embryo survival with their non-coated counterparts in both single- and double-stranded forms. Murine zygotes were microinjected with a large 30 kb alpha(S1)-casein/human lysozyme DNA construct and a small 5.5 kb beta-lactoglobulin/desaturase DNA construct using four different construct preparations for each gene. The preparations included recombinase A protein-coated, single- and double-stranded DNA constructs and non-coated, single- and double-stranded DNA constructs. Using conventional non-coated, double-stranded DNA constructs, we obtained a transgene integration efficiency of 1.5% (1352 embryos transferred produced 20 transgenic pups). The same double-stranded DNA constructs coated with recombinase A protein yielded a similar percentage of transgene integration (1.1%, 18/1697). Using single-stranded DNA, non-coated constructs produced a transgene integration rate of 0.5%, while none of the 1040 zygotes injected with recombinase A-coated constructs produced transgenic pups. While recombinase A protein coating produced no effect on embryo survival, litter size or pregnancy rate with double-stranded constructs, a detrimental effect was observed on embryo survival (P < 0.001) and pregnancy rate (P < 0.005) with recombinase A protein coating of single-stranded human lysozyme DNA constructs. A trend toward increased embryo survival (P = 0.054) with no difference in pregnancy rate (P > 0.05) was observed with the recombinase A protein coating of single-stranded desaturase constructs. These results suggest that recombinase A protein coating of single- and double-stranded DNA constructs produced no significant differences (P > 0.05) in the efficiency of generating transgenic mice with respect to the percentage of transgenic animals born.

Production and Processing of Milk from Transgenic Goats Expressing Human Lysozyme in the Mammary Gland

The potential for applying biotechnology to benefit animal agriculture and food production has long been speculated. The addition of human milk components with intrinsic antimicrobial activity and positive charge to livestock milk by genetic engineering has the potential to benefit animal health, as well as food safety and production. We generated one line of transgenic goats as a model for the dairy cow designed to express human lysozyme in the mammary gland. Here we report the characterization of the milk from 5 transgenic females of this line expressing human lysozyme in their milk at 270 microg/mL or 68% of the level found in human milk. Milk from transgenic animals had a lower somatic cell count, but the overall component composition of the milk and milk production were not different from controls. Milk from transgenic animals had a shorter rennet clotting time and increased curd strength. Milk of such nature may be of benefit to the producer by influencing udder health and milk processing.

Hot Topic: Using a Stearoyl-CoA Desaturase Transgene to Alter Milk Fatty Acid Composition

Stearoyl-CoA desaturase enzyme converts specific medium- and long-chain saturated fatty acids to their monounsaturated form. Transgenic goats expressing a bovine beta-lactoglobulin promoter-rat stearoyl-CoA desaturase cDNA construct in mammary gland epithelial cells were produced by pronuclear microinjection. The fatty acid composition of milk from 4 female transgenic founders was analyzed on d 7, 14, and 30 of their first lactation. In 2 animals, the expression of the transgene changed the overall fatty acid composition of the resulting milk fat to a less saturated and more monounsaturated fatty acid profile at d 7 of lactation; however, this effect diminished by d 30. In addition, one animal had an increased proportion of the rumen-derived monounsaturated fatty acid C18:1 trans11 converted by stearoyl-CoA desaturase to the conjugated linoleic acid isomer C18:2 cis9 trans11. Milk that has higher proportions of monounsaturated fatty acids and conjugated linoleic acid may have benefits for human cardiovascular health.

Expression of lysostaphin in milk of transgenic mice affects the growth of neonates

As an initial step towards enhancing mastitis resistance in dairy animals, we generated BLG-Lys transgenic mice that secrete lysostaphin, a potent antistaphylococcal protein, in their milk. In the current study, we continue our assessment of lysostaphin as a suitable antimicrobial protein for mastitis resistance and have investigated mammary gland development and function in three lines of transgenic mice. As the lines were propagated, there was a tendency for fewer BLG-Lys litters to survive to weaning (51% as compared to 90% for nontransgenic lines, p = 0.080). Nontransgenic pups fostered on dams from these three lines exhibited diminished growth rates during the first week of lactation. Rates of gain became comparable to pups on nontransgenic dams at later time points. Initial slow growth also resulted in decreased weaning weights for pups nursed by transgenic dams (15.35 +/- 0.27 g) when compared to pups delivered and nursed by nontransgenic dams (18.61 +/- 0.61 g; p < 0.001), but the effect was temporary, as similar weights were attained by adulthood. Milk yield at peak lactation was not different between BLG-Lys (0.79 +/- 0.33 g) and nontransgenic (0.91 +/- 0.38 g; p = 0.166) dams. Histological examination of the transgenic mammary glands during gestation revealed no differences when compared to control glands; however, at early lactational stages, the BLG-Lys glands exhibited less alveolar area than control glands and a delay in lobulo-alveolar maturation. The results clearly demonstrate reduced growth of neonates on BLG-Lys dams; whether the poor pup performance can be attributed to delayed mammary development or the gland development merely reflects reduced suckling stimuli from the pups remains to be determined.

Increased efficiency of transgenic livestock production

Production of transgenic livestock by pronuclear microinjection of DNA into fertilized zygotes suffers from the compounded inefficiencies of low embryo survival and low integration frequencies of the injected DNA into the genome. These inefficiencies are one of the major obstacles to the large-scale use of pronuclear microinjection techniques in livestock. We investigated exploiting the properties of recombinase proteins that allow them to bind DNA to generate transgenic animals via pronuclear microinjection. In theory, the use of recombinase proteins has the potential to generate transgenic animals with targeted changes, but in practice we found that the use of RecA recombinase-coated DNA increases the efficiency of transgenic livestock production. The use of RecA protein resulted in a significant increase in both embryo survival rates and transgene integration frequencies. Embryo survival rates were doubled in goats, and transgene integration was 11-fold higher in goats and three-fold higher in pigs when RecA protein-coated DNA was used compared with conventional DNA constructs without RecA protein coating. However, a large number of the transgenic founders generated with RecA protein-coated DNA were mosaic. The RecA protein coating of DNA is straightforward and can be applied to any species and any existing microinjection apparatus. These findings represent significant improvements on standard pronuclear microinjection methods by enabling the more efficient production of transgenic livestock.

Mammary expression of new genes to combat mastitis

Continual advances in the ability to produce transgenic animals make it likely that such animals will become important components of animal agriculture. The full benefit of the technology, and justification of its initial cost outlay, will be dependent on the establishment within these animals of new traits not easily achievable by other means. Potential applications include enhanced nutrient digestibility with reduced fecal losses, significantly altered milk composition with superior nutritional properties, and enhanced disease resistance. Our goal is to enhance mastitis resistance of dairy cows by enabling the cells of the mammary gland to secrete additional antibacterial proteins. Proof of concept has been obtained through experimentation with a transgenic mouse model. Three lines of mice were developed that produce varying levels of lysostaphin in their milk. This protein has potent anti-staphylococcal activity and its secretion into milk confers substantial resistance to infection caused by intramammary challenge with Staphylococcus aureus, a major mastitis pathogen. Additional antibacterial proteins are being sought that will complement lysostaphin. A potential benefit of transgenic application of antibacterial proteins is the concomitant sparing in the agricultural use of antibiotics currently used as human therapeutics. Antibacterial proteins, such as lysostaphin, are not typically used as injectable or oral therapeutics because of immune-mediated or digestive destruction of their activity. In contrast, the immune system of transgenic animals will not consider the transgenic protein as being foreign. In addition we are exploring the potential of involution or mastitis responsive promoter elements for use in subsequent transgenic experiments designed to restrict lysostaphin production to these important time points. It is anticipated that genomics will play a role in unveiling candidate genes whose promoter elements will enable desired temporal expression patterns. The transgenic approach to insertion of new genetic material into agriculturally important animals is feasible but requires extensive prior evaluation of the transgene and transgene product in model systems.

The use of recombinase proteins to generate transgenic large animals

The endogenous properties of recombinase proteins allow them to associate with and bind DNA to catalyze homologous recombination. These endogenous properties of cellular recombination enzymes may be useful to the field of transgenesis. The production of transgenic animals, in particular livestock, is an inefficient process by both conventional pronuclear microinjection techniques and nuclear transfer. Furthermore, the use of pronuclear microinjection is currently limited to the random addition of genes and does not allow for the replacement of an endogenous gene with a more desired one. The functions of cellular recombination enzymes have been exploited to develop a technique that is compatible with pronuclear microinjection and may make the process of generating transgenic livestock more efficient while also enabling the targeting of homologous chromosomal genes. In our hands, transgenic animals generated by the pronuclear microinjection of various recombinase protein-coated DNA fragments led to a higher than expected birth rate as well as transgene integration frequency. Most founder animals generated were likely mosaic, indicating that integration occurred after cell division. The presence of multiple related genes makes detection of any recombination event difficult. Overall, this technique is a straightforward, rapid, and efficient procedure that can be applied to any segment of DNA and any microinjection apparatus, and is less labor intensive than nuclear transfer.

Expression of functional recombinant human lysozyme in transgenic rice cell culture

Using particle bombardment-mediated transformation, a codon-optimized synthetic gene for human lysozyme was introduced into the calli of rice (Oryza sativa) cultivar Taipei 309. The expression levels of recombinant human lysozyme in the transformed rice suspension cell culture approached approximately 4% of total soluble protein. Recombinant human lysozyme was purified to greater than 95% homogeneity using a two-step chromatography process. Amino acid sequencing verified that the N-terminus of the mature recombinant human lysozyme was identical to native human lysozyme. This indicates that the rice RAmy3D signal peptide was correctly cleaved off from the human lysozyme preprotein by endogenous rice signal peptidase. Recombinant human lysozyme was found to have the same molecular mass, isoelectric point and specific activity as native human lysozyme. The bactericidal activity of recombinant human lysozyme was determined by turbidimetric assay using Micrococcus lysodeikticus in 96-well microtiter plates. The bactericidal activity of lysozyme on gram-negative bacteria was examined by adding purified lysozyme to mid-log phase cultures of E. coli strain JM109. In this study, significant bactericidal activity was observed after E. coli cells were exposed to recombinant human lysozyme for 60 min. Both native and recombinant human lysozyme displayed the same thermostability and resistance to degradation by low pH. The potential for using rice-derived lysozyme as an antimicrobial food supplement, particularly for infant formula and baby foods, is discussed.

Lysostaphin expression in mammary glands confers protection against staphylococcal infection in transgenic mice

Infection of the mammary gland, in addition to causing animal distress, is a major economic burden of the dairy industry. Staphylococcus aureus is the major contagious mastitis pathogen, accounting for approximately 15-30% of infections, and has proved difficult to control using standard management practices. As a first step toward enhancing mastitis resistance of dairy animals, we report the generation of transgenic mice that secrete a potent anti-staphylococcal protein into milk. The protein, lysostaphin, is a peptidoglycan hydrolase normally produced by Staphylococcus simulans. When the native form is secreted by transfected eukaryotic cells it becomes glycosylated and inactive. However, removal of two glycosylation motifs through engineering asparagine to glutamine codon substitutions enables secretion of Gln(125,232)-lysostaphin, a bioactive variant. Three lines of transgenic mice, in which the 5'-flanking region of the ovine beta-lactoglobulin gene directed the secretion of Gln(125,232)-lysostaphin into milk, exhibit substantial resistance to an intramammary challenge of 104 colony-forming units (c.f.u.) of S. aureus, with the highest expressing line being completely resistant. Milk protein content and profiles of transgenic and nontransgenic mice are similar. These results clearly demonstrate the potential of genetic engineering to combat the most prevalent disease of dairy cattle.

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.

A hybrid bovine beta-casein/bGH gene directs transgene expression to the lung and mammary gland of transgenic mice

We investigated spatial and temporal expression of bGH controlled by two different sizes (1.8 kb and 15 kb) of 5'-flanking sequences of the bovine beta-casein in transgenic mice. In the 1.8-kb promoter-containing mice, bGH expression was specifically confined to lung and mammary gland at lactation. While mammary gland expression was highly variable depending on the lines, lung expression was relatively constant with a high level in most lines. Moreover, this dual-tissue specificity of bGH expression was consistently retained in all of the 15 kb-promoter-containing mice, although a low ectopic expression was sometimes detected in salivary gland or brain. During mammary gland development in the 1.8-kb promoter-containing mice was mammary gland expression first detected at lactation, following the bovine rather than murine pattern of beta-casein expression. In contrast, lung expression was almost constant regardless of mammary gland developmental state or sex. Therefore, it can be concluded that a combination of the bovine beta-casein promoter and bGH gene directs a distinct dual-tissue specific bGH expression with different regulatory mechanisms between mammary gland and lung and as little as 1.8-kb promoter is sufficient for the proper regulation of the bovine beta-casein gene in mammary gland.

Transgenic animal bioreactors in biotechnology and production of blood proteins

The regulatory elements of genes used to target the tissue-specific expression of heterologous human proteins have been studied in vitro and in transgenic mice. Hybrid genes exhibiting the desired performance have been introduced into large animals. Complex proteins like protein C, factor IX, factor VIII, fibrinogen and hemoglobin, in addition to simpler proteins like alpha 1-antitrypsin, antithrombin III, albumin and tissue plasminogen activator have been produced in transgenic livestock. The amount of functional protein secreted when the transgene is expressed at high levels may be limited by the required posttranslational modifications in host tissues. This can be overcome by engineering the transgenic bioreactor to express the appropriate modifying enzymes. Genetically engineered livestock are thus rapidly becoming a choice for the production of recombinant human blood proteins.

Stable production of human insulin-like growth factor 1 (IGF-1) in the milk of hemi- and homozygous transgenic rabbits over several generations

One transgenic rabbit line was generated carrying a fusion gene consisting of the cDNA for human IGF-1 fused to a mammary gland specific expression cassette derived from bovine alpha-S1-casein sequences. Transgene expression was shown to be strictly tissue and lactation period specific. The transgenic rabbit line was bred for six generations. All transgenic animals showed stable production of biologically active IGF-1 over the generations and no apparent effect on the physiological or reproductive performance was observed. The absence of adverse effects on homozygous transgenic rabbits suggested the absence of insertional mutagenesis. Eight hemizygous transgenic offspring analysed produced on average 363 +/- 12 micrograms/ml (ranging from 223 +/- 61 to 484 +/- 39 micrograms/ml) mature human IGF-1 in their milk, whereas three homozygous animals produced on average 543 +/- 41 micrograms/ml (ranging from 360 +/- 15 to 678 +/- 80 micrograms/ml). Homozygous hulGF-1 females clearly showed a significantly increased production performance of the recombinant protein.

Growth, development and differentiation: a functional food science approach

Few other aspects of food supply and metabolism are of greater biological importance than the feeding of mothers during pregnancy and lactation, and of their infants and young children. Nutritional factors during early development not only have short-term effects on growth, body composition and body functions but also exert long-term effects on health, disease and mortality risks in adulthood, as well as development of neural functions and behaviour, a phenomenon called 'metabolic programming'. The interaction of nutrients and gene expression may form the basis of many of these programming effects and needs to be investigated in more detail. The relation between availability of food ingredients and cell and tissue differentiation and its possible uses for promoting health and development requires further exploration. The course of pregnancy, childbirth and lactation as well as human milk composition and the short- and long-term outcome of the child are influenced by the intake of foods and particularly micronutrients, e.g. polyunsaturated fatty acids, Fe, Zn and I. Folic acid supplementation from before conception through the first weeks of pregnancy can markedly reduce the occurrence of severe embryonic malformations; other potential benefits of modulating nutrient supply on maternal and child health should be further evaluated. The evaluation of dietary effects on child growth requires epidemiological and field studies as well as evaluation of specific cell and tissue growth. Novel substrates, growth factors and conditionally essential nutrients (e.g. growth factors, amino acids, polyunsaturated fatty acids) may be potentially useful as ingredients in functional foods and need to be assessed carefully. Intestinal growth, maturation, and adaptation as well as long-term function may be influenced by food ingredients such as oligosaccharides, gangliosides, high-molecular-mass glycoproteins, bile salt-activated lipase, pre- and probiotics. There are indications for some beneficial effects of functional foods on the developing immune response, for example induced by antioxidant vitamins, trace elements, fatty acids, arginine, nucleotides, and altered antigen contents in infant foods. Peak bone mass at the end of adolescence can be increased by dietary means, which is expected to be of long-term importance for the prevention of osteoporosis at older ages. Future studies should be directed to the combined effects of Ca and other constituents of growing bone, such as P, Mg and Zn, as well as vitamins D and K, and the trace elements F and B. Pregnancy and the first postnatal months are critical time periods for the growth and development of the human nervous system, processes for which adequate substrate supplies are essential. Early diet seems to have long-term effects on sensory and cognitive abilities as well as behaviour. The potential beneficial effects of a balanced supply of nutrients such as I, Fe, Zn and polyunsaturated fatty acids should be further evaluated. Possible long-term effects of early exposure to tastes and flavours on later food choice preferences may have a major impact on public health and need to be further elucidated. The use of biotechnology and recombinant techniques may offer the opportunity to include various bioactive substances in special dietary products, such as human milk proteins, peptides, growth factors, which may have beneficial physiological effects, particularly in infancy and early childhood.

Accurate spatial and temporal transgene expression driven by a 3.8-kilobase promoter of the bovine beta-casein gene in the lactating mouse mammary gland

The spatial, temporal, and hormonal pattern of expression of the beta-casein gene is highly regulated and confined to the epithelial cells of the lactating mammary gland. Previous studies have shown that 1.7 kb of the bovine beta-casein promoter were able to drive cell-specific and hormone-dependent expression to a mouse mammary cell line but failed to induce accurate expression to the mammary gland of transgenic mice. We investigated here the ability of 3.8 kb of the bovine beta-casein gene promoter to drive the expression of the human growth hormone (hGH) gene in transgenic mice. A Northern blot analysis using total RNA obtained from different tissues of lactating and nonlactating females revealed the presence of hGH mRNA only in the mammary gland of lactating females. hGH mRNA was not detectable in the mammary gland of virgin females or males. A developmental analysis showed that hGH mRNA only peaked on parturition, resembling more closely the bovine beta-casein temporal expression pattern rather than the murine. In situ hibridization studies performed on mammary gland sections showed that the cellular pattern of hGH expression was homogeneous in all lobules from heterozygous and homozygous transgenic mice. Silver grain counts on the tissue sections highly correlated with the hGH contents in the milk determined by radioimmunoassay (r = 0.996). Thus 3.8 kb of the bovine beta-casein promoter direct a high-level expression of a reporter gene to the lactating mammary gland of transgenic mice in a tissue-specific and developmentally regulated manner.

High-level expression of bovine alpha s1-casein in milk of transgenic mice

The bovine alpha s1-casein gene, isolated from a cosmid library, was introduced into the murine germline. Transgene expression occurred in all transgenic mice, and was confined to the lactating mammary gland. Half of the mouse lines (five out of ten) expressed at relatively high expression levels (> 1 mg ml-1). The highest levels of expression were obtained with a transgene containing 14.2 kb of 5' flanking sequence, in two cases expression levels comparable to (10 mg ml-1) or well above (20 mg ml-1) alpha s1-casein levels in bovine milk were obtained. Transcription initiation occurred at the same site in the bovine alpha s1-casein gene in transgenic mouse as in the cow. A marked induction of expression occurred at parturition rather than at mid-pregnancy, and thus resembled the bovine rather than the murine developmental expression pattern. Bovine alpha s1-casein specific immunoblotting and RIA were developed for characterization and quantification of the recombinant protein. Using these assays, the properties of the recombinant protein could not be distinguished from those of the natural bovine protein. In spite of the high-level tissue-specific and correctly regulated developmental expression of the transgene, expression levels were integration-site dependent. This may indicate that not all cis-acting regulatory elements involved in bovine alpha s1-casein expression were included in the transgene.

Recombinant protein production in transgenic animals

The engineering of animals for recombinant protein production has gone beyond the stage of identifying proper regulatory sequences. Efforts are now spent on the generation of transgenic animals that process heterologous proteins more efficiently. Another line of research is the development of strategies aimed at bypassing pronuclear microinjection.

Expression of a bovine kappa-CN cDNA in the mammary gland of transgenic mice utilizing a genomic milk protein gene as an expression cassette

Transgenic mice were produced by microinjection of a DNA construct composed of the bovine kappa-casein (kappa-CN) cDNA under the control of the goat beta-CN 5' promoter elements and 3' flanking regions into pronuclear-stage embryos. The gene construct targeted the expression of bovine kappa-CN RNA to the mammary gland and secretion of bovine kappa-CN in the milk. In the three lines studied (BC-7, BC-31 and BC-67) the transgene was stably integrated and propagated as a Mendelian locus. Expression of the bovine protein in lactating mice from the three transgenic lines was demonstrated by northern and western blots. In ten different tissues analysed by northern blotting, expression was confined to the mammary gland of lactating transgenic mice from line BC-7, with low-level expression also observed in the salivary gland of lines BC-31 and BC-67. Transgene expression in the mammary gland paralleled normal casein gene expression during lactation and was not observed in virgin females. The level of bovine kappa-CN mRNA expression on day 10 of lactation in hemizygous transgenic females in relation to endogenous mRNA of whey acid protein (WAP) gene expression was 14%, 69%, and 127% in lines BC-7, BC-31 and BC-67, respectively. No association between transgene copy number and expression was observed. The bovine kappa-CN concentration in milk on day 10 of lactation ranged from 0.94 to 3.85 mg of protein per ml of milk. The bovine kappa-CN expressed in mouse milk had the same molecular mass and immunoactivity with polyclonal antibodies as did kappa-CN from bovine milk. A high degree of variation in the production of bovine kappa-CN within each of the transgenic lines was observed.

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.

The effect of mammary gland expression of human lysozyme on the properties of milk from transgenic mice

Transgenic mice were used as model systems to evaluate the impact of human lysozyme expression in the mammary gland. We previously generated two lines of transgenic mice that express human lysozyme mRNA in the mammary gland under the tissue-specific and developmentally correct control of the bovine gene promoter for alpha s1-casein. Concentrations of human lysozyme protein in milk of transgenic mice varied from .25 to .71 micrograms/microliters of milk. Human lysozyme secreted into mouse milk retained its antimicrobial activity, as determined by a denaturing polyacrylamide gel activity assay. The physical and functional properties of the milk were also altered, because mouse milk containing human lysozyme had a 35% decrease in rennet clotting time, a smaller median micelle size (157 nm vs. 172 nm), and a 2.5- to 3-fold greater gel strength than control milk. From these results, we conclude that the use of transgenic animals producing lysozyme in the milk is feasible and potentially useful to the dairy industry.

Expression analysis of the individual bovine beta-, alpha s2- and kappa-casein genes in transgenic mice

To identify cis-acting regulatory elements involved in the regulation of expression of the casein genes, the bovine beta-, alpha s2- and kappa-casein genes were isolated from cosmid libraries and introduced into the murine germline. Bovine casein expression was analysed at the RNA and protein level. The bovine beta-casein gene, including 16 kb of 5'- and 8 kb of 3'-flanking region, appeared to be expressed in all 12 transgenic mouse lines analysed. In 50% of these lines expression levels in milk exceeded 1 mg/ml. Three lines displayed expression levels comparable with or well above (20 mg/ml) the beta-casein levels in bovine milk. Transgene expression was restricted to the mammary gland. Strong induction of expression occurred at parturition and thus resembled the bovine rather than the murine pattern. In spite of this high-level tissue-specific and developmentally regulated expression, beta-casein expression levels were integration-site-dependent, suggesting that not all elements involved in regulation of expression were included in this beta-casein clone. Neither the bovine alpha s2- nor the kappa-casein gene, including 8 kb and 5 kb of 5'- and 1.5 kb and 19 kb of 3'-flanking sequences respectively, were properly expressed in transgenic mice. However, they were transcribed in stably transfected mouse mammary epithelial cells. This indicates that regulatory elements required for high-level, mammary gland-specific expression are not present in the alpha s2- and kappa-casein clones used in this study and are probably located elsewhere in the casein gene locus.