In vitro transcription of the mouse whey acidic protein promoter is affected by upstream sequences

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.

Exon organization and sequence of the genes encoding alpha-lactalbumin and beta-lactoglobulin from the tammar wallaby (Macropodidae, Marsupialia)

Clones encompassing the genes encoding alpha-lactalbumin and beta-lactoglobulin were isolated from a tammar wallaby genomic library, the exons localized using end-labeled oligonucleotides and the DNA sequences determined. The tammar beta-lactoglobulin gene has the same 7 exon-6 intron structure as the sheep homologue. Potential binding sites for mammary gland-specific transcription factors were identified, on the basis of similarity to sites in the sheep gene, in the promoter region of the tammar beta-lactoglobulin gene. The tammar gene encoding alpha-lactalbumin appears to contain four introns rather than three as are present in the eutherian homologues, or the evolutionarily related lysozyme gene. The additional intron appears to occur within the 5' noncoding region of the tammar gene.

Negative regulatory element in the mammary specific whey acidic protein promoter

Expression of the whey acidic protein (WAP) gene is tightly regulated in a tissue and developmental stage specific manner, in that the WAP gene is exclusively expressed in the mammary gland during pregnancy and lactation. Using both deletion and competition analyses, evidence is provided for the existence of a negative regulatory element (NRE) in the WAP promoter located between -413 and -93 with respect to the WAP transcriptional initiation site. This NRE dramatically decreases transcription from linked heterologous promoter-reporter gene constructs. The activity of NRE requires WAP promoter sequences that are 230 bp apart since subfragments of the NRE fail to inhibit transcription of adjoining reporter genes. Nuclear extracts from different cell types, in which the WAP gene is not active, contain a protein or complex that specifically interacts with the entire NRE but not with subfragments of it. The contact points between this protein (NRE binding factor [NBF]) and the NRE element have been partially determined. Mutation of the implicated nucleotides severely reduces the ability of NBF to bind, and such mutated promoter fragments fail to alleviate transcriptional repression in competition experiments. This suggests that NBF binding to the NRE is at least in part responsible for the negative regulation of the WAP promoter. Since NBF is not detectable in the lactating mammary gland, where the WAP gene is expressed, we speculate that it may be a determinant of the expression spectrum of the WAP gene.

The activated mammary gland specific nuclear factor (MGF) enhances in vitro transcription of the beta-casein gene promoter

The hormonal induction of the beta-casein gene in mammary epithelial cells is dependent on the action of peptide and steroid hormones. Epidermal growth factor, insulin, glucocorticoids and prolactin act in a sequential manner to regulate the transcription of the gene. We have studied the hormonal requirements as well as the nuclear proteins which are involved in the induction process. In vitro transcription in cell free nuclear extracts has been used to demonstrate the central role of the mammary gland specific nuclear factor, MGF, in the mediation of the hormonal signals to the transcription machinery. A gene construct comprising 344 nucleotides of wild type beta-casein promoter sequence and a G-free cassette of 220 nucleotides was used to test the activity of nuclear extracts in the in vitro transcription experiments. A construct in which the proximal MGF binding site in the beta-casein promoter region has been inactivated by mutation and a construct regulated by the adenovirus major late promoter served as controls. Nuclear extracts were prepared from Sf insect cells, HeLa cells and mammary epithelial cells of lactating rats. Strong transcription of the wild type beta-casein promoter construct was observed in the mammary cell extract, weak transcription in the extracts of Sf insect cells and the HeLa cells. The mutation of the MGF binding site drastically reduced the in vitro transcription in the mammary gland cell extract. Beta-casein promoter activity was also compared in nuclear extracts from uninduced and lactogenic hormone induced HC11 mammary epithelial cells. Extracts from induced cells are more efficient in the support of beta-casein gene transcription.

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.

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.