[Nuclear organization and expression of milk protein genes]

Milk protein gene expression varies during the pregnancy/lactation cycle under the influence of lactogenic hormones which induce the activation of several transcription factors. Beyond this activation modifying the binding properties of these factors to their consensus sequences, their interactions with DNA is regulated by variations of the chromatin structure. In the nuclei of the mammary epithelial cell, the three dimensional organisation of the chromatin loops, located between matrix attachment regions, is now being studied. The main milk components are organised in supramolecular structures. Milk fat globules are made of a triglyceride core enwrapped by a tripartite membrane originating from various intracellular compartments. The caseins, the main milk proteins, form aggregates: the casein micelles. Their gradual aggregation in the secretory pathway is initiated as soon as from the endoplasmic reticulum. The mesostructures of the milk fat globule and of the casein micelle remain to be elucidated. Our goal is to make some progress into the understanding of the molecular and cellular mechanisms involved in the formation of these milk products.

Analysis of the efficiency of the rabbit whey acidic protein gene 5′ flanking region in controlling the expression of homologous and heterologous linked genes

For 10 years, the regulatory regions of the mouse and rabbit whey acidic protein gene have been used to express heterologous proteins in the milk of transgenic mice, as well as to produce pharmaceutical proteins, on a large scale, in the milk of transgenic livestock. To date, a broad range of expression levels have been detected, and elucidation of the structure-function relationship in these regulatory regions might help to achieve high levels of expression, reproducibly. An extended 5′ regulatory region (17·6 kb v. 6·3 kb) of the rabbit whey acidic promoter resulted in an increased frequency of rabbit whey acidic protein expression in transgenic mice. However, the expression levels were low compared with the high expression levels achieved in both transgenic mice and rabbits using the heterologous κ-casein in the 6·3 kb rabbit whey acidic protein 5′ regulatory region. These results underline the importance of the 3’ downstream regulatory regions, which still need to be better characterized in the whey acidic protein gene.

Interactions between the rabbitCSN1 gene and the nuclear matrix of stably transfected HC11 mammary epithelial cells vary with its level of expression

The expression of casein genes is specific to the mammary gland and maximal during lactation. However, among the numerous mammary cell lines described so far, only a few express some casein genes. The regulatory regions of casein genes have been largely described but the mechanisms explaining the mammary specific expression of these genes, and their silencing in most mammary cell lines, have not yet been fully elucidated. To test the hypothesis that the nuclear location of the casein genes may affect their expression, we transfected HC11 mouse mammary cell line with a 100 kb DNA fragment surrounding the rabbit alpha S1 casein gene. We derived stable clones which express or not the transfected rabbit casein gene, in the same cellular context, independently of the number of transgene copies. Metaphase spreads were prepared from the different clones and the transfected genes were localized. Unexpectedly, we observed that in the original HC11 cell line the number of chromosomes per metaphase spread is close to 80, suggesting that HC11 cells have undergone a duplication event, since the mouse karyotype is 2n = 40. In alpha S1 casein expressing cells, the expression level does not clearly correlate with a localization of the transfected DNA proximal to the centromeres or the telomeres. Analysis of the localization of the transfected DNA in nuclear halos allows us to conclude that when expressed, transfected DNA is more closely linked to the nuclear matrix. The next step will be to study the attachment of the endogenous casein gene in mammary nuclei during lactation.

Hormone-induced modifications of the chromatin structure surrounding upstream regulatory regions conserved between the mouse and rabbit whey acidic protein genes

The upstream regulatory regions of the mouse and rabbit whey acidic protein (WAP) genes have been used extensively to target the efficient expression of foreign genes into the mammary gland of transgenic animals. Therefore both regions have been studied to elucidate fully the mechanisms controlling WAP gene expression. Three DNase I-hypersensitive sites (HSS0, HSS1 and HSS2) have been described upstream of the rabbit WAP gene in the lactating mammary gland and correspond to important regulatory regions. These sites are surrounded by variable chromatin structures during mammary-gland development. In the present study, we describe the upstream sequence of the mouse WAP gene. Analysis of genomic sequences shows that the mouse WAP gene is situated between two widely expressed genes (Cpr2 and Ramp3). We show that the hypersensitive sites found upstream of the rabbit WAP gene are also detected in the mouse WAP gene. Further, they encompass functional signal transducer and activator of transcription 5-binding sites, as has been observed in the rabbit. A new hypersensitive site (HSS3), not specific to the mammary gland, was mapped 8 kb upstream of the rabbit WAP gene. Unlike the three HSSs described above, HSS3 is also detected in the liver, but similar to HSS1, it does not depend on lactogenic hormone treatments during cell culture. The region surrounding HSS3 encompasses a potential matrix attachment region, which is also conserved upstream of the mouse WAP gene and contains a functional transcription factor Ets-1 (E26 transformation-specific-1)-binding site. Finally, we demonstrate for the first time that variations in the chromatin structure are dependent on prolactin alone.

A distal region, hypersensitive to DNase I, plays a key role in regulating rabbit whey acidic protein gene expression

The aim of the present study was to identify the functional domains of the upstream region of the rabbit whey acidic protein (WAP) gene, which has been used with considerable efficacy to target the expression of several foreign genes to the mammary gland. We have shown that this region exhibits three sites hypersensitive to DNase I digestion in the lactating mammary gland, and that all three sites harbour elements which can bind to Stat5 in vitro in bandshift assays. However, not all hypersensitive regions are detected at all stages from pregnancy to weaning, and the level of activated Stat5 detected in the rabbit mammary gland is low except during lactation. We have studied the role of the distal site, which is only detected during lactation, in further detail. It is located within a 849 bp region that is required to induce a strong expression of the chloramphenicol acetyltransferase reporter gene in transfected mammary cells. Taken together, these results suggest that this region, centred around a Stat5-binding site and surrounded by a variable chromatin structure during the pregnancy-lactation cycle, may play a key role in regulating the expression of this gene in vivo. Furthermore, this distal region exhibits sequence similarity with a region located around 3 kb upstream of the mouse WAP gene. The existence of such a distal region in the mouse WAP gene may explain the differences in expression between 4.1 and 2.1 kb mouse WAP constructs.

Effect of rabbit kappa-casein expression on the properties of milk from transgenic mice

Transgenic mice were produced carrying the coding region of the rabbit kappa-casein gene linked to the upstream region of the rabbit whey acidic protein gene. Mice from the highest-expressing line produced 2.5 mg rabbit kappa-casein/ml in their milk. The foreign protein was associated with the casein micelles and altered micelle size, though in the high-expressing line rabbit kappa-casein also segregated into the whey fraction obtained after centrifuging the milk samples. Milk from transgenic mice had the same overall protein content as that from non-transgenic mice, except for the transgene product. However, litters fed with this transgenic mouse milk grew less well than litters given milk from non-transgenic mice. This reduction in growth was not related to changes in mammary gland structure or mammary cell morphology. Preliminary results indicated that milk from the transgenic mice had a higher viscosity.

High expression of the human hepatocarcinoma-intestine-pancreas/pancreatic-associated protein (HIP/PAP) gene in the mammary gland of lactating transgenic mice. Secretion into the milk and purification of the HIP/PAP lectin

The human hepatocarcinoma-intestine-pancreas/pancreatic-associated protein (HIP/PAP) gene was previously identified because of its increased expression in primary liver cancers and during the acute phase of pancreatitis. In normal tissues, HIP/PAP is expressed both in endocrine and exocrine cells of the intestine and pancreas. HIP/PAP is a lactose binding C-type lectin which acts as an adhesion molecule for rat hepatocytes. The aim of the work was to study the HIP/PAP secretory pathway and to produce high levels of HIP/PAP in the milk of lactating transgenic mice. In view of its lactose C-type lectin properties, we have studied the consequences of the expression of HIP/PAP on mammary epithelial cells. In homozygous mice, production reached 11.2 mg.mL-1 of milk. High levels of soluble and pure HIP/PAP (18.6 mg) were purified from 29 mL of milk. The purified protein was sequenced and the N-terminal amino acid of the mature HIP/PAP was identified as Glu27, thus localizing the site of cleavage of the signal peptide. The HIP/PAP transgene was only expressed in the mammary gland of lactating transgenic mice. HIP/PAP was detected by immunofluorescence in the whole gland, but labelling was heterogeneous between alveolar clusters, with strongly positive sparse cells. Using immuno electron microscopy, HIP/PAP was observed in all the compartments of the secretory pathway within the mammary epithelial cells. We provide evidence that HIP/PAP is secreted through the Golgi pathway. However, the number of distended Golgi saccules was increased when compared to that found in wild-type mouse mammary cells. These modifications could be related to HIP/PAP C-type lectin specific properties.

Polymorphism of the rabbit kappa kasein gene and its influence on performance traits

The rabbit kappa-casein encoding gene has previously been shown to possess two alleles. The two alleles do not differ in their coding region and in the accumulation levels of mRNA. However they differ greatly with respect to their intronic regions. The rearranged regions in the first and fourth introns were found to be inverse and complementary LINE sequences. The A allele was found to be more frequent in different European breeds. Correlation of the kappa-casein genotype with the breeding capacity in a New Zealand White rabbit stock has been examined.

Polymorphic insertions/deletions of both 1550nt and 100nt in two microsatellite-containing, LINE-related intronic regions of the rabbit kappa-casein gene

The most frequent allele of the rabbit kappa-casein (kappa-Cas)-encoding gene (A allele) has previously been shown to possess two sequences similar to those found in the 5' end of long interspersed repeated elements (LINE). Part of an inverted rabbit LINE is present in the first intron and part of a direct rabbit LINE in the fourth intron. We describe herewith a less frequent allele (B allele) that lacks both 100bp in the first intron and 1550bp in the fourth intron. It was not possible to identify any allele exhibiting only one of the deletions in a population of 55 rabbits. The 100bp present in the first intron of the A allele, but absent from the B allele, are located at the 5' end of the inverse complementary LINE and include the poly (T) track of the LINE. The 1550bp present in the fourth intron of the A allele, but absent from the B allele, include the entire direct LINE sequence. Therefore, the B allele only possesses one partial LINE sequence that is located in the first intron and is truncated when compared to the copy found in the first intron of the A allele. The B allele might thus be more recent than the A allele. Differences between the sequences of transcripts corresponding to each allele are limited to two silent mutations and three modifications in the 3' UTR. In the mammary glands of lactating rabbits, which are homozygous for both alleles, kappa-Cas mRNA accumulate to similar levels and are translated into identical kappa-Cas that are secreted at similar concentrations into milk.

Recombinant expression and secretion of a natural splicing variant containing the ectodomain of porcine LH receptor in HC11 mammary epithelial cells

Large-scale synthesis of active recombinant porcine luteinizing hormone/chorionic gonadotropin receptor (pLHR) is required for biophysical and structural studies. This study was undertaken to improve expression of the corresponding cDNA already obtained with a number of other systems, (i) by turning to cells from mammalian origin able to perform adequate glycosylation, (ii) by using an expression vector containing the acknowledged high-performance rabbit WAP gene upstream region together with transcription and translation stimulating sequences, and (iii) by expressing natural splicing variants. Selection of the transfected HC11 cells was performed in terms of pLHR expression using specific radioligand binding and immunoradiometric assays. Secretion of pLHR ectodomain into the culture medium of the HC11 clones was quantified, and reached 70 ng/ml, which represents the highest active amount ever produced. However, this level of expression was relatively low in comparison to that currently observed with bGH cDNA used as reporter gene. Additional investigations were performed in order to gain further insight into the limitation of the production of pLHR relative to bovine or human growth hormone using the same expression system. A high number of copies of cDNA in the genome of HC11 cells was found, provided that an antibiotic selection pressure was maintained to avoid drifting. The low mRNA levels detected for pLHR relative to hGH mRNAs correlate well with the relative protein production levels. They could arise from poor stability of mRNAs, a fact already observed for the natural receptor in gonadal cells. These results thus constitute a promising indicator for possible expression of pLHR in the milk of transgenic animals.

Structure of the rabbit kappa-casein encoding gene: expression of the cloned gene in the mammary gland of transgenic mice

The rabbit kappa-casein (kappa-Cas) encoding gene has been isolated as a series of overlapping DNA fragments cloned from a rabbit genomic library constructed in bacteriophage lambda EMBL3. The clones harboured the 7.5-kb gene flanked by about 2.1 kb upstream and 9 kb downstream sequences. The cloned gene is the most frequently occurring of two kappa-Cas alleles identified in New Zealand rabbits. Comparison of the corresponding domains in rabbit and bovine kappa-Cas shows that both genes comprise 5 exons and that the exon/intron boundary positions are conserved whereas the introns have diverged considerably. The first three introns are shorter in the rabbit, the second intron showing the greatest difference between the two species: 1.35 kb instead of 5.8 kb in the bovine gene. Repetitive sequence motives reminiscent of the rabbit C type repeat and the complementary inverted C type repeat were identified in the fourth and first introns, respectively. Transgenic mice were produced by microinjecting into mouse oocytes an isolated genomic DNA fragment which contained the entire kappa-Cas coding region, together with 2.1-kb 5' and 4.0-kb 3' flanking region. Expression of transgene rabbit kappa-Cas mRNA could be detected in the mammary gland of lactating transgenic mice and the production of rabbit kappa-Cas was detected in milk using species-specific antibodies. The cloned gene is thus functional.

Rabbit whey acidic protein gene upstream region controls high-level expression of bovine growth hormone in the mammary gland of transgenic mice

Transgenic mice were produced which secreted high levels of bGH into milk. The 6.3-kb upstream region of the rabbit whey acidic protein (rWAP) gene was linked to the structural part of the bovine growth hormone (bGH) gene, and the chimeric gene was radioimmunoassay into mouse oocytes. bGH was detected by radioimmunoassay in the milk of all resulting transgenic mice. bGH concentrations in milk varied from line to line, from 1.0-16 mg/ml. This expression was not correlated to the number of transgene copies. In all lines studied, the mammary gland was the major organ expressing bGH mRNA during lactation. bGH mRNA concentrations were barely detectable in the mammary gland of cyclic females; they increased during pregnancy. These results show that the upstream region of the rWAP gene harbors powerful regulatory elements which target high levels of bGH transgene expression to the mammary gland of lactating transgenic mice.

High level production of human growth hormone in the milk of transgenic mice: the upstream region of the rabbit whey acidic protein (WAP) gene targets transgene expression to the mammary gland

The 5' flanking region (6.3 kb) of the rabbit WAP (rWAP) gene possesses important regulatory elements. This region was linked to the human growth hormone (hGH) structural gene in order to target transgene expression to the mammary gland. Thirteen lines of transgenic mice were produced. Milk could be collected from six lines of transgenic mice. In five of them, hGH was present in the milk at high concentrations ranging from 4 to 22 mg ml-1. hGH produced by the mammary gland comigrated with hGH of human origin. It was biologically active, and through its prolactin-like activity induced lactogenesis when introduced into mammary culture media. Two of these mouse lines were studied further. hGH mRNA was only detected in the mammary gland during lactation. In the seven other transgenic lines, hGH was present in the blood of cyclic females. The prolactin-like effect of hGH in these mice probably induced female sterility, and milk could therefore not be obtained. In two lines studied in more detail, the mammary gland was the main organ producing hGH, even in cyclic mice. Low ectopic expression was detected in other organs which varied from one line to the other. This was probably due to the influence on the transgene of the site of integration into the mouse genome. In the 13 lines studied, high mammary-specific hGH expression was not correlated to the transgene copy number. The rWAP-hGH construct thus did not behave as an independent unit of transcription. However, it can be concluded that the 6.3 kb flanking region of the rWAP gene contains regulatory elements responsible for the strong mammary-specific expression of hGH transgene, and that it is a good candidate to control high levels of foreign protein gene expression in the mammary gland of lactating transgenic animals.

Characterization of rabbit kappa-casein cDNA: control of kappa-casein gene expression in vivo and in vitro

The rabbit kappa-casein cDNA was cloned and sequenced. One of the isolated clones included almost the entire 5' end, while another clone corresponded to the 3' end of the cDNA. No polyadenylation site was found and therefore this clone did not harbour the complete cDNA. The amino acid sequence of a full-length protein was deduced from the nucleotide sequence obtained for this partial cDNA. It revealed the presence of a chymosin cleavage site and five potential phosphorylation sites. Rabbit kappa-casein was compared with those already described in other species. The rabbit sequence is closer to the ovine than to the mouse sequence. This result supports the idea that Lagomorpha are not closer to Rodentia than to Artiodactyla. The cDNA described above was used to study kappa-casein gene expression in the rabbit mammary gland. This expression was induced primarily by prolactin in mammary gland organoids and was similar to alpha s1-casein gene expression in vivo. The kappa-casein gene present in the casein gene locus is thus subject to the same regulation as the alpha s1-casein gene, although it has evolved from a fibrinogen gene.

Structure of the gene encoding rabbit alpha s1-casein

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

Structure of the gene encoding rabbit beta-casein

The entire rabbit beta-casein-encoding gene and 400 bp upstream were sequenced. Eight introns, located essentially at a position similar to the corresponding gene in other species, were found. Strong homology with several casein-encoding genes from rabbit and from other species was observed in the upstream region of the gene. Repeated sequences of unknown function were also located within introns.