Impact of strong selection for the PrP major gene on genetic variability of four French sheep breeds(Open Access publication).

Effective selection on the PrP gene has been implemented since October 2001 in all French sheep breeds. After four years, the ARR "resistant" allele frequency increased by about 35% in young males. The aim of this study was to evaluate the impact of this strong selection on genetic variability. It is focussed on four French sheep breeds and based on the comparison of two groups of 94 animals within each breed: the first group of animals was born before the selection began, and the second, 3-4 years later. Genetic variability was assessed using genealogical and molecular data (29 microsatellite markers). The expected loss of genetic variability on the PrP gene was confirmed. Moreover, among the five markers located in the PrP region, only the three closest ones were affected. The evolution of the number of alleles, heterozygote deficiency within population, expected heterozygosity and the Reynolds distances agreed with the criteria from pedigree and pointed out that neutral genetic variability was not much affected. This trend depended on breed, i.e. on their initial states (population size, PrP frequencies) and on the selection strategies for improving scrapie resistance while carrying out selection for production traits.

Regulation by the extracellular matrix (ECM) of prolactin-induced alpha s1-casein gene expression in rabbit primary mammary cells: role of STAT5, C/EBP, and chromatin structure.

The aim of the present study was to understand how the extracellular matrix (ECM) regulates at the gene level the prolactin (Prl)-induced signal transducer and activator of transcription 5 (STAT5)-dependent expression of the alpha s1-casein gene in mammary epithelial cells. CCAAT enhancer binding proteins (C/EBPs) are assumed regulators of beta-casein gene expression. Rabbit primary mammary cells express alpha s1-casein gene when cultured on collagen and not on plastic. Similar C/EBPbeta, C/EBPdelta, STAT5, and Prl-activated STAT5 were found under all culture conditions. Thus the ECM does not act through C/EBPs or STAT5. This was confirmed by transfections of rabbit primary mammary cells by a construct sensitive to ovine prolactin (oPrl) and ECM (6i TK luc) encompassing STAT5 and C/EBP binding sites. The mutation of C/EBPs binding sites showed that these sites were not mandatory for Prl-induced expression of the construct. Interestingly, chromatin immunoprecipitation by the anti-acetylhistone H4 antibody (ChIP) showed that the ECM (and not Prl) maintained a high amount of histone H4 acetylation upstream of the alpha s1-casein gene especially at the level of a distal Prl- and ECM-sensitive enhancer. Alpha6 integrin (a membrane receptor of laminin, the principal active component of the mammary ECM) was found at the surface of cells cultured on collagen but not on plastic. In cells cultured on collagen in the presence of anti-alpha6 integrin antibody, Prl-induced transcription of the endogenous alpha s1-casein gene was significantly reduced, without modifying C/EBPs and STAT5. Besides, histone H4 acetylation was reduced. Thus, we propose that the ECM regulates rabbit alpha s1-casein protein expression by local modification of chromatin structure, independently of STAT5 and C/EBPs.

In vitro and in vivo effects of a multimerized alphas 1-casein enhancer on whey acidic protein gene promoter activity.

Experimental data obtained in previous works have led to postulate that enhancers increase the frequency of action of a linked promoter in a given cell and may have some insulating effects. The multimerized rabbit alpha s1-casein gene enhancer, the 6i multimer, was added upstream of the rabbit whey acidic protein gene (WAP) promoter (-6,300; +28 bp) fused to the firefly luciferase (luc) gene (6i WAP-luc construct). The 6i multimer increased reporter gene expression in mouse mammary HC11 cells. In transgenic mice, a very weak but significant increase was also observed. More noticeable, no silent lines were found when the 6i multimer was associated to the WAP-luc construct. This reflects the fact that the 6i multimer tends to prevent the silencing of the WAP-luc construct. After addition of the 5'HS4 insulator region from the chicken beta-globin locus upstream of the 6i multimer, similar luciferase levels were measured in 6i WAP-luc and 5'HS4 WAP-luc transgenic mice. Our present data and previous ones, which show that the 6i multimer has no insulating activity on a TK gene promoter construct indicate that the insulating activity of the 6i multimer is construct-dependent and not amplified by the 5'HS4 insulator.

The insulator effect of the 5'HS4 region from the beta-globin chicken locus on the rabbit WAP gene promoter activity in transgenic mice.

Previous studies have shown that the 5'HS4 DNaseI hypersensitive site of the chicken beta-globin locus is endowed with classic insulator activities: (i) it blocks the interaction between promoter and enhancers when it is inserted between them (ii) it confers expression of integrated foreign genes independent of their position in the chromatin. The aim of this present work was to determine whether the 5'HS4 element was able to stimulate the expression level and/or to increase the expression frequency of a luc+ reporter gene controlled by the rabbit WAP gene promoter. Two constructs with 5'HS4 insulator (p5'HS4-WAPluc) or without (pWAPluc) were introduced in mouse fertilised oocytes. All transgenic lines containing the 5'HS4 element (six lines) expressed the transgene whereas only two out of eight lines harbouring the pWAP-luc construct expressed the transgene to a significant level. Moreover, the mean level of expression was seven times higher in p5'HS4WAP-luc lines than in pWAP-luc lines. Even all these benefits on transgene expression, the 5'HS4 element did not confer a copy-dependent expression, did not decrease the ectopic expression of the reporter gene and did not decrease the variability of expression. Thus, the 5'HS4 element does not have all the properties of a perfect insulator on a construct containing the luc+ reporter gene controlled by the rabbit WAP promoter.

Transgenesis for the study and the control of lactation.

The study and the control of milk synthesis are required to decipher the mechanisms of gene expression, to improve milk production, to modify milk composition, to induce a resistance to diseases in the mammary gland and to produce recombinant proteins of pharmaceutical interest. Transgenesis has become a mandatory tool to reach these goals. The use of transgenesis is still limited by the difficulty of adding foreign genes in farm animals and mainly by replacing genes by homologous recombination. Transgene expression is also often ill-controlled. The present paper summarizes the current progress in this field with a particular emphasis on expression vectors for transgenes.

Effect of the rabbit alphas1-casein gene distal enhancer on the expression of a reporter gene in vitro and in vivo.

Several gene constructs containing the firefly luciferase gene and the herpes simplex virus thymidine kinase gene promoter (TK) were used to evaluate the transcriptional activity of the distal enhancer (-3442, -3285) of the rabbit alphas1-casein gene. Six copies of the enhancer (6i) were added upstream of the TK-luciferase construct in the presence or absence of the chicken beta-globin 5'HS4 insulator. The activity of the constructs was tested by transient transfection in CHO cells and in rabbit primary mammary cell cultured on plastic or on floating collagen. Constructs were also tested in stably transfected mouse mammary HC11 cells. In all cell types the multimerized alphas1-casein enhancer strongly stimulated luciferase gene expression in the presence of lactogenic hormones. It was also sensitive to the extracellular matrix in rabbit primary mammary cells. The constructs were used to generate transgenic mice. The 6i TK transgenic animals expressed the luciferase gene at very low levels irrespectively of the physiological state. No preferential expression in the mammary gland was observed. Addition of 5'HS4 insulator to the 6i TK construct did not prevent silencing in most of the transgenic lines. However, two lines expressed high luciferase levels specifically in the mammary gland. Our data suggest that 6i may confer, when insulated properly, a higher and mammary-specific expression to the TK promoter.

Effects of cryoprotectant and plunging temperature in liquid nitrogen on the in vitro and in vivo development of murine morulae

The effects of plunging temperature in liquid nitrogen and cryoprotectant dilution methods were evaluated for compacted mouse morulae frozen in 1.5 M ethylene-glycol (E), 1.5M propylene-glycol (P) or 1.4M glycerol (G). Morulae were equilibrated for 10 minutes in cryoprotectant solution and loaded into 0.25 ml straws with cryoprotectant solution in 3 columns (groups E1, P1, G1) or cryoprotectant in the center and PBS in the lateral columns (E2, P2). Straws were cooled at 0.5§C/min to -25 or -30§C and plunged into liquid nitrogen. Straws were thawed in water at 22§C for 20 seconds. Cryoprotectant was diluted in 3 steps for group G1 and in one step for groups E1 and P1 (direct transfer to PBS + 10 por cento FCS) and E2 and P2 (shaken to mix the 3 columns before transferring to PBS+ 10 por cento FCS). Plunging temperature had no significant effect on the proportion of morulae developed to blastocysts in vitro; this proportion was higher (p < 0.0001) in E1 (69.2 por cento) than in E2 (60.3 por cento), G1 (51.9 por cento) and combined for P1 and P2 (46.9 por cento). In second experiment, the proportion of transferred morulae that developed to viable fetuses was lower (p < 0.07) for E1-25 than for E1-30, G1-30, E2-25 or unfrozen (control) embryos (8.7, 20.0, 20.0, 17.4 and 19.8 por cento, respectively). In conclusion, the ethylene glycol diluted directly in PBS (E1) exhibit the highest rate of in vitro embryos development, but based on in vivo embryos development was more efficacious in plunging temperature at -30§C (E1-30)

Cryopresevation of mouse morulae in glycerol, sucrose and honeybee royal jelly

Compacted mouse morulae were frozen at 0.3§C/min. or 0.5§C/min. from -6§C to -24§C or -32§C in 10 por cento of glycerol plus different sucrose concentrations with or without 0.1 por cento of honeybee royal jelly. Embryos were thawed in water bath at 22§C for 20 seconds and cryoprotectant dilution was done in three steps. Embryos were cultured in Whitten's medium for 24, 48 and 72 hours at 37§C, 5 por cento of CO2 and 100por cento of humidity. The in vitro development ranged from 56.6 por cento to 100 por cento after 72 hours. Expanded blastocysts were transferred to pseudopregnant recipients on the third day of the estrous cycle. Viable fetuses rates for embryos frozen to -24 or -32§C at 0.3§C/minute in 10 por cento glycerol + 10 por cento sucrose, 10 por cento glycerol + 10 por cento sucrose + 0.1 por cento honeybee royal jelly, 10 por cento glycerol + 0.1 por cento honeybee royal jelly or 10 por cento glycerol were respectively: 28.1 por cento and 13.6 por cento, 48.7 por cento and 31.9 por cento, 28.6 por cento and 13.2 por cento, 20.0 por cento and 42.4 por cento. Viable fetuses for embryos frozen to -24§C or -32§C at 0.5§C/minute in 10 por cento glycerol + 10 por cento sucrose or 10 por cento glycerol + 10 por cento sucrose + 0.1 por cento honeybee royal jelly were respectively 29.0 por cento and 15.3 por cento, 48.8 por cento and 32.0 por cento. We can conclude that addition of 10 por cento sucrose to 10 por cento glycerol was efficient for embryo freezing at 0.3 or 0.5§C/minute and plunged in liquid nitrogen at -24§C. The honeybee royal jelly addition provided higher viable fetuses rates when embryos were cooled at 0.3 or 0.5§C/minute and plunged in liquid nitrogen at -24§C