Transgenic expression of human heme oxygenase-1 in pigs confers resistance against xenograft rejection during ex vivo perfusion of porcine kidneys.

BACKGROUND: The major immunological hurdle to successful porcine-to-human xenotransplantation is the acute vascular rejection (AVR), characterized by endothelial cell (EC) activation and perturbation of coagulation. Heme oxygenase-1 (HO-1) and its derivatives have anti-apoptotic, anti-inflammatory effects and protect against reactive oxygen species, rendering HO-1 a promising molecule to control AVR. Here, we report the production and characterization of pigs transgenic for human heme oxygenase-1 (hHO-1) and demonstrate significant protection in porcine kidneys against xenograft rejection in ex vivo perfusion with human blood and transgenic porcine aortic endothelial cells (PAEC) in a TNF-α-mediated apoptosis assay. METHODS: Transgenic and non-transgenic PAEC were tested in a TNF-α-mediated apoptosis assay. Expression of adhesion molecules (ICAM-1, VCAM-1, and E-selectin) was measured by real-time PCR. hHO-1 transgenic porcine kidneys were perfused with pooled and diluted human AB blood in an ex vivo perfusion circuit. MHC class-II up-regulation after induction with IFN-γ was compared between wild-type and hHO-1 transgenic PAEC. RESULTS: Cloned hHO-1 transgenic pigs expressed hHO-1 in heart, kidney, liver, and in cultured ECs and fibroblasts. hHO-1 transgenic PAEC were protected against TNF-α-mediated apoptosis. Real-time PCR revealed reduced expression of adhesion molecules like ICAM-1, VCAM-1, and E-selectin. These effects could be abrogated by the incubation of transgenic PAECs with the specific HO-1 inhibitor zinc protoporphorine IX (Zn(II)PPIX, 20 µm). IFN-γ induced up-regulation of MHC class-II molecules was significantly reduced in PAECs from hHO-1 transgenic pigs. hHO-1 transgenic porcine kidneys could successfully be perfused with diluted human AB-pooled blood for a maximum of 240 min (with and without C1 inh), while in wild-type kidneys, blood flow ceased after ∼60 min. Elevated levels of d-Dimer and TAT were detected, but no significant consumption of fibrinogen and antithrombin was determined. Microthrombi could not be detected histologically. CONCLUSIONS: These results are encouraging and warrant further studies on the biological function of heme oxygenase-I expression in hHO-1 transgenic pigs in the context of xenotransplantation.

Efficient generation of a biallelic knockout in pigs using zinc-finger nucleases.

Zinc-finger nucleases (ZFNs) are powerful tools for producing gene knockouts (KOs) with high efficiency. Whereas ZFN-mediated gene disruption has been demonstrated in laboratory animals such as mice, rats, and fruit flies, ZFNs have not been used to disrupt an endogenous gene in any large domestic species. Here we used ZFNs to induce a biallelic knockout of the porcine α1,3-galactosyltransferase (GGTA1) gene. Primary porcine fibroblasts were treated with ZFNs designed against the region coding for the catalytic core of GGTA1, resulting in biallelic knockout of ∼1% of ZFN-treated cells. A galactose (Gal) epitope counter-selected population of these cells was used in somatic cell nuclear transfer (SCNT). Of the resulting six fetuses, all completely lacked Gal epitopes and were phenotypically indistinguishable from the starting donor cell population, illustrating that ZFN-mediated genetic modification did not interfere with the cloning process. Neither off-target cleavage events nor integration of the ZFN-coding plasmid was detected. The GGTA1-KO phenotype was confirmed by a complement lysis assay that demonstrated protection of GGTA1-KO fibroblasts relative to wild-type cells. Cells from GGTA1-KO fetuses and pooled, transfected cells were used to produce live offspring via SCNT. This study reports the production of cloned pigs carrying a biallelic ZFN-induced knockout of an endogenous gene. These findings open a unique avenue toward the creation of gene KO pigs, which could benefit both agriculture and biomedicine.

Pluripotent stem cells and reprogrammed cells in farm animals.

Pluripotent cells are unique because of their ability to differentiate into the cell lineages forming the entire organism. True pluripotent stem cells with germ line contribution have been reported for mice and rats. Human pluripotent cells share numerous features of pluripotentiality, but confirmation of their in vivo capacity for germ line contribution is impossible due to ethical and legal restrictions. Progress toward derivation of embryonic stem cells from domestic species has been made, but the derived cells were not able to produce germ line chimeras and thus are termed embryonic stem-like cells. However, domestic animals, in particular the domestic pig (Sus scrofa), are excellent large animals models, in which the clinical potential of stem cell therapies can be studied. Reprogramming technologies for somatic cells, including somatic cell nuclear transfer, cell fusion, in vitro culture in the presence of cell extracts, in vitro conversion of adult unipotent spermatogonial stem cells into germ line derived pluripotent stem cells, and transduction with reprogramming factors have been developed with the goal of obtaining pluripotent, germ line competent stem cells from domestic animals. This review summarizes the present state of the art in the derivation and maintenance of pluripotent stem cells in domestic animals.

Oct4-enhanced green fluorescent protein transgenic pigs: a new large animal model for reprogramming studies.

The domesticated pig has emerged as an important tool for development of surgical techniques, advancement of xenotransplantation, creation of important disease models, and preclinical testing of novel cell therapies. However, germ line-competent pluripotent porcine stem cells have not yet been derived. This has been a major obstacle to genetic modification of pigs. The transcription factor Oct4 is essential for the maintenance of pluripotency and for reprogramming somatic cells to a pluripotent state. Here, we report the production of transgenic pigs carrying an 18 kb genomic sequence of the murine Oct4 gene fused to the enhanced green fluorescent protein (EGFP) cDNA (OG2 construct) to allow identification of pluripotent cells by monitoring Oct4 expression by EGFP fluorescence. Eleven viable transgenic piglets were produced by somatic cell nuclear transfer. Expression of the EGFP reporter construct was confined to germ line cells, the inner cell mass and trophectoderm of blastocysts, and testicular germ cells. Reprogramming of fibroblasts from these animals by fusion with pluripotent murine embryonic stem cells or viral transduction with human OCT4, SOX2, KLF4, and c-MYC cDNAs resulted in Oct4-EGFP reactivation. The OG2 pigs have thus proved useful for monitoring reprogramming and the induction and maintenance of pluripotency in porcine cells. In conclusion, the OG2 transgenic pigs are a new large animal model for studying the derivation and maintenance of pluripotent cells, and will be valuable for the development of cell therapy.

DNA methylation patterns reflect epigenetic reprogramming in bovine embryos.

To understand the epigenetic alterations associated with assisted reproduction technology (ART) and the reprogramming of gene expression that follows somatic cell nuclear transfer (SCNT), we screened a panel of 41 amplicons representing 25 developmentally important genes on 15 different chromosomes (a total of 1079 CpG sites). Methylation analysis was performed on DNA from pools of 80 blastocysts representing three classes of embryos. This revealed a subset of amplicons that distinguish between embryos developing in vivo, produced in vitro, or reconstructed by SCNT. Following SCNT, we observed massive epigenetic reprogramming evidenced by reduced levels of methylation in the resultant embryos. Analysis of data from the 28 most informative amplicons (hotspot loci), representing more than 523 individual CpG sites, we discovered subsets of amplicons with methylation patterns that were unique to each class of embryo and may indicate metastable epialleles. Analysis of eight genes with respect to mRNA expression did not reveal a direct correlation with DNA methylation levels. In conclusion, this approach revealed a subset of amplicons that can be used to evaluate blastocyst quality and reprogramming following SCNT, and can also be employed for the localization of the epigenetic control regions within individual genes and for more general studies of stem cell differentiation.

Preferential loss of porcine chromosomes in reprogrammed interspecies cell hybrids.

Fusion of terminally differentiated somatic cells with pluripotent embryonic stem cells has been proposed as model for reprogramming the somatic cell genome, and may contribute to our understanding of the underlying mechanisms of this epigenetic process. We established an interspecies cell fusion model using murine embryonic stem cells (ESCs) and porcine fibroblasts. These inter-species fusion experiments yielded much lower conversion efficiency rates than murine intraspecies fusion. Nevertheless, two double-resistant mouse-pig hybrid clones could be generated. Reactivation of the porcine OCT4 gene, an essential pluripotent stem cell marker, and demethylation of the porcine OCT4 promoter in hybrid clone 1, suggested successful reprogramming of porcine chromosomes. A rapid loss of porcine chromosomes was observed during the selection phase. Spectral karyotyping (SKY) analysis showed that fusion-hybrid clone 1 carried a tetraploid mouse chromosome complement with only few pig chromosomes and/or chromosomal fragments. Hybrid clone 2 had a diploid set of murine chromosomes complements and also contained an interspecies chromosome fusion product. Interspecies cell fusion results in hybrid cells that retained the complement of mouse chromosomes and preferentially lose porcine chromosomes during colony expansion. Neither species-specific chromosomal segregation nor reprogrammed diploid porcine cells were observed. These findings indicate that generation of reprogrammed pluripotent diploid cells by cell fusion may require additional supporting provisions.

DNA methylation in the IGF2 intragenic DMR is re-established in a sex-specific manner in bovine blastocysts after somatic cloning.

The recent identification of an intragenic differentially methylated region (DMR) within the last exon of the bovine Insulin-like growth factor 2 (IGF2) gene provides a diagnostic tool for in-depth investigation of bovine imprinting and regulatory mechanisms which are active during embryo development. Here, we used bisulfite sequencing to compare sex-specific DNA methylation patterns within this DMR in bovine blastocysts produced in vivo, by in vitro fertilization and culture, SCNT, androgenesis or parthenogenesis. In in vivo derived embryos, DNA methylation was removed from this intragenic DMR after fertilization, but partially replaced by the time the embryo reached the blastocyst stage. Among embryos developing in vivo, the level of DNA methylation was significantly lower in female than in male blastocysts. This sexual dimorphism was also found between parthenogenetic and androgenetic embryos, and followed the donor cell sex in SCNT derived blastocysts and is evidence for correct methylation reprogramming in SCNT embryos.

Transgenic expression of the human A20 gene in cloned pigs provides protection against apoptotic and inflammatory stimuli.

BACKGROUND: Porcine organs with transgenic expression of anti-apoptotic and anti-inflammatory genes like the human A20 gene (hA20), a tumor necrosis factor-alpha (TNF-alpha)-inducible gene, may control the acute vascular rejection (AVR) of porcine xenografts. The human A20 molecule possesses protective features against inflammatory and apoptotic stimuli in various cell types including endothelial cells, rendering it a promising candidate for transgenic pig production in the context of xenotransplantation. Here, we produced pigs transgenic for hA20 and investigated whether hA20-transgenic porcine aortic endothelial cells (PAECs) were resistant against the induction of apoptosis in vitro and to what extent hA20-transgenic porcine hearts were protected against ischemia/reperfusion (I/R) injury. METHODS: Porcine fetal fibroblasts (PFFs) were transfected with the vector pCAGGSEhA20-IRESNEO containing a chicken beta-actin/rabbit beta-globin (CAGGS)-promoter element, known to provide ubiquitous gene expression in both mice and pigs. Transfected PFFs were then used in somatic cell nuclear transfer (SCNT). Three hA20-transgenic pigs were killed for PAEC isolation and organ mRNA and protein expression analysis by reverse transcriptase-polymerase chain reaction (RT-PCR), Northern and Western Blotting. PAECs were tested for susceptibility to apoptosis after TNF-alpha challenging and triggering of the CD95(Fas)/CD95Ligand pathway. Five transgenic and three wild type animals were subjected to an I/R experiment followed by measurement of infarct size, myeloperoxidase (MPO) activity and subendocardial segmental shortening (SES) to assess protective effects of hA20 in the porcine myocardium. RESULTS: The hA20-transgenic pigs developed normally and expression of hA20 was found in skeletal muscle, heart and PAECs. Cultured human A20-transgenic PAECs showed significantly reduced apoptosis when compared to their wild type counterparts and were less susceptible to the induction of cell death by CD95(Fas)L. Only partial protection of hA20-transgenic pig hearts was observed after I/R. While infarct size did not differ between the two groups after ischemic assault, hA20-transgenic porcine hearts showed significantly lower MPO activity and better hemodynamic performance (determined as SES) than their wild type counterparts. CONCLUSIONS: The hA20 gene was for the first time functionally expressed in transgenic pigs. Although the CAGGS is a ubiquitous promoter element, expression was restricted to heart, skeletal muscle and PAECs of transgenic animals. Cultivated hA20-transgenic PAECs were protected against TNF-alpha-mediated apoptosis, and partially protected against CD95(Fas)L-mediated cell death; cardiomyocytes were partially protected in I/R. These findings reveal hA20 as a promising molecule for controlling AVR in multi-transgenic pigs for xenotransplantation studies.

The perspectives for porcine-to-human xenografts.

The shortage of donated human organs for transplantation continues to be a life threatening problem for patients suffering from complete organ failure. Although this gap is increasing due to the demographic changes in aging Western populations, it is generally accepted that international trading in human organ is not an ethical solution. Alternatives to the use of human organs for transplantation must be developed and these alternatives include stem cell therapy, artificial organs and organs from other species, i.e. xenografts. For practical reasons but most importantly because of its physiological similarity with humans, the pig is generally accepted as the species of choice for xenotransplantation. Nevertheless, before porcine organs can be used in human xenotransplantation, it is necessary to make a series of precise genetic modifications to the porcine genome, including the addition of genes for factors which suppress the rejection of transplanted porcine tissues and the inactivation or removal of undesirable genes which can only be accomplished at this time by targeted recombination and somatic nuclear transfer. This review will give an insight into the advances in transgenic manipulation and cloning in pigs--in the context of porcine-to-human xenotransplantation.

Production of viable pigs from fetal somatic stem cells.

Fetal somatic stem cells (FSSCs) are a novel type of somatic stem cells that have recently been discovered in primary fibroblast cultures from pigs and other species. The goal of the present study was to produce viable piglets from FSSCs. NT complexes were prepared from both FSSCs and porcine fetal fibroblasts (pFF) to permit comparison of these two donor cell types. FSSCs from isolated attached colonies were compared with pFF in their ability to form blastocysts upon use in NT. Fusion and cleavage rates were similar between the two groups, while blastocyst rates were significantly higher when using pFF as donor cells. FSSCs of three different size categories derived from dissociation of spheroids yielded similar results. The use of FSSCs of 15-20 microm in size yielded similar cleavage and blastocyst rates as fetal fibroblasts. In the final experiment NT complexes produced from FSSCs were transferred to foster mothers. After transfer to prepubertal gilts, three of seven recipients established pregnancies and delivered seven piglets, of which three piglets were viable and showed normal development. Results for the first time demonstrate that FSSCs are able to produce cloned embryos, and that pregnancies can be established and viable piglets can be produced.

Conserved molecular portraits of bovine and human blastocysts as a consequence of the transition from maternal to embryonic control of gene expression.

The present study investigated mRNA expression profiles of bovine oocytes and blastocysts by using a cross-species hybridization approach employing an array consisting of 15,529 human cDNAs as probe, thus enabling the identification of conserved genes during human and bovine preimplantation development. Our analysis revealed 419 genes that were expressed in both oocytes and blastocysts. The expression of 1,324 genes was detected exclusively in the blastocyst, in contrast to 164 in the oocyte including a significant number of novel genes. Genes indicative for transcriptional and translational control (ELAVL4, TACC3) were overexpressed in the oocyte, whereas cellular trafficking (SLC2A14, SLC1A3), proteasome (PSMA1, PSMB3), cell cycle (BUB3, CCNE1, GSPT1), and protein modification and turnover (TNK1, UBE3A) genes were found to be overexpressed in blastocysts. Transcripts implicated in chromatin remodeling were found in both oocytes (NASP, SMARCA2) and blastocysts (H2AFY, HDAC7A). The trophectodermal markers PSG2 and KRT18 were enriched 5- and 50-fold in the blastocyst. Pathway analysis revealed differential expression of genes involved in 107 distinct signaling and metabolic pathways. For example, phosphatidylinositol signaling and gluconeogenesis were prominent pathways identified in the blastocyst. Expression patterns in bovine and human blastocysts were to a large extent identical. This analysis compared the transcriptomes of bovine oocytes and blastocysts and provides a solid foundation for future studies on the first major differentiation events in blastocysts and identification of a set of markers indicative for regular mammalian development.

Gene expression and methylation patterns in cloned embryos.

A considerable proportion of the offspring, in particular in ruminants and mouse, born from nuclear transfer (NT)-derived and in vitro-produced (IVP) embryos are affected by multiple abnormalities, of which a high birthweight and an extended gestation length are the predominant features; a phenomenon that has been termed "Large Offspring syndrome" (LOS). According to a current hypothesis, LOS is caused by persistent aberrations of expression patterns of developmentally important genes starting as early as at the preimplantation stages. The underlying mechanisms are widely unknown at present, but epigenetic modifications of embryonic and fetal gene expression patterns, primarily caused by alterations in DNA methylation are thought to be involved in this syndrome. Appropriate DNA methylation is essential for regular transcription during mammalian development and differentiation. Sensitive reverse transcription polymerase chain reaction assays allow the study of messenger RNA (mRNA) expression levels of specific genes in single embryos. The methylation status of a specific gene can be assessed by bisulfite sequencing. Studies to unravel mRNA expression patterns from IVP- and NT-derived embryos have revealed numerous aberrations ranging from suppression of expression to de novo overexpression or more frequently to a significant upregulation or downregulation of a specific gene. mRNA expression patterns from in vivo-derived embryos are essential as the "physiological standard" against which the findings for IVP and NT-derived embryos are to be compared. Unraveling the underlying molecular mechanisms will contribute to the production of viable embryos and aid to improve biotechnologies applied to early mammalian embryos.

The bovine IGF2 gene is differentially methylated in oocyte and sperm DNA.

The insulin-like growth factor 2 gene (IGF2) encodes an essential growth factor and is imprinted in various mammalian species. Differentially methylated regions (DMRs) are often located within CpG islands and are critically involved in the regulation of monoallelic Igf2 expression in the mouse. Only partial sequence information is available for the bovine IGF2 gene and no DMR has currently been identified. The goal of this study was to identify a DMR within the bovine IGF2 gene as a prerequisite for further studies on gene-specific methylation patterns during preimplantation development. Here we describe the sequence analysis of a CpG-rich DNA fragment from the 5' untranslated region spanning exons and introns 4 and 5 and the identification of a previously unknown DMR in exon 10 of the bovine IGF2 gene. Bisulfite analysis revealed that this DMR is differentially methylated in mature oocytes and sperm. The identification of an intragenic DMR within a developmentally important gene such as the bovine IGF2 gene provides a useful tool to evaluate the methylation patterns of embryos derived in vivo and in vitro. Our study is the first report of a differentially methylated region in a bovine imprinted gene discovered by the analysis of female and male gametes.

Comparison of real-time polymerase chain reaction and end-point polymerase chain reaction for the analysis of gene expression in preimplantation embryos.

The aim of the present study was to compare real-time polymerase chain reaction (PCR) and end-point PCR with respect to their suitability for the analysis of gene expression in samples in which the number of cells is limited; for example, in studies of preimplantation embryonic development and to determine the variability of the real-time reverse transcription-PCR assay. The sensitivity, dynamic range and precision of both PCR systems were compared using a single mouse liver cDNA standard. The real-time system was 100-fold more sensitive than the end-point system and had a dynamic range of more than four orders of magnitude. The linear range for end-point PCR extended for two orders of magnitude using a fixed end-point of 31 cycles. The percentage standard error of the mean based on 30 replicates was 0.14% of the threshold cycle (Ct) value for the real-time system and 6.8% for the end-point fluorescence intensity. The coefficients of variation (CV) for reverse transcription combined with real-time analysis and the complete gene expression protocol consisting of mRNA isolation, reverse transcription and real-time PCR analysis were 0.6% and 1.4% of the Ct values, respectively. The present paper details, for the first time, measurement of the biological variation of individual mammalian oocytes. The CV was 1.8% of the Ct value for expression analysis of six bovine oocytes. The results are discussed in relation to the analysis of gene expression in preimplantation embryo development.

Development to the blastocyst stage of parthenogenetically activated in vitro matured porcine oocytes after solid surface vitrification (SSV).

The effects of solid surface vitrification (SSV) on viability and parthenogenetic development of in vitro matured (IVM) porcine oocytes was investigated in the present study. Cumulus-free IVM porcine oocytes were subjected either to SSV or SSV combined with a cytochalasin B (CB) pre-treatment (SSV+CB) or all steps of SSV but without cooling (toxicity control=TC; toxicity control with CB pre-treatment=TC+CB). Oocyte viability was evaluated by plasma membrane integrity and esterase activity measured by a combined staining with fluorescein diacetate, propidium iodide and Hoechst 33342. Surviving oocytes were parthenogenetically activated then cultured in vitro (IVC) for 6 days. The proportion of live oocytes after vitrification was significantly lower than that of the TC, TC+CB and the control groups, regardless of the CB pre-treatment. Treatment of oocytes with cryoprotectants did not decrease the rates of surviving oocytes. After activation of oocytes, the proportion of cleaved embryos was significantly higher in the SSV+CB (P<0.05) than that of the SSV group. Nevertheless, significantly more oocytes cleaved (P<0.05) in the TC, TC+CB and the control groups. On Day 6, the rate of blastocysts in the SSV and SSV+CB groups did not differ significantly. The number of oocytes developing to blastocyst and the mean number of blastomeres per embryo were significantly higher (P<0.05) in the TC, TC+CB and the control compared with that of the SSV and SSV+CB groups. To our knowledge, this is the first report on parthenogenetic development to blastocysts of porcine oocytes vitrified at the metaphase-II stage. Results indicate that the high concentrations of cryoprotectants were not harmful for in vitro development, and that CB pre-treatment may increase survival and development of SSV vitrified porcine oocytes.

Bovine ICM derived cells express the Oct4 ortholog.

The goal of this study was to define conditions for the successful isolation of embryonic stem cells from bovine blastocysts. Expression of the Pit-Oct-Unc (POU) transcription factor Oct4 was employed to monitor the pluripotent status of cultured cells. No expression of the previously identified bovine Oct4 pseudogene was found, and transcription of the Oct4 ortholog correlated with the proliferative potential of bovine ICM derived cells. Two methods to isolate pluripotent inner cell mass were compared; 90% of trypsin isolated ICMs formed growing cultures, whereas only 12%-23% of the ICMs isolated by immunosurgery attached and grew. Colony formation from complete blastocysts was 55%. The bovine ICM derived cells could be grown for 4-7 passages. However, Oct4 transcripts were only present in the primary cultures, indicating that the initial culture period of bovine ICM derived cells is critical and needs to be optimized to yield true ES cells. In contrast to bovine ICMs, murine ICMs yielded rapidly growing cells, which proliferated for more than 60 passages.

Epigenetic reprogramming throughout preimplantation development and consequences for assisted reproductive technologies.

Knowledge about preimplantation development is important both for basic reproductive biology and for practical applications, including livestock breeding and regenerative medicine. During preimplantation development, epigenetic modifications such as DNA methylation and histone modifications are involved in the regulation of imprinted and non-imprinted genes, in the initiation of X chromosome inactivation, and the adjustment of telomere length. The underlying events are particularly vulnerable to external factors. Characterization of expression profiles in in vivo-derived embryos of different developmental stages and understanding the mechanisms and dynamics underlying the reprogramming process are the first steps towards the analysis of the complex gene regulatory networks. They provide a baseline for the analysis of manipulated embryos of all mammalian species, including humans, to improve embryo technologies and related therapeutic applications.

From fibroblasts and stem cells: implications for cell therapies and somatic cloning.

Pluripotent embryonic stem cells (ESCs) from the inner cell mass of early murine and human embryos exhibit extensive self-renewal in culture and maintain their ability to differentiate into all cell lineages. These features make ESCs a suitable candidate for cell-replacement therapy. However, the use of early embryos has provoked considerable public debate based on ethical considerations. From this standpoint, stem cells derived from adult tissues are a more easily accepted alternative. Recent results suggest that adult stem cells have a broader range of potency than imagined initially. Although some claims have been called into question by the discovery that fusion between the stem cells and differentiated cells can occur spontaneously, in other cases somatic stem cells have been induced to commit to various lineages by the extra- or intracellular environment. Recent data from our laboratory suggest that changes in culture conditions can expand a subpopulation of cells with a pluripotent phenotype from primary fibroblast cultures. The present paper critically reviews recent data on the potency of somatic stem cells, methods to modify the potency of somatic cells and implications for cell-based therapies.

Isolation of murine and porcine fetal stem cells from somatic tissue.

Adult stem cells have been previously isolated from a variety of somatic tissues, including bone marrow and the central nervous system; however, contribution of these cells to the germ line has not been shown. Here we demonstrate that fetal somatic explants contain a subpopulation of somatic stem cells (FSSCs), which can be induced to display features of lineage-uncommitted stem cells. After injection into blastocysts, these cells give rise to a variety of cell types in the resultant chimeric fetuses, including those of the mesodermal lineage; they even migrate into the genital ridge. In vitro, FSSCs exhibit characteristics of embryonic stem cells, including extended self-renewal; expression of stem cell marker genes, such as Pou5f1 (Oct4), Stat3, and Akp2 (Tnap) and growth as multicellular aggregates. We report that fetal tissue contains somatic stem cells with greater potency than previously thought, which might form a new source of stem cells useful in somatic nuclear transfer and cell therapy.

Cross-species hybridisation of human and bovine orthologous genes on high density cDNA microarrays.

BACKGROUND: Cross-species gene-expression comparison is a powerful tool for the discovery of evolutionarily conserved mechanisms and pathways of expression control. The usefulness of cDNA microarrays in this context is that broad areas of homology are compared and hybridization probes are sufficiently large that small inter-species differences in nucleotide sequence would not affect the analytical results. This comparative genomics approach would allow a common set of genes within a specific developmental, metabolic, or disease-related gene pathway to be evaluated in experimental models of human diseases. The objective of this study was to investigate the feasibility and reproducibility of cross-species analysis employing a human cDNA microarray as probe. RESULTS: As a proof of principle, total RNA derived from human and bovine fetal brains was used as a source of labelled targets for hybridisation onto a human cDNA microarray composed of 349 characterised genes. Each gene was spotted 20 times representing 6,980 data points thus enabling highly reproducible spot quantification. Employing high stringency hybridisation and washing conditions, followed by data analysis, revealed slight differences in the expression levels and reproducibility of the signals between the two species. We also assigned each of the genes into three expression level categories- i.e. high, medium and low. The correlation co-efficient of cross hybridisation between the orthologous genes was 0.94. Verification of the array data by semi-quantitative RT-PCR using common primer sequences enabled co-amplification of both human and bovine transcripts. Finally, we were able to assign gene names to previously uncharacterised bovine ESTs. CONCLUSIONS: Results of our study demonstrate the harnessing and utilisation power of comparative genomics and prove the feasibility of using human microarrays to facilitate the identification of co-expressed orthologous genes in common tissues derived from different species.