The migratory pathways of the cells that form the endocardium, dorsal aortae, and head vasculature in the mouse embryo.

BACKGROUND: Vasculogenesis in amniotes is often viewed as two spatially and temporally distinct processes, occurring in the yolk sac and in the embryo. However, the spatial origins of the cells that form the primary intra-embryonic vasculature remain uncertain. In particular, do they obtain their haemato-endothelial cell fate in situ, or do they migrate from elsewhere? Recently developed imaging techniques, together with new Tal1 and existing Flk1 reporter mouse lines, have allowed us to investigate this question directly, by visualising cell trajectories live and in three dimensions. RESULTS: We describe the pathways that cells follow to form the primary embryonic circulatory system in the mouse embryo. In particular, we show that Tal1-positive cells migrate from within the yolk sac, at its distal border, to contribute to the endocardium, dorsal aortae and head vasculature. Other Tal1 positive cells, similarly activated within the yolk sac, contribute to the yolk sac vasculature. Using single-cell transcriptomics and our imaging, we identify VEGF and Apela as potential chemo-attractants that may regulate the migration into the embryo. The dorsal aortae and head vasculature are known sites of secondary haematopoiesis; given the common origins that we observe, we investigate whether this is also the case for the endocardium. We discover cells budding from the wall of the endocardium with high Tal1 expression and diminished Flk1 expression, indicative of an endothelial to haematopoietic transition. CONCLUSIONS: In contrast to the view that the yolk sac and embryonic circulatory systems form by two separate processes, our results indicate that Tal1-positive cells from the yolk sac contribute to both vascular systems. It may be that initial Tal1 activation in these cells is through a common mechanism.

Targeted deletion of p97 (VCP/CDC48) in mouse results in early embryonic lethality.

The highly conserved AAA ATPase p97 (VCP/CDC48) has well-established roles in cell cycle progression, proteasome degradation and membrane dynamics. Gene disruption in Saccromyces cerevisiae, Drosophila melanogaster and Trypanosoma brucei demonstrated that p97 is essential in unicellular and multicellular organisms. To explore the requirement for p97 in mammalian cell function and embryogenesis, we disrupted the p97 locus by gene targeting. Heterozygous p97+/- mice were indistinguishable from their wild-type littermates, whereas homozygous mutants did not survive to birth and died at a peri-implantation stage. These results show that p97 is an essential gene for early mouse development.

Truncation of the Mll gene in exon 5 by gene targeting leads to early preimplantation lethality of homozygous embryos.

The mixed lineage leukemia gene (MLL) was originally identified through its involvement in reciprocal translocations in leukemias. MLL codes for a large multidomain protein and bears homology to the Drosophila developmental control gene trithorax in two small domains in the amino terminal region, the central zinc finger domain and the carboxy SET domain. Like the Drosophila trx, MLL has also been shown to be a positive regulator of Hox gene expression. We have targeted Mll (the murine homologue of MLL) in exon 5 causing expression of three truncated in-frame Mll transcripts. These transcripts retain all or some of the AT hook motifs and the DMT domain. This mutant allele causes early in vivo preimplantation lethality of homozygous embryos prior to the 2-cell stage. Embryos cultured in vitro progress to the 2-cell stage, but further development is arrested. The heterozygotes exhibit mild skeletal defects as well as defects in some neuroectodermal derivatives.

Fibroblast growth factor receptor 2-IIIb acts upstream of Shh and Fgf4 and is required for limb bud maintenance but not for the induction of Fgf8, Fgf10, Msx1, or Bmp4.

Mice deficient for FgfR2-IIIb were generated by placing translational stop codons and an IRES-LacZ cassette into exon IIIb of FgfR2. Expression of the alternatively spliced receptor isoform, FgfR2-IIIc, was not affected in mice deficient for the IIIb isoform. FgfR2-IIIb(-/-) (lac)(Z) mice survive to term but show dysgenesis of the kidneys, salivary glands, adrenal glands, thymus, pancreas, skin, otic vesicles, glandular stomach, and hair follicles, and agenesis of the lungs, anterior pituitary, thyroid, teeth, and limbs. Detailed analysis of limb development revealed an essential role for FgfR2-IIIb in maintaining the AER. Its absence did not prevent expression of Fgf8, Fgf10, Bmp4, and Msx1, but did prevent induction of Shh and Fgf4, indicating that they are downstream targets of FgfR2-IIIb activation. In the absence of FgfR2-IIIb, extensive apoptosis of the limb bud ectoderm and mesenchyme occurs between E10 and E10.5, providing evidence that Fgfs act primarily as survival factors. We propose that FgfR2-IIIb is not required for limb bud initiation, but is essential for its maintenance and growth.

The nuclear receptor co-repressor nrip1 (RIP140) is essential for female fertility.

Ovulatory dysfunction is the commonest cause of female infertility. Here we show that the co-repressor nuclear-receptor-interacting protein 1 (Nrip1; encoded by the gene Nrip1) is essential for ovulation. Mice null for this protein are viable, but female mice are infertile because of complete failure of mature follicles to release the oocyte at ovulation. In contrast, luteinization proceeds normally, resulting in a phenotype closely resembling that of luteinized unruptured follicle syndrome, often associated with infertility in women. Therefore, whereas the pre-ovulatory surge of luteinizing hormone induces both ovulation and luteinization, the ability to suppress the action of nuclear receptors is essential for the coordinated control of ovarian function with the essential process of oocyte release dependent on the activity of the transcriptional co-repressor Nrip1 (RIP40).

Disruption of the developmentally regulated Rev3l gene causes embryonic lethality.

The REV3 gene encodes the catalytic subunit of DNA polymerase (pol) zeta, which can replicate past certain types of DNA lesions [1]. Saccharomyces cerevisiae rev3 mutants are viable and have lower rates of spontaneous and DNA-damage-induced mutagenesis [2]. Reduction in the level of Rev31, the presumed catalytic subunit of mammalian pol zeta, decreased damage-induced mutagenesis in human cell lines [3]. To study the function of mammalian Rev31, we inactivated the gene in mice. Two exons containing conserved DNA polymerase motifs were replaced by a cassette encoding G418 resistance and beta-galactosidase, under the control of the Rev3l promoter. Surprisingly, disruption of Rev3l caused mid-gestation embryonic lethality, with the frequency of Rev3l(-/-) embryos declining markedly between 9.5 and 12.5 days post coitum (dpc). Rev3l(-/-) embryos were smaller than their heterozygous littermates and showed retarded development. Tissues in many areas were disorganised, with significantly reduced cell density. Rev3l expression, traced by beta-galactosidase staining, was first detected during early somitogenesis and gradually expanded to other tissues of mesodermal origin, including extraembryonic membranes. Embryonic death coincided with the period of more widely distributed Rev3l expression. The data demonstrate an essential function for murine Rev31 and suggest that bypass of specific types of DNAlesions by pol zeta is essential for cell viability during embryonic development in mammals.

Fibroblast growth factor signalling and cyclin D1 function are necessary for normal mammary gland development during pregnancy. A transgenic mouse approach.

A number of growth factors, growth factor receptors and cell cycle regulatory proteins have been implicated in the genesis of mammary carcinomas both in animal models as well as in human breast tumour samples. Studies on the development of the mammary gland has revealed that several of the proto-oncogenes, or their closely related gene-family members, have a function in the normal growth and differentiation of the gland. In this review the role of fibroblast growth factor signalling and the critical requirement for the cell cycle regulator, cyclin D1 is discussed with respect to their normal function in mammary gland development and abnormal role in mammary carcinogenesis.

Uracil-DNA glycosylase (UNG)-deficient mice reveal a primary role of the enzyme during DNA replication.

Gene-targeted knockout mice have been generated lacking the major uracil-DNA glycosylase, UNG. In contrast to ung- mutants of bacteria and yeast, such mice do not exhibit a greatly increased spontaneous mutation frequency. However, there is only slow removal of uracil from misincorporated dUMP in isolated ung-/- nuclei and an elevated steady-state level of uracil in DNA in dividing ung-/- cells. A backup uracil-excising activity in tissue extracts from ung null mice, with properties indistinguishable from the mammalian SMUG1 DNA glycosylase, may account for the repair of premutagenic U:G mispairs resulting from cytosine deamination in vivo. The nuclear UNG protein has apparently evolved a specialized role in mammalian cells counteracting U:A base pairs formed by use of dUTP during DNA synthesis.

The p24 family member p23 is required for early embryonic development.

The p24 family of type I integral-membrane proteins, which are localised in the endoplasmic reticulum (ER), the intermediate compartment and the Golgi apparatus, are thought to function as receptors for cargo exit from the ER and in transport vesicle formation. Members of the p24 family have been found in a molecular complex and are enriched in COPI-coated vesicles, which are involved in membrane traffic between the ER and Golgi complex. Although expressed abundantly, simultaneous deletion of several family members does not appear to affect cell viability and protein secretion in yeast. In order to gain more insights into the physiological roles of different p24 proteins, we generated mice deficient in the expression of one family member, p23 (also called 24delta1, see for alternative nomenclature). In contrast to yeast genetics, in mice disruption of both p23 alleles resulted in early embryonic lethality. Inactivation of one allele led not only to reduced levels of p23 itself but also to reduced levels of other family members. The reduction in steady-state protein levels also induced structural changes in the Golgi apparatus, such as the formation of dilated saccules. The generation of mice deficient in p23 expression has revealed an essential and non-redundant role for p23 in the earliest stages of mammalian development. It has also provided genetic evidence for the participation of p24 family members in oligomeric complexes and indicates a structural role for these proteins in maintaining the integrity of the early secretory pathway.

An important role for the IIIb isoform of fibroblast growth factor receptor 2 (FGFR2) in mesenchymal-epithelial signalling during mouse organogenesis.

The fibroblast growth factor receptor 2 gene is differentially spliced to encode two transmembrane tyrosine kinase receptor proteins that have different ligand-binding specificities and exclusive tissue distributions. We have used Cre-mediated excision to generate mice lacking the IIIb form of fibroblast growth factor receptor 2 whilst retaining expression of the IIIc form. Fibroblast growth factor receptor 2(IIIb) null mice are viable until birth, but have severe defects of the limbs, lung and anterior pituitary gland. The development of these structures appears to initiate, but then fails with the tissues undergoing extensive apoptosis. There are also developmental abnormalities of the salivary glands, inner ear, teeth and skin, as well as minor defects in skull formation. Our findings point to a key role for fibroblast growth factor receptor 2(IIIb) in mesenchymal-epithelial signalling during early organogenesis.

Accumulation of premutagenic DNA lesions in mice defective in removal of oxidative base damage.

DNA damage generated by oxidant byproducts of cellular metabolism has been proposed as a key factor in cancer and aging. Oxygen free radicals cause predominantly base damage in DNA, and the most frequent mutagenic base lesion is 7,8-dihydro-8-oxoguanine (8-oxoG). This altered base can pair with A as well as C residues, leading to a greatly increased frequency of spontaneous G.C-->T.A transversion mutations in repair-deficient bacterial and yeast cells. Eukaryotic cells use a specific DNA glycosylase, the product of the OGG1 gene, to excise 8-oxoG from DNA. To assess the role of the mammalian enzyme in repair of DNA damage and prevention of carcinogenesis, we have generated homozygous ogg1(-/-) null mice. These animals are viable but accumulate abnormal levels of 8-oxoG in their genomes. Despite this increase in potentially miscoding DNA lesions, OGG1-deficient mice exhibit only a moderately, but significantly, elevated spontaneous mutation rate in nonproliferative tissues, do not develop malignancies, and show no marked pathological changes. Extracts of ogg1 null mouse tissues cannot excise the damaged base, but there is significant slow removal in vivo from proliferating cells. These findings suggest that in the absence of the DNA glycosylase, and in apparent contrast to bacterial and yeast cells, an alternative repair pathway functions to minimize the effects of an increased load of 8-oxoG in the genome and maintain a low endogenous mutation frequency.

Tumor burden and clonality in multiple intestinal neoplasia mouse/normal mouse aggregation chimeras.

Aggregation chimeras were formed between C57BL/6 mice heterozygous for the Apc(min) (Min) mutation and wild-type SWR mice, that differ in their Pla2g2a status, a modifier of Apc(min), and also in their resistance to intestinal polyp formation. Variation in the dolichos biflorus agglutinin-staining patterns of the intestines of these mouse strains was used to determine the chimeric composition of the intestine in individual mice and to examine the clonal composition of adenomas. Macroscopic adenoma numbers in chimeric mice were compared with the expected adenoma numbers based on the percentage of C57BL/6J-Apc(min/+) epithelium in individual mice. These results unexpectedly show that there was no apparent inhibitory effect of the SWR-derived (Pla2g2a wild-type) tissue on adenoma formation in the C57BL/6J-Apc(min/+) epithelium. This suggests that the main genetic modifiers of the Min phenotype act at a cellular or crypt-restricted level with no discernable systemic effect. All adenomas were seen to contain C57BL/6J-Apc(min/+)-derived epithelium, confirming that the germ-line mutation of the mApc gene is necessary to initiate tumorigenesis in this model system, and that the mApc gene acts in a cell autonomous fashion.

Cyclin B2-null mice develop normally and are fertile whereas cyclin B1-null mice die in utero.

Two B-type cyclins, B1 and B2, have been identified in mammals. Proliferating cells express both cyclins, which bind to and activate p34(cdc2). To test whether the two B-type cyclins have distinct roles, we generated lines of transgenic mice, one lacking cyclin B1 and the other lacking cyclin B2. Cyclin B1 proved to be an essential gene; no homozygous B1-null pups were born. In contrast, nullizygous B2 mice developed normally and did not display any obvious abnormalities. Both male and female cyclin B2-null mice were fertile, which was unexpected in view of the high levels and distinct patterns of expression of cyclin B2 during spermatogenesis. We show that the expression of cyclin B1 overlaps the expression of cyclin B2 in the mature testis, but not vice versa. Cyclin B1 can be found both on intracellular membranes and free in the cytoplasm, in contrast to cyclin B2, which is membrane-associated. These observations suggest that cyclin B1 may compensate for the loss of cyclin B2 in the mutant mice, and implies that cyclin B1 is capable of targeting the p34(cdc2) kinase to the essential substrates of cyclin B2.

Targeted disruption of the gene encoding DNA ligase IV leads to lethality in embryonic mice.

DNA ligase IV is the most recently identified member of a family of enzymes joining DNA strand breaks in mammalian cell nuclei [1] [2]. The enzyme occurs in a complex with the XRCC4 gene product [3], an interaction mediated via its unique carboxyl terminus [4] [5]. Cells lacking XRCC4 are hypersensitive to ionising radiation and defective in V(D)J recombination [3] [6], implicating DNA ligase IV in the pathway of nonhomologous end-joining (NHEJ) of DNA double-strand breaks mediated by XRCC4, the Ku70/80 heterodimer and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) in mammalian cells (reviewed in [7]). The phenotype of a null mutant of the Saccharomyces cerevisiae DNA ligase IV homologue indicates that the enzyme is non-essential and functions in yeast NHEJ [8] [9] [10]. Unlike other mammalian DNA ligases for which cDNAs have been characterised, DNA ligase IV is encoded by an intronless gene (LIG4). Here, we show that targeted disruption of LIG4 in the mouse leads to lethality associated with extensive apoptotic cell death in the embryonic central nervous system. Thus, unlike Ku70/80 and DNA-PKcs [11] [12] [13] [14], DNA ligase IV has an essential function in early mammalian development.

Exit of the pre-TCR from the ER/cis-Golgi is necessary for signaling differentiation, proliferation, and allelic exclusion in immature thymocytes.

A major issue is whether surface expression of the pre-TCR is necessary for signaling the development of immature thymocytes. To address this question, we generated transgenic mice expressing a TCRbeta chain that had a strong endoplasmic reticulum (ER) retrieval signal (TCRbetaER) and that was expressed intracellularly but failed to reach the cell surface. In TCRbetaER transgenic mice, there was a failure of allelic exclusion. Also, the transgene failed to rescue the developmental defects observed in TCRbeta-null mice. In contrast, TCRbeta transgenes with a mutant ER retrieval sequence or lacking this sequence signaled efficient allelic exclusion and suppressed the TCRbeta-/- defect. These data show that exit of the pre-TCR from the ER/cis-Golgi is required for progression through the double-negative thymocyte checkpoint.

Fibroblast growth factor receptor signalling has a role in lobuloalveolar development of the mammary gland.

We have used the mouse mammary tumor virus promoter to express two dominant negative (DN) fibroblast growth factor receptor (FGFR) isoforms in the mammary epithelium of transgenic mice. While expression of DN-FGFR1(IIIc) showed no discernible phenotype, a similar kinase negative form of FGFR2(IIIb) caused a marked impairment of lobuloalveolar development. The growth retardation was apparent by mid-pregnancy and persisted in the post-partum glands. Despite the substantial underdevelopment of the mammary gland there was a measurable lactational response, but it was insufficient to properly sustain the new-born pups. These findings demonstrate that fibroblast growth factor signalling is necessary for pregnancy dependent lobuloalveolar development of the mammary gland.

Cbfa1, a candidate gene for cleidocranial dysplasia syndrome, is essential for osteoblast differentiation and bone development.

We have generated Cbfa1-deficient mice. Homozygous mutants die of respiratory failure shortly after birth. Analysis of their skeletons revealed an absence of osteoblasts and bone. Heterozygous mice showed specific skeletal abnormalities that are characteristic of the human heritable skeletal disorder, cleidocranial dysplasia (CCD). These defects are also observed in a mouse Ccd mutant for this disease. The Cbfa1 gene was shown to be deleted in the Ccd mutation. Analysis of embryonic Cbfa1 expression using a lacZ reporter gene revealed strong expression at sites of bone formation prior to the earliest stages of ossification. Thus, the Cbfa1 gene is essential for osteoblast differentiation and bone formation, and the Cbfa1 heterozygous mouse is a paradigm for a human skeletal disorder.

Raf regulates positive selection.

T cell development is regulated by extracellular signals that mediate cellular proliferation and differentiation via specific signal transduction pathways. To determine the importance of the mitogen-activated protein kinase (MAP kinase) pathway in thymocyte development, we analyzed transgenic mice expressing dominant negative Raf (DN Raf) and a constitutively active v-Raf under the control of the p56lck proximal promoter. DN Raf had a profound effect on T cell receptor (TCR)-mediated signaling events as assessed by the inhibition of mitogen-induced proliferation of thymocytes in vitro. Overall thymocyte numbers were decreased by at most twofold from nontransgenic littermates. Positive selection was inhibited in DN Raf transgenic mice, as evidenced by both reduced numbers of mature thymocytes and a decrease in CD8+ thymocytes in female mice doubly transgenic for DN-Raf and a class I-restricted H-Y TCR. In contrast, the differentiation of double-positive thymocytes to single-positive thymocytes was enhanced in H-YTCR transgenic mice expressing constitutively active Raf (v-Raf). Thus, Raf regulates positive selection in the thymus.

Mice lacking cyclin D1 are small and show defects in eye and mammary gland development.

Using homologous recombination, mice lacking cyclin D1 were generated by replacing most of the first exon of the Cyl-1 gene with sequences encoding neomycin resistance. Cyl-1(-1-) mice were viable and fertile but consistently smaller than their heterozygous or wild-type littermates. The nullizygous animals also showed two distinctive abnormalities: a severe retinopathy caused by impaired development of all layers of the retina and, in the mammary gland during pregnancy, a marked reduction in acinar development accompanied by a failure to lactate. Approximately 50% of animals also had a malformation of the jaw that manifested itself as a misalignment of the incisor teeth. Mouse embryo fibroblasts isolated from 14 day nullizygous, heterozygous, or wild-type embryos and grown under standard conditions showed similar cell-cycle and growth characteristics. Thus although cyclin D1 kinase activity may facilitate G1 progression, it is not essential for the development of most tissues and organs, and only a few specialized cell lineages are demonstrably sensitive to its absence.

Subtle differences in antibody responses and hypermutation of lambda light chains in mice with a disrupted chi constant region.

Analysis of lambda light chain use in normal mice is made difficult by the dominant chi light chain repertoire. We produced mice rendered deficient in chi light chain expression by gene targeting and focused on questions concerned with the generation of lambda light chain diversity. Whilst these mice compensate the chi deficiency with increased lambda liters, and their Ig level is therefore not significantly reduced, they show major differences in immunization titers, germinal center (GC) development and somatic hypermutation. After immunization, using antigens that elicit a restricted IgL response in normal mice, we obtained in the chi-/- mice elevated primary antibody titers but a subsequent lack in titer increase after repeated antigen challenge. Analysis of the Peyer's patches (PP) revealed a dramatically reduced cell content with rather small but highly active GC. Flow cytometric analysis showed different cell populations in the PP with enriched peanut agglutinin (PNA)hi/CD45R(B220)+ B cells, implying that the apparent compensation for the lack of lambda light chain expression involves the GC microenvironment in cell selection, the initiation of hypermutation and high affinity expansion. The three V lambda genes, V1, V2 and Vx, are mutated in the GC B cells, but show no junctional diversity. In contrast, a reduced rate of V lambda hypermutation is found in the hybridoma antibodies, which appears to reflect a selection bias rather than structural constraints. However, mechanisms of somatic mutation and specificity selection can operate with equal efficiency on the few V lambda genes.