Dissecting Wnt/beta-catenin signaling during gastrulation using RNA interference in mouse embryos

Normal germ line establishment in mice carrying a deletion of the Ifitm/Fragilis gene family cluster

The family of interferon-inducible transmembrane proteins (Ifitm) consists of five highly sequence-related cell surface proteins, which are implicated in diverse cellular processes. Ifitm genes are conserved, widely expressed, and characteristically found in genomic clusters, such as the 67-kb Ifitm family locus on mouse chromosome 7. Recently, Ifitm1 and Ifitm3 have been suggested to mediate migration of early primordial germ cells (PGCs), a process that is little understood. To investigate Ifitm function during germ cell development, we used targeted chromosome engineering to generate mutants which either lack the entire Ifitm locus or carry a disrupted Ifitm3 gene only. Here we show that the mutations have no detectable effects on development of the germ line or on the generation of live young. Hence, contrary to previous reports, Ifitm genes are not essential for PGC migration. The Ifitm family is a striking example of a conserved gene cluster which appears to be functionally redundant during development.

The inhibitory effects of Disabled-2 (Dab2) on Wnt signaling are mediated through Axin

beta-Catenin-mediated Wnt signaling is essential in embryonic development and in adult tissues. Recent studies have demonstrated that Axin not only plays an important inhibitory role in coordinating beta-catenin degradation, but is itself degraded by the low-density-lipoprotein receptor-related protein (LRP)5/6 Wnt co-receptor. Here, we demonstrate that the endocytic adaptor molecule Disabled-2 (Dab2), which we have previously demonstrated to act as an inhibitor of beta-catenin signaling, interacts with Axin and prevents its interaction with and degradation by the LRP5 co-receptor, thereby increasing its half-life and stabilization. Dab2 levels induced during retinoic acid-induced differentiation of F9, or during transforming growth factor-beta-induced epithelial-mesenchymal transdifferentiation of mouse mammary epithelial cells result in the stabilization of Axin and concomitant inhibition of beta-catenin signaling. Ectopic expression of Dab2 in F9 cells as well as in transformed cell lines results in increased Axin expression and attenuation of Wnt-mediated signaling. We conclude that Dab2 may play an important role in the maintenance of the differentiated state and restrain Wnt-mediated proliferation through its association with and modulation of Axin.

The inhibitory effects of Disabled-2 (Dab2) on Wnt signaling are mediated through Axin

beta-Catenin-mediated Wnt signaling is essential in embryonic development and in adult tissues. Recent studies have demonstrated that Axin not only plays an important inhibitory role in coordinating beta-catenin degradation, but is itself degraded by the low-density-lipoprotein receptor-related protein (LRP)5/6 Wnt co-receptor. Here, we demonstrate that the endocytic adaptor molecule Disabled-2 (Dab2), which we have previously demonstrated to act as an inhibitor of beta-catenin signaling, interacts with Axin and prevents its interaction with and degradation by the LRP5 co-receptor, thereby increasing its half-life and stabilization. Dab2 levels induced during retinoic acid-induced differentiation of F9, or during transforming growth factor-beta-induced epithelial-mesenchymal transdifferentiation of mouse mammary epithelial cells result in the stabilization of Axin and concomitant inhibition of beta-catenin signaling. Ectopic expression of Dab2 in F9 cells as well as in transformed cell lines results in increased Axin expression and attenuation of Wnt-mediated signaling. We conclude that Dab2 may play an important role in the maintenance of the differentiated state and restrain Wnt-mediated proliferation through its association with and modulation of Axin.

Mouse mutants with neural tube closure defects and their role in understanding human neural tube defects

BACKGROUND

The number of mouse mutants and strains with neural tube closure defects (NTDs) now exceeds 190, including 155 involving known genes, 33 with unidentified genes, and eight "multifactorial" strains.

METHODS

The emerging patterns of mouse NTDs are considered in relation to the unknown genetics of the common human NTDs, anencephaly, and spina bifida aperta.

RESULTS

Of the 150 mouse mutants that survive past midgestation, 20% have risk of either exencephaly and spina bifida aperta or both, parallel to the majority of human NTDs, whereas 70% have only exencephaly, 5% have only spina bifida, and 5% have craniorachischisis. The primary defect in most mouse NTDs is failure of neural fold elevation. Most null mutations (>90%) produce syndromes of multiple affected structures with high penetrance in homozygotes, whereas the "multifactorial" strains and several null-mutant heterozygotes and mutants with partial gene function (hypomorphs) have low-penetrance nonsyndromic NTDs, like the majority of human NTDs. The normal functions of the mutated genes are diverse, with clusters in pathways of actin function, apoptosis, and chromatin methylation and structure. The female excess observed in human anencephaly is found in all mouse exencephaly mutants for which gender has been studied. Maternal agents, including folate, methionine, inositol, or alternative commercial diets, have specific preventative effects in eight mutants and strains.

CONCLUSIONS

If the human homologs of the mouse NTD mutants contribute to risk of common human NTDs, it seems likely to be in multifactorial combinations of hypomorphs and low-penetrance heterozygotes, as exemplified by mouse digenic mutants and the oligogenic SELH/Bc strain.

Trophoblast-specific gene manipulation using lentivirus-based vectors

The trophoblast layers of the mammalian placenta carry out many complex functions required to pattern the developing embryo and maintain its growth and survival in the uterine environment. Genetic disruption of many gene pathways can result in embryonic lethality because of placental failure, potentially confusing the interpretation of mouse knockout phenotypes. Development of tools to specifically and efficiently manipulate gene expression in the trophoblast lineage would greatly aid understanding of the relative roles of different genetic pathways in the trophoblast versus embryonic lineages. We show that short-term lentivirus-mediated infection of mouse blastocysts can lead to rapid expression of a green fluorescent protein (GFP) transgene specifically in the outer trophoblast progenitors and their later placental derivatives. Efficient trophoblast-specific gene knockdown can also be produced by lentivirus-mediated pol III-driven short hairpin RNA (shRNA) and efficient trophoblast-specific gene knockout by pol II-driven Cre recombinase lentiviral vectors. This lentivirus lineage-specific infection system thus facilitates both gain and loss of function studies during placental development in the mouse and potentially other mammalian species.

Gene function in mouse embryogenesis: get set for gastrulation

During early mouse embryogenesis, temporal and spatial regulation of gene expression and cell signalling influences lineage specification, embryonic polarity, the patterning of tissue progenitors and the morphogenetic movement of cells and tissues. Uniquely in mammals, the extraembryonic tissues are the source of signals for lineage specification and tissue patterning. Here we discuss recent discoveries about the lead up to gastrulation, including early manifestations of asymmetry, coordination of cell and tissue movements and the interactions of transcription factors and signalling activity for lineage allocation and germ-layer specification.

Transgenic RNAi: Accelerating and expanding reverse genetics in mammals

Reverse genetics in mammals has relied on gene targeting strategies and has mostly been limited to the mouse. Gene targeting through homologous recombination in mouse ES cells has drawbacks which include time, expense and complexity. Recently, a new approach has been developed based on RNA-interference (RNAi) which is simpler, faster, less expensive, and should be applicable to mammalian species other than mouse. The advent of RNAi is poised to accelerate the pace at which reverse genetics can be applied to study gene function in mammals.

Cloning of vertebrate Protogenin (Prtg) and comparative expression analysis during axis elongation

A murine cDNA encoding Protogenin, which belongs to the DCC/Neogenin family, was cloned in a screen performed to identify novel cDNAs regionally expressed in the neural plate. Isolation of the putative zebrafish orthologues allowed a comparative analysis of the expression patterns of Protogenin genes during embryogenesis in different vertebrate species. From mid-gastrulation to early somite stages, Protogenin expression is restricted to posterior neural plate and mesoderm, with an anterior limit at the level of the rhombencephalon in mouse, chicken, and zebrafish. During somitogenesis, the expression profiles in the three species share features in the neural tube but present also species-specific characteristics. The initiation of Protogenin expression just before somitogenesis and its maintenance in the neural tube and paraxial mesoderm during this process suggest a conserved role in axis elongation.

RNA interference in mice

Silencing of gene expression by RNA interference (RNAi) has become a powerful tool for functional genomics in mammalian cells. Furthermore, RNAi holds promise as a simple, fast and cost-effective approach to studying mammalian gene function in vivo and as a novel therapeutic approach. This review provides an overview of the progress of RNAi in vivo, with emphasis on systemic/local siRNA delivery, viral shRNA vectors, shRNA vector transgenic mice and conditional systems to control shRNA vectors. Taken together, the data from 80 in vivo studies show that RNAi is a useful tool that offers new opportunities for functional genomics in mice.

Analysis of mouse development with conditional mutagenesis

Explorations into the molecular embryology of the mouse have played a vital role in our understanding of the basic mechanisms of gene regulation that govern development and disease. In the last 15 years, these mechanisms have been analyzed with vastly greater precision and clarity with the advent of systems that allow the conditional control of gene expression. Typically, this control is achieved by silencing or activating the gene of interest with site-specific DNA recombination or transcriptional transactivation. In this review, I discuss the application of these technologies to mouse development, focusing on recent innovations and experimental designs that specifically aid the study of the mouse embryo.

Gene expression dynamics in deer antler: mesenchymal differentiation toward chondrogenesis

Annual re-growth of deer antler represents a unique example of complete organ regeneration. Because antler mesenchymal cells retain their embryonic capacity to develop into cartilage or bone, studying antler development provides a natural system to follow gene expression changes during mesenchymal differentiation toward chondrogenic/osteogenic lineage. To identify novel genes involved either in early events of mesenchymal cell specialization or in robust bone development, we have introduced a 3 K heterologous microarray set-up (deer cDNA versus mouse template). Fifteen genes were differentially expressed; genes for housekeeping, regulatory functions (components of different signaling pathways, including FGF, TGFbeta, Wnt), and genes encoding members of the Polycomb group were represented. Expression dynamics for genes are visualized by an expression logo. The expression profile of the gene C21orf70 of unknown function is described along with the effects when over-expressed; furthermore the nuclear localization of the cognate protein is shown. In this report, we demonstrate the particular advantage of the velvet antler model in bone research for: (1) identification of mesenchymal and precartilaginous genes and (2) targeting genes upregulated in robust cartilage development.

A complex oscillating network of signaling genes underlies the mouse segmentation clock

The segmental pattern of the spine is established early in development, when the vertebral precursors, the somites, are rhythmically produced from the presomitic mesoderm. Microarray studies of the mouse presomitic mesoderm transcriptome reveal that the oscillator associated with this process, the segmentation clock, drives the periodic expression of a large network of cyclic genes involved in cell signaling. Mutually exclusive activation of the notch-fibroblast growth factor and Wnt pathways during each cycle suggests that coordinated regulation of these three pathways underlies the clock oscillator.

Building the mouse gastrula: signals, asymmetry and lineages

The mouse embryo is built by assembling the progenitors of various tissue types into a body plan. Early postimplantation development involves the establishment of anatomical asymmetries and regionalized gene expression in the conceptus, the specification of tissue lineages, and the coordination of cell movement for correct positioning of the lineage progenitors before and at gastrulation. Recent findings reveal that Wnt and Tgfbeta signalling function is instrumental in delineating the anterior-posterior embryonic axis by defining the site of primitive streak formation and by directing the movement of the visceral endoderm. These signalling activities are also required for the specification of anterior and posterior fates of the epiblast cells and for the induction and navigation of the primordial germ cells.

RNAi in mice: a promising approach to decipher gene functions in vivo

RNA interference (RNAi) is a simple and powerful tool widely used to study gene functions in many species. Vector-based systems using RNA polymerase III promoters have been developed to achieve stable expression of small interfering RNA (siRNA) or small hairpin RNA (shRNA) in mammalian cells. Recent investigations demonstrated that when, combined with the Cre-loxP system, the vector-based RNAi can be used to achieve conditional or tissue specific knockdown of endogenous genes with high efficiency in mice. Here, we review these recent progresses and discuss the advantages, limitations and future development of this emerging technology.

Identification of the IFITM family as a new molecular marker in human colorectal tumors

We analyzed the expression profiles of intestinal adenomas from a new murine familial adenomatous polyposis model (Apc(delta14/+)) using suppression subtractive hybridization to identify novel diagnostic markers of colorectal carcinogenesis. We identified 18 candidate genes having increased expression levels in the adenoma. Subsequent Northern blotting, real-time reverse transcription-PCR, and in situ hybridization analysis confirmed their induction in beta-catenin-activated epithelial cells of murine adenomas. We showed that most of the genes also have altered expression levels in human colonic adenomas and carcinomas. We focused on the IFITM genes that encode IFN-inducible transmembrane proteins. Serial analyses of gene expression levels revealed high levels of expression in early and late intestinal neoplasm in both mice and humans. Using a conditional mouse model of Apc inactivation and a human colon carcinoma cell line, we showed that IFITM gene expression is rapidly induced after activation of the beta-catenin signaling. Using a large-scale analysis of human tumors, we showed that IFITM gene expression is significantly up-regulated specifically in colorectal tumors and thus may be a useful diagnostic tool in these tumors.

RNAi in embryonic stem cells

Embryonic stem (ES) cells are pluripotent cells that can be isolated and grown in vitro from the inner cell mass of blastocysts. Their potential to differentiate into any cell of the body makes them a promising starting material for cell therapy. Much progress has been made in recent years to develop ES cell differentiation protocols employing cocktails of certain growth factors or by using cell-type-restricted promoters driving the expression of selection markers or fluorescent proteins. However, little is known about the molecular details underlying the earliest processes of mammalian development. Genetic tools that provide novel insight into these processes would be very helpful to gain a better molecular understanding and to design better differentiation protocols. Recently, RNAi has emerged as a powerful technology to perform loss-of-function studies in mammalian cells. This technology should be ideal to identify and study genes required for ES cell self-renewal and differentiation. Here, we review the recent advances and challenges of RNAi research in ES cells and we provide a perspective on possible applications to enhance our understanding of ES cell self-renewal and early differentiation.

Gene expression profiling in the discovery, optimization and development of novel drugs: one universal screening platform

With recent advances in robotics and high-content screening and analysis methods, transcriptional profiling can now be utilized as a comprehensive forward chemical genomics platform for drug discovery, lead selection and lead optimization. It can be used to define the state of a cell on the basis of gene networks, and to search for drugs that can shift cellular states in a manner predicted at the genome level to be therapeutically beneficial. The treatment of cells with compounds produces transcriptional 'fingerprints' that reveal mechanism-of-action, and enable discrimination between individual compounds based on drug behaviors important to all phases of drug discovery and development.

IFITM/Mil/Fragilis Family Proteins IFITM1 and IFITM3 Play Distinct Roles in Mouse Primordial Germ Cell Homing and Repulsion

The family of interferon-induced transmembrane protein (Ifitm/mil/fragilis) genes encodes cell surface proteins that may modulate cell adhesion and influence cell differentiation. Mouse Ifitm1 and -3, which are expressed in primordial germ cells (PGCs), are implicated to have roles in germ cell development, but the specific functions have been unclear. Our results show that Ifitm1 activity is required for PGC transit from the mesoderm into the endoderm, and that it appears to act via a repulsive mechanism, such that PGCs avoid Ifitm1-expressing tissues. In contrast, Ifitm3, which is expressed in migratory PGCs, is sufficient to confer autonomous PGC-like homing properties to somatic cells. These guidance activities are mediated by the N-terminal extracellular domain of the specific IFITM, which cannot be substituted by that of another family member. Complex homo- and/or heterotypic intercellular interactions among various IFITMs in PGCs and neighboring cells may underpin coordinated germ cell guidance in mice.

How will RNAi facilitate drug development?

Development of effective drugs for treatment of human disease relies on identification of therapeutic molecular targets. The identification of targets to treat human disease has previously relied on genetic screens in model organisms, and less robust or lower throughput approaches in mammalian systems. RNA interference (RNAi) makes possible, for the first time, the use of large-scale functional genomics approaches for target identification in human cells. This remarkable breakthrough has the potential to influence every facet of the drug discovery process, and is poised to revolutionize drug development. Reports of RNAi screens for the identification of novel genes implicated in apoptosis, cell division, and drug resistance support the enormous promise of this technology. Here, we discuss the potential impact of RNAi screens on target identification and validation and consider issues that warrant caution when interpreting RNAi screening results.