Systematic screening for signaling molecules expressed during somitogenesis by the signal sequence trap method

Blood flow-mediated gene transfer and siRNA-knockdown in the developing vasculature in a spatio-temporally controlled manner in chicken embryos

We describe a method by which early developing vasculature can be gene-manipulated independently of the heart in a spatio-temporally controlled manner. Lipofectamine 2000 or 3000, an easy-to-use lipid reagent, has been found to yield a high efficiency of transfection when co-injected with GFP DNA within a critical range of lipid concentration. By exploiting developmentally changing patterns of vasculature and blood flow, we have succeed in controlling the site of transfection: injection with a lipid-DNA cocktail into the heart before or after the blood circulation starts results in a limited and widely spread patterns of transfection, respectively. Furthermore, a cocktail injection into the right dorsal aorta leads to transgenesis of the right half of embryonic vasculature. In addition, this method combined with the siRNA technique has allowed, for the first time, to knockdown the endogenous expression of VE-cadherin (also called Cdh5), which has been implicated in assembly of nasant blood vessels: when Cah5 siRNA is injected into the right dorsal aorta, pronounced defects in the right half of vasculature are observed without heart defects. Whereas infusion-mediated gene transfection method has previously been reported using lipid reagents that were elaborately prepared on their own, Lipofectamine is an easy-use reagent with no requirement of special expertise. The methods reported here would overcome shortcomings of conventional vascular-transgenic animals, such as mice and zebrafish, in which pan-endothelial enhancer-driven transgenesis often leads to the heart malformation, which, in turn, indirectly affects peripheral vasculature due to flow defects. Since a variety of subtypes in vasculature have increasingly been appreciated, the spatio-temporally controllable gene manipulation described in this study offers a powerful tool to understand how the vasculature is established at the molecular level.

Angiogenesis in the developing spinal cord: blood vessel exclusion from neural progenitor region is mediated by VEGF and its antagonists

Blood vessels in the central nervous system supply a considerable amount of oxygen via intricate vascular networks. We studied how the initial vasculature of the spinal cord is formed in avian (chicken and quail) embryos. Vascular formation in the spinal cord starts by the ingression of intra-neural vascular plexus (INVP) from the peri-neural vascular plexus (PNVP) that envelops the neural tube. At the ventral region of the PNVP, the INVP grows dorsally in the neural tube, and we observed that these vessels followed the defined path at the interface between the medially positioned and undifferentiated neural progenitor zone and the laterally positioned differentiated zone. When the interface between these two zones was experimentally displaced, INVP faithfully followed a newly formed interface, suggesting that the growth path of the INVP is determined by surrounding neural cells. The progenitor zone expressed mRNA of vascular endothelial growth factor-A whereas its receptor VEGFR2 and FLT-1 (VEGFR1), a decoy for VEGF, were expressed in INVP. By manipulating the neural tube with either VEGF or the soluble form of FLT-1, we found that INVP grew in a VEGF-dependent manner, where VEGF signals appear to be fine-tuned by counteractions with anti-angiogenic activities including FLT-1 and possibly semaphorins. These results suggest that the stereotypic patterning of early INVP is achieved by interactions between these vessels and their surrounding neural cells, where VEGF and its antagonists play important roles.

In vivo gene manipulations of epithelial cell sheets: a novel model to study epithelial-to-mesenchymal transition

Embryonic cells are classified into two types of cells by their morphology, epithelial and mesenchymal cells. During dynamic morphogenesis in development, epithelial cells often switch to mesenchymal by the process known as epithelial-to-mesenchymal transition (EMT). EMT is a central issue in cancer metastasis where epithelial-derived tumor cells are converted to mesenchymal with high mobility. Although many molecules have been identified to be involved in the EMT mostly by in vitro studies, in vivo model systems have been limited. We here established a novel model with which EMT can be analyzed directly in the living body. By an electroporation technique, we targeted a portion of the lateral plate mesoderm that forms epithelial cell sheets delineating the kidney region, called nephric coelomic epithelium (Neph-CE). Enhanced green fluorescent protein-electroporated Neph-CE retained the epithelial integrity without invading into the underling stroma (mesonephros). The Neph-CE transgenesis further allowed us to explore EMT inducers in vivo, and to find that Ras-Raf and RhoA signals were potent inducers. Live-imaging confocal microscopy revealed that during EMT processes cells started extending cellular protrusions toward the stroma, followed by translocation of their cell bodies. Furthermore, we established a long-term tracing of EMT-induced cells, which were dynamically relocated within the kidney stroma. The Neph-CE-transgenesis will open a way to study cellular and molecular mechanisms underlying EMT directly in actual body.

Notch signal is sufficient to direct an endothelial conversion from non-endothelial somitic cells conveyed to the aortic region by CXCR4

During the early formation of the dorsal aorta, the first-forming embryonic vessel in amniotes, a subset of somitic cells selected as presumptive angioblasts, migrates toward the dorsal aorta, where they eventually differentiate into endothelial cells. We have recently shown that these processes are controlled by Notch signals (Sato, Y., Watanabe, T., Saito, D., Takahashi, T., Yoshida, S., Kohyama, J., Ohata, E., Okano, H., and Takahashi, Y., 2008. Notch mediates the segmental specification of angioblasts in somites and their directed migration toward the dorsal aorta in avian embryos. Dev. Cell 14, 890-901.). Here, we studied a possible link between Notch and chemokine signals, SDF1/CXCR4, the latter found to be dominantly expressed in developing aorta/somites. Although CXCR4 overexpression caused a directed migration of somitic cells to the aortic region in a manner similar to Notch, no positive epistatic relationships between Notch and SDF1/CXCR4 were detected. After reaching the aortic region, the CXCR4-electroporated cells exhibited no endothelial character. Importantly, however, once provided with Notch activity, they could successfully be incorporated into developing vessels as endothelial cells. These findings were obtained combining the tetracycline-inducible gene expression method with the transposon-mediated stable gene transfer technique. We conclude that Notch activation is sufficient to direct naïve mesenchymal cells to differentiate into endothelial cells once the cells are conveyed to the aortic region.

Haemonchus contortus: evaluation of two signal sequence trapping systems for detection of secreted molecules

Given that signal sequences between secreted proteins of different species can be interchanged, it is reasonable to expect that both mammalian and yeast signal sequence trapping (SST) systems would secrete Haemonchus contortus proteins with similar efficiency and quality. To determine if H. contortus cDNAs that contain a signal sequence could re-establish secretion of a reporter protein, mammalian and yeast SST vectors were designed, 10 H. contortus genes selected, and their respective cDNAs cloned into these two SST vectors. The selected molecules included genes known to code for excretory/secretory or membrane-bound proteins as potential test 'positives', and genes known to code for non-secreted proteins as test 'negatives'. While differentiation between secretion and non-secretion was evident in both systems, the results indicated greater efficiency was achieved when the mammalian system was used. Therefore, mammalian SST using COS cells would be a more useful tool to screen H. contortus cDNA libraries for potential secreted and type-1 integral membrane proteins than yeast SST.

Analysis of Lrrn1 expression and its relationship to neuromeric boundaries during chick neural development

BACKGROUND

The Drosophila leucine-rich repeat proteins Tartan (TRN) and Capricious (CAPS) mediate cell affinity differences during compartition of the wing imaginal disc. This study aims to identify and characterize the expression of a chick orthologue of TRN/CAPS and examine its potential function in relation to compartment boundaries in the vertebrate central nervous system.

RESULTS

We identified a complementary DNA clone encoding Leucine-rich repeat neuronal 1 (Lrrn1), a single-pass transmembrane protein with 12 extracellular leucine-rich repeats most closely related to TRN/CAPS. Lrrn1 is dynamically expressed during chick development, being initially localized to the neural plate and tube, where it is restricted to the ventricular layer. It becomes downregulated in boundaries following their formation. In the mid-diencephalon, Lrrn1 expression prefigures the position of the anterior boundary of the zona limitans intrathalamica (ZLI). It becomes progressively downregulated from the presumptive ZLI just before the onset of expression of the signalling molecule Sonic hedgehog (Shh) within the ZLI. In the hindbrain, downregulation at rhombomere boundaries correlates with the emergence of specialized boundary cell populations, in which it is subsequently reactivated. Immunocolocalization studies confirm that Lrrn1 protein is endocytosed from the plasma membrane and is a component of the endosomal system, being concentrated within the early endosomal compartment.

CONCLUSION

Chick Lrrn1 is expressed in ventricular layer neuroepithelial cells and is downregulated at boundary regions, where neurogenesis is known to be delayed, or inhibited. The timing of Lrrn1 downregulation correlates closely with the activation of signaling molecule expression at these boundaries. This expression is consistent with the emergence of secondary organizer properties at boundaries and its endosomal localisation suggests that Lrrn1 may regulate the subcellular localisation of specific components of signalling or cell-cell recognition pathways in neuroepithelial cells.

Identification of oscillatory genes in somitogenesis from functional genomic analysis of a human mesenchymal stem cell model

During somitogenesis, oscillatory expression of genes in the notch and wnt signaling pathways plays a key role in regulating segmentation. These oscillations in expression levels are elements of a species-specific developmental mechanism. To date, the periodicity and components of the human clock remain unstudied. Here we show that a human mesenchymal stem/stromal cell (MSC) model can be induced to display oscillatory gene expression. We observed that the known cycling gene HES1 oscillated with a 5 h period consistent with available data on the rate of somitogenesis in humans. We also observed cycling of Hes1 expression in mouse C2C12 myoblasts with a period of 2 h, consistent with previous in vitro and embryonic studies. Furthermore, we used microarray and quantitative PCR (Q-PCR) analysis to identify additional genes that display oscillatory expression both in vitro and in mouse embryos. We confirmed oscillatory expression of the notch pathway gene Maml3 and the wnt pathway gene Nkd2 by whole mount in situ hybridization analysis and Q-PCR. Expression patterns of these genes were disrupted in Wnt3a(tm1Amc) mutants but not in Dll3(pu) mutants. Our results demonstrate that human and mouse in vitro models can recapitulate oscillatory expression observed in embryo and that a number of genes in multiple developmental pathways display dynamic expression in vitro.

Expression of Lrrn1 marks the prospective site of the zona limitans thalami in the early embryonic chicken diencephalon

An unknown chicken gene selected from a published substractive hybridization screen (GenBank Accession No. ; [Christiansen, J.H., Coles, E.G., Robinson, V., Pasini, A., Wilkinson, D.G., 2001. Screening from a subtracted embryonic chick hindbrain cDNA library: identification of genes expressed during hindbrain, midbrain and cranial neural crest development. Mech. Dev. 102, 119-133.]) was deemed of interest because of its dynamic pattern of expression across the forebrain and midbrain regions. A 528bp fragment cloned from early chick embryo cDNA and used for in situ hybridization corresponded to part of the 3' untranslated region of the chicken gene Leucine-rich repeat neuronal protein 1 (Lrrn1). The expression of this gene, mapped in the embryonic chick brain between stages HH10 and HH26, apparently preconfigures the zona limitans thalami site before overt formation of this boundary structure. Apart of colateral expression in the forebrain, midbrain and hindbrain basal plate, the most significant expression of Lrrn1 was found early on across the entire alar plate of midbrain and forebrain (HH10). This unitary domain soon divides at HH14 into a rostral part, across alar secondary prosencephalon and prospective alar prosomere 3 (prethalamus; caudal limit at the prospective zona limitans), and a caudal part in alar prosomere 1 (pretectum) and midbrain. The rostral forebrain domain later downregulates gradually most extratelencephalic signal of Lrrn1, but the rostral shell of zona limitans retains expression longer. Expression in the caudal alar domain also changes by downregulation within its pretectal subdomain. Caudally, the midbrain domain ends at the isthmo-mesencephalic junction throughout the studied period. Embryonic Lrrn1 signal also appears in the somites and in the otic vesicle.

A novel signal induces a segmentation fissure by acting in a ventral-to-dorsal direction in the presomitic mesoderm

We describe here a novel inductive action that operates during somitic segmentation in chicken embryos. We previously reported that the posterior border cells located at a next-forming boundary in the anterior end of the presomitic mesoderm (PSM) exhibit an inductive activity that acts on the anterior cells to cause the formation of a somitic fissure (Sato, Y., Yasuda, K., Takahashi, Y., 2002. Morphological boundary forms by a novel inductive event mediated by Lunatic fringe and Notch during somitic segmentation. Development 129, 3633-3644). In this study, we have found a second inductive action along the dorso-ventral (D-V) axis during fissure formation. When relocated into a non-segmenting region of PSM, the ventral-most cells taken from the presumptive boundary are sufficient to induce an ectopic fissure in host cells. The ventrally derived signal acts in a ventral-to-dorsal direction but not ventrally, regardless of where the ventral cells are placed. This directional signaling is governed, at least in part, by the signal-receiving cells of the PSM, which we found to be polarized along the D-V axis, and also by intimate cell-cell interactions. Finally, we have observed that morphological segmentation is able to rearrange the anterior and posterior regionalization of individual somites. These findings suggest that discrete unidirectional signals along both the antero-posterior and the D-V axes act coordinately to achieve the formation of the intersomitic fissure, and also that fissure formation is important for the fine-tuning of A-P regionalization in individual somites.

The NLRR gene family and mouse development: Modified differential display PCR identifies NLRR-1 as a gene expressed in early somitic myoblasts

During vertebrate embryogenesis, the somites form by segmentation of the trunk mesoderm, lateral to the neural tube, in an anterior to posterior direction. Analysis of differential gene expression during somitogenesis has been problematic due to the limited amount of tissue available from early mouse embryos. To circumvent these problems, we developed a modified differential display PCR technique that is highly sensitive and yields products that can be used directly as in situ hybridisation probes. Using this technique, we isolated NLRR-1 as a gene expressed in the myotome of developing somites but not in the presomitic mesoderm. Detailed expression analysis showed that this gene was expressed in the skeletal muscle precursors of the myotome, branchial arches and limbs as well as in the developing nervous system. Somitic expression occurs in the earliest myoblasts that originate from the dorsal lip in a pattern reminiscent of the muscle determination gene Myf5, but not at the ventral lip, indicating that NLRR-1 is expressed in a subset of myotome cells. The NLRR genes comprise a three-gene family encoding glycosylated transmembrane proteins with external leucine-rich repeats, a fibronectin domain, an immunoglobulin domain and short intracellular tails capable of mediating protein-protein interaction. Analysis of NLRR-3 expression revealed regulated expression in the neural system in developing ganglia and motor neurons. NLRR-2 expression appears to be predominately confined to the adult. The regulated embryonic expression and cellular location of these proteins suggest important roles during mouse development in the control of cell adhesion, movement or signalling.

Mesenchymal-epithelial transition during somitic segmentation is regulated by differential roles of Cdc42 and Rac1

Mesenchymal-epithelial transitions (MET) are crucial for vertebrate organogenesis. The roles of Rho family GTPases in such processes during actual development remain largely unknown. By electroporating genes into chick presomitic mesenchymal cells, we demonstrate that Cdc42 and Rac1 play important and different roles in the MET that generates the vertebrate somites. Presomitic mesenchymal cells, which normally contribute to both the epithelial and mesenchymal populations of the somite, were hyperepithelialized when Cdc42 signaling was blocked. Conversely, cells taking up genes that elevate Cdc42 levels remained mesenchymal. Thus, Cdc42 activity levels appear critical for the binary decision that defines the epithelial and mesenchymal somitic compartments. Proper levels of Rac1 are necessary for somitic epithelialization, since cells with activated or inhibited Rac1 failed to undergo correct epithelialization. Furthermore, Rac1 appears to be required for Paraxis to act as an epithelialization-promoting transcription factor during somitogenesis.