Congress Abstracts - Society for Developmental Biology

Program/Abstract # 493
A network responsible for lineage segregation of lateral dermomyotome progenitors into myotomal and vascular fates
Applebaum, Mordechai (The Hebrew Univ of Jerusalem, Israel) Ben-Yair, Raz (Massachusetts General Hospital, USA); Chaya,
Kalcheim (The Hebrew Univ of Jerusalem, Israel)
Lineage diversification from an apparently homogenous epithelium is a central event in development though the mechanisms
underlying such processes are only slowly being discovered. To this end we implemented the avian dermomyotome (DM) as a model
for studying lineage segregation. The DM is the dorsal remnant of the somite after the dissociation of its ventral aspect into the
sclerotome. The DM primarily generates skeletal muscle and dermis and its lateral domain also gives rise to endothelium and vascular
smooth muscle (vSM) of adjacent blood vessels. Focusing on the lateral DM domain, we uncovered a molecular network underlying
myotomal (skeletal muscle) vs. vascular (endothelial and vSM) fates. We found that Id2, Id3, FoxC2 and Snail1 are enriched in the
lateral DM and promote vascular at the expense of myotomal fates. These factors, together with Pax7 and Myf5, constitute a
regulatory network. Moreover, Notch signaling, previously shown to promote vascular fate, both regulates and is controlled by the
network components. In this context, we found that Notch has a biphasic activity. Short exposure periods induce cells to enter the
myotome, yet inhibit terminal differentiation into myofibers while maintaining Pax7 expression. Long exposure periods initiate a
Snail1-dependent epithelial-to-mesenchyme transition thus promoting the vascular fate at target sites. Thus, our data illustrate a gene
regulatory network through which the lateral DM segregates into its derivatives.
Program/Abstract # 494
The Notch pathway promotes vascular cell fates of multipotent Pax3+ progenitors, in the somite
Mayeuf, Alicia; Lagha, Mounia; Danckaert, Anne; Relaix, Frédéric (Inst Pasteur, France); Vincent, Stéphane (IGBMC, France);
Buckingham, Margaret (Inst Pasteur, France)
Multipotent Pax3-positive cells in the somites give rise to skeletal muscle and to cells of the vasculature. We had proposed that this
cell fate choice depends on the equilibrium between Pax3 and Foxc2. We now focus on somite derivatives that migrate to the limb,
which we characterise as Pax3-independent endothelial cells and Pax3-dependant myogenic progenitors. Using a conditional Pax3 NICD
allele, which leads to overactivation of Notch targets, we show a decrease in myogenic progenitors, with a corresponding increase in
vascular endothelial cells. The resulting deficit in myogenesis is compensated by a Notch-induced delay in muscle differentiation,
which is strikingly compensated during development when Pax3 is down-regulated. Based on in vivo manipulation of the timing of
NICD expression and on somite explant experiments, we show that Notch signalling affects the Pax3/Foxc2 balance and promotes
vascular cell fates, prior to migration to the limb, in multipotent Pax3-positive cells in the somite of the mouse embryo.
Program/Abstract # 495
Delineating the early molecular steps required for cardiac progenitor development in the zebrafish embryo.
Deshwar, Ashish R. (U of Toronto, Canada, Scott, Ian (Hosp for Sick Children, Canada)
The earliest molecular steps required for cardiac progenitor development remain unclear. Our lab has previously shown that in the
zebrafish embryo Gata5 and Smarcd3b are key players in this process, able to direct cells to the anterior lateral plate mesoderm and
drive development into all three major cardiac lineages. The signals that lie both up and downstream of these two genes remain
relatively un-characterized. Through the use of a conditional version of Gata5, it was found that it is required early in gastrulation to
drive cells to the heart. The study of several candidate genes at this same time point has revealed novel upstream regulators of Gata5,
possibly revealing a genetic hierarchy required for its activation. To identify downstream targets transcriptional profiling of
Gata5/Smarcd3b over-expressing embryos was performed and clusters of genes implicated in cell adhesion and migration were found
to be up-regulated. Further study of these genes may reveal novel regulators of cardiac development.
Program/Abstract # 496
Crosstalk between cell adhesion and cell fate specification during zebrafish gastrulation
Barone, Vanessa; Heisenberg, Carl-Philipp (Institute of Science and Technology, Austria)
During gastrulation blastoderm cells, forming an initially homogeneous population, differentiate and segregate into the three germ
layers: ectoderm, mesoderm and endoderm. Cell fate specification leads to differences in the adhesive properties between the
progenitor cell types, driving their segregation into the germ layers. However, much less is known about potential feedbacks of cell
adhesion on progenitor cell fate specification. To address such feedbacks, we study the crosstalk between progenitor cell fate
specification and cell adhesion. We monitor cell fate using zebrafish transgenic lines expressing GFP in specific germ layer progenitor
types, and characterize cell adhesion employing a Dual Pipette Assay. Specifically, we analyze the role of cell adhesion in the
specification of mesendoderm cell fates. Our results show that cell adhesion does not affect the specification of early panmesendoderm
progenitors, but instead is required for the maintenance of axial mesoderm (prechordal plate) progenitors. This could be
due to signaling directly from cell-cell adhesion complexes, and/or paracrine Nodal/TGFbeta signaling between adhering cells,
previously suggested to be essential for mesendoderm cell fate specification. To distinguish between these two possibilities, we
interfered with progenitor cell-cell adhesion using a neutralizing E-cadherin antibody, and with Nodal/TGFbeta signaling using Nodalreceptor
inhibitors. Our preliminary results show that both cell-cell adhesion and Nodal-signaling are required for prechordal plate cell
fate maintenance, suggesting a cooperative effect of cell adhesion and Nodal-signaling. Our future work will elucidate the molecular
and cellular mechanisms underlying this process.
142