Chang, Lisa L.

Kessler, Daniel S.

Elsevier - Publisher Connector 

FoxD3 regulation of mesoderm induction in the zebrafish embryo 

differentiated endoderm cells sorted circumferentially while the
undifferentiated F9 cells remained predominantly internal. This
indicates that the acquisition of a surface position is an intrinsic
property of endoderm epithelia. Disabled-2 (Dab2), an endocytic
adaptor protein that mediates the directional transport of clathrin-
coated cargos, is required for the spontaneous surface sorting and
positioning of the endoderm cells. When Dab2 expression was
compromised, the differentiated F9 cells no longer localized
correctly and were distributed throughout the interior of the EBs.
These results support a model where primitive endoderm cells are
first formed within the interior of the inner cell mass of the
preimplantation mouse blastocyst and are subsequently sorted to
the surface by aDab2-dependent mechanism.We propose that the
autonomous property of epithelial cells to generate polarity is the
factor responsible for surface positioning of epithelia.
doi:10.1016/j.ydbio.2007.03.608
Program/Abstract # 308
Thyrotropin-releasing hormone precursor—A novel
marker of the mouse definitive endoderm
Kristen D. McKnight 1, Pamela A. Hoodless 2
1 Genetics Graduate Program, University of British Columbia,
Vancouver, BC, Canada
2 Department of Medical Genetics, University of British
Columbia, BC, Canada
3 Terry Fox Laboratory, B.C. Cancer Agency, Vancouver, BC,
Canada
Gastrulation is one of the most critical events of embryogen-
esis, generating the three primary germ layers (endoderm,
mesoderm and ectoderm) that will give rise to the tissues of the
developing embryo. Of the germ layers the least is known about
the definitive endoderm (DE), which gives rise to the lungs,
digestive tract, liver and pancreas. This is due in large part to the
lack of genetic markers specific for the DE as many of the current
markers, including Cerl, Foxa2 and Sox17, are also expressed in
the visceral endoderm (VE), an extraembryonic tissue. Using
Affymetrix GeneChips and Serial Analysis of Gene Expression
(SAGE) we have identified a novel marker of the mouse DE—
Thyrotropin-releasing hormone precursor (Trh). We have char-
acterized the expression of Trh throughout mouse gastrulation and
early organogenesis stages using whole mount in situ hybridiza-
tion. Our expression data shows that Trh is expressed in newly
formed DE cells and is subsequently expressed in the entire DE
before becoming downregulated as the DE is patterned. The
dynamic expression pattern ofTrh is in accordancewith recent fate
mapping experiments detailing the movement of the DE during
gastrulation. Preliminary experiments suggest that Trh is absent
from the VE, being expressed in a mutually exclusive pattern with
Pem (a marker of the extraembryonic visceral endoderm). These
results point to Trh being an exclusive DE marker.
doi:10.1016/j.ydbio.2007.03.609
Program/Abstract # 309
FoxD3 regulation of mesoderm induction in the zebrafish
embryo
Lisa L. Chang, Daniel S. Kessler
Dept. of Cell and Developmental Biology, University of
Pennsylvania School of Medicine, Philadelphia, PA, USA
Nodal ligands are required for germ layer induction in
vertebrates. During zebrafish gastrulation, the expression
domain of the Nodal-related genes, Cyclops (Cyc) and Squint
(Sqt), overlaps that of FoxD3 in the shield, suggesting a possible
role for FoxD3 in mesoderm development. Overexpression of
FoxD3 results in expansion of Cyc expression and dorsal
mesoderm markers. Knockdown results in reduced expression
of these markers and 24-h embryos show a phenotype similar to
Nodal pathway mutants. To determine the functional interaction
of FoxD3 with the Nodal pathway we examined Antivin-
overexpressing and MZoep mutant embryos. FoxD3 does not
rescue or induce ectopic mesoderm indicating that FoxD3 is
dependent on a functional Nodal pathway for dorsal mesoderm
induction. A FoxD3 mutant, Sym1, where the mutation
inactivates the FoxD3 gene has been reported. The phenotype
shows craniofacial defects and delayed/reduced development of
chromatophores. From our results and our model for FoxD3
activity we predict early gastrulation deficiencies and defects in
tissues derived from dorsal mesoderm. Our preliminary results
indicate that the sym1 protein retains partial function as its
overexpression induces Cyc and dorsal mesoderm markers.
Future work will examine sym1 embryos for unappreciated
defects in mesodermal gene expression and axial development.
Results suggest that dorsal mesoderm induction is regulated, at
least in part, by FoxD3. We hypothesize that FoxD3 regulates
Nodal expression in the zebrafish shield by repressing a negative
regulator of Nodal expression, thus indirectly promoting Nodal
expression and mesoderm development.
doi:10.1016/j.ydbio.2007.03.610
Program/Abstract # 310
Genetic analysis of Fgf gene function in the limb
Francesca Mariani 1, Christina Ahn 1, David Ornitz 2,
Gail Martin 1
1 Dept. of Anatomy, University of California, San Francisco, CA
2 Dept. of Molecular Biology and Pharmacology,
Washington University, St. Louis, MO
The tetrapod limb emerges from the flank of the embryo as a
bud of mesenchyme encased in an epithelial hull. Substantial
growth and differentiation of this bud gives rise to a scaffold of
skeletal elements that can vary among species in their size,
number and shape. At the distal edge of the bud the apical
ectodermal ridge (AER), produces signals essential for limb
development. In the mouse, four Fgf genes, Fgf4, Fgf8, Fgf9
and Fgf17, are expressed specifically in the AER and may be the
critical genes that provide these essential signals. Using a genetic
403ABSTRACTS / Developmental Biology 306 (2007) 396–410


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FoxD3 regulation of mesoderm induction in the zebrafish embryo

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