Page 79 - Mouse Molecular Genetics

Full Abstracts
Program number is above title. Author in bold is the presenter.
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up-regulated in dimple embryos before morphological phenotypes are observed, suggesting that increased FGF signaling is
primarily responsible for the developmental defects of dimple mutants. Positional cloning revealed that dimple disrupts
SLC25A26, a transmembrane protein located in the inner mitochondrial membrane and responsible for the transport of S-
Adenosylmethionine (SAM). Preliminary characterization of mitochondrial function in dimple embryos indicates absence of
mitochondrial stress and normal rates of proliferation/apoptosis. Together, these results indicate that the gastrulation defects of
dimple mutants are not caused by an energetic imbalance, but rather to a role of mitochondria in modulating the signaling
pathways that control mouse gastrulation.
129
Wnt3a/b-catenin signalling maintains mesoderm progenitors through the Sp1-like zinc finger transcription factors Sp5
and Sp8. Mark Kennedy
1
,
William Dunty Jr
1
,
Kristin Biris
1
,
Kenneth Campbell
2
,
Terry Yamaguchi
1
. 1)
Cell and
Developmental Biology Laboratory, CCR, Frederick National Lab, Frederick, MD; 2) Division of Developmental Biology,
Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio.
During gastrulation and axis extension, bipotential epiblast stem cells residing near the primitive streak give rise to the neural
and paraxial mesoderm progenitors, the latter of which ultimately generate the axial skeleton, muscle, cartilage and dermis of the
trunk and tail. Wnt3a is expressed in the primitive streak where it is required for both the maintenance and differentiation of these
paraxial mesoderm progenitors, however the mechanisms by which Wnt3a/-catenin signaling regulates their fate remain unclear.
We transcriptionally profiled Wnt3a-/- mouse embryos to identify the gene regulatory network controlled by Wnt3a. This
approach identified the Sp1-like zinc finger transcription factors Sp5 and Sp8 as putative Wnt3a target genes. Analysis of Sp5/8
expression in Wnt3a null and mesoderm-specific T-Cre;-catenin conditional loss and gain-of-function embryos, as well as the
response of ES cells to recombinant Wnt3a in vitro, validated Sp5/8 as Wnt3a target genes. Surprisingly, Sp5 null mice are viable
without any overt mutant phenotype and Sp8-/- embryos have relatively mild paraxial mesoderm defects compared to Wnt3a-/-
embryos. Therefore, we hypothesized that these 2 highly related transcription factors may function redundantly in paraxial
mesoderm development. In support of this hypothesis, both Sp5/8 were determined to be expressed in paraxial mesoderm
progenitors. To test for redundancy between Sp5 and Sp8, we generated mesoderm-specific conditional Sp5/8 double mutants
using the T-cre driver. Interestingly, the loss of Sp5/8 activity in mesoderm progenitors led to severe posterior body truncations
that closely resemble Wnt3a-/- embryo phenotypes. Examination of E18.5 mutant skeletons indicates that only the anterior most
~15 vertebrae formed. In situ hybridization analysis revealed that the mesodermal stem cell (MSC) markers Brachyury and Fgf8
were down-regulated with the apparent loss of the MSC population before E9.5 in Tcre;Sp5/8 mutants. Together, these results
provide strong genetic evidence that Sp5 and Sp8 function as novel redundant effectors of the Wnt3a/ -catenin pathway during
paraxial mesoderm development. We are currently addressing the target genes of Sp5/8 in stem cells through ChIP-seq and
transcriptional profiling studies.
130
Dickkopf1, an antagonist for canonical Wnt signaling, regulates determination between neuroectoderm and surface
ectoderm from ectodermal cell lineage. Chiharu Kimura-Yoshida
,
Isao Matsuo. Dept. of Moelcular Embryology, Osaka
Medical Center for MCH, Izumi, Osaka, Japan.
Canonical Wnt signaling plays important roles for the forebrain development throughout vertebrates. Especially, the forebrain
induction of the vertebrates was essential to be free from canonical Wnt signaling. Dickkopf1 (Dkk1) is one of antagonists for
canonical Wnt signaling and Dkk1-deficient embryos show head-less phonotypes (Glinka et al., 1997; Mukhopadhyay et al.,
2001).
In order to know more precise function in forebrain development by Wnt and Dkk1, we have generated the Dkk1 over-
expressed transgenic mouse (CAG-Dkk1) and analyzed its phenotypes. Previous Xenopus experiments indicated that
microinjection of Xenopus Dkk1 gene caused the dorsalization and induced big-head (Glinka et al., 1997). Our Dkk1-expressing
mouse displayed larger forebrain and lacked fore/hindlimb. These phenotypes resemble that of Xenopus experiments and showed
opposite phenotypes of the Dkk1-deficient mouse (Mukhopadhyay et al., 2001). Notably, in Dkk1-expressing transgenic
embryos, molecular maker studies revealed that neural markers were ectopically localized in the surface ectoderm, demonstrating
that canonical Wnt antagonist induced the neuroectoderm dominantly, instead of surface ectoderm (epidermis). In contrast by
overexpressing Wnt or reducing Dkk1, surface ectoderm expanded, while neuroectodermal cells had decreased. These above
findings demonstrated canonical Wnt and its antagonist regulate determination of cell fate between surface ectoderm and
neuroectoderm in the mouse embryo. Now we are trying to identify target genes of canonical Wnt signaling, which controls the
commitment of surface ectoderm dominantly by means of DNA microarray.
131
Guts and gastrulation: dynamic cell behaviors driving the morphogenesis of the early mouse embryo.
Manuel Viotti,
Tara
Sharma
,
Maria Pulina, Gloria Kwon, Kat Hadjantonakis. Developmental Biology, Sloan-Kettering Institute, New York, NY.
During gastrulation changes in cell shape, movement and organization direct the formation of ectoderm, mesoderm, gut
endoderm and, the three germ layers of the embryo. In the mouse, gastrulation transforms a cup-shaped bilaminar epithelial
structure, comprising the epiblast and visceral endoderm into one containing three tissue layers, two epithelia (ectoderm and gut
endoderm) and intervening mesenchyme (the mesoderm). Live imaging combined with genetic labeling experiments have
revealed that the gut endoderm forms by a novel morphogenetic mechanism of widespread intercalation. Cells of the definitive
endoderm (DE) cell lineage originate in the epiblast, ingress through the primitive streak, migrate within the wings of mesoderm,