Page 80 - Mouse Molecular Genetics

Full Abstracts
Program number is above title. Author in bold is the presenter.
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and then egress into the overlying visceral endoderm (VE) resulting in formation of an epithelium on the surface of the embryo
containing cells from both embryonic (DE) and extra-embryonic (VE) lineages. In this way widespread egression of DE cells
leads to VE cell dispersal. This suggests that the segregation of embryonic and extra-embryonic tissue is not as strict as
previously believed and that a lineage previously believed to be exclusively extra-embryonic may contribute cellular descendants
to somatic tissues. Our studies further reveal that the egression of DE cells into the VE epithelium occurs concomitant with the de
novo assembly of a basement membrane at the gut endoderm/mesoderm interface. We propose that this basement membrane may
function to: (1) provide the traction force to facilitate DE cell egression into the VE, (2) prevent ingression of DE cells out of the
VE, and (3) segregate and compartmentalize the mesoderm and gut endoderm germ layers. The HMG domain containing protein
Sox17 is an evolutionarily conserved key regulator of gut endoderm formation. Embryos lacking Sox17 exhibit a failure in DE
cell specification and thus egression, and consequently a failure in VE cell dispersal. Notably, a basement membrane is never
assembled at the mesoderm/gut endoderm interface of Sox17 mutants. Our studies highlight a previously overlooked feature of
gastrulation: the transformation of a single BM (two tissue layer) containing pre-gastrula stage embryo to a two BM (three tissue
layer) containing embryo. Our observations reveal a link between de novo basement membrane assembly and germ layer
segregation with gut endoderm morphogenesis.
132
Lhx1
promotes prechordal plate formation in a cell non-autonomous manner. William Shawlot
1
,
Kenichiro Taniguchi
2
. 1)
The Dell Pediatric Research Institute, The University of Texas at Austin, Austin, TX; 2) Department of Cell and Developmental
Biology, The University of Michigan Medical School, Ann Arbor, MI.
The prechordal plate is an anterior midline endodermal structure that underlies the presumptive forebrain during vertebrate
embryogenesis. Perturbations that affect prechordal plate development cause holoprosencephaly and cyclopia in model vertebrate
organisms. In humans, holoprosencephaly is the most common defect in forebrain development with an incidence as high as 1 in
250
during embryogenesis. The molecular and cellular processes that mediate prechordal plate development in mammalian
embryos are not well understood.
Lhx1
encodes a homeobox transcription factor that is transiently expressed in the primitive
streak and the anterior mesendoderm of gastrulation-stage mouse embryos. We previously generated
Lhx1
-
deficient mice and
showed that
Lhx1
is absolutely required for anterior head formation and that chimeric embryos with reduced
Lhx1
activity display
holoprosencephalic-like phenotypes. Here to determine the short-term fate of
Lhx1
-
expressing cells in the anterior mesendoderm,
we used the perdurance of -galactosidase produced from the
Lhx1
lacZ
knock-in allele in which the
lacZ
gene is under the control
of
Lhx1
regulatory elements. We found that
Lhx1
-
expressing cells in the anterior endoderm contribute to the prechordal plate and
the rostral-most foregut endoderm. Molecular analysis of genes expressed in the prechordal plate and the rostral foregut
endoderm revealed that
Goosecoid
and
Dkk1
were not expressed in embryonic (E) 7.75
Lhx1
-/-
embryos and that the most
anterior domain of
FoxA2
expression in the midline was absent. Analysis of
lacZ
expression from the
Lhx1
locus in null embryos
revealed that X-gal staining was increased in the anterior mesendoderm suggesting that
Lhx1
directly or indirectly regulates its
own expression. Developmental analysis indicated that
Hhex
,
a marker for the newly emerging midline endoderm, was expressed
in a smaller domain in the anterior primitive streak region of E6.75
Lhx1
-/-
embryos. Chimera studies revealed that
lacZ
-
marked
Lhx1
-/-
cells were able to contribute to the prechordal plate and the foregut endoderm in the presence of wild-type cells.
Together these results suggest that
Lhx1
is necessary to correctly specify prechordal plate precursors in the anterior primitive
streak and that
Lhx1
acts in a cell non-autonomous manner. These results identify a gene pathway critical for prechordal plate
development that may aid in understanding the pathogenesis of holoprosencephaly in humans.
133
Conditional
Aurora A
deficiency differentially affects early mouse embryo patterning. Yeonsoo Yoon
1
,
Dale O. Cowley
2
,
Terry Van Dyke
3
,
Jaime A. Rivera-Pérez
1
. 1)
Department of Cell and Developmental Biology, University of Massachusetts
Medical School, Worcester, MA 01655, USA; 2) TransViragen, Inc., Research Triangle Park, NC 27709, USA; 3) Mouse Cancer
Genetics Program, National Cancer Institute, Frederick, MD 20892, USA.
Aurora A is a mitotic serine/threonine kinase, involved in centrosome maturation, spindle assembly and chromosome
segregation during the cell division cycle. Ablation of
Aurora A
in mice results in mitotic arrest and pre-implantation lethality,
preventing further investigation on the function of
Aurora A
at later stages of development. Here, we report the effects of
Aurora
A
ablation on embryo patterning at early post-implantation stages by tissue-specific ablation of
Aurora A
.
Conditional knockout
of
Aurora A
in the epiblast or visceral endoderm layers of the conceptus leads to apoptosis and embryo growth inhibition, causing
lethality and resorption at approximately E9.5. The effects on embryo patterning, however, depend on the tissue affected by the
mutation. Embryos with an epiblast ablation of
Aurora A
are able to properly establish the anteroposterior axis but do not proceed
through the gastrulation. In contrast, mutation of
Aurora A
in the visceral endoderm leads to posteriorization of the conceptus or
failure to elongate the anteroposterior axis. These results show that Aurora A is essential for proper embryo patterning in post-
implantation embryo and suggests that abnormal development of mutant embryos is linked to abnormal growth brought about by
a paucity of epiblast or visceral endoderm cells.