Abstract:
The Spemann organizer in amphibians gives rise to the anterior mesendoderm
(AME) and is capable of inducing neural tissues. This inductive activity is thought to
occur largely via the antagonism of Bone Morphogenetic Protein (BMP) signaling in the
organizer. In the mouse, BMP antagonists Chordin and Noggin function redundantly in
the AME and are required during forebrain maintenance. However, the timing of
forebrain initiation and the function of BMP antagonism in forebrain initiation remained
unclear prior to this study. In addition, the Transforming Growth Factor β (TGFβ) ligand
Nodal patterns the forebrain via its function in the anterior primitive streak (APS), the
precursor tissue of the AME. Whether BMP and Nodal signaling pathways interact has
not been previously investigated.
The goal of this dissertation was to investigate the cellular and molecular
mechanisms involved in early mammalian forebrain establishment by embryonic and
genetic manipulations. This study determined that forebrain initiation occurs during
early gastrulation and requires signals from the AVE and AME. The AVE was identified
as a source of active BMP antagonism in vivo, and the BMP antagonism supplied by
exogenous tissues was capable to promote forebrain initiation and maintenance in the
murine ectoderm. It is likely that BMP antagonism enhances forebrain gene expression
via inhibiting posteriorization. This study further identified a possible crosstalk between
BMP and Nodal signaling. Loss of Chordin or Noggin in combination with heterozygosity
for Nodal or Smad3 results in holoprosencephaly. Molecular analyses suggest that the
BMP-Nodal interaction occurs in the APS and/or the AME. Failure of this interaction
results in an imbalance of BMP and Nodal signal levels that devastate APS and AME
patterning during early forebrain establishment, ultimately leading to holoprosencephaly
in mid-gestation. This interaction is likely to occur extracellularly, possibly by formation
of a BMP-Nodal heteromeric complex. Furthermore, the spatiotemporal expression of
phospho-Smad1/5/8, an effector of BMP signaling pathway, was characterized during
early mouse embryogenesis. Distribution of phospho-Smad1/5/8 serves as a faithful
readout of BMP signaling activity and helps to better understand how BMPs are
involved in patterning early embryos. The implication of phospho-Smad1/5/8
expression in both wildtype and mutant embryos is also discussed.
