Segregating and Patterning Mesoderm from Endoderm: Emerging Roles for Hedgehog and FoxA
Abstract
One of the fundamental questions in developmental biology is how cells communicate
during embryonic development to pattern the animal with defined axes and correctly
placed organs. There are several key signal transduction pathways whose signaling
has been found to be crucial during this period in the life history of many model
organisms and whose functions have been well conserved between species. Two of those
are the Notch and Hedgehog signal transduction pathways. Previous work established
that the Notch pathway is important in the specification of mesoderm in the sea urchin
embryo. Here it is established that the Hedgehog pathway is important for mesoderm
patterning in the echinoderm embryo.In many animals, including the sea urchin, endomesoderm
is specified as a bipotential tissue which is then subdivided through cell signaling
to become endoderm and mesoderm. Notch signaling was found to be critical for that
dichotomy; endomesoderm that received the Notch signal becomes mesoderm, the remaining
endomesoderm becomes endoderm. Prior to this work, no functional roles for Hedgehog
signaling in the sea urchin had been defined, though this pathway is known to operate
in organisms throughout the animal kingdom. Here we find through analysis and comparison
of the sea urchin genome with cnidarians, arthropods, urochordates, and vertebrates
that key components and modifiers of the Notch and Hedgehog signaling pathways are
well conserved among metazoans. Many animals contain the full suite of genes that
constitute both pathways, and in deuterostomes the pathways operate in embryos to
mediate similar fate decisions. The Notch pathway, for example, is engaged in endomesoderm
gene regulatory networks and in neural functions. In the sea urchin RNA in situ hybridization
of Notch pathway members confirms that Notch functions sequentially in the vegetal-most
secondary mesenchyme cells and later in the endoderm.The Hh signaling pathway is essential
for patterning of many structures in vertebrates ranging from the nervous system,
chordamesoderm, and limb to endodermal organs. In the sea urchin, a basal deuterostome,
we show that Hedgehog (Hh) signaling participates in organizing the mesoderm. During
gastrulation expression of the Hh ligand is localized to the endoderm while the co-receptors
Patched (Ptc) and Smoothened (Smo) are expressed in the neighboring secondary mesoderm
and in the ventrolaterally clustered primary mesenchyme cells where skeletogenesis
initiates. Perturbations of Hh signaling cause embryos to develop with skeletal defects,
as well as inappropriate secondary mesoderm patterning, although initial specification
of secondary mesoderm occurs normally. Perturbations of Hedgehog pathway members altered
normal numbers of pigment and blastocoelar cells, randomized left-right signaling
in coelomic pouches, and resulted in disorganization of the circumesophageal muscle,
causing an inability to perform peristaltic movements. Together our data support the
requirement of Hh signaling in patterning each of the mesoderm subtypes in the sea
urchin embryo.Activation of the Hedgehog pathway requires FoxA acting upstream of
Hedgehog transcription, early in gastrulation. When FoxA is knocked-down there is
a loss of transcription of Hedgehog and Hh expression is expanded in embryos expressing
ectopic FoxA. In collaboration with another lab, we found that FoxA acts to repress
mesodermal genes within the endoderm as part of the endomesoderm dichotomy. If FoxA
expression is reduced by a morpholino, more endomesoderm cells become pigment and
other mesenchymal cell types, and less gut is specified. Conversely, when FoxA is
ectopically expressed, endoderm is increased at the expense of mesoderm. More specifically
we found through mosaic analysis that FoxA acts in a portion of the endomesoderm derived
from one of two tiers of vegetal cells at the 60 cell stage called the veg2 cells.
FoxA remains on in all endoderm and its territory of expression is superimposeable
with the location of Hh expression.The data we present here together with previous
studies suggest a model in which Notch signaling cues cells of the endomesoderm to
become mesoderm, while cells of the nascent endoderm upregulate FoxA. FoxA ensures
proper partitioning of endoderm from mesoderm by repressing mesoderm genes, as well
as positively regulating transcription of Hedgehog in the endoderm. The Ptc and Smo
transducing apparatus is separately expressed in mesoderm. Hh then signals to its
receptors in the mesoderm to convey patterning information of tissues derived from
that mesoderm. Thus, Hh, Ptc and Smo molecules diverge during specification then
converge during signaling to play important roles in mesoderm development in the sea
urchin.
Type
DissertationDepartment
BiologyPermalink
https://hdl.handle.net/10161/451Citation
Walton, Katherine Dempsey (2007). Segregating and Patterning Mesoderm from Endoderm: Emerging Roles for Hedgehog and
FoxA. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/451.Collections
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