Specification of mesodermal pattern in Xenopus laevis by interactions between Brachyury, noggin and Xwnt-8
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1Laboratory of Developmental Biology, National Institute for Medical Research, Ridgeway, Mill Hill, London, UK.
Abstract:
We demonstrate that the nuclear, sequence-specific DNA-binding protein Xbra causes dorsal mesodermal differentiation of animal cap ectoderm when co-expressed with the secreted proteins noggin and Xwnt-8. None of these molecules causes dorsal mesoderm formation when expressed alone. Co-expression of Xenopus Brachyury (Xbra) mRNA with noggin mRNA in animal caps specifies the main dorsal tissues, namely muscle, notochord and neural tissue. Co-expression of Xbra with Xwnt-8, in contrast, converts animal caps to muscle masses. We have previously shown that expression of Xbra alone in animal caps is sufficient to specify ventral mesoderm which expresses Xhox3 and low levels of muscle-specific actin. We now conclude that the putative transcription factor Xbra defines a cell state in the vertebrate embryo which can respond to diffusible dorsal signals such as noggin and Xwnt-8, resulting in dorsal mesodermal differentiation. In the absence of such dorsal signals Xbra causes ventral mesodermal differentiation. This state is only partially maintained after the mid-blastula transition as it permits the dorsal response to zygotically expressed noggin but it does not allow a dorsal response to zygotically expressed Xwnt-8, which elicits only ventral mesodermal differentiation.
Insights
The transcription factor Xbra induces dorsal mesodermal differentiation in Xenopus embryos when combined with noggin or Xwnt-8 signals. Without these signals, Xbra alone specifies ventral mesoderm.
Area of Science:
- Developmental Biology
- Molecular Biology
- Embryology
Background:
- The Xenopus Brachyury (Xbra) protein is a nuclear, sequence-specific DNA-binding protein.
- Understanding the role of Xbra in mesodermal differentiation is crucial for comprehending vertebrate embryonic development.
Purpose of the Study:
- To investigate the role of Xbra in dorsal mesodermal differentiation.
- To determine how Xbra interacts with signaling molecules like noggin and Xwnt-8 to specify cell fates.
Main Methods:
- Co-expression of Xenopus Brachyury (Xbra) mRNA with noggin or Xwnt-8 mRNA in animal caps.
- Analysis of resulting tissue types, including muscle, notochord, and neural tissue.
- Assessment of Xbra's role in ventral mesodermal specification.
Main Results:
- Co-expression of Xbra with noggin specifies dorsal tissues (muscle, notochord, neural).
- Co-expression of Xbra with Xwnt-8 results in muscle masses.
- Xbra alone specifies ventral mesoderm, expressing Xhox3 and muscle-specific actin.
- Xbra defines a cell state responsive to dorsal signals for differentiation.
Conclusions:
- Xbra acts as a key transcription factor defining a cell state in vertebrate embryos.
- The response of Xbra-expressing cells to dorsal signals (noggin, Xwnt-8) dictates dorsal mesodermal differentiation.
- In the absence of dorsal signals, Xbra promotes ventral mesodermal differentiation.