''Xbra'' is a homologue of

Brachyury T-box transcription factor T, also known as Brachyury protein, is encoded for in humans and other apes by the ''TBXT'' gene. Brachyury functions as a transcription factor within the T-box, T-box family of genes. Brachyury Sequence homology, homo ...

(T) gene for ''

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos'' = strange, πους, ''pous'' = foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described with ...

''. It is a transcription activator involved in vertebrate

gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals, the blastocyst, is reorganized into a two-layered or three-layered embryo known as ...

which controls posterior

mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical ...

patterning and

notochord The notochord is an elastic, rod-like structure found in chordates. In vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the verteb ...

differentiation by activating transcription of genes expressed throughout mesoderm. The effects of ''Xbra'' is concentration dependent where concentration gradient controls the development of specific types of mesoderm in ''Xenopus''. Xbra results of the expression of the FGF transcription factor, synthesized by the ventral

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastr ...

. So while ventral mesoderm is characterized by a high concentration of FGF and Xbra, the dorsal mesoderm is characterized by a reunion of two others transcription factors, Siamois and XnR, which activates the synthesis of Goosecoid Transcription Factor. Goosecoid enables the depletion of Xbra. In a nutshell, high concentrations of ''Xbra'' induce ventral mesoderm while low concentration stimulates the formation of a back. Posterior mesoderm development presents two types of cell behaviors,

cell migration Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryogenesis, embryonic development, wound healing and immune system, immune responses all require the orchestrated movemen ...

and

convergent extension Convergent extension (CE), sometimes called convergence and extension (C&E), is the process by which the tissue of an embryo is restructured to converge (narrow) along one axis and extend (elongate) along a perpendicular axis by cellular movement. ...

, in prechordal mesoderm and

chordamesoderm Axial mesoderm, or chordamesoderm, is the mesoderm in the embryo that lies along the central axis under the neural tube. * will give rise to notochord * starts as the notochordal process, whose formation finishes at day 20 in humans. * important ...

cells, respectively. Cell migration is exhibited by the prechordal mesoderm cells, resulting in the formation of the future anterior end. ''Xbra'' induces convergent extension which inhibits cell migration and rearranges the chordamesoderm cells into a structure that will later differentiate into notochord. As a result, ''Xbra'' acts as a switch to convert between these two behaviors. ''Xbra'' is able to activate itself indirectly, specifically for dorsal mesoderm, through FGF signaling while eFGF maintains ''Xbra'' expression, creating an autoregulatory loop. Inhibition of ''Xbra'' leads to abnormal patterning of mesoderm, such as shortened trunk. In a previous study, the activation domain of ''Xbra'' was replaced by repressor domain of Drosophila engrailed protein in order to form a dominant-interfering ''Xbra'' construct that would help to study the function and regulation of ''Xbra''. The injection of

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

encoding this construct has led to various birth defects such as defective

blastopore Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals, the blastocyst, is reorganized into a two-layered or three-layered embryo known as ...

closure and abnormal notochord differentiation in the developing embryo.

References

{{Reflist Transcription factors Xenopus