Anteroposterior neural tissue specification by activin-induced mesoderm

Abstract

The transforming growth factor ␤ superfamily member, activin, is able to induce mesodermal tissues in animal cap explants from Xenopus laevis blastula stage embryos. Activin can act like a morphogen of the dorsoventral axis in that lower doses induce more ventral, and higher doses more dorsal, tissue types. Activin has also previously been reported to induce neural tissues in animal caps. From cell mixing experiments it was inferred that this might be an indirect effect of induced mesoderm signaling to uninduced ectoderm. Here we demonstrate directly that neural tissues do indeed arise by the action of induced mesoderm on uninduced ectoderm. Dorsal mesoderm is itself subdivided into posterior and anterior domains in vivo, but this had not been demonstrated for induced mesoderm. We therefore tested whether different concentrations of activin recreate these different anteroposterior properties as well. We show that the anteroposterior positional value of induced mesoderm, including its neuroinductive properties, depends on the dose of activin applied to the mesoderm, with lower doses inducing more posterior and higher doses giving more anterior markers. We discuss the implications of these results for patterning signals and the relationship between anteroposterior and dorsoventral axes.

Comparison of induction during development between Xenopus tropicalis and Xenopus laevis

“…Differences in the amounts and peaks of expression of other marker genes (chd, cer and Xbra) between the two species suggested that there are different mechanisms to define expression levels in a germ layer-(tissue or position) or genespecific manner between Xenopus laevis and Xenopus laevis. Our experiments also revealed that activin-treated animal cap explants of Xenopus tropicalis can differentiate into mesodermal and endodermal tissues in a dose-dependent manner, as reported previously for Xenopus laevis (Ariizumi et al, 1991, Green et al, 1997, Green et al, 1992. This result indicates that there are common mechanisms of tissue differentiation, which are conserved among species.…”

“…This study demonstrated that animal cap cells dissected from late blastulae of Xenopus tropicalis have the same competence in response to activin as Xenopus laevis animal caps studied previously (Ariizumi et al, 1991, Green et al, 1997, Green et al, 1992. Organizer and mesoderm marker genes were expressed in the same temporal and dose-dependent pattern.…”

“…There have been many developmental studies of morphogen gradients and axes patterning (Bourillot et al, 2002, Dubrulle et al, 2001, Dyson and Gurdon, 1998, Fujii et al, 2002, Gamse and Sive, 2000, Gamse and Sive, 2001, Green et al, 1997, Green et al, 1992, Gurdon et al, 1996, Gurdon et al, 1999, Ninomiya et al, 2004, Tabata and Takei, 2004. Morphogen gradients were thought to regulate the positioning of cells in a developing embryo, as well as controlling gene expression and tissue-specific differentiation.…”

“…Morphogen gradients were thought to regulate the positioning of cells in a developing embryo, as well as controlling gene expression and tissue-specific differentiation. Traditionally, activin A (human recombinant activin A) treatment of animal caps from amphibian embryos such as Xenopus laevis were used in this research (Ariizumi and Asashima, 1995, Ariizumi et al, 1991, Asashima et al, 2000b, Green et al, 1997, Green et al, 1992, Gurdon et al, 1996, Gurdon et al, 1999, McDowell et al, 1997, Ryan et al, 2000. At low concentrations of activin, ventral mesoderm tissues such as coelomic epidermis and blood cells were induced in the animal cap explants.…”

Induction of tooth and eye by transplantation of activin A-treated, undifferentiated presumptive ectodermal Xenopus cells into the abdomen

“…Green et al, demonstrated that the activin A-treated dissociated animal cap cells interact with untreated animal caps cells, resulting in induction of neural tissues (Green et al, 1997). We demonstrated that, when cells treated with activin A were mixed with untreated cells, the activin A-treated (1 ng/ml) cells concentrated in a central mass of the aggregates and they effectively formed notochord (Kuroda et al, 2002, Kuroda et al, 1999.…”