Expansion-restriction of Spemann's organizer by an ADMP self-regulatory switch to scale the BMP gradient

XIPOU 2, a member of the class III POU-domain family, is expressed initially at mid-blastula transition (MBT) and during gastrulation in the entire marginal zone mesoderm, including Spemann's Organizer (the Organizer). To identify potential targets of XIPOU 2, the interaction of XIPOU 2 with other genes co-expressed in the Organizer was examined by microinjecting XIPOU 2's mRNA into the lineage of cells that contributes to the Organizer, head mesenchyme and prechordal plate. XIPOU 2 suppresses the expression of a number of dorsal mesoderm-specific genes, including gsc, Xlim-1, Xotx2, noggin and chordin, but not Xnot. As a consequence of the suppression of dorsal mesoderm gene expression, bone morphogenetic factor-4 (Bmp-4), a potent inducer of ventral mesoderm, is activated in the Organizer. Gsc is a potential target of XIPOU 2. XIPOU 2 is capable of binding a class III POU protein binding site (CATTAAT) that is located within the gsc promoter, in the activin-inducible (distal) element. Furthermore, XIPOU 2 suppresses the activation of the gsc promoter by activin signaling. At the neurula and tailbud stages, dorsoanterior structures are affected: embryos displayed micropthalmia and the loss of the first branchial arch, as detected by the expression of pax-6, Xotx2 and en-2. By examining events downstream from the Wnt and chordin pathways, we determined that XIPOU 2, when overexpressed, acts specifically in the Organizer, downstream from GSK-3beta of the Wnt pathway and upstream from chordin. The interference in dorsalizing events caused by XIPOU 2 was rescued by chordin. Thus, in addition to its direct neuralizing ability, in a different context, XIPOU 2 has the potential to antagonize dorsalizing events in the Organizer.

Download Full-text

The origins of primitive blood in Xenopus: implications for axial patterning

Development ◽

10.1242/dev.126.3.423 ◽

1999 ◽

Vol 126 (3) ◽

pp. 423-434 ◽

Cited By ~ 4

Author(s):

M.C. Lane ◽

W.C. Smith

Keyword(s):

Xenopus Laevis ◽

Marginal Zone ◽

Cell Stage ◽

Axial Patterning ◽

Xenopus Embryos ◽

Spemann's Organizer ◽

Dorsal Mesoderm

The marginal zone in Xenopus laevis is proposed to be patterned with dorsal mesoderm situated near the upper blastoporal lip and ventral mesoderm near the lower blastoporal lip. We determined the origins of the ventralmost mesoderm, primitive blood, and show it arises from all vegetal blastomeres at the 32-cell stage, including blastomere C1, a progenitor of Spemann's organizer. This demonstrates that cells located at the upper blastoporal lip become ventral mesoderm, not solely dorsal mesoderm as previously believed. Reassessment of extant fate maps shows dorsal mesoderm and dorsal endoderm descend from the animal region of the marginal zone, whereas ventral mesoderm descends from the vegetal region of the marginal zone, and ventral endoderm descends from cells located vegetal of the bottle cells. Thus, the orientation of the dorsal-ventral axis of the mesoderm and endoderm is rotated 90(degrees) from its current portrayal in fate maps. This reassessment leads us to propose revisions in the nomenclature of the marginal zone and the orientation of the axes in pre-gastrula Xenopus embryos.

Download Full-text

A New Look at Spemann’s Organizer

The Molecular Biology of Cell Determination and Cell Differentiation ◽

10.1007/978-1-4615-6817-9_4 ◽

1988 ◽

pp. 127-150

Author(s):

Johannes F. Holtfreter

Keyword(s):

Spemann's Organizer ◽

New Look

Download Full-text

Xenopus axis formation: induction of goosecoid by injected Xwnt-8 and activin mRNAs

Development ◽

10.1242/dev.118.2.499 ◽

1993 ◽

Vol 118 (2) ◽

pp. 499-507 ◽

Cited By ~ 13

Author(s):

H. Steinbeisser ◽

E.M. De Robertis ◽

M. Ku ◽

D.S. Kessler ◽

D.A. Melton

Keyword(s):

Uv Irradiation ◽

High Frequency ◽

Marginal Zone ◽

Homeobox Gene ◽

Dorsal Side ◽

Axis Formation ◽

Gene Products ◽

Turn On ◽

Xenopus Embryos ◽

Spemann's Organizer

In this study, we compare the effects of three mRNAs-goosecoid, activin and Xwnt-8- that are able to induce partial or complete secondary axes when injected into Xenopus embryos. Xwnt-8 injection produces complete secondary axes including head structures whereas activin and goosecoid injection produce partial secondary axes at high frequency that lack head structures anterior to the auditory vesicle and often lack notochord. Xwnt-8 can activate goosecoid only in the deep marginal zone, i.e., in the region in which this organizer-specific homeobox gene is normally expressed on the dorsal side. Activin B mRNA, however, can turn on goosecoid in all regions of the embryo. We also tested the capacity of these gene products to restore axis formation in embryos in which the cortical rotation was blocked by UV irradiation. Whereas Xwnt-8 gives complete rescue of anterior structures, both goosecoid and activin give partial rescue. Rescued axes including hindbrain structures up to level of the auditory vesicle can be obtained at high frequency even in the absence of notochord structures. The possible functions of Wnt-like and activin-like signals and of the goosecoid homeobox gene, and their order of action in the formation of Spemann's organizer are discussed.

Download Full-text

XIPOU 2, a noggin-inducible gene, has direct neuralizing activity

Development ◽

10.1242/dev.121.3.721 ◽

1995 ◽

Vol 121 (3) ◽

pp. 721-730 ◽

Cited By ~ 12

Author(s):

S.E. Witta ◽

V.R. Agarwal ◽

S.M. Sato

Keyword(s):

Cell Fate ◽

Neural Induction ◽

Mesoderm Induction ◽

Class Iii ◽

Cell Stage ◽

Neuronal Phenotype ◽

Pou Domain ◽

Spemann's Organizer ◽

Stage Embryo ◽

Synthetic Mrna

XIPOU 2, a member of the class III POU domain family, is expressed initially in Spemann's organizer, and later, in discrete regions of the developing nervous system in Xenopus laevis. XIPOU 2 may act downstream from initial neural induction events, since it is activated by the neural inducer, noggin. To determine if XIPOU 2 participates in the early events of neurogenesis, synthetic mRNA was microinjected into specific blastomeres of the 32-cell stage embryo. Misexpression of XIPOU 2 in the epidermis causes a direct switch in cell fate from an epidermal to a neuronal phenotype. In the absence of mesoderm induction, XIPOU 2 has the ability to induce a neuronal phenotype in uncommitted ectoderm. These data demonstrate the potential of XIPOU 2 to act as a master regulator of neurogenesis.

Download Full-text