Molecular mechanisms of cell-cell signaling by the Spemann-Mangold organizer - PubMed (original) (raw)

Review

Molecular mechanisms of cell-cell signaling by the Spemann-Mangold organizer

E M De Robertis et al. Int J Dev Biol. 2001.

Abstract

We review how studies on the first Spemann-Mangold organizer marker, the homeobox gene goosecoid, led to the discovery of secreted factors that pattern the vertebrate embryo. Microinjection of goosecoid mRNA formed secondary axes and recruited neighboring cells. These non-cell autonomous effects are mediated in part by the expression of secreted factors such as chordin, cerberus and Frzb-1. Unexpectedly, many of the molecules secreted by the Spemann-Mangold organizer turned out to be antagonists that bind growth factors in the extracellular space and prevent them from binding to their receptors. The case of chordin is reviewed in detail, for this molecule has provided biochemical insights into how patterning by Spemann's organizer can be regulated by diffusion and proteolytic control. The study of the BMP-binding repeats of Chordin, which are present in many extracellular proteins, may provide a new paradigm for how cell-cell signaling is regulated in the extracellular space not only in embryos, but also in adult tissues.

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Figures

Fig. 1. Organizer-specific genes that pattern the early Xenopus embryo

Schematic representation of a gastrula stage embryo showing the localization of the organizer in the dorsal marginal zone and its effect on patterning of all three germ layers, the endo-, meso- and ectoderm. The boxes show secreted and nuclear factors that are expressed in this region and that have been suggested to contribute to the function of the organizer.

Fig. 2. The organizer is a source of secreted antagonists that bind growth factors in the extracellular space

Three different types of extracellular modulators were been discovered in the organizer. All three have been shown to encode secreted inhibitors. Chordin, Noggin and Follistatin bind to BMPs and thereby inhibit them from activating their receptor. Frzb-1 and Dkk-1 antagonize Xwnt-8 in the extracellular space. Cerberus is a multivalent inhibitor of three different signals, BMPs, Xwnt-8 and the Xenopus Nodal-related mesoderm-inducing molecules (Xnr1, 2 and 4).

Fig. 3. Spemann-Mangold organizer secreted factors antagonize ventral signals provided by BMPs

BMPs are secreted by a wide region at the ventral side of the embryo and are antagonized by organizer secreted factors such as Chordin, Noggin and Follistatin (blue oval). These factors directly bind to BMPs in the extracellular space of ectoderm, mesoderm and endoderm and thereby pattern these three germ layers.

Fig. 4. Chordin mRNA induces secondary axes

(Upper panel) Ventral injection at the 8-cell stage lead to the formation of a twinned embryo, which contains eyes and cement gland. (Lower panel) A sagittal histological section of this embryo shows that it contains two notochords (ntc), neural tubes (ne) and gut (1° G and 2° G). Thus, injection of a single molecule can recapitulate the inductions mediated by organizer grafts.

Fig. 5. Model of BMP signal re-activation by Xolloid cleavage

BMP-4 binds to BMP receptors inducing the ventral pathway. Binding to Chordin blocks this signaling, whereas cleavage of Chordin by the Xolloid metalloprotease at two specific sites releases active BMP-4, re-establishing the ventralizing signal.

Fig. 6. xolloid mRNA ventralizes mesodermal pattern

Sagittal sections of tail bud stage embryos at the trunk level. The various dorso-ventral tissues are indicated in the lower panel. (A) Control. (B) Embryo after radial injection of xolloid mRNA at the four cell stage. Note the ventralization of the entire mesodermal layer in embryos expressing ectopic xolloid metalloprotease.

Fig. 7. Chordin is cleaved by Tolloid/xolloid/mTII-1 at two specific sites

Schematic drawing of Xenopus Chordin showing its signal peptide (gray box) and the BMP binding modules, the four cysteine-rich domains (CR1−4). The metalloprotease Tolloid/xolloid/mTII-1 cleaves the mature protein at two specific sites 29 amino acids downstream of CR1 and 16 amino acids downstream of the third CR repeat and thereby inactivates the protein. The recognition sequences for the protease and the relative positions of the cleavage sites are indicated. After experiments of Piccolo et al., 1997 and Scott et al., 1999.

Fig. 8. Hypothetical model showing that Chordin binds BMP with higher affinity than CR1

Chordin blocks BMP signaling efficiently (KD 3 × 10−10 M) probably because the presence of CR1 and CR3 provide a Chordin monomer with two high affinity sites for each BMP dimer. In contrast, the CR1 fragment produced by xolloid digestion (cleavage sites on Chordin are indicated by arrows) binds BMP-4 with a 10-fold lower affinity (KD 3 × 10−9 M) and is less efficient in blocking BMP signaling.

Fig. 9. CR domains present in procollagen IIA modulate BMP signaling. (A)

Sequence comparison of CR domains contained in different extracellular matrix proteins. Coll-CR, type IIA Xenopus procollagen; CR2, murine Chordin second repeat; Nel, rat nel; Pxdasin, Drosophila peroxidasin; C. eleg. EST, C. elegans hypothetical protein containing five procollagen-like domains (accession No. CAA94866). (B) Ventral injection of Xenopus procollagen IIA mRNA induces secondary axes. A construct encoding the splice variant Coll IIB lacking the CR domains is inactive in this assay (after Larraín et al., 2000). (C) Hypothetical model for the binding of BMP-4 to procollagen IIA triple helix. The cartoon shows how the presence of multiple CR domains in the procollagen triple helix could bind BMP-4 with high affinity, as is the case for Chordin. The question mark indicates the proposed protease that, like Tolloid, would release active BMPs from this extracellular reservoir of growth factors (see text) when required for tissue homeostasis.

Fig. 10. Chordin is required for dorsal-ventral patterning in zebrafish

The Chordino mutant phenotype is due to a loss-of-function of zebrafish chordin. In mutant embryos the tail is enlarged at the expense of head and anterior trunk. In situ analyses of mutant embryos demonstrated a reduction of the neural plate (marked by fkd3) and dorsal mesoderm (marked by shh), and an expansion of ventral mesoderm (marked by eve1) (Hammerschmidt et al., 1996). Therefore the antagonism between the Spemann-Mangold organizer secreted protein Chordin and BMP is required for the establishment of the dorsal-ventral polarity of ectoderm and mesoderm in zebrafish. Zebrafish photographs courtesy of Dr. Stefan Schulte-Merker (Tübingen).

Fig. 11. Loss of prosencephalon in chordin and noggin double mutants

(AB) [illegible] almost completely absent, whereas Krox-20 is expressed in the double mutant at the expected position. The white arrowhead in B marks the level of anterior-most expression of Shh. These double mutant mouse embryos lose the prosencephalon as well as the anterior notochord.

Fig. 12. Vertebrates and invertebrates share a common dorsal-ventral patterning system

Vertebrate xolloid blocks Chordin suppression of BMP-4 allowing ventralization (antineural); likewise in Drosophila, Tolloid blocks short-gastrulation suppression of Dpp allowing dorsalization (antineural) differentiation. This is a powerful argument in favor of the last common ancestor of protostomes and deuterostomes, the Urbilateria, having this dorso-ventral patterning system in place. During the course of evolution the dorsal axis has become inverted, as first proposed by French Zoologist Etienne Geoffroy Saint-Hilaire (1822).

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Molecular mechanisms of cell-cell signaling by the Spemann-Mangold organizer - PubMed (original) (raw)