Dynamic Development

CONTENTS

Main Page Dynamic Development

The Foundations of Developmental Biology

Gametogenesis

From Sperm and Egg to Embryo

Genetic Regulation of Development

Organizing the Multicellular Embryo

Generating Cell Diversity


Dynamic Development at a Glance


Learning Resources

Research Resources

The Developmental Biology Journal Club

Developmental Biology Tutorial

Embryonic Induction during Vertebrate Development: Regionalization of the Mesoderm

What causes diversity in the middle kingdom?

 

The mesoderm is a complex array of tissue-types in three primary domains: dorsal, ventrolateral, and ventral. Let's briefly review the sources of the signals that generate the regional diversity of the mesoderm.

A general mesoderm-inducing signal from the vegetal hemisphere induces overlying marginal zone cells to become ventral mesoderm. Within the Nieuwkoop center, signals are produced that induce the dorsal marginal zone cells to become axial and head mesoderm. The dorsal marginal zone functions as Spemann's organizer. One of the properties of Spemann's organizer is to dorsalize the adjacent lateral mesoderm.

As we have discussed, the Nieuwkoop center is apparently established by activation of a Wnt signaling pathway, of which ß-catenin is a critical component. Recently, evidence has been presented that a consequence of activation of the Wnt pathway is activation of the transcription factor Siamois, which is located downstream of ß-catenin (Carnac et al., 1996). One implication of these results may be that the signaling properties of cells of the Nieuwkoop center depend upon regulation of gene expression by Siamois. This would imply that Nieuwkoop signaling is dependent upon zygotic gene transcription, whereas earlier events, including dorsalization and initiation of mesoderm induction, would be dependent upon maternal messengers. This is a new way of thinking about mesoderm induction. Time will tell whether this dichotomy holds.

It is likely given the large number of factors that are capable of inducing mesoderm that the regional character of the mesoderm is a result of the combinatorial action of the endogenous correlates of these factors. We still cannot state unequivocally what the roles of these individual factors are in generating mesoderm, but we should examine their potential involvement. The messenger RNAs for Vg-1, BMP-2 , BMP-4, FGF-2, FGF-4, Wnt-11, and noggin are present in the egg. Thus, they are present during the time when mesoderm induction is initiated. As we shall discuss, noggin is also produced by zygotic gene expression and may play different roles at different times.

Vg-1

We have discussed extensively the likely involvement of Vg-1 in mesoderm induction. Kessler and Melton (1995) proposed that the rotation of the cortex at fertilization triggers localized processing of Vg1 precursor, which imparts on those cells functionality as the Nieuwkoop center. Mature Vg1 ligand secreted from the Nieuwkoop center would induce the Spemann organizer. The Spemann organizer, in turn, would induce neural tissue and also dorsalize lateral mesoderm.

BMPs

The bone morphogenetic proteins (BMPs) are TGF-ß-related proteins. BMP-4 induces ventral mesoderm, suppresses induction of dorsal mesoderm by activin and inhibits dorsoanterior development of embryos, suggesting that it is a ventralizing factor. The messengers encoding the BMPs are not localized in the vegetal hemisphere, which casts some doubt on their role in mesoderm induction. However, expression of a dominant-negative BMP-4 receptor blocked mesoderm induction by BMP-4 in isolated animal caps and prevented ventralization of embryos by BMP-4. Overexpression of the receptor in embryos caused mesoderm to be dorsalized. These results suggest that BMP-4 may be involved in ventralization of the mesoderm by counteracting the dorsalizing signals (for review, see Kessler and Melton, 1994).

FGFs

The fibroblast growth factors induce ventrolateral mesoderm. Expression of dominant-negative FGF receptor inhibits mesoderm induction in animal cap explants and causes defects in trunk and posterior development, while not affecting anterior development (Fig. 1). This result suggests that FGF is necessary, but not sufficient, for induction of trunk and posterior mesoderm (for review, see Kessler and Melton, 1994).

Figure 1 Effect on Xenopus embryo of overexpression of FGF (top) and of ablation of FGF activity (bottom). Photo courtesy of Jonathan Slack.

Noggin

Noggin mRNA is oogenic, but - like FGF and BMP mRNA - it is not localized to the vegetal hemisphere. However, zygotic expression of noggin is interesting because it begins at the mid-blastula transition (MBT) in the organizer region and later in the dorsal mid-line of the archenteron roof. This puts it in a position to play an important role in either dorsalization and/or neural induction (Slack, 1994). It is noteworthy that when ventral marginal zone tissue is exposed to noggin protein, it induces muscle (Smith et al., 1993). Thus, noggin is a possible candidate for the third signal in the three signal model.

Wnts

The Wnts are a diverse group of proteins related to the transforming int proteins of mammals and wingless in Drosophila. They were first implicated in Xenopus development by the demonstration that mouse wnt-1 overexpression could cause induction of ectopic dorsal axial structures (McMahon and Moon, 1989). This stimulated a search for the Xenopus homologues and discovery of a number of Xwnt genes, including Xwnt-11, whose transcript is localized to the vegetal hemisphere.

Combinatorial effects of factors involved in mesoderm induction

Brachyury (Xbra) is the Xenopus homolog of a mouse mutation that causes absence of notochord and posterior mesoderm. It encodes a nuclear protein that is a putative transcription factor. Brachyury in Xenopus is initially transcribed generally in the prospective mesoderm and later becomes restricted to the notochord and posterior mesoderm. Overexpression studies have shown that it specifies ventro-posterior mesoderm differentiation.

As we have discussed, noggin is expressed in the organizer and can dorsalize ventral mesoderm. Xwnt-8 on the other hand, is expressed zygotically in all mesoderm except the organizer and can ventralize dorsal tissue.

Thus, all three of these factors are expressed concurrently in the mesoderm: Xbra is expressed throughout and is co-expressed dorsally with noggin and ventrally with Xwnt-8. What, then might be the consequences of their co-expression? Remember, the brachyury protein is nuclear, whereas the other two are ligands. Thus, Xbra may be involved in mediating signaling events, whereas the others are candidates for signals.

Cunliffe and Smith (1994) demonstrated the effects of injecting the Xbra messenger and the plasmids encoding the other two factors, either alone or in combination (Fig. 4). The plasmids had the genes encoding these factors under the control of a cytoskeletal actin promoter, which becomes activated at the MBT. This allowed them to investigate how these factors might interact after the MBT. Xbra by itself specifies ventral mesoderm, and noggin by itself specifies neural ectoderm. However, in combination, Xbra synergizes with noggin to specify notochord and muscle. On the other hand, Xbra could not synergize with Xwnt-8 to specify dorsal mesoderm. These results suggest that specification of dorsal mesoderm may depend upon synergism between Xbra and noggin.

The ability of noggin by itself to induce neural tissue makes it a strong candidate for an endogenous neural inducer.

Cunliffe and Smith have proposed a model to explain how these factors may be involved in pattern specification during Xenopus development (Fig. 6).

Food for thought

Makoto Asashima and his associates at the University of Tokyo have used the capacity for activin to induce animal caps to form mesoderm to make organs and tissues in vitro (Roush, 1997). They have taken advantage of the differential effects of different concentrations of activin. Low doses (50 ng/ml) produced notochord, 75 ng/ml produced a beating heart, whereas 100 ng/ml produced a liver. Combining activin with other factors, such as retinoic acid and insulin-like growth factors, yielded pronephros, rudimentary eyes and ears. These intriguing results suggest the possibility that regeneration of organs (perhaps even human organs) in vitro may be a realistic possibility.


Learning Objectives

  • What is the proposed role of Siamois in establishment of the Spemann organizer?
  • How is cortical rotation thought to affect Vg-1 function?
  • What are the BMPs, and what is their proposed role in regionalization of the mesoderm? What evidence supports that role?
  • What role are the FGFs proposed to play in regionalization of the mesoderm? What evidence supports that role?
  • What is unique about the sources of Noggin mRNA in the embryo?
  • What are the effects of exposure of ventral marginal zone tissue to noggin protein? What is the significance of this result?
  • What is X-bra and what is its pattern of expression in the embryo?
  • How are X-bra, noggin and Xwnt-8 proposed to interact to specify regional pattern on mesoderm? Support your statement with experimental evidence.
  • How might a gradient of activin be involved in specifying regional pattern on mesoderm?


Digging Deeper:

Links to Related Material

See also Patterning of the Mesoderm by Activin in Zygote.

Recent Literature

Graff, J.M. 1997. Embryonic patterning: to BMP or not to BMP, that is the question. Cell 89: 171-174.

Maeda, R., Kobayashi, A., Sekine, R., Lin, J.-J., Kung, H.-f. and Maéno, M. 1997. Xmsx-1 modifies mesodermal tissue pattern along dorsoventral axis in Xenopus laevis embryo. Development 124: 2553-2560.

Isaacs, H.V. 1997. New perspectives on the role of the fibroblast growth factor family in amphibian development. Cell. Mol. Life Sci. 53: 350-361.

LaBonne, C. and Whitman, M. 1997. Localization of map kinase activity in early Xenopus embryos: implications for endogenous FGF signaling. Develop. Biol. 183: 9-20.

Joseph, E.M. and Melton, D.A., Xnr4: A Xenopus nodal-related gene expressed in the Spemann organizer. Develop. Biol. 184: 367-372.

Pownall, M.E., Tucker, A.S., Slack, J.M.W. and Isaacs, H.V. 1996. eFGF, Xcad3 and Hox genes form a molecular pathway that establishes the anteroposterior axis in Xenopus. Development 122: 3881-3892.

Tada, M., O'Reilly, M.-A.J. and Smith, J.C. 1997. Analysis of competence and of Brachyury autoinduction by use of hormone-inducible Xbra. Development 124: 2225-2234.

Tonegawa, A., Funayama, N., Ueno, N. and Takahashi, Y. 1997. Mesodermal subdivision along the mediolateral axis in chicken controlled by different concentrations of BMP-4. Development 124: 1975-1984.


References

Carnac, G., Kodjabachian, L., Gurdon, J.B. and Lemaire, P. 1996. The homeobox gene Siamois is a target of the Wnt dorsalisation pathway and triggers organiser activity in the absence of mesoderm. Development 122: 3055-3065.

Cunliffe, V. and J.C. Smith. 1994. Specification of mesodermal pattern in Xenopus laevis by interactions between Brachyury, noggin and Xwnt-8. EMBO J. 13: 349-359.

Dale, L., G. Matthews, and A. Colman. 1993. Secretion and mesoderm inducing activity of the TGF-beta-related domain of Xenopus Vg1. EMBO J. 12: 4471-4480.

Kessler, D.S. and D. A. Melton. 1994. Vertebrate embryonic induction: mesodermal and neural patterning. Science 266: 596-604.

Kessler, D. S. and Melton, D. A. 1995. Induction of dorsal mesoderm by soluble, mature
Vg1 protein. Development 121: 2155-2164.

McMahon, A.P. and R.T. Moon. 1989. Ectopic expression of the proto-oncogene int-1 in Xenopus embryos leads to duplication of the embryonic axis. Cell 58: 1075-.1084.

Roush, W. 1997. A developmental biology summit in the high country. Science 277: 639-640.

Slack, J.M.W. 1994. Inducing factors in Xenopus early embryos. Current Biology 4: 116-126.

Smith, W.C., A.K. Knecht, M. Wu and R.M. Harland. 1993. Secreted noggin protein mimics the Spemann organizer in dorsalizing Xenopus mesoderm. Nature 361: 547-549.


Dynamic Development at a Glance
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Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Thursday, October 8, 1998