Patterning of the Mesoderm by Activin

The patterning of the mesoderm may be accomplished by the TGF-beta-family member, activin. Activin proteins appears to be present maternally in the early Xenopus embryo, but their localization and concentrations are not well known. In the early 1990s, activin was seen as possibly playing a major role in patterning the mesoderm. However, support for this model dwindled when activin was not detected in the early embryo at levels significantly high enough to induce mesodermal genes. Recently, activin has come back into prominence because the low levels of activin may have been an isolation artifact. It appears that Xenopus oocytes do not make activin, but absorb it from the blood by pinocytosis. This activin is probably in an inactive complex, bound with both its inhibitor follistatin and with the yolk protein vitellogenin (Fukui et al., 1994; Uchiyama et al., 1994).

The presence of activin in the early embryo is extremely important, since activin is known to induce different genes and different structures at different doses. When added to animal cap blastomeres (which would normally become ectoderm), low levels of activin induce these cells to become coelomic epithelium and blood cells. Higher doses of activin induce the animal cap to become muscle tissue, and still higher levels of activin induces the expression of the Brachyury and goosecoid genes and causes the tissues to become notochord (Figures 1,2. Asashima et al., 1990; Sokol et al, 1990; Ariizumi et al., 1991; Green and Smith, 1992; Green et al., 1992). At even higher concentrations of activin, the animal cap can form heart and endodermal tissue (Ariizumi and Asashima, 1995).

Figure 1
Figure 1   Dissociated animal cap cells express different genes when exposed to different concentrations of activin. At low concentrations, they become epidermal cells (expressing keratin). However, after a sharp threshold, they begin expressing genes that characterize the posterior and lateral plate mesoderm (XlHbox6, Xhox3, Brachyury). If exposed to higher concentrations of activin, the cells become vacuolated and express notochord markers such as Mz15 and low amounts of goosecoid. The Brachyury gene is expressed in two concentrations: in low concentrations in posterior and lateral mesoderm and high concentrations in the notochord. (After Green et al., 1992.)

Figure 2
Figure 2   Not only does gene expression change in the activin A-induced animal caps, so do the structures. (After Asashima, 1994.)

Adding other factors to activin also produces interesting results. The addition of retinoic acid to the concentration of activin that usually induces the formation of notochord will cause the formation of kidney tubules (see Figure 2). When insulin-like growth factor is added to the concentration of activin that can produce muscles, ear and eye vesicles form.

The long-range gradient effects of activin have also been demonstrated by another assay. Gurdon and coworkers (1994, 1995) have placed activin-containing beads within sandwiches of competent animal cap ectoderm. After two hours, Xenopus Brachyury (Xbra) expression can be seen a few cell diameters from the beads. After four hours, no Xbra expression is seen near the beads, but a rim of Xbra expression is now seen near the margins of the explant. Also at 4 hr, goosecoid (Xgsc) message is now seen near the explant (Figure 3). By transplanting cells with an inactive activin receptor in between the beads and the region where Xbra is expressed in four hours, Gurdon and colleagues showed that the expression of Xbra is due to the passive diffusion of activin and not to any activin-dependent signal mediated by the intermediate cells. Thus, the expression of Xbra and Xgsc is spatially related to the source of the gradient. The material closest to the source (presumably having a higher concentration of activin) induces the Xgsc gene (which is typical of notochord) , while regions further away from the activin source (presumably seeing a lower activin concentration) activate the Xbra gene (characteristic of many mesodermal tissues). Moreover, cells that used to express Xbra can be induced to express Xgsc when higher amounts of activin become available.

Figure 3
Figure 3   Diagramatic representation of results showing that cells can switch their response to changing concentrations of activin. "Weak beads" incubated in 1.5 nM activin were implanted into animal cap "sandwiches" from blastulae. Two hours later some were fixed and stained for Xbra expression. Others, however, had their "weak beads removed and "strong beads" reimplanted into them. After two hours they were stained for either Xbra or Xgsc. (data from Gurdon et al., 1995.)

This demonstrates that activin can act in a gradient fashion to pattern the mesoderm. Ariizumi and Asashima (1995) have also documented, in a similar assay, that the time spent exposed to activin can have a similar effect. Thus, activin concentration differentials can be established by either distance from the source of activin or by time spent under the influence of activin. In either of these two manners, different activin concentrations may possibly help pattern the mesoderm.

While we still remain uncertain concerning the concentration and localization of activin the oocyte, it is probable that activin or some activin-like molecule is involved in patterning the mesoderm.

Literature Cited

Ariizumi, T. and Asashima, M. 1995. Control of the embryonic body plan by activin during amphibian development. Zool. Sci. 12: 509-521.

Ariizumi, T., Moriya, N., Uchiyama, H., and Asashima, M. 1991. Concentration-dependent inducing activity of activin A. Wilhelm Roux's Arch. Devel. Biol. 200: 230-233.

Asashima, M. 1994. Mesoderm induction during early amphibian development. Develop. Growth Differ. 36: 343-355.

Asashima, M., Nikano, H., Shimada, K., Kinoshita, K., Ishii, K., Shibai, H., and Veno, N. 1990. Mesodermal induction in early Xenopus embryos by acttivin A (erythroid differentiation factor). Wilhelm Roux Arch. Devel. Biol. 198: 330-335.

Fukui, A., Nakamura, T., Uchiyama, H., Sugino, K., Sugino, H., and Asashima, M. 1994. Identification of activins A, AB, and B and follistatin proteins in Xenopus oocytes. Devel. Biol. 163: 279-281.

Green, J. B. A. and Smith, J. C. 1992. Graded changes in dose of a Xenopus activin A homologue elicit stepwise transitions in embryonic cell fate. Nature 347: 391-394.

Green, J. B. A., New, H. V., and Smith, J. C. 1992. Responses of embryonic Xenopus cells to activin and FGF are separated by multiple dose thresholds and correspond to distinct axes in the mesoderm. Cell 71: 731-739.

Gurdon, J. B., Mitchell, A., and Mahony, D. 1994. Direct and continuous assessment by cells of their position in a morphogen gradient. Nature 376: 520-521.

Gurdon, J. B., Harger, P., Mitchell, A., and Lemaire, P. 1994. Activin signalling and response to a morphogen gradient. Nature 371: 487-492.

Sokol, S., Wong, G., and Melton, D. A. 1990. A mouse macrophage factor induces head structures and organizes body axis in Xenopus. Science 249: 561-564.

Uchiyama, H. Nakamura, T., Komazaki, S., Takio, K., Asashima, M., and Sugino, H. 1994. Localization of activin and follistatin proteins in the Xenopus oocyte. Biochem. Biophys.Res. Commun. 202: 484-489.

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