β-catenin plays a central role in setting up the head organizer in hydra

Abstract

In an adult hydra the head organizer, located in the hypostome, is constantly active in maintaining the structure of the animal in the context of its steady state tissue dynamics. Several Wnt genes, TCF, and elevated levels of beta-catenin are expressed in the hypostome as well as during the formation of a new organizer region in developing buds suggesting they play a role in the organizer. Transgenic hydra were generated in which a modified hydra beta-catenin gene driven by an actin promoter is continuously expressed at a high level throughout the animal. These animals formed heads and secondary axes in multiple locations along the body column. Transplantation experiments indicate they have a high and stable level of head organizer activity throughout the body columns. However, none of the Wnt genes are expressed in the body columns of these transgenic animals. Further, in alsterpaullone-treated animals, which results in a transient rise in head organizer activity throughout the body column, the time of expression of the Wnt genes is much shorter than the time of the elevated level of head inducing activity. These results for the first time provide direct functional evidence that beta-catenin plays a crucial role in the maintenance and activity of the head organizer and suggest that Wnt ligands may be required only for the initiation but not in maintenance of the organizer in Hydra.

Modeling pattern formation in hydra: a route to understanding essential steps in development

“…Remarkably, this overall tentacle formation does not occur in germlinetransformed animals with elevated b-catenin levels ( Fig. 6B; Gee et al, 2010). Presumably there is still a residual gradient in the competence that is sufficient for tentacle localization.…”

“…The long-ranging system is appropriate to integrate information over a larger part of the tissue to find the best place for activation, including that peak formation takes place preferentially at a marginal position. In contrast, the local peak allows, for instance, the formation of Gierer and Meinhardt, 1972 (Gee et al, 2010;see also Fig. 7E Gee et al, 2010, Dev.…”

“…A loop of Wnt3 and Brachyury is a possible (Meinhardt, 2012 (Gee et al, 2010). A Wnt-Brachyury loop is also known to be involved in posterior elongation in higher organisms (Martin and Kimelman, 2009).…”

“…After application of Alsterpaullone, a drug that stabilizes b-catenin (Bain et al, 2003), all cells obtain a high competence (Gee et al, 2010). After removal of Alsterpaullone, the tissue maintains a high competence for several days as examined by transplantation experiments while b-catenin returns much faster to normal.…”

“…Essential for their axial organization is the Wnt pathway Broun et al, 2005;Kusserow et al, 2005;Bosch 2007; Lengfeld et al, 2009;Plickert et al, 2006;Gee et al, 2010;Duffy et al, 2010). A comparison of the expression patterns of Wnt, Brachyury and several other genes in hydra and higher organisms suggests that the head of the hydra, in contrast to its usual naming, represents the most posterior and the foot the most anterior structure (Meinhardt, 2002).…”