The Wnt code: Deciphering early Wnt interactions in Hydra
Wnt signaling is an evolutionary old and metazoan-specific pathway. We have discovered this pathway in cnidarians, gastrula-shaped organisms at the base of metazoan evolution and sister group to bilaterians. Cnidarians exhibit the complete repertoire of Wnt ligands essential for the formation of the blastopore-like signaling center of polyps. This “head organizer” guides pattern formation and stem cell differentiation along the animal’s oral-aboral body axis (“Wnt Code”), which is reminiscent to the gradient of Wnt/β-Catenin signaling regulating anteroposterior patterning in vertebrates.
In the previous funding period, we studied receptor-ligand interactions and functions of specific Wnt ligands during regeneration. By interfering with Wnt functions by loss-of-function and gain of function approaches, we found highly specific “morphants” for Wnt2, 3, -5, -7, -9/10a, -9/10b, and -16 during head regeneration. They indicate a different response of different Wnts to the regeneration stimulus within the Wnt cascade. When we studied the role of the injury signal on the activation of Wnt genes we found that MAPK signaling activates Wnt3 and Wnt9/10a (i.e., the earliest Wnt genes) in a specific and antagonistic manner. We also analyzed the function of the extracellular matrix for Wnt signaling. Biophysical data indicated that the stiffness of the extracellular matrix was dramatically lowered in Hydra wildtype polyps at the sides of head organizer formation and in animals with activated canonical Wnt signaling. This was the starting point of a collaboration of the Holstein and Tanaka labs on the basis of which we apply for this joint project.
Now, we want to study the hierarchy of Wnts (“Wnt code”) in the Hydra head organizer and their interplay with physical cues. (i) To understand the activation of the Wnt-network of the Hydra head organizer in steady state animals and during regeneration we will analyze the activation range of Wnts with an emphasis on antagonistic Wnts (Wnt3 and -9/10a; Wnt5 vs -8) (with U. Engel; Z02). We will combine these studies with an analysis of the transcriptomic and proteomic landscape of the emerging head organizer (with J. Krijgsveld; Z04). (ii) To understand how biophysical (mechanical) and biochemical (Wnt signaling) cues interact, we will study the symmetry break in regenerates and in reaggregates under conditions of de novo pattern formation. By mathematical modelling (with A. Marciniak-Czochra; B05) we test the putative feedback loop between morphogen patterning and tissue mechanics. Our main goal here is to generate a generalized model that integrates the molecular network underlying Hydra’s Wnt code with biophysical dynamics during pattern formation. This will be crucial for the analysis of more complex bilaterian models.
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