Stem cells

Hydra is composed of three independent cell lineages, all of which contain large pools of either epithelial or small, set aside adult stem cells. They represent ancestral forms of prototypic stem cells in multicellular animals. Of particular interest is the interstitial cell lineage located in the intercellular spaces in the ectodermal tissue layer. Interstitial stem cells give rise to somatic cell types such as neurons, stinging cells and gland cells, as well as to both types of gametes. They exhibit a stochastic mode of decision making, whose molecular control mechanisms are unknown. We are interested in characterizing regulatory factors in order to gain a molecular understanding of the homeostatic balance of cell types during growth, regeneration, and sex-induced ageing. Our recent work has focused on Myc stem cell maintenance factors, their structural and functional conservation, and their biochemical properties in comparison to mammalian Myc proteins. Myc1 and Myc2 seem to act on interstitial stem cell self renewal, Myc3 seems to be involved in neurogenesis. In addition, we study the role of Wnt/beta-Catenin signaling in interstitial stem cell decision making. The accompanying scheme illustrates our current working hypothesis, which we intend to test in detail by functional interference experiments.

 

Regeneration

Hydra is one of the classic models for regeneration research exhibiting an unparalleled capacity to rebuild lost body parts due to the phenotypic and molecular plasticity of the remaining tissue. As an extreme case of regeneration, suspensions of dissociated cells can re-aggregate and self-organize into intact polyps within days. We were able to show that activation of the Wnt/beta-Catenin pathway is a hallmark of regeneration in Hydra, and this pathway seems to be required in regeneration responses throughout the animal kingdom. The action of Wnt/beta-Catenin signaling is complex, starts right after regeneration, and is poorly understood. There are proposed functions during initial wound healing phase, during the following molecular patterning phase, as well as in the final differentiation of the newly forming structures. We investigate the role of canonical and non-canonical Wnt signaling pathways during Hydra regeneration and re-aggregation at cellular and molecular levels.

 

Axial patterning

Under conditions of regular, asexual growth, adult Hydra polyps exhibit astonishing features usually found in animal embryos. All tissues are in a state of continuous replacement and movement. New polyps constantly develop by lateral budding. Experimentally, polyps can also form de novo by re-aggregation of cell suspensions. We are interested to understand the cellular mechanisms involved in creating the 3-dimensional shape of polyp bodies and the underlying molecular patterning system. Positional information is under control of an oral head organizer and a second signaling center located in the aboral foot. The head organizer is the major inductive signaling center and the equivalent of blastoporal organizers of higher animal gastrula stages, Wnt/beta-Catenin signaling has been demonstrated to represent the key element in the head organizer with axis inducing capacity upon transplantation to ectopic positions. Wnt/beta-Catenin signaling also activates non-canonical Wnt signaling at specific positions in the head and the lower body column, which then causes tentacle and bud evagination, respectively. Furthermore, Wnt-interacting, secreted proteins of the sFRP family are activated in the aboral signaling center. Currently, we try to understand in more detail the interactions between the two oral and aboral signaling centers and their combined action in setting up axial positional values in newly forming buds, aggregates and regenerating polyps.

 

Members of this work group are:

Bert Hobmayer (head)

Lucy Neumann (technical support)

Eilidh Charles (master student)

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