Jochen Rink - Scale and proportion during planarian regeneration

Past and present research

Our lab studies the astonishing regenerative abilities of planarian flatworms. Despite their complex internal anatomy (Fig A), these animals can regenerate completely even from tiny amputation fragments (Fig B). We are interested in the molecular mechanisms that orchestrate the transformation of such random tissue pieces back into the specific shape and proportions of the planarian body plan. Specifically, using the ability to initiate regeneration from precisely defined fragment geometries, we hope to eventually understand how developing tissues specify and measure axial dimensions. The underlying mechanisms remain largely unknown, despite their central importance in such important problems as organ size determination or the fascinating precision by which embryos develop into their species specific shape, size and proportions.

Our first goal was to identify the signaling pathways that shape the planarian body plan in the first place. To this end, we screened a gene library comprising planarian homologues of patterning molecules for regeneration defects in RNA-interference (RNAi) screens. So far, this approach has revealed some of the molecular circuitry by which tissue fragments decide whether to regenerate a head or a tail. The Wnt/βCatenin signaling pathway turned out to be crucial in this respect. Whenever bCatenin activity was reduced, amputation fragments regenerated heads in place of tails (Fig C). Whenever βCatenin activity was increased, animals regenerate tails instead of heads (Fig D). Since even non-regenerating animals treated with βCatenin(RNAi) started to sprout ectopic heads all along their body margins (Fig E), the tight control of βCatenin activity is of obvious importance to the animals. Searching for the upstream control mechanisms, we identified a positive feedback loop, whereby the Hedgehog (Hh) signaling pathway activates expression of the Wnt ligand(s) that in turn activate bCatenin at sites of tail formation. However, these results are not the last word in planarian anterior-posterior (A/P) specification, as we still do not understand what sets off the Hh/Wnt/βCatenin cascade at sites of tail formation. Apart from the A/P-axis, we and others have identified other signaling pathways that participate in the patterning of the planarian dorso-ventral (D/V) axis and the medio-lateral (M/L) axis. Together, these results allow a first glimpse at the molecular coordinate system that specifies and maintains the shape and proportions of the planarian body plan.

Experimentally, the components of the coordinate system now provide entry points into the scores of fascinating questions raised by the transformation of a tissue fragment back into a recognizable animal. For example, which cells are sending/responding to the polarity signals and how can missing signaling cell populations reemerge in amputation fragments? How do axial polarity signals interface with stem cell proliferation and differentiation in order to build and shape missing body parts? What is the design logic of the system, allowing for reconstruction of the whole irrespective of the starting point? And finally, how is the length of polarity axes determined and how is the coordinated scaling of multiple axes accomplished?

A: Examples of planarian organ systems. B: A planarium cut into 16 fragments (top), each of which regenerates back into a perfectly proportioned worm over the course of 14 days (bottom). C: Movie of a centre fragment regenerated under reduced βCatenin function (βCatenin(RNAi)). D: Movie of a centre fragment regenerated under increased βCatenin function (APC(RNAi)). E: Uncut βCatenin(RNAi) animal sprouting ectopic heads.

Future prospects and goals

Using genomic tools to obtain a systems perspective on polarity-dependent regeneration responses.
Investigating axial patterning in the cell biological landscape of the regeneration blastema.
Developing planarian stem cell cultures for in vitro manipulation and characterization

Selected publications