Chemical signalling is the primary means by which cells communicate in embryonic development. The underlying principle refers to a ligand-producing group of cells and a competent receiver group, which can respond to this signal because they express the specific receptor. The Wnt/Planar Cell Polarity (Wnt/PCP) signalling pathway controls tissue polarity and cell movement by activating several downstream cascades, including c-Jun N-terminal kinase (JNK) signalling. In the zebrafish embryo, the Wnt/PCP ligand Wnt5b binds to its bona fide receptor Ror2 to trigger the Wnt/PCP signalling cascade. However, it is still unclear how this lipophilic ligand is transported from a localised source through the aqueous extracellular space. This is essential because paracrine Wnt/PCP signalling restructures the embryonic tissue leading to convergence along one axis and extension along the perpendicular axis.
In this thesis, I show that Wnt5b, together with its cognate receptor Ror2, is loaded on signalling filopodia, better known as cytonemes. Wnt5b/Ror2 complex on producing cells can control the emergence of cytoneme and maintain the longer cytonemes. These cytonemes extend several tens of micrometres in the zebrafish gastrula. The Wnt5b/Ror2 complex is handed over from the cytoneme to the receiving cell. The transferred Wnt5b/Ror2 complex remains intact during transport and can trigger Wnt/PCP signalling in the receiving cells, regardless of whether the cell expresses functional receptors. On the tissue level, I further show that cytoneme-dependent spreading of active Wnt5b/Ror2 affects convergence and extension in the zebrafish gastrula. Therefore, I suggest that cytoneme-mediated transfer of ligand-receptor complexes is a vital mechanism for paracrine signalling in a tissue, even if the receiving cells are considered non-responsive by lacking a functional receptor. Thus my work challenges the long-standing concept of characterising responsive and non-responsive tissues based solely on the expression of the receptors.