Challenging cellular competence: Spreading of active ligand-receptor complexes by cytonemes

During the development of all organisms consisting of more than one cell, effective cellular communication is vital to allow development from a single fertilised egg cell to a complex adult body. Cell-to-cell communication is fundamental to establishing cell-type diversity, forming tissues and organs, and finally, the entire organism. One of the most basic principles of our current understanding of cell signalling is that signals are produced by one group of cells and received by another group. The producer cells generate a signal that is detected by the receiver cells using a specific receptor protein. The detection of this signal then leads to activation of a signalling network in the receiver cells causing a change in behaviour.

Wnt proteins are one class of these essential signalling molecules that orchestrate cell behaviour. Wnts regulate many critical cellular processes and are therefore fundamental to the development of early life. A vital signalling pathway within the Wnt signalling network is Wnt/PCP. Wnt/PCP coordinates the migration of cells and tissues by regulating cell speed, directionality, and adhesion. Wnt/PCP can also dictate the polarity of cells within a cell layer to control the position and orientation of cell extensions such as cilia, bristles and hairs. Therefore, Wnt/PCP signalling is fundamental to embryogenesis, and organ development. However, there are still significant gaps in our understanding of how the Wnt/PCP pathway functions.

In preparation for this proposal, we have revealed the existence of an unexpected cell-to-cell transport mechanism for Wnt signals. Specific finger-like cell membrane protrusions - also known as cytonemes - carry Wnt proteins and transport them to neighbouring cells. Disturbance in the formation of Wnt cytonemes leads to severe consequences during development such as malformation of tissues and severe embryogenic abnormalities. Therefore, understanding the system that governs cytonemes is fundamental for understanding how Wnt functions during embryogenesis and adult tissue self-regulation. This unique knowledge will provide the foundations to manipulate Wnt protein transport to control the activity of Wnt signalling cascades in regeneration and diseases.

Our proposal will address two key gaps in our understanding of Wnt/PCP signalling: (1) How Wnt/PCP signals are delivered from one cell to another and (2) how is the delivered message translated into a cellular response.

Based on preliminary work leading up to this proposal, we hypothesise that Wnt/PCP signals bind to specific receptors within the producer cells, which activates the formation of Wnt cytonemes. We further hypothesise that the active Wnt/PCP ligand-receptor complexes are transported along these cytonemes and are transferred to the receiver cell to regulate signalling. Understanding this mechanism is vitally important because it means that cells can respond to Wnt/PCP signals even if they do not express the relevant receptors. This new knowledge has the potential to fundamentally change our understanding of signal producers and signal receivers and will allow us to develop an advanced concept of signalling during embryogenesis.

The Wnt signalling network is fundamental to the development and tissue homeostasis in all multicellular organisms. Wnt proteins are produced in a source cell and act on the neighbouring cells to guide their behaviour. Despite four decades of Wnt research, it is currently unknown how these signalling proteins traffic between cells to activate the signalling pathway in the target cells to control tissue behaviour and organ formation. However, this knowledge is fundamental to control the Wnt network effectively during development, regeneration and disease.

Our work has revealed the existence of an unexpected transport mechanism for Wnt signals in vertebrates, which changes the way we understand signal spreading in tissues. Wnt and their receptors are rapidly loaded on thin cellular extensions in the source cells and are then transferred to target cells for signal activation. This means that receiving cells may be able to respond to signalling events even if they do not express the relevant receptor. In this project, we aim (1) to understand how the active Wnt/PCP signalling complex is transported, (2) how many ligands and receptors compromise an active complex, and (3) how these transferred complexes regulate PCP signalling, i.e. polarity and migration in the embryo. To address how this signalling system operates in an intact tissue, we will use a set of state-of-the-art microscopy and spectroscopy techniques in the living zebrafish embryo to map the localisation, determine the local activity, and study the function in vertebrate development.

This will deliver a comprehensive view of Wnt/PCP signalling coordinating vertebrate development. The results of this multiscale project will therefore provide a step-change in understanding how cells react upon an inducing signal to correctly interpret a ligand - a fresh knowledge, which will lead to an improved ability to control Wnt signalling in diseases and engineer new cellular functions.