Integration of cell-cell interactions and cell division by novel Dkk1 functions

Cells connect to each other by forming cell-cell contacts, or adhesions, between neighbouring cells. These adhesions are essential for maintaining tissue integrity and for communication between cells. When intercellular adhesion is compromised, cells may break free from their neighbours and become invasive, or cells relying on close contact for signal transmission, such as nerve cells, may lose their ability to signal between neighbouring cells. The consequence of such events is often associated with human disease.
Our research is centered on a molecule called Dkk1, a secreted protein that induces head formation during embryonic development. Dkk1 is frequently detected in several cancers with elevated levels correlating with disease progression and a poor prognosis, and is also associated with loss of signalling between nerve cells, called synaptic signalling, which is linked to Alzheimer's disease. The functional role of Dkk1 in these diseases is unresolved.
Using the zebrafish embryo as an in vivo model, we have recently reported that Dkk1 disrupts migration of groups of cells in the embryo due to reduced cell-cell adhesion, and that this occurs by a mechanism different from its known signalling function. We now propose to carry out experiments to gain insight into how Dkk1 reduces cell-cell adhesion by identifying interaction partners in vivo and study how they contribute to Dkk1's function in regulation of cell adhesion and cell behaviour. We have also found a pool of Dkk1 associated with structures that orchestrate cell division, which we propose to also further investigate. Insight into the mechanisms by which Dkk1 impacts cell behaviour and cell division will be crucial for understanding Dkk1's role in cancer and neurodegenerative disease.

We have very recently found that the signalling protein Dkk1 acts on cell-cell interaction via a molecular interaction independent from its known action.
We propose a research plan which will elucidate the molecular interactions by which Dkk1 directly impacts cell behaviour and cell division in vivo. The data from this research will be pertinent for several cancers, neurodegenerative- and cardiovascular disease, where upregulation of Dkk1 and loss of cell-cell contacts lie at the core of disease progression. Identifying interaction partners for Dkk1 in vivo is key to understanding the mechanistic details of how Dkk1 downregulates adhesion between cells. By identifying Fz-independent membrane receptors in complex with Dkk1, we will gain understanding of Dkk1-induced signalling events which locally modify cell-cell adhesion. We also propose to define these Dkk1-induced modifications at the molecular level by determining the contribution of specific myosin isoforms at these adhesive sites.
Our discovery of an intracellular centrosome associated pool of Dkk1 raises questions regarding the nature and function of these complexes and how they relate to membrane localised Dkk1. Imaging by electron microscopy will give us detailed data on the cellular structure(s) that Dkk1 associates with, and how these and other cellular components change in response to a sustained increase in Dkk1 levels. This analysis will give valuable insight into the function of intracellular Dkk1 and the cellular structures that are involved in generating the loss of adhesion phenotype related to disease.
Our finding that Dkk1 associates with the mitotic spindle puts it in a central position for control of mitotic progression. Our proposed experiments will determine the role of Dkk1 in this context and further our understanding of the consequences for timing and orientation of cell division when Dkk1 levels are elevated, which is of particular relevance to malignant disease.