How do the cadherins Fat and Dachsous control polarised cell behaviours during development?

The cells that make up the tissues and organs of our body often need to know left from right or front from back. They need to have this directional information as when tissues are forming cells need to move in the same direction or rearrange with their neighbours to produce organs of the correct shape. Also, for the correct function of some organs, such as the lungs, cells all need to produce cilia (hairs on the surface of cells) that point in the same direction. If this organised control of cells is lost then mistakes in the formation and function of tissues can occur leading to birth defects.

There are systems in place that provide this directional information to cells. One such system is called the Fat-Dachsous pathway. This consists of two large proteins that are localised at opposite ends of the cell providing a direction to the cells. What we don't fully understand is how cells "read out" this uneven distribution of Fat and Dachsous proteins resulting in changes within the cell.

The Fat-Dachsous pathway has been extensively studied in the fruit fly, which is commonly used by scientists to ask basic questions about how cells behave to form tissues. The Fat and Dachsous proteins in the fruit fly are similar to those in humans, so what we find out in flies may have significance for our understanding of these proteins in human disease.

We will carry out experiments to understand how cells read out Fat-Dachsous signals and how this influences how cells behave. Specifically we will look at networks of proteins in the cells called the actin and microtubule cytoskeletons - networks of fibres that give cells their structure. We will image proteins using microscopy techniques that allow us to measure amounts of proteins and how they are changing. We can then take Fat or Dachsous protein away or add it back and look at changes in the way proteins or cells behave. We will also look at new proteins that we think may respond to the distribution of Fat or Dachsous to cause changes in the actin and microtubule networks. Through our experiments we will understand how Fat/Dachsous protein distributions are understood by cells, leading to a range of cell behaviours, that are needed for the correct form and function of tissues.

The conserved Fat-Dachsous (Ft-Ds) signalling pathway is a key regulator of a range of cell behaviours required for tissue and organ morphogenesis. It is also important in human development, with pathway malfunction linked to congenital abnormalities.

Ft and Ds are atypical cadherins that bind heterophilically, becoming planar polarised within cells. This provides each cell with intracellular polarity cues, co-ordinated with the tissue axis, that are interpreted to produce a range of cellular behaviours including control of cell proliferation, orientation of cell division and cell rearrangements, polarisation of actin structures/hairs, alignment of the microtubule cytoskeleton and co-ordinated migration of cells.

We propose to use our model system Drosophila to dissect how cells interpret polarised Ft-Ds signals. We will focus on understanding cellular outputs independent of growth control, as this is an important but unexplored area broadly relevant to Ft-Ds function in all organisms. In previous studies interpretation of the cellular outputs of Ft-Ds signalling has been complicated by the fact that Ft-Ds influence both the actin and microtubule cytoskeletons, and by the cross-talk between these systems. Our study will use novel genetic manipulation techniques to modulate Ft and Ds, and downstream effector proteins, in a precise temporal and spatial manner - allowing us to reveal novel mechanistic insights into pathway signalling.

Specifically, we will address the mechanism by which Dachs regulates junctional tension, determining if Dachs acts via direct motor activity or via intermediary proteins. Furthermore, we will use quantitative live imaging to characterise the role of Ft-Ds in regulating the actin accumulation that leads to denticle/hair formation, and identify Dachs-independent effectors. Finally, we will determine if Ft-Ds regulate microtubule alignment either directly or indirectly through regulation of cell shape.

Beneficiaries:
1) Future patients suffering from heart defects. The proposed study is relevant to the understanding of human heath as the human homologues of Fat-Dachsous play an important role in the development of tissues and organs. Of particular relevance to human health is the role of Dachsous in heart development. Mutations in human Dachsous have been linked to mitral valve prolapse (MVP), a heart valve defect, which affects 1/40 in the population. Our study will help in the understanding of how this disease develops.
Basic biomedical research has a translation timescale of 20+ years. However, we will proactively seek out opportunities for collaborations with clinical researchers to build on the research in this proposal. Timescale: 1-3+ years.
2) Future patients suffering from cancer: The proposed study is most relevant to carcinomas, which derive from epithelial cells, covering 80% of known primary cancers. During this project we propose to study the function of known tumour suppressor molecules, Fat and Dachsous, which are mutated in breast, brain, gastric and oral cancers.
3) Health services: As we are seeking to understand the downstream cellular effectors of Fat-Dachsous signalling we may uncover additional genes that are relevant to MVP. Such knowledge will aid clinicians in classifying genetic variants as pathogenic in relation to MVP.
4) Those recruiting scientifically trained staff, including business, industrial and public sectors: Group members supported by this grant will improve their training, including transferable skills, e.g. project management and leadership skills. Additionally, we will supervise undergraduate/postgraduate students, contributing to their training in scientific experimentation, experimental design, data analysis, and transferable skills. Thus, this grant will contribute towards the health of UK science and higher education through developing expertise and training highly skilled researchers. Timescale: 1-3+ years.
5) The public: The team will communicate the importance, excitement and beauty of scientific research to the public, e.g. by presenting at our Institute-wide events such as Discovery Night, Researchers' Night and Festival of the Mind. We will seek to inspire the next generation of scientists by taking part in outreach in schools. Examining molecular and cellular behaviour in living animals provides beautiful images that effectively capture and communicate the concepts of biomedicine, which we will share at outreach events. Timescale: years 1-3.