Fat-PCP and branching morphogenesis

Organs such as the kidney, lungs, salivary, prostate and mammary glands consist of epithelial and mesenchymal cells. A large surface area of epithelial cells is required for organ function. For example, the kidney and lungs require a large surface area for the removal and concentration of excretory products from the blood stream and exchange of gases during breathing respectively. For many organs these large surface areas are generated during embryonic development by the continuous branching of epithelial tubules which allow an increase in the epithelial surface area within a compact space. This is clearly illustrated by the adult lungs which have a surface area of a tennis court. A defect in branching affects the function of the organ and can also have unexpected secondary consequences. For example, defects in kidney branching are the leading cause of hypertension (high blood pressure) in adults.
During development, the branching organs arise from epithelium and mesenchyme. The epithelium branches and this branching is controlled by signals from the mesenchyme. The epithelium grows and branches by combinations of signals that control proliferation and cell death. Additionally, there is cell movement and a change in cell shape. We have shown that two genes, Fat4 and Dchs1, that control the shape and orientation of cells, are essential for branching morphogenesis in the developing kidney. The proposal aims to determine how Fat4 and Dchs1 control kidney branching morphogenesis and if these proteins control branching morphogenesis in other organs. We will determine if Dchs1 and Fat4 function in the epithelium or mesenchyme and how loss or gain of Dchs1/Fat4 affects cell behaviours that contribute to branching morphogenesis.
The knowledge will contribute to our understanding of cell behaviours during the growth and morphogenesis of different organs. We will determine the role of two factors that we already know are important for this process but we currently do not understand why. The knowledge will give insight into how organs develop, how developmental birth defects arise, and has relevance in stem cell biology and tissue-engineering approaches in which damaged cells/organs are replaced by newly generated cells/organs.

How individual cells co-operate to form polarised and appropriately patterned structures is still a major question in embryogenesis. Yet, the establishment of the appropriate tissue architecture, for example during development of the branching organs such as the kidney and lung is clearly a prerequisite for normal organ function. Each branching organ is characterized by a unique shape which is determined during embryonic development by the directions and appropriate extension of the epithelial branches. Recently the planar cell polarity (PCP) pathways, which classically control the polarization of cells within an epithelium, have been shown to control branching morphogenesis in the kidney and the lungs. The cellular mechanisms are unknown. The proposal will determine how the newly identified Fat-PCP pathway, which includes Fat4, the receptor, and Dchs1, the ligand, regulates branching morphogenesis. We will focus on the kidney where we have shown that Fat-PCP is essential for branching morphogenesis (Mao et al. 2011). The kidney is also dysmorphic indicating altered polarity in the direction of tubule extension. Using tissue-specific conditional mouse mutants, reaggregate chimeric techniques and real-time imaging studies the proposal will determine (a) the tissue-specific requirements of Fat4 and Dchs1 in the developing kidney (b) how Fat-PCP influences cell behaviour during epithelial morphogenesis and if these influences are via PCP or an alternative pathway (c) how potential Drosophila modulators of Fat4/Dchs1 influence Fat4/Dchs1 activity in vertebrates and (d) construct a 3D model to show how Fat-PCP affects the direction of tubule elongation. We will also analyse, and extrapolate our findings, to the lungs and salivary glands where we have shown more subtle defects in Fat4 and Dchs1 mutants to determine which aspects of Fat-PCP signaling are conserved and how modulation of Fat-PCP may contribute to the diversity of branching morphogenesis.

The proposal addresses fundamental questions about cell behaviour during the establishment of the epithelial architecture of branching organs with a focus on the role of the newly identified Fat-PCP signalling pathway. To date just 2 papers, including our own published during a current BBSRC grant, have been published on Fat-PCP in vertebrates. We have provided a key advancement in our understanding of how this pathway functions in vertebrates, and its critical roles during many aspects of embryogenesis. Based on comparison with other signalling pathways, these two papers will be the first of many publications and we have set the foundations in understanding how this pathway functions, not only in development but post-natally and in disease. We have now created a new team with distinct expertise to analyse a novel and unexpected finding in the Dchs1 and Fat4 mutants. We build on a collaboration that provides us with crucial unpublished reagents and exploit new tools that we have generated to analyse cell polarity in real time. The proposal will provide new knowledge which advances the field and enhances research capacity by combining expertise and resources keeping the UK at the forefront of this new and emerging research area. The focus on the establishment of epithelial architecture, and the role of polarised cell behaviours, also ensures that this research has wide reaching implications from embryogenesis, post-natal growth, de-regulation during disease, mechanisms of epithelial wound healing to tissue regeneration.
The demonstration of the mechanism of PCP regulation of branching morphogenesis will be a significant contribution, if not leading contribution, to the field. The focus on the developing kidney, which is a model system for many developmental processes, from inductive interactions, branching morphogenesis, differentiation and the establishment of apical-basal polarity, will ensure manuscripts will be given high prominence amongst developmental, cell biologists and clinicians.
The proposal will train the post-doctoral research assistant in techniques at the forefront of the field such as real time imaging of cell movement and generation of 3D models, expose the young scientist to international and national collaborations which will enhance success and career progression, and the opportunity to publish high impact publications at the forefront of the research field. Training in a new research area will also provide the post-doctoral research assistant with increased opportunities for career progression -e.g. fellowships or an independent position - using data obtained and resources generated within the BBSRC funded position. These positions are not restricted to the developmental biology field but could be expanded for example, to tissue engineering, cell biology depending on the post-doctoral research assistants developing research interests. The skill set would also be applicable to non-academic research.
The proposal also has an economic and societal impact. It will increase our understanding of developmental birth defects and ultimately, disease processes, repair mechanisms following wounding (e.g.the ability of cells to organise appropriately during wound healing), how repair mechanisms may change with age, and stem cell/ tissue engineering approaches that will be used to regenerate organs. At a different level, we will establish 3D models of the developing branching organs. In future, these models, together with the "wet science" approaches described in the proposal to understand cell behaviour, will be steps forward towards the generation of computer models to factor in the effects of modulators of branching/Fat-PCP signalling ultimately reducing the number of animals needed for research and conforming with the 3R policy.