The Fat4-Dchs1 pathway: A new regulator of neuronal development and migration

Development of the brain involves the generation of neurons (the major population of cells in the brain) and their migration to their final destination. After reaching their targets, neurons make the connections that will eventually "wire" the brain. This project will determine the role of two proteins called Fat4 and Dchs1 during brain development. Specifically, we will determine how these proteins regulate the generation and migration of newborn neurons. We already know that Fat4 and Dchs1 are essential for human brain development as mutations in Fat4 and Dchs1 in humans result in syndromes characterized by brain abnormalities and intellectual disability.

The focus of this project will be on one region of the brain called the subventricular zone (SVZ), a thin tissue layer surrounding the walls of the lateral ventricle. This area represents the most prominent source of stem cells, which are able to generate new neurons both before and after birth. Abnormalities in SVZ neurogenesis are associated with cognitive decline, dementia and neurodegenerative diseases like Alzheimer's and Huntington's diseases.

We will determine how Fat4 and Dchs1 control the generation of new neurons from the SVZ and their migration to their final destination. We will analyse how SVZ neural stem cell proliferation and differentiation are affected when Fat4 or Dchs1 are not present. We will also clarify why SVZ-derived neurons fail to migrate properly in the absence of Fat4 and Dchs1. We hypothesize that Fat4 and Dchs1 are required to ensure the proper directed collective movement of SVZ-derived neurons. To investigate this, we will use time-lapse imaging of neuronal migration in brain slice cultures and examine abnormalities in neuronal motility and directionality.We will also examine which signalling cascades are triggered by Fat4 and Dchs1 to control the generation and migration of new neurons in the mammalian brain.

Our studies will identify crucial mechanisms in neural stem cell biology, which will not only help increase our understanding of brain development but also help design therapeutic strategies to repair the brain in trauma and disease.

The protocadherins, Fat4 and Dchs1, are a receptor-ligand pair in a new vertebrate pathway. We, and others, have shown that Fat4-Dchs1 have critical roles in embryogenesis, including brain development. Fat4-Dchs1 control Planar Cell Polarity (PCP), defined as the co-ordinated polarity of cells, and tissue growth through the Hippo pathway. We recently reported an essential role for Fat4-Dchs1 during tangential migration of the facial branchiomotor neurons. This identified Fat4-Dchs1 as a novel neuronal guidance cue. Our data also provided the first evidence in vertebrates that gradients of Fat activity may determine polarity as in Drosophila. Finally, we showed a novel intersection with another PCP pathway, the Fz-PCP pathway whereby Fat-PCP and Fz-PCP control cell polarity along orthogonal axes. Despite these advances, we lack a deep mechanistic understanding of how Fat4-Dchs1 control neuronal cell behaviours.
We now show that Fat4-Dchs1 are also required for tangential migration of neurons from the subventricular zone (SVZ), a major post-natal stem cell niche. Our data also indicate that Fat4-Dchs1 may also affect other aspects of neurogenesis, such as proliferation and differentiation. The SVZ is amenable to in vivo gain and loss of function studies and to ex vivo real-time imaging of cell behaviour, providing an excellent model system to dissect the roles, together with the cellular and intracellular mechanisms of Dchs1-Fat4 signalling, in neurogenesis. We will use genetically modified mouse lines, including mosaic analyses, in vivo postnatal electroporation and ex vivo confocal spinning disc imaging approaches to determine how Fat4-Dchs1 regulate (a) neural stem cell polarity and differentiation (b) tangential neuroblast migration (c) the role of PCP and the Hippo pathways and (d) intersection between the Fat-PCP and Fz-PCP pathways. Thus, this study will reveal the roles and mechanisms of the Fat4-Dchs1 pathway, a critical regulator of mammalian neurogenesis.

There are many levels at which the proposed research will have significant impact. The proposal addresses fundamental questions about how a newly identified signalling pathway, the Fat4-Dchs1 pathway, controls neurogenesis. An understanding of this pathway is essential as it has recently been shown that mutations in FAT4 and DCHS1 in humans result in a variety of abnormalities, including brain disorders. It is also very likely that de-regulation of Fat4-Dchs1 signalling is associated with post-natal neuronal degenerative orders. Thus, the proposal has direct implications for understanding brain development and neurological disorders.
We have also have shown that Fat4-Dchs1 regulate tangential neuronal migrations and the proposal aims to dissect the mechanisms of Fat4-Dchs1 signalling during tangential migrations. These are collective cell migrations that occur parallel to the ventricular layer. Collective cells migrations are important for many aspects of embryogenesis, and also occur during tissue repair and degenerative conditions. Thus our data adds a new regulator of these processes.
Additionally we have recently shown that cell polarity and directed neuronal cell migrations may be established by a gradient of Fat4-Dchs1 activity - this is the first implication that gradients establish polarised cell behaviour in vertebrates. The proposal will investigate if gradients of Fat activity also determine polarity and migration of the rostral migratory stream, neurons that arise from the subventricular zone.
We will also determine how Fat4-Dchs1 regulate stem cell development. An understanding of factors that direct neuronal stem cell proliferation and differentiation (together with migration) is essential to ultimately devise strategies for therapeutic repair.
All of the above are of significant interest to developmental biologists, cell biologists and clinicians and will have an impact on our understanding of embryogenesis, post-natal development and disease. The proposal, therefore, has an economic and societal impact.
To date just 6 papers have been published on this novel Fat4/Dchs1 pathway in vertebrates. This includes 2 of our own, published in a previous and a current BBSRC grant, which provided significant advancements on our understanding of how this pathway controls many aspects of embryogenesis. Based on comparison with the advances that have taken place upon the first identification of other signalling pathways, it is highly likely that these seminal papers will provide the springboard for many further publications, by both us and others; it is envisioned that these initial findings will set the foundations in our understanding of how this pathway functions, not only in development but post-natally and in disease.
The proposal will also train the post-doctoral research assistant (PDRA) in techniques at the forefront of the field such as cell biology, developmental biology, state of the art live confocal imaging and imaging analyses, expose the young scientist to international and national collaborations which will enhance success and career progression, and opportunity to publish high impact publications at the forefront of the research field. This training will provide the PDRA with opportunities for career progression (e.g. a fellowship or independent position). The skill set would also be applicable to non-academic research. The proposal will also introduce a newly established PI investigator to a new collaborative network and an emerging but important field of research.
In summary, the proposal will provide new knowledge in an important scientific field, will enhance research capacity by combining distinct expertise and resources within the UK and with a HHMI collaborator, keeping the UK at the forefront of this new and emerging research area.