Parallel notch activation mechanisms provide robustness to developmental patterning in Drosophila

Multicellular organisms like humans and fruit flies, develop from the proliferation of a single cell. An important problem in biology is to understand how it is that the millions of cells present in the adult body arise. These cells have to perform many different functions in diverse tissues and therefore they become differently specialised, for example into heart cells or brain cells. It is an important challenge in biology to understand, not only how this specialisation occurs, but also how each specialised cell arises in the correct place at the correct time of development. This is not only important from the point of view of adding to our fundamental biological knowledge, but it has important implications for our understanding of diseases like cancer, heart disease and dementia. This is because the signals that control the development of the organism are often signals that go wrong when we are suffering such medical problems. If we can understand how these signals function normally in development then this will provide essential insight into how to correct defects in these signals when they go wrong. For example sometimes cancer is caused when a signal is switched on permanently by a mutation, rather being regulated on and off. If we can find a way to block such signals without harming its normal functions, then it might be possible to cure particular cancers. It is obviously difficult both practically and ethically, to find out the answers to these problems by experimenting on humans. However, because all living organisms today have evolved from a common ancestor, it is possible to understand problems related to human biology and medicine, by studying other organisms. The fruit fly is one powerful model system that is often used to understand problems of signalling and development. The major signals that control fruit fly development are conserved with humans, including many of the signals that are associated with human diseases such as cancer. It is a wonderful model organism because it is so easy to manipulate by making mutations in its genes. This means we can study signals in intact tissue rather than trying to deduce their function by studying isolated cells in a dish. The latter approach is also very useful but can give misleading answers unless it can be combined with the investigation of signalling in real tissues. Studying fruit flies has resulted in the identification of many signals that are misregulated in human cancers. Our aim is to investigate a critical signal that is mediated by a protein called Notch. This protein is used many times in development and in many different tissues. One of the most well known uses is during the development of the brain. The function of Notch is to ensure that correctly specialised cells arise in the proper places in the body. My research recently found out that our understanding of how this receptor works is incomplete. This is very important to know because this protein is used many times in development and it is misregulated in many types of cancer. We have now discovered that there is a way for Notch to signal that was not known before. This programme aims to understand this new signal by identifying its required components and finding out what they do during development of the fruit fly brain, and other tissues. The work has an enormous potential to help find ways to cure diseases like cancer. It may also help find ways to solve problems associated with aging and help us to lead healthier lives for longer. This is because Notch regulates stem cells and these contribute to keeping our tissues and organs healthy over our lifespan. It also functions in the brain cells to help us remember events for a long time. A decline in the ability to form memories is a problem often associated with age-related conditions such as dementia. Understanding how Notch works may help lead to solutions to these challenging problems in the future.

A critical and conserved developmental signal is mediated by the Notch receptor which is utilised in numerous key decisions and patterning processes. The Notch signal refines, to a single cell resolution, more broad patterns laid down by long-range signals. DSL-domain ligands activate Notch by causing proteolytic release of the Notch intracellular domain. The latter translocates to the nucleus to regulate gene expression. However, recent work from my group, using the Drosophila model organism, shows a proportion of Notch signalling depends on a DSL ligand-independent signal, that regulates the trafficking of Notch to the lysosome membrane. We propose that parallel pathways of Notch activation contribute to developmental robustness during cell fate patterning. To characterise the novel pathway further, we need to identify the remaining components and their functions. We will determine the full contribution of the alternative Notch pathway to development by identifying any redundancy, or parallel pathways by which Notch traffics to the lysosome. We will understand the impact of endocytic sorting on Notch signalling, by investigating how the pathway can be switched between positive and negative outcomes, and the relationship between the endocytosis-induced and the DSL ligand-induced signals. Our hypothesis is that these are distinct routes to activation rather than being part of the same linear pathway. However we will rigorously test this hypothesis by comparing the structure/function requirements of Notch in the two pathways, and the requirements for the pathway components. The outcomes from the above will enable us to generate more realistic models of the Notch signalling network during pattern formation, exploiting an existing collaboration to generate predictive computer simulations. This more comprehensive understanding will inevitably be informative for the ability to manipulate Notch signal activity for therapeutic purposes.