Molecular mechanisms of axon guidance in Drosophila

During development, growing axons are guided to their appropriate targets by extracellular cues, the response to which depends crucially on the repertoire of receptors expressed by each axon and the intracellular mechanisms that transduce and regulate the response to these cues. In the past this research area has been significantly advanced by the identification of several major families of guidance cues and their cognate receptors and the elucidation of many aspects of the molecular mechanisms of axon guidance. However, globally we still have only a rudimentary understanding of how these axon guidance processes are coordinated to specify the precise wiring of billions of cells in thousands of tracts in the brain. To fully understand this process it will be necessary to identify additional genes involved in wiring, define their expression patterns over the course of development and elucidate their cellular functions and molecular interactions. Through genetic and bioinformatic approaches we have identified further molecules that regulate the development of connectivity in the embryonic central nervous system (CNS) of Drosophila. Currently we are focusing our investigations on additional genes that contribute to guiding axons towards the midline of the CNS.

Drosophila provides a model system to identify the normal and pathological roles of proteins associated with neurodegenerative disease. We are using the benefits of Drosophila to increase our understanding of gene products associated with Alzheimer’s and Batten disease. The Alzheimer’s work has focused on the critical role played by Tau hyperphosphorylation in Alzheimer’s disease pathology where it has become clear that a small number of kinases are responsible for the majority of the phosphorylation sites on tau in AD brain. We have established Drosophila as a whole animal assay to investigate which of these are responsible for the generation of toxic forms of tau in vivo. This makes use of humanised Drosophila that express human tau and specific identified human kinases to create an AD-like pathology. Using this system we are beginning to discriminate the specific involvement of the different kinases alone or together in the creation of toxic forms of tau. Batten disease, also known as neuronal ceroid lipofuscinoses (NCLs), describes a group of at least nine fatal monogenetic neurodegenerative disorders that primarily affect infants and children. The genes mutated in several forms of the disorder have been identified recently, but very little is known about the precise roles of these gene products in normal neuronal tissue and how their mutation contributes to the disease. We have begun to combat this by investigating the role of the transmembrane proteins Cln7 and Cln3, which are affected in the most common forms of NCL. We have identified that the Drosophila Cln3 shares many properties with the vertebrate form, it is localised to the endosomal-lysosomal compartment in many cell type and found at the synapse. We are investigating the roles these proteins play in the normal function and development of the neuromuscular junction. We are also near completion of a large scale screen to identify genes that modify Cln3 activity and this has provided us with a significant amount of information on the possible molecular pathways that are affected by changes in Cln3 activity.