Functional genetics of trophic support in neuron-glia interactions in the central nervous system of Drosophila

One of the most devastating burdens to human health is spinal cord injury, which occurs primarily through car or sport accidents and it wrecks peoples?
lives with paralysis than can affect all four limbs. The medical challenge is to promote the regeneration of the spinal cord, to allow functional recovery. The efforts to promote regeneration have been partly hampered by the fact that upon injury the cellular environment becomes inhibitory to axonal growth. One challenge is thus to turn the cellular environment into one that will promote axonal growth. However, this requires knowledge of how axons interact with glia during growth, how to eliminate secondary death of neurons and glia and how to promote the re-establishment of myelinated, functional neural networks. Unfortunately, little is still understood of the underlying molecular mechanisms that while keeping neurons and glia alive contribute to the restoration of functional axonal connections. Similarly, in neurodegenerative diseases (i.e. Multiple Sclerosis, Parkinson?s disease, Alzheimer?s), neurons and glia die and the promotion of neuronal and glial survival is a strategy to alleviate these diseases. However, it is unknown how to implement the restoration of normal neuronal connections and function from cells that are maintained alive. For the last five years, I have been studying how neuron-glia interactions link the promotion of neuronal and glial survival with the formation of axonal trajectories. I study these processes during development of the nervous system, as this knowledge is absolutely essential to work out how they can be implemented or manipulated upon injury or disease. I use the Drosophila equivalent of the spinal cord as a model system because it is possible to study and manipulate individual neurons and glia with single cell resolution, and in vivo, something not yet possible in vertebrate models. Furthermore, working with Drosophila is very cheap, progress is fast because the fly life cycle is short and experimentation with flies does not raise ethical concerns. Findings in Drosophila are totally relevant to the understanding of the human spinal cord and brain. Drosophila has long been one of the most powerful genetical model organisms to work out molecular pathways underlying universal cellular functions. Many important molecules involved in the development of the human brain and disrupted in human cancers were discovered in Drosophila. The fruit-fly is also a powerful model organism to model neurodegeneration and neurotrophic interactions are known to occur in flies. With this proposal, I aim to gain from Drosophila information of how a homologue of a trophic factor already known to exist in humans functions in axon guidance and targeting during development.
Using the power of Drosophila genetics and of the sequenced genomes, I also aim to discover novel trophic factors that maintain neurons alive, which I anticipate ultimately will be relevant for humans too.

Recovery of neuronal function following spinal cord injury requires the re-establishment of axonal trajectories, synaptic connections and glial populations and the maintenance of neuronal and glial survival. Demyelinating and neurodegenerative diseases are also characterised by glial and neuronal death. One approach to cure spinal cord injury and neurodegenerative diseases is to understand the mechanisms that maintain cells alive and that promote axon growth.
Neuron-glia interactions are needed for axon guidance and to regulate the number of neurons and glia necessary for function. Cell number is adjusted through the non-autonomous control of cell survival and cell proliferation. The same molecules may implement trophic support and axon guidance. Neurotrophins are produced by the target to promote neuronal survival, they are involved in neuron-glia interactions and in axon guidance. Thus, neurotrophins are ideal molecules to implement repair.
However, multiple trophic factors present in limiting amounts are thought to maintain cell survival in the central nervous system. Thus, most likely trophic factors remain to be discovered.
Analysis of neuron-glia interactions during axon guidance in the developing spinal cord is technically challenging: the use of a more amenable model system is compelling. Drosophila has been used as a powerful model system to study axon guidance and neurodegeneration. In the Drosophila equivalent of the spinal cord, the ventral nerve cord, the trophic neuron-glia interactions taking place during axon guidance are well known. I have means of manipulating neurons and glia with single cell resolution, non-invasively, in vivo and in real time.
I aim to: (1) Analyse the in vivo function of the Drosophila BDNF homologue. I will study its roles in the control of cell survival and proliferation, axon guidance and fasciculation. I will also study genetic interactions, e.g. with neuregulin. This will tell me whether neurotrophins function in comparable ways in vertebrates and in insects and I anticipate it will reveal in vivo roles not yet known from vertebrates. (2) I will search for novel trophic factors, using suppressor genetic screens. The trophic function of an unknown gene is tested by its ability to rescue a mutant phenotype of extensive neuronal or glial death. I will use the GAL4/EP and GS systems to drive selectively in neurons or glia the expression of genes downstream of UAS inserted randomly throughout the genome. The sequenced genome and material available from the BDGP will provide molecular access to the candidate genes.