03-P069 Development of the oculomotor system and modelling Duane’s syndrome in zebrafish

Eye movements in vertebrates are controlled by six extraocular muscles innervated by three of the cranial nerves – the oculomotor, trochlear and abducens. Incorrect development of this wiring network can lead to eye movement disorders, such as the congenital condition Duane retraction syndrome (DRS), which results in squint (strabismus). In DRS patients, the abducens nerve is often absent, and the oculomotor nerve branches to an inappropriate muscle target. How the program of neural projections to the extraocular muscles is regulated in normal or abnormal development is mostly unknown. However, our recent studies have identified mutations in the signalling molecule alpha2-chimaerin as being causal in DRS, and have shown that mutant forms of alpha2-chimaerin lead to striking axon guidance defects of the oculomotor nerve1. We are mapping axon projections to the extraocular muscles in normal development and Duane syndrome, using the zebrafish embryo as a model system. Timelapse movies of oculomotor axon guidance (using the Islet1-GFP line) show a stereotyped sequence of growth and branching to muscle targets, which differs from the pattern previously identified in the chick embryo. We are also using DNA injection or electroporation into oculomotor neurons and immunohistochemistry for nerve, muscle and/or synaptic components to precisely map the development of the ocular motor system. Fluorescently-tagged alpha2-chimaerin constructs harbouring known human mutations will then be expressed in oculomotor and/or abducens neurons in the zebrafish in order to determine how the axon projection pattern is disrupted, using a combination of live imaging and imaging of fixed embryos.