Neurotrophic factors are messengers for communication between neurons. They regulate neuronal differentiation and may be instrumental in the formation, stabilization and plasticity of synapses. Targeting of neurotrophic factors to their proper intracellular destination is essential for trophic signaling. The application's central aim is to elucidate pathways of internalized neurotrophins and to reveal mechanisms of their axonal transport and release from synapses in the visual system. The proposed studies will utilize an in-vivo system in which the transport of trophic factors can be quantified. The developing visual system (retinotectal and isthmo-optic projections) of chick embryos is a unique model which allows the controlled introduction of iodinated trophic factors into the posterior chamber of the eye and the study of retrograde and anterograde transport to distant targets. Experimental techniques have been designed to determine the mechanisms of release at the axon terminus and the function and significance of axodendritic transfer of neurotrophins. Specifically, the proposed studies will identify the source of neurotrophin-3 in the retina and use crosslinkers and antibodies to determine which receptors bind neurotrophins during anterograde and retrograde transport. The organelles in which internalized neurotrophins are sorted and in which anterograde transport and release takes place will be characterized at the ultrastructural level. The hypothesis that neurotrophins are co-localized with neurotransmitters or with neuropeptides in synaptic vesicles will be tested. The mechanisms of release at the axon terminus will be explored by measuring neurotrophin content in synaptosomes after treatment with pharmacological substances. It will be further examined if anterogradely transported neurotrophins promote synaptogenesis. The combination of molecular, pharmacological and ultrastructural approaches in an advantageous model system will allow the investigator to answer questions about trafficking of neurotrophins which are crucial to an understanding of how these factors may regulate neuronal survival as well as synaptic plasticity in the developing visual system and other parts to the brain. Neurotrophins are important regulators during the development of the visual system as well as potential therapeutic agents in degenerative retinal disease and after optic nerve injury.
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