Regulation of axonal arbor and synaptic dynamics by the apoptotic pathway: Interactions with the cytoskeleton, downstream substrates and upstream regulators

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The formation and maintenance of appropriate connections between neurones are critical to the normal functioning of the nervous system. Defects in these processes can lead to disorders and alterations in behaviour. In the central nervous system (CNS), synapse formation depends on the regulated elaboration and interaction of pre- and post-synaptic axonal and dendritic branches. Arborisation and synaptogenesis are hence critical in ensuring the proper pattern of synaptic connections and at the same time in defining the complex and compartmentalised geometry of neurones. This compartmentalisation enables the partitioning of cellular signaling pathways, which is critical especially if signaling cascades involved have pleiotropic effects or that could even harm the cell as a whole. I have recently shown that caspases, key mediators of apoptosis, are activated locally at branch points of young, dynamic axonal arbors and are key in maintaining axonal and synaptic dynamics during arbor growth. For this, I took advantage of zebrafish larvae, which owing to their small size, transparency and genetic malleability, offer an unprecedented opportunity to study neuronal cell biology in vivo in the vertebrate CNS. The specific focus on the retinotectal projection, which connects the retina to the midbrain tectum, has long served as an important developmental model of synaptic specificity. In this application, I propose to address an important gap in our knowledge about the role of local signaling pathways in arbor dynamics: We do not understand how caspases mediate branch dynamics, which must involve modulation of the cytoskeleton, what cell biology translates such cytoskeletal changes into morphogenic events or how it is regulated by upstream signals to avoid cell death. It is also not known whether neural activity-independent and activity-dependent mechanisms regulating arbor growth share similar mechanisms. Via a combination of in vivo live imaging of arborisation and synapse formation, manipulation of gene function and visual stimulation patterns, I propose to study the cell and molecular biology of caspases during developmental and experience-dependent arbor and synaptic dynamics. Specifically I plan: 1.) To define the underlying cytoskeletal dynamics of arborisation and synaptogenesis and to identify how and where caspase activation promotes the cytoskeletal changes underlying arbor and presynaptic remodeling. 2.) To identify downstream caspase targets and how they modify the cytoskeleton. 3.) To determine how caspase activity is dynamically and locally regulated in arbors via the extrinsic or intrinsic apoptosis pathways and visual experience. My studies will provide key mechanistic insights into neuronal arborisation and synapse formation and my research will identify a set of cellular and molecular components and principles in neurones, which may be transferred to nonapoptotic processes in other cellular contexts.