Role of Myelin in Spinal Cord Regeneration

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DESCRIPTION (provided by applicant): One of the major obstacles to axonal regeneration in the adult CNS is inhibitors associated with myelin, such as MAG. However, axonal growth can be encouraged in an inhibitory environment both in culture and in vivo if the neuronal cAMP levels are elevated, either with analogues such as db cAMP or by priming neurons with neurotrophins. One situation where spontaneous CNS axon regeneration does occur is of dorsal root ganglion axons if the peripheral branch of the same neuron, the dorsal root ganglion (DRG) neuron, is lesioned one week before - the conditioning lesion (CL) effect, which is cAMP dependent. Both the cAMP and the CL effects are dependent on transcription and one gen that is up-regulated is for the enzyme Arginase I (Arg I), which is key in the synthesis of polyamines. The polyamine, spermidine, can overcome inhibition by MAG in culture and promotes optic nerve regeneration in vivo. Furthermore, spermidine promotes regeneration by activating the kinase CDK5 by hypusinating the eukaryotic initiation factor 5A (eIF5A), resulting in an increase in translation of the CDK5 activator p35. In Aim 1a the CDK5 substrates that are activated in response to polyamine will be identified and characterized for their role in overcoming inhibition and promoting regeneration in vivo. Aim 1b will address the cross-talk between the neurotrophin and MAG signaling pathway, focusing on the ability of MAG to block the activation of the small GTPase, Rap1, by neurotrophin. Strong preliminary data suggest that both the cAMP and CL effects require local translation in the axon to promote regeneration in an inhibitory environment. In Aim 2 mRNA and microRNAs that increase in the processes after both a CL and treatment with db cAMP will be identified. In Aim 3 those mRNAs and microRNAs that increase after both conditions will be characterized (over-expression, knock-down) for a possible role in promoting regeneration in the presence of MAG in culture and in promoting regeneration in vivo. Through the experiments described in this proposal not only will our understanding be advanced of the mechanism of action of agents known to promote regeneration in vivo but novel agents will be identified. This in turn will reveal novel targets for intervention and drug development to promote axonal regeneration in vivo.