Axonal Transport of mRNA for Mitochondrial Proteins

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Neurons have more extended and complex shapes than any other cell and consequently face a far greater challenge in distributing and maintaining mitochondria throughout their arbors. Neurons can last a lifetime, but proteins turn over rapidly. Mitochondria, therefore, need constant rejuvenation of their protein components no matter how far they are from the soma where the genes for most mitochondrial proteins reside. Transport of mitochondria from soma to periphery may be one means of rejuvenating the peripheral population, but mounting evidence indicates that local protein synthesis in axons and dendrites may also supply mitochondrial needs. This may be particularly true for proteins with very short half-lives; proteins that would be unlikely to survive the long trip down an axon. One such protein is PINK1, whose half-life is estimated to be on the order of a few minutes. Constant synthesis and degradation of PINK1 is an essential feature of current models for PINK1 function in mitochondrial quality control. Consistent with this model, we have found that blocking proteins synthesis selectively in axons prevents the local activation of the PINK1/Parkin pathway for mitophagy and that PINK1 mRNA is enriched in axons. The current proposal is based on these findings and also the observation that PINK1 mRNA colocalizes with mitochondria in axons and dendrites and is present on moving mitochondria. We have therefore hypothesized 1) the existence of a mechanism to localize PINK1 mRNA, and potentially many other transcripts for mitochondrial proteins, to the surface of the mitochondrion and 2) that mRNA for PINK1 is transported into axons and dendrites by virtue of its association with mitochondria. We have therefore proposed to identify the sequences within the PINK1 transcript that are required for its association with mitochondria, to identify the protein factors that mediate that association, and to determine if the association is required for the presence of the transcript in axons and for the local induction of PINK1- dependent mitophagy. We further propose to determine whether a similar mechanism operates for other proteins and is necessary for preserving mitochondrial and axonal health. Because defects in mitochondrial transport and mitophagy are implicated in Parkinson's and other neurodegenerative disorders, it is necessary to understand how a neuron can preserve mitochondrial health in a vast arbor and whether the transport of mRNA on mitochondria is part of that mechanism.