Final Report Summary - NEUROGROWTH (Axonuclear Communication in Neuronal Growth Control) Cell size homeostasis is one of the most fundamental aspects of biology, with distinctsize ranges for individual cell types. Neurons are the largest known cells, with complex andhighly polarized morphologies. Moreover, axonal lengths impose a significant delay betweentranscription and biosynthesis in the cell body and delivery of the components necessary forgrowth and maintenance to the axon. The large dimensions of a growing neuron require activetransport by molecular motors for transfer of signals between neurites and cell body. In thisproject, we showed that motor-dependent messenger RNA (mRNA) localization regulatesneuronal growth and cycling cell size. We identified the RNA-binding protein nucleolin as anessential factor in importin beta1 mRNA transport to neuronal axons and to the cellularperiphery in fibroblasts. We showed that depletion of importin beta1 from axons by a 3’ UTRknockout (KO) or by sequestration of nucleolin enhances neuronal outgrowth, concomitantlywith a subcellular shift in protein synthesis. Similar perturbations affect the morphology andsize of fibroblasts in culture (Perry et al. 2016). Thus, the subcellular localization of nucleolinassociatedmRNAs regulates axon length and cell size and growth control mechanisms.The link between subcellular localization of protein synthesis and neuronal length sensingmotivated us to examine how protein synthesis is initiated locally in neurons. We found thatmTOR (mechanistic target of rapamycin), a central regulator of protein synthesis, was upregulatedand activated in injured axons, due to local translation of mTOR mRNA. This mRNAwas transported into axons by the cell size–regulating RNA-binding protein nucleolin.Furthermore, mTOR controlled its own local translation and that of retrograde injury signalingmolecules such as importin beta1 and STAT3 (signal transducer and activator of transcription3) in injured axons. Deletion of the mTOR 3′ untranslated region (3′UTR) in mice reducedmTOR in axons and decreased local translation after nerve injury and decreased proprioceptiveneuronal survival after nerve injury. Thus, mRNA localization enables spatiotemporal controlof mTOR pathways regulating local translation and long-range intracellular signaling(Terenzio et al., 2018).Perry et al., 2016, Nucleolin-Mediated RNA Localization Regulates Neuron Growth andCycling Cell Size. Cell Reports 16, 1664–1676.Terenzio et al., 2018, Locally translated mTOR controls axonal local translation in nerveinjury. Science 359, 1416–1421.References citedAhmad AH, Ismail Z. (2002) c-fos and its Consequences in Pain. 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