TAU MODIFIES RIBOSOMAL DYNAMICS SHIFTING TRANSLATIONAL PROFILES IN AD

A major challenge in the field of tauopathies is identifying the molecular mechanisms by which tau causes neuronal dysfunction. Tau-ribosome complexes were identified over thirty years ago as predominant structures in neurons of Alzheimer's disease (AD) brains. However, the consequence of tau-ribosome complexes remains unknown. We recently determined that tauopathic models and AD brains differentially translate pools of mRNA compared to non-tauopathic controls. Therefore, we hypothesized that tau drives ribosome's affinity for discrete transcripts. We measured changes in the synthesis of proteins involved in translation by coupling microarrays and proteomics in the TET/OFF rTg4510 tau transgenic mice. We focused on newly synthesized proteins by tagging growing polypeptide chains with puromycin, a tRNA analog. We injected puromycin 30min before harvesting the brain. We then isolated puromycinylated proteins by IP and identified them using LC-MS/MS. We then validated our results using Alzheimer's disease brains by IP:WB of the ribosomal protein S6 among other 5’TOP mRNAs, which mediate production of proteins involved in translation. We determined that rTg4510 mice strongly shifted gene expression pools, and that this shift returned to normal when tau expression was suppressed using doxycycline. Strikingly, a subset of transcripts that were modified by tau expression were involved in promoting translation. Nascent proteomics confirmed that tau suppression rescued synthesis of translationally related peptides. These data suggested that tau impairs an inherent mechanism of translational selectivity. Therefore, we validated these results in human Alzheimer's brains by focusing on rpS6, a mediator of translational switch. rpS6 promotes translation of 5'TOP mRNAs, which are transcripts expressed during cell growth. We found that in AD brains, tau forms robust complexes with rpS6 and showed translational deficiencies in these transcripts. We identified a novel mechanism of cellular dysfunction by which pathological tau disturbs translation. Pathological tau diminishes ribosomal affinity for 5’TOP mRNAs. As a result, basic brain functions such as memory formation, consolidation, and retrieval are substantially impaired. This work advances the field by filling the gap in understanding of tau-mediated ribosomal dysfunction. This knowledge will serve to better characterize the link between tau and memory impairment in order to develop novel therapeutic strategies.