Regulatory functions of hnRNP R for local translation in axons: roles in motoneuron development and neurodegeneration

Neurons critically rely on precise spatial and temporal control of protein synthesis for axon growth and the establishment and maintenance of neuromuscular connections. RNA-binding proteins (RBPs) play vital roles in axon growth by regulating the axonal localization and translation of mRNAs encoding proteins involved in cytoskeletal components and other proteins required for axon development. Dysfunction of RBPs can cause neurological disorders such as amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD). In this study, we investigated the function of the neuronal RBP heterogeneous nuclear ribonucleoprotein R (hnRNP R), which is crucial for axon growth in motoneurons. hnRNP R is abundant in the nervous system and primarily localized in the cell body of neuronal cells but is also found in axons and growth cones. In the first part of the study, we report that cultured motoneurons from hnRNP R knockout mice exhibit axon growth defects accompanied by reduced synthesis of cytoskeletal and synaptic components in axons without affecting their axonal transcript level. We present evidence that hnRNP R is associated with translation initiation complexes in axons and facilitates axonal protein synthesis by promoting O-GlcNAcylation of eIF4G through interaction with O-linked β-N-acetylglucosamine (O-GlcNAc) transferase (Ogt). Furthermore, we show that restoring axonal O-GlcNAc levels rescued local protein synthesis and axon growth defects of hnRNP R knockout motoneurons. Additionally, we observed that hnRNP R knockout mice display denervated neuromuscular junctions and impaired motor behavior. Our findings pinpoint that hnRNP R regulates the local synthesis of essential factors necessary for axon growth and the formation of neuromuscular innervations. In the second part of this study, we show that hnRNP R mediates the axonal localization of the microtubule-associated protein tau (Mapt) mRNA encoding tau protein through binding to the 3' untranslated region (3' UTR) of Mapt mRNA. Tau as a microtubule bundler, regulating microtubule assembly. However, in AD, tau is hyperphosphorylated and plays a crucial role in the pathogenesis of AD and other tauopathies by altering tau's normal function, promoting its polymerization and aggregation into neurofibrillary tangles (NFTs). Our data reveal that hnRNP R loss selectively lowers the level of Mapt mRNA and tau protein in axons but not neuronal cell bodies. We found that 5xFAD mice, an AD mouse model, deficient in hnRNP R exhibit reduced plaque and phospho-tau burden in the brain. In addition, both mouse and human neurons treated with antisense oligonucleotides (ASOs) to prevent hnRNP R binding to Mapt mRNA show a similar reduction in axonal Mapt mRNA and tau protein levels as in hnRNP R-deficient neurons. Lastly, intracerebroventricular injection of a MAPT-ASO ameliorates the phospho-tau and plaque load in 5xFAD mice. As tau pathology occurs first in the axons of affected neurons, our data suggest that lowering tau selectively in axons represents an innovative therapeutic approach for the treatment of AD and other tauopathies.