TDP-43 transports ribosomal protein mRNA to regulate axonal local translation in neuronal axons
Mislocalization and abnormal deposition of TDP-43 into the cytoplasm (TDP-43 proteinopathy) is a hallmark in neurons of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). However, the pathogenic mechanism of the diseases linked to TDP-43 is largely unknown. We hypothesized that the failure of mRNA transport to neuronal axons by TDP-43 may contribute to neurodegeneration in ALS and FTLD, and sought to examine the function of TDP-43 by identifying its target mRNA for axonal transport. We found that mRNAs related to translational function including ribosomal proteins (RPs) were decreased by shRNA-based TDP-43 knock-down in neurites of cortical neurons. TDP-43 binds to and transports the RP mRNAs through their 5′ untranslated region, which contains a common 5′ terminal oligopyrimidine tract motif and a downstream GC-rich region. We showed by employing in vitro and in vivo models that the RP mRNAs were translated and incorporated into native ribosomes locally in axons to maintain functionality of axonal ribosomes, which is required for local protein synthesis in response to stimulation and stress to axons. We also found that RP mRNAs were reduced in the pyramidal tract of sporadic ALS cases harboring TDP-43 pathology. Our results elucidated a novel function of TDP-43 to control transport of RP mRNAs and local translation by ribosomes to maintain morphological integrity of neuronal axons, and proved the influence of this function of TDP-43 on neurodegeneration in ALS and FTLD associated with TDP-43 proteinopathy.