RTP801 INTERACTS WITH THE tRNA LIGASE COMPLEX AND DYSREGULATES ITS RNA LIGASE ACTIVITY IN ALZHEIMER'S DISEASE

RTP801/REDD1 is a stress-responsive protein overexpressed in neurodegenerative diseases such as Alzheimer's disease (AD) that contributes to cognitive deficits and neuroinflammation. Here we found that RTP801 interacts with HSPC117, DDX1, and CGI-99, three members of the tRNA ligase complex (tRNA-LC), which ligates the excised exons of intron-containing tRNAs and the mRNA exons of the transcription factor XBP1 during the unfolded protein response (UPR). We also found that RTP801 modulates the mRNA ligase activity of the complex in vitro, since RTP801 knockdown promoted XBP1 splicing and the expression of its transcriptional target, SEC24D. On the contrary, RTP801 overexpression inhibited the splicing of XBP1. In this line, in human AD postmortem hippocampal samples, where RTP801 protein levels are upregulated, we found that XBP1 splicing dramatically decreased. In the 5xFAD mouse model of AD, silencing RTP801 expression in hippocampal neurons promoted Xbp1 splicing and prevented the accumulation of intron-containing pre-tRNAs. Finally, the tRNA-enriched fraction obtained from 5xFAD mice promoted abnormal dendritic arborization in cultured hippocampal neurons, and RTP801 silencing in the source neurons prevented this phenotype. Altogether, these results show that elevated RTP801 impairs RNA processing in vitro and in vivo, in the context of AD and suggest that RTP801 inhibition could be a promising therapeutic approach. Overall design: To study the effect of RTP801 on tRNA processing, we specifically silenced RTP801 in the hippocampal neurons of 6-month-old wild type (WT) and 5xFAD mice using shRNA (shRTP801, or shControl (shCT)) and analyzed the pool of hippocampal tRNAs by Hydro-tRNA-seq