RTCB-1 mediates neuroprotection via XBP-1 mRNA splicing in the unfolded protein response pathway

Parkinson's disease (PD), the second most prevalent neurodegenerative disorder, is characterized by the degeneration of dopamine (DA) neurons and age-dependent formation of protein inclusions that contain the α-synuclein (α-syn) protein. RNA interference (RNAi) screening using Caenorhabditis elegans identified RTCB-1, an uncharacterized gene product, as one of several significant modifiers of α-syn protein misfolding. RTCB-1 is the worm ortholog of the human HSPC117 protein, a component of RNA trafficking granules in mammalian neurons. Here we show that RTCB-1 protects C. elegans DA neurons from age-dependent degeneration induced by human α-syn. Moreover, neuronal-specific RNAi depletion of rtcb-1 enhanced α-syn-induced degeneration. Similar results were obtained when worms were exposed to the DA neurotoxin 6-hydroxydopamine. HSPC117 has been characterized recently as an essential subunit of the human tRNA splicing ligase complex. tRNA ligases have alternative functions in RNA repair and nonconventional mRNA splicing events. For example, in yeast, unconventional splicing of HAC1, a transcription factor that controls the unfolded protein response (UPR), is mediated by a tRNA ligase. In C. elegans, we demonstrate that RTCB-1 is necessary for xbp-1 (worm homolog of HAC1) mRNA splicing. Moreover, using a RNA ligase-dead mutant, we determine that the ligase activity of worm RTCB-1 is required for its neuroprotective role, which, in turn, is mediated through XBP-1 in the UPR pathway. Collectively, these studies highlight the mechanistic intersection of RNA processing and proteostasis in mediating neuroprotection.