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. Conversely, RTP801 overexpression inhibited the splicing of XBP1. Similarly, in human AD postmortem hippocampal samples, where RTP801 is upregulated, we found that XBP1 splicing was 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.

RTP801 regulates motor cortex synaptic transmission and learning.

BACKGROUND: RTP801/REDD1 is a stress-regulated protein whose upregulation is necessary and sufficient to trigger neuronal death in in vitro and in vivo models of Parkinson's and Huntington's diseases and is up regulated in compromised neurons in human postmortem brains of both neurodegenerative disorders. Indeed, in both Parkinson's and Huntington's disease mouse models, RTP801 knockdown alleviates motor-learning deficits. RESULTS: We investigated the physiological role of RTP801 in neuronal plasticity and we found RTP801 in rat, mouse and human synapses. The absence of RTP801 enhanced excitatory synaptic transmission in both neuronal cultures and brain slices from RTP801 knock-out (KO) mice. Indeed, RTP801 KO mice showed improved motor learning, which correlated with lower spine density but increased basal filopodia and mushroom spines in the motor cortex layer V. This paralleled with higher levels of synaptosomal GluA1 and TrkB receptors in homogenates derived from KO mice motor cortex, proteins that are associated with synaptic strengthening. CONCLUSIONS: Altogether, these results indicate that RTP801 has an important role modulating neuronal plasticity and motor learning. They will help to understand its role in neurodegenerative disorders where RTP801 levels are detrimentally upregulated.

Loss of NEDD4 contributes to RTP801 elevation and neuron toxicity: implications for Parkinson's disease.

Parkinson's disease (PD) is a disorder characterized by the degeneration of certain neuronal populations in the central and peripheral nervous system. One of the hallmarks of the disease is the toxic accumulation of proteins within susceptible neurons due to major impairment in the degradation/clearance protein systems.RTP801 is a pro-apoptotic protein that is sufficient and necessary to induce neuronal death in cellular and animal models of PD. RTP801 is also upregulated in sporadic and parkin mutant PD brains. Here, we report the role of NEDD4, an E3 ligase involved in α-synuclein degradation and PD pathogenesis, in the regulation of RTP801 protein levels and toxicity. NEDD4 polyubiquitinates RTP801 in a cell-free system and in cellular cultures, and they interact physically. NEDD4 conjugates K63-ubiquitin chains to RTP801 and targets it for degradation. NEDD4 regulates RTP801 protein levels in both cultured cells and in the brain tissue. NEDD4 levels are diminished in nigral neurons from human PD brains. Interestingly, neurotoxin 6-OHDA decreases dramatically NEDD4 protein expression but elevates RTP801 protein levels. Moreover, NEDD4 protects neuronal PC12 cells from both 6-OHDA and RTP801-induced toxicity. In primary cortical neurons, NEDD4 knockdown toxicity is mediated by RTP801 since the double knockdown of RTP801 and NEDD4 abrogates the loss of phospho Ser473-Akt and the appearance of caspase-cleaved spectrin fragments.Thus, NEDD4 ligase regulates RTP801 and is sensitive to PD-associated oxidative stress. This suggests that NEDD4 loss of function in PD could contribute importantly into neuronal death by elevating RTP801.

RTP801 Is Involved in Mutant Huntingtin-Induced Cell Death.

RTP801 expression is induced by cellular stress and has a pro-apoptotic function in non-proliferating differentiated cells such as neurons. In several neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease, elevated levels of RTP801 have been observed, which suggests a role for RTP801 in neuronal death. Neuronal death is also a pathological hallmark in Huntington's disease (HD), an inherited neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin gene. Currently, the exact mechanisms underlying mutant huntingtin (mhtt)-induced toxicity are still unclear. Here, we investigated whether RTP801 is involved in (mhtt)-induced cell death. Ectopic exon-1 mhtt elevated RTP801 mRNA and protein levels in nerve growth factor (NGF)-differentiated PC12 cells and in rat primary cortical neurons. In neuronal PC12 cells, mhtt also contributed to RTP801 protein elevation by reducing its proteasomal degradation rate, in addition to promoting RTP801 gene expression. Interestingly, silencing RTP801 expression with short hairpin RNAs (shRNAs) blocked mhtt-induced cell death in NGF-differentiated PC12 cells. However, RTP801 protein levels were not altered in the striatum of Hdh(Q7/Q111) and R6/1 mice, two HD models that display motor deficits but not neuronal death. Importantly, RTP801 protein levels were elevated in both neural telencephalic progenitors differentiated from HD patient-derived induced pluripotent stem cells and in the putamen and cerebellum of human HD postmortem brains. Taken together, our results suggest that RTP801 is a novel downstream effector of mhtt-induced toxicity and that it may be relevant to the human disease.

RTP801/REDD1: a stress coping regulator that turns into a troublemaker in neurodegenerative disorders.

Mechanistic target of Rapamycin (mTOR) pathway regulates essential processes directed to preserve cellular homeostasis, such as cell growth, proliferation, survival, protein synthesis and autophagy. Importantly, mTOR pathway deregulation has been related to many diseases. Indeed, it has become a hallmark in neurodegenerative disorders, since a fine-tuned regulation of mTOR activities is crucial for neuron function and survival. RTP801/REDD1/Dig2 has become one of the most puzzling regulators of mTOR. Although the mechanism is not completely understood, RTP801 inactivates mTOR and Akt via the tuberous sclerosis complex (TSC1/TSC2) in many cellular contexts. Intriguingly, RTP801 protects dividing cells from hypoxia or H2O2-induced apoptosis, while it sensitizes differentiated cells to stress. Based on experimental models of Parkinson's disease (PD), it has been proposed that at early stages of the disease, stress-induced RTP801 upregulation contributes to mTOR repression, in an attempt to maintain cell function and viability. However, if RTP801 elevation is sustained, it leads to neuron cell death by a sequential inhibition of mTOR and Akt. Here, we will review RTP801 deregulation of mTOR in a context of PD and other neurodegenerative disorders.