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.

Motor skill learning modulates striatal extracellular vesicles' content in a mouse model of Huntington's disease.

Huntington's disease (HD) is a neurological disorder caused by a CAG expansion in the Huntingtin gene (HTT). HD pathology mostly affects striatal medium-sized spiny neurons and results in an altered cortico-striatal function. Recent studies report that motor skill learning, and cortico-striatal stimulation attenuate the neuropathology in HD, resulting in an amelioration of some motor and cognitive functions. During physical training, extracellular vesicles (EVs) are released in many tissues, including the brain, as a potential means for inter-tissue communication. To investigate how motor skill learning, involving acute physical training, modulates EVs crosstalk between cells in the striatum, we trained wild-type (WT) and R6/1 mice, the latter with motor and cognitive deficits, on the accelerating rotarod test, and we isolated their striatal EVs. EVs from R6/1 mice presented alterations in the small exosome population when compared to WT. Proteomic analyses revealed that striatal R6/1 EVs recapitulated signaling and energy deficiencies present in HD. Motor skill learning in R6/1 mice restored the amount of EVs and their protein content in comparison to naïve R6/1 mice. Furthermore, motor skill learning modulated crucial pathways in metabolism and neurodegeneration. All these data provide new insights into the pathogenesis of HD and put striatal EVs in the spotlight to understand the signaling and metabolic alterations in neurodegenerative diseases. Moreover, our results suggest that motor learning is a crucial modulator of cell-to-cell communication in the striatum.

Increased Phospho-AKT in Blood Cells from LRRK2 G2019S Mutation Carriers.

The purpose of this study was to investigate whether  differential phosphorylation states of blood markers can identify patients with LRRK2 Parkinson's disease (PD). We assessed phospho(P)-Ser-935-LRRK2 and P-Ser-473-AKT levels in peripheral blood cells from patients with G2019S LRRK2-associated PD (L2PD, n = 31), G2019S LRRK2 non-manifesting carriers (L2NMC, n = 26), idiopathic PD (iPD, n = 25), and controls (n = 40, total n = 122). We found no differences at P-Ser-935-LRRK2 between groups but detected a specific increase of P-Ser-473-AKT levels in all G2019S carriers, either L2PD or L2NMC, absent in iPD. Although insensitive to LRRK2 inhibition, our study identifies P-Ser-473-AKT as an endogenous candidate biomarker for peripheral inflammation in G2019S carriers using accessible blood cells. ANN NEUROL 2022;92:888-894.

Tcf20 deficiency is associated with increased liver fibrogenesis and alterations in mitochondrial metabolism in mice and humans.

BACKGROUND & AIMS: Transcription co-activator factor 20 (TCF20) is a regulator of transcription factors involved in extracellular matrix remodelling. In addition, TCF20 genomic variants in humans have been associated with impaired intellectual disability. Therefore, we hypothesized that TCF20 has several functions beyond those described in neurogenesis, including the regulation of fibrogenesis. METHODS: Tcf20 knock-out (Tcf20-/- ) and Tcf20 heterozygous mice were generated by homologous recombination. TCF20 gene genotyping and expression was assessed in patients with pathogenic variants in the TCF20 gene. Neural development was investigated by immufluorescense. Mitochondrial metabolic activity was evaluated with the Seahorse analyser. The proteome analysis was carried out by gas chromatography mass-spectrometry. RESULTS: Characterization of Tcf20-/- newborn mice showed impaired neural development and death after birth. In contrast, heterozygous mice were viable but showed higher CCl4 -induced liver fibrosis and a differential expression of genes involved in extracellular matrix homeostasis compared to wild-type mice, along with abnormal behavioural patterns compatible with autism-like phenotypes. Tcf20-/- embryonic livers and mouse embryonic fibroblast (MEF) cells revealed differential expression of structural proteins involved in the mitochondrial oxidative phosphorylation chain, increased rates of mitochondrial metabolic activity and alterations in metabolites of the citric acid cycle. These results parallel to those found in patients with TCF20 pathogenic variants, including alterations of the fibrosis scores (ELF and APRI) and the elevation of succinate concentration in plasma. CONCLUSIONS: We demonstrated a new role of Tcf20 in fibrogenesis and mitochondria metabolism in mice and showed the association of TCF20 deficiency with fibrosis and metabolic biomarkers in humans.

Differential Phospho-Signatures in Blood Cells Identify LRRK2 G2019S Carriers in Parkinson's Disease.

BACKGROUND: The clinicopathological phenotype of G2019S LRRK2-associated Parkinson's disease (L2PD) is similar to idiopathic Parkinson's disease (iPD), and G2019S LRRK2 nonmanifesting carriers (L2NMCs) are at increased risk for development of PD. With various therapeutic strategies in the clinical and preclinical pipeline, there is an urgent need to identify biomarkers that can aid early diagnosis and patient enrichment for ongoing and future LRRK2-targeted trials. OBJECTIVE: The objective of this work was to investigate differential protein and phospho-protein changes related to G2019S mutant LRRK2 in peripheral blood mononuclear cells from G2019S L2PD patients and G2019S L2NMCs, identify specific phospho-protein changes associated with the G2019S mutation and with disease status, and compare findings with patients with iPD. METHODS: We performed an unbiased phospho-proteomic study by isobaric label-based mass spectrometry using peripheral blood mononuclear cell group pools from a LRRK2 cohort from Spain encompassing patients with G2019S L2PD (n = 20), G2019S L2NMCs (n = 20), healthy control subjects (n = 30), patients with iPD (n = 15), patients with R1441G L2PD (n = 5), and R1441G L2NMCs (n = 3) (total N = 93). RESULTS: Comparing G2019S carriers with healthy controls, we identified phospho-protein changes associated with the G2019S mutation. Moreover, we uncovered a specific G2019S phospho-signature that changes with disease status and can discriminate patients with G2019S L2PD, G2019S L2NMCs, and healthy controls. Although patients with iPD showed a differential phospho-proteomic profile, biological enrichment analyses revealed similar changes in deregulated pathways across the three groups. CONCLUSIONS: We found a differential phospho-signature associated with LRRK2 G2019S for which, consistent with disease status, the phospho-profile from PD at-risk G2019S L2NMCs was more similar to healthy controls than patients with G2019S L2PD with the manifested disease. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

R1441G but not G2019S mutation enhances LRRK2 mediated Rab10 phosphorylation in human peripheral blood neutrophils.

Heterozygous gain-of-kinase function variants in LRRK2 (leucine-rich repeat kinase 2) cause 1-2% of all cases of Parkinson's disease (PD) albeit with incomplete and age-dependent penetrance. All pathogenic LRRK2 mutations reside within the two catalytic domains of LRRK2-either in its kinase domain (e.g. G2019S) with modest effect or its ROC-COR GTPase domain (e.g. R1441G/H) with large effect on LRRK2 kinase activity. We have previously reported assays to interrogate LRRK2 kinase pathway activity in human bio-samples measuring phosphorylation of its endogenous substrate Rab10, that mirrors LRRK2 kinase activation status. Here, we isolated neutrophils from fresh peripheral blood from 101 participants including 42 LRRK2 mutation carriers (21 with the G2019S and 21 with the R1441G mutations), 27 patients with idiopathic PD, and 32 controls. Using a dual approach, LRRK2 dependent Rab10 phosphorylation at Threonine 73 (pRab10Thr73) was measured by quantitative multiplexed immunoblotting for pRab10Thr73/total Rab10 as well as targeted mass-spectrometry for absolute pRab10Thr73 occupancy. We found a significant over fourfold increase in pRab10Thr73 phosphorylation in carriers of the LRRK2 R1441G mutation irrespective of clinical disease status. The effect of the LRRK2 G2019S mutation did not reach statistical significance. Furthermore, we show that LRRK2 phosphorylation at Serine 935 is not a marker for LRRK2 kinase activity in human neutrophils. When analysing pRab10Thr73 phosphorylation in post-mortem brain samples, we observed overall high variability irrespective of clinical and LRRK2 mutation status and attributed this mainly to the adverse effect of the peri- and post-mortem period on the stability of posttranslational modifications such as protein phosphorylation. Overall, in vivo LRRK2 dependent pRab10Thr73 phosphorylation in human peripheral blood neutrophils is a specific, robust and promising biomarker for significant LRRK2 kinase hyperactivation, as with the LRRK2 R1441G mutation. Additional readouts and/or assays may be needed to increase sensitivity to detect modest LRRK2 kinase activation, as with the LRRK2 G2019S mutation. Our assays could be useful for patient stratification and target engagement studies for LRRK2 kinase inhibitors.

Increased translation as a novel pathogenic mechanism in Huntington's disease.

Huntington's disease is a neurodegenerative disorder caused by a CAG repeat expansion in exon 1 of the huntingtin gene. Striatal projection neurons are mainly affected, leading to motor symptoms, but molecular mechanisms involved in their vulnerability are not fully characterized. Here, we show that eIF4E binding protein (4E-BP), a protein that inhibits translation, is inactivated in Huntington's disease striatum by increased phosphorylation. Accordingly, we detected aberrant de novo protein synthesis. Proteomic characterization indicates that translation specifically affects sets of proteins as we observed upregulation of ribosomal and oxidative phosphorylation proteins and downregulation of proteins related to neuronal structure and function. Interestingly, treatment with the translation inhibitor 4EGI-1 prevented R6/1 mice motor deficits, although corticostriatal long-term depression was not markedly changed in behaving animals. At the molecular level, injection of 4EGI-1 normalized protein synthesis and ribosomal content in R6/1 mouse striatum. In conclusion, our results indicate that dysregulation of protein synthesis is involved in mutant huntingtin-induced striatal neuron dysfunction.

SNCA and mTOR Pathway Single Nucleotide Polymorphisms Interact to Modulate the Age at Onset of Parkinson's Disease.

BACKGROUND: Single nucleotide polymorphisms (SNPs) in the α-synuclein (SNCA) gene are associated with differential risk and age at onset (AAO) of both idiopathic and Leucine-rich repeat kinase 2 (LRRK2)-associated Parkinson's disease (PD). Yet potential combinatory or synergistic effects among several modulatory SNPs for PD risk or AAO remain largely underexplored. OBJECTIVES: The mechanistic target of rapamycin (mTOR) signaling pathway is functionally impaired in PD. Here we explored whether SNPs in the mTOR pathway, alone or by epistatic interaction with known susceptibility factors, can modulate PD risk and AAO. METHODS: Based on functional relevance, we selected a total of 64 SNPs mapping to a total of 57 genes from the mTOR pathway and genotyped a discovery series cohort encompassing 898 PD patients and 921 controls. As a replication series, we screened 4170 PD and 3014 controls available from the International Parkinson's Disease Genomics Consortium. RESULTS: In the discovery series cohort, we found a 4-loci interaction involving STK11 rs8111699, FCHSD1 rs456998, GSK3B rs1732170, and SNCA rs356219, which was associated with an increased risk of PD (odds ratio = 2.59, P < .001). In addition, we also found a 3-loci epistatic combination of RPTOR rs11868112 and RPS6KA2 rs6456121 with SNCA rs356219, which was associated (odds ratio = 2.89; P < .0001) with differential AAO. The latter was further validated (odds ratio = 1.56; P = 0.046-0.047) in the International Parkinson's Disease Genomics Consortium cohort. CONCLUSIONS: These findings indicate that genetic variability in the mTOR pathway contributes to SNCA effects in a nonlinear epistatic manner to modulate differential AAO in PD, unraveling the contribution of this cascade in the pathogenesis of the disease. © 2019 International Parkinson and Movement Disorder Society.

Chelerythrine promotes Ca2+-dependent calpain activation in neuronal cells in a PKC-independent manner.

BACKGROUND: Chelerythrine is widely used as a broad range protein kinase C (PKC) inhibitor, but there is controversy about its inhibitory effect. Moreover, it has been shown to exert PKC-independent effects on non-neuronal cells. METHODS: In this study we investigated possible off-target effects of chelerythrine on cultured cortical rodent neurons and a neuronal cell line. RESULTS: We found that 10µM chelerythrine, a commonly used concentration in neuronal cultures, reduces PKC and cAMP-dependent protein kinase substrates phosphorylation in mouse cultured cortical neurons, but not in rat primary cortical neurons or in a striatal cell line. Furthermore, we found that incubation with chelerythrine increases pERK1/2 levels in all models studied. Moreover, our results show that chelerythrine promotes calpain activation as assessed by the cleavage of spectrin, striatal-enriched protein tyrosine phosphatase and calcineurin A. Remarkably, chelerythrine induces a concentration-dependent increase in intracellular Ca2+ levels that mediates calpain activation. In addition, we found that chelerythrine induces ERK1/2- and calpain-independent caspase-3 activation that can be prevented by the Ca2+ chelator BAPTA-AM. CONCLUSIONS: This is the first report showing that chelerythrine promotes Ca2+-dependent calpain activation in neuronal cells, which has consequences for the interpretation of studies using this compound. GENERAL SIGNIFICANCE: Chelerythrine is still marketed as a specific PKC inhibitor and extensively used in signal transduction studies. We believe that the described off-target effects should preclude its use as a PKC inhibitor in future works.

ATF4 protects against neuronal death in cellular Parkinson's disease models by maintaining levels of parkin.

Parkinson's disease (PD) is a common neurodegenerative disorder, for which there are no effective disease-modifying therapies. The transcription factor ATF4 (activating transcription factor 4) is induced by multiple PD-relevant stressors, such as endoplasmic reticulum stress and oxidative damage. ATF4 may exert either protective or deleterious effects on cell survival, depending on the paradigm. However, the role of ATF4 in the pathogenesis of PD has not been explored. We find that ATF4 levels are increased in neuromelanin-positive neurons in the substantia nigra of a subset of PD patients relative to controls. ATF4 levels are also upregulated in neuronal PC12 cells treated with the dopaminergic neuronal toxins 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenylpyridinium (MPP+). To explore the role of ATF4 in cell survival in PD-relevant contexts, we either silenced or overexpressed ATF4 in cellular models of PD. In neuronal PC12 cells, silencing of ATF4 enhanced cell death in response to either 6-OHDA or MPP+. Conversely, overexpression of ATF4 reduced cell death caused by dopaminergic neuronal toxins. ATF4 was also protective against 6-OHDA-induced death of cultured mouse ventral midbrain dopaminergic neurons. We further show that parkin, a gene associated with autosomal recessive PD, plays a critical role in ATF4-mediated protection. After treatment with 6-OHDA or MPP+, parkin protein levels fall, despite an increase in mRNA levels. ATF4 silencing exacerbates the toxin-induced reduction of parkin, whereas ATF4 overexpression partially preserves parkin levels. Finally, parkin silencing blocked the protective capacity of ATF4. These results indicate that ATF4 plays a protective role in PD through the regulation of parkin.

Preserved VPS13A distribution and expression in Huntington's disease: divergent mechanisms of action for similar movement disorders?

VPS13A disease and Huntington's disease (HD) are two basal ganglia disorders that may be difficult to distinguish clinically because they have similar symptoms, neuropathological features, and cellular dysfunctions with selective degeneration of the medium spiny neurons of the striatum. However, their etiology is different. VPS13A disease is caused by a mutation in the VPS13A gene leading to a lack of protein in the cells, while HD is due to an expansion of CAG repeat in the huntingtin (Htt) gene, leading to aberrant accumulation of mutant Htt. Considering the similarities of both diseases regarding the selective degeneration of striatal medium spiny neurons, the involvement of VPS13A in the molecular mechanisms of HD pathophysiology cannot be discarded. We analyzed the VPS13A distribution in the striatum, cortex, hippocampus, and cerebellum of a transgenic mouse model of HD. We also quantified the VPS13A levels in the human cortex and putamen nucleus; and compared data on mutant Htt-induced changes in VPS13A expression from differential expression datasets. We found that VPS13A brain distribution or expression was unaltered in most situations with a decrease in the putamen of HD patients and small mRNA changes in the striatum and cerebellum of HD mice. We concluded that the selective susceptibility of the striatum in VPS13A disease and HD may be a consequence of disturbances in different cellular processes with convergent molecular mechanisms already to be elucidated.

STAT3 and REDD1: an unconventional story of gene repression.

The non-canonical functions of the transcription factor STAT3 have been poorly studied in comparison to its canonical mechanisms of gene expression activation. Here, Köhler et al. put the spotlight on a novel unconventional repressing mechanism of STAT3 over the REDD1 gene, named DDIT4. These findings are crucial to expand the knowledge of the stress-induced short-lived REDD1 protein that inactivates mTOR and the consequences of this fine-tuned regulation in the context of pathological conditions such as cancer or neurodegenerative diseases.

Neuron-derived extracellular vesicles contain synaptic proteins, promote spine formation, activate TrkB-mediated signalling and preserve neuronal complexity.

Extracellular vesicles (EVs) play an important role in intercellular communication as carriers of signalling molecules such as bioactive miRNAs, proteins and lipids. EVs are key players in the functioning of the central nervous system (CNS) by influencing synaptic events and modulating recipient neurons. However, the specific role of neuron-to-neuron communication via EVs is still not well understood. Here, we provide evidence that primary neurons uptake neuron-derived EVs in the soma, dendrites, and even in the dendritic spines, and carry synaptic proteins. Neuron-derived EVs increased spine density and promoted the phosphorylation of Akt and ribosomal protein S6 (RPS6), via TrkB-signalling, without impairing the neuronal network activity. Strikingly, EVs exerted a trophic effect on challenged nutrient-deprived neurons. Altogether, our results place EVs in the spotlight for synaptic plasticity modulation as well as a possible therapeutic tool to fight neurodegeneration.

RTP801 mediates transneuronal toxicity in culture via extracellular vesicles.

Extracellular vesicles (EVs) play a crucial role in intercellular communication, participating in the paracrine trophic support or in the propagation of toxic molecules, including proteins. RTP801 is a stress-regulated protein, whose levels are elevated during neurodegeneration and induce neuron death. However, whether RTP801 toxicity is transferred trans-neuronally via EVs remains unknown. Hence, we overexpressed or silenced RTP801 protein in cultured cortical neurons, isolated their derived EVs (RTP801-EVs or shRTP801-EVs, respectively), and characterized EVs protein content by mass spectrometry (MS). RTP801-EVs toxicity was assessed by treating cultured neurons with these EVs and quantifying apoptotic neuron death and branching. We also tested shRTP801-EVs functionality in the pathologic in vitro model of 6-Hydroxydopamine (6-OHDA). Expression of RTP801 increased the number of EVs released by neurons. Moreover, RTP801 led to a distinct proteomic signature of neuron-derived EVs, containing more pro-apoptotic markers. Hence, we observed that RTP801-induced toxicity was transferred to neurons via EVs, activating apoptosis and impairing neuron morphology complexity. In contrast, shRTP801-EVs were able to increase the arborization in recipient neurons. The 6-OHDA neurotoxin elevated levels of RTP801 in EVs, and 6-OHDA-derived EVs lost the mTOR/Akt signalling activation via Akt and RPS6 downstream effectors. Interestingly, EVs derived from neurons where RTP801 was silenced prior to exposing them to 6-OHDA maintained Akt and RPS6 transactivation in recipient neurons. Taken together, these results suggest that RTP801-induced toxicity is transferred via EVs, and therefore, it could contribute to the progression of neurodegenerative diseases, in which RTP801 is involved.

CD300f immune receptor contributes to healthy aging by regulating inflammaging, metabolism, and cognitive decline.

Emerging evidence suggests that immune receptors may participate in many aging-related processes such as energy metabolism, inflammation, and cognitive decline. CD300f, a TREM2-like lipid-sensing immune receptor, is an exceptional receptor as it integrates activating and inhibitory cell-signaling pathways that modulate inflammation, efferocytosis, and microglial metabolic fitness. We hypothesize that CD300f can regulate systemic aging-related processes and ultimately healthy lifespan. We closely followed several cohorts of two strains of CD300f-/- and WT mice of both sexes for 30 months and observed an important reduction in lifespan and healthspan in knockout mice. This was associated with systemic inflammaging, increased cognitive decline, reduced brain glucose uptake observed by 18FDG PET scans, enrichment in microglial aging/neurodegeneration phenotypes, proteostasis alterations, senescence, increased frailty, and sex-dependent systemic metabolic changes. Moreover, the absence of CD300f altered macrophage immunometabolic phenotype. Taken together, we provide strong evidence suggesting that myeloid cell CD300f immune receptor contributes to healthy aging.

RTP801/REDD1 regulates the timing of cortical neurogenesis and neuron migration.

The generation, differentiation, and migration of newborn neurons are critical features of normal brain development that are subject to both extracellular and intracellular regulation. However, the means of such control are only partially understood. Here, we show that expression of RTP801/REDD1, an inhibitor of mTOR (mammalian target of rapamycin) activation, is regulated during neuronal differentiation and that RTP801 functions to influence the timing of both neurogenesis and neuron migration. RTP801 levels are high in embryonic cortical neuroprogenitors, diminished in newborn neurons, and low in mature neurons. Knockdown of RTP801 in vitro and in vivo accelerates cell cycle exit by neuroprogenitors and their differentiation into neurons. It also disrupts migration of rat newborn neurons to the cortical plate and results in the ectopic localization of mature neurons. On the other hand, RTP801 overexpression delays neuronal differentiation. These findings suggest that endogenous RTP801 plays an essential role in temporal control of cortical development and in cortical patterning.

Rapamycin protects against neuron death in in vitro and in vivo models of Parkinson's disease.

We report that rapamycin, an allosteric inhibitor of certain but not all actions of the key cellular kinase mammalian target of rapamycin (mTOR), protects neurons from death in both cellular and animal toxin models of Parkinson's disease (PD). This protective action appears to be attributable to blocked translation of RTP801/REDD1/Ddit4, a protein that is induced in cell and animal models of PD and in affected neurons of PD patients and that causes neuron death by leading to dephosphorylation of the survival kinase Akt. In support of this mechanism, in PD models, rapamycin spares phosphorylation of Akt at a site critical for maintenance of its survival-promoting activity. The capacity of rapamycin to provide neuroprotection in PD models appears to arise from its selective suppression of some but not all actions of mTOR, as indicated by the contrasting finding that Torin1, a full catalytic mTOR inhibitor, is not protective and induces Akt dephosphorylation and neuron death.

Akt as a victim, villain and potential hero in Parkinson's disease pathophysiology and treatment.

There are two major purposes of this essay. The first is to summarize existing evidence that irrespective of the initiating causes, neuron death and degeneration in Parkinson's disease (PD) are due to the common feature of failure of signaling by Akt, a kinase involved in neuron survival and maintenance of synaptic contacts. The second is to consider possible means by which such a failure of Akt signaling might be benignly prevented or reversed in neurons affected by PD, so as to treat PD symptoms, block disease progression, and potentially, promote recovery.

RTP801/REDD1 contributes to neuroinflammation severity and memory impairments in Alzheimer's disease.

RTP801/REDD1 is a stress-regulated protein whose upregulation is necessary and sufficient to trigger neuronal death. Its downregulation in Parkinson's and Huntington's disease models ameliorates the pathological phenotypes. In the context of Alzheimer's disease (AD), the coding gene for RTP801, DDIT4, is responsive to Aß and modulates its cytotoxicity in vitro. Also, RTP801 mRNA levels are increased in AD patients' lymphocytes. However, the involvement of RTP801 in the pathophysiology of AD has not been yet tested. Here, we demonstrate that RTP801 levels are increased in postmortem hippocampal samples from AD patients. Interestingly, RTP801 protein levels correlated with both Braak and Thal stages of the disease and with GFAP expression. RTP801 levels are also upregulated in hippocampal synaptosomal fractions obtained from murine 5xFAD and rTg4510 mice models of the disease. A local RTP801 knockdown in the 5xFAD hippocampal neurons with shRNA-containing AAV particles ameliorates cognitive deficits in 7-month-old animals. Upon RTP801 silencing in the 5xFAD mice, no major changes were detected in hippocampal synaptic markers or spine density. Importantly, we found an unanticipated recovery of several gliosis hallmarks and inflammasome key proteins upon neuronal RTP801 downregulation in the 5xFAD mice. Altogether our results suggest that RTP801 could be a potential future target for theranostic studies since it could be a biomarker of neuroinflammation and neurotoxicity severity of the disease and, at the same time, a promising therapeutic target in the treatment of AD.

RTP801/REDD1 Is Involved in Neuroinflammation and Modulates Cognitive Dysfunction in Huntington's Disease.

RTP801/REDD1 is a stress-regulated protein whose levels are increased in several neurodegenerative diseases such as Parkinson's, Alzheimer's, and Huntington's diseases (HD). RTP801 downregulation ameliorates behavioral abnormalities in several mouse models of these disorders. In HD, RTP801 mediates mutant huntingtin (mhtt) toxicity in in vitro models and its levels are increased in human iPSCs, human postmortem putamen samples, and in striatal synaptosomes from mouse models of the disease. Here, we investigated the role of RTP801 in the hippocampal pathophysiology of HD. We found that RTP801 levels are increased in the hippocampus of HD patients in correlation with gliosis markers. Although RTP801 expression is not altered in the hippocampus of the R6/1 mouse model of HD, neuronal RTP801 silencing in the dorsal hippocampus with shRNA containing AAV particles ameliorates cognitive alterations. This recovery is associated with a partial rescue of synaptic markers and with a reduction in inflammatory events, especially microgliosis. Altogether, our results indicate that RTP801 could be a marker of hippocampal neuroinflammation in HD patients and a promising therapeutic target of the disease.