Tau interactome mapping based identification of Otub1 as Tau deubiquitinase involved in accumulation of pathological Tau forms in vitro and in vivo
Dysregulated proteostasis is a key feature of a variety of neurodegenerative disorders. In Alzheimer’s disease (AD), progression of symptoms closely correlates with spatiotemporal progression of Tau aggregation, with “early” oligomeric Tau forms rather than mature neurofibrillary tangles (NFTs) considered to be pathogenetic culprits. The ubiquitin–proteasome system (UPS) controls degradation of soluble normal and abnormally folded cytosolic proteins. The UPS is affected in AD and is identified by genomewide association study (GWAS) as a risk pathway for AD. The UPS is determined by balanced regulation of ubiquitination and deubiquitination. In this work, we performed isobaric tags for relative and absolute quantitation (iTRAQ)-based Tau interactome mapping to gain unbiased insight into Tau pathophysiology and to identify novel Tau-directed therapeutic targets. Focusing on Tau deubiquitination, we here identify Otub1 as a Tau-deubiquitinating enzyme. Otub1 directly affected Lys48-linked Tau deubiquitination, impairing Tau degradation, dependent on its catalytically active cysteine, but independent of its noncanonical pathway modulated by its N-terminal domain in primary neurons. Otub1 strongly increased AT8-positive Tau and oligomeric Tau forms and increased Tau-seeded Tau aggregation in primary neurons. Finally, we demonstrated that expression of Otub1 but not its catalytically inactive form induced pathological Tau forms after 2 months in Tau transgenic mice in vivo, including AT8-positive Tau and oligomeric Tau forms. Taken together, we here identified Otub1 as a Tau deubiquitinase in vitro and in vivo, involved in formation of pathological Tau forms, including small soluble oligomeric forms. Otub1 and particularly Otub1 inhibitors, currently under development for cancer therapies, may therefore yield interesting novel therapeutic avenues for Tauopathies and AD.
KeywordsTau Tau oligomerization Tau pathology Alzheimer’s disease Interactome mapping Otub1 Ubiquitination
Alzheimer’s disease (AD) is the most prevalent form of dementia, being characterized by progressive accumulation of amyloid plaques and neurofibrillary tangles, composed of aggregated Aβ and hyperphosphorylated Tau, respectively [9, 69]. Over the last decade, Tau has emerged as an important therapeutic target. This is underscored by (i) the close correlation of Tau pathology with the progression of symptoms and (ii) the existence of a family of Tauopathies which are neurodegenerative disorders characterized by Tau aggregation spatiotemporally correlating with brain dysfunction and associated symptoms, as well as, most importantly, (iii) the identification of mutations in Tau causally linked to Tauopathies, demonstrating a causal executive role of Tau in Tauopathies. Although Tau aggregation is closely linked to symptom progression, smaller oligomeric Tau forms rather than mature full-blown neurofibrillary tangles (NFTs) are generally considered to be pathogenetic culprits in the disease process [6, 11, 12, 15, 20, 33, 37, 39, 40, 59, 60, 67, 70]. Accumulation of misfolded and aggregated proteins is a key feature of a variety of neurodegenerative disorders, raising interest in understanding clearance and degradation mechanisms of these pathogenetic culprits [7, 22, 28, 35, 42, 43]. The UPS selectively degrades normal and abnormally folded soluble proteins, which are tagged by ubiquitin for elimination , while the autophagic–lysosomal system (ALS) mainly degrades large protein aggregates or inclusions and organelles. With smaller soluble Tau forms considered to be toxic culprits, the UPS represents an important target for therapeutic interventions and is the main focus of this work.
The UPS is reported to be downregulated in various neurodegenerative disorders, with increased proteasome activity shown to be beneficial in related disease models [15, 28, 42, 52]. The presence of ubiquitinated Tau in pathological lesions of AD provided the first lead implicating the UPS in AD and Tauopathies [49, 50]. In AD patients’ brain, paired helical filaments (PHFs) were found to be associated with impaired proteasome activity in a brain-region specific way [15, 28, 34, 35]. Furthermore, a ubiquitin mutant called UBB+1, with a 19-AA extension, has been identified in neurons from AD patients and suppresses UPS function [38, 45, 71]. UBB+1-induced proteasome dysfunction resulted in Tau pathology and related neuronal dysfunction in transgenic mice expressing UBB+1 . Accumulating evidence indicates that the UPS is implicated in clearance of different Tau forms and its dysfunction is closely related to Tau aggregation and accumulation [1, 13, 14, 15, 16, 25, 46, 57, 68]. Conversely, Tau accumulation was recently shown to impair proteasomal degradation, suggesting a vicious circle . Degradation of target proteins through the UPS is regulated by ubiquitination, with differently linked (Lys6, Lys11, Lys48, and Lys63) polyubiquitinated Tau identified in AD brains and models [13, 49, 50, 51, 56]. Different Tau forms can be ubiquitinated by E3 ligases CHIP [57, 63], TRAF6 , and MARCH7 , with particularly CHIP/Hsp70-dependent polyubiquitination characterized in detail in vitro and in vivo. Ubiquitination is highly dynamic and reversible by the action of deubiquitinating enzymes known as deubiquitinases (DUBs), hence being determined by balanced regulation between ubiquitination and deubiquitination. While Tau ubiquitination has been studied intensively, its deubiquitination remains less well explored.
Deubiquitinases are attracting increasing interest as therapeutic targets. Otub1 is an ovarian-tumor domain cysteine protease deubiquitinating enzyme  with strong preference for Lys48-linked polyubiquitin chains over Lys11-, Lys29-, or Lys63-linked polyubiquitin chains in the canonical pathway, dependent on its highly conserved active-site cysteine (C91) [17, 48, 74]. Recently, a noncanonical pathway of Otub1, involved in DNA double-strand break response by modulating Lys63 polyubiquitin chain formation, was highlighted [31, 53, 77, 78]. Otub1 has been implicated in a broad range of cellular pathways such as IL-1β-induced inflammation and stabilization of proteins including c-IAP1, p53, TRAF3/6, SMAD2/3, and RhoA, indicating that it is an important regulator in different physiological processes [23, 26, 32, 44, 66], while its role in the nervous system remains unknown.
In this study, we performed iTRAQ-based Tau interactome mapping to identify Tau-interacting proteins with Tau-modifying potential as therapeutic targets. This highlighted Otub1 as a potential entry point to better understand the link between the UPS and Tauopathies. We demonstrate here for the first time that Tau is deubiquitinated by Otub1. We demonstrate that Otub1 regulates levels of Lys48-linked ubiquitin-conjugated Tau and increases pathological forms of Tau in vitro and in vivo.
Materials and methods
TauP301S (PS19) mice  were bred and used in our laboratory as reported previously [64, 65, 72]. The mice bred in our laboratory develop Tau pathology and a neurodegenerative phenotype starting at around ~10–11 months. In this study, stereotactic injections were performed at P0 and age-matched littermates were used for analysis 2 months postinjection. At the age of 2 months, no Tau pathology or AT8-positive staining is detected in TauP301S mice in our colony. All experiments were performed in compliance with protocols approved by the UCLouvain Ethical Committee for Animal Welfare.
Reagents and antibodies
MG132 and cycloheximide were purchased from Sigma. Phosphatase inhibitor (PhosSTOP™) and protease inhibitor cocktails (cOmplete™, Mini, EDTA-free) were obtained from Roche. Primary antibodies used in this study included antibodies directed against human Tau (Dako), Otub1, Ubiquitin-Lys48-specific, Ubiquitin-Lys63-specific (Abcam), Phospho-Tau (Ser202/Thr205) AT8, Phospho-Tau (Thr231) AT180, Phospho-Tau (Thr212/Ser214) AT100 (Thermo Fisher Scientific), and Oligomeric-Tau T22 (Merck), applied in combination with appropriate horseradish peroxidase (HRP) or Alexa Fluor® (488/568/647) coupled secondary antibodies.
Cell culture and transfection
Human kidney-derived QBI-293 were originally obtained from QBiogene (Carlsbad, CA, USA) and HEK293 cells from American Type Culture Collection (ATCC), and were cultured according to the manufacturer’s protocol. All cells were maintained at 37 °C in humidified atmosphere with 5% CO2. Cells were transfected with plasmids expressing Otub1, USP9x, USP5 or empty vector using FuGENE® 6 (Roche) or short interfering RNAs (siRNAs) using Lipofectamine 2000 (Invitrogen) according to the manufacturer’s protocol. For analysis of the effects on Tau or on Tau-seeded Tau aggregation, transfection was performed 24 h before starting the assay.
In vivo gene delivery
Adeno-associated viral (AAV) vectors expressing wild-type Otub1 fused with a green fluorescent protein (GFP) tag (AAV–Otub1) or expressing GFP (AAV–GFP) driven by neuron-specific Syn promoter were generated using a previously described plasmid (Addgene, plasmid #26976). Mutant Otub1 C91A and N-terminal truncation (N-T) were generated by molecular cloning starting from the WT-Otub1 construct. AAV viruses were produced in HEK-AAV cells using AAV-DJ8 kit (Cell Biolabs, Inc.), and subsequently purified and concentrated by iodixanol-based ultracentrifugation. AAV titer was tested using a QuickTiter™ AAV Quantitation Kit (Cell Biolabs, Inc). For P0 injection, each mouse was injected into the lateral ventricles of both cerebral hemispheres with 4.2 × 109 total viral particles per side, TauP301S transgenic mice were euthanized, and brains were dissected as described previously following transcardial flushing and analyzed at 2 months postinjection [64, 65, 72].
Immunoprecipitation and Western blot analysis
To detect Tau ubiquitination, primary neurons were infected with AAV–Otub1 (wild type; catalytically dead mutant C91A; N-terminal truncation) or AAV–GFP, five days after infection, cells were washed with phosphate-buffered saline (PBS) and lysed in TGN lysis buffer (50 mM Tris HCl, pH 7.5, 200 mM NaCl, 50 mM sodium β-glycerophosphate, 1% Tween 20, 0.2% NP40) containing phosphatase inhibitor (PhosSTOP™; Roche) and protease inhibitor cocktails (cOmplete™, Mini, EDTA-free; Roche) at 4 °C for 30 min on a wheel rotor, and spun at 12,000×g for 15 min. Following preclearing without antibodies at 4 °C for 1 h, the supernatants were incubated with specific antibodies for 1 h at 4 °C, followed by incubation with protein A-Sepharose beads at 4 °C for 45 min. Following stringent washing with TGN buffer and PBS, immunoprecipitated proteins were analyzed by immunoblotting.
For Western blot analysis, cells were washed twice with PBS and extracted for 30 min at 4 °C with Triton lysis buffer (1% Triton X-100, 50 mM Tris, 150 mM NaCl, pH 7.6) containing protease and phosphatase inhibitors, and centrifuged at 12,000×g for 15 min at 4 °C to remove insoluble material. Protein content was determined by BCA Protein Assay kit (Thermo Fisher Scientific, Waltham, MA, USA). Samples (10 μg) were separated using precast 8% Tris–glycine gels or 4–12% Bis–Tris gels (MOPS running; Invitrogen) and transferred to polyvinylidene difluoride membranes. Immunoblotting was performed using the indicated primary antibodies with corresponding secondary antibodies, and developed using ECL kit (PerkinElmer, Waltham, MA, USA).
Tau aggregation assay
Tau PFFs (synthetic preformed fibrils), referred to as “Tau seeds,” were generated as described previously [24, 65, 72]. Truncated human Tau fragments bearing a proaggregation mutant (Tau-P301L) containing the four-repeat domain [K18; Q244-E372 (4RTau)], N-terminally Myc-tagged were produced in Escherichia coli (TEBU-Bio). Tau-PFFs were obtained by incubation of Tau fragments (66 μM) at 37 °C for 5 days in presence of heparin (133 μM) in 100 mM ammonium acetate buffer (pH 7.0), spun down (100,000×g, 1 h, 4 °C), resuspended to 333 μM, and sonicated before use.
In vitro Tau aggregation assay in HEK293 cells: PFF-induced Tau aggregation in vitro was performed essentially as described previously [24, 65, 72]. Sonicated Tau seeds were diluted in 100 mM ammonium acetate buffer (pH 7.0) to 10 µM solution and sonicated with 8 pulses/30% amplitude and added to the cells using BioPORTER® (AMS Biotechnology, Milton, UK) according to previously described protocol. To detect Tau aggregation, cells were fixed with 4% paraformaldehyde (PFA) containing 2% sucrose and 1% Triton X-100 for 15 min to remove soluble proteins. After washing with PBS, aggregated Tau-GFP was analyzed microscopically. To detect the effect of Otub1, USP5, and USP9x overexpression or knockdown on Tau aggregation, stably Tau-expressing QBI-293 cells were transfected with plasmid and empty vector or siRNA in 24-well plates. At 24 h after transfection, the growth medium was replaced with OptiMEM, and sonicated Tau seeds (10 μM) were added to BioPORTER® single-use tubes and added to cells. Three days after seeding, Tau-GFP aggregation was measured by microscopy.
In vitro Tau aggregation assay in primary neurons was performed as described previously . Tau seeds (10 nM) were added at DIV3 and DIV6 to primary cortical neuronal cultures (PNC) from P0 heterozygous TauP301S pups. To detect the effect of Otub1 on Tau aggregation in primary neurons, AAV–Otub1 and AAV–GFP infections were carried out at DIV3, Tau seeds (10 nM) were added at DIV6 and DIV9, and cells were fixed at DIV12 with 4% PFA containing 2% sucrose and 1% Triton X-100 for 15 min to remove soluble proteins. After washing with PBS, aggregated Tau-GFP was detected under microscope.
Immunofluorescence microscopy assay
At indicated times after infection, cells were washed by phosphate-buffered saline (PBS) for three times and fixed with 4% paraformaldehyde in PBS for 15 min at room temperature (RT). For stringent extraction to detect aggregated Tau, 1% Triton X-100 (4% PFA, 2% sucrose) was used. Cells were briefly permeabilized in PBS/0.2% Triton X-100, then blocked in blocking solution (PBS containing 10% fetal calf serum and 0.1% Triton X-100). Primary and secondary antibody incubations were performed in blocking solution overnight at 4 °C or 1 h at RT using the indicated antibodies and goat anti-mouse IgG1 or goat anti-rabbit IgG1 secondary antibody coupled to Alexa® 488, Alexa® 568 or Alexa® 647. Cells were visualized using a digital inverted fluorescence microscope (EVOS-xl auto microscope).
Tau interactome mapping
Total mouse brain homogenates were used for coimmunoprecipitation in combination with three different polyclonal anti-Tau antibodies and control antibodies that were cross-linked to Sepharose beads. Mass spectrometry procedures were performed essentially as previously reported . A detailed description of the Tau interactome mapping procedure is provided in the Supplemental Experimental Procedures.
Tau interactome mapping identifies Tau-interacting proteins in mouse brain, including Otub1 as a potential Tau modifier
Otub1 increases Tau stability and increases Tau aggregation in a well-characterized cellular Tau aggregation model
Since Otub1 is a deubiquitinase, known to regulate the stability of many crucial proteins in different cellular processes [23, 26, 32, 44, 66], we assessed whether Otub1 affected Tau stability by measuring Tau half-life following addition of the protein synthesis inhibitor cycloheximide (CHX). Knockdown of Otub1 increased Tau turnover, correlating with the decrease in expression of Otub1 obtained by the different siRNAs (Fig. 2c). Taken together, our data indicate that Otub1, a deubiquitinating enzyme, is a novel positive regulator of Tau aggregation and Tau stability in vitro, in a nonneuronal cell line.
Otub1 increases total Tau concentration and Tau phosphorylation in primary neurons
Otub1 enhances Tau oligomerization and Tau-seeded Tau aggregation in primary neurons
To measure the effect of Otub1 expression on oligomeric Tau forms, we performed Western blotting analysis in nonreducing conditions, following AAV-driven expression of Otub1 and GFP, in absence of Tau seeding. This revealed a significant increase of oligomeric Tau forms in AAV–Otub1-infected neurons compared with AAV–GFP-infected neurons (Fig. 4c). In addition, we found robust induction of a (homo- or heterotypic) Tau complex (Tauc), which was strongly detected in AAV–Otub1-infected neurons but absent in AAV–GFP-infected neurons, indicating clearcut Otub1-induced modulation of Tau in neurons. The induction of oligomeric Tau forms was further confirmed using dot-blot analysis with T22 antibody (Fig. 4d), revealing a significant increase in oligomeric Tau following expression of Otub1.
Taken together, our data demonstrate that Otub1, identified as a Tau-interacting protein in the Tau interactome mapping, modulates Tau, by increasing Tau-seeded Tau aggregation, by formation of a Otub1-induced Tau complex Tauc, and by increasing the concentration of oligomeric Tau forms in primary neurons.
Otub1 but not Otub1-C91A impairs Tau degradation by removing Lys48-linked polyubiquitin chains from Tau in primary neurons
To confirm the effect of Lys48-linked polyubiquitination change on Tau degradation, TauP301S neurons were infected with AAV–Otub1–GFP or AAV–GFP, and subsequently treated with protein synthesis inhibitor cycloheximide (CHX) for different durations up to 36 h to quantify the rate of Tau turnover using Western blot. We found that Tau degradation was significantly impaired in primary neurons infected with Otub1 WT, but not with catalytically dead mutant C91A, compared with GFP control. These results indicate that Otub1 regulates Tau degradation, dependent on its catalytic activity (Fig. 5c).
Taken together, our data prove that Otub1 can remove Lys48-linked polyubiquitin chains from Tau in a catalytic activity-dependent manner, implicating the canonical pathway. Consequently, Otub1-dependent deubiquitination inhibits Tau degradation through the proteasome pathway and prolongs its half-life, leading to accumulation of pathological forms of Tau.
Otub1 promotes Tau phosphorylation and oligomerization in vivo in Tau transgenic mice
The effect of Otub1 on Tau phosphorylation and oligomerization in vivo is dependent on its catalytic function
To further analyze the requirement for catalytically active Otub1 for induction of oligomeric Tau forms, brain extracts of AAV-infected TauP301S mice were biochemically analyzed 2 months postinjection, under nonreducing condition. This revealed significantly increased oligomeric Tau forms, following expression of wild-type Otub1, but not following expression of GFP or of the catalytically inactive form of Otub1 (C91A) (Fig. 8c). This was further confirmed by dot-blot analysis using T22 antibody recognizing oligomeric Tau forms (Fig. 8d). These data indicate that expression of catalytically active Otub1 is required for accumulation of phosphorylated Tau and oligomeric Tau forms.
Noteworthy, to analyze whether endogenous Otub1 is modulated in conditions of accumulating pathological Tau forms, we took advantage of our microarray analysis performed for a different project in which we analyzed time-dependent changes in gene expression in hippocampus following Tau-seeded Tau aggregation in Tau transgenic mice. Interestingly, Otub1 expression is gradually downregulated with increasing Tau-seeded Tau aggregation (Fig. S4a), which could be considered as a protective mechanism. Furthermore, analysis of hippocampal gene expression in aging mice, using publicly available data (NCBI database GDS2082) , indicates a significant increase in Otub1 expression with aging (Fig. S4b). The latter suggests that increased expression of Otub1 with aging might contribute to increased risk for accumulation of pathological Tau forms with aging.
Accumulation of aberrantly folded proteins is common to many neurodegenerative disorders, including AD, making (dys)regulation of proteostasis not only a potentially implicated pathogenetic mechanism, but most importantly an attractive therapeutic target. In AD, symptom progression strongly correlates with progression of Tau pathology, with early, soluble, oligomeric forms of Tau generally considered as toxic culprits. Aberrant protein degradation occurs through either the ALS or UPS, with the former mainly involved in degradation of large insoluble protein aggregates, while the latter is preferentially involved in clearance of smaller soluble forms, being the focus of this work. In view of the importance of the UPS in AD, Tau-ubiquitinating enzymes have been identified, while Tau deubiquitination has received less attention. Starting from a Tau interactome mapping, we identified here for the first time Otub1 as a novel Tau deubiquitinase, affecting accumulation of pathological forms of Tau in vitro and in vivo.
Here, we present a proteome-wide screening approach to identify Tau-interacting proteins, as a basis to gain insight into Tau pathophysiology and to identify Tau modifiers with therapeutic potential. Proteome-wide screening approaches are increasingly used—in analogy to genome-wide screening approaches—to gain novel insights from an unbiased starting point for drug discovery and fundamental science [3, 10, 27, 58, 62]. We used an iTRAQ-based approach to identify proteins interacting with endogenous Tau from mouse brain. The validity of our approach is reflected in the identification of several well-characterized Tau-interacting proteins and the current identification of Otub1 as a Tau deubiquitinase. The presented Tau interactome provides important information, extending beyond the current work, providing the basis for novel insights into the pathological role of Tau and to identify novel Tau-directed targets.
Here, we validated this approach and identified for the first time Otub1 as a novel Tau deubiquitinase, based on the Tau interactome map. Accumulating evidence implicates dysregulated proteostasis and a dysregulated UPS in AD, as highlighted in the “Introduction.” Different types of polyubiquitination of Tau, including Lys48 and Lys63 linked, have been identified in AD patients by mass spectrometry and immunological analysis [13, 49, 50, 51, 56]. Interestingly, we here identified Otub1 as a Tau deubiquitinase which decreases the levels of Lys48- but not Lys63-linked polyubiquitin chains on Tau, increasing Tau stability in neurons. Our finding is in line with previous reports showing that Otub1 has preference for Lys48-linked polyubiquitination [17, 48, 74] and functions as a regulator of protein turnover [23, 26, 32, 44, 66]. Mutation of the catalytically conserved cysteine (C91) abolished its role in Tau polyubiquitination, further confirming the crucial role of its catalytic capacity for its involvement in the regulation of Lys48 polyubiquitination on Tau. Otub1 has also been found to have a noncanonical role in DNA damage response by impeding Lys63 polyubiquitin chain formation [53, 77, 78], but no effect of Otub1 on Lys63 polyubiquitination of Tau nor implication of the N-terminal part of Otub1 was demonstrated.
Notably, the effect of polyubiquitination modification is determinant for the fate of its substrate. Lys48 polyubiquitin chains act as a proteasomal degradative signal, while Lys63-linked ubiquitin modification has been proposed to contribute to biogenesis of inclusions and clearance of larger aggregates by autophagy [36, 55, 68, 80]. Although soluble Tau forms as well as large insoluble aggregated Tau forms can be polyubiquitinated [13, 49, 50], higher-order protein aggregations are unlikely to pass through the narrow proteasome opening, and they are normally sequestrated into inclusions or aggresomes and cleared through the ALS. Recently, several lines of evidence have indicated selective autophagy using Lys63-linked polyubiquitin chains as a cargo recognition signal [36, 55, 68, 80]. In our experiments, we found that Otub1 only influences Lys48 polyubiquitination but not Lys63-linked polyubiquitin chains of Tau, and therefore would be rather involved in clearance of smaller soluble forms of Tau through the proteasome.
These findings are further strengthened by our assessment of the effect of Otub1 on Tau metabolism. We demonstrate that Otub1-dependent Tau deubiquitination is linked to accumulation of Tau phosphorylated at the pathologically relevant AT8 epitope—used for Braak staging—and accumulation of oligomeric soluble forms of Tau, which have been considered to be toxic culprits. Hence, overexpressed Otub1, which regulates Lys48-linked polyubiquitination of Tau, promotes accumulation of abnormally phosphorylated Tau and oligomeric Tau, both in Tau transgenic mice and in primary neurons. These findings are in line with previously published findings. Depletion of CHIP in Tau mice leads to significant accumulation of ubiquitin-negative, and phosphorylated soluble Tau species . In a nonneuronal cell line which overexpresses the longest human Tau isoform, proteasome inhibitor treatment stabilizes phosphorylated and aggregated Tau species which arise from Tau phosphatase PPA inactivation and normally decay within 24 h . In a triple transgenic mouse model, amyloid β-peptide (Aβ) immunotherapy leads to clearance of early soluble Tau forms by the UPS, but not of late Tau aggregates . Along the same vein, molecular chaperon FKBP51, together with Hsp90, increase Tau oligomer formation by inhibiting Tau degradation through the proteasome in a mouse model . In line with these reports, our work corroborates regulation of the early pathological Tau forms by the UPS, including deubiquitination as an important modulator.
Our data are important in view of the fact that early oligomeric forms of Tau are generally considered to be toxic culprits in AD and related Tauopathies. We have previously shown in a Tau seeding assay in in vitro and in vivo models that neuronal dysfunction correlated strongly with early pathological forms of Tau rather than with full-blown mature NFTs . Previous studies using elegant approaches have indicated that the Tau aggregation process and early pathological forms rather than full-blown NFTs correlate with neuronal dysfunction [6, 11, 12, 15, 33, 37, 39, 40, 59, 60, 67]. The detrimental role of Tau oligomers in neuronal function and behavior was further demonstrated using acute injection of oligomeric Tau and immunization with anti-Tau oligomeric antibodies [11, 12, 39, 40]. Taken together, these data highlight the importance of targeting early oligomeric Tau forms as pathogenetic culprits, and hence the importance of the demonstrated effect of Otub1 on soluble oligomeric forms of Tau.
Interestingly, we show that the conserved catalytic cysteine (C91) of Otub1 is crucial for accumulation of hyperphosphorylated Tau and oligomeric Tau species. This finding is not only important as an internal control but also in the context of drug development, providing a druggable target. It must be noted that upstream modulators of Otub1, mechanisms regulating Otub1 activity, expression and probably the binding of Otub1 to chaperone molecules must be considered as potential therapeutic targets as well. Regulation of key proteins via recruitment of distinct E3 ligases or DUBs mediated by specific adaptor proteins is common in many cellular pathways. While the mechanisms upstream of Otub1 remain unknown, they may provide additional therapeutic targets. Inhibition of deubiquitinases as therapeutic targets is attracting increasing attention for different diseases, including cancer. Our findings identifying Otub1 as a Tau deubiquitinase affecting pathological forms of Tau in vitro and in vivo may therefore yield new perspectives for therapy.
Taken together, in this work we present a Tau interactome map, which yields a basis for novel insights into Tau biology and its pathological role, validated by and extending beyond our current work. We furthermore provide novel insight into the link between the UPS and AD and open new avenues for development of small-molecule inhibitors specifically targeting accumulation of pathological forms of Tau in AD and related Tauopathies.
This work was supported by the Belgian Fonds National pour la Recherche Scientifique–Fonds de la Recherche Scientifique (FNRS-FRS; Qualified Researcher, Impulse Financing, Research Credits), by Interuniversity Attraction Poles Programme–Belgian State–Belgian Science Policy, by the Belgian Fonds de la Recherche Scientifique Médicale, by the Queen Elisabeth Medical Foundation of Belgium (QEMF-FMRE), by the Stichting Alzheimer Onderzoek (SAO), by the Institute for the Promotion of Innovation by Science and Technology (IWT) in Flanders (IWT O&O, currently FWO), Belgium, and by Stellar funding of Janssen Research Foundation. We would like to thank Markus Boesche, Carola Doce, Frank Fischer, and Melanie Jundt for their technical expertise.
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