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Harnessing the immune system to clear protein aggregates is emerging as a promising approach to treat various neurodegenerative diseases. In Alzheimer's disease (AD), several clinical trials are ongoing using active and passive immunotherapy targeting the amyloid-β (Aβ) peptide. Limited emphasis has been put into clearing tau/tangle pathology, another major hallmark of the disease. Recent findings from the first Aβ vaccination trial suggest that this approach has limited effect on tau pathology and that Aβ plaque clearance may not halt or slow the progression of dementia in individuals with mild-to-moderate AD. To assess within a reasonable timeframe if targeting tau pathology with immunotherapy could prevent cognitive decline, we developed a new model with accelerated tangle development. It was generated by crossing available strains that express all six human tau isoforms and the M146L presenilin mutation. Here we show that this unique approach completely prevents severe cognitive impairment in three different tests. This remarkable effect correlated well with extensive clearance of abnormal tau within the brain. Overall, our findings indicate that immunotherapy targeting pathological tau is very feasible for tauopathies, and should be assessed in clinical trials in the near future.
Immunotherapies targeting the amyloid-β (Aβ) peptide in AD are currently in several clinical trials, with a few having advanced into Phase III based on some promising findings (Kerchner and Boxer, 2010). In the AN-1792 trial, Aβ plaque clearance had limited effect on tau pathology (Nicoll et al., 2003;Ferrer et al., 2004;Masliah et al., 2005a;Holmes et al., 2008;Serrano-Pozo et al., 2010;Boche et al., 2010b;Boche et al., 2010a), which emphasizes the need for therapy that specifically targets this other major hallmark of the disease. Furthermore, recent findings from this trial indicate that plaque clearance did not appear to halt or slow the progression of dementia once it was well underway, suggesting that alternative targets are needed at this stage of the disease (Holmes et al., 2008). Targeting Aβ and tau simultaneously should also improve therapeutic efficacy because these pathologies are likely synergistic (Sigurdsson, 2009). Recent reports that extracellular tau is important for the anatomical spread of tau pathology strengthen as well the feasibility of effectively reducing these lesions (Frost et al., 2009;Clavaguera et al., 2009).
A key feature of any promising experimental treatment for AD is to prevent or attenuate cognitive decline. This issue has been difficult to assess in available tangle mouse models, either because of their tangle-related motor impairments or late onset of tau pathology. Therefore, we developed a tangle model for cognitive testing by crossing htau mice (Andorfer et al., 2003) with a model carrying the human presenilin 1 (PS1) M146L mutation (Duff et al., 1996). The htau mice express unmutated human tau without mouse tau and the new model was maintained on a mouse tau knockout background. These mice have an earlier onset, at or before 2 months of age, and more rapid progression of tau pathology than the htau mice, while the distribution is similar with extensive involvement of hippocampal and cortical regions. These features render this model ideally suited for efficient screening of tau-targeting therapy.
Here we report that tau immunotherapy prevents cognitive decline in several tests in the htau/PS1 model that was associated with reduction in pathological tau within the brain.
Tau peptides were synthesized and purified at the Keck facility (Yale University) as described (Sigurdsson et al., 2001). Highly purified full length human Tau441 (2N/4R) was generously provided by Oligomerix Inc.
The htau model (Jackson Labs, Stock 004808, (Andorfer et al., 2005)) was crossed with a model that expresses the PS1 M146L human mutation (Duff et al., 1996). htau mice express unmutated human tau protein on a null mouse tau background and develop tau pathology and tangles with age. The new htau X PS1 model on a mouse tau knockout background (mtau KO), referred to as htau/PS1, has an earlier onset and more aggressive progression of tau pathology than the htau model. The mice (3–4 months of age) received 100 µg of Tau379-408[P-Ser396, 404] intraperitoneally (i.p.) in 100 µl alum adjuvant (Adju-Phos, Brenntag Biosector) with the first 3 injections every 2 weeks. Subsequent administration was at monthly intervals. The peptide was mixed with the adjuvant overnight at 4°C to allow it to adsorb onto the aluminum phosphate adjuvant. The control groups received adjuvant alone. Those were 1) htau/PS1, 2) htau/PS1/mtau and 3) htau mice. As described above, the htau/PS1 and htau mice are on a mouse tau KO background. At 7–8 months the mice went through extensive behavioral testing and were subsequently killed for tissue analyses at 8–9 months of age. All the mouse lines were on the same genetic background as they are littermates from the same colony. According to Jackson Labs, the htau mice that we received had been maintained on a BL6 × 129S4 × SW × DBA2/J background. The PS1 Tg mice were originally SW × B6D2F1 and then bred for several years on a BL6/SJL background. We decided to add the htau/PS1/mtau and htau groups as additional controls because our preliminary analysis indicated that these models had less pathology than the htau/PS1 model.
Determined by 1:200 dilution of plasma using an ELISA assay as we have described previously (Sigurdsson et al., 2001;Sigurdsson et al., 2004), in which the immunogen, its unphosphorylated sequence, or the full-length recombinant tau peptide were coated onto microtiter wells (Immulon 2HB, Thermo Electron Corp., Milford, MA). The signal was detected by a goat anti-mouse IgG or IgM linked to a horseradish peroxidase (Pierce), and tetramethyl benzidine (Pierce) was the substrate. The mice were bled prior to immunization (T0), one week after the third- (T1) and fourth injection (T2), and at the end of the study (Tf).
Following behavioral testing, mice were deeply anesthetized with ketamine/xylazine (250 mg/50 mg per kg body weight, i.p.). The brain was subsequently removed without perfusion and processed as previously described (Sigurdsson et al., 1996). The left hemisphere was snap frozen and stored at −80°C until processed for Western blots. Coronal brain sections (40 µm) of the right hemisphere were saved for histological staining with 1) mouse monoclonal tau antibodies that stain pathological tau [PHF1 (1:1000, recognizes phosphorylated serines 396 and 404 of tau protein (Otvos, Jr. et al., 1994), and shows only minimal axonal staining in normal human brain, generously provided by Peter Davies) and AT8 (1:500, recognizes tau phosphorylated at serine 202 and adjacent sites (Goedert et al., 1993), Pierce); 2) rabbit polyclonal antibody against glial fibrillary acidic protein (GFAP, 1:500 Dako) in astrocytes, and; 3) tomatolectin (10 ug/ml, Vector Laboratories) to detect microglia. The sectioned series were placed in ethylene glycol cryoprotectant and stored at −20°C until used for immunohistochemistry.
To verify that reduced tau immunoreactivity in immunized mice was not caused by epitope masking, unmasking procedure was performed as described previously (Asuni et al., 2007).
Tau pathology in brain sections was quantified blindly with the Bioquant system as described previously (Asuni et al., 2007). Every 10th section, randomly chosen, was stained with each antibody. The measurement was the percent of area in the measurement field (200X) of the pyriform cortex that was occupied by reaction product. This brain region was chosen as it contained prominent tau pathology. Five sections were analyzed per animal.
The assessment of the tomato lectin (microglia) stained sections was based on a semiquantitative analysis of the extent of microgliosis throughout the brain (0, predominantly resting microglia; 1+, a few ramified and/or phagocytic microglia; 2+, moderate number of ramified/phagocytic microglia; 3+, numerous ramified/phagocytic microglia). The rating of the GFAP sections was based on the complexity of astrocytic branching throughout the brain (1+, resting astrocytes, few processes; 2+, reactive astrocytes, moderate branching; 3+, reactive astrocytes, extensive branching).
Brain tissue was homogenized in a buffer containing 0.1 mM 2-(N-morpholino) ethanosulfonic acid, 0.5 mM MgSO4, 1 mM EGTA, 2 mM dithiothreitol, pH 6.8, 0.75 mM NaCl, 2 mM phenylmethyl sulfonyl fluoride, Complete mini protease inhibitor mixture (1 tablet in 10 ml of water; Roche) and phosphatase inhibitors (20 mM NaF and 0.5 mM sodium orthovanadate). The homogenate was then centrifuged (20,000×g) for 30 min at 4°C to separate a soluble cytosolic fraction (supernatant 1) and insoluble fraction (pellet 1). The pellet was resuspended in the same volume of buffer without protease and phosphatase inhibitors, but that contained 1% (v/v) Triton X-100 and 0.25% (w/v) deoxycholate sodium and ultracentrifuged at 50,000 for 30 min to obtain a detergent-extracted supernatant 2 that was analyzed as insoluble fraction. Supernatant 1 and 2 were heated at 100 °C for 5 min and the same amount of protein was electrophoresed on 12 % (w/v) polyacrylamide gel. The blots were blocked in 5% non-fat milk with 0.1% Tween-20 in TBS, and incubated with different antibodies overnight, and then washed and incubated at RT for 1 h with peroxidase-conjugated, anti-mouse or anti-rabbit IgG. Subsequently, the bound antibodies [PHF1, CP13 (P-Ser202, generously provided by Peter Davies, B19 (total tau, generously provided by Jean-Pierre Brion), anti-actin] were detected by ECL (Pierce). Densitometric analysis of immunoblots were performed by NIH Image J program and the levels of pathological tau was normalized relative to actin and total tau protein.
The objective was to evaluate the effects of the vaccination on selected sensorimotor and cognitive behaviors. The animals were tested in the month before tissue harvesting at the end of the study.
The htau model was recently reported to develop cognitive deficits with age (Polydoro et al., 2009), but the M146L PS1 model does not appear to develop memory impairments (Sadowski et al., 2004). Our main focus was on cognitive testing in the htau/PS1 model to determine if: 1) It had impairments in learning and memory, and if; 2) The immunotherapy targeting pathological tau would prevent or attenuate possible cognitive deficits. Sensorimotor tests were used to verify that any measured differences in cognition could not be explained by poor motor performance and hence limited maze navigation. The tests were: 1. Locomotor activity, 2. Motor and reflex behaviors: a) Traverse beam test, b) Accelerating rotarod. 3. Learning and memory tests: a) Radial maze learning and retention, b) Closed-Field Symmetrical Maze, and c) Object Recognition. We routinely use all these tests in our mouse immunotherapy studies, using procedures previously described (Sigurdsson et al., 2004;Asuni et al., 2006;Asuni et al., 2007).
Prior to testing, the mice were adapted to the room with lights on for 15 min.
Exploratory locomotor activity was recorded in a circular open field activity chamber (70 cm in diameter). A video camera mounted above the chamber automatically recorded horizontal movements in the open field in each dimension (i.e., x, y, and two z planes). Total distance was measured in centimeters (cm) traveled and is defined as sequential movement interruptions of the animal (white) measured relative to the background (black). The duration of the behavior was timed for 15 min. Results are reported based on distance traveled (cm), mean resting time, and velocity (mean and maximum) of the animal.
The animals were placed onto the rod (diameter 3.6 cm) apparatus to assess differences in motor coordination and balance by measuring fore- and hindlimb motor coordination and balance (Rotarod 7650 accelerating model; Ugo Basile, Biological Research Apparatus, Varese, Italy). This procedure was designed to assess motor behavior without a practice confound. The animals were habituated to the apparatus by receiving training sessions of two trials, sufficient to reach a baseline level of performance. Then the mice were tested a further 3 times, with increasing speed. During habituation, the rotarod was set at 1.0 rpm, which was gradually raised every 30 sec, and was also wiped clean with 30% ethanol solution after each session. A soft foam cushion was placed beneath the apparatus to prevent potential injury from falling. Each animal was tested for three sessions, with each session separated by 15 min, and measures were taken for rotarod speed when the animals fell or inverted (by clinging) from the top of the rotating barrel.
This task tests balance and general motor coordination and function integration. Mice were assessed by measuring their ability to traverse a narrow wooden beam to reach a goal box. The mice were placed on a 1.1 cm wide beam that is 50.8 cm long and suspended 30 cm above a padded surface by two identical columns. Attached at each end of the beam is a shaded goal box. Mice were placed on the beam in a perpendicular orientation to habituate, and were then monitored for a maximum of 60 sec. The number of foot slips each mouse had before falling or reaching the goal box were recorded for each of four successive trials. Errors are defined as footslips and were recorded numerically. To prevent injury from falling, a soft foam cushion was always kept underneath the beam. Animals that fell off were placed back in their position prior to the fall.
Apparatus is an 8-arm elevated radial maze constructed from Plexiglas. Each arm is 35 cm long and 7 cm wide with a water cup 1 cm in diameter positioned at the end of each arm. Sidewalls 15cm high extend 12 cm into each arm to prevent animals from crossing between arms. The central area is an octagonal shaped hub 14 cm in diameter. Clear Plexiglas guillotine doors, operated remotely by a pulley system control access to the arms. The maze is elevated 75 cm above floor level and situated in a room in which several distinctive objects of a constant location serve as extra maze cues. Prior to testing, mice were adapted for 5 days. During this period, the mice received 0.1% saccharine in water for 1 hour per day and were then adapted 16 hours later to access the sugar solution from a cup placed at the end of each arm. The first two days of adaptation were performed in a Y-maze which the mice were allowed to explore freely. The subsequent three days of adaptation were performed in the radial arm maze, in which the doors were raised and lowered periodically to accustom the animals to the sound associated with their operation. The same water deprivation schedule was maintained during the 9 day testing period. The mice maintain good health on this schedule. Each testing trial was begun by placing the mouse in the central area and raising all doors. When an arm was entered all doors were lowered. After the mouse consumed the saccharine water, the door to that arm was raised allowing the mouse to return to the central arena. After a 5 sec interval, the next trial was initiated by again raising all of the doors simultaneously. This procedure was continued until the animal had entered all 8 arms or until 10 min has elapsed. Daily acquisition sessions were continued for 9 days. The number of errors (entries to previously visited arms) and time to complete each session were recorded.
This apparatus is a rectangular field 30 cm square with 9 cm high walls divided into 36, 9.5 cm squares and covered by a clear Plexiglas top. Endboxes, each 11 × 16 × 9 cm, are situated at diagonal corners of the field. The symmetrical maze is a modification of the Hebb–Williams and Rabinovitch–Rosvold type of tests, as we have discussed previously (Asuni et al., 2006). Briefly, the main difference is that each end-compartment functions as both a startbox and a goalbox, and the mice run in opposite direction on alternate trials, thereby eliminating intertrial handling. The barriers are placed in the field in symmetrical patterns, so that mice face the same turns going in either direction within a given problem. Prior to testing, the mice were adapted to a water restriction schedule (2 h daily access to water). The mice were given two adaptation sessions prior to the beginning of testing. In the first session, all animals were given saccharine flavored water in the goal box for 10 min. In session 2, they were placed in the start chamber and permitted to explore the field and enter the goal box where water reward (0.05 mL) was available. When the mice were running reliably from the start chamber to the goal box, they were given three practice sessions on simple problems where one or two barriers were placed in different positions in the field so as to obstruct direct access to the goal box. Formal testing consisted of the presentation of three problems graded in difficulty based on our data (Asuni et al., 2006) and published norms for mice. One problem was presented per day and the mice were given five trials on each problem with an intertrial interval of 2 min. Performance was scored manually by the same observer in terms of errors (i.e. entries and reentries into designated error zones) and time to complete each trial.
The spontaneous object recognition test that was utilized measures deficits in short term memory, and was conducted in a square-shaped open-field box (48 cm square, with 18 cm high walls constructed from black Plexiglas), raised 50 cm from the floor. The light intensity was set to 30 lx. On the day before the tests, mice were individually habituated in a session in which they were allowed to explore the empty box for 15 min. During training sessions, two novel objects were placed at diagonal corners in the open field and the animal was allowed to explore for 15 min. For any given trial, the objects in a pair were 10 cm high, and composed of the same material so that they could not readily be distinguished by olfactory cues. The time spent exploring each object was recorded by a tracking system (San Diego Instruments), and at the end of the training phase, the mouse was removed from the box for the duration of the retention delay (RD = 3 h). Normal mice remember a specific object after a delay of 3 h and spend the majority of their time investigating the novel object during the retention trial. During retention tests, the animals were placed back into the same box, in which one of the previous familiar objects used during training was replaced by a novel object, and allowed to explore freely for 6 min. A different object pair was used for each trial for a given animal, and the order of exposure to object pairs as well as the designated sample and novel objects for each pair were counterbalanced within and across groups. The time spent exploring the novel and familiar objects was recorded for the 6 min. The percentage Short Term Memory score is the time spent exploring any one of the two objects (training session) compared to the novel one (retention session).
All the data was analyzed with GraphPad Prism 5.01, except the radial arm maze data that was analyzed with Statistica 6. The amount of tau aggregates on western blots, the immunoreactivity on brain sections within the pyriform cortex, the locomotor activity measurements (distance, Vmax, Vmean, rest time) and the object recognition test were analyzed by one-way ANOVA test and Neuman Keuls post hoc test. When the data failed at least two out of three normality tests (KS-, D’Agostino & Pearson omnibus-, and Shapiro-Wilk normality tests) non-parametric Kruskal-Wallis test was used, followed by Dunn’s post hoc analysis. The data from the radial arm maze, traverse beam and rotarod were analyzed by two-way ANOVA repeated measures and a Neuman Keuls (Statistica) or a Bonferroni (GraphPad) post hoc tests. Correlation between behavioral outcome and tau pathology as assessed by immunohistochemistry or Western blotting was analyzed by Pearson r correlation or Spearman rank correlation if the data failed at least two out of three normality test.
Beside significant cognitive differences between treated and control htau/PS1 mice in the CFSM, post-hoc analysis also revealed significant differences between some of the other groups but those differences are not relevant to the tau immunotherapy and are therefore not detailed.
Mice immunized with the Tau379-408[P-Ser396, 404] immunogen in alum adjuvant developed good IgG antibody response against the immunogen (Figure 1A). Those antibodies recognized its unphosphorylated version as well (Figure 1A). The immunogen was selected based on its overall immunogenicity and its AD phospho-epitope. It was therefore expected that the polyclonal response would include antibodies against regions outside the phospho-epitope. Three non-immunized control groups were included that received adjuvant alone: 1) Identical htau/PS1 mice on a mouse tau knock-out background; 2) htau/PS1 mice that expressed mouse tau (htau/PS1/mtau), and; 3) htau littermates without mouse tau. Low binding to recombinant tau was observed in both controls and immunized mice, comparable to control levels against the immunogenic epitope. We have previously detected such autoantibodies in the JNPL3 P310L model (Asuni et al., 2007). IgM response was less pronounced as expected but was of a similar pattern as the IgG response. Plasma from the immunized mice recognized tau pathology in AD and mouse tissue (data not shown), as we observed previously with this immunogen in a different mouse model (Asuni et al., 2007).
The htau/PS1 mice were clearly cognitively impaired based on our historical norms for wild-type mice in these tests. The other transgenic control groups showed equal or lesser deficits, with the htau/PS1/mtau group showing less overall decline than the htau group. The tau immunotherapy prevented cognitive decline in all three tests that were employed: 1) The Radial Arm Maze (RAM; two-way ANOVA repeated measures, p<0.0001, Figure 1B); 2) Object Recognition Test (ORT; one-way ANOVA, p=0.005, Figure 1C), and; and;3)3) Closed Field Symmetrical Maze (CFSM; one-way ANOVA, Maze I: p<0.001, Maze II: p<0.0001, Maze III: p<0.01, Figure 1D), all of which we have used extensively in other models (Sigurdsson et al., 2004;Asuni et al., 2006;Asuni et al., 2007). In the RAM and the CFSM, the immunized htau/PS1 mice performed better than the control htau/PS1 mice on all the days (RAM; p<0.01 – 0.001) and in all the mazes that were of increasing complexity, as indicated by the number of errors (note that the Y axis scale differs; CFSM Maze I: p<0.01, Mazes II, III: p<0.001). In the ORT, post hoc analysis revealed that the immunized htau/PS1 mice had better short-term memory than identical control mice (p<0.01). It is well established by us and others that cognitively normal mice spend about 70% of their time with the new object compared to the old object (Asuni et al., 2007). The immunized htau/PS1 mice did not differ significantly from their non-immunized identical control mice in any of the sensorimotor tasks, and all the groups appeared to have normal motor functions based on our experience with wild-type mice in these tests (rotarod, traverse beam, locomotor activity; see Suppl. Figure 1). These findings indicate that the cognitive improvements observed following the immunization cannot be explained by sensorimotor effects, which further strengthens our results.
Immunohistochemical staining with PHF1 and AT8 antibodies revealed pronounced tau pathology, primarily in the htau/PS1 controls and to a lesser and comparable degree in the other three groups (Figure 2A–H). PHF1 immunoreactivity (IR) was generally more pronounced than AT8 staining, and its quantitative analysis indicated significant difference between the groups (Figure 2I; one-way ANOVA, p<0.01). The therapy reduced PHF1 reactive tau aggregates by 57% in the pyriform cortex (p<0.01), compared to identical controls. This brain region has previously been chosen for analysis in the htau model (Andorfer et al., 2005) because of its prominent pathology. However, the immunotherapy appeared to reduce tau pathology throughout the brain. The htau mice had similar degree and distribution of tau pathology as the htau/PS1/mtau mice and the immunized htau/PS1 mice. No significant difference was observed between immunized and identical controls in degree of microgliosis [htau/PS1 immunized: 1.7 ± 0.2 (average rating ± SEM); htau/PS1 controls: 1.8 ± 0.1) or astrogliosis (htau/PS1 immunized: 1.6 ± 0.1; htau/PS1 controls: 1.6 ± 0.1), suggesting that gradual removal of tau aggregates is not associated with gliosis. In the other control groups, a similar degree of microglial (htau/PS1/mtau: 1.9 ± 0.1; htau: 1.4 ± 0.1) and astroglial (htau/PS1/mtau: 1.8 ± 0.1; htau: 1.8 ± 0.1) activation was observed. The regional pattern of tau pathology was similar as described previously for the htau model (Andorfer et al., 2003), with prominent cortical and hippocampal involvement, but more severe in the htau/PS1 model at the age analyzed. A time course study of the progression of brain pathology in the htau/PS1 model is underway.
For Western blot analysis, total tau was measured with polyclonal B19 antibody whereas pathological tau was detected with monoclonal PHF1 and CP13 antibodies. Levels of pathological tau were normalized with actin ((33–4) and total tau levels (Suppl. Figures 2–3). Actin levels did not differ significantly between the groups (Figures 3–4A–D, whereas levels of soluble PHF1 stained tau were significantly decreased (43%, p<0.001) in the immunized mice compared to their identical controls (Figure 3E). A trend was observed for a decrease (22%) in insoluble PHF1 reactive tau (Figure 3F). Further analysis indicated a very strong trend for the immunotherapy to reduce the ratio of PHF1/actin by 35% and 42% in the soluble and insoluble fractions, respectively (Figure 3G, H). Likewise, the ratio of CP13/actin in the insoluble fraction was reduced by 29% (Figure 4H). Levels of total soluble and insoluble tau did not differ significantly between the groups (Suppl. Figure 2A–D).Further analysis indicated a significant 45% reduction (p<0.05) in the soluble fraction ratio of PHF1/B19 in the treated group, compared to identical controls, and a very strong trend in the same direction (43%) in the insoluble fraction (Suppl Figure 2G, H). Likewise, the ratio of CP13/B19 in the insoluble fraction was reduced by 26% (Suppl. Figure 3H). Together, these findings indicate that pathological tau was preferentially being cleared by the immunotherapy
Importantly, the cognitive improvements correlated well with reduction in PHF1 stained tau aggregates assessed by immunohistochemistry. Significant correlation was observed in all three memory tests (RAM (last day of testing analyzed): r=0.36, p=0.01; CFSM: Maze I, r=0.33, p=0.02; Maze III, r=0.40, p=0.01; ORT: r=−0.31, p=0.03). Less consistent correlations were observed between the Western blot fractions and cognitive outcome that varied depending on the fraction (soluble, insoluble) tau antibody (PHF1, CP13), the protein used for normalizing the data (total tau, actin), and the cognitive test (data not shown). These findings indicate that tau pathology on histological sections rather than Western blots may predict cognitive outcome. Overall, these results strongly demonstrate the feasibility of tau immunotherapy for AD and related tauopathies.
We have previously shown that: 1) Antibodies against this immunogen recognize both pathological tau in human AD brain and tangle Tg mouse brain (Asuni et al., 2007); 2) When injected into the carotid artery, these antibodies enter the brain, bind to pathological tau within neurons, and; 3) This type of therapy reduces the amount of these aggregates and improves motor performance in a tangle mouse model, JNPL3, containing the P301L tau mutation (Asuni et al., 2007). The severe motor impairments that develop as tau pathology advances in the homozygous JNPL3 mice makes it impossible to thoroughly assess their cognition as this requires extensive maze navigation. Hence, the important question if tau immunotherapy could prevent cognitive decline remained unanswered. We now show in a new model, htau/PS1, that this type of treatment can indeed prevent cognitive impairments. The htau model does develop spatial memory deficits by 12 months of age, which supports our results and interpretations (Polydoro et al., 2009). Recent findings by others also strengthen the feasibility of tau immunotherapy (Boimel et al., 2010).
Our present findings clearly show a good relationship between antibody titer, the amount of tau aggregates in the brain and performance on various cognitive tasks, indicating that anti-tau antibodies have at least a major role in the therapeutic outcome. As tau is mostly found intracellularly, it has been an elusive target. However, numerous studies have shown neuronal uptake of antibodies, and it appears to be accelerated under pathological conditions (Sigurdsson, 2008). Furthermore, other intracellular protein aggregates, α-synuclein and Aβ, can be targeted by immunotherapy (Masliah et al., 2005b;Tampellini et al., 2007). Clearance of extracellular tau may also reduce associated pathology, and indirectly facilitate removal of intracellular tau (Sigurdsson, 2009), as it may be important for the spread of tau pathology throughout the brain (Frost et al., 2009;Clavaguera et al., 2009). Previously, tau has been reported to be secreted in cell culture and to have an extracellular function (Gomez-Ramos et al., 2008), that may become aberrant if it accumulates. Together, these reports and our prior and current findings provide a strong support for the feasibility of harnessing the immune system to target and clear pathological tau in AD and related tauopathies.
Normal tau and at least certain species of non-aggregated hyperphosphorylated tau are likely degraded by the proteosome system as other soluble proteins (Petrucelli et al., 2004), whereas upon aggregation under pathological conditions, clearance through the autophagy/lysosomal system should be favored. Indeed, prevalent lysosomal and autophagic vesicles have been detected by ultrastructural analysis in the JNPL3 P301L tangle mouse model (Lin et al., 2003), as well as in neuronal cultures that express various tau mutations (Lim et al., 2001). Interestingly, early pathological changes have been observed in the lysosomal system in AD (Nixon, 2007), which may be a primary event of unknown origin or secondary to intracellular aggregation of Aβ and/or tau. Regardless of the exact cause, antibody-mediated disassembly of these aggregates should facilitate their lysosomal clearance and thereby promote neuronal health. In further support of this scenario, lysosomal tau is detected in AD and control brains (Ikeda et al., 1998), inhibition of lysosomes increases tau levels (Bendiske and Bahr, 2003), macroautophagy is likely involved in tau clearance (Hamano et al., 2008;Berger et al., 2006), and lysosomal processing has most recently been shown to influence tau aggregation and clearance in an inducible cellular model of tauopathy (Wang et al., 2009). Furthermore, antibodies have been visualized in lysosomes by immunoelectroscopy (Meeker et al., 1987).
Potential side effects of tau immunotherapy could be associated with clearance of normal tau but total tau levels were not reduced in the treated animals. Astrogliosis, a sensitive indicator of neurotoxicity, did not appear to be increased in the immunized mice, further supporting the safety of the approach. As the antibodies will mainly clear extracellular tau aggregates and/or be primarily taken up into diseased neurons with accumulated tau, those entities should be preferentially targeted. This clearance was not associated with enhanced microgliosis which may be explained by gradual removal over an extended period. Intracellularly, the antibodies would be processed through the endosomal/autophagic/lysosomal system and, therefore, solely interact with tau aggregates clogging the same pathway whereas normal soluble tau would primarily be located in the cytosol and hence not accessible. However, some clearance of healthy tau may not be detrimental as tau knock-out animals appear remarkably normal indicating that other microtubule associated protein(s) can perform similar functions (Denk and Wade-Martins, 2009). Furthermore, reducing normal endogenous tau has been shown to ameliorate Aβ induced dysfunction in transgenic mice (Roberson et al., 2007).
It is not known how clearance of tau pathology influences Aβ but immunotherapeutic clearance of Aβ in humans or mice has modest effect on tau pathology (Oddo et al., 2004;Serrano-Pozo et al., 2010;Boche et al., 2010b;Boche et al., 2010a), and CSF levels of tau were reported to be reduced in antibody responders in the halted AN-1792 trial (Gilman et al., 2005). For cognitive improvement in Aβ mouse models, it seems more important to remove early stage Aβ assemblies than plaques (Lambert et al., 1998). A similar concept may apply to tau pathology, as suppression of transgenic tau expression improves memory in a mouse model despite stable levels of NFT (Santacruz et al., 2005), and synaptic loss precedes tangle formation in a different mouse model (Yoshiyama et al., 2007). Together, these findings support that clearing early stage pathological tau is likely beneficial, and these smaller intra- and extracellular assemblies should also be easier to clear than late stage NFT. We are currently assessing if tau pathology can be reversed in older mice.
The PS1 M146L model we employed to generate the htau/PS1 mice does not develop tangles, at least up to 8 months of age, when bred with Tg2576 Aβ plaque mice (Holcomb et al., 1998), but tau hyperphosphorylation has been reported in this APP/PS1 cross at 6 months (Kurt et al., 2003). Other studies have shown tau hyperphosphorylation in PS1 I213T knock-in mice that becomes evident in the hippocampus around 7 months (Tanemura et al., 2006), and in the spinal cord of PS1δE9 mice at 5 months (Lazarov et al., 2007). Additional models that express one isoform of wild-type human tau (0N3R) and PS1 M146L with or without APP mutations (Boutajangout et al., 2002;Boutajangout et al., 2004), show a somatodendritic accumulation of phosphorylated tau without tangle formation that is not enhanced by Aβ plaque development. Overall, the tau pathology in these models appears to be less than in the htau/PS1 cross that we generated on a mouse tau knock-out background. These differences can be explained by their expression of mouse tau that seems to interfere with aggregation of human tau (Andorfer et al., 2003), as well as limited if any expression of human tau isoforms. Autopsy findings are more variable with some studies suggesting that presenilin mutations may enhance tangle pathology compared to sporadic AD whereas others show no difference (Shepherd et al., 2009). The exact mechanism for how PS1 mutations may enhance tau phosphorylation is not known but it is likely to involve GSK-3β. PS1 can form a complex with this enzyme (Takashima et al., 1998;Tesco and Tanzi, 2000), and PS1 mutations have been shown to increase GSK-3β activity in cultures and animal models (Takashima et al., 1998;Weihl et al., 1999;Pigino et al., 2003;Baki et al., 2004;Tanemura et al., 2006).
Overall, we have demonstrated that immunotherapy targeting pathological tau reduces its aggregates within the brain in a novel AD tangle mouse model, and importantly prevents cognitive deterioration, indicating that it has a great potential as a therapy for AD and related tauopathies.
Supported by NIH grants AG032611, AG020197, the Alzheimer’s Association, and the Irma T. Hirschl Charitable Trust. NYU patent pending on tau immunotherapy. We thank Oligomerix Inc. for providing purified recombinant tau protein.