Abstract
Background:
Alterations in the methionine cycle and abnormal tau phosphorylation are implicated in many neurodegenerative diseases, including Alzheimer’s disease and frontotemporal dementia. rTg4510 mice express mutant human P301L tau and are a model of tau hyperphosphorylation. The cognitive deficit seen in these animals correlates with a burden of hyperphosphorylated tau and is a model to test therapies aimed at lowering phosphorylated tau.
Objective:
This study aimed to increase protein phosphatase 2A activity through supplementation of S-adenosylmethionine and analyze the effect on spatial memory and tau in treated animals.
Methods:
6-month-old rTg4510 mice were treated with 100 mg/kg S-adenosylmethionine by oral gavage for 3 weeks. Spatial recognition memory was tested in the Y-maze. Alterations to phosphorylated tau and protein phosphatase 2A were explored using immunohistochemistry, western blot, and enzyme-linked immunosorbent assays.
Results:
Treatment with S-adenosylmethionine increased the Y-maze novel arm exploration time and increased both the expression and activity of protein phosphatase 2A. Furthermore, treatment reduced the number of AT8 positive neurons and reduced the expression of phosphorylated tau (Ser202/Thr205). S-adenosylmethionine contributes to multiple pathways in neuronal homeostasis and neurodegeneration.
Conclusion:
This study shows that supplementation with S-adenosylmethionine stabilizes the heterotrimeric form of PP2A resulting in an increase the enzymatic activity, a reduced level of pathological tau, and improved cognition.
INTRODUCTION
Tau is a protein with many isoforms that are produced by alternative splicing of the MAPT gene. The most well-established physiological role for tau is as a microtubule stabilizer, and this key function of tau is regulated by its phosphorylation state [1, 2]. There is a physiological equilibrium of tau and phosphorylated tau (pTau), regulated by a balance of kinase and phosphatase activity. A disruption of this balance and the subsequent shift in the equilibrium of tau and pTau results in abnormal tau phosphorylation and is suggested to contribute to tau aggregation and the formation of tau aggregates. Tau is known to be hyperphosphorylated and aggregated into neurofibrillary tangles (NFTs) in many neurological conditions, including Alzheimer’s disease (AD), frontotemporal dementia, corticobasal degeneration, Pick’s disease, and progressive supranuclear palsy [3]. Furthermore, mutations in tau are known to initiate frontotemporal dementia with parkinsonism type-17 [4]. Collectively these diseases are often referred to as the tauopathies.
Normally, tau contains 2–3 phosphate groups per molecule of protein; however, in AD there is reported to be 9–10 moles of phosphate per mole of tau [5, 6]. This abnormal level of phosphorylation is associated with a loss of normal tau function, a gain of toxic function, and aggregation into paired helical filaments [7 –9]. This highlights the crucial role of tau dephosphorylation in modulating tau function, which is primarily regulated by protein phosphatase 2A (PP2A) in the brain [10]. PP2A is the predominant phosphatase, accounting for more than 70% of phosphatase activity in the brain, and in the AD brain, PP2A activity is reduced by half [11]. PP2A exists as a heterodimer, with the structural A subunit and the catalytic C subunit; the fully active form of the enzyme is a heterotrimeric assembly, with the A/C and regulatory B subunit. The activity of PP2A is regulated by phosphorylation, methylation, and the binding of endogenous inhibitors. In the absence of a methylation event, the PP2A(A/C) subunits are unable to bind to the PP2A(B) subunit and this renders the enzyme inactive [12].
S-adenosylmethionine (SAMe) is the primary methyl donor for methyltransferase enzymes in the cell, including PP2A [13]. In AD patients, SAMe levels are reduced by up to 80% in both the cerebrospinal fluid and brain [14, 15]. It is likely that this reduction results in decreased methylation and de-activation of PP2A which in turn leads to the hyperphosphorylation of tau that occurs in disease. However, whether an increase in pTau is a contributor to disease pathogenesis, or simply a marker of the disease remains contentious. The rTg4510 mice are a transgenic model of tauopathy that express a form of mutant human tauP301L, predominantly in the forebrain regions that can be repressed by the administration of doxycycline (Dox) [16, 17]. These mice have progressive hyperphosphorylation of tau and eventually develop NFTs. These mice have been previously reported to have spatial memory deficits, coinciding with reductions in PP2A [18]. Utilizing this in vivo model of tau hyperphosphorylation, we sought to test the potential therapeutic benefit of SAMe supplementation, as well as the biological consequences on PP2A and tau.
METHODS
Animals
The rTg4510 (n = 15; M:9, F:6) and wild-type (WT) control (n = 23; M:10, F:13) mice are on an FVBN/129 background and were a kind gift from The Mayo Foundation for Medical Education. Mice were generated by crossing FBV/N mice expressing human tau containing the MAPT P301L mutation downstream of a tetracycline operon-responsive element (TRE) with 129SVeV mice expressing a tetracycline controlled transactivator (tTA) under control of the Ca2 + calmodulin-dependent protein kinase II (CaMKII) promoter [16, 17]. These mice constitutively express human tauP301L until transgene expression is inactivated by the administration of dox; however, dox inactivation was not utilized in this study. Transgene expression is largely restricted to the forebrain due to the CaMKIIα promoter.
Animals were housed in transparent, individually ventilated cages under a 12 h light/dark cycle. Rodent chow and water were available ad libitum. All experimentation was approved by The Florey Animal Ethics Committee (AEC number: 15–092) and conformed to the Australia National Health and Medical Research Council published code for animal research.
Drug treatment, behavior, and tissue analysis were performed on rTg4510 and WT animals; however, there were no changes in treated WT animals, so data is not shown.
Drug administration
6-month-old rTg4510 and WT control mice were treated with either 100 mg/kg SAMe (#A7007; Sigma Aldrich, Germany) or dH2O daily for 21 days. The drug was administered via oral gavage using a blunt 23-gauge needle. SAMe was stored in powder form at –80°C and dissolved in dH2O 30 min before dosing each day. 100 mg/kg has been previously demonstrated as a safe dose that is able to cross the blood-brain barrier [19].
Y-maze
Spatial recognition memory was investigated in a two-trial Y-maze task as previously reported [20]. Briefly, three identical arms (starting arm, familiar arm, novel arm) of the Y-maze (arms: 59.5 (L)×7.5 (W)×15.5 (H) cm; radially arranged) were randomly assigned a visual cue mounted at the distal end of the arm. Mice were placed in the Y-shaped maze for two trials (1 h inter-trial interval). In the first trial (acquisition), mice were able to explore the start arm and the familiar arm for 10 min. For the second trial (retention), mice were able to explore all three arms for 5 min. Exploratory behavior was recorded by an overhead video and analyzed using the TopScan system (CleverSys Inc.; USA).
Brain tissue collection and preparation
Animals were anesthetized 30 min post-gavage (100 mg/kg intraperitoneal pentobarbitone; Virbac; Australia) and transcardially perfused with Dulbecco’s phosphate-buffered saline (D-PBS) (#14190136; Gibco, Aus). Brains were then removed and the right hemisphere microdissected, frozen on dry ice, and stored at –80°C. The left hemisphere was placed in 50 mL 4% paraformaldehyde for post-fixation. After 24 h brains were transferred to 30% white sugar (CSR, Aus) solution (in D-PBS) and stored at 4°C overnight. Brains were then transferred to fresh 30% white sugar solution and stored at 4°C for one week before being snap-frozen with isopentane and stored at –20°C.
AT8 immunohistochemistry
30μm thick sections (1 : 10) were processed for immunohistochemical (IHC) analysis. Sections were removed from the freezer and allowed to air dry at room temperature (RT) overnight (ON). The next day sections were heated at 62°C for 10 min, then microwaved in a citric acid buffer (pH 6.0) for 2 min. After cooling to RT, sections were washed in PBS and incubated in blocking solution (5 mL NGS, 1500μL Triton×(10%), 43.5 mL PBS) for 30 min at RT. Sections were then incubated in 1 : 1000 AT8 antibody (#MN1020; ThermoFisher, Aus) ON at RT. After washing with PBS, sections were incubated in biotin-labeled secondary antibodies (3 h, RT). Sections were washed, incubated in avidin peroxidase (1 h, RT), then incubated in 3,3’-Diaminobenzidine (DAB) (20 min, RT). The immunoreaction was detected using 3% H2O2. Sections were washed, dehydrated in ethanol, immersed in xylene, and coverslipped.
Stereological estimates of the number of AT8+ tau neurons were quantitated using StereoInvestigator (version 11.06.2, MBF Bioscience, USA). The whole hippocampus (including the dentate gyrus and Cornu Ammonis (CA) subfields –CA1, CA2, and CA3) and the cortex in the same sections were traced. The average volume of the hippocampus and cortex (mean±SD) for vehicle-treated animals is 1.49×106 μm2±4.99×105 μm2 and 1.03 ×106 μm2±3.79×105 μm2, respectively. The average volume of the hippocampus and cortex (mean±SD) for SAMe treated animals is 1.37×106 μm2±5.67×105 μm2 and 1.09×106 μm2±4.66×105 μm2, respectively. The quantitation represents the total number of positive cells counted divided by the area (the area is: total area of the region traced/area of the counting frame; where the counting frame is 30μm (X-axis) by 3μm (Y-axis)), normalized to the average of vehicle-treated counts.
AT8 ELISA
Hippocampus and cortex tissues were sonicated in D-PBS with phosphatase (#4906837001, PhosSTOP Phosphatase Inhibitor Cocktail; Roche Diagnostics, Aus) and protease inhibitors (#4906837001, Complete Mini Protease Inhibitor Cocktail; Roche Diagnostics, Aus). Protein concentrations were quantified with the BCA Protein Assay Kit (#23225; Pierce, USA). An ELISA plate was precoated with carbonate coating buffer containing 1 : 1000 anti-human Tau polyclonal antibody (#A0024; Dako, USA) ON at 4°C. The next day the plate was blocked with SuperBlock™ T20 (TBS) blocking buffer (#37515; ThermoFisher, Aus). The plate was washed in Tris-buffered saline (TBS-T) and allowed to air dry (2 h, RT), then 1.5μg of the sample was added to each well for incubation (2 h, RT). The plate was then washed thoroughly and incubated in 1 : 2000 AT8 antibody (#MN1020; ThermoFisher, Aus) (2 h, RT). Finally, samples were incubated in 1 : 500 secondary antibody (1 h, RT) and detected using the QuantaBlu™ Fluorogenic Peroxidase Substrate Kit (#15169; ThermoFisher, Aus). Excitation was induced at 325 nm and emission was detected at 420 nm after immediately after stop solution was added using a microplate reader.
Protein phosphatase 2A activity
The PP2A enzyme activity assay was performed according to the manufacturer’s instructions (#17-313; Millipore, Germany). Briefly, hippocampus and cortex tissues were sonicated in buffer (20 mM imidazole-HCl, 2 mM EDTA, 2 mM EGTA, 10μg/mL aprotinin leupeptin, 10μg/ml pepstatin, 1 mM benzamidine, and 1 mM PMSF) and centrifuged at 2000×g for 5 min at 4°C. Protein concentrations were quantified with the BCA Protein Assay Kit (#23225; Pierce, USA). Immunoprecipitation of PP2A was performed by incubating lysate containing 500 mg of protein with 4 mg of anti-PP2A(C) antibody and 40 mL of protein A-agarose slurry for 2 h at 4°C with constant rocking. Immunoprecipitated samples were washed in TBS and Ser/Thr buffer (50 mM Tris-HCl (pH 7.0) and 100 mM CaCl2), followed by resuspension in Ser/Thr buffer. 60μL of diluted phosphopeptide and 20μL of Ser/Thr assay buffer were then added to beads for 10 min at 30°C in a shaking incubator to initiate the reaction. After brief centrifugation 25μL of lysate supernatant was transferred to a 96-well microtiter plate. The reaction was terminated by the addition of malachite green phosphate detection solution for 10–15 min at RT and free phosphate was quantified by measuring the absorbance at 650 nm using a microplate reader.
S-adenosylmethionine ELISA
Hippocampus and cortex tissue was sonicated in D-PBS with phosphatase and protease inhibitors. Protein concentrations were quantified with the BCA Protein Assay Kit. SAMe levels were quantified using an S-Adenosylmethionine ELISA Kit (#STA-671-C; Cell Biolabs, USA) according to manufacturer’s instructions. Briefly, a pre-coated ELISA plate was prepared by adding SAMe conjugate (1 : 100) to each well (ON, 4°C). The next day, the plate was blocked for 1 h at RT, washed, then the wells were incubated with pre-prepared standards or samples (200μg) (1 h, RT). After washing, secondary antibody HRP conjugate was added to each well (1 h, RT). Finally, substrate solution was added to each well (20 min, RT). Absorbance was read at 450 nm immediately after the addition of stop enzyme.
Immunoblots
Hippocampus and cortex tissues were sonicated in D-PBS with phosphatase and protease inhibitors. Protein concentrations were quantified with the BCA Protein Assay Kit Levels of PP2A(A), PP2A(B), PP2A(C), and total tau were determined by western blotting. Equivalent amounts of proteins were resolved on 4–20% Tris-HCl SDS-PAGE gels and transferred to nitrocellulose membranes. Membranes were blocked in 5% skim milk (Diploma, Aus) (1 h, RT) then incubated in either PP2A(A) (1 : 1000, #2039; CST, USA), PP2A(B) (1 : 1000, #4953; CST, USA), PP2A(C) (1 : 1000, #2038; CST, USA) or human Tau (1 : 2000, #A0024; Dako, USA) ON at 4°C. Membranes were washed in PBS-T (7 min X 3, RT), and incubated with IRDye® secondary antibody (Li-Cor, Aus) (2 h, RT). Visualization of bands was performed on the Odyssey® Fc system (Li-Cor, Aus).
Statistical analysis
For all statistical analyses, the software package GraphPad Prism (version 6.05 for Windows) was used. Y-maze data are presented as mean±standard error of the mean (SEM), all other analysis is presented by box and whisker plots with whiskers representing min-max. Detail including statistical test, replicate number, experimental repeats, and significance are reported in the figure legends. Data were analyzed by unpaired Students T-tests for comparison between groups. p < 0.05 was considered statistically significant.
RESULTS
S-adenosylmethionine levels were increased in the cortex and hippocampus of treated rTg4510 mice
The concentration of SAMe was decreased in the cortex (19.4%; p = 0.002) and the hippocampus (21.1%; p < 0.0001) of untreated rTg4510 mice compared to WT controls (Fig. 1). Oral supplementation of SAMe in the rTg4510 mice resulted in higher brain levels in both the cortex (14.8%; p = 0.04; Fig. 1a) and the hippocampus (21.6%; p = 0.0008; Fig. 1b), indicating the molecule had reached the brain regions of interest. It was hypothesized that once in the brain, SAMe would stabilize the active heterotrimer form of PP2A.
Oral S-adenosylmethionine treatment increases the concentration of SAMe in the brain. a) The relative concentration of SAMe in cortical homogenate from WT, treated and untreated rTg4510 mice. b) The relative concentration of SAMe in hippocampal homogenate from WT, treated and untreated rTg4510 mice. Analysis by one-way ANOVA. ELISA samples performed in duplicate, N = 6 per group. ∗ p < 0.05, ∗∗ p < 0.01.
S-adenosylmethionine increases PP2A activity and PP2A(B) and (C) protein levels in the hippocampus and cortex of rTg4510 mice
The enzymatic activity of PP2A was measured in both cortex and hippocampal tissue and shown to be decreased in rTg4510 mice, compared to WT controls in both the cortex (17.5%; p = 0.01) and the hippocampus (18.6%; p = 0.03) (Fig. 1a, b). PP2A activity was increased by 29.6% in the cortex (p = 0.001; Fig. 2a) and 31.0% in the hippocampus (p = 0.005; Fig. 2b).
Treatment with S-adenosylmethionine increases the activity of PP2A in rTg4510 mice. a) Relative PP2A activity in cortical homogenate and b) hippocampal homogenate from WT, treated and untreated rTg4510 mice. c) Representative cortical immunoblot probed for PP2A(A), PP2A(B), and PP2A(C). d) Quantification of cortical PP2A immunoblots, normalized to β-actin, then calculated relative to vehicle-treated. e) Representative hippocampal immunoblot probed for PP2A(A), PP2A(B), and PP2A(C). f) Quantification of hippocampal PP2A immunoblots, normalized to β-actin, then calculated relative to vehicle-treated. Analysis by one-way ANOVA. PP2A activity ELISA samples performed in triplicate, N = 6 per group. Western blot analyzed by one-way ANOVA from 3 independent repeats, N = 6 per group. ∗ p < 0.05, ∗∗ p < 0.01, ∗∗∗ p < 0.001.
PP2A subunit protein levels were analyzed using western blot. With respect to the cortical tissue there is a significant reduction in PP2A(A), (B), and (C) in untreated rTg4510 mice compared to WT controls (31.5% p = 0.04; 38.1% p < 0.0001; 22.9% p = 0.002; respectively). There were significant increases in PP2A(B) (28.4%; p = 0.03) and PP2A(C) (38.0%; p = 0.0002) (Fig. 2d). With respect to the hippocampal tissue, there is a significant reduction in PP2A(B) in untreated rTg4510 mice compared to WT controls (22.2% p = 0.002). With respect to the hippocampal tissue, there was a non-significant 21.4% increase in the level of PP2A(A), and a significant 20.2% increase in PP2A(B) (p = 0.02), and a 26.9% increase in PP2A(C) (p = 0.006) (Fig. 2f). This increase in activity and protein subunit levels is consistent with SAMe treatment stabilizing the active PP2A complex. As PP2A is the phosphatase responsible for the dephosphorylation of tau, with an increase in PP2A activity it was hypothesized that there would be a reduction in the levels of phosphorylated tau in the cortex and the hippocampus.
S-adenosylmethionine reduces AT8+ neurons and AT8 protein levels in the cortex of rTg4510 mice
Cortex and hippocampal tissue were analyzed by IHC, western blot, and ELISA utilizing the AT8 antibody. This antibody recognizes a form of tau that is phosphorylated at Ser202 and Thr205 which is enriched in paired helical filaments, a precursor to the formation of NFTs. Compared to untreated mice, there was a 31.2% reduction in the number of AT8 positive neurons in the cortex of SAMe treated rTg4510 mice as visualized by IHC (p = 0.05; Fig. 3a, b), and a 25.0% reduction in the hippocampus, although this was not statistically significant (p = 0.21; Fig. 3a, b). Similar to the IHC results, when utilizing an ELISA for AT8 the concentration of pTau was shown to be significantly reduced by 24.1% in the cortical tissue (p = 0.05; Fig. 3c), but not significantly reduced in the hippocampus (13.7%; p = 0.49; Fig. 3c). There was no significant change in the level of soluble tau as determined by immunoblot, in the cortex (Fig. 3d, e) or the hippocampus (Fig. 3f, g).
Treatment with S-adenosylmethionine significantly reduced the number of AT8 positive cells and the level of phosphorylated tau in the cortex but not in the hippocampus of rTg4510 mice. a) Representative images of AT8+ stained brain sections from rTg4510 untreated and SAMe treated mice, arrows indicating AT8 positive structures. b) Quantification of neuronal counts from cortical and hippocampal AT8+ brain sections. c) Relative AT8 pTau level in cortical and hippocampal homogenate from treated and untreated rTg4510 tissue as measured by ELISA. d) Representative cortical immunoblot probed for soluble human tau. e) Quantification of cortical soluble tau western blot, normalized to β-actin, then calculated relative to vehicle-treated. f) Representative hippocampal immunoblot probed for soluble human tau. g) Quantification of hippocampal soluble tau immunoblot. Analysis by unpaired Student’s T-test. Neuronal counting performed on 5 sections per animal, N = 6 per group. ELISA samples performed in triplicate, N = 6 per group. Western blot analyzed by unpaired T-test from 3 independent repeats, N = 6 per group. ∗ p < 0.05.
S-adenosylmethionine rescues the spatial memory deficit in rTg4510 mice
SAMe was increased in the cortex and hippocampus which lead to an increase in the activity and expression of PP2A subunits, and a subsequent reduction in cortical AT8+ neurons. The level of pTau has been previously correlated with a hippocampal memory deficit in the rTg450 mice and in concordance with the literature, rTg4510 mice demonstrate an impairment in spatial recognition memory in this study, as indicated by reduced exploration time in the novel arm of the Y-maze compared to WT controls (p = 0.01; Fig. 4). Treatment of rTg4510 mice with 100 mg/kg of SAMe daily for three weeks resulted in a significant increase in the investigation time of the novel arm of the Y-maze (p = 0.01; Fig. 4).
Treatment with S-adenosylmethionine improved the spatial recognition memory of rTg4510 mice demonstrated by an increase in time spent exploring the novel arm. Analysis by two-way ANOVA, with Sidak test for multiple comparisons, ∗ p < 0.05, N = 12 (WT), N = 7 in both rTg4510 groups.
DISCUSSION
The rTg450 mice conditionally express mutant human P301L tau and are used widely as a model of progressive tau dysfunction and hyperphosphorylation [16]. It has been previously demonstrated that there is a spatial memory deficit in the rTg4510 mice which is rescued by turning off TauP301L through the administration of doxycycline (Dox), reducing the overall tau burden and subsequent aggregation [21]. In this study, treatment with SAMe improved spatial memory deficits, as measured by the Y-maze, in 7-month-old rTg4510 mice. As well as cognitive rescue, there was a significant reduction in the level of phosphorylated tau in the treated animals. As total tau levels were unchanged in the animals, it suggests that it is the AT8+ tau that underlies cognitive deficits.
The phosphorylation state of tau is tightly regulated by a balance of kinase and phosphatase activity. The phosphatase responsible for dephosphorylation of tau is protein phosphatase 2A (PP2A), which has been implicated in the pathogenesis of AD [22, 23]. There was an increase in PP2A activity and PP2A(B) and (C) protein expression in SAMe treated animals in this study. The results are consistent with previous work demonstrating that modulation of PP2A with the metal-binding compounds Cu(GTSM) and PBT2 will induce a reduction in p-Tau levels and consequent cognitive rescue in rTg4510 animals [18, 24].
PP2A is a holoenzyme that exists in a heterodimeric and heterotrimeric assembly, consisting of a structural A subunit, a regulatory B subunit, and a catalytic C subunit. PP2A(A) and PP2A(C) exist in a dimeric assembly and require the methylation of the Leu-309 residue on the C terminal of PP2A(C) to enable the formation and stabilization of the heterotrimer with PP2A(B) [25]. This methylation event occurs via the action of PP2A methyltransferase (PPMT), which uses SAMe as a substrate and methyl donor [26].
SAMe is a ubiquitous methyl group donor with a role in DNA methylation, membrane stability, and synthesis of neurotransmitters [27, 28]. SAMe synthesis is part of a cycle in which methionine is converted to SAMe, which is used as a substrate for methylation reactions resulting in the degradation product S-adenosylhomocysteine (SAH). SAH is then hydrolyzed to homocysteine and it can then be remethylated to methionine. This re-methylation is dependent on the derivatives of folate and vitamin B12 as cofactors [29]. Impairments in any aspect of this methionine cycle will inhibit SAMe-dependent reactions, and it has been shown that folate deficiency induces accumulation of homocysteine resulting in decreased formation and catabolism of SAMe [30, 31]. Plasma levels of SAH are elevated in AD, and the elevation reflects an increased risk of the development of AD [32 –34]. Elevated SAH results in decreased PP2A methylation, which is associated with PP2A(B) subunit downregulation, and subsequent accumulation of pTau [35], and knockout of PP2A(B) in WT animals results in tauopathy [36]. Furthermore, PPMT expression and PP2A methylation are both downregulated in AD [37] (Fig. 5a).
Disruptions in the methionine cycle in Alzheimer’s disease and the effect of S-adenosylmethionine supplementation. a) Reductions in key components of the methionine cycle (grey text) resulting in increased phosphorylated tau level. b) SAMe supplementation is hypothesized to increase PP2A activation by increasing methylation, leading to a reduction in pTau. GSK-3β, glycogen synthase kinase 3 beta; HCY, homocysteine; Met, methionine; SAH, S-adenosylhomocysteine; SAMe, S-adenosylmethionine; PP2A, protein phosphatase 2A; PPMT, protein phosphatase 2A methyltransferase; VitB12, vitamin B12.
As seen in this study, supplementation of SAMe resulted in higher protein levels of PP2A(B) and PP2A(C) subunits, this leads to an increase in enzymatic activity of PP2A (Fig. 5b). This is in line with recent findings by Ma et al., who demonstrated that increasing PP2A activity and methylation of Leu-309 on the PP2A(C) subunit with Cornel Iridoid Glycoside reduced tau phosphorylation and improved memory deficits in the rTg4510 mice [38]. A summary of the utility of SAMe to date is reviewed in [39]. More specifically, dietary supplementation of SAMe delayed tau pathology in the 3×Tg-AD mice, a model that develops NFTs and extracellular Aβ plaques [40]. Furthermore, Phase I and Phase II results of a nutraceutical formulation, including SAMe, have demonstrated improved cognitive performance in individuals with AD [41 –43]. The same formulation has also improved cognitive performance in people with mild cognitive impairment, and in individuals with no cognitive difficulties [44, 45].
Disruptions in the methionine cycle and abnormal tau have been implicated in the pathogenesis of neurodegenerative diseases. SAMe contributes to multiple pathways in neuronal homeostasis that are directly relevant to age-related neurodegeneration [46, 47]. As SAMe is widely available and approved for use in humans, SAMe represents a pragmatic therapeutic option for diseases where aberrant phosphorylation of tau occurs, such as AD. This study suggests that SAMe can increase the enzymatic activity and stabilization of the heterotrimeric form of PP2A and reduce the level of pathological tau resulting in cognitive improvement, suggesting that clinical evaluation of SAMe is warranted.
Footnotes
ACKNOWLEDGMENTS
The Florey Institute of Neuroscience and Mental Health acknowledges the strong support from the Victorian Government and in particular the funding from the Operational Infrastructure Support Grant. We acknowledge the assistance of A/Prof Laura Jacobson in this research, the Alzheimer’s Association for a Fellowship (AARF-18-566256) to L.J.V and the Australian Research Training Scholarship for financial support of L.C.B.
