As a person’s symptoms of Alzheimer’s disease emerge and worsen, tau tangles spread inexorably through the cortex. What unfortunate series of events pushes this microtubule-binding protein into a cataclysmic state? In a paper published November 10 in Cell, researchers led by Judith Steen at Boston Children’s Hospital addressed this question by mapping and quantifying myriad post-translational modifications of the tau protein extracted from more than 90 human brain samples. From truncation to phosphorylation, from acetylation to ubiquitination, the researchers identified modifications that distinguished people with AD from controls. They described an ordered piling-on of alterations that tracked with disease stage. Some of these same modifications adorn tau species that seed aggregation. Ubiquitination stood out as a potential culprit. In all, the findings could help researchers zero in on which species of tau to target therapeutically, and suggest different targets for different stages of disease.

  • Proteomics of tau extracted from human brain maps tau post-translational modifications on largest scale yet.
  • Specific changes track with tau’s aggregation propensity and disease stage.
  • As modifications pile on, they neutralize tau’s charge, goading aggregation.

“This work represents the first unbiased and systematic effort to provide qualitative and quantitative profiling of the diversity of the tau proteoform and how it changes during the progression of Alzheimer’s disease,” commented Hilal Lashuel of École Polytechnique Fédérale de Lausanne, Switzerland. “It reaffirms our previous calls for embracing the complexity of tau as a necessary step to deciphering the tau PTM code and understanding its role in health and disease.”

Tau is notoriously manifold. Even before the MAPT gene is stitched into protein, alternative splicing renders different isoforms that contain either three or four microtubule-binding domains. After translation, proteases have their way with the protein, nipping off various segments of the N- and C-termini. What’s left is subject to a dizzying array of post-translational modifications. Phosphorylation is the best-known, and a vast literature spanning more than 30 years has tied hyperphosphorylated forms to pathology, mostly with the help of antibodies trained against phospho-epitopes of tau (Grundke-Iqbal et al., 1986; Goedert et al., 1992). However, not only do the types of phosphorylated tau species differ from person to person, but other modifications such as acetylation, ubiquitination, methylation, and glycosylation glom onto the protein as well. How do all of these alterations influence tau’s structure, and most importantly, its tendency to aggregate and wreak havoc in the brain?

Dressed from N to C. Different modifications of tau bedeck the length of the protein. Tau proteoforms also differ based on how many N-terminal inserts they contain (0N, 1N, or 2N) and whether they contain three or four microtubule-binding domains (3R or 4R). The C-terminus can also become truncated. [Courtesy of Wesseling et al., Cell, 2020.]

Co-first authors Hendrik Wesseling and Waltraud Mair and colleagues grappled with these questions in a proteomic magnum opus of mapping and quantifying tau PTMs. The researchers extracted tau from the BA39 angular gyrus—a region typically burdened with moderate tau pathology in late-stage disease—from postmortem brain samples of 49 AD patients and 42 controls. The controls were matched to cases by age, sex, and postmortem interval. They ran complementary analyses for another brain region—the frontal gyrus—on 10 AD and nine control brain samples.

Using a mass spectrometry technique called FLEXITau they had previously developed, the scientists first quantified total amounts of soluble and insoluble tau (Mair et al., 2016). People with AD had less soluble but 100 times more insoluble tau than did controls. Soluble tau, whether it came from controls or people with AD, was more likely to be full-length, containing both its N-terminal domains and all four microtubule-binding domains (2N4R). In contrast, the insoluble fraction tended to lack the N-terminal domains, and was enriched for the 0N4R isoform.

Turning their focus to insoluble tau, the researchers next used mass spec to detect post-translational modifications along the length of the protein. Among all samples, they detected a total of 95, including 55 phosphorylated, 17 ubiquitinated, 19 acetylated, and four methylated sites. The vast majority of acetylations and ubiquitinations crowded within the microtubule-binding domain, and some residues carried both modifications.

With this PTM catalog in place, the researchers next asked which ones correlated with disease. They calculated the proportion of tau molecules harboring each modification within a given brain sample. Steen emphasized that measuring the frequency and extent—as opposed to merely the presence or absence—of each modification is key to understanding its connection to disease.

Phosphorylation sites clustered in the proline-rich region of tau, as well as its C-terminus. While some residues in this region were phosphorylated in both control and AD samples, the number of residues that were phosphorylated, and the proportion of tau molecules that harbored each phosphorylated residue, was much higher in AD samples than controls. Truncation of the C-terminus, following the microtubule-binding domain, was also a feature that predominated in tau from AD brains and from some controls.

Curiously, some residues in the PRR, including T181, were phosphorylated at high frequency in about 20 percent to 40 percent of clinical and pathological controls, suggesting they may have started the transition toward tauopathy even though they had no tangles or symptoms yet when they died.

Ubiquitination and acetylation centered in the microtubule-binding domain (MBD). These modifications were virtually absent in controls, but bedecked a high percentage of tau molecules in people with AD. Acetylation mostly riddled the R4 domain, while ubiquitination predominated in R1–R3. One lysine residue, K311, was struck with a double whammy of both modifications in people with AD. It is the last residue in the infamous VQIVYK hexapeptide, which plays a pivotal role in snapping tau fibrils into place (van Bergen et al., 2000; research timeline). 

Piling On. Map of PTMs along tau’s N- to its C-terminus for Alzheimer’s (top) and control (bottom). The Y-axis depicts the average frequency of each modification in each sample; type of modification is color-coded. Common antibodies specific for some modifications are designated at top of bars. Heatmap color also denotes extent of modification of each tau peptide. [Courtesy of Wesseling et al., Cell, 2020.]

The investigators analyzed the data in many ways to tease out connections between PTMs and disease. One analysis used a statistical technique called hierarchical clustering to group samples based on their tau PTM profiles. Four groups sprang up. The first had the fewest PTMs, with some singly phosphorylated tau proteins, on residues T181, S231, or S235. Most samples in this group came from asymptomatic controls.

A second group was heterogeneous, comprising 10 AD patients with advanced neuropathology and 16 controls with little to no neuropathology. Tau from samples in this group was phosphorylated on more sites and with higher frequency than in the first group. Interestingly, a subset of the samples in this second group also had ubiquitination in the MBD, and all of these came from symptomatic patients. The finding suggests that ubiquitination may be the defining feature of tau from people with symptomatic AD.

The last two clusters consisted entirely of samples from symptomatic patients. Both of these clusters had extensive PTMs, including ubiquitination and acetylation of the MBD. The second of the two clusters had the most PTMs. Notably, the extent of tau PTMs in these two symptomatic clusters correlated with neuropathological staging—most of the samples in the first cluster came from brains ranked at Braak stage V, while the second cluster was predominated by brains staged at Braak VI.

Together, the findings from this cluster analysis suggested that phosphorylation may start in an early presymptomatic stage of disease and ramp up over time, while ubiquitination and acetylation of the MBD primarily occur in symptomatic stages.

The scientists also ranked specific tau PTMs for their association with clinical diagnosis, reporting that ubiquitination of residues K311 and K317, as well as phosphorylation on residues T217 and S262, best distinguished tau in people with AD from controls.

Finally, the researchers investigated which of these PTMs might goad tau into aggregating. To do this, they referenced data from a previous study, in which they had fractionated tau proteins based on size and solubility, and tested each fraction for its seeding activity in cellular assays (Takeda et al., 2015). In that paper, they had reported that low-molecular-weight forms of soluble tau poorly seeded aggregation, while still soluble but high-molecular-weight oligomers, and insoluble fibrils, sparked tau aggregation with gusto. They now report that the fractions of tau that seed the best also happen to contain the most disease-associated PTMs.

Putting their data together, the scientists proposed a model of how tau might collect modifications that propel it toward aggregation. First off, the isoforms of tau lacking their N-terminal inserts—0N4R and 0N3R—are most aggregation-prone to begin with. Tau undergoes a cascade of PTMs, including cleavage of its C-terminus, phosphorylation of key residues within the PRR, and ubiquitination and acetylation of the MBD. The researchers proposed that the negative charge bestowed by phosphorylation of the PRR effectively neutralizes the positive charge of the lysine-loaded MBD, in much the same way that heparin goads tau fibrillization. Ubiquitination and acetylation of the MBD further neutralize the region, removing kinetic barriers to tau filament formation. The negatively charged, phosphorylated PRR could even fold back onto the MBD, forming a hairpin structure capable of fibril formation (Jeganathan et al., 2006). 

The Making of a Fibril? In this model, 0N and 4R isoforms sustain a cascade of PTMs, including C terminus cleavage, negatively charged phosphorylation in the PRR, followed by charge-neutralizing acetylation and ubiquitination in the MBR. These progressive steps could facilitate tau fibrillization and AD progression. [Courtesy of Wesseling et al., Cell, 2020.]

“This study is an incredible resource of tau PTMs, both from people with AD and controls,” commented Gail Johnson of the University of Rochester in New York. Because the study included a large number of samples, the researchers were able to tease out a tau PTM disease signature amidst daunting heterogeneity, she added. Johnson said the discovery of ubiquitinated sites in the MBD that correlate with seeding competency and with disease should spur increased focus on the role of this modification in tauopathy. The findings suggest the tau PTM signature is the product of cooperative modification of epitopes that ultimately lead to pathological aggregation, Johnson said.

Lashuel viewed the model the authors proposed as overly simplistic. He noted past studies showing that some phosphorylation events actually slow aggregation, and that ubiquitin residues in the MBD are bulky, likely hindering fibrillization. Lashuel said the model is at odds with the cryo-EM structure of tau fibrils extracted from AD brain, which was devoid of such PTMs in the MBD (Jul 2017 news). A more recent study that employed both cryo-EM and mass spectrometry suggested that such modifications, including ubiquitination and acetylation, may play a pivotal role in spinning fibrils of tau in people with AD as well as corticobasal degeneration (Feb 2020 news).

Khalid Iqbal of New York State Institute for Basic Research in Developmental Disabilities, Staten Island, commended the authors for their comprehensive detection and quantification of so many tau PTMs. He said the findings largely confirm early antibody studies. He noted, however, that the paper does not demonstrate which PTMs drive tau’s pathological aggregation, and questioned whether modifications beyond the earliest phosphorylation events played a functional role. Past research reported that phosphorylation on specific tau residues was sufficient to drive tau to ditch microtubules and form aggregates (Liu et al., 2007). “The study does not give information about which PTM sites are critical for therapeutic targeting or disease biomarkers,” Iqbal said.

Nicolas Barthélemy of Washington University, St. Louis, made a similar point. He noted that the aggregation process may expose certain residues in the MBD to processes like ubiquitination, meaning that some modifications could be a consequence, rather than the cause, of aggregation. Steen acknowledged that the snapshot of tau PTMs in the study could not decipher cause and effect relationships, and hypothesized that bidirectional feedback between structural and biochemical changes likely exists.

Barthélemy also said that while the FLEXITau approach enables detection of different PTMs across the tau protein in an unbiased manner, the method has a drawback in that it relies on the disappearance of unmodified tau peptides to infer the presence of potential modifications within a sample. Therefore, it could mistake a truncation for a modification, he said. Barthélemy uses a different mass spectrometry-based technique to detect phospho-epitopes of tau in brain, blood, and CSF. While his approach is highly sensitive and is forming the basis of emerging fluid biomarker tests, it also requires that researchers decide a priori which phospho-epitopes to pursue.

Might detection of tau with ubiquitinated and/or acetylated sites in in the MBD boost the sensitivity of fluid biomarkers? Not necessarily, Barthélemy said. Ubiquitinated sites are tricky to detect with his technique, and most tau in the blood and CSF is C-terminally truncated, lacking the ubiquitinated MBD. On this point, researchers led by Dennis Selkoe at Brigham and Women’s Hospital, Boston, just reported that an N-terminal fragment of tau detected in blood predicts future cognitive decline (Dec 2020 news). 

Carlo Condello at the University of California, San Francisco, considers the paper a tremendous resource for the field. He views the massive dataset as a guidebook that will inform future studies investigating the functional consequences of certain PTMs. He said researchers could attempt to re-create different modifications in model systems to test if they play a role in aggregation and/or propagation. Future analyses could check for PTMs in other regions of the brain that develop tau pathology earlier or later in disease, which would simulate a timeline of PTMs.

The findings mesh with a recent study led by Bradley Hyman, Massachusetts General Hospital in Charlestown, who co-authored the current paper. It tied aggregation-prone oligomeric forms of tau, phosphorylated on specific residues, to faster progression in AD (Jun 2020 news). 

Eckhard Mandelkow of the German Center for Neurodegenerative Diseases in Bonn pointed out that the PTM profile of tau extracted from postmortem brain samples might not reflect the profile in living cells. During the postmortem interval, kinases become inactive while phosphatases continue their work. Using a technique called native state mass spectrometry to analyze tau in cultured cells, Mandelkow and colleagues recently reported that normal human tau was amply phosphorylated (Drepper et al., 2020). He views Steen’s findings—which quantified differences in tau PTMs between AD and control brains—as complementary.

How might this PTM playbook steer therapeutic targeting of tau? Steen and colleagues argue that the findings caution against targeting tau’s N-terminus. The warning dovetails with previous studies, which have reported that antibodies specific for tau’s mid-region—not those trained against the N-terminus—block tau’s seeding activity (Apr 2018 conference news). However, because N-terminal antibodies tend to have the highest affinity for tau and also clear neurofibrillary tangles in model systems, several have entered clinical development (such as semorinemab; tilavonemab, zagotenemab, and gosuranemab). 

Einar Sigurdsson of New York University agreed about avoiding the N-terminus. Steen and colleagues also say the best species to target might change as disease progresses. Sigurdsson suggested that tau targets should be prominent in AD but also detected, to some extent, in controls. “As alluded to in this article, these are likely to be the earliest pathological epitopes and the control subjects that have them in low amounts may simply be in their early stages of developing tauopathy,” he wrote. “Epitopes that are specific to AD/tauopathy are less attractive for targeting in large part because they occur late in the disease, as nicely documented in this article.”—Jessica Shugart


  1. This is an impressive proteomic study providing a large list of tau post PTMs identified in different brain extracts including AD brain aggregates containing insoluble tau. The authors performed a semi-quantitative analysis measuring the frequency of detection of the different PTMs identified in brain extracts and the absolute quantitation of numerous tau peptides across the protein. To my knowledge, this is the largest brain cohort investigating AD tau by proteomics to date.

    It confirms previous reports on phosphorylation enrichment in AD insoluble tau mid-domain (i.e. pT181, pT217) and C-terminus (pS404). It also confirms extensive modification of the AD insoluble tau microtubule binding domain by methylation, acetylation, and ubiquitination.

    The FlexiTau method provides an indirect quantitation of tau modification across the protein. The PTMs’ abundance is inferred from the relative decrease in unmodified peptide levels compared to the other parts of the protein. There is some limitation to determining with precision the modification occupancy for each tau residue, and which PTMs contribute the most to the decreased level. Notably, the extent of modification reported could be affected in part by truncation.

    For example, an enrichment of the MTBR region (core of tau aggregation) and a partial degradation of the fuzzy coat (the soluble part surrounding the core) could contribute to a relative decrease in the abundance of the peptides still accessible to proteolysis in AD insoluble tau (Fitzpatrick, 2017). I note low molecular-weight tau characterized in this study is remarkably similar to the profile we measured in cerebrospinal fluid (CSF) using an analogous mass spectrometry (MS) quantitative approach (Barthélemy et al., 2016). As in CSF, N-terminus and mid domain are enriched compared to MTBR and C-terminus, likely due to degradation of the protein domain after 224 cleavage (Cicognola et al., 2019).

    An alternative MS method to measure tau modification occupancy is to quantify simultaneously modified and unmodified peptides. However, this approach is difficult to apply to all of the tremendous number of PTMs reported by the Steen group without considerable effort in designing MS assays for each reported PTM.

    We have currently applied this method to tau phosphorylation monitoring in brain, CSF, and plasma. We confirmed, for example, a significant enrichment of tau species phosphorylated on T181, S202, T217 or T231 in insoluble tau, consistent with the results reported by Wesseling et al.

    However, Wesseling et al did not find differences in soluble tau when AD and controls were compared. Conversely, we found hyperphosphorylation in soluble tau from AD brain on T111, T153, T205 or T217 (Horie et al., 2020). The same is observed in AD CSF (Barthélemy, 2019). These PTMs are typically modified in soluble brain tau and CSF with a low phosphorylation occupancy (<5 percent). Thus, discrepancy between the two reports could be due to differences in sensitivity between the two MS strategies employed.


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  2. This work represents the first unbiased and systematic effort to provide qualitative and quantitative profiling of the diversity of the Tau proteoforms, and how they change during the progression of AD. It illustrates the power of the bottom-up approach to large data analysis based on unbiased biochemical data acquisition to generate clinically meaningful clusters within the subject cohorts. It represents a significant advance that paves the way for future studies to decipher the Tau PTM code. It guides efforts to develop new working hypotheses, novel tools and assays to elucidate roles of PTMs in regulating Tau aggregation, seeding and pathology spreading. The authors should be commended for their efforts and courage to tackle complex, fundamental questions.

    This study reinforces the complexity of the Tau PTMs. It reaffirms our previous calls for embracing the complexity of Tau as a necessary step to deciphering the Tau PTM code and understanding its role in health and disease (Haj-Yahya and Lashuel, 2018; Haj-Yahya et al., 2020). This study yielded several interesting novel findings. Firstly, the cumulative number of the detected phosphorylation sites on Tau was much lower than the predicted number of sites, 55 out of possible 85. The clustering of the PTMs in specific domains, and the presence of multiple PTMs throughout the sequence, suggest that the Tau PTM code is a combinatorial code that involves complex crosstalk and the interplay between different PTMs in disease (Haj-Yahya and Lashuel, 2018; Haj-Yahya et al., 2020). In addition, it underscores the crucial point that Tau is not a single protein, but six unique proteins, in both their associations with differential pathology development, as well as their distinct PTM profiles.

    However, we emphasize that the PTM frequency and maps the investigators provided are still incomplete. Firstly, they were based on single-time-point snapshots in different patients, therefore the longitudinal alterations of PTM profiles are not known for any specific individual to robustly establish the temporal sequence of PTM occurrence. Secondly, maps represented the cumulative occurrence of the PTMs, which was derived from the detection of predominantly individual PTMs on the Tau peptide fragments. This lacks insight into the co-occurrence of PTMs along broader regions of the same protein molecule, and does not provide the information on the isoform origin, apart from several peptides containing isoform-differentiating regions at N- and C-terminus, but not for the domains common to all six isoforms. Furthermore, it is intriguing that several other PTMs reported to occur on Tau were not reported, discussed, or addressed in the proposed model. These include nitration, oxidation, tyrosine phosphorylation, O-GlcNacylation, as well as N-glycosylation. It is unclear whether these PTMs were absent in the samples, under the detection limit of the experimental methods, or the data analysis was not optimized for detection of these modifications. 

    It is difficult to gain insight into how the various PTM patterns evolve over time, or when the different PTMs are introduced during oligomerization and aggregation of Tau on the pathway to neurofibrillary tangle formation. The authors attempted to address this by correlating the PTM patterns and disease stages, but this remains a correlation based on a small number of cases.

    For example, the authors emphasized that the underrepresentation of modified N-terminal domain could suggest it may not significantly influence tau seeding activity, and thus antibodies targeting this domain are “predicted to be less-effective therapeutic agents for AD.” This interpretation illustrates the problem of reconstructing a process based on looking only at the end product. The existing data do not rule out important roles of PTMs in the N-terminal domain in the initiation and modulation of Tau oligomerization and aggregation, as well as its synergistic roles with other PTMs alongside Tau molecule (Ait-Bouziad et al., 2020).  It is also plausible that interactions between the N-terminus and central domains of Tau may influence their folding and the susceptibility to modification of the polyproline-rich region and MBD at the early stages of Tau aggregation.

    The authors also found that peptides bearing phosphorylated S262 were highly enriched in the sarcosyl-insoluble fractions of AD patients, which by the definition of the brain lysate preparation protocol used should not have contained NFTs or PHF-enriched Tau material (Mair et al., 2016; Greenberg and Davies, 1990). In vitro, we have shown that phosphorylation at S262 inhibited rather than promoted Tau aggregation process (Haj-Yahya et al., 2020), raising the possibility of enrichment of the pS262 Tau species in the sarcosyl-extracted fraction, not recruitment into NFT aggregates in AD.

    Two models were presented in this work. The first, proposing successive PTM addition with disease progression stages, is likely to apply only to the pool of relatively dynamic and semi-soluble sarcosyl-extractable Tau monomeric, oligomeric, or multimeric species. The study detected clustering of several PTMs in the microtubule-binding repeat domains. However, these modifications were not detected in the experimental data obtained from cryogenic electron microscopy coupled with mass spectrometry analyses of the structures of Tau filaments isolated from the brains of patients with AD and tauopathies (Arakhamia et al., 2020). This underscores the diversity in PTM patterns of different Tau species, at the level of Tau the monomer, oligomer, and high-order fibrillar assembly.

    Indeed, many of the PTMs detected in this work are expected to inhibit rather than promote Tau aggregation. For example, the authors explained the effects of acetylation and ubiquitination on the basis of charge neutralization. Introducing a bulky protein such as ubiquitin at a single or multiple lysine residue in the aggregation-prone MTBR domain is more likely to interfere with the onset and the process of Tau aggregation. This is what we have seen in our work with α-synuclein and mutant Huntingtin, where mono-ubiquitination was shown to stabilize the monomeric proteins and inhibit its aggregation.

    The second model suggests that phosphorylation or hyperphosphorylation of Tau drives its aggregation. This is not a universal phenomenon, as we and others have shown using site-specifically phosphorylated proteins. There are likely some phosphorylation patterns that promote aggregation, but this has to be further validated experimentally.

    Nonetheless, the data provided here represent an essential starting point for developing testable models, including the ones proposed by the authors. This could be achieved using the methodologies we recently developed for reconstructing Tau isoforms from scratch, and for producing homogenous proteins that are site-specifically modified at single or multiple residues. Using these methods, we were able for the first time to investigate the role of site-specific acetylation at K280 and investigate the role of cross-talk between different phosphorylation sites (Haj-Yahya and Lashuel, 2018; Haj-Yahya et al., 2020). The ability to generate these site-specifically modified proteins allows for more precise mapping of the role of PTMs in regulating Tau function in health and disease.

    The findings presented in this work must be independently validated in larger cohorts of patients, and across broader brain regions specifically associated with AD dementia, before we start to rethink our approaches to developing Tau-targeted diagnostics and therapeutics. Future studies are needed to refine the distribution maps of the Tau PTMs, with an emphasis on mapping co-occurring PTMs, as well as their Tau isoform origin, cross-talk between the PTMs in each domain, and the interplay between PTMs in different protein regions.

    This exciting work highlights that it is time we embrace the complexity of Tau. This is essential for bridging the widening gap between what is being studied in vitro using unmodified recombinant proteins, and the complexity of Tau species in the brain.


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  3. The extensive data in this article is an outstanding resource for the field. It nicely confirms several known features of the progression of tauopathy in AD and adds new information to refer to for ongoing and future studies.

    With regard to therapies, I agree, as we and others have previously discussed in articles on this topic, that a personalized treatment based on epitope profile that relates to the stage of the disease may be feasible in the future. However, I think the focus should be to target epitopes that are prominent in AD and other tauopathies, but are also found to some extent in normal subjects.

    As alluded to in this article, these are likely to be the earliest pathological epitopes and the control subjects who have them in low amounts may simply be in their early stages of developing tauopathy. Epitopes that are specific to AD/tauopathy are less attractive for targeting in large part because they occur late in the disease, as nicely documented in this article. These late epitopes are most prominent in insoluble tau, which is not very accessible to treatment and anyway probably better left alone.

    Lastly, the point the authors make about N-terminal tau not being a good target for therapy relates to what we have mentioned over the years, because this region has been shown previously by several laboratories to be missing in many forms of pathological tau. It is reasonable to target normal epitopes as long as they are also prominent in pathological tau.

    It will be interesting to see if the epitope profiles in this article will correlate with efficacy of the many tau immunotherapies in clinical trials. Given the complexity of the topic, the fact that antibodies against the same region can differ greatly in their efficacy, and considering that seeding and toxicity do not necessarily go hand in hand, the answer is unlikely to be straightforward.

  4. This paper represents a major advance in the understanding of molecular changes in neuronal tau protein during the progression of Alzheimer‘s disease. This was made possible in large part by advances in mass spectroscopy methods, making use of calibration of peptide masses with regard to isotope-labeled peptides, embedded in the FLEXITAU procedures described by the authors previously (Mair et al., 2016; for recent updates on methods see Schlaffner et al., 2020). This was now applied to cohorts of AD patient brains and non-AD controls. The analysis revealed multiple modifications along the tau chain, mostly phosphorylation, acetylation, and ubiquitination, occurring in different tau isoforms, derived from different states of aggregation (tau monomers, oligomers, and fibers), and changing with the progression of the disease. As such, the paper presents a "high resolution" view into the Braak stages that were originally based on silver-stained neurofibrillary tangles (Braak and Braak, 1991). Wesseling et al. is in several ways complementary to recent studies by Barthélemy, Bateman and colleagues on tau protein from CSF (Barthélemy et al., 2019). 

    Here we will restrict our comments to a comparison with a recent study of tau protein by "native MS" to analyze intact, uncleaved tau, and combined with subsequent LC-MS/MS of tryptic peptides from the same protein (Drepper et al., 2020). Our study was done on the eukaryotic cell line Sf9, expressing the full-length human tau isoform (termed 2N4R, 441 residues, UniprotKB 10636-F). It revealed that tau, which was rapidly purified from cells to minimize enzymatic modifications, has a high extent of phosphorylation at Ser/Thr residues, ~8±5 Pi per tau molecule in a bell-shaped distribution, with states separated by 1 unit of Pi.

    Other modifications, i.e. phosphorylation at Tyr, acetylation, ubiquination, are minor, at or below the limit of detection. When phosphatases are inhibited by okadaic acid before cell preparation, the extent of phosphorylation rises to 14±6. Subsequent peptide analysis reveals ~51 phosphorylation sites out of 85 potential sites, with 90 percent sequence coverage. In agreement with Wesseling et al., the P-sites are preferentially located in the basic C-terminal half of tau, i.e. residues 150-441. Thus, much of the basic character of tau, caused by the dominant Lys+Arg residues, is neutralized by acidic phospho-Ser or -Thr. Phosphorylation is prominent at sites considered to be diagnostic for AD-tau, i.e. the epitopes of antibodies AT-8, AT-270, PHF-1 etc. Our conclusion is that phosphorylation is high and is the dominant modification of tau in a healthy eukaryotic cell. This reflects a strong phosphorylation "tone," as revealed by a natively unfolded protein like tau, in agreement with previous studies (Tepper et al., 2014; Mair et al., 2016). 

    How does this compare with the common notions in the AD field that tau is "hyperphosphorylated" in AD, and that hyperphorylation leads to fibrous aggregation of tau, i.e. paired helical filaments (PHFs), coalescing into neurofibrillary tangles (NFTs)? This view stems from early investigations showing that tau obtained from AD patient brains is highly phosphorylated (~10 Pi/molecule), in contrast to normal adult human brain tau (~2 Pi/molecule) (Ksiezak-Reding et al., 1992; Köpke et al., 1993; Kenessey and Yen, 1993). This led to the idea that hyperphosphorylation of tau is the cause of the pathologic conversion of soluble into aggregated tau at advanced age.

    As a consequence, many labs investigated phosphorylation sites, corresponding protein kinases, and protein phosphatases. The aim was to find early diagnostic markers of AD, and to develop therapeutic approaches to reduce tau aggregation, either by inhibiting the responsible kinases or by activating phosphatases. Early diagnostic markers were indeed found, most recently phospho-peptides of tau in CSF and blood (Bateman et al., 2020; Moscoso et al., 2020). However, modulation of tau phosphorylation via kinases or PPases has not led to treatments.

    It turns out that there may be a problem in the concept that pathological hyperphosphorylation leads to aggregation. Tau is an unusually hydrophilic and disordered protein that remains soluble in vitro at <200 µM irrespective of phosphorylation (Tepper et al., 2014). However, tau readily forms aggregates in the presence of polyanions like sulphated glycosaminoglycans or RNA of various origins (Goedert et al., 1996; Kampers et al., 1996). This suggests that polyanionic co-factors—rather than phosphorylation—could be the cause of aggregation. In fact, there has been a string of papers indicating that normal cellular tau is phosphorylated at many sites that were initially considered to be AD-specific (Matsuo et al., 1994; Sergeant et al., 1995; Gartner et al., 1998Wang et al., 2016). 

    Alas, this did not change the predominant view of pathological hyperphosphorylation causing tau aggregation. The likely explanation for the ostensibly low phosphorylation of normal adult tau lies in the fact that tau's state of phosphorylation is extremely sensitive to preparation conditions. As shown 12 years ago (Planel et al., 2007), even a small decrease in temperature during anesthesia generates enhanced phosphorylation because PPases lose activity faster than kinases. More importantly to AD, though, at time of death, kinases lose their activity more quickly because the co-factor ATP is depleted, whereas PPases continue to operate because they do not require ATP. As a result, tau from normal adult brains, prepared after a postmortem delay of many hours, is mostly dephosphorylated. Tau from fetal brain or from brain operations is prepared rapidly and is still in a high state of phosphorylation (Kenessey and Yen, 1993). By contrast, tau from AD brain remains in a high state of phosphorylation, most likely because many sites become inaccessible to PPases in tau‘s fibrous or oligomeric states (Schneider et al., 1999). 

    What conclusions can one draw from these observations?

    (1) There is room for doubt that tau phosphorylation is causally involved in the progression of AD, as most evidence is only correlative. This might explain the failure of treatments aimed at modulating kinases or PPases.

    (2) Certain phosphorylation sites of tau are valid early markers of AD, but not necessarily in a mechanistic sense; they might merely reflect the neuronal phosphorylation tone, and the protection of oligomeric or polymeric tau against PPases during the postmortem period. Thus, these early markers may help to identify other non-tau factors involved in the onset of AD, e.g. changes in energy metabolism, microglia activation, etc.

    (3) If the phosphorylation tone is altered in the course of AD, this will affect not only tau but multiple other proteins whose roles may have escaped attention so far. 

    (4) Phosphorylation is the major post-translational modification of tau in healthy cells, e.g. Sf9 cells known for their resistance to toxicity and radiation. As an intrinsically disordered protein with many Ser/Thr sites, tau is a faithful sensor of the phosphorylation tone, but the functional significance of sites is a different matter. Thus the involvement of multiple other modifications besides phosphorylation, occurring in different tau isoforms presented by Wesseling et al., 2020, and their integration into the cascade of AD progression, opens a new perspective for the identification of other, non-tau factors involved in the onset of tau aggregation and AD progression.

    Acknowledgements: We thank Dr. Emmanuel Planel (Laval Univ., Quebec) for insightful discussions.


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Paper Citations

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  2. . Tau proteins of Alzheimer paired helical filaments: abnormal phosphorylation of all six brain isoforms. Neuron. 1992 Jan;8(1):159-68. PubMed.
  3. . FLEXITau: Quantifying Post-translational Modifications of Tau Protein in Vitro and in Human Disease. Anal Chem. 2016 Apr 5;88(7):3704-14. Epub 2016 Mar 7 PubMed.
  4. . Assembly of tau protein into Alzheimer paired helical filaments depends on a local sequence motif ((306)VQIVYK(311)) forming beta structure. Proc Natl Acad Sci U S A. 2000 May 9;97(10):5129-34. PubMed.
  5. . Neuronal uptake and propagation of a rare phosphorylated high-molecular-weight tau derived from Alzheimer's disease brain. Nat Commun. 2015 Oct 13;6:8490. PubMed.
  6. . Global hairpin folding of tau in solution. Biochemistry. 2006 Feb 21;45(7):2283-93. PubMed.
  7. . Site-specific effects of tau phosphorylation on its microtubule assembly activity and self-aggregation. Eur J Neurosci. 2007 Dec;26(12):3429-36. PubMed.
  8. . A combinatorial native MS and LC-MS/MS approach reveals high intrinsic phosphorylation of human Tau but minimal levels of other key modifications. J Biol Chem. 2020 Dec 25;295(52):18213-18225. Epub 2020 Oct 26 PubMed.

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