Is Tau Phosphorylation All Bad?
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Hyperphosphorylation causes tau to form toxic aggregates that underlie a range of tauopathies, including Alzheimer’s. So goes the conventional wisdom in Alzheimer’s disease research. But could some tau phosphorylation be protective? In the November 18 Science, researchers led by Lars Ittner, University of New South Wales, Sydney, claim that by phosphorylating tau at threonine 205, mitogen-activated protein kinase p38γ prevents formation of a postsynaptic complex that mediates Aβ-induced excitotoxicity. In cell and mouse models, boosting p38γ levels protected against Aβ and seizures, while knocking out the enzyme left neurons vulnerable.
“This is an interesting study that suggests the field may have to change the way they think about tau phosphorylation in Alzheimer's disease,” said Joe Lewcock from Denali Therapeutics, South San Francisco. “It is a nice addition to our understanding of tau function and will surely provide a starting point for a range of future work.”
Dendritic Locale.
The kinase p38γ (green) co-localizes with the postsynaptic marker PSD-95 (red, top) but not with presynaptic synaptophysin (red, bottom). The dendritic marker MAP2 is blue. [Courtesy of Science/AAAS.]
Ittner’s group previously reported that tau plays a crucial role in Aβ excitotoxicity (Jul 2010 news on Ittner et al., 2010). Tau targets Fyn kinase to the synapse, where the enzyme phosphorylates a subunit of the NMDA receptor. This strengthens the interaction between NMDA and PSD-95, making the synapse hyperactive. Ittner wondered if the neuron had a way to regulate this process to prevent Aβ toxicity. Ittner’s brother, first author Arne Ittner, a molecular biologist who joined the lab in 2010, had worked extensively on p38 kinases. Since p38—specifically the γ isoform—is known to interact with PSD-95, he suggested they test if the p38 kinases regulate the process. Certain p38 isoforms—β, γ, and δ, but not α—can hyperphosphorylate tau (Goedert et al., 1997).
To see how the individual isoforms of p38 contribute to the hyperexcitability caused by Aβ, the Ittners knocked out each one individually in wild-type mice, then induced seizures with the GABA(A) receptor antagonist pentylenetetrazole (PTZ). Deleting only the p38γ isoform hastened and intensified seizures. If the researchers knocked out p38γ in APP23 mice, which overexpress human APP with the Swedish mutation, the animals became more sensitive to these electrical imbalances in the brain. These mice also died younger, developed earlier and worse memory problems, and had more disrupted brain network activity than APP23 mice with active p38γ. The authors did not cross APP23 mice with any of the other p38 knockouts.
This p38γ effect seemed to depend on tau. Knocking out tau in APP23 mice did away with any ill effects of expunging p38γ. At the same time, elevated tau expression in p38γ knockouts brought on more seizures. All signs pointed to a mechanism whereby p38γ dampened tau’s ability to mediate Aβ toxicity. Interestingly, APP23 mice, APPNL-G-F knock-in mice, and postmortem tissue from AD patients all had lower levels of the enzyme compared to their respective controls.
In keeping with the idea that p38γ might regulate PSD-95/tau/Fyn complexes, the researchers found that the kinase concentrated in dendritic spines in primary hippocampal neurons (see image above). They found that raising the level of p38γ in human embryonic kidney cells reduced levels of PSD-95/tau/Fyn. Expressing a constitutively active form of p38γ, p38γCA, in the brain of APP23 mice diminished the complexes there, as well. In addition, constitutively active p38γ protected cultured neurons from Aβ toxicity and suppressed PTZ-induced seizures in wild-type mice. This constitutively active kinase restored coordinated electrical activity that is compromised in APP23 animals, and improved their performance in the Morris water maze (see image below).
Better Navigation.
APP23 mice that express constitutively active p38γ (right) find a submerged platform sooner than control littermates (left). [Courtesy of Science/AAAS.]
How does p38γ modify tau? Based on experiments carried out in both test tubes and mice, p38γ phosphorylates tau at threonine 205 (T205). If the researchers mimicked this phosphorylation by replacing that threonine with negatively charged glutamic acid, tau interacted less with PSD-95, but its relationship with Fyn was unaffected.
Taken together, the results suggest that phosphorylating tau at T205 protects against Aβ-induced excitotoxicity, countering the idea that all tau phosphorylation is damaging. “This is the first example of a specific tau phosphorylation that has beneficial effects in the context of Aβ toxicity,” said Lars Ittner. “It challenges the dogma that all tau phosphorylation causes neuronal toxicity.” He suggested T205 might be one of many functional tau phosphorylation sites and speculated that tau phosphorylation at T205 prevents Aβ toxicity early in Alzheimer’s disease process, but that this protection becomes overwhelmed by later hyperphosphorylation. “We should rethink tau modification and examine in detail how phosphorylation at specific sites affects physiological function and disease,” Ittner told Alzforum.
Could this discovery have therapeutic implications? Possibly, said Ittner. Perhaps a drug could be developed to enhance this type of phosphorylation, he proposed. Lewcock thought this would be difficult. “From a therapeutic perspective, a wealth of data is presented supporting p38γ as the key kinase regulating tau T205 phosphorylation, though unfortunately the development of compounds that act as kinase activators has proven far less tractable than [developing] kinase inhibitors,” he said.
Ittner also advised caution about developing p38 inhibitors or other drugs that might inadvertently block p38γ. “I would be concerned about inhibiting p38 generally,” he said. “I would strongly urge people to look for specific inhibitors that do not inhibit the γ form of p38,” said Ittner. One p38 inhibitor called VX-745 has been tested in Phase 2 trials of people with mild cognitive impairment due to AD or mild AD (Dec 2014 conference news).
Researchers are also developing selective p38α MAPK inhibitors for AD and other neurodegenerative diseases (Roy et al., 2015). In microglia, this isoform regulates the release of pro-inflammatory cytokines in response to stressors such as Aβ42.—Gwyneth Dickey Zakaib
References
News Citations
- Honolulu: The Missing Link? Tau Mediates Aβ Toxicity at Synapse
- New Ideas for Alzheimer’s Treatment: What’s on Offer in 2015?
Research Models Citations
Therapeutics Citations
Paper Citations
- Ittner LM, Ke YD, Delerue F, Bi M, Gladbach A, van Eersel J, Wölfing H, Chieng BC, Christie MJ, Napier IA, Eckert A, Staufenbiel M, Hardeman E, Götz J. Dendritic function of tau mediates amyloid-beta toxicity in Alzheimer's disease mouse models. Cell. 2010 Aug 6;142(3):387-97. Epub 2010 Jul 22 PubMed.
- Goedert M, Hasegawa M, Jakes R, Lawler S, Cuenda A, Cohen P. Phosphorylation of microtubule-associated protein tau by stress-activated protein kinases. FEBS Lett. 1997 Jun 2;409(1):57-62. PubMed.
- Roy SM, Grum-Tokars VL, Schavocky JP, Saeed F, Staniszewski A, Teich AF, Arancio O, Bachstetter AD, Webster SJ, Van Eldik LJ, Minasov G, Anderson WF, Pelletier JC, Watterson DM. Targeting human central nervous system protein kinases: An isoform selective p38αMAPK inhibitor that attenuates disease progression in Alzheimer's disease mouse models. ACS Chem Neurosci. 2015 Apr 15;6(4):666-80. Epub 2015 Feb 23 PubMed.
Further Reading
News
- New Target for Tauopathies? Blocking Nuak1 Could Reduce Tau Build-Up
- Tipping the Balance Toward Four-Repeat Tau Exacerbates Toxicity in Mice
- Stress Granule Protein Entwines and Misfolds Tau
- ROCK’n the Tau Pathway?
- Inventory of Tau Modifications Hints at Undiscovered Functions
- Could Kink in Tau Lead to Neurodegeneration?
Primary Papers
- Ittner A, Chua SW, Bertz J, Volkerling A, van der Hoven J, Gladbach A, Przybyla M, BI m m, van Hummel A, Stevens CH, Ippati S, Suh LS, Macmillan A, Sutherland G, Kril JJ, Silva AP, Mackay J, Poljak A, Delerue F, Ke YD, Ittner LM. Site-specific phosphorylation of tau inhibits amyloid-b toxicity in Alzheimer’s mice. Science. 2016 Nov 18; 354(6314):904-8.
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Comments
Denali Therapeutics
This is an interesting study that suggests the field may have to change the way they think about tau phosphorylation in Alzheimer's disease. Tau phosphorylation has historically been thought of as a contributor to neurofibrillary tangle formation and disease-associated tau toxicity, but in this study Ittner et al. identify that phosphorylation of tau on threonine 205 may actually serve a protective role. This is of note as phosphorylation of tau at this site has often been used as a measure of disease pathology and is the recognition site of the commonly used “AT8” tau antibody as well many commonly used reagents.
The authors identify p38γ, a previously underappreciated isoform of p38, as a tau kinase that preferentially phosphorylates T205. Surprisingly, loss of p38γ enhanced rather than suppressed tau-based toxicity. This could be explained through the observation that phosphorylation of T205 resulted in dissociation of tau/fyn/PSD-95 complexes, which was identified as a mechanism of tau-based neurotoxicity by the authors in a previous study. From a therapeutic perspective, a wealth of data is presented supporting p38γ as the key kinase regulating tau T205 phosphorylation, though unfortunately the development of compounds that act as kinase activators has proven far less tractable than kinase inhibitors. Therefore, more work is likely needed in order to translate these discoveries into development of new treatments for Alzheimer's disease.
Overall the study is comprehensive, well-designed, and contains a number of loss-of-function and gain-of-function experiments with both tau and p38γ that solidify their conclusions. It is a nice addition to our understanding of tau function and will surely provide a starting point for a range of future work.
New York Institute for Basic Research in Developmental Disabilities
This is a well-done study. Although the authors have not pointed it out, one very intriguing possible conclusion that can be drawn is that the Aβ drug trials, especially immunotherapies, continue to be negative because tau in AD brain is hyperphosphorylated at Thr 205, along with several other sites, which protects AD brain from Aβ-induced neurotoxicity and hence removal of Aβ will not have any beneficial therapeutic effect.
I believe a serious problem with this present study, and for that matter for many other such studies on the etiopathogenic relationship between Aβ and tau pathologies, is the use of highly artificial overexpression/knockout transgenic mice, which can grossly alter the quantitative effects and hence the outcome.
Shanghai Qiangrui Biotech Co., Ltd
The hyperphosphorylation of tau has long been proposed to contribute to the tau pathology in Alzheimer’s disease and other tauopathies. However, owing to the number and heterogeneity of phosphorylation sites on tau, investigating the exact role of phosphorylation of tau in neurodegeneration proves to be a challenge. Although hyperphosphorylation of tau is generally regarded as a culprit of neurodegeneration, hyperphosphorylation of tau occurs in other transient situations (e.g., hibernation, stress) without causing lasting side effects. In their recent paper, the Ittner brothers provide evidence that the phosphorylation of tau at Thr205 by P38γ can even be protective because it can suppress excitotoxicity induced by Aβ or PTZ. This is because the phosphorylation at Thr205 of tau disrupts the formation of NMDA-Receptor/PSD-95/tau/Fyn complexes, which mediate Aβ or PTZ induced excitotoxicity. This interesting result expands on the earlier studies of the Ittner and Götz team (Ittner et al., 2010), which assigned a physiological role to the small fraction of tau found in dendrites (which is otherwise mainly axonal). The Thr205 residue is one of several Ser-Pro or Thr-Pro motifs in tau that are targeted by several proline-directed kinases involved in cellular signaling pathways, including those of the JNK family. This type of phosphorylation has been under intense scrutiny, and therefore the current study can be compared to others, leaving several issues to be clarified in the future:
(1) Mondragon-Rodriguez and colleagues reported that phosphorylation of tau can suppress excitotoxicity and thus can serve as a regulatory mechanism to prevent NMDA receptor overexcitation (Mondragon-Rodriguez et al., 2012). These authors focused on phosphorylation of tau at the phospho-epitopes AT180, AT8, or AT100, also representing Ser-Pro or Thr-Pro sites, which reduces tau's interaction with PSD-95. This is at variance with the current study demonstrating that only phosphorylation at Thr205 reduces the association of tau with PSD-95 and Fyn.
(2) The current study showed that the kinase P38γ phosphorylates tau at Thr205 and to a lesser extent at Ser199 (included in the phospho-epitope AT8, a widely used antibody to characterize phosphorylated tau), but not at Ser 202 (included in the phospho-epitope AT8) and hardly any at Ser396 and Ser404 (epitopes of PHF1, another commonly used antibody against phospho-tau). On the other hand, several earlier studies (for instance, Buee-Scherrer and Goedert, 2002; Goedert et al., 1997) revealed that P38γ phosphorylates tau not only at AT8 sites, but also strongly at the PHF1-epitope pSer396/pSer404. It is not clear whether this discrepancy is due to the difference of antibodies used in these studies, since the current Ittner paper used antibodies with epitopes of a single phosphorylation site (pT205, pS202 etc.), while the other studies used antibodies against a combination of multiple phosphorylation sites (e.g. AT8, PHF1).
The present study also shows that the P38γ level is reduced in old APP23 mice but not in young APP23 mice (Fig.S10), compared to wild-type mice. On the other hand, the APP23 mice display enhanced epileptiform activity at four months, when there is no P38γ reduction. Therefore, there must be mechanisms independent of P38γ underlying the Aβ-induced hyperexcitotoxicity in the young APP mice whose nature deserves attention in future experiments.
References:
Buée-Scherrer V, Goedert M. Phosphorylation of microtubule-associated protein tau by stress-activated protein kinases in intact cells. FEBS Lett. 2002 Mar 27;515(1-3):151-4. PubMed.
Goedert M, Hasegawa M, Jakes R, Lawler S, Cuenda A, Cohen P. Phosphorylation of microtubule-associated protein tau by stress-activated protein kinases. FEBS Lett. 1997 Jun 2;409(1):57-62. PubMed.
Mondragón-Rodríguez S, Trillaud-Doppia E, Dudilot A, Bourgeois C, Lauzon M, Leclerc N, Boehm J. Interaction of Endogenous Tau Protein with Synaptic Proteins Is Regulated by N-Methyl-D-aspartate Receptor-dependent Tau Phosphorylation. J Biol Chem. 2012 Sep 14;287(38):32040-53. PubMed.
Ittner LM, Ke YD, Delerue F, Bi M, Gladbach A, van Eersel J, Wölfing H, Chieng BC, Christie MJ, Napier IA, Eckert A, Staufenbiel M, Hardeman E, Götz J. Dendritic function of tau mediates amyloid-beta toxicity in Alzheimer's disease mouse models. Cell. 2010 Aug 6;142(3):387-97. Epub 2010 Jul 22 PubMed.
Max Planck Institute for Biology of Ageing
This is a very interesting paper. I find this study even more intriguing because pT205 is one of the two epitopes recognized by the AT8 antibody, whose staining has been shown to be increased in tauopathy brains in a plethora of studies. I wonder what the authors' opinion on this issue is.
University of North Carolina- Chapel Hill
This paper is a very important step toward identifying the kinases and site-specific post-translational events that determine tau toxicity.
These findings are highly suggestive that tau hyperphosphorylation is not the whole story in terms of AD pathogenesis. Furthermore, this study suggests that complex site-specific modifications of tau could either promote or prevent toxicity. This concept could have major implications for how we therapeutically target tau in AD patients.
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