Does Norepinephrine Metabolite DOPEGAL Turn Tau Toxic?
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What makes tau toxic? In the March 24 Nature Structural & Molecular Biology, researchers led by Keqiang Ye at Shenzhen Institute of Advanced Technology, China, suggest a new culprit, a norepinephrine metabolite known as DOPEGAL, which is produced only in the locus coeruleus (LC). The authors found that DOPEGAL covalently binds tau at lysine residue 353. From there, it promotes tau phosphorylation, truncation, and aggregation. When the authors suppressed DOPEGAL production in tauopathy model mice, few LC tau aggregates formed, and they did not spread to other brain regions. Moreover, LC neurons remained healthy, and the mice stayed sharp in memory tests. The findings may help explain why tau pathology begins in the LC in aging and Alzheimer’s disease, Ye told Alzforum.
- DOPEGAL binding to tau stimulates its phosphorylation, cleavage, and aggregation.
- It also promotes the spread of tau aggregates from LC to other brain regions.
- In mice, blocking DOPEGAL binding preserved neurons and memory.
“This is a very important paper, placing norepinephrine and the locus coeruleus center stage in Alzheimer’s disease,” James Rowe at Cambridge University, U.K., wrote to Alzforum. “This study highlights a potential route to treatment that might be both symptomatic and disease-modifying.”
Previous research linked degeneration of the LC to cognitive decline, as well as to cortical plaques and tangles (Sep 2019 news; Sep 2021 news). Researchers have long known that about threefold more DOPEGAL accumulates in the LC of AD than control brain, though the role of this metabolite was unclear (Burke et al., 1999). Recently, while working at Emory University School of Medicine in Atlanta, Ye reported that DOPEGAL triggers the protease asparagine endopeptidase (AEP) to snip tau at residue Asn368, creating a fragment prone to aggregate and spread through the brain (Kang et al., 2020).
Preventing Degeneration. In the LC of 9-month-old P301S mice (top), tau-DOPEGAL adducts (red) accumulate, and neurons (green) degenerate. In mice treated with an MAO-A inhibitor to prevent DOPEGAL formation (bottom), ß neurons stay healthy. Nuclei are blue. [Courtesy of Kang et al., Nature Structural & Molecular Biology.]
To figure out how DOPEGAL does this, the authors combined DOPEGAL and tau in vitro and analyzed the resulting structure by mass spectrometry. This revealed the covalent bond at Lys353. What effect does this molecular mating have? In cell-free preparations, adding DOPEGAL to tau fragments accelerated fibrillization and made the aggregates more compact and resistant to proteolysis. In addition, adding DOPEGAL to a mix of full-length tau and AEP doubled the cleavage rate, showing that DOPEGAL also promotes tau truncation. When Lys353 was mutated to an arginine, abolishing DOPEGAL binding, the metabolite had no effect on tau aggregation or cleavage.
Next, first author Seong Su Kang at Emory studied HEK293 cells expressing tau-repeat domains, which are prone to aggregate. Adding DOPEGAL to these cells triggered tau phosphorylation and aggregation. When the cells were pretreated with tau preformed fibrils (PFFs) to induce pathology, DOPEGAL further accelerated fibrillization. It also facilitated the spread of aggregates to new cells, with about twice as many cells acquiring them. As expected, Lys353Arg tau was unaffected by DOPEGAL, with no increase in aggregation or seeding.
Other experiments tied DOPEGAL to toxicity and neurodegeneration. In primary neurons treated with PFFs, adding DOPEGAL doubled the number of apoptotic cells. In P301S mice, injecting DOPEGAL into the LC triggered degeneration, with the number of dying neurons going from about 5 to 40 percent. The same thing happened when the authors injected monoamine oxidase A (MAO-A), the enzyme that converts norepinephrine into DOPEGAL. DOPEGAL spiked, leading to tau aggregation and cell death.
Could blocking DOPEGAL binding ameliorate tauopathy? The authors used two approaches to test this. In the first, they overexpressed either wild-type or Lys353Arg tau in the LC of tau knockout mice. Exogenous wild-type tau spread beyond the LC to the entorhinal cortex and hippocampus, and caused synapse loss, neuron death, and memory problems. Lys353Arg tau stayed put, and mouse memory stayed keen. The findings suggest that when DOPEGAL cannot bind tau, tau does not turn toxic.
In the second approach, the authors treated 6-month-old P301S mice with the MAO-A inhibitor clorgyline for three months. Treatment lowered DOPEGAL and prevented tau phosphorylation and truncation, while preserving LC neurons and memory (see image above).
Do the findings apply to people? In one sign that they might, the authors found fivefold higher amounts of tau-DOPEGAL adducts in the LC of AD brains compared to age-matched controls. This correlated with truncated and aggregated tau.
Drugs that selectively inhibit MAO-A might have therapeutic potential for preventing AD, Ye noted. MAO inhibitors have been used for decades to treat depression, but have been largely supplanted by newer drugs due to their side effect profile and interactions with foods and other drugs. Clorgyline is FDA-approved but was never marketed, and is used only as a research tool.
Ye thinks a better approach might be to inhibit AEP. In one experiment, deleting AEP in P301S mice lowered the amount of tau-DOPEGAL as well, suggesting a positive feedback loop between DOPEGAL binding and AEP cleavage. “Blockage of AEP may break the feed-forward chain during aging,” Ye wrote to Alzforum. He noted that AEP also cleaves amyloid precursor protein, so suppressing it could have broad effects on AD pathology. He is looking for AEP inhibitors with good clinical properties (Zhang et al., 2016).—Madolyn Bowman Rogers
References
News Citations
- Tiny Brain Structure Plays Big Role in Memory
- Is a Waning Locus Coeruleus an Early Sign of Alzheimer’s Disease?
Research Models Citations
Paper Citations
- Burke WJ, Li SW, Schmitt CA, Xia P, Chung HD, Gillespie KN. Accumulation of 3,4-dihydroxyphenylglycolaldehyde, the neurotoxic monoamine oxidase A metabolite of norepinephrine, in locus ceruleus cell bodies in Alzheimer's disease: mechanism of neuron death. Brain Res. 1999 Jan 23;816(2):633-7. PubMed.
- Kang SS, Liu X, Ahn EH, Xiang J, Manfredsson FP, Yang X, Luo HR, Liles LC, Weinshenker D, Ye K. Norepinephrine metabolite DOPEGAL activates AEP and pathological Tau aggregation in locus coeruleus. J Clin Invest. 2020 Jan 2;130(1):422-437. PubMed.
- Zhang Z, Xie M, Ye K. Asparagine endopeptidase is an innovative therapeutic target for neurodegenerative diseases. Expert Opin Ther Targets. 2016 Oct;20(10):1237-45. Epub 2016 May 13 PubMed.
Further Reading
Primary Papers
- Kang SS, Meng L, Zhang X, Wu Z, Mancieri A, Xie B, Liu X, Weinshenker D, Peng J, Zhang Z, Ye K. Tau modification by the norepinephrine metabolite DOPEGAL stimulates its pathology and propagation. Nat Struct Mol Biol. 2022 Apr;29(4):292-305. Epub 2022 Mar 24 PubMed.
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Comments
Cambridge University
This is a very important paper, placing norepinephrine and the locus coeruleus center stage in Alzheimer’s disease (and perhaps in other tauopathies where the LC is also vulnerable). The LC is small—just 50,000 neurons in adult humans—but these cells each project widely throughout the brain.
Previous work has shown the very early involvement of the locus coeruleus in the tau pathology of AD; the secondary effects of NE on neuroinflammatory cascades in AD; and the effect of LC/NE loss on cognition, as the basis of noradrenergic clinical trials like ADMET and NORAD.
But this study is different. It addresses the questions of (i) why is the LC an origin of tauopathy and (ii) how might this promote the propagation of pathology? The answers are important for their therapeutic potential.
Kang et al. demonstrate several mechanisms by which the NE metabolite DOPEGAL may interact and facilitate AD pathogenesis. It selectively stimulates formation of fibrils via a single residue, Lys353, and these fibrils are proteinase resistant; it increases intraneuronal tau pathology, with enhanced seeding efficacy; it does so under the partial control of MAO-A; and it triggers synaptotoxic and cytotoxic pathways.
Of particular value is the team’s approach to bridge all the way from structural biology, via HEK293 cell models, primary neurons, and transgenic (P301S) mice to postmortem human brain tissue. This is a model approach. The authors also demonstrate proof of concept that inhibition of MAO-A and modification of Lys353 reduces the formation and spread of tau pathology. An important step, which guides therapeutic options. And their multilevel approach mitigates concerns about the non-physiological conditions of some of the biochemical and cellular studies.
Although the MAO-A inhibitor used here differs from those already in clinical practice (e.g., for antidepressant effects), this study highlights a potential route to treatment that might be both symptomatic and disease-modifying. Clearly experimental medicine studies would be needed to evaluate this in people with—or at risk of—AD. Such studies would need to give special consideration to the stage of the disease at which such DOPEGAL-based treatments might be expected to work.
While the LC-NE story is important to understand and treat Alzheimer’s, Kang et al. have written an exciting new chapter.
Boston University Chobanian & Avedisian School of Medicine
This is an important finding that supports an interaction between adduct formation and protein aggregation in AD. DOPEGAL is metabolized by aldehyde reductase and aldehyde dehydrogenase. Future studies should be designed to investigate the relationship between enzymatic polymorphisms and AD risk particularly with respect to age at onset, which would be expected a priori to be younger in those individuals who do not effectively metabolize DOPEGAL.
References:
Kamino K, Nagasaka K, Imagawa M, Yamamoto H, Yoneda H, Ueki A, Kitamura S, Namekata K, Miki T, Ohta S. Deficiency in mitochondrial aldehyde dehydrogenase increases the risk for late-onset Alzheimer's disease in the Japanese population. Biochem Biophys Res Commun. 2000 Jun 24;273(1):192-6. PubMed.
Yu RL, Tan CH, Lu YC, Wu RM. Aldehyde dehydrogenase 2 is associated with cognitive functions in patients with Parkinson's disease. Sci Rep. 2016 Jul 25;6:30424. PubMed.
Katholieke Universiteit Leuven, Department of Imaging and Pathology, Laboratory of Neuropathology
Kang et al. found that the primary residue Lys 353 of the τ protein reacts with the norepinephrine metabolite DOPEGAL. This interaction stimulates the accumulation of abnormal phosphorylated τ (p-τ), as well as its aggregation and propagation. This process was inhibited by replacing the lysine residue by arginine (Lys353Arg-τ). The interaction of τ with DOPEGAL was studied in HEK293 and SH-SY5Y/HA-τ cells. Aggregation and propagation effects were studied in MAPT transgenic mice using an Lys353-DOPEGAL antibody. Upregulation of the locus coeruleus MAO activity using AAV-MAO-A led to an increased production of DOPEGAL and p-τ. Treatment with an MAO-A inhibitor reduced DOPEGAL production and improved synaptic integrity. After inducing seeding by injecting fibrillar τ and Lys353Arg-τ into the brains of MAPT transgenic mice, the authors showed that only the “regular” τ carrying the Lys353 induced seeding and propagation but not Lys353Arg-τ.
This is a very elegant study demonstrating for the first time why the locus coeruleus is the brain region affected earliest by p-τ pathology (Braak and Del Tredici , 2011). In principle, the interaction with the norepinephrine metabolite DOPEGAL seems to be essential for the abnormal phosphorylation, seeding, and propagation; at least it is a prerequisite. Whether other factors will also contribute to make real AD pathology out of p-τ accumulation, or whether this is the key event that will in any case cause AD, still needs to be clarified.
On the one hand, the stage-like propagation process starting in the locus coeruleus and ending in the primary visual cortex argues in favor of AD being initiated with this first DOPEGAL-driven p-τ accumulation. On the other hand, locus coeruleus p-τ pathology was observed in nearly all cases over the age of 40 years (Braak et al., 2011), whereas higher Braak NFT stages and amyloid pathology were restricted to less than 40 percent even in the age group 90-100 years (Braak et al., 2011) indicating that not everyone exhibiting p-τ in the locus coeruleus will develop AD during life. Moreover, p-τ accumulation occurs physiologically in the status of hibernation and disappears after arousal (Arendt et al., 2003). These two points argue in favor of the hypothesis that p-τ accumulation per se is not sufficient to proceed to AD in any case, but it is very likely a prerequisite enabling the AD process to be started.
Probably Aβ may play an accelerating role here, as it aggravates p-τ pathology and its propagation (Gomes et al., 2019; Götz et al., 2001; Lewis et al., 2001), probably via a PrPC-related mechanism (Corbett et al., 2020; Gomes et al., 2019). This is integrated in the concept of primary age-related tauopathy (PART) developing into AD and being a prerequisite of this disease (Jellinger et al., 2015; Spires-Jones et al., 2017).
The potential role of norepinephrine metabolism in this process will be an important factor in understanding p-τ and its potential physiological/pathological roles. In hibernating animals, the production of p-τ is physiological and reversible (Arendt et al., 2003). If there is a border between reversible p-τ and AD p-τ, it is currently being discussed that this border may be influenced by the different phosphorylation sites of τ becoming phosphorylated at certain steps of the τ pathology development (Aragão Gomes et al., 2021).
Whether stress and sleep have an impact on p-τ formation, and whether the generation of p-τ has a sleep/stress-related role, needs to be further analyzed and will teach us about lifestyle influence on τ pathology. The role of MAO-A as shown by Kang et al., and the involvement of p-τ in the hibernation process of the brain, could argue in favor of this hypothesis.
The distribution pattern of AD-related granulovacuolar degeneration in regions related to the chronic stress response and to sleep wakefulness, all receiving input from the locus coeruleus (Thal et al., 2011), may support the link to AD. The reason for this is that granulovacuolar degeneration has been reported to be induced by p-τ (Wiersma et al. 2019) and appears to be critically involved in AD-related neuron loss via necroptosis (Koper et al., 2020).
References:
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Arendt T, Stieler J, Strijkstra AM, Hut RA, Rüdiger J, Van der Zee EA, Harkany T, Holzer M, Härtig W. Reversible paired helical filament-like phosphorylation of tau is an adaptive process associated with neuronal plasticity in hibernating animals. J Neurosci. 2003 Aug 6;23(18):6972-81. PubMed.
Braak H, Del Tredici K. The pathological process underlying Alzheimer's disease in individuals under thirty. Acta Neuropathol. 2011 Feb;121(2):171-81. PubMed.
Braak H, Thal DR, Ghebremedhin E, Del Tredici K. Stages of the pathologic process in Alzheimer disease: age categories from 1 to 100 years. J Neuropathol Exp Neurol. 2011 Nov;70(11):960-9. PubMed.
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Gomes LA, Hipp SA, Rijal Upadhaya A, Balakrishnan K, Ospitalieri S, Koper MJ, Largo-Barrientos P, Uytterhoeven V, Reichwald J, Rabe S, Vandenberghe R, von Arnim CA, Tousseyn T, Feederle R, Giudici C, Willem M, Staufenbiel M, Thal DR. Aβ-induced acceleration of Alzheimer-related τ-pathology spreading and its association with prion protein. Acta Neuropathol. 2019 Dec;138(6):913-941. Epub 2019 Aug 14 PubMed.
Götz J, Chen F, van Dorpe J, Nitsch RM. Formation of neurofibrillary tangles in P301l tau transgenic mice induced by Abeta 42 fibrils. Science. 2001 Aug 24;293(5534):1491-5. PubMed.
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Koper MJ, Van Schoor E, Ospitalieri S, Vandenberghe R, Vandenbulcke M, von Arnim CA, Tousseyn T, Balusu S, De Strooper B, Thal DR. Necrosome complex detected in granulovacuolar degeneration is associated with neuronal loss in Alzheimer's disease. Acta Neuropathol. 2020 Mar;139(3):463-484. Epub 2019 Dec 4 PubMed.
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