30 Jul 2021

Scientists had previously reported that removing senescent glia from mouse models of tauopathy protected them from neurodegeneration. But what made those glia senescent to begin with? In the July 20 Cell Reports, researchers led by Rakez Kayed, University of Texas, Galveston, claim it was tau. They detail how tau oligomers inflamed astrocytes in culture, prompting them to expel a protein called high mobility group box 1. HMGB1 then led adjacent cells down the path to senescence. Inhibiting HMGB1 release prevented this culture of corruption and, in mouse models of tauopathy, it not only reduced senescent astrocytes but also the amount of tau oligomers and tangles. The animals’ short-term memory also improved.

“Their data show HMGB1 secretion as a readout of astrocytic pathology and emphasize the important role of astrocytes in tauopathies,” Kenneth Kosik, University of California, Santa Barbara, told Alzforum.

HMGB1 is a nuclear protein involved in DNA replication and repair. Its appearance in the cytosol signals cellular senescence. Released by glia, it can activate nearby cells to crank out inflammatory cytokines, ultimately damaging tissue through a process called senescence-associated inflammation (Davalos et al., 2013). 

Senescence Cycle. In both human and mouse brain tissue, tau oligomers (TauO) stir astrocytes. These cells then release HMGB1, setting off senescence in nearby cells in a paracrine fashion. Ultimately, neuroinflammation ensues and tangles accumulate. Ethyl pyruvate (EP) and glycyrrhizic acid (GA) prevent release of HMGB1 in mice, reducing the number of senescent astrocytes, and quelling neuroinflammation and tau pathology. [Courtesy of Gaikwad et al., Cell Reports, 2021.]

Kayed and colleagues previously found HMGB1 in the cytosol of astrocytes that were surrounded by tau oligomers in AD and FTD postmortem tissue (Nilson et al., 2017). Did HMGB1 relocalization within astrocytes indeed indicate senescence and contribute to tau pathology?

To find out, first author Sagar Gaikwad and colleagues examined frontal cortex tissue taken postmortem from eight people who had had AD, six people who had FTD, and eight age-matched controls. He identified astrocytes by their expression of glial fibrillary acidic protein (GFAP), oligomers of tau by their affinity for the TOMA antibody, and senescent cells by binding of the antibody D3W8G, which recognizes the tumor-suppressor protein p16. The latter is a marker of senescence-associated cell cycle arrest. Generally, cells are considered senescent if they highly express p16, have high β-galactosidase (β-gal) activity, and spew cytokines (Childs et al., 2017). 

In the AD and FTD samples, 75 percent of astrocytes were senescent and had oligomers of tau within or nearby (see image below). Gaikwad found HMGB1 in their cytoplasm and clumps of the histone protein γH2AX in their nuclei. This histone is a marker of DNA damage caused by senescence.

Senescent Astrocytes. In cortex tissue from people who had AD (top) or FTD (middle), astrocytes (green) that accumulated the senescent marker p16 (red) lay adjacent to tau oligomers (pink). Control tissue (bottom) had neither senescent cells nor oligomers. [Courtesy of Gaikwad et al., Cell Reports, 2021.]

Did the oligomers cause senescence? To find out, the researchers cultured astrocytes from healthy wild-type mice and treated them with oligomers made from recombinant human tau. The astrocytes took up the oligomers. Eleven days later, HMGB1 had turned up in the cytoplasm, ultimately escaping into the culture medium (see image below). Seventy percent of the tau-exposed astrocytes also expressed p16 and had high β-gal activity.

Leaky HMGB1. Cultured astrocytes (top), normally constrain HMGB1(red) to the nucleus (blue). On addition of tau oligomers (bottom), HMGB1 leaked into the cytoplasm (white arrows). [Courtesy of Gaikwad et al., Cell Reports, 2021.]

Senescence can spread from cell to cell via paracrine inflammatory signals (Acosta et al., 2013). Did the secreted HMGB1 act this way? Gaikwad and colleagues replaced media from healthy astrocytes with that from senescent ones. The healthy cells became senescent, as evident by high p16 expression and β-gal activity. The β-gal activity also increased when the scientists treated cultured astrocytes with recombinant HMGB1 directly, but not if they also added an anti-HMGB1 antibody. This indicates that the HMGB1 in the senescent cell medium likely spread senescence to nearby cells.

Could preventing the HMGB1 secretion in the first place protect astrocytes from tau oligomers? The scientists pretreated cultured astrocytes with ethyl pyruvate (EP) and glycyrrhizic acid (GA), two inhibitors of HMGB1 release, then they added tau oligomers. After 11 days, cells pretreated with either molecule still had high levels of HMGB1 in the cytosol, but the cultures had fewer p16-positive astrocytes. The authors interpreted this as evidence that EP and GA had prevented secretion of HMGB1 and therefore reduced senescence in the cultures. Cultures pretreated with both inhibitors had even fewer senescent astrocytes.

The scientists wondered if these inhibitors also quelled inflammation. They measured levels of HMGB1 and cytokines in the astrocyte medium. While oligomer-exposed astrocytes spewed lots of HMGB1 and inflammatory cytokines, including interleukin-6 and tumor necrosis factor-α, cells treated with both inhibitors secreted significantly less of each.

Finally, would inhibiting HMBG1 release from cells have any benefit in the brain? To find out, Gaikwad treated 12-month-old hTau mice with EP and GA three times a week for eight weeks. These mice express six isoforms of human tau and have significant tangles, gliosis, neurodegeneration, and cognitive problems by 1 year of age. Compared to vehicle-treated mice, inhibitor-treated mice were more curious about new objects and environments, hinting that their short-term memory had improved.

In keeping with this, the treated mice had fewer tangles and less p-tau in the hippocampus (see image below). They also had higher levels of postsynaptic PSD95 and presynaptic synapsin I in their brains than did control hTau mice, and more neurons as judged by NeuN staining. The findings indicate that the inhibitors preserved synapses and prevented neuron loss. Tau oligomer levels, p16-expressing astrocytes, and astrocytes with cytoplasmic HMGB1 were all lower in the treated animals as well, suggesting fewer senescent cells. Those astrocytes that did express senescent markers had fewer γH2AX foci, indicating less DNA damage.

Preventing HMGB1 secretion also calmed astrogliosis and neuroinflammation in the mice. Brain slices had less GFAP expression and lower concentrations of IL-6 and TNF-α. Interestingly, inhibitor treatment also lowered the amount of HMGB1 in the animals’ blood.

Tackling Tangles. By 14 months, the hippocampi of hTau mice (top) have amassed neurofibrillary tangles (green). Mice treated with HMGB1 release inhibitors from 12 months of age and had 75 percent less tangle pathology (bottom row). Nuclei are stained blue. [Courtesy of Gaikwad et al., Cell Reports, 2021.]

Shane Liddelow, New York University, noted that different brain regions were studied in mice and people, making direct comparison difficult. "There might not be a difference, but comparing rodent cortical tissue to that from people would have alleviated this concern," he wrote to Alzforum.

Dezhi Liao, University of Minnesota, Minneapolis, thought this senescence hypothesis was an interesting adjunct to the amyloid cascade hypothesis, which contends that pathogenic Aβ triggers brain problems by triggering production of oligomers of tau. “Instead, the authors propose a novel hypothesis that tau oligomers induce astrocyte senescence by increasing the release of HMGB1,” he wrote to Alzforum (full comment below). However, he cautioned that the mouse used does not recapitulate all the pathophysiology of the AD brain. “It will be interesting for further studies to test how the proposed astrocyte pathway fits in the big picture of the pathophysiology of tauopathies,” he added.

However, some questioned if cultured astrocytes behave the same way as those in the brain, while others wondered about the nature of the astrocyte senescence. This paper is built on the assumption that astrocytes can be senescent, a concept that is currently debated, noted Alberto Serrano-Pozo, Massachusetts General Hospital, Charlestown. He and 80 other researchers recently penned a consensus statement on the vagaries of astrocyte nomenclature (Escartin et al., 2021). “We suggest caution about extending the concept of senescence to astrocytes based upon the expression of cell senescence marker p16INK4A, increased β-galactosidase activity, and secretion of cytokines because the core definition of senescence (that is, irreversible cell-cycle arrest in proliferative cells) may not apply to astrocytes, which are essentially post-mitotic cells that rarely divide in healthy tissue,” they wrote.—Chelsea Weidman Burke

Comments

1. • Dezhi Liao
     The University of Minnesota
   • Posted: 30 Jul 2021

   In the classical amyloid hypothesis, pathogenic Aβ species impair brain functions and structures through tau hyperphosphorylation and oligomerization. In this interesting paper, the authors instead propose a novel hypothesis that tau oligomers induce senescence of astrocytes by increasing the release of HMGB1. The hypothesis is highly novel and has a great translational significance as it unravels a new drug target.

   A huge amount of literature supports that soluble tau oligomers in neurons impair synaptic and cognitive functions. Treatment with HMGB1 inhibitors here probably affect both neurons and glia. Moreover, the aged human tau transgenic mice used here do not contain all regulatory elements of human tau genes and have no change in Aβ. Tau expression levels and the proportion of all six isoforms of tau proteins may not be the same as in human brains. Therefore, the model will probably not recapture all aspects of the cellular changes that occur in Alzheimer’s disease and other neurodegenerative diseases.

   It will be interesting for further studies to test how the proposed astrocyte pathway fits in the big picture of the pathophysiology of tauopathies.

   View all comments by Dezhi Liao

Make a Comment

To make a comment you must login or register.

References

Research Models Citations

1. htau

Paper Citations

1. Davalos AR, Kawahara M, Malhotra GK, Schaum N, Huang J, Ved U, Beausejour CM, Coppe JP, Rodier F, Campisi J. p53-dependent release of Alarmin HMGB1 is a central mediator of senescent phenotypes. J Cell Biol. 2013 May 13;201(4):613-29. Epub 2013 May 6 PubMed.
2. Nilson AN, English KC, Gerson JE, Barton Whittle T, Nicolas Crain C, Xue J, Sengupta U, Castillo-Carranza DL, Zhang W, Gupta P, Kayed R. Tau Oligomers Associate with Inflammation in the Brain and Retina of Tauopathy Mice and in Neurodegenerative Diseases. J Alzheimers Dis. 2017;55(3):1083-1099. PubMed.
3. Childs BG, Gluscevic M, Baker DJ, Laberge RM, Marquess D, Dananberg J, van Deursen JM. Senescent cells: an emerging target for diseases of ageing. Nat Rev Drug Discov. 2017 Oct;16(10):718-735. Epub 2017 Jul 21 PubMed.
4. Acosta JC, Banito A, Wuestefeld T, Georgilis A, Janich P, Morton JP, Athineos D, Kang TW, Lasitschka F, Andrulis M, Pascual G, Morris KJ, Khan S, Jin H, Dharmalingam G, Snijders AP, Carroll T, Capper D, Pritchard C, Inman GJ, Longerich T, Sansom OJ, Benitah SA, Zender L, Gil J. A complex secretory program orchestrated by the inflammasome controls paracrine senescence. Nat Cell Biol. 2013 Aug;15(8):978-90. Epub 2013 Jun 16 PubMed.
5. Escartin C, Galea E, Lakatos A, O'Callaghan JP, Petzold GC, Serrano-Pozo A, Steinhäuser C, Volterra A, Carmignoto G, Agarwal A, Allen NJ, Araque A, Barbeito L, Barzilai A, Bergles DE, Bonvento G, Butt AM, Chen WT, Cohen-Salmon M, Cunningham C, Deneen B, De Strooper B, Díaz-Castro B, Farina C, Freeman M, Gallo V, Goldman JE, Goldman SA, Götz M, Gutiérrez A, Haydon PG, Heiland DH, Hol EM, Holt MG, Iino M, Kastanenka KV, Kettenmann H, Khakh BS, Koizumi S, Lee CJ, Liddelow SA, MacVicar BA, Magistretti P, Messing A, Mishra A, Molofsky AV, Murai KK, Norris CM, Okada S, Oliet SH, Oliveira JF, Panatier A, Parpura V, Pekna M, Pekny M, Pellerin L, Perea G, Pérez-Nievas BG, Pfrieger FW, Poskanzer KE, Quintana FJ, Ransohoff RM, Riquelme-Perez M, Robel S, Rose CR, Rothstein JD, Rouach N, Rowitch DH, Semyanov A, Sirko S, Sontheimer H, Swanson RA, Vitorica J, Wanner IB, Wood LB, Wu J, Zheng B, Zimmer ER, Zorec R, Sofroniew MV, Verkhratsky A. Reactive astrocyte nomenclature, definitions, and future directions. Nat Neurosci. 2021 Mar;24(3):312-325. Epub 2021 Feb 15 PubMed.

Further Reading

News

1. Does Astrocyte Tau Cause Dementia? 13 Nov 2020
2. Are Tauopathies Caused by Neuronal and Glial Senescence? 28 Sep 2018
3. Plaques Age Glial Precursors, Stoking Inflammation 11 Apr 2019
4. Squelching ApoE in Astrocytes of Tau-Ravaged Mice Dampens Degeneration 10 Apr 2021
5. Toxic Tau: Who Are You, and Where Are You From? 20 Dec 2018