Two Paths for TREM2-Positive Microglia: DAM or Senescence?

Numerous studies have cast TREM2-expressing microglia as good guys in the Alzheimer’s brain, but new research suggests some play a darker role. In the April 18 Nature Neuroscience, researchers led by Michal Schwartz and Valery Krizhanovsky at the Weizmann Institute of Science, Rehovot, Israel, reported that some TREM2-positive microglia in an amyloidosis mouse model become senescent. Eliminating these cells cooled neuroinflammation and improved memory, indicating they harm the brain. Like disease-associated microglia (DAM), senescent microglia require TREM2, with few forming in TREM2 knockouts. Senescent microglia could be distinguished from DAM by their protein signature, but not easily by gene expression, explaining why they have been overlooked in previous transcriptomic studies.

  • Senescent microglia in amyloidosis mice express high levels of TREM2.
  • They have distinct protein profiles from disease-associated microglia (DAM).
  • The two TREM2 subtypes are hard to distinguish in transcriptomic studies.

“This study suggests that TREM2 displays a dual activity in microglia, which should be carefully considered when contemplating TREM2 as a therapeutic target,” the authors noted.

Marco Colonna at Washington University School of Medicine, St. Louis, called the paper insightful, and said it raises several mechanistic questions for future study, such as how TREM2 expression leads to senescence. “Additionally, it underscores the value of integrating protein data alongside transcriptional data to account for potential disparities,” Colonna wrote (comment below).

Close Yet Aloof. In the 5xFAD mouse cortex, senescent microglia (red) occur near (gold circles) but do not surround plaques (pink) as do healthy microglia (green). [Courtesy of Rachmian et al., Nature Neuroscience.]

Diego Gómez-Nicola at the University of Southampton, U.K., had previously reported that DAM could turn senescent (Jun 2021 news). Another study found senescent microglia that were unable to transition into a DAM state in amyloidosis mice (May 2023 news). Still, little is known about these cells and how they form.

To further characterize senescent microglia, Schwartz and colleagues focused on their protein profile, rather than gene expression. First author Noa Rachmian isolated microglia from the brains of 2-year-old 5xFAD mice, then incubated the cells with antibodies labeled with heavy-metal ions. These antibodies were directed against 33 senescence factors and key microglial proteins. The cells were then shot through a mass spectrometer one at a time to quantify each targeted protein.

To the authors’ surprise, microglia that expressed senescent markers also expressed TREM2, ApoE, and other proteins characteristic of activated microglia. However, they had a distinct profile from DAM, expressing some homeostatic proteins as well, such as TMEM119, P2RY12, and CX3CR1. Moreover, senescent microglia expressed about twice as much TREM2 as did DAM, again indicating they represent a distinct cell population. In immunostained cortex, the senescent cells appeared near amyloid plaques, but did not surround them as do DAM (image at right).

TREM2 expression correlated with the amount of senescent proteins, causing the authors to wonder if TREM2 might help trigger this cell state. Supporting this, when the authors crossed 5xFAD mice with TREM2 knockouts, the offspring accumulated less than half as many senescent microglia, amounting to about 7 percent of all microglia, compared with 18 percent in 5xFAD mice.

Gómez-Nicola said the dependence of senescent microglia on TREM2 is a new finding, and speculated it might be linked to TREM2’s effects on microglial survival and proliferation (Feb 2015 news; Apr 2017 conference news). “Senescent microglia may be more susceptible to the evolving AD-like pathology, therefore becoming more dependent on mechanisms of survival,” Gómez-Nicola wrote (comment below).

Why have TREM2-positive senescent microglia not been widely reported before? Most studies characterizing microglial subtypes have been transcriptional. When the authors reanalyzed previous microglial RNA-Seq datasets, they detected a population of senescent microglia, but found they were hard to discern from DAM. Protein signatures can differ from gene-expression profiles due to post-translational modifications, protein stability, and the different sensitivities of the methods, they noted.

Senescent cells are typically harmful, and that was the case here as well. The authors injected 11-month-old 5xFAD mice with the senolytic drug ABT-737 for three days, then analyzed their brains two weeks later. The number of senescent microglia had been cut nearly in half, while DAM were unchanged. Inflammatory cytokines were suppressed, while the mice’s ability to recognize a new object bounced back to that of wild-types. Another senolytic combination, dasatinib plus quercetin, is being tested in AD patients (Dec 2021 news; Gonzales et al., 2023).—Madolyn Bowman Rogers

Comments

    • User Profile Image Marco Colonna
      Washington University School of Medicine
    • Posted:

    This insightful paper by the Schwartz lab delves into the analysis of microglia in Alzheimer’s disease (AD) and aging models, shedding light on intriguing findings. The key discovery revolves around identifying two distinct subsets of microglia within the TREM2-positive group: one associated with disease (DAM) and the other with senescence. Interestingly, despite their similar transcriptional profiles, they can be differentiated at the protein level using CγTOF. Specifically, senescence microglia express markers such as p16, p21, p53, and γH2AX. Both subsets rely, at least partially, on TREM2 for their differentiation. Additionally, treating mice with a senolytic Bcl2 inhibitor shows promise in improving behavioral traits in both senescent and Alzheimer's disease-afflicted mice.

    This paper carries particular weight due to its exploration of the connection between aging and Alzheimer's disease. Additionally, it underscores the value of integrating protein data alongside transcriptional data to account for potential disparities. The paper also prompts several compelling questions for future research:

    What mechanisms drive distinct protein expression patterns despite identical transcriptional signatures? Post-translational modifications could be a factor.

    Do senolytic drugs primarily target microglia, or do they affect other glial cells or neurons as well?

    How does TREM2 sustain two distinct differentiation pathways? Are TREM2-mediated signals distinct?

    It will be important to establish the TREM2 signaling pathways in aging microglia versus DAM.

    View all comments by Marco Colonna

  2. This article serves to validate and confirm a series of findings from previous papers, highlighting the presence of senescent microglia in AD-like pathology and their DAM-like identity.

    Previous studies from our group, using the APP/PS1 model (Hu et al., 2021), evidenced the presence of senescent microglia in the brain during AD pathology, with specific signs of replicative senescence. We found these cells to display a DAM phenotype, and both senescent and DAM microglia were reduced when limiting the proliferation of microglia at early stages of the pathology. Interestingly, we found this intervention ameliorated AD-like pathology, a similar finding to that reported in the present article when using senolytic treatment. The beneficial impact of senolytics in ameliorating microglial senescence and AD-like pathology has also been reported in the past (Bussian et al., 2018) and the evaluation of senolytics as a possible therapy for AD is underway (Gonzales et al., 2023). Having the additional validation in this article will ensure the body of literature is strongly supporting this mechanism.

    Another aspect covered in this article is the presence of DAM-like cells with signs of senescence in the aging brain, and this also serves as validation of previous findings by and perhaps hints at common mechanisms to attain a senescent state in microglia through aging and chronic neuropathology (Matsudaira et al. 2023).

    A novel aspect covered in this article is the dependence of senescent microglia on TREM2 function, evidenced by the absence of senescent microglia in TREM2-/- mice. Of course, one plausible explanation is the dependence of TREM2 for microglial survival (Wang et al., 2015). Senescent microglia may be more susceptible to the evolving AD-like pathology, therefore becoming more dependent on mechanisms of survival such as TREM2.

    Overall, this article replicates and confirms several findings reported in the last decade, always useful when building a body of knowledge around a potential target for AD.

    References:

    Hu Y, Fryatt GL, Ghorbani M, Obst J, Menassa DA, Martin-Estebane M, Muntslag TA, Olmos-Alonso A, Guerrero-Carrasco M, Thomas D, Cragg MS, Gomez-Nicola D. Replicative senescence dictates the emergence of disease-associated microglia and contributes to Aβ pathology. Cell Rep. 2021 Jun 8;35(10):109228. PubMed.

    Bussian TJ, Aziz A, Meyer CF, Swenson BL, van Deursen JM, Baker DJ. Clearance of senescent glial cells prevents tau-dependent pathology and cognitive decline. Nature. 2018 Oct;562(7728):578-582. Epub 2018 Sep 19 PubMed.

    Gonzales MM, Garbarino VR, Kautz TF, Palavicini JP, Lopez-Cruzan M, Dehkordi SK, Mathews JJ, Zare H, Xu P, Zhang B, Franklin C, Habes M, Craft S, Petersen RC, Tchkonia T, Kirkland JL, Salardini A, Seshadri S, Musi N, Orr ME. Senolytic therapy in mild Alzheimer's disease: a phase 1 feasibility trial. Nat Med. 2023 Oct;29(10):2481-2488. Epub 2023 Sep 7 PubMed.

    Matsudaira T, Nakano S, Konishi Y, Kawamoto S, Uemura K, Kondo T, Sakurai K, Ozawa T, Hikida T, Komine O, Yamanaka K, Fujita Y, Yamashita T, Matsumoto T, Hara E. Cellular senescence in white matter microglia is induced during ageing in mice and exacerbates the neuroinflammatory phenotype. Commun Biol. 2023 Jun 23;6(1):665. PubMed.

    Wang Y, Cella M, Mallinson K, Ulrich JD, Young KL, Robinette ML, Gilfillan S, Krishnan GM, Sudhakar S, Zinselmeyer BH, Holtzman DM, Cirrito JR, Colonna M. TREM2 lipid sensing sustains the microglial response in an Alzheimer's disease model. Cell. 2015 Mar 12;160(6):1061-71. Epub 2015 Feb 26 PubMed.

    View all comments by Diego Gómez-Nicola

  3. In this article, Schwartz’s group elegantly demonstrated the presence of senescent microglia in aged mice and in the 5xFAD Alzheimer’s disease (AD) mouse model by using CyTOF. While expressing senescent proteins, such as CD38, senescent microglia surprisingly show higher expression of TREM2 and APOE, the proteins associated with disease associated microglia (DAM) or microglia with a neurodegenerative phenotype (Keren-Shaul et al., 2017; Krasemann et al., 2017). Interestingly, these senescent cells are mostly unassociated with amyloid plaques, another distinction from the known DAM phenotype.

    The effects of TREM2 knockout on amyloid pathology are known to be divergent: protective in early stages but detrimental in the later stages (Jay et al., 2017). This new finding could add another layer of TREM2 dependency in terms of microglial phenotypic diversity, as deletion of TREM2 reduces senescent microglial population. It is still unclear, however, what kind of pathology, other than amyloid plaque formation, may help development of senescent microglia, which are not associated with plaques, and how these microglia may contribute to disease progression in 5xFAD mice as well as in human AD brains.

    On the latter, one hint could be their high expression of Cd9 and Cd74, suggesting increased extracellular vesicle secretion (Clayton  et al., 2021). They observed a reduction in the number of senescent microglia by senolysis drug, ABT-737, which ameliorates cognitive deficit and neuroinflammation in 5XFAD mice at later stages of pathology. Since the reduction is less than 50 percent, the effect of more efficient elimination of senescent microglia should be investigated.

    References:

    Keren-Shaul H, Spinrad A, Weiner A, Matcovitch-Natan O, Dvir-Szternfeld R, Ulland TK, David E, Baruch K, Lara-Astaiso D, Toth B, Itzkovitz S, Colonna M, Schwartz M, Amit I. A Unique Microglia Type Associated with Restricting Development of Alzheimer's Disease. Cell. 2017 Jun 15;169(7):1276-1290.e17. Epub 2017 Jun 8 PubMed.

    Krasemann S, Madore C, Cialic R, Baufeld C, Calcagno N, El Fatimy R, Beckers L, O'Loughlin E, Xu Y, Fanek Z, Greco DJ, Smith ST, Tweet G, Humulock Z, Zrzavy T, Conde-Sanroman P, Gacias M, Weng Z, Chen H, Tjon E, Mazaheri F, Hartmann K, Madi A, Ulrich JD, Glatzel M, Worthmann A, Heeren J, Budnik B, Lemere C, Ikezu T, Heppner FL, Litvak V, Holtzman DM, Lassmann H, Weiner HL, Ochando J, Haass C, Butovsky O. The TREM2-APOE Pathway Drives the Transcriptional Phenotype of Dysfunctional Microglia in Neurodegenerative Diseases. Immunity. 2017 Sep 19;47(3):566-581.e9. PubMed.

    Jay TR, Hirsch AM, Broihier ML, Miller CM, Neilson LE, Ransohoff RM, Lamb BT, Landreth GE. Disease Progression-Dependent Effects of TREM2 Deficiency in a Mouse Model of Alzheimer's Disease. J Neurosci. 2017 Jan 18;37(3):637-647. PubMed.

    Clayton K, Delpech JC, Herron S, Iwahara N, Ericsson M, Saito T, Saido TC, Ikezu S, Ikezu T. Plaque associated microglia hyper-secrete extracellular vesicles and accelerate tau propagation in a humanized APP mouse model. Mol Neurodegener. 2021 Mar 22;16(1):18. PubMed. Correction.

    View all comments by Seiko Ikezu

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References

News Citations

  1. DAMned to Death? Microglia May Proliferate to Senescence
  2. When Autophagy Stops, Microglia Sour into Senescence
  3. TREM2 Buoys Microglial Disaster Relief Efforts in AD and Stroke
  4. New Evidence Confirms TREM2 Binds Aβ, Drives Protective Response
  5. Young ApoE4 Carriers Have Reversed AD Proteomic Signature

Research Models Citations

  1. 5xFAD (B6SJL)

Therapeutics Citations

  1. Dasatinib + Quercetin

Paper Citations

  1. Gonzales MM, Garbarino VR, Kautz TF, Palavicini JP, Lopez-Cruzan M, Dehkordi SK, Mathews JJ, Zare H, Xu P, Zhang B, Franklin C, Habes M, Craft S, Petersen RC, Tchkonia T, Kirkland JL, Salardini A, Seshadri S, Musi N, Orr ME. Senolytic therapy in mild Alzheimer's disease: a phase 1 feasibility trial. Nat Med. 2023 Oct;29(10):2481-2488. Epub 2023 Sep 7 PubMed.

Further Reading

Primary Papers

  1. Rachmian N, Medina S, Cherqui U, Akiva H, Deitch D, Edilbi D, Croese T, Salame TM, Ramos JM, Cahalon L, Krizhanovsky V, Schwartz M. Identification of senescent, TREM2-expressing microglia in aging and Alzheimer's disease model mouse brain. Nat Neurosci. 2024 Jun;27(6):1116-1124. Epub 2024 Apr 18 PubMed.

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