. Microglia use TAM receptors to detect and engulf amyloid β plaques. Nat Immunol. 2021 May;22(5):586-594. Epub 2021 Apr 15 PubMed.

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  1. Mer and Axl are phagocytic receptors expressed on microglia. During AD, Axl is highly upregulated in activated microglia, while Mer has a high baseline expression. In this exciting paper, Huang et al. presented provocative results showing that deficiency in Mer and Axl results in less microglia activation and fewer dense-core plaques in an AD mouse model. Contrary to our intuition that microglia help clear Aβ plaques, these results suggest that microglial phagocytosis promotes formation of dense-core plaque. Moreover, this study suggests that when it comes to therapeutics, interventions that reduce microglia phagocytosis may be effective in the treatment of AD.

    One interesting question raised by this study is which form of Aβ is neurotoxic. This study finds that dense-core plaques are neurotoxic; however, other studies found that fibrillar plaques are. We are also intrigued by the parallels and differences between the Axl-/- Mer-/- model and the TREM2-deficient model. We saw a similar phenotype of more diffuse plaques in the 5xFAD amyloid mouse model deficient in Trem2. Moreover, similar to the defective microglial clustering observed in Trem2-/- amyloid mouse models, Axl-/- Mer-/- microglia fail to encapsulate plaques in APP/PS1 mice. And yet, despite some reduction in DAM gene expression in the Axl-/- Mer-/- mice, the DAM transcription cluster is adequately present as revealed by single-cell RNA-Seq, unlike the almost complete absence of DAM clusters after TREM2 ablation.

    We also noted that TREM2 expression is not affected by genetic ablation of Axl and Mer.

    Altogether, these results suggest that TAM receptors and TREM2 have different functions, with TREM2 being necessary but not sufficient for activation of DAM signature. The fact that Axl-/- Mer-/- microglia are less activated compared to wild-type microglia in the APP/PS1 background is likely due to these microglia being farther away from plaques. Then the interesting question remains as to why the TAM receptor-deficient microglia are distant from plaques. What are the signals driving microglia migration toward the amyloid deposits?

    View all comments by Yingyue Zhou
  2. The TAM receptors Axl and Mer play an instrumental role in phagocytosis of cellular debris in the periphery of apoptotic cells generated during adult neurogenesis, as shown previously by Dr Lemke’s group (Fourgeaud et al., 2016). Axl is one of the hallmark genes upregulated in DAM/MGnD microglia subset in Aβ models (Keren-Shaul et al., 2017; Krasemann et al., 2017). We showed, in addition, that Axl protein is enriched in Stat1+ microglia surrounding plaques as part of innate interferon response, and that microglia wrapping around neuritic plaques in human brains also positively express AXL protein (Roy et al., 2020). 

    Here, Huang et al. set out to assess the role of TAM receptors in mouse models of AD. They show that Axl expression increases with age specifically in plaque-associated microglia in APP/PS1 mice. Along with the increase in Axl, they observe an upregulation of its ligand Gas6 and of the co-ligand phosphotidylserine in areas surrounding plaques. Using two photon in vivo live imaging, they show that APP/PS1;Axl-/-Mertk-/- microglia are deficient in detecting, binding, and phagocytizing amyloid plaques, highlighting the importance of these receptors for microglia to engage with plaques. Interestingly, between the two TAM receptors, Mer seems to be primarily accountable for plaque uptake and dense-core modification by microglia, which leaves an interesting question of any functional involvement of Axl around the plaques, besides promoting Gas6 deposition.

    Unexpectedly, the authors observe a drastic and specific reduction of compact plaques in APP/PS1 animals lacking the TAM receptors. Based on their single-cell transcriptomics data, they found genes related to DAM, lipid metabolism, and MHC class II antigen presentation were blunted in APP/PS1;Axl-/-Mertk-/- compared to APP/PS1. However, they didn’t observe any changes in cytokine and chemokine expression, emphasizing that the phenotypes they report are not a result of dampened neuroinflammation.

    Overall, the authors propose a model in which phosphotidylserine-Gas6-Axl complex activates TAM driving phagocytosis of plaques. Once internalized, the plaques are transferred to lysosomes, where the acidic environment promotes the aggregation of insoluble and protease resistant fibrils. These can be delivered via exocytosis or by cell death and contribute to formation and growth of dense-core plaques. In this regard, it would be interesting to understand if dead cells accrue preferentially in aged Mer-/-Axl-/- brain, thus contributing to certain aspects of the phenotypic observations in conjunction with neuritic plaques.

    This is a landmark study that links the core molecular machinery of phagocytes to amyloid pathology. The model the authors put forward is provocative and challenges our current understanding of microglia and plaque interaction, which surely will inspire further investigation and deeper dissection in the future.

    References:

    . TAM receptors regulate multiple features of microglial physiology. Nature. 2016 Apr 14;532(7598):240-244. Epub 2016 Apr 6 PubMed.

    . 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.

    . The TREM2-APOE Pathway Drives the Transcriptional Phenotype of Dysfunctional Microglia in Neurodegenerative Diseases. Immunity. 2017 Sep 19;47(3):566-581.e9. PubMed.

    . Type I interferon response drives neuroinflammation and synapse loss in Alzheimer disease. J Clin Invest. 2020 Apr 1;130(4):1912-1930. PubMed.

    View all comments by Hui Zheng
  3. Huang et al. report, in an elegant and detailed study, the central roles played by the TAM receptors Axl and Mer in amyloidogenic models of AD. There are a few surprises that will provoke re-evaluation of exactly how microglia act to actively influence the deposition and remodeling of amyloid plaques in the brains of murine models of AD. The most remarkable finding is that genetic inactivation of both Mer and Axl results in a paradoxical 35 percent decrease in the number of dense-core, thioS-positive plaques in 12-month-old mice. This effect is due principally to the action of Mer, and preferentially affects small plaques.

    The authors posit that dense-core plaque formation requires its “construction” and compaction from more diffusely organized fibrillar forms of Aβ. This is rather different from the conventional view of microglial phagocytic trimming and remodeling of diffuse plaques, which was well documented in Yuan et al. and other publications (Yuan et al., 2016). They note that it had been suggested that plaques might arise from death of microglia with deposition of their undigested phagocytic Aβ cargo, analogous to the earlier suggestion that neurons bearing intraneuronal Aβ found in these models die and then seed plaque formation (Moon et al., 2012). It remains unclear exactly how the receptor’s phagocytic functionality is modulated to drive compact plaque formation.

    The study clearly documents that loss of Axl and Mer dramatically reduces the ability of microglia to detect amyloid deposits and mount a complex cellular response, including changes in morphology, gene expression, and phagocytosis. Remarkably, the loss of Axl and Mer elicits effects on amyloid phagocytosis and plaque density that are much larger than that of TREM2.

    In addition, the manuscript adds interesting detail to the involvement of these TAM receptors in several aspects of disease pathogenesis in the animal models. These include the preferential expression of Axl in plaque-associated microglia, reduction in plaque-associated microglia, suppression of proliferation, increased neuritic dystrophy in the Axl/Mer-deficient mice, as well as an increase in cerebral amyloid angiopathy.

    Overall, this study adds considerable detail into the actions of the TAM receptors and reinforces an understanding of their importance in microglial biology in AD. The caliber of the work is terrific, a characteristic feature of the work from the Lemke lab. 

    References:

    . Intracellular Amyloid-β Accumulation in Calcium-Binding Protein-Deficient Neurons Leads to Amyloid-β Plaque Formation in Animal Model of Alzheimer's Disease. J Alzheimers Dis. 2012 Jan 1;29(3):615-28. PubMed.

    . TREM2 Haplodeficiency in Mice and Humans Impairs the Microglia Barrier Function Leading to Decreased Amyloid Compaction and Severe Axonal Dystrophy. Neuron. 2016 May 18;90(4):724-39. PubMed.

    View all comments by Gary Landreth
  4. This paper by Huang et al. reports a number of interesting observations that provide significant insights into the role of microglia in Alzheimer’s disease and into the mechanism of plaque biogenesis that have a number of implications for the amyloid hypothesis and amyloid-based therapeutic strategies.

    The authors report that the TAM system (Tyr3, Axl, Mer), which mediates the phagocytosis and degradation of cellular debris from dead and dying cells, is required for the recognition and recruitment of microglia to plaques, and that it promotes the formation of dense-core and neuritic plaques. Because phosphatidylserine (PS), an essential co-ligand for the TAM receptors, is only found on the cytosolic leaflet of normal cell membranes or on the surface of apoptotic and dying cells and intracellular vesicular debris, this suggests that a substantial amount of the aggregated precursor amyloid Aβ for neuritic plaques is contained in vesicles derived from dying neurons as we have previously suggested (Pensalfini et al., 2014; Sosna et al., 2018). This has a number of implications for the amyloid hypothesis, for the significance of neuritic plaques, and for the therapeutic targeting of amyloid plaques. 

    If neuritic plaques are derived from amyloid that had first aggregated within neurons and was released as a consequence of cell death, then plaque formation is the result of microglial accumulation and processing of “dystrophic neurites” that contain lysosome- and autophagosome-related vesicles packed with material reactive with aggregation-specific monoclonal antibodies for Aβ, and not the other way around. If neuritic plaques arise from dead and dying neurons, then these plaques really do represent “tombstone markers” of antecedent pathology, so targeting their removal maybe analogous to removing the trash after your house burns down. It looks much nicer, but it does not really help your living situation.

    A prediction of this model is that the remaining plaques in the APP/PS1Axl−/−Mertk−/- mice would contain more obvious neuronal remnants and markers in the “trash” that is not removed by microglia. If neuritic plaque amyloid aggregates initially within neurons, this could also explain why γ-secretase inhibitors (GSIs), which inhibit the secretion of Aβ from neurons, exacerbate cognitive decline in human clinical trials, since inhibiting Aβ secretion may lead to intraneuronal Aβ aggregation (Pensalfini et al., 2014). 

    Although these studies define a novel role for TAM receptors in neuritic and dense-core plaque formation, this may not be the only role of microglia in plaque biogenesis. In the double-knockout APP/PS1Axl−/−Mertk−/- mice, dense-core plaque formation was inhibited by less than 50 percent, but if microglia are ablated during the entire time period of intraneuronal amyloid accumulation and plaque formation, both intraneuronal amyloid and all types of plaques are inhibited by 90 percent (Sosna et al., 2018). 

    References:

    . Intracellular amyloid and the neuronal origin of Alzheimer neuritic plaques. Neurobiol Dis. 2014 Nov;71:53-61. Epub 2014 Aug 1 PubMed.

    . Early long-term administration of the CSF1R inhibitor PLX3397 ablates microglia and reduces accumulation of intraneuronal amyloid, neuritic plaque deposition and pre-fibrillar oligomers in 5XFAD mouse model of Alzheimer's disease. Mol Neurodegener. 2018 Mar 1;13(1):11. PubMed.

    View all comments by Charles Glabe
  5. Microglia respond to amyloid plaques by converting into a disease-associated state (DAM) that depends on TREM2. The DAM signature comprises a large set of genes including AXL, known to be involved in clearing apoptotic cells in which phosphatidylserine is exposed on the surface. Huang et al. now show that AXL/MERTK is involved not only in clearing cells undergoing apoptosis, but also in clearing amyloid plaques.

    They find that microglia phagocytose amyloid decorated with phosphatidylserine in an AXL/MERTK-dependent manner. Surprisingly, they find that AXL/MERTK-dependent phagocytosis of amyloid does not inhibit but rather promotes the formation of dense-core plaques. The phenotype the authors describe is surprisingly similar to TREM2-deficient microglia arguing that AXL/MERTK are essential executing molecules of the DAM program in amyloid plaque clearance. Previously, the increase of dense-core plaques has been shown to be a function of TREM2-dependent APOE secretion. However, in this study the authors propose that AXL/MERTK-dependent phagocytosis of Aβ may lead to aggregation within lysosomes, which is followed by the subsequent release of indigestible amyloid, which may form the core of the plaques.

    This is an important study not only for our understanding of phagocytic receptors in microglia, but in particular for our understanding their role in amyloid clearance. In future studies, it will be interesting to understand the role of phosphatidylserine in Aβ deposition, and to explore the role of seeding factors released or induced after phagocytosis.

    View all comments by Mikael Simons
  6. This is a very interesting study, demonstrating that TAM receptors, such as Axl and Mertk, are required for microglia phagocytosis of Aβ, resulting in the formation of dense-core plaques (for review describing differences between dense-core and diffuse plaques, see DeTure and Dickson, 2019). However, the identification of Mertk and not Axl as an essential receptor in microglial phagocytosis was recently described (Damisah et al., 2020). Moreover, we previously showed that expression of Mertk is significantly suppressed and expression of Axl is induced in Aβ-plaque associated microglia (see Krasemann et al., 2017, and mouse RNA-Seq data therein below). Thus, it is not clear why the authors mainly focused on Mertk/Axl double knockouts.

    In addition, there is no discrimination between the contribution of microglia and peripheral monocytes to pathology, as the authors used global genetic deletion. Furthermore, it seems that DAM genes are slightly downregulated in the double knockouts, although the authors interpreted that there was no change in DAM signature. Overall, the fact that microglial phagocytosis associated with increased plaque burden is an interesting observation that requires further investigation to determine whether microglial phagocytosis is beneficial or detrimental in AD.

    References:

    . The neuropathological diagnosis of Alzheimer's disease. Mol Neurodegener. 2019 Aug 2;14(1):32. PubMed.

    . Astrocytes and microglia play orchestrated roles and respect phagocytic territories during neuronal corpse removal in vivo. Sci Adv. 2020 Jun;6(26):eaba3239. Epub 2020 Jun 26 PubMed.

    . The TREM2-APOE Pathway Drives the Transcriptional Phenotype of Dysfunctional Microglia in Neurodegenerative Diseases. Immunity. 2017 Sep 19;47(3):566-581.e9. PubMed.

    View all comments by Oleg Butovsky
  7. This elegant paper provides substantial evidence for a critical role of microglia in building up dense plaques in Aβ overproducing transgenic mice. Though the direct translatability of results obtained using the aggressive APP/PS1-based animal model to pathogenesis in sporadic AD might be limited, the results support the notion that plaque dissolution strategies, or impeding dense plaque formation, should not be a primary goal for therapies targeting Aβ.

    Indeed, early but often-overlooked studies have revealed poor correlation between cognitive decline and dense plaque load in AD brains (Terry et al., 1991), and the lack of clinical efficacy of merely dissolving plaques was established in the early AN1792 clinical trial (Bayer et al., 2005Nicoll et al., 2019). 

    Based on these antecedents and on the current paper from the Lemke group, we are encouraged to focus our attention on early misfolded oligomeric species as the central molecular entities responsible for the pathogenic role of Aβ in AD (Cline et al., 2018). From a mechanistic perspective, the optimal profile for future Aβ therapies in prevention or treatment of spontaneous AD should be the neutralization of early misfolded oligomers with high selectivity, as opposed to physiological Aβ monomer turnover (Hillen, 2019) or plaque dissolution. 

    In the past, the main argument against developing those antibodies was the unavailability of practical biomarkers to take such an antibody to a decision point in early clinical studies with a  limited number of patients, time, and cost. With the breakthrough biomarker developments in the tau area (e.g., Barthélemy et al., 2020) during the last two years, we should now have reliable and easy blood-based readouts to test the mechanistic efficacy of moving these early aberrant tau species induced by Aβ oligomers via conformer-specific antibodies in a relevant clinical setting. Highly Aβ oligomer-specific antibodies and vaccines have been described preclinically (Hillen et al., 2010Gibbs et al., 2019). 

    References:

    . Physical basis of cognitive alterations in Alzheimer's disease: synapse loss is the major correlate of cognitive impairment. Ann Neurol. 1991 Oct;30(4):572-80. PubMed.

    . Evaluation of the safety and immunogenicity of synthetic Abeta42 (AN1792) in patients with AD. Neurology. 2005 Jan 11;64(1):94-101. PubMed.

    . Persistent neuropathological effects 14 years following amyloid-β immunization in Alzheimer's disease. Brain. 2019 Jul 1;142(7):2113-2126. PubMed.

    . The Amyloid-β Oligomer Hypothesis: Beginning of the Third Decade. J Alzheimers Dis. 2018;64(s1):S567-S610. PubMed.

    . The Beta Amyloid Dysfunction (BAD) Hypothesis for Alzheimer's Disease. Front Neurosci. 2019;13:1154. Epub 2019 Nov 7 PubMed.

    . Cerebrospinal fluid phospho-tau T217 outperforms T181 as a biomarker for the differential diagnosis of Alzheimer's disease and PET amyloid-positive patient identification. Alzheimers Res Ther. 2020 Mar 17;12(1):26. PubMed.

    . Generation and therapeutic efficacy of highly oligomer-specific beta-amyloid antibodies. J Neurosci. 2010 Aug 4;30(31):10369-79. PubMed.

    . A Rationally Designed Humanized Antibody Selective for Amyloid Beta Oligomers in Alzheimer's Disease. Sci Rep. 2019 Jul 8;9(1):9870. PubMed.

    View all comments by William Klein
  8. This is elegant and intriguing work. As one possible next step, it might be important to determine whether macrophage-like cells derived from circulating monocytes express the same constellation of TAM receptors, and behave in the same manner, toward diffuse Aβ deposits.

    View all comments by Steve Barger

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