Meilandt WJ, Ngu H, Gogineni A, Lalehzadeh G, Lee SH, Srinivasan K, Imperio J, Wu T, Weber M, Kruse AJ, Stark KL, Chan P, Kwong M, Modrusan Z, Friedman BA, Elstrott J, Foreman O, Easton A, Sheng M, Hansen DV. Trem2 Deletion Reduces Late-Stage Amyloid Plaque Accumulation, Elevates the Aβ42:Aβ40 Ratio, and Exacerbates Axonal Dystrophy and Dendritic Spine Loss in the PS2APP Alzheimer's Mouse Model. J Neurosci. 2020 Feb 26;40(9):1956-1974. Epub 2020 Jan 24 PubMed.
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Biomedizinisches Centrum (BMC), Biochemie & Deutsches Zentrum für Neurodegenerative Erkrankungen (DZNE)
Washington University School of Medicine
This is an intriguing study highlighting the importance of TREM2-dependent microglial activity in protecting against neurotoxic pathology associated with β-amyloidosis. Unique to this study is their use of PS2APP mice to examine sex- and age-dependent Trem2 functions in the context of amyloid pathology. The authors confirm several fundamental effects of Trem2 deficiency, such as impaired microglial response to amyloid plaques, downregulation of proliferative microglia markers typically associated with disease-associated microglia (DAM)/microglia of neurodegenerative phenotype (Keren-Shaul et al., 2017; Krasemann et al., 2017), and formation of diffuse, fussy plaques which appear to promote neuritic dystrophy and axonal damage.
Previous studies examining Trem2 function in amyloid pathology have reported conflicting results, likely due to the differences in β-amyloidosis mouse models, disease stage, and experimental methods used. The current paper’s findings are fully in line with our recent results showing that Trem2 deletion increases amyloid plaque seeding at an earlier age but reduces the rate of amyloid plaque accumulation at later ages (Parhizkar et al., 2019). The authors' findings further underscore sex-dependent changes in plaque load, Aβ42/Aβ40 ratio, as well as an abundance of soluble fibrillar Aβ oligomers observed more prominently in Trem2-deficient females.
Meilandt et al. also described the novel and surprising finding that plaque-associated ApoE is elevated in aged female PS2APP mice (but not in males) upon loss of Trem2 expression, and that microglial ApoE expression in PS2APP/Trem2-/- mice is significantly upregulated. The authors suggest that this is due to impaired lipid clearance by Trem2-deficient microglia and that ApoE is induced by Aβ-driven neuropathology in a largely Trem2-independent manner in the PS2APP mouse model, similar to that reported in 5XFAD mice (Keren-Shaul et al., 2017). This is in clear contrast to previous reports by us and others highlighting a reduction of plaque- and microglia-associated ApoE in Trem2-deficient 5xFAD mice (Zhou et al., 2020), APPPS1 mice (Kraseman et al, 2017; Mazaheri et al., 2018; Parhizkar et al., 2019), and in AD patients with different loss-of-Trem2-function variants (Parhizkar et al., 2019).
The authors argue that the discrepancy may arise due to differences in sampling and technical procedures, or differences in immunohistochemistry protocols. To that end, we would like to emphasize that we confirmed our immunohistochemical data by unbiased proteomic mass spectrometry analyses of microglia isolated from APPPS1 mice, as well as western blotting of formic-acid extracts from not only APPPS1 mice, but also AD patients with and without Trem2 coding variants (Parhizkar et al., 2019).
It also appears surprising that in PS2APP/TREM2+/+ mice extremely little, or almost no, plaque-associated ApoE was detected, although in the original description of this model (Richards et al., 2003), as well as in all other models, robust plaque-associated ApoE was consistently reported. The authors do not discuss additional evidence from Trem2-independent experimental models—such as sustained microglia depletion via herpes simplex thymidine kinase in APPPS1 mice (Parhizkar et al., 2019), or CSF1R inhibitor treatment in 5xFAD mice (Spangenberg et al., 2019)—which fully confirms that plaque surrounding microglia are a major source of plaque-associated ApoE. Additionally, Trem2-dependent upregulation of microglial ApoE appears to play a major role in a mouse model of tauopathy (Leyns et al., 2017).
It would be interesting to know if the Trem2-deficient microglia associated with the observed ApoE surge, share a similar microglial signature to the DAM microglia reported by Keren-Shaul et al., or whether these display an altogether unique profile mainly responding to plaque-associated neuropathology.
Overall, this study underlines a detailed understanding of Trem2 function and amyloid pathology using a different amyloidosis model, providing further insights into the beneficial aspects of microglia in AD.
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.
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Richards JG, Higgins GA, Ouagazzal AM, Ozmen L, Kew JN, Bohrmann B, Malherbe P, Brockhaus M, Loetscher H, Czech C, Huber G, Bluethmann H, Jacobsen H, Kemp JA. PS2APP transgenic mice, coexpressing hPS2mut and hAPPswe, show age-related cognitive deficits associated with discrete brain amyloid deposition and inflammation. J Neurosci. 2003 Oct 1;23(26):8989-9003. PubMed.
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OHSU
Since the identification of Alzheimer’s disease-associated variants in the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) gene in 2013 (Guerreiro et al., 2013; Jonsson et al., 2013), many labs have focused their attention on understanding TREM2 biology. Among all variants identified in genome-wide association studies, TREM2 variants conferred the strongest risk for developing late-onset AD, suggesting an important role for TREM2 in the process of AD onset or progression. TREM2 is exclusively expressed on myeloid cells (Jay et al., 2015), including microglia within the brain, which also indicated that altering the function of microglia can actively promote neurodegeneration. Therefore, understanding how TREM2 functions in microglia promised to provide insight into a long-standing question in the field: How do microglia participate in AD pathogenesis and progression? To approach this question, our lab (Jay et al., 2015; Jay et al., 2017) and others (Ulrich et al., 2014; Wang et al., 2015; Wang et al., 2016; Yuan et al., 2016; Parhizkar et al., 2019) have previously examined microglial function and pathology in TREM2-deficient mouse models of AD. In this new study, Meilandt and colleagues continue to add to this line of work, examining loss of TREM2 in the PS2APP mouse model in which expression of human PSEN2 and APP genes carrying AD-causing mutations results in progressive amyloid deposition. This paper confirms previous work, showing that TREM2 deficiency impairs amyloid-associated changes in microglial gene expression, limits microglial association with plaques, leads to changes in plaque structure, and enhances plaque-associated neuritic dystrophy. In addition, this paper provides new insight into our understanding of microglial responses to, and regulation of, AD pathology.
Consistent with previous reports, the authors show that loss of TREM2 impairs accumulation of microglia around plaques (Ulrich et al., 2014; Jay et al., 2015; Wang et al., 2015) and attenuates upregulation of disease-associated genes in response to pathology (Wang et al., 2015; Keren-Shaul et al., 2017). Largely, previous studies have focused on ways in which TREM2 deficiency limits microglial responses in AD, leading to the idea that TREM2 could be required for microglia to sense or respond to amyloid pathology (Jay et al., 2017). Interestingly, it is clear in this study, as well as from the authors’ analysis of previous datasets, that TREM2-deficient mice still exhibit substantial reductions in the expression of homeostatic microglial genes, comparable to those that occur in mice expressing TREM2. This suggests that TREM2 is not required for microglia to recognize that pathology is occurring, as they still engage in some of the expected transcriptional changes. Further, these findings demonstrate that the downregulation of homeostatic genes and upregulation of disease-associated genes are separable processes, dependent on distinct mechanisms. This represents an important insight into the general mechanisms by which microglia coordinate responses to stimuli.
This study also provides a possible mechanism linking the loss of TREM2 with attenuated microglial proliferation in AD, a phenomenon which had been previously reported (Wang et al., 2016; Jay et al., 2017). Their sequencing results suggest a link between TREM2 and Wnt signaling as a possible regulatory pathway instructing proliferation of microglia in AD. This is interesting, given the previously identified roles for TREM2 as a regulator of osteoclast proliferation, where it was found to interact with the Wnt-β-catenin pathway and CSF-1 to regulate appropriate osteoclast numbers (Otero et al., 2012). Perhaps there will be more to learn about TREM2 by analogy to these earlier studies focused on its role in other macrophage-derived populations.
In addition to examining how loss of TREM2 influences microglial gene expression and function, this paper also adds to the existing literature examining the effects of TREM2 deficiency on AD pathology in different AD models and stages of disease progression. Our previous work rectified previous findings in the field by demonstrating that loss of TREM2 can result in differential effects on amyloid deposition at different disease stages, reducing pathology early and exacerbating it late in disease (Jay et al., 2017). Instead, Meilandt and colleagues find that there is a reduction in amyloid pathology at these late time points. In our work, we found that as pathology progresses, TREM2-deficient mice start to exhibit reduced protein levels of transgenic APP (Jay et al., 2017), which could ultimately result in reduced amyloid deposition simply through reducing pathogenic protein production. This could be an explanation for why the authors see a reduction in amyloid pathology in the very advanced disease time points they examined here. There have also been inconsistencies in the observed amyloid levels observed in patients with AD-associated TREM2 variants, with some finding an increase in amyloid plaque density in TREM2-variant carriers relative to noncarriers (Roussos et al., 2015), while others found no differences based on TREM2 genotype (Rosenthal et al., 2015). Collectively, studies in mouse models have clearly demonstrated that altering microglial function by manipulating TREM2 expression can influence amyloid levels, but it still remains an open question whether TREM2 variants confer AD risk through altering amyloid accumulation.
In addition to plaque deposition, this study also finds changes in the levels of soluble amyloid beta (Aβ) species in TREM2 deficient mice. While similar changes in soluble peptides and effects on specific Aβ species have previously been reported (Jay et al., 2015; Wang et al., 2016), it remains a mystery how altering microglia function would result in these effects. Either microglia must normally regulate the neuronally derived enzymes that result in differential cleavage of these peptides, or TREM2 must selectively regulate microglial clearance of specific peptide species. It will be interesting to elucidate these mechanisms in future work.
The authors also demonstrate that loss of TREM2 results in an increase in neuritic dystrophy around plaques, consistent with previous reports in other AD models (Wang et al., 2015; Wang et al., 2016; Yuan et al., 2016; Jay et al., 2017). The authors interpret these results as TREM2 promoting enhanced formation of dystrophic neurites around plaques. An alternative explanation is that the formation of these dystrophic neurites is unchanged, but rather that their clearance is impaired with TREM2 deficiency. Indeed, the increase in neuritic dystrophy was not correlated with enhanced synaptic loss near plaques, which would be consistent with limited changes in the overall extent of neuronal damage. More work will be required to distinguish these two possibilities.
Using sparsely labeled fluorescent neuronal reporters, the authors beautifully show a reduction in dendritic spines specifically in the vicinity of plaques in PS2APP mice. This reduction in spines was not dependent on TREM2, suggesting that their loss can occur in the absence of activated, plaque-associated microglia. In fact, TREM2 expression on microglia appeared to be slightly protective against synaptic loss. However, as the authors point out, other work does clearly show that microglia can contribute to synapse loss in AD (Hong et al., 2016; Spangenberg et al., 2016). While it is currently unclear how these findings fit in with this existing literature, it does add an important piece to solve the puzzle of how microglia regulate amyloid pathology, neuritic damage, and synaptic loss in AD.
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