Researchers know that ApoE4 and Trem2 can worsen neurodegeneration in mouse models of tauopathy. Do these proteins conspire to exacerbate pathology? It turns out that ApoE4 causes much of the damage alone, according to scientists led by David Holtzman, Washington University School of Medicine in St. Louis. In the November 10 Neuron, they reported that despite knocking out TREM2, APOE4 drove a type of microgliosis that worsened tau pathology and brain atrophy. The microglia ramped up phagocytosis and production of lysosomal proteins, and their lysosomes swelled with lipids.
- In mice, APOE4 exacerbates tau tangles and brain atrophy.
- Knocking out TREM2 made these pathologies worse.
- ApoE4 causes microglial lysosomes to swell with lipids.
“This study emphasizes the importance of TREM2-independent microglial mechanisms underpinning AD risk and pathogenesis,” wrote Michelle Ehrlich and Sam Gandy of Mount Sinai School of Medicine in New York. To Jessica Rexach of the University of California, Los Angeles, the study pointed to the complex role microglia play in disease pathogenesis. “The findings are consistent with a more nuanced view of microglia in neurodegeneration that is beginning to emerge,” she wrote.
Holtzman and colleagues previously reported that in the P301S model of tauopathy, human ApoE4 drove the accumulation of phosphorylated tau, lysosome dysfunction in microglia, and brain atrophy (Sep 2017 news). On the other hand, knocking out TREM2 quelled neurodegeneration in mice carrying the same tau mutation and their own endogenous ApoE (Oct 2017 news). Given these proteins’ prominent roles in microglial responses to disease, the scientists predicted that knocking out TREM2 would also protect mice expressing human APOE4. To the authors’ surprise, it did not.
First author Maud Gratuze crossed P301S mice with Apoe knockouts or a knock-in strain carrying the human APOE4 gene, and then she knocked out TREM2 in the crosses. This created four strains: TREM2 mice expressing either no Apoe (TREM2/Apoe-KO) or APOE4 (TREM2/E4), and their respective TREM2 knockouts (TREM2-KO/Apoe-KO and TREM2-KO/E4). She examined brain tissue from 9.5-month-old animals because, at that age, P301S mice have extensive tangles, gliosis, and neuronal loss in the hippocampus and cortex.
Despite knocking out TREM2, pathology worsened in APOE4 mice. In the hippocampus and cortex, TREM2-KO/E4 animals accumulated more phospho-tau, as measured by the AT8 and AT180 antibodies, and more tangles, as measured by MC1, than all other mouse strains. Likewise, in the TREM2-KO/E4 mice, these regions shrank 15 percent more than in TREM2/E4 mice and 30 percent more than in Apoe knockouts that did or did not have TREM2 (see image below). The authors concluded that APOE4 worsens tauopathy and atrophy even when TREM2 is absent.
Tangles and Atrophy. Compared to P301S tauopathy mice lacking Apoe (left), P301S mice carrying APOE4 (middle) had more neurofibrillary tangles in the hippocampus and cortex (top), and worse atrophy in the same regions (bottom). Knocking out TREM2 worsened pathology further (right). [Courtesy of Gratuze et al., Neuron, 2022.]
Could faulty microglia be to blame for the damage? The researchers analyzed microglial transcriptomes from the four mouse strains after RNA-Seq of PU.1-positive single nuclei isolated from hippocampal tissue. The 25,000 total microglial nuclei clustered into 10 subgroups. Three subclusters expressed homeostatic genes, such as CX3CR1 and CSF1R. Compared to Apoe knockouts, APOE4 tauopathy mice had fewer of these homeostatic microglia, and their TREM2 knockout counterparts had even less.
One of the clusters caught Gratuze’s eye because APOE4-positive P301S mice mustered twice as many of them as did APOE knockouts, regardless of TREM2 expression. These cells, dubbed Tau/ApoE4 reactive microglia (TERM), overexpressed some disease-associated microglia (DAM) genes typically upregulated in the presence of amyloid. Among these, LPL and CTSB, encoding a lipoprotein lipase and a lysosomal protease, respectively, hinted that something might be awry with their lysosomes. TERMs expressed more of the lysosomal damage marker galectin-3, and their lysosomes were stuffed with lipids. These results suggest that, in the context of tau pathology, ApoE4 activates DAM-like microglia yet hobbles their lysosomes, and it does so independently of Trem2. “This work supports an important shift away from considering microglia only with respect to amyloid plaques, or as a single ‘disease-associated’ state, to instead considering the multiple interactions between microglia and other aspects of [neurodegenerative] disease,” Rexach wrote.
Holtzman was struck by how severe the lysosomal abnormalities were in TREM2-KO/E4 mice. “They are more pronounced than those in amyloidosis mice,” he told Alzforum. “Trem2 can’t prevent the lysosomal damage caused by ApoE4.”
APOE4 microglia also ramped up phagocytosis. They engulfed four times as many PSD-95-positive synapses as microglia lacking APOE, independent of TREM2 expression. Such damage mirrors the thinned synapses seen in E4 mice, suggesting that their dysfunctional microglia drive neurodegeneration. “ApoE4 plays a more substantial role than Trem2 in tau-driven degeneration, at least in this tauopathy mouse model,” concluded David Hansen, Brigham Young University, Provo, Utah. William Meilandt of Genentech, South San Francisco, California, found the results compelling, noting how clearly damaging ApoE4 is in the presence of tau and absence of TREM2 (comments below).
Might similar lysosome changes happen in people carrying APOE4 and loss-of-function TREM2 mutations? The researchers analyzed published snRNA-Seq data from cortical tissue of 15 volunteers from Wash U’s Knight Alzheimer’s Disease Research Center and nine from the ROSMAP cohort (Brase et al., 2021). All had had AD and carried at least one copy of APOE4. Fourteen carried two copies of the common variant of TREM2; 10 had one copy of either R47H or R62H variants that associate with AD. Reactive microglia in the risk variant carriers expressed higher levels of genes involved in lysosome function. In the face of poor TREM2 function, it appears that ApoE4 drives similar lysosome dysfunction in microglia from people.
All told, knocking out TREM2 in tauopathy mice carrying human APOE4 exacerbated tau pathology, microgliosis, and neurodegeneration—a direct contrast with knocking out TREM2 in the P301S mice expressing normal mouse Apoe. “The question of why TREM2 deletion would be significantly protective in Apoe wild-type mice, inconsequential in Apoe knockout mice, and slightly detrimental in APOE4 knock-in mice remains unanswered,” Hansen wrote. Rexach believes the findings behoove further analysis. “This work clearly establishes a relationship between APOE4, tau pathology, and the lysosome/phagosome that opens the door for continued study,” she wrote. Holtzman and colleagues are looking for ways to reverse the lipid accumulation in these microglia.—Chelsea Weidman Burke
Research Models Citations
- ApoE4 Makes All Things Tau Worse, From Beginning to End
- Changing With the Times: Disease Stage Alters TREM2 Effect on Tau
Mutation Interactive Images Citations
- Brase L, You SF, del Aguila J, Dai Y, Novotny BC, Soriano-Tarraga C, Dykstra T, Fernandez MV, Budde JP, Bergmann K, Morris JC, Bateman RJ, Perrin RJ, McDade E, Xiong C, Goate A, Farlow M, Chhatwal JP, Schofield P, Chui H, Dominantly Inherited Alzheimer Network (DIAN), Sutherland GT, Kipnis J, Karch CM, Benitez BA, Cruchaga C, Harari O. A landscape of the genetic and cellular heterogeneity in Alzheimer disease. medRxiv 2021.11.30. medRxiv
- Gratuze M, Schlachetzki JC, D'Oliveira Albanus R, Jain N, Novotny B, Brase L, Rodriguez L, Mansel C, Kipnis M, O'Brien S, Pasillas MP, Lee C, Manis M, Colonna M, Harari O, Glass CK, Ulrich JD, Holtzman DM. TREM2-independent microgliosis promotes tau-mediated neurodegeneration in the presence of ApoE4. Neuron. 2023 Jan 18;111(2):202-219.e7. Epub 2022 Nov 10 PubMed.