Wu X, Miller JA, Lee BT, Wang Y, Ruedl C.
Reducing microglial lipid load enhances β amyloid phagocytosis in an Alzheimer's disease mouse model.
Sci Adv. 2025 Feb 7;11(6):eadq6038. Epub 2025 Feb 5
PubMed.
Reducing microglial lipid load enhances Aβ phagocytosis in an Alzheimer’s disease mouse model
Wu and co-authors investigate the role of lipid-droplet-accumulating microglia (LDAM) and border-associated macrophages (BAMs) in an APP-KI model, which harbors a humanized Aβ sequence containing the Swedish, Beyreuther/Iberian, and Arctic mutations. The experiments were conducted in 6-month-old mice, an age at which memory deficits are observed in this model (Saito et al., 2014). By integrating multi-omics approaches, including transcriptomic and lipidomic profiling, the authors document how LD formation influences microglial function. Notably, genetic deletion of FIT2 which is a known regulator of lipid droplet biogenesis, restored microglial phagocytosis and enhanced Aβ clearance, suggesting a potential therapeutic avenue.
Targeting FIT2 effectively reduced LD formation, appeared to restore microglial phagocytosis, and improved plaque clearance in APP-KI mice. Interestingly, CD11c+ microglia in FIT2-deficient APP-KI mice exhibited a downregulation of disease-associated microglial (DAM) genes, including Trem2, Apoe, and Cd9, which may be a consequence of reduced amyloid burden. However, these microglia retained high expression of MHC II genes, particularly Cd74. Notably, Cd74 and Axl were previously reported to show higher expression in LD-low microglia compared to LD-high microglia (Marschallinger et al., 2020).
Furthermore, ACSL1, an enzyme that catalyzes the conversion of free fatty acids (FFAs) into Acyl-CoAs, plays a crucial role in lipid droplet biogenesis and has been implicated in microglial dysfunction in AD (Haney et al., 2024). The elevated FFA levels observed in FIT2-deficient microglia warrant further investigation, particularly in the context of lipid metabolism and microglial homeostasis.
From a therapeutic standpoint, targeting FIT2 would likely present significant challenges due to its higher expression in neurons and astrocytes than in microglia within the brain. Notably, homozygous FITM2 mutations have been associated with deafness-dystonia syndrome, motor regression, ichthyosis, and sensory neuropathy, raising concerns about potential adverse effects of systemic FIT2 inhibition (Zazo Seco et al., 2017). Another critical question is whether peripheral immune cells exhibit distinct FIT2-regulated pathways and whether FIT2 modulation could be CX3CR1-specific. Addressing these questions will be essential for the development of targeted therapeutic strategies that minimize unintended consequences while effectively modulating FIT2.
Another key question emerging from this and other studies on LDs in mice is their relevance to human brain aging and AD, as well as the role different ApoE isoforms play. Previous research from our lab, building on Alzheimer’s original observations of LDs in AD, suggests that APOE4 carriers may be more prone to LD formation and more susceptible to their toxic effects. However, further studies are needed to determine in which cell types LDs accumulate in humans, the molecular processes driving LD formation, and how these deposits interfere with cellular functions.
Overall, this study elegantly highlights the importance of microglial lipid droplets in amyloid pathology and provides novel insights into their role in microglial function, lipid metabolism, and disease progression.
References:
Saito T, Matsuba Y, Mihira N, Takano J, Nilsson P, Itohara S, Iwata N, Saido TC.
Single App knock-in mouse models of Alzheimer's disease.
Nat Neurosci. 2014 May;17(5):661-3. Epub 2014 Apr 13
PubMed.
Marschallinger J, Iram T, Zardeneta M, Lee SE, Lehallier B, Haney MS, Pluvinage JV, Mathur V, Hahn O, Morgens DW, Kim J, Tevini J, Felder TK, Wolinski H, Bertozzi CR, Bassik MC, Aigner L, Wyss-Coray T.
Lipid-droplet-accumulating microglia represent a dysfunctional and proinflammatory state in the aging brain.
Nat Neurosci. 2020 Feb;23(2):194-208. Epub 2020 Jan 20
PubMed.
Correction.
Haney MS, Pálovics R, Munson CN, Long C, Johansson PK, Yip O, Dong W, Rawat E, West E, Schlachetzki JC, Tsai A, Guldner IH, Lamichhane BS, Smith A, Schaum N, Calcuttawala K, Shin A, Wang YH, Wang C, Koutsodendris N, Serrano GE, Beach TG, Reiman EM, Glass CK, Abu-Remaileh M, Enejder A, Huang Y, Wyss-Coray T.
APOE4/4 is linked to damaging lipid droplets in Alzheimer's disease microglia.
Nature. 2024 Apr;628(8006):154-161. Epub 2024 Mar 13
PubMed.
Zazo Seco C, Castells-Nobau A, Joo SH, Schraders M, Foo JN, van der Voet M, Velan SS, Nijhof B, Oostrik J, de Vrieze E, Katana R, Mansoor A, Huynen M, Szklarczyk R, Oti M, Tranebjærg L, van Wijk E, Scheffer-de Gooyert JM, Siddique S, Baets J, de Jonghe P, Kazmi SA, Sadananthan SA, van de Warrenburg BP, Khor CC, Göpfert MC, Qamar R, Schenck A, Kremer H, Siddiqi S.
A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy.
Dis Model Mech. 2017 Feb 1;10(2):105-118. Epub 2016 Dec 15
PubMed.
Although the sequestering of lipids within intracellular lipid droplets protects against lipid oxidation, excess lipid droplet accumulation is associated with aging and disease. In AD, microglia accumulate excess lipid droplets, which leads to reduced functionality, such as phagocytosis, promoting AD neuropathogenesis. While many research groups, including our own, have been interested in understanding the mechanisms leading to lipid droplet accumulation, in this study, Wu et al. focus on strategies that reduce LDs to increase microglial phagocytosis and prevent AD pathology.
The authors use cell sorting to confirm that lipid-droplet-containing microglia increase with aging and disease, in this case, a murine model of amyloid accumulation (APP-KI). In further support, untargeted lipidomics showed that microglia isolated from APP-KI mice contained more neutral lipids typically associated with LDs (e.g., triglycerides and cholesterol esters). I also found it interesting that the very-long-chain polyunsaturated fatty acid and eicosanoid precursor arachidonic acid was depleted in microglia from the APP-KI mice, highlighting potential deficits in the inflammatory response. It would also be interesting to probe the saturation index of lipids within the microglia, especially given recent papers highlighting increased unsaturated lipids in (pathology-associated) lipid droplets.
To further ascertain the phenotype of microglia in response to increased amyloid load, the authors use single-cell transcriptomics, identifying lipid-accumulating disease-associated microglia (DAM)-like signatures (e.g., expressing ApoE, LPL, Trem2, and Cst7) that have also been observed in other AD models (e.g., 5xFAD). Going forward, methodological advances permitting, performing lipidomic and transcriptomic analysis on the same cells will advance our mechanistic understanding of microglial lipid metabolism and lipid droplet formation.
Chasing a therapeutic intervention, Wu and colleagues genetically ablate FIT2 (CX3CR1; FIT2flox), a protein that binds to triglycerides and diacylglycerols in the endoplasmic reticulum to increase lipid droplet formation. Microglia-specific FIT2 depletion in APP-KI mice resulted in reduced microglial lipid droplet accumulation, triglyceride composition, expression of DAM-like genes (e.g., Plin2, ApoE, Trem2, and Cts7), and amyloid load. FIT2 depletion also results in increased microglial phagocytosis ex vivo, and in vitro, supporting the idea that reducing lipid droplet accumulation in microglia could preserve functionality and rescue AD-like pathology.
These exciting proof-of-concept data support a growing body of literature highlighting microglial lipid metabolism as a rational, and clinically relevant, target to improve AD pathology. However, it is important to reiterate that at baseline lipid droplets serve to protect the cell. Therefore, selectively inhibiting lipid droplet formation in subsets of microglia and at specific disease stages will be an attractive strategy going forward.
This study investigates the role of the protein FIT2, a key protein in lipid droplet biogenesis, in LD accumulation within microglia and BAMs in an AD mouse model. They show that LDs accumulate with age and AD progression in the two cell types in the CNS and that this is exacerbated by a high-fat diet. Other key findings include that FIT2 deficiency reduces LD formation in microglia and BAMs, leading to changes in their lipid metabolism and gene-expression profiles. Furthermore, FIT2 depletion enhances microglial phagocytosis, increasing the uptake of E. coli bioparticles and Aβ oligomers in vitro, as well as reducing Aβ accumulation in the brain tissue.
This study is novel and has the following strengths:
Identifies FIT2 as a key driver of LD accumulation in microglia and BAMs, which impaired their phagocytic function and clearance of amyloid in an AD mouse model.
Suggests the potential for a therapeutic intervention by reducing FIT2 expression which resulted in enhanced phagocytic function in microglia and BAMs, suggesting this as a possible strategy to improve Aβ clearance in AD.
Integrates single-cell transcriptomic and lipidomic analyses of microglia, to identify specific microglial lipid-associated phenotypes and lipid profiles in an AD mouse model setting with the potential to provide new mechanistic insights. Importantly, certain microglial clusters had similar transcriptomic profiles to foam cells, well-characterized lipid-rich macrophages found in atherosclerotic plaques in cardiovascular diseases.
Another interesting aspect is related to the impact of diet (high-fat diet) in modulating LD accumulation in microglia and BAMs, which could have relevant clinical implications.
Limitations include:
Lack of in vivo functional validation of these findings: while FIT2 deficiency improved microglial functions and lowered Ab deposition, the study does not clearly demonstrate whether this leads to in vivo cognitive or behavioural improvement in the AD model.
It seems the role of Ab accumulation in AD progression and cognitive decline is currently a matter of intense debate.
The study does not explore broader effects of FIT2 depletion in vivo of this ubiquitously expressed protein. Interfering with its function could have systemic and long-term effects as well as impact other cell types beyond microglia; this could hinder its clinical translation.
In conclusion, this study provides important insights into the metabolic dysfunction of microglia in AD, as a result of altered lipid-homeostasis. Furthermore, it identifies FIT2 as a promising target for intervention. However, further in vivo and human studies are needed to confirm its therapeutic potential and to understand long-term effects of interfering with its function in the brain.
This study contributes to a growing body of research on the role of microglial lipid metabolism in Alzheimer’s disease. Important studies from the labs of Tony Wyss-Coray, Li-Huei Tsai, Mathew Blurton-Jones, Gaurav Chopra, Erik Musiek, and many others showed that amyloidosis induces a shift in microglial lipid metabolism and lipidomic profile, as well as an increase in lipid-droplet-accumulating microglia near sites of pathology. A common theme in these studies is that these microglia have reduced phagocytic capabilities and a neurotoxic effect.
Christiane Ruedl’s research group shows that this phenomenon is not limited to microglia but also seen in brain macrophages of the meninges. This study goes another step further, showing us that reducing LDs in microglia/macrophages via genetic depletion of FIT2 not only restores their phagocytic activity but also reduces amyloid burden in APP knock-in.
In this study Wu et al. make an important contribution to a trajectory of recent papers on the damaging role of lipid-droplet-accumulating microglia (LDAM). In 2020, transcriptomic and functional studies of LDAM in the aged mouse brain established this microglia subset as a dysfunctional and proinflammatory state (Marschallinger et al., 2020). This was followed by studies showing a similar lipid-accumulating state in human microglia in the AD context (Claes et al., 2021; Victor et al., 2022; Sun et al., 2023; Haney et al., 2023). This study establishes a functional assessment of LDAM in AD models in vivo.
Here, Wu et al. thoroughly characterize and functionally test lipid droplet (LD) accumulation in microglia and border-associated macrophages (BAMs). They do this by comparing 2-month and 6-month APP-KI mice to age-matched WT mice. Specifically, they isolate lipid-accumulating microglia and BAMs by FACS for single-cell and lipidomic characterization and observe more LD in the microglia and BAMs of older mice, which was further exacerbated in the AD model with a majority of LDAMs surrounding Aβ plaques in these animals. Additionally, they test the effects of a high-fat diet which led to an even greater increase in LD accumulation with age, especially in the APP-KI mice. They reduce LDs in a macrophage-specific manner by crossing the Cx3cr1CreERT2 mouse strain with the Fit2fl/fl mouse strain into the APP-KI background. This lowering of LDs led to a functional rescue in microglial phagocytic and efferocytic activity as well as decreased Aβ plaque load in vivo.
This study is an important leap forward on the topic of LDs in AD for several reasons:
This is the first demonstration that lowering LDs in microglia in vivo reduces AD pathology and rescues microglia function. Previously, reduced function of LDAM has been indicated in vitro (Marschallinger et al., 2020; Victor et al., 2022; Haney et al., 2023) but not in in vivo AD models. This is a big step forward in determining that the LDAM state plays a detrimental role in brain function, as opposed to a benign or protective role. This reaffirms that LDAM are a new therapeutic target in AD that could not only reduce amyloid pathology but restore microglial function.
This is another study confirming that amyloid induces cytosolic lipid droplets (as defined by the gold standard LD marker Plin2) in microglia and macrophages. This can be contrasted with lipid-accumulating microglia caused by lysosomal lipid accumulation due to phagocytosis of myelin debris.
Some studies have shown that the lipids accumulating in microglial lipid droplets are mainly Triglycerides (TGs) (Marschallinger et al., 2020; Prakash et al., 2023; Haney et al., 2023) whereas others have seen primarily Cholesterol esters (CE) (Xia et al., 2022; Litvinchuk et al., 2024). This has led to questions as to whether these differences are due to methods or AD models. In this APP-KI model the predominant change in microglia lipidome appears to be in TGs and this is reversed in the conditional Fit2 KO. This may support a model of microglial lipid changes in AD being LDs formed in microglia as opposed to phagocytosed myelin.
Lastly, this is an important single-cell RNA-Seq and lipidomic resource of LDAM and LD+ BAMs in an AD mouse model. This will be important for comparison with other mouse models of disease with LDAMs and with human transcriptomes.
This study raises the following:
One question is that there seems to be a striking difference between the transcriptome of these APP LDAM and aged mouse LDAM as well as human AD LDAM. An in-depth comparison of all these different LDAM states would be interesting.
Lowering LDs by conditional Fit2 KO has striking functional benefits whereas the transcriptomic effects appeared subtle. While qPCR data showed no significant increase in the pro-inflammatory signature of these cells, it may be informative to do a more direct comparison to DAM datasets to determine whether lowering LDses push MG toward a more homeostatic or DAM-like state.
Fit2 KO changed expression of chemokines such as Ccl3, Ccl4, Ccl5. This has also recently been reported as a feature of LDAM (Marschallinger et al., 2020; Haney et al., 2023). Does the Fit2 KO and LDAM reduction also reduce peripheral immune cell infiltration in AD mouse models?
Recent studies have shown harmful effects of LDAM on neurons (Victor et al., 2022; Haney et al., 2023), likely mediated by the transfer of lipids made in microglia via APOE, which has been shown to induce lipofuscin and tau accumulation in an APOE variant-dependent manner (Guo et al., 2025). Does lowering LDs by Fit2 KO impact neuronal function and lipid accumulation in AD models?
While this paper demonstrates that a high-fat diet can further increase LDAMs, several other studies have shown that a high-fat diet can accelerate cognitive decline in models of AD and aging (Sah et al., 2017; Gannon et al., 2022; Liang et al., 2023). Do LDAMs directly contribute to this behavioral phenotype? Are cognitive/behavioral phenotypes rescued in the conditional Fit2 KO mice?
Overall, this is a very exciting study on this newly identified microglial state and the pathogenic role these cells play in AD.
References:
Claes C, Danhash EP, Hasselmann J, Chadarevian JP, Shabestari SK, England WE, Lim TE, Hidalgo JL, Spitale RC, Davtyan H, Blurton-Jones M.
Plaque-associated human microglia accumulate lipid droplets in a chimeric model of Alzheimer's disease.
Mol Neurodegener. 2021 Jul 23;16(1):50.
PubMed.
Gannon OJ, Robison LS, Salinero AE, Abi-Ghanem C, Mansour FM, Kelly RD, Tyagi A, Brawley RR, Ogg JD, Zuloaga KL.
High-fat diet exacerbates cognitive decline in mouse models of Alzheimer's disease and mixed dementia in a sex-dependent manner.
J Neuroinflammation. 2022 May 14;19(1):110.
PubMed.
Guo JL, Braun D, Fitzgerald GA, Hsieh YT, Rougé L, Litvinchuk A, Steffek M, Propson NE, Heffner CM, Discenza C, Han SJ, Rana A, Skuja LL, Lin BQ, Sun EW, Davis SS, Balasundar S, Becerra I, Dugas JC, Ha C, Hsiao-Nakamoto J, Huang F, Jain S, Kung JE, Liau NP, Mahon CS, Nguyen HN, Nguyen N, Samaddar M, Shi Y, Tatarakis D, Tian Y, Zhu Y, Suh JH, Sandmann T, Calvert ME, Arguello A, Kane LA, Lewcock JW, Holtzman DM, Koth CM, Di Paolo G.
Decreased lipidated ApoE-receptor interactions confer protection against pathogenicity of ApoE and its lipid cargoes in lysosomes.
Cell. 2025 Jan 9;188(1):187-206.e26. Epub 2024 Nov 11
PubMed.
Haney MS, Pálovics R, Munson CN, Long C, Johansson P, Yip O, Dong W, Rawat E, West E, Schlachetzki JC, Tsai A, Guldner IH, Lamichhane BS, Smith A, Schaum N, Calcuttawala K, Shin A, Wang YH, Wang C, Koutsodendris N, Serrano GE, Beach TG, Reiman EM, Glass CK, Abu-Remaileh M, Enejder A, Huang Y, Wyss-Coray T.
APOE4/4 is linked to damaging lipid droplets in Alzheimer's microglia.
bioRxiv. 2023 Jul 25;
PubMed.
Liang Z, Gong X, Ye R, Zhao Y, Yu J, Zhao Y, Bao J.
Long-Term High-Fat Diet Consumption Induces Cognitive Decline Accompanied by Tau Hyper-Phosphorylation and Microglial Activation in Aging.
Nutrients. 2023 Jan 3;15(1)
PubMed.
Litvinchuk A, Suh JH, Guo JL, Lin K, Davis SS, Bien-Ly N, Tycksen E, Tabor GT, Remolina Serrano J, Manis M, Bao X, Lee C, Bosch M, Perez EJ, Yuede CM, Cashikar AG, Ulrich JD, Di Paolo G, Holtzman DM.
Amelioration of Tau and ApoE4-linked glial lipid accumulation and neurodegeneration with an LXR agonist.
Neuron. 2024 Feb 7;112(3):384-403.e8. Epub 2023 Nov 22
PubMed.
Neuron
Marschallinger J, Iram T, Zardeneta M, Lee SE, Lehallier B, Haney MS, Pluvinage JV, Mathur V, Hahn O, Morgens DW, Kim J, Tevini J, Felder TK, Wolinski H, Bertozzi CR, Bassik MC, Aigner L, Wyss-Coray T.
Lipid-droplet-accumulating microglia represent a dysfunctional and proinflammatory state in the aging brain.
Nat Neurosci. 2020 Feb;23(2):194-208. Epub 2020 Jan 20
PubMed.
Correction.
Prakash P, Manchanda P, Paouri E, Bisht K, Sharma K, Wijewardhane PR, Randolph CE, Clark MG, Fine J, Thayer EA, Crockett A, Gasmi N, Stanko S, Prayson RA, Zhang C, Davalos D, Chopra G.
Amyloid β Induces Lipid Droplet-Mediated Microglial Dysfunction in Alzheimer's Disease.
bioRxiv. 2023 Jun 6;
PubMed.
Sah SK, Lee C, Jang JH, Park GH.
Effect of high-fat diet on cognitive impairment in triple-transgenic mice model of Alzheimer's disease.
Biochem Biophys Res Commun. 2017 Nov 4;493(1):731-736. Epub 2017 Sep 1
PubMed.
Sun N, Victor MB, Park YP, Xiong X, Scannail AN, Leary N, Prosper S, Viswanathan S, Luna X, Boix CA, James BT, Tanigawa Y, Galani K, Mathys H, Jiang X, Ng AP, Bennett DA, Tsai LH, Kellis M.
Human microglial state dynamics in Alzheimer's disease progression.
Cell. 2023 Sep 28;186(20):4386-4403.e29.
PubMed.
Victor MB, Leary N, Luna X, Meharena HS, Scannail AN, Bozzelli PL, Samaan G, Murdock MH, von Maydell D, Effenberger AH, Cerit O, Wen HL, Liu L, Welch G, Bonner M, Tsai LH.
Lipid accumulation induced by APOE4 impairs microglial surveillance of neuronal-network activity.
Cell Stem Cell. 2022 Aug 4;29(8):1197-1212.e8.
PubMed.
Xia D, Lianoglou S, Sandmann T, Calvert M, Suh JH, Thomsen E, Dugas J, Pizzo ME, DeVos SL, Earr TK, Lin CC, Davis S, Ha C, Leung AW, Nguyen H, Chau R, Yulyaningsih E, Lopez I, Solanoy H, Masoud ST, Liang CC, Lin K, Astarita G, Khoury N, Zuchero JY, Thorne RG, Shen K, Miller S, Palop JJ, Garceau D, Sasner M, Whitesell JD, Harris JA, Hummel S, Gnörich J, Wind K, Kunze L, Zatcepin A, Brendel M, Willem M, Haass C, Barnett D, Zimmer TS, Orr AG, Scearce-Levie K, Lewcock JW, Di Paolo G, Sanchez PE.
Novel App knock-in mouse model shows key features of amyloid pathology and reveals profound metabolic dysregulation of microglia.
Mol Neurodegener. 2022 Jun 11;17(1):41.
PubMed.
Comments
Stanford University Medical School
Stanford University
Reducing microglial lipid load enhances Aβ phagocytosis in an Alzheimer’s disease mouse model
Wu and co-authors investigate the role of lipid-droplet-accumulating microglia (LDAM) and border-associated macrophages (BAMs) in an APP-KI model, which harbors a humanized Aβ sequence containing the Swedish, Beyreuther/Iberian, and Arctic mutations. The experiments were conducted in 6-month-old mice, an age at which memory deficits are observed in this model (Saito et al., 2014). By integrating multi-omics approaches, including transcriptomic and lipidomic profiling, the authors document how LD formation influences microglial function. Notably, genetic deletion of FIT2 which is a known regulator of lipid droplet biogenesis, restored microglial phagocytosis and enhanced Aβ clearance, suggesting a potential therapeutic avenue.
Targeting FIT2 effectively reduced LD formation, appeared to restore microglial phagocytosis, and improved plaque clearance in APP-KI mice. Interestingly, CD11c+ microglia in FIT2-deficient APP-KI mice exhibited a downregulation of disease-associated microglial (DAM) genes, including Trem2, Apoe, and Cd9, which may be a consequence of reduced amyloid burden. However, these microglia retained high expression of MHC II genes, particularly Cd74. Notably, Cd74 and Axl were previously reported to show higher expression in LD-low microglia compared to LD-high microglia (Marschallinger et al., 2020).
Furthermore, ACSL1, an enzyme that catalyzes the conversion of free fatty acids (FFAs) into Acyl-CoAs, plays a crucial role in lipid droplet biogenesis and has been implicated in microglial dysfunction in AD (Haney et al., 2024). The elevated FFA levels observed in FIT2-deficient microglia warrant further investigation, particularly in the context of lipid metabolism and microglial homeostasis.
From a therapeutic standpoint, targeting FIT2 would likely present significant challenges due to its higher expression in neurons and astrocytes than in microglia within the brain. Notably, homozygous FITM2 mutations have been associated with deafness-dystonia syndrome, motor regression, ichthyosis, and sensory neuropathy, raising concerns about potential adverse effects of systemic FIT2 inhibition (Zazo Seco et al., 2017). Another critical question is whether peripheral immune cells exhibit distinct FIT2-regulated pathways and whether FIT2 modulation could be CX3CR1-specific. Addressing these questions will be essential for the development of targeted therapeutic strategies that minimize unintended consequences while effectively modulating FIT2.
Another key question emerging from this and other studies on LDs in mice is their relevance to human brain aging and AD, as well as the role different ApoE isoforms play. Previous research from our lab, building on Alzheimer’s original observations of LDs in AD, suggests that APOE4 carriers may be more prone to LD formation and more susceptible to their toxic effects. However, further studies are needed to determine in which cell types LDs accumulate in humans, the molecular processes driving LD formation, and how these deposits interfere with cellular functions.
Overall, this study elegantly highlights the importance of microglial lipid droplets in amyloid pathology and provides novel insights into their role in microglial function, lipid metabolism, and disease progression.
References:
Saito T, Matsuba Y, Mihira N, Takano J, Nilsson P, Itohara S, Iwata N, Saido TC. Single App knock-in mouse models of Alzheimer's disease. Nat Neurosci. 2014 May;17(5):661-3. Epub 2014 Apr 13 PubMed.
Marschallinger J, Iram T, Zardeneta M, Lee SE, Lehallier B, Haney MS, Pluvinage JV, Mathur V, Hahn O, Morgens DW, Kim J, Tevini J, Felder TK, Wolinski H, Bertozzi CR, Bassik MC, Aigner L, Wyss-Coray T. Lipid-droplet-accumulating microglia represent a dysfunctional and proinflammatory state in the aging brain. Nat Neurosci. 2020 Feb;23(2):194-208. Epub 2020 Jan 20 PubMed. Correction.
Haney MS, Pálovics R, Munson CN, Long C, Johansson PK, Yip O, Dong W, Rawat E, West E, Schlachetzki JC, Tsai A, Guldner IH, Lamichhane BS, Smith A, Schaum N, Calcuttawala K, Shin A, Wang YH, Wang C, Koutsodendris N, Serrano GE, Beach TG, Reiman EM, Glass CK, Abu-Remaileh M, Enejder A, Huang Y, Wyss-Coray T. APOE4/4 is linked to damaging lipid droplets in Alzheimer's disease microglia. Nature. 2024 Apr;628(8006):154-161. Epub 2024 Mar 13 PubMed.
Zazo Seco C, Castells-Nobau A, Joo SH, Schraders M, Foo JN, van der Voet M, Velan SS, Nijhof B, Oostrik J, de Vrieze E, Katana R, Mansoor A, Huynen M, Szklarczyk R, Oti M, Tranebjærg L, van Wijk E, Scheffer-de Gooyert JM, Siddique S, Baets J, de Jonghe P, Kazmi SA, Sadananthan SA, van de Warrenburg BP, Khor CC, Göpfert MC, Qamar R, Schenck A, Kremer H, Siddiqi S. A homozygous FITM2 mutation causes a deafness-dystonia syndrome with motor regression and signs of ichthyosis and sensory neuropathy. Dis Model Mech. 2017 Feb 1;10(2):105-118. Epub 2016 Dec 15 PubMed.
View all comments by Andy TsaiUniversity of Colorado Anschutz Medical Campus
Although the sequestering of lipids within intracellular lipid droplets protects against lipid oxidation, excess lipid droplet accumulation is associated with aging and disease. In AD, microglia accumulate excess lipid droplets, which leads to reduced functionality, such as phagocytosis, promoting AD neuropathogenesis. While many research groups, including our own, have been interested in understanding the mechanisms leading to lipid droplet accumulation, in this study, Wu et al. focus on strategies that reduce LDs to increase microglial phagocytosis and prevent AD pathology.
The authors use cell sorting to confirm that lipid-droplet-containing microglia increase with aging and disease, in this case, a murine model of amyloid accumulation (APP-KI). In further support, untargeted lipidomics showed that microglia isolated from APP-KI mice contained more neutral lipids typically associated with LDs (e.g., triglycerides and cholesterol esters). I also found it interesting that the very-long-chain polyunsaturated fatty acid and eicosanoid precursor arachidonic acid was depleted in microglia from the APP-KI mice, highlighting potential deficits in the inflammatory response. It would also be interesting to probe the saturation index of lipids within the microglia, especially given recent papers highlighting increased unsaturated lipids in (pathology-associated) lipid droplets.
To further ascertain the phenotype of microglia in response to increased amyloid load, the authors use single-cell transcriptomics, identifying lipid-accumulating disease-associated microglia (DAM)-like signatures (e.g., expressing ApoE, LPL, Trem2, and Cst7) that have also been observed in other AD models (e.g., 5xFAD). Going forward, methodological advances permitting, performing lipidomic and transcriptomic analysis on the same cells will advance our mechanistic understanding of microglial lipid metabolism and lipid droplet formation.
Chasing a therapeutic intervention, Wu and colleagues genetically ablate FIT2 (CX3CR1; FIT2flox), a protein that binds to triglycerides and diacylglycerols in the endoplasmic reticulum to increase lipid droplet formation. Microglia-specific FIT2 depletion in APP-KI mice resulted in reduced microglial lipid droplet accumulation, triglyceride composition, expression of DAM-like genes (e.g., Plin2, ApoE, Trem2, and Cts7), and amyloid load. FIT2 depletion also results in increased microglial phagocytosis ex vivo, and in vitro, supporting the idea that reducing lipid droplet accumulation in microglia could preserve functionality and rescue AD-like pathology.
These exciting proof-of-concept data support a growing body of literature highlighting microglial lipid metabolism as a rational, and clinically relevant, target to improve AD pathology. However, it is important to reiterate that at baseline lipid droplets serve to protect the cell. Therefore, selectively inhibiting lipid droplet formation in subsets of microglia and at specific disease stages will be an attractive strategy going forward.
View all comments by Kimberley BruceWashington University in St. Louis
This study investigates the role of the protein FIT2, a key protein in lipid droplet biogenesis, in LD accumulation within microglia and BAMs in an AD mouse model. They show that LDs accumulate with age and AD progression in the two cell types in the CNS and that this is exacerbated by a high-fat diet. Other key findings include that FIT2 deficiency reduces LD formation in microglia and BAMs, leading to changes in their lipid metabolism and gene-expression profiles. Furthermore, FIT2 depletion enhances microglial phagocytosis, increasing the uptake of E. coli bioparticles and Aβ oligomers in vitro, as well as reducing Aβ accumulation in the brain tissue.
This study is novel and has the following strengths:
Limitations include:
In conclusion, this study provides important insights into the metabolic dysfunction of microglia in AD, as a result of altered lipid-homeostasis. Furthermore, it identifies FIT2 as a promising target for intervention. However, further in vivo and human studies are needed to confirm its therapeutic potential and to understand long-term effects of interfering with its function in the brain.
View all comments by Laura PiccioCUNY Advanced Science Research Center
This study contributes to a growing body of research on the role of microglial lipid metabolism in Alzheimer’s disease. Important studies from the labs of Tony Wyss-Coray, Li-Huei Tsai, Mathew Blurton-Jones, Gaurav Chopra, Erik Musiek, and many others showed that amyloidosis induces a shift in microglial lipid metabolism and lipidomic profile, as well as an increase in lipid-droplet-accumulating microglia near sites of pathology. A common theme in these studies is that these microglia have reduced phagocytic capabilities and a neurotoxic effect.
Christiane Ruedl’s research group shows that this phenomenon is not limited to microglia but also seen in brain macrophages of the meninges. This study goes another step further, showing us that reducing LDs in microglia/macrophages via genetic depletion of FIT2 not only restores their phagocytic activity but also reduces amyloid burden in APP knock-in.
View all comments by Pinar AyataUniversity of Pennsylvania
University of Pennsylvania
In this study Wu et al. make an important contribution to a trajectory of recent papers on the damaging role of lipid-droplet-accumulating microglia (LDAM). In 2020, transcriptomic and functional studies of LDAM in the aged mouse brain established this microglia subset as a dysfunctional and proinflammatory state (Marschallinger et al., 2020). This was followed by studies showing a similar lipid-accumulating state in human microglia in the AD context (Claes et al., 2021; Victor et al., 2022; Sun et al., 2023; Haney et al., 2023). This study establishes a functional assessment of LDAM in AD models in vivo.
Here, Wu et al. thoroughly characterize and functionally test lipid droplet (LD) accumulation in microglia and border-associated macrophages (BAMs). They do this by comparing 2-month and 6-month APP-KI mice to age-matched WT mice. Specifically, they isolate lipid-accumulating microglia and BAMs by FACS for single-cell and lipidomic characterization and observe more LD in the microglia and BAMs of older mice, which was further exacerbated in the AD model with a majority of LDAMs surrounding Aβ plaques in these animals. Additionally, they test the effects of a high-fat diet which led to an even greater increase in LD accumulation with age, especially in the APP-KI mice. They reduce LDs in a macrophage-specific manner by crossing the Cx3cr1CreERT2 mouse strain with the Fit2fl/fl mouse strain into the APP-KI background. This lowering of LDs led to a functional rescue in microglial phagocytic and efferocytic activity as well as decreased Aβ plaque load in vivo.
This study is an important leap forward on the topic of LDs in AD for several reasons:
This study raises the following:
Overall, this is a very exciting study on this newly identified microglial state and the pathogenic role these cells play in AD.
References:
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