Lipid droplets accumulate in microglia in mouse and cell models of amyloidosis but scientists are not sure why. Evidence suggests that Aβ helps create the glut, but now scientists led by Lingyan Shi and Xu Chen at the University of California, San Diego, report that mutant tau lends a hand. In the April 23 Cell Metabolism, they show that, in mouse, fly, and cell models of tauopathy, neurons shuttle lipids to microglia, where they form droplets and trigger neuroinflammation. Neuronal AMP-activated protein kinase modulates this process, the authors found. Knocking it out exacerbated lipid droplets in the glia, while activating the kinase alleviated them. However, it’s unclear if this lipid regulation is linked to phosphorylation of tau or of the many other AMPK substrates.

  • In models of tauopathy, neurons produce more lipid yet metabolize less.
  • Instead, they pass the lipids to microglia.
  • AMP kinase attenuates this process.

“[This] work beautifully demonstrates how upregulated lipid synthesis and downregulated lipid turnover leads to pathological lipid accumulation in both neurons and glia in multiple tauopathy model systems,” wrote Priyanka Narayan and Roxan Stephenson, National Institutes of Health in Bethesda, Maryland.

Amita Sehgal at the University of Pennsylvania in Philadelphia was excited by these findings. “[They] bolster the growing body of work that neurons transfer lipids to glia, most likely to alleviate their own oxidative damage,” she wrote. Earlier this year, Sehgal reported that, in wild-type fruit flies, neurons offload oxidized lipids to glia, which clear the fat droplets during sleep (Feb 2024 news).

In the aging mouse hippocampus, the number of lipid droplet-accumulating microglia tick up. These poorly clear debris, while spewing inflammatory cytokines (Aug 2019 news). Both Aβ and APOE4 exacerbate microglial lipid droplet accumulation (Mar 2024 news).

To see what happens in tauopathy models, first author Yajuan Li used Raman spectroscopy to measure lipids within hippocampal tissue of PS19 mice. Raman spectroscopy uses two lasers to jiggle molecular bonds, allowing scientists to measure vibrational energies emitted and chart the distribution of molecules in cells without needing to stain for lipids.

Within the hippocampi of 9-month-old PS19 mice, about three months after neurofibrillary tangles had formed, lipid droplets accumulated, mainly in microglia (image below) and especially in those surrounding synapses containing phospho-tau inclusions. Droplet load tightly correlated with hippocampal neuron loss. Lipid-filled microglia appeared to be under oxidative stress, as indicated by a high FAD-to-NADH ratio. These are among the major redox coenzymes, and measuring both gives a more complete snapshot of mitochondrial burden than measuring either alone (Chance et al., 1979). Being in oxidized and reduced forms, respectively, a high FAD/NADH ratio indicates a more oxidative state.

Fatty Microglia. In PS19 hippocampal tissue, lipid droplets (light blue puncta) mostly accumulated in microglia (purple, first panel), though some occurred in astrocytes (purple, second panel). Oligodendrocytes (purple, third panel) and neurons (purple, fourth panel) had almost none. Arrows indicate lipid droplets within cells. [Courtesy of Li et al., Cell Metabolism, 2024.]

Suspecting the microglial lipids came from neurons, the authors tested if human iPSC-derived neurons carrying the frontotemporal dementia V337M variant of tau could pass lipids to mouse microglia. Within 24 hours of bathing in conditioned media from tauopathy neurons, but not wild-type neurons, microglia accumulated lipid droplets (image below). Their FAD/NADH ratio inched up, they poorly phagocytosed fluorescent beads, and they ramped up expression of inflammatory cytokines—all features of droplet-laden microglia seen in AD models. “This study adds a new source—neurons—as a cause of lipid-droplet accumulating microglia in neurodegeneration,” wrote Michael Haney, University of Pennsylvania (comment below).

Lipid Media? Unlike microglia grown in control media (left), those grown in media from tauopathy iPSC neurons (right) accumulated lipid droplets (blue puncta). [Courtesy of Li et al., Cell Metabolism, 2024.]

Curiously, the V337M tau neurons made more lipids, and degraded fewer, than did wild-type neurons, as determined by isotope labeling. They also had 10 times more lipid droplets and 1.4-fold higher FAD/NADH ratio. The authors believe that tauopathy perturbs lipid processing, and that in the brain, neurons protect themselves by offloading the excess fats to microglia.

To track that lipid turnover, the scientists turned to a fruit fly model of tauopathy, using deuterium oxide labeling to enhance lipid detection by Raman spectroscopy. D2O introduced in their chow incorporated into lipids, which then cleared when the scientists deprived the flies of food. Microglia in tauopathy flies accumulated larger lipid droplets than glia in wild-type flies yet barely cleared lipids when food was withdrawn, whereas wild-type flies halved their deuterium-lipid load.

This enhanced lipid synthesis and impaired breakdown aligns with altered gene expression patterns that Li and colleagues found when analyzing published RNA-Seq data (Sjöstedt et al., 2020). Compared to control tissue, AD brains upregulated genes involved in lipid synthesis and transfer, such as LPIN1 and ABCA1. Among the most downregulated was PRKAA, which encodes the α subunit of AMP kinase.

This protein caught the researchers’ attention because AMPK is crucial for lipid metabolism. It is also a tau kinase. Notably, in the cortices of 9-month-old PS19 mice, there was less phosphorylated AMPK, the active form, than in control brain. V337M iPSC neurons also had less p-AMPK and less phosphorylated AMPK substrates than did wild-type neurons, suggesting that AMPK is less active in tauopathy brains and neurons.

Did AMPK affect lipid droplet formation? Indeed, giving wild-type iPSC neurons an AMPK inhibitor ramped up expression of lipid synthesis genes, and droplets accumulated. Knocking out AMPK in 5-month-old PS19 tauopathy mice, which do not yet have tangles, did the same.

Death by Fat. In tauopathy neurons (gray), downregulated AMP kinase causes lipid droplets (LDs, yellow) to accumulate, forcing the cells to export the fats to microglia (blue). This triggers oxidative stress, suppresses phagocytosis, and ramps up production of neuroinflammatory cytokines. [Courtesy of Li et al., Cell Metabolism, 2024.]

In contrast, giving V337M tau neurons a small molecule that activates AMPK, or overexpressing the kinase in tauopathy flies, downregulated lipogenesis gene expression, accelerated lipid metabolism, and reduced lipid droplet load. Taken together, these results suggest that AMPK prevents lipid synthesis and promotes their clearance.

All told, tauopathy spurs neurons to make more lipid and break down less, inundating them with fat that they offload to microglia as droplets. This, in turn, weakens microglia (image above).

“The findings highlight that targeting pathways that cause lipid accumulation in various cell types in neurodegenerative diseases is an important field for therapeutic development,” wrote Ole Isacson and Penny Hallett of Harvard Medical School, Boston (comment below).

Chen said they are exploring pathways beyond AMPK that regulate lipid droplets to see if molecules downstream, or specific to microglia, might be good drug targets.—Chelsea Weidman Burke

Comments

  1. The work from Li et al. beautifully demonstrates how upregulated lipid synthesis and downregulated lipid turnover leads to pathological lipid accumulation in both neurons and glia in multiple tauopathy model systems—iPSC-derived cells as well as transgenic flies and mice. This work uses an inventive combination of label-free imaging and metabolic labelling assays to clearly establish its findings in multiple model systems. This study contributes to the growing body of work cementing lipid accumulation as a central pathology of AD and related dementias.

    In addition to adding more support to the idea that lipid accumulation is a signature of proinflammatory microglia (Haney et al., 2024; Victor et al., 2022; Prakash et al., 2023; Stephenson et al., 2024), this work adds a new mechanistic understanding of the origins of glial lipid accumulation—that it arises from unsaturated fatty acid transfer from neurons. A few questions that arise from this are: 1) What is it about the tauopathy models that leads to the increased lipid droplets in the first place? 2) Why do lipid droplets accumulate in human neurons (iPSC-derived) and not (visibly) in neurons within the tauopathy mouse models? 3) Why is there regional specificity to LD accumulation, and why do microglia near neurons with hyperphosphorylated tau accumulate LDs more than other microglia? And finally: 4) How could we bolster the capacity of microglia and astrocytes to handle these lipids?

    Li et al. demonstrate that increasing lipid turnover through AMPK signaling alleviates lipid accumulation in neurons and its non-cell-autonomous consequences in microglia, namely lipid accumulation and immune activation. This provides a new non-cell-autonomous approach to alleviate neuroinflammatory states, which accompany many neurodegenerative diseases. It will be interesting to explore whether these same mechanisms are at play in other neurodegenerative diseases that display lipid accumulation, including α-synucleinopathies and APOE4-linked diseases (Fanning et al., 2019). 

    Multiple studies have profiled how microglial lipid accumulation impacts neurons—both their signaling activity and tau phosphorylation (Haney et al., 2024; Victor et al., 2022)—as well as astrocyte reactivity (Guttenplan et al., 2021), but Li et al. flip that paradigm to show how neuronal lipids lead to lipid accumulation and inflammatory activation in microglia. Together, these studies paint a picture of neuronal and microglial lipid accumulation feeding bidirectionally to create a perfect pathological storm. However, this study from Li et al. offers one way to target neurons and perhaps break this cycle.

    References:

    . Lipidomic Analysis of α-Synuclein Neurotoxicity Identifies Stearoyl CoA Desaturase as a Target for Parkinson Treatment. Mol Cell. 2019 Mar 7;73(5):1001-1014.e8. Epub 2018 Dec 4 PubMed.

    . Neurotoxic reactive astrocytes induce cell death via saturated lipids. Nature. 2021 Nov;599(7883):102-107. Epub 2021 Oct 6 PubMed.

    . 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.

    . Amyloid β Induces Lipid Droplet-Mediated Microglial Dysfunction in Alzheimer's Disease. bioRxiv. 2023 Jun 6; PubMed.

    . Triglyceride metabolism controls inflammation and APOE4-associated disease states in microglia. 2024 Apr 13 10.1101/2024.04.11.589145 (version 1) bioRxiv.

    . Lipid accumulation induced by APOE4 impairs microglial surveillance of neuronal-network activity. Cell Stem Cell. 2022 Aug 4;29(8):1197-1212.e8. PubMed.

  2. Accumulation of lipids in glia was first noted as a hallmark of Alzheimer’s disease when it was first described over 100 years ago, but only recently has there been a number of studies characterizing this phenomenon at a molecular and mechanistic level (Marschallinger et al., 2020; Sienski et al., 2021; Claes et al., 2021; Victor et al., 2022; Prakash et al., 2023; Windham et al., 2023; Haney et al., 2024). A common thread of these studies is that these lipid accumulations are lipid droplets (LDs) found primarily in microglia but also astrocytes and that these cells have a dysfunctional or damaging phenotype. Lipid-droplet-accumulating glia have been described in the context of brain injury, brain aging, and neurodegenerative disease. In the context of Alzheimer’s disease, recent studies have used amyloid models to show that an inflammatory microglial response to amyloid plaques could be a driver of lipid-droplet-accumulating microglia (LDAM) (Prakash et al., 2023Haney et al., 2024). 

    This study from Li et al. is exciting because it illustrates that similar lipid droplet accumulations occur in a tauopathy mouse model (PS19), with the highest proportion of LDs, 70 percent, observed in microglia. The lipid droplet-accumulating microglia are mostly observed near phospho-tau pathology. Consistent with previous reports, the LD-accumulating microglia show increased inflammatory cytokines and impaired phagocytosis. A great aspect of this study is the authors pair deuterium oxide (D2O) labeling with Raman scattering (SRS) microscopy in fly models and in vitro cell culture models to trace lipid turnover and transfer between cells. This is very exciting because there is likely a high degree of transfer of these accumulating lipids between glial cells to neurons and vice-versa, but this is difficult to measure. The authors show that in iPSCs lines with an FTD-associated V337M tau mutation, neurons transfer lipids to microglia and induce microglial LD accumulation, pro-inflammatory responses, and impaired phagocytosis, a cellular phenotype of LDAM consistent with previous reports (Marschallinger et al., 2020). 

    This is interesting because it has always been a bit of a mystery as to what is the source of the lipids that accumulate in microglia under these conditions. There could be cell-autonomous or non-cell-autonomous causes for lipid accumulation or a combination of both. Previous reports have shown that innate immune triggers, such as LPS or fibrillar Ab are sufficient to induce lipid droplets in vitro and in vivo (Marschallinger et al., 2020; Prakash et al., 2023; Haney et al., 2024) and there are also reports indicating that phagocytosed myelin debris is a major source of lipid accumulation in microglia in models of demyelination (Nugent et al., 2020; Cantuti-Castelvetri el al., 2018). This study adds a new source—neurons—as a cause of lipid droplet accumulating microglia in neurodegeneration.

    Given the cellular complexity of the CNS and how neurodegenerative diseases unfold over long periods of time, all these mechanisms could be occurring sequentially, or simultaneously, and bi-directionally. Questions remain as to the mechanisms and timing of how these lipids are trafficked between these cell types and about which cell types are resilient versus vulnerable to these lipids. Excitingly, this study concludes by showing that, through targeting AMPK, lipid levels can be reduced through autophagy, and microglial state and function restored, which is consistent with similar approaches to lower lipid droplet levels through autophagy related pathways (Haney et al., 2024). This indicates, that while there is much to be learned about the cellular mechanisms underlying lipid accumulation and trafficking in neurodegenerative disease, pharmacological interventions could revert these cellular states and restore function.

    References:

    . 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.

    . Plaque-associated human microglia accumulate lipid droplets in a chimeric model of Alzheimer's disease. Mol Neurodegener. 2021 Jul 23;16(1):50. PubMed.

    . Lipid accumulation induced by APOE4 impairs microglial surveillance of neuronal-network activity. Cell Stem Cell. 2022 Aug 4;29(8):1197-1212.e8. PubMed.

    . APOE4 disrupts intracellular lipid homeostasis in human iPSC-derived glia. Sci Transl Med. 2021 Mar 3;13(583) PubMed.

    . APOE traffics to astrocyte lipid droplets and modulates triglyceride saturation and droplet size. bioRxiv. 2023 Apr 29; PubMed.

    . Amyloid β Induces Lipid Droplet-Mediated Microglial Dysfunction in Alzheimer's Disease. bioRxiv. 2023 Jun 6; PubMed.

    . 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.

    . TREM2 Regulates Microglial Cholesterol Metabolism upon Chronic Phagocytic Challenge. Neuron. 2020 Mar 4;105(5):837-854.e9. Epub 2020 Jan 2 PubMed.

    . Defective cholesterol clearance limits remyelination in the aged central nervous system. Science. 2018 Feb 9;359(6376):684-688. Epub 2018 Jan 4 PubMed.

  3. Using sophisticated SRS (stimulated Raman scattering) imaging analyses combined with mouse and fly tauopathy in vivo models, or in vitro in human neurons carrying a V337M Tau mutation, the authors show that tauopathy is associated with an accumulation of lipid droplets in microglia and human neurons, and that pathways for lipid synthesis (lipogenesis) are increased and lipid mobilization is decreased. Moreover, microglial cells in vitro accumulated lipids, and increased inflammatory cytokine production, when exposed to conditioned media from V337M tau human neurons. Based on the analysis of AMP-AD RNA-Seq datasets, the authors found a downregulation of PRKAA (which encodes the alpha subunit of AMPK), in AD brain. Genetic and pharmacological manipulation of AMPK pathways using both in vitro and in vivo approaches demonstrated AMPK regulation of lipid droplets in neurons and microglia, and of inflammatory molecules. Using AMPK activation or inhibition/deletion as tools to manipulate lipid turnover, the findings highlight that targeting pathways that cause lipid accumulations in various cell types in neurodegenerative diseases is an important field for therapeutic development.

    A key finding of this study is the demonstration that tauopathy, lipid disturbances, and neuroinflammation are highly interconnected. Our point of view is that microglial activation occurs in many different neurodegenerative diseases, and we have shown that neuroinflammation and complement activation is associated with brain lipid accumulation (Rocha et al., 2015; Connolly et al., 2023). We and others have also shown that lipid abnormalities, including those associated with aging, precede protein accumulation and neuronal degeneration (Rocha et al., 2015; Brekk et al., 2020; Cooper et al., 2024). 

    It is not clear why lipids accumulate in the tauopathy models, although lipid droplet accumulation is observed in human neurons following a-synuclein overexpression (Fanning et al., 2019). One consideration not discussed, and sometimes overlooked, is the critical normal function of tau in microtubule stabilization and intracellular transport, on which neurons depend. Indeed, tau abnormalities and axonal transport deficits are central to many neurodegenerative diseases.

    Another question to consider is why the expression of PRKAA is decreased in the brain in AD. This could be a physiological adaptation and part of a network of feedback mechanisms in response to metabolic and lipid disturbances in the AD brain, which becomes pathological over time.

    Overall, this elegant work highlights the complex lipid networks that are at the core of AD, PD, LBD, and FTD, and highlights the need for us to understand these systems and intervene in a meaningful way for patients.

    References:

    . Upregulating β-hexosaminidase activity in rodents prevents α-synuclein lipid associations and protects dopaminergic neurons from α-synuclein-mediated neurotoxicity. Acta Neuropathol Commun. 2020 Aug 6;8(1):127. PubMed.

    . Loss of Lipid Carrier ApoE Exacerbates Brain Glial and Inflammatory Responses after Lysosomal GBA1 Inhibition. Cells. 2023 Nov 2;12(21) PubMed.

    . Upstream lipid and metabolic systems are potential causes of Alzheimer's disease, Parkinson's disease and dementias. FEBS J. 2022 Sep 27; PubMed.

    . Lipidomic Analysis of α-Synuclein Neurotoxicity Identifies Stearoyl CoA Desaturase as a Target for Parkinson Treatment. Mol Cell. 2019 Mar 7;73(5):1001-1014.e8. Epub 2018 Dec 4 PubMed.

    . Sustained Systemic Glucocerebrosidase Inhibition Induces Brain α-Synuclein Aggregation, Microglia and Complement C1q Activation in Mice. Antioxid Redox Signal. 2015 Aug 20;23(6):550-64. Epub 2015 Jul 29 PubMed.

  4. I’m excited about these findings, which bolster the growing body of work that neurons transfer lipids to glia, most likely to alleviate their own oxidative damage, and that lipids accumulate in Alzheimer’s brains. The role for AMPK described here is intriguing, although, as the authors note, this can have varied effects. While astrocytes can also be recipients of lipids from neurons, it appears that in the context of neurodegeneration at least, microglia dominate. How microglia normally process those lipids remains a question of interest.

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References

News Citations

  1. While a Fly Sleeps, Its Glia Burn Neuronal Lipids to Refresh the Brain
  2. Newly Identified Microglia Contain Lipid Droplets, Harm Brain
  3. Paper Alert: APOE4 Packs on Lipid Droplets in Microglia

Research Models Citations

  1. Tau P301S (Line PS19)

Mutations Citations

  1. MAPT V337M

Paper Citations

  1. . Oxidation-reduction ratio studies of mitochondria in freeze-trapped samples. NADH and flavoprotein fluorescence signals. J Biol Chem. 1979 Jun 10;254(11):4764-71. PubMed.
  2. . An atlas of the protein-coding genes in the human, pig, and mouse brain. Science. 2020 Mar 6;367(6482) PubMed.

Further Reading

No Available Further Reading

Primary Papers

  1. . Microglial lipid droplet accumulation in tauopathy brain is regulated by neuronal AMPK. Cell Metab. 2024 Apr 16; PubMed.