27 Feb 2024

How does a good night’s sleep re-energize the brain? At least in fruit flies, by burning damaged lipids. So claim scientists led by Amita Sehgal at the University of Pennsylvania in Philadelphia. In the February 15 Nature Neuroscience, they reported that during waking, neurons offload oxidized lipids to glia, which store them as droplets. Later, as the insects sleep, glia burn off the fat, and any decrepit mitochondria in glia or neurons are cleared to ready the flies for a new day. Interrupting this neuron-glia lipid shuttle interrupts sleep as well.

“[This] study is a beautiful example of the complex interaction of astrocytes and neurons [and] is exciting because it expands neuroglia interactions into sleep,” Maiken Nedergaard, University of Rochester Medical Center, New York, wrote to Alzforum.

This work may help scientists understand links between poor sleep and pathologies in Alzheimer’s disease brain, for example the known habit of microglia to accumulate lipid droplets (LDs) in response to Aβ (Aug 2019 news; Apr 2023 conference news).

“The connection between sleep and AD was obvious but it could never be linked to a molecular pathway,” Rik van der Kant, Vrije University, Amsterdam, told Alzforum. “This paper provides a novel way of connecting sleep to lipid homeostasis, which is directly linked to AD.”

Hugo Bellen and colleagues at the Baylor College of Medicine in Houston had discovered the same neuron-glia lipid shuttle in flies carrying AD risk genes, finding their glia bloated with lipid droplets (Moulton et al., 2021). “I thought it was a pathological phenomenon, but Seghal’s paper shows that this shuttle is physiological,” Bellen told Alzforum. “We now know that glial LD formation, which is a highly conserved pathway across multiple species, is promoted by neuronal activity and that glial LDs are consumed during sleep in preparation for another day of activity,” Bellen and colleagues wrote in a Nature Neuroscience News & Views article.

To understand how sleep supports brain metabolism, first author Paula Haynes let Drosophila go about their normal days—i.e., with lights on and off for 12 hours at a time—or she sleep-deprived them for a night by shaking the vials they are kept in every 30 seconds. She studied flies either just after turning the lights on, two hours later, just after turning the lights off, or in the middle of their night. To analyze the poppy seed-sized fly brain, Haynes added fluorescent reporters of reactive oxygen species (ROS), lipid metabolism, and mitochondrial health, i.e., mitophagy, and then imaged the brain under a confocal microscope.

Neuronal activity demands copious energy, generated through mitochondrial oxidative phosphorylation, hence the authors measured oxidative damage in neurons and glia using fluorescent redox proteins, expressed in mitochondria under cell-specific promoters. These proteins switch colors when oxidized, reflecting accumulating ROS. Surprisingly, cortex glia, not neurons, built up mitochondrial ROS during the day (image below). Fruit flies have four types of glia: cortex glia that wrap around neuron bodies, ensheathing glia that surround neuronal projections, blood-brain barrier glia, and astrocytes. They do not have microglia analogous to those of the immune cell lineage found in mammals.

ROS can oxidize lipids. Indeed, lipid droplets that lit up with a fluorescent antibody against peroxidated lipid byproducts accumulated in the cortex glia, matching the distribution of ROS. The droplets peaked in the middle of the night then troughed right around waking, suggesting that the glia accumulate these peroxidated lipids during the day and clear them overnight.

Peroxidated lipids are generated by ROS inside neurons. How did they end up in glia? The authors surmised they were brought there courtesy of apolipoproteins. Flies express ApoE orthologs NLaz in neurons and GLaz in glia. When Haynes knocked down either, glia formed fewer lipid droplets. Neurons did not accumulate lipid droplets. Further, neuronal but not glial mitochondria became oxidized. This suggests that neurons were indeed the source of the peroxidated lipids and that, by shuttling them to glia, neurons protected their own mitochondria from oxidative stress.

To test this idea, Haynes knocked down neuronal Drp1, an ROS-activated mitochondrial damage response protein. Without Drp1 in neurons, glia accumulated fewer lipid droplets by the end of the day, suggesting that mitochondrial damage control drives the neuron-glia lipid shuttle. In contrast, sans Drp1, glia couldn’t metabolize lipid droplets overnight as usual, suggesting they need mitochondria to power lipid catabolism. All told, the data suggest that mitochondrial activity drives the wake/sleep cycle that transfers oxidized lipids from neurons to glia for breakdown. “We propose that a mitochondrial lipid metabolic cycle between neurons and glia reflects a fundamental function of sleep relevant for brain energy homeostasis,” wrote the authors.

Would tinkering with the proteins in this cycle mess with sleep? Indeed, flies without NLaz, GLaz, or Drp1 in either neurons or glia cell slept less, and the shut-eye they got was fragmented. The same happened when the scientists suppressed genes that break down fatty acids, in glia.

Interrupted sleep cycles have been linked to an increased risk of AD and worse amyloid pathology (Feb 2017 news; Apr 2018 news). Even cognitively normal older adults with one or two copies of the AD risk gene APOE4 have disrupted sleep (Blackman et al., 2022). In fruit flies expressing human APOE4 instead of GLaz, glia poorly accumulated lipid droplets, which hastened neurodegeneration (Liu et al., 2017). Julia TCW of Boston University takes these results, and Sehgal’s, to mean that ApoE4 hobbles the neuron-glia lipid shuttle, which might triggers sleep problems and neuronal damage in people.

Russell Swerdlow, Kansas University Medical Center, Kansas City, noted that sleep troubles seem to start early in the AD trajectory; he attributes this primarily to floundering mitochondria. “Sleep disruption may prove to be less of a driver of AD, and more a consequence of mitochondrial strain,” he wrote (full comment below).—Chelsea Weidman Burke

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References

News Citations

1. Newly Identified Microglia Contain Lipid Droplets, Harm Brain 23 Aug 2019
2. Dysregulated Lipid Metabolism Comes to the Fore at AD/PD 27 Apr 2023
3. A Change in Sleep Habits from Normal to Long: Harbinger of Dementia? 25 Feb 2017
4. Does Amyloid Accumulate After a Single Sleepless Night? 18 Apr 2018

Paper Citations

1. Moulton MJ, Barish S, Ralhan I, Chang J, Goodman LD, Harland JG, Marcogliese PC, Johansson JO, Ioannou MS, Bellen HJ. Neuronal ROS-induced glial lipid droplet formation is altered by loss of Alzheimer's disease-associated genes. Proc Natl Acad Sci U S A. 2021 Dec 28;118(52) PubMed.
2. Blackman J, Love S, Sinclair L, Cain R, Coulthard E. APOE ε4, Alzheimer's disease neuropathology and sleep disturbance, in individuals with and without dementia. Alzheimers Res Ther. 2022 Mar 30;14(1):47. PubMed.
3. Liu L, MacKenzie KR, Putluri N, Maletić-Savatić M, Bellen HJ. The Glia-Neuron Lactate Shuttle and Elevated ROS Promote Lipid Synthesis in Neurons and Lipid Droplet Accumulation in Glia via APOE/D. Cell Metab. 2017 Nov 7;26(5):719-737.e6. Epub 2017 Sep 28 PubMed.

Further Reading

News

1. Some Neurons Enlist Glia to Take Care of Their Trash 25 Jun 2014
2. Sleep: Too Little, or Too Much, Foreshadows Brain Shrinkage 11 May 2022
3. Getting Too Little Sleep? You May Be Accumulating Amyloid. 10 Sep 2021
4. Faulty Brain Clocks Precede Alzheimer’s, May Drive Pathology 14 Feb 2018