How oligodendrocytes use fatty acid metabolism as an energy reserve to maintain axonal function and myelin stability when glucose is scarce, offering insights into neurodegenerative disease mechanisms.
Study: Oligodendroglial fatty acid metabolism as a central nervous system energy reserve. Image Credit: Shot4Sell/Shutterstock.com
In a recent study published in Nature Neuroscience, a group of researchers investigated how oligodendroglial lipid metabolism provides an energy reserve for maintaining axonal function and myelin homeostasis under conditions of glucose deprivation in the central nervous system.
Background
In vertebrates, oligodendrocytes produce myelin to facilitate saltatory conduction and provide metabolic substrates like lactate or pyruvate for axonal Adenosine triphosphate (ATP) generation. This support is essential when myelin limits axonal access to extracellular nutrients.
Oligodendrocytes also perform fatty acid (FA) β-oxidation in mitochondria and peroxisomes, using FAs for energy or synthesis. Reduced glucose levels affect FA metabolism and myelin turnover, contributing to white matter abnormalities in neurodegenerative diseases.
Further research is needed to clarify how oligodendroglial FA metabolism supports myelin maintenance and axonal function under different metabolic conditions, especially in the context of neurodegeneration.
About the study
All mice used in the present study were bred on a C57 Black 6 (C57BL/6) (a common inbred strain of laboratory mouse) background, with the exception of Aldehyde Dehydrogenase 1 Family Member L1 - Green Fluorescent Protein (Aldh1l1-GFP) mice.
They were housed under standard conditions with a 12-hour day/night cycle, free access to food and water, and a controlled environment at 22°C and 30-70% humidity.
Transgenic mice were generated in-house following standard protocols. To visualize autophagosomes in oligodendrocytes, an Monomeric Tag Red Fluorescent Protein - Monomeric Wasabi Green Fluorescent Protein - Microtubule-associated protein 1A/1B-light chain 3 (mTagRFP-mWasabi-LC3) (a tandem fluorescent construct used for autophagy research) construct was placed under the control of the 2',3'-Cyclic Nucleotide 3'-Phosphodiesterase (Cnp)(an enzyme expressed in oligodendrocytes) promoter.
Genotyping was carried out using specific primers and a polymerase chain reaction (PCR) program. Eight other mouse lines were also genotyped as previously described, targeting various cell types and pathways, such as microglia, astrocytes, oligodendrocyte precursor cells, and oligodendrocytes.
Homozygous mutants were compared with control genotypes in various experiments to assess gene function.
Reagents were sourced from Merck unless otherwise noted. The artificial cerebrospinal fluid (aCSF) solution used for optic nerve incubation and electrophysiological recordings contained specific components, including glucose and sucrose, for osmolarity maintenance.
Various inhibitors for metabolic and autophagy pathways were freshly prepared and added to the aCSF at specific concentrations.
Optic nerves were prepared from mice following cervical dislocation, dissected, and incubated in aCSF at 37°C. Electrophysiological recordings and survival analysis were performed to measure cellular and axonal responses under different metabolic conditions. Data were analyzed using software such as Fiji and Imaris, and statistical analysis was conducted with Excel or GraphPad Prism 9.
Study results
In this study, fully myelinated optic nerves from 2-month-old transgenic mice expressing fluorescent proteins in oligodendrocytes or astrocytes were analyzed under glucose deprivation conditions. Optic nerves were incubated in aCSF at 37°C with varying glucose concentrations (10 mM, 0 mM, or low glucose).
After 24 hours, fluorescence analysis was used to assess cell survival. Surprisingly, the majority of oligodendrocytes remained healthy even in the absence of glucose, while over 70% of astrocytes had died.
Oligodendrocyte precursor cells (OPCs) and microglia were also unaffected. These results indicate that oligodendrocytes rely on a pre-existing energy reserve, potentially via FA metabolism.
When optic nerves were incubated with one mM glucose, a concentration insufficient for axonal conduction, all cells survived for at least 24 hours.
Additionally, providing 3-hydroxybutyrate as an alternative energy source prevented cell death, suggesting that energy metabolism, rather than glucose deprivation alone, was responsible for cell survival.
Inhibitors of reactive oxygen species (ROS) did not enhance cell survival, indicating that ROS generation was not the cause of cell death. However, under severe hypoxia and glucose deprivation, extensive cell death was observed, affecting all cell types.
To investigate whether FAs could serve as an energy reserve, nerves were incubated without glucose and treated with 4-bromocrotonic acid (4-Br), a mitochondrial FA β-oxidation inhibitor.
This significantly reduced cell survival, supporting the hypothesis that FA metabolism via β-oxidation sustains energy production. Interestingly, inhibiting peroxisomal β-oxidation with thioridazine did not affect cell survival, suggesting that mitochondrial β-oxidation compensates for the loss of peroxisomal function.
Electron microscopy revealed that energy deprivation led to structural changes in the myelin sheath, with increased g-ratios and vesicular demyelination, possibly due to myelin degradation via autophagy.
Proteomic analysis confirmed alterations in FA metabolism, with an increase in enzymes involved in FA and ketone body metabolism. The study also observed increased autophagosome formation in oligodendrocytes under glucose deprivation, indicating a basal level of autophagy that is upregulated during starvation.
Lastly, inhibition of β-oxidation in oligodendrocytes via genetic manipulation or pharmacological means impaired axonal function, demonstrating that FA metabolism supports axonal conduction under low glucose conditions.
Conclusions
To summarize, the combined in vitro and in vivo data suggest an extended model of myelin dynamics, where oligodendrocytes support axonal function not only through glycolysis but also by metabolizing FAs.
The study's key experiment involved analyzing axonal conductivity and ATP levels in myelinated optic nerves under low glucose conditions and using metabolic inhibitors. These experiments revealed that reduced glucose availability in oligodendrocytes leads to gradual myelin loss.
The study also found that while FA metabolism supports axonal function, it cannot fully compensate for glucose. Thus, oligodendroglial FA metabolism helps prevent axon degeneration during metabolic stress.
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