APPROXIMATELY 75% of mammalian lipid species are found exclusively in neural tissues, playing a crucial role in the morphological and functional diversity of the central nervous system (CNS), which includes neurons and glial cells. As essential components of neural cells, lipids data allows us to understand the human brain and its associated diseases.
In this article, we discuss the lipid profiles of neurons and glial cells: oligodendrocytes, astrocytes, and microglia. We further talk about the influence of lipid metabolism on myelination and remyelination. Finally, we look closer at the role of lipids in particular diseases. We investigate the lipid metabolism in multiple sclerosis, Alzheimer’s disease, and Parkinson’s disease.
Neurons and glial cells. Lipid profiles.
The CNS comprises electrically excitable neurons and supportive glial cells. Glial cells include oligodendrocytes, astrocytes, and microglia. In CNS, oligodendrocytes are responsible for generating myelin, which covers neuronal axons. Astrocytes process neurotransmitters, shape synaptic circuits, and support the blood-brain barrier. Microglia are key to immune responses and brain homeostasis, significantly contributing to both myelination and remyelination. Myelination enhances the speed of nerve impulse transmission, while remyelination facilitates the repair of damaged myelin sheaths in the CNS.
Lipids and lipid metabolism are crucial for the structure and function of brain tissue, directly impacting brain health. Specific lipid classes and individual lipid species are more dominant in particular cell types. For instance, neurons and myelin have high cholesterol levels, while galactosylceramides predominantly accumulate in oligodendrocyte membranes during myelination. Different lipids are essential for the specific functions of various neural cells, although many of these relationships are still not fully understood.
Researchers from the German Center for Neurodegenerative Diseases created a detailed lipidomic profile of the mouse brain to establish a foundational dataset for further neurobiological studies. They used lipidomics analysis to examine the lipid composition of neurons and various glial cell types.
Lipid class composition of mouse central nervous system cell types. Average distribution of the 20 most abundant lipid classes in primary cell cultures of neurons, oligodendrocytes, microglia, and astrocytes.
D Fitzner et al., Cell Reports (2020), 10.1016/j.celrep.2020.108132
CNS neurons’ lipid profiles were characterized by elevated levels of phosphatidylcholine, phosphatidylethanolamine, and cholesterol. Oligodendrocytes exhibited a distinct lipid profile due to their role in myelin production, enriched with sulfatides and hexosylceramides, which are essential components of myelin. Microglia contained high levels of phosphatidylglycerols and specific sphingomyelins almost absent in neurons and oligodendrocytes, while astrocytes had increased amounts of phosphatidylserine, phosphatidylinositol, and diacylglycerols.
The research explored whether each neural cell type utilizes specific lipid metabolism pathways. By integrating newly identified lipid profiles of neurons and glial cells with existing proteomics data, unique lipid metabolism profiles were revealed for each cell type. This deeper understanding of the brain’s cellular processes and lipid involvement can help researchers understand better critical functions such as neurotransmitter release, cellular signaling, and the maintenance of the blood-brain barrier.
Ganglioside lipidomics in CNS developmental myelination
Ganglioside composition changes during myelination is not well understood. To investigate this, researchers, including scientists from Lipotype, analyzed the ganglioside lipidome in mice during CNS development and adulthood. They also examined the ganglioside content in mice lacking genes necessary for synthesizing most ganglioside species.
Gangliosides are structurally diverse and their synthesis is a regulated and sequential process. It begins with the production of glycosphingolipid precursors in the endoplasmic reticulum, followed by the glycosylation of ceramide in the Golgi apparatus. This process produces glucosylceramide (GlcCer), which is then converted into lactosylceramide (LacCer), serving as a precursor for complex glycosphingolipid synthesis.
Synthesis of gangliosides.Gangliosides are synthesized from ceramide through the addition of sugar groups and sialic acid as shown on the pathway scheme. Ganlioside synthesis is impaired in mice lacking genes responsible for synthesizing most ganglioside species. These species are highlighted in grey. The main brain gangliosides are highlighted in red.
Arends et al., iScience 2022, 25 (11), 105323, 10.1016/j.isci.2022.105323
Scientists developed a mass spectrometry technique that allows for the quantification of gangliosides in small tissue samples during CNS myelination. Traditional methods, such as liquid chromatography-mass spectrometry (LC-MS), are effective for quantifying gangliosides but require larger samples and extensive extraction procedures. To assess changes in lipid composition during CNS developmental myelination, they examined mouse brains at various stages: early myelination, ongoing myelination nearing completion, and adulthood (6 months), when myelination is fully developed.
Using their new method, the researchers identified and quantified ganglioside levels throughout developmental myelination in mouse brain, finding the highest quantities in GD1 species, followed by GT1. Lower levels were noted for GM1 and GQ1, with GD3, GT2, GD2, and GM3 being the least abundant. Less complex ganglioside species like GD2, GD3, and GT2 were notably present during the early and nearly complete myelination stages. More complex species, such as GD1 and GT1, primarily appeared in adult samples. The most abundant gangliosides identified were GD1 36:1; 2 and GT1 36:1; 2 at the early myelination stage and adulthood, while GD1 36:2; 2 was prominent during early myelination.
The alteration in ganglioside content within the murine brain during developmental stages. The composition of gangliosides in brain during early myelination, nearly complete myelination, and adult stages. A sample size of 4 for each age group was used. All data are presented as mean ± standard deviation.
Arends et al., iScience 2022, 25 (11), 105323, 10.1016/j.isci.2022.105323
Further analysis of CNS development focused on the lipid composition of the optic nerve at various developmental stages. This tissue choice, primarily composed of axons and oligodendrocyte lineage cells, allowed researchers to narrow down the gangliosides specifically involved in axo-glial interactions. Additionally, they measured ganglioside content in mice lacking genes responsible for synthesizing most ganglioside species, confirming a nearly total absence of gangliosides in their brains. Surprisingly, the overall lipid composition in these mice was similar to that of control mice.
Overall, the researchers developed and applied a quantitative mass spectrometry-based shotgun lipidomics method designed for targeted ganglioside analysis in small samples like optic nerve or other CNS samples. The compatibility of this shotgun approach with broader global lipidomic profiling techniques marks a significant technical advancement.
Lipid metabolism impacts remyelination
Remyelination is a regenerative process that can occur following damage to the central nervous system. However, in conditions such as multiple sclerosis and Alzheimer’s disease, remyelination is often not functioning correctly during particular stages of the disease.
As mentioned before, microglia serve as the innate immune cells of the brain and spinal cord, playing a crucial role in the inflammatory response and in the repair of myelin following demyelinating injuries. Understanding the mechanisms of remyelination and the remodelling of lipid composition is vital for developing interventions aimed at reconstructing the myelin sheath. When the CNS is injured, myelin debris is produced from the myelin sheath that surrounds axons. This debris includes fragments of myelin membranes, cellular debris, and various lipids such as cholesterol, phospholipids, ceramides, and other sphingolipids.
Cholesterol cannot be degraded by lysosomal enzymes during the clearance of myelin debris. Instead, cholesterol is transferred from late endosomes to the endoplasmic reticulum (ER). The ER is unable to manage excessive free cholesterol, leading to its esterification into cholesteryl esters, which are then stored in lipid droplets. Exposure to cholesterol is crucial for microglial macrophages to effectively uptake myelin debris. When the process of lipid droplet formation is impaired, microglia struggle to resolve demyelinating lesions, which disrupts the regenerative response necessary for remyelination.
Gouna and Simons explored the function of the triggering receptor expressed on myeloid cells 2 (TREM2) within the metabolic pathways that is necessary for remyelination. Specifically, they focused on the clearance of myelin debris in cell cultures, mouse brain, and spinal tissue. Their lipidomic analysis indicated that phagocytes lacking TREM2 not only failed to produce cholesterol esters but also exhibited a reduction in triacylglycerol synthesis. Moreover, the accumulation of phosphatidic acid in TREM2-deficient phagocytes suggests a decrease in the activity of lipin phosphatidic acid phosphatase, which main function is to catalyze the conversion of phosphatidic acid into diacylglycerol. Lipin is unique among TAG-synthesizing enzymes, as its activity is regulated by its movement between the ER and the cytosol.
Comparison of lipid profiles in TREM2-KO and WT cells and ER stress presence: A Untargeted lipidomics analysis of WT and TREM2 KO cells cultured in serum-free media or with myelin for 8 or 24 h. B and C Analysis of ER stress by quantification of phosphorylated eIF2α (B) and phosphorylated JNK (C), measured by Western blot. **, P < 0.01; ***, P < 0.001.
Gouna et al., Journal of Experimental Medicine (2021), doi:10.1084/jem.20210227
TREM2-deficient phagocytes can internalize myelin debris but do not gather the necessary metabolic responses for effective remyelination in CNS, leading to an accumulation of free cholesterol over time. This buildup exposes them to cholesterol-induced cellular stress, and the authors noted signs of endoplasmic reticulum (ER) stress in these phagocytes loaded with myelin debris.
This study focuses on the role of phagocytes in clearing excess cholesterol resulting from demyelinating injuries of CNS. Processes like cholesterol efflux and esterification are crucial for resolving innate immune inflammation and supporting remyelination.
Lipases and Parkinson’s disease
Parkinson’s disease and other synucleinopathies are characterized by the accumulation of a protein called α-Synuclein, which forms abnormal structures known as Lewy bodies and Lewy neurites. Lewy bodies are clusters of aggregated α-Synuclein found within nerve cells, and Lewy neurites are abnormal neurites containing granular material and α-Synuclein filaments. Both of these structures are associated with synucleinopathies, including Parkinson’s disease and dementia with Lewy bodies.
The small protein α-Synuclein is present in nerve cells, playing a role in both normal cellular functions and disease processes. It interacts with lipids in cell membranes and can disrupt lipid balance within cells. Excess production of α-Synuclein leads to the lipid droplets formation, contributing to the development of Lewy bodies. Alterations in lipid droplets have been linked to the toxicity associated with α-Synuclein and impairments in cellular transport mechanisms. Additionally, mutations in genes related to lipid and fatty acid metabolism have been linked to an increased risk of Parkinson’s disease.
Fanning and Selkoe have identified a promising therapeutic target in the form of a triacylglycerol lipase known as LIPE, which influences lipid metabolism by breaking down TAGs. They discovered that LIPE is involved in regulating the levels of unsaturated fatty acids incorporated into phospholipids. This regulation is significant because alterations in lipid composition can affect how α-Synuclein interacts with cell membranes. Modifying these interactions through targeting LIPE may suggest a new approach for treating Parkinson’s disease.
Both genetic and pharmacological reductions in LIPE’s metabolic activity have been shown to decrease the abnormal accumulation of α-Synuclein in cells and lessen the unfolded protein response. Reduced lipase activity correlates with lower levels of particular fatty acids, including 18:1n9, 16:1n9, 16:0, and 18:1n7. Notably, the decrease in 18:1n9 is particularly significant, as it is a highly abundant fatty acid.
Regulation of fatty acid composition with LIPE inhibition. Upper image: LIPE knockdown decreases monounsaturated FAs. Lower image: LIPE pharmacological inhibition decreases monounsaturated FAs in α-Synuclein-expressing cells. Red and blue heatmap is a representation of a given FA species relative amount. Saturated/unsaturated status indicated by white/black squares.
Fanning et al., npj Parkinson’s Disease (2022) 74, 10.1038/s41531-022-00335-6.
The findings from both genetic and pharmacological experiments indicate that reducing the LIPE activity can lower levels of unsaturated fatty acids and reduce the accumulation of α-Synuclein protein in brain cells. These results suggest that slowing down the lipid degradation process by targeting LIPE may be a promising approach for developing new therapies for Parkinson’s disease.
Changes in membrane composition can influence α-Synuclein interactions with membranes, leading to protein aggregation and cell toxicity. This study suggests that LIPE can be a potential therapeutic target for synucleinopathies. LIPE plays a key role in regulating the levels of unsaturated fatty acids in phospholipids, which are crucial for α-Synuclein’s interaction with membranes.
Inhibiting LIPE activity was found to reduce α-Synuclein accumulation in membrane-rich cytoplasmic inclusions and lower levels of Parkinson’s disease-associated phosphorylated and insoluble α-Synuclein. These findings suggest that targeting fatty acid metabolism, particularly by modulating phospholipid-incorporated fatty acids, offers a promising therapeutic approach for treating synucleinopathies and can offer insights into other CNS diseases.
Lipid biomarkers for multiple sclerosis
Multiple sclerosis (MS) is a chronic inflammatory and neurodegenerative CNS disease in which the immune system attacks the myelin sheath that protects neurons in the brain and spinal cord. This myelin damage disrupts communication between the central nervous system and the rest of the body, leading to various neurological symptoms. While there is no cure for MS, treatments are available to help manage symptoms and reduce the frequency of relapses.
Research has shown that lipid metabolism plays a crucial role in MS, as lipids are essential for the structure, function, and health of neural cells. Changes in lipid metabolism have been observed in MS patients, suggesting that lipids may directly or indirectly influence the progression of the disease. Detailed molecular analysis of lipids could help identify specific biomarkers, leading to faster diagnosis, improved treatment options, and more effective disease monitoring.
Changes in the lipid profile are a feature of MS development and progression. Disruptions in lipid metabolism can affect myelin integrity and contribute to neurodegeneration in MS. Researchers from the German Center for Neurodegenerative Diseases conducted a lipidomics analysis of blood plasma samples from 73 monozygotic twins, where only one twin had MS. They analyzed 243 distinct lipids in blood plasma for further investigation into lipid changes associated with the disease to suggest novel disease biomarkers.
Altered lipid classes and species in monozygotic twins discordant for multiple sclerosis. A On the lipid class level, PC O- (***) and PE O- (**) are significantly decreased in twins with MS (dark blue = healthy twin, light blue = twin with MS). B Number of significantly altered lipid species (dark green) as parts of the whole corresponding lipid class.
Horst Penkert et al., Annals of Clinical and Translational Neurology (2020), 10.1002/acn3.51216
The study found no significant differences in total lipid content between the twin with multiple sclerosis (MS) and the healthy twin. On a molecular level, however, twins with MS had reduced levels of ether phosphatidylcholine (PC O-) and ether phosphatidylethanolamine (PE O-) lipids, with the most affected lipids being PC O- containing long polyunsaturated fatty acids (PUFAs). These lipids could potentially serve as blood plasma biomarkers for MS. Ether lipids can bind to a transcription factor that regulates genes involved in metabolism and anti-inflammatory functions, suggesting altered lipid signaling in MS. Importantly, this lipid profile alteration was observed even in patients receiving and not receiving treatment, confirming the lipid changes were disease-related rather than treatment-induced.
The high variability in MS between individuals makes predicting disease progression difficult, highlighting the need for accessible biomarkers. Identifying specific lipid profiles as biomarkers for MS could improve early diagnosis, help monitor disease progression, and enhance treatment strategies. Lipid biomarkers could not only aid in a more reliable diagnosis but also in tracking disease development and treatment responses in neurodegenerative and neuroinflammatory conditions.
Multiomics profiling of Alzheimer’s disease
Alzheimer’s disease is characterized by the gradual degeneration of neurons, resulting in disrupted communication between them. Though it is often linked to aging, the exact cause of Alzheimer’s is still unknown. Alzheimer’s disease patients memory loss, difficulty in thinking and decision-making, and changes in behavior.
Nowadays, Alzheimer’s disease is managed with pharmaceutical support and behavioral interventions, but there is no proper cure. One potential target for drug therapy is the tau protein, which forms disruptive fibrils in the brains of Alzheimer’s patients. A deeper understanding of the molecular pathways involved in the disease could lead to improved diagnostics and therapies. Multiomics technologies, which analyze proteins, lipids, and other molecules in tissue samples, are useful for identifying these pathways in CNS by comparing molecular changes in both healthy and diseased states.
Scientists from the Nestlé Institute of Health Sciences applied multiomics and statistical analysis to study cerebrospinal fluid from 120 participants with Alzheimer’s disease, aged 55 and older, over a period of 36 months. Participants were grouped into a control, group of patients with mild cognitive dysfunction, and patients with Alzheimer’s disease, based on psychiatric assessments and ELISA assays for key Alzheimer’s biomarkers like beta-amyloid 1-42 (Aβ1-42), tau, and phosphorylated tau (P-tau).
The multiomics approach, which included proteomics, metabolomics, and lipidomics, identified 82 molecules associated with Alzheimer’s biomarkers and revealed 37 metabolites, 29 proteins, and 5 lipids in cerebrospinal fluid correlated with psychiatric evaluations, offering new insights into the molecular basis of this CNS disease.
Multiomics Factor Analysis. Overview of the trained Multiomics Factor Analysis model showing variance (R2) within the cohort explained by A each analytical approach and B latent factors.
Clark et al., Alz Res Therapy (2021), 10.1186/s13195-021-00814-7
To reduce the complexity of the multiomics data, the researchers applied multiomics factor analysis, which identified five latent factors that explained the observed variance between participants. Each molecule group accounted for portions of this variance. Latent factors 1 and 2 (LF1 and LF2) appeared across most multiomics analyses, and factors 3, 4, and 5 (LF3, LF4, and LF5) contributed less overall but captured variance in specific datasets. Alzheimer’s biomarkers accounted for 38.5% of the variance, proteins 39.8%, lipids and neuroinflammation markers each for 10.3%, one-carbon metabolites for 9%, and other metabolites for 3.7%. Each latent factor had unique correlations with both multiomics analytes and Alzheimer’s biomarkers, suggesting they may represent distinct molecular pathways.
The study also derived two molecular signatures — each consisting of four molecules — that improved predictive models for Alzheimer’s disease and cognitive decline, underscoring the role of lipid pathway alterations in the disease. This study highlights how multiomics analyses including lipidomics analysis can reveal underlying cellular and molecular pathways of CNS diseases, improving the prediction and understanding of Alzheimer’s pathology.
How can lipidomics be used in the studies of the central nervous system?
Lipidomics analysis is crucial in central and peripheral nervous system research, helping to understand the role of lipids in brain structure, functions, and its changes during neurodegenerative diseases. As discussed in this article, lipidomics analysis also plays a vital role in advancing our understanding of lipid profiles in neurons and glial cells, particularly oligodendrocytes, astrocytes, and microglia. This approach is essential for studying lipid metabolism’s influence on myelination and remyelination processes, as well as its involvement in neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and multiple sclerosis. By identifying lipid imbalances and specific lipid classes and species linked to disease progression, lipidomics provides knowledge that can help in developing targeted therapies and improving disease management.
Lipotype Lipidomics is a powerful tool for studying lipids, offering detailed insights into the complex lipid composition of the brain and the role of lipid metabolism in neurodegenerative diseases. Lipidomics can identify alterations in lipid pathways that may contribute to disease processes, such as those seen in Alzheimer’s disease, Parkinson’s disease, and multiple sclerosis.
