FTY720 decreases ceramides levels in the brain and prevents memory impairments in a mouse model of familial Alzheimer's disease expressing APOE4

The protection mediated by the bioactive sphingolipid sphingosine-1-phosphate (S1P) declines during Alzheimer's disease (AD) progression, especially in patients carrying the apolipoprotein E ε4 (APOE4) isoform. The drug FTY720 mimics S1P bioactivity, but its efficacy in treating AD is unclear. Two doses of FTY720 (0.1 mg / kg and 0.5 mg / kg daily) were given by oral gavage for 15 weeks to transgenic mouse models of familial AD carrying human apolipoprotein E (APOE) APOE3 (E3FAD) or APOE4 (E4FAD). After 12 weeks of treatment, animals were subjected to behavioral tests for memory, locomotion, and anxiety. Blood was withdrawn at different time points and brains were collected for sphingolipids analysis by mass spectrometry, gene expression by RT-PCR and Aβ quantification by ELISA. We discovered that low levels of S1P in the plasma is associated with a higher probability of failing the memory test and that FTY720 prevents memory impairments in E4FAD. The beneficial effect of FTY720 was induced by a shift of the sphingolipid metabolism in the brain towards a lower production of toxic metabolites, like ceramide d18:1/16:0 and d18:1/22:0, and reduction of amyloid-β burden and inflammation. In conclusion, we provide further evidence of the druggability of the sphingolipid system in AD.

Function of ceramide transfer protein for biogenesis and sphingolipid composition of extracellular vesicles

The formation of extracellular vesicles (EVs) is induced by the sphingolipid ceramide. How this pathway is regulated is not entirely understood. Here, we report that the ceramide transport protein (CERT) mediates a non-vesicular transport of ceramide between the endoplasmic reticulum (ER) and the multivesicular endosome at contact sites. The process depends on the interaction of CERT's PH domain with PI4P generated by PI4KIIα at endosomes. Furthermore, a complex is formed between the START domain of CERT, which carries ceramide, and the Tsg101 protein, which is part of the endosomal sorting complex required for transport (ESCRT-I). Inhibition of ceramide biosynthesis reduces CERT-Tsg101 complex formation. Overexpression of CERT increases EV secretion while its inhibition reduces EV formation and the concentration of ceramides and sphingomyelins in EVs. In conclusion, we discovered a function of CERT in regulating the sphingolipid composition and biogenesis of EVs, which links ceramide to the ESCRT-dependent pathway.

CERTL reduces C16 ceramide, amyloid-β levels, and inflammation in a model of Alzheimer's disease

BACKGROUND

Dysregulation of ceramide and sphingomyelin levels have been suggested to contribute to the pathogenesis of Alzheimer's disease (AD). Ceramide transfer proteins (CERTs) are ceramide carriers which are crucial for ceramide and sphingomyelin balance in cells. Extracellular forms of CERTs co-localize with amyloid-β (Aβ) plaques in AD brains. To date, the significance of these observations for the pathophysiology of AD remains uncertain.

METHODS

A plasmid expressing CERTL, the long isoform of CERTs, was used to study the interaction of CERTL with amyloid precursor protein (APP) by co-immunoprecipitation and immunofluorescence in HEK cells. The recombinant CERTL protein was employed to study interaction of CERTL with amyloid-β (Aβ), Aβ aggregation process in presence of CERTL, and the resulting changes in Aβ toxicity in neuroblastoma cells. CERTL was overexpressed in neurons by adeno-associated virus (AAV) in a mouse model of familial AD (5xFAD). Ten weeks after transduction, animals were challenged with behavior tests for memory, anxiety, and locomotion. At week 12, brains were investigated for sphingolipid levels by mass spectrometry, plaques, and neuroinflammation by immunohistochemistry, gene expression, and/or immunoassay.

RESULTS

Here, we report that CERTL binds to APP, modifies Aβ aggregation, and reduces Aβ neurotoxicity in vitro. Furthermore, we show that intracortical injection of AAV, mediating the expression of CERTL, decreases levels of ceramide d18:1/16:0 and increases sphingomyelin levels in the brain of male 5xFAD mice. CERTL in vivo over-expression has a mild effect on animal locomotion, decreases Aβ formation, and modulates microglia by decreasing their pro-inflammatory phenotype.

CONCLUSION

Our results demonstrate a crucial role of CERTL in regulating ceramide levels in the brain, in amyloid plaque formation and neuroinflammation, thereby opening research avenues for therapeutic targets of AD and other neurodegenerative diseases.

Altered sphingolipid function in Alzheimer's disease; a gene regulatory network approach

Sphingolipids (SLs) are bioactive lipids involved in various important physiological functions. The SL pathway has been shown to be affected in several brain-related disorders, including Alzheimer's disease (AD). Recent evidence suggests that epigenetic dysregulation plays an important role in the pathogenesis of AD as well. Here, we use an integrative approach to better understand the relationship between epigenetic and transcriptomic processes in regulating SL function in the middle temporal gyrus of AD patients. Transcriptomic analysis of 252 SL-related genes, selected based on GO term annotations, from 46 AD patients and 32 healthy age-matched controls, revealed 103 differentially expressed SL-related genes in AD patients. Additionally, methylomic analysis of the same subjects revealed parallel hydroxymethylation changes in PTGIS, GBA, and ITGB2 in AD. Subsequent gene regulatory network-based analysis identified 3 candidate genes, that is, SELPLG, SPHK1 and CAV1 whose alteration holds the potential to revert the gene expression program from a diseased towards a healthy state. Together, this epigenomic and transcriptomic approach highlights the importance of SL-related genes in AD, and may provide novel biomarkers and therapeutic alternatives to traditionally investigated biological pathways in AD.

Immunofluorescence Labeling of Lipid-Binding Proteins CERTs to Monitor Lipid Raft Dynamics

Fluorescence microscopy is a powerful and widely used tool in molecular biology. Over the years, the discovery and development of lipid-binding fluorescent probes has established new research possibilities to investigate lipid composition and dynamics in the cell. For instance, fluorescence microscopy has allowed the investigation of lipid localization and density in specific cell compartments such as membranes or organelles. Often, the characteristics and the composition of lipid-enriched structures are determined by analyzing the distribution of a fluorescently labeled lipid probe, which intercalates in lipid-enriched platforms, or specifically binds to parts of the lipid molecule. However, in many cases antibodies targeting proteins have higher specificity and are easier to generate. Therefore, we propose to use both antibodies targeting lipid transporters and lipid binding probes to better monitor lipid membrane changes. As an example, we visualize lipid rafts using the fluorescently labeled-B-subunit of the cholera toxin in combination with antibodies targeting ceramide-binding proteins CERTs, central molecules in the metabolism of sphingolipids.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.