Impact of 7-ketocholesterol and very long chain fatty acids on oligodendrocyte lipid membrane organization: Evaluation via LAURDAN and FAMIS spectral image analysis

Crosstalk between Lipid Rafts and Aging: New Frontiers for Delaying Aging

With the rapid aging in the global population, delay of aging has become a hot research topic. Lipid rafts (LRs) are microdomains in the plasma membrane that contain sphingolipids and cholesterol. Emerging evidence indicates an interesting interplay between LRs and aging. LRs and their components are altered with aging. Further, the aging process is strongly influenced by LRs. In recent years, LRs and their component signaling molecules have been recognized to affect aging by interfering with its hallmarks. Therefore, targeting LRs is a promising strategy to delay aging.

Different effects of oxysterols on a model lipid raft - Langmuir monolayer study complemented with theoretical calculations

Three oxysterols (7β-hydroxycholesterol; 7β-OH, 7-ketocholesterol; 7-K and 25-hydroxycholesterol, 25-OH) differing in the site of oxidation (ring system versus chain) and kind of polar group (hydroxyl versus carbonyl) were studied in lipid raft environment using the Langmuir monolayer technique complemented with theoretical calculations. Experiments were performed for the unmodified raft system, composed of sphingomyelin (SM) and cholesterol (Chol), and in the next step the raft was modified by the incorporation of oxysterol in different proportions. In the examined three-component system (Chol:SM:oxysterol), apart from interactions between the lipid raft components, the affinity of Chol to its oxidized derivatives also plays an important role. 25-OH was found to enhance interactions between SM and Chol and thus stabilize the raft, contrary to 7β-OH and 7-K, which exterted the fluidizing effect as well as the destabilization of the raft. Different action of oxysterols on model raft was observed. 7β-OH and 7-K, which are highly potent inducers of cell dath caused raft destabilization, while 25-OH, which is the least toxic of the investigated oxysterols, was found to stabilize the raft.

Plasma Membrane Lipids: An Important Binding Site for All Lipoprotein Classes

Cholesterol is one of the main constituents of plasma membranes; thus, its supply is of utmost importance. This review covers the known mechanisms of cholesterol transfer from circulating lipoprotein particles to the plasma membrane, and vice versa. To achieve homeostasis, the human body utilizes cellular de novo synthesis and extracellular transport particles for supply of cholesterol and other lipids via the blood stream. These lipoprotein particles can be classified according to their density: chylomicrons, very low, low, and high-density lipoprotein (VLDL, LDL, and HDL, respectively). They deliver and receive their lipid loads, most importantly cholesterol, to and from cells by several redundant routes. Defects in one of these pathways (e.g., due to mutations in receptors) usually are not immediately fatal. Several redundant pathways, at least temporarily, compensate for the loss of one or more of them, but the defects trigger systemic diseases, such as atherosclerosis later on. Recently, intracellular membrane–membrane contact sites were shown to be involved in intracellular cholesterol transfer and the plasma membrane itself has been proposed to act as a binding site for lipoprotein-mediated cargo unloading.

Prevention by Dietary Polyphenols (Resveratrol, Quercetin, Apigenin) Against 7-Ketocholesterol-Induced Oxiapoptophagy in Neuronal N2a Cells: Potential Interest for the Treatment of Neurodegenerative and Age-Related Diseases

The Mediterranean diet is associated with health benefits due to bioactive compounds such as polyphenols. The biological activities of three polyphenols (quercetin (QCT), resveratrol (RSV), apigenin (API)) were evaluated in mouse neuronal N2a cells in the presence of 7-ketocholesterol (7KC), a major cholesterol oxidation product increased in patients with age-related diseases, including neurodegenerative disorders. In N2a cells, 7KC (50 µM; 48 h) induces cytotoxic effects characterized by an induction of cell death. When associated with RSV, QCT and API (3.125; 6.25 µM), 7KC-induced toxicity was reduced. The ability of QCT, RSV and API to prevent 7KC-induced oxidative stress was characterized by a decrease in reactive oxygen species (ROS) production in whole cells and at the mitochondrial level; by an attenuation of the increase in the level and activity of catalase; by attenuating the decrease in the expression, level and activity of glutathione peroxidase 1 (GPx1); by normalizing the expression, level and activity of superoxide dismutases 1 and 2 (SOD1, SOD2); and by reducing the decrease in the expression of nuclear erythroid 2-like factor 2 (Nrf2) which regulates antioxidant genes. QCT, RSV and API also prevented mitochondrial dysfunction in 7KC-treated cells by counteracting the loss of mitochondrial membrane potential (ΨΔm) and attenuating the decreased gene expression and/or protein level of AMP-activated protein kinase α (AMPKα), sirtuin 1 (SIRT1) and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) implicated in mitochondrial biogenesis. At the peroxisomal level, QCT, RSV and API prevented the impact of 7KC by counteracting the decrease in ATP binding cassette subfamily D member (ABCD)3 (a peroxisomal mass marker) at the protein and mRNA levels, as well as the decreased expresssion of genes associated with peroxisomal biogenesis (Pex13, Pex14) and peroxisomal β-oxidation (Abcd1, Acox1, Mfp2, Thiolase A). The 7KC-induced decrease in ABCD1 and multifunctional enzyme type 2 (MFP2), two proteins involved in peroxisomal β-oxidation, was also attenuated by RSV, QCT and API. 7KC-induced cell death, which has characteristics of apoptosis (cells with fragmented and/or condensed nuclei; cleaved caspase-3; Poly(ADP-ribose) polymerase (PARP) fragmentation) and autophagy (cells with monodansyl cadaverine positive vacuoles; activation of microtubule associated protein 1 light chain 3–I (LC3-I) to LC3-II, was also strongly attenuated by RSV, QCT and API. Thus, in N2a cells, 7KC induces a mode of cell death by oxiapoptophagy, including criteria of OXIdative stress, APOPTOsis and autoPHAGY, associated with mitochondrial and peroxisomal dysfunction, which is counteracted by RSV, QCT, and API reinforcing the interest for these polyphenols in prevention of diseases associated with increased 7KC levels.

Mitochondrial dysfunctions in 7-ketocholesterol-treated 158N oligodendrocytes without or with α-tocopherol: Impacts on the cellular profil of tricarboxylic cycle-associated organic acids, long chain saturated and unsaturated fatty acids, oxysterols, cholesterol and cholesterol precursors

In multiple sclerosis (MS) a process of white matter degradation leading to demyelination is observed. Oxidative stress, inflammation, apoptosis, necrosis and/or autophagy result together into a progressive loss of oligodendrocytes. 7-ketocholesterol (7KC), found increased in the cerebrospinal fluid of MS patients, triggers a rupture of RedOx homeostasis associated with mitochondrial dysfunctions, aptoptosis and autophagy (oxiapoptophagy) in cultured murine oligodendrocytes (158N). α-tocopherol is able to mild the alterations induced by 7KC partially restoring the cellular homeostasis. In presence of 7KC, the amount of adherent 158N cells was decreased and oxidative stress was enhanced. An increase of caspase-3 and PARP degradation (evidences of apoptosis), and an increased LC3-II/LC3-I ratio (criterion of autophagy), were detected. These events were associated with a decrease of the mitochondrial membrane potential (ΔΨ_m) and by a decrease of oxidative phosphorylation revealed by reduced NAD⁺ and ATP. The cellular lactate was higher while pyruvate, citrate, fumarate, succinate (tricarboxylic acid (TCA) cycle intermediates) were significantly reduced in exposed cells, suggesting that an impairment of mitochondrial respiratory functions could lead to an increase of lactate production and to a reduced amount of ATP and acetyl-CoA available for the anabolic pathways. The concentration of sterol precursors lathosterol, lanosterol and desmosterol were significantly reduced together with satured and unsatured long chain fatty acids (C16:0 - C18:0, structural elements of membrane phospholipids). Such reductions were milder with α-tocopherol. It is likely that the cell death induced by 7KC is associated with mitochondrial dysfunctions, including alterations of oxidative phosphorylation, which could result from lipid anabolism dysfunctions, especially on TCA cycle intermediates. A better knowledge of mitochondrial associated dysfunctions triggered by 7KC will contribute to bring new information on the demyelination processes which are linked with oxidative stress and lipid peroxidation, especially in MS.

Laurdan monitors different lipids content in eukaryotic membrane during embryonic neural development

We describe a method based on fluorescence-lifetime imaging microscopy (FLIM) to assess the fluidity of various membranes in neuronal cells at different stages of development [day 12 (E12) and day 16 (E16) of gestation]. For the FLIM measurements, we use the Laurdan probe which is commonly used to assess membrane water penetration in model and in biological membranes using spectral information. Using the FLIM approach, we build a fluidity scale based on calibration with model systems of different lipid compositions. In neuronal cells, we found a marked difference in fluidity between the internal membranes and the plasma membrane, being the plasma membrane the less fluid. However, we found no significant differences between the two cell groups, E12 and E16. Comparison with NIH3T3 cells shows that the plasma membranes of E12 and E16 cells are significantly more fluid than the plasma membrane of the cancer cells.

Oxidative stress modulates mitochondrial failure and cyclophilin D function in X-linked adrenoleukodystrophy

A common process associated with oxidative stress and severe mitochondrial impairment is the opening of the mitochondrial permeability transition pore, as described in many neurodegenerative diseases. Thus, inhibition of mitochondrial permeability transition pore opening represents a potential target for inhibiting mitochondrial-driven cell death. Among the mitochondrial permeability transition pore components, cyclophilin D is the most studied and has been found increased under pathological conditions. Here, we have used in vitro and in vivo models of X-linked adrenoleukodystrophy to investigate the relationship between the mitochondrial permeability transition pore opening and redox homeostasis. X-linked adrenoleukodystrophy is a neurodegenerative condition caused by loss of function of the peroxisomal ABCD1 transporter, in which oxidative stress plays a pivotal role. In this study, we provide evidence of impaired mitochondrial metabolism in a peroxisomal disease, as fibroblasts in patients with X-linked adrenoleukodystrophy cannot survive when forced to rely on mitochondrial energy production, i.e. on incubation in galactose. Oxidative stress induced under galactose conditions leads to mitochondrial damage in the form of mitochondrial inner membrane potential dissipation, ATP drop and necrotic cell death, together with increased levels of oxidative modifications in cyclophilin D protein. Moreover, we show increased expression levels of cyclophilin D in the affected zones of brains in patients with adrenomyeloneuropathy, in spinal cord of a mouse model of X-linked adrenoleukodystrophy (Abcd1-null mice) and in fibroblasts from patients with X-linked adrenoleukodystrophy. Notably, treatment with antioxidants rescues mitochondrial damage markers in fibroblasts from patients with X-linked adrenoleukodystrophy, including cyclophilin D oxidative modifications, and reverses cyclophilin D induction in vitro and in vivo. These findings provide mechanistic insight into the beneficial effects of antioxidants in neurodegenerative and non-neurodegenerative cyclophilin D-dependent disorders.

The Relationship between fenestrations, sieve plates and rafts in liver sinusoidal endothelial cells

Fenestrations are transcellular pores in endothelial cells that facilitate transfer of substrates between blood and the extravascular compartment. In order to understand the regulation and formation of fenestrations, the relationship between membrane rafts and fenestrations was investigated in liver sinusoidal endothelial cells where fenestrations are grouped into sieve plates. Three dimensional structured illumination microscopy, scanning electron microscopy, internal reflectance fluorescence microscopy and two-photon fluorescence microscopy were used to study liver sinusoidal endothelial cells isolated from mice. There was an inverse distribution between sieve plates and membrane rafts visualized by structured illumination microscopy and the fluorescent raft stain, Bodipy FL C5 ganglioside GM1. 7-ketocholesterol and/or cytochalasin D increased both fenestrations and lipid-disordered membrane, while Triton X-100 decreased both fenestrations and lipid-disordered membrane. The effects of cytochalasin D on fenestrations were abrogated by co-administration of Triton X-100, suggesting that actin disruption increases fenestrations by its effects on membrane rafts. Vascular endothelial growth factor (VEGF) depleted lipid-ordered membrane and increased fenestrations. The results are consistent with a sieve-raft interaction, where fenestrations form in non-raft lipid-disordered regions of endothelial cells once the membrane-stabilizing effects of actin cytoskeleton and membrane rafts are diminished.

Induction of mitochondrial changes associated with oxidative stress on very long chain fatty acids (C22:0, C24:0, or C26:0)-treated human neuronal cells (SK-NB-E)

In Alzheimer's disease, lipid alterations point towards peroxisomal dysfunctions. Indeed, a cortical accumulation of saturated very long chain fatty acids (VLCFAs: C22:0, C24:0, C26:0), substrates for peroxisomal β-oxidation, has been found in Alzheimer patients. This study was realized to investigate the effects of VLCFAs at the mitochondrial level since mitochondrial dysfunctions play crucial roles in neurodegeneration. On human neuronal SK-NB-E cells treated with C22:0, C24:0, or C26:0 (0.1-20 μM; 48 h), an inhibition of cell growth and mitochondrial dysfunctions were observed by cell counting with trypan blue, MTT assay, and measurement of mitochondrial transmembrane potential (Δψ(m)) with DiOC(6)(3). A stimulation of oxidative stress was observed with DHE and MitoSOX used to quantify superoxide anion production on whole cells and at the mitochondrial level, respectively. With C24:0 and C26:0, by Western blotting, lower levels of mitochondrial complexes III and IV were detected. After staining with MitoTracker and by transmission electron microscopy used to study mitochondrial topography, mass and morphology, major changes were detected in VLCFAs treated-cells: modification of the cytoplasmic distribution of mitochondria, presence of large mitochondria, enhancement of the mitochondrial mass. Thus, VLCFAs can be potential risk factors contributing to neurodegeneration by inducing neuronal damages via mitochondrial dysfunctions.

α-Tocopherol impairs 7-ketocholesterol-induced caspase-3-dependent apoptosis involving GSK-3 activation and Mcl-1 degradation on 158N murine oligodendrocytes

In important and severe neurodegenerative pathologies, 7-ketocholesterol, mainly resulting from cholesterol autoxidation, may contribute to dys- or demyelination processes. On various cell types, 7-ketocholesterol has often been shown to induce a complex mode of cell death by apoptosis associated with phospholipidosis. On 158N murine oligodendrocytes treated with 7-ketocholesterol (20 μg/mL corresponding to 50 μM, 24-48 h), the induction of a mode of cell death by apoptosis characterised by the occurrence of cells with condensed and/or fragmented nuclei, caspase activation (including caspase-3) and internucleosomal DNA fragmentation was observed. It was associated with a loss of transmembrane mitochondrial potential (ΔΨm) measured with JC-1, with a dephosphorylation of Akt and GSK3 (especially GSK3β), and with degradation of Mcl-1. With α-tocopherol (400 μM), which was capable of counteracting 7-ketocholesterol-induced apoptosis, Akt and GSK3β dephosphorylation were inhibited as well as Mcl-1 degradation. These data underline that the potential protective effects of α-tocopherol against 7-ketocholesterol-induced apoptosis do not depend on the cell line considered, and that the cascade of events (Akt/GSK3β/Mcl-1) constitutes a link between 7-ketocholesterol-induced cytoplasmic membrane dysfunctions and mitochondrial depolarisation leading to apoptosis.