My journey into the world of sphingolipids and sphingolipidoses

Analysis of lipid storage in postmortem brains of patients with amaurotic idiocy led to the recognition of five lysosomal ganglioside storage diseases and identification of their inherited metabolic blocks. Purification of lysosomal acid sphingomyelinase and ceramidase and analysis of their gene structures were the prerequisites for the clarification of Niemann-Pick and Farber disease. For lipid catabolism, intraendosomal vesicles are formed during the endocytotic pathway. They are subjected to lipid sorting processes and were identified as luminal platforms for cellular lipid and membrane degradation. Lipid binding glycoproteins solubilize lipids from these cholesterol poor membranes and present them to water-soluble hydrolases for digestion. Biosynthesis and intracellular trafficking of lysosomal hydrolases (hexosaminidases, acid sphingomyelinase and ceramidase) and lipid binding and transfer proteins (GM2 activator, saposins) were analyzed to identify the molecular and metabolic basis of several sphingolipidoses. Studies on the biosynthesis of glycosphingolipids yielded the scheme of Combinatorial Ganglioside Biosynthesis involving promiscuous glycosyltransferases. Their defects in mutagenized mice impair brain development and function.

Development of an assay for the intermembrane transfer of cholesterol by Niemann-Pick C2 protein

Niemann-Pick type C disease is an inherited fatal disorder characterized by the accumulation of unesterified cholesterol and other lipids in the endosomal/lysosomal compartment. Two independent genes responsible for this neurodegenerative disorder have been identified, but the precise functions of the encoded Niemann-Pick C1 (NPC1) and C2 (NPC2) proteins are not yet known. We developed a cell-free assay for measuring intermembrane lipid transport and examined the ability of bovine NPC2 (bNPC2) for intermembrane cholesterol transfer. NPC2 specifically extracts cholesterol from phospholipid bilayers and catalyzes intermembrane transfer to acceptor vesicles in a dose- and time-dependent manner. This transfer activity is dependent on temperature, pH, ionic strength, lipid composition of the model membranes, and the ratio of donor to acceptor vesicles. In model membranes, the presence of the lysosomal anionic phospholipids bis(monooleoylglycero)phosphate and phosphatidyl inositol significantly stimulated cholesterol transfer by NPC2, whereas bis(monomyristoylglycero)phosphate, phosphatidyl serine, and phosphatidic acid had no effect. Moreover, ceramide stimulated cholesterol transfer slightly, whereas sphingomyelin reduced cholesterol transfer rates. With our assay system we identified for the first time the ability of other lysosomal proteins, most notably the GM2-activator protein, to mediate intermembrane cholesterol transfer. This assay system promises to be a valuable tool for further quantitative and mechanistic studies of protein-mediated lipid transfer.

Increased cholesterol in Aβ-positive nerve terminals from Alzheimer's disease cortex

Synapse loss in Alzheimer's disease (AD) is poorly understood but evidence suggests it is a key pathological event. In order to precisely detect stable synaptic changes, we have developed methods for flow cytometry analysis of synaptosomes prepared from cryopreserved AD samples, and have previously shown that amyloid-beta (Abeta) accumulates in surviving presynaptic terminals in AD cortex. In the present experiments we have examined amyloid-containing terminals in more detail, first dual labeling synaptosomes from AD cortex for Abeta and a series of markers, and then using quadrant analysis to compare amyloid-positive and amyloid-negative terminals. Amyloid-positive synaptosomes were larger in size than amyloid-negatives (p<0.007), and significant increases were observed in mean fluorescence for the lipid raft markers cholesterol (27%; p<0.0005) and GM1 ganglioside (24%; p<0.005). SNAP-25 immunofluorescence was increased by 31% (p<0.0001) in amyloid-bearing terminals, consistent with a sprouting response to amyloid accumulation. These results suggest that Abeta accumulation in synaptic terminals may underly dysfunction prior to or independent of extracellular amyloid deposition.

Oxidative stress generated by hemorrhagic shock recruits Toll-like receptor 4 to the plasma membrane in macrophages

Oxidative stress generated by ischemia/reperfusion is known to prime inflammatory cells for increased responsiveness to subsequent stimuli, such as lipopolysaccharide (LPS). The mechanism(s) underlying this effect remains poorly elucidated. These studies show that alveolar macrophages recovered from rodents subjected to hemorrhagic shock/resuscitation expressed increased surface levels of Toll-like receptor 4 (TLR4), an effect inhibited by adding the antioxidant N-acetylcysteine to the resuscitation fluid. Consistent with a role for oxidative stress in this effect, in vitro H₂O₂ treatment of RAW 264.7 macrophages similarly caused an increase in surface TLR4. The H₂O₂-induced increase in surface TLR4 was prevented by depleting intracellular calcium or disrupting the cytoskeleton, suggesting the involvement of receptor exocytosis. Further, fluorescent resonance energy transfer between TLR4 and the raft marker GM1 as well as biochemical analysis of the raft components demonstrated that oxidative stress redistributes TLR4 to lipid rafts in the plasma membrane. Preventing the oxidant-induced movement of TLR4 to lipid rafts using methyl-β-cyclodextrin precluded the increased responsiveness of cells to LPS after H₂O₂ treatment. Collectively, these studies suggest a novel mechanism whereby oxidative stress might prime the responsiveness of cells of the innate immune system.

Amphotropic murine leukemia virus is preferentially attached to cholesterol-rich microdomains after binding to mouse fibroblasts

Background

We have recently shown that amphotropic murine leukemia virus (A-MLV) can enter the mouse fibroblast cell line NIH3T3 via caveola-dependent endocytosis. But due to the size and omega-like shape of caveolae it is possible that A-MLV initially binds cells outside of caveolae. Rafts have been suggested to be pre-caveolae and we here investigate whether A-MLV initially binds to its receptor Pit2, a sodium-dependent phosphate transporter, in rafts or caveolae or outside these cholesterol-rich microdomains.

Results

Here, we show that a high amount of cell-bound A-MLV was attached to large rafts of NIH3T3 at the time of investigation. These large rafts were not enriched in caveolin-1, a major structural component of caveolae. In addition, they are rather of natural occurrence in NIH3T3 cells than a result of patching of smaller rafts by A-MLV. Thus cells incubated in parallel with vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped MLV particles showed the same pattern of large rafts as cells incubated with A-MLV, but VSV-G pseudotyped MLV particles did not show any preference to attach to these large microdomains.

Conclusion

The high concentration of A-MLV particles bound to large rafts of NIH3T3 cells suggests a role of these microdomains in early A-MLV binding events.

The cholesterol-dependent cytolysin listeriolysin O aggregates rafts via oligomerization

The pore-forming toxin listeriolysin O (LLO) is the main virulence factor of Listeria monocytogenes. LLO is known to act as a pseudo cytokine/chemokine, which induces a broad spectrum of host responses that ultimately influences the outcome of listeriosis. In the present study we demonstrate that LLO is a potent aggregator of lipid rafts. LLO was found to aggregate the raft associated molecules GM1, the GPI-anchored proteins CD14 and CD16 as well as the tyrosine kinase Lyn. Abrogation of the cytolytic activity of LLO by cholesterol pretreatment was found not to interfere with LLO's ability to aggregate rafts or trigger tyrosine phosphorylation in cells. However, a monoclonal antibody that blocks the oligomerization of LLO was found to inhibit rafts' aggregation as well as the induction of tyrosine phosphorylation. This implies that rafts aggregation by LLO which is independent of cytolytic activity, is due to the oligomerization of its membrane bound toxin monomers. Thus, LLO most likely induces signalling through the coaggregation of rafts' associated receptors, kinases and adaptors.

Principles of lysosomal membrane digestion: stimulation of sphingolipid degradation by sphingolipid activator proteins and anionic lysosomal lipids

Sphingolipids and glycosphingolipids are membrane components of eukaryotic cell surfaces. Their constitutive degradation takes place on the surface of intra-endosomal and intra-lysosomal membrane structures. During endocytosis, these intra-lysosomal membranes are formed and prepared for digestion by a lipid-sorting process during which their cholesterol content decreases and the concentration of the negatively charged bis(monoacylglycero)phosphate (BMP)--erroneously also called lysobisphosphatidic acid (LBPA)--increases. Glycosphingolipid degradation requires the presence of water-soluble acid exohydrolases, sphingolipid activator proteins, and anionic phospholipids like BMP. The lysosomal degradation of sphingolipids with short hydrophilic head groups requires the presence of sphingolipid activator proteins (SAPs). These are the saposins (Saps) and the GM2 activator protein. Sphingolipid activator proteins are membrane-perturbing and lipid-binding proteins with different specificities for the bound lipid and the activated enzyme-catalyzed reaction. Their inherited deficiency leads to sphingolipid- and membrane-storage diseases. Sphingolipid activator proteins not only facilitate glycolipid digestion but also act as glycolipid transfer proteins facilitating the association of lipid antigens with immunoreceptors of the CD1 family.

Structural membrane alterations in Alzheimer brains found to be associated with regional disease development; increased density of gangliosides GM1 and GM2 and loss of cholesterol in detergent-resistant membrane domains

The formation of neurotoxic beta-amyloid fibrils in Alzheimer's disease (AD) is suggested to involve membrane rafts and to be promoted, in vitro, by enriched concentrations of gangliosides, particularly GM1, and the cholesterol therein. In our study, the presence of rafts and their content of the major membrane lipids and gangliosides in the temporal cortex, reflecting late stages of AD pathology, and the frontal cortex, presenting earlier stages, has been investigated. Whole tissue and isolated detergent-resistant membrane fractions (DRMs) were analysed from 10 AD and 10 age-matched control autopsy brains. DRMs from the frontal cortex of AD brains contained a significantly higher concentration (micromol/micromol glycerophospholipids), of ganglioside GM1 (22.3 +/- 4.6 compared to 10.3 +/- 6.4, p <0.001) and GM2 (2.5 +/- 1.0 compared to 0.55 +/- 0.3, p <0.001). Similar increases of these gangliosides were also seen in DRMs from the temporal cortex of AD brains, which, in addition, comprised significantly lower proportions of DRMs. Moreover, these remaining rafts were depleted in cholesterol (from 1.5 +/- 0.2 to 0.6 +/- 0.3 micromol/micromol glycerophospholipids, p <0.001). In summary, we found an increased proportion of GM1 and GM2 in DRMs, and accelerating plaque formation at an early stage, which may gradually lead to membrane raft disruptions and thereby affect cellular functions associated with the presence of such membrane domains.

Decrease of P-Glycoprotein Activity in K562/ADR Cells by M?CD and Filipin and Lack of Effect Induced by Cholesterol Oxidase Indicate That This Transporter Is Not Located in Rafts

The effect of low-density membrane domains on function of the plasma membrane transporter P-glycoprotéine (P-gp), involved in multidrug resistance (MDR) phenotype, has been investigated in K562/ADR cells. To this end we reversibly altered the cholesterol content of K562/ADR cells by using methyl-beta-cyclodextrin as a cholesterol chelator and conversely we repleted them through incubation with cholesterol in culture medium. We also used the cholesterol-binding fluorochrome filipin and cholesterol oxidase. Our data show that either cholesterol depletion or complex formation with filipin resulted in a strong decrease of P-gp activity. However, when cells were incubated with cholesterol oxidase that are known to disrupt rafts, no modification of the P-gp activity was observed. In addition, using a free-detergent methodology to separate by ultracentrifugation, "light," "heavy," and "extra heavy" fractions we show that no P-gp is found in the "light" fraction where rafts are usually detected. Altogether, our data strongly suggest that, in this cell line, P-gp is not localized in rafts.