Perfringolysin O, a cholesterol-binding cytolysin, as a probe for lipid rafts.
ABSTRACT Gaining an understanding of the structural and functional roles of cholesterol in membrane lipid rafts is a critical issue in studies on cellular signaling and because of the possible involvement of lipid rafts in various diseases. We have focused on the potential of perfringolysin O (theta-toxin), a cholesterol-binding cytolysin produced by Clostridium perfringens, as a probe for studies on membrane cholesterol. We prepared a protease-nicked and biotinylated derivative of perfringolysin O (BCtheta) that binds selectively to cholesterol in cholesterol-rich microdomains of cell membranes without causing membrane lesions. Since the domains fulfill the criteria of lipid rafts, BCtheta can be used to detect cholesterol-rich lipid rafts. This is in marked contrast to filipin, another cholesterol-binding reagent, which binds indiscriminately to cell cholesterol. Using BCtheta, we are now searching for molecules that localize specifically in cholesterol-rich lipid rafts. Recently, we demonstrated that the C-terminal domain of perfringolysin O, domain 4 (D4), possesses the same binding characteristics as BCtheta. BIAcore analysis showed that D4 binds specifically to cholesterol with the same binding affinity as the full-size toxin. Cell-bound D4 is recovered predominantly from detergent-insoluble, low-density membrane fractions where raft markers, such as cholesterol, flotillin and Src family kinases, are enriched, indicating that D4 also binds selectively to lipid rafts. Furthermore, a green fluorescent protein-D4 fusion protein (GFP-D4) was revealed to be useful for real-time monitoring of cholesterol in lipid rafts in the plasma membrane. In addition, the expression of GFP-D4 in the cytoplasm might allow the investigations of intracellular trafficking of lipid rafts. The simultaneous visualization of lipid rafts in plasma membranes and inside cells might help in gaining a total understanding of the dynamic behavior of lipid rafts.
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ABSTRACT: Purpose: To investigate the effects of Bruch's membrane (BrM) neutral lipid deposition in mouse models and its significance to aging and age-related macular degeneration it is essential to reliable detect small quantities of neutral lipids including esterified cholesterol (EC). In chorioretinal sections and BrM wholemounts we tested a novel fluorescent cholesterol marker based on the bacterial toxin Perfringolysin O (PFO) and compared results to those obtained with the classic cholesterol dye filipin. Methods: An engineered plasmid containing the specific cholesterol binding domain (D4) of PFO fused to green fluorescent protein (GFP) was expressed in cultured E. coli, isolated, purified, and concentrated. A total of 150 BrM-choroid wholemounts and chorioretinal sections of 11 to 13-month-old ApoEnull mice were prepared and stained with PFO/D4-GFP or filipin for EC. Samples were examined by epifluorescence microscopy. Results: The fluorescence intensity of PFO/D4-GFP was strong, stable, and if small quantities of EC were present superior to filipin. In all specimens we could sharply locate the PFO/D4-GFP signal to BrM. A semi-quantitative evaluation of BrM lipid deposition is possible by measuring PFO/D4-GFP fluorescence intensity. Conclusions: PFO/D4-GFP allowed a robust and direct detection of EC in aged murine BrM. In wholemount samples its strong and stable fluorescence facilitated a semi-quantitative evaluation of BrM's EC content over a large area. The patterns of EC deposition in murine BrM wholemounts are comparable to findings in human BrM wholemounts. PFO/D4-GFP could be an important tool for investigating the effects of BrM lipid deposition in mouse models.Investigative Ophthalmology & Visual Science 07/2014; · 3.66 Impact Factor
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ABSTRACT: Cholesterol-dependent cytolysins (CDCs) constitute a family of pore forming toxins secreted by Gram-positive bacteria. These toxins form transmembrane pores by inserting a large β-barrel into cholesterol-containing membrane bilayers. Binding of water-soluble CDCs to the membrane triggers the formation of oligomers containing 35-50 monomers. The coordinated insertion of more than seventy β-hairpins into the membrane requires multiple structural conformational changes. Perfringolysin O (PFO), secreted by Clostridium perfringens, has become the prototype for the CDCs. In this chapter, we will describe current knowledge on the mechanism of PFO cytolysis, with special focus on cholesterol recognition, oligomerization, and the conformational changes involved in pore formation.Sub-cellular biochemistry 01/2014; 80:63-81.
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ABSTRACT: Background: Niemann-Pick disease type C (NPC) is caused by defects in cholesterol efflux from lysosomes due to mutations of genes coding for NPC1 and NPC2 proteins. As a result, massive accumulation of unesterified cholesterol in late endosomes/lysosomes is observed. At the level of the organism these cholesterol metabolism disorders are manifested by progressive neurodegeneration and hepatosplenomegaly. Until now filipin staining of cholesterol deposits in cells has been widely used for NPC diagnostics. In this report we present an alternative method for cholesterol visualization and estimation using a cholesterol-binding bacterial toxin, perfringolysin O. Methods: To detect cholesterol deposits, a recombinant probe, perfringolysin O fused with glutathione S-transferase (GST-PFO) was prepared. GST-PFO followed by labeled antibodies or streptavidin was applied for immunofluorescence and immunoelectron microscopy to analyze cholesterol distribution in cells derived from NPC patients. The identity of GST-PFO–positive structures was revealed by a quantitative analysis of their colocalization with several organelle markers. Cellular ELISA using GST-PFO was developed to estimate the level of unesterified cholesterol in NPC cells. Results: GST-PFO recognized cholesterol with high sensitivity and selectivity, as demonstrated by a protein/lipid overlay assay and surface plasmon resonance analysis. When applied to stain NPC cells, GST-PFO decorated abundant deposits of cholesterol in intracellular vesicles that colocalized with filipin-positive structures. These cholesterol deposits were resistant to 0.05%-0.2% Triton X-100 used for cells permeabilization in the staining procedure. GST-PFO-stained organelles were identified as late endosomes/lysosomes based on their colocalization with LAMP-1 and lysobisphosphatidic acid. On the other hand, GST-PFO did not colocalize with markers of the Golgi apparatus, endoplasmic reticulum, peroxisomes or with actin filaments. Only negligible GST-PFO staining was seen in fibroblasts of healthy individuals. When applied to cellular ELISA, GST-PFO followed by anti-GST-peroxidase allowed a semiquantitative analysis of cholesterol level in cells of NPC patients. Binding of GST-PFO to NPC cells was nearly abolished after extraction of cholesterol with methyl-β-cyclodextrin.Orphanet Journal of Rare Diseases 04/2014; 9:64. · 3.96 Impact Factor