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Ceramide in Bacterial Infections and Cystic Fibrosis

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Ceramide in Bacterial Infections and Cystic Fibrosis Article in press - uncorrected proof Biol. Chem., Vol. 389, pp. 1371–1379, November 2008 • Copyright ᮊ by Walter de Gruyter • Berlin • New York. DOI 10.1515/BC.2008.162 Review Ceramide in bacterial infections and cystic fibrosis Heike Grassme´ , Katrin Anne Becker, ordered phase and thus form distinct domains in the cell Yang Zhang and Erich Gulbins* membrane (Simons and Ikonen, 1997; Brown and Lon- don, 1998). Results from these studies indicate that Department of Molecular Biology, University of sphingolipids and cholesterol associate via hydrophilic Duisburg-Essen, Hufelandstrasse 55, D-45122 Essen, and hydrophobic interactions, resulting in an organized Germany membrane domain with a higher melting point than other * Corresponding author phospholipids in model membranes investigated in bio- e-mail: [email protected] physical experiments (Simons and Ikonen, 1997; Brown and London, 1998). In particular, the head groups of (gly- co)sphingolipids and the hydroxyl group of cholesterol Abstract seem to stabilize these domains via hydrophilic interac- tions, whereas the acyl chains of sphingolipids and the Ceramide is formed by the activity of sphingomyelinases, sterol ring form hydrophobic van der Waals’ interactions by degradation of complex sphingolipids, reverse cera- (for a review see Kolesnick et al., 2000). In addition, cho- midase activity or de novo synthesized. The formation of lesterol seems to stabilize these domains by filling the ceramide within biological membranes results in the for- void spaces between bulky sphingolipids, and thus mation of large ceramide-enriched membrane domains. extraction of cholesterol results in destruction of these These domains serve the spatial and temporal organi- small membrane domains (Xu et al., 2001; Megha et al., zation of receptors and signaling molecules. The acid 2006). It was suggested that this lateral organization of sphingomyelinase-ceramide system plays an important the cell membrane resulted in the formation of sphingo- role in the infection of mammalian host cells with lipid- and cholesterol-enriched membrane domains, also bacterial pathogens such as Neisseria gonorrhoeae, named rafts (Simons and Ikonen, 1997). However, at Escherichia coli, Staphylococcus aureus, Listeria mono- present only indirect evidence exists to support the pres- cytogenes, Salmonella typhimurium and Pseudomonas ence of rafts in cells under in vivo conditions, for instance aeruginosa. Ceramide and ceramide-enriched membrane at physiological temperature, and the concept of rafts is platforms are also involved in the induction of apoptosis still somewhat controversial. in infected cells, such as in epithelial and endothelial cells after infection with Pseudomonas aeruginosa and Staphy- Sphingomyelinases and ceramide-enriched lococcus aureus, respectively. Finally, ceramide-enriched membrane domains membrane platforms are critical regulators of the release Ceramide-enriched membrane platforms are membrane of pro-inflammatory cytokines upon infection. The domains that are either generated from rafts or inde- diverse functions of ceramide in bacterial infections pendent of rafts (Kolesnick et al., 2000; Gulbins and suggest that ceramide and ceramide-enriched membrane Kolesnick, 2003). Here we focus on the generation of domains are key players in host responses to many path- ceramide by the activity of sphingomyelinases that ogens and thus are potential novel targets to treat hydrolyze sphingomyelin, the most prevalent sphingolipid infections. in the cell membrane (Quintern et al., 1989; Calderon and Keywords: acid sphingomyelinase; bacteria; ceramide; DeVries, 1997). Sphingomyelin is an amide ester of a cystic fibrosis; virus. hydrophobic ceramide moiety and a hydrophilic phos- phorylcholine headgroup. Ceramide is composed of a D-erythro-sphingosine and a fatty acid containing 2–32 Ceramide, microdomains and ceramide- carbon atoms in the acyl chain (Hakomori, 1983; Rabio- enriched membrane platforms net et al., 2008). Sphingomyelinases are characterized by their pH optimum, with acid, neutral and alkaline sphin- Biophysical aspects of membrane domains gomyelinase identified (Quintern et al., 1989; Chatterjee et al., 1999; Duan et al., 2003). Since sphingomyelin is In 1972, Singer and Nicolson suggested a random dis- predominantly present in the outer leaflet of the cell tribution of lipids and proteins in the cell membrane membrane (Calderon and DeVries, 1997), the hydrolysis named the fluid mosaic model of the cell membrane of sphingomyelin results in ceramide-enriched mem- (Singer and Nicolson, 1972), although they did not rule brane domains that are primarily in the outer leaflet of the out some ordering of the membrane. This model also cell membrane, or in general in anti-cytoplasmic leaflets suggested that the membrane exists in a fluid-disordered of cellular membranes. It remains to be determined status. However, biophysical studies in the last 15 years whether a relevant exchange of ceramide between the revised this model and suggested the formation of small outer and inner leaflets of the cell membrane occurs domains in the cell membrane that exist in a liquid- under in vivo conditions, but Johnston and Johnston 2008/174 Article in press - uncorrected proof 1372 H. Grassme´ et al. (2008) demonstrated that hydrolysis of sphingomyelin to resulted in the rapid formation of large ceramide-enriched ceramide in a bilayer promotes flip-flop of ceramide, membrane platforms that were visualized by fluores- which has also been shown to occur in erythrocytes cence microscopy of giant vesicles (Holopainen et al., (Lopez-Montero et al., 2005). Several studies demon- 1998; Nurminen et al., 2002). These platforms sponta- strated that ceramide in the outer leaflet of the cell neously formed small vesicles that budded into giant membrane is formed by hydrolysis of sphingomyelin by vesicles. It should be noted that the formation of these the activity of acid sphingomyelinase, which is present in ceramide-enriched membrane platforms did not require vesicles, mostly lysosomes and secretory lysosomes, in any staining with antibodies and, most importantly, did mammalian cells (Schissel et al., 1996, 1998; Grassme not require any other cellular components such as the et al., 2001a). The sorting of acid sphingomyelinase cytoskeleton. These studies suggest that ceramide- between different compartments seems to be controlled enriched membrane platforms spontaneously form in cell by its glycosylation pattern, which determines transport membranes owing to the biophysical properties of cera- of mannose-6-phosphate-modified acid sphingomyeli- mide, although these data certainly do not exclude the nase into lysosomes, whereas complex-glycosylated possibility that in vivo formation of ceramide-enriched sphingomyelinase is secreted upon stimulation, for membrane domains also involves the cytoskeleton and/ instance via interleukin (IL)-1 receptors (Schissel et al., or other components of the cell. In vitro studies also indi- 1996, 1998). Several studies demonstrated that stimuli cate that even small amounts of ceramide incorporated such as CD95, DR5 and CD40 or infection with some into rafts transfer fluid phospholipid layers into a gel-like pathogenic bacteria and viruses mobilize intracellular phase (Veiga et al., 1999). Atomic force microscopy vesicles, a process that results in exposure of acid sphin- studies confirmed the phase separation in artificial gomyelinase on the outer leaflet of the cell membrane C16-ceramide-enriched glycerol-phospholipid/cholesterol (Grassme et al., 2001b, 2002, 2003a,b, 2005; Cremesti membranes (ten Grotenhuis et al., 1996). et al., 2001; Dumitru and Gulbins, 2006). The activity of A recent study (Johnston and Johnston, 2008) used a the acid sphingomyelinase results in hydrolysis of sphin- combination of atomic force microscopy and total inter- gomyelin and generation of ceramide. Ceramide exhibits nal reflection fluorescence to directly visualize clustering special biophysical properties that mediate association of of small membrane domains into larger domains in arti- ceramide molecules and the formation of small cera- ficial membranes composed of 1,2-dioleoyl-sn-glycero- mide-enriched microdomains (Holopainen et al., 1998; 3-phosphocholine/sphingomyelin/cholesterol mixtures Kolesnick et al., 2000; Nurminen et al., 2002). These upon treatment with Bacillus cereus sphingomyelinase to microdomains spontaneously fuse to larger domains and generate ceramide or incubation with C -ceramide. finally large ceramide-enriched membrane domains, also 16 Atomic force microscopy permits investigation of the named platforms, that can reach a width of up to 5 mm nanoscale organization of a membrane, whereas total (Gulbins and Kolesnick, 2003). Accumulation of ceramide internal reflection fluorescence permits improved analysis within rafts results in a change in the biophysical prop- of the dynamics of distinct membrane domains. The erties of these domains. Thus, the formation of ceramide studies revealed that enzymatic hydrolysis of sphingo- increases the hydrophobicity and changes the fluidity of myelin to ceramide in model membranes resulted in very membrane domains, since ceramide molecules are tight- ly packed and form highly ordered microdomains. Fur- rapid reorganization of the membrane, clustering of thermore, ceramide accumulation may change the domains and the formation of larger distinct domains that membrane height and very likely results in a change in presumably correspond to ceramide-enriched
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