title: Vectorial budding of vesicles by asymmetrical enzymatic formation of ceramide in giant liposomes.
abstract: Sphingomyelin is an abundant component of eukaryotic membranes. A specific enzyme, sphingomyelinase can convert this lipid to ceramide, a central second messenger in cellular signaling for apoptosis (programmed cell death), differentiation, and senescence. We used microinjection and either Hoffman modulation contrast or fluorescence microscopy of giant liposomes composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), N-palmitoyl-sphingomyelin (C16:0-SM), and Bodipy-sphingomyelin as a fluorescent tracer (molar ratio 0.75:0.20:0.05, respectively) to observe changes in lipid lateral distribution and membrane morphology upon formation of ceramide. Notably, in addition to rapid domain formation (capping), vectorial budding of vesicles, i.e., endocytosis and shedding, can be induced by the asymmetrical sphingomyelinase-catalyzed generation of ceramide in either the outer or the inner leaflet, respectively, of giant phosphatidylcholine/sphingomyelin liposomes. These results are readily explained by 1) the lateral phase separation of ceramide enriched domains, 2) the area difference between the adjacent monolayers, 3) the negative spontaneous curvature, and 4) the augmented bending rigidity of the ceramide-containing domains, leading to membrane invagination and vesiculation of the bilayer.
authors.affiliation: Helsinki Biophysics and Biomembrane Group, Department of Medical Chemistry, Institute of Biomedicine, University of Helsinki, Finland.
authors.forename: J M
authors.initials: JM
authors.lastname: Holopainen
authors.forename: M I
authors.initials: MI
authors.lastname: Angelova
authors.forename: P K
authors.initials: PK
authors.lastname: Kinnunen
mesh_terms: D001409:Bacillus cereus; D001896:Boron Compounds / Q000737:chemistry; D002518:Ceramides / Q000737:chemistry*; D005456:Fluorescent Dyes; D007700:Kinetics; D008081:Liposomes / Q000737:chemistry*; D008845:Microinjections; D010713:Phosphatidylcholines / Q000737:chemistry; D013108:Sphingomyelin Phosphodiesterase / Q000737:chemistry; D013109:Sphingomyelins / Q000737:chemistry
chemical_list: C095489:4,4-difluoro-4-bora-3a,4a-diaza-s-indacene; D001896:Boron Compounds; D002518:Ceramides; D005456:Fluorescent Dyes; D008081:Liposomes; D010713:Phosphatidylcholines; D013109:Sphingomyelins; D013108:Sphingomyelin Phosphodiesterase