Holopainen J M, Angelova M I, Kinnunen P K
Helsinki Biophysics and Biomembrane Group, Department of Medical Chemistry, Institute of Biomedicine, University of Helsinki, Finland.
Biophys J. 2000 Feb;78(2):830-8. doi: 10.1016/S0006-3495(00)76640-9.
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.
鞘磷脂是真核细胞膜的一种丰富成分。一种特定的酶，鞘磷脂酶可将这种脂质转化为神经酰胺，神经酰胺是细胞凋亡（程序性细胞死亡）、分化和衰老的细胞信号传导中的一种核心第二信使。我们使用显微注射以及对由1-硬脂酰-2-油酰-sn-甘油-3-磷酸胆碱（SOPC）、N-棕榈酰鞘磷脂（C16:0-SM）和作为荧光示踪剂的Bodipy-鞘磷脂（摩尔比分别为0.75:0.20:0.05）组成的巨型脂质体进行霍夫曼调制对比度或荧光显微镜观察，以观察神经酰胺形成时脂质横向分布和膜形态的变化。值得注意的是，除了快速形成结构域（封帽）外，巨型磷脂酰胆碱/鞘磷脂脂质体的外层或内层分别通过不对称鞘磷脂酶催化生成神经酰胺可诱导囊泡的矢量出芽，即内吞作用和脱落。这些结果可以通过以下几点轻松解释：1）富含神经酰胺的结构域的横向相分离；2）相邻单层之间的面积差异；3）负的自发曲率；4）含神经酰胺结构域增强的弯曲刚性，导致膜内陷和双层膜形成囊泡。
