title: Sorting of an internalized plasma membrane lipid between recycling and degradative pathways in normal and Niemann-Pick, type A fibroblasts.
abstract: We examined the metabolism and intracellular transport of a fluorescent sphingomyelin analogue, N-(N-[6-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]caproyl])- sphingosylphosphorylcholine (C6-NBD-SM), in both normal and Niemann-Pick, type A (NP-A) human skin fibroblast monolayers. C6-NBD-SM was integrated into the plasma membrane bilayer by transfer of C6-NBD-SM monomers from liposomes to cells at 7 degrees C. The cells were washed, and within 3 min of warming to 37 degrees C, both normal and NP-A fibroblasts had internalized C6-NBD-SM from the plasma membrane, resulting in a punctate pattern of intracellular fluorescence. Rates for C6-NBD-SM internalization and transport from intracellular compartments to the plasma membrane (recycling) were similar for normal and NP-A cells. With increasing time at 37 degrees C, internalized C6-NBD-SM accumulated in the lysosomes of NP-A fibroblasts, while normal fibroblasts showed increasing Golgi apparatus fluorescence with no observable lysosomal labeling. Since NP-A fibroblasts lack lysosomal (acid) sphingomyelinase (A-SMase), this result suggested that hydrolysis of C6-NBD-SM prevented its accumulation in the lysosomes of normal fibroblasts during its transport along the degradative pathway. We used the amount of C6-NBD-SM hydrolysis by A-SMase in normal cells as a measure of C6-NBD-SM transported from the cell surface to the lysosomes. After a lag period, C6-NBD-SM was delivered to the lysosomes at a rate of approximately 8%/h. This rate was approximately 18-19 fold slower than the rate of C6-NBD-SM recycling from intracellular compartments to the plasma membrane. Thus, small amounts of C6-NBD-SM were transported along the degradative pathway, while most endocytosed C6-NBD-SM was sorted for transport along the plasma membrane recycling pathway.
authors.affiliation: Carnegie Institution of Washington, Department of Embryology, Baltimore, Maryland 21210.
authors.forename: M
authors.initials: M
authors.lastname: Koval
authors.forename: R E
authors.initials: RE
authors.lastname: Pagano
mesh_terms: D009327:4-Chloro-7-nitrobenzofurazan / Q000031:analogs & derivatives / Q000378:metabolism*; D001692:Biological Transport; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D005347:Fibroblasts / Q000378:metabolism / Q000648:ultrastructure; D005456:Fluorescent Dyes; D006056:Golgi Apparatus / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D008247:Lysosomes / Q000378:metabolism / Q000648:ultrastructure; D008563:Membrane Lipids / Q000378:metabolism*; D009542:Niemann-Pick Diseases / Q000378:metabolism*; D010069:Oxadiazoles / Q000378:metabolism*; D012016:Reference Values; D012867:Skin / Q000378:metabolism*; D013109:Sphingomyelins / Q000378:metabolism*
chemical_list: D005456:Fluorescent Dyes; D008563:Membrane Lipids; C041065:N-4-nitrobenzo-2-oxa-1,3-diazoleaminocaproyl sphingosylphosphorylcholine; D010069:Oxadiazoles; D013109:Sphingomyelins; D009327:4-Chloro-7-nitrobenzofurazan