Abstract
(Englisch)
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Endocytic organelles are composed of a mosaic of structural and functional regions, which consist of specialized protein-lipid domains and of protein complexes associated to specific membrane lipids1. The late endosome is a multivesicular organelle, and its limiting and internal membranes exhibit different protein and lipid composition. The very abundant glycoproteins Lamp1 and Lamp2 are exclusively found on the limiting membrane, while internal membranes contain transiting molecules of some receptors, tetraspanins2, and high amounts of an unusual phospholipid, lysobisphosphatidic acid (LBPA)3. We have shown that these LBPA-membranes regulate both protein and cholesterol transport, and conversely that cholesterol accumulation interferes with both protein transport3-6 and motility7. Hence, LBPA-membranes may have turnpike functions in late endosome dynamics1. Interestingly, MLN64, a transmembrane protein that shares homology with the cholesterol binding domain of the steroidogenic acute regulatory protein (StAR), is also restricted to the limiting membrane8, but its function is unclear. Internal membranes originate from invaginations, but the mechanisms are not known. Conversely, nothing is known about the molecular mechanisms that may regulate proteins export from internal membranes. Human B lymphocytes secrete microvesicles (exosomes) that contain mature MHC-II molecules and tetraspanins and can stimulate T lymphocytes in vitro 2. Exosomes are believed to be derived from the internal membranes of the endocytic multivesicular compartments containing MHC-II molecules in antigen-presenting cells, perhaps via fusion with the plasma membrane. Our data show that both exosomes and internal membranes of late endosomes contain similar constituents, including LBPA. We have generated a monoclonal antibody against LBPA, which interferes, when endocytosed, with LBPA-membrane functions in transport3,4,7, and selectively modulates the exosomal pathway in human B-lymphocytes. Strikingly, we find that the exosomal blood content is changed in sera from patients with the so-called anti-phospholipid syndrome (aPL). Since our previous studies indicated that aPL antibodies recognize LBPA and act much like our monoclonal antibody3-5, it is tempting to believe that LBPA-containing exosomes play a role in the complex clinical picture associated with this auto-immune disorder.
1. Gruenberg, J. The endocytic pathway: a mosaic of domains. Nature Reviews Molecular Cell Biology 2, 721-30 (2001); 2. Escola, J.M., et al. Selective Enrichment of Tetraspan Proteins on the Internal Vesicles of Multivesicular Endosomes and on Exosomes Secreted by Human B- lymphocytes. J Biol Chem 273, 20121-7 (1998); 3. Kobayashi, T., et al. A lipid associated with the antiphospholipid syndrome regulates endosome structure/function. Nature 392, 193-197 (1998); 4. Kobayashi, T., et al. Late endosomal membranes rich in lysobisphosphatidic acid regulate cholesterol transport. Nature Cell Biology 1, 113-118 (1999); 5. Galve de Rochemonteix, B., et al. Interaction of anti-phospholipid antibodies with late endosomes of human endothelial cells. Arterioscler Thromb Vasc Biol. 20, 563-574 (2000); 6. Kobayashi, K., et al. The tetraspanin CD63/lamp3 cycles between endocytic and secretory compartments in human endothelial cells. Mol. Biol. Cell 11, 1829-1843 (2000); 7. Lebrand, C., et al. Late endosome motility depends on lipids via the small GTPase Rab7. Embo J (in press), (2002); 8. Alpy, F., et al. The StAR homolog MLN64 a late endosomal cholesterol binding protein. J Biol Chem 276, 4261-4269 (2000).
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