The tetraspanin CD63/lamp3 cycles between endocytic and secretory compartments in human endothelial cells

In the present study, we show that in human endothelial cells the tetraspan in CD63/lamp3 distributes predominantly to the internal membranes of multiv esicular-multilamellar late endosomes, which contain the unique lipid lysob isphosphatidic acid. Some CD63/lamp3 is also present in Weibel-Palade bodie s, the characteristic secretory organelle of these cells. We find that CD63 /lamp3 molecules can be transported from late endosomes to Weibel-Palade bo dies and thus that CD63/lamp3 cycles between endocytic and biosynthetic com partments; however, movement of CD63/lamp3 is much slower than that of P-se lectin, which is known to cycle between plasma membrane and Weibel-Palade b odies. When cells are treated with U18666A, a drug that mimics the Niemann- Pick type C syndrome, both proteins accumulate in late endosomes and fail t o reach Weibel-Palade bodies efficiently, suggesting that P-selectin, like CD63/lamp3, cycles via late endosomes. Our data suggest that CD63/lamp3 par titions preferentially within late endosome internal membranes, thus causin g its accumulation, and that this mechanism contributes to CD63/lamp3 reten tion in late endosomes; however, our data also indicate that the protein ca n eventually escape from these internal membranes and recycle toward Weibel -Palade bodies to be reused. Our observations thus uncover the existence of a selective trafficking route from late endosomes to Weibel-Palade bodies.