Sphingolipid microdomains mediate CD38 internalization: topography of the endocytosis

Plasma membranes of several cell types contain specialized microdomains (or lipid rafts) enriched in sphingolipids, cholesterol, sphingomyelin, and glycosyl-phosphatidylinositol-anchored proteins. These membrane domains are characterized by detergent insolubility at low temperatures and low buoyant density. Human CD38 is the prototype of a gene family encoding surface molecules endowed with multiple functional activities. The endocytosis of the human CD38 molecule has been investigated in normal lymphocytes and in a number of leukemia- and lymphoma-derived cell lines demonstrating that internalization after CD38 ligation is a reproducible event involving only a fraction of the whole amount of the surface molecule. This study reports the results obtained by conventional, confocal, and electron microscopy on the effects induced by the engagement of the molecule with agonistic mAb, reproducing the signals mediated by its natural ligand. The results demonstrate that the endocytosis induced as consequence of CD38 ligation is preceded by a thorough rearrangement of the cell surface with formation of glycosphingolipid- and cholesterol-rich plasma membrane microdomains. These data suggest that specialized raft microdomains might be the plasma membrane structure through which CD38 translocates at intracellular level. The CD38/lipid interactions during the coated pit formation trigger a process that generate membrane curvature, considered as the first step of CD38 endocytosis. Moreover, ultrastructural studies show that early CD38(+) endosomes are pleiomorphic and contain cisternal and vesicular regions. Late endosomes exhibit a complex organisation, containing uncoupled CD38-ligand multivesicular- or multilamellar-regions.