title: Cell surface-expressed phosphatidylserine and annexin A5 open a novel portal of cell entry.
abstract: Expression of phosphatidylserine (PtdSer) at the cell surface is part of the membrane dynamics of apoptosis. Expressed phosphatidylserine functions as an "eat me" flag toward phagocytes. Here, we report that the expressed phosphatidylserine forms part of a hitherto undescribed pinocytic pathway. Annexin A5, a phosphatidylserine-binding protein, binds to and polymerizes through protein-protein interactions on membrane patches expressing phosphatidylserine. The two-dimensional protein network of annexin A5 at the surface prevents apoptotic body formation without interfering with the progression of apoptosis as demonstrated by activation of caspase-3, PtdSer exposure, and DNA fragmentation. The annexin A5 protein network bends the membrane patch nanomechanically into the cell and elicits budding, endocytic vesicle formation, and cytoskeleton-dependent trafficking of the endocytic vesicle. Annexin A1, which binds to PtdSer without forming a two-dimensional protein network, does not induce the formation of endocytic vesicles. This novel pinocytic pathway differs from macropinocytosis, which is preceded by membrane ruffling and actin polymerization. We clearly showed that actin polymerization is not involved in budding and endocytic vesicle formation but is required for intracellular trafficking. The phosphatidylserine-annexin A5-mediated pinocytic pathway is not restricted to cells in apoptosis. We demonstrated that living tumor cells can take up substances through this novel portal of cell entry. This opens new avenues for targeted drug delivery and cell entry.
authors.affiliation: Department of Biochemistry, Cardiovascular Research Institute Maastricht, PO Box 616, 6200 MD Maastricht, the Netherlands.
authors.forename: Heidi
authors.initials: H
authors.lastname: Kenis
authors.forename: Hugo
authors.initials: H
authors.lastname: van Genderen
authors.forename: Abdel
authors.initials: A
authors.lastname: Bennaghmouch
authors.forename: Hilde A
authors.initials: HA
authors.lastname: Rinia
authors.forename: Peter
authors.initials: P
authors.lastname: Frederik
authors.forename: Jagat
authors.initials: J
authors.lastname: Narula
authors.forename: Leo
authors.initials: L
authors.lastname: Hofstra
authors.forename: Chris P M
authors.initials: CP
authors.lastname: Reutelingsperger
mesh_terms: D017304:Annexin A5 / Q000737:chemistry / Q000378:metabolism*; D017209:Apoptosis; D002462:Cell Membrane / Q000378:metabolism*; D003460:Crystallization; D006367:HeLa Cells; D006801:Humans; D019169:Jurkat Cells; D008954:Models, Biological; D010718:Phosphatidylserines / Q000378:metabolism*; D010873:Pinocytosis / Q000502:physiology*; D011485:Protein Binding
chemical_list: D017304:Annexin A5; D010718:Phosphatidylserines