After shrinkage apoptotic cells expose internal membrane-derived epitopes on their plasma membranes

Abstract

Apoptosis and phagocytosis of apoptotic cells are crucial processes. At best the phagocytic machinery detects and swallows all apoptotic cells in a way that progression to secondary necrosis is avoided. Otherwise, inflammation and autoimmune diseases may occur. Most apoptotic cells are phagocytosed instantaneously in a silent fashion; however, some dying cells escape their clearance. If the cells are not cleared early, they lose membranes due to extensive shedding of membrane surrounded vesicles (blebbing) and shrink. It is unclear how apoptotic cells compensate their massive loss of plasma membrane. Here, we demonstrate that endoplasmic reticulum-(ER) resident proteins (calnexin, the KDEL receptor and a dysfunctional immunoglobulin heavy chain) were exposed at the surfaces of shrunken late apoptotic cells. Additionally, these cells showed an increased binding of lectins, which recognize sugar structures predominantly found as moieties of incompletely processed proteins in ER and Golgi. In addition the ER resident lipophilic ER-Trackert Blue-White DPX, and internal GM1 were observed to translocate to the cell surfaces during late apoptosis. We conclude that during blebbing of apoptotic cells the surface membrane loss is substituted by immature membranes from internal stores. This mechanism explains the simultaneous appearance of preformed recognition structures for several adaptor proteins known to be involved in clearance of dead cells.

Molecular mechanisms of ATP secretion during immunogenic cell death

“…Indeed, a substantial fraction of cells responding to chemotherapy in vitro exposed LAMP1 on the cell surface as it externalized PS residues, in line with the hypothesis that lysosomes fuse with the plasma membrane during the apoptotic process. 55 In this setting, the depletion of essential autophagic factors including AT5, ATG7 and BCN1 failed to affect the translocation of LAMP1 to the outer leaflet of the plasma membrane, meaning that autophagy is not required for lysosomal exocytosis. In sharp contrast, the knockdown of PANX1 prevented LAMP1 from translocating to the plasma membrane, and vice versa LAMP1 depletion interfered with the externalization of PANX1.…”

From regulation of dying cell engulfment to development of anti-cancer therapy

“…17 Another aspect is the blebbing of apoptotic cells, implying an increase in the membrane surface area provided from the cell's internal membrane stores, such as the ER. In support of this notion, Franz et al 18 showed that apoptotic cells during cellular shrinkage expose calnexin, an ER integral membrane protein. Interestingly, the ER of phagocytes also supplies additional membrane required for phagocytosis, 19 indicating a possible homotypic interaction between surfaces of dying cells and phagocytes.…”

Apoptosis-dependent Externalization and Involvement in Apoptotic Cell Clearance of DmCaBP1, an Endoplasmic Reticulum Protein of Drosophila

“…Unlike DmCaBP1, these proteins seemed to directly move from the ER to the cell surface without being released from cells. Franz et al (33) showed that several ER-residing proteins, which were not analyzed in this study, are exposed at the surface of apoptotic cells. Therefore, the externalization or surface exposure of ER proteins could be a general feature in apoptotic cells, although the mechanism of relocation may vary from protein to protein.…”