Phosphatidylserine externalization, “necroptotic bodies” release, and phagocytosis during necroptosis

Abstract

Necroptosis is a regulated, nonapoptotic form of cell death initiated by receptor-interacting protein kinase-3 (RIPK3) and mixed lineage kinase domain-like (MLKL) proteins. It is considered to be a form of regulated necrosis, and, by lacking the “find me” and “eat me” signals that are a feature of apoptosis, necroptosis is considered to be inflammatory. One such “eat me” signal observed during apoptosis is the exposure of phosphatidylserine (PS) on the outer plasma membrane. Here, we demonstrate that necroptotic cells also expose PS after phosphorylated mixed lineage kinase-like (pMLKL) translocation to the membrane. Necroptotic cells that expose PS release extracellular vesicles containing proteins and pMLKL to their surroundings. Furthermore, inhibition of pMLKL after PS exposure can reverse the process of necroptosis and restore cell viability. Finally, externalization of PS by necroptotic cells drives recognition and phagocytosis, and this may limit the inflammatory response to this nonapoptotic form of cell death. The exposure of PS to the outer membrane and to extracellular vesicles is therefore a feature of necroptotic cell death and may serve to provide an immunologically-silent window by generating specific “find me” and “eat me” signals.

Myelopoiesis embraces its inner weakness

“…Apoptosis, as well as the regulated non-apoptotic forms of cell death (necroptosis and pyroptosis), can influence hematopoiesis following infection or in the setting of bone- marrow transplantation 13 . The demonstration that annexin V staining of hematopoietic stem and progenitor cells is diminished in the absence of OPN following transplantation or infection might also be consistent with descriptions of annexin V staining of cells undergoing non-apoptotic cell death 14,15 . iOPN and sOPN might also regulate the expression of regulators of these non-apoptotic forms of cell death, in addition to that of genes encoding apoptotic molecules such as survivin and Bcl-x L .…”

Neutrophil–macrophage cooperation and its impact on tissue repair

“…Furthermore, molecules released by neutrophils such as neutrophil gelatinase‐associated lipocalin can enhance efferocytosis by increasing the expression of the phosphatidylserine receptor MER proto‐oncogene, tyrosine kinase on macrophages . More recently, necroptosis, a regulated, nonapoptotic form of necrotic cell death has been shown to cause expression of “eat me signals” by neutrophils, thus enhancing efferocytosis by macrophages and improving the outcome after myocardial injury …”

To the edge of cell death and back

“…Similar to apoptosis, necroptosis can likewise be reversed . Although necrosis used to be considered as an ‘accidental’ cell death beyond any regulation, studies have revealed a comprehensive signaling pathway resulting in the form of necrosis called necroptosis .…”

“…After MLKL phosphorylation, necroptotic cells undergo a series of biological events, including Ca 2+ influx, phosphatidylserine (PS) externalization, and the final compromise of PM integrity. If RIPK3 or MLKL is inactivated at the PS exposure stage, some of the necroptotic cells can fully recover . These dying cells can reattach to the culture surface, regain normal morphology, re‐establish PS asymmetry, lower intracellular Ca 2+ to the resting level, and inactivate or dephosphorylate MLKL.…”

“…For human necroptotic cells, the MLKL inhibitor necrosulfonamide (NSA) was used to allow cell recovery. Since NSA binds to a cysteine only present in human MLKL, NSA cannot resuscitate murine necroptotic cells . The other approach takes advantage of (tandem) FKBP domain (Fv)‐mediated dimerization.…”