Depletion of RIPK3 or MLKL blocks TNF-driven necroptosis and switches towards a delayed RIPK1 kinase-dependent apoptosis.

In human cells, the RIPK1-RIPK3-MLKL-PGAM5-Drp1 axis drives tumor necrosis factor (TNF)-induced necroptosis through mitochondrial fission, but whether this pathway is conserved among mammals is not known. To answer this question, we analyzed the presence and functionality of the reported necroptotic axis in mice. As in humans, knockdown of receptor-interacting kinase-3 (RIPK3) or mixed lineage kinase domain like (MLKL) blocks TNF-induced necroptosis in L929 fibrosarcoma cells. However, repression of either of these proteins did not protect the cells from death, but instead induced a switch from TNF-induced necroptosis to receptor-interacting kinase-1 (RIPK1) kinase-dependent apoptosis. In addition, although mitochondrial fission also occurs during TNF-induced necroptosis in L929 cells, we found that knockdown of phosphoglycerate mutase 5 (PGAM5) and dynamin 1 like protein (Drp1) did not markedly protect the cells from TNF-induced necroptosis. Depletion of Pink1, a reported interactor of both PGAM5 and Drp1, did not affect TNF-induced necroptosis. These results indicate that in these murine cells mitochondrial fission and Pink1 dependent processes, including Pink-Parkin dependent mitophagy, apparently do not promote necroptosis. Our data demonstrate that the core components of the necrosome (RIPK1, RIPK3 and MLKL) are crucial to induce TNF-dependent necroptosis both in human and in mouse cells, but the associated mechanisms may differ between the two species or cell types.

Dying for a cause: NETosis, mechanisms behind an antimicrobial cell death modality.

Neutrophil extracellular traps (NETs) are chromatin structures loaded with antimicrobial molecules. They can trap and kill various bacterial, fungal and protozoal pathogens, and their release is one of the first lines of defense against pathogens. In vivo, NETs are released during a form of pathogen-induced cell death, which was recently named NETosis. Ex vivo, both dead and viable neutrophils can be stimulated to release NETs composed of either nuclear or mitochondrial chromatin, respectively. In certain pathological conditions, NETs are associated with severe tissue damage or certain auto-immune diseases. This review describes the recent progress made in the identification of the mechanisms involved in NETosis and discusses its interplay with autophagy and apoptosis.

Parabutoporin, a cationic amphipathic peptide from scorpion venom: much more than an antibiotic.

Parabutoporin (PP) from the South African scorpion Parabuthus schlechteri is a 45-mer lysine-rich and cysteine-free peptide. At micromolar concentrations it has antimicrobial effects against G+ and G- bacteria and is antifungal as well. However, at submicromolar concentrations, parabutoporin also directly interferes with cellular functions of the human innate immune system, especially polymorphonuclear neutrophils (PMN): parabutoporin acts as a chemoattractant for neutrophils, induces their degranulation, while delaying constitutive neutrophil apoptosis. In addition, it potently inhibits induced superoxide production. Different signalling pathways regulating these biochemical processes were identified as targets of parabutoporin. Therefore, parabutoporin is a well documented scorpion venom peptide with immuno-regulatory properties beyond its antibiotic effects.