Necrotic Cells Actively Attract Phagocytes through the Collaborative Action of Two Distinct PS-Exposure Mechanisms

Abstract

Necrosis, a kind of cell death closely associated with pathogenesis and genetic programs, is distinct from apoptosis in both morphology and mechanism. Like apoptotic cells, necrotic cells are swiftly removed from animal bodies to prevent harmful inflammatory and autoimmune responses. In the nematode Caenorhabditis elegans, gain-of-function mutations in certain ion channel subunits result in the excitotoxic necrosis of six touch neurons and their subsequent engulfment and degradation inside engulfing cells. How necrotic cells are recognized by engulfing cells is unclear. Phosphatidylserine (PS) is an important apoptotic-cell surface signal that attracts engulfing cells. Here we observed PS exposure on the surface of necrotic touch neurons. In addition, the phagocytic receptor CED-1 clusters around necrotic cells and promotes their engulfment. The extracellular domain of CED-1 associates with PS in vitro. We further identified a necrotic cell-specific function of CED-7, a member of the ATP-binding cassette (ABC) transporter family, in promoting PS exposure. In addition to CED-7, anoctamin homolog-1 (ANOH-1), the C. elegans homolog of the mammalian Ca2+-dependent phospholipid scramblase TMEM16F, plays an independent role in promoting PS exposure on necrotic cells. The combined activities from CED-7 and ANOH-1 ensure efficient exposure of PS on necrotic cells to attract their phagocytes. In addition, CED-8, the C. elegans homolog of mammalian Xk-related protein 8 also makes a contribution to necrotic cell-removal at the first larval stage. Our work indicates that cells killed by different mechanisms (necrosis or apoptosis) expose a common “eat me” signal to attract their phagocytic receptor(s); furthermore, unlike what was previously believed, necrotic cells actively present PS on their outer surfaces through at least two distinct molecular mechanisms rather than leaking out PS passively.

Exposure of phosphatidylserine on the cell surface

“…Dominant mutations in mec-4 (the core subunit of a mechanically gated sodium channel) in touch neurons in C. elegans cause hyper channel activity, which results in excess Ca 2+ influx and necrotic cell death. 82,83 Li et al 58 recently showed that necrotic cells in mec-4 nematodes expose PtdSer by the action of ANOH1 (a TMEM16 homolog) and CED7 (an ABCA1-transporter homolog). TNF-induced necroptosis and bacteria-induced pyroptosis are mediated by calcium influx.…”

Programmed Cell Death DuringCaenorhabditis elegansDevelopment

“…These observations are confirmed by other studies in C. elegans Nawa et al 2012) and by a mammalian study in which inactivation of CDC50A, a cofactor for the human TAT-1 homolog ATP11C, causes ectopic PS exposure in living cells and their phagocytosis by macrophages (Segawa et al 2014). The engulfment of living cells by phagocytes in the tat-1 mutants is blocked by lf mutations in PS receptor family 1 (psr-1) and ced-1, two phagocyte receptors that recognize surface-exposed PS and act in two major phagocytosis pathways (Zhou et al 2001b;Wang et al 2003;DarlandRansom et al 2008;Wang et al 2010;Li et al 2015;Yang et al 2015), suggesting that externalized PS can serve as an eat me signal for both engulfment pathways. Mitochondrial elimination: As described above, mitochondria play an important role in regulating cell death execution in C. elegans.…”

“…TTR-52 binds both PS and the extracellular domain of CED-1 in vitro and likely functions as a bridging molecule that mediates recognition of apoptotic cells by cross-linking the exposed PS eat me signal with the engulfment receptor CED-1 ( Figure 2B). In addition, a recent study suggests that the extracellular region of CED-1 could directly bind PS in vitro, when fused to GST (Li et al 2015).…”

Programmed cell clearance: From nematodes to humans

“…More recent studies have provided evidence that PS exposure serves as a common “eat-me” signal for cells dying by necrosis and apoptosis [39]. Indeed, in contrast to common belief, PS was actively exposed on the outer surface of touch neurons dying by necrosis prior to the disintegration of the plasma membrane.…”