C. elegans mitochondrial factor WAH-1 promotes phosphatidylserine externalization in apoptotic cells through phospholipid scramblase SCRM-1

Abstract

Externalization of phosphatidylserine, which is normally restricted to the inner leaflet of plasma membrane, is a hallmark of mammalian apoptosis. It is not known what activates and mediates the phosphatidylserine externalization process in apoptotic cells. Here, we report the development of an annexin V-based phosphatidylserine labelling method and show that a majority of apoptotic germ cells in Caenorhabditis elegans have surface-exposed phosphatidylserine, indicating that phosphatidylserine externalization is a conserved apoptotic event in worms. Importantly, inactivation of the gene encoding either the C. elegans apoptosis-inducing factor (AIF) homologue (WAH-1), a mitochondrial apoptogenic factor, or the C. elegans phospholipid scramblase 1 (SCRM-1), a plasma membrane protein, reduces phosphatidylserine exposure on the surface of apoptotic germ cells and compromises cell-corpse engulfment. WAH-1 associates with SCRM-1 and activates its phospholipid scrambling activity in vitro. Thus WAH-1, after its release from mitochondria during apoptosis, promotes plasma membrane phosphatidylserine externalization through its downstream effector, SCRM-1.

Adenine nucleotide translocase: a component of the phylogenetically conserved cell death machinery

“…18 WAH-1 turned out to collaborate with another mitochondrial factor, CPS-6 (the nematode ortholog of endonuclease G), which reportedly co-translocates with WAH-1 to the nucleus where the WAH-1-CPS-6 complex mediates DNA degradation, 18 as well as with the phospholipid scramblase SCRM-1, a plasma membrane protein that favors the exposure of phosphatidylserine (PS) on the surface of apoptotic cells. 19 These observations were in line with the facts that AIF can acquire a nuclease function in mammals and yeast cells (by interacting with proteins from the cyclophilin family), 20 and can stimulate PS exposure in murine cells 17 (Figure 1). These results indirectly revealed the importance of MOMP (which is required for the release of WAH-1/AIF and CPS-6/endonuclease G from the mitochondrial intermembrane space) in the developmental cell death program of C. elegans.…”

Living with death: the evolution of the mitochondrial pathway of apoptosis in animals

“…Most obviously, neither MOMP nor cytochrome c plays a role in initiation of apoptosis in the nematode, although the mitochondrially localized proteins CPS-6 and WAH-1 (nematode homologs of mammalian endonuclease-G and AIF, respectively) have been suggested to contribute to DNA degradation and phosphatydilserine externalization later in the apoptotic program. [16][17][18][19] As noted, CED-4 lacks the WD-domain repeats present in APAF-1 that are necessary for cytochrome c binding, and the release of CED-4 from sequestration by CED-9 appears to be sufficient to cause formation of the nematode apoptosome. Furthermore, despite early reports to the contrary 20,21 it is now accepted that mammalian Bcl-2 homologs do not functionally interact with APAF-1.…”

The Active Role of Corpse Engulfment Pathways During Cell Competition

“…Phosphatidylserine exposure on the outer membrane leaflet is a conserved feature of apoptosis, and recognized by mammalian macrophage and during corpse engulfment in C. elegans. [16][17][18][19] The so-called psr gene was identified by expression-cloning the target of an engulfment-blocking antibody, 20 but is now thought to encode a nuclear protein, and recent studies have convincingly demonstrated that it is not required for corpse engulfment by either mouse fibroblasts or Drosophila hemocytes. 21,22 Meanwhile, other mechanisms of recognizing phosphatidylserine have been uncovered in mammals.…”