Using Phosphatidylserine Exposure on Apoptotic Cells to Stimulate Myoblast Fusion

Abstract Procoagulant platelets exhibit hallmark features of apoptotic cells, including membrane blebbing, microvesiculation, and phosphatidylserine (PS) exposure. Although platelets possess many well-known apoptotic regulators, their role in regulating the procoagulant function of platelets is unclear. To clarify this, we investigated the consequence of removing the essential mediators of apoptosis, Bak and Bax, or directly inducing apoptosis with the BH3 mimetic compound ABT-737. Treatment of platelets with ABT-737 triggered PS exposure and a marked increase in thrombin generation in vitro. This increase in procoagulant function was Bak/Bax- and caspase-dependent, but it was unaffected by inhibitors of platelet activation or by chelating extracellular calcium. In contrast, agonist-induced platelet procoagulant function was unchanged in Bak−/−Bax−/− or caspase inhibitor–treated platelets, but it was completely eliminated by extracellular calcium chelators or inhibitors of platelet activation. These studies show the existence of 2 distinct pathways regulating the procoagulant function of platelets.

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Apoptotic Killing and Phagocytosis of Host Cells by the Parasite Entamoeba histolytica

Infection and Immunity ◽

10.1128/iai.71.2.964-972.2003 ◽

2003 ◽

Vol 71 (2) ◽

pp. 964-972 ◽

Cited By ~ 117

Author(s):

Christopher D. Huston ◽

Douglas R. Boettner ◽

Vanessa Miller-Sims ◽

William A. Petri,

Keyword(s):

Host Cell ◽

Entamoeba Histolytica ◽

Caspase 3 ◽

Apoptotic Cell ◽

Cell Killing ◽

Jurkat Cells ◽

Host Cells ◽

Apoptotic Cells ◽

Outer Leaflet ◽

Phosphatidylserine Exposure

ABSTRACT The ability of Entamoeba histolytica to kill and phagocytose host cells correlates with parasite virulence. This study addressed the role of apoptotic cell killing and host cell phosphatidylserine exposure in the subsequent phagocytosis of Jurkat T cells by E. histolytica. Ingested host cells were apoptotic, as evidenced by the activation of caspase 3 in 88% ± 3% (mean and standard deviation [SD] of the mean) of Jurkat cells engulfed by E. histolytica; ingested cells without detectable active caspase 3 were already disrupted and partially digested. That apoptotic cell killing preceded phagocytosis was supported by the demonstration that a higher percentage of amebae ingested apoptotic cells than ingested healthy cells (62% ± 7% versus 30% ± 9%, respectively [mean and SD]) (P = 0.008). E. histolytica also ingested apoptotic Jurkat cells more rapidly than necrotic control cells (8.5% ± 0.4% versus 3.5% ± 0.7%, respectively [mean and SD]) (P < 0.001). The inhibition of amebic cytotoxicity with d-galactose (which blocks the amebic Gal/GalNAc lectin) blocked the phagocytosis of healthy cells by greater than 80%, providing further evidence that apoptosis preceded engulfment. In contrast, d-galactose blocked the phagocytosis of already apoptotic cells by only 40%, implicating an additional host ligand (besides d-galactose) in amebic engulfment of apoptotic cells. The most characteristic surface change on apoptotic cells is phosphatidylserine exposure. Consistent with a role for host cell phosphatidylserine exposure in amebic ingestion of killed cells, Jurkat cell phosphatidylserine was exposed during incubation with E. histolytica (27% ± 1% [mean and SD] specific increase at 30 min) (the P value versus the control was 0.0003). Approximately 50% more amebae ingested viable Jurkat cells expressing phosphatidylserine on the outer leaflet of the plasma membrane than ingested control cells (30.3% ± 2.2% versus 19.8% ± 1.9%, respectively [mean and SD]) (P = 0.003). By analogy with phagocytic clearance during apoptosis in metazoans, amebic apoptotic host cell killing followed by phagocytosis may limit inflammation and enable amebae to evade the host immune response.

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