Correlation between the expression of lipids in the inner membrane complex of Toxoplasma gondii and its virulent phenotypes

Abstract

Toxoplasma gondii, a highly prevalent obligate apicomplexan parasite, is important in clinical and/or veterinary medicine. Based on its virulence in laboratory mouse strains, T. gondii is divided into three lineages. In this study, we examined and compared the distributions of the glycerophospholipids phosphatidylserine (PtdSer) and phosphatidylethanolamine (PtdEtn) and a glycosphingolipid GM3 in type I RH and type II PLK strains of T. gondii (highly and moderately virulent phenotypes, respectively). The gold labeling density of PtdSer in the exoplasmic leaflet (EF) of the inner membrane complex (IMC) in the RH strain was significantly higher than that in the PLK strain. PtdEtn labeling was significantly higher in the EF than in the cytoplasmic leaflet (PF) of the middle and inner IMC membranes in the RH strain. In contrast, PtdEtn labeling was low in the PLK strain. The gold labeling density of GM3, localized in the EF of the IMC membrane, was much higher in the RH strain than that in the PLK strain. GM3 is a major raft component and both PtdSer and PtdEtn affect the maintenance of raft microdomains, which are important for gliding motility when T. gondii enters mammalian host cells. Raft microdomains have significant roles in the virulence of T. gondii. Therefore, we suggest that the PtdSer and PtdEtn expression levels are correlated with the virulent phenotypes of T. gondii. Furthermore, we demonstrated that PtdSer was absent in the PF of the inner IMC membrane but present in the PF of the middle IMC membrane. This suggests that a barrier-like mechanism that prevents the diffusion of PtdSer exists in the PF between these two membranes.

Phosphatidylserine synthase in the endoplasmic reticulum of Toxoplasma is essential for its lytic cycle in human cells

“…In T. gondii tachyzoites, while PtdThr and PtdIns are implicated in regulating calcium signaling and homeostasis in tachyzoites ( 4 , 7 ), PtdOH and PtdSer promote microneme exocytosis and gliding motility in response to calcium signaling ( 5 , 8 ), The PSS-depleted mutant also suggests a role of PtdSer in regulating the locomotion of tachyzoites. A recent study deployed electron microscopy (quick-freeze-fracture replica labeling) to determine the distribution of PtdSer and PtdEtn in parasite membrane leaflets ( 42 ). The authors observed that both lipids are primarily localized in the luminal leaflet of the inner membrane complex (IMC), located just underneath the plasma membrane.…”

“…Konishi et al. found that PtdSer is mainly confined to the cytoplasmic leaflet of the middle IMC membrane, while it was absent in the cytoplasmic leaflet of the inner IMC membrane ( 42 ). The data suggest a barrier-like mechanism preventing diffusion of PtdSer in the cytoplasmic leaflets of the two membranes.…”

“…PtdThr is a structural analog of PtdSer, which complicates the interpretation of PtdSer-related works described above. Our preliminary work has revealed that tools such as PtdSer-specific biosensors (Lact-C2-GFP, ( 45 )) and electron microscopy ( 42 ) cannot discriminate between PtdSer and PtdThr. Furthermore, albeit PtdThr can now be synthesized ( 46 ), its research-grade probes are not commercially available.…”