Ordered domains key to Entamoeba virulence

A report in the latest issue of Infection and Immunity presents evidence that not only are lipid rafts present in the membrane of the protozoan parasite Entamoeba histolytica but they also contribute to the virulence of this pathogen.

The view of the plasma membrane as a homogeneous structure was dispelled long ago with the 'fluid mosaic' concept, and further modified more recently with the discovery of lipid rafts — tightly ordered, cholesterol- and glycosphingolipid-enriched microdomains, which, although the methods for their detection remain controversial, have been shown to be involved in many vital cellular functions, including signal transduction, adhesion and secretion. E. histolytica is the causative agent of amoebic dysentery and estimates suggest that up to 100,000 people die each year and some 50 million people are symptomatically infected, almost all in developing countries. After ingestion of cysts in contaminated food or water, E. histolytica excystation occurs in the small intestine, releasing trophozoites that migrate to the large intestine, adhere to the colonic mucus mainly through the interaction of a galactose/N-acetyl galactosamine (Gal/GalNAc)-specific lectin with host glycoconjugates and then obtain nutrients via the endocytic pathway.

To investigate whether the E. histolytica membrane contained raft-like domains, Laughlin et al. stained trophozoites with two lipid analogues — one (DiIC₁₆) that preferentially partitions into ordered domains and another (FAST-DiI) that partitions into more fluid domains. The staining pattern indicated that tightly ordered domains were indeed present. To aid further interpretation of the staining pattern, the experiment was repeated but the trophozoites were first treated with raft-disrupting agents that remove (MBCD) or sequester (Filipin) cholesterol from the membrane. This treatment affected DiIC₁₆ but not FAST-DiI binding, suggesting that DiIC₁₆ co-localized with cholesterol-rich, raft-like domains.

To investigate the physiological function of the rafts, Laughlin et al. went on to look at the effects of raft-disrupting agents on pinocytosis (the uptake of fluids or solutes), secretion and adhesion to host cells. MBCD treatment significantly inhibited fluid-phase pinocytosis but had no effect on the secretion of cysteine proteases, suggesting that raft-like domains are involved in pinocytosis but that the secretion of cysteine proteases is raft independent. MBCD treatment also inhibited the adhesion of trophozoites to a mammalian cell monolayer and further work revealed that the Gal/GalNAc-specific lectin that has an important role in E. histolytica adhesion is enriched in cholesterol-containing membrane fractions; these results suggest that the adhesion of E. histolytica to host cells is also raft dependent.

This work is the first report of the presence of lipid rafts in the E. histolytica membrane. The authors hope that future analysis of these regions will provide a further insight into E. histolytica pathogenesis.