The bittersweet interface of parasite and host:: Lectin-carbohydrate interactions during human invasion by the parasite Entamoeba histolytica

Entamoeba histolytica, as its name suggests, is an enteric parasite with a remarkable ability to lyse host tissues. However, the interaction of the parasite with the host is more complex than solely destruction and invasion. It is at the host-parasite interface that cell-signaling events commit the parasite to (a) commensal, noninvasive infection, (b) developmental change from trophozoite to cyst, or (c) invasion and potential death of the human host. The molecule central to these processes is an amebic cell surface protein that recognizes the sugars galactose (Gal) and N-acetylgalactosamine (GalNAc) on the surface of host cells. Engagement of the Gal/GalNAc lectin to the host results in cytoskeletal reorganization in the parasite. The parasite cytoskeleton regulates the extracellular adhesive activity of the lectin and recruits to the host-parasite interface factors required for parasite survival within its host. If the parasite lectin attaches to the host mucin glycoproteins lining the intestine, the result is commensal infection. In contrast, attachment of the lectin to a host cell surface glycoprotein leads to lectin-induced host cell calcium transients, caspase activation, and destruction via apoptosis. Finally, trophozoite quorum sensing via the lectin initiates the developmental pathway resulting in encystment. The structure and function of the lectin that controls these divergent cell biologic processes are the subject of this review.