Cell Death in Trichomonads

Abstract

Programmed cell death (PCD) is not confined to mammals and is extremely widespread, possibly universal, in multicellular animals. It is now evident that PCD also occurs in single-celled organisms and it is an important feature of host-pathogen relationships. Protists are capable of eliciting an apoptotic response in several circumstances. Studies are in course in order to establish whether these organism share some or all of the effectors and regulators common to multicellular PCD or have evolved their own divergent pathways. Trichomonads are amitochondrial protists that inhabit different ecological niches. Among them Tritrichomonas foetus, a cattle parasite and Trichomonas vaginalis, a human parasite, are the most important because they cause trichomoniasis, a sexually transmitted disease. They do not possess mitochondria, but harbor another type of membrane-bounded organelle, an unusual anaerobic energy-producing organelle called hydrogenosome. Studies of cell death in trichomonads are under way in order to establish whether the hydrogenosome could represent an alternative to mitochondria whether these organisms possess all caspase activities and which conditions lead trichomonads to cell death. In these organisms the known “mitochondrial cell death machinery” is supposed to be distinct from mitochondrial eukaryotes. The presence of a cell death program in trichomonads suggests the existence either of a dependent or independent caspase-like execution pathway in such organisms. Dramatic changes in trichomonad morphology are observed when the cells are under stress, such as after drug treatment and nutrient depletion. These changes include intense plasma membrane and nuclear envelope blebbing, nucleus fragmentation, an abnormal number of oversized vacuoles and altered hydrogenosomes. DNA fragmentation, exposure of phosphatidylserine (PS) in the outer leaflet of the plasma membrane, hydrogenosomal membrane potential dissipation, are features observed in apoptotic cells. Trichomonads also present autophagic processes, observed when altered hydrogenosomes, misshapen flagella, abnormal cellular elements and tubulin precipitates are located in autophagic vacuoles, which are limited by a double or multiple concentric membrane. In all stress situations, trichomonads form pseudocysts, cells with internalized flagella. Different forms of cell death, such as apoptosis, autophagy and necrosis have been shown to exist in trichomonads and so the possibility of the existence of different pathways to cell death in trichomonads is raised.