A04: The massively expanded endocytotic machinery of Trichomonas vaginalis

Trichomonas vaginalis infects 3% of the world’s population annually. The parasite’s biology is remarkable and its overarching role within the vaginal microflora not well understood. Apart from causing trichomoniasis, the parasite alters the vaginal microflora through phagocytosing here thriving bacteria and fungi — and even macrophages. It has neither been explored how the parasite recognizes different prey, which we hypothesize is based on the membrane identity mediated by prey-specific surface molecules, nor how the material is processed once endo- or phagocytosed. This eukaryote metabolises substrate rapidly and is able to synthesise a new daughter cell from scavenged material (including human tissue and cells) in as little as three hours. Attaching to the host cell surface, or other microbes, and subsequent uptake of substrate occurs within only minutes upon recognition of the prey’s surface membrane. The parasites own plasma membrane dynamics upon different prey recognition differs significantly, too, because yeast and bacterial cells are completely engulfed by the parasite and phagocytosed, but human tissue is largely endocytosed by the amoeboid form of virulent strains. How is this prey diversity recognized and the different morphologies achieved? Furthermore, could the diversity of the parasite’s surface also be associated with evading the host’s immune system?

            Preliminary data has revealed that Trichomonas encodes hundreds of different proteins integrated into the plasma membrane through a single C-terminal transmembrane-spanning tail. Many are united by conserved C-terminal motifs that are all recognized by the (clathrin-mediated) endocytosis machinery. The very last three amino acids of some of these tails are furthermore conspicuous, as they terminate with a conserved motif usually associated with a kinase-related molecular switch. In addition to the complexity of the plasma membrane proteome, the machinery for downstream packaging and transport of the absorbed cargo is massively expanded, too. More than 290 Rab proteins of small GTPases and many dozen adaptins are encoded by the parasite’s 160-170 Mb large genome. Analyses have uncovered that a large set of the membrane proteins exposed on the parasite surface, and thought to assist in host tissue binding, stem from other mucosal pathogens. Among them are the BspA proteins (Bacteroides surface protein A) — with more than 900 members the largest family — and the Pmp proteins (polymorphic membrane proteins) of mostly chlamydial origin. Members of the Trichomonadida family not only infect humans. There are dozens of other species that infect all kinds of vertebrate hosts including farm animals (infected e.g. by Trichomitus batrachorum) and birds (infected e.g. by T. gallinae). Hence, one aim of this proposal is to commence comparative studies on a set of Trichomonadida species in order to identify human-specific surface molecules that mediate cell-cell interaction.

            In more detail, as part of the CRC initiative, our group will characterise the surface proteome of the parasite in combination with the endocytotic machinery as both, in concert, are key virulence factors with regard to pathogenicity. We will: (i) identify the parasite’s surface molecules and gene families encoding proteins involved in prey recognition and endo/phagocytosis and sort them according to higher-ranking groups. The simultaneous comparison to other parasitic Trichomonadida species will allow the identification of gene families (or their expansion) specific to the human pathogen, (ii) characterize individual and abundant candidates involved in the recognition of different prey and its subsequent processing and recycling [this will include the expression of prime candidates of virulent (i.e. adherent) strains in non-virulent (i.e. non-adherent) strains], and (iii) determine whether surface membrane and protein recycling could potentially be involved in evading the human immune system. Our group will complement the CRC-initiative by working on a non-model organism and parasite of an understudied eukaryotic protist group, providing the expertise to characterize eukaryotic cell biology in the light of evolution. Our work package will characterise the most complex endo/phagocytosis machinery so far identified, which evolved to scavenge from a multitude of different cell types, herby making Trichomonas the most successful eukaryotic pathogen transmitted through sex.

Figure 1: The two major phenotypes of Trichomonas vaginalis. S.B. Gould et al., International Journal for Parasitology 43 (2013) 707–719

Project leader:
PD Dr. Sven Gould, undefined email, undefinedInstitute of Molecular Evolution

Researcher: Maria Handrichundefined email

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