Atypical post-translational modification and targeting of a Schistosoma mansoni surface receptor, a member of the transforming growth factor beta receptor family of cell surface receptors

Schistosoma mansoni: TGF-beta signaling pathways

Schistosome parasites have co-evolved an intricate relationship with their human and snail hosts as well as a novel interplay between the adult male and female parasites. We review the role of the TGF-beta signaling pathway in parasite development, host-parasite interactions and male-female interactions. The data to date support multiple roles for the TGF-beta signaling pathway throughout schistosome development, in particular, in the tegument which is at the interface with the host and between the male and female schistosome, development of vitelline cells in female worms whose genes and development are regulated by a stimulus from the male schistosome and embryogenesis of the egg. The human ligand TGF-beta1 has been demonstrated to regulate the expression of a schistosome target gene that encodes a gynecophoric canal protein in the schistosome worm itself. Studies on signaling in schistosomes opens a new era for investigation of host-parasite and male-female interactions.

Proteins exposed at the adult schistosome surface revealed by biotinylation

The human blood-dwelling parasite Schistosoma mansoni can survive in the hostile host environment for decades and must therefore display effective strategies to evade the host immune responses. The surface of the adult worm is covered by a living syncytial layer, the tegument, bounded by a complex multilaminate surface. This comprises a normal plasma membrane overlain by a secreted bilayer, the membranocalyx. Recent proteomic studies have identified constituents of the tegument, but their relative locations remain to be established. We labeled the most exposed surface proteins using two impermeant biotinylation reagents that differed only in length. We anticipated that the two reagents would display distinct powers of penetration, thereby producing a differential labeling pattern. The labeled proteins were recovered by streptavidin affinity and identified by tandem mass spectrometry. A total of 28 proteins was identified, 13 labeled by a long form reagent and the same 13 plus a further 15 labeled by a short form reagent. The parasite proteins included membrane enzymes, transporters, and structural proteins. The short form reagent additionally labeled some cytosolic and cytoskeletal proteins, the latter being constituents of the intracellular spines. Only a single secreted protein was labeled, implying a location between the plasma membrane and the membranocalyx or as part of the latter. Four host proteins, three immunoglobulin heavy chains and C3c/C3dg, a fragment of complement C3, were labeled by both reagents indicating their exposed situation. The presence of the degraded complement C3 implicates inhibition of the classical pathway as a major element of the immune evasion strategy, whereas the recovery of only one truly secreted protein points to the membranocalyx acting primarily as an inert protective barrier between the immune system and the tegument plasma membrane. Collectively the labeled parasite proteins merit investigation as potential vaccine candidates.

The cellular distribution and stage-specific expression of two dynein light chains from the human blood fluke Schistosoma japonicum

Schistosomes are pathogenic helminth parasites of human portal veins. Their body wall is a highly active syncytial tegument involved in an array of host interactions. The cytoskeletal organization and dynamics of this syncytium are poorly understood, but predominant motor components are the LC8 class of cytoplasmic dynein light chains (DLC). Four LC8 members occur in schistosomes, two of which are expressed in the tegument. Here, we describe the cytoplasmic distribution, stage-specific expression and cellular location of two diverse LC8 molecules of Schistosoma japonicum. SjDLC1 was detected in surface-membrane specific extracts of adult worms and was shown by quantitative immuno-electron microscopy to predominate along heptalaminate membranes of the worm surface. SjDLC3 also occurs in the tegument, but was shown to be present in basal layers of the tegument and did not preferentially co-localize with particular membrane components. SjDLC3 was also detected in the gastrodermis. SjDLC1 is expressed only in mammalian-parasitic stages, whereas SjDLC3 is expressed throughout the life-cycle. The data suggest that SjDLC1 is preferentially located to the host-interactive distal parasite membrane, and plays a role in surface membrane dynamics, while SjDLC3 is a ubiquitous motor component of schistosome epithelia of all stages.

Tegumental expression of a novel type II receptor serine/threonine kinase (SmRK2) in Schistosoma mansoni

The TGF-beta family of receptor serine/threonine kinases (RSTKs) is responsible for a diverse array of functions in metazoans. Here, we describe the isolation of SmRK2, a type II RSTK expressed in schistosomula and adult stages of Schistosoma mansoni. Based on amino acid sequence homology, SmRK2 is most closely related to the Activin type II receptor subset of RSTKs. SmRK2 appears to be expressed as three different transcripts: one encoding a full-length receptor with 5'- and 3'-untranslated regions (UTRs) (SmRK2), a second encoding a longer form containing no 3'-UTR and no stop codon (SmRK2a), and a third truncated variant (SmRK2b), which contains sequence encoding the first 53 amino acids of the N-terminal extracellular domain followed by an inserted 10 residue hydrophobic domain. Using an anti-peptide antibody raised against a partial extracellular domain sequence common to all three isoforms, SmRK2 was localized predominantly to the tegumental surface of the parasites. We hypothesize that SmRK2 is the receptor partner for the previously reported type I RSTK SmRK1 (or SmTbetaR1) and that together these proteins constitute a receptor system for receiving signals from the mammalian host.

The guanine protein coupled receptor rhodopsin is developmentally regulated in the free-living stages of Schistosoma mansoni

Schistosoma mansoni parasites inhabit three distinct environments including water, intermediate molluscan hosts, and definitive vertebrate hosts. Determining how schistosomes interact with these environments may be one mechanism by which suitable vaccines or novel chemotherapeutic targets will be identified. Towards this end, we describe the identification of a 36-kDa S. mansoni protein that shares extensive sequence similarity to light absorbing rhodopsin guanine protein coupled receptors (GPCRs). This protein, S. mansoni rhodopsin (SmRHO), is the first molecularly characterized GPCR described in schistosomes. Sequence analysis reveals that SmRHO shares extensive phylogenetic conservation among rhodopsins/opsins expressed in water-dwelling invertebrates, possibly indicative of orthology. We demonstrate here that SmRHO is expressed in the free-living, light responsive miracidia and cercaria stages and is down-regulated in the adult, vertebrate residing forms. Moreover, we show that SmRHO is localized to sub-tegumental structures found towards the anterior end of cercariae. As SmRHO may be implicated in schistosome photoreception processes, we have begun a search for additional parasite encoded GPCR super-family members, which may be associated with chemoreception, chemotaxis, and olfaction. Identifying and characterizing new GPCRs may uncover hidden aspects of parasite biology useful towards the development of novel intervention strategies.