Proteins Exposed at the Adult Schistosome Surface Revealed by Biotinylation

Abstract

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. Molecular & Cellular Proteomics 5:347-356, 2006.The trematode Schistosoma mansoni is a long lived parasite of the human hepatic portal system, infecting millions of people in Africa and South and Central America (1). Infection of the mammalian host occurs by direct cercarial penetration through the skin followed by transformation into schistosomula. These then undertake a protracted intravascular migration to the hepatic portal vein where they begin to feed on blood, mature, and pair. The male carries the female up the mesenteric blood vessels to the gut wall where she begins to deposit eggs. A proportion of these pass through the tissue and reach the gut lumen to continue the life cycle, whereas others are washed downstream to the liver where they initiate the granulomatous inflammation that characterizes the disease. That schistosomes can survive for 30 -40 years in humans (2) attests to their possession of an effective immune evasion strategy.The mature worm is covered by a syncytial cytoplasmic layer, the tegument, attached to underlying cell bo...

Schistosomes—proteomics studies for potential novel vaccines and drug targets

“…In an attempt to determine which components of the tegument apical membrane are exposed to the host environment, Braschi and Wilson [47] performed an incubation of intact S. mansoni adult worms with two non cell-permeable biotinylation reagents of variable carbon chain length and later recovered the biotinylated proteins using a streptavidin column. A total of 24 schistosome proteins were identified and divided into two groups, one with 9 proteins that were labeled by both reagents with long and short carbon chain, which must represent the more exposed proteins in the tegument, and another group with 15 less exposed proteins that were labeled only by the short carbon chain reagent.…”

“…Although it cannot be ruled out that detection of some of these proteins can be derived from simple contamination of samples with host tissue, detection of host proteins in independent experiments with S. mansoni tegument [47] and egg hatching fluid [53] reinforce the idea that schistosomes might have some mechanism to actively capture host proteins. It has been proposed that presence of these host proteins in schistosome tissues could reflect an escape mechanism from host defense attacks [52].…”

Schistosome apyrase SmATPDase1, but not SmATPDase2, hydrolyses exogenous ATP and ADP

“…To assess the level of target gene suppression post-siRNA treatment, RNA was isolated from parasites and cDNA was synthesized using 1 μg of total RNA, an oligo(dT) 20 primer and Superscript III RT (Invitrogen, CA). Gene expression was measured by quantitative real time PCR (qRT-PCR), using custom TaqMan gene expression systems from Applied Biosystems, CA.…”

“…Unlike SmATPDase1, which has been immunolocalized at the outer border of the tegument at the host/parasite interface [19,20], SmATPDase 2 is found "in some internal cellular structure of the tegument syncytium" from where it has been hypothesized to be secreted into the exterior environment [18]. In this circumstance, SmATPDase2 might perform a similar function to that of SmATPDase1; by degrading exogenous ATP and ADP, this externalized enzyme too may be important in dampening host purinergic signalling pathways and/or in providing purine metabolites suitable for easy uptake as nourishment for the worms.…”

Schistosome vaccines: a critical appraisal

“…We have also characterised the surface of the adult worm tegument (Braschi & Wilson 2006, Braschi et al 2006a, as a prelude to the same approach with lung schistosomula, the targets of the immunity elicited in mice by attenuated cercariae. The proteins regurgitated by the adult worm in the course of blood feeding have also been investigated (Hall 2005).…”