Developmentally regulated expression, alternative splicing and distinct sub-groupings in members of the Schistosoma mansoni venom allergen-like (SmVAL) gene family.

BACKGROUND: The Sperm-coating protein/Tpx-1/Ag5/PR-1/Sc7 (SCP/TAPS) domain is found across phyla and is a major structural feature of insect allergens, mammalian sperm proteins and parasitic nematode secreted molecules. Proteins containing this domain are implicated in diverse biological activities and may be important for chronic host/parasite interactions. RESULTS: We report the first description of an SCP/TAPS gene family (Schistosoma mansoni venom allergen-like (SmVALs)) in the medically important Platyhelminthes (class Trematoda) and describe individual members' phylogenetic relationships, genomic organization and life cycle expression profiles. Twenty-eight SmVALs with complete SCP/TAPS domains were identified and comparison of their predicted protein features and gene structures indicated the presence of two distinct sub-families (group 1 & group 2). Phylogenetic analysis demonstrated that this group 1/group 2 split is zoologically widespread as it exists across the metazoan sub-kingdom. Chromosomal localisation and PCR analysis, coupled to inspection of the current S. mansoni genomic assembly, revealed that many of the SmVAL genes are spatially linked throughout the genome. Quantitative lifecycle expression profiling demonstrated distinct SmVAL expression patterns, including transcripts specifically associated with lifestages involved in definitive host invasion, transcripts restricted to lifestages involved in the invasion of the intermediate host and transcripts ubiquitously expressed. Analysis of SmVAL6 transcript diversity demonstrated statistically significant, developmentally regulated, alternative splicing. CONCLUSION: Our results highlight the existence of two distinct SCP/TAPS protein types within the Platyhelminthes and across taxa. The extensive lifecycle expression analysis indicates several SmVAL transcripts are upregulated in infective stages of the parasite, suggesting that these particular protein products may be linked to the establishment of chronic host/parasite interactions.

Plasmodium falciparum likely encodes the principal anion channel on infected human erythrocytes.

Invasion by the human malaria parasite, Plasmodium falciparum, is associated with marked yet selective increases in red blood cell (RBC) membrane permeability. We previously identified an unusual voltage-dependent ion channel, the plasmodial surface anion channel (PSAC), which may account for these increases. Since then, controversy has arisen about whether there are additional parasite-induced anion channels on the RBC membrane and whether these channels are parasite-encoded proteins or the result of modifications of an endogenous host protein. Here, we used genetically divergent parasite isolates and quantitative transport measurements to examine these questions. Our studies indicate that PSAC alone can adequately account for the increased permeability of infected RBCs to key solutes. Two distinct parasite isolates, grown in RBCs from a single donor, exhibit channel activity with measurably different voltage-dependent gating, a finding difficult to reconcile with simple activation or modification of a host protein. Instead, this difference in channel gating can be conservatively explained by a small number of polymorphisms in a parasite gene that encodes PSAC. The absence of known eukaryotic ion channel homologues in the completed P falciparum genome suggests a novel channel gene, and substantiates PSAC as a target for antimalarial development.

Extracellular lysines on the plasmodial surface anion channel involved in Na+ exclusion.

The human malaria parasite, Plasmodium falciparum, induces an unusual ion channel, the plasmodial surface anion channel (PSAC), on its host red blood cell (RBC) membrane. PSAC has a broad selectivity with permeability to anions, sugars, amino acids, purines, and certain vitamins, suggesting a role in nutrient acquisition by the intracellular parasite. Permeating solutes cover a range of molecular sizes and may be either neutral or carry a net negative or positive charge. Despite this broad selectivity, PSAC must efficiently exclude Na+ to avoid osmotic lysis of infected RBCs in the bloodstream. Here, we used amine-reactive N-hydroxysulfosuccinimide esters to probe PSAC's unusual selectivity. PSAC permeation rates, measured with both a kinetic osmotic lysis assay and single-channel patch-clamp, irreversibly decrease after treatment with these reagents. Sequential labelings with different esters and the effects of their chain length suggest that PSAC has multiple lysine residues near its extracellular pore mouth and that inhibition occurs via steric hindrance of its pore by the amide-linked side chain. When combined with the effects of pH on permeation, these findings implicate a combination of cation repulsion by pore mouth charges and a weak binding site for permeant solutes in PSAC's broad selectivity yet effective exclusion of Na+.

A two-compartment model of osmotic lysis in Plasmodium falciparum-infected erythrocytes.

We recently identified a voltage-dependent anion channel on the surface of human red blood cells (RBCs) infected with the malaria parasite, Plasmodium falciparum. This channel, the plasmodial erythrocyte surface anion channel (PESAC), likely accounts for the increased permeability of infected RBCs to various small solutes, as assessed quantitatively with radioisotope flux and patch-clamp studies. Whereas this increased permeability has also been studied by following osmotic lysis of infected cells in permeant solutes, these experiments have been limited to qualitative comparisons of lysis rates. To permit more quantitative examination of lysis rates, we have developed a mathematical model for osmotic fragility of infected cells based on diffusional uptake via PESAC and the two-compartment geometry of infected RBCs. This model, combined with a simple light scattering assay designed to track osmotic lysis precisely, produced permeability coefficients that match both previous isotope flux and patch-clamp estimates. Our model and light scattering assay also revealed Michaelian kinetics for inhibition of PESAC by furosemide, suggesting a 1:1 stoichiometry for their interaction.