Development of a sensitive real-time quaking-induced conversion (RT-QuIC) assay for application in prion-infected blood.

Efforts to prevent human-to-human transmission of variant Creutzfeldt-Jakob disease (vCJD) by contaminated blood would be aided by the development of a sensitive diagnostic test that could be routinely used to screen blood donations. As blood samples from vCJD patients are extremely rare, here we describe the optimisation of real-time quaking-induced conversion (RT-QuIC) for detection of PrPSc (misfolded prion protein, a marker of prion infection) in blood samples from an established large animal model of vCJD, sheep experimentally infected with bovine spongiform encephalopathy (BSE). Comparative endpoint titration experiments with RT-QuIC, miniaturized bead protein misfolding cyclic amplification (mb-PMCA) and intracerebral inoculation of a transgenic mouse line expressing sheep PrP (tgOvARQ), demonstrated highly sensitive detection of PrPSc by RT-QuIC in a reference sheep brain homogenate. Upon addition of a capture step with iron oxide beads, the RT-QuIC assay was able to detect PrPSc in whole blood samples from BSE-infected sheep up to two years before disease onset. Both RT-QuIC and mb-PMCA also demonstrated sensitive detection of PrPSc in a reference vCJD-infected human brain homogenate, suggesting that either assay may be suitable for application to human blood samples. Our results support the further development and evaluation of RT-QuIC as a diagnostic or screening test for vCJD.

The Schistosoma mansoni tegumental allergen protein, SmTAL1: Binding to an IQ-motif from a voltage-gated ion channel and effects of praziquantel.

SmTAL1 is a calcium binding protein from the parasitic worm, Schistosoma mansoni. Structurally it is comprised of two domains - an N-terminal EF-hand domain and a C-terminal dynein light chain (DLC)-like domain. The protein has previously been shown to interact with the anti-schistosomal drug, praziquantel (PZQ). Here, we demonstrated that both EF-hands in the N-terminal domain are functional calcium ion binding sites. The second EF-hand appears to be more important in dictating affinity and mediating the conformational changes which occur on calcium ion binding. There is positive cooperativity between the four calcium ion binding sites in the dimeric form of SmTAL1. Both the EF-hand domain and the DLC-domain dimerise independently suggesting that both play a role in forming the SmTAL1 dimer. SmTAL1 binds non-cooperatively to PZQ and cooperatively to an IQ-motif from SmCav1B, a voltage-gated calcium channel. PZQ tends to strengthen this interaction, although the relationship is complex. These data suggest the hypothesis that SmTAL1 regulates at least one voltage-gated calcium channel and PZQ interferes with this process. This may be important in the molecular mechanism of this drug. It also suggests that compounds which bind SmTAL1, such as six from the Medicines for Malaria Box identified in this work, may represent possible leads for the discovery of novel antagonists.

The Mechanism of Action of Praziquantel: Can New Drugs Exploit Similar Mechanisms?

Praziquantel (PZQ) is the drug of choice for treating infection with worms from the genus Schistosoma. The drug is effective, cheap and has few side effects. However, despite its use in millions of patients for over 40 years its molecular mechanism of action remains elusive. Early studies demonstrated that PZQ disrupts calcium ion homeostasis in the worm and the current consensus is that it antagonises voltage-gated calcium channels. It is hypothesised that disruption of these channels results in uncontrolled calcium ion influx leading to uncontrolled muscle contraction and paralysis. However, other experimental studies have suggested a role for myosin regulatory light chains and adenosine uptake in the drug's mechanism of action. Assuming voltage-gated calcium channels do represent the main molecular target of PZQ, the precise binding site for the drug remains to be identified. Unlike other commonly used anti-parasitic drugs, there are few definitive reports of resistance to PZQ in the literature. The lack of knowledge about PZQ's molecular mechanism(s) undermines our ability to predict how resistance might arise and also hinder our attempts to develop alternative antischistosomal drugs which exploit the same target(s). Some PZQ derivatives have been identified which also kill or paralyse schistosomes in culture. However, none of these are in widespread clinical use. There is a pressing need for fundamental research into the molecular mechanism( s) of action of PZQ. Such research would enable new avenues for antischsistosomal drug discovery.

Characterization of Calcium-Binding Proteins from Parasitic Worms.

Parasitic diseases caused by helminths (worms) represent a major burden on humanity with hundreds of millions of people infected worldwide. However, there are relatively few drugs to treat these diseases, and resistance is emerging to some of these. Therefore, there is a pressing need to characterize proteins from helminths as potential drug targets. Calcium signalling proteins represent attractive targets due to the vital nature of properly regulated calcium-mediated signalling and the presence of unusual calcium-binding proteins in helminths. Here we present methods to characterize these proteins in terms of their ion-binding properties, drug-binding properties, and oligomeric state, including a method to correct for the effects of non-spherical proteins in analytical gel filtration. In addition we present an overview of their recombinant expression and purification and methods to predict their structures.

The Mechanism of Action of Praziquantel: Six Hypotheses.

Despite being one of the most commonly used drugs, the molecular mechanism of action of the anthelmintic praziquantel remains unknown. There are some unusual features of this drug. Critically, widespread resistance to praziquantel has not developed despite decades of use. Here, we set out some challenges in praziquantel research and propose some provocative hypotheses to address these. We suggest that praziquantel may have multiple pharmacologically relevant targets and the effects on these may synergise to produce an overall, detrimental effect on the parasite. Praziquantel also acts on a number of host proteins and we propose that these actions are important in the drug's overall mechanism. Although the drug is largely used in the treatment of human and domestic animal worm infections, there is a considerable "grey literature" along with some academic studies which may have been overlooked. It appears that praziquantel may be effective against hydra. It may also be effective against some unicellular parasites such as Giardia spp. Further, scientific work on these understudied areas may be useful in understanding the molecular mechanism in Trematoda. The lack of widespread resistance suggests that praziquantel may act, at least in part, on a protein-protein interaction. Altered drug metabolism or enhanced drug efflux are the most likely ways resistance may arise. There is a critical need to understand the biochemical pharmacology of this drug in order to inform the discovery of the next generation of anthelmintic drugs.

Calmodulins from Schistosoma mansoni: Biochemical analysis and interaction with IQ-motifs from voltage-gated calcium channels.

The trematode Schistosoma mansoni is a causative agent of schistosomiasis, the second most common parasitic disease of humans after malaria. Calcium homeostasis and calcium-mediated signalling pathways are of particular interest in this species. The drug of choice for treating schistosomiasis, praziquantel, disrupts the regulation of calcium uptake and there is interest in exploiting calcium-mediated processes for future drug discovery. Calmodulin is a calcium sensing protein, present in most eukaryotes. It is a critical regulator of processes as diverse as muscle contraction, cell division and, partly through interaction with voltage-gated calcium channels, intra-cellular calcium concentrations. S. mansoni expresses two highly similar calmodulins - SmCaM1 and SmCaM2. Both proteins interact with calcium, manganese, cadmium (II), iron (II) and lead ions in native gel electrophoresis. These ions also cause conformational changes in the proteins resulting in the exposure of a more hydrophobic surface (as demonstrated by anilinonaphthalene-8-sulfonate fluorescence assays). The proteins are primarily dimeric in the absence of calcium ions, but monomeric in the presence of this ion. Both SmCaM1 and SmCaM2 interact with a peptide corresponding to an IQ-motif derived from the α-subunit of the voltage-gated calcium channel SmCav1B (residues 1923-1945). Both proteins bound with slightly higher affinity in the presence of calcium ions. However, there was no difference between the affinities of the two proteins for the peptide. This interaction could be antagonised by chlorpromazine and trifluoperazine, but not praziquantel or thiamylal. Interestingly no interaction could be detected with the other three IQ-motifs identified in S. mansoni voltage-gated ion calcium channels.

The tegumental allergen-like proteins of Schistosoma mansoni: A biochemical study of SmTAL4-TAL13.

Schistosoma mansoni, like other trematodes, expresses a number of unusual calcium binding proteins which consist of an EF-hand domain joined to a dynein light chain-like (DLC-like) domain by a flexible linker. These proteins have been implicated in host immune responses and drug binding. Three members of this protein family from S. mansoni (SmTAL1, SmTAL2 and SmTAL3) have been well characterised biochemically. Here we characterise the remaining family members from this species (SmTAL4-13). All of these proteins form homodimers and all except SmTAL5 bind to calcium and manganese ions. SmTAL9, 10 and 11 also bind to magnesium ions. The antischistosomal drug, praziquantel interacts with SmTAL4, 5 and 8. Some family members also bind to calmodulin antagonists such as chlorpromazine and trifluoperazine. Molecular modelling suggests that all ten proteins adopt similar overall folds with the EF-hand and DLC-like domains folding discretely. Bioinformatics analyses suggest that the proteins may fall into two main categories: (i) those which bind calcium ions reversibly at the second EF-hand and may play a role in signalling (SmTAL1, 2, 8 and 12) and (ii) those which bind calcium ions at the first EF-hand and may play either signalling or structural roles (SmTAL7, 9, 10 and 13). The remaining proteins include those which do not bind calcium ions (SmTAL3 and 5) and three other proteins (SmTAL4, 6 and 11). The roles of these proteins are less clear, but they may also have structural roles.

FhCaBP1 (FH22): A Fasciola hepatica calcium-binding protein with EF-hand and dynein light chain domains.

FH22 has been previously identified as a calcium-binding protein from the common liver fluke, Fasciola hepatica. It is part of a family of at least four proteins in this organism which combine an EF-hand containing N-terminal domain with a C-terminal dynein light chain-like domain. Here we report further biochemical properties of FH22, which we propose should be renamed FhCaBP1 for consistency with other family members. Molecular modelling predicted that the two domains are linked by a flexible region and that the second EF-hand in the N-terminal domain is most likely the calcium ion binding site. Native gel electrophoresis demonstrated that the protein binds both calcium and manganese ions, but not cadmium, magnesium, strontium, barium, cobalt, copper(II), iron (II), nickel, zinc, lead or potassium ions. Calcium ion binding alters the conformation of the protein and increases its stability towards thermal denaturation. FhCaBP1 is a dimer in solution and calcium ions have no detectable effect on the protein's ability to dimerise. FhCaBP1 binds to the calmodulin antagonists trifluoperazine and chlorpromazine. Overall, the FhCaBP1's biochemical properties are most similar to FhCaBP2 a fact consistent with the close sequence and predicted structural similarity between the two proteins.

A mysterious family of calcium-binding proteins from parasitic worms.

There is a family of proteins from parasitic worms which combine N-terminal EF-hand domains with C-terminal dynein light chain-like domains. Data are accumulating on the biochemistry and cell biology of these proteins. However, little is known about their functions in vivo Schistosoma mansoni expresses 13 family members (SmTAL1-SmTAL13). Three of these (SmTAL1, SmTAL2 and SmTAL3) have been subjected to biochemical analysis which demonstrated that they have different molecular properties. Although their overall folds are predicted to be similar, small changes in the EF-hand domains result in differences in their ion binding properties. Whereas SmTAL1 and SmTAL2 are able to bind calcium (and some other) ions, SmTAL3 appears to be unable to bind any divalent cations. Similar biochemical diversity has been seen in the CaBP proteins from Fasciola hepatica Four family members are known (FhCaBP1-4). All of these bind to calcium ions. However, FhCaBP4 dimerizes in the presence of calcium ions, FhCaBP3 dimerizes in the absence of calcium ions and FhCaBP2 dimerizes regardless of the prevailing calcium ion concentration. In both the SmTAL and FhCaBP families, the proteins also differ in their ability to bind calmodulin antagonists and related drugs. Interestingly, SmTAL1 interacts with praziquantel (the drug of choice for treating schistosomiasis). The pharmacological significance (if any) of this finding is unknown.

Fasciola hepatica calcium-binding protein FhCaBP2: structure of the dynein light chain-like domain.

The common liver fluke Fasciola hepatica causes an increasing burden on human and animal health, partly because of the spread of drug-resistant isolates. As a consequence, there is considerable interest in developing new drugs to combat liver fluke infections. A group of potential targets is a family of calcium-binding proteins which combine an N-terminal domain with two EF-hand motifs and a C-terminal domain with predicted similarity to dynein light chains (DLC-like domain). The function of these proteins is unknown, although in several species, they have been localised to the tegument, an important structure at the host-parasite interface. Here, we report the X-ray crystal structure of the DLC-like domain of F. hepatica calcium-binding protein 2 (FhCaBP2), solved using single-wavelength anomalous diffraction and refined at 2.3 Å resolution in two different crystal forms. The FhCaBP2 DLC-like domain has a structure similar to other DLC domains, with an anti-parallel ß-sheet packed against an α-helical hairpin. Like other DLC domains, it dimerises through its ß2-strand, which extends in an arch and forms the fifth strand in an extended ß-sheet of the other monomer. The structure provides molecular details of the dimerisation of FhCaBP2, the first example from this family of parasite proteins.

FhCaBP2: a Fasciola hepatica calcium-binding protein with EF-hand and dynein light chain domains.

FhCaBP2 is a Fasciola hepatica protein which belongs to a family of helminth calcium-binding proteins which combine an N-terminal domain containing two EF-hand motifs and a C-terminal dynein light chain-like (DLC-like) domain. Its predicted structure showed two globular domains joined by a flexible linker. Recombinant FhCaBP2 interacted reversibly with calcium and manganese ions, but not with magnesium, barium, strontium, copper (II), colbalt (II), iron (II), nickel, lead or potassium ions. Cadmium (II) ions appeared to bind non-site-specifically and destabilize the protein. Interaction with either calcium or magnesium ions results in a conformational change in which the protein's surface becomes more hydrophobic. The EF-hand domain alone was able to interact with calcium and manganese ions; the DLC-like domain was not. Alteration of a residue (Asp-58 to Ala) in the second EF-hand motif in this domain abolished ion-binding activity. This suggests that the second EF-hand is the one responsible for ion-binding. FhCaBP2 homodimerizes and the extent of dimerization was not affected by calcium ions or by the aspartate to alanine substitution in the second EF-hand. The isolated EF-hand and DLC-like domains are both capable of homodimerization. FhCaBP2 interacted with the calmodulin antagonists trifluoperazine, chlorpromazine, thiamylal and W7. Interestingly, while chlorpromazine and thiamylal interacted with the EF-hand domain (as expected), trifluoperazine and W7 bound to the DLC-like domain. Overall, FhCaBP2 has distinct biochemical properties compared with other members of this protein family from Fasciola hepatica, a fact which supports the hypothesis that these proteins have different physiological roles.

Comparative biochemical analysis of three members of the Schistosoma mansoni TAL family: Differences in ion and drug binding properties.

The tegumental allergen-like (TAL) proteins from Schistosoma mansoni are part of a family of calcium binding proteins found only in parasitic flatworms. These proteins have attracted interest as potential drug or vaccine targets, yet comparatively little is known about their biochemistry. Here, we compared the biochemical properties of three members of this family: SmTAL1 (Sm22.6), SmTAL2 (Sm21.7) and SmTAL3 (Sm20.8). Molecular modelling suggested that, despite similarities in domain organisation, there are differences in the three proteins' structures. SmTAL1 was predicted to have two functional calcium binding sites and SmTAL2 was predicted to have one. Despite the presence of two EF-hand-like structures in SmTAL3, neither was predicted to be functional. These predictions were confirmed by native gel electrophoresis, intrinsic fluorescence and differential scanning fluorimetry: both SmTAL1 and SmTAL2 are able to bind calcium ions reversibly, but SmTAL3 is not. SmTAL1 is also able to interact with manganese, strontium, iron(II) and nickel ions. SmTAL2 has a different ion binding profile interacting with cadmium, manganese, magnesium, strontium and barium ions in addition to calcium. All three proteins form dimers and, in contrast to some Fasciola hepatica proteins from the same family; dimerization is not affected by calcium ions. SmTAL1 interacts with the anti-schistosomal drug praziquantel and the calmodulin antagonists trifluoperazine, chlorpromazine and W7. SmTAL2 interacts only with W7. SmTAL3 interacts with the aforementioned calmodulin antagonists and thiamylal, but not praziquantel. Overall, these data suggest that the proteins have different biochemical properties and thus, most likely, different in vivo functions.