Summary Cerebral malaria is a major complication of Plasmodium falciparum infection in children. The pathogenesis of cerebral malaria involves vascular inflammation, immune stimulation and obstruction of cerebral capillaries. Platelets have a prominent role in both immune responses and vascular obstruction. We now demonstrate that the platelet derived chemokine, platelet factor 4 (PF4)/CXCL4, promotes the development of experimental cerebral malaria. Plasmodium infected red blood cells (RBC) activated platelets independent of vascular effects, resulting in increased plasma PF4. PF4 or CXCR3 null mice had less ECM, decreased brain T-cell recruitment, and platelet depletion or aspirin treatment reduced the development of ECM. We conclude that Plasmodium infected RBC can activate platelets and platelet derived PF4 then contributes to immune activation and T-cell trafficking as part of the pathogenesis of ECM.

Structure-based drug design is underway to inhibit the S100B-p53 interaction as a strategy for treating malignant melanoma. X-ray crystallography was used here to characterize an interaction between Ca 2+ -S100B and a target, TRTK-12, which binds to the p53 binding site on S100B. The structures of Ca 2+ -S100B (1.5 Å resolution) and S100B-Ca 2+ -TRTK12 (2.0 Å resolution) determined here indicate that the S100B-Ca 2+ -TRTK12 complex is dominated by an interaction between Trp-7 of TRTK-12 and a hydrophobic binding pocket exposed on Ca 2+ -S100B involving residues in helices 2 & 3 and loop 2. As with a S100B-Ca 2+ -p53 peptide complex, TRTK-12 binding to Ca 2+ -S100B was found to increase the proteins Ca 2+ ion binding affinity. One explanation for this effect was that peptide binding introduced a structural change that increased the number of Ca 2+ ligands and/or improved Ca 2+ ion coordination geometry of S100B. This possibility was ruled out when the structures of S100B-Ca 2+ -TRTK12 and S100B-Ca 2+ were compared and calcium ion coordination by the protein was found to be nearly identical in both EF-hand calcium-binding domains (RMSD=0.19). On the other hand, B-factors for residues in EF2 of Ca 2+ -S100B were found to be significantly lowered with TRTK-12 bound. This result is consistent with NMR 15 N relaxation studies that showed that TRTK-12 binding eliminated dynamic properties observed in Ca 2+ -S100B. Such a loss of protein motion may also provide an explanation for how calcium ion binding affinity is increased upon binding a target. Lastly, it follows that any small molecule inhibitor bound to Ca 2+ -S100B would also have to cause an increase in calcium ion binding affinity to be effective therapeutically inside a cell, so these data need to be considered in future drug-design studies involving S100B.

Abstract-Platelets