Rozprawy doktorskie na temat „Erythrocytes invasion”

Sialic acids are acidic sugars that terminate glycan chains on proteins or lipids on vertebrate cell surfaces. They vary greatly in structure, presentation and amount, all of which are important physiologically, but can also impact the tissue and host tropism of diverse pathogens. Parasites of the genus Plasmodium cause malaria, a disease characterized by a cyclical process of parasite invasion of host erythrocytes, growth and replication and fresh invasion of new erythrocytes. During erythrocyte invasion – an event central to malaria pathogenesis – proteins on the surface of the parasite, known as invasion ligands, bind to specific erythrocyte receptors, many of which are sialylated. In this dissertation, we determined how sialic acid variation impacts erythrocyte invasion by the zoonotic parasite, Plasmodium knowlesi and the most virulent human parasite, Plasmodium falciparum. For studies on P. knowlesi, we determined if Neu5Gc, a sialic acid that is absent in humans but present in most other primates, is a major determinant of parasite tropism. We used the recently described ex vivo erythrocyte culture system to transgenically express the CMAH enzyme, responsible for production of Neu5Gc. P. knowlesi showed significantly increased invasion of Neu5Gc-expressing human erythrocytes, providing evidence that loss of Neu5Gc in humans restricts P. knowlesi invasion of human erythrocytes. We then biochemically characterized two P. knowlesi invasion ligands of the EBL family and found they specifically bind Neu5Gc. These ligands potentially mediate Neu5Gc-dependent invasion of human and macaque erythrocytes. We finally showed that in natural human infections, P. knowlesi can adapt to infect erythrocytes independently of sialic acid. We also studied the use of sialic acid-containing erythrocyte receptors by P. falciparum using the ex vivo erythrocyte culture system. We determined the importance in invasion of glycophorin B (GPB), receptor for P. falciparum invasion ligand, EBL-1, and one of the highly sialylated receptors on the erythrocyte surface. We specifically knocked down gene expression of GPB as well as two well characterized receptors involved in P. falciparum invasion – GPA, the largest contributor to erythrocyte sialic acid and GPC, another sialylated receptor. Invasion assays using P. falciparum laboratory strains and field isolates revealed that GPB is a dominant receptor in P. falciparum invasion, of comparable importance to GPA.
Biological Sciences in Public Health

The human immune-regulatory protein, complement receptor type 1 (CR1, CD35), occurs on erythrocytes where it serves as the immune adherence receptor. It interacts with C3b, C4b, C1q and mannan-binding lectin (MBL). It additionally binds the Plasmodium falciparum protein, Rh4, in the non-sialic acid-dependent erythrocye-invasion pathway, and is also important for rosetting, via an interaction with P. falciparum erythrocyte membrane protein 1 (PfEMP1). A C3b/C4b, and PfEMP1 binding site lies in CCP modules 15-17 (out of 30 in CR1), while polymorphisms that afford advantage to some populations in dealing with severe malaria occur in CCPs 24-25, begging the question central to this thesis – do these polymorphism modulate function, and if so how? We hypothesized that the CR1 architecture apposes CCPs 15-17 and CCPs 24-25 using the exceptionally long linker between CCPs 21 and 22 as a hinge, thus polymorphic variants in CCPs 24-25 modulate functionality in CCPs 15-17. To test this, a panel of recombinant CR1 protein fragments (CCPs 21, 21-22, 20-23, 15-17, 17, 10-11, 17-25, 15-25 and 24-25) were produced in Pichia pastoris along with polymorphic forms of the relevant constructs. After purification, biophysical and biological methods were used to assess whether the linker does indeed act as a hinge, and the comparative abilities of the CCPs 15-25 variants (along with soluble CR1 (sCR1), CCPs 1-3 and the panel of CR1 fragments) to interact with a range of ligands were measured. We found no evidence from NMR for face-to-face contacts between CCPs 21 and 22 that would be consistent with the long linker permitting a 180-degree bend between them. Indeed, based on scattering and analytical ultracentrifugation data, CCPs 20-23 form an extended rather than a bent-back structure. All of the four Knops blood-group variants of the CCPs 15-25 proteins produced similar results according to dynamic light scattering and AUC indicating no structural difference or change in self-association state between variants. In addition, based on the data collected from surface plasmon resonance (SPR), ELISA and fluid-phase cofactor (for factor I) assays, there were no evidence of any difference between the polymorphic forms with respect to their interactions with C3b, C4b, C1q and MBL. Only weak interaction was observed for sCR1, and all CCPs 15-25 variants, with the relevant part of PfEMP1, and there was no measurable difference amongst the variants in disrupting rosettes. The sCR1-Rh4.9 interaction was confirmed by SPR; affinities measured between the binding domain of Rh4 and the panel of CR1 fragments identified CCPs 1-3 (site 1) as the main interaction site. It seemed unlikely therefore that CCPs 24 and 25 could modulate Rh4 binding; indeed none of the four CR1 15-25 variants bound Rh4.9 appreciably. Thus we concluded that allotypic variations in CCPs 24-25 have no measurable effect on the architecture as well as binding of CR1 to its host or parasite ligands The inferred selective pressure acting on these variants likely arise from some other (i.e. besides malaria) geographically localised infectious diseases.

Erythrocyte invasion is a key step in the Plasmodium life cycle. This process is tightly regulated, involving the sequential release of specialised apical secretory organelles – the micronemes, rhoptries and dense granules. These organelles contain proteins required for invasion and establishment of the parasitophorous vacuole, but most of the proteins remain uncharacterised. The aim of this project was to uncover novel proteins with a role in invasion by the human malaria parasite Plasmodium falciparum merozoites. I identified proteins using the following selection criteria: a) expression in the schizont/merozoite form of the parasite; b) conservation across the genus; c) the presence of a signal peptide and d) one or more transmembrane (TM) domains. A list of 64 proteins was identified, and filtered further based on novelty, presence in the merozoite proteome, expression in other life cycle stages, and difficulty of study. Five proteins were selected, and I produced recombinant protein and raised antibodies against three, which I used to identify the sub-cellular location of the protein within the parasite. The proteins appear to reside in either the rhoptries or the endoplasmic reticulum of the merozoite. Attempts were made to epitope-tag and delete all 3 genes, with a focus on one protein, the type IV Hsp40, PF11_0443. This protein contains two TM domains and is expressed during schizogony. By immunofluorescence it is present in the ER of early schizonts, before accumulating at the apex of merozoites in a rhoptry location. Immunoprecipitation experiments indicated that the protein binds known rhoptry proteins and other chaperones. The protein has been epitope-tagged but attempts to delete the gene by genetic recombination were unsuccessful. The gene is conserved in Plasmodium spp. and there are orthologues in higher eukaryotes, but it is absent from other Apicomplexa. Current studies are focused on the role of this protein in erythrocyte invasion.

Malaria is a devastating disease that continues to affect millions of people worldwide every year. Specifically, Plasmodium falciparum is the most common human malaria parasite, particularly in sub-Saharan Africa. P. falciparum causes the most malignant and debilitating symptoms with the highest mortality and complication rates. Even with the worldwide efforts of many researchers and organizations, the road to discovering a vaccine has been difficult and challenging. Due do to the improvements in in vitro liver stage assays as well as rodent models of mammalian malaria, pre-erythrocytic stages of malaria have become a more accessible target for experimental studies. These vaccine candidates target Plasmodium sporozoites in the liver and liver stages to prevent development to the blood-stage forms, which is responsible for the debilitating symptoms of the disease. Scanning electron microscopy has been used for decades to provide insight on the morphology and topography of specimens, which cannot be seen through a light microscope. The purpose of this study was to analyze the morphology of sporozoites with some target antibodies. Sporozoites have previously shown uncharacterized appearances and development in an immunofluorescent stain at different concentrations of particular antibodies. With this further understanding on the morphological impact few of the target antibodies have on sporozoites through scanning electron microscopy, further grasp can be acquired.

The aim of this study was to investigate the genetic basis of selectivity in invasion of the red blood cells by the human malaria parasite Plasmodium falciparum. Multiple invasions of a single host red blood cell by more than one merozoite, which can be described or assessed in terms of the selectivity index (SI), has been reported to be related to the severity of malaria disease. In this study, selectivity index, defined as the ratio of the number of multiply-infected red cells observed to that expected from random invasion, as modelled by a Poisson distribution was determined for certain clones of P.falciparum. SI was measured under static and shaking culturing conditions for P. falciparum clones 3D7 and HB3 and 18 progeny clones derived from a genetic cross between these two parasite clones. P. falciparum clone 3D7 was found to have a significantly lower SI than HB3 under both static and shaking culture conditions. There was no relationship between SI and days in continuous culture for clone 3D7 under shaking and static conditions; the phenotype therefore appears to be stable over time. The genetic basis of the difference in selectivity index between P. falciparum clones 3D7 and HB3 was investigated in progeny clones from a cross between these two clones, to ascertain the inheritance pattern of the phenotype. Under static conditions, ten progeny clones had a selectivity index lower than either parent, one progeny clone had higher selectivity index than both parent, and six progeny clones had selectivity index intermediate between the parents . Under shaking conditions, fifteen progeny clones were observed to have a selectivity index lower than either parent. These observations suggest the involvement of more than one parasite gene in selectivity index. A Quantitative Trait Locus (QTL) analysis was performed in order to identify genomic regions influencing SI in the progeny clones. The highest LOD score of 5.06 was obtained for a QTL on chromosome 13 for SI measured in parasites cultured under shaking conditions. This QTL denoted, PF_SI_1, extends for approximately 100kb on chromosome 13 and contains 19 open reading frames. This finding indicates the presence of a gene or genes on chromosome 13 that influence the parasite’s selection of erythrocytes for invasion.

Malaria is a global pandemic that affects millions of people each year. It is a parasitic infection caused by the Plasmodium family, with Plasmodium falciparum being the most virulent strain. Malaria is transmitted to humans by the female Anopheles mosquito. The parasite undergoes two different cycles of its life cycle within the human host: the liver and intraerythrocytic life cycle. The latter consists of an asexual and sexual cycle. The intraerythrocytic cycle is perhaps the most important stage of the parasite's life cycle as it promotes the spread of the disease within and between hosts. The focus of this investigation was aimed at the invasion process of the merozoites into the erythrocytes. The Plasmodium merozoite utilises a cascade of proteins during the erythrocyte invasion process, which is a swift action that takes place in approximately 30 seconds. A number of surface proteins are expressed during merozoite development and are distributed along the merozoite surfaces to assist with attachment and invasion, the most crucial being MSP-1, AMA-1 and RON-2. MSP-1 and AMA-1 are vital targets for the development of malaria vaccines. AMA-1 is the central target protein of this investigation as it plays an essential role in the invasion process. AMA-1 commits the merozoite to invade the erythrocyte, as it assists the RON proteins in the formation of an irreversible tight-junction with the membrane of the erythrocyte. Antibodies, specific to AMA-1, bind to the protein, which prevents the formation of the tight junction and inhibits the invasion of the merozoite into the erythrocyte, therefore preventing the spread of the disease. However, before invasion, AMA-1 undergoes a number of proteolytic processes. It is synthesized as an 83 kDa (AMA-183) precursor protein in the apical organelle of the merozoite. This is then cleaved at the N-terminus to give rise to a 66 kDa (AMA-166) fragment, which is secreted onto the surface of the merozoite. The AMA-166 fragment is then cleaved into either a 48 kDa (AMA-148) or 44 kDa (AMA-144) fragment. One of these three fragments is then used by the merozoite for erythrocyte invasion. The aim of this investigation was to isolate and characterise each of the fragments of the Plasmodium falciparum AMA-1 (PfAMA-1) protein using the 3D7 lab strain of P. falciparum and to visualise the merozoite-erythrocyte invasion process, to possibly identify which of the AMA-1 fragments are involved in the invasion process. In order to achieve this large clusters of merozoites from sorbitol-synchronised cultures were isolated. Schizonts were isolated from culture by magnetic separation and incubated with E64 to prevent the release of merozoites. Merozoites that were required for the isolation of PfAMA-1 were harvested from the schizonts by saponin lysis, then homogenised, separated by SDS-PAGE and digested for LC-MS/MS analysis. Merozoites that were required for the visualisation procedures were not incubated with E64, to allow natural egression from the erythrocyte. The transmission electron microscopy results produced clear images of the merozoiteerythrocyte invasion process and the positioning of PfAMA-1 on the merozoite, before and after schizont rupture, was visualised from results obtained from confocal microscopy. Then PfAMA-1 was identified in isolated merozoite samples by LC-MS/MS analysis. However, due to its low abundance, isolation of high enough concentrations of PfAMA-1 to characterise its different fragments was not achieved. Further investigation into the development of the culturing and isolating methods could help in future projects aimed at isolating higher concentrations of merozoite proteins from synchronised cultures with a lower merozoite egression window period, in order to accomplish detailed analysis on invading proteins for the future development of treatments against malaria.
Dissertation (MSc)--University of Pretoria, 2016.
Pharmacology
MSc
Unrestricted

A conserved acto-myosin motor complex is implicated in parasite motility and host invasion by a wide variety of Apicomplexan zoites. Until recently, only actin, myosin A (MyoA) and its putative light chain (MLC1 in Toxoplasma gondii) or myosin tail domain interacting protein (MTIP) in Plasmodium spp., had been identified as central to the function of this motor. Identification of two further components in T. gondii, the gliding-associated proteins (GAP45 and GAP50), has provided a valuable insight into how the motor may be anchored in the inner membrane complex (IMC) that lies below the plasma membrane. Results presented here demonstrate that Plasmodium falciparum (Pf)GAP45 and PfGAP50 are expressed and co-localise with PfMTIP at the periphery of merozoites. Both GAPs are found to be in complex with PfMyoA, and PfMTIP. Pulse-chase experiments indicate that the motor complex is assembled in two stages. PfGAP50 is incorporated after the formation of a ternary complex comprising PfGAP45, PfMyoA and PfMTIP. PfGAP45 is shown to be N-myristoylated and palmitoylated and may therefore function as a linker protein tethering the motor to the outer leaflet of IMC. Additionally, PfGAP45 is phosphorylated by calmodulin-dependent protein kinase 1 (CDPK1); a process that may be important in the regulation of the motor. Recombinant PfGAP50 is a well-ordered protein, whereas PfGAP45 has a low content of secondary structure. Potential interaction of GAPs with other motor components has been examined. Co-immunoprecipitation experiments, circular dichroism (CD) and fluorescence spectroscopic analyses have not provided any evidence of direct interaction with any other motor proteins.

Malaria is a disease caused by the protozoan parasite Plasmodium where the species that causes the most severe form of malaria in humans is known as Plasmodium falciparum. At least 40% of the global population is at risk of contracting malaria with 627 000 people dying as a result of this disease in 2012. Approximately 90% of all malaria deaths occur in sub-Saharan Africa, where approximately every 30 seconds a young child dies, making malaria the leading cause of death in children under the age of five years old. The malaria parasite has a complex life cycle utilising both invertebrate and vertebrate hosts across sexual and asexual stages. The erythrocyte invasion stage of the life cycle in the human whereby the invasive merozoite form of the parasite enters the erythrocyte is a central and essential step, and it is during this stage that the clinical symptoms of malaria manifest themselves. Merozoites invade erythrocytes utilising multiple, highly specific receptor-ligand interactions in a series of co-ordinated events. The aim of this study was to better understand the interactions occurring between the merozoite and erythrocyte during invasion by using modern, cutting-edge proteomic techniques. This was done in the hope of laying the foundation for the discovery of new key therapeutic targets for antimalarial drug and vaccine-based strategies, as there is currently no commercially available antimalarial vaccine and no drug to which the parasite has not at least started showing resistance. In this study healthy human erythrocytes were treated separately with different protein-altering enzymes and chemicals being trypsin, the potent oxidant sodium periodate (NaIO4), the amine cross-linker tris(2-chloroethyl)amine hydrochloride (TCEA) and the thiol cross-linker 1,11-bis(maleimido)triethylene glycol (BM(PEG)3). The resulting erythrocyte protein alterations were visualised as protein band differences on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE), where treated and untreated control erythrocyte ghost protein fingerprints were visually compared to one another. The protein bands showing differences between treated and control samples were in-gel digested using trypsin then sequenced by liquid chromatography tandem mass spectrometry (LC-MS/MS) and identified using proteomics-based software. In this way, the erythrocyte proteins altered by each enzyme/chemical treatment were identified. Malaria invasion assays were performed where each treatment group of erythrocytes as well as the control erythrocytes were incubated separately with schizont stage malaria parasites for the duration of one complete life cycle. Using fluorescent staining and flow cytometry, the invasion inhibition efficiency for each treatment group was evaluated. By utilising these methods, the identification and the relative importance of the erythrocyte proteins involved in the invasion process were determined. Protein fingerprints of control and treated erythrocyte ghosts were visualised and optimised on SDS PAGE where induced protein band differences were successfully identified by LC-MS/MS. It was found that each treatment altered erythrocyte proteins with changes found in Band 3, actin, phosphoglycerate kinase 1, spectrin alpha, spectrin beta, ankyrin, haemoglobin, Bands 4.1 and 4.2, glycophorin A and stomatin. The invasion assays revealed that TCEA inhibited invasion to the greatest extent as compared to the other treatments, followed by BM(PEG)3 and trypsin. Sodium periodate-treated erythrocytes could not be assessed using the invasion assay due to auto-haemolysis. Band 3, glycophorin A, Band 4.1 and stomatin appear to be of higher relative importance in the invasion process as compared to the other altered erythrocyte proteins. These results confirmed the known roles of spectrin alpha, spectrin beta, glycophorin A, Band 3 and Band 4.1 in invasion, and suggested that ankyrin, Band 4.2 and stomatin may also be involved. This study highlighted the potential that modern, cutting-edge proteomic techniques provide when applied to previous comparative studies found in older literature, as previously unidentified proteins that can be involved in invasion were revealed. These results can be used as a foundation in future studies in order to identify new key targets for the development of new antimalarial drug- and vaccine-based strategies, with the hope of preventing the suffering of the millions of malaria-inflicted people worldwide, and ultimately eradicating this deadly disease.
Dissertation (MSc)--University of Pretoria, 2014.
tm2015
Pharmacology
MSc
Unrestricted

Invasion of host erythrocytes is an essential stage in the life cycle of Plasmodium parasites and in development of the pathology of malaria. The stages of erythrocyte invasion, including initial contact, apical reorientation, junction formation, and active invagination, are directed by the coordinated release of specialised apical organelles and their parasite protein contents. Among these proteins, and central to invasion by all species, are two parasite protein families, the reticulocyte-binding protein homologue (RH) and the erythrocyte-binding like (EBL) proteins, that mediate host-parasite interactions. RH5 from Plasmodium falciparum (PfRH5) is the only member of either family demonstrated to be necessary for erythrocyte invasion in all tested strains, through its interaction with the erythrocyte surface protein basigin. Indeed, antibodies targeting either PfRH5 or basigin can block parasite invasion with high efficiency in vitro, making PfRH5 an excellent candidate for a vaccine to protect against the most deadly form of malaria. Here I present crystal structures of PfRH5 in complex with basigin and with two distinct inhibitory antibodies. This is the first structure of any RH protein, revealing a novel fold in which two three-helical bundles come together to form a kite-like architecture. The two immunoglobulin domains of basigin and the inhibitory antibodies bind to one tip of the kite. These findings provide the first structural insights into erythrocyte binding by the Plasmodium RH protein family and identify novel inhibitory epitopes to guide the design of a new generation of vaccines against the blood-stage parasite.

Plasmodium falciparum is a protozoan parasite responsible for causing the most severe form of malaria in humans. This species is responsible for over 90% of malaria mortalities which occur predominantly in Africa. An increase in drug resistant parasites in recent years is threatening the progress made against malaria and thus new antimalarial drugs and vaccines are needed to combat this disease. During the intraerythrocytic phase, merozoites egress from mature schizonts to invade new uninfected erythrocytes. Glycophosphatidylinositol (GPI) -anchored proteins cover most of the exterior surface of the merozoite prior to invasion, while other GPI-anchored proteins are released onto the merozoite surface through apical organelle secretions. These proteins are involved in interactions with erythrocytes and are thought to be vital to erythrocyte invasion. GPI-anchored proteins have also been implicated as a cause of pathogenic symptoms and activation of immune components. These proteins are then released or cleaved to enable merozoite entry into the erythrocyte. Several enzymes are thought to be involved in their cleavage including the serine proteases subtilisin-like proteases (SUB) 1 and 2, and phosphatidylinositol-phospholipase C (PIPLC); GPI-anchored proteins are also generally sensitive to phospholipase A2 (PLA2). Cleaved proteins are released into the host blood system, while uncleaved proteins are carried into the erythrocyte during invasion. Merozoites have a limited period in which they retain invasive capacity. A previous lack of available techniques that are specifically adapted to merozoite analysis has resulted in an incomplete understanding of invasion and GPI-anchored protein involvement in invasion. This study aimed to determine how GPI-anchored proteins on the merozoite surface are altered in the invasive phase, and explore the possibility of using merozoite GPI-anchored proteins as potential drug targets to block erythrocyte invasion. Optimised methods of in vitro parasite culturing which produce highly synchronised merozoites was essential to this study. Parasite culturing techniques were optimised by utilising low haematocrit cultures with frequent culture splitting and optimised synchronisation. The “Malarwheel” is a tool that was developed for this research to provide a means for scheduling sorbitol treatments and MACs isolations. This tool and optimised culturing methods enabled large volumes of highly synchronised invasive merozoites to be harvested. Four compounds (vanadate, edelfosine, dioctyl sodium sulfosuccinate (DSS), and gentamicin) suspected to interfere with GPIanchored cleavage or processes were screened on intraerythrocytic stages and merozoites. Antimalarial and anti-invasive properties of these compounds were screened by modified malaria SYBR Green I-based fluorescence (MSF) assay and merozoite invasion assays (MIA) respectively. DSS and gentamicin showed limited potential as antimalarials or as anti-invasive agents. Vanadate and edelfosine both showed antimalarial and anti-invasive activity, while edelfosine was the most potent anti-invasive agent at physiological concentrations. The merozoite GPI-anchored proteome was analysed by sodium dodecyl sulphatepolyacrylamide gel electrophoresis (SDS-PAGE) followed by complete gel lane analyses conducted by liquid chromatography-tandem mass spectrometry (LC-MS/MS) on soluble and pelleted merozoite proteins in samples from either invasive or non-invasive merozoites. Thirteen known or predicted GPI-anchored proteins were identified in samples. Several changes were identified in merozoite GPI-anchored proteins between the invasive phase and after its completion, and minor differences were observed following treatment with edelfosine. Edelfosine showed partial inhibition of erythrocyte invasion, however, the primary cause of inhibition cannot be directly related to interferences with GPI-anchored proteins. These results suggest that GPIanchored proteins are controlled by various complex processes, and are cleaved or processed by diverse mechanisms during the invasive phase. These mechanisms may be controlled by multiple signals which effect proteins or groups of proteins in specific ways. These signals may be influenced by “checkpoints” during invasion processes including the time period after egress from schizonts, and possibly the recognition of erythrocyte targets. These methods and results provide a foundation for future research to enable culturing of P. falciparum parasites specifically for merozoite research, and to identify merozoite proteins active during the invasive phase. These results confirm and challenge previous ideas reported in literature on the GPI-anchored processes of merozoites and further characterise less studied GPIanchored proteins. The results suggest that the processes controlling GPI-anchored proteins may be more complex than previously thought. These results form a basis to further identify and characterise GPI-anchored proteins in the aim to develop antimalarial medications and vaccines that target merozoites and their GPI-anchored processes.
Dissertation (MSc)--University of Pretoria, 2017.
Pharmacology
MSc
Unrestricted

Malaria is the deadliest parasitosis worldwide, causing 216 million cases and 445.000 casualties in 2016. The disease is caused by Plasmodium parasites that develop and grow inside human erythrocytes. Among them, Plasmodium falciparum is the deadliest one. The human protein Rac1 is a GTPase involved in the development of several cancers and essential in the invasion of the host cell by many intracellular pathogens, including Toxoplasma gondii, which belongs to the same phylum as P. falciparum. Rac1 has been extensively studied and several cell permeable inhibitors of the GTPase have already been developed. In order to investigate a possible role of Rac1 in malaria infection, we analysed Rac1 subcellular localization in P. falciparum infected erythrocytes by immuno-fluorescence assays. We showed that during invasion of the host cell, Rac1 is recruited to the site of parasite entrance and is activated by the parasite. During the intraerythrocytic growth of the parasite, Rac1 is further recruited from the cell membrane to the parasitophorous vacuole membrane . These data suggest that Rac1 may play a role in P. falciparum infection of human erythrocytes.To confirm these data, we performed invasion assays on wt erythrocytes from donor, in the presence of two different Rac1-specific inhibitors, further confirming the role of the GTPase in parasite invasion of the host cell. Both the Rac1 inhibitors show an effect also on parasite growth. Finally, 9 different chemical inhibitors of Rac1 were tested on synchronous P. falciparum cultures. These reduced parasite growth with a half minimal inhibitory concentration (IC50) below 40 µM, indicating that Rac1 is a druggable target . Two among the inhibitors showed nanomolar IC50. Rac1 may be thus considered an interesting target for the development of novel anti-malarial drugs. with the advantage of being a well studied protein, with a known x-ray crystal structure and several specific inhibitors on commerce. Moreover, therapies targeting the host instead of the parasite reduce the probability of resistance insurgence, a critical issue for currently available drugs.

Babesia microti is a tickborne hemoprotozoan parasite that causes the disease babesiosis in humans. Babesia microti Apical Membrane Antigen-1 (AMA-1) is a micronemal protein suspected to play a role in erythrocyte invasion. To investigate interaction between AMA-1 and the host cell, the ectodomain region of the B. microti ama-1 gene was cloned into an expression vector, expressed as a histidine-tagged fusion protein, and used to probe red blood cell membrane proteins in far Western blot assays. The B. microti ama-1 ectodomain, which excludes the signal peptide and the transmembrane region of the open reading frame, was amplified from a cloned gene sequence. The AMA-1 ectodomain is a membrane bound polypeptide that extends into the extracellular space and is most likely to interact or initiate interaction with the host red blood cell surface receptor(s). The amplicon was ligated into a protein expression vector to produce a 58.1 kDa recombinant His-tagged fusion protein, which was confirmed by Western blot analysis. The recombinant B. microti AMA-1 fusion protein was enriched on nickel affinity columns and then used to probe mouse, human and horse red blood cell membrane proteins in far Western blot assays. Babesia microti AMA-1 consistently reacted strongly with a protein migrating at 49 kDa. A similar reaction occurred between the B. microti AMA-1 and horse red blood cell membrane proteins, suggesting that similar interacting proteins of this size are shared by red blood cells from the three species. The B. microti AMA-1 may bind to red blood cell membrane sialic-acid groups, as shown for other Babesia spp. This may explain the signal at the 49 kDa position observed between B. microti AMA-1 and red blood cell membrane proteins from three different species. Further studies may determine if the binding epitopes of the red blood cell binding partner at this position vary and contribute to the specificity of each parasite AMA-1 for their respective host cells.