A Plasmodium berghei putative serine-threonine kinase 2 (PBANKA_0311400) is required for late liver stage development and timely initiation of blood stage infection.

In Plasmodium, protein kinases govern key biological processes of the parasite life cycle involved in the establishment of infection, dissemination and sexual reproduction. The rodent malaria model P lasmodium berghei encodes for 66 putative eukaryotic protein kinases (ePKs) as identified through modelling domain signatures and are highly conserved in Plasmodium falciparum We report here the functional characterisation of a putative serine-threonine kinase P BANKA_0311400 identified in this kinome analysis and designate it as Pbstk2 To elucidate its role, we knocked out Pbstk2 locus and performed a detailed phenotypic analysis at different life cycle stages. The Pbstk2 knockout (KO) was not compromised in asexual blood stage propagation, transmission and development in the mosquito vector. The Pbstk2 KO produced viable salivary gland sporozoites that successfully transformed into exo-erythrocytic forms (EEFs) and were morphologically indistinguishable from wild-type GFP (WT GFP) with regard to size and shape until 48 h. An intravenous dose of 1×103 Pbstk2 KO sporozoites in C57BL/6 mice failed to establish blood stage infection and a higher dose of 5X103 showed a 2-3 day delay in prepatency as compared to WT GFP parasites. Consistent with such an observation, analysis of in vitro EEF development at 62 h revealed that the hepatic merozoite numbers were reduced to nearly 40% as compared to WT GFP and showed meagre expression of MSP1. Our studies provide evidence for the role of PbSTK2 in late liver stage development and for the successful establishment of a timely blood stage infection.

Plasmodium berghei sporozoite specific genes- PbS10 and PbS23/SSP3 are required for the development of exo-erythrocytic forms.

Plasmodium sporozoites are infective forms of the parasite to mammalian hepatocytes. Sporozoite surface or secreted proteins likely play an important role in recognition, invasion and successful establishment of hepatocyte infection. By approaches of reverse genetics, we report the functional analysis of two Plasmodium berghei (Pb) sporozoite specific genes- PbS10 and PbS23/SSP3 that encode for proteins with a putative signal peptide. The expression of both genes was high in oocyst and salivary gland sporozoite stages as compared to other life cycle stages and PbS23/SSP3 protein was detected in salivary gland sporozoites. Both mutants were indistinguishable to wild-type parasites with regard to asexual growth in RBC, ability to complete sexual reproduction and form sporozoites in vector host. While the sporozoite stage of both mutants were able to glide and invade hepatocytes normally in vitro and in vivo, PbS10 mutants suffered growth attenuation at an early stage while PbS23/SSP3 mutants manifested defect during late exo-erythrocytic form maturation. Interestingly, both mutants gave rare breakthrough infections, suggesting that while both were critical for liver stage development, their depletion did not completely abrogate blood stage infection. These findings have important implications for weakening sporozoites by multiple gene attenuation towards the generation of a safe whole organism vaccine.

Plasmodium berghei plasmepsin VIII is essential for sporozoite gliding motility.

Plasmodium aspartic proteases, termed plasmepsins (PMs) play many critical roles such as haemoglobin degradation, cleavage of PEXEL proteins and sporozoite development in the parasite life cycle. Most of the plasmepsins are well characterized, however the role of PM VIII in Plasmodium remains unknown. Here, we elucidate the functions of PM VIII (PBANKA_132910) in the rodent malaria parasite Plasmodium berghei (Pb). By targeted gene deletion, we show that PbPM VIII is critical for sporozoite egress from an oocyst and gliding motility, which is a prerequisite for the invasion of salivary glands and subsequent transmission to the vertebrate host.

Modulation of host cell SUMOylation facilitates efficient development of Plasmodium berghei and Toxoplasma gondii.

SUMOylation is a reversible post translational modification of proteins that regulates protein stabilization, nucleocytoplasmic transport, and protein-protein interactions. Several viruses and bacteria modulate host SUMOylation machinery for efficient infection. Plasmodium sporozoites are infective forms of malaria parasite that invade mammalian hepatocytes and transforms into exoerythrocytic forms (EEFs). Here, we show that during EEF development, the distribution of SUMOylated proteins in host cell nuclei was significantly reduced and expression of the SUMOylation enzymes was downregulated. Plasmodium EEFs destabilized the host cytoplasmic protein SMAD4 by inhibiting its SUMOylation. SUMO1 overexpression was detrimental to EEF growth, and insufficiency of the only conjugating enzyme Ubc9/E2 promoted EEF growth. The expression of genes involved in suppression of host cell defense pathways during infection was reversed during SUMO1 overexpression, as revealed by transcriptomic analysis. The inhibition of host cell SUMOylation was also observed during Toxoplasma infection. We provide a hitherto unknown mechanism of regulating host gene expression by Apicomplexan parasites through altering host SUMOylation.

A Novel and Conserved Plasmodium Sporozoite Membrane Protein SPELD is Required for Maturation of Exo-erythrocytic Forms.

Plasmodium sporozoites are the infective forms of malaria parasite to vertebrate host and undergo dramatic changes in their transcriptional repertoire during maturation in mosquito salivary glands. We report here the role of a novel and conserved Plasmodium berghei protein encoded by PBANKA_091090 in maturation of Exo-erythrocytic Forms (EEFs) and designate it as Sporozoite surface Protein Essential for Liver stage Development (PbSPELD). PBANKA_091090 was previously annotated as PB402615.00.0 and its transcript was recovered at maximal frequency in the Serial Analysis of the Gene Expression (SAGE) of Plasmodium berghei salivary gland sporozoites. An orthologue of this transcript was independently identified in Plasmodium vivax sporozoite microarrays and was designated as Sporozoite Conserved Orthologous Transcript-2 (scot-2). Functional characterization through reverse genetics revealed that PbSPELD is essential for Plasmodium liver stage maturation. mCherry transgenic of PbSPELD localized the protein to plasma membrane of sporozoites and early EEFs. Global microarray analysis of pbspeld ko revealed EEF attenuation being associated with down regulation of genes central to general transcription, cell cycle, proteosome and cadherin signaling. pbspeld mutant EEFs induced pre-erythrocytic immunity with 50% protective efficacy. Our studies have implications for attenuating the human Plasmodium liver stages by targeting SPELD locus.

Genetic ablation of plasmoDJ1, a multi-activity enzyme, attenuates parasite virulence and reduces oocyst production.

Malaria parasites must respond to stresses and environmental signals to perpetuate efficiently during their multistage development in diverse environments. To gain insights into the parasite's stress response mechanisms, we investigated a conserved Plasmodium protein, which we have named plasmoDJ1 on the basis of the presence of a putative cysteine protease motif of the DJ-1/PfpI superfamily, for its activities, potential to respond to stresses and role in parasite development. PlasmoDJ1 is expressed in all intraerythrocytic stages and ookinetes. Its expression was increased 7-9-fold upon heat shock and oxidative stress due to H2O2 and artemisinin; its expression in a stress-sensitive Escherichia coli mutant conferred tolerance against oxidative stress, indicating that plasmoDJ1 has the potential to sense and/or protect from stresses. Recombinant plasmoDJ1 efficiently neutralized H2O2, facilitated renaturation of denatured citrate synthase and showed protease activity, indicating that plasmoDJ1 is a multi-activity protein. Mutation of the catalytic cysteine residue, but not other residues, reduced H2O2-neutralization activity by ~90% and significantly decreased chaperone and protease activities, indicating that these activities are intrinsic to plasmoDJ1. The plasmoDJ1 gene knockout in Plasmodium berghei ANKA attenuated virulence and reduced oocyst production, suggesting a major role for plasmoDJ1 in parasite development, which probably depends on its multiple activities.

Genetic ablation of plasmoDJ1, a multi-activity enzyme, attenuates parasite virulence and reduces oocyst production.

Malaria parasites must respond to stresses and environmental signals to perpetuate efficiently during their multistage development in diverse environments. To gain insights into the parasite's stress response mechanisms, we investigated a conserved Plasmodium protein, which we have named plasmoDJ1 on the basis of the presence of a putative cysteine protease motif of the DJ-1/PfpI superfamily, for its activities, potential to respond to stresses and role in parasite development. PlasmoDJ1 is expressed in all intraerythrocytic stages and ookinetes. Its expression was increased 7-9-fold upon heat shock and oxidative stress due to H2O2 and artemisinin; its expression in a stress-sensitive Escherichia coli mutant conferred tolerance against oxidative stress, indicating that plasmoDJ1 has the potential to sense and/or protect from stresses. Recombinant plasmoDJ1 efficiently neutralized H2O2, facilitated renaturation of denatured citrate synthase and showed protease activity, indicating that plasmoDJ1 is a multi-activity protein. Mutation of the catalytic cysteine residue, but not other residues, reduced H2O2-neutralization activity by ~90% and significantly decreased chaperone and protease activities, indicating that these activities are intrinsic to plasmoDJ1. The plasmoDJ1 gene knockout in Plasmodium berghei ANKA attenuated virulence and reduced oocyst production, suggesting a major role for plasmoDJ1 in parasite development, which probably depends on its multiple activities.

Gene disruption reveals a dispensable role for plasmepsin VII in the Plasmodium berghei life cycle.

Plasmepsins (PM), aspartic proteases of Plasmodium, comprises a family of ten proteins that perform critical functions in Plasmodium life cycle. Except VII and VIII, functions of the remaining plasmepsin members have been well characterized. Here, we have generated a mutant parasite lacking PM VII in Plasmodium berghei using reverse genetics approach. Systematic comparison of growth kinetics and infection in both mosquito and vertebrate host revealed that PM VII depleted mutants exhibited no defects in development and progressed normally throughout the parasite life cycle. These studies suggest a dispensable role for PM VII in Plasmodium berghei life cycle.