Research Groups
Mammalian Biology: Malaria
Research Interests and Description
Staff Research Scientist: Chetan Chitnis, PhD
Group Leader: Virander Chauhan
Research Interests
Receptor-ligand interactions in erythrocyte invasion by malaria parasites. Signaling mechanism involved in host cell invasion by malaria parasites. Malaria vaccine development.
Description of Research
Signaling mechanisms during erythrocyte invasion by malaria parasites
Parasite proteins that mediate interactions with erythrocyte receptors during invasion are commonly localized in membrane bound apical organelles referred to as micronemes and rhoptries. The sequence, signals and mechanisms that lead to the timely release of these parasite proteins to the merozoite surface during invasion are not known. We have demonstrated that secretion of microneme and rhoptry proteins to the merozoite surface during invasion is a two-step process. In the first step, exposure of merozoites to a low K+ environment as found in blood plasma triggers a rise in cytosolic Ca2+ through a phospholipase C (PLC) pathway. Rise in cytosolic Ca2+ leads to the release of microneme proteins such as EBA175 and AMA1 to the merozoite surface. In the second step, binding of released parasite ligands such as EBA175 to its receptor on erythrocytes, glycophorin A, restores basal cytosolic Ca2+ levels and triggers release of rhoptry proteins. These studies have identified, for the first time, the external signals and sequence for apical organelle release during erythrocyte invasion by Plasmodium merozoites. We are now exploring the role of another second messenger, cAMP, in microneme release and invasion. Exposure of P. falciparum merozoites to a low K+ environment leads to a rise in cAMP, which activates protein kinase A (PKA). The soluble adenyl cyclase (ACa) inhibitor, KH7, inhibits both cAMP increase and microneme release implicating cAMP as a second messenger in the process of microneme release. The mechanism leading to the rise in cAMP following exposure of merozoites to low K+ is under investigation. PKA inhibitors such as the inhibitory peptide PI inhibit microneme release implicating PKA in invasion processes. The phosphorylation substrates of PKA in merozoites leading to microneme release remain to be identified. Understanding the signaling pathways that lead to a rise in cAMP and Ca2+ and release of microneme proteins may open avenues for design of inhibitors that block microneme release and inhibit erythrocyte invasion.
Malaria vaccine development
There is an urgent need to develop vaccines against P. falciparum and P. vivax malaria. Our malaria vaccine development efforts have focused on targeting merozoite stage antigens that play a role in erythrocyte invasion by Plasmodium merozoites. The first generation P. falciparum vaccine candidate JAIVAC-1 developed at ICGEB is based on a physical mixture of the receptor-binding domain, PfF2, of the 175 kD erythrocyte binding antigen (EBA175) and the C-terminal conserved region of P. falciparum merozoite surface protein-1 (PfMSP119). Recombinant PfF2 and PfMSP119 produced in E. coli, purified by chromatography and formulated with adjuvant Montanide ISA720 is currently being tested in a Phase I clinical trial to evaluate safety and immunogenicity in healthy adults. The decision to proceed with JAIVAC-1 to Phase II trials to evaluate efficacy will be made based on the safety and immunogenicity data from the Phase I trial.
In addition, we are also developing a vaccine for P. vivax malaria based on the receptor-binding domain, region II, of P. vivax Duffy binding protein (PvDBPII). PvDBPII mediates interaction with the Duffy antigen on erythrocytes, which is essential for invasion by P. vivax. Laboratory and field studies have been used to develop the rationale for a vaccine based on recombinant PvDBPII. Naturally acquired binding inhibitory antibodies against PvDBPII protect against P. vivax infection. Recombinant PvDBPII has been shown to be immunogenic and elicits cross-reactive binding inhibitory antibodies that block receptor-binding by a wide range of polymorphic PvDBPII domains. Antibodies against PvDBPII also block erythrocyte invasion by P. vivax isolates. These data support the development of a recombinant vaccine for P. vivax based on PvDBPII. Methods to produce recombinant PvDBPII in its correctly folded conformation have been developed and will be used for production under cGMP for use in clinical development.
Recent Publications
malERA Consultative Group on Vaccines, Alonso, P.L., Ballou, R., Brown, G., Chitnis, C., Loucq, C., Moorthy, V., Saul, A., Wirth, D. 2011. A research agenda for malaria eradication: vaccines. PLoS Med 8, e1000398 PubMed link
Singh, S., Alam, M., Pal-Bhowmick, I., Brzostowski, Chitnis, C.E. 2010. Distinct external signals trigger sequential release of apical organelles during erythrocyte invasion by malaria parasites. PLoS Pathog 6, e1000746 PubMed link
Mayor, A., Rovira-Vallbona, E., Srivastava, A., Sharma, S.K., Pati, S.S., Puyol, L., Quinto, L., Bassat, Q., Machevo, S., Mandomando, I., Chauhan, V.S., Alonso, P.L., Chitnis, C.E. 2009. Functional and immunological characterization of a Duffy binding-like alpha domain from Plasmodium falciparum erythrocyte membrane protein 1 that mediates rosetting. Infect Immun 77, 3857-3863 PubMed link
Biswas, A.K., Hafiz, A., Banerjee, B., Kim, K., S., Datta, K., Chitnis, C.E. 2008. Plasmodium falciparum uses gC1qR/HABP1/p32 as a receptor to bind to vascular endothelium and for platelet-mediated clumping. PLoS Pathog 3, e130 PubMed link
Chitnis, C.E., Sharma, A. 2008. Targeting the Plasmodium vivax Duffy-binding protein. Trends Parasitol 24, 29-34 PubMed link
King, C.L., Michon, P., Shakri, A.R., Marcotty, A., Stanisic, D., Zimmerman, P.A., Cole-Tobian, J.L., Mueller, I., Chitnis, C.E. 2008. Naturally acquired Duffy-binding protein-specific binding inhibitory antibodies confer protection from blood-stage Plasmodium vivax infection. Proc Natl Acad Sci USA 105, 8363-8368 PubMed link
