Characterization of AMA1-RON2L complex with native gel electrophoresis and capillary isoelectric focusing

Rhoptry neck protein 2 (RON2) binds to the hydrophobic groove of apical membrane antigen 1 (AMA1), an interaction essential for invasion of red blood cells (RBCs) by Plasmodium falciparum (Pf) parasites. Vaccination with AMA1 alone has been shown to be immunogenic, but unprotective even against homologous challenge in human trials. However, the AMA1-RON2L (L is referred to as the loop region of RON2 peptide) complex is a promising candidate, as preclinical studies with Freund's adjuvant have indicated complete protection against lethal challenge in mice and superior protection against virulent infection in Aotus monkeys. To prepare for clinical trials of the AMA1-RON2L complex, identity and integrity of the candidate vaccine must be assessed, and characterization methods must be carefully designed to not dissociate the delicate complex during evaluation. In this study, we developed a native Tris-glycine gel method to separate and identify the AMA1-RON2L complex, which was further identified and confirmed by Western blotting using anti-AMA1 monoclonal antibodies (mAbs 4G2 and 2C2) and anti-RON2L polyclonal Ab coupled with mass spectrometry. The formation of complex was also confirmed by Capillary Isoelectric Focusing (cIEF). A short-term (48 h and 72 h at 4°C) stability study of AMA1-RON2L complex was also performed. The results indicate that the complex was stable for 72 h at 4°C. Our research demonstrates that the native Tris-glycine gel separation/Western blotting coupled with mass spectrometry and cIEF can fully characterize the identity and integrity of the AMA1-RON2L complex and provide useful quality control data for the subsequent clinical trials.

cAMP-dependent protein kinase regulates secretion of apical membrane antigen 1 (AMA1) in Plasmodium yoelii

Malaria remains a heavy global burden on human health, and it is important to understand the molecular and cellular biology of the parasite to find targets for drug and vaccine development. The mouse malaria model is an essential tool to characterize the function of identified molecules; however, robust technologies for targeted gene deletions are still poorly developed for the widely used rodent malaria parasite, Plasmodium yoelii. To overcome this problem, we established a DiCre-loxP inducible knockout (iKO) system in P. yoelii, which showed more than 80% excision efficacy of the target locus and more than 90% reduction of locus transcripts 24 h (one cell cycle) after RAP administration. Using this developed system, cAMP-dependent protein kinase (PKAc) was inducibly disrupted and the phenotypes of the resulting PKAc-iKO parasites were analyzed. We found that PKAc-iKO parasites showed severe growth and erythrocyte invasion defects. We also found that disruption of PKAc impaired the secretion of AMA1 in P. yoelii, in contrast to a report showing no role of PKAc in AMA1 secretion in P. falciparum. This discrepancy may be related to the difference in the timing of AMA1 distribution to the merozoite surface, which occurs just after egress for P. falciparum, but after several minutes for P. yoelii. Secretions of PyEBL, Py235, and RON2 were not affected by the disruption of PKAc in P. yoelii. PyRON2 was already secreted to the merozoite surface immediately after merozoite egress, which is inconsistent with the current model that RON2 is injected into the erythrocyte cytosol. Further investigations are required to understand the role of RON2 exposed on the merozoite surface.