Probing Translational Regulation by the Malaria Parasite Plasmodium falciparum: Applying a Novel In Vitro Assay to Identify Genetic Determinants of Regulation and Identify Small Molecules Exploiting P. falciparum Translation as a Drug Target
- Author(s): Sheridan, Christine Moore
- Advisor(s): DeRisi, Joseph L
- Weissman, Jonathan S
- et al.
Over half of all pregnancies worldwide occur in malaria endemic regions. Placental malaria, a serious condition caused by the malaria parasite Plasmodium falciparum, occurs when malaria-infected red blood cells adhere to the tissue of the placenta, with potentially devastating consequences for both mother and infant. Placental malaria infections are responsible for approximately 30% of preventable low birth weight newborns, 20% of stillbirths, and 200,000 infant deaths per year in Africa alone. Placental malaria infection is mediated by VAR2CSA, a P. falciparum protein that is expressed by the parasite only when in a pregnant woman, and translationally repressed outside of pregnancy. However, the mechanisms by which this repression and expression occur or, indeed, how the parasite senses when its host is pregnant are unknown. Elucidation of the genetic determinants of this specific translational regulation could provide insight for therapeutic development for placental infection. Additionally, further study of overall translation and its pharmacologic inhibition under “normal” circumstances may help identify novel therapies for malaria in general.
Utilizing a novel in vitro translation system derived from P. falciparum cultures, I have shown that synthesis of VAR2CSA is repressed under normal conditions, and that multiple elements in the 5’ untranslated region of the var2csa gene contribute to this repression. Further, this repression occurs only in P. falciparum, and not mammalian in vitro translation systems, indicating a P. falciparum-specific mechanism of inhibition. Importantly, I have found that circulating factors present in maternal serum during the first and second trimesters of pregnancy relieve repression of VAR2CSA translation, identifying two placental enzymes as candidate factors. Both enzymes serve to alter the pH of the microenvironment and, in fact, increasing pH in the P. falciparum in vitro translation system mimics the increase in VAR2CSA production induced by pregnant serum. Separately, I have utilized this in vitro translation system to identify inhibitors of translation among clinically approved antimalarial drugs and found that none utilize this mechanism of action. Importantly, this disproved the recent assertion that mefloquine inhibits translation, while also underscoring the therapeutic potential for targeting the translational apparatus as a novel and orthogonal mechanism of action.