gms | German Medical Science

The transmission of the malaria parasite Plasmodium falciparum from the human host to the mosquito vector during a blood meal is mediated by sexual precursor cells, the dormant gametocytes. When entering the mosquito midgut, the gametocytes become activated by environmental stimuli, which include a drop of temperature and the contact of the parasite with the mosquito-derived molecule xanthurenic acid (XA). Gametocyte activation initiates gametogenesis, and within 15 min, the crescent gametocytes round up and egress from the enveloping red blood cell (RBC), while forming male and female gametes. It was the aim of this study to investigate in detail the initial contact of the intraerythrocytic gametocytes with XA and to identify additional factors involved in gametocyte activation. A combination of exflagellation (= microgametogenesis) inhibition assays and mass spectrometric analyses were used to identify the modes of action of different external and intrinsic factors of gametocyte activation. We show that RBCs accumulate XA in their cytosol independent from infection. Inhibitor studies suggest that XA uptake by the RBCs is mediated by organic anion transporter peptide (OATP)-type transporters. When life gametocytes were liberated from the enveloping RBC, they remained sensitive to XA-induced activation, indicating that the XA receptor is located on the gametocyte plasma membrane. We also showed that the ionophores nigericin and valinomycin are able to induce exflagellation in the absence of XA, indicating that a change in RBC potassium levels plays a role in gametogenesis. Interestingly, the herbicide fluridone, which was shown to inhibit the synthesis of the egress molecule absisic acid (ABA) in Toxoplasma gondii, is able to block exflagellation in vitro, and fluridone-mediated inhibition of gametogenesis can be reversed by external addition of ABA. Our data demonstrate that XA is being taken up by RBC transporters of the OATP-type and indicate that the XA-receptor is located on the gametocyte plasma membrane. Additional factors playing a role for the induction of gametogenesis include the host cell potassium levels and the intrinsic molecule ABA. This molecule is known to induce the production of the second-messenger cADPR, which in turn controls release of calcium from internal stores during egress. Due to the importance of calcium in egress and the previously shown involvement of phospholipase C in gametocyte activation, we are currently currently investigating the potential involvement of plasmodial G protein-coupled receptors in perceiving the XA signal.