Figure 8.

A proposed model for the effects of PfATP4 inhibitors in P. falciparum schizonts. (A) The outcomes of PfATP4 inhibition differ according to schizont age. PfATP4 inhibitors cause early- to mid-stage schizonts to accumulate Na⁺ and osmotically swell. This results in lysis or leakage of host RBCs and therefore the release of nanoluciferase into the culture medium. When administered to late schizonts, PfATP4 inhibitors arrest egress without inducing substantial lysis or leakage of host RBCs. After cGMP elevation and the deployment of PfPKG, mature schizonts are no longer susceptible to PfATP4 inhibitors and egress can proceed in the presence of these compounds. (B) PfATP4 inhibitors act upstream of the cGMP/PfPKG checkpoint to inhibit egress. During merozoite development, intra-parasitic cGMP levels are stabilised by the opposing activities of guanylyl cyclase α (PfGCα) and phosphodiesterases (PDEs) An as-yet unidentified endogenous stimulus induces the functional upregulation of PfGCα in late schizonts, resulting in cGMP elevation and PfPKG activation. A PDE inhibitor, zaprinast, prevents cGMP degradation and therefore induces cGMP elevation and egress without the requirement for an increase in the rate of cGMP production, thereby obviating the need for the endogenous egress stimulus. Co-administration of zaprinast overcomes the egress-inhibitory effect of PfATP4 inhibitors. This supports a model wherein PfATP4 inhibitors block egress by preventing the functional upregulation of PfGCα.