Plasmodium falciparum evades mosquito immunity by disrupting JNK-mediated apoptosis of invaded midgut cells

Abstract

The malaria parasite, Plasmodium, must survive and develop in the mosquito vector to be successfully transmitted to a new host. The Plasmodium falciparum Pfs47 gene is critical for malaria transmission. Parasites that express Pfs47 (NF54 WT) evade mosquito immunity and survive, whereas Pfs47 knockouts (KO) are efficiently eliminated by the complement-like system. Two alternative approaches were used to investigate the mechanism of action of Pfs47 on immune evasion. First, we examined whether Pfs47 affected signal transduction pathways mediating mosquito immune responses, and show that the Jun-N-terminal kinase (JNK) pathway is a key mediator of Anopheles gambiae antiplasmodial responses to P. falciparum infection and that Pfs47 disrupts JNK signaling. Second, we used microarrays to compare the global transcriptional responses of A. gambiae midguts to infection with WT and KO parasites. The presence of Pfs47 results in broad and profound changes in gene expression in response to infection that are already evident 12 h postfeeding, but become most prominent at 26 h postfeeding, the time when ookinetes invade the mosquito midgut. Silencing of 15 differentially expressed candidate genes identified caspase-S2 as a key effector of Plasmodium elimination in parasites lacking Pfs47. We provide experimental evidence that JNK pathway regulates activation of caspases in Plasmodium-invaded midgut cells, and that caspase activation is required to trigger midgut epithelial nitration. Pfs47 alters the cell death pathway of invaded midgut cells by disrupting JNK signaling and prevents the activation of several caspases, resulting in an ineffective nitration response that makes the parasite undetectable by the mosquito complement-like system.

Malaria parasite Pfs47 disrupts JNK signaling to escape mosquito immunity

“…In the absence of Pfs47, JNK signaling promotes cell death mediated by the induction of forkhead box O, caspase (CASP)-L1, and several downstream effector caspases, including CASP-S2 (1). When CASP-S2 is silenced, the activation of HPX2 and NOX5 transcripts is attenuated, demonstrating that cell death initiated by caspase activation is required for midgut epithelial nitration (1). Taken together, these data suggest that in the absence of Pfs47, ookinete invasion activates the JNK pathway and downstream signaling events that lead to increased caspase activity, epithelial nitration, immune recognition, and ultimately parasite death (Fig.…”

“…It is also possible that the role of Pfs47 is passive, either in masking immune recognition of parasite surface proteins, or by serving as a ligand for an alternate ookinete invasion pathway. As shown by Ramphul et al (1), Pfs47 is an important determinant of success of parasite development in the mosquito host. We may now look forward to future reports detailing how Pfs47 mediates immune evasion by malaria parasites in more detail.…”

“…Also present on the surface of ookinetes (5), the expression of Pfs47 can have significant effects on parasite infection as early as 12 h postblood feeding, well before midgut invasion. In addition, Pfs47 dramatically alters midgut transcriptional responses to ookinete invasion (1). Likely mediated in part by evading JNK As shown by Ramphul et al, Pfs47 is an important determinant of success of parasite development in the mosquito host.…”

“…Thus, understanding the parasite life cycle in the mosquito host is of prime importance. In PNAS, Ramphul et al (1) show that a protein, Pfs47, expressed in the mosquito stages of the malaria parasite can disrupt mosquito JNK signaling to render parasites "invisible" to the mosquito immune system, and in this way increase its survival and chances of transmission.…”

“…Not only do Ramphul et al (1) demonstrate that a parasite protein, Pfs47, can modulate immune recognition and signaling in its mosquito host, but they also integrate existing knowledge of ookinete midgut invasion to provide a more cohesive understanding of the mosquito immune responses to Plasmodium. Observations that ookinete invasion promotes cell death (8) and epithelial nitration (6, 9) have been made previously, yet Ramphul et al (1) now formally connect these mosquito responses.…”