Date Published: November 29, 2018
Publisher: Public Library of Science
Author(s): Lattha Souvannaseng, Lewis Vibul Hun, Heather Baker, John M. Klyver, Bo Wang, Nazzy Pakpour, Jordan M. Bridgewater, Eleonora Napoli, Cecilia Giulivi, Michael A. Riehle, Shirley Luckhart, David S. Schneider.
Malaria is a global health concern caused by infection with Plasmodium parasites. With rising insecticide and drug resistance, there is a critical need to develop novel control strategies, including strategies to block parasite sporogony in key mosquito vector species. MAPK signaling pathways regulated by extracellular signal-regulated kinases (ERKs) and the stress-activated protein kinases (SAPKs) c-Jun N-terminal kinases (JNKs) and p38 MAPKs are highly conserved across eukaryotes, including mosquito vectors of the human malaria parasite Plasmodium falciparum. Some of these pathways in mosquitoes have been investigated in detail, but the mechanisms of integration of parasite development and mosquito fitness by JNK signaling have not been elucidated. To this end, we engineered midgut-specific overexpression of MAPK phosphatase 4 (MKP4), which targets the SAPKs, and used two potent and specific JNK small molecule inhibitors (SMIs) to assess the effects of JNK signaling manipulations on Anopheles stephensi fecundity, lifespan, intermediary metabolism, and P. falciparum development. MKP4 overexpression and SMI treatment reduced the proportion of P. falciparum-infected mosquitoes and decreased oocyst loads relative to controls. SMI-treated mosquitoes exhibited no difference in lifespan compared to controls, whereas genetically manipulated mosquitoes exhibited extended longevity. Metabolomics analyses of SMI-treated mosquitoes revealed insights into putative resistance mechanisms and the physiology behind lifespan extension, suggesting for the first time that P. falciparum-induced JNK signaling reduces mosquito longevity and increases susceptibility to infection, in contrast to previously published reports, likely via a critical interplay between the invertebrate host and parasite for nutrients that play essential roles during sporogonic development.
The etiologic agents of malaria are protozoan parasites in the genus Plasmodium and are responsible for 216 million new cases and 445,000 deaths worldwide in 2016 . Artemisinin-based combination therapies (ACTs) have been adopted as first-line treatments of uncomplicated and severe Plasmodium falciparum malaria in many countries with concomitant reductions in the global malaria burden . Unfortunately, artemisinin-resistant malaria parasites have been detected in five countries in Southeast Asia  and spread of these strains to Africa or the Indian subcontinent could be disastrous. Anopheles stephensi, one of the major vectors of malaria in the Indian subcontinent and Middle East, is well adapted to urban areas and feeds aggressively on humans. The appearance of A. stephensi in Djibouti, Horn of Africa, has been linked to a recent resurgence of severe Plasmodium falciparum malaria . More recently, A. stephensi has been detected in Sri Lanka, where it has never been reported, raising concerns regarding vulnerability of this country to reintroduction of malaria . Therefore, continued efforts in the development of novel strategies and tools to curb malaria transmission, including those focused on the mosquito host, are still required.
With complementary experimental approaches, we have demonstrated that moderate inhibition of JNK signaling in the A. stephensi midgut extends lifespan and enhances resistance to the human malaria parasite P. falciparum. Resistance was independent of effects on NF-κB-dependent innate immunity, adding to the list of similar observations in A. stephensi that highlight the importance of alternative signaling pathways and intermediary metabolism in mediating anti-parasite resistance [14, 20, 22]. The phenotypic effects of JNK inhibition may be due in part to inhibition of IIS in the A. stephensi midgut, which we have previously associated with lifespan extension and resistance to parasite infection and which also reaffirms our observations of IIS-MAPK signaling cross-talk in the A. stephensi midgut [20, 22].