The salivary protein Saglin facilitates efficient midgut colonization of Anopheles mosquitoes by malaria parasites

Author summaryFemale mosquitoes rely on blood feeding to acquire sufficient nutrients for egg development. Because of the importance of this process mosquitoes evolved salivary proteins with a broad range of functions acting as blood thinners, anti-coagulants and immunosuppressants. The effect of these proteins on the blood at the bite site directly influences the size of the blood bolus a female takes up in a given time frame. Both, time of feeding and bolus size, are important parameters for fecundity and survival. Recent studies have shown that a significant proportion of salivated proteins is re-ingested during feeding and becomes part of the blood meal. Here we investigated the salivary protein Saglin which has been previously suggested as putative receptor mediating malaria parasite entry into the salivary gland. By engineering a loss-of-function mutant in An. coluzzi we could show that the absence of Saglin impairs the development of parasite stages in the blood meal of the rodent malaria parasite P. berghei and the human malaria parasite P. falciparum lowering the parasite burden of subsequent stages and preventing efficient transmission at low infection densities. Furthermore, we could show that Saglin is present in the blood meal after feeding possibly indicating a previously overlooked parasite-vector interaction. Malaria is caused by the unicellular parasite Plasmodium which is transmitted to humans through the bite of infected female Anopheles mosquitoes. To initiate sexual reproduction and to infect the midgut of the mosquito, Plasmodium gametocytes are able to recognize the intestinal environment after being ingested during blood feeding. A shift in temperature, pH change and the presence of the insect-specific compound xanthurenic acid have been shown to be important stimuli perceived by gametocytes to become activated and proceed to sexual reproduction. Here we report that the salivary protein Saglin, previously proposed to be a receptor for the recognition of salivary glands by sporozoites, facilitates Plasmodium colonization of the mosquito midgut, but does not contribute to salivary gland invasion. In mosquito mutants lacking Saglin, Plasmodium infection of Anopheles females is reduced, resulting in impaired transmission of sporozoites at low infection densities. Interestingly, Saglin can be detected in high amounts in the midgut of mosquitoes after blood ingestion, possibly indicating a previously unknown host-pathogen interaction between Saglin and midgut stages of Plasmodium. Furthermore, we were able to show that saglin deletion has no fitness cost in laboratory conditions, suggesting this gene would be an interesting target for gene drive approaches.