Anopheles gambiae PGRPLC-Mediated Defense against Bacteria Modulates Infections with Malaria Parasites

Abstract

Recognition of peptidoglycan (PGN) is paramount for insect antibacterial defenses. In the fruit fly Drosophila melanogaster, the transmembrane PGN Recognition Protein LC (PGRP-LC) is a receptor of the Imd signaling pathway that is activated after infection with bacteria, mainly Gram-negative (Gram−). Here we demonstrate that bacterial infections of the malaria mosquito Anopheles gambiae are sensed by the orthologous PGRPLC protein which then activates a signaling pathway that involves the Rel/NF-κB transcription factor REL2. PGRPLC signaling leads to transcriptional induction of antimicrobial peptides at early stages of hemolymph infections with the Gram-positive (Gram+) bacterium S taphylococcus aureus, but a different signaling pathway might be used in infections with the Gram− bacterium Escherichia coli. The size of mosquito symbiotic bacteria populations and their dramatic proliferation after a bloodmeal, as well as intestinal bacterial infections, are also controlled by PGRPLC signaling. We show that this defense response modulates mosquito infection intensities with malaria parasites, both the rodent model parasite, Plasmodium berghei, and field isolates of the human parasite, Plasmodium falciparum. We propose that the tripartite interaction between mosquito microbial communities, PGRPLC-mediated antibacterial defense and infections with Plasmodium can be exploited in future interventions aiming to control malaria transmission. Molecular analysis and structural modeling provided mechanistic insights for the function of PGRPLC. Alternative splicing of PGRPLC transcripts produces three main isoforms, of which PGRPLC3 appears to have a key role in the resistance to bacteria and modulation of Plasmodium infections. Structural modeling indicates that PGRPLC3 is capable of binding monomeric PGN muropeptides but unable to initiate dimerization with other isoforms. A dual role of this isoform is hypothesized: it sequesters monomeric PGN dampening weak signals and locks other PGRPLC isoforms in binary immunostimulatory complexes further enhancing strong signals.

Author Summary

Recognition of peptidoglycan on the bacteria cell wall triggers insect immune responses. The fruit fly PGRPLC receptor protein senses the presence of peptidoglycan and activates a pathway that mediates resistance to bacterial infections, mainly Gram-negative. We show that the PGRPLC receptor of the malaria vector mosquito Anopheles gambiae can also sense infections of the hemolymph (the mosquito blood) or the gut with bacteria of both Gram types and thereby activate a pathway that confers resistance to these infections. PGRPLC and its downstream responses also control the numbers of symbiotic bacteria that are mostly found in the mosquito gut where they drastically proliferate after a female mosquito takes a bloodmeal. Importantly, when the bloodmeal is infected with malaria parasites, the defense reaction that the mosquito mounts against proliferating bacteria also eliminate a large number of parasites. These mechanisms are largely elucidated using a rodent malaria parasite, but we also show that they significantly affect the intensities of mosquito infections with Plasmodium falciparum parasites found in the blood of children in sub-Saharan Africa.