Abstract
BackgroundApolipophorin-III (ApoLp-III) is known to play an important role in lipid transport and innate immunity in lepidopteran insects. However, there is no evidence of involvement of ApoLp-IIIs in the immune responses of dipteran insects such as Drosophila and mosquitoes.Methodology/Principal FindingsWe report the molecular and functional characterization of An. gambiae apolipophorin-III (AgApoLp-III). Mosquito ApoLp-IIIs have diverged extensively from those of lepidopteran insects; however, the predicted tertiary structure of AgApoLp-III is similar to that of Manduca sexta (tobacco hornworm). We found that AgApoLp-III mRNA expression is strongly induced in the midgut of An. gambiae (G3 strain) mosquitoes in response to Plasmodium berghei infection. Furthermore, immunofluorescence stainings revealed that high levels of AgApoLp-III protein accumulate in the cytoplasm of Plasmodium-invaded cells and AgApoLp-III silencing increases the intensity of P. berghei infection by five fold.ConclusionThere are broad differences in the midgut epithelial responses to Plasmodium invasion between An. gambiae strains. In the G3 strain of An. gambiae AgApoLp-III participates in midgut epithelial defense responses that limit Plasmodium infection.
Highlights
Lipophorins are protein-lipid complexes that transport lipid in the aqueous environment of the insect hemolymph [1,2]
In the G3 strain of An. gambiae AgApoLp-III participates in midgut epithelial defense responses that limit Plasmodium infection
High-density lipophorin (HDLp) transforms into low-density lipophorin (LDLp) as it takes up diacylglycerol (DAG) and the lipid content of the particle increases
Summary
Lipophorins are protein-lipid complexes that transport lipid in the aqueous environment of the insect hemolymph [1,2]. Apolipophorin-1 (apoLp-I, ,250 kDa), apolipophorin-II (apoLp-II, ,70–80 kDa), and apolipophorin-III (apoLp-III, ,18 kDa), are present in insect lipophorins [2]. ApoLp-I and apoLp-II are generated by proteolytic cleavage of a common precursor protein called preproapolipophorin [3,4] and form the ‘‘non-exchangeable’’ lipid-binding protein core of the lipophorin particle [2]. ApoLp-III is an ‘‘exchangeable’’ lipid-free hemolymph protein that changes conformation reversibly, as it binds to the LDL surface to prevent lipids in the particle from coming into contact with the aqueous environment [2]. Apolipophorin-III (ApoLp-III) is known to play an important role in lipid transport and innate immunity in lepidopteran insects. There is no evidence of involvement of ApoLp-IIIs in the immune responses of dipteran insects such as Drosophila and mosquitoes
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