Secretion of secretory phospholipase A2 into Spodoptera exigua larval midgut lumen and its role in lipid digestion

In insects, lipid digestion is controversial because insects have no bile salts to solubilize dietary lipids. One hypothesis is that a secretory type of phospholipase A(2) (sPLA(2)) provides lysophospholipid (LPL) from dietary phospholipids (PLs). We identified a sPLA(2), Se-sPLA(2), in beet armyworm, Spodoptera exigua, that hydrolyses PLs at sn-2. Our goal was to investigate its role in lipid digestion. Se-sPLA(2) was expressed in the entire alimentary canal. Incubating the isolated midgut in a cell culture medium led to secretion of Se-sPLA(2) and other proteins. Ex vivo RNA interference (RNAi) of Se-sPLA(2) expression in isolated midgut culture led to significantly decreased Se-sPLA(2) secretion into the medium. Feeding double-stranded RNA specific to Se-sPLA(2) to larvae suppressed sPLA(2) activity in gut contents. A recombinant Se-sPLA(2) was susceptible to benzylideneacetone (BZA), a specific PLA(2) inhibitor. After feeding BZA to larvae, we recorded significant decreases in gut content sPLA(2) activity, body growth and total haemolymph lipid contents. RNAi against Se-sPLA(2) resulted in reduced digestibility. Addition of a specific LPL, 1-palmitoyl-sn-glycero-3-phosphocholine, to BZA-treated larvae rescued digestibility and larval growth. These results strongly bolster our hypothesis that Se-sPLA(2) secreted from the midgut acts in lipid digestion by providing necessary LPL to solubilize dietary neutral lipids.