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

Authors: SAJJADIAN, S - Andong National University; VATANPARAST, M - Andong National University; Stanley, David; KIM, Y - Andong National University

Submitted to: Insect Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2019
Publication Date: 4/12/2019
Citation: Sajjadian, S.M., Vatanparast, M., Stanley, D.W., Kim, Y. 2019. Secretion of secretory phospholipase A2 into Spodoptera exigua larval midgut lumen and its role in lipid digestion. Insect Molecular Biology. https://doi.org/10.1111/imb.12588.
DOI: https://doi.org/10.1111/imb.12588

Interpretive Summary: Chemical insecticides are effective pest insect management tools, however, negative effects, including environmental contamination and insect resistance to the insecticides attend use of these products. These negative effects drive research into alternative insect management technologies such as biological control based on microbes, such as bacteria. Bacteria applied to crop plants may be taken into pest insects during normal feeding. The problem is the fate of dietary bacteria within insect digestive tracts is virtually unknown. Bacteria are surrounded by fatty (lipid) membranes and there is virtually no information on how insects digest dietary lipids or bacteria-associated lipid membranes. To gain new information on lipid digestion, we identified an enzyme that is secreted into digestive tracks of beet armyworms, serious crop pests that have spread all around the globe. Here, we report on the properties of the enzyme. Inhibiting the enzyme with a specific drug or silencing the gene that encodes the enzyme led to reduced enzyme activity, reduced digestion and reduced body growth. This new information will be used by scientists at the international level to improve understanding of digestive processes in pest insects and improve microbe-based biological control programs. This information will ultimately benefit consumers with improved pest control technologies and sustained production of healthy, high quality foods.

Technical Abstract: In insects, lipid digestion in the midgut is controversial because insects have no bile salts to solubilize dietary neutral lipids. One hypothesis is that a secretory type of phospholipase A2 (sPLA2) releases a free fatty acid and a lysophospholipid (LPL) from dietary phospholipids (PLs). We identified a sPLA2, Se-sPLA2, in the beet armyworm, Spodoptera exigua that hydrolyses the sn-2 fatty acid from PLs. Our goal was to investigate a midgut Se-sPLA2 and its role in digestion in S. exigua larvae. Se-sPLA2 was expressed in the entire alimentary canal. Incubating the isolated midgut in a cell culture medium led to secretion of several proteins with PLA2 activity. After separating the proteins on SDS-PAGE gels, LC-MS/MS and bioinformatics analyses of a 24 kDa protein revealed a Se-sPLA2. Ex vivo RNA interference (RNAi) of Se-sPLA2 expression in isolated midgut, led to significantly decreased Se-sPLA2 secretion into the culture medium. Feeding double-stranded RNA specific to Se-sPLA2 suppressed gene expression in the midgut and sPLA2 activity in gut contents. We prepared a recombinant Se-sPLA2 in a bacterial expression system, which has sPLA2 activity that is susceptible to benzylideneacetone (BZA), a specific PLA2 inhibitor. After feeding BZA to larvae, we recorded significant decreases in gut content sPLA2 activity, body growth and total hemolymph lipid contents. RNAi treatment against Se-sPLA2 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-sPLA2 secreted from the midgut acts in lipid digestion by providing necessary LPL to solubilize dietary neutral lipids.