Abstract
Insect feeding behavior is regulated by many intrinsic factors, including hemolymph nutrient levels. Adipokinetic hormone (AKH) is a peptide factor that modulates hemolymph nutrient levels and regulates the nutritional state of insects by triggering the transfer of lipids into the hemolymph. We recently demonstrated that RNA interference (RNAi)-mediated knockdown of the AKH receptor (AKHR) reduces hemolymph lipid levels, causing an increase in the feeding frequency of the two-spotted cricket, Gryllus bimaculatus. This result indicated that reduced hemolymph lipid levels might motivate crickets to feed. In the present study, to elucidate whether hemolymph lipid levels contribute to insect feeding behavior, we attempted to manipulate hemolymph lipid levels via the lipophorin (Lp)-mediated lipid transferring system in G. bimaculatus. Of the constituent proteins in Lp, we focused on apolipophorin-III (GrybiApoLp-III) because of its possible role in facilitating lipid mobilization. First, we used RNAi to reduce the expression of GrybiApoLp-III. RNAi-mediated knockdown of GrybiApoLp-III had little effect on basal hemolymph lipid levels and the amount of food intake. In addition, hemolymph lipid levels remained static even after injecting AKH into GrybiApoLp-IIIRNAi crickets. These observations indicated that ApoLp-III does not maintain basal hemolymph lipid levels in crickets fed ad libitum, but is necessary for mobilizing lipid transfer into the hemolymph following AKH stimulation. Second, Lp (containing lipids) was injected into the hemolymph to induce a temporary increase in hemolymph lipid levels. Consequently, the initiation of feeding was delayed in a dose-dependent manner, indicating that increased hemolymph lipid levels reduced the motivation to feed. Taken together, these data validate the importance of basal hemolymph lipid levels in the control of energy homeostasis and for regulating feeding behavior in crickets.
Highlights
The feeding behavior of insects has been extensively investigated, in the migratory locust, Locusta migratoria [1,2,3]
We recently demonstrated that RNA interference (RNAi)-mediated knockdown of the Adipokinetic hormone (AKH) receptor (AKHR) reduces hemolymph lipid levels, causing an increase in the feeding frequency of the two-spotted cricket, Gryllus bimaculatus
As AKH is a peptide hormone facilitating the mobilization of lipids from the fat body to hemolymph, we investigated the contribution of GrybiApoLp-III to AKH-stimulated lipid mobilization
Summary
The feeding behavior of insects has been extensively investigated, in the migratory locust, Locusta migratoria [1,2,3]. The feeding behaviors of L. migratoria occur at regular intervals [2], and they are assumed to result from a repertoire of physiologically important causal factors [3, 4]. The physiological relationship between feeding behavior and these causal factors has been investigated, limited information is available at the molecular level. Our research group has focused on how the nutrient content in the hemolymph affects feeding behavior in crickets at the molecular level. Hemolymph nutrient levels are influenced by the equilibration between the utilization of stored nutrients and ingested nutrients/metabolites [9]. The fat body stores metabolic fuels, such as glycogen and triacylglycerol, and synthesizes the hemolymph sugar, trehalose [12]. Hemolymph nutrient and metabolite levels might be indicators for maintaining nutrient levels, possibly reflecting the demand for nutrients
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