Regulation of hemolymph trehalose titers by insulin signaling in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae)

A disaccharide, trehalose, is a main hemolymph sugar of the legume pod borer, Maruca vitrata larvae, but its titers fluctuated with feeding activity. During diurnal feeding in the photophase, hemolymph trehalose remained at a relatively low level (69 mM) and increased (98 mM) during scotophase. Starvation significantly increased the hemolymph trehalose level, in which the elevation of trehalose titers was dependent on the non-feeding period. The down-regulation of the trehalose level during the active feeding period seemed to result from mediation of the insulin/IGF signal (IIS). Injection of a porcine insulin suppressed the trehalose level in a dose-dependent manner. Genes associated with IIS of M. vitrata were predicted from its larval transcriptome, and their expression was confirmed in different developmental stages and tissues. All seven IIS genes selected were expressed in all developmental stages and different tissues. Silencing of four IIS genes (insulin receptor, Forkhead box O, a serine-threonine protein kinase, target of rapamycin) by RNA interference significantly modulated the hemolymph trehalose level. Starvation treatment changed expression of two trehalose metabolism-associated genes (trehalose phosphate synthase (TPS) and trehalase (TRE)) as well as the IIS genes. Silencing of TPS or TRE expression significantly down- or up-regulated the hemolymph trehalose level, respectively. In addition, silencing of IIS genes altered both TPS and TRE expression, indicating a functional link between IIS and trehalose metabolism. These results suggest that nutrients obtained from feeding activate IIS of M. vitrata, which then down-regulates the hemolymph trehalose level by altering trehalose metabolism.