Apolipophorin-II/I Contributes to Cuticular Hydrocarbon Transport and Cuticle Barrier Construction in Locusta migratoria

Abstract

Apolipophorins are carrier proteins that bind lipids and mediate their transport from tissue to tissue in animals. Apolipophorin I and II (apoLp-II/I) are the major apolipophorins in insects. The implication of apoLp-II/I in cuticle lipid-barrier formation in insects has not been addressed to date. In the present study, we investigated the function of apoLp-II/I in the migratory locust <i>Locusta migratoria</i> (<i>LmapoLp-II/I</i>). During the development of fifth instar nymphs, <i>LmapoLp-II/I</i> transcript levels increased until mid-instar, and then decreased gradually until molting to the adult stage. We found that <i>LmapoLp-II/I</i> was predominately expressed in the fat body and the integument including oenocytes and epidermal cells. Immunodetection experiments revealed that LmapoLp-I mainly localized in the cytoplasm of oenocytes and epidermal cells. Silencing of <i>LmapoLp-II/I</i> caused molting defects in nymphs. Importantly, RNA interference against <i>LmapoLp-II/I</i> resulted in a significant decrease in the content of cuticle surface lipids including alkanes and methyl alkanes. Cuticular permeability was significantly enhanced in these nymphs in Eosin Y penetration assays. By consequence, desiccation resistance and insecticide tolerance of ds<i>LmapoLp-II/I</i>-treated locusts were reduced. Taken together, our results indicate that <i>LmapoLp-II/I</i> is involved in the transport and deposition of surface-cuticular lipids that are crucial for maintaining normal cuticle barrier function in <i>L. migratoria</i>.