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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 Locusta migratoria (LmapoLp-II/I). During the development of fifth instar nymphs, LmapoLp-II/I transcript levels increased until mid-instar, and then decreased gradually until molting to the adult stage. We found that LmapoLp-II/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 LmapoLp-II/I caused molting defects in nymphs. Importantly, RNA interference against LmapoLp-II/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 dsLmapoLp-II/I-treated locusts were reduced. Taken together, our results indicate that LmapoLp-II/I is involved in the transport and deposition of surface-cuticular lipids that are crucial for maintaining normal cuticle barrier function in L. migratoria.
Highlights
Apolipoproteins are carrier proteins that bind lipids to form lipoprotein particles called lipophorins
The coding sequence of LmapoLp-II/I is 10,143 bp long and encodes a protein of 3380 amino acids containing a signal peptide, a lipoprotein N-terminal domain (LPD_N), a von Willebrand factor type D (VWD) domain, a C8 domain and two domains (DUF1943 and DUF1081) with unknown function (Supplementary Figure S1A)
Phylogenetic analysis using full-length amino acid sequences from other insect species revealed that LmapoLp-II/I clustered with the orthologs of Blattaria (Supplementary Figure S1B)
Summary
Apolipoproteins are carrier proteins that bind lipids to form lipoprotein particles called lipophorins. The major apolipoproteins (apo) are generally divided into four categories including apoA, apoB, apoC, and apoE (Mahley et al, 1984; Liu et al, 2019). These proteins are important components of the blood plasma, and mediate trafficking of various lipids between producing and consuming organs (Su and Peng, 2020). Apolipoproteins were referred to as apolipophorins (apoLp) (Beenakkers et al, 1981). Apolipophorins are the main components of insect lipophorin particles. Lipophorin (Lp) was isolated and purified from the hemolymph of various insects such as Hyalophora cecropia, Philosamia cynthia, Cuticle Barrier Function of Apolipophorin-II/I
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