Molecular characterization of the mosquito vitellogenin receptor reveals unexpected high homology to the Drosophila yolk protein receptor.

Abstract

The mosquito (Aedes aegypti) vitellogenin receptor (AaVgR) is a large membrane-bound protein (214 kDa when linearized) that mediates internalization of vitellogenin, the major yolk-protein precursor, by oocytes during egg development. We have cloned and sequenced two cDNA fragments encompassing the entire coding region of AaVgR mRNA, to our knowledge the first insect VgR sequence to be reported. The 7.3-kb AaVgR mRNA is present only in female germ-line cells and is abundant in previtellogenic oocytes, suggesting that the AaVgR gene is expressed early in oocyte differentiation. The deduced amino acid sequence predicts a 202.7-kDa protein before posttranslational processing. The AaVgR is a member of the low density lipoprotein receptor superfamily, sharing significant homology with the chicken (Gallus gallus) VgR and particularly the Drosophila melanogaster yolk protein receptor, in spite of a very different ligand for the latter. Distance-based phylogenetic analyses suggest that the insect VgRIyolk protein receptor lineage and the vertebrate VgR/low density lipoprotein receptor lineage diverged before the bifurcation of nematode and deuterostome lines.The developing embryo of an oviparous animal draws practically all of its requisite nutrients from a cache of proteins, lipids, and carbohydrates stored within the egg as yolk. Yolkprotein precursors are synthesized extraovarially and transported to the developing egg where they are specifically recognized and bound by membrane-spanning cell-surface receptors. Receptor-ligand complexes accumulate in clathrincoated pits, which pinch-off into the cytoplasm, a fundamental process ubiquitous among cells for internalizing macromolecules referred to as receptor-mediated endocytosis (1, 2). The insect oocyte provides an excellent system for studying receptor-mediated endocytosis because of the high intensity of protein uptake. Mosquitoes are especially useful models in this regard, because a tightly regulated series of physiological events associated with egg maturation is synchronized across all primary oocytes by a blood meal.In the yellow fever mosquito Aedes aegypti (Aa), oocyte size increases more than 300-fold within 36 h of a blood meal (3), largely through the specific accumulation of the major yolkprotein precursor vitellogenin (AaVg). This impressive biological feat depends on the proper interaction ofAaVg with its receptor (AaVgR) on the oocyte surface. In addition to its exceptional value as a model for studying receptor-mediated endocytosis, this system is also a promising target for future novel control strategies. For example, interruption of the receptor-ligand interaction would block egg development, and theAaVgR could serve as a target for an antimosquito vaccine (4). A prerequisite to successful manipulation of this system is a thorough understanding of the proteins involved: their structures, interactions, regulation, and expression. Meaningful progress on all of these fronts hinges on knowledge of the primary structures of both AaVg, which we r...

Regulation of the Vitellogenin Receptor duringDrosophila melanogasterOogenesis

“…This was subsequently confirmed by the cloning of a very closely related vitellogenin receptor from another dipteran, the mosquito Aedes aegypti (Sappington et al, 1996). The similarity of vitellogenin receptors from insects to birds suggests conserved mecha- nisms for regulating yolk uptake into eggs.…”

Isolation and Characterization of Rhamnose-binding Lectins from Eggs of Steelhead Trout (Oncorhynchus mykiss) Homologous to Low Density Lipoprotein Receptor Superfamily

“…Although the VTG receptor has been isolated from the egg of rainbow trout (46, 47), we do not know the relationship between the VTG receptor and STL2 because of the lack of sequence information of the fish VTG receptor. However, it is not probable that STL2 is derived from the VTG receptor, since STL does not have the consensus sequences for the modular repeats characteristic of LDL receptor superfamily members as revealed with the mosquito VTG receptor (48). In mammals, VTG receptors localized in coated pits on the surface of growthcomponent oocytes are able to accumulate the VTG and other ligands in the yolk with high concentrations (42).…”

The Endocrine Basis for Reproductive Life-history Trade-offs during the Previtellogenic Resting Stage in the Yellow Fever Mosquito, Aedes aegypti

“…The period after a blood meal in mosquitoes is marked by intense vitellogenin receptor, lipophorin receptor and heavy-chain clathrin synthesis by the oocyte and nurse cells (Sappington et al, 1996;Sun et al, 2000;Cheon et al, 2001;Cho and Raikhel, 2001;Seo et al, 2003), as well as massive endocytosis of yolk components (Reviewed in Dhadialla and Raikhel, 1992). Since endocytosis and protein synthesis are both energy-dependent processes, it is not unreasonable to suggest that a pool of previtellogenic oocyte lipids supports the energetic demands of the growing oocyte during at least the beginning stages of vitellogenesis.…”

“…While oocyte quality may mean different things depending on the observational context as well as the specific developmental features of an organism, Thomson et al, define oocyte quality broadly as "…[the] intrinsic genetic, epigenetic and cytoplasmic characteristics required for the completion of development and capability of producing a normal offspring" (2010). We examined likely indicators of a follicles ability to complete vitellogenic development (i.e., "quality") such as: oocyte neutral lipid content, lipophorin receptor expression (AaLpRov) (Cheon et al, 2001;Seo et al, 2003;Sun et al, 2000), vitellogenin receptor expression (AaVgR) (Sappington et al, 1996;Cho and Raikhel, 2001), heavy-chain clathrin expression (AaCHC) as well as 60s ribosomal protein expression subunit L32 (rpL32). These markers of mosquito oocyte quality, while certainly not meant to be an exhaustive inventory, begin to illustrate how previtellogenic nutrition and JH synthesis can affect vitellogenic resorption by altering an oocyte's "…intrinsic developmental potential" (Gilchrist et al, 2008).…”

“…After a blood meal, the oocytes and nurse cells show rapid increases in the synthesis of components required for the endocytosis of yolk proteins and lipids such as the: lipophorin receptor (Cheon et al, 2001;Seo et al, 2003;Sun et al, 2000), vitellogenin receptor (Sappington et al, 1996;Cho and Raikhel, 2001), and heavy-chain clathrin (Kokoza and Raikhel, 1997a;. The synthesis of endocytotic components in the follicles occurs in parallel with the synthesis and transfer of yolk components by the fat body (Roth and Porter, 1964;Koller and Raikhel, 1991;reviewed in Raikhel and Dhadialla, 1992).…”