In egg-laying species, such as the chicken, the mode of transport of lipoprotein particles from the capillary plasma to endocytic receptors on the oocyte surface is largely unknown. Here we show by molecular characterization that the large prominent heparan sulfate proteoglycan of extracellular matrices, termed perlecan or HSPG2 (the product of the hspg2 gene), is a component of ovarian follicles that may participate in this process. However, although normally a major HSPG of basement membranes or basal laminae, in chicken follicles, perlecan is absent from the membranous structure between the theca interna and granulosa cell layers, which to date has been considered a bona fide basement membrane. Rather, the protein is localized in the extracellular matrix of theca externa cells, which produce this HSPG. Furthermore, in chicken testes, perlecan is localized in the peritubular spaces but in less organized fashion than the classical basement membrane components, agrin and laminin. All five domains and structural hallmarks of chicken perlecan (4071 residues) have been conserved in its mammalian counterparts. We have produced the recombinant domain II (containing low density lipoprotein (LDL) receptor-like binding repeats) of chicken perlecan and demonstrate its capacity to bind LDL and very low density lipoprotein (VLDL), apolipoprotein B-containing lipoproteins ultimately destined for uptake into oocytes via members of the low density lipoprotein receptor family. Binding to perlecan heparan sulfate side chains may facilitate the interaction of lipoproteins with domain II. Based on the current results and on domain-domain interactions revealed by recent ultrastructural investigations of the LDL receptor, nidogen, and laminin (Rudenko, G

Low levels of expression and sluggish sterol-mediated regulation have been likely reasons for the failure to molecularly characterize a bona fide LDL receptor (LDLR) in egg-laying species to date. The overall structure of the chicken LDLR, delineated here by cDNA cloning, has been conserved in evolution, since hallmark properties of mammalian LDLRs are already present in the avian protein.The chicken receptor appears to prefer LDL over VLDL as ligand, in compliance with its main role in providing lipoprotein-derived cholesterol for steroid production in ovarian follicular cells. This is also compatible with the fact that estrogen administration increased hepatic LDLR expression in roosters despite dramatically stimulated VLDL production. In cultured chicken embryo fibroblasts, expression of the receptor was induced by incubation with cholesterol synthesis inhibitors such as a statin. Furthermore, preincubation of induced cells with a specific anti-receptor antibody blocks LDL endocytosis, demonstrating that the receptor is ligand-endocytosis competent. Finally, the distribution of LDLRs among the extraoocytic cell populations lends support to a three-cell model for estrogen production within the ovarian follicle.In summary, the molecular characterization of the first avian LDLR reveals novel information about evolutionary, structural, and functional aspects of members of the supergene family of LDLR-related proteins. -Hummel, S., E. G. Lynn, A. Osanger, S. Hirayama, J. Nimpf, and W. J. Schneider. One of the main tasks of females of oviparous (egg-laying) species is to coordinate synthetic and metabolic pathways for the production of eggs, i.e., their reproductive effort. In the domesticated chicken ( Gallus gallus ), lipoprotein metabolism in the laying hen (LH) is under stringent hormonal control, which assures that the massive lipoprotein flow required for oocyte growth can be met without compromising systemic lipid homeostasis (1, 2). Chickens lack apolipoprotein B-48 (apoB-48) and apoE; furthermore, only apoA-I, but not apoA-II, is synthesized by this species. Our past studies in the avian system have shown that hepatically derived triglyceride-rich VLDL particles harboring apoB-100 and the unique protein apo-VLDL-II (3) as main functional surface components are directed toward two sets of receptors belonging to the supergene family of the LDL receptor (LDLR) and LDLR relatives (LRs). One set of LRs is expressed almost exclusively and at high levels in oocytes; these receptors recognize, in addition to apoB-100 of VLDL, the lipophosphoglycoprotein vitellogenin, which together with VLDL constitutes the vast majority of yolk. The other set of LRs is synthesized in cells and tissues other than the oocytes; receptors of this group bind apoB, but not vitellogenin, and are generally expressed at low levels compared with receptor genes directly involved in oocyte growth (2,4,5).The yolk precursor receptors thus far identified and characterized are the avian homolog of the mammalian socalled VLDLR (6), in c...

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