Chicken oocyte growth is mediated by an eight ligand binding repeat member of the LDL receptor family.

Abstract

Deposition of the yolk mass components of chicken oocytes, very low density lipoprotein (VLDL) and vitellogenin (VTG), is mediated by a 95 kDa plasma membrane protein, termed VLDL/VTG receptor (VLDL/VTGR). Molecular characterization of the VLDL/VTGR revealed that it is a member of the LDLR gene superfamily, and harbours eight complement‐type, cysteine‐rich ligand binding repeats at the N‐terminus. This ligand binding domain structure is the hallmark of the recently discovered mammalian so‐called VLDLRs, whose true physiological function remains to be elucidated. Northern blot analysis revealed that this receptor is expressed almost exclusively in oocytes, with very much lower levels of hybridizing transcripts present in heart and skeletal muscle. Heterologous expression of the cloned receptor demonstrated its ability to bind both VLDL and VTG. The receptor gene is located on the avian sex chromosome Z, in agreement with the sex linkage of a single‐gene defect in animals that fail to reproduce because of the lack of expression of functional VLDL/VTGR. In situ hybridization analysis of oocytes suggested that VLDL/VTGR mRNA may relocalize during oocyte growth. Thus, the current study has identified and characterized the first non‐mammalian VLDLR. Its key role in avian reproduction and extremely high evolutionary conservation shed new light on VLDLR function in mammals, which also express the gene in ovaries.

An oocytic membrane receptor for biotin‐binding protein

“…Our results suggest that BBP, like certain other lipoproteins and vitamin transporters, enters the oocyte via a receptor-mediated process. These data are consistent with the observation of a high density of endocytic clathrin-coated pits in the oocyte vitelline membranes of birds and other oviparous species which, in turn, indicates that receptor-mediated endocytosis is the major process involved in oocyte growth [25][26][27].…”

Very low density lipoprotein receptor binds apolipoprotein E2/2 as well as apolipoprotein E3/3

“…A similar relaxed specificity toward different isoforms of human apoE has been observed in a 95-kDa oocyte-specific receptor to vitellogenin (VTG), which also binds human and rabbit apoE with high affinity [19]. This receptor was recently identified as a VLDL receptor in chicken oocyte, and termed the VLDL/VTG receptor [20].…”

Multiple Involvement of Clusterin in Chicken Ovarian Follicle Development

“…Up to now, the only receptor known to interact with clusterin is mammalian megalin, a large member of the LDLR gene family (12). LR8, a well characterized receptor present on the chicken oocyte plasma membrane (17,19,22) has been known thus far to bind and internalize VLDL, VTG, ␣ 2 -macroglobulin, and riboflavin-binding protein/VTG complexes (57), i.e. molecules essential for the normal growth and development of the chicken embryo.…”

“…Thus, based on our previous work describing LDLR family members in the chicken (17)(18)(19)(20), we have become interested in the biology of clusterin in this species. All presently known receptors belonging to the LDLR gene family are represented in this avian model (19,21,22), allowing a thorough investiga-tion into their possible roles in clusterin metabolism.…”