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.