Multiple involvement of clusterin in chicken ovarian follicle development - Binding to two oocyte-specific members of the low density lipoprotein receptor gene family

The interaction of the female germ cell with somatic cells during the devel opment of the ovarian follicle in the chicken provides a prime system to st udy gene expression. Here, we have uncovered the involvement of clusterin, the function(s) of which is still poorly understood, in this complex proces s. As revealed by molecular cloning, chicken clusterin is a 428-residue pro tein that migrates at 70 kDa on SDS-polyacrylamide gel electrophoresis and possesses most of the structural features of its mammalian successors. Howe ver, in contrast to mammalian clusterin, the chicken protein appears not to be cleaved intracellularly into a disulfide-linked heterodimer; possibly a s a consequence thereof, it is not secreted constitutively and is absent fr om the circulation, where most of clusterin is found in mammals. In the ova ry, clusterin is a major product of the somatic granulosa cells, in a patte rn correlating with the developmental phases of individual follicles. In th at, transcript levels are high not only at onset of vitellogenesis, but als o in atretic follicles and in the postovulatory follicle sac, ie. in situat ions characterized by apoptotic events. Yolk of growing oocytes contains a 43-kDa truncated form of clusterin that does not appear to be synthesized w ithin the oocyte, Rather, we here show for the first time that 70-kDa clust erin interacts not only with megalin, but also with two chicken oocyte-spec ific members of the low density lipoprotein receptor (LDLR) gene family. Th ese receptors, termed LDLR-related protein with eight ligand binding repeat s (LR8) and LDLR-related protein (380 kDa), likely internalize granulosa ce ll-derived 70-kDa clusterin, which may subsequently be processed to the 43- kDa product. Thus, chicken clusterin could serve as a marker for follicular atresia and resorption, and, based on its ability to bind several other pr oteins, it may serve as carrier for the receptor-mediated endocytosis into oocytes of components important for embryonic development, two hitherto unk nown functions of this intriguing protein.