Multiple Involvement of Clusterin in Chicken Ovarian Follicle Development

Abstract

The interaction of the female germ cell with somatic cells during the development of the ovarian follicle in the chicken provides a prime system to study gene expression. Here, we have uncovered the involvement of clusterin, the function(s) of which is still poorly understood, in this complex process. As revealed by molecular cloning, chicken clusterin is a 428-residue protein that migrates at 70 kDa on SDS-polyacrylamide gel electrophoresis and possesses most of the structural features of its mammalian successors. However, in contrast to mammalian clusterin, the chicken protein appears not to be cleaved intracellularly into a disulfide-linked heterodimer; possibly as a consequence thereof, it is not secreted constitutively and is absent from the circulation, where most of clusterin is found in mammals. In the ovary, clusterin is a major product of the somatic granulosa cells, in a pattern correlating with the developmental phases of individual follicles. In that, transcript levels are high not only at onset of vitellogenesis, but also in atretic follicles and in the postovulatory follicle sac, i.e. in situations characterized by apoptotic events. Yolk of growing oocytes contains a 43-kDa truncated form of clusterin that does not appear to be synthesized within the oocyte. Rather, we here show for the first time that 70-kDa clusterin interacts not only with megalin, but also with two chicken oocyte-specific members of the low density lipoprotein receptor (LDLR) gene family. These receptors, termed LDLR-related protein with eight ligand binding repeats (LR8) and LDLR-related protein (380 kDa), likely internalize granulosa cell-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 proteins, it may serve as carrier for the receptor-mediated endocytosis into oocytes of components important for embryonic development, two hitherto unknown functions of this intriguing protein.

Clusterin facilitates in vivo clearance of extracellular misfolded proteins

“…Previous work has shown that CLU and CLU-Ab complexes (specifically) can be cleared by receptor-mediated endocytosis, via the cell surface receptor megalin, and are degraded in lysosomes [21]. A number of other studies have also suggested that CLU and CLU-ligand complexes can be cleared by the same route [39,43]. Data presented here also strongly implicates the classical route of receptor-mediated endocytosis and lysosomal degradation in the clearance of CLU-client protein complexes.…”

Early intervention in the 3xTg-AD mice with an amyloid β-antibody fragment ameliorates first hallmarks of Alzheimer disease

“…26 These complexes are removed by interaction with megalin (LRP-2) 41,42 and other receptors of the LDLR family. 43 Since exposition of hydrophobic residues of the Aβ:scFv-h3D6 complex has been reported, 12 it is tempting to propose that clusterin could have internalized the complex and, as a consequence, both clusterin and Aβ-oligomers would be decreased in the extracellular fraction of treated animals.…”

Variation in yolk precursor receptor mRNA expression is a key determinant of reproductive phenotype in the zebra finch (Taeniopygia guttata)

“…Finally, and more generally, it is becoming clear that both yolk precursor receptors bind and transport nonlipoproteins to the yolk (e.g. Mac Lachlan et al, 1994;Mahon et al, 1999). Variation in yolk precursor receptor expression, as we have documented here, might therefore provide an important mechanism mediating inter-individual variation in 'maternal effects', i.e.…”