Clusterin is an intriguing glycoprotein that, despite its discovery some 20 years ago and an immense volune of scientific papers dedicated to it since, continues to remain an enigma. The predominant form of clusterin is a disulfide-linked, heterodimeric glycoprotein of 75-80 kDa that is constitutively expressed in almost all mammalian tissues and is a major protein secreted into biological fluids. In addition, a truncated form of clusterin that escapes secretion and is instead targeted to the nucleus, has also been reported. Aside from its presence in normal, biological processes such as sperm maturation, lipid transport and apoptosis, clusterin has also been identified in various pathological processes that include Alzheimer's disease, cancer and atherosclerosis. Its purported involvement in both physiological and pathophysiological processes and almost ubiquitous tissue distribution has made it seemingly difficult to clarify a true role for this
protein. The main uncertainty that surrounds clusterin is whether it genuinely is a multifunctional protein or serves a single, primary function. To obtain a more clear understanding of clusterin's true role in such diverse processes, a study of clusterin at both the molecular and protein level in the chicken was undertaken. In particular, the interaction of the female germ cell with somatic cells during the development of the ovarian follicle in the chicken provided a prime system to study clusterin expression. Here, the involvement of clusterin in this complex process has been uncovered. As revealed by molecular cloning, chicken clusterin is a 428-residue protein that migrates at 70 kDa on SDS-polyacrylanude 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 that correlates 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 contain a 43-kDa truncated form of clusterin that does not appear to be synthesized within the oocyte. Rather, it is shown here for the first time that 70-kDa clusterin interacts not only with megalin and low density lipoprotein receptor-related protein (LRP), 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.