Lipoprotein receptors in oocyte growth

The transport and removal of lipoproteins from the blood in the laying hen is of particular interest because it is a system in which the massive transport of lipid to one organ (the ovary) coexists with regulatory mechanisms for the control of lipid homeostasis in extraovarian tissues. In order to achieve this dual task, the laying hen expresses dichotomous receptor-mediated pathways. On one hand, very low density lipoproteins (VLDL) and vitellogenin (VTG), which together form over 95% of the lipid in a fully grown oocyte (i.e., an egg yolk), are transported into oocytes via a 95-kDa receptor protein. This receptor, termed oocyte VLDL/VTG receptor, is exclusively produced in growing oocytes and is absent from somatic cells. It shows a high degree of structural similarity to other members of the so-called low density lipoprotein (LDL) receptor family, but in contrast to the LDL receptor, its expression is not suppressed by sterols. On the other hand, somatic cells, but not oocytes, synthesize a 130-kDa receptor that recognizes VLDL-derived, cholesterol-rich lipoproteins. This receptor is the functional analogue to the mammalian LDL receptor in that it mediates, at least in part, extraoocytic lipid homeostasis. The somatic LDL receptor of the chicken recognizes apolipoprotein (apo)B, but not VTG, in accordance with VTG's exclusive routing to growing oocytes. Within oocytes, both apoB of VLDL and VTG undergo limited specific postendocytic proteolytic processing. Recent studies have shown that this breakdown of macromolecular plasma precursor molecules is catalyzed by an endosomal form of cathepsin D and is a key event in the formation of yolk, the major nutrient source for the developing embryo.