Ovary

Receptor-Mediated Chicken Oocyte Growth: Differential Expression of Endophilin Isoforms in Developing Follicles

Abstract

Receptor-mediated endocytosis of yolk precursors via clathrin-coated structures is the key mechanism underlying rapid chicken oocyte growth. In defining oocyte-specific components of clathrin-mediated events, we have to date identified oocyte-specific yolk transport receptors, but little is known about the oocytes' supporting endocytic machinery. Important proteins implicated in clathrin-mediated endocytosis and recycling are the endophilins, which thus far have been studied primarily in synaptic vesicle formation; as a different highly active endocytic system, we exploit rapidly growing chicken oocytes. Molecular characterization of the chicken endophilins I, II, and III revealed that their mammalian counterparts have been highly conserved. All chicken endophilins interact via their SH3 domain with the avian dynamin and synaptojanin homologues, and thus share key functional properties of mammalian endophilins. The genes show different expression patterns : as in mammals, expression in the liver is low to undetectable, and high in the brain; in ovarian follicles harboring oocytes rapidly growing via receptor- mediated endocytosis, levels of endophilins II and III, but not of endophilin I, are high. Immunohistochemical analysis of follicles demonstrated that endophilin II is mainly present in the theca interna, but endophilin III predominates within the oocyte proper. Moreover, in a chicken strain with impaired oocyte growth and absence of egg-laying due to a genetic defect in the receptor for yolk endocytosis, endophilin III is diminished in oocytes, while endophilin III levels in the brain and endophilin II localization to thecal cells are unaltered. Thus, these studies reveal that the endophilins differentially contribute to oocyte endocytosis and development.