RETINOL IN AVIAN OOGENESIS - MOLECULAR-PROPERTIES OF THE CARRIER PROTEIN

Normal embryo development in oviparous (egg-laying) species requires t he coordinated targeting to growing oocytes of nutrients and regulator y molecules such as retinol, the precursor of active retinoids. Serum retinol-binding protein (RBP) is the major carrier protein for retinol in the circulatory system of vertebrates. In oviparous animals, RBP i s thought to function in the delivery of retinol to yolk, in analogy t o other important nutrients and vitamins known to accumulate within th e oocyte. Here, immunoelectron microscopy revealed that RBP indeed acc umulates in yolk, in particular in the electron-lucent phase of yolk o rganelles known to harbor other serum-derived yolk proteins and their receptors. To gain understanding of the RBP-mediated serum-to-yolk tra nsport of retinol, we have characterized the chicken carrier protein a t the molecular level. The essential function of RBPs is emphasized by the first known avian RBP structure, which confirms that these vitami n carriers are among the most highly conserved serum proteins known. I nterestingly, by analysis of RBP hepatic RNA and serum protein levels, we identified a unique property of chicken RBP relative to other know n RBPs and yolk precursors, i.e., the absence of estrogen induction. O ne cause of the observed reduction in RBP RNA is an estrogen-dependent decrease of RBP gene transcription. Furthermore, Northern blot analys is of tissues of the hen demonstrated a lack of RBP synthesis by the o ocyte or other ovarian cells, confirming the exogenous (hepatic) origi n of yolk RBP. These results provide strong evidence that chicken RBP is an essential serum-to-yolk vitamin carrier with certain properties different from those of other such transporters.