Developmental regulation of insulin-like growth factor binding protein-2 in chick embryo serum and vitreous humor

The chick embryo is a useful vertebrate model for studying developmental embryogenesis. Insulin-like growth factor I (IGF-I), a potent mitogen, is thought to contribute to the general growth of the embryo as an endocrine factor, and as a paracrine factor to the development of the early embryo and of specific organs such as the eye. Recent data suggest that a family of at least six IGF binding proteins (IGFBPs) complex IGF-I and modulate its biological actions. In the present study, we examine the expression of IGFBPs in chicken serum and vitreous humor at different stages of embryonic development, and compare it with that of IGF-I. As determined by ligand blotting, the predominant IGFBP in chick serum and vitreous humor between embryonic days 4 and 22 (E4-E22) is a 30 kDa IGFBP. This IGFBP was specifically immunoprecipitated by a polyclonal antiserum raised against rat IGFBP-2, the predominant IGFBP in fetal human and rat serum. Although IGFBP-2 is present in both chick fluids at all times examined, serum IGFBP-2 increased progressively between E10-E22, whereas vitreous IGFBP-2 was highest during eye organogenesis (E4-E8). This suggests that vitreous IGFBP-2 is synthesized locally. Like serum IGFBP-2, levels of immunoreactive IGF-I in serum are higher in the second week of embryogenesis than the first. Despite this correlation, changes in IGFBP-2 do not appear to be regulated by IGF-I: (a) serum IGF-I decreases after day 15, whereas IGFBP-2 levels remain stable until hatching; (b) vitreous IGF-I, like serum IGF-I, is higher in the second week of embryogenesis, whereas vitreous IGFBP-2 is highest in the first week; (c) embryos cultured ex ovo express IGFBP-2 at E15-E19, although they lack the normal mid-embryogenesis surge in IGF-I. We conclude that vitreous IGFBP-2 is synthesized locally in the eye, and that the expression of IGFBP-2 in chick embryos is not directly regulated by IGF-I.