Insulin and IGF-I binding in developing chick neural retina and pigment epithelium: A characterization of binding and structural differences

Abstract

We have characterized insulin and insulin-like growth factor I (IGF-I) binding sites in developing chick retina and pigment epithelium (10- and 14-day embryonic, and 2-week post-hatched). For comparison, binding sites in brain and liver were also examined. Both the retina and pigment epithelium (PE) contain separate, specific, high affinity binding sites for insulin and IGF-I. In both tissues, IGF-I binding exceeds insulin binding by two to threefold. Insulin and IGF-I binding in the retina is four to six times greater than in PE. Insulin and IGF-I binding in the retina and PE exhibit independent developmental regulation. In the retina, the number of binding sites decreases by approximately 50% between embryonic day 10 and 2 weeks post-hatching. In the PE, binding decreases slightly between embryonic day 10 and 14 and then, in the 2-week post-hatched chick, increases threefold. Insulin receptor binding subunits in the retina and brain are similar in that both are neuraminidase insensitive and have apparent molecular weights of 116 kD. In contrast, binding subunits in the PE and liver have higher molecular weights (about 126 kD), and are sensitive to neuraminidase. At the embryonic stages examined, the levels of retinal insulin and IGF-I binding exceed those of the brain by five to 13-fold. Taken together, these data suggest that the retina is a major target of insulin and IGF-I and that the binding of these growth factors is developmentally regulated.