Demonstration and structural comparison of receptors for insulin-like growth factor-I and -II (IGF-I and -II) in brain and blood-brain barrier.

Specific receptors for insulin-like growth factors (IGF) I and II on microvessel-free rat brain cell membranes (RBCM) and in the microvessels that constitute the blood-brain barrier (BBB) were identified and characterized by means of affinity cross-linking techniques and specific anti-receptor antibodies. Two different models of BBB were examined: isolated rat brain capillaries and cultured bovine brain microvessel endothelial cells. Cross-linking with 125-I-IGF-I, followed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), revealed an alpha subunit of apparent Mr = 138,000 in both BBB preparations, compared to 120,000 in RBCM. Cross-linking was inhibited by unlabeled IGF and insulin, but not by antibody directed against the IGF-II receptor. When 125-I-IGF-II was cross-linked, followed by SDS-PAGE under reducing conditions, a major band of apparent Mr = 250,000 was identified in RBCM and both BBB preparations. This band, which migrated with an approximately equivalent Mr in both brain and BBB membranes, was inhibited by unlabeled IGF and by antibody specific for the IGF-II receptor. Thus, both rat and bovine brain microvessels possess classical Type I and II IGF receptors. While the alpha subunit of the Type I receptor of brain is smaller than that of the BBB, the Type II receptor of brain and BBB appear to be structurally and immunologically identical.