Spatial polarization of insulin-like growth factor receptors on the human syncytiotrophoblast

The expression of IGF receptors on die maternal-facing, microvillous membrane (MVM) surface and the fetal-facing, basal membrane (BM) surface of the syncytiotrophoblast was studied using standard ligand binding assays, covalent crosslinking techniques, and immunoblot analysis. Scatchard analysis of [I-125]IGF-I and -II binding revealed the presence of both high and low affinity binding sites associated with each membrane preparation that did not clearly distinguish between the two membrane preparations. Cross-linking analysis, however, demonstrated type I and type II IGF receptors associated primarily with MVM, suggesting that nonreceptor binding sites may contribute to total membrane binding. Ligand blot analysis revealed that BM are uniquely associated with 29- and 24-kD IGF binding proteins (IGFBPs). [I-125]QAYL-IGF-1, having reduced affinity for IGFBPs, was therefore used to study receptor-specific binding. Approximately 5-fold more type I IGF receptors were shown to be associated with MVM than BM by Scatchard and crosslinking analyses. This was confirmed by immunoblot analysis. By contrast, immunoblot analysis revealed approximately 50-100% more type II IGF receptor protein associated with BM, whereas cross-linking to [I-125]IGF-II revealed a MVM predominance. In the presence of 5 mM mannose 6-phosphate, however, a substantial increase in [I-125]TGF-II cross-linked to the type Il IGF receptor was observed in BM but not MVM, consistent with immunoblot analysis. These data demonstrate that type I and unoccupied type II IGF receptors are expressed primarily on the maternal-facing, MVM surface of the syncytiotrophoblast.