FUNCTIONAL-PROPERTIES OF POLYCLONAL ANTIBODIES RAISED AGAINST THE N-TERMINUS REGION (RESIDUES 9-30) OF THE FOLLICLE-STIMULATING-HORMONE (FSH) RECEPTOR - SIGNIFICANCE OF THIS RECEPTOR REGION IN FSH RECOGNITION AND SIGNAL-TRANSDUCTION

In this study, we raised polyclonal antibodies in rabbits against a synthetic peptide corresponding to a unique region of the FSH receptor, residues 9-30, with no sequence homology to receptors for LH and TSH, and examined their characteristics relevant to receptor function. Binding of [I-125]human (h) FSH to membrane-bound receptors was inhibited in a concentration-dependent manner by the anti-FSH receptor-(9-30) peptide antibody. Preimmune serum had no effect. Line-weaver-Burke plot analysis of [I-125]hFSH binding to membrane receptors in the presence or absence of antireceptor peptide antibody indicated that the antibody effectively competed with FSH at a hormone-binding site on the receptor. Also, antireceptor peptide antibody, but not preimmune serum, inhibited the ability of FSH to stimulate the conversion of androstenedione to estradiol in cultured immature rat Sertoli cells. Stimulation of estradiol synthesis by Sertoli cells caused by cholera toxin or forskolin (which are known to act through the G(s)-protein and catalytic unit of adenylate cyclase, respectively) was not inhibited by antireceptor peptide antibody. Indirect immunofluorescence staining of cultured rat Sertoli cells showed binding of antibody to plasma membrane receptor. No fluorescent staining of receptor was observed when cells were incubated with preimmune serum or antireceptor peptide antibody in the presence of excess receptor-(9-30) peptide or hFSH. These results were consistent with specific labeling of membrane-bound FSH receptors by anti-receptor-(9-30) peptide antibody. When detergent-solubilized membrane preparations from rat Sertoli cells were fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions and then subjected to Western blot analysis, antireceptor peptide antibody, but not preimmune rabbit serum, specifically recognized intact FSH receptor. Although the antireceptor peptide antibody occupied the N-terminus 9-30 epitope region in the FSH receptor, it did not induce postbinding events, such as receptor patching (aggregation), as shown by indirect immunofluorescence staining of rat Sertoli cells and the estradiol response. In contrast, a polyclonal antibody against the FSH holoreceptor capable of interacting with multiple epitopes on the receptor could induce FSH-like effects, such as receptor patching and estradiol response in Sertoli cells. In conclusion, antibody raised against the N-terminus region (9-30) of the FSH receptor recognized intact FSH receptor, inhibited FSH binding, and behaved as an antagonist, suggesting that this N-terminus epitope region of the receptor is involved in hormone binding. Moreover, the differential effects of this antibody and antibody raised against the FSH holo-receptor on postbinding events suggest that the N-terminus (9-30) region of the FSH receptor alone may not be directly involved in signal transduction. The site-directed antibodies reported herein should offer a useful probe for further study of FSH receptor structure, biosynthesis, and mode of action.