Identification of an S-antigen-like molecule in human choroid plexus and cerebrospinal fluid

Phosphorylation uncouples the gastrin-releasing peptide receptor from G(q)

Previous work on the desensitization of G protein-coupled receptors has focused on the role of arrestin binding following receptor phosphorylation. We have examined the hypothesis that phosphorylation alone contributes to desensitization. In this study we demonstrate that for the G(q)-coupled gastrin-releasing peptide receptor (GRP-R), phosphorylation by GRK2 to a stoichiometry of approximately 1 mol PO(4)/mol GRP-R is sufficient in the absence of arrestin to reduce the rate of receptor catalyzed G protein activation by approximately 80%. Furthermore, GRP-Rs exposed in vivo to agonist are rapidly phosphorylated to a similar stoichiometry and are desensitized to a similar degree. Finally, the molecular mechanism for both in vitro GRK2-induced and in vivo agonist-induced desensitization is primarily a decrease in the maximum velocity (V(max)) for the catalysis of guanine nucleotide exchange by the GRP-R rather than a change in the affinity of the receptor for the alpha(q) or betagamma subunits. Based on these results, we suggest that, for some G protein-coupled receptors, phosphorylation has a role in desensitization that is independent of arrestin.

Immunodetection of an arrestin-like protein in human retinal pigment epithelium

The goal of this study was to determine if an arrestin/S-antigen-like protein is produced by human retinal pigment epithelial (HRPE) cells maintained in tissue culture. Arrestin immunoreactivity was examined in fixed, monolayer cultures of HRPE and on immunoblots of SDS-PAGE separations of whole cell lysates of HRPE using five monoclonal antibodies (mAbs A2G5, A9C6, 3C4.2, 3D1.2 and 5c6.47) that bind to different epitopes in bovine retinal S-antigen. Monolayers of HRPE cells showed immunoreactivity with four of the mAbs though the relative staining intensity varied among mAbs and donors. For example, mAb A2G5 which historically shows very limited crossreactivity among species, reacted strongly with cells from one donor, moderately with cells from a second donor and only weakly with other donor cultures examined. mAb 3D1.2 showed no reactivity with HRPE cells. Immunoblots of SDS-PAGE separations of whole cell lysates of HRPE established from ten different donors confirmed the presence of an arrestin-related polypeptide that comigrated with retinal arrestin. These results demonstrate the presence of an arrestin-like protein in HRPE cells which have been maintained in tissue culture. Though the function of this arrestin homologue in HRPE is not yet established, it could play a role in the downregulation of receptor and/or transport protein activity.

Immunochemical evaluation of S-antigen of rabbit pineal gland

Purified S-antigen of photoreceptor cells induces experimental autoimmune uveoretinitis (EAU) and experimental autoimmune pinealitis (EAP) in laboratory animals. However, in rabbits, S-antigen induces only EAU without EAP. To evaluate this difference, the authors studied immunochemical reactivity of rabbit pineal gland with a panel of anti-S-antigen monoclonal antibodies (MAb). Rabbit pineal gland reacted with the MAbs by ELISA and immunoblot but not by immunohistochemistry. In contrast, rabbit retina like guinea pig retina, guinea pig pineal gland and bovine retina reacted with these MAbs by immunohistochemistry as well as by ELISA and immunoblot. Also, S-antigen purified from rabbit retina reacted as did bovine and guinea pig S-antigen. Therefore, S-antigen in situ in rabbit pineal gland is different from S-antigen of rabbit retina and different from S-antigen of pineal gland and retina of other species. Just as the MAbs did not react with S-antigen in rabbit pineal gland, it is possible that S-antigen activated lymphocytes may not recognize S-antigen in rabbit pineal gland and thereby not induce EAP.

S-antigen immunoreactivity in tumors of the choroid plexus

Using monoclonal antibodies (MAbs) directed against retinal S-antigen, the authors demonstrated the presence of S-antigen immunoreactivity in six of six (100%) tumors of the human choroid plexus, a tissue rich in β-adrenergic receptor activity. An anti-retinal S-antigen MAb F4C1, showed S-antigen immunoreactivity in one papilloma of the choroid plexus and in five of five (100%) carcinomas of the choroid plexus. In contrast, a papillary ependymoma of the spinal cord, an astrocytoma, an oligodendroglioma, metastatic oat cell carcinoma and metastatic adenocarcinoma to the brain did not stain. Our study indicates that MAb F4C1 may prove to be a useful immunohistochemical marker of tumors originating from the choroid plexus.