Searching for specific binding sites of the secretory glycoproteins of thesubcommissural organ

The molecular organization of Reissner's fiber (RF), the structure of its p roteins, and the permanent turnover of these proteins are all facts support ing the possibility that RF may perform multiple functions. There is eviden ce that CSF-soluble RF-glycoproteins may occur under physiological conditio ns. The present investigation was designed to investigate the probable exis tence within the CNS of specific binding sites for RF-glycoproteins. Three experimental protocols were used: (1) immunocytochemistry of the CNS of bov ine fetuses using anti-idiotypic antibodies, raised against monoclonal anti bodies developed against bovine RF-glycoproteins; (2) in vivo binding of th e RF glycoproteins, perfusing into the rat CSF 125I-labeled RF-glycoprotein s, or grafting SCO into a lateral ventricle of the rat; (3) in vitro bindin g of unlabeled RF-glycoproteins to rat and bovine choroid plexuses maintain ed in culture. One of the anti-idiotypic antibody generated by a Mab raised against RF-glycoproteins binds to choroidal cells. Furthermore, binding of RF-glycoproteins to the rat choroid plexus was obtained when: (1) the chor oid plexus was cultured in the presence of unlabeled RF-glycoproteins; (2) the concentration of soluble RF-glycoproteins in the CSF was increased by i sografting SCOs into a lateral ventricle; (3) radiolabeled glycoproteins we re perfused into the ventricular CSF. This evidence suggests that the apica l plasma membrane of the ependymal cells of the choroid plexus has specific binding sites for RF-glycoproteins, of unknown functional significance. Th e radiolabeled RF-glycoproteins perfused into the rat CSF also bound to the paraventricular thalamic nucleus, the floor of the Sylvian aqueduct and of the rostral half of the fourth ventricle, and title meninges of the brain and spinal cord. The labeling of the paraventricular thalamic nucleus point s to a functional relationship between this nucleus and the SCO. The possib ility that; the SCO may be a component of the circadian timing system is di scussed. Microsc. Res. Tech. 52:541-551, 2001. (C) 2001 Wiley-Liss, Inc.