THE PRESENCE OF TRANSTHYRETIN IN RAT EPENDYMAL CELLS IS DUE TO ENDOCYTOSIS AND NOT SYNTHESIS

The presence and synthesis of transthyretin, a major carrier protein o f thyroxine in rat cerebrospinal fluid, was investigated in choroid pl exus epithelial cells and ependymal cells by immunocytochemistry, in s itu hybridization, and analysis by Northern and Western blot using a s pecific oligonucleotide probe and a specific polyclonal antibody to tr ansthyretin. Choroid plexus epithelial cells expressed transthyretin a t high levels in developing rat cerebral hemispheres and in cultured c ells. These cells secreted transthyretin into the cerebrospinal fluid. In the developing rat brain transthyretin was present in the cytoplas m of ependymal cells, in vesicles in contact with the apical membrane and in cilia. In ependymal cell cultures this protein was particularly abundant in the cilia of these cells. In contrast, ependymal cells di d not synthesize transthyretin. It is postulated that transthyretin is transported to ependymal cells from the cerebrospinal fluid by endocy tosis. (C) 1998 Elsevier Science B.V. All rights reserved.