Localization of immunoreactive transthyretin (prealbumin) and of transthyretin mRNA in fetal and adult rat brain

Abstract

We used a combination of immunohistochemical and molecular-biological techniques to investigate the localization of transthyretin (TTR) in the brains of adult and fetal rats. The immunohistochemical studies employed antibodies purified by immunosorbent affinity chromatography, permitting the specific staining and localization of TTR using the unlabeled peroxidase-antiperoxidase method. TTR mRNA levels were measured by Northern-blot analysis of poly (A+) RNA, followed by hybridization to 32P-labeled TTR cDNA; TTR mRNA was localized in brain tissue sections by in situ hybridization. Immunoreactive TTR was found to be specifically localized in the choroid plexus epithelial cells of adult rat brain. High levels of TTR mRNA were found in poly (A+) RNA samples obtained from the choroid plexus. In addition, the specific localization of TTR mRNA in the epithelial cells of the choroid plexus was demonstrated by in situ hybridization. Neither immunoreactive TTR nor TTR mRNA were found in other regions of adult rat brains. The levels of TTR mRNA in the choroid plexus were at least 30 times higher than those observed in the adult liver. Immunoreactive TTR was observed in the brains of fetal rats on as early as the 11th day of gestation. This immunoreactive TTR was localized in the tela choroidea, the developmental forerunner of the choroid plexus. Immunoreactive TTR was also observed in the fetal choroid plexus as it began to form (14th day of gestation) as well as in the more completely developed choroid plexus (18th day of gestation). Finally, TTR mRNA was detectable in the brain of fetal rats at 16 days of gestation, and in the head of fetal rats at 14 days of gestation. Taken together, these data strongly suggest that the choroid plexus epithelium synthesizes TTR de novo and secretes it into the cerebrospinal fluid, and that TTR synthesis begins in the developing fetal rat brain before the formation of the choroid plexus (in the tela choroidea). The role that TTR plays in the brain and cerebrospinal fluid of the developing and adult rat remains to be determined.