The immunocytochemical localization of cellular retinol-binding protein (CRBP), of plasma retinol-binding protein (RBP), and of plasma transthyretin (TTR) was studied in rat liver and kidney. The studies employed normal rats, retinol-deficient rats, and rats fed excess retinol . Antisera were prepared in rabbits against purified rat CRBP, RBP, and TTR . The primary antibodies and goat anti-rabbit IgG were purified by immunosorbent affinity chromatography, using the respective pure antigen coupled to Sepharose as the immunosorbent . This procedure effectively removed cross-reactive and heterophile antibodies, which permitted the specific staining and localization of each antigen by the unlabeled peroxidase-antiperoxidase method . CRBP was found to be localized in two cell types in the liver, the parenchymal cells and the fat-storing cells . Diffuse cytoplasmic staining for CRBP was seen in all the parenchymal cells . Much more intense staining for CRBP was seen in the fat-storing cells . The prominence of the CRBP-positive fat-storing cells changed markedly with vitamin A status . Thus, these cells were most prominent, and appeared most numerous, in liver from rats fed excess retinol . Both RBP and TTR were localized within liver parenchymal cells . The intensity of RBP staining increased markedly in retinol-deficient rat liver, consistent with previous biochemical observations . With the methods employed, specific staining for RBP or TTR was not seen in cells other than the parenchymal cells . In the kidney, all three proteins (CRBP, RBP, and TTR) were localized in the proximal convoluted tubules of the renal cortex . Staining for RBP was much more intense in normal kidney than in kidney from retinol-deficient rats. These findings reflect the fact that RBP in the tubules represents filtered and reabsorbed RBP . The pattern of specific staining for CRBP among the various tubules was very similar to that seen for RBP on adjacent, serial sections of kidney . The function of CRBP in the kidney is not known .It is now well established that specific binding proteins for retinol exist in plasma and in the intracellular compartment in a number of tissues. These proteins play important roles in the metabolism of vitamin A. Thus, retinol-binding protein (RBP)`, the specific plasma transport protein for vitamin A, transports retinol from its storage site in the liver to peripheral target tissues in the body. A different protein, cellular retinolbinding protein (CRBP) is found within cells in many tissues.Since the initial isolation of human RBP in 1968 (1), 'Abbreviations used in this paper are : CRBP, cellular retinol-binding protein ; RBP, retinol-binding protein ; TTR, transthyretin . 1696extensive studies in many laboratories have provided considerable information about the structure, metabolism, and biological roles of RBP (see references 2-4 for recent reviews) . RBP, with a molecular weight close to 21,000 and one binding site for retinol, is synthesized in (5) and secreted by the liver . RBP in plasma stro...

The immunohistochemical localization of cellular retinol-binding protein (CRBP) was studied in rat testis and epididymis. Parallel studies were also carried out on the localization of plasma retinol-binding protein (RBP) and transthyretin (TTR) in testis. The studies employed antibodies purified by immunosorbent affinity chromatography, permitting the specific staining and localization of each antigen by the unlabeled peroxidase-antiperoxidase method. For RBP and TTR, specific immune staining was found in the interstitial spaces between the seminiferous tubules, and not in the tubules themselves. In contrast, strong specific immune staining for CRBP was found in the seminiferous tubules, with a striking localization within Sertoli cells. Moreover, a distinct cyclic variation of specific staining for CRBP within Sertoli cells was observed during the spermatogenic cycle. This cyclic variation was seen with regard to both the intensity of staining and to the anatomic distribution of CRBP within the Sertoli cells. Within the epididymis CRBP was selectively localized to the proximal portion of the caput epididymidis, with variations in intensity of the staining of the epithelium of the ducts in different histological zones. Specific immune staining for CRBP was very weak or absent in the other portions of the epididymis. These results were confirmed by radioimmunoassay. Vitamin A-deficient rats showed markedly reduced specific immune staining for CRBP in both testes and epididymides, and greatly reduced levels of CRBP in these tissues on radioimmunoassay. These studies on the localization of CRBP provide information concerning the specific cells and anatomic loci within the testis and epididymis where retinol may be playing an important role in sperm formation and maturation.

Three polyadenylated RNAs, 9, 7.3, and 3.5 kilobases long, of a human endogenous retrovirus, ERV3, are abundant in human placental chorion, representing about 0.03 to 0.05% of the total mRNA. We characterized the structure of these mRNAs by Northern blot and S1 nuclease mapping analyses. We found that all three RNAs were spliced mRNAs that lacked 5.9 kilobases of proviral sequence, including the gag gene and most of the pol gene. In contrast to the transcription pattern usual for other retroviruses, the transcription pattern of the ERV3 provirus did not include a genome-length mRNA. All three of the ERV3 mRNAs initiated transcription at the same point in the 5' long terminal repeat (LTR) and contained identical splice junctions in the provirus. The 3.5-kilobase RNA was a typical subgenomic proviral mRNA, with its polyadenylation site in the 3' LTR. The two larger ERV3 mRNAs, however, extended through the polyadenylation site in the 3' LTR and were spliced at a second position approximately 370 nucleotides downstream from the 3' LTR. This finding suggests that when the ERV3 retrovirus integrated at this genomic locus in an ancestor of humans, it integrated within or adjacent to a cellular gene.

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