Expression of cellular retinol-binding protein and lecithin-retinol acyltransferase in developing rat testis

Vitamin A deficiency in mammals results in the loss of germ cells on the adluminal side of the blood-testis barrier, suggesting a need for vitamin A that would be supplied by the surrounding Sertoli cells. Cellular retinol-binding protein (CRBP) and lecithin-retinol acyltransferase (LRAT) are two proteins found in Sertoli cells that are known to be involved in vitamin A trafficking. To clarify the role of these two proteins in the delivery of vitamin A to developing germ cells, we have examined changes in their cell-specific expression during the onset of puberty in the rat. In adult rats, Sertoli cell expression of CRBP varies with the cycle of the seminiferous epithelium. Here, we demonstrate that differences in the intensity of CRBP immunoreactivity are detectable in Sertoli cells of different tubules as early as postnatal Day 4, prior to the onset of meiosis. This indicates that variable expression of CRBP by Sertoli cells is established independently of late germ cells and may anticipate the cyclical variation seen in the adult. We further demonstrate that the specific activity of LRAT in rat testis increases tenfold between postnatal Days 20 and 35. This increase is attributable to the appearance of post-meiotic germ cells: the LRAT activity of microsomes prepared from a round spermatid-enriched cell fraction from post-pubertal rat testis could account for the majority of the LRAT activity observed in the whole testis. The presence of LRAT activity within adluminal germ cells suggests that they receive vitamin A as retinol and synthesize the retinyl esters that have been shown to be present in mature sperm.