MR 06. Amnionless in Mouse Testis

In testis, vitamin B₁₂ (cobalmin)-deficiency resulted in atrophy of seminiferous tubules and aplasia of sperms and spermatids. Vitamin B₁₂ is involved in DNA synthesis as it participates in methionine synthase followed by thymidine synthesis. Amnionless (Amn), a transmembrane protein directs endocytosis of cubilin with its ligand, contributing intrinsic factor-vitamin B₁₂ absorption. Amn is considerably expressed in kidney, small intestine, placenta, and testis in mouse. In an effort to understand vitamin B₁₂ transport to the testicular cells, we examined the Amn gene expression in the developing mouse testis and Leydig cells. Meanwhile, change in Amn expression in Leydig cell hyperplasia by flutamide, an antiandrogen was examined in testis. In mouse testis, Amn mRNA levels were low until 14 days post partum (pp), markedly increased at puberty onwards. In the interstitium, Amn mRNA levels were low until 14 days pp, markedly increased together with 3β-hydroxysteroid dehydrogenase type VI mRNA at puberty (28 days pp), and further increased at 56 days pp. AMN immunoreactivity was found weakly in Sertoli cells throughout the testis development. Early spermatocytes showed strong AMN immunoreactivity at 14 days pp onwards, suggesting that AMN participates in the early meiosis. In Leydig cells, AMN immunoreactivity was not found until 14 days pp but strongly expressed at 28 days pp onwards. Quantitative image analysis revealed that AMN immunoreactivity in Leydig cells was increased by 2.8 fold between 28 and 56 days pp. Mean size of AMN-positive Leydig cells was increased by 1.8 fold between 14 and 28 days pp and 1.5 fold between 28 and 56 days pp. This suggests positive relationship between AMN expression and functional differentiation of adult Leydig cells. Oral administration of flutamide (100 mg/kg/day) for 10 days induced a Leydig cell hyperplasia in which Amn mRNA and AMN immunoreactivity were increased, suggesting that AMN may participate in the pathogenesis of Leydig cell hyperplasia. In adult Leydig cells primary culture, flutamide increased Amn mRNA expression, suggesting that Amn gene expression is negatively coupled with androgen receptor activation. In conclusion, AMN may participate in the meiosis of early spermatocytes and functional differentiation of adult Leydig cells via mediating the vitamin B₁₂ transport in mouse testis. In addition, AMN may participate in the Leydig cell hyperplasia via promoting cell proliferation in Leydig cell via stimulating DNA synthesis. This is the first report on the AMN expression in male germ cells and somatic cells in mammalian testis.