Molecular and functional characterization of sex hormone binding globulin in zebrafish

SHBG (sex hormone binding globulin) transports androgens and estrogens in the blood of vertebrates including fish. Orthologs of SHBG in fish are poorly defined, and we have now obtained a zebrafish SHBG cDNA and characterized the zebrafish SHBG gene and protein through molecular biological, biochemical, and informatics approaches. Amino-terminal analysis of zebrafish SHBG indicated that its deduced precursor sequence includes a 25-residue secretion polypeptide and exhibits 22-27% homology with mammalian SHBG sequences and 41% with a deduced fugufish SHBG sequence. The 356-residue mature zebrafish SHBG (39,243 Da) sequence comprises a tandem repeat of laminin G-like domains typical of SHBG sequences; contains three N-glycosylation sites; and exists as a 105,000 +/- 8700 Da homodimer. Zebrafish SHBG exhibits a high affinity and specificity for sex steroids. An RT-PCR indicated that SHBG mRNA first appears in zebrafish larva, and SHBG mRNA was localized within the liver and gut at this stage of development by whole-mount in situ hybridization. In adult fish, SHBG mRNA was found in liver, testis, and gut. In the liver, immunoreactive SHBG was present in hepatocytes and concentrated in intrahepatic bile duct cells, whereas in the testis it was confined to cells surrounding the seminiferous tubule cysts. In the intestine, immunoreactive SHBG was present in the stroma and epithelial cells of the villous projections and the surrounding muscle. The production and presence of SHBG in the gut of developing and adult zebrafish suggests a novel role for this protein in regulating sex steroid action at this site.