Overexpression of androgen-binding protein/sex hormone-binding globulin in male transgenic mice: tissue distribution and phenotypic disorders

The rat androgen-binding protein/sex hormone-binding globulin (ABP/SHBG) gene in transgenic mice was previously shown to be specifically expressed in the testes. This study verifies a Sertoli cell location of ABP and translation of testicular ABP mRNA in the transgenic mice by dihydrotestosterone (DHT)-binding assays and immunohistochemistry. DHT-binding activities in the testis and epididymis of the hemizygous transgenic mice were elevated 20-fold as compared to activity in the wild-type tissues. DHT-binding activities were also elevated in blood plasma at least 25- to 50-fold in the transgenic mice; binding was undetectable in the plasma from control mice. Immunohistochemical analysis revealed that the transgenic testicular ABP was primarily in the cytoplasm of Sertoli cells and lumen of the seminiferous tubules. In some tubules, intense staining also was associated with spermatids. After transport to the epididymis, there were large amounts of immunoreactive ABP internalized in the epithelium of the initial segment and proximal caput. The increased levels of plasma and testicular ABP had no effect on levels of testosterone; there was a 30-fold range of plasma and testicular testosterone levels in the wild-type and transgenic mice. Increased ABP levels in the transgenic mice were associated with structural and functional abnormalities in the testis. Abnormal spermatogenesis resulted in extensive structural changes in the transgenic testis; the degree of the defect varied from near normality to the loss of most germ cells. In the affected mice, seminiferous tubules had smaller diameters and decreased numbers of germ cells, particularly in the spermatid stages of differentiation. Pyknotic nuclei and multinucleated cells were associated with the spermatids in the defective tubules, but not in the wild-type tubules. Consequently, mice with the spermatogenic disorder had reduced epididymal sperm numbers. The variable spermatogenic disorder was associated with variable male fertility. The homozygous transgenic male and female mice also had a serious motor dysfunction affecting their hind limbs. This study demonstrates how the transgenic mouse model can be used to study ABP's function, and the data support several hypotheses on its function in the testis and epididymis.