Identification of a Nuclear Localization Signal in Activin/Inhibin βA Subunit; Intranuclear βA in Rat Spermatogenic Cells

Activin is a dimeric glycoprotein hormone that was initially characterized by its ability to stimulate pituitary FSH secretion and was subsequently recognized as a growth factor with diverse biological functions in a large variety of tissues. In the testis, activin has been implicated in the auto/paracrine regulation of spermatogenesis through its cognate cell membrane receptors on Sertoli and germ cells. In this study we provide evidence for intranuclear activin/inhibin β A subunit and show its distribution in the rat seminiferous epithelium. We have shown by transient expression in HeLa cells of β-galactosidase fusion proteins that the β A subunit precursor contains a functional nuclear localization signal within the lysine-rich sequence corresponding to amino acids 231-244. In all stages of the rat seminiferous epithelial cycle, an intense immunohistochemical staining of nuclear β A was demonstrated in intermediate or type B spermatogonia or primary spermatocytes in their initial stages of the first meiotic prophase, as well as in pachytene spermatocytes and elongating spermatids primarily in stages IX-XII. In some pachytene spermatocytes, the pattern of β A immunoreactivity was consistent with the characteristic distribution of pachytene chromosomes. In the nuclei of round spermatids, β A immunoreactivity was less intense, and in late spermatids it was localized in the residual cytoplasm, suggesting disposal of β A before spermatozoal maturation. Immunoblot analysis of a protein extract from isolated testicular nuclei revealed a nuclear β A species with a molecular mass of approximately 24 kDa, which is more than 1.5 times that of the mature activin β A subunit present in activin dimers. These results suggest that activin/inhibin β A may elicit its biological functions through two parallel signal transduction pathways, one involving the dimeric molecule and cell surface receptors and the other an alternately processed β A sequence acting directly within the nucleus. According to our immunohistochemical data, β A may play a significant role in the regulation of nuclear functions during meiosis and spermiogenesis.