Molecular mechanisms of activin B-induced spermatogenesis

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The purpose of this study is to understand the molecular mechanisms of gonadotropin-induced spermatogenesis in the Japanese eel (Anguilla japonica). With introduction of the expression vector containing activin betaB subunit cDNA into mammalian CHO cells, these cells secreted a 22kDa protein into the medium. Purified recombinant protein (eel activin B) had erythroid differentiation factor activity. Addition of eel activin B to the culture medium induced proliferation of spermatogonia, producing late type B spermatogonia, within 15 days in the same manner of 11-ketotestosterone. Taken together, it is concluded that activin B produced by Sertoli cells under the influence of gonadotropin and 11-ketotestosterone plays an important role in the regulation of spermatogenesis in fish testes. We have also cloned cDNAs encoding ell activin type I and II receptors from an eel testis cDNA library. Northern blot analysis reveals that mRNA transcripts of activin type I and II receptors are expressed in testes of eels prior to gonadotropin injection. In situ hybridization shows that these receptors are localized in germ cells (spermatogonia and spermatocytes) and Sertoli cells, providing evidence that activin B may act directly on the germ cells. Several cDNAs encoding G1/S cyclins (A1, A2, D1, D2, E1, E2) have been cloned from eel testes. None of these G1/S cyclin transcripts were present in testes of eels before hormonal induction of spermatogenesis. The following sequence of expression of these cyclins was observed in testes after gonadotropin injection. : E1 (G1/S) *A2, B1, B2*A1 (spermatocytes).