Nóbrega, R.H., de Souza Morais, R.D., Crespo, D., de Waal, P.P., de França, L.R., Schulz, R.W., Bogerd, J. (2015) Fsh stimulates spermatogonial proliferation and differentiation in zebrafish via Igf3. Endocrinology. 156(10):3804-17.
Growth factors modulate germ line stem cell self-renewal and differentiation behavior. We investigate the effects of insulin-like growth factor 3 (Igf3), a fish-specific member of the igf family. Follicle-stimulating hormone (Fsh) increased in a steroid-independent manner the number and mitotic index of single type A undifferentiated (Aund) spermatogonia and of clones of type A differentiating (Adiff) spermatogonia in adult zebrafish testis. All four igf gene family members in zebrafish are expressed in the testis but in tissue culture only igf3 transcript levels increased in response to recombinant zebrafish Fsh. This occurred in a 3',5'-cyclic AMP (cAMP)/protein kinase A (PKA)-dependent manner, in line with the results of studies on the igf3 gene promoter. Igf3 protein was detected in Sertoli cells. Recombinant zebrafish Igf3 increased the mitotic index of type Aund and type Adiff spermatogonia and up-regulated the expression of genes related to spermatogonial differentiation and entry into meiosis, but Igf3 did not modulate testicular androgen release. An Igf receptor inhibitor blocked these effects of Igf3. Importantly, the Igf receptor inhibitor also blocked Fsh-induced spermatogonial proliferation. We conclude that Fsh stimulated Sertoli cell production of Igf3, which promoted via Igf receptor signaling spermatogonial proliferation and differentiation and their entry into meiosis. Since previous work showed that Fsh also released spermatogonia from an inhibitory signal by down-regulating anti-Müllerian hormone (Amh) and by stimulating androgen production, we can now present a model, in which Fsh orchestrates the activity of stimulatory (Igf3, androgens) and inhibitory (Amh) signals to promote spermatogenesis.