Igf Binding Proteins Protect Undifferentiated Spermatogonia in the Zebrafish Testis Against Excessive Differentiation.

IGF binding proteins (IGFBPs) modulate the availability of IGFs for their cognate receptors. In zebrafish testes, IGF3 promotes the proliferation and differentiation of type A undifferentiated (A) spermatogonia, and igf3 expression is strongly elevated by FSH but also responds to T. Here we report the effects of FSH and T on igfbp transcript levels in adult zebrafish testis. We then examined T and FSH effects on zebrafish spermatogenesis and explored the relevance of IGFBPs in modulating these T or FSH effects, using a primary tissue culture system for adult zebrafish testis. T up-regulated igfbp1a and igfbp3 expression, whereas FSH reduced igfbp1a transcript levels. To quantify effects on spermatogenesis, we determined the mitotic index and relative section areas occupied by A, type A differentiating, or type B spermatogonia. In general, T and FSH stimulated spermatogonial proliferation and increased the areas occupied by spermatogonia, suggesting that both self-renewal and differentiating divisions were stimulated. Preventing IGF/IGFBP interaction by NBI-31772 further increased T- or FSH-induced spermatogonial proliferation. However, under these conditions the more differentiated type A differentiating and B spermatogonia occupied larger surface areas at the expense of the area held by A spermatogonia. Clearly decreased nanos2 transcript levels are in agreement with this finding, and reduced amh expression may have facilitated spermatogonial differentiation. We conclude that elevating IGF3 bioactivity by blocking IGFBPs shifted T- or FSH-induced signaling from stimulating spermatogonial self-renewal as well as differentiation toward predominantly stimulating spermatogonial differentiation, which leads to a depletion of type A spermatogonia.