Hormonal Control of Meiosis Initiation in the Testis from Japanese Newt, Cynops pyrrhogaster

Abstract

Meiosis is an event that occurs prerequisitely and specifically in gametogenesis. However, the mechanisms of conversion from mitosis to meiosis are poorly understood. I will review the results so far obtained by us using newt testis as a model system, and discuss about the extrinsic mechanism(s) controlling the conversion from mitosis to meiosis. In the newt spermatogonia enter meiosis in the 8th generation after 7 mitotic divisions. We developed organ and reaggregate culture systems with a chemically defined medium in which porcine follicle-stimulating hormone (pFSH) promotes spermatogonial proliferation and differentiation into primary spermatocytes. Human recombinant stem cell factor (RhSCF) in vitro stimulates the spermatogonial proliferation and progression to the 7th generation, but not the differentiation into primary spermatocytes; instead they die of apoptosis. The reason why rhSCF does not stimulate meiosis entrance seems to be due to the low level expression of c-kit protein at the 7th generation of spermatogonia. Ovine PRL induces apoptosis in the 7th generation of spermatogonia in vivo and in vitro. Incubation of newts at low temperature causes spermatogonial apoptosis by the elevation of plasma PRL titer. In the absence of FSH in organ culture spermatogonia can progress until the 7th generation, but the 8th generation never appear due to the apoptosis. Altogether there seems to be a regulatory checkpoint for entrance into meiosis in the 7th generation. Spermatogonia could circumvent the checkpoint by the influence of some factor(s) produced by Sertoli cells upon activation by FSH. Trial to isolate factor(s) responsible for the meiosis-initiation is now underway.

Reduced Expression of an RNA-binding Protein by Prolactin Leads to Translational Silencing of Programmed Cell Death Protein 4 and Apoptosis in Newt Spermatogonia

“…In the testis of the Japanese red-bellied newt, Cynops pyrrhogaster, primary spermatogonia proliferate through seven mitotic divisions, and then in the 8th generation the spermatogonia differentiate into primary spermatocytes and initiate meiosis in the spring when the ambient temperature is high; in contrast, in the autumn when the ambient temperature is low, the spermatogonia often undergo apoptosis in the 7th generation, resulting in the cessation of spermatogenesis (2). These findings suggest the existence of molecular mechanisms regulating the fate of 7th generation spermatogonia.…”

“…The germ cells are in close contact with somatic Sertoli cells in a spermatogenic cyst, the smallest unit of the testis. The testis consists of lobules in successive zones arranged along a cephalocaudal axis, in which spermatogenesis proceeds synchronously (2,6). Therefore, we can dissect zones containing particular spermatogenic stages and identify the generation of spermatogonia by counting their numbers in a cyst.…”

Promotion of spermatogonial proliferation by neuregulin 1 in newt (Cynops pyrrhogaster) testis

“…Effect of nNRG1 antibody on the spermatogonial proliferation induced by nNRG1 and FSH FSH induces newt spermatogonial proliferation and their differentiation through its receptor expressed on Sertoli cells (Abé , 2004). In order to examine whether FSH promotes spermatogonial proliferation by activating Sertoli cells to produce nNRG1, antibody against EGF-like domain of nNRG1 (antinNRG1-EGF) was added to the reaggregated culture of testicular cells.…”

“…In newt testis, we have demonstrated that FSH alone is sufficient to stimulate spermatogonial proliferation and differentiation into primary spermatocytes through the receptor expressed on Sertoli cells in both organ and reaggregate cultures (Abé and Ji, 1994;Abé , 2004), but not in the culture of spermatogonia alone (Ito and Abé , 1999). In addition, we reported that SCF and IGF-I, both of which are upregulated in response to FSH (unpublished data; Yamamoto et al, 2001), induce spermatogonial proliferation (Abé et al, 2002) and differentiation (Nakayama et al, 1999), respectively, in the organ culture.…”

“…In contrast, clonally derived germ cells are enclosed in a cyst, the smallest unit of the testis, and surrounded by Sertoli cells within the lobules in newt testis (Callard, 1991;Abé , 2004). Therefore, newt testis is an appropriate model to isolate germ cells at a specific stage and study the effect of various factors on specific stage of spermatogenesis.…”