Mechanism of meiosis initiation in spermatogonia

减数分裂起始的机制尚未阐明。我们发现蝾螈精巢精原细胞在第7代存在一个减数分裂起始的检查点<th>，并提出减数分裂起始受FSH和催乳素浓度比调节的假说。本研究从蝾螈睾丸组织cDNA微阵列中筛选到促卵泡激素（FSH）表达上调的neuregulin（NRG）基因，并将其克隆到支持细胞（Sertoli cells）中。结果表明，Ig型NRG在支持细胞中表达，而CRD型mRNA在支持细胞和生殖细胞中表达。FSH对Ig型mRNA的表达有明显的促进作用，但对CRD型mRNA的表达无明显影响。将NRG功能域EGF样结构域的重组蛋白加入到小鼠睾丸组织培养中，刺激精原细胞增殖，发现NRG mRNA在小鼠睾丸中有表达，随着发育的进行，NRG 1 mRNA在支持细胞中的表达水平逐渐降低，而NRG 3 mRNA在支持细胞和生殖细胞中的表达水平逐渐升高。将NRG 1和3重组蛋白加入到仅存在A型精原细胞的睾丸器官培养物中，导致促进精原细胞增殖和初级精母细胞特异性SPO11 mRNA的表达。FSH刺激NRGs mRNA的表达。这些结果表明，NRGs作为作用于精原细胞的重要因子，在减数分裂起始中起着关键作用。

The mechanism underlying meiosis initiation has yet to be clarified. We have shown that there is a checkpoint of meiosis initiation in the 7^<th> generation of spermatogonia in newt testis and presented a hypothesis that meiosis initiation is regulated by the concentration ratio of FSH and prolactin. It was thought that meiosis initiation requires Sertoli cells, upon stimulation by FSH, produce some factor(s) that circumvent apoptosis and act on spermatogonia.Hence we screened cDNA microarray derived from newt testis and identified neuregulin (NRG) as one of the FSH-upregulated clones. It was revealed that Ig-type NRG is expressed in Sertoli cells, while CRD-type mRNA is expressed in both Sertoli and germ cells. FSH stimulates the expression of Ig-type mRNA highly, but not that of CRD-type. When recombinant protein of the EGF-like domain which is a functional domain of NRG is added to organ culture of the testis, it stimulated the spermatogonial proliferation.Then we found that NRG mRNA is expressed in mouse testis; NRG1 mRNA level expressed in Sertoli cells decreases as development proceeds, while NRG 3 mRNA level expressed both in Sertoli and germ cells increases. Recombinant proteins of NRG1 and 3 were added to organ culture of testis from 6dpp when only A-type spermatogonia are present, resulting in promotion of spermatogonial proliferation and expression of SPO11 mRNA that is specific to primary spermatocytes. FSH stimulated the expression of NRGs mRNA. These results indicate that NRGs play a pivotal role in meiosis initiation as important factors to act on spermatogonia.