SUMMARYEpigenetic modifications influence gene expression and chromatin remodeling. In embryonic pluripotent stem cells, these epigenetic modifications have been extensively characterized; by contrast, the epigenetic events of tissue-specific stem cells are poorly understood. Here, we define a new epigenetic shift that is crucial for differentiation of murine spermatogonia toward meiosis. We have exploited a property of incomplete cytokinesis, which causes male germ cells to form aligned chains of characteristic lengths, as they divide and differentiate. These chains revealed the stage of spermatogenesis, so the epigenetic differences of various stages could be characterized. Single, paired and medium chain-length spermatogonia not expressing Kit (a marker of differentiating spermatogonia) showed no expression of Dnmt3a2 and Dnmt3b (two de novo DNA methyltransferases); they also lacked the transcriptionally repressive histone modification H3K9me2. By contrast, spermatogonia consisting of ~8-16 chained cells with Kit expression dramatically upregulated Dnmt3a2/3b expression and also displayed increased H3K9me2 modification. To explore the function of these epigenetic changes in spermatogonia in vivo, the DNA methylation machinery was destabilized by ectopic Dnmt3b expression or Np95 ablation. Forced Dnmt3b expression induced expression of Kit; whereas ablation of Np95, which is essential for maintaining DNA methylation, interfered with differentiation and viability only after spermatogonia become Kit positive. These data suggest that the epigenetic status of spermatogonia shifts dramatically during the Kit-negative to Kit-positive transition. This shift might serve as a switch that determines whether spermatogonia self-renew or differentiate.

Heterochromatin-related epigenetic mechanisms, such as DNA methylation, facilitate pairing of homologous chromosomes during the meiotic prophase of mammalian spermatogenesis. In pro-spermatogonia, de novo DNA methylation plays a key role in completing meiotic prophase and initiating meiotic division. However, the role of maintenance DNA methylation in the regulation of meiosis, especially in the adult, is not well understood. Here, we reveal that NP95 (also known as UHRF1) and DNMT1 – two essential proteins for maintenance DNA methylation – are co-expressed in spermatogonia and are necessary for meiosis in male germ cells. We find that Np95- or Dnmt1-deficient spermatocytes exhibit spermatogenic defects characterized by synaptic failure during meiotic prophase. In addition, assembly of pericentric heterochromatin clusters in early meiotic prophase, a phenomenon that is required for subsequent pairing of homologous chromosomes, is disrupted in both mutants. Based on these observations, we propose that DNA methylation, established in pre-meiotic spermatogonia, regulates synapsis of homologous chromosomes and, in turn, quality control of male germ cells. Maintenance DNA methylation, therefore, plays a role in ensuring faithful transmission of both genetic and epigenetic information to offspring.