Sycp1 Is Not Required for Subtelomeric DNA Double-Strand Breaks but Is Required for Homologous Alignment in Zebrafish Spermatocytes

In meiotic prophase I, homologous chromosomes are bound together by the synaptonemal complex, in which two axial elements are connected by transverse filaments and central element proteins. In human and zebrafish spermatocytes, homologous recombination and assembly of the synaptonemal complex initiate predominantly near telomeres. In mice, synapsis is not required for meiotic double-strand breaks (DSBs) and homolog alignment but is required for DSB repair; however, the interplay of these meiotic events in the context of peritelomeric bias remains unclear. In this study, we identified a premature stop mutation in the zebrafish gene encoding the transverse filament protein Sycp1. In sycp1 mutant zebrafish spermatocytes, axial elements were formed and paired at chromosome ends between homologs during early to mid-zygonema. However, they did not synapse, and their associations were mostly lost in late zygotene- or pachytene-like stages. In sycp1 mutant spermatocytes, γH2AX signals were observed, and Dmc1/Rad51 and RPA signals appeared predominantly near telomeres, resembling wild-type phenotypes. We observed persistent localization of Hormad1 along the axis in sycp1 mutant spermatocytes, while the majority of Iho1 signals appeared and disappeared with kinetics similar to those in wild-type spermatocytes. Notably, persistent Iho1 foci were observed in spo11 mutant spermatocytes, suggesting that Iho1 dissociation from axes occurs in a DSB-dependent manner. Our results demonstrated that Sycp1 is not required for peritelomeric DSB formation but is necessary for complete pairing of homologs in zebrafish meiosis.

LncRNA-Gm2044 is transcriptionally activated by A-MYB and regulates Sycp1 expression as a miR-335-3p sponge in mouse spermatocyte-derived GC-2spd(ts) cells

Long noncoding RNAs (lncRNAs) have been shown to execute key roles in spermatogenesis. However, little is known about how lncRNAs gene expression is itself regulated in the germ cells of testis. We previously demonstrated that high expression of lncRNA-Gm2044 exists in spermatocytes and can regulate male germ cell proliferation. Here, the transcriptional regulation of lnRNA-Gm2044 expression in spermatocytes and the downstream signaling were further explored. A bioinformatics assessment predicted two potential binding-sites for the spermatocyte-specific transcription factor A-MYB in the promoter region of lncRNA-Gm2044. Our results proved that the transcription factor A-MYB promotes the expression of lncRNA-Gm2044 in mouse spermatocyte-derived GC-2spd(ts) cells. ChIP and luciferase assays verified that A-MYB mainly binds to the distal promoter region (-819 bp relative to the transcription start site) of lncRNA-Gm2044 and regulates lncRNA-Gm2044 expression through the -819 bp binding-site. In addition, we confirmed that lncRNA-Gm2044 functions as a miR-335-3p sponge to enhance the levels of miR-335-3p's direct target protein, Sycp1. Furthermore, A-MYB can up-regulate Sycp1 expression and down-regulate GC-2spd(ts) cell proliferation by activating its target, lncRNA-Gm2044. Overexpression of lncRNA-Gm2044 or knockdown of miR-335-3p can, at least partially, rescue the effects of A-MYB on Sycp1 expression and GC-2spd(ts) cell proliferation.Taken together, our results provide new information on the mechanistic roles of lncRNA-miRNA in transcription factor A-MYB regulation of spermatocyte function.

Sohlh1 is required for synaptonemal complex formation by transcriptionally regulating meiotic genes during spermatogenesis in mice

Gonad-specific transcription factor spermatogenesis- and oogenesis-specific helix-loop-helix transcription factor 1 (SOHLH1) plays a key role in the transcriptional regulation of the expression of differentiating spermatogonial genes. However, its role in spermatocytes (meiotic male germ cells) remains largely unknown. In this study, Sohlh1 knockout (KO) male mice displayed meiotic defects at the zygotene stage during spermatogenesis. Microarray analyses identified 66 upregulated genes and 139 downregulated genes in Sohlh1 KO testes compared with those in wild-type testes at postnatal Day 7.5. Among many of the downregulated genes, Sycp1 and Sycp3, which encode synaptonemal complex proteins 1 and 3 (SYCP1 and SYCP3), respectively, were significantly reduced in Sohlh1 knockout mice. Transmission electron microscopy revealed no formation of the synaptonemal complex in Sohlh1 KO spermatocytes. Luciferase reporter and chromatin-immunoprecipitation assays demonstrated that SOHLH1 enhanced the expression of the Sycp1 and Sycp3 genes by binding the -1276, -708, and -94 basepairs (bp) E-boxes upstream of the Sycp1 promoter and the -64 and -43 bp E-boxes upstream of the Sycp3 promoter. Our data suggest that SOHLH1 transcriptionally regulates the expression of many target genes critical for the meiotic phase of spermatogenesis.

A promoter fragment of the sycp1 gene is sufficient to drive transgene expression in male and female meiotic germ cells in zebrafish

The synaptonemal complex protein 1 (Sycp1) is required for the formation of crossovers that occurs during the meiotic prophase. The tissue and cell-specific expression pattern of the Sycp1 protein have been studied in mammals and fish, but data on the corresponding transcript remain scarce. In this report, we described for the first time in zebrafish the tissue- and cell-specific expression pattern of the sycp1 gene. In ovary, the expression of the sycp1 transcript was restricted to the early primary oocytes. In testis, the sycp1 transcript was observed in primary spermatocytes in agreement with a previous report describing the localization of the Sycp1 protein in those cells. Unexpectedly, sycp1 transcript expression remained high in spermatids. To gain insight on the genomic region responsible for the sycp1 gene expression pattern, we generated four independent Dr_sycp1:eGFP transgenic zebrafish lines carrying the -1482/+338 gene fragment fused to the enhanced green fluorescent protein reporter gene. We demonstrate that this promoter fragment contains the information required for the cell-specific expression of the endogenous sycp1 gene in males and in females. However, the GFP protein and its associated fluorescence persist in spermatozoa and maturing oocytes. The Dr_sycp1:eGFP zebrafish lines have the potential to be valuable models to trace meiosis onset in zebrafish and constitute the first transgenic lines expressing the GFP reporter protein only in the male meiotic and postmeiotic cells in fish.