Transposons present an acute challenge to the germline, and mechanisms that repress their activity are essential for transgenerational genomic integrity. LINE1 (L1) is the most successful retrotransposon and is epigenetically repressed by CpG DNA methylation. Here, we identify two additional important mechanisms by which L1 is repressed during spermatogenesis. We demonstrate that the Piwi protein Mili and the piRNA pathway are required to posttranscriptionally silence L1 in meiotic pachytene cells even in the presence of normal L1 DNA methylation. Strikingly, in the absence of both a functional piRNA pathway and DNA methylation, L1 elements are normally repressed in mitotic stages of spermatogenesis. Accordingly, we find that the euchromatic repressive histone H3 dimethylated lysine 9 modification cosuppresses L1 expression therein. We demonstrate the existence of multiple epigenetic mechanisms that in conjunction with the piRNA pathway sequentially enforce L1 silencing and genomic stability during mitotic and meiotic stages of adult spermatogenesis.

SUMMARY Hematopoietic stem cells (HSCs) are maintained in a perivascular niche in bone marrow, in which leptin receptor+ (LepR) stromal cells and endothelial cells synthesize factors required for HSC maintenance, including stem cell factor (SCF). An important question is why LepR+ cells are one hundred times more frequent than HSCs. Here, we show that SCF from LepR+ cells is also necessary to maintain many c-kit+-restricted hematopoietic progenitors. Conditional deletion of Scf from LepR+ cells depleted common myeloid progenitors (CMPs), common lymphoid progenitors (CLPs), granulocyte-macrophage progenitors (GMPs), megakaryocyte-erythrocyte progenitors (MEPs), pre-megakaryocyte-erythrocyte progenitors (PreMegEs), and colony-forming units-erythroid (CFU-Es), as well as myeloid and erythroid blood cells. This was not caused by HSC depletion, as many other restricted progenitors were unaffected. Moreover, Scf deletion from endothelial cells depleted HSCs, but not progenitors. Early erythroid progenitors were closely associated with perisinusoidal LepR+ cells. This reveals cellular specialization within the niche: SCF from LepR+ cells is broadly required by HSCs and restricted progenitors while SCF from endothelial cells is required mainly by HSCs.

In mammals, hematopoietic stem cells (HSCs) engage in hematopoiesis throughout adult life within the bone marrow, where they produce the mature cells necessary to maintain blood cell counts and immune function. In the bone marrow and spleen, HSCs are sustained in perivascular niches (microenvironments) associated with sinusoidal blood vessels-specialized veins found only in hematopoietic tissues. Endothelial cells and perivascular leptin receptor + stromal cells produce the known factors required to maintain HSCs and many restricted progenitors in the bone marrow. Various other cells synthesize factors that maintain other restricted progenitors or modulate HSC or niche function. Recent studies identified new markers that resolve some of the heterogeneity among stromal cells and refine the localization of restricted progenitor niches. Other recent studies identified ways in which niches regulate HSC function and hematopoiesis beyond growth factors. We summarize the current understanding of hematopoietic niches, review recent progress, and identify important unresolved questions.

Male fertility requires the continuous production of high quality motile spermatozoa in abundance. Alterations in all three metrics cause oligoasthenoteratozoospermia, the leading cause of human sub/infertility. Post-mitotic spermatogenesis inclusive of several meiotic stages and spermiogenesis (terminal spermatozoa differentiation) are transcriptionally inert, indicating the potential importance for the post-transcriptional microRNA (miRNA) gene-silencing pathway therein. We found the expression of miRNA generating enzyme Dicer within spermatogenesis peaks in meiosis with critical functions in spermatogenesis. In an expression screen we identified two miRNA loci of the miR-34 family (miR-34b/c and miR-449) that are specifically and highly expressed in post-mitotic male germ cells. A reduction in several miRNAs inclusive of miR-34b/c in spermatozoa has been causally associated with reduced fertility in humans. We found that deletion of both miR34b/c and miR-449 loci resulted in oligoasthenoteratozoospermia in mice. MiR-34bc/449-deficiency impairs both meiosis and the final stages of spermatozoa maturation. Analysis of miR-34bc−/−;449−/− pachytene spermatocytes revealed a small cohort of genes deregulated that were highly enriched for miR-34 family target genes. Our results identify the miR-34 family as the first functionally important miRNAs for spermatogenesis whose deregulation is causal to oligoasthenoteratozoospermia and infertility.

Carrieri et al. identify a novel population of Miwi2-expressing undifferentiated spermatogonia. This population behave as transit-amplifying cells during homeostasis, but retain facultative stem cell activity and is critical for regenerative spermatogenesis.

BackgroundRepression of retrotransposons is essential for genome integrity and the development of germ cells. Among retrotransposons, the establishment of CpG DNA methylation and epigenetic silencing of LINE1 (L1) elements and the intracisternal A particle (IAP) endogenous retrovirus (ERV) is dependent upon the piRNA pathway during embryonic germ cell reprogramming. Furthermore, the Piwi protein Mili, guided by piRNAs, cleaves expressed L1 transcripts to post-transcriptionally enforce L1 silencing in meiotic cells. The loss of both DNA methylation and the Mili piRNA pathway does not affect L1 silencing in the mitotic spermatogonia where histone H3 lysine 9 dimethylation (H3K9me2) is postulated to co-repress these elements.ResultsHere we show that the histone H3 lysine 9 dimethyltransferase G9a co-suppresses L1 elements in spermatogonia. In the absence of both a functional piRNA pathway and L1 DNA methylation, G9a is both essential and sufficient to silence L1 elements. In contrast, H3K9me2 alone is insufficient to maintain IAP silencing in spermatogonia. The loss of all three repressive mechanisms has a major impact on spermatogonial populations inclusive of spermatogonial stem cells, with the loss of all germ cells observed in a high portion of seminiferous tubules.ConclusionsOur study identifies G9a-mediated H3K9me2 as a novel and important L1 repressive mechanism in the germ line. We also demonstrate fundamental differences in the requirements for the maintenance of L1 and IAP silencing during adult spermatogenesis, where H3K9me2 is sufficient to maintain L1 but not IAP silencing. Finally, we demonstrate that repression of retrotransposon activation in spermatogonia is important for the survival of this population and testicular homeostasis.

Several developmental stages of spermatogenesis are transcriptionally quiescent which presents major challenges associated with the regulation of gene expression. Here we identify that the zygotene to pachytene transition is not only associated with the resumption of transcription but also a wave of programmed mRNA degradation that is essential for meiotic progression. We explored whether terminal uridydyl transferase 4- (TUT4-) or TUT7-mediated 3′ mRNA uridylation contributes to this wave of mRNA degradation during pachynema. Indeed, both TUT4 and TUT7 are expressed throughout most of spermatogenesis, however, loss of either TUT4 or TUT7 does not have any major impact upon spermatogenesis. Combined TUT4 and TUT7 (TUT4/7) deficiency results in embryonic growth defects, while conditional gene targeting revealed an essential role for TUT4/7 in pachytene progression. Loss of TUT4/7 results in the reduction of miRNA, piRNA and mRNA 3′ uridylation. Although this reduction does not greatly alter miRNA or piRNA expression, TUT4/7-mediated uridylation is required for the clearance of many zygotene-expressed transcripts in pachytene cells. We find that TUT4/7-regulated transcripts in pachytene spermatocytes are characterized by having long 3′ UTRs with length-adjusted enrichment for AU-rich elements. We also observed these features in TUT4/7-regulated maternal transcripts whose dosage was recently shown to be essential for sculpting a functional maternal transcriptome and meiosis. Therefore, mRNA 3′ uridylation is a critical determinant of both male and female germline transcriptomes. In conclusion, we have identified a novel requirement for 3′ uridylation-programmed zygotene mRNA clearance in pachytene spermatocytes that is essential for male meiotic progression.

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