Mammalian spermatogenesis contributes a constant production of large numbers of spermatozoa, which is achieved by a cyclically regulated program known as the seminiferous epithelial cycle. Sertoli ...cells, functionally unique somatic cells, create a microenvironment to support the continuous differentiation of germ cells especially through the formation of a blood-testis barrier (BTB). The BTB is essential for maintaining homeostasis in seminiferous tubules and opens transiently at stages VII-VIII to ensure constant differentiation of spermatogenic cells. However, it is poorly understood how the dynamic organization of BTB is regulated. In our current study, we find that the overexpression of a dominant-negative form of RARα (dnRARα) in Sertoli cells disrupts the BTB at stages VII-XII and causes the large-scale apoptosis of differentiating germ cells. These abnormal events are found to be associated with cyclical gene expression changes in Sertoli cells, which can be represented by abnormal activation and repression of genes showing peaks of expression during stages I-VI and VII-XII, respectively. We find that one such gene, Ocln, encoding a tight junction component, partly contributes to the BTB disruption caused by dnRARα. Taken together, our data suggest that the cyclical activation of RA signaling in Sertoli cells during stages VII-XII contributes to a periodic organization of the BTB through changes in stage-dependent gene expression.
Coordination of stem cell fate is regulated by extrinsic niche signals and stem cell intrinsic factors. In mammalian testes, spermatogonial stem cells maintain constant production of abundant ...spermatozoa by alternating between self‐renewal and differentiation at regular intervals according to a periodical program known as the seminiferous epithelial cycle. Although retinoic acid (RA) signaling has been suggested to direct the cyclical differentiation of spermatogonial stem cells, it remains largely unclear how their cycle‐dependent self‐renewal/proliferation is regulated. Here, we show that MEK/ERK signaling contributes to the cyclical activity of spermatogonial stem cells. We found that ERK1/2 is periodically activated in Sertoli cells during the stem cell self‐renewal/proliferation phase, and that MEK/ERK signaling is required for the stage‐related expression of the critical niche factor GDNF. In addition, ERK1/2 is activated in GFRα1‐positive spermatogonial stem cells under the control of GDNF and prevent them from being differentiated. These results suggest that MEK/ERK signaling directly and indirectly maintains spermatogonial stem cells by mediating a signal that promotes their periodical self‐renewal/proliferation. Conversely, RA signaling directly and indirectly induces differentiation of spermatogonial stem cells. We propose that temporally regulated activations of RA signaling and a signal regulating MEK/ERK antagonistically coordinates the cycle‐related activity of spermatogonial stem cells. Stem Cells 2013;31:2517–2527
In many adult tissues, homeostasis relies on self-renewing stem cells that are primed for differentiation. The reconciliation mechanisms of these characteristics remain a fundamental question in stem ...cell biology. We propose that regulation at the post-transcriptional level is essential for homeostasis in murine spermatogonial stem cells (SSCs). Here, we show that Nanos2, an evolutionarily conserved RNA-binding protein, works with other cellular messenger ribonucleoprotein (mRNP) components to ensure the primitive status of SSCs through a dual mechanism that involves (1) direct recruitment and translational repression of genes that promote spermatogonial differentiation and (2) repression of the target of rapamycin complex 1 (mTORC1), a well-known negative pathway for SSC self-renewal, by sequestration of the core factor mTOR in mRNPs. This mechanism links mRNA turnover to mTORC1 signaling through Nanos2-containing mRNPs and establishes a post-transcriptional buffering system to facilitate SSC homeostasis in the fluctuating environment within the seminiferous tubule.
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•Cytoplasmic mRNP formation is critical for spermatogonial stem cell (SSC) maintenance•Nanos2 promotes condensation of cytoplasmic mRNPs•Nanos2/mRNPs repress mTORC1 activity by trapping mTOR in cytoplasmic mRNPs•Nanos2-resistant Sohlh2 is sufficient to trigger the differentiation of SSCs
Spermatogonial stem cells (SSCs) can self-renew, contributing to continuous spermatozoa production. Zhou et al. implicate Nanos2 in the repression of murine spermatogonia differentiation. Nanos2 mediates direct translational repression of differentiation-related transcripts and inhibits mTORC1 signaling by sequestrating mTOR protein in mRNPs, thereby providing a buffering system to facilitate SSC homeostasis.
Stem cells are maintained by both stem cell‐extrinsic niche signals and stem cell‐intrinsic factors. During murine spermatogenesis, glial cell line‐derived neurotrophic factor (GDNF) signal emanated ...from Sertoli cells and germ cell‐intrinsic factor NANOS2 represent key regulators for the maintenance of spermatogonial stem cells. However, it remains unclear how these factors intersect in stem cells to control their cellular state. Here, we show that GDNF signaling is essential to maintain NANOS2 expression, and overexpression of Nanos2 can alleviate the stem cell loss phenotype caused by the depletion of Gfra1, a receptor for GDNF. By using an inducible Cre‐loxP system, we show that NANOS2 expression is downregulated upon the conditional knockout (cKO) of Gfra1, while ectopic expression of Nanos2 in GFRA1‐negative spermatogonia does not induce de novo GFRA1 expression. Furthermore, overexpression of Nanos2 in the Gfra1‐cKO testes prevents precocious differentiation of the Gfra1‐knockout stem cells and partially rescues the stem cell loss phenotypes of Gfra1‐deficient mice, indicating that the stem cell differentiation can be suppressed by NANOS2 even in the absence of GDNF signaling. Taken together, we suggest that NANOS2 acts downstream of GDNF signaling to maintain undifferentiated state of spermatogonial stem cells. STEM CELLS 2012; 30:280–291.
Repressive H3K27me3 and active H3K4me2/3 together form bivalent chromatin domains, molecular hallmarks of developmental potential. In the male germline, these domains are thought to persist into ...sperm to establish totipotency in the next generation. However, it remains unknown how H3K27me3 is established on specific targets in the male germline. Here, we demonstrate that a germline-specific Polycomb protein, SCML2, binds to H3K4me2/3-rich hypomethylated promoters in undifferentiated spermatogonia to facilitate H3K27me3. Thus, SCML2 establishes bivalent domains in the male germline of mice. SCML2 regulates two major classes of bivalent domains: Class I domains are established on developmental regulator genes that are silent throughout spermatogenesis, while class II domains are established on somatic genes silenced during late spermatogenesis. We propose that SCML2-dependent H3K27me3 in the male germline prepares the expression of developmental regulator and somatic genes in embryonic development.
During spermatogenesis, a large number of germline genes essential for male fertility are coordinately activated. However, it remains unknown how timely activation of this group of germline genes is ...accomplished. Here we show that Polycomb-repressive complex 1 (PRC1) directs timely activation of germline genes during spermatogenesis. Inactivation of PRC1 in male germ cells results in the gradual loss of a stem cell population and severe differentiation defects, leading to male infertility. In the stem cell population, RNF2, the dominant catalytic subunit of PRC1, activates transcription of
, which codes for a transcription factor essential for subsequent spermatogenic differentiation. Furthermore, RNF2 and SALL4 together occupy transcription start sites of germline genes in the stem cell population. Once differentiation commences, these germline genes are activated to enable the progression of spermatogenesis. Our study identifies a novel mechanism by which Polycomb directs the developmental process by activating a group of lineage-specific genes.
Spermatogenesis involves the progressive reorganization of heterochromatin. However, the mechanisms that underlie the dynamic remodeling of heterochromatin remain unknown. Here, we identify SCML2, a ...germline-specific Polycomb protein, as a critical regulator of heterochromatin organization in spermatogenesis. We show that SCML2 accumulates on pericentromeric heterochromatin (PCH) in male germ cells, where it suppresses PRC1-mediated monoubiquitylation of histone H2A at Lysine 119 (H2AK119ub) and promotes deposition of PRC2-mediated H3K27me3 during meiosis. In postmeiotic spermatids, SCML2 is required for heterochromatin organization, and the loss of SCML2 leads to the formation of ectopic patches of facultative heterochromatin. Our data suggest that, in the absence of SCML2, the ectopic expression of somatic lamins drives this process. Furthermore, the centromere protein CENP-V is a specific marker of PCH in postmeiotic spermatids, and SCML2 is required for CENP-V localization on PCH. Given the essential functions of PRC1 and PRC2 for genome-wide gene expression in spermatogenesis, our data suggest that heterochromatin organization and spermatogenesis-specific gene expression are functionally linked. We propose that SCML2 coordinates the organization of heterochromatin and gene expression through the regulation of Polycomb complexes.
Gametogenesis is dependent on the expression of germline-specific genes. However, it remains unknown how the germline epigenome is distinctly established from that of somatic lineages. Here we show ...that genes commonly expressed in somatic lineages and spermatogenesis-progenitor cells undergo repression in a genome-wide manner in late stages of the male germline and identify underlying mechanisms. SCML2, a germline-specific subunit of a Polycomb repressive complex 1 (PRC1), establishes the unique epigenome of the male germline through two distinct antithetical mechanisms. SCML2 works with PRC1 and promotes RNF2-dependent ubiquitination of H2A, thereby marking somatic/progenitor genes on autosomes for repression. Paradoxically, SCML2 also prevents RNF2-dependent ubiquitination of H2A on sex chromosomes during meiosis, thereby enabling unique epigenetic programming of sex chromosomes for male reproduction. Our results reveal divergent mechanisms involving a shared regulator by which the male germline epigenome is distinguished from that of the soma and progenitor cells.
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•SCML2 regulates global silencing of somatic/progenitor genes in the male germline•SCML2 is a germline-specific subunit of Polycomb repressive complex 1•SCML2 establishes H2A ubiquitination to silence somatic/progenitor genes on autosomes•SCML2 prevents H2A ubiquitination on meiotic sex chromosomes
Hasegawa et al. show that SCML2, a germline-specific subunit of a Polycomb repressive complex 1, mediates genome-wide repression of genes commonly expressed in somatic lineages and spermatogenesis-progenitor cells in late stages of the male germline. SCML2 acts by differentially regulating histone H2A ubiquitination on autosomes versus meiotic sex chromosomes.
The male germline transcriptome changes dramatically during the mitosis-to-meiosis transition to activate late spermatogenesis genes and to transiently suppress genes commonly expressed in somatic ...lineages and spermatogenesis progenitor cells, termed somatic/progenitor genes.
These changes reflect epigenetic regulation. Induction of late spermatogenesis genes during spermatogenesis is facilitated by poised chromatin established in the stem cell phases of spermatogonia, whereas silencing of somatic/progenitor genes during meiosis and postmeiosis is associated with formation of bivalent domains which also allows the recovery of the somatic/progenitor program after fertilization. Importantly, during spermatogenesis mechanisms of epigenetic regulation on sex chromosomes are different from autosomes: X-linked somatic/progenitor genes are suppressed by meiotic sex chromosome inactivation without deposition of H3K27me3.
Our results suggest that bivalent H3K27me3 and H3K4me2/3 domains are not limited to developmental promoters (which maintain bivalent domains that are silent throughout the reproductive cycle), but also underlie reversible silencing of somatic/progenitor genes during the mitosis-to-meiosis transition in late spermatogenesis.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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