BIOLOGY OF REPRODUCTION 85, 219–220 (2011) DOI 10.1095/biolreprod.111.093971
World of Reproductive Biology Charlotte Schubert, Ph.D., Science Writer Silencing Sex Chromosomes in Spermatocytes
The findings show how a similar molecular pathway underlies both the response of somatic cells to DNA damage and the inactivation of chromosomes during male meiosis. The findings also broaden the role of cH2AX, which is also implicated in the repair of damaged sperm DNA after fertilization and in DNA demethylation in early embryos.
MDC1 directs chromosome-wide silencing of the sex chromosomes in male germ cells. Yosuke Ichijima, Misako Ichijima, Zhenkun Lou, Andre´ Nussenzweig, R. Daniel Camerini-Otero, Junjie Chen, Paul R. Andreassen, and Satoshi H. Namekawa. Genes & Dev 2011; 25:959–971. doi: 10.1101/gad.2030811
BOR–Papers in Press published 18 May 2011 DOI: 10.1095/biolreprod.111.093385
During meiosis in the male germline, the poorly-paired X and Y chromosomes do not synapse fully—instead they undergo chromosome-wide gene inactivation. A report in Genes and Development examines how factors involved in the DNA damage response mediate this silencing, dubbed meiotic sex chromosome inactivation (MSCI). DNA damage response factors have previously been implicated in MSCI in mammals. These factors include cH2AX (phosphorylated histone H2AX), which marks the entire X and Y chromosome during silencing. Another factor is ATR (ataxia telangiectasia and Rad3related), a kinase that senses single-stranded DNA at the site of DNA damage in somatic cells. ATR also marks the entire X and Y chromosomes during silencing and catalyzes the phosphorylation of H2AX. To flesh out the mechanism of sex chromosome inactivation, Yosuke Ichijima et al. took a close look at MDC1 (mediator of DNAdamage checkpoint-1), which is known to bind cH2AX, and an ATRcontaining complex during DNA repair. The researchers found that mice deficient in MDC1 had several defects in sex chromosome inactivation—specifically, cH2AX and ATR failed to completely mark the entire sex chromosome in spermatocytes and silencing was impaired (see accompanying figure). These and other experiments led the authors to propose a two-step model of MSCI. During the first step, ATR and its associated factors recognize unsynapsed areas of the X and Y chromosomes, and ATR phosphorylates H2AX. This first step occurs independently of MDC1. In the second step, MDC1 facilitates the spread of ATR, cH2AX, and other factors to the entire sex chromosome chromatin, perhaps through a feedback loop involving ATR.
Sperm or Egg: Clearing the FOG
Germ cells in many species face a big decision during development: whether to turn into an oocyte or sperm. Researchers have now uncovered how a key regulator helps make this decision in the C. elegans germline. C. elegans are found as males, which make only sperm, and as hermaphrodites. Hermaphrodites make sperm transiently, during larval development, before switching over to oocyte production. Specification of the sperm cell fate is known to involve FOG-3, the immediate target of the TRA-1 transcription factor; for instance, in FOG-3 mutant males and hermaphrodites, germ cells that normally differentiate as sperm are transformed into oocytes. Myon-Hee Lee and Kyung Won Kim et al. speculated that the phosphorylation status of FOG-3 affects how it operates, given clues such as consensus docking and phospho-acceptor sites for ERK. To test their hypothesis, the researchers created various FOG-3 transgenes mutated at the phosphorylation sites and assessed their ability to rescue spermatogenesis in fog-3 mutant worms. Their experiments suggest that unphosphorylated FOG-3 initiates spermatogenesis and phosphorylated FOG-3 maintains spermatogenesis. Still unanswered are how FOG-3 operates mechanistically and which factors directly regulate it (the data did not point to a clear role for a C. elegans ERK homolog). The findings are published in the Proceedings of the National Academy of Sciences. FOG-3 is a member of a family of proteins also found in vertebrates, the TOB family. Multiple members of this family are expressed in mammalian testes; their function in mammals presents an exciting question for future experimentation. BOR–Papers in Press published 25 May 2011 DOI: 10.1095/biolreprod.111.093500
A Scaffold for Germ Cell Development RanBPM is essential for mouse spermatogenesis and oogenesis. Sandrine Puverel, Colleen Barrick, Susanna Dolci, Vincenzo Coppola, Lino Tessarolla. Development 2011; 138:2511–2521. doi: 10.1242/dev.062505
Geneticists have uncovered a role for a scaffolding protein, RanBPM, in spermatogenesis and oogenesis in mice. RanBPM (Ran binding protein 9) links cell surface receptors with their intracellular signaling pathways. It is known to bind more than 45 different proteins, involved in a variety of functions. To date, the physiological function of RanBPM has been unclear.
MDC1 (red) accumulates throughout the chromatin of the X and Y chromosome during meiotic prophase (prophase chromosomes marked in green by SCP3; synaptonemal complex protein 3). Photo credit: Yosuke Ichijima.
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Phosphorylation state of a Tob/BTG protein, FOG-3, regulates initiation and maintenance of the Caenorhabditis elegans sperm fate program. Myon-Hee Lee, Kyung Won Kim, Clinton T. Morgan, Dyan E. Morgan, and Judith Kimble. Proc Natl Acad Sci U S A 2011; 108:9125–9130. doi: 10.1073/pnas.1106027108
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BOR–Papers in Press published 2 June 2011 DOI: 10.1095/biolreprod.111.093765
DMRT1: Not Just for Boys DMRT1 promotes oogenesis by transcriptional activation of Stra8 in the mammalian fetal ovary. Anthony D. Krentz, Mark W. Murphy, Aaron L. Sarver, Michael D. Griswold, Vivian J. Bardwell, David Zarkower. Dev Biol 2011; published online 20 May 2011; doi:10. 1016/j.ydbio.2011.05.658.
A protein with a well-established role in the differentiation of the testes, DMRT1, is now shown to operate in the fetal ovary in mice. DMRT1 (doublesex and mab-3 related transcription factor 1) belongs to a family of conserved sexual regulators, having in common
a zinc-finger like DNA-binding motif (DM domain). In the mouse, DMRT1 is expressed in the gonadal primordium in both sexes, becoming restricted to the testes in early embryogenesis, shortly after sex determination. In the testes, DMRT1 performs several functions including keeping meiosis in check during spermatogonial proliferation and differentiation. DMRT1 performs this function by repressing the meiotic initiator STRA8 (stimulated by retinoic acid gene 8). STRA8 also operates in females, but it does so much earlier— STRA8 prompts female germ cells to enter meiosis during early embryonic development. The cells remain arrested in the diplotene stage of prophase I until after puberty. Anthony Krentz et al. now report in Developmental Biology that, in contrast to its repressive function in males, DMRT1 helps to activate STRA8 in female germ cells. To show this, the researchers took a close look at mice deficient in Dmrt1. Previous analyses had found that such mice are fertile, but Krentz et al. found defects in ovarian development. In particular, the mice had drastically reduced expression of STRA8, and defects in meiotic prophase—for instance, proteins that normally cling to chromosomes during meiotic prophase were abnormally localized. What’s more, the mice had reduced numbers of ovarian follicles— possibly due to culling of germ cells damaged by poor meiotic regulation, speculate the researchers. The researchers went on to provide evidence that DMRT1 directly activates STRA8. The researchers speculate that DMRT1 may operate in concert with retinoic acid, another factor known to promote STRA8 expression; consistent with this idea, the researchers found that DMRT1 binds the Stra8 promoter near a pair of Retinoic Acid Response Elements. Expression of DAZL (deleted in azoospermia-like), another factor that prompts female STRA8 expression, was unaffected in mice deficient in DMRT1. Future experiments should address how DMRT1 may function with DAZL and retinoic acid, as well as the mechanism behind the different effects of DMRT1 in male and female germ cells. The findings hint that DMRT1 may have a role in female infertility or premature ovarian failure in people.
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BOR–Papers in Press published 8 June 2011 DOI: 10.1095/biolreprod.111.093930
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Sandrine Puverel et al. created mice deficient in RanBPM (official symbol RANBP9). In Development, they report that both male and female mice lacking the gene were sterile. Further analysis suggested that oocytes and spermatocytes reached the pachytene stage of meiosis I normally, but that differentiation was arrested at the end of prophase I. In females this timing coincided with the normal dictyate arrest, but in the RanBPM null mutants, arrest was followed by apoptotic demise. The result was premature ovarian failure. In females, the few oocytes that managed to develop in the mutant mice appeared normal, suggesting that RanBPM is not essential for functions later in germ cell differentiation. In contrast, the protein seems to be required for maintaining spermatogonial stem cells in males. RanBPM was expressed in Leydig and Sertoli cells, but further experiments with chimeric mice generated with RanBPM-deficient cells suggested that the protein acts autonomously in germ cells, where it is also expressed. Only a few other molecules are known to affect germ cell proliferation in the testis, primarily the tyrosine kinase receptor Kit. It is possible that RanBPM interacts with KIT, given that KIT is expressed in germ cells, and that the scaffolding protein is known to partner with other tyrosine kinase receptors. This study points to RanBPM as an important regulator of germ cell meiosis in both males and females.
