RESUMO
The double bromodomain and extra-terminal domain (BET) proteins are critical epigenetic readers that bind to acetylated histones in chromatin and regulate transcriptional activity and modulate changes in chromatin structure and organization. The testis-specific BET member, BRDT, is essential for the normal progression of spermatogenesis as mutations in the Brdt gene result in complete male sterility. Although BRDT is expressed in both spermatocytes and spermatids, loss of the first bromodomain of BRDT leads to severe defects in spermiogenesis without overtly compromising meiosis. In contrast, complete loss of BRDT blocks the progression of spermatocytes into the first meiotic division, resulting in a complete absence of post-meiotic cells. Although BRDT has been implicated in chromatin remodeling and mRNA processing during spermiogenesis, little is known about its role in meiotic processes. Here we report that BRDT is an essential regulator of chromatin organization and reprograming during prophase I of meiosis. Loss of BRDT function disrupts the epigenetic state of the meiotic sex chromosome inactivation in spermatocytes, affecting the synapsis and silencing of the X and Y chromosomes. We also found that BRDT controls the global chromatin organization and histone modifications of the chromatin attached to the synaptonemal complex. Furthermore, the homeostasis of crossover formation and localization during pachynema was altered, underlining a possible epigenetic mechanism by which crossovers are regulated and differentially established in mammalian male genomes. Our observations reveal novel findings about the function of BRDT in meiosis and provide insight into how epigenetic regulators modulate the progression of male mammalian meiosis and the formation of haploid gametes.
Assuntos
Cromatina/genética , Epigênese Genética/genética , Meiose/fisiologia , Proteínas Nucleares/genética , Cromossomos Sexuais/genética , Animais , Cromatina/metabolismo , Montagem e Desmontagem da Cromatina , Pareamento Cromossômico/genética , Troca Genética , Regulação da Expressão Gênica , Histonas/genética , Histonas/metabolismo , Infertilidade Masculina/genética , Masculino , Camundongos Knockout , Proteínas Nucleares/metabolismo , Espermatócitos/patologia , Espermatócitos/fisiologia , Testículo/citologia , Testículo/fisiologiaRESUMO
In contrast to most hematopoietic lineages, megakaryocytes (MKs) can derive rapidly and directly from hematopoietic stem cells (HSCs). The underlying mechanism is unclear, however. Here, we show that DNA damage induces MK markers in HSCs and that G2 arrest, an integral part of the DNA damage response, suffices for MK priming followed by irreversible MK differentiation in HSCs, but not in progenitors. We also show that replication stress causes DNA damage in HSCs and is at least in part due to uracil misincorporation in vitro and in vivo. Consistent with this notion, thymidine attenuated DNA damage, improved HSC maintenance, and reduced the generation of CD41+ MK-committed HSCs. Replication stress and concomitant MK differentiation is therefore one of the barriers to HSC maintenance. DNA damage-induced MK priming may allow rapid generation of a lineage essential to immediate organismal survival, while also removing damaged cells from the HSC pool.
Assuntos
Diferenciação Celular , Dano ao DNA , Células-Tronco Hematopoéticas , Megacariócitos , Células-Tronco Hematopoéticas/metabolismo , Células-Tronco Hematopoéticas/citologia , Animais , Camundongos , Megacariócitos/metabolismo , Megacariócitos/citologia , Trombopoese , Pontos de Checagem da Fase G2 do Ciclo Celular , Camundongos Endogâmicos C57BL , HumanosRESUMO
Hematopoietic stem cells (HSCs) reside in the bone marrow (BM), can self-renew, and generate all cells of the hematopoietic system. 1 Most hematopoietic lineages arise through successive, increasingly lineage-committed progenitors. In contrast, megakaryocytes (MKs), hyperploid cells that generate platelets essential to hemostasis, can derive rapidly and directly from HSCs. 2 The underlying mechanism is unknown however. Here we show that DNA damage and subsequent arrest in the G2 phase of the cell cycle rapidly induce MK commitment specifically in HSCs, but not in progenitors, through an initially predominantly post-transcriptional mechanism. Cycling HSCs show extensive replication-induced DNA damage associated with uracil misincorporation in vivo and in vitro . Consistent with this notion, thymidine attenuated DNA damage, rescued HSC maintenance and reduced the generation of CD41 + MK-committed HSCs in vitro . Similarly, overexpression of the dUTP-scavenging enzyme, dUTPase, enhanced in vitro maintenance of HSCs. We conclude that a DNA damage response drives direct megakaryopoiesis and that replication stress-induced direct megakaryopoiesis, at least in part caused by uracil misincorporation, is a barrier to HSC maintenance in vitro . DNA damage-induced direct megakaryopoiesis may allow rapid generation of a lineage essential to immediate organismal survival, while simultaneously removing damaged HSCs and potentially avoiding malignant transformation of self-renewing stem cells.