Department of Genetics and Development, Columbia University Medical Center, New York, New York 10032, USA.
Dev Biol. 2011 Dec 15;360(2):358-68. doi: 10.1016/j.ydbio.2011.10.005. Epub 2011 Oct 12.
Mice homozygous for a mutation (Brdt(∆BD1/∆BD1)) lacking the first bromodomain of Brdt, a testis-specific member of the BET family of double-bromodomain containing proteins, are sterile and exhibit profound defects in chromatin remodeling during spermiogenesis. We have now observed that a prominent feature of the aberrant spermatid nuclei is a fragmented chromocenter, a structure comprised of peri-centromeric heterochromatin. There was a concomitant increase in the levels of heterochromatin protein 1 alpha (Hp1α), suggesting that the presence of multiple chromocenters was correlated with a spread of heterochromatin beyond the normal centromeric region. Brdt protein was normally present throughout the nucleus but was excluded from the chromocenter. A more densely staining region of Brdt protein appeared to separate sirtuin 1 (Sirt1) protein from contact with the chromocenter. Although still nuclear, this unique localization of Brdt protein was lost in Brdt(∆BD1/∆BD1) mutant spermatids and Brdt and Sirt1 overlapped around the chromocenters. There was also ectopic localization of the H1 histone family, member N, testis-specific (H1fnt) protein in Brdt(∆BD1/∆BD1) round spermatids, which may be linked to the previously reported loss of polarized localization of peri-nuclear heterochromatin foci. The extent of chromocenter fragmentation was more severe and penetrant in mutant testes on a pure 129Sv/Ev as compared to a pure C57Bl/6 background. Indeed, all aspects of the mutant phenotype were more severe on the 129Sv/Ev background. Contrary to previous studies in genetic models where fragmented chromocenters were observed in spermatids, the Brdt(∆BD1/∆BD1) mutant spermatids do not undergo apoptosis (on either background). These observations suggest that the first bromodomain of Brdt is critical in the formation and/or maintenance of an intact chromocenter and implicate this structure in proper remodeling of the chromatin architecture of the sperm head.
Brdt（∆BD1/∆BD1）纯合突变小鼠缺失 Brdt 的第一个溴结构域，Brdt 是 BRD 家族双溴结构域蛋白的一个睾丸特异性成员，其表现为不育，并在精子发生过程中出现严重的染色质重塑缺陷。我们现在观察到，异常精母细胞核的一个显著特征是染色质中心的碎片化，该结构由着丝粒周围异染色质组成。异染色质蛋白 1α（Hp1α）的水平同时升高，表明多个染色质中心的存在与异染色质在正常着丝粒区域之外的扩散有关。Brdt 蛋白正常存在于整个核中，但被排除在染色质中心之外。Brdt 蛋白出现更密集染色的区域似乎将 Sirtuin 1（Sirt1）蛋白与染色质中心隔离开来。尽管仍位于核内，但 Brdt（∆BD1/∆BD1）突变精母细胞中的这种独特定位丢失了，Brdt 和 Sirt1 在染色质中心周围重叠。在 Brdt（∆BD1/∆BD1）圆形精母细胞中，H1 组蛋白家族成员 N，睾丸特异性（H1fnt）蛋白也存在异位定位，这可能与先前报道的核周异染色质焦点极化定位的丢失有关。与纯 C57Bl/6 背景相比，在纯 129Sv/Ev 背景下，突变睾丸中染色质中心碎片化的程度更严重、更普遍。事实上，在 129Sv/Ev 背景下，突变表型的各个方面都更为严重。与先前在遗传模型中观察到碎片化染色质中心的研究相反，Brdt（∆BD1/∆BD1）突变精母细胞不会发生凋亡（在任何背景下都不会）。这些观察结果表明，Brdt 的第一个溴结构域对于完整染色质中心的形成和/或维持至关重要，并暗示该结构在精子头部染色质结构的适当重塑中发挥作用。
