The first bromodomain of the testis-specific double bromodomain protein Brdt is required for chromocenter organization that is modulated by genetic background.
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.
Department of Genetics and Development, Columbia University Medical Center, New York, New York 10032, USA.
Binyamin D
BD
Berkovits
Debra J
DJ
Wolgemuth
D000818:Animals; D002843:Chromatin / Q000378:metabolism*; D042002:Chromatin Assembly and Disassembly; D003433:Crosses, Genetic; D005455:Fluorescent Antibody Technique; D006657:Histones / Q000235:genetics / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009154:Mutation; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D056564:Sirtuin 1 / Q000235:genetics / Q000378:metabolism; D013087:Spermatids / Q000378:metabolism*; D013091:Spermatogenesis; D013737:Testis / Q000378:metabolism*; D000092468:Bromodomain Containing Proteins
D002843:Chromatin; D006657:Histones; D009687:Nuclear Proteins; D056564:Sirtuin 1; C518475:BRDT protein, mouse; D000092468:Bromodomain Containing Proteins; C470545:Sirt1 protein, mouse