The first bromodomain of Brdt, a testis-specific member of the BET sub-family of double-bromodomain-containing proteins, is essential for male germ cell differentiation

Abstract

*Brdt is a testis-specific member of the distinctive BET sub-family of bromodomain motif-containing proteins, a motif that binds acetylated lysines and is implicated in chromatin remodeling. Its expression is restricted to the germ line, specifically to pachytene and diplotene spermatocytes and early spermatids. Targeted mutagenesis was used to generate mice carrying a mutant allele of Brdt, Brdt ⌬BD1, which lacks only the first of the two bromodomains that uniquely characterize BET proteins. Homozygous Brdt ⌬BD1/⌬BD1 mice were viable but males were sterile, producing fewer and morphologically abnormal sperm. Aberrant morphogenesis was first detected in step 9 elongating spermatids, and those elongated spermatids that were formed lacked the distinctive foci of heterochromatin at the peri-nuclear envelope. Quantitative reverse transcription (RT)-PCR showed threefold increased levels of histone H1t (Hist1h1t) in Brdt ⌬BD1/⌬BD1 testes and chromatin immunoprecipitation revealed that Brdt protein, but not Brdt ⌬BD1 protein, was associated with the promoter of H1t. Intracytoplasmic sperm injection suggested that the DNA in the Brdt ⌬BD1 mutant sperm could support early embryonic development and yield functional embryonic stem cells. This is the first demonstration that deletion of just one of the two bromodomains in members of the BET sub-family of bromodomain-containing proteins has profound effects on in vivo differentiation.

The bromodomain interaction module

“…Affinity for histone H3 peptides has also been demonstrated although it appears to be systematically stronger in the case of the C-terminal domains suggesting a mode of cooperativity between the tandem modules, either binding to the same nucleosome on different histone tails at the same time, or bridging adjacent nucleosomes by binding on histone H3 of the first and histone H4 of the second. It is however noteworthy that deletion of the first BRD in BRDT is sufficient to cause male sterility in mice by blocking development of mature sperm [50]. A link between BET proteins and increased expression of inflammatory genes via the transcriptional activation of NF-jB occurs through their interaction with the acetylated RelA subunit of NF-jB [51] as evidenced in the case of BRD4.…”

Parental epigenetic control of embryogenesis: a balance between inheritance and reprogramming?

“…Hyperacetylated histones are recognized by the bromodomain containing protein Brdt [78], [79]. Deletion of the first bromodomain of Brdt in mice results in abnormal chromatin remodeling and male infertility [80]. Brdt-mutant sperm is, however, capable of supporting normal embryonic development when used for intracytoplasmic sperm injection (ICSI) [80].…”

“…Deletion of the first bromodomain of Brdt in mice results in abnormal chromatin remodeling and male infertility [80]. Brdt-mutant sperm is, however, capable of supporting normal embryonic development when used for intracytoplasmic sperm injection (ICSI) [80]. Histones are also ubiquitinated in elongating spermatids [81].…”

Transcription and post-transcriptional regulation of spermatogenesis

“…Acetylated lysine residues are bound by a structural motif called a bromodomain (Yang 2004 including several HATs and transcription coactivators, most of which are involved in transcriptional regulation and/or chromatin remodelling (Zeng & Zhou 2002;Mujtaba et al 2007). Among these is bromodomain testis-specific protein (BRDT), a member of the bromodomains and extraterminal (BET) subfamily, all of which have two bromodomains and a region of homology in the C-terminus (Florence & Faller 2001;Shang et al 2007). BRDT is abundantly expressed in the nuclei of primary spermatocytes and round spermatids, and thus it is a good candidate to regulate transcription in germ cells.…”