UBR2 mediates transcriptional silencing during spermatogenesis via histone ubiquitination

Abstract

Ubiquitination of histones provides an important mechanism regulating chromatin remodeling and gene expression. Recent studies have revealed ubiquitin ligases involved in histone ubiquitination, yet the responsible enzymes and the function of histone ubiquitination in spermatogenesis remain unclear. We have previously shown that mice lacking the ubiquitin ligase UBR2, one of the recognition E3 components of the N-end rule proteolytic pathway, are infertile associated with meiotic arrest at prophase I. We here show that UBR2 localizes to meiotic chromatin regions, including unsynapsed axial elements linked to chromatin inactivation, and mediates transcriptional silencing via the ubiquitination of histone H2A. UBR2 interacts with the ubiquitin conjugating enzyme HR6B and its substrate H2A and promotes the HR6B-H2A interaction and the HR6B-to-H2A transfer of ubiquitin. UBR2 and ubiquitinated H2A (uH2A) spatiotemporally mark meiotic chromatin regions subject to transcriptional silencing, and UBR2-deficient spermatocytes fail to induce the ubiquitination of H2A during meiosis. UBR2-deficient spermatocytes are profoundly impaired in chromosomewide transcriptional silencing of genes linked to unsynapsed axes of the X and Y chromosomes. Our findings suggest that insufficiency in UBR2-dependent histone ubiquitination triggers a pachytene checkpoint system, providing a new insight into chromatin remodeling and gene expression regulation. A pre-N-degron of a stable protein can be converted into an Ndegron through N-terminal modifications, such as deamidation, oxidation, and arginylation (3-5). UBR1 and UBR2 are really interesting new gene (RING) finger E3s that share size (200 kDa), sequence (46% identity), and enzymatic specificities to type-1 (Arg, Lys, and His; basic) and type-2 (Phe, Leu, Trp, Tyr, and Ile; bulky hydrophobic) destabilizing N-terminal residues (3, 5, 6). Regardless of the similarity in biochemical properties, deficiency of UBR1 and UBR2 causes completely different null phenotypes in mammals. Mutations of human UBR1 cause Johanson-Blizzard syndrome (JBS), an autosomal recessive disorder with multiple developmental abnormalities including exocrine pancreatic insufficiency; knockout of mouse UBR1 also results in metabolic abnormalities such as JBS-like exocrine pancreatic defects (reviewed in ref. 2). UBR2 −∕− male mice are infertile associated with germ cell apoptosis and arrest of spermatocytes at meiotic prophase I, whereas UBR2 −∕− female mice show reduced fertility and partial lethality (5). Besides these functions, the N-end rule pathway plays a role in a variety of processes, including cardiovascular system, DNA repair, neurogenesis, olfaction, sensing oxygen and nitric oxide, G-protein signaling, chromosome segregation, peptide import, selective degradation of misfolded proteins, and apoptosis (reviewed in ref. 2; 3-8).Spermatogenesis involves one of the most dramatic chromatinremodeling processes, including synapsis and transcriptional silencing. During the pachytene stage of meiosis, the X and Y ...

Identification and characteristics of the testes-specific gene, Ccdc38, in mice

“…Spermatogenesis involves various histone modifications including ubiquitination, phosphorylation, methylation and acetylation (17). A previous study indicated that a high quantity of uH2A was detected in the pachytene stage of spermatocytes, by immunoblot and immunohistochemical analysis of wild-type mice testes (18).…”

RNF8 regulates active epigenetic modifications and escape gene activation from inactive sex chromosomes in post-meiotic spermatids

“…PRC1 mediates chromatin compaction (Francis et al 2004), and RNF2-mediated monoubiquitination of H2A is associated with gene silencing. Based on these views, ubiquitinated H2A on meiotic sex chromosomes has been considered to have functions in gene silencing (Baarends et al 2005;An et al 2010). In contrast, we revealed a novel role for ubiquitinated H2A in gene activation in our study.…”

The arginylation branch of the N-end rule pathway positively regulates cellular autophagic flux and clearance of proteotoxic proteins

“…19 These outcomes indicate that a single component of the N-end rule pathway may be involved in multiple layers of regulatory circuits, probably in temporal cooperation with various downstream regulators of the pathway, such as the UBR proteins. The biological functions of these UBR proteins include the regulation of cardiovascular development, 17 chromosomal stability, 20 spermatogenesis, 21 oxygen sensing, 22 muscle wasting, [23][24][25][26] proteotoxic protein clearance, 27 hypertension, 28 neuronal tube formation, 29 bacterial/viral virulence, 30 and apoptosis. 31 More recent studies have suggested that the Arg/N-end rule pathway is also implicated in autophagy and mitophagy.…”