Cell Autonomous and Nonautonomous Function of CUL4B in Mouse Spermatogenesis

Abstract

CUL4B ubiquitin ligase belongs to the cullin-RING ubiquitin ligase family. Although sharing many sequence and structural similarities, CUL4B plays distinct roles in spermatogenesis from its homologous protein CUL4A. We previously reported that genetic ablation of Cul4a in mice led to male infertility because of aberrant meiotic progression. In the present study, we generated Cul4b germ cell-specific conditional knock-out (Cul4b Protein degradation via the ubiquitin-proteasome system plays a critical role during mammalian spermatogenesis. Timely removal of outlived proteins is crucial to ensure progression through different phases of spermatogenesis including mitotic, meiotic divisions, and postmeiotic morphogenesis. The ubiquitin-proteasome system selects its targets via a group of E3 ubiquitin ligases, which upon interaction with E2 ubiquitin-conjugating enzymes tag substrates for proteasomal degradation by the 26S proteasome. The largest mammalian E3 ligase family is the CRL (cullin-RING finger ubiquitin ligase) family, which includes eight members. Each cullin family member serves as a molecular scaffold and forms an E3 ligase complex with substrate-recruiting receptors and a linker protein (1). The Cul4b gene, located on the X chromosome, shares extensive sequence homology and functional redundancy with the other CRL4 family member, Cul4a. Both proteins employ DDB1 (DNA damage-binding protein 1) as the linker protein, which in turn recruits common or distinct DDB1-CUL4 associated factors for substrate binding. The DDB1-CUL4-mediated protein modification/degradation is involved in critical cellular processes including DNA replication, DNA repair, cell cycle control, and histone modifications (2).Male infertility accounts for ϳ40 -50% of infertility cases in humans (3, 4). Many factors contribute to male infertility; however, the vast majority of male infertility cases are associated with oligozoospermia (decreased sperm number), asthenozoospermia (reduced sperm motility), and/or teratozoospermia (abnormal sperm morphology). In the mammalian testis, a single pluripotent spermatogonial stem cell (SSC) 2 undergoes several rounds of mitotic divisions followed by meiotic divisions and complex postmeiotic morphogenesis, eventually giving rise to a cohort of mature spermatozoa. We have previously reported that the CRL4 proteins exhibited complementary expression patterns in adult mouse testis, where CUL4A was predominantly detected in primary spermatocytes, whereas CUL4B was highly expressed in Sertoli cells, spermatogonia, and spermatids (5). The absence of CUL4B in spermatocytes is likely due to meiotic sex chromosome inactivation, a transient X chromosome inactivation caused by sex chromosome condensation and subsequent gene silencing (6). However, the

Deletion of DDB1- and CUL4- associated factor-17 (Dcaf17) gene causes spermatogenesis defects and male infertility in mice

“…In several studies, it was shown that the CUL4 ubiquitin ligases (CUL4A and CUL4B) play essential roles in male germ cell development. Both Cul4A and Cul4B mutant male mice were infertile 22 – 25 . Although CUL4A and CUL4B share sequence and structural similarities, both the proteins play diverse roles in spermatogenesis 23 – 25 , 47 .…”

“…Although CUL4A and CUL4B share sequence and structural similarities, both the proteins play diverse roles in spermatogenesis 23 – 25 , 47 . Disruption of Cul4A in mice resulted in male infertility due to abnormal meiotic progression, whereas disruption of Cul4B in mice caused male infertility because of aberrant post-meiotic sperm development 22 – 25 . Macroscopic features of Cul4A KO mice testes showed two fold reductions in the size and the weight of the Cul4A −/− testis compared to that of wild type 25 .…”

“…E3 ubiquitin ligases (E3s) are diverse enzymes with modular, single protein or multi-protein complex structures that provide specificity toward a variety of substrates for the ubiquitination 19 – 21 . Cullin (CUL) 4 A and CUL4B, members of cullin-RING finger E3 ligase (CRL) sub-family, have been shown to play distinct and essential roles during spermatogenesis 22 – 25 . Both the proteins have displayed complementary expression patterns in adult mouse testis, where CUL4A is mainly present in meiosis-stage spermatocytes, while CUL4B is predominantly expressed in Sertoli cells, spermatogonia and spermatids 24 .…”

“…Disruption of Cul4A caused male infertility because of the defective meiotic progression 24 , 25 . Whereas, the male infertility phenotype resulted from Cul4B deletion was due to abnormal post-meiotic sperm development 22 , 23 .…”

Dysregulation of CUL4A and CUL4B Ubiquitin Ligases in Lung Cancer

“…Strikingly, cul4a null mice are hyper-resistant to UV-induced skin cancer (15). CUL4A and CUL4B are also expressed at different stages of spermatogenesis, and germ line deletion of either cul4 gene results in male infertility (24,27). Therefore, CUL4A and CUL4B share overlapping functions in growth and survival, while also playing distinct roles in diverse cellular processes.…”