The role of dead-end in germ-cell tumor development

Germ cell migration-Evolutionary issues and current understanding

In many organisms, primordial germ cells (PGCs) are specified at a different location than where the gonad forms, meaning that PGCs must migrate toward the gonad within the early developing embryo. Following species-specific paths, PGCs can be passively carried by surrounding tissues and also perform active migration. When PGCs actively migrate through and along a variety of embryonic structures in different organisms, they adopt an ancestral robust migration mode termed "amoeboid motility", which allows cells to migrate within diverse environments. In this review, we discuss the possible significance of the PGC migration process in facilitating the evolution of animal body shape. In addition, we summarize the latest findings relevant for the molecular and cellular mechanisms controlling the movement and the directed migration of PGCs in different species.

Pluripotent cell derivation from male germline cells by suppression of Dmrt1 and Trp53

Diploid germ cells are thought to have pluripotency potential. We recently described a method to derive pluripotent stem cells (PSCs) from cultured spermatogonial stem cells (SSCs) by depleting Trp53 and Dmrt1, both of which are known suppressors of teratomas. In this study, we used this technique to analyze the effect of this protocol in deriving PSCs from the male germline at different developmental stages. We collected primordial germ cells (PGCs), gonocytes and spermatogonia, and the cells were transduced with lentiviruses expressing short hairpin RNA against Dmrt1 and/or Trp53. We found that PGCs are highly susceptible to reprogramming induction and that only Trp53 depletion was sufficient to induce pluripotency. In contrast, gonocytes and spermatogonia were resistant to reprogramming by double knockdown of Dmrt1 and Trp53. PSCs derived from PGCs contributed to chimeras produced by blastocyst injection, but some of the embryos showed placenta-only phenotypes suggestive of epigenetic abnormalities of PGC-derived PSCs. These results show that PGCs and gonocytes/spermatogonia have distinct reprogramming potential and also suggest that fresh and cultured SSCs do not necessarily have the same properties.

Mouse dead end 1-β interacts with c-Jun and stimulates activator protein 1 transactivation

Dead end 1 (DND1), important for maintaining the viability of primordial germ cells, is the first protein containing an RNA recognition motif that has been directly implicated as a heritable cause of spontaneous tumorigenesis. In the present study, c-Jun was identified through yeast two-hybrid screening of a 10.5-day old mouse embryo cDNA library as one of the proteins which interact with DND1-β. The interaction between DND1-β and c-Jun was demonstrated to occur by glutathione S‑transferase pull‑down and co-immunoprecipitation. Using confocal microscopy, DND1-β was found to be specifically expressed in GC-1 spermatogonia cells, mainly in the nuclei. When transfected into GC-1 cells, DND1-β and c-Jun were demonstrated to be co-localized principally in the nuclei. Furthermore, in a dual luciferase reporter assay, the transcriptional activity of activator protein 1 was demonstrated to be significantly increased by co-transfection with DND1-β and c-Jun plasmids in GC-1 cells. The identification and confirmation of an additional protein interacting with DND1-β facilitates the investigation of the functions and molecular mechanisms of DND1.

Regulation of pluripotency in male germline stem cells by Dmrt1

Spermatogonial stem cells (SSCs) have the potential to acquire pluripotency under specific culture conditions. Takashima et al. report that global DNA hypomethylation triggered by Dnmt1 depletion induces pluripotent cell derivation. Dnmt1 depletion down-regulates Dmrt1, a gene involved in sexual differentiation. Dmrt1 depletion up-regulates Sox2, which in turn up-regulates Oct4 and produces pluripotent cells. These results suggest that the Dmrt1–Sox2 axis plays a crucial role in repression of SSC pluripotency.

Spermatogonial stem cells (SSCs) present the potential to acquire pluripotency under specific culture conditions. However, the frequency of pluripotent cell derivation is low, and the mechanism of SSC reprogramming remains unknown. In this study, we report that induction of global DNA hypomethylation in germline stem (GS) cells (cultured SSCs) induces pluripotent cell derivation. When DNA demethylation was triggered by Dnmt1 depletion, GS cells underwent apoptosis. However, GS cells were converted into embryonic stem (ES)-like cells by double knockdown of Dnmt1 and p53. This treatment down-regulated Dmrt1, a gene involved in sexual differentiation, meiosis, and pluripotency. Dmrt1 depletion caused apoptosis of GS cells, but a combination of Dmrt1 and p53 depletion also induced pluripotency. Functional screening of putative Dmrt1 target genes revealed that Dmrt1 depletion up-regulates Sox2. Sox2 transfection up-regulated Oct4 and produced pluripotent cells. This conversion was enhanced by Oct1 depletion, suggesting that the balance of Oct proteins maintains SSC identity. These results suggest that spontaneous SSC reprogramming is caused by unstable DNA methylation and that a Dmrt1–Sox2 cascade is critical for regulating pluripotency in SSCs.

Maternal Dead-End1 is required for vegetal cortical microtubule assembly during Xenopus axis specification

Vertebrate axis specification is an evolutionarily conserved developmental process that relies on asymmetric activation of Wnt signaling and subsequent organizer formation on the future dorsal side of the embryo. Although roles of Wnt signaling during organizer formation have been studied extensively, it is unclear how the Wnt pathway is asymmetrically activated. In Xenopus and zebrafish, the Wnt pathway is triggered by dorsal determinants, which are translocated from the vegetal pole to the future dorsal side of the embryo shortly after fertilization. The transport of dorsal determinants requires a unique microtubule network formed in the vegetal cortex shortly after fertilization. However, molecular mechanisms governing the formation of vegetal cortical microtubule arrays are not fully understood. Here we report that Dead-End 1 (Dnd1), an RNA-binding protein required for primordial germ cell development during later stages of embryogenesis, is essential for Xenopus axis specification. We show that knockdown of maternal Dnd1 specifically interferes with the formation of vegetal cortical microtubules. This, in turn, impairs translocation of dorsal determinants, the initiation of Wnt signaling, organizer formation, and ultimately results in ventralized embryos. Furthermore, we found that Dnd1 binds to a uridine-rich sequence in the 3'-UTR of trim36, a vegetally localized maternal RNA essential for vegetal cortical microtubule assembly. Dnd1 anchors trim36 to the vegetal cortex in the egg, promoting high concentrations of Trim36 protein there. Our work thus demonstrates a novel and surprising function for Dnd1 during early development and provides an important link between Dnd1, mRNA localization, the microtubule cytoskeleton and axis specification.

Expression of RNA-binding proteins DND1 and FXR1 in the porcine ovary, and during oocyte maturation and early embryo development

The porcine oocyte and early embryo are transcriptionally quiescent following germinal vesicle breakdown in the oocyte and prior to activation of the embryonic genome, at approximately the 4-cell stage of development. Despite a lack of new transcription, mRNA and protein repertoires are subject to regulation during this time. One potential mechanism of regulation is through the functional activity of miRNAs and/or the presence of specific RNA-binding proteins. Both DND1 (dead end homolog 1) and FXR1 (fragile-X-mental retardation-related protein 1) are RNA-binding proteins that have been demonstrated to impact miRNA-mediated, post-transcriptional gene regulation. The objective was to characterize the presence and the expression changes in DND1 and FXR1 during pig oocyte maturation and early embryo development. DND1 and FXR1 expression were evaluated in oocytes and cumulus cells during meiotic progression and in 4-cell stage embryos using quantitative RT-PCR, Western blot analysis, and immunostaining. These data demonstrate DND1 and FXR1 mRNA are expressed in the maturing oocyte and early in vitro-fertilized embryos, with significantly less DND1 in 4-cell stage embryos as compared to germinal vesicle and metaphase II-arrested oocytes. Based on immunohistochemistry, DND1 protein abundance is greater in secondary follicles in comparison to primary and tertiary follicles. Using ribonucleoprotein immunoprecipitation from germinal vesicle-stage oocytes, DND1 was demonstrated to interact with several mRNAs associated with pluripotency. This work provides a better understanding of the biological relevance of DND1 and FXR1 during female gametogenesis and embryo development in pigs.

Screening for germline DND1 mutations in testicular cancer patients

Although several observations suggest that a strong genetic predisposition to developing testicular germ cell tumors (TGCT) exists, no associated, highly penetrant germline mutations have been identified so far. In the 129/Sv mouse strain, a germline mutation in the DND1 gene has been shown to strongly increase the TGCT risk. We screened 272 men with TGCT (89% sporadic cases, 11% familial) for germline mutations in the human homologue of DND1. A single nucleotide substitution c.657C > G (p.Asp219Glu) was observed in a non-familial case of testicular embryonal carcinoma. The variant was also present in the patient’s asymptomatic father and two brothers, but not observed in 210 control chromosomes. The wild type DND1 allele was not lost in the patient’s tumor. In silico analysis of the variant predicts it to be non-pathogenic. We conclude that germline DND1 mutations are unlikely to contribute significantly to human testicular germ cell tumor susceptibility. The role of human DND1 in normal physiology and disease, however, is still virtually unknown and it therefore warrants further research.

Mouse apolipoprotein B editing complex 3 (APOBEC3) is expressed in germ cells and interacts with dead-end (DND1)

BACKGROUND

The dead-end (Dnd1) gene is essential for maintaining the viability of germ cells. Inactivation of Dnd1 results in sterility and testicular tumors. The Dnd1 encoded protein, DND1, is able to bind to the 3'-untranslated region (UTR) of messenger RNAs (mRNAs) to displace micro-RNA (miRNA) interaction with mRNA. Thus, one function of DND1 is to prevent miRNA mediated repression of mRNA. We report that DND1 interacts specifically with APOBEC3. APOBEC3 is a multi-functional protein. It inhibits retroviral replication. In addition, recent studies show that APOBEC3 interacts with cellular RNA-binding proteins and to mRNA to inhibit miRNA-mediated repression of mRNA.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that DND1 specifically interacts with another cellular protein, APOBEC3. We present our data which shows that DND1 co-immunoprecipitates APOBEC3 from mammalian cells and also endogenous APOBEC3 from mouse gonads. Whether the two proteins interact directly remains to be elucidated. We show that both DND1 and APOBEC3 are expressed in germ cells and in the early gonads of mouse embryo. Expression of fluorescently-tagged DND1 and APOBEC3 indicate they localize to the cytoplasm and when DND1 and APOBEC3 are expressed together in cells, they sequester near peri-nuclear sites.

CONCLUSIONS/SIGNIFICANCE

The 3'-UTR of mRNAs generally encode multiple miRNA binding sites as well as binding sites for a variety of RNA binding proteins. In light of our findings of DND1-APOBEC3 interaction and taking into consideration reports that DND1 and APOBEC3 bind to mRNA to inhibit miRNA mediated repression, our studies implicate a possible role of DND1-APOBEC3 interaction in modulating miRNA-mediated mRNA repression. The interaction of DND1 and APOBEC3 could be one mechanism for maintaining viability of germ cells and for preventing germ cell tumor development.