Mouse Dazl and its novel splice variant functions in translational repression of target mRNAs in embryonic stem cells

Risk assessment and environmental consequences of the use of the Allium-derived compound propyl-propane thiosulfonate (PTSO) in agrifood applications

The organosulfur compound propyl-propane thiosulfonate (PTSO), mainly found in Allium cepa, has a promising use in the agrifood industry. To confirm its safety for livestock, consumers, and environment, toxicological assessment is needed. In this regard, endocrine-disrupting chemicals (EDCs) are in the spotlight of research. Therefore, as part of the risk assessment of PTSO, in the present work, an in vivo study was performed in mice exposed to PTSO to investigate its potential reproductive toxicity considering fertility, genetic and endocrine endpoints. Five-weeks-old CD1 mice (80 males, 80 females) were exposed for 11 or 16 weeks (males or females, respectively) to different doses of PTSO (0, 14, 28 and 55 mg PTSO/kg b.w./day; 20 animals per group and sex) through the food pellets. No clinical observations or mortality and no changes in absolute organ weights and relative organ weights/body weight or brain ratios occurred during the study. The estrous cycle did not undergo any significant toxicologically relevant change. Most of the sex hormones displayed normal values. Some alterations in the expression of some genes related to reproduction is only observed in females, but they do not appear to have consequences in the development of sex organs. Docking results showed the impossibility of stable binding to estrogen and androgen receptors. Considering all the results obtained, the safe profile of PTSO can be confirmed for different agrifood applications at the conditions assayed.

SRSF10 is essential for progenitor spermatogonia expansion by regulating alternative splicing

Alternative splicing expands the transcriptome and proteome complexity and plays essential roles in tissue development and human diseases. However, how alternative splicing regulates spermatogenesis remains largely unknown. Here, using a germ cell-specific knockout mouse model, we demonstrated that the splicing factor Srsf10 is essential for spermatogenesis and male fertility. In the absence of SRSF10, spermatogonial stem cells can be formed, but the expansion of Promyelocytic Leukemia Zinc Finger (PLZF)-positive undifferentiated progenitors was impaired, followed by the failure of spermatogonia differentiation (marked by KIT expression) and meiosis initiation. This was further evidenced by the decreased expression of progenitor cell markers in bulk RNA-seq, and much less progenitor and differentiating spermatogonia in single-cell RNA-seq data. Notably, SRSF10 directly binds thousands of genes in isolated THY+ spermatogonia, and Srsf10 depletion disturbed the alternative splicing of genes that are preferentially associated with germ cell development, cell cycle, and chromosome segregation, including Nasp, Bclaf1, Rif1, Dazl, Kit, Ret, and Sycp1. These data suggest that SRSF10 is critical for the expansion of undifferentiated progenitors by regulating alternative splicing, expanding our understanding of the mechanism underlying spermatogenesis.

Primordial Germ Cell Development in the Poeciliid, Gambusia holbrooki, Reveals Shared Features Between Lecithotrophs and Matrotrophs

Metazoans exhibit two modes of primordial germ cell (PGC) specification that are interspersed across taxa. However, the evolutionary link between the two modes and the reproductive strategies of lecithotrophy and matrotrophy is poorly understood. As a first step to understand this, the spatio-temporal expression of teleostean germ plasm markers was investigated in Gambusia holbrooki, a poecilid with shared lecitho- and matrotrophy. A group of germ plasm components was detected in the ovum suggesting maternal inheritance mode of PGC specification. However, the strictly zygotic activation of dnd-β and nanos1 occurred relatively early, reminiscent of models with induction mode (e.g., mice). The PGC clustering, migration and colonisation patterns of G. holbrooki resembled those of zebrafish, medaka and mice at blastula, gastrula and somitogenesis, respectively-recapitulating features of advancing evolutionary nodes with progressive developmental stages. Moreover, the expression domains of PGC markers in G. holbrooki were either specific to teleost (vasa expression in developing PGCs), murine models (dnd spliced variants) or shared between the two taxa (germline and somatic expression of piwi and nanos1). Collectively, the results suggest that the reproductive developmental adaptations may reflect a transition from lecithotrophy to matrotrophy.

Distinct roles of haspin in stem cell division and male gametogenesis

The kinase haspin phosphorylates histone H3 at threonine-3 (H3T3ph) during mitosis. H3T3ph provides a docking site for the Chromosomal Passenger Complex at the centromere, enabling correction of erratic microtubule-chromosome contacts. Although this mechanism is operational in all dividing cells, haspin-null mice do not exhibit developmental anomalies, apart from aberrant testis architecture. Investigating this problem, we show here that mouse embryonic stem cells that lack or overexpress haspin, albeit prone to chromosome misalignment during metaphase, can still divide, expand and differentiate. RNA sequencing reveals that haspin dosage affects severely the expression levels of several genes that are involved in male gametogenesis. Consistent with a role in testis-specific expression, H3T3ph is detected not only in mitotic spermatogonia and meiotic spermatocytes, but also in non-dividing cells, such as haploid spermatids. Similarly to somatic cells, the mark is erased in the end of meiotic divisions, but re-installed during spermatid maturation, subsequent to methylation of histone H3 at lysine-4 (H3K4me₃) and arginine-8 (H3R8me₂). These serial modifications are particularly enriched in chromatin domains containing histone H3 trimethylated at lysine-27 (H3K27me₃), but devoid of histone H3 trimethylated at lysine-9 (H3K9me₃). The unique spatio-temporal pattern of histone H3 modifications implicates haspin in the epigenetic control of spermiogenesis.

Germ cell determination and the developmental origin of germ cell tumors

In each generation, the germline is tasked with producing somatic lineages that form the body, and segregating a population of cells for gametogenesis. During animal development, when do cells of the germline irreversibly commit to producing gametes? Integrating findings from diverse species, we conclude that the final commitment of the germline to gametogenesis - the process of germ cell determination - occurs after primordial germ cells (PGCs) colonize the gonads. Combining this understanding with medical findings, we present a model whereby germ cell tumors arise from cells that failed to undertake germ cell determination, regardless of their having colonized the gonads. We propose that the diversity of cell types present in these tumors reflects the broad developmental potential of migratory PGCs.

In vitro derived female hPGCLCs are unable to complete meiosis in embryoid bodies

The fundamental question about the functionality of in vitro derived human primordial germ cell-like cells remains unanswered, despite ongoing research in this area. Attempts have been made to imitate the differentiation of human primordial germ cells (hPGCs) and meiocytes in vitro from human pluripotent stem cells (hPSCs). A defined system for developing human haploid cells in vitro is the challenge that scientists face to advance the knowledge of human germ cell development. To develop human primordial germ cell-like cells (hPGCLCs) from human pluripotent stem cells (hPSCs) that are capable of giving rise to haploid cells, we applied a sequential induction protocol via the early mesodermal push of female human embryonic and induced pluripotent stem cells. BMP4-induced early mesoderm-like cells showed significant alterations in their expression profiles toward early (PRDM1 and NANOS3) and late (VASA and DAZL) germ cell markers. Furthermore, using retinoic acid (RA), we induced hPGCLCs in embryoid bodies and identified positive staining for the meiotic initiation marker STRA8. Efforts to find the cells exhibiting progression to meiosis were unsuccessful. The validation by the expression of SCP3 did not correspond to the natural pattern. Regarding the 20-day meiotic induction, the derived hPGCLCs containing two X-chromosomes were unable to complete the meiotic division. We observed the expression of the oocyte marker PIWIL1 and PIWIL4. RNAseq analysis and cluster dendrogram showed a similar clustering of hPGCLC groups and meiotic like cell groups as compared to previously published data. This reproducible in vitro model for deriving hPGCLCs provides opportunities for studying the molecular mechanisms involved in the specification of hPGCs. Moreover, our results will support a further elucidation of gametogenesis and meiosis of female hPGCs.

The RNA-binding protein DAZL functions as repressor and activator of mRNA translation during oocyte maturation

Deleted in azoospermia-like (DAZL) is an RNA-binding protein critical for gamete development. In full-grown oocytes, the DAZL protein increases 4-fold during reentry into the meiotic cell cycle. Here, we have investigated the functional significance of this accumulation at a genome-wide level. Depletion of DAZL causes a block in maturation and widespread disruption in the pattern of ribosome loading on maternal transcripts. In addition to decreased translation, DAZL depletion also causes translational activation of a distinct subset of mRNAs both in quiescent and maturing oocytes, a function recapitulated with YFP-3′UTR reporters. DAZL binds to mRNAs whose translation is both repressed and activated during maturation. Injection of recombinant DAZL protein in DAZL-depleted oocytes rescues the translation and maturation to MII. Mutagenesis of putative DAZL-binding sites in these mRNAs mimics the effect of DAZL depletion. These findings demonstrate that DAZL regulates translation of maternal mRNAs, functioning both as the translational repressor and activator during oocyte maturation.

The RNA binding protein DAZL plays a critical role during germ cell development. Here the authors provide evidence that DAZL functions both as activator and repressor of translation during oocyte maturation in mouse.

The Function of Pre-mRNA Alternative Splicing in Mammal Spermatogenesis

Alternative pre-mRNA splicing plays important roles in co-transcriptional and post-transcriptional regulation of gene expression functioned during many developmental processes, such as spermatogenesis. The studies focusing on alternative splicing on spermatogenesis supported the notion that the development of testis is regulated by a higher level of alternative splicing than other tissues. Here, we aim to review the mechanisms underlying alternative splicing, particularly the splicing variants functioned in the process of spermatogenesis and the male infertility. There are five points regarding the alternative splicing including ⅰ) a brief introduction of alternative pre-mRNA splicing; ⅱ) the alternative splicing events in spermatogenesis-associated genes enriched in different stages of spermatogenesis; ⅲ) the mechanisms of alternative splicing regulation, such as splicing factors and m⁶A demethylation; ⅳ) the splice site recognition and alternative splicing, including the production and degradation of abnormal transcripts caused by gene variations and nonsense-mediated mRNA decay, respectively; ⅴ) abnormal alternative splicing correlated with male infertility. Taking together, this review highlights the impacts of alternative splicing and splicing variants in mammal spermatogenesis and provides new insights of the potential application of the alternative splicing into the therapy of male infertility.

DAZL Regulates Germ Cell Survival through a Network of PolyA-Proximal mRNA Interactions

The RNA binding protein DAZL is essential for gametogenesis, but its direct in vivo functions, RNA targets, and the molecular basis for germ cell loss in Dazl-null mice are unknown. Here, we mapped transcriptome-wide DAZL-RNA interactions in vivo, revealing DAZL binding to thousands of mRNAs via polyA-proximal 3′ UTR interactions. In parallel, fluorescence-activated cell sorting and RNA-seq identified mRNAs sensitive to DAZL deletion in male germ cells. Despite binding a broad set of mRNAs, integrative analyses indicate that DAZL post-transcriptionally controls only a subset of its mRNA targets, namely those corresponding to a network of genes that are critical for germ cell proliferation and survival. In addition, we provide evidence that polyA sequences have key roles in specifying DAZL-RNA interactions across the transcriptome. Our results reveal a mechanism for DAZL-RNA binding and illustrate that DAZL functions as a master regulator of a post-transcriptional mRNA program essential for germ cell survival.

Combining transgenic mice, FACS, and multiple RNA-profiling methods, Zagore et al. show that DAZL binds thousands of mRNAs via GUU sites upstream of polyA tails. Loss of DAZL results in decreased mRNA levels for a network of genes that are essential for germ cell proliferation and differentiation.

DDX5 plays essential transcriptional and post-transcriptional roles in the maintenance and function of spermatogonia

Mammalian spermatogenesis is sustained by mitotic germ cells with self-renewal potential known as undifferentiated spermatogonia. Maintenance of undifferentiated spermatogonia and spermatogenesis is dependent on tightly co-ordinated transcriptional and post-transcriptional mechanisms. The RNA helicase DDX5 is expressed by spermatogonia but roles in spermatogenesis are unexplored. Using an inducible knockout mouse model, we characterise an essential role for DDX5 in spermatogonial maintenance and show that Ddx5 is indispensable for male fertility. We demonstrate that DDX5 regulates appropriate splicing of key genes necessary for spermatogenesis. Moreover, DDX5 regulates expression of cell cycle genes in undifferentiated spermatogonia post-transcriptionally and is required for cell proliferation and survival. DDX5 can also act as a transcriptional co-activator and we demonstrate that DDX5 interacts with PLZF, a transcription factor required for germline maintenance, to co-regulate select target genes. Combined, our data reveal a critical multifunctional role for DDX5 in regulating gene expression programmes and activity of undifferentiated spermatogonia.

Sustained sperm production is dependent on activity of undifferentiated spermatogonia. Here, the authors demonstrate an essential role for RNA helicase DDX5 in maintenance of spermatogonia in adults through control of gene transcription plus RNA processing and export.

DAZL is a master translational regulator of murine spermatogenesis

Expression of DAZ-like (DAZL) is a hallmark of vertebrate germ cells, and is essential for embryonic germ cell development and differentiation, yet the gametogenic function of DAZL has not been fully characterized and most of its in vivo direct targets remain unknown. We showed that postnatal stage-specific deletion of Dazl in mouse germ cells did not affect female fertility, but caused complete male sterility with gradual loss of spermatogonial stem cells, meiotic arrest and spermatid arrest. Using the genome-wide high-throughput sequencing of RNAs isolated by cross-linking immunoprecipitation and mass spectrometry approach, we found that DAZL bound to a large number of testicular mRNA transcripts (at least 3008) at the 3'-untranslated region and interacted with translation proteins including poly(A) binding protein. In the absence of DAZL, polysome-associated target transcripts, but not their total transcripts, were significantly decreased, resulting in a drastic reduction of an array of spermatogenic proteins and thus developmental arrest. Thus, DAZL is a master translational regulator essential for spermatogenesis.

BCAS2 is involved in alternative mRNA splicing in spermatogonia and the transition to meiosis

Breast cancer amplified sequence 2 (BCAS2) is involved in multiple biological processes, including pre-mRNA splicing. However, the physiological roles of BCAS2 are still largely unclear. Here we report that BCAS2 is specifically enriched in spermatogonia of mouse testes. Conditional disruption of Bcas2 in male germ cells impairs spermatogenesis and leads to male mouse infertility. Although the spermatogonia appear grossly normal, spermatocytes in meiosis prophase I and meiosis events (recombination and synapsis) are rarely observed in the BCAS2-depleted testis. In BCAS2 null testis, 245 genes are altered in alternative splicing forms; at least three spermatogenesis-related genes (Dazl, Ehmt2 and Hmga1) can be verified. In addition, disruption of Bcas2 results in a significant decrease of the full-length form and an increase of the short form (lacking exon 8) of DAZL protein. Altogether, our results suggest that BCAS2 regulates alternative splicing in spermatogonia and the transition to meiosis initiation, and male fertility.

Breast cancer amplified sequence 2 (BCAS2) is involved in pre-mRNA splicing but its physiological role is unclear. Here, the authors find BCAS2 enriched in mice spermatogonia in the testes, and BCAS2 deletion in germ cells alters alternative splicing of spermatogenesis-related genes, causing male infertility.

Over Expression of NANOS3 and DAZL in Human Embryonic Stem Cells

The mechanisms underlying human germ cell development are largely unknown, partly due to the scarcity of primordial germ cells and the inaccessibility of the human germline to genetic analysis. Human embryonic stem cells can differentiate to germ cells in vitro and can be genetically modified to study the genetic requirements for germ cell development. Here, we studied NANOS3 and DAZL, which have critical roles in germ cell development in several species, via their over expression in human embryonic stem cells using global transcriptional analysis, in vitro germ cell differentiation, and in vivo germ cell formation assay by xenotransplantation. We found that NANOS3 over expression prolonged pluripotency and delayed differentiation. In addition, we observed a possible connection of NANOS3 with inhibition of apoptosis. For DAZL, our results suggest a post-transcriptional regulation mechanism in hES cells. In addition, we found that DAZL suppressed the translation of OCT4, and affected the transcription of several genes associated with germ cells, cell cycle arrest, and cell migration. Furthermore, DAZL over expressed cells formed spermatogonia-like colonies in a rare instance upon xenotransplantation. These data can be used to further elucidate the role of NANOS3 and DAZL in germ cell development both in vitro and in vivo.

Roles of RNA-binding Proteins and Post-transcriptional Regulation in Driving Male Germ Cell Development in the Mouse

Tissue development and homeostasis are dependent on highly regulated gene expression programs in which cell-specific combinations of regulatory factors determine which genes are expressed and the post-transcriptional fate of the resulting RNA transcripts. Post-transcriptional regulation of gene expression by RNA-binding proteins has critical roles in tissue development-allowing individual genes to generate multiple RNA and protein products, and the timing, location, and abundance of protein synthesis to be finely controlled. Extensive post-transcriptional regulation occurs during mammalian gametogenesis, including high levels of alternative mRNA expression, stage-specific expression of mRNA variants, broad translational repression, and stage-specific activation of mRNA translation. In this chapter, an overview of the roles of RNA-binding proteins and the importance of post-transcriptional regulation in male germ cell development in the mouse is presented.

Snail Is a Direct Target of Hypoxia-inducible Factor 1α (HIF1α) in Hypoxia-induced Endothelial to Mesenchymal Transition of Human Coronary Endothelial Cells

Endothelial to mesenchymal transition (EndMT) was originally described in heart development where the endocardial endothelial cells that line the atrioventricular canal undergo an EndMT to form the endocardial mesenchymal cushion that later gives rise to the septum and mitral and tricuspid valves. In the postnatal heart specifically, endothelial cells that originate from the endocardium maintain increased susceptibility to undergo EndMT as remnants from their embryonic origin. Such EndMT involving adult coronary endothelial cells contributes to microvascular rarefaction and subsequent chronification of hypoxia in the injured heart, ultimately leading to cardiac fibrosis. Although in most endothelial beds hypoxia induces tip cell formation and sprouting angiogenesis, here we demonstrate that hypoxia is a stimulus for human coronary endothelial cells to undergo phenotypic changes reminiscent of EndMT via a mechanism involving hypoxia-inducible factor 1α-induced activation of the EndMT master regulatory transcription factor SNAIL. Our study adds further evidence for the unique susceptibility of endocardium-derived endothelial cells to undergo EndMT and provides novel insights into how hypoxia contributes to progression of cardiac fibrosis. Additional studies may be required to discriminate between distinct sprouting angiogenesis and EndMT responses of different endothelial cells populations.

Characterisation of the deleted in azoospermia like (Dazl)-green fluorescent protein mouse model generated by a two-step embryonic stem cell-based strategy to identify pluripotent and germ cells

The deleted in azoospermia like (Dazl) gene is preferentially expressed in germ cells; however, recent studies indicate that it may have pluripotency-related functions. We generated Dazl-green fluorescent protein (GFP) transgenic mice and assayed the ability of Dazl-driven GFP to mark preimplantation embryo development, fetal, neonatal and adult tissues, and in vitro differentiation from embryonic stem cells (ESCs) to embryoid bodies (EBs) and to primordial germ cell (PGC)-like cells. The Dazl-GFP mice were generated by a two-step ESC-based strategy, which enabled primary and secondary screening of stably transfected clones before embryo injection. During preimplantation embryo stages, GFP was detected from the zygote to blastocyst stage. At Embryonic Day (E) 12.5, GFP was expressed in gonadal ridges and in neonatal gonads of both sexes. In adult mice, GFP expression was found during spermatogenesis from spermatogonia to elongating spermatids and in the cytoplasm of oocytes. However, GFP mRNA was also detected in other tissues harbouring multipotent cells, such as the intestine and bone marrow. Fluorescence was maintained along in vitro Dazl-GFP ESC differentiation to EBs, and in PGC-like cells. In addition to its largely known function in germ cell development, Dazl could have an additional role in pluripotency, supporting these transgenic mice as a valuable tool for the prospective identification of stem cells from several tissues.

RNA-binding proteins in pluripotency, differentiation, and reprogramming

Embryonic stem cell maintenance, differentiation, and somatic cell reprogramming require the interplay of multiple pluripotency factors, epigenetic remodelers, and extracellular signaling pathways. RNA-binding proteins (RBPs) are involved in a wide range of regulatory pathways, from RNA metabolism to epigenetic modifications. In recent years we have witnessed more and more studies on the discovery of new RBPs and the assessment of their functions in a variety of biological systems, including stem cells. We review the current studies on RBPs and focus on those that have functional implications in pluripotency, differentiation, and/or reprogramming in both the human and mouse systems.

Stage-dependent DAZL localization in stallion germ cells

Deleted in azoospermia-like (DAZL) is used as a germ cell marker in several species, including mice, rats, pigs, rhesus monkeys, bulls, and humans. Our objectives with this study were to investigate DAZL expression in stallion germ cells by using immunofluorescence, immunocytochemistry, and western blotting, and to determine the effects of reproductive stage and breeding season on the DAZL-positive cell population in seminiferous tubule cross sections. Testes were obtained during routine castration procedures at a large animal clinic and routine field service castration. The reproductive stage of the stallions was classified as pre-pubertal (<1 yr), pubertal (1-1.5 yr), post-pubertal (2-3 yr), or adult (4-8 yr). Using immunofluorescent staining, we showed that DAZL is localized to the cytoplasm of some, but not all, spermatogonia in pre-pubertal and pubertal horses. In the post-pubertal and adult testes, DAZL immunostaining was observed in spermatogonia proximal to the basement membrane of seminiferous tubules; however, few spermatogonia attached to the basement membrane were not immunolabeled. DAZL immunostaining was also observed in primary spermatocytes, but not in secondary spermatocytes, spermatids, or spermatozoa. DAZL protein was not detected in Leydig, Sertoli, or myoid cells of the testes at any reproductive stage. The immunocytochemistry analysis showed that DAZL immunolabeling was also localized to the cytoplasm of isolated germ cells such as spermatogonia or primary spermatocytes. We conclude that DAZL can be used as a marker of pre-meiotic germ cells in stallions.

Conservation and function of Dazl in promoting the meiosis of goat male germline stem cells

Dazl (deleted in azoospermia-like) is a conserved gene in mammalian meiosis, which encodes RNA binding protein required for spermatocyte meiosis. Up to date, the expression and function of Dazl in the goat testis are unknown. The objectives of this study were to investigate the expression pattern of Dazl in dairy goat testis and their function in male germline stem cells (mGSCs). The results first revealed that the expression level of Dazl in adult testes was significantly higher than younger and immature goats, and azoospermia and male intersex testis. The dairy goat Dazl is highly conserved analysed by several online and bioinformatics software, respectively. Over-expression of Dazl promoted the expression of meiosis-related genes in dairy goat mGSCs. The expression of Stra8 was up-regulated by over-expression of Dazl analysed by Luciferase reporter assay. Taken together, results suggest the Dazl plays an important role in dairy goat spermatogenesis and that over-expression of Dazl may promote Stra8 expression in dairy goat mGSCs.