Autonomous transition into meiosis of mouse fetal germ cells in vitro and its inhibition by gp130-mediated signaling

Visualizing histone H4K20me1 in knock-in mice expressing the mCherry-tagged modification-specific intracellular antibody

During development and differentiation, histone modifications dynamically change locally and globally, associated with transcriptional regulation, DNA replication and repair, and chromosome condensation. The level of histone H4 Lys20 monomethylation (H4K20me1) increases during the G2 to M phases of the cell cycle and is enriched in facultative heterochromatin, such as inactive X chromosomes in cycling cells. To track the dynamic changes of H4K20me1 in living cells, we have developed a genetically encoded modification-specific intracellular antibody (mintbody) probe that specifically binds to the modification. Here, we report the generation of knock-in mice in which the coding sequence of the mCherry-tagged version of the H4K20me1-mintbody is inserted into the Rosa26 locus. The knock-in mice, which ubiquitously expressed the H4K20me1-mintbody, developed normally and were fertile, indicating that the expression of the probe does not disturb the cell growth, development, or differentiation. Various tissues isolated from the knock-in mice exhibited nuclear fluorescence without the need for fixation. The H4K20me1-mintbody was enriched in inactive X chromosomes in developing embryos and in XY bodies during spermatogenesis. The knock-in mice will be useful for the histochemical analysis of H4K20me1 in any cell types.

Mechanisms of meiosis initiation and meiotic prophase progression during spermatogenesis

Meiosis is a critical step for spermatogenesis and oogenesis. Meiosis commences with pre-meiotic S phase that is subsequently followed by meiotic prophase. The meiotic prophase is characterized by the meiosis-specific chromosomal events such as chromosome recombination and homolog synapsis. Meiosis initiator (MEIOSIN) and stimulated by retinoic acid gene 8 (STRA8) initiate meiosis by activating the meiotic genes by installing the meiotic prophase program at pre-meiotic S phase. This review highlights the mechanisms of meiotic initiation and meiotic prophase progression from the point of the gene expression program and its relevance to infertility. Furthermore, upstream pathways that regulate meiotic initiation will be discussed in the context of spermatogenic development, indicating the sexual differences in the mode of meiotic entry.

Isolation and Purification of Viable PGCs from Mouse Embryos

In all organisms with sexual reproduction, sperm and oocytes derive from embryonic precursors termed primordial germ cells (PGCs) which pass on genetic information to subsequent generations. Studies aimed to unravel PGC development at molecular level in mammals can be traced at the early 1980s and were hampered by the difficulty in obtaining both sufficient quantities and purity of PGCs. For many laboratories, the isolation and purification methods of PGCs at different stages from embryos are the most shortcut and affordable tool to study many aspects of their development at cellular and molecular levels. In the present chapter, I focus on immunomagnetic cell sorting (MACS) and fluorescence-activated cell sorting (FACS) methods used in my laboratory for the purification of mouse PGCs from 10.5 to 12.5 dpc embryos before their differentiation in oogonia/oocytes in female and prospermatogonia in male.

Loss of Dead end1 induces testicular teratomas from primordial germ cells that failed to undergo sexual differentiation in embryonic testes

Spontaneous testicular teratomas (STTs) are tumours comprising a diverse array of cell and tissue types, which are derived from pluripotent stem-like cells called embryonal carcinoma cells (ECCs). Although mouse ECCs originate from primordial germ cells (PGCs) in embryonic testes, the molecular basis underlying ECC development remains unclear. This study shows that the conditional deletion of mouse Dead end1 (Dnd1) from migrating PGCs leads to STT development. In Dnd1-conditional knockout (Dnd1-cKO) embryos, PGCs colonise the embryonic testes but fail to undergo sexual differentiation; subsequently, ECCs develop from a portion of the PGCs. Transcriptomic analyses reveal that PGCs not only fail to undergo sexual differentiation but are also prone to transformation into ECCs by upregulating the expression of marker genes for primed pluripotency in the testes of Dnd1-cKO embryos. Thus, our results clarify the role of Dnd1 in developing STTs and developmental process of ECC from PGC, providing novel insights into pathogenic mechanisms of STTs.

Determination and analysis of sex ratios in heme oxygenase 1 gene targeted mouse embryos

Heme oxygenase 1 or Hmox1 enzyme is involved in catalyzing the first and rate-limiting step in heme breakdown reactions. Many studies have reported a partial lethality of Hmox1 knockout mice obtained from heterozygous breeding pairs. Similar results were obtained in our transgenic mice colony and a sex specific bias was observed in the favor of males in the adult mice. Hmox1 independent factors which could have caused this bias were initially analyzed and it was found that those factors were not a reason behind this anomaly. Certain studies involving gene knockout hinted toward a prenatal or neonatal lethality of female knockout mice embryos or pups, respectively. In order to check if this bias was occurring in embryonic stages, that is, either if mutant female embryos were dying or if heterozygous mothers were not carrying embryos to term, we analyzed the sex-ratios in mid- and late-gestational ages (9.5-13.5 dpc and 14.5-18.5 dpc, respectively). Our results did not indicate any significant difference in the sex ratios in embryonic stages; hence, it was concluded that females are not dying in embryonic stages. It can be speculated that these deaths were probably occurring at neonatal age. More studies are required to confirm that the lack of Hmox1 gene products is the sole reason for this female lethality.

The Beginning of Meiosis in Mammalian Female Germ Cells: A Never-Ending Story of Intrinsic and Extrinsic Factors

Meiosis is the unique division of germ cells resulting in the recombination of the maternal and paternal genomes and the production of haploid gametes. In mammals, it begins during the fetal life in females and during puberty in males. In both cases, entering meiosis requires a timely switch from the mitotic to the meiotic cell cycle and the transition from a potential pluripotent status to meiotic differentiation. Revealing the molecular mechanisms underlying these interrelated processes represents the essence in understanding the beginning of meiosis. Meiosis facilitates diversity across individuals and acts as a fundamental driver of evolution. Major differences between sexes and among species complicate the understanding of how meiosis begins. Basic meiotic research is further hindered by a current lack of meiotic cell lines. This has been recently partly overcome with the use of primordial-germ-cell-like cells (PGCLCs) generated from pluripotent stem cells. Much of what we know about this process depends on data from model organisms, namely, the mouse; in mice, the process, however, appears to differ in many aspects from that in humans. Identifying the mechanisms and molecules controlling germ cells to enter meiosis has represented and still represents a major challenge for reproductive medicine. In fact, the proper execution of meiosis is essential for fertility, for maintaining the integrity of the genome, and for ensuring the normal development of the offspring. The main clinical consequences of meiotic defects are infertility and, probably, increased susceptibility to some types of germ-cell tumors. In the present work, we report and discuss data mainly concerning the beginning of meiosis in mammalian female germ cells, referring to such process in males only when pertinent. After a brief account of this process in mice and humans and an historical chronicle of the major hypotheses and progress in this topic, the most recent results are reviewed and discussed.

Mechanism of initiation of meiosis in mouse germ cells

Meiosis is critical for germ cell development in multicellular organisms. Initiation of meiosis coincides with pre-meiotic S phase, which is followed by meiotic prophase, a prolonged G2 phase that ensures numerous meiosis-specific chromosome events. Meiotic prophase is accompanied by robust alterations of gene expression. In mouse germ cells, MEIOSIN and STRA8 direct cell cycle switch from mitosis to meiosis. MEIOSIN and STRA8 coordinate meiotic initiation with cell cycle, by activating the meiotic genes to have meiotic prophase program installed at S phase. This review mainly focuses on the mechanism of meiotic initiation in mouse germ cells from the viewpoint of the transcription of meiotic genes. Furthermore, signaling pathways that regulate meiotic initiation will be discussed in the context of germ cell development, pointing out the sexual differences in the mode of meiotic initiation.

Instructing Mouse Germ Cells to Adopt a Female Fate

BACKGROUND

Germ cells are critical for the survival of our species. They are the only cells that undergo meiosis - the reductive form of cell division that is necessary for genetic reassortment of chromosomes and production of the haploid gametes, the sperm and eggs. Remarkably, the initial female/male fate decision in fetal germ cells does not depend on whether they are chromosomally XX or XY; rather, initial sexual fate is imposed by influences from the surrounding tissue. In mammals, the female germline is particularly precious: despite recent suggestions that germline stem cells exist in the ovary, it is still generally accepted that the ovarian reserve is finite, and its size is dependant on germ cells of the fetal ovary initiating meiosis in a timely manner.

SUMMARY

Prior to 2006, evidence suggested that gonadal germ cells initiate meiotic prophase I by default, but more recent data support a key role for the signalling molecule retinoic acid (RA) in instructing female germ cell fate. Newer findings also support a key meiosis-inducing role for another signalling molecule, bone morphogenic protein (BMP). Nonetheless, many questions remain.

KEY MESSAGES

Here, we review knowledge thus far regarding extrinsic and intrinsic determinants of a female germ cell fate, focusing on the mouse model.

Tesmin, Metallothionein-Like 5, is Required for Spermatogenesis in Mice†

In mammals, more than 2000 genes are specifically or abundantly expressed in testis, but gene knockout studies revealed several are not individually essential for male fertility. Tesmin (Metallothionein-like 5; Mtl5) was originally reported as a testis-specific transcript that encodes a member of the cysteine-rich motif containing metallothionein family. Later studies showed that Tesmin has two splicing variants and both are specifically expressed in male and female germ cells. Herein, we clarified that the long (Tesmin-L) and short (Tesmin-S) transcript forms start expressing from spermatogonia and the spermatocyte stage, respectively, in testis. Furthermore, while Tesmin-deficient female mice are fertile, male mice are infertile due to arrested spermatogenesis at the pachytene stage. We were able to rescue the infertility with a Tesmin-L transgene, where we concluded that TESMIN-L is critical for meiotic completion in spermatogenesis and indispensable for male fertility.

NANOS2 suppresses the cell cycle by repressing mTORC1 activators in embryonic male germ cells

During murine germ cell development, male germ cells enter the mitotically arrested G0 stage, which is an initial step of sexually dimorphic differentiation. The male-specific RNA-binding protein NANOS2 has a key role in suppressing the cell cycle in germ cells. However, the detailed mechanism of how NANOS2 regulates the cell cycle remains unclear. Using single-cell RNA sequencing (scRNA-seq), we extracted the cell cycle state of each germ cell in wild-type and Nanos2-KO testes and revealed that Nanos2 expression starts in mitotic cells and induces mitotic arrest. We identified Rheb, a regulator of mTORC1, and Ptma as possible targets of NANOS2. We propose that repression of the cell cycle is a primary function of NANOS2 and that it is mediated via the suppression of mTORC1 activity through the repression of Rheb in a post-transcriptional manner.

WNT signaling in pre-granulosa cells is required for ovarian folliculogenesis and female fertility

In mammalian ovaries, immature oocytes are reserved in primordial follicles until their activation for potential ovulation. Precise control of primordial follicle activation (PFA) is essential for reproduction, but how this is achieved is unclear. Here, we show that canonical wingless-type MMTV integration site family (WNT) signaling is pivotal for pre-granulosa cell (pre-GC) activation during PFA. We identified several WNT ligands expressed in pre-GCs that act in an autocrine manner. Inhibition of WNT secretion from pre-GCs/GCs by conditional knockout (cKO) of the wntless (Wls) gene led to female infertility. In Wls cKO mice, GC layer thickness was greatly reduced in growing follicles, which resulted in impaired oocyte growth with both an abnormal, sustained nuclear localization of forkhead box O3 (FOXO3) and reduced phosphorylation of ribosomal protein S6 (RPS6). Constitutive stabilization of β-catenin (CTNNB1) in pre-GCs/GCs induced morphological changes of pre-GCs from a squamous into a cuboidal form, though it did not influence oocyte activation. Our results reveal that canonical WNT signaling plays a permissive role in the transition of pre-GCs to GCs, which is an essential step to support oocyte growth.

Intercellular bridges coordinate the transition from pluripotency to meiosis in mouse fetal oocytes

Meiosis is critical to generating oocytes and ensuring female fertility; however, the mechanisms regulating the switch from mitotic primordial germ cells to meiotic germ cells are poorly understood. Here, we implicate intercellular bridges (ICBs) in this state transition. We used three-dimensional in toto imaging to map meiotic initiation in the mouse fetal ovary and revealed a radial geometry of this transition that precedes the established anterior-posterior wave. Our studies reveal that appropriate timing of meiotic entry across the ovary and coordination of mitotic-meiotic transition within a cyst depend on the ICB component Tex14, which we show is required for functional cytoplasmic sharing. We find that Tex14 mutants more rapidly attenuate the pluripotency transcript Dppa3 upon meiotic initiation, and Dppa3 mutants undergo premature meiosis similar to Tex14 Together, these results lead to a model that ICBs coordinate and buffer the transition from pluripotency to meiosis through dilution of regulatory factors.

Identification of regulatory elements required for Stra8 expression in fetal ovarian germ cells of the mouse

In mice, the entry of germ cells into meiosis crucially depends on the expression of stimulated by retinoic acid gene 8 (Stra8). Stra8 is expressed specifically in pre-meiotic germ cells of females and males, at fetal and postnatal stages, respectively, but the mechanistic details of its spatiotemporal regulation are yet to be defined. In particular, there has been considerable debate regarding whether retinoic acid is required, in vivo, to initiate Stra8 expression in the mouse fetal ovary. We show that the distinctive anterior-to-posterior pattern of Stra8 initiation, characteristic of germ cells in the fetal ovary, is faithfully recapitulated when 2.9 kb of the Stra8 promoter is used to drive eGFP expression. Using in vitro transfection assays of cutdown and mutant constructs, we identified two functional retinoic acid responsive elements (RAREs) within this 2.9 kb regulatory element. We also show that the transcription factor DMRT1 enhances Stra8 expression, but only in the presence of RA and the most proximal RARE. Finally, we used CRISPR/Cas9-mediated targeted mutation studies to demonstrate that both RAREs are required for optimal Stra8 expression levels in vivo.

Dissecting the initiation of female meiosis in the mouse at single-cell resolution

Meiosis is one of the most finely orchestrated events during gametogenesis with distinct developmental patterns in males and females. However, the molecular mechanisms involved in this process remain not well known. Here, we report detailed transcriptome analyses of cell populations present in the mouse female gonadal ridges (E11.5) and the embryonic ovaries from E12.5 to E14.5 using single-cell RNA sequencing (scRNA seq). These periods correspond with the initiation and progression of meiosis throughout the first stage of prophase I. We identified 13 transcriptionally distinct cell populations and 7 transcriptionally distinct germ cell subclusters that correspond to mitotic (3 clusters) and meiotic (4 clusters) germ cells. By analysing cluster-specific gene expression profiles, we found four cell clusters correspond to different cell stages en route to meiosis and characterized their detailed transcriptome dynamics. Our scRNA seq analysis here represents a new important resource for deciphering the molecular pathways driving female meiosis initiation.

Expression and possible roles of extracellular signal-related kinases 1-2 (ERK1-2) in mouse primordial germ cell development

In the present work, we described the expression and activity of extracellular signal-related kinases 1-2 (ERK1-2) in mouse primordial germ cells (PGCs) from 8.5–14.5 days post coitum (dpc) and investigated whether these kinases play a role in regulating the various processes of PGC development. Using immunofluorescence and immunoblotting to detect the active phosphorylated form of ERK1-2 (p-ERK1-2), we found that the kinases were present in most proliferating 8.5–10.5 dpc PGCs, low in 11.5 dpc PGCs, and progressively increasing between 12.5–14.5 dpc both in female and male PGCs. In vitro culture experiments showed that inhibiting activation of ERK1-2 with the MEK-specific inhibitor U0126 significantly reduced the growth of 8.5 dpc PGCs in culture but had little effect on 11.5–12.5 dpc PGCs. Moreover, we found that the inhibitor did not affect the adhesion of 11.5 dpc PGCs, but it significantly reduced their motility features onto a cell monolayer. Further, while the ability of female PGCs to begin meiosis was not significantly affected by U0126, their progression through meiotic prophase I was slowed down. Notably, the activity of ERK1-2 was necessary for maintaining the correct expression of oocyte-specific genes crucial for germ cells survival and the formation of primordial follicles.

Maternal dietary imbalance between omega-6 and omega-3 fatty acids triggers the offspring's overeating in mice

The increasing prevalence of obesity and its effects on our society warrant intensifying basic animal research for understanding why habitual intake of highly palatable foods has increased due to recent global environmental changes. Here, we report that pregnant mice that consume a diet high in omega-6 (n-6) polyunsaturated fatty acids (PUFAs) and low in omega-3 (n-3) PUFAs (an n-6^high/n-3^low diet), whose n-6/n-3 ratio is approximately 120, induces hedonic consumption in the offspring by upregulating the midbrain dopaminergic system. We found that exposure to the n-6^high/n-3^low diet specifically increases the consumption of palatable foods via increased mesolimbic dopamine release. In addition, neurodevelopmental analyses revealed that this induced hedonic consumption is programmed during embryogenesis, as dopaminergic neurogenesis is increased during in utero access to the n-6^high/n-3^low diet. Our findings reveal that maternal consumption of PUFAs can have long-lasting effects on the offspring’s pattern for consuming highly palatable foods.

Sakayori et al. show that feeding pregnant mice with a diet high in omega-6 polyunsaturated fatty acids (PUFAs) and low in omega-3 PUFAs triggers hedonic consumption in the offspring by increasing its dopaminergic neurogenesis. This study suggests that maternal consumption of diets with unbalanced PUFAs contributes to the offspring’s overconsumption of foods.

Retinoic acid-stimulated ERK1/2 pathway regulates meiotic initiation in cultured fetal germ cells

In murine fetal germ cells, retinoic acid (RA) is an extrinsic cue for meiotic initiation that stimulates transcriptional activation of the Stimulated by retinoic acid gene 8 (Stra8), which is required for entry of germ cells into meiotic prophase I. Canonically, the biological activities of RA are mediated by nuclear RA receptors. Recent studies in somatic cells found that RA noncanonically stimulates intracellular signal transduction pathways to regulate multiple cellular processes. In this study, using a germ cell culture system, we investigated (1) whether RA treatment activates any mitogen-activated protein kinase (MAPK) pathways in fetal germ cells at the time of sex differentiation, and (2) if this is the case, whether the corresponding RA-stimulated signaling pathway regulates Stra8 expression in fetal germ cells and their entry into meiosis. When XX germ cells at embryonic day (E) 12.5 were cultured with RA, the extracellular-signal-regulated kinase (ERK) 1/2 pathway was predominantly activated. MEK1/2 inhibitor (U0126) treatment suppressed the mRNA expressions of RA-induced Stra8 and meiotic marker genes (Rec8, Spo11, Dmc1, and Sycp3) in both XX and XY fetal germ cells. Furthermore, U0126 treatment dramatically reduced STRA8 protein levels and numbers of meiotic cells among cultured XX and XY fetal germ cells even in the presence of RA. Taken together, our results suggest the novel concept that the RA functions by stimulating the ERK1/2 pathway and that this activity is critical for Stra8 expression and meiotic progression in fetal germ cells.

Blastocyst transfer in mice alters the placental transcriptome and growth

Assisted reproduction technologies (ART) are becoming increasingly common. Therefore, how these procedures influence gene regulation and feto-placental development are important to explore. Here, we assess the effects of blastocyst transfer on mouse placental growth and transcriptome. C57Bl/6 blastocysts were transferred into uteri of B6D2F1 pseudopregnant females and dissected at embryonic day 10.5 for analysis. Compared to non-transferred controls, placentas from transferred conceptuses weighed less even though the embryos were larger on average. This suggested a compensatory increase in placental efficiency. RNA-sequencing of whole male placentas revealed 543 differentially expressed genes (DEGs) after blastocyst transfer: 188 and 355 genes were down-regulated and up-regulated, respectively. DEGs were independently validated in male and female placentas. Bioinformatic analyses revealed that DEGs represented expression in all major placental cell types and included genes that are critical for placenta development and/or function. Furthermore, the direction of transcriptional change in response to blastocyst transfer implied an adaptive response to improve placental function to maintain fetal growth. Our analysis revealed that CpG methylation at regulatory regions of two DEGs was unchanged in female transferred placentas and that DEGs had fewer gene-associated CpG islands (within ~20 kb region) compared to the larger genome. These data suggested that altered methylation at proximal promoter regions might not lead to transcriptional disruption in transferred placentas. Genomic clustering of some DEGs warrants further investigation of long-range, cis-acting epigenetic mechanisms including histone modifications together with DNA methylation. We conclude that embryo transfer, a protocol required for ART, significantly impacts the placental transcriptome and growth.

Function of leukaemia inhibitory factor in spermatogenesis of a teleost fish, the medaka Oryzias latipes

In response to gonadotropins and androgens, testicular cells produce various molecules that control proper proliferation and differentiation of spermatogenic cells through their paracrine and autocrine actions. However, molecules functioning downstream of the hormonal stimulation are poorly understood. Leukaemia inhibitory factor (Lif) is known to maintain the pluripotency of stem cells including embryonic stem cells and primordial germ cells at least in vitro, but its actual roles in vivo remain to be elucidated. To clarify the function of Lif in teleost (medaka) testes, we examined the effects of Lif on spermatogenesis in a newly established cell culture system using a cell line (named Mtp1) derived from medaka testicular somatic cells as feeder cells. We found that addition of baculovirus-produced recombinant medaka Lif to the culture medium or co-culture with Lif-overexpressing Mtp1 cells increased the number of spermatogonia. In situ hybridization and immunohistochemical analyses of the medaka testes showed that mRNAs and proteins of Lif are expressed in spermatogonia and the surrounding Sertoli cells, with higher expression levels in type A (undifferentiated) spermatogonia than in type B (differentiated) spermatogonia. Our findings suggest that Lif regulates spermatogonial cell proliferation in the medaka.

Usp26 mutation in mice leads to defective spermatogenesis depending on genetic background

Spermatogenesis is a reproductive system process that produces sperm. Ubiquitin specific peptidase 26 (USP26) is an X chromosome-linked deubiquitinase that is specifically expressed in the testes. It has long been controversial whether USP26 variants are associated with human male infertility. Thus, in the present study, we introduced a mutation into the Usp26 gene in mice and found that Usp26 mutant males backcrossed to a DBA/2 background, but not a C57BL/6 background, were sterile or subfertile and had atrophic testes. These findings indicate that the effects of the Usp26 mutation on male reproductive capacity were influenced by genetic background. Sperm in the cauda epididymis of Usp26 mutant mice backcrossed to a DBA/2 background were decreased in number and showed a malformed head morphology compared to those of wild-type mice. Additionally, histological examinations of the testes revealed that the number of round and elongated spermatids were dramatically reduced in Usp26 mutant mice. The mutant mice exhibited unsynapsed chromosomes in pachynema and defective chiasma formation in diplonema, which presumably resulted in apoptosis of metaphase spermatocytes and subsequent decrease of spermatids. Taken together, these results indicate that the deficiencies in fertility and spermatogenesis caused by mutation of Usp26 were dependent on genetic background.

Gene Expression in the Supporting Cells at the Onset of Meiosis in Rabbit Gonads

Subsequent to somatic gonadal sexual differentiation, germ cells enter meiosis or mitotic arrest in the ovary or testis, respectively. Among mice, these processes occur almost synchronically in fetal gonads and depend, among other factors, on the levels of retinoic acid (RA). In contrast to those in mice, rabbit germ cells enter meiosis or mitotic arrest after birth and coexist with proliferating germ cells. Here, we studied the somatic cell context in which germ cells enter meiosis or mitotic arrest in the rabbit. Using confocal immunofluorescence and real-time PCR, we studied the expression profiles of ALDH1A1 and ALDH1A2 and, comprising 2 genes required for RA synthesis, 2 meiosis markers STRA8 and SYCP3 as well as 2 genes involved in meiosis inhibition, CYP26B1 and NANOS2. We found that although both meiosis and mitotic arrest initiate after birth, these 2 processes are regulated in a way similar to the human fetal gonad. Current results reinforce the value of the neonatal rabbit gonad as an alternative experimental model for analyzing the direct effect of environmental factors during critical stages of germ cell establishment.

Spiny rat SRY lacks a long Q-rich domain and is not stable in transgenic mice

BACKGROUND

Although Tokudaia muenninki has multiple extra copies of the Sry gene on the Y chromosome, loss of function of these sequences is indicated. To examine the Sry gene function for sex determining in T. muenninki, we screened a BAC library and identified a clone (SRY26) containing complete SRY coding and promoter sequences.

RESULTS

SRY26 showed high identity to mouse and rat SRY. In an in vitro reporter gene assay, SRY26 was unable to activate testis-specific enhancer of Sox9. Four lines of BAC transgenic mice carrying SRY26 were generated. Although the embryonic gonads of XX transgenic mice displayed sufficient expression levels of SRY26 mRNA, these mice exhibited normal female phenotypes in the external and internal genitalia, and up-regulation of Sox9 was not observed. Expression of the SRY26 protein was confirmed in primate-derived COS7 cells transfected with a SRY26 expression vector. However, the SRY26 protein was not expressed in the gonads of BAC transgenic mice.

CONCLUSIONS

Overall, these results support a previous study demonstrated a long Q-rich domain plays essential roles in protein stabilization in mice. Therefore, the original aim of this study, to examine the function of the Sry gene of this species, was not achieved by creating TG mice.

Sexually dimorphic germ cell identity in mammals

Germ cells are the stem cells of the species. Thus, it is critical that we have a good understanding of how they are specified, how the somatic cells instruct and support them, how they commit to one or other sex, and how they ultimately develop into functional gametes. Here, we focus on specifics of how sexual fate is determined during fetal life. Because the majority of relevant experimental work has been done using the mouse model, we focus on that species. We review evidence regarding the identity of instructive signals from the somatic cells, and the molecular responses that occur in germ cells in response to those extrinsic signals. In this way we aim to clarify progress to date regarding the mechanisms underlying the mitotic to meiosis switch in germ cells of the fetal ovary, and those involved in adopting and securing male fate in germ cells of the fetal testis.

Dual functions for the ssDNA-binding protein RPA in meiotic recombination

Meiotic recombination permits exchange of genetic material between homologous chromosomes. The replication protein A (RPA) complex, the predominant ssDNA-binding complex, is required for nearly all aspects of DNA metabolism, but its role in mammalian meiotic recombination remains unknown due to the embryonic lethality of RPA mutant mice. RPA is a heterotrimer of RPA1, RPA2, and RPA3. We find that loss of RPA1, the largest subunit, leads to disappearance of RPA2 and RPA3, resulting in the absence of the RPA complex. Using an inducible germline-specific inactivation strategy, we find that loss of RPA completely abrogates loading of RAD51/DMC1 recombinases to programmed meiotic DNA double strand breaks, thus blocking strand invasion required for chromosome pairing and synapsis. Surprisingly, loading of MEIOB, SPATA22, and ATR to DNA double strand breaks is RPA-independent and does not promote RAD51/DMC1 recruitment in the absence of RPA. Finally, inactivation of RPA reduces crossover formation. Our results demonstrate that RPA plays two distinct roles in meiotic recombination: an essential role in recombinase recruitment at early stages and an important role in promoting crossover formation at later stages.

The non-canonical SMC protein SmcHD1 antagonises TAD formation and compartmentalisation on the inactive X chromosome

The inactive X chromosome (Xi) in female mammals adopts an atypical higher-order chromatin structure, manifested as a global loss of local topologically associated domains (TADs), A/B compartments and formation of two mega-domains. Here we demonstrate that the non-canonical SMC family protein, SmcHD1, which is important for gene silencing on Xi, contributes to this unique chromosome architecture. Specifically, allelic mapping of the transcriptome and epigenome in SmcHD1 mutant cells reveals the appearance of sub-megabase domains defined by gene activation, CpG hypermethylation and depletion of Polycomb-mediated H3K27me3. These domains, which correlate with sites of SmcHD1 enrichment on Xi in wild-type cells, additionally adopt features of active X chromosome higher-order chromosome architecture, including A/B compartments and partial restoration of TAD boundaries. Xi chromosome architecture changes also occurred following SmcHD1 knockout in a somatic cell model, but in this case, independent of Xi gene derepression. We conclude that SmcHD1 is a key factor in defining the unique chromosome architecture of Xi.

The inactive X chromosome (Xi) has an atypical structure, with global loss of TADs, A/B compartments and formation of mega-domains. Here the authors show that the non-canonical SMC family protein, SmcHD1, important for developmental gene silencing on Xi, antagonises TAD formation and compartmentalization on the Xi in a transcription independent way.

Chimeric analysis with newly established EGFP/DsRed2-tagged ES cells identify HYDIN as essential for spermiogenesis in mice

The CRISPR/Cas9 system can efficiently introduce biallelic mutations in ES cells (ESCs), and its application with fluorescently-tagged ESCs enables phenotype analysis in chimeric mice. We have utilized ESCs that express EGFP in the cytosol and acrosome [EGR-G101 129S2 × (CAG/Acr-EGFP) B6] in previous studies; however, the EGFP signal in the sperm cytosol is weak and the signal in the acrosome is lost after the acrosome reaction, precluding analysis between wild type and ESC derived spermatozoa. In this study, we established an ESC line from RBGS (Red Body Green Sperm) transgenic mice [B6D2-Tg (CAG/Su9-DsRed2, Acr3-EGFP) RBGS002Osb] whose spermatozoa exhibit green fluorescence in the acrosome and red fluorescence in the mitochondria within the flagellar midpiece that is retained after the acrosome reaction. We utilized these new ESCs to analyze HYDIN, which is reported to function in sperm motility in humans. Analysis of Hydin-disrupted spermatozoa in mice is difficult as Hydin-mutant mice (hy3) die within 3 weeks, before sexual maturation, due to hydrocephaly. To circumvent the early lethality of the whole-body knockout, we disrupted Hydin in RBGS-ESCs and generated chimeric mice, which survived into sexual maturity. Hydin-disrupted spermatozoa obtained from the chimeric mice possessed short tails and were immotile. When we injected Hydin-disrupted spermatozoa into oocytes, heterozygous pups were obtained, which suggests that the genome of Hydin-disrupted spermatozoa can produce viable pups. Consequently, RBGS-ESCs can be a useful tool for screening and analysis of male-fertility related genes in chimeric mice.

Exit from Naive Pluripotency Induces a Transient X Chromosome Inactivation-like State in Males

A hallmark of naive pluripotency is the presence of two active X chromosomes in females. It is not clear whether prevention of X chromosome inactivation (XCI) is mediated by gene networks that preserve the naive state. Here, we show that robust naive pluripotent stem cell (nPSC) self-renewal represses expression of Xist, the master regulator of XCI. We found that nPSCs accumulate Xist on the male X chromosome and on both female X chromosomes as they become NANOG negative at the onset of differentiation. This is accompanied by the appearance of a repressive chromatin signature and partial X-linked gene silencing, suggesting a transient and rapid XCI-like state in male nPSCs. In the embryo, Xist is transiently expressed in males and in females from both X chromosomes at the onset of naive epiblast differentiation. In conclusion, we propose that XCI initiation is gender independent and triggered by destabilization of naive identity, suggesting that gender-specific mechanisms follow, rather than precede, XCI initiation.

MicroRNA dynamics at the onset of primordial germ and somatic cell sex differentiation during mouse embryonic gonad development

In mammals, commitment and specification of germ cell lines involves complex programs that include sex differentiation, control of proliferation, and meiotic initiation. Regulation of these processes is genetically controlled by fine-tuned mechanisms of gene regulation in which microRNAs (miRNAs) are involved. We have characterized, by small-RNA-seq and bioinformatics analyses, the miRNA expression patterns of male and female mouse primordial germ cells (PGCs) and gonadal somatic cells at embryonic stages E11.5, E12.5, and E13.5. Differential expression analyses revealed differences in the regulation of key miRNA clusters such as miR-199-214, miR-182-183-96, and miR-34c-5p, whose targets have defined roles during gonadal sexual determination in both germ and somatic cells. Extensive analyses of miRNA sequences revealed an increase in noncanonical isoforms on PGCs at E12.5 and dramatic changes of 3' isomiR expression and 3' nontemplate nucleotide additions in female PGCs at E13.5. Additionally, RT-qPCR analyses of genes encoding proteins involved in miRNA biogenesis and 3' nucleotide addition uncovered sexually and developmentally specific expression, characterized by the decay of Drosha, Dgcr8, and Xpo5 expression along gonadal development. These results demonstrate that miRNAs, their isomiRs, and miRNA machinery are differentially regulated and participate actively in gonadal sexual differentiation in both PGCs and gonadal somatic cells.

SOX30 is required for male fertility in mice

Male infertility is a major and growing problem and, in most cases, the specific root cause is unknown. Here we show that the transcription factor SOX30 plays a critical role in mouse spermatogenesis. Sox30-null mice are healthy and females are fertile, but males are sterile. In the absence of Sox30 meiosis initiates normally in both sexes but, in males, germ cell development arrests during the post-meiotic round spermatid period. In the mutant testis, acrosome and axoneme development are aberrant, multinucleated germ cells (symplasts) form and round spermatids unable to process beyond step 3 of spermiogenesis. No elongated spermatids nor spermatozoa are produced. Thus, Sox30 represents a rare example of a gene for which loss of function results in a complete arrest of spermatogenesis at the onset of spermiogenesis. Our results suggest that SOX30 mutations may underlie some instances of unexplained non-obstructive azoospermia in humans.

PARI Regulates Stalled Replication Fork Processing To Maintain Genome Stability upon Replication Stress in Mice

DNA replication is frequently perturbed by intrinsic, as well as extrinsic, genotoxic stress. At damaged forks, DNA replication and repair activities require proper coordination to maintain genome integrity. We show here that PARI antirecombinase plays an essential role in modulating the initial response to replication stress in mice. PARI is functionally dormant at replisomes during normal replication, but upon replication stress, it enhances nascent-strand shortening that is regulated by RAD51 and MRE11. PARI then promotes double-strand break induction, followed by new origin firing instead of replication restart. Such PARI function is apparently obstructive to replication but is nonetheless physiologically required for chromosome stability in vivo and ex vivo Of note, Pari-deficient embryonic stem cells exhibit spontaneous chromosome instability, which is attenuated by differentiation induction, suggesting that pluripotent stem cells have a preferential requirement for PARI that acts against endogenous replication stress. PARI is a latent modulator of stalled fork processing, which is required for stable genome inheritance under both endogenous and exogenous replication stress in mice.

Genetic sex determination of mice by simplex PCR

Background

Investigating fetal development in mice necessitates the determination of fetal sex. However, whilst the sex of adult and juvenile mice can be readily distinguished from anogenital distance, the sex of fetal and neonatal mice cannot be identified visually. Instead, genetic sex must be determined by PCR amplification of X chromosome genes with divergent Y chromosome gametologs. Existing simplex PCR methods are confounded by small size differences between amplicons, amplification of unexpected products, and biased amplification of the shorter amplicon.

Results

Primers were designed flanking an 84 bp deletion of the X-linked Rbm31x gene relative to its Y-linked gametolog Rbm31y. A single product was amplified from XX samples, with two products amplified from XY samples. Amplicons were resolved by gel electrophoresis for 20 min, with unbiased amplification of both products observed in XY samples.

Conclusion

This method achieves rapid and unequivocal genetic sex determination of mice in low volume PCR reactions, reducing reagent usage and simultaneously eliminating shortcomings of previous methods.

The epigenetic modifier Fam208a is required to maintain epiblast cell fitness

Gastrulation initiates with the formation of the primitive streak, during which, cells of the epiblast delaminate to form the mesoderm and definitive endoderm. At this stage, the pluripotent cell population of the epiblast undergoes very rapid proliferation and extensive epigenetic programming. Here we show that Fam208a, a new epigenetic modifier, is essential for early post-implantation development. We show that Fam208a mutation leads to impaired primitive streak elongation and delayed epithelial-to-mesenchymal transition. Fam208a mutant epiblasts had increased expression of p53 pathway genes as well as several pluripotency-associated long non-coding RNAs. Fam208a mutants exhibited an increase in p53-driven apoptosis and complete removal of p53 could partially rescue their gastrulation block. This data demonstrates a new in vivo function of Fam208a in maintaining epiblast fitness, establishing it as an important factor at the onset of gastrulation when cells are exiting pluripotency.

Round Spermatid Injection Rescues Female Lethality of a Paternally Inherited Xist Deletion in Mouse

In mouse female preimplantation embryos, the paternal X chromosome (Xp) is silenced by imprinted X chromosome inactivation (iXCI). This requires production of the noncoding Xist RNA in cis, from the Xp. The Xist locus on the maternally inherited X chromosome (Xm) is refractory to activation due to the presence of an imprint. Paternal inheritance of an Xist deletion (XpΔXist) is embryonic lethal to female embryos, due to iXCI abolishment. Here, we circumvented the histone-to-protamine and protamine-to-histone transitions of the paternal genome, by fertilization of oocytes via injection of round spermatids (ROSI). This did not affect initiation of XCI in wild type female embryos. Surprisingly, ROSI using ΔXist round spermatids allowed survival of female embryos. This was accompanied by activation of the intact maternal Xist gene, initiated with delayed kinetics, around the morula stage, resulting in Xm silencing. Maternal Xist gene activation was not observed in ROSI-derived males. In addition, no Xist expression was detected in male and female morulas that developed from oocytes fertilized with mature ΔXist sperm. Finally, the expression of the X-encoded XCI-activator RNF12 was enhanced in both male (wild type) and female (wild type as well as XpΔXist) ROSI derived embryos, compared to in vivo fertilized embryos. Thus, high RNF12 levels may contribute to the specific activation of maternal Xist in XpΔXist female ROSI embryos, but upregulation of additional Xp derived factors and/or the specific epigenetic constitution of the round spermatid-derived Xp are expected to be more critical. These results illustrate the profound impact of a dysregulated paternal epigenome on embryo development, and we propose that mouse ROSI can be used as a model to study the effects of intergenerational inheritance of epigenetic marks.

Quantitative analysis of male germline stem cell differentiation reveals a role for the p53-mTORC1 pathway in spermatogonial maintenance

p53 protects cells from DNA damage by inducing cell-cycle arrest upon encountering genomic stress. Among other pathways, p53 elicits such an effect by inhibiting mammalian target of rapamycin complex 1 (mTORC1), the master regulator of cell proliferation and growth. Although recent studies have indicated roles for both p53 and mTORC1 in stem cell maintenance, it remains unclear whether the p53-mTORC1 pathway is conserved to mediate this process under normal physiological conditions. Spermatogenesis is a classic stem cell-dependent process in which undifferentiated spermatogonia undergo self-renewal and differentiation to maintain the lifelong production of spermatozoa. To better understand this process, we have developed a novel flow cytometry (FACS)-based approach that isolates spermatogonia at consecutive differentiation stages. By using this as a tool, we show that genetic loss of p53 augments mTORC1 activity during early spermatogonial differentiation. Functionally, loss of p53 drives spermatogonia out of the undifferentiated state and causes a consistent expansion of early differentiating spermatogonia until the stage of preleptotene (premeiotic) spermatocyte. The frequency of early meiotic spermatocytes is, however, dramatically decreased. Thus, these data suggest that p53-mTORC1 pathway plays a critical role in maintaining the homeostasis of early spermatogonial differentiation. Moreover, our FACS approach could be a valuable tool in understanding spermatogonial differentiation.

Methods for the Study of Gonadal Development

Current knowledge on gonadal development and sex determination is the product of many decades of research involving a variety of scientific methods from different biological disciplines such as histology, genetics, biochemistry, and molecular biology. The earliest embryological investigations, followed by the invention of microscopy and staining methods, were based on histological examinations. The most robust development of histological staining techniques occurred in the second half of the nineteenth century and resulted in structural descriptions of gonadogenesis. These first studies on gonadal development were conducted on domesticated animals; however, currently the mouse is the most extensively studied species. The next key point in the study of gonadogenesis was the advancement of methods allowing for the in vitro culture of fetal gonads. For instance, this led to the description of the origin of cell lines forming the gonads. Protein detection using antibodies and immunolabeling methods and the use of reporter genes were also invaluable for developmental studies, enabling the visualization of the formation of gonadal structure. Recently, genetic and molecular biology techniques, especially gene expression analysis, have revolutionized studies on gonadogenesis and have provided insight into the molecular mechanisms that govern this process. The successive invention of new methods is reflected in the progress of research on gonadal development.

Lentiviral Vector-Mediated Complementation Restored Fetal Viability but Not Placental Hyperplasia in Plac1-Deficient Mice

The X-linked Plac1 gene is maternally expressed in trophoblast cells during placentation, and its disruption causes placental hyperplasia and intrauterine growth restriction. In contrast, Plac1 is also reported to be one of the upregulated genes in the hyperplastic placenta generated by nuclear transfer. However, the effect of overexpressed Plac1 on placental formation and function remained unaddressed. We complemented the Plac1 knockout placental dysfunction by lentiviral vector-mediated, placenta-specific Plac1 transgene expression. Whereas fetal development and the morphology of maternal blood sinuses in the labyrinth zone improved, placental hyperplasia remained, with an expanded the junctional zone that migrated and encroached into the labyrinth zone. Further experiments revealed that wild-type placenta with transgenically expressed Plac1 resulted in placental hyperplasia without the encroaching of the junctional zone. Our findings suggest that Plac1 is involved in trophoblast cell proliferation, differentiation, and migration. Its proper expression is required for normal placentation and fetal development.

Exposure to Brefeldin A promotes initiation of meiosis in murine female germ cells

In mammals, ontogenesis starts from a fusion of spermatozoon and oocyte, which are produced by reductive nuclear division of a diploid germ cell in a specialised but complex biological process known as meiosis. However, little is known about the mechanism of meiotic initiation in germ cells, although many factors may be responsible for meiosis both in male and female gonads. In this study, 11.5 days post coitum (dpc) female fetal mouse genital ridges were cultured in vitro with exposure to Brefeldin A (BFA) for 6h, and the changes in meiosis were detected. Synaptonemal-complex analysis implied that BFA played a positive role in meiosis initiation and this hypothesis was confirmed by quantitative PCR of meiosis-specific genes: stimulated by retinoic acid gene 8 (Stra8) and deleted in a zoospermia-like (DAZL). At the same time, mRNA expression of retinoic acid synthetase (Raldh2) and retinoic acid (RA) receptors increased in female gonads with in vitro exposure to BFA. Transplanting genital ridges treated with BFA into the kidney capsule of immunodeficient mice demonstrated that the development capacity of female germ cells was normal, while formation of primordial follicles was seen to be a result of accelerated meiosis after exposure to BFA. In conclusion, the study indicated that BFA stimulated meiosis initiation partly by RA signalling and then promoted the development of follicles.

Licensing of primordial germ cells for gametogenesis depends on genital ridge signaling

In mouse embryos at mid-gestation, primordial germ cells (PGCs) undergo licensing to become gametogenesis-competent cells (GCCs), gaining the capacity for meiotic initiation and sexual differentiation. GCCs then initiate either oogenesis or spermatogenesis in response to gonadal cues. Germ cell licensing has been considered to be a cell-autonomous and gonad-independent event, based on observations that some PGCs, having migrated not to the gonad but to the adrenal gland, nonetheless enter meiosis in a time frame parallel to ovarian germ cells -- and do so regardless of the sex of the embryo. Here we test the hypothesis that germ cell licensing is cell-autonomous by examining the fate of PGCs in Gata4 conditional mutant (Gata4 cKO) mouse embryos. Gata4, which is expressed only in somatic cells, is known to be required for genital ridge initiation. PGCs in Gata4 cKO mutants migrated to the area where the genital ridge, the precursor of the gonad, would ordinarily be formed. However, these germ cells did not undergo licensing and instead retained characteristics of PGCs. Our results indicate that licensing is not purely cell-autonomous but is induced by the somatic genital ridge.

During embryonic development, stem cell-like primordial germ cells travel across the developing embryo to the genital ridge, which gives rise to the gonad. Around the time of their arrival, the primordial germ cells gain the capacity to undertake sexual specialization and meiosis—a process called germ cell licensing. Based on the observation that meiosis and sexual differentiation can occur when primordial germ cells stray into the area of the adrenal gland, the primordial germ cell has been thought to be responsible for its own licensing. We tested this notion by examining the licensing process in mutant mouse embryos that did not form a genital ridge. We discovered that in the absence of the genital ridge, primordial germ cells migrate across the developing embryo properly, but instead of undergoing licensing, these cells retain their primordial germ cell characteristics. We conclude that licensing of embryonic primordial germ cells for gametogenesis is dependent on signaling from the genital ridge.

Rapid reprogramming of epigenetic and transcriptional profiles in mammalian culture systems

Background

The DNA methylation profiles of mammalian cell lines differ from those of the primary tissues from which they were derived, exhibiting increasing divergence from the in vivo methylation profile with extended time in culture. Few studies have directly examined the initial epigenetic and transcriptional consequences of adaptation of primary mammalian cells to culture, and the potential mechanisms through which this epigenetic dysregulation occurs is unknown.

Results

We demonstrate that adaptation of mouse embryonic fibroblasts to cell culture results in a rapid reprogramming of epigenetic and transcriptional states. We observed global 5-hydroxymethylcytosine (5hmC) erasure within three days of culture initiation. Loss of genic 5hmC was independent of global 5-methylcytosine (5mC) levels and could be partially rescued by addition of vitamin C. Significantly, 5hmC loss was not linked to concomitant changes in transcription. Discrete promoter-specific gains of 5mC were also observed within seven days of culture initiation. Against this background of global 5hmC loss we identified a handful of developmentally important genes that maintained their 5hmC profile in culture, including the imprinted loci Gnas and H19. Similar outcomes were identified in the adaption of CD4⁺ T cells to culture.

Conclusions

We report a dramatic and novel consequence of adaptation of mammalian cells to culture in which global loss of 5hmC occurs, suggesting rapid concomitant loss of methylcytosine dioxygenase activity. The observed epigenetic and transcriptional re-programming occurs much earlier than previously assumed, and has significant implications for the use of cell lines as faithful mimics of in vivo epigenetic and physiological processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0576-y) contains supplementary material, which is available to authorized users.

The "virgin birth", polyploidy, and the origin of cancer

Recently, it has become clear that the complexity of cancer biology cannot fully be explained by somatic mutation and clonal selection. Meanwhile, data have accumulated on how cancer stem cells or stemloids bestow immortality on tumour cells and how reversible polyploidy is involved. Most recently, single polyploid tumour cells were shown capable of forming spheroids, releasing EMT-like descendents and inducing tumours in vivo. These data refocus attention on the centuries-old embryological theory of cancer. This review attempts to reconcile seemingly conflicting data by viewing cancer as a pre-programmed phylogenetic life-cycle-like process. This cycle is apparently initiated by a meiosis-like process and driven as an alternative to accelerated senescence at the DNA damage checkpoint, followed by an asexual syngamy event and endopolyploid-type embryonal cleavage to provide germ-cell-like (EMT) cells. This cycle is augmented by genotoxic treatments, explaining why chemotherapy is rarely curative and drives resistance. The logical outcome of this viewpoint is that alternative treatments may be more efficacious - either those that suppress the endopolyploidy-associated ‘life cycle’ or, those that cause reversion of embryonal malignant cells into benign counterparts. Targets for these opposing strategies are components of the same molecular pathways and interact with regulators of accelerated senescence.

SHP-2 deletion in postmigratory neural crest cells results in impaired cardiac sympathetic innervation

Autonomic innervation is an essential component of cardiovascular regulation that is first established from the neural crest (NC) lineage in utero and continues developing postnatally. Although in vitro studies have indicated that SH2-containing protein tyrosine phosphatase 2 (SHP-2) is a signaling factor critical for regulating sympathetic neuron differentiation, this has yet to be shown in the complex in vivo environment of cardiac autonomic innervation. Targeting SHP-2 within postmigratory NC lineages resulted in a fully penetrant mouse model of diminished sympathetic cardiac innervation and concomitant bradycardia. Immunohistochemistry of the sympathetic nerve marker tyrosine hydroxylase revealed a progressive loss of adrenergic ganglionic neurons and reduction of cardiac sympathetic axon density in Shp2 cKOs. Molecularly, Shp2 cKOs exhibit lineage-specific suppression of activated phospo-ERK1/2 signaling but not of other downstream targets of SHP-2 such as pAKT. Genetic restoration of the phosphorylated-extracellular signal-regulated kinase (pERK) deficiency via lineage-specific expression of constitutively active MEK1 was sufficient to rescue the sympathetic innervation deficit and its physiological consequences. These data indicate that SHP-2 signaling specifically through pERK in postmigratory NC lineages is essential for development and maintenance of sympathetic cardiac innervation postnatally.

Characterization of the skeletal fusion with sterility (sks) mouse showing axial skeleton abnormalities caused by defects of embryonic skeletal development

The development of the axial skeleton is a complex process, consisting of segmentation and differentiation of somites and ossification of the vertebrae. The autosomal recessive skeletal fusion with sterility (sks) mutation of the mouse causes skeletal malformations due to fusion of the vertebrae and ribs, but the underlying defects of vertebral formation during embryonic development have not yet been elucidated. For the present study, we examined the skeletal phenotypes of sks/sks mice during embryonic development and the chromosomal localization of the sks locus. Multiple defects of the axial skeleton, including fusion of vertebrae and fusion and bifurcation of ribs, were observed in adult and neonatal sks/sks mice. In addition, we also found polydactyly and delayed skull ossification in the sks/sks mice. Morphological defects, including disorganized vertebral arches and fusions and bifurcations of the axial skeletal elements, were observed during embryonic development at embryonic day 12.5 (E12.5) and E14.5. However, no morphological abnormality was observed at E11.5, indicating that defects of the axial skeleton are caused by malformation of the cartilaginous vertebra and ribs at an early developmental stage after formation and segmentation of the somites. By linkage analysis, the sks locus was mapped to an 8-Mb region of chromosome 4 between D4Mit331 and D4Mit199. Since no gene has already been identified as a cause of malformation of the vertebra and ribs in this region, the gene responsible for sks is suggested to be a novel gene essential for the cartilaginous vertebra and ribs.

Regulation of mesodermal precursor production by low-level expression of B1 Sox genes in the caudal lateral epiblast

High expression of the B1 Sox genes, Sox2 and Sox3, is associated with the development of definitive neural primordia, the neural plates, in early stage embryos. However, in the caudal lateral epiblast (CLE) where axial stem cells reside, Sox2 and Sox3 are expressed at low levels, together with Brachyury. Because axial stem cells are the bipotential precursors of the neural plate and paraxial mesoderm, we investigated the possibility that low-level B1 Sox expression in CLE may regulate the fate of axial stem cells. We combined the genetic conditions of Sox3-null and Sox2 N1 enhancer homozygous deletion (Sox2(ΔN1/ΔN1)) to decrease B1 Sox expression in CLE. At 5-7 somite stages of mouse embryogenesis, these genetic manipulations caused approximately 30% higher production of paraxial mesodermal precursors, resulting in the development of larger somites. Analysis of mitotic cell populations suggested that decrease of B1 Sox expression in CLE does not activate cell proliferation but promotes cell migration into the mesodermal compartment. Thus, the low-level B1 Sox expression in CLE regulates axial stem cells to adjust the production of paraxial mesoderm precursors to an appropriate level.

Notch pathway regulates female germ cell meiosis progression and early oogenesis events in fetal mouse

A critical process of early oogenesis is the entry of mitotic oogonia into meiosis, a cell cycle switch regulated by a complex gene regulatory network. Although Notch pathway is involved in numerous important aspects of oogenesis in invertebrate species, whether it plays roles in early oogenesis events in mammals is unknown. Therefore, the rationale of the present study was to investigate the roles of Notch signaling in crucial processes of early oogenesis, such as meiosis entry and early oocyte growth. Notch receptors and ligands were localized in mouse embryonic female gonads and 2 Notch inhibitors, namely DAPT and L-685,458, were used to attenuate its signaling in an in vitro culture system of ovarian tissues from 12.5 days post coitum (dpc) fetus. The results demonstrated that the expression of Stra8, a master gene for germ cell meiosis, and its stimulation by retinoic acid (RA) were reduced after suppression of Notch signaling, and the other meiotic genes, Dazl, Dmc1, and Rec8, were abolished or markedly decreased. Furthermore, RNAi of Notch1 also markedly inhibited the expression of Stra8 and SCP3 in cultured female germ cells. The increased methylation status of CpG islands within the Stra8 promoter of the oocytes was observed in the presence of DAPT, indicating that Notch signaling is probably necessary for maintaining the epigenetic state of this gene in a way suitable for RA stimulation. Furthermore, in the presence of Notch inhibitors, progression of oocytes through meiosis I was markedly delayed. At later culture periods, the rate of oocyte growth was decreased, which impaired subsequent primordial follicle assembly in cultured ovarian tissues. Taken together, these results suggested new roles of the Notch signaling pathway in female germ cell meiosis progression and early oogenesis events in mammals.

Generation of precise point mutation mice by footprintless genome modification

Point mutation mice are a key tool in the study of biological functions of genomic DNA sequences and the creation of human disease models. These mice are produced by homologous recombination combined with site-specific recombinase, which allows removal of drug selection cassettes. However, the methods currently available leave ectopic sequences in the "inactive" intron region of the targeted genome in addition to the desired mutation. Since recent research suggests that the intron region may actually have some functionality, these sequences could potentially interfere with neighboring gene expression and, as a result, affect the mouse phenotype. To completely avoid this issue, we used the PiggyBac transposon to remove selection cassettes and achieve precise genome modification without leaving behind a footprint. This PiggyBac system allowed us to successfully generate mice carrying an artificially introduced W(v) point mutation in the Kit gene, and these mice were confirmed to have phenotypes identical to spontaneous W(v) mutation mice. Generation of W(v) -mutation corrected mice was also possible, and phenotypes were completely restored. Our footprintless genome modification technology can generate precise point mutation mice with only the desired mutation, and they reflect an accurate phenotype that makes these mice a reliable and "worry-free" research resource.

A mutation in the nuclear pore complex gene Tmem48 causes gametogenesis defects in skeletal fusions with sterility (sks) mice

Skeletal fusions with sterility (sks) is an autosomal recessive mutation of mouse that results in male and female sterility because of defects in gametogenesis. The mutants also have skeletal malformations with fused vertebrae and ribs. We examined testicular phenotypes of sks/sks mice to investigate the defects in spermatogenesis. Histological and immunocytochemical analyses and expression analyses of the marker genes demonstrated that spermatogenesis is arrested at mid to late pachytene stage of meiotic prophase with defective synapsis of the homologous chromosomes. Next, we determined the precise chromosomal localization of the sks locus on a 0.3-Mb region of mouse chromosome 4 by linkage analysis. By sequencing the positional candidate genes in this region and whole exome sequencing, we found a GG to TT nucleotide substitution in exon 6 of the Tmem48 gene that encodes a putative transmembrane protein with six transmembrane domains. The nucleotide substitution causes aberrant splicing, which deletes exon 6 of the Tmem48 transcript. Specific expression of TMEM48 was observed in germ cells of males and females. Furthermore, the phenotypes of the sks mutant were completely rescued by the transgenesis of a genomic fragment containing the wild-type Tmem48 gene. These findings indicate that the Tmem48 mutation is responsible for the gametogenesis defects and skeletal malformations in the sks mice. The TMEM48 protein is a nuclear membrane protein comprising the nuclear pore complex; its exact function in the nuclear pore complex is still unknown. Our finding suggested that the nuclear pore complex plays an important role in mammalian gametogenesis and skeletal development.

Molecular characteristics of malignant ovarian germ cell tumors and comparison with testicular counterparts: implications for pathogenesis

This review focuses on the molecular characteristics and development of rare malignant ovarian germ cell tumors (mOGCTs). We provide an overview of the genomic aberrations assessed by ploidy, cytogenetic banding, and comparative genomic hybridization. We summarize and discuss the transcriptome profiles of mRNA and microRNA (miRNA), and biomarkers (DNA methylation, gene mutation, individual protein expression) for each mOGCT histological subtype. Parallels between the origin of mOGCT and their male counterpart testicular GCT (TGCT) are discussed from the perspective of germ cell development, endocrinological influences, and pathogenesis, as is the GCT origin in patients with disorders of sex development. Integrated molecular profiles of the 3 main histological subtypes, dysgerminoma (DG), yolk sac tumor (YST), and immature teratoma (IT), are presented. DGs show genomic aberrations comparable to TGCT. In contrast, the genome profiles of YST and IT are different both from each other and from DG/TGCT. Differences between DG and YST are underlined by their miRNA/mRNA expression patterns, suggesting preferential involvement of the WNT/β-catenin and TGF-β/bone morphogenetic protein signaling pathways among YSTs. Characteristic protein expression patterns are observed in DG, YST and IT. We propose that mOGCT develop through different developmental pathways, including one that is likely shared with TGCT and involves insufficient sexual differentiation of the germ cell niche. The molecular features of the mOGCTs underline their similarity to pluripotent precursor cells (primordial germ cells, PGCs) and other stem cells. This similarity combined with the process of ovary development, explain why mOGCTs present so early in life, and with greater histological complexity, than most somatic solid tumors.