Eukaryotic translation initiation factor 4G (EIF4G) is an important scaffold protein in the translation initiation complex. In mice, mutation of the Eif4g3 gene causes male infertility, with arrest of meiosis at the end of meiotic prophase. This study documents features of the developmental expression and subcellular localization of EIF4G3 that might contribute to its highly specific role in meiosis and spermatogenesis. Quite unexpectedly, EIF4G3 is located in the nucleus of spermatocytes, where it is highly enriched in the XY body, the chromatin domain formed by the transcriptionally inactive sex chromosomes. Moreover, many other, but not all, translation-related proteins are also localized in the XY body. These unanticipated observations implicate roles for the XY body in controlling mRNA metabolism and/or "poising" protein translation complexes before the meiotic division phase in spermatocytes.
SMC complexes include three major classes: cohesin, condensin, and SMC5/6. However, the localization pattern and genetic requirements for the SMC5/6 complex during mammalian oogenesis had not previously been examined. In mouse oocytes, the SMC5/6 complex is enriched at the pericentromeric heterochromatin, and also localizes along chromosome arms during meiosis. The infertility phenotypes of females with a Zp3-Cre-driven conditional knockout (cKO) of Smc5 demonstrated that maternally expressed SMC5 protein is essential for early embryogenesis. Interestingly, protein levels of SMC5/6 complex components in oocytes decline as wild-type females age. When SMC5/6 complexes were completely absent in oocytes during meiotic resumption, homologous chromosomes failed to segregate accurately during meiosis I. Despite what appears to be an inability to resolve concatenation between chromosomes during meiosis, localization of topoisomerase II alpha to bivalents was not affected; however, localization of condensin along the chromosome axes was perturbed. Taken together, these data demonstrate that the SMC5/6 complex is essential for the formation of segregation-competent bivalents during meiosis I, and findings suggest that age-dependent depletion of the SMC5/6 complex in oocytes could contribute to increased incidence of oocyte aneuploidy and spontaneous abortion in aging females.
ABSTRACT The development of tools to manipulate the mouse genome, including knockout and transgenic technology, revolutionized our ability to explore gene function in mammals. Moreover, for genes that are expressed in multiple tissues or at multiple stages of development, the use of tissue-specific expression of the Cre recombinase allows gene function to be perturbed in specific cell types and/or at specific times. However, it is well known that putative tissue-specific promoters often drive unanticipated “off target” expression. In our efforts to explore the biology of the male reproductive tract, we unexpectedly found that expression of Cre in the central nervous system resulted in recombination in the epididymis, a tissue where sperm mature for ~1-2 weeks following the completion of testicular development. Remarkably, we not only observed reporter expression in the epididymis when Cre expression was driven from neuron-specific transgenes, but also when Cre expression in the brain was induced from an AAV vector carrying a Cre expression construct. A surprisingly wide range of Cre drivers – including six different neuronal promoters as well as the adipose-specific AdipoQ Cre promoter – exhibited off target recombination in the epididymis, with a subset of drivers also exhibiting unexpected activity in other tissues such as the reproductive accessory glands. Finally, using a combination of parabiosis and serum transfer experiments, we find evidence supporting the hypothesis that Cre may be trafficked from its cell of origin to the epididymis through the circulatory system. Together, our findings should motivate extreme caution when interpreting conditional alleles, and suggest the exciting possibility of inter-tissue RNA or protein trafficking in modulation of reproductive biology.
The most common genetic disorder in humans, trisomy, is caused predominantly by errors in chromosome segregation during oogenesis. Isolated mouse oocytes resuming meiosis and progressing to metaphase II in vitro have recently been used to assess targets, aneugenic potential and sensitivity of oocytes to chemical exposures. In order to extend in vitro maturation tests to earlier stages of oogenesis, an in vitro assay with mouse preantral follicle cultures has been established. It permits the identification of direct and also indirect effects of environmental chemicals on the somatic compartment, the follicle and theca cells, that may lead to disturbances of oocyte growth, maturation and chromosome segregation. Early preantral follicles from prepubertal female mice are cultured in microdroplets for 12 days under strictly controlled conditions. The follicle‐enclosed oocytes resume maturation, develop to metaphase II and become in vitro ovulated within 16 h after a physiological ovulatory stimulus with recombinant human gonadotrophins and epidermal growth factor. These oocytes grown and matured in vitro possess normal barrel‐shaped spindles with well‐aligned chromosomes. Their chromosomes segregate with high fidelity during anaphase I. The model aneugen colchicine induced a meiotic arrest and aneuploidy in these in vitro grown, follicle‐enclosed oocytes in a dose‐dependent manner, comparable to in vivo tests. Therefore, preantral follicle culture appears to provide an effective and reliable method to assess the influences of environmental mutagens, pharmaceutical agents and potentially endocrine disrupting chemicals on the fidelity of female meiosis.
Male infertility contributes to roughly half of human couple infertility, yet causes, which can be genetic and/or environmental, are poorly understood. One obstacle is that we have not yet identified the full repertoire of genes whose function is required for male fertility. Targeted mutagenesis strategies, such as gene knock-outs, are a useful approach to defining genetic requirements for male fertility, but some prior knowledge of the targeted gene is necessary. However, in cases when infertility was an unexpected outcome of gene disruption, such strategies have led to the identification of genes previously unsuspected to play a role in fertility. In contrast to gene-directed approaches such as knock-outs and gene traps, phenotype-driven approaches do not rely on previous knowledge of gene identity and are almost wholly unbiased. The ReproGenomics Program at The Jackson Laboratory (http://reprogenomics.jax.org/) identifies novel genes required for fertility. Phenotype analyses for male fertility are based on breeding performance and/or gonad size; practical considerations dictate a strategy of screening for infertility rather than reduced fecundity. This program has identified phenotypes affecting male germ cell differentiation and function from spermatogonia to ejaculated sperm, providing valuable resources for dissection of developmental pathways at all stages. One recovered mutation affects survival of germ cells at the onset of adult Sertoli cell function. Another affects progress of spermatocytes out of meiotic prophase I; the affected germ cells do not exhibit competence to undergo the meiotic divisions. A large cluster of recovered mutations affect the post-meiotic spermiogenic differentiation of germ cells. These models for human syndromes of oligoasthenoteratozoospermia provide interesting insight into the etiology and clinical management of these syndromes. Many of these mutations have been distributed to the scientific community. The mutated gene has been identified for about 20% of the mutations produced. The staff of ReproGenomics is actively pursuing the positional cloning of some and also collaborates with outside end users to facilitate fine mapping and positional cloning of several mutations. Mice carrying any of the mutations are available to any interested scientists, and the ReproGenomics Program maintains a cyropreserved sperm bank for all mutations recovered and regionally mapped. To date, considering mutations that affect males and/or females, the ReproGenomics Program has produced roughly 10% of all mutations known to affect fertility in mice, providing novel experimental approaches for resolving problems of infertility in the clinic. (Supported by the NIH, HD42137)
The single and/or combination use of immune checkpoint blockade therapies in human infectious diseases and cancer are rapidly expanding. Despite early efforts, substantial uncertainty remains about the safety and efficacy of immune checkpoint blockade in some populations. Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and T-cell immunoglobulin mucin-3 (Tim-3) are the major targetable co-inhibitory receptors on T cells. Here we showed that in animal studies, treatment with either CTLA-4- or Tim-3-blocking antibody caused greater susceptibility to fetal loss with altered cytokine profiles by decidual CD4+T (dCD4+T) cells. CTLA-4 and Tim-3 pathways appeared to play key roles in maintaining maternal-fetal tolerance by regulating the function of dCD4+T cells. In addition, the abnormality in number and functionality of dCTLA-4+Tim-3+CD4+T cells was associated with miscarriage. These findings underscored the important roles of the CTLA-4 and Tim-3 pathways in regulating dCD4+T cells function and maintaining normal pregnancy. Our study also emphasized the importance of careful consideration of reproductive safety when choosing immune checkpoint blockade therapies in real world clinical care.
Offspring affected by sperm small RNAs Paternal dietary conditions in mammals influence the metabolic phenotypes of offspring. Although prior work suggests the involvement of epigenetic pathways, the mechanisms remains unclear. Two studies now show that altered paternal diet affects the level of small RNAs in mouse sperm. Chen et al. injected sperm transfer RNA (tRNA) fragments from males that had been kept on a high-fat diet into normal oocytes. The progeny displayed metabolic disorders and concomitant alteration of genes in metabolic pathways. Sharma et al. observed the biogenesis and function of small tRNA-derived fragments during sperm maturation. Further understanding of the mechanisms by which progeny are affected by parental exposure may affect human diseases such as diet-induced metabolic disorders. Science , this issue p. 397 , p. 391
Beyond the haploid genome, mammalian sperm carry a payload of epigenetic information with the potential to modulate offspring phenotypes. Recent studies show that the small RNA repertoire of sperm is remodeled during post-testicular maturation in the epididymis. Epididymal maturation has also been linked to changes in the sperm methylome, suggesting that the epididymis might play a broader role in shaping the sperm epigenome. Here, we characterize the genome-wide methylation landscape in seven germ cell populations from throughout the male reproductive tract. We find very few changes in the cytosine methylation landscape between testicular germ cell populations and cauda epididymal sperm, demonstrating that the sperm methylome is stable throughout post-testicular maturation. Although our sequencing data suggested that caput epididymal sperm exhibit a highly unusual methylome, follow-up studies revealed that this resulted from contamination of caput sperm by extracellular DNA. Extracellular DNA formed web-like structures that ensnared sperm, and was present only in sperm samples obtained from the caput epididymis and vas deferens of virgin males. Curiously, contaminating extracellular DNA was associated with citrullinated histone H3, potentially resulting from a PAD-driven genome decondensation process. Taken together, our data emphasize the stability of cytosine methylation in mammalian sperm, and identify a surprising, albeit transient, period during which sperm are associated with extracellular DNA.
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