Ultra-High Resolution 3D Imaging of Whole Cells.

Fluorescence nanoscopy, or super-resolution microscopy, has become an important tool in cell biological research. However, because of its usually inferior resolution in the depth direction (50-80 nm) and rapidly deteriorating resolution in thick samples, its practical biological application has been effectively limited to two dimensions and thin samples. Here, we present the development of whole-cell 4Pi single-molecule switching nanoscopy (W-4PiSMSN), an optical nanoscope that allows imaging of three-dimensional (3D) structures at 10- to 20-nm resolution throughout entire mammalian cells. We demonstrate the wide applicability of W-4PiSMSN across diverse research fields by imaging complex molecular architectures ranging from bacteriophages to nuclear pores, cilia, and synaptonemal complexes in large 3D cellular volumes.

Applying "gold standards" to in-vitro-derived germ cells.

Germ cells are the ultimate stem cells, and reports of their in vitro derivation generate excitement due to potential applications in reproductive medicine. To date, there is no firm evidence that meiosis, the hallmark of gametogenesis, can be faithfully replicated outside of the gonad. We propose benchmarks for evaluating in vitro derivation of germ cells, facilitating realization of their potential.

Reproductive genomics of the mouse: implications for human fertility and infertility.

Genetic analyses of mammalian gametogenesis and fertility have the potential to inform about two important and interrelated clinical areas: infertility and contraception. Here, we address the genetics and genomics underlying gamete formation, productivity and function in the context of reproductive success in mammalian systems, primarily mouse and human. Although much is known about the specific genes and proteins required for meiotic processes and sperm function, we know relatively little about other gametic determinants of overall fertility, such as regulation of gamete numbers, duration of gamete production, and gamete selection and function in fertilization. As fertility is not a binary trait, attention is now appropriately focused on the oligogenic, quantitative aspects of reproduction. Multiparent mouse populations, created by complex crossing strategies, exhibit genetic diversity similar to human populations and will be valuable resources for genetic discovery, helping to overcome current limitations to our knowledge of mammalian reproductive genetics. Finally, we discuss how what we know about the genomics of reproduction can ultimately be brought to the clinic, informing our concepts of human fertility and infertility, and improving assisted reproductive technologies.

New allele of mouse DNA/RNA helicase senataxin causes meiotic arrest and infertility.

In brief: A new allele of the senataxin gene Setxspcar3 causes meiotic arrest of spermatocytes with aberrant DNA damage and accumulation of R-loops. Abstract: An unbiased screen for discovering novel mouse genes for fertility identified the spcar3, spermatocyte arrest 3, mutant phenotype. The spcar3 mutation identified a new allele of the Setx gene, encoding senataxin, a DNA/RNA helicase that regulates transcription termination by resolving DNA/RNA hybrid R-loop structures. The Setxspcar3 mutant mice exhibit male infertility and female subfertility. Histology of the Setxspcar3 mutant testes revealed the absence of spermatids and mature spermatozoa in the seminiferous tubules. Cytological analysis of chromosome preparations of the Setxspcar3 mutant spermatocytes revealed normal synapsis, but aberrant DNA damage in the autosomes, defective formation of the sex body, and arrest of meiosis in mid-prophase. Additionally, Setxspcar3 testicular cells exhibit abnormal accumulation of R-loops. Transient expression assays identified regions of the senataxin protein required for sub-nuclear localization. Together, these results not only confirm that senataxin is required for normal meiosis and spermatogenesis but also provide a new resource for the determination of its role in maintaining R-loop formation and genome integrity.

SMC5/6 is required for the formation of segregation-competent bivalent chromosomes during meiosis I in mouse oocytes.

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 have 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 IIα 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.

ENU-induced mutant allele of Dnah1, ferf1, causes abnormal sperm behavior and fertilization failure in mice.

Given attention to both contraception and treatment of infertility, there is a need to identify genes and sequence variants required for mammalian fertility. Recent unbiased mutagenesis strategies have expanded horizons of genetic control of reproduction. Here we show that male mice homozygous for the ethyl-nitroso-urea-induced ferf1 (fertilization failure 1) mutation are infertile, producing apparently normal sperm that does not fertilize oocytes in standard fertilization in vitro fertilization assays. The ferf1 mutation is a single-base change in the Dnah1 gene, encoding an axoneme-associated dynein heavy chain, and previously associated with male infertility in both mice and humans. This missense mutation causes a single-amino-acid change in the DNAH1 protein in ferf1 mutant mice that leads to abnormal sperm clumping, aberrant sperm motility, and the inability of sperm to penetrate the oocyte's zona pellucida; however, the ferf1 mutant sperm is competent to fertilize zona-free oocytes. Taken together, the various mutations affecting the DNAH1 protein in both mouse and human produce a diversity of phenotypes with both subtle and considerable differences. Thus, future identification of the interacting partners of DNAH1 might lead to understanding its unique function among the sperm dyneins.

Hybrid Sterility with Meiotic Metaphase Arrest in Intersubspecific Mouse Crosses.

Although organisms belonging to different species and subspecies sometimes produce fertile offspring, a hallmark of the speciation process is reproductive isolation, characterized by hybrid sterility (HS) due to failure in gametogenesis. In mammals, HS is usually exhibited by males, the heterogametic sex. The phenotypic manifestations of HS are complex. The most frequently observed are abnormalities in both autosomal and sex chromosome interactions that are linked to meiotic prophase arrest or postmeiotic spermiogenesis aberrations and lead to defective or absent gametes. The aim of this study was to determine the HS phenotypes in intersubspecific F1 mice produced by matings between Mus musculus molossinus-derived strains and diverse Mus musculus domesticus-inbred laboratory mouse strains. Most of these crosses produced fertile F1 offspring. However, when female BALB/cJ (domesticus) mice were mated to male JF1/MsJ (molossinus) mice, the (BALBdomxJF1mol)F1 males were sterile, whereas the (JF1molxBALBdom)F1 males produced by the reciprocal crossings were fertile; thus the sterility phenotype was asymmetric. The sterile (BALBdomxJF1mol) F1 males exhibited a high rate of meiotic metaphase arrest with misaligned chromosomes, probably related to a high frequency of XY dissociation. Intriguingly, in the sterile (BALBdomxJF1mol)F1 males we observed aberrant allele-specific expression of several meiotic genes, that play critical roles in important meiotic events including chromosome pairing. Together, these observations of an asymmetrical HS phenotype in intersubspecific F1 males, probably owing to meiotic defects in the meiotic behavior of the XY chromosomes pair and possibly also transcriptional misregulation of meiotic genes, provide new models and directions for understanding speciation mechanisms in mammals.

Meiosis: the chromosomal foundation of reproduction.

Meiosis is the chromosomal foundation of reproduction, with errors in this important process leading to aneuploidy and/or infertility. In this review celebrating the 50th anniversary of the founding of the Society for the Study of Reproduction, the important chromosomal structures and dynamics contributing to genomic integrity across generations are highlighted. Critical unsolved biological problems are identified, and the advances that will lead to their ultimate resolution are predicted.

Nuclear localization of EIF4G3 suggests a role for the XY body in translational regulation during spermatogenesis in mice.

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.

Unpackaging the genetics of mammalian fertility: strategies to identify the "reproductive genome".

Gene mutations, including different alleles of the same gene, are tremendously useful in deconstructing complex developmental systems, such as reproduction, into component molecular pathways. For this reason, great effort has been devoted in the past three decades to biased (reverse genetic) and unbiased (forward genetic) searches for new genes that impact mammalian reproduction and fertility. These efforts have more recently been complemented with international efforts to systematically mutate all mouse genes and to determine their phenotypes (essentially a hybrid of forward and reverse genetics). Here, we survey the available data on the relative productivity of these approaches in identifying fertility genes, estimate the number of protein-coding genes essential for fertility of males and females, and predict the next major directions in the genetics of reproduction and fertility.

Regulatory complexity revealed by integrated cytological and RNA-seq analyses of meiotic substages in mouse spermatocytes.

BACKGROUND: The continuous and non-synchronous nature of postnatal male germ-cell development has impeded stage-specific resolution of molecular events of mammalian meiotic prophase in the testis. Here the juvenile onset of spermatogenesis in mice is analyzed by combining cytological and transcriptomic data in a novel computational analysis that allows decomposition of the transcriptional programs of spermatogonia and meiotic prophase substages. RESULTS: Germ cells from testes of individual mice were obtained at two-day intervals from 8 to 18 days post-partum (dpp), prepared as surface-spread chromatin and immunolabeled for meiotic stage-specific protein markers (STRA8, SYCP3, phosphorylated H2AFX, and HISTH1T). Eight stages were discriminated cytologically by combinatorial antibody labeling, and RNA-seq was performed on the same samples. Independent principal component analyses of cytological and transcriptomic data yielded similar patterns for both data types, providing strong evidence for substage-specific gene expression signatures. A novel permutation-based maximum covariance analysis (PMCA) was developed to map co-expressed transcripts to one or more of the eight meiotic prophase substages, thereby linking distinct molecular programs to cytologically defined cell states. Expression of meiosis-specific genes is not substage-limited, suggesting regulation of substage transitions at other levels. CONCLUSIONS: This integrated analysis provides a general method for resolving complex cell populations. Here it revealed not only features of meiotic substage-specific gene expression, but also a network of substage-specific transcription factors and relationships to potential target genes.

Nuclear localization of PRDM9 and its role in meiotic chromatin modifications and homologous synapsis.

Developmental progress of germ cells through meiotic phases is closely tied to ongoing meiotic recombination. In mammals, recombination preferentially occurs in genomic regions known as hotspots; the protein that activates these hotspots is PRDM9, containing a genetically variable zinc finger (ZNF) domain and a PR-SET domain with histone H3K4 trimethyltransferase activity. PRDM9 is required for fertility in mice, but little is known about its localization and developmental dynamics. Application of spermatogenic stage-specific markers demonstrates that PRDM9 accumulates in male germ cell nuclei at pre-leptonema to early leptonema but is no longer detectable in nuclei by late zygonema. By the pachytene stage, PRDM9-dependent histone H3K4 trimethyl marks on hotspots also disappear. PRDM9 localizes to nuclei concurrently with the deposition of meiotic cohesin complexes, but is not required for incorporation of cohesin complex proteins into chromosomal axial elements, or accumulation of normal numbers of RAD51 foci on meiotic chromatin by late zygonema. Germ cells lacking PRDM9 exhibit inefficient homology recognition and synapsis, with aberrant repair of meiotic DNA double-strand breaks and transcriptional abnormalities characteristic of meiotic silencing of unsynapsed chromatin. Together, these results on the developmental time course for nuclear localization of PRDM9 establish its direct window of function and demonstrate the independence of chromosome axial element formation from the concurrent PRDM9-mediated activation of recombination hotspots.

Genetics of mammalian meiosis: regulation, dynamics and impact on fertility.

Meiosis is an essential stage in gamete formation in all sexually reproducing organisms. Studies of mutations in model organisms and of human haplotype patterns are leading to a clearer understanding of how meiosis has adapted from yeast to humans, the genes that control the dynamics of chromosomes during meiosis, and how meiosis is tied to gametic success. Genetic disruptions and meiotic errors have important roles in infertility and the aetiology of developmental defects, especially aneuploidy. An understanding of the regulation of meiosis, coupled with advances in genomics, may ultimately allow us to diagnose the causes of meiosis-based infertilities, more wisely apply assisted reproductive technologies, and derive functional germ cells.

Dynamic localization of SMC5/6 complex proteins during mammalian meiosis and mitosis suggests functions in distinct chromosome processes.

Four members of the structural maintenance of chromosome (SMC) protein family have essential functions in chromosome condensation (SMC2/4) and sister-chromatid cohesion (SMC1/3). The SMC5/6 complex has been implicated in chromosome replication, DNA repair and chromosome segregation in somatic cells, but its possible functions during mammalian meiosis are unknown. Here, we show in mouse spermatocytes that SMC5 and SMC6 are located at the central region of the synaptonemal complex from zygotene until diplotene. During late diplotene both proteins load to the chromocenters, where they colocalize with DNA Topoisomerase IIα, and then accumulate at the inner domain of the centromeres during the first and second meiotic divisions. Interestingly, SMC6 and DNA Topoisomerase IIα colocalize at stretched strands that join kinetochores during the metaphase II to anaphase II transition, and both are observed on stretched lagging chromosomes at anaphase II following treatment with Etoposide. During mitosis, SMC6 and DNA Topoisomerase IIα colocalize at the centromeres and chromatid axes. Our results are consistent with the participation of SMC5 and SMC6 in homologous chromosome synapsis during prophase I, chromosome and centromere structure during meiosis I and mitosis and, with DNA Topoisomerase IIα, in regulating centromere cohesion during meiosis II.

Meiosis arrest female 1 (MARF1) has nuage-like function in mammalian oocytes.

Orderly regulation of meiosis and protection of germline genomic integrity from transposable elements are essential for male and female gamete development. In the male germline, these processes are ensured by proteins associated with cytoplasmic nuage, but morphologically similar germ granules or nuage have not been identified in mammalian female germ cells. Indeed, many mutations affecting nuage-associated proteins such as PIWI and tudor domain containing proteins 5 and 7 (TDRD5/7) can result in failure of meiosis, up-regulation of retrotransposons, and infertility only in males and not in females. We recently identified MARF1 (meiosis arrest female 1) as a protein essential for controlling meiosis and retrotransposon surveillance in oocytes; and in contrast to PIWI-pathway mutations, Marf1 mutant females are infertile, whereas mutant males are fertile. Here we put forward the hypothesis that MARF1 in mouse oocytes is a functional counterpart of the nuage-associated components of spermatocytes. We describe the developmental pattern of Marf1 expression and its roles in retrotransposon silencing and protection from DNA double-strand breaks. Analysis of MARF1 protein domains compared with PIWI and TDRD5/7 revealed that these functional similarities are reflected in remarkable structural analogies. Thus, functions that in the male germline require protein interactions and cooperative scaffolding are combined in MARF1, allowing a single molecule to execute crucial activities of meiotic regulation and protection of germline genomic integrity.

A-MYB (MYBL1) transcription factor is a master regulator of male meiosis.

The transcriptional regulation of mammalian meiosis is poorly characterized, owing to few genetic and ex vivo models. From a genetic screen, we identify the transcription factor MYBL1 as a male-specific master regulator of several crucial meiotic processes. Spermatocytes bearing a novel separation-of-function allele (Mybl1(repro9)) had subtle defects in autosome synapsis in pachynema, a high incidence of unsynapsed sex chromosomes, incomplete double-strand break repair on synapsed pachytene chromosomes and a lack of crossing over. MYBL1 protein appears in pachynema, and its mutation caused specific alterations in expression of diverse genes, including some translated postmeiotically. These data, coupled with chromatin immunoprecipitation (ChIP-chip) experiments and bioinformatic analysis of promoters, identified direct targets of MYBL1 regulation. The results reveal that MYBL1 is a master regulator of meiotic genes that are involved in multiple processes in spermatocytes, particularly those required for cell cycle progression through pachynema.

Mutation of Eif4g3, encoding a eukaryotic translation initiation factor, causes male infertility and meiotic arrest of mouse spermatocytes.

The ENU-induced repro8 mutation was identified in a screen to uncover genes that control mouse gametogenesis. repro8 causes male-limited infertility, with failure of spermatocytes to exit meiotic prophase via the G2/MI transition. The repro8 mutation is in the Eif4g3 gene, encoding eukaryotic translation initiation factor 4, gamma 3. Mutant germ cells appear to execute events of meiotic prophase normally, and many proteins characteristic of the prophase-to-metaphase transition are not obviously depleted. However, activity of CDC2A (CDK1) kinase is dramatically reduced in mutant spermatocytes. Strikingly, HSPA2, a chaperone protein for CDC2A kinase, is absent in mutant spermatocytes in spite of the presence of Hspa2 transcript, consistent with the observation that the repro8 phenotype is markedly similar to the phenotype of the Hspa2 knockout. Thus, EIF4G3 is required for HSPA2 translation in spermatocytes, a finding that provides the first genetic evidence for selective translational control of meiotic exit in mammalian spermatocytes.

t-SNARE Syntaxin2 (STX2) is implicated in intracellular transport of sulfoglycolipids during meiotic prophase in mouse spermatogenesis.

Syntaxin2 (STX2), also known as epimorphin, is a member of the SNARE family of proteins, with expression in various types of cells. We previously identified an ENU-induced mutation, repro34, in the mouse Stx2 gene. The Stx2(repro34) mutation causes male-restricted infertility due to syncytial multinucleation of spermatogenic cells during meiotic prophase. A similar phenotype is also observed in mice with targeted inactivation of Stx2, as well as in mice lacking enzymes involved in sulfoglycolipid synthesis. Herein we analyzed expression and subcellular localization of STX2 and sulfoglycolipids in spermatogenesis. The STX2 protein localizes to the cytoplasm of germ cells at the late pachytene stage. It is found in a distinct subcellular pattern, presumably in the Golgi apparatus of pachytene/diplotene spermatocytes. Sulfoglycolipids are produced in the Golgi apparatus and transported to the plasma membrane. In Stx2(repro34) mutants, sulfoglycolipids are aberrantly localized in both pachytene/diplotene spermatocytes and in multinucleated germ cells. These results suggest that STX2 plays roles in transport and/or subcellular distribution of sulfoglycolipids. STX2 function in the Golgi apparatus and sulfoglycolipids may be essential for maintenance of the constriction between neighboring developing spermatocytes, which ensures ultimate individualization of germ cells in later stages of spermatogenesis.