Sexually dimorphic DNA damage responses and mutation avoidance in the mouse germline.

Germ cells specified during fetal development form the foundation of the mammalian germline. These primordial germ cells (PGCs) undergo rapid proliferation, yet the germline is highly refractory to mutation accumulation compared with somatic cells. Importantly, while the presence of endogenous or exogenous DNA damage has the potential to impact PGCs, there is little known about how these cells respond to stressors. To better understand the DNA damage response (DDR) in these cells, we exposed pregnant mice to ionizing radiation (IR) at specific gestational time points and assessed the DDR in PGCs. Our results show that PGCs prior to sex determination lack a G1 cell cycle checkpoint. Additionally, the response to IR-induced DNA damage differs between female and male PGCs post-sex determination. IR of female PGCs caused uncoupling of germ cell differentiation and meiotic initiation, while male PGCs exhibited repression of piRNA metabolism and transposon derepression. We also used whole-genome single-cell DNA sequencing to reveal that genetic rescue of DNA repair-deficient germ cells (Fancm-/- ) leads to increased mutation incidence and biases. Importantly, our work uncovers novel insights into how PGCs exposed to DNA damage can become developmentally defective, leaving only those genetically fit cells to establish the adult germline.

Female-biased embryonic death from inflammation induced by genomic instability.

Genomic instability can trigger cellular responses that include checkpoint activation, senescence and inflammation1,2. Although genomic instability has been extensively studied in cell culture and cancer paradigms, little is known about its effect during embryonic development, a period of rapid cellular proliferation. Here we report that mutations in the heterohexameric minichromosome maintenance complex-the DNA replicative helicase comprising MCM2 to MCM73,4-that cause genomic instability render female mouse embryos markedly more susceptible than males to embryonic lethality. This bias was not attributable to X chromosome-inactivation defects, differential replication licensing or X versus Y chromosome size, but rather to 'maleness'-XX embryos could be rescued by transgene-mediated sex reversal or testosterone administration. The ability of exogenous or endogenous testosterone to protect embryos was related to its anti-inflammatory properties5. Ibuprofen, a non-steroidal anti-inflammatory drug, rescued female embryos that contained mutations in not only the Mcm genes but also the Fancm gene; similar to MCM mutants, Fancm mutant embryos have increased levels of genomic instability (measured as the number of cells with micronuclei) from compromised replication fork repair6. In addition, deficiency in the anti-inflammatory IL10 receptor was synthetically lethal with the Mcm4Chaos3 helicase mutant. Our experiments indicate that, during development, DNA damage associated with DNA replication induces inflammation that is preferentially lethal to female embryos, because male embryos are protected by high levels of intrinsic testosterone.

Practical strategies for robust and inexpensive imaging of aqueous-cleared tissues.

Lightsheet microscopy offers an ideal method for imaging of large (mm-cm scale) biological tissues rendered transparent via optical clearing protocols. However the diversity of clearing technologies and tissue types, and how these are adapted to the microscope can make tissue mounting complicated and somewhat irreproducible. Tissue preparation for imaging can involve glues and or equilibration in a variety of expensive and/or proprietary formulations. Here we present practical advice for mounting and capping cleared tissues in optical cuvettes for macroscopic imaging, providing a standardised 3D cell that can be imaged routinely and relatively inexpensively. We show that acrylic cuvettes cause minimal spherical aberration with objective numerical apertures less than 0.65. Furthermore, we describe methods for aligning and assessing the light sheets, discriminating fluorescence from autofluorescence, identifying chromatic artefacts due to differential scattering and removing streak artefacts such that they do not confound downstream 3D object segmentation analyses, with mouse embryo, liver and heart imaging as demonstrated examples.

A reporter mouse for in vivo detection of DNA damage in embryonic germ cells.

Maintaining genome integrity in the germline is essential for survival and propagation of a species. In both mouse and human, germ cells originate during fetal development and are hypersensitive to both endogenous and exogenous DNA damaging agents. Currently, mechanistic understanding of how primordial germ cells respond to DNA damage is limited in part by the tools available to study these cells. We developed a mouse transgenic reporter strain expressing a 53BP1-mCherry fusion protein under the control of the Oct4ΔPE embryonic germ cell-specific promoter. This reporter binds sites of DNA double strand breaks (DSBs) on chromatin, forming foci. Using ionizing radiation as a DNA DSB-inducing agent, we show that the transgenic reporter expresses specifically in the embryonic germ cells of both sexes and forms DNA damage induced foci in both a dose- and time-dependent manner. The dynamic time-sensitive and dose-sensitive DNA damage detection ability of this transgenic reporter, in combination with its specific expression in embryonic germ cells, makes it a versatile and valuable tool for increasing our understanding of DNA damage responses in these unique cells.

Nuclear RNR-α antagonizes cell proliferation by directly inhibiting ZRANB3.

Since the origins of DNA-based life, the enzyme ribonucleotide reductase (RNR) has spurred proliferation because of its rate-limiting role in de novo deoxynucleoside-triphosphate (dNTP) biosynthesis. Paradoxically, the large subunit, RNR-α, of this obligatory two-component complex in mammals plays a context-specific antiproliferative role. There is little explanation for this dichotomy. Here, we show that RNR-α has a previously unrecognized DNA-replication inhibition function, leading to growth retardation. This underappreciated biological activity functions in the nucleus, where RNR-α interacts with ZRANB3. This process suppresses ZRANB3's function in unstressed cells, which we show to promote DNA synthesis. This nonreductase function of RNR-α is promoted by RNR-α hexamerization-induced by a natural and synthetic nucleotide of dA/ClF/CLA/FLU-which elicits rapid RNR-α nuclear import. The newly discovered nuclear signaling axis is a primary defense against elevated or imbalanced dNTP pools that can exert mutagenic effects irrespective of the cell cycle.

Analysis of a mouse germ cell tumor model establishes pluripotency-associated miRNAs as conserved serum biomarkers for germ cell cancer detection.

Malignant testicular germ cells tumors (TGCTs) are the most common solid cancers in young men. Current TGCT diagnostics include conventional serum protein markers, but these lack the sensitivity and specificity to serve as accurate markers across all TGCT subtypes. MicroRNAs (miRNAs) are small non-coding regulatory RNAs and informative biomarkers for several diseases. In humans, miRNAs of the miR-371-373 cluster are detectable in the serum of patients with malignant TGCTs and outperform existing serum protein markers for both initial diagnosis and subsequent disease monitoring. We previously developed a genetically engineered mouse model featuring malignant mixed TGCTs consisting of pluripotent embryonal carcinoma (EC) and differentiated teratoma that, like the corresponding human malignancies, originate in utero and are highly chemosensitive. Here, we report that miRNAs in the mouse miR-290-295 cluster, homologs of the human miR-371-373 cluster, were detectable in serum from mice with malignant TGCTs but not from tumor-free control mice or mice with benign teratomas. miR-291-293 were expressed and secreted specifically by pluripotent EC cells, and expression was lost following differentiation induced by the drug thioridazine. Notably, miR-291-293 levels were significantly higher in the serum of pregnant dams carrying tumor-bearing fetuses compared to that of control dams. These findings reveal that expression of the miR-290-295 and miR-371-373 clusters in mice and humans, respectively, is a conserved feature of malignant TGCTs, further validating the mouse model as representative of the human disease. These data also highlight the potential of serum miR-371-373 assays to improve patient outcomes through early TGCT detection, possibly even prenatally.

Oocyte Elimination Through DNA Damage Signaling from CHK1/CHK2 to p53 and p63.

Eukaryotic organisms have evolved mechanisms to prevent the accumulation of cells bearing genetic aberrations. This is especially crucial for the germline, because fecundity and fitness of progeny would be adversely affected by an excessively high mutational incidence. The process of meiosis poses unique problems for mutation avoidance because of the requirement for SPO11-induced programmed double-strand breaks (DSBs) in recombination-driven pairing and segregation of homologous chromosomes. Mouse meiocytes bearing unrepaired meiotic DSBs or unsynapsed chromosomes are eliminated before completing meiotic prophase I. In previous work, we showed that checkpoint kinase 2 (CHK2; CHEK2), a canonical DNA damage response protein, is crucial for eliminating not only oocytes defective in meiotic DSB repair (e.g., Trip13Gt mutants), but also Spo11-/- oocytes that are defective in homologous chromosome synapsis and accumulate a threshold level of spontaneous DSBs. However, rescue of such oocytes by Chk2 deficiency was incomplete, raising the possibility that a parallel checkpoint pathway(s) exists. Here, we show that mouse oocytes lacking both p53 (TRP53) and the oocyte-exclusive isoform of p63, TAp63, protects nearly all Spo11-/- and Trip13Gt/Gt oocytes from elimination. We present evidence that checkpoint kinase I (CHK1; CHEK1), which is known to signal to TRP53, also becomes activated by persistent DSBs in oocytes, and to an increased degree when CHK2 is absent. The combined data indicate that nearly all oocytes reaching a threshold level of unrepaired DSBs are eliminated by a semiredundant pathway of CHK1/CHK2 signaling to TRP53/TAp63.

SKP1 drives the prophase I to metaphase I transition during male meiosis.

The meiotic prophase I to metaphase I (PI/MI) transition requires chromosome desynapsis and metaphase competence acquisition. However, control of these major meiotic events is poorly understood. Here, we identify an essential role for SKP1, a core subunit of the SKP1-Cullin-F-box (SCF) ubiquitin E3 ligase, in the PI/MI transition. SKP1 localizes to synapsed chromosome axes and evicts HORMAD proteins from these regions in meiotic spermatocytes. SKP1-deficient spermatocytes display premature desynapsis, precocious pachytene exit, loss of PLK1 and BUB1 at centromeres, but persistence of HORMAD, γH2AX, RPA2, and MLH1 in diplonema. Strikingly, SKP1-deficient spermatocytes show sharply reduced MPF activity and fail to enter MI despite treatment with okadaic acid. SKP1-deficient oocytes exhibit desynapsis, chromosome misalignment, and progressive postnatal loss. Therefore, SKP1 maintains synapsis in meiosis of both sexes. Furthermore, our results support a model where SKP1 functions as the long-sought intrinsic metaphase competence factor to orchestrate MI entry during male meiosis.

Whole Mount Immunofluorescence and Follicle Quantification of Cultured Mouse Ovaries.

Research in the field of mammalian reproductive biology often involves evaluating the overall health of ovaries and testes. Specifically, in females, ovarian fitness is often assessed by visualizing and quantifying follicles and oocytes. Because the ovary is an opaque three-dimensional tissue, traditional approaches require laboriously slicing the tissue into numerous serial sections in order to visualize cells throughout the entire organ. Furthermore, because quantification by this method typically entails scoring only a subset of the sections separated by the approximate diameter of an oocyte, it is prone to inaccuracy. Here, a protocol is described that instead utilizes whole organ tissue clearing and immunofluorescence staining of mouse ovaries to visualize follicles and oocytes. Compared to more traditional approaches, this protocol is advantageous for visualizing cells within the ovary for numerous reasons: 1) the ovary remains intact throughout sample preparation and processing; 2) small ovaries, which are difficult to section, can be examined with ease; 3) cellular quantification is more readily and accurately achieved; and 4) the whole organ imaged.

SMG-ly knocking out gene expression in specific cells: an educational primer for use with "a novel strategy for cell-autonomous gene knockdown in caenorhabditis elegans defines a cell-specific function for the G-protein subunit GOA-1".

A recent article by Maher et al. in GENETICS introduces an alternative approach to cell-type-specific gene knockdown in Caenorhabditis elegans, using nonsense-mediated decay. This strategy has the potential to be applicable to other organisms (this strategy requires that animals can survive without nonsense-mediated decay-not all can). This Primer article provides a guide and resource for educators and students by describing different gene knockdown methodologies, by assisting with the technically difficult portions of the Maher et al. article, and by providing conceptual questions relating to the article.