A widely employed germ cell marker is an ancient disordered protein with reproductive functions in diverse eukaryotes

How germ granules promote germ cell fate

Germ cells are the only cells in the body capable of giving rise to a new organism, and this totipotency hinges on their ability to assemble membraneless germ granules. These specialized RNA and protein complexes are hallmarks of germ cells throughout their life cycle: as embryonic germ granules in late oocytes and zygotes, Balbiani bodies in immature oocytes, and nuage in maturing gametes. Decades of developmental, genetic and biochemical studies have identified protein and RNA constituents unique to germ granules and have implicated these in germ cell identity, genome integrity and gamete differentiation. Now, emerging research is defining germ granules as biomolecular condensates that achieve high molecular concentrations by phase separation, and it is assigning distinct roles to germ granules during different stages of germline development. This organization of the germ cell cytoplasm into cellular subcompartments seems to be critical not only for the flawless continuity through the germline life cycle within the developing organism but also for the success of the next generation.

What are the DNA lesions underlying formaldehyde toxicity?

Formaldehyde is a highly reactive organic compound. Humans can be exposed to exogenous sources of formaldehyde, but formaldehyde is also produced endogenously as a byproduct of cellular metabolism. Because formaldehyde can react with DNA, it is considered a major endogenous source of DNA damage. However, the nature of the lesions underlying formaldehyde toxicity in cells remains vastly unknown. Here, we review the current knowledge of the different types of nucleic acid lesions that are induced by formaldehyde and describe the repair pathways known to counteract formaldehyde toxicity. Taking this knowledge together, we discuss and speculate on the predominant lesions generated by formaldehyde, which underly its natural toxicity.

Recurrent Duplication and Diversification of a Vital DNA Repair Gene Family Across Drosophila

Maintaining genome integrity is vital for organismal survival and reproduction. Essential, broadly conserved DNA repair pathways actively preserve genome integrity. However, many DNA repair proteins evolve adaptively. Ecological forces like UV exposure are classically cited drivers of DNA repair evolution. Intrinsic forces like repetitive DNA, which also imperil genome integrity, have received less attention. We recently reported that a Drosophila melanogaster-specific DNA satellite array triggered species-specific, adaptive evolution of a DNA repair protein called Spartan/MH. The Spartan family of proteases cleave hazardous, covalent crosslinks that form between DNA and proteins ("DNA-protein crosslink repair"). Appreciating that DNA satellites are both ubiquitous and universally fast-evolving, we hypothesized that satellite DNA turnover spurs adaptive evolution of DNA-protein crosslink repair beyond a single gene and beyond the D. melanogaster lineage. This hypothesis predicts pervasive Spartan gene family diversification across Drosophila species. To study the evolutionary history of the Drosophila Spartan gene family, we conducted population genetic, molecular evolution, phylogenomic, and tissue-specific expression analyses. We uncovered widespread signals of positive selection across multiple Spartan family genes and across multiple evolutionary timescales. We also detected recurrent Spartan family gene duplication, divergence, and gene loss. Finally, we found that ovary-enriched parent genes consistently birthed functionally diverged, testis-enriched daughter genes. To account for Spartan family diversification, we introduce a novel mechanistic model of antagonistic coevolution that links DNA satellite evolution and adaptive regulation of Spartan protease activity. This framework promises to accelerate our understanding of how DNA repeats drive recurrent evolutionary innovation to preserve genome integrity.

Germline cell de novo mutations and potential effects of inflammation on germline cell genome stability

Uncontrolled pathogenic genome mutations in germline cells might impair adult fertility, lead to birth defects or even affect the adaptability of a species. Understanding the sources of DNA damage, as well as the features of damage response in germline cells are the overarching tasks to reduce the mutations in germline cells. With the accumulation of human genome data and genetic reports, genome variants formed in germline cells are being extensively explored. However, the sources of DNA damage, the damage repair mechanisms, and the effects of DNA damage or mutations on the development of germline cells are still unclear. Besides exogenous triggers of DNA damage such as irradiation and genotoxic chemicals, endogenous exposure to inflammation may also contribute to the genome instability of germline cells. In this review, we summarized the features of de novo mutations and the specific DNA damage responses in germline cells and explored the possible roles of inflammation on the genome stability of germline cells.

GCNA is a histone binding protein required for spermatogonial stem cell maintenance

Recycling and de-novo deposition of histones during DNA replication is a critical challenge faced by eukaryotic cells and is coordinated by histone chaperones. Spermatogenesis is highly regulated sophisticated process necessitating not only histone modification but loading of testis specific histone variants. Here, we show that Germ Cell Nuclear Acidic protein (GCNA), a germ cell specific protein in adult mice, can bind histones and purified GCNA exhibits histone chaperone activity. GCNA associates with the DNA replication machinery and supports progression through S-phase in murine undifferentiated spermatogonia (USGs). Whilst GCNA is dispensable for embryonic germ cell development, it is required for the maintenance of the USG pool and for long-term production of sperm. Our work describes the role of a germ cell specific histone chaperone in USGs maintenance in mice. These findings provide a mechanistic basis for the male infertility observed in patients carrying GCNA mutations.

Potassium perchlorate effects on primordial germ cells of developing medaka larvae

Perchlorate is a chemical compound commonly used in military artillery and equipment. It has been detected in drinking water, air, soil, and breast milk. Exposure of humans to perchlorate can occur in the theater of war and areas adjacent to military training grounds. A high concentration of perchlorate has been found to affect reproduction in vertebrates, including fish. However, whether environmental concentrations of perchlorate can affect primordial germ cells (PGCs), the founders of sperm and eggs, is not clearly understood. In the present study, we examined the effects of 0, 10, 100, and 1000 μg/L potassium perchlorate exposure on the embryonic development of medaka and their PGCs. Perchlorate exposure delayed hatching time, reduced heartbeat, inhibited migration of PGCs, and increased developmental deformities in the larvae. The 10 and 20 mg/L concentrations of perchlorate were lethal to embryos, whereas vitamin C co-treatment (1 mg/L) completely blocked perchlorate-induced mortality. RNA-seq analysis of isolated PGCs showed a non-linear pattern in expression profiles of differentially altered genes. Significantly upregulated genes were found in PGCs from the 10 and 1000 μg/L groups, whereas the 100 μg/L groups showed the highest number of significantly downregulated genes. Gene ontology analysis predicted differentially expressed genes to be involved in proteolysis, metabolic processes, peptides activity, hydrolase activity, and hormone activity. Among the cellular components, extracellular, intracellular, sarcoplasmic, and 6-phosphofructokinase and membrane-bounded processes were affected. Ingenuity Pathway Analysis of PGC transcriptomes revealed thyroid hormone signaling to be affected by all concentrations of perchlorate. The present results suggested that perchlorate affected the development of medaka larvae and vitamin C was able to ameliorate perchlorate-induced embryo mortality. Additionally, perchlorate altered the global transcriptional network in PGCs in a non-linear fashion suggesting its potential effects on developing germ cells and fertility.

The germ cell-specific RNA binding protein RBM46 is essential for spermatogonial differentiation in mice

Control over gene expression is exerted, in multiple stages of spermatogenesis, at the post-transcriptional level by RNA binding proteins (RBPs). We identify here an essential role in mammalian spermatogenesis and male fertility for 'RNA binding protein 46' (RBM46). A highly evolutionarily conserved gene, Rbm46 is also essential for fertility in both flies and fish. We found Rbm46 expression was restricted to the mouse germline, detectable in males in the cytoplasm of premeiotic spermatogonia and meiotic spermatocytes. To define its requirement for spermatogenesis, we generated Rbm46 knockout (KO, Rbm46-/-) mice; although male Rbm46-/- mice were viable and appeared grossly normal, they were infertile. Testes from adult Rbm46-/- mice were small, with seminiferous tubules containing only Sertoli cells and few undifferentiated spermatogonia. Using genome-wide unbiased high throughput assays RNA-seq and 'enhanced crosslinking immunoprecipitation' coupled with RNA-seq (eCLIP-seq), we discovered RBM46 could bind, via a U-rich conserved consensus sequence, to a cohort of mRNAs encoding proteins required for completion of differentiation and subsequent meiotic initiation. In summary, our studies support an essential role for RBM46 in regulating target mRNAs during spermatogonia differentiation prior to the commitment to meiosis in mice.

CHD4 acts as a critical regulator in the survival of spermatogonial stem cells in mice

Spermatogenesis is sustained by homeostatic balance between the self-renewal and differentiation of spermatogonial stem cells (SSCs), which is dependent on the strict regulation of transcription factor and chromatin modulator gene expression. Chromodomain helicase DNA-binding protein 4 (CHD4) is highly expressed in SSCs but roles in mouse spermatogenesis are not fully understood. Here, we report that the germ-cell-specific deletion of Chd4 resulted in complete infertility in male mice, with rapid loss of SSCs and excessive germ cell apoptosis. Chd4-knockdown in cultured SSCs also promoted the expression of apoptosis-related genes and thereby activated the tumor necrosis factor signaling pathway. Mechanistically, CHD4 occupies the genomic regulatory region of key apoptosis-related genes including Jun and Nfkb1. Together, our findings reveal the determinant role of CHD4 in SSCs survival in vivo, which will offer insight into the pathogenesis of male sterility and potential novel therapeutic targets.

Targeting DNA-Protein Crosslinks via Post-Translational Modifications

Covalent binding of proteins to DNA forms DNA-protein crosslinks (DPCs), which represent cytotoxic DNA lesions that interfere with essential processes such as DNA replication and transcription. Cells possess different enzymatic activities to counteract DPCs. These include enzymes that degrade the adducted proteins, resolve the crosslinks, or incise the DNA to remove the crosslinked proteins. An important question is how DPCs are sensed and targeted for removal via the most suited pathway. Recent advances have shown the inherent role of DNA replication in triggering DPC removal by proteolysis. However, DPCs are also efficiently sensed and removed in the absence of DNA replication. In either scenario, post-translational modifications (PTMs) on DPCs play essential and versatile roles in orchestrating the repair routes. In this review, we summarize the current knowledge of the mechanisms that trigger DPC removal via PTMs, focusing on ubiquitylation, small ubiquitin-related modifier (SUMO) conjugation (SUMOylation), and poly (ADP-ribosyl)ation (PARylation). We also briefly discuss the current knowledge gaps and emerging hypotheses in the field.

Mechanisms and Regulation of DNA-Protein Crosslink Repair During DNA Replication by SPRTN Protease

DNA-protein crosslinks (DPCs) are deleterious DNA lesions that occur when proteins are covalently crosslinked to the DNA by the action of variety of agents like reactive oxygen species, aldehydes and metabolites, radiation, and chemotherapeutic drugs. Unrepaired DPCs are blockades to all DNA metabolic processes. Specifically, during DNA replication, replication forks stall at DPCs and are vulnerable to fork collapse, causing DNA breakage leading to genome instability and cancer. Replication-coupled DPC repair involves DPC degradation by proteases such as SPRTN or the proteasome and the subsequent removal of DNA-peptide adducts by nucleases and canonical DNA repair pathways. SPRTN is a DNA-dependent metalloprotease that cleaves DPC substrates in a sequence-independent manner and is also required for translesion DNA synthesis following DPC degradation. Biallelic mutations in SPRTN cause Ruijs-Aalfs (RJALS) syndrome, characterized by hepatocellular carcinoma and segmental progeria, indicating the critical role for SPRTN and DPC repair pathway in genome maintenance. In this review, we will discuss the mechanism of replication-coupled DPC repair, regulation of SPRTN function and its implications in human disease and cancer.

The Requirement of Ubiquitin C-Terminal Hydrolase L1 (UCHL1) in Mouse Ovarian Development and Fertility †

Ubiquitin C-Terminal Hydrolase L1 (UCHL1) is a de-ubiquitinating enzyme enriched in neuronal and gonadal tissues known to regulate the cellular stores of mono-ubiquitin and protein turnover. While its function in maintaining proper motor neuron function is well-established, investigation into its role in the health and function of reproductive processes is only just beginning to be studied. Single-cell-sequencing analysis of all ovarian cells from the murine perinatal period revealed that Uchl1 is very highly expressed in the developing oocyte population, an observation which was corroborated by high levels of oocyte-enriched UCHL1 protein expression in oocytes of all stages throughout the mouse reproductive lifespan. To better understand the role UCHL1 may be playing in oocytes, we utilized a UCHL1-deficient mouse line, finding reduced number of litters, reduced litter sizes, altered folliculogenesis, morphologically abnormal oocytes, disrupted estrous cyclicity and apparent endocrine dysfunction in these animals compared to their wild-type and heterozygous littermates. These data reveal a novel role of UCHL1 in female fertility as well as overall ovarian function, and suggest a potentially essential role for the ubiquitin proteasome pathway in mediating reproductive health. Summary sentence: Ubiquitin C-Terminal Hydrolase L1 (UCHL1) is required for proper ovarian folliculogenesis, estrous cyclicity, and fertility in the female mouse.

idpr: A package for profiling and analyzing Intrinsically Disordered Proteins in R

Intrinsically disordered proteins (IDPs) and intrinsically disordered regions (IDRs) are proteins or protein-domains that do not have a single native structure, rather, they are a class of flexible peptides that can rapidly adopt multiple conformations. IDPs are quite abundant, and their dynamic characteristics provide unique advantages for various biological processes. The field of “unstructured biology” has emerged, in part, because of numerous computational studies that had identified the unique characteristics of IDPs and IDRs. The package ‘idpr’, short for Intrinsically Disordered Proteins in R, implements several R functions that match the established characteristics of IDPs to protein sequences of interest. This includes calculations of residue composition, charge-hydropathy relationships, and predictions of intrinsic disorder. Additionally, idpr integrates several amino acid substitution matrices and calculators to supplement IDP-based workflows. Overall, idpr aims to integrate tools for the computational analysis of IDPs within R, facilitating the analysis of these important, yet under-characterized, proteins. The idpr package can be downloaded from Bioconductor (https://bioconductor.org/packages/idpr/).

DNA-Protein Crosslinks and Their Resolution

Covalent DNA-protein crosslinks (DPCs) are pervasive DNA lesions that interfere with essential chromatin processes such as transcription or replication. This review strives to provide an overview of the sources and principles of cellular DPC formation. DPCs are caused by endogenous reactive metabolites and various chemotherapeutic agents. However, in certain conditions DPCs also arise physiologically in cells. We discuss the cellular mechanisms resolving these threats to genomic integrity. Detection and repair of DPCs require not only the action of canonical DNA repair pathways but also the activity of specialized proteolytic enzymes-including proteases of the SPRTN/Wss1 family-to degrade the crosslinked protein. Loss of DPC repair capacity has dramatic consequences, ranging from genome instability in yeast and worms to cancer predisposition and premature aging in mice and humans. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Multiple 9-1-1 complexes promote homolog synapsis, DSB repair, and ATR signaling during mammalian meiosis

DNA damage response mechanisms have meiotic roles that ensure successful gamete formation. While completion of meiotic double-strand break (DSB) repair requires the canonical RAD9A-RAD1-HUS1 (9A-1-1) complex, mammalian meiocytes also express RAD9A and HUS1 paralogs, RAD9B and HUS1B, predicted to form alternative 9-1-1 complexes. The RAD1 subunit is shared by all predicted 9-1-1 complexes and localizes to meiotic chromosomes even in the absence of HUS1 and RAD9A. Here, we report that testis-specific disruption of RAD1 in mice resulted in impaired DSB repair, germ cell depletion, and infertility. Unlike Hus1 or Rad9a disruption, Rad1 loss in meiocytes also caused severe defects in homolog synapsis, impaired phosphorylation of ATR targets such as H2AX, CHK1, and HORMAD2, and compromised meiotic sex chromosome inactivation. Together, these results establish critical roles for both canonical and alternative 9-1-1 complexes in meiotic ATR activation and successful prophase I completion.

Phylogenetic convergence of phase separation and mitotic function in the disordered protein BuGZ

Intrinsically disordered proteins (IDPs) effect biological function despite their sequence-encoded lack of preference for stable three-dimensional structure. Among their many functions, IDPs form membraneless cellular compartments through liquid-liquid phase separation (LLPS), also termed biomolecular condensation. The extent to which LLPS has been evolutionarily selected remains largely unknown, as the complexities of IDP evolution hamper progress. Unlike structured proteins, rapid sequence divergence typical of IDPs confounds inference of their biophysical or biological functions from comparative sequence analyses. Here, we leverage mitosis as a universal eukaryotic feature to interrogate condensate evolutionary history. We observe that evolution has conserved the ability for six homologs of the mitotic IDP BuGZ to undergo LLPS and to serve the same mitotic function, despite low sequence conservation. We also observe that cellular context may tune LLPS. The phylogenetic correlation of LLPS and mitotic function in one protein raises the possibility of an ancient evolutionary interplay between LLPS and biological function, dating back at least 1.6 billion years to the last common ancestor of plants and animals. This article is protected by copyright. All rights reserved.

Proteolysis in Reproduction: Lessons From Gene-Modified Organism Studies

The physiological roles of proteolysis are not limited to degrading unnecessary proteins. Proteolysis plays pivotal roles in various biological processes through cleaving peptide bonds to activate and inactivate proteins including enzymes, transcription factors, and receptors. As a wide range of cellular processes is regulated by proteolysis, abnormalities or dysregulation of such proteolytic processes therefore often cause diseases. Recent genetic studies have clarified the inclusion of proteases and protease inhibitors in various reproductive processes such as development of gonads, generation and activation of gametes, and physical interaction between gametes in various species including yeast, animals, and plants. Such studies not only clarify proteolysis-related factors but the biological processes regulated by proteolysis for successful reproduction. Here the physiological roles of proteases and proteolysis in reproduction will be reviewed based on findings using gene-modified organisms.

Building RNA-protein germ granules: insights from the multifaceted functions of DEAD-box helicase Vasa/Ddx4 in germline development

The segregation and maintenance of a dedicated germline in multicellular organisms is essential for species propagation in the sexually reproducing metazoan kingdom. The germline is distinct from somatic cells in that it is ultimately dedicated to acquiring the "totipotency" and to regenerating the offspring after fertilization. The most striking feature of germ cells lies in the presence of characteristic membraneless germ granules that have recently proven to behave like liquid droplets resulting from liquid-liquid phase separation (LLPS). Vasa/Ddx4, a faithful DEAD-box family germline marker highly conserved across metazoan species, harbors canonical DEAD-box motifs and typical intrinsically disordered sequences at both the N-terminus and C-terminus. This feature enables it to serve as a primary driving force behind germ granule formation and helicase-mediated RNA metabolism (e.g., piRNA biogenesis). Genetic ablation of Vasa/Ddx4 or the catalytic-dead mutations abolishing its helicase activity led to sexually dimorphic germline defects resulting in either male or female sterility among diverse species. While recent efforts have discovered pivotal functions of Vasa/Ddx4 in somatic cells, especially in multipotent stem cells, we herein summarize the helicase-dependent and -independent functions of Vasa/Ddx4 in the germline, and discuss recent findings of Vasa/Ddx4-mediated phase separation, germ granule formation and piRNA-dependent retrotransposon control essential for germline development.

Pathogenic variations in Germ Cell Nuclear Acidic Peptidase (GCNA) are associated with human male infertility

Infertility affects one in six couples, half of which are caused by a male factor. Male infertility can be caused by both, qualitative and quantitative defects, leading to Oligo- astheno-terato-zoospermia (OAT; impairment in ejaculate sperm cell concentration, motility and morphology). Azoospermia defined as complete absence of sperm cells in the ejaculation. While hundreds of genes are involved in spermatogenesis the genetic etiology of men's infertility remains incomplete.We identified a hemizygous stop gain pathogenic variation (PV) in the X-linked Germ Cell Nuclear Acidic Peptidase (GCNA), in an Azoospermic patient by exome sequencing. Assessment of the prevalence of pathogenic variations in this gene in infertile males by exome sequence data of 11 additional unrelated patients identified a probable hemizygous causative missense PV in GCNA in a severe OAT patient. Expression of GCNA in the patients' testes biopsies and the stage of spermatogonial developmental arrest were determined by immunofluorescence and immunohistochemistry. The Azoospermic patient presented spermatogenic maturation arrest with an almost complete absence of early and late primary spermatocytes and thus the complete absence of sperm. GCNA is critical for genome integrity and its loss results in genomic instability and infertility in Drosophila, C. elegans, zebrafish, and mouse. PVs in GCNA appear to be incompatible with male fertility in humans as well: A stop-gain PV caused Azoospermia and a missense PV caused severe OAT with very low fertilization rates and no pregnancy in numerous IVF treatments.

Primordial Germ Cell Specification in Vertebrate Embryos: Phylogenetic Distribution and Conserved Molecular Features of Preformation and Induction

The differentiation of primordial germ cells (PGCs) occurs during early embryonic development and is critical for the survival and fitness of sexually reproducing species. Here, we review the two main mechanisms of PGC specification, induction, and preformation, in the context of four model vertebrate species: mouse, axolotl, Xenopus frogs, and zebrafish. We additionally discuss some notable molecular characteristics shared across PGC specification pathways, including the shared expression of products from three conserved germline gene families, DAZ (Deleted in Azoospermia) genes, nanos-related genes, and DEAD-box RNA helicases. Then, we summarize the current state of knowledge of the distribution of germ cell determination systems across kingdom Animalia, with particular attention to vertebrate species, but include several categories of invertebrates - ranging from the "proto-vertebrate" cephalochordates to arthropods, cnidarians, and ctenophores. We also briefly highlight ongoing investigations and potential lines of inquiry that aim to understand the evolutionary relationships between these modes of specification.

Mechanism and function of DNA replication-independent DNA-protein crosslink repair via the SUMO-RNF4 pathway

DNA‐protein crosslinks (DPCs) obstruct essential DNA transactions, posing a serious threat to genome stability and functionality. DPCs are proteolytically processed in a ubiquitin‐ and DNA replication‐dependent manner by SPRTN and the proteasome but can also be resolved via targeted SUMOylation. However, the mechanistic basis of SUMO‐mediated DPC resolution and its interplay with replication‐coupled DPC repair remain unclear. Here, we show that the SUMO‐targeted ubiquitin ligase RNF4 defines a major pathway for ubiquitylation and proteasomal clearance of SUMOylated DPCs in the absence of DNA replication. Importantly, SUMO modifications of DPCs neither stimulate nor inhibit their rapid DNA replication‐coupled proteolysis. Instead, DPC SUMOylation provides a critical salvage mechanism to remove DPCs formed after DNA replication, as DPCs on duplex DNA do not activate interphase DNA damage checkpoints. Consequently, in the absence of the SUMO‐RNF4 pathway cells are able to enter mitosis with a high load of unresolved DPCs, leading to defective chromosome segregation and cell death. Collectively, these findings provide mechanistic insights into SUMO‐driven pathways underlying replication‐independent DPC resolution and highlight their critical importance in maintaining chromosome stability and cellular fitness.

Identification and characterization of the antigen recognized by the germ cell mAb TRA98 using a human comprehensive wet protein array

TRA98 is a rat monoclonal antibody (mAb) which recognizes a specific antigen in the nuclei of germ cells. mAb TRA98 has been used to understand the mechanism of germ cell development and differentiation in many studies. In mice, the antigen recognized by mAb TRA98 or GCNA1 has been reported to be a GCNA gene product, but despite the demonstration of the immunoreactivity of this mAb in human testis and sperm in 1997, the antigen in humans remains unknown, as of date. To identify the human antigen recognized by mAb TRA98, a human comprehensive wet protein array was developed containing 19,446 proteins derived from human cDNAs. Using this array, it was found that the antigen of mAb TRA98 is not a GCNA gene product, but nuclear factor-κB activating protein (NKAP). In mice, mAb TRA98 recognized both the GCNA gene product and NKAP. Furthermore, conditional knockout of Nkap in mice revealed a phenotype of Sertoli cell-only syndrome. Although NKAP is a ubiquitously expressed protein, NKAP recognized by mAb TRA98 in mouse testis was SUMOylated. These results suggest that NKAP undergoes modifications, such as SUMOylation in the testis, and plays an important role in spermatogenesis.

Protein-based condensation mechanisms drive the assembly of RNA-rich P granules

Germ granules are protein-RNA condensates that segregate with the embryonic germline. In Caenorhabditis elegans embryos, germ (P) granule assembly requires MEG-3, an intrinsically disordered protein that forms RNA-rich condensates on the surface of PGL condensates at the core of P granules. MEG-3 is related to the GCNA family and contains an N-terminal disordered region (IDR) and a predicted ordered C-terminus featuring an HMG-like motif (HMGL). We find that MEG-3 is a modular protein that uses its IDR to bind RNA and its C-terminus to drive condensation. The HMGL motif mediates binding to PGL-3 and is required for co-assembly of MEG-3 and PGL-3 condensates in vivo. Mutations in HMGL cause MEG-3 and PGL-3 to form separate condensates that no longer co-segregate to the germline or recruit RNA. Our findings highlight the importance of protein-based condensation mechanisms and condensate-condensate interactions in the assembly of RNA-rich germ granules.

Variants in GCNA, X-linked germ-cell genome integrity gene, identified in men with primary spermatogenic failure

Male infertility impacts millions of couples yet, the etiology of primary infertility remains largely unknown. A critical element of successful spermatogenesis is maintenance of genome integrity. Here, we present a genomic study of spermatogenic failure (SPGF). Our initial analysis (n = 176) did not reveal known gene-candidates but identified a potentially significant single-nucleotide variant (SNV) in X-linked germ-cell nuclear antigen (GCNA). Together with a larger follow-up study (n = 2049), 7 likely clinically relevant GCNA variants were identified. GCNA is critical for genome integrity in male meiosis and knockout models exhibit impaired spermatogenesis and infertility. Single-cell RNA-seq and immunohistochemistry confirm human GCNA expression from spermatogonia to elongated spermatids. Five identified SNVs were located in key functional regions, including N-terminal SUMO-interacting motif and C-terminal Spartan-like protease domain. Notably, variant p.Ala115ProfsTer7 results in an early frameshift, while Spartan-like domain missense variants p.Ser659Trp and p.Arg664Cys change conserved residues, likely affecting 3D structure. For variants within GCNA's intrinsically disordered region, we performed computational modeling for consensus motifs. Two SNVs were predicted to impact the structure of these consensus motifs. All identified variants have an extremely low minor allele frequency in the general population and 6 of 7 were not detected in > 5000 biological fathers. Considering evidence from animal models, germ-cell-specific expression, 3D modeling, and computational predictions for SNVs, we propose that identified GCNA variants disrupt structure and function of the respective protein domains, ultimately arresting germ-cell division. To our knowledge, this is the first study implicating GCNA, a key genome integrity factor, in human male infertility.

Germ cell determination and the developmental origin of germ cell tumors

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

DNA-Histone Cross-Links: Formation and Repair

The nucleosome is a stretch of DNA wrapped around a histone octamer. Electrostatic interactions and hydrogen bonds between histones and DNA are vital for the stable organization of nucleosome core particles, and for the folding of chromatin into more compact structures, which regulate gene expression via controlled access to DNA. As a drawback of tight association, under genotoxic stress, DNA can accidentally cross-link to histone in a covalent manner, generating a highly toxic DNA-histone cross-link (DHC). DHC is a bulky lesion that can impede DNA transcription, replication, and repair, often with lethal consequences. The chemotherapeutic agent cisplatin, as well as ionizing and ultraviolet irradiations and endogenously occurring reactive aldehydes, generate DHCs by forming either stable or transient covalent bonds between DNA and side-chain amino groups of histone lysine residues. The mechanisms of DHC repair start to unravel, and certain common principles of DNA-protein cross-link (DPC) repair mechanisms that participate in the removal of cross-linked histones from DNA have been described. In general, DPC is removed via a two-step repair mechanism. First, cross-linked proteins are degraded by specific DPC proteases or by the proteasome, relieving steric hindrance. Second, the remaining DNA-peptide cross-links are eliminated in various DNA repair pathways. Delineating the molecular mechanisms of DHC repair would help target specific DNA repair proteins for therapeutic intervention to combat tumor resistance to chemotherapy and radiotherapy.

DNA-protein crosslink proteases in genome stability

Proteins covalently attached to DNA, also known as DNA-protein crosslinks (DPCs), are common and bulky DNA lesions that interfere with DNA replication, repair, transcription and recombination. Research in the past several years indicates that cells possess dedicated enzymes, known as DPC proteases, which digest the protein component of a DPC. Interestingly, DPC proteases also play a role in proteolysis beside DPC repair, such as in degrading excess histones during DNA replication or controlling DNA replication checkpoints. Here, we discuss the importance of DPC proteases in DNA replication, genome stability and their direct link to human diseases and cancer therapy.

How to fix DNA-protein crosslinks

Proteins that act on DNA, or are in close proximity to it, can become inadvertently crosslinked to DNA and form highly toxic lesions, known as DNA-protein crosslinks (DPCs). DPCs are generated by different chemotherapeutics, environmental or endogenous sources of crosslinking agents, or by lesions on DNA that stall the catalytic cycle of certain DNA processing enzymes. These bulky adducts impair processes on DNA such as DNA replication or transcription, and therefore pose a serious threat to genome integrity. The large diversity of DPCs suggests that there is more than one canonical mechanism to repair them. Indeed, many different enzymes have been shown to act on DPCs by either processing the protein, the DNA or the crosslink itself. In addition, the cell cycle stage or cell type are likely to dictate pathway choice. In recent years, a detailed understanding of DPC repair during S phase has started to emerge. Here, we review the current knowledge on the mechanisms of replication-coupled DPC repair, and describe and also speculate on possible pathways that remove DPCs outside of S phase. Moreover, we highlight a recent paradigm shifting finding that indicates that DPCs are not always detrimental, but can also play a protective role, preserving the genome from more deleterious forms of DNA damage.

Premature ovarian insufficiency in the XO female mouse on the C57BL/6J genetic background

In humans, all but 1% of monosomy 45.X embryos die in utero and those who reach term suffer from congenital abnormalities and infertility termed Turner's syndrome (TS). By contrast, XO female mice on various genetic backgrounds show much milder physical defects and normal fertility, diminishing their value as an animal model for studying the infertility of TS patients. In this article, we report that XO mice on the C57BL/6J (B6) genetic background showed early oocyte loss, infertility or subfertility and high embryonic lethality, suggesting that the effect of monosomy X in the female germline may be shared between mice and humans. First, we generated XO mice on either a mixed N2(C3H.B6) or B6 genetic background and compared the number of oocytes in neonatal ovaries; N2.XO females retained 45% of the number of oocytes in N2.XX females, whereas B6.XO females retained only 15% of that in B6.XX females. Second, while N2.XO females were as fertile as N2.XX females, both the frequency of delivery and the total number of pups delivered by B6.XO females were significantly lower than those by B6.XX females. Third, after mating with B6 males, both N2.XO and B6.XO females rarely produced XO pups carrying paternal X chromosomes, although a larger percentage of embryos was found to be XO before implantation. Furthermore, B6.XO females delivered 20% XO pups among female progeny after mating with C3H males. We conclude that the impact of monosomy X on female mouse fertility depends on the genetic background.

Aberrant and constitutive expression of FOXL2 impairs ovarian development and functions in mice

Development and functions of the ovary rely on appropriate signaling and communication between various ovarian cell types. FOXL2, a transcription factor that plays a key role at different stages of ovarian development, is associated with primary ovarian insufficiency and ovarian cancer as a result of its loss-of-function or mutations. In this study, we investigated the impact of aberrant, constitutive expression of FOXL2 in somatic cells of the ovary. Overexpression of FOXL2 that started during fetal life resulted in defects in nest breakdown and consequent formation of polyovular follicles. Granulosa cell differentiation was impaired and recruitment and differentiation of steroidogenic theca cells was compromised. As a consequence, adult ovaries overexpressing FOXL2 exhibited defects in compartmentalization of granulosa and theca cells, significant decreased steroidogenesis and lack of ovulation. These findings demonstrate that fine-tuned expression of FOXL2 is required for proper folliculogenesis and fertility.

When the Family Treasure Is a Doormat

Germ cells carry genetic information to the next generation, necessitating special attention to their genome maintenance. Two new studies in this issue of Developmental Cell (Bhargava et al., 2020; Dokshin et al., 2020) reveal an essential function of germ cell-specific protein GCNA in the genome maintenance of germ cells.

Meiosis occurs normally in the fetal ovary of mice lacking all retinoic acid receptors

Gametes are generated through a specialized cell differentiation process, meiosis, which, in ovaries of most mammals, is initiated during fetal life. All-trans retinoic acid (ATRA) is considered as the molecular signal triggering meiosis initiation. In the present study, we analyzed female fetuses ubiquitously lacking all ATRA nuclear receptors (RAR), obtained through a tamoxifen-inducible cre recombinase-mediated gene targeting approach. Unexpectedly, mutant oocytes robustly expressed meiotic genes, including the meiotic gatekeeper STRA8. In addition, ovaries from mutant fetuses grafted into adult recipient females yielded offspring bearing null alleles for all Rar genes. Thus, our results show that RAR are fully dispensable for meiotic initiation, as well as for the production of functional oocytes. Assuming that the effects of ATRA all rely on RAR, our study goes against the current model according to which meiosis is triggered by endogenous ATRA in the developing ovary. It therefore revives the search for the meiosis-inducing substance.

Contrasted evolutionary trajectories of plant transcription factors

Because of their prominent roles in plant development, transcription factors (TF) play central roles as drivers of innovation in the evolution of the green lineage (viridiplantae). The advent of massive sequencing combined with comparative genetics/genomics allows a rigorous investigation of how TF families have contributed to plant diversification from charophyte algae to bryophytes to angiosperms. Here, we review recent progress on TF family reconstruction and the identification of distantly related TFs present throughout the evolutionary timeline from algae to angiosperms. These data provide examples of contrasting evolutionary trajectories of TF families and illustrate how conserved TFs adopt diverse roles over the course of evolution.

Function and evolution of the DNA-protein crosslink proteases Wss1 and SPRTN

Covalent DNA-protein crosslinks (DPCs) are highly toxic DNA adducts, which interfere with faithful DNA replication. The proteases Wss1 and SPRTN degrade DPCs and have emerged as crucially important DNA repair enzymes. Their protective role has been described in various model systems ranging from yeasts, plants, worms and flies to mice and humans. Loss of DPC proteases results in genome instability, cellular arrest, premature ageing and cancer predisposition. Here we discuss recent insights into the function and molecular mechanism of these enzymes. Furthermore, we present an in-depth phylogenetic analysis of the Wss1/SPRTN protease continuum. Remarkably flexible domain architectures and constantly changing protein-protein interaction motifs indicate ongoing evolutionary dynamics. Finally, we discuss recent data, which suggest that further partially-overlapping proteolytic systems targeting DPCs exist in eukaryotes. These new developments raise interesting questions regarding the division of labour between different DPC proteases and the mechanisms and principles of repair pathway choice.

GCNA Interacts with Spartan and Topoisomerase II to Regulate Genome Stability

GCNA proteins are expressed across eukarya in pluripotent cells and have conserved functions in fertility. GCNA homologs Spartan (DVC-1) and Wss1 resolve DNA-protein crosslinks (DPCs), including Topoisomerase-DNA adducts, during DNA replication. Here, we show that GCNA mutants in mouse and C. elegans display defects in genome maintenance including DNA damage, aberrant chromosome condensation, and crossover defects in mouse spermatocytes and spontaneous genomic rearrangements in C. elegans. We show that GCNA and topoisomerase II (TOP2) physically interact in both mice and worms and colocalize on condensed chromosomes during mitosis in C. elegans embryos. Moreover, C. elegans gcna-1 mutants are hypersensitive to TOP2 poison. Together, our findings support a model in which GCNA provides genome maintenance functions in the germline and may do so, in part, by promoting the resolution of TOP2 DPCs.

GCNA Preserves Genome Integrity and Fertility Across Species

The propagation of species depends on the ability of germ cells to protect their genome from numerous exogenous and endogenous threats. While these cells employ ubiquitous repair pathways, specialized mechanisms that ensure high-fidelity replication, chromosome segregation, and repair of germ cell genomes remain incompletely understood. We identified Germ Cell Nuclear Acidic Peptidase (GCNA) as a conserved regulator of genome stability in flies, worms, zebrafish, and human germ cell tumors. GCNA contains an acidic intrinsically disordered region (IDR) and a protease-like SprT domain. In addition to chromosomal instability and replication stress, Gcna mutants accumulate DNA-protein crosslinks (DPCs). GCNA acts in parallel with the SprT domain protein Spartan. Structural analysis reveals that while the SprT domain is needed to limit DNA damage, the IDR imparts significant function. This work shows that GCNA protects germ cells from various sources of damage, providing insights into conserved mechanisms that promote genome integrity across generations.

Mammalian germ cells are determined after PGC colonization of the nascent gonad

Mammalian primordial germ cells (PGCs) are induced in the embryonic epiblast, before migrating to the nascent gonads. In fish, frogs, and birds, the germline segregates even earlier, through the action of maternally inherited germ plasm. Across vertebrates, migrating PGCs retain a broad developmental potential, regardless of whether they were induced or maternally segregated. In mammals, this potential is indicated by expression of pluripotency factors, and the ability to generate teratomas and pluripotent cell lines. How the germline loses this developmental potential remains unknown. Our genome-wide analyses of embryonic human and mouse germlines reveal a conserved transcriptional program, initiated in PGCs after gonadal colonization, that differentiates germ cells from their germline precursors and from somatic lineages. Through genetic studies in mice and pigs, we demonstrate that one such gonad-induced factor, the RNA-binding protein DAZL, is necessary in vivo to restrict the developmental potential of the germline; DAZL's absence prolongs expression of a Nanog pluripotency reporter, facilitates derivation of pluripotent cell lines, and causes spontaneous gonadal teratomas. Based on these observations in humans, mice, and pigs, we propose that germ cells are determined after gonadal colonization in mammals. We suggest that germ cell determination was induced late in embryogenesis-after organogenesis has begun-in the common ancestor of all vertebrates, as in modern mammals, where this transition is induced by somatic cells of the gonad. We suggest that failure of this process of germ cell determination likely accounts for the origin of human testis cancer.

Interplay between Caspase 9 and X-linked Inhibitor of Apoptosis Protein (XIAP) in the oocyte elimination during fetal mouse development

Mammalian female fertility is limited by the number and quality of oocytes in the ovarian reserve. The number of oocytes is finite since all germ cells cease proliferation to become oocytes in fetal life. Moreover, 70-80% of the initial oocyte population is eliminated during fetal and neonatal development, restricting the ovarian reserve. Why so many oocytes are lost during normal development remains an enigma. In Meiotic Prophase I (MPI), oocytes go through homologous chromosome synapsis and recombination, dependent on formation and subsequent repair of DNA double strand breaks (DSBs). The oocytes that have failed in DSB repair or synapsis get eliminated mainly in neonatal ovaries. However, a large oocyte population is eliminated before birth, and the cause or mechanism of this early oocyte loss is not well understood. In the current paper, we show that the oocyte loss in fetal ovaries was prevented by a deficiency of Caspase 9 (CASP9), which is the hub of the mitochondrial apoptotic pathway. Furthermore, CASP9 and its downstream effector Caspase 3 were counteracted by endogenous X-linked Inhibitor of Apoptosis (XIAP) to regulate the oocyte population; while XIAP overexpression mimicked CASP9 deficiency, XIAP deficiency accelerated oocyte loss. In the CASP9 deficiency, more oocytes were accumulated at the pachytene stage with multiple γH2AFX foci and high LINE1 expression levels, but with normal levels of synapsis and overall DSB repair. We conclude that the oocytes with LINE1 overexpression were preferentially eliminated by CASP9-dependent apoptosis in balance with XIAP during fetal ovarian development. When such oocytes were retained, however, they get eliminated by a CASP9-independent mechanism during neonatal development. Thus, the oocyte is equipped with multiple surveillance mechanisms during MPI progression to safe-guard the quality of oocytes in the ovarian reserve.

The testicular soma of Tsc22d3 knockout mice supports spermatogenesis and germline transmission from spermatogonial stem cell lines upon transplantation

Spermatogonial stem cells (SSCs) are adult stem cells that are slowly cycling and self-renewing. The pool of SSCs generates very large numbers of male gametes throughout the life of the individual. SSCs can be cultured in vitro for long periods of time, and established SSC lines can be manipulated genetically. Upon transplantation into the testes of infertile mice, long-term cultured mouse SSCs can differentiate into fertile spermatozoa, which can give rise to live offspring. Here, we show that the testicular soma of mice with a conditional knockout (conKO) in the X-linked gene Tsc22d3 supports spermatogenesis and germline transmission from cultured mouse SSCs upon transplantation. Infertile males were produced by crossing homozygous Tsc22d3 floxed females with homozygous ROSA26-Cre males. We obtained 96 live offspring from six long-term cultured SSC lines with the aid of intracytoplasmic sperm injection. We advocate the further optimization of Tsc22d3-conKO males as recipients for testis transplantation of SSC lines.

Membraneless organelles: P granules in Caenorhabditis elegans

Membraneless organelles are distinct compartments within a cell that are not enclosed by a traditional lipid membrane and instead form through a process called liquid-liquid phase separation. Examples of these non-membrane-bound organelles include nucleoli, stress granules, P bodies, pericentriolar material and germ granules. Many recent studies have used Caenorhabditis elegans germ granules, known as P granules, to expand our understanding of the formation of these unique cellular compartments. From this work, we know that proteins with intrinsically disordered regions (IDRs) play a critical role in the process of phase separation. IDR phase separation is further tuned through their interactions with RNA and through protein modifications such as phosphorylation and methylation. These findings from C elegans, combined with work done in other model organisms, continue to provide insight into the formation of membraneless organelles and the important role they play in compartmentalizing cellular processes.

SUMOylation promotes protective responses to DNA-protein crosslinks

DNA‐protein crosslinks (DPCs) are highly cytotoxic lesions that obstruct essential DNA transactions and whose resolution is critical for cell and organismal fitness. However, the mechanisms by which cells respond to and overcome DPCs remain incompletely understood. Recent studies unveiled a dedicated DPC repair pathway in higher eukaryotes involving the SprT‐type metalloprotease SPRTN/DVC1, which proteolytically processes DPCs during DNA replication in a ubiquitin‐regulated manner. Here, we show that chemically induced and defined enzymatic DPCs trigger potent chromatin SUMOylation responses targeting the crosslinked proteins and associated factors. Consequently, inhibiting SUMOylation compromises DPC clearance and cellular fitness. We demonstrate that ACRC/GCNA family SprT proteases interact with SUMO and establish important physiological roles of Caenorhabditis elegans GCNA‐1 and SUMOylation in promoting germ cell and embryonic survival upon DPC formation. Our findings provide first global insights into signaling responses to DPCs and reveal an evolutionarily conserved function of SUMOylation in facilitating responses to these lesions in metazoans that may complement replication‐coupled DPC resolution processes.

Amplification of a broad transcriptional program by a common factor triggers the meiotic cell cycle in mice

The germ line provides the cellular link between generations of multicellular organisms, its cells entering the meiotic cell cycle only once each generation. However, the mechanisms governing this initiation of meiosis remain poorly understood. Here, we examined cells undergoing meiotic initiation in mice, and we found that initiation involves the dramatic upregulation of a transcriptional network of thousands of genes whose expression is not limited to meiosis. This broad gene expression program is directly upregulated by STRA8, encoded by a germ cell-specific gene required for meiotic initiation. STRA8 binds its own promoter and those of thousands of other genes, including meiotic prophase genes, factors mediating DNA replication and the G1-S cell-cycle transition, and genes that promote the lengthy prophase unique to meiosis I. We conclude that, in mice, the robust amplification of this extraordinarily broad transcription program by a common factor triggers initiation of meiosis.

A rapidly evolved domain, the SCML2 DNA-binding repeats, contributes to chromatin binding of mouse SCML2†

Genes involved in sexual reproduction diverge rapidly as a result of reproductive fitness. Here, we identify a novel protein domain in the germline-specific Polycomb protein SCML2 that is required for the establishment of unique gene expression programs after the mitosis-to-meiosis transition in spermatogenesis. We term this novel domain, which is comprised of rapidly evolved, DNA-binding repeat units of 28 amino acids, the SCML2 DNA-binding (SDB) repeats. These repeats are acquired in a specific subgroup of the rodent lineage, having been subjected to positive selection in the course of evolution. Mouse SCML2 has two DNA-binding domains: one is the SDB repeats and the other is an RNA-binding region, which is conserved in human SCML2. For the recruitment of SCML2 to target loci, the SDB repeats cooperate with the other functional domains of SCML2 to bind chromatin. The cooperative action of these domains enables SCML2 to sense DNA hypomethylation in an in vivo chromatin environment, thereby enabling SCML2 to bind to hypomethylated chromatin. We propose that the rapid evolution of SCML2 is due to reproductive adaptation, which has promoted species-specific gene expression programs in spermatogenesis.

DNA protein crosslink proteolysis repair: From yeast to premature ageing and cancer in humans

DNA-protein crosslinks (DPCs) are a specific type of DNA lesion consisting of a protein covalently and irreversibly bound to DNA, which arise after exposure to physical and chemical crosslinking agents. DPCs can be bulky and thereby pose a barrier to DNA replication and transcription. The persistence of DPCs during S phase causes DNA replication stress and genome instability. The toxicity of DPCs is exploited in cancer therapy: many common chemotherapeutics kill cancer cells by inducing DPC formation.

Recent work from several laboratories discovered a specialized repair pathway for DPCs, namely DPC proteolysis (DPCP) repair. DPCP repair is carried out by replication-coupled DNA-dependent metalloproteases: Wss1 in yeast and SPRTN in metazoans. Mutations in SPRTN cause premature ageing and liver cancer in humans and mice; thus, defective DPC repair has great clinical ramifications. In the present review, we will revise the current knowledge on the mechanisms of DPCP repair and on the regulation of DPC protease activity, while highlighting the most significant unresolved questions in the field. Finally, we will discuss the impact of faulty DPC repair on disease and cancer therapy.

The P Granules of C. elegans: A Genetic Model for the Study of RNA-Protein Condensates

P granules are RNA/protein condensates in the germline of Caenorhabditis elegans. Genetic analyses have begun to identify the proteins that regulate P granule assembly in the cytoplasm of zygotes. Among them, the RGG-domain protein PGL-3, the intrinsically disordered protein MEG-3, and the RNA helicase LAF-1 all bind and phase separate with RNA in vitro. We discuss how RNA-induced phase separation, competition with other RNA-binding proteins, and reversible phosphorylation contribute to the asymmetric localization of P granules in the cytoplasm of newly fertilized embryos. P granules contain RNA silencing complexes that monitor the germline transcriptome and may provide an RNA memory of germline gene expression across generations.

Yao HH, Jensen P. A new mouse line for cell ablation by diphtheria toxin subunit A controlled by a Cre-dependent FLEx switch

Recombinase responsive mouse lines expressing diphtheria toxin subunit A (DTA) are well established tools for targeted ablation of genetically defined cell populations. Here we describe a new knock-in allele at the Gt(Rosa)26Sor locus that retains the best features of previously described DTA alleles-including a CAG promoter, attenuated mutant DTA cDNA, and ubiquitous EGFP labeling-with the addition of a Cre-dependent FLEx switch for tight control of expression. The FLEx switch consists of two pairs of antiparallel lox sites requiring Cre-mediated recombination for inversion of the DTA to the proper orientation for transcription. We demonstrate its utility by Cre-dependent ablation of both a broad domain in the embryonic nervous system and a discrete population of cells in the fetal gonads. We conclude that this new DTA line is useful for targeted ablation of genetically-defined cell populations.

Yao HH, Jensen P. A new mouse line for cell ablation by diphtheria toxin subunit A controlled by a Cre-dependent FLEx switch

Recombinase responsive mouse lines expressing diphtheria toxin subunit A (DTA) are well established tools for targeted ablation of genetically defined cell populations. Here we describe a new knock-in allele at the Gt(Rosa)26Sor locus that retains the best features of previously described DTA alleles-including a CAG promoter, attenuated mutant DTA cDNA, and ubiquitous EGFP labeling-with the addition of a Cre-dependent FLEx switch for tight control of expression. The FLEx switch consists of two pairs of antiparallel lox sites requiring Cre-mediated recombination for inversion of the DTA to the proper orientation for transcription. We demonstrate its utility by Cre-dependent ablation of both a broad domain in the embryonic nervous system and a discrete population of cells in the fetal gonads. We conclude that this new DTA line is useful for targeted ablation of genetically-defined cell populations.

A new mouse line for cell ablation by diphtheria toxin subunit A controlled by a Cre-dependent FLEx switch

Recombinase responsive mouse lines expressing diphtheria toxin subunit A (DTA) are well established tools for targeted ablation of genetically defined cell populations. Here we describe a new knock-in allele at the Gt(Rosa)26Sor locus that retains the best features of previously described DTA alleles-including a CAG promoter, attenuated mutant DTA cDNA, and ubiquitous EGFP labeling-with the addition of a Cre-dependent FLEx switch for tight control of expression. The FLEx switch consists of two pairs of antiparallel lox sites requiring Cre-mediated recombination for inversion of the DTA to the proper orientation for transcription. We demonstrate its utility by Cre-dependent ablation of both a broad domain in the embryonic nervous system and a discrete population of cells in the fetal gonads. We conclude that this new DTA line is useful for targeted ablation of genetically-defined cell populations.