When boundaries benefit science and society.

In the rapidly moving field of stem cell and embryo research, research questions often sit at the intersection of scientific inquiry and ethical considerations. The International Society for Stem Cell Research (ISSCR) produces guidelines to help navigate decisions in this area. For Cell's 50th Anniversary Focus on Developmental Biology, scientific editor Sarah Geisler discussed the importance of the ISSCR guidelines on stem cell and embryo research for both the stem cell community and the broader public with Amander Clark, Robin Lovell-Badge, and Janet Rossant, who have been involved in the ongoing evolution of the guidelines. A lightly edited transcript of their conversation is shared here.

Nervous System Regionalization Entails Axial Allocation before Neural Differentiation.

Neural induction in vertebrates generates a CNS that extends the rostral-caudal length of the body. The prevailing view is that neural cells are initially induced with anterior (forebrain) identity; caudalizing signals then convert a proportion to posterior fates (spinal cord). To test this model, we used chromatin accessibility to define how cells adopt region-specific neural fates. Together with genetic and biochemical perturbations, this identified a developmental time window in which genome-wide chromatin-remodeling events preconfigure epiblast cells for neural induction. Contrary to the established model, this revealed that cells commit to a regional identity before acquiring neural identity. This "primary regionalization" allocates cells to anterior or posterior regions of the nervous system, explaining how cranial and spinal neurons are generated at appropriate axial positions. These findings prompt a revision to models of neural induction and support the proposed dual evolutionary origin of the vertebrate CNS.

Establishing and maintaining fertility: the importance of cell cycle arrest.

Development of the ovary or testis is required to establish reproductive competence. Gonad development relies on key cell fate decisions that occur early in embryonic development and are actively maintained. During gonad development, both germ cells and somatic cells proliferate extensively, a process facilitated by cell cycle regulation. This review focuses on the Cip/Kip family of cyclin-dependent kinase inhibitors (CKIs) in mouse gonad development. We particularly highlight recent single-cell RNA sequencing studies that show the heterogeneity of cyclin-dependent kinase inhibitors. This diversity highlights new roles for cell cycle inhibitors in controlling and maintaining female fertility.

An end-to-end pipeline based on open source deep learning tools for reliable analysis of complex 3D images of ovaries.

Computational analysis of bio-images by deep learning (DL) algorithms has made exceptional progress in recent years and has become much more accessible to non-specialists with the development of ready-to-use tools. The study of oogenesis mechanisms and female reproductive success has also recently benefited from the development of efficient protocols for three-dimensional (3D) imaging of ovaries. Such datasets have a great potential for generating new quantitative data but are, however, complex to analyze due to the lack of efficient workflows for 3D image analysis. Here, we have integrated two existing open-source DL tools, Noise2Void and Cellpose, into an analysis pipeline dedicated to 3D follicular content analysis, which is available on Fiji. Our pipeline was developed on larvae and adult medaka ovaries but was also successfully applied to different types of ovaries (trout, zebrafish and mouse). Image enhancement, Cellpose segmentation and post-processing of labels enabled automatic and accurate quantification of these 3D images, which exhibited irregular fluorescent staining, low autofluorescence signal or heterogeneous follicles sizes. In the future, this pipeline will be useful for extensive cellular phenotyping in fish or mammals for developmental or toxicology studies.

Hypomorphic and dominant-negative impact of truncated SOX9 dysregulates Hedgehog-Wnt signaling, causing campomelia.

Haploinsufficiency for SOX9, the master chondrogenesis transcription factor, can underlie campomelic dysplasia (CD), an autosomal dominant skeletal malformation syndrome, because heterozygous Sox9 null mice recapitulate the bent limb (campomelia) and some other phenotypes associated with CD. However, in vitro cell assays suggest haploinsufficiency may not apply for certain mutations, notably those that truncate the protein, but in these cases in vivo evidence is lacking and underlying mechanisms are unknown. Here, using conditional mouse mutants, we compared the impact of a heterozygous Sox9 null mutation (Sox9+/-) with the Sox9+/Y440X CD mutation that truncates the C-terminal transactivation domain but spares the DNA-binding domain. While some Sox9+/Y440X mice survived, all Sox9+/- mice died perinatally. However, the skeletal defects were more severe and IHH signaling in developing limb cartilage was significantly enhanced in Sox9+/Y440X compared with Sox9+/-. Activating Sox9Y440X specifically in the chondrocyte-osteoblast lineage caused milder campomelia, and revealed cell- and noncell autonomous mechanisms acting on chondrocyte differentiation and osteogenesis in the perichondrium. Transcriptome analyses of developing Sox9+/Y440X limbs revealed dysregulated expression of genes for the extracellular matrix, as well as changes consistent with aberrant WNT and HH signaling. SOX9Y440X failed to interact with ß-catenin and was unable to suppress transactivation of Ihh in cell-based assays. We propose enhanced HH signaling in the adjacent perichondrium induces asymmetrically localized excessive perichondrial osteogenesis resulting in campomelia. Our study implicates combined haploinsufficiency/hypomorphic and dominant-negative actions of SOX9Y440X, cell-autonomous and noncell autonomous mechanisms, and dysregulated WNT and HH signaling, as the cause of human campomelia.

SOX9 and SOX10 control fluid homeostasis in the inner ear for hearing through independent and cooperative mechanisms.

The in vivo mechanisms underlying dominant syndromes caused by mutations in SRY-Box Transcription Factor 9 (SOX9) and SOX10 (SOXE) transcription factors, when they either are expressed alone or are coexpressed, are ill-defined. We created a mouse model for the campomelic dysplasia SOX9Y440X mutation, which truncates the transactivation domain but leaves DNA binding and dimerization intact. Here, we find that SOX9Y440X causes deafness via distinct mechanisms in the endolymphatic sac (ES)/duct and cochlea. By contrast, conditional heterozygous Sox9-null mice are normal. During the ES development of Sox9Y440X/+ heterozygotes, Sox10 and genes important for ionic homeostasis are down-regulated, and there is developmental persistence of progenitors, resulting in fewer mature cells. Sox10 heterozygous null mutants also display persistence of ES/duct progenitors. By contrast, SOX10 retains its expression in the early Sox9Y440X/+ mutant cochlea. Later, in the postnatal stria vascularis, dominant interference by SOX9Y440X is implicated in impairing the normal cooperation of SOX9 and SOX10 in repressing the expression of the water channel Aquaporin 3, thereby contributing to endolymphatic hydrops. Our study shows that for a functioning endolymphatic system in the inner ear, SOX9 regulates Sox10, and depending on the cell type and target gene, it works either independently of or cooperatively with SOX10. SOX9Y440X can interfere with the activity of both SOXE factors, exerting effects that can be classified as haploinsufficient/hypomorphic or dominant negative depending on the cell/gene context. This model of disruption of transcription factor partnerships may be applicable to congenital deafness, which affects ∼0.3% of newborns, and other syndromic disorders.

iPS cells: mapping the policy issues.

Given the explosion of research on induced pluripotent stem (iPS) cells, it is timely to consider the various ethical, legal, and social issues engaged by this fast-moving field. Here, we review issues associated with the procurement, basic research, and clinical translation of iPS cells.

Somatic sex reprogramming of adult ovaries to testes by FOXL2 ablation.

In mammals, the transcription factor SRY, encoded by the Y chromosome, is normally responsible for triggering the indifferent gonads to develop as testes rather than ovaries. However, testis differentiation can occur in its absence. Here we demonstrate in the mouse that a single factor, the forkhead transcriptional regulator FOXL2, is required to prevent transdifferentiation of an adult ovary to a testis. Inducible deletion of Foxl2 in adult ovarian follicles leads to immediate upregulation of testis-specific genes including the critical SRY target gene Sox9. Concordantly, reprogramming of granulosa and theca cell lineages into Sertoli-like and Leydig-like cell lineages occurs with testosterone levels comparable to those of normal XY male littermates. Our results show that maintenance of the ovarian phenotype is an active process throughout life. They might also have important medical implications for the understanding and treatment of some disorders of sexual development in children and premature menopause in women.

The rates of adult neurogenesis and oligodendrogenesis are linked to cell cycle regulation through p27-dependent gene repression of SOX2.

Cell differentiation involves profound changes in global gene expression that often has to occur in coordination with cell cycle exit. Because cyclin-dependent kinase inhibitor p27 reportedly regulates proliferation of neural progenitor cells in the subependymal neurogenic niche of the adult mouse brain, but can also have effects on gene expression, we decided to molecularly analyze its role in adult neurogenesis and oligodendrogenesis. At the cell level, we show that p27 restricts residual cyclin-dependent kinase activity after mitogen withdrawal to antagonize cycling, but it is not essential for cell cycle exit. By integrating genome-wide gene expression and chromatin accessibility data, we find that p27 is coincidentally necessary to repress many genes involved in the transit from multipotentiality to differentiation, including those coding for neural progenitor transcription factors SOX2, OLIG2 and ASCL1. Our data reveal both a direct association of p27 with regulatory sequences in the three genes and an additional hierarchical relationship where p27 repression of Sox2 leads to reduced levels of its downstream targets Olig2 and Ascl1. In vivo, p27 is also required for the regulation of the proper level of SOX2 necessary for neuroblasts and oligodendroglial progenitor cells to timely exit cell cycle in a lineage-dependent manner.

SOX2 is required independently in both stem and differentiated cells for pituitary tumorigenesis in p27-null mice.

P27, a cell cycle inhibitor, is also able to drive repression of Sox2 This interaction plays a crucial role during development of p27-/- pituitary tumors because loss of one copy of Sox2 impairs tumorigenesis [H. Li et al., Cell Stem Cell 11, 845-852 (2012)]. However, SOX2 is expressed in both endocrine and stem cells (SCs), and its contribution to tumorigenesis in either cell type is unknown. We have thus explored the cellular origin and mechanisms underlying endocrine tumorigenesis in p27-/- pituitaries. We found that pituitary hyperplasia is associated with reduced cellular differentiation, in parallel with increased levels of SOX2 in stem and endocrine cells. Using conditional loss-of-function and lineage tracing approaches, we show that SOX2 is required cell autonomously in p27-/- endocrine cells for these to give rise to tumors, and in SCs for promotion of tumorigenesis. This is supported by studies deleting the Sox2 regulatory region 2 (Srr2), the target of P27 repressive action. Single cell transcriptomic analysis further reveals that activation of a SOX2-dependent MAPK pathway in SCs is important for tumorigenesis. Altogether, our data highlight different aspects of the role of SOX2 following loss of p27, according to cellular context, and uncover an unexpected SOX2-dependent tumor-promoting role for SCs. Our results imply that targeting SCs, in addition to tumor cells, may represent an efficient antitumoral strategy in certain contexts.

Primary sex determination in birds depends on DMRT1 dosage, but gonadal sex does not determine adult secondary sex characteristics.

In birds, males are the homogametic sex (ZZ) and females the heterogametic sex (ZW). Primary sex determination is thought to depend on a sex chromosome gene dosage mechanism, and the most likely sex determinant is the Z chromosome gene Doublesex and Mab-3-Related Transcription factor 1 (DMRT1). To clarify this issue, we used a CRISPR-Cas9-based monoallelic targeting approach and sterile surrogate hosts to generate birds with targeted mutations in the DMRT1 gene. The resulting chromosomally male (ZZ) chicken with a single functional copy of DMRT1 developed ovaries in place of testes, demonstrating the avian sex-determining mechanism is based on DMRT1 dosage. These ZZ ovaries expressed typical female markers and showed clear evidence of follicular development. However, these ZZ adult birds with an ovary in place of testes were indistinguishable in appearance to wild-type adult males, supporting the concept of cell-autonomous sex identity (CASI) in birds. In experiments where estrogen synthesis was blocked in control ZW embryos, the resulting gonads developed as testes. In contrast, if estrogen synthesis was blocked in ZW embryos that lacked DMRT1, the gonads invariably adopted an ovarian fate. Our analysis shows that DMRT1 is the key sex determination switch in birds and that it is essential for testis development, but that production of estrogen is also a key factor in primary sex determination in chickens, and that this production is linked to DMRT1 expression.

Oestrogen in the chick embryo can induce chromosomally male ZZ left gonad epithelial cells to form an ovarian cortex that can support oogenesis.

In chickens, the embryonic ovary differentiates into two distinct domains before meiosis: a steroidogenic core (the female medulla), overlain by the germ cell niche (the cortex). The differentiation of the medulla is a cell-autonomous process based on chromosomal sex identity (CASI). In order to address the extent to which cortex differentiation depends on intrinsic or extrinsic factors, we generated models of gonadal intersex by mixing ZW (female) and ZZ (male) cells in gonadal chimeras, or by altering oestrogen levels of ZW and ZZ embryos. We found that CASI does not apply to the embryonic cortex. Both ZW and ZZ cells can form the cortex and this can happen independently of the phenotypic sex of the medulla as long as oestrogen is provided. We also show that the cortex-promoting activity of oestrogen signalling is mediated via estrogen receptor alpha within the left gonad epithelium. However, the presence of a medulla with an 'intersex' or male phenotype may compromise germ cell progression into meiosis, causing cortical germ cells to remain in an immature state in the embryo.

Testis formation in XX individuals resulting from novel pathogenic variants in Wilms' tumor 1 (WT1) gene.

Sex determination in mammals is governed by antagonistic interactions of two genetic pathways, imbalance in which may lead to disorders/differences of sex development (DSD) in human. Among 46,XX individuals with testicular DSD (TDSD) or ovotesticular DSD (OTDSD), testicular tissue is present in the gonad. Although the testis-determining gene SRY is present in many cases, the etiology is unknown in most SRY-negative patients. We performed exome sequencing on 78 individuals with 46,XX TDSD/OTDSD of unknown genetic etiology and identified seven (8.97%) with heterozygous variants affecting the fourth zinc finger (ZF4) of Wilms' tumor 1 (WT1) (p.Ser478Thrfs*17, p.Pro481Leufs*15, p.Lys491Glu, p.Arg495Gln [x3], p.Arg495Gly). The variants were de novo in six families (P = 4.4 × 10-6), and the incidence of WT1 variants in 46,XX DSD is enriched compared to control populations (P < 1.8 × 10-4). The introduction of ZF4 mutants into a human granulosa cell line resulted in up-regulation of endogenous Sertoli cell transcripts and Wt1Arg495Gly/Arg495Gly XX mice display masculinization of the fetal gonads. The phenotype could be explained by the ability of the mutated proteins to physically interact with and sequester a key pro-ovary factor ß-CATENIN, which may lead to up-regulation of testis-specific pathway. Our data show that unlike previous association of WT1 and 46,XY DSD, ZF4 variants of WT1 are a relatively common cause of 46,XX TDSD/OTDSD. This expands the spectrum of phenotypes associated with WT1 variants and shows that the WT1 protein affecting ZF4 can function as a protestis factor in an XX chromosomal context.

CRISPR babies: a view from the centre of the storm.

Towards the end of November 2018, news broke that the Chinese researcher He Jiankui had created the world's first genome-edited babies. This came shortly before the start of the Second International Summit on Human Genome Editing, where researchers, ethicists and others concerned with regulation, social issues and public engagement from around the world gathered to discuss the latest advances in the field. In this Spotlight, I provide my perspective on the events that occurred shortly prior to and at the summit, where He Jiankui gave an account of his activities. I also discuss what was wrong with his approach and how, after more research and with appropriate regulation, clinical applications of germline genome editing in humans may be justifiable.

Pathogenic variants in RNPC3 are associated with hypopituitarism and primary ovarian insufficiency.

PURPOSE: We aimed to investigate the molecular basis underlying a novel phenotype including hypopituitarism associated with primary ovarian insufficiency. METHODS: We used next-generation sequencing to identify variants in all pedigrees. Expression of Rnpc3/RNPC3 was analyzed by in situ hybridization on murine/human embryonic sections. CRISPR/Cas9 was used to generate mice carrying the p.Leu483Phe pathogenic variant in the conserved murine Rnpc3 RRM2 domain. RESULTS: We described 15 patients from 9 pedigrees with biallelic pathogenic variants in RNPC3, encoding a specific protein component of the minor spliceosome, which is associated with a hypopituitary phenotype, including severe growth hormone (GH) deficiency, hypoprolactinemia, variable thyrotropin (also known as thyroid-stimulating hormone) deficiency, and anterior pituitary hypoplasia. Primary ovarian insufficiency was diagnosed in 8 of 9 affected females, whereas males had normal gonadal function. In addition, 2 affected males displayed normal growth when off GH treatment despite severe biochemical GH deficiency. In both mouse and human embryos, Rnpc3/RNPC3 was expressed in the developing forebrain, including the hypothalamus and Rathke's pouch. Female Rnpc3 mutant mice displayed a reduction in pituitary GH content but with no reproductive impairment in young mice. Male mice exhibited no obvious phenotype. CONCLUSION: Our findings suggest novel insights into the role of RNPC3 in female-specific gonadal function and emphasize a critical role for the minor spliceosome in pituitary and ovarian development and function.

High SOX9 Maintains Glioma Stem Cell Activity through a Regulatory Loop Involving STAT3 and PML.

Glioma stem cells (GSCs) are critical targets for glioma therapy. SOX9 is a transcription factor with critical roles during neurodevelopment, particularly within neural stem cells. Previous studies showed that high levels of SOX9 are associated with poor glioma patient survival. SOX9 knockdown impairs GSCs proliferation, confirming its potential as a target for glioma therapy. In this study, we characterized the function of SOX9 directly in patient-derived glioma stem cells. Notably, transcriptome analysis of GSCs with SOX9 knockdown revealed STAT3 and PML as downstream targets. Functional studies demonstrated that SOX9, STAT3, and PML form a regulatory loop that is key for GSC activity and self-renewal. Analysis of glioma clinical biopsies confirmed a positive correlation between SOX9/STAT3/PML and poor patient survival among the cases with the highest SOX9 expression levels. Importantly, direct STAT3 or PML inhibitors reduced the expression of SOX9, STAT3, and PML proteins, which significantly reduced GSCs tumorigenicity. In summary, our study reveals a novel role for SOX9 upstream of STAT3, as a GSC pathway regulator, and presents pharmacological inhibitors of the signaling cascade.

Gonadal supporting cells acquire sex-specific chromatin landscapes during mammalian sex determination.

Cis-regulatory elements are critical for the precise spatiotemporal regulation of genes during development. However, identifying functional regulatory sites that drive cell differentiation in vivo has been complicated by the high numbers of cells required for whole-genome epigenetic assays. Here, we identified putative regulatory elements during sex determination by performing ATAC-seq and ChIP-seq for H3K27ac in purified XX and XY gonadal supporting cells before and after sex determination in mice. We show that XX and XY supporting cells initiate sex determination with similar chromatin landscapes and acquire sex-specific regulatory elements as they commit to the male or female fate. To validate our approach, we identified a functional gonad-specific enhancer downstream of Bmp2, an ovary-promoting gene. This work increases our understanding of the complex regulatory network underlying mammalian sex determination and provides a powerful resource for identifying non-coding regulatory elements that could harbor mutations that lead to Disorders of Sexual Development.