Mutation of p53 increases the competitive ability of pluripotent stem cells.

During development, the rate of tissue growth is determined by the relative balance of cell division and cell death. Cell competition is a fitness quality-control mechanism that contributes to this balance by eliminating viable cells that are less fit than their neighbours. The mutations that confer cells with a competitive advantage and the dynamics of the interactions between winner and loser cells are not well understood. Here, we show that embryonic cells lacking the tumour suppressor p53 are 'super-competitors' that eliminate their wild-type neighbours through the direct induction of apoptosis. This elimination is context dependent, as it does not occur when cells are pluripotent and it is triggered by the onset of differentiation. Furthermore, by combining mathematical modelling and cell-based assays we show that the elimination of wild-type cells is not through competition for space or nutrients, but instead is mediated by short-range interactions that are dependent on the local cell neighbourhood. This highlights the importance of the local cell neighbourhood and the competitive interactions within this neighbourhood for the regulation of proliferation during early embryonic development.

DRP1 levels determine the apoptotic threshold during embryonic differentiation through a mitophagy-dependent mechanism.

The changes that drive differentiation facilitate the emergence of abnormal cells that need to be removed before they contribute to further development or the germline. Consequently, in mice in the lead-up to gastrulation, ∼35% of embryonic cells are eliminated. This elimination is caused by hypersensitivity to apoptosis, but how it is regulated is poorly understood. Here, we show that upon exit of naive pluripotency, mouse embryonic stem cells lower their mitochondrial apoptotic threshold, and this increases their sensitivity to cell death. We demonstrate that this enhanced apoptotic response is induced by a decrease in mitochondrial fission due to a reduction in the activity of dynamin-related protein 1 (DRP1). Furthermore, we show that in naive pluripotent cells, DRP1 prevents apoptosis by promoting mitophagy. In contrast, during differentiation, reduced mitophagy levels facilitate apoptosis. Together, these results indicate that during early mammalian development, DRP1 regulation of mitophagy determines the apoptotic response.

Cell competition acts as a purifying selection to eliminate cells with mitochondrial defects during early mouse development.

Cell competition is emerging as a quality-control mechanism that eliminates unfit cells in a wide range of settings from development to the adult. However, the nature of the cells normally eliminated by cell competition and what triggers their elimination remains poorly understood. In mice, 35% of epiblast cells are eliminated before gastrulation. Here we show that cells with mitochondrial defects are eliminated by cell competition during early mouse development. Using single-cell transcriptional profiling of eliminated mouse epiblast cells, we identify hallmarks of cell competition and mitochondrial defects. We demonstrate that mitochondrial defects are common to a range of different loser cell types and that manipulating mitochondrial function triggers cell competition. Moreover, we show that in the mouse embryo, cell competition eliminates cells with sequence changes in mt-Rnr1 and mt-Rnr2, and that even non-pathological changes in mitochondrial DNA sequences can induce cell competition. Our results suggest that cell competition is a purifying selection that optimizes mitochondrial performance before gastrulation.

Evolution of an Amniote-Specific Mechanism for Modulating Ubiquitin Signaling via Phosphoregulation of the E2 Enzyme UBE2D3.

Genetic variation in the enzymes that catalyze posttranslational modification of proteins is a potentially important source of phenotypic variation during evolution. Ubiquitination is one such modification that affects turnover of virtually all of the proteins in the cell in addition to roles in signaling and epigenetic regulation. UBE2D3 is a promiscuous E2 enzyme, which acts as an ubiquitin donor for E3 ligases that catalyze ubiquitination of developmentally important proteins. We have used protein sequence comparison of UBE2D3 orthologs to identify a position in the C-terminal α-helical region of UBE2D3 that is occupied by a conserved serine in amniotes and by alanine in anamniote vertebrate and invertebrate lineages. Acquisition of the serine (S138) in the common ancestor to modern amniotes created a phosphorylation site for Aurora B. Phosphorylation of S138 disrupts the structure of UBE2D3 and reduces the level of the protein in mouse embryonic stem cells (ESCs). Substitution of S138 with the anamniote alanine (S138A) increases the level of UBE2D3 in ESCs as well as being a gain of function early embryonic lethal mutation in mice. When mutant S138A ESCs were differentiated into extraembryonic primitive endoderm, levels of the PDGFRα and FGFR1 receptor tyrosine kinases were reduced and primitive endoderm differentiation was compromised. Proximity ligation analysis showed increased interaction between UBE2D3 and the E3 ligase CBL and between CBL and the receptor tyrosine kinases. Our results identify a sequence change that altered the ubiquitination landscape at the base of the amniote lineage with potential effects on amniote biology and evolution.

Author Correction: P53 and mTOR signalling determine fitness selection through cell competition during early mouse embryonic development.

The original version of this article contained an error in the spelling of Juan Pedro Martinez-Barbera, which was incorrectly given as Juan Pedro Martinez Barbera. This error has now been corrected in both the PDF and HTML versions of the Article.

P53 and mTOR signalling determine fitness selection through cell competition during early mouse embryonic development.

Ensuring the fitness of the pluripotent cells that will contribute to future development is important both for the integrity of the germline and for proper embryogenesis. Consequently, it is becoming increasingly apparent that pluripotent cells can compare their fitness levels and signal the elimination of those cells that are less fit than their neighbours. In mammals the nature of the pathways that communicate fitness remain largely unknown. Here we identify that in the early mouse embryo and upon exit from naive pluripotency, the confrontation of cells with different fitness levels leads to an inhibition of mTOR signalling in the less fit cell type, causing its elimination. We show that during this process, p53 acts upstream of mTOR and is required to repress its activity. Finally, we demonstrate that during normal development around 35% of cells are eliminated by this pathway, highlighting the importance of this mechanism for embryonic development.

Cell Competition and Its Role in the Regulation of Cell Fitness from Development to Cancer.

Cell competition is a cell fitness-sensing mechanism conserved from insects to mammals that eliminates those cells that, although viable, are less fit than their neighbors. An important implication of cell competition is that cellular fitness is not only a cell-intrinsic property but is also determined relative to the fitness of neighboring cells: a cell that is of suboptimal fitness in one context may be "super-fit" in the context of a different cell population. Here we discuss the mechanisms by which cell competition measures and communicates cell fitness levels and the implications of this mechanism for development, regeneration, and tumor progression.

Competitive interactions eliminate unfit embryonic stem cells at the onset of differentiation.

A fundamental question in developmental biology is whether there are mechanisms to detect stem cells with mutations that, although not adversely affecting viability, would compromise their ability to contribute to further development. Here, we show that cell competition is a mechanism regulating the fitness of embryonic stem cells (ESCs). We find that ESCs displaying defective bone morphogenetic protein signaling or defective autophagy or that are tetraploid are eliminated at the onset of differentiation by wild-type cells. This elimination occurs in an apoptosis-dependent manner and is mediated by secreted factors. Furthermore, during this process, we find that establishment of differential c-Myc levels is critical and that c-Myc overexpression is sufficient to induce competitive behavior in ESCs. Cell competition is, therefore, a process that allows recognition and elimination of defective cells during the early stages of development and is likely to play important roles in tissue homeostasis and stem cell maintenance.

Coordination of cell proliferation and anterior-posterior axis establishment in the mouse embryo.

During development, the growth of the embryo must be coupled to its patterning to ensure correct and timely morphogenesis. In the mouse embryo, migration of the anterior visceral endoderm (AVE) to the prospective anterior establishes the anterior-posterior (A-P) axis. By analysing the distribution of cells in S phase, M phase and G2 from the time just prior to the migration of the AVE until 18 hours after its movement, we show that there is no evidence for differential proliferation along the A-P axis of the mouse embryo. Rather, we have identified that as AVE movements are being initiated, the epiblast proliferates at a much higher rate than the visceral endoderm. We show that these high levels of proliferation in the epiblast are dependent on Nodal signalling and are required for A-P establishment, as blocking cell division in the epiblast inhibits AVE migration. Interestingly, inhibition of migration by blocking proliferation can be rescued by Dkk1. This suggests that the high levels of epiblast proliferation function to move the prospective AVE away from signals that are inhibitory to its migration. The finding that initiation of AVE movements requires a certain level of proliferation in the epiblast provides a mechanism whereby A-P axis development is coordinated with embryonic growth.

Correct patterning of the primitive streak requires the anterior visceral endoderm.

Anterior-posterior axis specification in the mouse requires signalling from a specialised extra-embryonic tissue called the anterior visceral endoderm (AVE). AVE precursors are induced at the distal tip of the embryo and move to the prospective anterior. Embryological and genetic analysis has demonstrated that the AVE is required for anterior patterning and for correctly positioning the site of primitive streak formation by inhibiting Nodal activity. We have carried out a genetic ablation of the Hex-expressing cells of the AVE (Hex-AVE) by knocking the Diphtheria toxin subunit A into the Hex locus in an inducible manner. Using this model we have identified that, in addition to its requirement in the anterior of the embryo, the Hex-AVE sub-population has a novel role between 5.5 and 6.5dpc in patterning the primitive streak. Embryos lacking the Hex-AVE display delayed initiation of primitive streak formation and miss-patterning of the anterior primitive streak. We demonstrate that in the absence of the Hex-AVE the restriction of Bmp2 expression to the proximal visceral endoderm is also defective and expression of Wnt3 and Nodal is not correctly restricted to the posterior epiblast. These results, coupled with the observation that reducing Nodal signalling in Hex-AVE ablated embryos increases the frequency of phenotypes observed, suggests that these primitive streak patterning defects are due to defective Nodal signalling. Together, our experiments demonstrate that the AVE is not only required for anterior patterning, but also that specific sub-populations of this tissue are required to pattern the posterior of the embryo.

An early developmental role for miRNAs in the maintenance of extraembryonic stem cells in the mouse embryo.

The two first cell fate decisions taken in the mammalian embryo generate three distinct cell lineages: one embryonic, the epiblast, and two extraembryonic, the trophoblast and primitive endoderm. miRNAs are essential for early development, but it is not known if they are utilized in the same way in these three lineages. We find that in the pluripotent epiblast they inhibit apoptosis by blocking the expression of the proapoptotic protein Bcl2l11 (Bim) but play little role in the initiation of gastrulation. In contrast, in the trophectoderm, miRNAs maintain the trophoblast stem cell compartment by directly inhibiting expression of Cdkn1a (p21) and Cdkn1c (p57), and in the primitive endoderm, they prevent differentiation by maintaining ERK1/2 phosphorylation through blocking the expression of Mapk inhibitors. Therefore, we show that there are fundamental differences in how stem cells maintain their developmental potential in embryonic and extraembryonic tissues through miRNAs.

Differences in the epigenetic and reprogramming properties of pluripotent and extra-embryonic stem cells implicate chromatin remodelling as an important early event in the developing mouse embryo.

BACKGROUND: During early mouse development, two extra-embryonic lineages form alongside the future embryo: the trophectoderm (TE) and the primitive endoderm (PrE). Epigenetic changes known to take place during these early stages include changes in DNA methylation and modified histones, as well as dynamic changes in gene expression. RESULTS: In order to understand the role and extent of chromatin-based changes for lineage commitment within the embryo, we examined the epigenetic profiles of mouse embryonic stem (ES), trophectoderm stem (TS) and extra-embryonic endoderm (XEN) stem cell lines that were derived from the inner cell mass (ICM), TE and PrE, respectively. As an initial indicator of the chromatin state, we assessed the replication timing of a cohort of genes in each cell type, based on data that expressed genes and acetylated chromatin domains, generally, replicate early in S-phase, whereas some silent genes, hypoacetylated or condensed chromatin tend to replicate later. We found that many lineage-specific genes replicate early in ES, TS and XEN cells, which was consistent with a broadly 'accessible' chromatin that was reported previously for multiple ES cell lines. Close inspection of these profiles revealed differences between ES, TS and XEN cells that were consistent with their differing lineage affiliations and developmental potential. A comparative analysis of modified histones at the promoters of individual genes showed that in TS and ES cells many lineage-specific regulator genes are co-marked with modifications associated with active (H4ac, H3K4me2, H3K9ac) and repressive (H3K27me3) chromatin. However, in XEN cells several of these genes were marked solely by repressive modifications (such as H3K27me3, H4K20me3). Consistent with TS and XEN having a restricted developmental potential, we show that these cells selectively reprogramme somatic cells to induce the de novo expression of genes associated with extraembryonic differentiation. CONCLUSIONS: These data provide evidence that the diversification of defined embryonic and extra-embryonic lineages is accompanied by chromatin remodelling at specific loci. Stem cell lines from the ICM, TE and PrE can each dominantly reprogramme somatic cells but reset gene expression differently, reflecting their separate lineage identities and increasingly restricted developmental potentials.

BMP signalling inhibits premature neural differentiation in the mouse embryo.

The specification of a subset of epiblast cells to acquire a neural fate constitutes the first step in the generation of the nervous system. Little is known about the signals required for neural induction in the mouse. We have analysed the role of BMP signalling in this process. We demonstrate that prior to gastrulation, Bmp2/4 signalling via Bmpr1a maintains epiblast pluripotency and prevents precocious neural differentiation of this tissue, at least in part by maintaining Nodal signalling. We find that during gastrulation, BMPs of the 60A subgroup cooperate with Bmp2/4 to maintain pluripotency. The inhibition of neural fate by BMPs is independent of FGF signalling, as inhibition of FGF signalling between 5.5 and 7.5 days post-coitum does not block neural differentiation in the mouse embryo. Together, our results demonstrate that inhibition of BMP signalling has a central role during neural induction in mammals and suggest that FGFs do not act as neural inducers in the post-implantation mouse embryo.