Developmental and stem cell biology's bright future.

The next 50 years of developmental biology will illuminate exciting new discoveries but are also poised to provide solutions to important problems society faces. Ten scientists whose work intersects with developmental biology in various capacities tell us about their vision for the future.

Depletion of Mettl3 in cholinergic neurons causes adult-onset neuromuscular degeneration.

Motor neuron (MN) demise is a hallmark of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Post-transcriptional gene regulation can control RNA's fate, and defects in RNA processing are critical determinants of MN degeneration. N6-methyladenosine (m6A) is a post-transcriptional RNA modification that controls diverse aspects of RNA metabolism. To assess the m6A requirement in MNs, we depleted the m6A methyltransferase-like 3 (METTL3) in cells and mice. METTL3 depletion in embryonic stem cell-derived MNs has profound and selective effects on survival and neurite outgrowth. Mice with cholinergic neuron-specific METTL3 depletion display a progressive decline in motor behavior, accompanied by MN loss and muscle denervation, culminating in paralysis and death. Reader proteins convey m6A effects, and their silencing phenocopies METTL3 depletion. Among the m6A targets, we identified transactive response DNA-binding protein 43 (TDP-43) and discovered that its expression is under epitranscriptomic control. Thus, impaired m6A signaling disrupts MN homeostasis and triggers neurodegeneration conceivably through TDP-43 deregulation.

Complete human day 14 post-implantation embryo models from naive ES cells.

The ability to study human post-implantation development remains limited owing to ethical and technical challenges associated with intrauterine development after implantation1. Embryo-like models with spatially organized morphogenesis and structure of all defining embryonic and extra-embryonic tissues of the post-implantation human conceptus (that is, the embryonic disc, the bilaminar disc, the yolk sac, the chorionic sac and the surrounding trophoblast layer) remain lacking1,2. Mouse naive embryonic stem cells have recently been shown to give rise to embryonic and extra-embryonic stem cells capable of self-assembling into post-gastrulation structured stem-cell-based embryo models with spatially organized morphogenesis (called SEMs)3. Here we extend those findings to humans using only genetically unmodified human naive embryonic stem cells (cultured in human enhanced naive stem cell medium conditions)4. Such human fully integrated and complete SEMs recapitulate the organization of nearly all known lineages and compartments of post-implantation human embryos, including the epiblast, the hypoblast, the extra-embryonic mesoderm and the trophoblast layer surrounding the latter compartments. These human complete SEMs demonstrated developmental growth dynamics that resemble key hallmarks of post-implantation stage embryogenesis up to 13-14 days after fertilization (Carnegie stage 6a). These include embryonic disc and bilaminar disc formation, epiblast lumenogenesis, polarized amniogenesis, anterior-posterior symmetry breaking, primordial germ-cell specification, polarized yolk sac with visceral and parietal endoderm formation, extra-embryonic mesoderm expansion that defines a chorionic cavity and a connecting stalk, and a trophoblast-surrounding compartment demonstrating syncytium and lacunae formation. This SEM platform will probably enable the experimental investigation of previously inaccessible windows of human early post implantation up to peri-gastrulation development.

Single-nucleus multiomic mapping of m6A methylomes and transcriptomes in native populations of cells with sn-m6A-CT.

N6-methyladenosine (m6A) RNA modification plays important roles in the governance of gene expression and is temporally regulated in different cell states. In contrast to global m6A profiling in bulk sequencing, single-cell technologies for analyzing m6A heterogeneity are not extensively established. Here, we developed single-nucleus m6A-CUT&Tag (sn-m6A-CT) for simultaneous profiling of m6A methylomes and transcriptomes within a single nucleus using mouse embryonic stem cells (mESCs). m6A-CT is capable of enriching m6A-marked RNA molecules in situ, without isolating RNAs from cells. We adapted m6A-CT to the droplet-based single-cell omics platform and demonstrated high-throughput performance in analyzing nuclei isolated from thousands of cells from various cell types. We show that sn-m6A-CT profiling is sufficient to determine cell identity and allows the generation of cell-type-specific m6A methylome landscapes from heterogeneous populations. These indicate that sn-m6A-CT provides additional dimensions to multimodal datasets and insights into epitranscriptomic landscape in defining cell fate identity and states.

m6A RNA modification regulates innate lymphoid cell responses in a lineage-specific manner.

Innate lymphoid cells (ILCs) can quickly switch from a quiescent state to an active state and rapidly produce effector molecules that provide critical early immune protection. How the post-transcriptional machinery processes different stimuli and initiates robust gene expression in ILCs is poorly understood. Here, we show that deletion of the N6-methyladenosine (m6A) writer protein METTL3 has little impact on ILC homeostasis or cytokine-induced ILC1 or ILC3 responses but significantly diminishes ILC2 proliferation, migration and effector cytokine production and results in impaired antihelminth immunity. m6A RNA modification supports an increase in cell size and transcriptional activity in activated ILC2s but not in ILC1s or ILC3s. Among other transcripts, the gene encoding the transcription factor GATA3 is highly m6A methylated in ILC2s. Targeted m6A demethylation destabilizes nascent Gata3 mRNA and abolishes the upregulation of GATA3 and ILC2 activation. Our study suggests a lineage-specific requirement of m6A for ILC2 responses.

LIS1 RNA-binding orchestrates the mechanosensitive properties of embryonic stem cells in AGO2-dependent and independent ways.

Lissencephaly-1 (LIS1) is associated with neurodevelopmental diseases and is known to regulate the molecular motor cytoplasmic dynein activity. Here we show that LIS1 is essential for the viability of mouse embryonic stem cells (mESCs), and it governs the physical properties of these cells. LIS1 dosage substantially affects gene expression, and we uncovered an unexpected interaction of LIS1 with RNA and RNA-binding proteins, most prominently the Argonaute complex. We demonstrate that LIS1 overexpression partially rescued the extracellular matrix (ECM) expression and mechanosensitive genes conferring stiffness to Argonaute null mESCs. Collectively, our data transforms the current perspective on the roles of LIS1 in post-transcriptional regulation underlying development and mechanosensitive processes.

Stabilization of hESCs in two distinct substates along the continuum of pluripotency.

A detailed understanding of the developmental substates of human pluripotent stem cells (hPSCs) is needed to optimize their use in cell therapy and for modeling early development. Genetic instability and risk of tumorigenicity of primed hPSCs are well documented, but a systematic isogenic comparison between substates has not been performed. We derived four hESC lines in naive human stem cell medium (NHSM) and generated isogenic pairs of NHSM and primed cultures. Through phenotypic, transcriptomic, and methylation profiling, we identified changes that arose during the transition to a primed substate. Although early NHSM cultures displayed naive characteristics, including greater proliferation and clonogenic potential compared with primed cultures, they drifted toward a more primed-like substate over time, including accumulation of genetic abnormalities. Overall, we show that transcriptomic and epigenomic profiling can be used to place human pluripotent cultures along a developmental continuum and may inform their utility for clinical and research applications.

Human primed and naïve PSCs are both able to differentiate into trophoblast stem cells.

The recent derivation of human trophoblast stem cells (TSCs) from placental cytotrophoblasts and blastocysts opened opportunities for studying the development and function of the human placenta. Recent reports have suggested that human naïve, but not primed, pluripotent stem cells (PSCs) retain an exclusive potential to generate TSCs. Here we report that, in the absence of WNT stimulation, transforming growth factor ß (TGF-ß) pathway inhibition leads to direct and robust conversion of primed human PSCs into TSCs. The resulting primed PSC-derived TSC lines exhibit self-renewal, can differentiate into the main trophoblast lineages, and present RNA and epigenetic profiles that are indistinguishable from recently established TSC lines derived from human placenta, blastocysts, or isogenic human naïve PSCs expanded under human enhanced naïve stem cell medium (HENSM) conditions. Activation of nuclear Yes-associated protein (YAP) signaling is sufficient for this conversion and necessary for human TSC maintenance. Our findings underscore a residual plasticity in primed human PSCs that allows their in vitro conversion into extra-embryonic trophoblast lineages.

Recent insights into mammalian natural and synthetic ex utero embryogenesis.

Research on early postimplantation mammalian development has been limited by the small size and intrauterine confinement of the developing embryos. Owing to the inability to observe and manipulate living embryos at these stages in utero, the establishment of robust ex utero embryo-culture systems that capture prolonged periods of mouse development has been an important research goal. In the last few years, these methods have been significantly improved by the optimization and enhancement of in vitro culture systems sustaining embryo development during peri-implantation stages for several species, and more recently, proper growth of natural mouse embryos from pregastrulation to late organogenesis stages and of embryonic stem cell (ES)-derived synthetic embryo models until early organogenesis stages. Here, we discuss the most recent ex utero embryo-culture systems established to date for rodents, nonhuman primates, and humans. We emphasize their technical aspects and developmental timeframe and provide insights into the new opportunities that these methods will contribute to the study of natural and synthetic mammalian embryogenesis and the stem-cell field.

Pride in STEM worldwide.

Cell asked LGBTQ+ scientists around the world about how their identity shapes their experiences in STEM. Here we share six unique perspectives of researchers highlighting how their area of expertise, research focus, institutions, and geographical location have played a role in this regard. We thank them for sharing their voices and continued efforts toward making science more inclusive.

Perceived exertion and dyspnea while cycling during a hypoxic and hyperoxic placebo.

Rating of perceived exertion (RPE) is used to subjectively quantify the perception of physical activity, breathlessness or dyspnea, and leg discomfort (RPElegs) during exercise. However, it is unknown how dyspnea or RPElegs can be influenced by expectations. Thirty healthy, active participants (19 males, 11 females) completed five, 5-minute submaximal cycling trials at 60% peak work rate. We deceived participants by telling them they were inspiring different hypoxic and hyperoxic gases, when in fact they breathed room air. Cardiorespiratory variables were similar between the trials, however, dyspnea and RPElegs evaluated with a Borg scale changed in a dose-response manner. When participants believed they were breathing 15% O2, they significantly increased dyspnea +0.70 ± 0.2 units (p = 0.03) compared to room air, whereas RPElegs was unchanged +0.35 ± 0.1 units (p = 0.70). When participants believed they were breathing 15% O2, they significantly increased dyspnea +1.05 ± 0.4 units (p = 0.003) compared to 23% hyperoxic condition, whereas RPElegs was unchanged +0.35 ± 0.1 units (p = 0.70). We found that dyspnea during exercise is susceptible to expectancy, without any accompanying physiological changes. Given coaches and clinicians use perceived exertion to prescribe exercise intensity and evaluate treatments, our findings show that the effect of expectations must be considered when interpreting sensations of breathlessness.

Embryo model completes gastrulation to neurulation and organogenesis.

Embryonic stem (ES) cells can undergo many aspects of mammalian embryogenesis in vitro1-5, but their developmental potential is substantially extended by interactions with extraembryonic stem cells, including trophoblast stem (TS) cells, extraembryonic endoderm stem (XEN) cells and inducible XEN (iXEN) cells6-11. Here we assembled stem cell-derived embryos in vitro from mouse ES cells, TS cells and iXEN cells and showed that they recapitulate the development of whole natural mouse embryo in utero up to day 8.5 post-fertilization. Our embryo model displays headfolds with defined forebrain and midbrain regions and develops a beating heart-like structure, a trunk comprising a neural tube and somites, a tail bud containing neuromesodermal progenitors, a gut tube, and primordial germ cells. This complete embryo model develops within an extraembryonic yolk sac that initiates blood island development. Notably, we demonstrate that the neurulating embryo model assembled from Pax6-knockout ES cells aggregated with wild-type TS cells and iXEN cells recapitulates the ventral domain expansion of the neural tube that occurs in natural, ubiquitous Pax6-knockout embryos. Thus, these complete embryoids are a powerful in vitro model for dissecting the roles of diverse cell lineages and genes in development. Our results demonstrate the self-organization ability of ES cells and two types of extraembryonic stem cells to reconstitute mammalian development through and beyond gastrulation to neurulation and early organogenesis.

Post-gastrulation synthetic embryos generated ex utero from mouse naive ESCs.

In vitro cultured stem cells with distinct developmental capacities can contribute to embryonic or extraembryonic tissues after microinjection into pre-implantation mammalian embryos. However, whether cultured stem cells can independently give rise to entire gastrulating embryo-like structures with embryonic and extraembryonic compartments remains unknown. Here, we adapt a recently established platform for prolonged ex utero growth of natural embryos to generate mouse post-gastrulation synthetic whole embryo models (sEmbryos), with both embryonic and extraembryonic compartments, starting solely from naive ESCs. This was achieved by co-aggregating non-transduced ESCs, with naive ESCs transiently expressing Cdx2 or Gata4 to promote their priming toward trophectoderm and primitive endoderm lineages, respectively. sEmbryos adequately accomplish gastrulation, advance through key developmental milestones, and develop organ progenitors within complex extraembryonic compartments similar to E8.5 stage mouse embryos. Our findings highlight the plastic potential of naive pluripotent cells to self-organize and functionally reconstitute and model the entire mammalian embryo beyond gastrulation.

YTHDF2 suppresses the plasmablast genetic program and promotes germinal center formation.

Antibody-mediated immunity is initiated by B cell differentiation into multiple cell subsets, including plasmablast, memory, and germinal center (GC) cells. B cell differentiation trajectories are determined by transcription factors, yet very few mechanisms that specifically determine early B cell fates have been described. Here, we report a post-transcriptional mechanism that suppresses the plasmablast genetic program and promotes GC B cell fate commitment. Single-cell RNA-sequencing analysis reveals that antigen-specific B cell precursors at the pre-GC stage upregulate YTHDF2, which enhances the decay of methylated transcripts. Ythdf2-deficient B cells exhibit intact proliferation and activation, whereas differentiation into GC B cells is blocked. Mechanistically, B cells require YTHDF2 to attenuate the plasmablast genetic program during GC seeding, and transcripts of key plasmablast-regulating genes are methylated and bound by YTHDF2. Collectively, this study reveals how post-transcriptional suppression of gene expression directs appropriate B cell fate commitment during initiation of the adaptive immune response.

Long-term follow up of systemic rituximab therapy as first-line and salvage therapy for idiopathic orbital inflammation and review of the literature.

PURPOSE: To report long-term outcomes of systemic rituximab therapy for idiopathic orbital inflammation (IOI) as both primary and salvage therapy and to review the English literature. METHODS: A retrospective review of four consecutive biopsy-proven IOI cases managed with systemic rituximab including demographics, management, and outcomes, and review of English literature, were performed. Primary outcome measures included resolution of symptoms, recurrence, and length of follow up. RESULTS: Of four cases, systemic rituximab was the first-line therapy in two cases and salvage therapy in two cases. The mean age of the patients was 62 years (range, 50-68 years). The orbit was involved in three cases and extraocular muscle in one case. Systemic rituximab (1 g weekly for 4 weeks) was given for one session in three patients and for 12 sessions in 1 patient. All four patients responded with the resolution of all symptoms without recurrence after at least 5 years of follow up. Review of the literature showed systemic rituximab had provided clinical improvement at shorter follow up in 14 of 15 cases when used as a salvage therapy. CONCLUSIONS: Systemic rituximab therapy seems to be an effective therapy for IOI as salvage or first-line therapy with long-term clinical durability.

SUMOylation of linker histone H1 drives chromatin condensation and restriction of embryonic cell fate identity.

The fidelity of the early embryonic program is underlined by tight regulation of the chromatin. Yet, how the chromatin is organized to prohibit the reversal of the developmental program remains unclear. Specifically, the totipotency-to-pluripotency transition marks one of the most dramatic events to the chromatin, and yet, the nature of histone alterations underlying this process is incompletely characterized. Here, we show that linker histone H1 is post-translationally modulated by SUMO2/3, which facilitates its fixation onto ultra-condensed heterochromatin in embryonic stem cells (ESCs). Upon SUMOylation depletion, the chromatin becomes de-compacted and H1 is evicted, leading to totipotency reactivation. Furthermore, we show that H1 and SUMO2/3 jointly mediate the repression of totipotent elements. Lastly, we demonstrate that preventing SUMOylation on H1 abrogates its ability to repress the totipotency program in ESCs. Collectively, our findings unravel a critical role for SUMOylation of H1 in facilitating chromatin repression and desolation of the totipotent identity.

Control of Foxp3 induction and maintenance by sequential histone acetylation and DNA demethylation.

Regulatory T (Treg) cells play crucial roles in suppressing deleterious immune response. Here, we investigate how Treg cells are mechanistically induced in vitro (iTreg) and stabilized via transcriptional regulation of Treg lineage-specifying factor Foxp3. We find that acetylation of histone tails at the Foxp3 promoter is required for inducing Foxp3 transcription. Upon induction, histone acetylation signals via bromodomain-containing proteins, particularly targets of inhibitor JQ1, and sustains Foxp3 transcription via a global or trans effect. Subsequently, Tet-mediated DNA demethylation of Foxp3 cis-regulatory elements, mainly enhancer CNS2, increases chromatin accessibility and protein binding, stabilizing Foxp3 transcription and obviating the need for the histone acetylation signal. These processes transform stochastic iTreg induction into a stable cell fate, with the former sensitive and the latter resistant to genetic and environmental perturbations. Thus, sequential histone acetylation and DNA demethylation in Foxp3 induction and maintenance reflect stepwise mechanical switches governing iTreg cell lineage specification.

Ex Utero Culture of Mouse Embryos from Pregastrulation to Advanced Organogenesis.

Postimplantation mammalian embryo culture methods have been generally inefficient and limited to brief periods after dissection out of the uterus. Platforms have been recently developed for highly robust and prolonged ex utero culture of mouse embryos from egg-cylinder stages until advanced organogenesis. These platforms enable appropriate and faithful development of pregastrulating embryos (E5.5) until the hind limb formation stage (E11). Late gastrulating embryos (E7.5) are grown in rotating bottles in these settings, while extended culture from pregastrulation stages (E5.5 or E6.5) requires a combination of static and rotating bottle cultures. In addition, sensitive regulation of O2 and CO2 concentration, gas pressure, glucose levels, and the use of a specific ex utero culture medium are critical for proper embryo development. Here, a detailed step-by-step protocol for extended ex utero mouse embryo culture is provided. The ability to grow normal mouse embryos ex utero from gastrulation to organogenesis represents a valuable tool for characterizing the effect of different experimental perturbations during embryonic development.