James O. S. Hackland

The vertebrate nervous system develops into functionally distinct domains along the body axis ¹ . Developmental studies implicate multiple signaling pathways in coordinating this process, but the order of specification events, sufficiency of individual signals, and molecular basis of competency remain unclear ^2,3 , leaving the field without a unified model of neuraxis patterning. Enteric nervous system (ENS) and midbrain dopaminergic (mDA) progenitors have reached clinical and preclinical testing ^4–11 , yet neither is produced at full purity, and yields vary markedly across human pluripotent stem cell (hPSC) lines, reflecting incomplete control of axial identity. Here we use single-cell genetic fate mapping to demonstrate that hPSCs can commit to anterior or posterior cranial fate within the first 24 hours of differentiation, before neural induction. Building on this finding, we show that the duration of a single saturating WNT pulse is sufficient to specify three “first-order” domains: anterior cranial (0 h), posterior cranial (24 h) and trunk (48 h), each pre-neural and competent for CNS, neural crest and epidermal lineages. Nested “second-order” domains are specified within first-order domains in response to WNT, FGF, and retinoic acid. Mechanistically, an anterior gene regulatory network repressing WNT targets is irreversibly shut down during the transition to posterior identities. Applying this binary WNT patterning logic, we generate mDA neurons and ENS progenitors robustly across hPSC lines and establish an assembloid model combining first-order progenitors to produce all major regions of the nervous system.

Embryonic development follows a conserved sequence of events across species, yet the pace of development is highly variable and particularly slow in humans. Species-specific developmental timing is largely recapitulated in stem cell models, suggesting a cell-intrinsic clock. Here we use directed differentiation of human embryonic stem cells into neuroectoderm to perform a whole-genome CRISPR-Cas9 knockout screen and show that the epigenetic factors Menin and SUZ12 modulate the speed of PAX6 expression during neural differentiation. Genetic and pharmacological loss-of-function of Menin or SUZ12 accelerate cell fate acquisition by shifting the balance of H3K4me3 and H3K27me3 at bivalent promoters, thereby priming key developmental genes for faster activation upon differentiation. We further reveal a synergistic interaction of Menin and SUZ12 in modulating differentiation speed. The acceleration effects were observed in definitive endoderm, cardiomyocyte and neuronal differentiation paradigms, pointing to chromatin bivalency as a general driver of timing across germ layers and developmental stages.

Summary: The enteric nervous system (ENS) is derived primarily from the vagal neural crest, a migratory multipotent cell population emerging from the dorsal neural tube between somites 1-7. Defects in the development and function of the ENS give rise to a range of disorders, termed enteric neuropathies and include conditions such as Hirschsprung’s disease. Little is known about the signalling that specifies early ENS progenitors. This has, thus far, limited progress in the generation of enteric neurons from human Pluripotent Stem Cells (hPSCs) that could provide a useful tool for disease modelling and regenerative medicine. We describe the efficient and accelerated generation of ENS progenitors from hPSCs, revealing that retinoic acid is critical for the acquisition of both vagal axial identity and early ENS progenitor specification. These ENS progenitors generate enteric neuronsin vitroand followingin vivotransplantation, achieving long-term colonisation of the ENS in adult mice. Thus, hPSC-derived ENS progenitors may provide the basis for cell therapy for defects in the ENS.

In the 20 years since the first human pluripotent stem cell (hPSC) lines were established, there have been a plethora of protocols developed that allow us to generate a wide range of human cell types in vitro. Efforts to achieve a greater degree of specificity and efficiency in generating desired cell types have resulted in increasingly complex approaches. The magnitude and timing of signals has become key, and the concept of a “fully defined” system is a forever sought‐after goal with shifting goalposts. This overview discusses two related approaches that can be used to deliver a tightly regulated, intermediate‐strength signal, and which can also manage the impact of endogenous signaling variation and enable a switch away from bovine serum albumin–containing medium to a better‐defined system without suffering a subsequent loss of robustness or efficiency. The approaches, referred to as top‐down inhibition and baseline activation, were developed to deliver intermediate levels of BMP and WNT signaling during neural crest induction from hPSC, but could be applied to a variety of other signals and differentiation systems. © 2019 by John Wiley & Sons, Inc.

The WNT/β-CATENIN pathway is critical for neural crest (NC) formation. However, the effects of the magnitude of the signal remains poorly defined. Here we evaluate the consequences of WNT magnitude variation in a robust model of human NC formation. This model is based on human embryonic stem cells induced by WNT signaling through the small molecule CHIR9902. In addition to its known effect on NC formation, we find that the WNT signal modulates the anterior-posterior axial identity of NCCs in a dose dependent manner, with low WNT leading to anterior OTX+, HOX-NC, and high WNT leading to posterior OTX−, HOX+ NC. Differentiation tests of posterior NC confirm expected derivatives including posterior specific adrenal derivatives, and display partial capacity to generate anterior ectomesenchymal derivatives. Furthermore, unlike anterior NC, posterior NC transit through a TBXT+/SOX2+ neuromesodermal precursor-like intermediate. Finally, we analyze the contributions of other signaling pathways in posterior NC formation, and suggest a critical role for FGF in survival/proliferation, and a requirement of BMP for NC maturation. As expected RA and FGF are able to modulate HOX expression in the posterior NC, but surprisingly, RA supplementation prohibits anterior, but only reduces, posterior NC formation. This work reveals for the first time that the amplitude of WNT signaling can modulate the axial identity of NC cells in humans.

WNT/β-CATENIN signaling is critical for neural crest (NC) formation, yet magnitude effects of the signal remain ill-defined. Using a robust model of human NC formation based on human pluripotent stem cells (hPSCs), we expose that the WNT signal modulates the axial identity of NCs in a dose dependent manner, with low WNT leading to anterior OTX+, HOX- NC, and high WNT leading to posterior OTX-, HOX+ NC. Differentiation tests of posterior NC confirm expected derivatives including posterior specific adrenal derivatives, and display partial capacity to generate anterior ectomesenchymal derivatives. Furthermore, unlike anterior NC, posterior NC exhibit a transient TBXT+/SOX2+ neuromesodermal precursor-like intermediate. Finally, we analyze the contributions of other signaling pathways in posterior NC formation, which suggest a critical role for FGF in survival/proliferation, and a requirement of BMP for NC maturation. As expected Retinoic Acid (RA) and FGF are able to modulate HOX expression in the posterior NC. Surprisingly, early RA supplementation prohibits NC formation. This work reveals for the first time that the amplitude of WNT signaling can modulate the axial identity of NC cells in humans.

The neural crest (NC) is a multipotent embryonic cell population that generates distinct cell types in an axial position-dependent manner. The production of NC cells from human pluripotent stem cells (hPSCs) is a valuable approach to study human NC biology. However, the origin of human trunk NC remains undefined and current in vitro differentiation strategies induce only a modest yield of trunk NC cells. Here we show that hPSC-derived axial progenitors, the posteriorly-located drivers of embryonic axis elongation, give rise to trunk NC cells and their derivatives. Moreover, we define the molecular signatures associated with the emergence of human NC cells of distinct axial identities in vitro. Collectively, our findings indicate that there are two routes toward a human post-cranial NC state: the birth of cardiac and vagal NC is facilitated by retinoic acid-induced posteriorisation of an anterior precursor whereas trunk NC arises within a pool of posterior axial progenitors.

The neural crest (NC) is a multipotent embryonic cell population generating distinct cell types in an axial position-dependent manner. The production of NC cells from human pluripotent stem cells (hPSCs) is a valuable approach to study human NC biology. However, the origin of human trunk NC remains undefined and therefore current in vitro differentiation strategies induce only a modest yield of trunk NC cells. Here we show that hPSC-derived axial progenitors, the posteriorly-located drivers of embryonic axis elongation, give rise to trunk NC cells and their derivatives. Moreover, we define the molecular signatures associated with the emergence of human NC cells of distinct axial identities in vitro. Collectively, our findings indicate that there are two routes toward a human post-cranial NC state: the birth of cardiac and vagal NC is facilitated by retinoic acid-induced posteriorisation of an anterior precursor whereas trunk NC arises within a pool of posterior axial progenitors.