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.

Publisher Correction: c-Maf controls immune responses by regulating disease-specific gene networks and repressing IL-2 in CD4+ T cells.

In the version of this article initially published, the Supplementary Data file was an incorrect version. The correct version is now provided. The error has been corrected in the HTML and PDF version of the article.

c-Maf controls immune responses by regulating disease-specific gene networks and repressing IL-2 in CD4+ T cells.

The transcription factor c-Maf induces the anti-inflammatory cytokine IL-10 in CD4+ T cells in vitro. However, the global effects of c-Maf on diverse immune responses in vivo are unknown. Here we found that c-Maf regulated IL-10 production in CD4+ T cells in disease models involving the TH1 subset of helper T cells (malaria), TH2 cells (allergy) and TH17 cells (autoimmunity) in vivo. Although mice with c-Maf deficiency targeted to T cells showed greater pathology in TH1 and TH2 responses, TH17 cell-mediated pathology was reduced in this context, with an accompanying decrease in TH17 cells and increase in Foxp3+ regulatory T cells. Bivariate genomic footprinting elucidated the c-Maf transcription-factor network, including enhanced activity of NFAT; this led to the identification and validation of c-Maf as a negative regulator of IL-2. The decreased expression of the gene encoding the transcription factor RORγt (Rorc) that resulted from c-Maf deficiency was dependent on IL-2, which explained the in vivo observations. Thus, c-Maf is a positive and negative regulator of the expression of cytokine-encoding genes, with context-specific effects that allow each immune response to occur in a controlled yet effective manner.

ERK1/2 signalling dynamics promote neural differentiation by regulating chromatin accessibility and the polycomb repressive complex.

Fibroblast growth factor (FGF) is a neural inducer in many vertebrate embryos, but how it regulates chromatin organization to coordinate the activation of neural genes is unclear. Moreover, for differentiation to progress, FGF signalling must decline. Why these signalling dynamics are required has not been determined. Here, we show that dephosphorylation of the FGF effector kinase ERK1/2 rapidly increases chromatin accessibility at neural genes in mouse embryos, and, using ATAC-seq in human embryonic stem cell derived spinal cord precursors, we demonstrate that this occurs genome-wide across neural genes. Importantly, ERK1/2 inhibition induces precocious neural gene transcription, and this involves dissociation of the polycomb repressive complex from key gene loci. This takes place independently of subsequent loss of the repressive histone mark H3K27me3 and transcriptional onset. Transient ERK1/2 inhibition is sufficient for the dissociation of the repressive complex, and this is not reversed on resumption of ERK1/2 signalling. Moreover, genomic footprinting of sites identified by ATAC-seq together with ChIP-seq for polycomb protein Ring1B revealed that ERK1/2 inhibition promotes the occupancy of neural transcription factors (TFs) at non-polycomb as well as polycomb associated sites. Together, these findings indicate that ERK1/2 signalling decline promotes global changes in chromatin accessibility and TF binding at neural genes by directing polycomb and other regulators and appears to serve as a gating mechanism that provides directionality to the process of differentiation.

Precision of tissue patterning is controlled by dynamical properties of gene regulatory networks.

During development, gene regulatory networks allocate cell fates by partitioning tissues into spatially organised domains of gene expression. How the sharp boundaries that delineate these gene expression patterns arise, despite the stochasticity associated with gene regulation, is poorly understood. We show, in the vertebrate neural tube, using perturbations of coding and regulatory regions, that the structure of the regulatory network contributes to boundary precision. This is achieved, not by reducing noise in individual genes, but by the configuration of the network modulating the ability of stochastic fluctuations to initiate gene expression changes. We use a computational screen to identify network properties that influence boundary precision, revealing two dynamical mechanisms by which small gene circuits attenuate the effect of noise in order to increase patterning precision. These results highlight design principles of gene regulatory networks that produce precise patterns of gene expression.

Heads or tails: making the spinal cord.

The central nervous system contains a vast array of cell types that are produced along the length of the rostrocaudal axis. This diversity in cell identity is established during embryonic development, and ensures that physiologically distinct cell types develop in the appropriate position in the body. Understanding how this cellular diversity arises remains a major challenge central to the field of developmental biology. In more recent years, approaches using pluripotent embryonic stem cells (ESCs) as in vitro models of development have revealed many insights into nervous system regionalisation. Here, we outline advances in the directed differentiation of ESCs, focusing on the generation of the spinal cord. We discuss the regionalisation events that impact the caudal part of the nervous system, highlighting general principles underpinning rostrocaudal differences within the mammalian body plan.

The route to spinal cord cell types: a tale of signals and switches.

Understanding the mechanisms that control induction and elaboration of the vertebrate central nervous system (CNS) requires an analysis of the extrinsic signals and downstream transcriptional networks that assign cell fates in the correct space and time. We focus on the generation and patterning of the spinal cord. We summarize evidence that the origin of the spinal cord is distinct from the anterior regions of the CNS. We discuss how this affects the gene regulatory networks and cell state transitions that specify spinal cord cell subtypes, and we highlight how the timing of extracellular signals and dynamic control of transcriptional networks contribute to the correct spatiotemporal generation of different neural cell types.

Patched1 is required in neural crest cells for the prevention of orofacial clefts.

Defects such as cleft lip with or without cleft palate (CL/P) are among the most common craniofacial birth defects in humans. In many cases, the underlying molecular and cellular mechanisms that result in these debilitating anomalies remain largely unknown. Perturbed hedgehog (HH) signalling plays a major role in craniofacial development, and mutations in a number of pathway constituents underlie craniofacial disease. In particular, mutations in the gene encoding the major HH receptor and negative regulator, patched1 (PTCH1), are associated with both sporadic and familial forms of clefting, yet relatively little is known about how PTCH1 functions during craniofacial morphogenesis. To address this, we analysed the consequences of conditional loss of Ptch1 in mouse neural crest cell-derived facial mesenchyme. Using scanning electron microscopy (SEM) and live imaging of explanted facial primordia, we captured defective nasal pit invagination and CL in mouse embryos conditionally lacking Ptch1. Our analysis demonstrates interactions between HH and FGF signalling in the development of the upper lip, and reveals cell-autonomous and non-autonomous roles mediated by Ptch1. In particular, we show that deletion of Ptch1 in the facial mesenchyme alters cell morphology, specifically in the invaginating nasal pit epithelium. These findings highlight a critical link between the neural crest cells and olfactory epithelium in directing the morphogenesis of the mammalian lip and nose primordia. Importantly, these interactions are critically dependent on Ptch1 function for the prevention of orofacial clefts.

Patched 1 is a crucial determinant of asymmetry and digit number in the vertebrate limb.

The vertebrate hedgehog receptor patched 1 (Ptc1) is crucial for negative regulation of the sonic hedgehog (Shh) pathway during anterior-posterior patterning of the limb. We have conditionally inactivated Ptc1 in the mesenchyme of the mouse limb using Prx1-Cre. This results in constitutive activation of hedgehog (Hh) signalling during the early stages of limb budding. Our data suggest that variations in the timing and efficiency of Cre-mediated excision result in differential forelimb and hindlimb phenotypes. Hindlimbs display polydactyly (gain of digits) and a molecular profile similar to the Gli3 mutant extra-toes. Strikingly, forelimbs are predominantly oligodactylous (displaying a loss of digits), with a symmetrical, mirror-image molecular profile that is consistent with re-specification of the anterior forelimb to a posterior identity. Our data suggest that this is related to very early inactivation of Ptc1 in the forelimb perturbing the gene regulatory networks responsible for both the pre-patterning and the subsequent patterning stages of limb development. These results establish the importance of the downstream consequences of Hh pathway repression, and identify Ptc1 as a key player in limb patterning even prior to the onset of Shh expression.

Sox2 levels regulate the chromatin occupancy of WNT mediators in epiblast progenitors responsible for vertebrate body formation.

WNT signalling has multiple roles. It maintains pluripotency of embryonic stem cells, assigns posterior identity in the epiblast and induces mesodermal tissue. Here we provide evidence that these distinct functions are conducted by the transcription factor SOX2, which adopts different modes of chromatin interaction and regulatory element selection depending on its level of expression. At high levels, SOX2 displaces nucleosomes from regulatory elements with high-affinity SOX2 binding sites, recruiting the WNT effector TCF/ß-catenin and maintaining pluripotent gene expression. Reducing SOX2 levels destabilizes pluripotency and reconfigures SOX2/TCF/ß-catenin occupancy to caudal epiblast expressed genes. These contain low-affinity SOX2 sites and are co-occupied by T/Bra and CDX. The loss of SOX2 allows WNT-induced mesodermal differentiation. These findings define a role for Sox2 levels in dictating the chromatin occupancy of TCF/ß-catenin and reveal how context-specific responses to a signal are configured by the level of a transcription factor.

Inactivation of Patched1 in the mouse limb has novel inhibitory effects on the chondrogenic program.

The bones of the vertebrate limb form by the process of endochondral ossification, whereby limb mesenchyme condenses to form an intermediate cartilage scaffold that is then replaced by bone. Although Indian hedgehog (IHH) is known to control hypertophic differentiation of chondrocytes during this process, the role of hedgehog signaling in the earlier stages of chondrogenesis is less clear. We have conditionally inactivated the hedgehog receptor Ptc1 in undifferentiated limb mesenchyme of the mouse limb using Prx1-Cre, thus inducing constitutively active ligand-independent hedgehog signaling. In addition to major patterning defects, we observed a marked disruption to the cartilage elements in the limbs of Prx1-Cre:Ptc1(c/c) embryos. Using an in vitro micromass culture system we show that this defect lies downstream of mesenchymal cell condensation and likely upstream of chondrocyte differentiation. Despite early increases in levels of chondrogenic genes, soon after mesenchymal condensation the stromal layer of Prx1-Cre:Ptc1(c/c)-derived micromass cultures is characterized by a loss of cell integrity, which is associated with increased cell death and a striking decrease in Alcian blue staining cartilage nodules. Furthermore, inhibition of the hedgehog pathway activation using cyclopamine was sufficient to essentially overcome this chondrogenic defect in both micromass and ex vivo explant assays of Prx1-Cre:Ptc1(c/c) limbs. These data demonstrate for the first time the inhibitory effect of cell autonomously activated hedgehog signaling on chondrogenesis, and stress the importance of PTC1 in maintaining strict control of signaling levels during this phase of skeletal development.

Building consensus in neuromesodermal research: Current advances and future biomedical perspectives.

The development of the vertebrate body axis relies on the activity of different populations of axial progenitors, including neuromesodermal progenitors. Currently, the term 'Neuromesodermal progenitors' is associated with various definitions. Here, we use distinct terminologies to highlight advances in our understanding of this cell type at both the single-cell and population levels. We discuss how these recent insights prompt new opportunities to address a range of biomedical questions spanning cancer metastasis, congenital disorders, cellular metabolism, regenerative medicine, and evolution. Finally, we outline some of the major unanswered questions and propose future directions at the forefront of neuromesodermal research.

The metalloendopeptidase gene Pitrm1 is regulated by hedgehog signaling in the developing mouse limb and is expressed in muscle progenitors.

Pitrm1 is a zinc metalloendopeptidase that has been implicated in Alzheimer's disease and mitochondrial peptide degradation, but to date no major role in embryonic development has been documented. In a screen for genes regulated by hedgehog signaling in the mouse limb, we showed that expression of Pitrm1 is upregulated in response to loss of the Gli3 transcription factor. Here we confirm spatial changes in Pitrm1 expression in the Gli3 mutant mouse limb and examine Pitrm1 expression in Shh null and Ptch1 conditional deletion mouse mutants. In wild-type mice, Pitrm1 is expressed in a number of developing tissues known to be patterned by Sonic hedgehog, including the limbs, face, cortex, hippocampus, cerebellum, tectum, sub-mandibular gland, lung, genital tubercle, hair follicles, and the enamel knot of the teeth. Additionally, Pitrm1 is expressed in Pax3-expressing myoblast progenitors in the limb, the dermomyotome, and developing muscles of the face and torso.

Mutations in DZIP1L, which encodes a ciliary-transition-zone protein, cause autosomal recessive polycystic kidney disease.

Autosomal recessive polycystic kidney disease (ARPKD), usually considered to be a genetically homogeneous disease caused by mutations in PKHD1, has been associated with ciliary dysfunction. Here, we describe mutations in DZIP1L, which encodes DAZ interacting protein 1-like, in patients with ARPKD. We further validated these findings through loss-of-function studies in mice and zebrafish. DZIP1L localizes to centrioles and to the distal ends of basal bodies, and interacts with septin2, a protein implicated in maintenance of the periciliary diffusion barrier at the ciliary transition zone. In agreement with a defect in the diffusion barrier, we found that the ciliary-membrane translocation of the PKD proteins polycystin-1 and polycystin-2 is compromised in DZIP1L-mutant cells. Together, these data provide what is, to our knowledge, the first conclusive evidence that ARPKD is not a homogeneous disorder and further establish DZIP1L as a second gene involved in ARPKD pathogenesis.

Unmasking the ciliopathies: craniofacial defects and the primary cilium.

Over the past decade, the primary cilium has emerged as a pivotal sensory organelle that acts as a major signaling hub for a number of developmental signaling pathways. In that time, a vast number of proteins involved in trafficking and signaling have been linked to ciliary assembly and/or function, demonstrating the importance of this organelle during embryonic development. Given the central role of the primary cilium in regulating developmental signaling, it is not surprising that its dysfunction results in widespread defects in the embryo, leading to an expanding class of human congenital disorders known as ciliopathies. These disorders are individually rare and phenotypically variable, but together they affect virtually every vertebrate organ system. Features of ciliopathies that are often overlooked, but which are being reported with increasing frequency, are craniofacial abnormalities, ranging from subtle midline defects to full-blown orofacial clefting. The challenge moving forward is to understand the primary mechanism of disease given the link between the primary cilium and a number of developmental signaling pathways (such as hedgehog, platelet-derived growth factor, and WNT signaling) that are essential for craniofacial development. Here, we provide an overview of the diversity of craniofacial abnormalities present in the ciliopathy spectrum, and reveal those defects in common across multiple disorders. Further, we discuss the molecular defects and potential signaling perturbations underlying these anomalies. This provides insight into the mechanisms leading to ciliopathy phenotypes more generally and highlights the prevalence of widespread dysmorphologies resulting from cilia dysfunction.

Expression of the NET family member Zfp503 is regulated by hedgehog and BMP signaling in the limb.

The NET/Nlz family of zinc finger transcription factors contribute to aspects of developmental growth and patterning across evolutionarily diverse species. To date, however, these molecules remain largely uncharacterized in mouse and chick. We previously reported that limb bud expression of Zfp503, the mouse orthologue of zebrafish nlz2/znf503, is dependent on Gli3. Here, we show that Zfp503/Znf503 is expressed in a restricted pattern during mouse and chick embryogenesis, with particularly dynamic expression in the developing limbs, face, somites, and brain. We also add to our previous data on Gli3 regulation by showing that the anterior domain of Zfp503 expression in the mouse limb is responsive to genetic and nongenetic manipulation of hedgehog signaling. Finally, we demonstrate that posterior expression of Znf503 in the chick limb is responsive to bone morphogenetic protein (BMP) signaling, indicating that Zfp503/Znf503 may act at the nexus of multiple signaling pathways in development.