Transcriptional targets of TWIST1 in the cranial mesoderm regulate cell-matrix interactions and mesenchyme maintenance.

TWIST1, a basic helix-loop-helix transcription factor is essential for the development of cranial mesoderm and cranial neural crest-derived craniofacial structures. We have previously shown that, in the absence of TWIST1, cells within the cranial mesoderm adopt an abnormal epithelial configuration via a process reminiscent of a mesenchymal to epithelial transition (MET). Here, we show by gene expression analysis that loss of TWIST1 in the cranial mesoderm is accompanied by a reduction in the expression of genes that are associated with cell-extracellular matrix interactions and the acquisition of mesenchymal characteristics. By comparing the transcriptional profiles of cranial mesoderm-specific Twist1 loss-of-function mutant and control mouse embryos, we identified a set of genes that are both TWIST1-dependent and predominantly expressed in the mesoderm. ChIP-seq was used to identify TWIST1-binding sites in an in vitro model of a TWIST1-dependent mesenchymal cell state, and the data were combined with the transcriptome data to identify potential target genes. Three direct transcriptional targets of TWIST1 (Ddr2, Pcolce and Tgfbi) were validated by ChIP-PCR using mouse embryonic tissues and by luciferase assays. Our findings reveal that the mesenchymal properties of the cranial mesoderm are likely to be regulated by a network of TWIST1 targets that influences the extracellular matrix and cell-matrix interactions, and collectively they are required for the morphogenesis of the craniofacial structures.

The mesenchymal architecture of the cranial mesoderm of mouse embryos is disrupted by the loss of Twist1 function.

The basic helix-loop-helix transcription factor Twist1 is a key regulator of craniofacial development. Twist1-null mouse embryos exhibit failure of cephalic neural tube closure and abnormal head development and die at E11.0. To dissect the function of Twist1 in the cranial mesoderm beyond mid-gestation, we used Mesp1-Cre to delete Twist1 in the anterior mesoderm, which includes the progenitors of the cranial mesoderm. Deletion of Twist1 in mesoderm cells resulted in loss and malformations of the cranial mesoderm-derived skeleton. Loss of Twist1 in the mesoderm also resulted in a failure to fully segregate the mesoderm and the neural crest cells, and the malformation of some cranial neural crest-derived tissues. The development of extraocular muscles was compromised whereas the differentiation of branchial arch muscles was not affected, indicating a differential requirement for Twist1 in these two types of craniofacial muscle. A striking effect of the loss of Twist1 was the inability of the mesodermal cells to maintain their mesenchymal characteristics, and the acquisition of an epithelial-like morphology. Our findings point to a role of Twist1 in maintaining the mesenchyme architecture and the progenitor state of the mesoderm, as well as mediating mesoderm-neural crest interactions in craniofacial development.

Fetal growth delay caused by loss of non-canonical imprinting is resolved late in pregnancy and culminates in offspring overgrowth.

Germline epigenetic programming, including genomic imprinting, substantially influences offspring development. Polycomb Repressive Complex 2 (PRC2) plays an important role in Histone 3 Lysine 27 trimethylation (H3K27me3)-dependent imprinting, loss of which leads to growth and developmental changes in mouse offspring. In this study, we show that offspring from mouse oocytes lacking the PRC2 protein Embryonic Ectoderm Development (EED) were initially developmentally delayed, characterised by low blastocyst cell counts and substantial growth delay in mid-gestation embryos. This initial developmental delay was resolved as offspring underwent accelerated fetal development and growth in late gestation resulting in offspring that were similar stage and weight to controls at birth. The accelerated development and growth in offspring from Eed-null oocytes was associated with remodelling of the placenta, which involved an increase in fetal and maternal tissue size, conspicuous expansion of the glycogen-enriched cell population, and delayed parturition. Despite placental remodelling and accelerated offspring fetal growth and development, placental efficiency, and fetal blood glucose levels were low, and the fetal blood metabolome was unchanged. Moreover, while expression of the H3K27me3-imprinted gene and amino acid transporter Slc38a4 was increased, fetal blood levels of individual amino acids were similar to controls, indicating that placental amino acid transport was not enhanced. Genome-wide analyses identified extensive transcriptional dysregulation and DNA methylation changes in affected placentas, including a range of imprinted and non-imprinted genes. Together, while deletion of Eed in growing oocytes resulted in fetal growth and developmental delay and placental hyperplasia, our data indicate a remarkable capacity for offspring fetal growth to be normalised despite inefficient placental function and the loss of H3K27me3-dependent genomic imprinting.

Regionalized Twist1 activity in the forelimb bud drives the morphogenesis of the proximal and preaxial skeleton.

Development of the mouse forelimb bud depends on normal Twist1 activity. Global loss of Twist1 function before limb bud formation stops limb development and loss of Twist1 throughout the mesenchyme after limb bud initiation leads to polydactyly, the ulnarization or loss of the radius and malformations and reductions of the shoulder girdle. Here we show that conditional deletion of Twist1 by Mesp1-Cre in the mesoderm that migrates into the anterior-proximal part of the forelimb bud results in the development of supernumerary digits and carpals, the acquisition of ulna-like characteristics by the radius and malformations of the humerus and scapula. The mirror-like duplications and posteriorization of pre-axial tissues are preceded by disruptions to anterior-posterior Shh, Bmp and Fgf signaling gradients and dysregulation of transcription factors that regulate anterior-posterior limb patterning.

Elucidation of Gene Expression Patterns in the Craniofacial Tissues of Mouse Embryos by Wholemount In Situ Hybridization.

Analysis of animal models allows a deeper understanding of craniofacial development in health and diseases of humans. Wholemount in situ hybridization (WISH) is an informative technique to visualize gene expression in tissues across the developmental stages of embryos. The principle of WISH is based on the complementary binding (hybridization) of the DNA/RNA probe to the target transcript. The bound probe can then be visualized by an enzymatic color reaction to delineate the expression pattern of transcripts within a tissue. Here we describe an optimized method to perform in situ hybridization in mouse embryos.

Visualization of the Cartilage and Bone Elements in the Craniofacial Structures by Alcian Blue and Alizarin Red Staining.

Craniofacial morphogenesis is underpinned by orchestrated growth and form-shaping activity of skeletal and soft tissues in the head and face. Disruptions during development can lead to dysmorphology of the skull, jaw, and the pharyngeal structures. Developmental disorders can be investigated in animal models to elucidate the molecular and cellular consequences of the morphogenetic defects. A first step in determining the disruption in the development of the head and face is to analyze the phenotypic features of the skeletal tissues. Examination of the anatomy of bones and cartilage over time and space will identify structural defects of head structures and guide follow-up analysis of the molecular and cellular attributes associated with the defects. Here we describe a protocol to simultaneously visualize the cartilage and bone elements by Alcian blue and Alizarin red staining, respectively, of wholemount specimens in mouse models.

Breaking constraint of mammalian axial formulae.

The vertebral column of individual mammalian species often exhibits remarkable robustness in the number and identity of vertebral elements that form (known as axial formulae). The genetic mechanism(s) underlying this constraint however remain ill-defined. Here, we reveal the interplay of three regulatory pathways (Gdf11, miR-196 and Retinoic acid) is essential in constraining total vertebral number and regional axial identity in the mouse, from cervical through to tail vertebrae. All three pathways have differing control over Hox cluster expression, with heterochronic and quantitative changes found to parallel changes in axial identity. However, our work reveals an additional role for Hox genes in supporting axial elongation within the tail region, providing important support for an emerging view that mammalian Hox function is not limited to imparting positional identity as the mammalian body plan is laid down. More broadly, this work provides a molecular framework to interrogate mechanisms of evolutionary change and congenital anomalies of the vertebral column.

Transient Polycomb activity represses developmental genes in growing oocytes.

BACKGROUND: Non-genetic disease inheritance and offspring phenotype are substantially influenced by germline epigenetic programming, including genomic imprinting. Loss of Polycomb Repressive Complex 2 (PRC2) function in oocytes causes non-genetically inherited effects on offspring, including embryonic growth restriction followed by post-natal offspring overgrowth. While PRC2-dependent non-canonical imprinting is likely to contribute, less is known about germline epigenetic programming of non-imprinted genes during oocyte growth. In addition, de novo germline mutations in genes encoding PRC2 lead to overgrowth syndromes in human patients, but the extent to which PRC2 activity is conserved in human oocytes is poorly understood. RESULTS: In this study, we identify a discrete period of early oocyte growth during which PRC2 is expressed in mouse growing oocytes. Deletion of Eed during this window led to the de-repression of 343 genes. A high proportion of these were developmental regulators, and the vast majority were not imprinted genes. Many of the de-repressed genes were also marked by the PRC2-dependent epigenetic modification histone 3 lysine 27 trimethylation (H3K27me3) in primary-secondary mouse oocytes, at a time concurrent with PRC2 expression. In addition, we found H3K27me3 was also enriched on many of these genes by the germinal vesicle (GV) stage in human oocytes, strongly indicating that this PRC2 function is conserved in the human germline. However, while the 343 genes were de-repressed in mouse oocytes lacking EED, they were not de-repressed in pre-implantation embryos and lost H3K27me3 during pre-implantation development. This implies that H3K27me3 is a transient feature that represses a wide range of genes in oocytes. CONCLUSIONS: Together, these data indicate that EED has spatially and temporally distinct functions in the female germline to repress a wide range of developmentally important genes and that this activity is conserved in the mouse and human germlines.

Wnt signaling promotes AChR aggregation at the neuromuscular synapse in collaboration with agrin.

Wnt proteins regulate the formation of central synapses by stimulating synaptic assembly, but their role at the vertebrate neuromuscular junction (NMJ) is unclear. Wnt3 is expressed by lateral motoneurons of the spinal cord during the period of motoneuron-muscle innervation. Using gain- and loss-of-function studies in the chick wing, we demonstrate that Wnt signaling is necessary for the formation of acetylcholine receptor (AChR) clusters without affecting muscle growth. Similarly, diaphragms from Dishevelled-1 mutant mice with deficiency in Wnt signaling exhibit defects in cluster distribution. In cultured myotubes, Wnt3 increases the number and size of AChR clusters induced by agrin, a nerve-derived signal critical for NMJ development. Wnt3 does not signal through the canonical Wnt pathway to induce cluster formation. Instead, Wnt3 induces the rapid formation of unstable AChR micro-clusters through activation of Rac1, which aggregate into large clusters only in the presence of agrin. Our data reveal a role for Wnts in post-synaptic assembly at the vertebrate NMJ by enhancing agrin function through Rac1 activation.

Requirement for Twist1 in frontonasal and skull vault development in the mouse embryo.

Using a Cre-mediated conditional deletion approach, we have dissected the function of Twist1 in the morphogenesis of the craniofacial skeleton. Loss of Twist1 in neural crest cells and their derivatives impairs skeletogenic differentiation and leads to the loss of bones of the snout, upper face and skull vault. While no anatomically recognizable maxilla is formed, a malformed mandible is present. Since Twist1 is expressed in the tissues of the maxillary eminence and the mandibular arch, this finding suggests that the requirement for Twist1 is not the same in all neural crest derivatives. The effect of the loss of Twist1 function is not restricted to neural crest-derived bones, since the predominantly mesoderm-derived parietal and interparietal bones are also affected, presumably as a consequence of lost interactions with neural crest-derived tissues. In contrast, the formation of other mesodermal skeletal derivatives such as the occipital bones and most of the chondrocranium are not affected by the loss of Twist1 in the neural crest cells.

Out of sight, out of mind? Germ cells and the potential impacts of epigenomic drugs.

Epigenetic modifications, including DNA methylation and histone modifications, determine the way DNA is packaged within the nucleus and regulate cell-specific gene expression. The heritability of these modifications provides a memory of cell identity and function. Common dysregulation of epigenetic modifications in cancer has driven substantial interest in the development of epigenetic modifying drugs. Although these drugs have the potential to be highly beneficial for patients, they act systemically and may have "off-target" effects in other cells such as the patients' sperm or eggs. This review discusses the potential for epigenomic drugs to impact on the germline epigenome and subsequent offspring and aims to foster further examination into the possible effects of these drugs on gametes. Ultimately, the information gained by further research may improve the clinical guidelines for the use of such drugs in patients of reproductive age.

Regulatory landscape of the Hox transcriptome.

Precise regulation of Hox gene activity is essential to achieve proper control of animal embryonic development and to avoid generation of a variety of malignancies. This is a multilayered process, including complex polycistronic transcription, RNA processing, microRNA repression, long noncoding RNA regulation and sequence-specific translational control, acting together to achieve robust quantitative and qualitative Hox protein output. For many such mechanisms, the Hox cluster gene network has turned out to serve as a paradigmatic model for their study. In this review, we discuss current knowledge of how the different layers of post-transcriptional regulation and the production of a variety of noncoding RNA species control Hox output, and how this shapes formation of developmental systems that are reproducibly patterned by complex Hox networks.

Smchd1 regulates long-range chromatin interactions on the inactive X chromosome and at Hox clusters.

The regulation of higher-order chromatin structure is complex and dynamic, and a full understanding of the suite of mechanisms governing this architecture is lacking. Here, we reveal the noncanonical SMC protein Smchd1 to be a novel regulator of long-range chromatin interactions in mice, and we add Smchd1 to the canon of epigenetic proteins required for Hox-gene regulation. The effect of losing Smchd1-dependent chromatin interactions has varying outcomes that depend on chromatin context. At autosomal targets transcriptionally sensitive to Smchd1 deletion, we found increased short-range interactions and ectopic enhancer activation. In contrast, the inactive X chromosome was transcriptionally refractive to Smchd1 ablation, despite chromosome-wide increases in short-range interactions. In the inactive X, we observed spreading of trimethylated histone H3 K27 (H3K27me3) domains into regions not normally decorated by this mark. Together, these data suggest that Smchd1 is able to insulate chromatin, thereby limiting access to other chromatin-modifying proteins.

Dataset of TWIST1-regulated genes in the cranial mesoderm and a transcriptome comparison of cranial mesoderm and cranial neural crest.

This article contains data related to the research article entitled "Transcriptional targets of TWIST1 in the cranial mesoderm regulate cell-matrix interactions and mesenchyme maintenance" by Bildsoe et al. (2016) [1]. The data presented here are derived from: (1) a microarray-based comparison of sorted cranial mesoderm (CM) and cranial neural crest (CNC) cells from E9.5 mouse embryos; (2) comparisons of transcription profiles of head tissues from mouse embryos with a CM-specific loss-of-function of Twist1 and control mouse embryos collected at E8.5 and E9.5; (3) ChIP-seq using a TWIST1-specific monoclonal antibody with chromatin extracts from TWIST1-expressing MDCK cells, a model for a TWIST1-dependent mesenchymal state.

Analyses of the differentiation potential of satellite cells from myoD-/-, mdx, and PMP22 C22 mice.

BACKGROUND: Sporadic and sometimes contradictory studies have indicated changes in satellite cell behaviour associated with the progressive nature of human Duchenne muscular dystrophy (DMD). Satellite cell proliferation and number are reportedly altered in DMD and the mdx mouse model. We recently found that satellite cells in MSVski transgenic mice, a muscle hypertrophy model showing progressive muscle degeneration, display a severe ageing-related differentiation defect in vitro. We tested the hypothesis that similar changes contribute to the gradual loss of muscle function with age in mdx and PMP22 mice, a model of human motor and sensory neuropathy type 1A (HMSN1A). METHODS: Single extensor digitorum longus muscle fibres were cultured from mdx and PMP22 mice and age- and genetic background-matched controls. Mice at several ages were compared with regard to the differentiation of satellite cells, assayed as the proportion of desmin-expressing cells that accumulated sarcomeric myosin heavy chain. RESULTS: Satellite cells of 2 month, 6 month, and 12 month old mdx mice were capable of differentiating to a similar extent to age-matched wild type control animals in an in vitro proliferation/differentiation model. Strikingly, differentiation efficiency in individual 6 month and 12 month old mdx animals varies to a much higher extent than in age-matched controls, younger mdx animals, or PMP22 mice. In contrast, differentiation of myoblasts from all myoD null mice assayed was severely impaired in this assay system. The defect in satellite cell differentiation that occurs in some mdx animals arises from a delay in differentiation that is not overcome by IGF-1 treatment at any phase of cultivation. CONCLUSION: Overall, a defect in satellite cell differentiation above that arising through normal ageing does not occur in mdx or PMP22 mouse models of human disease. Nonetheless, the impaired differentiation of satellite cells from some mdx animals suggests that additional factors, environmental or epigenetic, may lead to deteriorating muscle repair through poor differentiation of satellite cells in genetically predisposed individuals.

Hedgehog can drive terminal differentiation of amniote slow skeletal muscle.

BACKGROUND: Secreted Hedgehog (Hh) signalling molecules have profound influences on many developing and regenerating tissues. Yet in most vertebrate tissues it is unclear which Hh-responses are the direct result of Hh action on a particular cell type because Hhs frequently elicit secondary signals. In developing skeletal muscle, Hhs promote slow myogenesis in zebrafish and are involved in specification of medial muscle cells in amniote somites. However, the extent to which non-myogenic cells, myoblasts or differentiating myocytes are direct or indirect targets of Hh signalling is not known. RESULTS: We show that Sonic hedgehog (Shh) can act directly on cultured C2 myoblasts, driving Gli1 expression, myogenin up-regulation and terminal differentiation, even in the presence of growth factors that normally prevent differentiation. Distinct myoblasts respond differently to Shh: in some slow myosin expression is increased, whereas in others Shh simply enhances terminal differentiation. Exposure of chick wing bud cells to Shh in culture increases numbers of both muscle and non-muscle cells, yet simultaneously enhances differentiation of myoblasts. The small proportion of differentiated muscle cells expressing definitive slow myosin can be doubled by Shh. Shh over-expression in chick limb bud reduces muscle mass at early developmental stages while inducing ectopic slow muscle fibre formation. Abundant later-differentiating fibres, however, do not express extra slow myosin. Conversely, Hh loss of function in the limb bud, caused by implanting hybridoma cells expressing a functionally blocking anti-Hh antibody, reduces early slow muscle formation and differentiation, but does not prevent later slow myogenesis. Analysis of Hh knockout mice indicates that Shh promotes early somitic slow myogenesis. CONCLUSIONS: Taken together, the data show that Hh can have direct pro-differentiative effects on myoblasts and that early-developing muscle requires Hh for normal differentiation and slow myosin expression. We propose a simple model of how direct and indirect effects of Hh regulate early limb myogenesis.

Timed deletion of Twist1 in the limb bud reveals age-specific impacts on autopod and zeugopod patterning.

Twist1 encodes a transcription factor that plays a vital role in limb development. We have used a tamoxifen-inducible Cre transgene, Ubc-CreERT2, to generate time-specific deletions of Twist1 by inducing Cre activity in mouse embryos at different ages from embryonic (E) day 9.5 onwards. A novel forelimb phenotype of supernumerary pre-axial digits and enlargement or partial duplication of the distal radius was observed when Cre activity was induced at E9.5. Gene expression analysis revealed significant upregulation of Hoxd10, Hoxd11 and Grem1 in the anterior half of the forelimb bud at E11.5. There is also localized upregulation of Ptch1, Hand2 and Hoxd13 at the site of ectopic digit formation, indicating a posterior molecular identity for the supernumerary digits. The specific skeletal phenotypes, which include duplication of digits and distal zeugopods but no overt posteriorization, differ from those of other Twist1 conditional knockout mutants. This outcome may be attributed to the deferment of Twist1 ablation to a later time frame of limb morphogenesis, which leads to the ectopic activation of posterior genes in the anterior tissues after the establishment of anterior-posterior anatomical identities in the forelimb bud.

Regionalisation of the endoderm progenitors and morphogenesis of the gut portals of the mouse embryo.

This fate-mapping study reveals that the progenitors of all major parts of the embryonic gut are already present in endoderm of the early-head-fold to early-somite stage (1-9 somites) mouse embryo. The anterior endoderm contributes primarily to the anterior intestinal portal of the early-organogenesis stage (16-19 somites) embryo. Endoderm cells around and lateral to the node are allocated to the open "midgut" region of the embryonic gut. The posterior (post-nodal) endoderm contributes not only to the posterior intestinal portal but also the open "midgut". Descendants of the posterior endoderm span a length of the gut from the level of the 3rd-5th somites to the posterior end of the embryonic gut. The formation of the anterior and posterior intestinal portals is accompanied by similar repertoires of morphogenetic tissue movement. We also discovered that cells on contralateral sides of the anterior endoderm are distributed asymmetrically to the dorsal and ventral sides of the anterior intestinal portal, heralding the acquisition of laterality by the embryonic foregut.

Fate-Mapping Technique: Grafting Fluorescent Cells into Gastrula-Stage Mouse Embryos at 7-7.5 Days Post-coitum.

INTRODUCTIONAnalysis of the developmental fate of cell populations in different parts of the embryo enables the construction of fate maps. These reveal the organization of the body plan and can presage the expression of molecular characteristics of cell lineages and the formation of body parts. The efficacy of fate-mapping techniques is critically dependent on their ability to track the cells and all their descendants without compromising the development of the embryo. Cell grafting involves isolating a population of genetically tagged cells from a transgenic embryo and grafting them to a defined site in a nontransgenic host embryo. Tissue colonization is analyzed using a genetic tag (e.g., a fluorescent protein that can be visualized noninvasively) that allows tracking of the transplanted cells and their descendants in the host embryo throughout development in culture. Alternatively, a lacZ transgene can be used to localize graft-derived cells histologically. Differentiation of the graft-derived cells can be studied by examining the expression of molecular markers by in situ hybridization of gene transcripts or immunohistochemical detection of lineage-specific proteins. This protocol describes how to graft cells isolated from a donor embryo into the germ layer of a wild-type host mouse embryo at 7-7.5 days post-coitum (dpc).

Fate-mapping technique: using carbocyanine dyes for vital labeling of cells in gastrula-stage mouse embryos cultured in vitro.

INTRODUCTIONThe allocation of different progenitor populations to embryonic structures can be visualized by tracking the distribution of cells to specific tissues in the live embryo. A critical prerequisite for cell tracking is to identify unambiguously the progenitors and their descendants during morphogenesis. This can be achieved by using molecular markers that are expressed from transgenes integrated into the genome or as episomal DNA constructs, or by tagging the cells with exogenous markers that are incorporated into the cell membrane or cytoplasmic components of the cells. These labels can be introduced by dye-labeling the membrane, injecting marker enzyme into the cytoplasm, or integrating reporter constructs by transfection or electroporation. This protocol describes how to label cells in the endoderm (which, at this stage of development, is the superficial tissue layer) of live mouse embryos at 7.0-7.5 days post-coitum (dpc), using two carbocyanine dyes (DiI and DiO).