Heparan Sulfate Chain-Conjugated Laminin-E8 Fragments Advance Paraxial Mesodermal Differentiation Followed by High Myogenic Induction from hiPSCs

Human-induced pluripotent stem cells (hiPSCs) have great therapeutic potential. The cell source differentiated from hiPSCs requires xeno-free and robust methods for lineage-specific differentiation. Here, a system is described for differentiating hiPSCs on new generation laminin fragments (NGLFs), a recombinant form of a laminin E8 fragment conjugated to the heparan sulfate chains (HS) attachment domain of perlecan. Using NGLFs, hiPSCs are highly promoted to direct differentiation into a paraxial mesoderm state with high-efficiency muscle lineage generation. HS conjugation to the C-terminus of Laminin E8 fragments brings fibroblast growth factors (FGFs) bound to the HS close to the cell surface of hiPSCs, thereby facilitating stronger FGF signaling pathways stimulation and initiating HOX gene expression, which triggers the paraxial mesoderm differentiation of hiPSCs. This highly efficient differentiation system can provide a roadmap for paraxial mesoderm development and an infinite source of myocytes and muscle stem cells for disease modeling and regenerative medicine.

Dermomyotome-derived endothelial cells migrate to the dorsal aorta to support hematopoietic stem cell emergence

Development of the dorsal aorta is a key step in the establishment of the adult blood-forming system, since hematopoietic stem and progenitor cells (HSPCs) arise from ventral aortic endothelium in all vertebrate animals studied. Work in zebrafish has demonstrated that arterial and venous endothelial precursors arise from distinct subsets of lateral plate mesoderm. Here, we profile the transcriptome of the earliest detectable endothelial cells (ECs) during zebrafish embryogenesis to demonstrate that tissue-specific EC programs initiate much earlier than previously appreciated, by the end of gastrulation. Classic studies in the chick embryo showed that paraxial mesoderm generates a subset of somite-derived endothelial cells (SDECs) that incorporate into the dorsal aorta to replace HSPCs as they exit the aorta and enter circulation. We describe a conserved program in the zebrafish, where a rare population of endothelial precursors delaminates from the dermomyotome to incorporate exclusively into the developing dorsal aorta. Although SDECs lack hematopoietic potential, they act as a local niche to support the emergence of HSPCs from neighboring hemogenic endothelium. Thus, at least three subsets of ECs contribute to the developing dorsal aorta: vascular ECs, hemogenic ECs, and SDECs. Taken together, our findings indicate that the distinct spatial origins of endothelial precursors dictate different cellular potentials within the developing dorsal aorta.

Come together over me: Cells that form the dermatocranium and chondrocranium in mice

Most bone develops either by intramembranous ossification where bone forms within a soft connective tissue, or by endochondral ossification by way of a cartilage anlagen or model. Bones of the skull can form endochondrally or intramembranously or represent a combination of the two types of ossification. Contrary to the classical definition of intramembranous ossification, we have previously described a tight temporo-spatial relationship between cranial cartilages and dermal bone formation and proposed a mechanistic relationship between chondrocranial cartilage and dermal bone. Here, we further investigate this relationship through an analysis of how cells organize to form cranial cartilages and dermal bone. Using Wnt1-Cre2 and Mesp1-Cre transgenic mice, we determine the derivation of cells that comprise cranial cartilages from either cranial neural crest (CNC) or paraxial mesoderm (PM). We confirm a previously determined CNC-PM boundary that runs through the hypophyseal fenestra in the cartilaginous braincase floor and identify four additional CNC-PM boundaries in the chondrocranial lateral wall, including a boundary that runs along the basal and apical ends of the hypochiasmatic cartilage. Based on the knowledge that as osteoblasts differentiate from CNC- and PM-derived mesenchyme, the differentiating cells express the transcription factor genes RUNX2 and osterix (OSX), we created a new transgenic mouse line called R2Tom. R2Tom mice carry a tdTomato reporter gene joined with an evolutionarily well-conserved enhancer sequence of RUNX2. R2Tom mice crossed with Osx-GFP mice yield R2Tom;Osx-GFP double transgenic mice in which various stages of osteoblasts and their precursors are detected with different fluorescent reporters. We use the R2Tom;Osx-GFP mice, new data on the cell derivation of cranial cartilages, histology, immunohistochemistry, and detailed morphological observations combined with data from other investigators to summarize the differentiation of cranial mesenchyme as it forms condensations that become chondrocranial cartilages and associated dermal bones of the lateral cranial wall. These data advance our previous findings of a tendency of cranial cartilage and dermal bone development to vary jointly in a coordinated manner, promoting a role for cranial cartilages in intramembranous bone formation.

Single-cell chromatin accessibility profiling of cell-state-specific gene regulatory programs during mouse organogenesis

In mammals, early organogenesis begins soon after gastrulation, accompanied by specification of various type of progenitor/precusor cells. In order to reveal dynamic chromatin landscape of precursor cells and decipher the underlying molecular mechanism driving early mouse organogenesis, we performed single-cell ATAC-seq of E8.5-E10.5 mouse embryos. We profiled a total of 101,599 single cells and identified 41 specific cell types at these stages. Besides, by performing integrated analysis of scATAC-seq and public scRNA-seq data, we identified the critical cis-regulatory elements and key transcription factors which drving development of spinal cord and somitogenesis. Furthermore, we intersected accessible peaks with human diseases/traits-related loci and found potential clinical associated single nucleotide variants (SNPs). Overall, our work provides a fundamental source for understanding cell fate determination and revealing the underlying mechanism during postimplantation embryonic development, and expand our knowledge of pathology for human developmental malformations.

BMP signaling is enhanced intracellularly by FHL3 controlling WNT-dependent spatiotemporal emergence of the neural crest

The spatiotemporal coordination of multiple morphogens is essential for embryonic patterning yet poorly understood. During neural crest (NC) formation, dynamic bone morphogenetic protein (BMP), fibroblast growth factor (FGF), and WNT signals cooperate by acting on mesoderm and ectoderm. Here, we show that Fhl3, a scaffold LIM domain protein, modulates BMP gradient interpretation during NC induction. During gastrulation, low BMP signaling neuralizes the neural border (NB) ectoderm, while Fhl3 enhances Smad1 intracellular response in underlying paraxial mesoderm, triggering the high WNT8 signals needed to pattern the NB. During neurulation, fhl3 activation in NC ectoderm promotes simultaneous high BMP and BMP-dependent WNT activity required for specification. Mechanistically, Fhl3 interacts with Smad1 and promotes Smad1 binding to wnt8 promoter in a BMP-dependent manner. Consequently, differential Fhl3 expression in adjacent cells ensures a finely tuned coordination of BMP and WNT signaling at several stages of NC development, starting by positioning the NC-inducing mesoderm center under competent NB ectoderm.

In vitro systems: A new window to the segmentation clock

Segmental organization of the vertebrate body plan is established by the segmentation clock, a molecular oscillator that controls the periodicity of somite formation. Given the dynamic nature of the segmentation clock, in vivo studies in vertebrate embryos pose technical challenges. As an alternative, simpler models of the segmentation clock based on primary explants and pluripotent stem cells have recently been developed. These ex vivo and in vitro systems enable more quantitative analysis of oscillatory properties and expand the experimental repertoire applicable to the segmentation clock. Crucially, by eliminating the need for model organisms, in vitro models allow us to study the segmentation clock in new species, including our own. The human oscillator was recently recapitulated using induced pluripotent stem cells, providing a window into human development. Certainly, a combination of in vivo and in vitro work holds the most promising potential to unravel the mechanisms behind vertebrate segmentation.

Development of pluripotent stem cell-based human tenocytes

Human pluripotent stem cells (PSCs) are used as a platform for therapeutic purposes such as cell transplantation therapy and drug discovery. Another motivation for studying PSCs is to understand human embryogenesis and development. All cell types that make up the body tissues develop through defined trajectories during embryogenesis. For example, paraxial mesoderm is considered to differentiate into several cell types including skeletal muscle cells, chondrocytes, osteocytes, dermal fibroblasts, and tenocytes. Tenocytes are fibroblast cells that constitute the tendon. The step-wise narrowing fate decisions of paraxial mesoderm in the embryo have been modeled in vitro using PSCs; however, deriving tenocytes from human-induced PSCs and their application in cell therapy have long been challenging. PSC-derived tenocytes can be used for a source of cell transplantation to treat a damaged or ruptured tendon due to injury, disorder, or aging. In this review, we discuss the latest research findings on the use of PSCs for studying the biology of tenocyte development and their application in therapeutic settings.

Five transcriptional factors reprogram fibroblast into myogenic lineage cells via paraxial mesoderm stage

It is hard to supply satellite cells as a cell source for therapy of muscle degenerative disease since the sampling of muscle tissue is very invasive to a patient with muscular disease. Direct conversion allows us to get specific cell types by transduction of defined transcriptional factors. To induce myogenic direct conversion, we transduced five transcriptional factors including Pax3, Sox2, Klf4, c-Myc, and Esrrb into mouse embryonic fibroblasts. We found that the transduction of the five transcriptional factors induced myogenic direct conversion of fibroblast. We revealed that the transduced cells with the five transcriptional factors were converted to myogenic lineage cells through a paraxial mesoderm-like stage. The expression level of myogenic-related genes of the transduced cells gradually increased as the passage increased. The induced myogenic lineage cells differentiated into muscle fibers in virto and in vivo. The current study revealed that the five transcription factors generated myogenic lineage cells from fibroblast via a paraxial mesoderm stage. The induced myogenic lineage cells may have a potential being applied as cell source for degenerative muscle disease.

Paraxial Mesoderm Is the Major Source of Lymphatic Endothelium

Endothelial cells (ECs), which line blood and lymphatic vessels, are generally described to come from the lateral plate mesoderm despite experimental evidence for a broader source of origin, including the paraxial mesoderm (PXM). Current dogma suggests that following specification from mesoderm, local environmental cues establish the distinct molecular and functional characteristics of ECs in different vascular beds. Here we present evidence to challenge this view, showing that lymphatic EC fate is imprinted during transition through the PXM lineage. We show that PXM-derived cells form the lymphatic endothelium of multiple organs and tissues, with a more restricted contribution to blood vessel endothelium. By deleting Prox1 specifically in PXM-derived cells, we show that this lineage is indispensable for lymphatic vessel development. Collectively, our data establish lineage history as a critical determinant of EC specialization, a finding with broad implications for our understanding of vascular development and heterogeneity.

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Lineage history is a key determinant of endothelial cell specialization

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Endothelial cells arising from paraxial mesoderm preferentially form lymphatic vessels

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Paraxial-mesoderm-derived endothelial cells are essential for lymphatic development

Pre-transplantational Control of the Post-transplantational Fate of Human Pluripotent Stem Cell-Derived Cartilage

Cartilage pellets generated from ectomesenchymal progeny of human pluripotent stem cells (hPSCs) in vitro eventually show signs of commitment of chondrocytes to hypertrophic differentiation. When transplanted subcutaneously, most of the surviving pellets were fully mineralized by 8 weeks. In contrast, treatment with the adenylyl cyclase activator, forskolin, in vitro resulted in slightly enlarged cartilage pellets containing an increased proportion of proliferating immature chondrocytes that expressed very low levels of hypertrophic/terminally matured chondrocyte-specific genes. Forskolin treatment also enhanced hyaline cartilage formation by reducing type I collagen gene expression and increasing sulfated glycosaminoglycan accumulation in the developed cartilage. Chondrogenic mesoderm from hPSCs and dedifferentiated nasal chondrocytes responded similarly to forskolin. Furthermore, forskolin treatment in vitro increased the frequency at which the cartilage pellets maintained unmineralized chondrocytes after subcutaneous transplantation. Thus, the post-transplantational fate of chondrocytes originating from hPSC-derived chondroprogenitors can be controlled during their genesis in vitro.

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Forskolin/cAMP suppresses/delays BMP-induced chondrocyte maturation in vitro

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Forskolin supports chondrocyte proliferation and hyaline chondrogenesis in vitro

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Forskolin suppresses osteogenesis and BMP signaling gene expression in cartilage

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In vitro forskolin treatment improves in vivo maintenance of uncalcified cartilage

Encephalocele development from a congenital meningocele: case report

A fetal MRI study obtained at 21 weeks' gestation revealed a suboccipital meningocele without hydrocephalus. One day after term birth, MRI demonstrated an acquired cerebellar encephalocele, and MRI obtained 5 months later showed progressive enlargement of the encephalocele, still without obvious hydrocephalus. The patient underwent an operation in which an external ventricular drain was placed, the grossly normal cerebellum was reduced into the posterior fossa without resection, and the dural defect was closed. The drain was weaned out over 5 days, and no ventriculoperitoneal shunt was placed. Postoperative MR images revealed normal cerebellum and no hydrocephalus. The patient is developmentally normal. Meningocele and encephalocele are embryologically distinct. An acquired encephalocele could develop from hydrocephalus (which was not present in this case), or secondary to the lower resistance to expansion into the dural defect of the meningocele relative to the resistance to expansion of the fetal skull. The cerebellar tissue was normal in this case, and was thus preserved. The developmental prognosis is excellent. To the authors' knowledge, this is the first reported case of this occurrence. It is important to differentiate between congenital and acquired encephalocele etiologies, because resection of the cerebellar tissue in an acquired encephalocele (as is routinely done in cases of congenital encephalocele) would be expected to result in neurological deficits.

A Gradient of Glycolytic Activity Coordinates FGF and Wnt Signaling during Elongation of the Body Axis in Amniote Embryos

Mammalian embryos transiently exhibit aerobic glycolysis (Warburg effect), a metabolic adaptation also observed in cancer cells. The role of this particular type of metabolism during vertebrate organogenesis is currently unknown. Here, we provide evidence for spatiotemporal regulation of glycolysis in the posterior region of mouse and chicken embryos. We show that a posterior glycolytic gradient is established in response to graded transcription of glycolytic enzymes downstream of fibroblast growth factor (FGF) signaling. We demonstrate that glycolysis controls posterior elongation of the embryonic axis by regulating cell motility in the presomitic mesoderm and by controlling specification of the paraxial mesoderm fate in the tail bud. Our results suggest that glycolysis in the tail bud coordinates Wnt and FGF signaling to promote elongation of the embryonic axis.

Time course and side-by-side analysis of mesodermal, pre-myogenic, myogenic and differentiated cell markers in the chicken model for skeletal muscle formation

The chicken is a well-established model for amniote (including human) skeletal muscle formation because the developmental anatomy of chicken skeletal muscle matches that of mammals. The accessibility of the chicken in the egg as well as the sequencing of its genome and novel molecular techniques have raised the profile of this model. Over the years, a number of regulatory and marker genes have been identified that are suited to monitor the progress of skeletal myogenesis both in wildtype and in experimental embryos. However, in the various studies, differing markers at different stages of development have been used. Moreover, contradictory results on the hierarchy of regulatory factors are now emerging, and clearly, factors need to be able to cooperate. Thus, a reference paper describing in detail and side-by-side the time course of marker gene expression during avian myogenesis is needed. We comparatively analysed onset and expression patterns of the key markers for the chicken immature paraxial mesoderm, for muscle-competent cells, for cells committed to myogenesis and for cells entering terminal differentiation. We performed this analysis from stages when the first paraxial mesoderm is being laid down to the stage when mesoderm formation comes to a conclusion. Our data show that, although the sequence of marker gene expression is the same at the various stages of development, the timing of the expression onset is quite different. Moreover, marker gene expression in myogenic cells being deployed from the dorsomedial and ventrolateral lips of the dermomyotome is different from those being deployed from the rostrocaudal lips, suggesting different molecular programs. Furthermore, expression of Myosin Heavy Chain genes is overlapping but different along the length of a myotube. Finally, Mef2c is the most likely partner of Mrf proteins, and, in contrast to the mouse and more alike frog and zebrafish fish, chicken Mrf4 is co-expressed with MyoG as cells enter terminal differentiation.

Zebrafish nephrogenesis is regulated by interactions between retinoic acid, mecom, and Notch signaling

The zebrafish pronephros provides a conserved model to study kidney development, in particular to delineate the poorly understood processes of how nephron segment pattern and cell type choice are established. Zebrafish nephrons are divided into distinct epithelial regions that include a series of proximal and distal tubule segments, which are comprised of intercalated transporting epithelial cells and multiciliated cells (MCC). Previous studies have shown that retinoic acid (RA) regionalizes the renal progenitor field into proximal and distal domains and that Notch signaling later represses MCC differentiation, but further understanding of these pathways has remained unknown. The transcription factor mecom (mds1/evi1 complex) is broadly expressed in renal progenitors, and then subsequently marks the distal tubule. Here, we show that mecom is necessary to form the distal tubule and to restrict both proximal tubule formation and MCC fate choice. We found that mecom and RA have opposing roles in patterning discrete proximal and distal segments. Further, we discovered that RA is required for MCC formation, and that one mechanism by which RA promotes MCC fate choice is to inhibit mecom. Next, we determined the epistatic relationship between mecom and Notch signaling, which limits MCC fate choice by lateral inhibition. Abrogation of Notch signaling with the γ-secretase inhibitor DAPT revealed that Notch and mecom did not have additive effects in blocking MCC formation, suggesting that they function in the same pathway. Ectopic expression of the Notch signaling effector, Notch intracellular domain (NICD), rescued the expansion of MCCs in mecom morphants, indicating that mecom acts upstream to induce Notch signaling. These findings suggest a model in which mecom and RA arbitrate proximodistal segment domains, while MCC fate is modulated by a complex interplay in which RA inhibition of mecom, and mecom promotion of Notch, titrates MCC number. Taken together, our studies have revealed several essential and novel mechanisms that control pronephros development in the zebrafish.

Crosstalk between Fgf and Wnt signaling in the zebrafish tailbud

Fibroblast growth factor (Fgf) and Wnt signaling are necessary for the intertwined processes of tail elongation, mesodermal development and somitogenesis. Here, we use pharmacological modifiers and time-resolved quantitative analysis of both nascent transcription and protein phosphorylation in the tailbud, to distinguish early effects of signal perturbation from later consequences related to cell fate changes. We demonstrate that Fgf activity elevates Wnt signaling by inhibiting transcription of the Wnt antagonists dkk1 and notum1a. PI3 kinase signaling also increases Wnt signaling via phosphorylation of Gsk3β. Conversely, Wnt can increase signaling within the Mapk branch of the Fgf pathway as Gsk3β phosphorylation elevates phosphorylation levels of Erk. Despite the reciprocal positive regulation between Fgf and Wnt, the two pathways generally have opposing effects on the transcription of co-regulated genes. This opposing regulation of target genes may represent a rudimentary relationship that manifests as out-of-phase oscillation of Fgf and Wnt target genes in the mouse and chick tailbud. In summary, these data suggest that Fgf and Wnt signaling are tightly integrated to maintain proportional levels of activity in the zebrafish tailbud, and this balance is important for axis elongation, cell fate specification and somitogenesis.

Transitions between epithelial and mesenchymal states and the morphogenesis of the early mouse embryo

Multicellular organisms arise from the generation of different cell types and the organization of cells into tissues and organs. Cells of metazoa display two main phenotypes, the ancestral epithelial state and the recent mesenchymal derivative. Epithelial cells are usually stationary and reside in two-dimensional sheets. By contrast mesenchymal cells are loosely packed and can move to new positions, thereby providing a vehicle for cell rearrangement, dispersal and novel cell-cell interactions. Transitions between epithelial and mesenchymal states drive key morphogenetic events in the early vertebrate embryo, including gastrulation, germ layer formation and somitogenesis. The cell behaviors and molecular mechanisms promoting transitions between these two states in the early mouse embryo are discussed in this review.

The bHLH regulator pMesogenin1 is required for maturation and segmentation of paraxial mesoderm

Paraxial mesoderm in vertebrates gives rise to all trunk and limb skeletal muscles, the trunk skeleton, and portions of the trunk dermis and vasculature. We show here that germline deletion of mouse pMesogenin1, a bHLH class gene specifically expressed in developmentally immature unsegmented paraxial mesoderm, causes complete failure of somite formation and segmentation of the body trunk and tail. At the molecular level, the phenotype features dramatic loss of expression within the presomitic mesoderm of Notch/Delta pathway components and oscillating somitic clock genes that are thought to control segmentation and somitogenesis. Subsequent patterning and specification steps for paraxial mesoderm also fail, leading to a complete absence of all trunk paraxial mesoderm derivatives, which include skeletal muscle, vertebrae, and ribs. We infer that pMesogenin1 is an essential upstream regulator of trunk paraxial mesoderm development and segmentation.