Transient and lineage-restricted requirement of Ebf3 for sternum ossification
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ABSTRACT: Osteoblasts arise from bone-surrounding connective tissue containing tenocytes and fibroblasts. Lineages of these cell populations and mechanisms of their differentiation are not well understood. Screening enhancer-trap lines of zebrafish allowed us to identify Ebf3 as a transcription factor marking tenocytes and connective tissue cells in skeletal muscle of embryos. Knockout of Ebf3 in mice had no effect on chondrogenesis but led to sternum ossification defects as a result of defective generation of Runx2+ pre-osteoblasts. Conditional and temporal Ebf3 knockout mice revealed requirements of Ebf3 in the lateral plate mesenchyme cells (LPMs), especially in tendon/muscle connective tissue cells, and a stage-specific Ebf3 requirement at embryonic day 9.5-10.5. Upregulated expression of connective tissue markers, such as Egr1/2 and Osr1, increased number of Islet1+ mesenchyme cells, and downregulation of gene expression of the Runx2 regulator Shox2 in Ebf3-deleted thoracic LPMs suggest crucial roles of Ebf3 in the onset of lateral plate mesoderm differentiation towards osteoblasts forming sternum tissues.
Project description:Purpose : Positional information driving limb muscle patterning is considered to be contained in lateral plate mesoderm-derived tissues, such as tendon or muscle connective tissue and not in myogenic cells themselves. The current consensus is that myogenic cells originate from somites, while connective tissue fibroblasts originate from the lateral plate mesoderm. We challenged this model by cell and genetic lineage tracing experiments and identified that a subpopulation of limb myogenic cells did not originate from somite or Pax3 lineage, but rather originated from the lateral plate mesoderm and were derived from Osr1 and Scx lineages. Results: Analysis of single-cell RNA-sequencing data obtained from limb cells at successive developmental stages identified a subpopulation of cells displaying a dual muscle and connective tissue signature, in addition to independent muscle and connective tissue populations. Active BMP signalling was detected in this junctional cell sub-population and at the tendon/muscle interface in developing limbs. BMP gain- and loss-of-function experiments performed in vivo and in vitro showed that this signalling pathway regulated a fibroblast-to-myoblast conversion. Conclusions: We propose that localised BMP signalling converts a subset of lateral plate mesoderm-derived fibroblasts to a myogenic fate and establishes a boundary of fibroblast-derived myonuclei at the tendon/muscle interface to control the muscle pattern during limb development and myotendinous formation.
Project description:The lung mesenchyme plays important roles in lung development and is affected in many respiratory diseases, yet relatively little is known about the biology of lung mesenchymal progenitors. We sought to establish an induced pluripotent stem cell (iPSC)-based model to study lung mesenchyme development and epithelial-mesenchymal interactions. We generated a mouse iPSC line carrying a lung mesenchyme-specific reporter/tracer to establish a protocol for differentiation into lung mesenchymal progenitors. We derived lung mesenchyme from iPSCs both by directed differentiation via a lateral plate mesodermal progenitor state (induced lung mesenchyme, iLM), and by co-development during lung epithelial differentiation (co-developed lung mesenchyme, cLM). We found that directed differentiation via a lateral plate mesoderm progenitor was not only more efficient, but also yielded engineered lung mesenchymal cells that were more similar to primary lung mesenchyme from day 12.5 mouse embryos, as determined by single cell RNAseq. Our iPSC-derived population will provide an inexhaustible source of cells for studying lung development, modeling diseases, and developing therapeutics.
Project description:Tendon is a highly organized, dense connective tissue that has been demonstrated to have very little turnover. In spite of the low turnover, tendon can grow in response to loading, which may take place primarily at the periphery. Tendon injuries and recurrence of injuries are common in both human and animal in sports. It is unclear why some areas of the tendon are more susceptible to such injury and whether this is due to intrinsic regional differences in extracellular matrix (ECM) production or tissue turnover. This study aimed to compare populations of tenocytes derived from the tendon core and periphery. Tenocytes were isolated from equine superficial digital flexor tendons (SDFT), and the proliferation capacity was determined. ECM production was characterized by immuno- and histological staining and by liquid chromatography-mass spectrometry-based proteomics. Core and periphery SDFT cultures exhibited comparable proliferation rates and had very similar proteome profiles, but showed biological variation in collagen type I deposition. In conclusion, the intrinsic properties of tenocytes from different regions of the tendon are very similar and other factors in the tissue may contribute to how specific areas respond to loading or injury.
Project description:The lateral plate mesoderm (LPM) is a transient tissue that produces a diverse range of differentiated structures, including the limbs. However, the molecular mechanisms that drive early LPM specification and development are poorly understood. In this study, we utilize single-cell transcriptomics to define the cell-fate decisions directing LPM specification, subdivision, and early initiation of the forelimb mesenchyme in chicken embryos. We establish a transcriptional atlas and global cell-cell signalling interactions in progenitor, transitional and mature cell types throughout the developing forelimb field.
Project description:The transcriptional regulator Runx2 has essential roles in chondrocytes and osteoblasts, central to the coordinated development of cartilage and bone. However, the regulatory mechanisms underlying Runx2’s roles in skeletal programming are not well understood. Here, we performed an integrative analysis of Runx2–DNA binding and chromatin accessibility in vivo and identified cell type-distinct chromatin accessibility underlying Runx2 roles in osteoblasts and chondrocytes.
Project description:The transcriptional regulator Runx2 has essential roles in chondrocytes and osteoblasts, central to the coordinated development of cartilage and bone. However, the regulatory mechanisms underlying Runx2’s roles in skeletal programming are not well understood. Here, we performed an integrative analysis of Runx2–DNA binding and chromatin accessibility in vivo and identified cell type-distinct chromatin accessibility underlying Runx2 roles in osteoblasts and chondrocytes.
Project description:Osteoblasts require substantial amounts of energy to synthesize bone matrix and coordinate the mineralization of the skeleton. This study analyzed the effect of mitochondrial dysfunction on bone formation in mouse limbs. The limb mesenchyme-specific Tfam knockout (Tfamf/f;Prx1-Cre: Tfam-cKO) mice were analyzed morphologically, and histologically and gene expression in the limb bones were assessed by in situ hybridization, quantitative real-time PCR and RNA sequencing. Moreover, we analyzed mitochondrial function of osteoblasts in Tfam-cKO mice by mitochondrial membrane potential assay and transmission electron microscopic (TEM) observations. We investigated the pathogenesis of spontaneous bone fractures by immunohistochemical analysis, TEM observations and biomechanical examination. The forelimbs in Tfam-cKO mice were significantly shortened from birth and occurred spontaneous fractures within the first week after birth, resulting in severe limb deformities. Histologically, bone hypoplasia with decrease of matrix mineralization was apparent, and the expressions of type Ⅰ collagen and osteocalcin were decreased in the osteoblasts of Tfam-cKO mice although Runx2 expression was unchanged. Decreased type Ⅰ collagen deposition and mineralization in the matrix of the limb bones in Tfam-cKO mice was associated with marked mitochondrial dysfunction. Biomechanical analysis showed significantly lower Young’s modulus and hardness due to poor apatite orientation in the bone tissue of Tfam-cKO mice. The mice with limb mesenchyme-specific Tfam deletion exhibited spontaneous limb bone fractures, resulting in severe limb deformities. Their bone fragility was caused by poor apatite orientation due to impaired osteoblasts differentiation and maturation.
Project description:Different bones of the skeleton originate from three distinct embryonic lineages. Osteoblasts derived from bones of different embryonic origin displayed cell intrinsic difference. Osteocytes are terminally differentiated osteoblasts which displays a unique genetic makeup and a distinct morphology. It is not know if osteocytes display a cell intrinsic differences depeding on their origin. To understand the difference in the gene expression profiles of osteocytes of three different embryonic origins we examined osteocytes from three different bones neural crest origin-frontal bone, paraxial mesoderm origin-parietal bone, and lateral plate mesoderm- femur bone.