Dchs1-Fat4 regulation of osteogenic differentiation in mouse

IL-32 aggravates metabolic disturbance in human nucleus pulposus cells by activating FAT4-mediated Hippo/YAP signaling

Extracellular matrix (ECM) metabolism disorders in the inflammatory microenvironment play a key role in the pathogenesis of intervertebral disc degeneration (IDD). Interleukin-32 (IL-32) has been reported to be involved in the progression of various inflammatory diseases; however, it remains unclear whether it participates in the matrix metabolism of nucleus pulposus (NP) cells. Therefore, this study aimed to investigate the mechanism of IL-32 on regulating the ECM metabolism in the inflammatory microenvironment. RNA-seq was used to identify aberrantly expressed genes in NP cells in the inflammatory microenvironment. Western blotting, real-time quantitative PCR, immunohistochemistry and immunofluorescence analysis were performed to measure the expression of IL-32 and metabolic markers in human NP tissues or NP cells treated with or without tumor necrosis factor-α (TNF-α). In vivo, an adeno-associated virus overexpressing IL-32 was injected into the caudal intervertebral discs of rats to assess its effect on IDD. Proteins interacting with IL-32 were identified via immunoprecipitation and mass spectrometry. Lentivirus overexpressing IL-32 or knocking down Fat atypical cadherin 4 (FAT4), yes-associated protein (YAP) inhibitor-Verteporfin (VP) were used to treat human NP cells, to explore the pathogenesis of IL-32. Hippo/YAP signaling activity was verified in human NP tissues. IL-32 expression was significantly upregulated in degenerative NP tissues, as indicated in the clinical samples. Furthermore, IL-32 was remarkably overexpressed in TNF-α-induced degenerative NP cells. IL-32 overexpression induced IDD progression in the rat model. Mechanistically, the elevation of IL-32 in the inflammatory microenvironment enhanced its interactions with FAT4 and mammalian sterile 20-like kinase1/2 (MST1/2) proteins, prompting MST1/2 phosphorylation, and activating the Hippo/YAP signaling pathway, causing matrix metabolism disorder in NP cells. Our results suggest that IL-32 mediates matrix metabolism disorders in NP cells in the inflammatory micro-environment via the FAT4/MST/YAP axis, providing a theoretical basis for the precise treatment of IDD.

A high-calcium environment induced ectopic calcification of renal interstitial fibroblasts via TFPI-2-DCHS1-ALP/ENPP1 axis to participate in Randall's plaque formation

Randall's plaques (RP) serve as anchoring sites for calcium oxalate (CaOx) stones, but the underlying mechanism remains unclear. Renal interstitium with a high-calcium environment is identified as pathogenesis of RP formation where the role of human renal interstitial fibroblasts (hRIFs) was highlighted. Our study aims to elucidate the potential mechanism by which a high-calcium environment drives ectopic calcification of hRIFs to participate in RP formation. Alizarin Red staining demonstrated calcium nodules in hRIFs treated with high-calcium medium. Utilizing transcriptome sequencing, tissue factor pathway inhibitor-2 (TFPI-2) was found to be upregulated in high-calcium-induced hRIFs and RP tissues, and TFPI-2 promoted high-calcium-induced calcification of hRIFs. Subsequently, the downstream regulator of TFPI2 was screened by transcriptome sequencing analysis of hRIFs with TFPI-2 knockdown or overexpressed. Dachsous Cadherin Related 1 (DCHS1) knockdown was identified to suppress the calcification of hRIFs enhanced by TFPI-2. Further investigation revealed that TFPI-2/DCHS1 axis promoted high-calcium-induced calcification of hRIFs via disturbing the balance of ENPP1/ALP activities, but without effect on the canonical osteogenic markers, such as osteopontin (OPN), osteogenic factors runt-related transcription factor 2 (RUNX2), bone morphogenetic protein 2 (BMP2). In summary, our study mimicked the high-calcium environment observed in CaOx stone patients with hypercalciuria, and discovered that the high-calcium drove ectopic calcification of hRIFs via a novel TFPI-2-DCHS1-ALP/ENPP1 pathway rather than adaption of osteogenic phenotypes to participate in RP formation.

Favorable impact of PD1/PD-L1 antagonists on bone remodeling: an exploratory prospective clinical study and ex vivo validation

BACKGROUND

Skeletal morbidity in patients with cancer has a major impact on the quality of life, and preserving bone health while improving outcomes is an important goal of modern antitumor treatment strategies. Despite their widespread use in early disease stages, the effects of immune checkpoint inhibitors (ICIs) on the skeleton are still poorly defined. Here, we initiated a comprehensive investigation of the impact of ICIs on bone health by longitudinal assessment of bone turnover markers in patients with cancer and by validation in a novel bioengineered 3D model of bone remodeling.

METHODS

An exploratory longitudinal study was conducted to assess serum markers of bone resorption (C-terminal telopeptide, CTX) and formation (procollagen type I N-terminal propeptide, PINP, and osteocalcin, OCN) before each ICI application (programmed cell death 1 (PD1) inhibitor or programmed death-ligand 1 (PD-L1) inhibitor) for 6 months or until disease progression in patients with advanced cancer and no evidence of bone metastases. To validate the in vivo results, we evaluated osteoclast (OC) and osteoblast (OB) differentiation on treatment with ICIs. In addition, their effect on bone remodeling was assessed by immunohistochemistry, confocal microscopy, and proteomics analysis in a dynamic 3D bone model.

RESULTS

During the first month of treatment, CTX levels decreased sharply but transiently. In contrast, we observed a delayed increase of serum levels of PINP and OCN after 4 months of therapy. In vitro, ICIs impaired the maturation of preosteoclasts by inhibiting STAT3/NFATc1 signaling but not JNK, ERK, and AKT while lacking any direct effect on osteogenesis. However, using our bioengineered 3D bone model, which enables the simultaneous differentiation of OB and OC precursor cells, we confirmed the uncoupling of the OC/OB activity on exposure to ICIs by demonstrating impaired OC maturation along with increased OB differentiation.

CONCLUSION

Our study indicates that the inhibition of the PD1/PD-L1 signaling axis interferes with bone turnover and may exert a protective effect on bone by indirectly promoting osteogenesis.

Genome-wide identification and spatiotemporal expression analysis of cadherin superfamily members in echinoderms

BACKGROUND

Cadherins are calcium-dependent transmembrane cell-cell adhesion proteins that are essential for metazoan development. They consist of three subfamilies: classical cadherins, which bind catenin, protocadherins, which contain 6-7 calcium-binding repeat domains, and atypical cadherins. Their functions include forming adherens junctions, establishing planar cell polarity (PCP), and regulating cell shape, proliferation, and migration. Because they are basal deuterostomes, echinoderms provide important insights into bilaterian evolution, but their only well-characterized cadherin is G-cadherin, a classical cadherin that is expressed by many embryonic epithelia. We aimed to better characterize echinoderm cadherins by conducting phylogenetic analyses and examining the spatiotemporal expression patterns of cadherin-encoding genes during Strongylocentrotus purpuratus development.

RESULTS

Our phylogenetic analyses conducted on two echinoid, three asteroid, and one crinoid species identified ten echinoderm cadherins, including one deuterostome-specific ortholog, cadherin-23, and an echinoderm-specific atypical cadherin that possibly arose in an echinoid-asteroid ancestor. Catenin-binding domains in dachsous-2 orthologs were found to be a deuterostome-specific innovation that was selectively lost in mouse, while those in Fat4 orthologs appeared to be Ambulacraria-specific and were selectively lost in non-crinoid echinoderms. The identified suite of echinoderm cadherins lacks vertebrate-specific innovations but contains two proteins that are present in protostomes and absent from mouse. The spatiotemporal expression patterns of four embryonically expressed cadherins (fat atypical cadherins 1 and 4, dachsous-2, and protocadherin-9) were dynamic and mirrored the expression pattern of Frizzled 5/8, a non-canonical Wnt PCP pathway receptor protein essential for archenteron morphogenesis.

CONCLUSIONS

The echinoderm cadherin toolkit is more similar to that of an ancient bilaterian predating protostomes and deuterostomes than it is to the suite of cadherins found in extant vertebrates. However, it also appears that deuterostomes underwent several cadherin-related innovations. Based on their similar spatiotemporal expression patterns and orthologous relationships to PCP-related and tumor-suppressing proteins, we hypothesize that sea urchin cadherins may play a role in regulating the shape and growth of embryonic epithelia and organs. Future experiments will examine cadherin expression in non-echinoid echinoderms and explore the functions of cadherins during echinoderm development.

Absence of TRIC-B from type XIV Osteogenesis Imperfecta osteoblasts alters cell adhesion and mitochondrial function - A multi-omics study

Osteogenesis Imperfecta (OI) is a heritable collagen-related bone dysplasia characterized by bone fractures, growth deficiency and skeletal deformity. Type XIV OI is a recessive OI form caused by null mutations in TMEM38B, which encodes the ER membrane intracellular cation channel TRIC-B. Previously, we showed that absence of TMEM38B alters calcium flux in the ER of OI patient osteoblasts and fibroblasts, which further disrupts collagen synthesis and secretion. How the absence of TMEM38B affects osteoblast function is still poorly understood. Here we further investigated the role of TMEM38B in human osteoblast differentiation and mineralization. TMEM38B-null osteoblasts showed altered expression of osteoblast marker genes and decreased mineralization. RNA-Seq analysis revealed that cell-cell adhesion was one of the most downregulated pathways in TMEM38B-null osteoblasts, with further validation by real-time PCR and Western blot. Gap and tight junction proteins were also decreased by TRIC-B absence, both in patient osteoblasts and in calvarial osteoblasts of Tmem38b-null mice. Disrupted cell adhesion decreased mutant cell proliferation and cell cycle progression. An important novel finding was that TMEM38B-null osteoblasts had elongated mitochondria with altered fusion and fission markers, MFN2 and DRP1. In addition, TMEM38B-null osteoblasts exhibited a significant increase in superoxide production in mitochondria, further supporting mitochondrial dysfunction. Together these results emphasize the novel role of TMEM38B/TRIC-B in osteoblast differentiation, affecting cell-cell adhesion processes, gap and tight junction, proliferation, cell cycle, and mitochondrial function.

ScRNA-Seq Reveals a Distinct Osteogenic Progenitor of Alveolar Bone

The metabolism and remodeling of alveolar bone are the most active among the whole skeletal system, which is related to the biological characteristics and heterogeneity of the bone mesenchymal stromal cells (MSCs). However, there is a lack of systematic description of the heterogeneity of MSC-derived osteoblastic lineage cells as well as their distinct osteogenic differentiation trajectory of alveolar bone. In this study, we constructed a single-cell atlas of the mouse alveolar bone cells through single-cell RNA sequencing (scRNA-seq). Remarkably, by comparing the cell compositions between the alveolar bone and long bone, we uncovered a previously undescribed cell population that exhibits a high expression of protocadherin Fat4 (Fat4⁺ cells) and is specifically enriched around alveolar bone marrow cavities. ScRNA-seq analysis indicated that Fat4⁺ cells may initiate a distinct osteogenic differentiation trajectory in the alveolar bone. By isolating and cultivating Fat4⁺ cells in vitro, we demonstrated that they possess colony-forming, osteogenic, and adipogenic capabilities. Moreover, FAT4 knockdown could significantly inhibit the osteogenic differentiation of alveolar bone MSCs. Furthermore, we revealed that the Fat4⁺ cells exhibit a core transcriptional signature consisting of several key transcription factors, such as SOX6, which are involved in osteogenesis, and further demonstrated that SOX6 is required for the efficient osteogenic differentiation of the Fat4⁺ cells. Collectively, our high-resolution single-cell atlas of the alveolar bone reveals a distinct osteogenic progenitor that may contribute to the unique physiological characteristics of alveolar bone.

The deubiquitinase UCHL1 negatively controls osteoclastogenesis by regulating TAZ/NFATC1 signalling

The ubiquitin‒proteasome system (UPS) plays a key role in maintaining protein homeostasis and bone remodelling. However, the role of deubiquitinating enzymes (DUBs) in bone resorption is still not well defined. Here, we identified the deubiquitinase ubiquitin C-terminal hydrolase 1 (UCHL1) as a negative regulator of osteoclastogenesis by using the GEO database, proteomic analysis, and RNAi. Osteoclast-specific UCHL1 conditional knockout mice exhibited a severe osteoporosis phenotype in an ovariectomized model. Mechanistically, UCHL1 deubiquitinated and stabilized the transcriptional coactivator with PDZ-binding motif (TAZ) at the K46 residue, thereby inhibiting osteoclastogenesis. The TAZ protein underwent K48-linked polyubiquitination, which was degraded by UCHL1. As a substrate of UCHL1, TAZ regulates NFATC1 through a nontranscriptional coactivator function by competing with calcineurin A (CNA) for binding to NFATC1, which inhibits NFATC1 dephosphorylation and nuclear transport to impede osteoclastogenesis. Moreover, overexpression of UCHL1 locally alleviated acute and chronic bone loss. These findings suggest that activating UCHL1 may serve as a novel therapeutic approach targeting bone loss in various bone pathological states.

Ectomesenchymal Six1 controls mandibular skeleton formation

Craniofacial development requires intricate cooperation between multiple transcription factors and signaling pathways. Six1 is a critical transcription factor regulating craniofacial development. However, the exact function of Six1 during craniofacial development remains elusive. In this study, we investigated the role of Six1 in mandible development using a Six1 knockout mouse model (Six1 ^-/- ) and a cranial neural crest-specific, Six1 conditional knockout mouse model (Six1 ^f/f ; Wnt1-Cre). The Six1 ^-/- mice exhibited multiple craniofacial deformities, including severe microsomia, high-arched palate, and uvula deformity. Notably, the Six1 ^f/f ; Wnt1-Cre mice recapitulate the microsomia phenotype of Six1 ^-/- mice, thus demonstrating that the expression of Six1 in ectomesenchyme is critical for mandible development. We further showed that the knockout of Six1 led to abnormal expression of osteogenic genes within the mandible. Moreover, the knockdown of Six1 in C3H10 T1/2 cells reduced their osteogenic capacity in vitro. Using RNA-seq, we showed that both the loss of Six1 in the E18.5 mandible and Six1 knockdown in C3H10 T1/2 led to the dysregulation of genes involved in embryonic skeletal development. In particular, we showed that Six1 binds to the promoter of Bmp4, Fat4, Fgf18, and Fgfr2, and promotes their transcription. Collectively, our results suggest that Six1 plays a critical role in regulating mandibular skeleton formation during mouse embryogenesis.

Creb5 coordinates synovial joint formation with the genesis of articular cartilage

While prior work has established that articular cartilage arises from Prg4-expressing perichondrial cells, it is not clear how this process is specifically restricted to the perichondrium of synovial joints. We document that the transcription factor Creb5 is necessary to initiate the expression of signaling molecules that both direct the formation of synovial joints and guide perichondrial tissue to form articular cartilage instead of bone. Creb5 promotes the generation of articular chondrocytes from perichondrial precursors in part by inducing expression of signaling molecules that block a Wnt5a autoregulatory loop in the perichondrium. Postnatal deletion of Creb5 in the articular cartilage leads to loss of both flat superficial zone articular chondrocytes coupled with a loss of both Prg4 and Wif1 expression in the articular cartilage; and a non-cell autonomous up-regulation of Ctgf. Our findings indicate that Creb5 promotes joint formation and the subsequent development of articular chondrocytes by driving the expression of signaling molecules that both specify the joint interzone and simultaneously inhibit a Wnt5a positive-feedback loop in the perichondrium.

Cranium growth, patterning and homeostasis

Craniofacial development requires precise spatiotemporal regulation of multiple signaling pathways that crosstalk to coordinate the growth and patterning of the skull with surrounding tissues. Recent insights into these signaling pathways and previously uncharacterized progenitor cell populations have refined our understanding of skull patterning, bone mineralization and tissue homeostasis. Here, we touch upon classical studies and recent advances with an emphasis on developmental and signaling mechanisms that regulate the osteoblast lineage for the calvaria, which forms the roof of the skull. We highlight studies that illustrate the roles of osteoprogenitor cells and cranial suture-derived stem cells for proper calvarial growth and homeostasis. We also discuss genes and signaling pathways that control suture patency and highlight how perturbing the molecular regulation of these pathways leads to craniosynostosis. Finally, we discuss the recently discovered tissue and signaling interactions that integrate skull and cerebrovascular development, and the potential implications for both cerebrospinal fluid hydrodynamics and brain waste clearance in craniosynostosis.

Planar cell polarity-dependent asymmetric organization of microtubules for polarized positioning of the basal body in node cells

For left-right symmetry breaking in the mouse embryo, the basal body must become positioned at the posterior side of node cells, but the precise mechanism for this has remained unknown. Here, we examined the role of microtubules (MTs) and actomyosin in this basal body positioning. Exposure of mouse embryos to agents that stabilize or destabilize MTs or F-actin impaired such positioning. Active myosin II was detected at the anterior side of node cells before the posterior shift of the basal body, and this asymmetric activation was lost in Prickle and dachsous mutant embryos. The organization of basal-body associated MTs (baMTs) was asymmetric between the anterior and posterior sides of node cells, with anterior baMTs extending horizontally and posterior baMTs extending vertically. This asymmetry became evident after polarization of the PCP core protein Vangl1 and before the posterior positioning of the basal body, and it also required the PCP core proteins Prickle and dachsous. Our results suggest that the asymmetry in baMT organization may play a role in correct positioning of the basal body for left-right symmetry breaking.

Emerging Mechanisms of Growth and Patterning Regulation by Dachsous and Fat Protocadherins

Dachsous (Ds) and Fat are evolutionarily conserved cell adhesion molecules that play a critical role in development of multiple organ systems, where they coordinate tissue growth and morphogenesis. Much of our understanding of Ds-Fat signaling pathway comes from studies in Drosophila, where they initiate a signaling pathway that regulate growth by influencing Hippo signaling and morphogenesis by regulating Planar Cell Polarity (PCP). In this review, we discuss recent advances in our understanding of the mechanisms by which Ds-Fat signaling pathway regulates these critical developmental processes. Further, we discuss the progress in our understanding about how they function in mammals.

DCHS1, Lix1L, and the Septin Cytoskeleton: Molecular and Developmental Etiology of Mitral Valve Prolapse

Mitral valve prolapse (MVP) is a common cardiac valve disease that often progresses to serious secondary complications requiring surgery. MVP manifests as extracellular matrix disorganization and biomechanically incompetent tissues in the adult setting. However, MVP has recently been shown to have a developmental basis, as multiple causal genes expressed during embryonic development have been identified. Disease phenotypes have been observed in mouse models with human MVP mutations as early as birth. This study focuses on the developmental function of DCHS1, one of the first genes to be shown as causal in multiple families with non-syndromic MVP. By using various biochemical techniques as well as mouse and cell culture models, we demonstrate a unique link between DCHS1-based cell adhesions and the septin-actin cytoskeleton through interactions with cytoplasmic protein Lix1-Like (LIX1L). This DCHS1-LIX1L-SEPT9 axis interacts with and promotes filamentous actin organization to direct cell-ECM alignment and valve tissue shape.

Bone marrow derived stromal cells from myelodysplastic syndromes are altered but not clonally mutated in vivo

The bone marrow (BM) stroma in myeloid neoplasms is altered and it is hypothesized that this cell compartment may also harbor clonal somatically acquired mutations. By exome sequencing of in vitro expanded mesenchymal stromal cells (MSCs) from n = 98 patients with myelodysplastic syndrome (MDS) and n = 28 healthy controls we show that these cells accumulate recurrent mutations in genes such as ZFX (n = 8/98), RANK (n = 5/98), and others. MDS derived MSCs display higher mutational burdens, increased replicative stress, senescence, inflammatory gene expression, and distinct mutational signatures as compared to healthy MSCs. However, validation experiments in serial culture passages, chronological BM aspirations and backtracking of high confidence mutations by re-sequencing primary sorted MDS MSCs indicate that the discovered mutations are secondary to in vitro expansion but not present in primary BM. Thus, we here report that there is no evidence for clonal mutations in the BM stroma of MDS patients.

Bone marrow-derived mesenchymal stroma cells (MSCs) in myeloid neoplasia have been hypothesized to carry somatic mutations and contribute to pathogenesis. Here the authors analyse ex-vivo cultures and primary MSCs derived from patients with myelodysplastic syndromes, finding functional alterations but no evidence of clonal mutations.

Emerging roles for the YAP/TAZ transcriptional regulators in brain tumour pathology and targeting options

The transcriptional co-activators Yes-associated protein 1/transcriptional co-activator with PDZ-binding motif (YAP/TAZ) have emerged as significant regulators of a wide variety of cellular and organ functions with impact in early embryonic development, especially during the expansion of the neural progenitor cell pool. YAP/TAZ signalling regulates organ size development, tissue homeostasis, wound healing and angiogenesis by participating in a complex network of various pathways. However, recent evidence suggests an association of these physiologic regulatory effects of YAP/TAZ with pro-oncogenic activities. Herein, we discuss the physiological functions of YAP/TAZ as well as the extensive network of signalling pathways that control their expression and activity, leading to brain tumour development and progression. Furthermore, we describe current targeting approaches and drug options including direct YAP/TAZ and YAP-TEA domain transcription factor (TEAD) interaction inhibitors, G-protein coupled receptors (GPCR) signalling modulators and kinase inhibitors, which may be used to successfully attack YAP/TAZ-dependent tumours.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Requirement of FAT and DCHS protocadherins during hypothalamic-pituitary development

Pituitary developmental defects lead to partial or complete hormone deficiency and significant health problems. The majority of cases are sporadic and of unknown cause. We screened 28 patients with pituitary stalk interruption syndrome (PSIS) for mutations in the FAT/DCHS family of protocadherins that have high functional redundancy. We identified seven variants, four of which putatively damaging, in FAT2 and DCHS2 in six patients with pituitary developmental defects recruited through a cohort of patients with mostly ectopic posterior pituitary gland and/or pituitary stalk interruption. All patients had growth hormone deficiency and two presented with multiple hormone deficiencies and small glands. FAT2 and DCHS2 were strongly expressed in the mesenchyme surrounding the normal developing human pituitary. We analyzed Dchs2-/- mouse mutants and identified anterior pituitary hypoplasia and partially penetrant infundibular defects. Overlapping infundibular abnormalities and distinct anterior pituitary morphogenesis defects were observed in Fat4-/- and Dchs1-/- mouse mutants but all animal models displayed normal commitment to the anterior pituitary cell type. Together our data implicate FAT/DCHS protocadherins in normal hypothalamic-pituitary development and identify FAT2 and DCHS2 as candidates underlying pituitary gland developmental defects such as ectopic pituitary gland and/or pituitary stalk interruption.

Localization of YAP activity in developing skeletal rudiments is responsive to mechanical stimulation

BACKGROUND

Normal skeletal development, in particular ossification, joint formation and shape features of condyles, depends on appropriate mechanical input from embryonic movement but it is unknown how such physical stimuli are transduced to alter gene regulation. Hippo/Yes-Associated Protein (YAP) signalling has been shown to respond to the physical environment of the cell and here we specifically investigate the YAP effector of the pathway as a potential mechanoresponsive mediator in the developing limb skeleton.

RESULTS

We show spatial localization of YAP protein and of pathway target gene expression within developing skeletal rudiments where predicted biophysical stimuli patterns and shape are affected in immobilization models, coincident with the period of sensitivity to movement, but not coincident with the expression of the Hippo receptor Fat4. Furthermore, we show that under reduced mechanical stimulation, in immobile, muscle-less mouse embryos, this spatial localization is lost. In culture blocking YAP reduces chondrogenesis but the effect differs depending on the timing and/or level of YAP reduction.

CONCLUSIONS

These findings implicate YAP signalling, independent of Fat4, in the transduction of mechanical signals during key stages of skeletal patterning in the developing limb, in particular endochondral ossification and shape emergence, as well as patterning of tissues at the developing synovial joint.