Analysis of chondrogenesis using micromass cultures of limb mesenchyme

Craniofacial studies in chicken embryos confirm the pathogenicity of human FZD2 variants associated with Robinow syndrome

Robinow syndrome is a rare disease caused by variants of seven WNT pathway genes. Craniofacial features include widening of the nasal bridge and jaw hypoplasia. We used the chicken embryo to test whether two missense human FZD2 variants (1301G>T, p.Gly434Val; 425C>T, p.Pro142Lys) were sufficient to change frontonasal mass development. In vivo, the overexpression of retroviruses with wild-type or variant human FZD2 inhibited upper beak ossification. In primary cultures, wild-type and variant human FZD2 significantly inhibited chondrogenesis, with the 425C>T variant significantly decreasing activity of a SOX9 luciferase reporter compared to that for the wild type or 1301G>T. Both variants also increased nuclear shuttling of β-catenin (CTNNB1) and increased the expression of TWIST1, which are inhibitory to chondrogenesis. In canonical WNT luciferase assays using frontonasal mass cells, the variants had dominant-negative effects on wild-type FZD2. In non-canonical assays, the 425C>T variant failed to activate the reporter above control levels and was unresponsive to exogenous WNT5A. This is the first single amino acid change to selectively alter ligand binding in a FZD receptor. Therefore, FZD2 missense variants are pathogenic and could lead to the altered craniofacial morphogenesis seen in Robinow syndrome.

Cellular taxonomy of Hic1+ mesenchymal progenitor derivatives in the limb: from embryo to adult

Tissue development and regeneration rely on the cooperation of multiple mesenchymal progenitor (MP) subpopulations. We recently identified Hic1 as a marker of quiescent MPs in multiple adult tissues. Here, we describe the embryonic origin of appendicular Hic1⁺ MPs and demonstrate that they arise in the hypaxial somite, and migrate into the developing limb at embryonic day 11.5, well after limb bud initiation. Time-resolved single-cell-omics analyses coupled with lineage tracing reveal that Hic1⁺ cells generate a unique MP hierarchy, that includes both recently identified adult universal fibroblast populations (Dpt⁺, Pi16⁺ and Dpt⁺ Col15a1⁺) and more specialised mesenchymal derivatives such as, peri and endoneurial cells, pericytes, bone marrow stromal cells, myotenocytes, tenocytes, fascia-resident fibroblasts, with limited contributions to chondrocytes and osteocytes within the skeletal elements. MPs endure within these compartments, continue to express Hic1 and represent a critical reservoir to support post-natal growth and regeneration.

The collagen receptor, discoidin domain receptor 2, functions in Gli1-positive skeletal progenitors and chondrocytes to control bone development

Discoidin Domain Receptor 2 (DDR2) is a collagen-activated receptor kinase that, together with integrins, is required for cells to respond to the extracellular matrix. Ddr2 loss-of-function mutations in humans and mice cause severe defects in skeletal growth and development. However, the cellular functions of Ddr2 in bone are not understood. Expression and lineage analysis showed selective expression of Ddr2 at early stages of bone formation in the resting zone and proliferating chondrocytes and periosteum. Consistent with these findings, Ddr2⁺ cells could differentiate into hypertrophic chondrocytes, osteoblasts, and osteocytes and showed a high degree of colocalization with the skeletal progenitor marker, Gli1. A conditional deletion approach showed a requirement for Ddr2 in Gli1-positive skeletal progenitors and chondrocytes but not mature osteoblasts. Furthermore, Ddr2 knockout in limb bud chondroprogenitors or purified marrow-derived skeletal progenitors inhibited chondrogenic or osteogenic differentiation, respectively. This work establishes a cell-autonomous function for Ddr2 in skeletal progenitors and cartilage and emphasizes the critical role of this collagen receptor in bone development.

Caspase-9 inhibition decreases expression of Mmp9 during chondrogenesis

Besides cell death, caspase-9 participates in non-apoptotic events, including cell differentiation. To evaluate a possible impact on the expression of chondrogenic/osteogenic factors, a caspase-9 inhibitor was tested in vitro. For this purpose, mouse forelimb-derived micromass cultures, the most common chondrogenic in vitro model, were used. The following analyses were performed based on polymerase chain reaction (PCR) arrays and real-time PCR. The expression of several chondrogenesis-related genes was shown to be altered, some of which may impact chondrogenic differentiation (Bmp4, Bmp7, Sp7, Gli1), mineral deposition (Alp, Itgam) or the remodelling of the extracellular matrix (Col1a2, Mmp9) related to endochondral ossification. From the cluster of genes with altered expression, Mmp9 showed the most significant decrease in expression, of more than 50-fold. Additionally, we determined the possible impact of caspase-9 downregulation on the expression of other Mmp genes. A mild increase in Mmp14 was observed, but there was no change in the expression of other studied Mmp genes (-2, -3, -8, -10, -12, -13). Interestingly, inhibition of Mmp9 in micromasses led to decreased expression of some chondrogenic markers related to caspase-9. These samples also showed a decreased expression of caspase-9 itself, suggesting a bidirectional regulation of these two enzymes. These results indicate a specific impact of caspase-9 inhibition on the expression of Mmp9. The localisation of these two enzymes overlaps in resting, proliferative and pre-hypertrophic chondrocytes during in vivo development, which supports their multiple functions, either apoptotic or non-apoptotic. Notably, a coincidental expression pattern was identified in Pik3cg, a possible candidate for Mmp9 regulation.

Caspase Inhibition Affects the Expression of Autophagy-Related Molecules in Chondrocytes

Objective. Caspases, cysteine proteases traditionally associated with apoptosis and inflammation, have recently been identified as important regulators of autophagy and reported within the growth plate, a cartilaginous part of the developing bone. The aim of this research was to identify novel autophagy-related molecules affected by inhibition of pro-apoptotic caspases in chondrocytes. Design. Chondrocyte micromasses derived from mouse limb buds were treated with pharmacological inhibitors of caspases. Autophagy-related gene expression was examined and possible novel molecules were confirmed by real-time polymerase chain reaction and immunocytofluorescence. Individual caspases inhibitors were used to identify the effect of specific caspases. Results. Chondrogenesis accompanied by caspase activation and autophagy progression was confirmed in micromass cultures. Expression of several autophagy-associated genes was significantly altered in the caspases inhibitors treated groups with the most prominent decrease for Pik3cg and increase of Tnfsf10. The results showed the specific pro-apoptotic caspases that play a role in these effects. Importantly, use of caspase inhibitors mimicked changes triggered by an autophagy stimulator, rapamycin, linking loss of caspase activity to an increase in autophagy. Conclusion. Caspase inhibition significantly affects regulation of autophagy-related genes in chondrocytes cultures. Detected markers are of importance in diagnostics and thus the data presented here open new perspectives in the field of cartilage development and degradation.

Ablation of Proliferating Osteoblast Lineage Cells After Fracture Leads to Atrophic Nonunion in a Mouse Model

Nonunion is defined as the permanent failure of a fractured bone to heal, often necessitating surgical intervention. Atrophic nonunions are a subtype that are particularly difficult to treat. Animal models of atrophic nonunion are available; however, these require surgical or radiation-induced trauma to disrupt periosteal healing. These methods are invasive and not representative of many clinical nonunions where osseous regeneration has been arrested by a "failure of biology". We hypothesized that arresting osteoblast cell proliferation after fracture would lead to atrophic nonunion in mice. Using mice that express a thymidine kinase (tk) "suicide gene" driven by the 3.6Col1a1 promoter (Col1-tk), proliferating osteoblast lineage cells can be ablated upon exposure to the nucleoside analog ganciclovir (GCV). Wild-type (WT; control) and Col1-tk littermates were subjected to a full femur fracture and intramedullary fixation at 12 weeks age. We confirmed abundant tk+ cells in fracture callus of Col-tk mice dosed with water or GCV, specifically many osteoblasts, osteocytes, and chondrocytes at the cartilage-bone interface. Histologically, we observed altered callus composition in Col1-tk mice at 2 and 3 weeks postfracture, with significantly less bone and more fibrous tissue. Col1-tk mice, monitored for 12 weeks with in vivo radiographs and micro-computed tomography (μCT) scans, had delayed bone bridging and reduced callus size. After euthanasia, ex vivo μCT and histology showed failed union with residual bone fragments and fibrous tissue in Col1-tk mice. Biomechanical testing showed a failure to recover torsional strength in Col1-tk mice, in contrast to WT. Our data indicates that suppression of proliferating osteoblast-lineage cells for at least 2 weeks after fracture blunts the formation and remodeling of a mineralized callus leading to a functional nonunion. We propose this as a new murine model of atrophic nonunion. © 2021 American Society for Bone and Mineral Research (ASBMR).

Epigenetic Mechanisms Mediating Cell State Transitions in Chondrocytes

Epigenetic modifications play critical roles in regulating cell lineage differentiation, but the epigenetic mechanisms guiding specific differentiation steps within a cell lineage have rarely been investigated. To decipher such mechanisms, we used the defined transition from proliferating (PC) into hypertrophic chondrocytes (HC) during endochondral ossification as a model. We established a map of activating and repressive histone modifications for each cell type. ChromHMM state transition analysis and Pareto-based integration of differential levels of mRNA and epigenetic marks revealed that differentiation-associated gene repression is initiated by the addition of H3K27me3 to promoters still carrying substantial levels of activating marks. Moreover, the integrative analysis identified genes specifically expressed in cells undergoing the transition into hypertrophy. Investigation of enhancer profiles detected surprising differences in enhancer number, location, and transcription factor binding sites between the two closely related cell types. Furthermore, cell type-specific upregulation of gene expression was associated with increased numbers of H3K27ac peaks. Pathway analysis identified PC-specific enhancers associated with chondrogenic genes, whereas HC-specific enhancers mainly control metabolic pathways linking epigenetic signature to biological functions. Since HC-specific enhancers show a higher conservation in postnatal tissues, the switch to metabolic pathways seems to be a hallmark of differentiated tissues. Surprisingly, the analysis of H3K27ac levels at super-enhancers revealed a rapid adaption of H3K27ac occupancy to changes in gene expression, supporting the importance of enhancer modulation for acute alterations in gene expression. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Murine Limb Bud Organ Cultures for Studying Musculoskeletal Development

The biological signals that coordinate the three-dimensional outgrowth and patterning of the vertebrate limb bud have been well delineated. These include a number of vital embryonic signaling pathways, including the fibroblast growth factor, WNT, transforming growth factor, and hedgehog. Collectively these signals converge on multiple progenitor populations to drive the formation of a variety of tissues that make up the limb musculoskeletal system, such as muscle, tendon, cartilage, stroma, and bone. The basic mechanisms regulating the commitment and differentiation of diverse limb progenitor populations has been successfully modeled in vitro using high density primary limb mesenchymal or micromass cultures. However, this approach is limited in its ability to more faithfully recapitulate the assembly of progenitors into organized tissues that span the entire musculoskeletal system. Other biological systems have benefitted from the development and availability of three-dimensional organoid cultures which have transformed our understanding of tissue development, homeostasis and regeneration. Such a system does not exist that effectively models the complexity of limb development. However, limb bud organ cultures while still necessitating the use of collected embryonic tissue have proved to be a powerful model system to elucidate the molecular underpinning of musculoskeletal development. In this methods article, the derivation and use of limb bud organ cultures from murine limb buds will be described, along with strategies to manipulate signaling pathways, examine gene expression and for longitudinal lineage tracking.

Atoh8 acts as a regulator of chondrocyte proliferation and differentiation in endochondral bones

Atonal homolog 8 (Atoh8) is a transcription factor of the basic helix-loop-helix (bHLH) protein family, which is expressed in the cartilaginous elements of endochondral bones. To analyze its function during chondrogenesis we deleted Atoh8 in mice using a chondrocyte- (Atoh8flox/flox;Col2a1-Cre) and a germline- (Atoh8flox/flox;Prx1-Crefemale) specific Cre allele. In both strains, Atoh8 deletion leads to a reduced skeletal size of the axial and appendicular bones, but the stages of phenotypic manifestations differ. While we observed obviously shortened bones in Atoh8flox/flox;Col2a1-Cre mice only postnatally, the bones of Atoh8flox/flox;Prx1-Crefemale mice are characterized by a reduced bone length already at prenatal stages. Detailed histological and molecular investigations revealed reduced zones of proliferating and hypertrophic chondrocytes. In addition, Atoh8 deletion identified Atoh8 as a positive regulator of chondrocyte proliferation. As increased Atoh8 expression is found in the region of prehypertrophic chondrocytes where the expression of Ihh, a main regulator of chondrocyte proliferation and differentiation, is induced, we investigated a potential interaction of Atoh8 function and Ihh signaling. By activating Ihh signaling with Purmorphamine we demonstrate that Atoh8 regulates chondrocyte proliferation in parallel or downstream of Ihh signaling while it acts on the onset of hypertrophy upstream of Ihh likely by modulating Ihh expression levels.

17-DMAG regulates p21 expression to induce chondrogenesis in vitro and in vivo

Cartilage degeneration after injury affects a significant percentage of the population, including those that will go on to develop osteoarthritis (OA). Like humans, most mammals, including mice, are incapable of regenerating injured cartilage. Interestingly, it has previously been shown that p21 (Cdkn1a) knockout (p21^-/-) mice demonstrate auricular (ear) cartilage regeneration. However, the loss of p21 expression is highly correlated with the development of numerous types of cancer and autoimmune diseases, limiting the therapeutic translation of these findings. Therefore, in this study, we employed a screening approach to identify an inhibitor (17-DMAG) that negatively regulates the expression of p21. We also validated that this compound can induce chondrogenesis in vitro (in adult mesenchymal stem cells) and in vivo (auricular cartilage injury model). Furthermore, our results suggest that 17-DMAG can induce the proliferation of terminally differentiated chondrocytes (in vitro and in vivo), while maintaining their chondrogenic phenotype. This study provides new insights into the regulation of chondrogenesis that might ultimately lead to new therapies for cartilage injury and/or OA.

SOX9 is dispensable for the initiation of epigenetic remodeling and the activation of marker genes at the onset of chondrogenesis

SOX9 controls cell lineage fate and differentiation in major biological processes. It is known as a potent transcriptional activator of differentiation-specific genes, but its earliest targets and its contribution to priming chromatin for gene activation remain unknown. Here, we address this knowledge gap using chondrogenesis as a model system. By profiling the whole transcriptome and the whole epigenome of wild-type and Sox9-deficient mouse embryo limb buds, we uncover multiple structural and regulatory genes, including Fam101a, Myh14, Sema3c and Sema3d, as specific markers of precartilaginous condensation, and we provide evidence of their direct transactivation by SOX9. Intriguingly, we find that SOX9 helps remove epigenetic signatures of transcriptional repression and establish active-promoter and active-enhancer marks at precartilage- and cartilage-specific loci, but is not absolutely required to initiate these changes and activate transcription. Altogether, these findings widen our current knowledge of SOX9 targets in early chondrogenesis and call for new studies to identify the pioneer and transactivating factors that act upstream of or along with SOX9 to prompt chromatin remodeling and specific gene activation at the onset of chondrogenesis and other processes.

No evidence for a direct role of HLA-B27 in pathological bone formation in axial SpA

Objective

The strong genetic association between HLA-B27 and ankylosing spondylitis has been known for over 40 years. HLA-B27 positivity is possibly associated with severity of ankylosis. We studied the in vitro and in vivo impact of HLA-B27 in models of chondrogenesis and osteogenesis.

Methods

Different in vitro differentiation systems were used to mimic endochondral and direct bone formation. ATDC5 cells and primary human periosteum-derived cells (hPDCs) were transduced with lentiviral vectors expressing HLA-B27 or HLA-B7. These cells and limb bud cells (from HLA-B27 transgenic and wild-type (WT) mice) were cultured in micromasses. To study direct osteogenesis in hPDCs, cells were cultured as monolayers and stimulated with osteogenic media. Chondrogenesis (COL2, ACAN, COL10) and osteogenesis (OSC, ALP, RUNX2) marker expression was studied by quantitative RT-PCR. Colorimetric tests were performed to measure proteoglycans, mineralization and collagens. Collagen antibody-induced arthritis (CAIA) was induced in HLA-B27 transgenic and WT mice. Clinical scoring and µCTs were performed. Statistical analyses were performed by two-way ANOVA.

Results

There was no difference in chondrogenesis markers or in colorimetric tests between HLA-B27⁺ and HLA-B7⁺ micromasses. Expression of osteogenesis markers and Alizarin red staining was comparable in the HLA-B27⁺ and the HLA-B7⁺ hPDCs in monolayers. HLA-B27 transgenic mice showed more severe arthritis compared with WT mice in the CAIA model. µCT analysis showed no increased bone formation in HLA-B27 transgenic mice.

Conclusion

HLA-B27 seems to enhance joint inflammation in the CAIA model. We could not document a direct effect of HLA-B27 on chondrogenesis or osteogenesis.

A potential translational approach for bone tissue engineering through endochondral ossification

Bone defect repair is a significant clinical challenge in orthopedic surgery. Despite tremendous efforts, the majority of the current bone tissue engineering strategies depend on bone formation via intramembranous ossification (IO), which often results in poor vascularization and limited-area bone regeneration. Recently, there has been increasing interest in exploring bone regeneration through a cartilage-mediated process similar to endochondral ossification (EO). This method is advantageous because long bones are originally developed through EO and moreover, vascularization is an inherent step of this process. Therefore, it may be possible to effectively employ the EO method for the repair and regeneration of large and segmental bone defects. Although a number of studies have demonstrated engineered bone formation through EO, there are no approaches aiming for their clinical translation. In this study, we propose a strategy modeled after the U.S. Food and Drug Administration (FDA) approved autologus chondrocyte implantation (ACI) procedure. In its implementation, we concentrated human bone marrow aspirate via a minimally manipulated process and demonstrated the potential of human bone marrow derived cells for in vitro pre-cartilage template formation and bone regeneration in vivo.

The tumor suppressor BTG1 is expressed in the developing digits and regulates skeletogenic differentiation of limb mesodermal progenitors in high density cultures

In the developing limb, differentiation of skeletal progenitors towards distinct connective tissues of the digits is correlated with the establishment of well-defined domains of Btg1 gene expression. Zones of high expression of Btg1 include the earliest digit blastemas, the condensing mesoderm at the tip of the growing digits, the peritendinous mesenchyme, and the chondrocytes around the developing interphalangeal joints. Gain- and loss-of function experiments in micromass cultures of skeletal progenitors reveal a negative influence of Btg1 in cartilage differentiation accompanied by up-regulation of Ccn1, Scleraxis and PTHrP. Previous studies have assigned a role to these factors in the aggregation of progenitors in the digit tips (Ccn1), in the differentiation of tendon blastemas (Scleraxis) and repressing hypertrophic cartilage differentiation (PTHrP). Overexpression of Btg1 up-regulates the expression of retinoic acid and thyroid hormone receptors, but, different from other systems, the influence of BTG1 in connective tissue differentiation appears to be independent of retinoic acid and thyroid hormone signaling.

Two novel disease-causing variants in BMPR1B are associated with brachydactyly type A1

Brachydactyly type A1 is an autosomal dominant disorder primarily characterized by hypoplasia/aplasia of the middle phalanges of digits 2–5. Human and mouse genetic perturbations in the BMP-SMAD signaling pathway have been associated with many brachymesophalangies, including BDA1, as causative mutations in IHH and GDF5 have been previously identified. GDF5 interacts directly as the preferred ligand for the BMP type-1 receptor BMPR1B and is important for both chondrogenesis and digit formation. We report pathogenic variants in BMPR1B that are associated with complex BDA1. A c.975A>C (p.(Lys325Asn)) was identified in the first patient displaying absent middle phalanges and shortened distal phalanges of the toes in addition to the significant shortening of middle phalanges in digits 2, 3 and 5 of the hands. The second patient displayed a combination of brachydactyly and arachnodactyly. The sequencing of BMPR1B in this individual revealed a novel c.447-1G>A at a canonical acceptor splice site of exon 8, which is predicted to create a novel acceptor site, thus leading to a translational reading frameshift. Both mutations are most likely to act in a dominant-negative manner, similar to the effects observed in BMPR1B mutations that cause BDA2. These findings demonstrate that BMPR1B is another gene involved with the pathogenesis of BDA1 and illustrates the continuum of phenotypes between BDA1 and BDA2.