Genesis and morphogenesis of limb synovial joints and articular cartilage

Sox, Fox, and Lmx1b binding sites differentially regulate a Gdf5-Associated regulatory region during elbow development

Introduction: The articulating ends of limb bones have precise morphology and asymmetry that ensures proper joint function. Growth differentiation factor 5 (Gdf5) is a secreted morphogen involved in cartilage and bone development that contributes to the architecture of developing joints. Dysregulation of Gdf5 results in joint dysmorphogenesis often leading to progressive joint degeneration or osteoarthritis (OA). The transcription factors and cis-regulatory modules (CRMs) that regulate Gdf5 expression are not well characterized. We previously identified a Gdf5-associated regulatory region (GARR) that contains predicted binding sites for Lmx1b, Osr2, Fox, and the Sox transcription factors. These transcription factors are recognized factors involved in joint morphogenesis and skeletal development. Methods: We used in situ hybridization to Gdf5, Col2A1, and the transcription factors of interest in developing chicken limbs to determine potential overlap in expression. We further analyzed scRNA-seq data derived from limbs and knees in published mouse and chicken datasets, identifying cells with coexpression of Gdf5 and the transcription factors of interest. We also performed site-directed mutatgenesis of the predicted transcription factor binding sites in a GARR-reporter construct and determined any change in activity using targeted regional electroporation (TREP) in micromass and embryonic chicken wing bioassays. Results: Gdf5 expression overlapped the expression of these transcription factors during joint development both by in situ hybridization (ISH) and scRNA-seq analyses. Within the GARR CRM, mutation of two binding sites common to Fox and Sox transcripstion factors reduced enhancer activity to background levels in micromass cultures and in ovo embryonic chicken wing bioassays, whereas mutation of two Sox-only binding sites caused a significant increase in activity. These results indicate that the Fox/Sox binding sites are required for activity, while the Sox-only sites are involved in repression of activity. Mutation of Lmx1b binding sites in GARR caused an overall reduction in enhancer activity in vitro and a dorsal reduction in ovo. Despite a recognized role for Osr2 in joint development, disruption of the predicted Osr2 site did not alter GARR activity. Conclusion: Taken together, our data indicates that GARR integrates positive, repressive, and asymmetrical inputs to fine-tune the expression of Gdf5 during elbow joint development.

Modeling human skeletal development using human pluripotent stem cells

Chondrocytes and osteoblasts differentiated from induced pluripotent stem cells (iPSCs) will provide insights into skeletal development and genetic skeletal disorders and will generate cells for regenerative medicine applications. Here, we describe a method that directs iPSC-derived sclerotome to chondroprogenitors in 3D pellet culture then to articular chondrocytes or, alternatively, along the growth plate cartilage pathway to become hypertrophic chondrocytes that can transition to osteoblasts. Osteogenic organoids deposit and mineralize a collagen I extracellular matrix (ECM), mirroring in vivo endochondral bone formation. We have identified gene expression signatures at key developmental stages including chondrocyte maturation, hypertrophy, and transition to osteoblasts and show that this system can be used to model genetic cartilage and bone disorders.

Single-cell RNA sequencing in orthopedic research

Although previous RNA sequencing methods have been widely used in orthopedic research and have provided ideas for therapeutic strategies, the specific mechanisms of some orthopedic disorders, including osteoarthritis, lumbar disc herniation, rheumatoid arthritis, fractures, tendon injuries, spinal cord injury, heterotopic ossification, and osteosarcoma, require further elucidation. The emergence of the single-cell RNA sequencing (scRNA-seq) technique has introduced a new era of research on these topics, as this method provides information regarding cellular heterogeneity, new cell subtypes, functions of novel subclusters, potential molecular mechanisms, cell-fate transitions, and cell‒cell interactions that are involved in the development of orthopedic diseases. Here, we summarize the cell subpopulations, genes, and underlying mechanisms involved in the development of orthopedic diseases identified by scRNA-seq, improving our understanding of the pathology of these diseases and providing new insights into therapeutic approaches.

Building a Co-ordinated Musculoskeletal System: The Plasticity of the Developing Skeleton in Response to Muscle Contractions

The skeletal musculature and the cartilage, bone and other connective tissues of the skeleton are intimately co-ordinated. The shape, size and structure of each bone in the body is sculpted through dynamic physical stimuli generated by muscle contraction, from early development, with onset of the first embryo movements, and through repair and remodelling in later life. The importance of muscle movement during development is shown by congenital abnormalities where infants that experience reduced movement in the uterus present a sequence of skeletal issues including temporary brittle bones and joint dysplasia. A variety of animal models, utilising different immobilisation scenarios, have demonstrated the precise timing and events that are dependent on mechanical stimulation from movement. This chapter lays out the evidence for skeletal system dependence on muscle movement, gleaned largely from mouse and chick immobilised embryos, showing the many aspects of skeletal development affected. Effects are seen in joint development, ossification, the size and shape of skeletal rudiments and tendons, including compromised mechanical function. The enormous plasticity of the skeletal system in response to muscle contraction is a key factor in building a responsive, functional system. Insights from this work have implications for our understanding of morphological evolution, particularly the challenging concept of emergence of new structures. It is also providing insight for the potential of physical therapy for infants suffering the effects of reduced uterine movement and is enhancing our understanding of the cellular and molecular mechanisms involved in skeletal tissue differentiation, with potential for informing regenerative therapies.

miR-200b-3p/ERG/PTHrP axis mediates the inhibitory effect of ethanol on the differentiation of fetal cartilage into articular cartilage

PURPOSE

This study aims to further explore cartilage development in prenatal ethanol exposure (PEE) offspring at different times to explore the specific time points and mechanism of ethanol-induced fetal cartilage dysplasia.

METHODS

On gestational day (GD)14, GD17, and GD20, PEE fetal cartilage was evaluated by morphological analysis. RT-qPCR, immunohistochemistry, and immunofluorescence were used to detect the expression of cartilage marker genes and their regulatory factors. Bone marrow mesenchymal stem cells (BMSCs) were used to explore the effect of ethanol on the differentiation of chondrocytes. Additionally, we used inhibitors, overexpression plasmids and a luciferase reporter assay on GD17 chondrocytes to verify the mechanism.

RESULTS

PEE significantly reduced cartilage matrix content and the expression of marker genes on GD17 and GD20 but had no effect on GD14. The inhibition of chondrogenic differentiation by PEE mainly occurred on GD14-17. Furthermore, the expression of miR-200b-3p was increased, while that of ERG and PTHrP was markedly reduced in PEE fetal cartilage. In vitro, ethanol (30-120 mM) inhibited the differentiation of BMSCs into chondrocytes in a concentration-dependent manner, accompanied by strong expression of miR-200b-3p and low expression of ERG and PTHrP. Moreover, PTHLH and ERG overexpressed, as well as a miR-200b-3p inhibitor reversed the inhibitory effect of ethanol on the differentiation of fetal chondrocytes. Furthermore, miR-200b-3p could target and negatively regulate ERG.

CONCLUSIONS

PEE can significantly inhibit the development of articular cartilage, especially during articular cartilage formation. The mechanism is related to the decreased differentiation of fetal cartilage into articular cartilage mediated by the miR-200b-3p/ERG/PTHrP axis.

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.

Identification of candidate enhancers controlling the transcriptome during the formation of interphalangeal joints

The formation of the synovial joint begins with the visible emergence of a stripe of densely packed mesenchymal cells located between distal ends of the developing skeletal anlagen called the interzone. Recently the transcriptome of the early synovial joint was reported. Knowledge about enhancers would complement these data and lead to a better understanding of the control of gene transcription at the onset of joint development. Using ChIP-sequencing we have mapped the H3-signatures H3K27ac and H3K4me1 to locate regulatory elements specific for the interzone and adjacent phalange, respectively. This one-stage atlas of candidate enhancers (CEs) was used to map the association between these respective joint tissue specific CEs and biological processes. Subsequently, integrative analysis of transcriptomic data and CEs identified new putative regulatory elements of genes expressed in interzone (e.g., GDF5, BMP2 and DACT2) and phalange (e.g., MATN1, HAPLN1 and SNAI1). We also linked such CEs to genes known as crucial in synovial joint hypermobility and osteoarthritis, as well as phalange malformations. These analyses show that the CE atlas can serve as resource for identifying, and as starting point for experimentally validating, putative disease-causing genomic regulatory regions in patients with synovial joint dysfunctions and/or phalange disorders, and enhancer-controlled synovial joint and phalange formation.

In vivo potency assay for the screening of bioactive molecules on cartilage formation

Cartilage regeneration is a priority in medicine for the treatment of osteoarthritis and isolated cartilage defects. Several molecules with potential for cartilage regeneration are under investigation. Unfortunately, in vitro chondrogenesis assays do not always predict the stability of the newly formed cartilage in vivo. Therefore, there is a need for a stringent, quantifiable assay to assess in vivo the capacity of molecules to promote the stable formation of cartilage that is resistant to calcification and endochondral bone formation. We developed an ectopic cartilage formation assay (ECFA) that enables one to assess the capacity of bioactive molecules to support cartilage formation in vivo using cartilage organoids. The ECFA predicted good clinical outcomes when used as a quality control for efficacy of chondrocyte preparations before implantation in patients with cartilage defects. In this assay, articular chondrocytes from human donors or animals are injected either intramuscularly or subcutaneously in nude mice. As early as 2 weeks later, cartilage organoids can be retrieved. The size of the implants and their degree of differentiation can be assessed by histomorphometry, immunostainings of molecular markers and real-time PCR. Mineralization can be assessed by micro-computed tomography or by staining. The effects of molecules on cartilage formation can be tested following the systemic administration of the molecule in mice previously injected with chondrocytes, or after co-injection of chondrocytes with cell lines overexpressing and secreting the protein of interest. Here we describe the ECFA procedure, including steps for harvesting human and bovine articular cartilage, isolating primary chondrocytes, preparing overexpression cell lines, injecting the cells intramuscularly and retrieving the implants. This assay can be performed by technicians and researchers with appropriate animal training within 3 weeks.

Dcx expression defines a subpopulation of Gdf5 + cells with chondrogenic potentials in E12.5 mouse embryonic limbs

Growth differentiation factor 5 (Gdf5) and doublecortin (Dcx) genes are both expressed in joint interzone cells during synovial joint development. In this study, we re-analyzed the single cell RNA-sequencing data (Gene Expression Omnibus GSE151985) generated from Gdf5 ⁺ cells of mouse knee joints at embryonic stages of E12.5, E13.5, E14.5, and E15.5, with a new focus on Dcx. We found that Dcx expression was enriched in clusters of Gdf5 ⁺ cells, with high expression levels of pro-chondrogenic genes including sex determining region Y-box transcription factor 5 (Sox5), Sox6, Sox9, Gdf5, versican, matrilin 4, collagen type II α 1 chain (Col2a1), Col9a1, Col9a2, and Col9a3 at E12.5. Dcx ⁺ and Dcx ^- cells had differential gene expression profiles. The up-regulated genes in Dcx ⁺ vs. Dcx ^- cells at E12.5 and E13.5 were enriched in chondrocyte differentiation and cartilage development, whereas those genes up-regulated at E14.5 and E15.5 were enriched in RNA splicing, protein stability, cell proliferation, and cell growth. Gene expression profiles in Dcx ⁺ cells showed rapid daily changes from E12.5 to E15.5, with limited number of genes shared across the time period. Expression of Gdf5, Sox5, Sox6, melanoma inhibitory activity, noggin, odd-skipped related transcription factor 2, matrilin 4, and versican was positively correlated with Dcx expression. Our results demonstrate that Dcx expression defines a subpopulation of Gdf5 ⁺ cells with chondrogenic potentials in E12.5 mouse embryonic limbs.

A Direct Comparison of Node and Element-Based Finite Element Modeling Approaches to Study Tissue Growth

Finite element analysis is a useful tool to model growth of biological tissues and predict how growth can be impacted by stimuli. Previous work has simulated growth using node-based or element-based approaches, and this implementation choice may influence predicted growth, irrespective of the applied growth model. This study directly compared node-based and element-based approaches to understand the isolated impact of implementation method on growth predictions by simulating growth of a bone rudiment geometry, and determined what conditions produce similar results between the approaches. We used a previously reported node-based approach implemented via thermal expansion and an element-based approach implemented via osmotic swelling, and we derived a mathematical relationship to relate the growth resulting from these approaches. We found that material properties (modulus) affected growth in the element-based approach, with growth completely restricted for high modulus values relative to the growth stimulus, and no restriction for low modulus values. The node-based approach was unaffected by modulus. Node- and element-based approaches matched marginally better when the conversion coefficient to relate the approaches was optimized based on the results of initial simulations, rather than using the theoretically predicted conversion coefficient (median difference in node position 0.042 cm versus 0.052 cm, respectively). In summary, we illustrate here the importance of the choice of implementation approach for modeling growth, provide a framework for converting models between implementation approaches, and highlight important considerations for comparing results in prior work and developing new models of tissue growth.

Effect of Skeletal Paracrine Signals on the Proliferation of Interzone Cells

OBJECTIVE

Articular cartilage in mammals has limited intrinsic capacity to repair structural defects, a fact that contributes to the chronic and progressive nature of osteoarthritis. In contrast, Mexican axolotl salamanders have demonstrated the remarkable ability to spontaneously and completely repair large joint cartilage lesions, a healing process that involves interzone cells in the intraarticular space. Furthermore, interzone tissue transplanted into skeletal defects in the axolotl salamander demonstrates a multi-differentiation potential. Cellular and molecular mechanisms of this repair process remain unclear. The objective of this study was to examine whether paracrine mitogenic signals are an important variable in the interaction between interzone cells and the skeletal microenvironment.

DESIGN

The paracrine regulation of the proliferation of equine interzone cells was evaluated in an in vitro co-culture system. Cell viability and proliferation were measured in equine fetal interzone cells after exposure to conditioned medium from skeletal and nonskeletal primary cell lines. Steady-state expression was determined for genes encoding 37 putative mitogens secreted by cells that generated the conditioned medium.

RESULTS

All experimental groups of conditioned media elicited a mitogenic response in interzone cells. Fetal anlage chondrocytes (P < 0.0001) and dermal fibroblasts (P < 0.0001) conditioned medium showed a significantly higher mitogenic potential compared with interzone cells. Conditioned medium from bone marrow-derived cells elicited a significantly higher proliferative response relative to that from young adult articular chondrocytes (P < 0.0001) or dermal fibroblasts (P < 0.0001). Sixteen genes had expression patterns consistent with the functional proliferation assays.

CONCLUSIONS

The results indicate a mitogenic effect of skeletal paracrine signals on interzone cells.

Kinetics of Gene Expression Changes in Equine Fetal Interzone and Anlagen Cells Over 14 Days of Induced Chondrogenesis

Within developing synovial joints, interzone and anlagen cells progress through divergent chondrogenic pathways to generate stable articular cartilage and transient hypertrophic anlagen cartilage, respectively. Understanding the comparative cell biology between interzone and anlagen cells may provide novel insights into emergent cell-based therapies to support articular cartilage regeneration. The aim of this study was to assess the kinetics of gene expression profiles in these skeletal cell lines after inducing chondrogenesis in culture. Interzone and anlagen cells from seven equine fetuses were isolated and grown in a TGF-β1 chondrogenic inductive medium. Total RNA was isolated at ten time points (0, 1.5, 3, 6, 12, 24, 48, 96, 168, and 336 h), and gene expression for 93 targeted gene loci was measured in a microfluidic RT-qPCR system. Differential transcriptional responses were observed as early as 1.5 h after the initiation of chondrogenesis. Genes with functional annotations that include transcription regulation responded to the chondrogenic stimulation earlier (1.5–96 h) than genes involved in signal transduction (1.5–336 h) and the extracellular matrix biology (3–336 h). Between interzone and anlagen cell cultures, expression levels of 73 out of the 93 targeted genes were not initially different at 0 h, but 47 out of the 73 genes became differentially expressed under the chondrogenic stimulation. While interzone and anlagen cells are both chondrogenic, they display clear differences in response to the same TGF-β1 chondrogenic stimulation. This study provides new molecular insight into a timed sequence of the divergent developmental fates of interzone and anlagen cells in culture over 14 days.

Tissue interactions are indispensable for cavity formation and disc separation in the temporomandibular joint

Purpose: Our previous study found that in the temporomandibular joint (TMJ) of the K14-cre; Ctnnb1^ex3f mouse embryo, the morphogenesis of glenoid fossa was interrupted by the dislocated condyle. This observation suggested that the formation of the glenoid fossa required tissue interactions with condylar mesenchyme. The purpose of this study was to clarify if the interactions between other components are essential for TMJ morphogenesis.Materials and methods: We examined the gross morphology, histology, cell proliferation, and gene expression in the developing TMJ of K14-cre; Ctnnb1^ex3f mice by whole-mount bone and cartilage staining, Azon staining, BrdU labeling, and in situ hybridization, respectively.Results: In K14-cre; Ctnnb1^ex3f mice, the zygomatic arch was misconnected to the mandibular bone by ectopic bone formation, which disrupted the attachment of temporalis to the mandibular bone and joint capsule formation. Although the initiation and differentiation of the condylar cartilage were slightly impacted, the K14-cre; Ctnnb1^ex3f TMJ lacked joint cavities and separated disc, suggesting that the tissue interactions between the joint capsule and the TMJ were indispensable for the cavity formation and disc separation. The ectopic activation of Gli2 in the cells occupying the cavities, and the enhanced PTHrP transcription in the condylar perichondrium of the K14-cre; Ctnnb1^ex3f TMJ suggested that the disrupted interactions between the joint capsule and the TMJ impaired cavity formation and disc separation by altering Hh signaling.Conclusion: Joint capsule formation was essential for cavity formation and disc separation during TMJ development.

Children's school footwear: The impact of fit on foot function, comfort and jump performance in children aged 8 to 12 years

BACKGROUND

There is a common perception that poorly fitting footwear will negatively impact a child's foot, however, there is limited evidence to support this.

AIM

To determine the effect of shoe size on foot motion, perceived footwear comfort and fit during walking, maximal vertical jump height and maximal standing broad jump distance in children aged 8-12 years.

METHODS

Fourteen participants completed 3D walking gait analysis and jumping tasks in three different sizes of school shoes (one size bigger, fitted for size, one size smaller). In-shoe motion of the hindfoot, midfoot and 1st metatarsophalangeal joint (1st MTPJ) were calculated using a multi-segment kinematic foot model. Physical performance measures were calculated via maximal vertical jump and maximal standing broad jump. Perceived footwear comfort and fit (heel, toes and overall) was assessed using a 100 mm visual analog scale (VAS). Differences were compared between shoe sizes using repeated measures ANOVA, post-hoc tests and effect sizes (Cohen's d).

RESULTS

Compared to the fitted footwear, the smaller sizing restricted hindfoot eversion (-2.5°, p = 0.021, d = 0.82), 1st MTPJ dorsiflexion (-3.9°, p = 0.012, d = 0.54), and compared to the bigger footwear, smaller sizing restricted sagittal plane midfoot range-of-motion during walking (-2.5°, p = 0.047, d = 0.59). The fitted footwear was rated as more comfortable overall with the smaller size rated as too tight in both the heel (mean difference 11.5 mm, p = 0.042, d = 0.58) and toes (mean difference 12.1 mm, p = 0.022, d = 0.59), compared to the fitted size. Vertical and standing broad jump distance were not impacted by footwear size (p = 0.218-0.836).

SIGNIFICANCE

Footwear that is too small restricts foot motion during walking in children aged 8-12 years. Jump performance was not affected. Children were able to recognise shoes that were not correctly matched to their foot length, reinforcing that comfort is an important part of the fitting process.

Exploration of changes in spatial chondrocyte organisation in human osteoarthritic cartilage by means of 3D imaging

Using two-dimensional top-down view microscopy, researchers have recently described chondrocytes as being spatially arranged in distinct patterns such as strings, double strings, and small and large clusters. Because of the seeming association of these changes with tissue degeneration, they have been proposed as an image-based biomarker for early osteoarthritis (OA) staging. The aim of our study was to investigate the spatial arrangement of chondrocytes in human articular cartilage in a 3D fashion and to evaluate the 3D changes of these patterns in the context of local tissue destruction. Decalcified femoral condyle resections from the load-bearing area were analysed in 3D for their spatial chondrocyte organisation by means of fluorescence microscopy and synchrotron-radiation micro-computed tomography (SR-µCT). In intact cartilage chondrocyte strings can be found in the superficial, transitional and deep zones. The proposed pattern changes accompanying tissue destruction could be located not just along the surface but also through all layers of cartilage. Each spatial pattern was characterised by a different cellular density (the only exception being between single and double strings with p = 0.062), with cellular density significantly increasing alongside the increase in local tissue degeneration as defined by the chondrocyte patterns. We can thus corroborate that the proposed cellular spatial changes are a three-dimensional function of local tissue degeneration, underlining their relevance as an image-based biomarker for the early diagnosis and description of OA.

Clinical trial registration number: Project number of the ethics committee of the University of Tübingen:171/2014BO2.

Rejuvenated Stem/Progenitor Cells for Cartilage Repair Using the Pluripotent Stem Cell Technology

It is widely accepted that chondral defects in articular cartilage of adult joints are never repaired spontaneously, which is considered to be one of the major causes of age-related degenerative joint disorders, such as osteoarthritis. Since mobilization of subchondral bone (marrow) cells and addition of chondrocytes or mesenchymal stromal cells into full-thickness defects show some degrees of repair, the lack of self-repair activity in adult articular cartilage can be attributed to lack of reparative cells in adult joints. In contrast, during a fetal or embryonic stage, joint articular cartilage has a scar-less repair activity, suggesting that embryonic joints may contain cells responsible for such activity, which can be chondrocytes, chondroprogenitors, or other cell types such as skeletal stem cells. In this respect, the tendency of pluripotent stem cells (PSCs) to give rise to cells of embryonic characteristics will provide opportunity, especially for humans, to obtain cells carrying similar cartilage self-repair activity. Making use of PSC-derived cells for cartilage repair is still in a basic or preclinical research phase. This review will provide brief overviews on how human PSCs have been used for cartilage repair studies.

Embryonic movement stimulates joint formation and development: Implications in arthrogryposis multiplex congenita

Arthrogryposis multiplex congenita (AMC) is a heterogeneous syndrome where multiple joints have reduced range of motion due to contracture formation prior to birth. A common cause of AMC is reduced embryonic movement in utero. This reduction in embryonic movement can perturb molecular mechanisms and signaling pathways involved in the formation of joints during development. The absence of mechanical stimuli can impair joint cavitation, resulting in joint fusion, and ultimately eliminate function. In turn, mechanical stimuli are critical for proper joint formation during development and for mitigating AMC. Studies in experimental animal models have provided a greater understanding on the molecular pathophysiology of congenital contracture formation as a consequence of embryonic immobilization. Elucidation of how the mechanical signaling environment is transduced to initiate a biological response will be necessary to gain a deeper understanding of how mechanical stimuli are intertwined in the molecular regulation of joint development.

Contribution of neural crest-derived stem cells and nasal chondrocytes to articular cartilage regeneration

Due to poor self-regenerative potential of articular cartilage, stem cell-based regeneration becomes a hopeful approach for the treatment of articular cartilage defects. Recent studies indicate that neural crest-derived cells (NCDCs) have the potential for repairing articular cartilage with even greater chondrogenic capacity than mesoderm-derived cells (MDCs): a conventional stem cell source for cartilage regeneration. Given that NCDCs originate from a different germ layer in the early embryo compared with MDCs that give rise to articular cartilage, a mystery remains regarding their capacity for articular cartilage regeneration. In this review, we summarize the similarities and differences between MDCs and NCDCs including articular and nasal chondrocytes in cell origin, anatomy, and chondrogenic differentiation and propose that NCDCs might be promising cell origins for articular cartilage regeneration.

Maturating Articular Cartilage Can Induce Ectopic Joint-Like Structures in Neonatal Mice

Abstract

Osteoarthritis is a huge health burden to our society. Seeking for potential ways to induce regeneration of articular cartilage (AC) that is intrinsically limited, we focused on the interaction between two opposing joints. To evaluate the role of the interaction of opposing regions of AC for joint maturation, we amputated digits at the distal interphalangeal level without injuring the articular surface of the intermediate phalanx (P2) and observed that the zonal organization of AC was defective. We then removed the P2 bone without injuring the articular surface of the proximal phalanx (P1), and the remaining part of the digit was amputated near the distal interphalangeal level. The distribution pattern of type II collagen and proteoglycan 4 (PRG4) suggested that maturation of AC in P1 was delayed. These two experiments suggested that an interaction between the opposing AC in a joint is necessary for maturation of the zonal organization of AC in neonatal digits. To test if an interaction of the joints is sufficient to induce articular cartilage, a proximal fragment of P2 was resected, inverted, and put back into the original location. Newly formed cartilage was induced at the interface region between the AC of the inverted graft and the cut edge of the distal part of P2. Type II collagen and PRG4 were expressed in the ectopic cartilage in a similar manner to normal AC, indicating that neonatal AC can induce ectopic joint-like structures in mice comparable with what has been reported in newts and frogs. These results suggest that the neonatal joint could be a source of inductive signals for regeneration of AC.

Lay Summary

In this study, we experimentally show that neonatal mice appear to have the capacity to regenerate articular cartilage (AC) in digits. It is already known that mice can regenerate a digit tip after amputation, but do not regenerate in response to amputations at more proximal levels. Therefore, it has been thought that mammalian joint structures are non-regenerative. However, we found that normal digit AC can induce AC-like structures in a non-joint region when it is placed next to the cut edge of a bone, suggesting that the normal AC has regenerative capacity in certain situations in neonatal mice.

Future Works

Joint disorders are a huge health problem of our society. The results of this study suggest that neonatal AC could be a potential source of inductive signals for regeneration of AC. The discovery of these inductive signals will aid in developing regenerative therapies of a joint in human.

Quantifying the tolerance of chick hip joint development to temporary paralysis and the potential for recovery

BACKGROUND

Abnormal fetal movements are implicated in joint pathologies such as arthrogryposis and developmental dysplasia of the hip (DDH). Experimentally induced paralysis disrupts joint cavitation and morphogenesis leading to postnatal abnormalities. However, the developmental window(s) most sensitive to immobility-and therefore the best time for intervention-have never been identified. Here, we systematically vary the timing and duration of paralysis during early chick hip joint development. We then test whether external manipulation of immobilized limbs can mitigate the effects of immobility.

RESULTS

Timing of paralysis affected the level of disruption to joints, with paralysis periods between embryonic days 4 and 7 most detrimental. Longer paralysis periods produced greater disruption in terms of failed cavitation and abnormal femoral and acetabular geometry. External manipulation of an immobilized limb led to more normal morphogenesis and cavitation compared to un-manipulated limbs.

CONCLUSIONS

Temporary paralysis is detrimental to joint development, particularly during days 4 to 7. Developmental processes in the very early stages of joint development may be critical to DDH, arthrogryposis, and other joint pathologies. The developing limb has the potential to recover from periods of immobility, and external manipulation provides an innovative avenue for prevention and treatment of developmental joint pathologies.

What do we know about bone morphogenetic proteins and osteochondroprogenitors in inflammatory conditions?

Osteochondroprogenitors are crucial for embryonic bone development and postnatal processes such as bone repair in response to fracture injury, and their dysfunction may contribute to insufficient repair of structural damage in inflammatory arthritides. In the fracture healing, the early inflammatory phase is crucial for normal callus development and new bone formation. This process involves a complex interplay of many molecules and cell types, responsible for recruitment, expansion and differentiation of osteochondroprogenitor populations. In inflammatory arthritides, inflammation induces bone resorption and causes insufficient bone formation, which leads to local and systemic bone loss. While bone loss is a predominant feature in rheumatoid arthritis, inflammation also induces pathologic bone formation at enthesial sites in seronegative spondyloarthropathies. Bone morphogenetic proteins (BMP) are involved in cell proliferation, differentiation and apoptosis, and have fundamental roles in maintenance of postnatal bone homeostasis. They are crucial regulators of the osteochondroprogenitor pool and drive their proliferation, differentiation, and lifespan during bone regeneration. In this review, we summarize the effects of inflammation on osteochondroprogenitor populations during fracture repair and in inflammatory arthritides, with special focus on inflammation-mediated modulation of BMP signaling. We also present data in which we describe a population of murine synovial osteochondroprogenitor cells, which are reduced in arthritis, and characterize their expression of genes involved in regulation of bone homeostasis, emphasizing the up-regulation of BMP pathways in early progenitor subset. Based on the presented data, it may be concluded that during an inflammatory response, innate immune cells induce osteochondroprogenitors by providing signals for their recruitment, by producing BMPs and other osteogenic factors for paracrine effects, and by secreting inflammatory cytokines that may positively regulate osteogenic pathways. On the other hand, inflammatory cells may secrete cytokines that interfere with osteogenic pathways, proapoptotic factors that reduce the pool of osteochondroprogenitor cells, as well as BMP and Wnt antagonists. The net effect is strongly context-dependent and influenced by the local milieu of cells, cytokines, and growth factors. Further elucidation of the interplay between inflammatory signals and BMP-mediated bone formation may provide valuable tools for therapeutic targeting.

A single cell transcriptional atlas of early synovial joint development

Synovial joint development begins with the formation of the interzone, a region of condensed mesenchymal cells at the site of the prospective joint. Recently, lineage-tracing strategies have revealed that Gdf5-lineage cells native to and from outside the interzone contribute to most, if not all, of the major joint components. However, there is limited knowledge of the specific transcriptional and signaling programs that regulate interzone formation and fate diversification of synovial joint constituents. To address this, we have performed single cell RNA-Seq analysis of 7329 synovial joint progenitor cells from the developing murine knee joint from E12.5 to E15.5. By using a combination of computational analytics, in situ hybridization and in vitro characterization of prospectively isolated populations, we have identified the transcriptional profiles of the major developmental paths for joint progenitors. Our freely available single cell transcriptional atlas will serve as a resource for the community to uncover transcriptional programs and cell interactions that regulate synovial joint development.

Human pluripotent stem cell-derived chondroprogenitors for cartilage tissue engineering

The cartilage of joints, such as meniscus and articular cartilage, is normally long lasting (i.e., permanent). However, once damaged, especially in large animals and humans, joint cartilage is not spontaneously repaired. Compensating the lack of repair activity by supplying cartilage-(re)forming cells, such as chondrocytes or mesenchymal stromal cells, or by transplanting a piece of normal cartilage, has been the basis of therapy for biological restoration of damaged joint cartilage. Unfortunately, current biological therapies face problems on a number of fronts. The joint cartilage is generated de novo from a specialized cell type, termed a 'joint progenitor' or 'interzone cell' during embryogenesis. Therefore, embryonic chondroprogenitors that mimic the property of joint progenitors might be the best type of cell for regenerating joint cartilage in the adult. Pluripotent stem cells (PSCs) are expected to differentiate in culture into any somatic cell type through processes that mimic embryogenesis, making human (h)PSCs a promising source of embryonic chondroprogenitors. The major research goals toward the clinical application of PSCs in joint cartilage regeneration are to (1) efficiently generate lineage-specific chondroprogenitors from hPSCs, (2) expand the chondroprogenitors to the number needed for therapy without loss of their chondrogenic activity, and (3) direct the in vivo or in vitro differentiation of the chondroprogenitors to articular or meniscal (i.e., permanent) chondrocytes rather than growth plate (i.e., transient) chondrocytes. This review is aimed at providing the current state of research toward meeting these goals. We also include our recent achievement of successful generation of "permanent-like" cartilage from long-term expandable, hPSC-derived ectomesenchymal chondroprogenitors.

Single cell RNA-sequencing reveals cellular heterogeneity and trajectories of lineage specification during murine embryonic limb development

The coordinated spatial and temporal regulation of gene expression in the murine hindlimb determines the identity of mesenchymal progenitors and the development of diversity of musculoskeletal tissues they form. Hindlimb development has historically been studied with lineage tracing of individual genes selected a priori, or at the bulk tissue level, which does not allow for the determination of single cell transcriptional programs yielding mature cell types and tissues. To identify the cellular trajectories of lineage specification during limb bud development, we used single cell mRNA sequencing (scRNA-seq) to profile the developing murine hindlimb between embryonic days (E)11.5-E18.5. We found cell type heterogeneity at all time points, and the expected cell types that form the mouse hindlimb. In addition, we used RNA fluorescence in situ hybridization (FISH) to examine the spatial locations of cell types and cell trajectories to understand the ancestral continuum of cell maturation. This data provides a resource for the transcriptional program of hindlimb development that will support future studies of musculoskeletal development and generate hypotheses for tissue regeneration.

Effects of 5-aza-2´-deoxycytidine on primary human chondrocytes from osteoarthritic patients

Chondrocytes, comparable to many cells from the connective tissue, dedifferentiate and end up in a similar fibroblastoid cell type, marked by the loss of the specific expression pattern. Here, chondrocytes isolated from osteoarthritic (OA) patients were investigated. The OA chondrocytes used in this work were not affected by the loss of specific gene expression upon cell culture. The mRNA levels of known cartilage markers, such as SOX5, SOX6, SOX9, aggrecan and proteoglycan 4, remained unchanged. Since chondrocytes from OA and healthy tissue show different DNA methylation patterns, the underlying mechanisms of cartilage marker maintenance were investigated with a focus on the epigenetic modification by DNA methylation. The treatment of dedifferentiated chondrocytes with the DNA methyltransferase inhibitor 5-aza-2´-deoxycytidine (5-aza-dC) displayed no considerable impact on the maintenance of marker gene expression observed in the dedifferentiated state, while the chondrogenic differentiation capacity was compromised. On the other hand, the pre-cultivation with 5-aza-dC improved the osteogenesis and adipogenesis of OA chondrocytes. Contradictory to these effects, the DNA methylation levels were not reduced after treatment for four weeks with 1 μM 5-aza-dC. In conclusion, 5-aza-dC affects the differentiation capacity of OA chondrocytes, while the global DNA methylation level remains stable. Furthermore, dedifferentiated chondrocytes isolated from late-stage OA patients represent a reliable cell source for in vitro studies and disease models without the need for additional alterations.

Evolutionary Selection and Constraint on Human Knee Chondrocyte Regulation Impacts Osteoarthritis Risk

During human evolution, the knee adapted to the biomechanical demands of bipedalism by altering chondrocyte developmental programs. This adaptive process was likely not without deleterious consequences to health. Today, osteoarthritis occurs in 250 million people, with risk variants enriched in non-coding sequences near chondrocyte genes, loci that likely became optimized during knee evolution. We explore this relationship by epigenetically profiling joint chondrocytes, revealing ancient selection and recent constraint and drift on knee regulatory elements, which also overlap osteoarthritis variants that contribute to disease heritability by tending to modify constrained functional sequence. We propose a model whereby genetic violations to regulatory constraint, tolerated during knee development, lead to adult pathology. In support, we discover a causal enhancer variant (rs6060369) present in billions of people at a risk locus (GDF5-UQCC1), showing how it impacts mouse knee-shape and osteoarthritis. Overall, our methods link an evolutionarily novel aspect of human anatomy to its pathogenesis.

A dot-stripe Turing model of joint patterning in the tetrapod limb

Iterative joints are a hallmark of the tetrapod limb, and their positioning is a key step during limb development. Although the molecular regulation of joint formation is well studied, it remains unclear what controls the location, number and orientation (i.e. the pattern) of joints within each digit. Here, we propose the dot-stripe mechanism for joint patterning, comprising two coupled Turing systems inspired by published gene expression patterns. Our model can explain normal joint morphology in wild-type limbs, hyperphalangy in cetacean flippers, mutant phenotypes with misoriented joints and suggests a reinterpretation of the polydactylous Ichthyosaur fins as a polygonal joint lattice. By formulating a generic dot-stripe model, describing joint patterns rather than molecular joint markers, we demonstrate that the insights from the model should apply regardless of the biological specifics of the underlying mechanism, thus providing a unifying framework to interrogate joint patterning in the tetrapod limb.

Attenuation of Hypertrophy in Human MSCs via Treatment with a Retinoic Acid Receptor Inverse Agonist

In vitro chondrogenically differentiated mesenchymal stem cells (MSCs) have a tendency to undergo hypertrophy, mirroring the fate of transient “chondrocytes” in the growth plate. As hypertrophy would result in ossification, this fact limits their use in cartilage tissue engineering applications. During limb development, retinoic acid receptor (RAR) signaling exerts an important influence on cell fate of mesenchymal progenitors. While retinoids foster hypertrophy, suppression of RAR signaling seems to be required for chondrogenic differentiation. Therefore, we hypothesized that treatment of chondrogenically differentiating hMSCs with the RAR inverse agonist, BMS204,493 (further named BMS), would attenuate hypertrophy. We induced hypertrophy in chondrogenic precultured MSC pellets by the addition of bone morphogenetic protein 4. Direct activation of the RAR pathway by application of the physiological RAR agonist retinoic acid (RA) further enhanced the hypertrophic phenotype. However, BMS treatment reduced hypertrophic conversion in hMSCs, shown by decreased cell size, number of hypertrophic cells, and collagen type X deposition in histological analyses. BMS effects were dependent on the time point of application and strongest after early treatment during chondrogenic precultivation. The possibility of modifing hypertrophic cartilage via attenuation of RAR signaling by BMS could be helpful in producing stable engineered tissue for cartilage regeneration.

Prospective isolation of chondroprogenitors from human iPSCs based on cell surface markers identified using a CRISPR-Cas9-generated reporter

Background

Articular cartilage shows little or no capacity for intrinsic repair, generating a critical need of regenerative therapies for joint injuries and diseases such as osteoarthritis. Human-induced pluripotent stem cells (hiPSCs) offer a promising cell source for cartilage tissue engineering and in vitro human disease modeling; however, off-target differentiation remains a challenge during hiPSC chondrogenesis. Therefore, the objective of this study was to identify cell surface markers that define the true chondroprogenitor population and use these markers to purify iPSCs as a means of improving the homogeneity and efficiency of hiPSC chondrogenic differentiation.

Methods

We used a CRISPR-Cas9-edited COL2A1-GFP knock-in reporter hiPSC line, coupled with a surface marker screen, to identify a novel chondroprogenitor population. Single-cell RNA sequencing was then used to analyze the distinct clusters within the population. An unpaired t test with Welch’s correction or an unpaired Kolmogorov-Smirnov test was performed with significance reported at a 95% confidence interval.

Results

Chondroprogenitors expressing CD146, CD166, and PDGFRβ, but not CD45, made up an average of 16.8% of the total population. Under chondrogenic culture conditions, these triple-positive chondroprogenitor cells demonstrated decreased heterogeneity as measured by single-cell RNA sequencing with fewer clusters (9 clusters in unsorted vs. 6 in sorted populations) closer together. Additionally, there was more robust and homogenous matrix production (unsorted: 1.5 ng/ng vs. sorted: 19.9 ng/ng sGAG/DNA; p < 0.001) with significantly higher chondrogenic gene expression (i.e., SOX9, COL2A1, ACAN; p < 0.05).

Conclusions

Overall, this study has identified a unique hiPSC-derived subpopulation of chondroprogenitors that are CD146⁺/CD166⁺/PDGFRβ⁺/CD45⁻ and exhibit high chondrogenic potential, providing a purified cell source for cartilage tissue engineering or disease modeling studies.

Evidence of vasculature and chondrocyte to osteoblast transdifferentiation in craniofacial synovial joints: Implications for osteoarthritis diagnosis and therapy

Temporomandibular joint osteoarthritis (TMJ OA) leads to permanent cartilage destruction, jaw dysfunction, and compromises the quality of life. However, the pathological mechanisms governing TMJ OA are poorly understood. Unlike appendicular articular cartilage, the TMJ has two distinct functions as the synovial joint of the craniofacial complex and also as the site for endochondral jaw bone growth. The established dogma of endochondral bone ossification is that hypertrophic chondrocytes undergo apoptosis, while invading vasculature with osteoprogenitors replace cartilage with bone. However, contemporary murine genetic studies support the direct differentiation of chondrocytes into osteoblasts and osteocytes in the TMJ. Here we sought to characterize putative vasculature and cartilage to bone transdifferentiation using healthy and diseased TMJ tissues from miniature pigs and humans. During endochondral ossification, the presence of fully formed vasculature expressing CD31⁺ endothelial cells and α-SMA⁺ vascular smooth muscle cells were detected within all cellular zones in growing miniature pigs. Arterial, endothelial, venous, angiogenic, and mural cell markers were significantly upregulated in miniature pig TMJ tissues relative to donor matched knee meniscus fibrocartilage tissue. Upon surgically creating TMJ OA in miniature pigs, we discovered increased vasculature and putative chondrocyte to osteoblast transformation dually marked by COL2 and BSP or RUNX2 within the vascular bundles. Pathological human TMJ tissues also exhibited increased vasculature, while isolated diseased human TMJ cells exhibited marked increased in vasculature markers relative to control 293T cells. Our study provides evidence to suggest that the TMJ in higher order species are in fact vascularized. There have been no reports of cartilage to bone transdifferentiation or vasculature in human-relevant TMJ OA large animal models or in human TMJ tissues and cells. Therefore, these findings may potentially alter the clinical management of TMJ OA by defining new drugs that target angiogenesis or block the cartilage to bone transformation.

Osteoarthritis from evolutionary and mechanistic perspectives

Developmental osteogenesis and the pathologies associated with tissues that normally are mineralized are active areas of research. All of the basic cell types of skeletal tissue evolved in early aquatic vertebrates. Their characteristics, transcription factors, and signaling pathways have been conserved, even as they adapted to the challenge imposed by gravity in the transition to terrestrial existence. The response to excess mechanical stress (among other factors) can be expressed in the pathologic phenotype described as osteoarthritis (OA). OA is mediated by epigenetic modification of the same conserved developmental gene networks, rather than by gene mutations or new chemical signaling pathways. Thus, these responses have their evolutionary roots in morphogenesis. Epigenetic channeling and heterochrony, orchestrated primarily by microRNAs, maintain the sequence of these responses, while allowing variation in their timing that depends at least partly on the life history of the individual.

The Roles of Indian Hedgehog Signaling in TMJ Formation

The temporomandibular joint (TMJ) is an intricate structure composed of the mandibular condyle, articular disc, and glenoid fossa in the temporal bone. Apical condylar cartilage is classified as a secondary cartilage, is fibrocartilaginous in nature, and is structurally distinct from growth plate and articular cartilage in long bones. Condylar cartilage is organized in distinct cellular layers that include a superficial layer that produces lubricants, a polymorphic/progenitor layer that contains stem/progenitor cells, and underlying layers of flattened and hypertrophic chondrocytes. Uniquely, progenitor cells reside near the articular surface, proliferate, undergo chondrogenesis, and mature into hypertrophic chondrocytes. During the past decades, there has been a growing interest in the molecular mechanisms by which the TMJ develops and acquires its unique structural and functional features. Indian hedgehog (Ihh), which regulates skeletal development including synovial joint formation, also plays pivotal roles in TMJ development and postnatal maintenance. This review provides a description of the many important recent advances in Hedgehog (Hh) signaling in TMJ biology. These include studies that used conventional approaches and those that analyzed the phenotype of tissue-specific mouse mutants lacking Ihh or associated molecules. The recent advances in understanding the molecular mechanism regulating TMJ development are impressive and these findings will have major implications for future translational medicine tools to repair and regenerate TMJ congenital anomalies and acquired diseases, such as degenerative damage in TMJ osteoarthritic conditions.

MicroRNA-377-3p alleviates IL-1β-caused chondrocyte apoptosis and cartilage degradation in osteoarthritis in part by downregulating ITGA6

Increasing evidence indicates that altered expression of microRNAs (miRNAs) is associated with osteoarthritis (OA) progression. In our study, we demonstrated that miR-377-3p is underexpressed in OA-affected cartilage and IL-1β-treated chondrocytes. Overexpression of miR-377-3p enhanced chondrocyte proliferation and restrained apoptosis and signs of cartilage matrix degradation and of an inflammatory response. Furthermore, ITGA6 was identified as a target gene of miR-377-3p. The latter was found to directly bind to the 3' untranslated region (3'UTR) of ITGA6 mRNA and downregulate ITGA6. In addition, ITGA6 expression was high in OA-affected tissues and negatively correlated with miR-77-3p expression. Overexpression of ITGA6 reversed the effects of miR-377-3p on IL-1β-caused chondrocyte apoptosis, cartilage matrix degradation, and the inflammatory response. Moreover, bioinformatic analysis and a luciferase assay indicated that miR-377-3p expression is regulated by long noncoding RNA NEAT1, which binds to miR-377-3p and inactivates it. We showed that NEAT1 was highly expressed in OA-affected cartilage, negatively correlated with miR-377-3p levels, and positively correlated with ITGA6 levels. These findings provide information for the development of future treatments of OA, suggesting that miR-377-3p may be a therapeutic target in OA.

Lgr5 and Col22a1 Mark Progenitor Cells in the Lineage toward Juvenile Articular Chondrocytes

The synovial joint forms from a pool of progenitor cells in the future region of the joint, the interzone. Expression of Gdf5 and Wnt9a has been used to mark the earliest cellular processes in the formation of the interzone and the progenitor cells. However, lineage specification and progression toward the different tissues of the joint are not well understood. Here, by lineage-tracing studies we identify a population of Lgr5+ interzone cells that contribute to the formation of cruciate ligaments, synovial membrane, and articular chondrocytes of the joint. This finding is supported by single-cell transcriptome analyses. We show that Col22a1, a marker of early articular chondrocytes, is co-expressed with Lgr5+ cells prior to cavitation as an important lineage marker specifying the progression toward articular chondrocytes. Lgr5+ cells contribute to the repair of a joint defect with the re-establishment of a Col22a1-expressing superficial layer.

Mechanisms of synovial joint and articular cartilage development

Articular cartilage is formed at the end of epiphyses in the synovial joint cavity and permanently contributes to the smooth movement of synovial joints. Most skeletal elements develop from transient cartilage by a biological process known as endochondral ossification. Accumulating evidence indicates that articular and growth plate cartilage are derived from different cell sources and that different molecules and signaling pathways regulate these two kinds of cartilage. As the first sign of joint development, the interzone emerges at the presumptive joint site within a pre-cartilage tissue. After that, joint cavitation occurs in the center of the interzone, and the cells in the interzone and its surroundings gradually form articular cartilage and the synovial joint. During joint development, the interzone cells continuously migrate out to the epiphyseal cartilage and the surrounding cells influx into the joint region. These complicated phenomena are regulated by various molecules and signaling pathways, including GDF5, Wnt, IHH, PTHrP, BMP, TGF-β, and FGF. Here, we summarize current literature and discuss the molecular mechanisms underlying joint formation and articular development.

Diffusion of fluoroquinolones into equine fetal fluids did not induce fetal lesions after enrofloxacin treatment in early gestation

While recent work demonstrated that enrofloxacin and ciprofloxacin reach the fetoplacental unit without causing obvious lesions in the 9-month-old equine fetus or resulting foal, many practitioners still hesitate to prescribe a fluoroquinolone during pregnancy. Since early gestation is a critical time for fetal skeletal development, if fluoroquinolones are chondrotoxic to the fetus at any point during gestation, this period would be important. The aim of this study was to assess the effects of 2 weeks' exposure to enrofloxacin on the equine fetus between 46 and 60 days gestation. Twelve pregnancies from nine healthy mares were allocated into two groups: untreated (n=7), or treatment (7.5mg/kg enrofloxacin, PO×14days, n=6). Abortion was induced with prostaglandin 24h after the last enrofloxacin dose, or on the equivalent day of gestation for untreated mares. Four of nine mares were rebred for a second cycle and were assigned to the opposite treatment to serve as their own controls. Fetal fluids from treated mares were analysed for enrofloxacin and ciprofloxacin concentrations. Fetal organs (heart, lungs, spleen, kidney, and liver) and limbs were examined histopathologically. Enrofloxacin and ciprofloxacin diffused to the fetal fluids during early gestation and did not result in detectable abnormalities in the fetus after 14 days of treatment. While current research does not determine long-term foal outcomes, enrofloxacin may be useful for select bacterial infections in pregnant mares.

TGFβ/BMP Signaling Pathway in Cartilage Homeostasis

Cartilage homeostasis is governed by articular chondrocytes via their ability to modulate extracellular matrix production and degradation. In turn, chondrocyte activity is regulated by growth factors such as those of the transforming growth factor β (TGFβ) family. Members of this family include the TGFβs, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs). Signaling by this protein family uniquely activates SMAD-dependent signaling and transcription but also activates SMAD-independent signaling via MAPKs such as ERK and TAK1. This review will address the pivotal role of the TGFβ family in cartilage biology by listing several TGFβ family members and describing their signaling and importance for cartilage maintenance. In addition, it is discussed how (pathological) processes such as aging, mechanical stress, and inflammation contribute to altered TGFβ family signaling, leading to disturbed cartilage metabolism and disease.

A standardized autopsy protocol for arthrogryposis (multiple congenital contractures)

Arthrogryposis multiplex congenita (AMC) describes disorders with multiple joint contractures that arise from neurological, neuromuscular, or mechanical origin. Although impaired fetal movement is the typical clinical presentation, the etiology underlying this phenotype for a number of conditions remains unknown. In an effort to better characterize and define the etiologies underlying these disorders, we recommend a standardized autopsy protocol that will allow for appropriate diagnosis and a methodical approach for examination that will facilitate subsequent study by investigators across disciplines. To further support investigation, we have also established an AMC autopsy registry to bank tissue obtained at autopsy for subsequent study.

Re-examining osteoarthritis therapy from a developmental biologist's perspective

Osteoarthritis is the most prevalent musculoskeletal disorder and one for which there is no disease modifying therapy available at present. Our current understanding of the disease mechanism of osteoarthritis is limited owing to a lacuna of knowledge about the development and maintenance of articular cartilage that is affected during osteoarthritis. All current therapeutic strategies aim at countering inflammation which though mitigates pain but does not arrest the progressive degeneration of articular cartilage. During osteoarthritis, articular cartilage expresses markers for transient cartilage differentiation. Moreover, blocking transient cartilage differentiation is sufficient for halting the progression of experimental osteoarthritis. A developmental biology inspired approach that combines restoration of tissue microenvironment, supplementation with engineered cartilage and built in mechanism to prevent transient cartilage differentiation could be an avenue for developing a disease modifying therapy for osteoarthritis.

BMPs, TGFβ, and border security at the interzone

Synovial joints enable movement and protect the integrity of the articular cartilage. Joints form within skeletal condensations destined to undergo chondrogenesis. The suppression of this chondrogenic program in the interzone is the first morphological sign of joint formation. While we have a fairly good understanding of the essential roles of BMP and TGFβ family members in promoting chondrogenic differentiation in developing skeletal elements, we know very little about how BMP activity is suppressed specifically within the interzone, a crucial step in joint development. The function of the BMP ligand Gdf5 has been especially difficult to decipher. On the one hand, Gdf5 is required to promote chondrogenesis of articular elements. On the other hand, Gdf5 is highly expressed in the joint interzone where chondrogenesis must be suppressed for the formation of many joints. Here we review the evidence that BMP signaling must be suppressed within the joint interzone for joint morphogenesis to progress, and consider how Gdf5 exerts its divergent effects on chondrogenesis and joint formation. We also consider how TGFβ signaling impacts formation of the interzone. Finally, we propose a model whereby Gdf5 exerts distinct effects in the interzone vs. surrounding cartilage based on the repertoire of BMP receptors available in these tissues. Understanding how BMP antagonists and counteracting TGFβ signals intersect with Gdf5 to sculpt the joint interzone is essential for understanding the origin of osteoarthritis and other diseases of joint tissues.

Chondrogenic differentiation potential of adult and fetal equine cell types

OBJECTIVE

To determine the chondrogenic potential of cells derived from interzone tissue, the normal progenitor of articular cartilage during fetal development, compared to that of adult bone marrow-derived and adipose-derived mesenchymal cell isolates. The objective of this study was to compare the chondrogenic potential of fetal musculoskeletal progenitor cells to adult cell types, which are currently used therapeutically to facilitate joint cartilage repair in equine clinical practice. The hypothesis tested was that cells derived from interzone tissue have a chondrogenic potential that exceeds that of adult bone marrow-derived and adipose-derived mesenchymal cell isolates.

STUDY DESIGN

In vitro study.

ANIMALS

Six young adult horses (15-17 months of age) and 6 equine fetuses aged 45-46 days of gestation.

METHODS

Three-dimensional pellet cultures were established under chondrogenic conditions with fresh, primary cells isolated from adult (articular cartilage, bone marrow, adipose, dermis) and fetal (interzone, skeletal anlagen cartilage, dermis) tissues. Cellular morphology, pellet architecture, and proteoglycan synthesis were assessed in the pellet cultures. Steady state levels of ACAN (aggrecan core protein), COL2A1 (collagen type II), and COL1A1 (collagen type I) messenger RNA (mRNA) were compared among these cell types as pellet cultures and monolayer cultures.

RESULTS

Adult articular chondrocytes, fetal interzone cells, and fetal anlage cells generated the largest pellets under these chondrogenic culture conditions. Pellets derived from adult articular chondrocytes and fetal anlage cells had the highest scores on a neocartilage grading scale. Fetal anlage and adult articular chondrocyte pellets had low steady-state levels of COL1A mRNA but high COL2A1 expression. Anlage chondrocyte pellets also had the highest expression of ACAN.

CONCLUSION

Adult articular chondrocytes, fetal interzone cells, and fetal anlage chondrocytes exhibited the highest chondrogenic potential. In this study, adult adipose-derived cells exhibited very limited chondrogenesis, and bone marrow-derived cells had limited and variable chondrogenic potential.

CLINICAL SIGNIFICANCE

Additional investigation of the high chondrogenic potential of fetal interzone cells and anlage chondrocytes to advance cell-based therapies in diarthrodial joints is warranted.

Step-Wise Chondrogenesis of Human Induced Pluripotent Stem Cells and Purification Via a Reporter Allele Generated by CRISPR-Cas9 Genome Editing

The differentiation of human induced pluripotent stem cells (hiPSCs) to prescribed cell fates enables the engineering of patient-specific tissue types, such as hyaline cartilage, for applications in regenerative medicine, disease modeling, and drug screening. In many cases, however, these differentiation approaches are poorly controlled and generate heterogeneous cell populations. Here, we demonstrate cartilaginous matrix production in three unique hiPSC lines using a robust and reproducible differentiation protocol. To purify chondroprogenitors (CPs) produced by this protocol, we engineered a COL2A1-GFP knock-in reporter hiPSC line by CRISPR-Cas9 genome editing. Purified CPs demonstrated an improved chondrogenic capacity compared with unselected populations. The ability to enrich for CPs and generate homogenous matrix without contaminating cell types will be essential for regenerative and disease modeling applications. Stem Cells 2019;37:65-76.

Roles of Ihh signaling in chondroprogenitor function in postnatal condylar cartilage

Condylar articular cartilage in mouse temporomandibular joint develops from progenitor cells near the articulating surface that proliferate, undergo chondrogenesis and mature into hypertrophic chondrocytes. However, it remains unclear how these processes are regulated, particularly postnatally. Here we focused on the apical polymorphic layer rich in progenitors and asked whether the phenotype and fate of the cells require signaling by Indian hedgehog (Ihh) previously studied in developing long bones. In condyles in newborn mice, the apical polymorphic/progenitor cell layer was ~10 cell layer-thick and expressed the articular matrix marker Tenascin-C (Tn-C), and the underlying thick cell layer expressed Tn-C as well as the chondrogenic master regulator Sox9. By 1 month, condylar cartilage had gained its full width, but became thinner along its main longitudinal axis and displayed hypertrophic chondrocytes. By 3 months, articular cartilage consisted of a 2-3 cell layer-thick zone of superficial cells and chondroprogenitors expressing both Tn-C and Sox9 and a bottom zone of chondrocytes displaying vertical matrix septa. EdU cell tracing in juvenile mice revealed that conversion of chondroprogenitors into chondrocytes and hypertrophic chondrocytes required about 48 and 72 h, respectively. Notably, EdU injection in 3 month-old mice labeled both progenitors and maturing chondrocytes by 96 h. Conditional ablation of Ihh in juvenile/early adult mice compromised chondroprogenitor organization and function and led to reduced chondroprogenitor and chondrocyte proliferation. The phenotype of mutant condyles worsened over time as indicated by apoptotic chondrocyte incidence, ectopic chondrocyte hypertrophy, chondrocyte column derangement and subchondral bone deterioration. In micromass cultures of condylar apical cells, hedgehog (Hh) treatment stimulated chondrogenesis and alkaline phosphatase (APase) activity, while treatment with HhAntag inhibited both. Our findings indicate that the chondroprogenitor layer is continuously engaged in condylar growth postnatally and its organization and functioning depend on hedgehog signaling.

MicroRNA-19a promotes cell viability and migration of chondrocytes via up-regulating SOX9 through NF-κB pathway

BACKGROUND

Osteoarthritis (OA), as a degenerative disease, is a major problem in ageing populations. To better understand the underlying mechanisms in the pathogenesis of OA, this study was undertaken to investigate the role of microRNA (miR)-19a in chondrocytes.

METHODS

Expression of the members of miR-17-92 cluster in synovium from OA patients and non-OA patients were measured by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). miR-19a was abnormal expressed in human chondrocyte line (CHON-001 and T-C/28 cells) and primary human chondrocytes by transient transfection. Cell viability, migration and apoptosis were determined by CCK-8 assay, wound healing assay, and flow cytometry, respectively. Expression of apoptosis related factors was measured by western blot. Transcription factor SOX9 expression and activity of NF-κB pathway were also assessed by western blot.

RESULTS

Levels of miR-19a and other five members of miR-17-92 cluster were down-regulated in OA patients' synovium compare with non-OA. miR-19a overexpression promoted cell viability and migration of chondrocytes, while miR-19a suppression promoted cell apoptosis, and inhibited cell viability and migration. miR-19a direct up-regulated expression of SOX9, and thus affecting cell viability and migration. miR-19a promoted activation of NF-κB signaling pathway to up-regulate SOX9 expression.

CONCLUSION

miR-19a was down-regulated in synovium form OA patients. miR-19a could promote cell viability and migration of chondrocyte via positively regulating SOX9 expression through NF-κB signaling pathway. This study might provide the novel strategy for clinical treatment of OA caused by chondrocyte function degradation.

Extracellular matrix content and WNT/β-catenin levels of cartilage determine the chondrocyte response to compressive load

During osteoarthritis (OA)-development extracellular matrix (ECM) molecules are lost from cartilage, thus changing gene-expression, matrix synthesis and biomechanical competence of the tissue. Mechanical loading is important for the maintenance of articular cartilage; however, the influence of an altered ECM content on the response of chondrocytes to loading is not well understood, but may provide important insights into underlying mechanisms as well as supplying new therapies for OA. Objective here was to explore whether a changing ECM-content of engineered cartilage affects major signaling pathways and how this alters the chondrocyte response to compressive loading. Activity of canonical WNT-, BMP-, TGF-β- and p38-signaling was determined during maturation of human engineered cartilage and followed after exposure to a single dynamic compression-episode. WNT/β-catenin- and pSmad1/5/9-levels declined with increasing ECM-content of cartilage. While loading significantly suppressed proteoglycan-synthesis and ACAN-expression at low ECM-content this catabolic response then shifted to an anabolic reaction at high ECM-content. A positive correlation was observed between GAG-content and load-induced alteration of proteoglycan-synthesis. Induction of high β-catenin levels by the WNT-agonist CHIR suppressed load-induced SOX9- and GAG-stimulation in mature constructs. In contrast, the WNT-antagonist IWP-2 was capable of attenuating load-induced GAG-suppression in immature constructs. In conclusion, either ECM accumulation-associated or pharmacologically induced silencing of WNT-levels allowed for a more anabolic reaction of chondrocytes to physiological loading. This is consistent with the role of proteoglycans in sequestering WNT-ligands in the ECM, thus reducing WNT-activity and also provides a novel explanation of why low WNT-activity in cartilage protects from OA-development in mechanically overstressed cartilage.

Mechanical regulation of musculoskeletal system development

During embryogenesis, the musculoskeletal system develops while containing within itself a force generator in the form of the musculature. This generator becomes functional relatively early in development, exerting an increasing mechanical load on neighboring tissues as development proceeds. A growing body of evidence indicates that such mechanical forces can be translated into signals that combine with the genetic program of organogenesis. This unique situation presents both a major challenge and an opportunity to the other tissues of the musculoskeletal system, namely bones, joints, tendons, ligaments and the tissues connecting them. Here, we summarize the involvement of muscle-induced mechanical forces in the development of various vertebrate musculoskeletal components and their integration into one functional unit.

Kruppel-like factor 4 regulates matrix metalloproteinase and aggrecanase gene expression in chondrocytes

Kruppel-like factor 4 (KLF4) is a zinc finger transcription factor that plays crucial roles during the development and maintenance of multiple organs. We and others have previously shown that KLF4 is involved in bone modeling and remodeling but roles played by KLF4 during skeletogenesis are still not fully understood. Here, we show that KLF4 is expressed in the epiphyseal growth plate and articular chondrocytes. Most articular chondrocytes expressed KLF4 in embryos but it localized only in a subset of superficial zone cells in postnatal mice. When KLF4 was overexpressed in chondrocytes in vitro, it severely repressed chondrocytic gene expressions. Global gene expression profiling of KLF4-transduced chondrocytes revealed matrix degrading proteinases of the matrix metalloproteinase and disintegrin and metalloproteinase with thrombospondin-1 domain families within the group of upregulated genes. Proteinase induction by KLF4 was alleviated by Trichostatin A treatment suggesting the possible involvement of epigenetic mechanisms on proteinase induction by KLF4. These results indicate the possible involvement of KLF4 in physiological and pathological aspects during cartilage development and maintenance.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

Extracellular matrix alterations in senescent cells and their significance in tissue homeostasis

Normal cells after a defined number of successive divisions or after exposure to genotoxic stresses are becoming senescent, characterized by a permanent growth arrest. In addition, they secrete increased levels of pro-inflammatory and catabolic mediators, collectively termed "senescence-associated secretory phenotype". Furthermore, senescent cells exhibit an altered expression and organization of many extracellular matrix components, leading to specific remodeling of their microenvironment. In this review we present the current knowledge on extracellular matrix alterations associated with cellular senescence and critically discuss certain characteristic examples, highlighting the ambiguous role of senescent cells in the homeostasis of various tissues under both normal and pathologic conditions.