Chondrogenesis and osteogenesis are one continuous developmental and lineage defined biological process

The effects of dietary loading on the transdifferentiation of condylar chondrocytes

INTRODUCTION

Transdifferentiation of chondrocytes into bone cells explains most condylar growth during prenatal and early postnatal stages, but the mechanisms regulating chondrocyte transdifferentiation during late postnatal growth remain unknown. This study aimed to quantify the effects of dietary loading on chondrocyte-derived osteogenesis during late postnatal condylar growth.

METHODS

Two compound mouse lines were used to trace the fate of chondrocyte lineage in vivo. Twelve 3-week-old male Aggrecan-CreERT2 (AcanLineage); R26RTdTomato; 2.3 Col10a1-GFP and twelve 3-week-old male Col10a1-Cre (Col10a1Lineage); R26RTdTomato; 2.3Col1a1-GFP were randomly divided into experimental (soft-food diet, n = 6) and control (hard-food diet, n = 6) groups and kept for 6 weeks. One time, tamoxifen injections were given to AcanLineage mice at 3 weeks. Radiographic, microcomputed tomographic, and histomorphometric analyses were performed.

RESULTS

Radiologic analysis showed that mice with a soft-food diet had smaller mandible lengths as well as decreased bone volume and density for their condylar process. Histologically, mice with soft diets had reduced activity in chondrocyte proliferation and maturation compared with the controls. Cell lineage tracing results showed the number of AcanLineage-derived bone cells (293.8 ± 39.8 vs 207.1 ± 44.6; P = 0.005), as well as total bone cells (445.6 ± 31.7 vs 360.7 ± 46.9; P = 0.004), was significantly higher in the hard-diet group than in the soft-diet group, whereas the number of non-AcanLineage-derived bone cells was not significantly different among groups (P = 0.938). Col10a1Lineage mice showed the same trend.

CONCLUSIONS

Dietary loading directly affects condyle chondrogenesis and chondrocyte transdifferentiation, which alters the extent of condylar growth and remodeling.

3D bio scaffold support osteogenic differentiation of mesenchymal stem cells

The regeneration of osteochondral lesions by tissue engineering techniques is challenging due to the lack of physicochemical characteristics and dual-lineage (osteogenesis and chondrogenesis). A scaffold with better mechanical properties and dual lineage capability is required for the regeneration of osteochondral defects. In this study, a hydrogel prepared from decellularized human umbilical cord tissue was developed and evaluated for osteochondral regeneration. Mesenchymal stem cells (MSCs) isolated from the umbilical cord were seeded with hydrogel for 28 days, and cell-hydrogel composites were cultured in basal and osteogenic media. Alizarin red staining, quantitative polymerase chain reaction, and immunofluorescent staining were used to confirm that the hydrogel was biocompatible and capable of inducing osteogenic differentiation in umbilical cord-derived MSCs. The findings demonstrate that human MSCs differentiated into an osteogenic lineage following 28 days of cultivation in basal and osteoinductive media. The expression was higher in the cell-hydrogel composites cultured in osteoinductive media, as evidenced by increased levels of messenger RNA and protein expression of osteogenic markers as compared to basal media cultured cell-hydrogel composites. Additionally, calcium deposits were also observed, which provide additional evidence of osteogenic differentiation. The findings demonstrate that the hydrogel is biocompatible with MSCs and possesses osteoinductive capability in vitro. It may be potentially useful for osteochondral regeneration.

BMP signaling maintains auricular chondrocyte identity and prevents microtia development by inhibiting protein kinase A

Elastic cartilage constitutes a major component of the external ear, which functions to guide sound to the middle and inner ears. Defects in auricle development cause congenital microtia, which affects hearing and appearance in patients. Mutations in several genes have been implicated in microtia development, yet, the pathogenesis of this disorder remains incompletely understood. Here, we show that Prrx1 genetically marks auricular chondrocytes in adult mice. Interestingly, BMP-Smad1/5/9 signaling in chondrocytes is increasingly activated from the proximal to distal segments of the ear, which is associated with a decrease in chondrocyte regenerative activity. Ablation of Bmpr1a in auricular chondrocytes led to chondrocyte atrophy and microtia development at the distal part. Transcriptome analysis revealed that Bmpr1a deficiency caused a switch from the chondrogenic program to the osteogenic program, accompanied by enhanced protein kinase A activation, likely through increased expression of Adcy5/8. Inhibition of PKA blocked chondrocyte-to-osteoblast transformation and microtia development. Moreover, analysis of single-cell RNA-seq of human microtia samples uncovered enriched gene expression in the PKA pathway and chondrocyte-to-osteoblast transformation process. These findings suggest that auricle cartilage is actively maintained by BMP signaling, which maintains chondrocyte identity by suppressing osteogenic differentiation.

Compensatory growth and recovery of cartilage cytoarchitecture after transient cell death in fetal mouse limbs

A major question in developmental and regenerative biology is how organ size and architecture are controlled by progenitor cells. While limb bones exhibit catch-up growth (recovery of a normal growth trajectory after transient developmental perturbation), it is unclear how this emerges from the behaviour of chondroprogenitors, the cells sustaining the cartilage anlagen that are progressively replaced by bone. Here we show that transient sparse cell death in the mouse fetal cartilage is repaired postnatally, via a two-step process. During injury, progression of chondroprogenitors towards more differentiated states is delayed, leading to altered cartilage cytoarchitecture and impaired bone growth. Then, once cell death is over, chondroprogenitor differentiation is accelerated and cartilage structure recovered, including partial rescue of bone growth. At the molecular level, ectopic activation of mTORC1 correlates with, and is necessary for, part of the recovery, revealing a specific candidate to be explored during normal growth and in future therapies.

The salamander limb: a perfect model to understand imperfect integration during skeletal regeneration

Limb regeneration in salamanders is achieved by a complex coordination of various biological processes and requires the proper integration of new tissue with old. Among the tissues found inside the limb, the skeleton is the most prominent component, which serves as a scaffold and provides support for locomotion in the animal. Throughout the years, researchers have studied the regeneration of the appendicular skeleton in salamanders both after limb amputation and as a result of fracture healing. The final outcome has been widely seen as a faithful re-establishment of the skeletal elements, characterised by a seamless integration into the mature tissue. The process of skeletal integration, however, is not well understood, and several works have recently provided evidence of commonly occurring flawed regenerates. In this Review, we take the reader on a journey through the course of bone formation and regeneration in salamanders, laying down a foundation for critically examining the mechanisms behind skeletal integration. Integration is a phenomenon that could be influenced at various steps of regeneration, and hence, we assess the current knowledge in the field and discuss how early events, such as tissue histolysis and patterning, influence the faithful regeneration of the appendicular skeleton.

The Origin and Fate of Chondrocytes: Cell Plasticity in Physiological Setting

PURPOSE OF REVIEW

Here, we discuss the origin of chondrocytes, their destiny, and their plasticity in relationship to bone growth, articulation, and formation of the trabeculae. We also consider these processes from a biological, clinical, and evolutionary perspective.

RECENT FINDINGS

Chondrocytes, which provide the template for the formation of most bones, are responsible for skeletal growth and articulation during postnatal life. In recent years our understanding of the fate of these cells has changed dramatically. Current evidence indicates a paradoxical situation during skeletogenesis, with some cells of mesenchymal condensation differentiating directly into osteoblasts, whereas others of the same kind give rise to highly similar osteoblasts via a complex process of differentiation involving several chondrocyte intermediates. The situation becomes even more paradoxical during postnatal growth when stem cells in the growth plate produce differentiated, functional progenies, which thereafter presumably dedifferentiate into another type of stem cell. Such a remarkable transition from one cell type to another under postnatal physiological conditions provides a fascinating example of cellular plasticity that may have valuable clinical implications.

Harnessing human adipose-derived stromal cell chondrogenesis in vitro for enhanced endochondral ossification

Endochondral ossification (ECO), the major ossification process during embryogenesis and bone repair, involves the formation of a cartilaginous template remodelled into a functional bone organ. Adipose-derived stromal cells (ASC), non-skeletal multipotent progenitors from the stromal vascular fraction (SVF) of human adipose tissue, were shown to recapitulate ECO and generate bone organs in vivo when primed into a hypertrophic cartilage tissue (HCT) in vitro. However, the reproducibility of ECO was limited and the major triggers remain unknown. We studied the effect of the expansion of cells and maturation of HCT on the induction of the ECO process. SVF cells or expanded ASC were seeded onto collagen sponges, cultured in chondrogenic medium for 3-6 weeks and implanted ectopically in nude mice to evaluate their bone-forming capacities. SVF cells from all tested donors formed mature HCT in 3 weeks whereas ASC needed 4-5 weeks. A longer induction increased the degree of maturation of the HCT, with a gradually denser cartilaginous matrix and increased mineralization. This degree of maturation was highly predictive of their bone-forming capacity in vivo, with ECO achieved only for an intermediate maturation degree. In parallel, expanding ASC also resulted in an enrichment of the stromal fraction characterized by a rapid change of their proteomic profile from a quiescent to a proliferative state. Inducing quiescence rescued their chondrogenic potential. Our findings emphasize the role of monolayer expansion and chondrogenic maturation degree of ASC on ECO and provides a simple, yet reproducible and effective approach for bone formation to be tested in specific clinical models.

Short-term effects of mechanical loading on the transdifferentiation of condylar chondrocytes

INTRODUCTION

Transdifferentiation of chondrocytes into bone cells explains most of the prenatal and early postnatal condylar growth, but its role during later postnatal growth and the mechanisms regulating transdifferentiation remain unknown. This study aimed to quantify the effects of mechanical loading on chondrocyte-derived osteogenesis during late postnatal condylar growth using a short-term mandibular laterotrusion model.

METHODS

Thirty 4-week-old Aggrecan-CreERT2, R26RtdTomato, and 2.3Col1a1-GFP compound mice received tamoxifen injections and were divided into control and experimental groups. Appliances were bonded to shift the mandibles of the experimental mice for 5 days, causing protrusion and retrusion of the right and left condyles, respectively. Radiographic, microcomputed tomographic, and histomorphometric analyses were performed.

RESULTS

The experimental and control groups showed substantial transdifferentiation of chondrocytes into bone cells. The experimental mice developed asymmetric mandibles, with the protrusive side significantly longer than the retrusive side. The protrusive condyles showed significantly increased chondrogenesis and greater numbers of chondrocyte-derived osteogenic cells, especially in the posterior third. The opposite effects were seen on the retrusive side.

CONCLUSIONS

Transdifferentiation of chondrocytes into bone cells occurs during late postnatal condylar growth. Laterotrusion regulates condylar chondrogenesis and chondrocyte transdifferentiation, which alters the amount and direction of condylar growth. Our study demonstrated that chondrocytes are key players in condylar bone formation and should be the focus of studies to control and further understand condylar growth.

Multiple roles of ALK3 in osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease characterized by progressive cartilage degradation, synovial membrane inflammation, osteophyte formation, and subchondral bone sclerosis. Pathological changes in cartilage and subchondral bone are the main processes in OA. In recent decades, many studies have demonstrated that activin-like kinase 3 (ALK3), a bone morphogenetic protein receptor, is essential for cartilage formation, osteogenesis, and postnatal skeletal development. Although the role of bone morphogenetic protein (BMP) signalling in articular cartilage and bone has been extensively studied, many new discoveries have been made in recent years around ALK3 targets in articular cartilage, subchondral bone, and the interaction between the two, broadening the original knowledge of the relationship between ALK3 and OA. In this review, we focus on the roles of ALK3 in OA, including cartilage and subchondral bone and related cells. It may be helpful to seek more efficient drugs or treatments for OA based on ALK3 signalling in future.

The emerging studies on mesenchymal progenitors in the long bone

Mesenchymal progenitors (MPs) are considered to play vital roles in bone development, growth, bone turnover, and repair. In recent years, benefiting from advanced approaches such as single-cell sequence, lineage tracing, flow cytometry, and transplantation, multiple MPs are identified and characterized in several locations of bone, including perichondrium, growth plate, periosteum, endosteum, trabecular bone, and stromal compartment. However, although great discoveries about skeletal stem cells (SSCs) and progenitors are present, it is still largely obscure how the varied landscape of MPs from different residing sites diversely contribute to the further differentiation of osteoblasts, osteocytes, chondrocytes, and other stromal cells in their respective destiny sites during development and regeneration. Here we discuss recent findings on MPs' origin, differentiation, and maintenance during long bone development and homeostasis, providing clues and models of how the MPs contribute to bone development and repair.

Enhancement of mesenchymal stem cells' chondrogenic potential by type II collagen-based bioscaffolds

BACKGROUND

Osteoarthritis (OA) is a common degenerative chronic disease accounting for physical pain, tissue stiffness and mobility restriction. Current therapeutic approaches fail to prevent the progression of the disease considering the limited knowledge on OA pathobiology. During OA progression, the extracellular matrix (ECM) of the cartilage is aberrantly remodeled by chondrocytes. Chondrocytes, being the main cell population of the cartilage, participate in cartilage regeneration process. To this end, modern tissue engineering strategies involve the recruitment of mesenchymal stem cells (MSCs) due to their regenerative capacity as to promote chondrocyte self-regeneration.

METHODS AND RESULTS

In the present study, we evaluated the role of type II collagen, as the main matrix macromolecule in the cartilage matrix, to promote chondrogenic differentiation in two MSC in vitro culture systems. The chondrogenic differentiation of human Wharton's jelly- and dental pulp-derived MSCs was investigated over a 24-day culture period on type II collagen coating to improve the binding affinity of MSCs. Functional assays, demonstrated that type II collagen promoted chondrogenic differentiation in both MSCs tested, which was confirmed through gene and protein analysis of major chondrogenic markers.

CONCLUSIONS

Our data support that type II collagen contributes as a natural bioscaffold enhancing chondrogenesis in both MSC models, thus enhancing the commitment of MSC-based therapeutic approaches in regenerative medicine to target OA and bring therapy closer to the clinical use.

Effect of Hydrogen Oxide-Induced Oxidative Stress on Bone Formation in the Early Embryonic Development Stage of Chicken

The current study aimed to monitor the impact of H₂O₂-induced oxidative stress on avian bone formation during the early stage of embryonic development. Fertilized Cobb broiler eggs were divided into five treatment groups and micro-injected with varying concentrations of H₂O₂, i.e., control (PBS; 0 nM), 10 nM, 30 nM, 100 nM, and 300 nM, on embryonic day 3, with continued incubation thereafter. The treatment concentrations were selected based on the level of lipid peroxidation and the survival rate of embryo. Embryos were collected at 6 h, 24 h, 48 h, and 72 h post-injection. The mRNA expression levels of apoptotic markers, antioxidant enzymes, and early bone formation gene markers were measured. The results showed that the microinjection of H₂O₂ altered the expression pattern of antioxidant enzymes' mRNA during early embryogenesis and decreased the expression of COL1A2 and COL2A1 at 6 h and 24 h post-injection. Decreased expression of BMP, BGLAP, and RUNX2 was observed 48 h post-injection. Additionally, a shorter embryo length was observed in the 100 nM and 300 nM H₂O₂ treatment groups 72 h post-injection. In conclusion, H₂O₂-induced oxidative stress suppressed the expression of bone formation gene markers, with chronic effects on avian embryonic development.

Fabrication and Bioactivity of Peptide-Conjugated Biomaterial Tissue Engineering Constructs

Tissue engineering combines materials engineering, cells and biochemical factors to improve, restore or replace various types of biological tissues. A nearly limitless combination of these strategies can be combined, providing a means to augment the function of a number of biological tissues such as skin tissue, neural tissue, bones, and cartilage. Compounds such as small molecule therapeutics, proteins, and even living cells have been incorporated into tissue engineering constructs to influence biological processes at the site of implantation. Peptides have been conjugated to tissue engineering constructs to circumvent limitations associated with conjugation of proteins or incorporation of cells. This review highlights various contemporary examples in which peptide conjugation is used to overcome the disadvantages associated with the inclusion of other bioactive compounds. This review covers several peptides that are commonly used in the literature as well as those that do not appear as frequently to provide a broad scope of the utility of the peptide conjugation technique for designing constructs capable of influencing the repair and regeneration of various bodily tissues. Additionally, a brief description of the construct fabrication techniques encountered in the covered examples and their advantages in various tissue engineering applications is provided.

Tendon Cells Root Into (Instead of Attach to) Humeral Bone Head via Fibrocartilage-Enthesis

Large joints are composed of two closely linked cartilages: articular cartilage (AC; rich in type II collagen, a well-studied tissue) and fibrocartilaginous enthesis (FE; rich in type I collagen, common disorder sites of enthesopathy and sporting injuries, although receiving little attention). For many years, both cartilages were thought to be formed by chondrocytes, whereas tendon, which attaches to the humeral bone head, is primarily considered as a completely different connective tissue. In this study, we raised an unconventional hypothesis: tendon cells directly form FE via cell transdifferentiation. To test this hypothesis, we first qualitatively and quantitatively demonstrated distinct differences between AC and FE in cell morphology and cell distribution, mineralization status, extracellular matrix (ECM) contents, and critical ECM protein expression profiles using comprehensive approaches. Next, we traced the cell fate of tendon cells using Scx^Lin (a tendon specific Cre Scx^CreERT2; R26R-tdTomato line) with one-time tamoxifen induction at early (P3) or young adult (P28) stages and harvested mice at different development ages, respectively. Our early tracing data revealed different growth events in tendon and FE: an initial increase but gradual decrease in the Scx^Lin tendon cells and a continuous expansion in the Scx^Lin FE cells. The young adult tracing data demonstrated continuous recruitment of Scx^Lin cells into FE expansion during P28 and P56. A separate tracing line, 3.2 Col 1^Lin (a so-called "bone-specific" line), further confirmed the direct contribution of tendon cells for FE cell formation, which occurred in days but FE ECM maturation (including high levels of SOST, a potent Wnt signaling inhibitor) took weeks. Finally, loss of function data using diphtheria toxin fragment A (DTA) in Scx^Lin cells demonstrated a significant reduction of Scx^Lin cells in both tendons and FE cells, whereas the gain of function study (by stabilizing β-catenin in Scx^Lin tendon cells via one-time injection of tamoxifen at P3 and harvesting at P60) displayed great expansion of both Scx^Lin tendon and FE mass. Together, our studies demonstrated that fibrocartilage is an invaded enthesis likely originating from the tendon via a quick cell transdifferentiation mechanism with a lengthy ECM maturation process. The postnatally formed fibrocartilage roots into existing cartilage and firmly connects tendon and bone instead of acting as a simple attachment site as widely believed. We believe that this study will stimulate more intense exploring in this understudied area, especially for patients with enthesopathy and sporting injuries.

Gain-of-Function of FGFR3 Accelerates Bone Repair Following Ischemic Osteonecrosis in Juvenile Mice

Bone collapse, bone deformity, and a long treatment period are major clinical problems associated with juvenile ischemic osteonecrosis (JIO). Accelerating the process of bone repair in JIO is expected to shorten the treatment duration and better maintain morphology. We previously indicated that both bone formation and resorption were accelerated following distraction osteogenesis-mediated limb lengthening in genetically engineered mutant mice with a gain-of-function mutation in fibroblast growth factor receptor 3 (FGFR3) gene (i.e., Fgfr3 mice). The purpose of this study was to investigate the role of FGFR3 in the bone repair process following surgically induced ischemic osteonecrosis in the mutant mice. Epiphyseal deformity was less in the Fgfr3 mice compared to the wild-type mice at 6 weeks following ischemic osteonecrosis in skeletally immature age. Assessment of the morphology by micro-computed tomography (CT) revealed that the trabecular bone volume was increased in the Fgfr3 mice. Dynamic bone histomorphometry revealed increased rates of bone formation and mineral apposition in the Fgfr3 mice at 4 weeks post-surgery. The number of tartrate-resistant acid phosphatase (TRAP)-positive cells rapidly increased, and the numbers of TdT-mediated dUTP nick-end labeling (TUNEL)-positive cells rapidly decreased in the Fgfr3 mice. Vascular endothelial growth factor (VEGF) expression was increased at the earlier phase post-surgery in the Fgfr3 mice. The activation of FGFR3 signaling shortens the time needed for bone repair after ischemic osteonecrosis by accelerating revascularization, bone resorption, and new bone formation. Our findings are clinically relevant as a new potential strategy for the treatment of JIO.

CTR9 drives osteochondral lineage differentiation of human mesenchymal stem cells via epigenetic regulation of BMP-2 signaling

Cell fate determination of human mesenchymal stem/stromal cells (hMSCs) is precisely regulated by lineage-specific transcription factors and epigenetic enzymes. We found that CTR9, a key scaffold subunit of polymerase-associated factor complex (PAFc), selectively regulates hMSC differentiation to osteoblasts and chondrocytes, but not to adipocytes. An in vivo ectopic osteogenesis assay confirmed the essentiality of CTR9 in hMSC-derived bone formation. CTR9 counteracts the activity of Enhancer Of Zeste 2 (EZH2), the epigenetic enzyme that deposits H3K27me3, in hMSCs. Accordingly, CTR9 knockdown (KD) hMSCs gain H3K27me3 mark, and the osteogenic differentiation defects of CTR9 KD hMSCs can be partially rescued by treatment with EZH2 inhibitors. Transcriptome analyses identified bone morphology protein-2 (BMP-2) as a downstream effector of CTR9. BMP-2 secretion, membrane anchorage, and the BMP-SMAD pathway were impaired in CTR9 KD MSCs, and the effects were rescued by BMP-2 supplementation. This study uncovers an epigenetic mechanism engaging the CTR9-H3K27me3-BMP-2 axis to regulate the osteochondral lineage differentiation of hMSCs.

How osteogenic is dexamethasone?—effect of the corticosteroid on the osteogenesis, extracellular matrix, and secretion of osteoclastogenic factors of jaw periosteum-derived mesenchymal stem/stromal cells

Dexamethasone (dexa) is commonly used to stimulate osteogenic differentiation of mesenchymal stem/stromal cells (MSCs) in vitro. However, it is paradoxical that glucocorticoids (GCs) such as dexa lead to bone loss and increased fracture risk in patients undergoing glucocorticoid therapy, causing glucocorticoid-induced osteoporosis (GIOP). In a recent publication, we demonstrated that osteogenic differentiation of progenitor cells isolated from jaw periosteal tissue (JPCs) does not depend on dexa, if the medium is supplemented with human platelet lysate (hPL) instead of fetal bovine serum (FBS). This allows the in vitro conditions to be much closer to the natural situation in vivo and enables us to compare osteogenic differentiation with and without dexa. In the present study, we demonstrate that the absence of dexa did not reduce mineralization capacity, but instead slightly improved the osteogenic differentiation of jaw periosteal cells. On the other hand, we show that dexa supplementation strongly alters the gene expression, extracellular matrix (ECM), and cellular communication of jaw periosteal cells. The secretome of periosteal cells previously treated with an osteogenic medium with and without dexa was used to investigate the changes in paracrine secretion caused by dexa. Dexa altered the secretion of several cytokines by jaw periosteal cells and strongly induced osteoclast differentiation of peripheral blood mononuclear cells (PBMCs). This study demonstrates how dexa supplementation can influence the outcome of in vitro studies and highlights a possible role of periosteal cells in the pathogenesis of glucocorticoid-induced osteoporosis. The methods used here can serve as a model for studying bone formation, fracture healing, and various pathological conditions such as (glucocorticoid-induced) osteoporosis, osteoarthritis, bone cancer, and others, in which the interactions of osteoblasts with surrounding cells play a key role.

Novel Peptides Derived from Sea Cucumber Intestine Promotes Osteogenesis by Upregulating Integrin-Mediated Transdifferentiation of Growth Plate Chondrocytes to Osteoblasts

The sea cucumber intestine is a major by-product of sea cucumber processing and contains high levels of protein. In this study, we isolated and identified 28 novel osteogenic peptides from sea cucumber intestinal hydrolysis by the activity-tracking method for the first time. In vitro experimental results showed that compared with high molecular weight, the peptides from sea cucumber intestine (SCIP) with molecular weight <3 kDa more significantly promoted the proliferation and mineralized nodules of MC3T3-E1 cell and exhibited potential integrin binding capacity. In vivo experimental results showed that the SCIP supplement significantly increased the longitudinal bone length and elevated the height of the growth plate (especially the hypertrophic zone, 37.2%, p < 0.01) in adolescent mice. Further, immunofluorescence labeling results indicated that the SCIP supplement increased chondrocyte transdifferentiate to osteoblast in the growth plate close to the diaphysis. Mechanistically, transcriptome analysis revealed that the SCIP supplement induced the dedifferentiation of chondrocyte to osteoprogenitor cell via integrin-mediated histone acetylation and then redifferentiated to osteoblast via integrin-mediated Wnt/β-catenin signaling. These results reported for the first time that sea cucumber intestine had the potential to develop into a dietary supplement for promoting osteogenic, and provide new evidence for the mechanism of dietary promotes chondrocyte to osteoblast transdifferentiation.

Independent Control of Molecular Weight, Concentration, and Stiffness of Hyaluronic Acid Hydrogels

Hyaluronic acid (HA) hydrogels have been used for a multitude of applications, perhaps most notably for tissue engineering and regenerative medicine, owing to the versatility of the polymer and its tunable nature. Various groups have investigated the impact of hydrogel parameters (e.g., molecular weight, concentration, stiffness, etc.) in vitro and in vivo to achieve desired material performance characteristics. A limitation in the literature to date has been that altering one hydrogel parameter (a 'manipulated variable') to achieve a given hydrogel characteristic (a 'controlled variable') changes two variables at a time (e.g., altering molecular weight and/or concentration to investigate cell response to stiffness). Therefore, if cell responses differ, it may be possible that more than one variable caused the changes in observed responses. In the current study, we leveraged thiol-ene click chemistry with a crosslinker to develop a method that minimizes material performance changes and permitted multiple material properties to be independently held constant to evaluate a single variable at a time. Independent control was accomplished by tuning the concentration of crosslinker to achieve an effectively constant stiffness for different HA hydrogel molecular weights and polymer concentrations. Specific formulations were thereby identified that enabled the molecular weight (76 - 1550 kDa), concentration (2 - 10%), or stiffness (~1 - 350 kPa) to be varied while the other two were held constant, a key technical achievement. The response of rat mesenchymal stem cells to varying molecular weight, concentration, and stiffness demonstrated consistent upregulation of osteocalcin gene expression. The methodology presented to achieve independent control of hydrogel parameters may potentially be adopted by others for alternative hydrogel polymers, cell types, or cell culture medium compositions to minimize confounding variables in experimental hydrogel designs.

Trp53 controls chondrogenesis and endochondral ossification by negative regulation of TAZ activity and stability via β-TrCP-mediated ubiquitination

Transformation-related protein 53 (Trp53) is a critical regulator of cell fate determination by controlling cell proliferation and differentiation. Ablation of Trp53 signaling in osteoblast lineages significantly promotes osteogenesis, bone formation, and bone remodeling. However, how Trp53 regulates chondrogenesis and endochondral bone formation is undefined. In this study, we found that Trp53 expression gradually decreased in tibia growth plates during embryonic development in vivo and during chondrogenesis in vitro. By deleting Trp53 in chondrocyte lineage using Col2-Cre transgenic line, we found that loss of Trp53 in chondrocytes significantly increased growth plate growth and bone formation by increasing chondrocyte proliferation, matrix production and maturation, and bone dynamic formation rate. Mechanistically, our data revealed loss of Trp53 significantly promoted TAZ transcriptional activity through inhibition of TAZ phosphorylation and nuclear translocation, whereas its activity was pronouncedly inhibited after forced expression of Trp53. Furthermore, Co-IP data demonstrated that Trp53 associated with TAZ. Moreover, Trp53 decreased the stability of TAZ protein and promoted its degradation through β-TrCP-mediated ubiquitination. Ablation of TAZ in Col2-Cre;Trp53^f/f mice rescued the phenotypes of enhanced chondrogenesis and bone formation caused by Trp53 deletion. Collectively, this study revealed that Trp53 modulates chondrogenesis and endochondral ossification through negative regulation of TAZ activity and stability, suggesting that targeting Trp53 signaling may be a potential strategy for fracture healing, heterotopic ossification, arthritis, and other bone diseases.

Distribution, structure, and mineralization of calcified cartilage remnants in hard antlers

Antlers are paired deciduous bony cranial appendages of deer that undergo a regular cycle of growth, death and casting, and constitute the most rapidly growing bones in mammals. Antler growth occurs in an appositional mode and involves a modified form of endochondral ossification. In endochondral bones, calcified cartilage is typically a transient tissue that is eventually completely replaced by bone tissue. We studied the distribution and characteristics of calcified cartilage in hard antlers from three deer species (Capreolus capreolus, Cervus elaphus, Dama dama), i.e., in antlers from which the skin (velvet) had been shed. Remnants of calcified cartilage were regularly present as part of the trabecular framework in the late formed, distal antler portions in all three species, whereas this tissue was largely or completely missing in the more proximal antler portions. The presence of calcified cartilage remnants in the distal antler portions is attributed to the limited antler lifespan of only a few months, which is also the reason for the virtual lack of bone remodeling in antlers. The calcified cartilage matrix was more highly mineralized than the antler bone matrix. Mineralized deposits were observed in some chondrocyte lacunae and occasionally also in osteocyte lacunae, a phenomenon that has not previously been reported in antlers. Using synchrotron radiation-induced X-ray fluorescence (SR-XRF) mapping, we further demonstrated increased zinc concentrations in cement lines, along the inner borders of incompletely formed primary osteons, along the walls of partly or completely mineral-occluded chondrocyte and osteocyte lacunae, and in intralacunar mineralized deposits. The present study demonstrates that antlers are a promising model for studying the mineralization of cartilage and bone matrices and the formation of mineralized deposits in chondrocyte and osteocyte lacunae.

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Remnants of calcified cartilage are regularly present in hard antlers of deer.

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Preservation of calcified cartilage is caused by the short lifespan of antlers.

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Calcified cartilage of antlers is more highly mineralized than antler bone.

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Mineralized deposits were observed in chondrocyte and osteocyte lacunae of antlers.

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SR-XRF showed increased Zn-concentration in cement lines and intralacunar deposits.

The Emerging Role of Cell Transdifferentiation in Skeletal Development and Diseases

The vertebrate musculoskeletal system is known to be formed by mesenchymal stem cells condensing into tissue elements, which then differentiate into cartilage, bone, tendon/ligament, and muscle cells. These lineage-committed cells mature into end-stage differentiated cells, like hypertrophic chondrocytes and osteocytes, which are expected to expire and to be replaced by newly differentiated cells arising from the same lineage pathway. However, there is emerging evidence of the role of cell transdifferentiation in bone development and disease. Although the concept of cell transdifferentiation is not new, a breakthrough in cell lineage tracing allowed scientists to trace cell fates in vivo. Using this powerful tool, new theories have been established: (1) hypertrophic chondrocytes can transdifferentiate into bone cells during endochondral bone formation, fracture repair, and some bone diseases, and (2) tendon cells, beyond their conventional role in joint movement, directly participate in normal bone and cartilage formation, and ectopic ossification. The goal of this review is to obtain a better understanding of the key roles of cell transdifferentiation in skeletal development and diseases. We will first review the transdifferentiation of chondrocytes to bone cells during endochondral bone formation. Specifically, we will include the history of the debate on the fate of chondrocytes during bone formation, the key findings obtained in recent years on the critical factors and molecules that regulate this cell fate change, and the role of chondrocyte transdifferentiation in skeletal trauma and diseases. In addition, we will also summarize the latest discoveries on the novel roles of tendon cells and adipocytes on skeletal formation and diseases.

[Latest Research Findings on the Regulation of Bone Homeostasis by ALK3]

Bone remodeling, which is well orchestrated by osteogenesis of osteoblasts and osteoclastogenesis of osteoclasts, maintains the homeostasis of osteal development and metabolism under physiological conditions. Bone morphogenetic protein receptor type 1A, also known as activin receptor-like kinase 3 (ALK3), which exists on cytomembrane, is one of the key receptors of BMP factors, and is an important "gateway" that regulates the entrance of BMP signaling into cells in order to perform biological functions. The roles of BMP signaling in bone remodeling have been extensively studied. Many new discoveries have been reported in recent years through research based on transgenic mice models and focused on ALK3 as targets, shedding new light on the regulations of bone remodeling, cartilage and joint development, and the occurrence and treatment of bone-related diseases. Established understanding has been expanded, but new challenges on existing clinical application of BMPs also appeared. Hence, we reviewed recent studies on ALK3's involvement in bone formation and bone resorption, analyzed its mechanism of action in bone regulation, summarized the roles of ALK3 in the development of cartilage and temporomandibular joint, and reported the latest progress in treatment in preclinical studies, intending to provide references for subsequent studies and clinical applications in the future.

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.

A Transgenic Rat for Noninvasive Assessment of Chondrogenesis in Vivo

OBJECTIVE

To support the preclinical evaluation of therapeutics that target chondrogenesis, our goal was to generate a rat strain that can noninvasively report endogenous chondrogenic activity.

DESIGN

A transgene was constructed in which the dual expression of bioluminescent (firefly luciferase) and fluorescent (mCherry) reporters is controlled by regulatory sequences from rat Col2a1. Candidate lines were established on a Lewis background and characterized by serial bioluminescence imaging as well as ex vivo measurement of molecular reporter levels in several tissues. The sensitivity and specificity of the reporter strain were assessed in models of orthotopic and ectopic chondrogenesis.

RESULTS

Substantial bioluminescence signal was detected from cartilaginous regions, including the appendicular synovial joints, spine, sternum, nose, and pinnae. Bioluminescent radiance was intense at 1 month of age, rapidly declined with continued development, yet remained detectable in 2-year-old animals. Explant imaging and immunohistochemistry confirmed that both molecular reporters were localized to cartilage. Implantation of wild-type bone marrow stromal cells into osteochondral defects made in both young adult and aged reporter rats led to a time-dependent elevation of intra-articular reporter activity concurrent with cartilaginous tissue repair. To stimulate ectopic, endochondral bone formation, bone morphogenetic protein 2 was overexpressed in the gastrocnemius muscle, which led to bioluminescent signal that closely preceded heterotopic ossification.

CONCLUSIONS

This strain can help develop strategies to stimulate cartilage repair and endochondral bone formation or to inhibit chondrogenesis associated with heterotopic ossification.

Cross-talk between EGFR and BMP signals regulates chondrocyte maturation during endochondral ossification

BACKGROUND

Progressive maturation of growth plate chondrocytes drives long bone growth during endochondral ossification. Signals from the epidermal growth factor receptor (EGFR), and from bone morphogenetic protein-2 (BMP2), are required for normal chondrocyte maturation. Here, we investigated cross-talk between EGFR and BMP2 signals in developing and adult growth plates.

RESULTS

Using in vivo mouse models of conditional cartilage-targeted EGFR or BMP2 loss, we show that canonical BMP signal activation is increased in the hypertrophic chondrocytes of EGFR-deficient growth plates; whereas EGFR signal activation is increased in the reserve, prehypertrophic and hypertrophic chondrocytes of BMP2-deficient growth plates. EGFR-deficient chondrocytes displayed increased BMP signal activation in vitro, accompanied by increased expression of IHH, COL10A1, and RUNX2. Hypertrophic differentiation and BMP signal activation were suppressed in normal chondrocyte cultures treated with the EGFR ligand betacellulin, effects that were partially blocked by simultaneous treatment with BMP2 or a chemical EGFR antagonist.

CONCLUSIONS

Cross-talk between EGFR and BMP2 signals occurs during chondrocyte maturation. In the reserve and prehypertrophic zones, BMP2 signals unilaterally suppress EGFR activity; in the hypertrophic zone, EGFR and BMP2 signals repress each other. This cross-talk may play a role in regulating chondrocyte maturation in developing and adult growth plates.

Postembryonic development and aging of the appendicular skeleton in Ambystoma mexicanum

BACKGROUND

The axolotl is a key model to study appendicular regeneration. The limb complexity resembles that of humans in structure and tissue components; however, axolotl limbs develop postembryonically. In this work, we evaluated the postembryonic development of the appendicular skeleton and its changes with aging.

RESULTS

The juvenile limb skeleton is formed mostly by Sox9/Col1a2 cartilage cells. Ossification of the appendicular skeleton starts when animals reach a length of 10 cm, and cartilage cells are replaced by a primary ossification center, consisting of cortical bone and an adipocyte-filled marrow cavity. Vascularization is associated with the ossification center and the marrow cavity formation. We identified the contribution of Col1a2-descendants to bone and adipocytes. Moreover, ossification progresses with age toward the epiphyses of long bones. Axolotls are neotenic salamanders, and still ossification remains responsive to l-thyroxine, increasing the rate of bone formation.

CONCLUSIONS

In axolotls, bone maturation is a continuous process that extends throughout their life. Ossification of the appendicular bones is slow and continues until the complete element is ossified. The cellular components of the appendicular skeleton change accordingly during ossification, creating a heterogenous landscape in each element. The continuous maturation of the bone is accompanied by a continuous body growth.

DEC1 deficiency results in accelerated osteopenia through enhanced DKK1 activity and attenuated PI3KCA/Akt/GSK3β signaling

BACKGROUND

Human differentiated embryonic chondrocyte expressed gene 1 (DEC1) has been implicated in enhancing osteogenesis, a desirable outcome to counteract against deregulated bone formation such as retarded bone development, osteopenia and osteoporosis.

METHODS AND RESULTS

DEC1 knockout (KO) and the age-matched wild-type (WT) mice were tested for the impact of DEC1 deficiency on bone development and osteopenia as a function of age. DEC1 deficiency exhibited retarded bone development at the age of 4 weeks and osteopenic phenotype in both 4- and 24-week old mice. However, the osteopenia was more severe in the 24-week age groups. Mechanistically, DEC1 deficiency downregulated the expression of bone-enhancing genes such as Runx2 and β-catenin accompanied by upregulating DKK1, an inhibitor of the Wnt/β-catenin signaling pathway. Consistently, DEC1 deficiency favored the attenuation of the integrated PI3KCA/Akt/GSK3β signaling, a pathway targeting β-catenin for degradation. Likewise, the attenuation was greater in the 24-week age group. These changes, however, were reversed by in vivo treatment with lithium chloride, a stabilizer of β-catenin, and confirmed by gain-of-function study with DEC1 transfection into DEC1 KO bone marrow mesenchymal stem cells and loss-of-function study with siDEC1 lentiviral infection into the corresponding WT cells.

CONCLUSION

DEC1 is a positive regulator with a broad activity spectrum in both bone development and maintenance, and the osteopenic phenotype accelerated by DEC1 deficiency is achieved by enhanced DKK1 activity and attenuated PI3KCA/Akt/GSK3β signaling.

High resolution 3D structures of mineralized tissues in health and disease

A thorough knowledge of the structures of healthy mineralized tissues, such as bone or cartilage, is key to understanding the pathological changes occurring during disease. Such knowledge enables the underlying mechanisms that are responsible for pathology to be pinpointed. One high-resolution 3D method in particular - focused ion beam-scanning electron microscopy (FIB-SEM) - has fundamentally changed our understanding of healthy vertebrate mineralized tissues. FIB-SEM can be used to study demineralized matrix, the hydrated components of tissue (including cells) using cryo-fixation and even untreated mineralized tissue. The latter requires minimal sample preparation, making it possible to study enough samples to carry out studies capable of detecting statistically significant differences - a pre-requisite for the study of pathological tissues. Here, we present an imaging and characterization strategy for tissue structures at different length scales, describe new insights obtained on healthy mineralized tissues using FIB-SEM, and suggest future research directions for both healthy and diseased mineralized tissues.

TAZ is required for chondrogenesis and skeletal development

Chondrogenesis is a major contributor to skeletal development and maintenance, as well as bone repair. Transcriptional coactivator with PDZ-binding motif (TAZ) is a key regulator of osteogenesis and adipogenesis, but how TAZ regulates chondrogenesis and skeletal development remains undefined. Here, we found that TAZ expression is gradually increased during chondrogenic differentiation. Deletion of TAZ in chondrocyte lineage impaired articular and growth plate, as well as the bone development in TAZ-deficient mice. Consistently, loss of TAZ impaired fracture healing. Mechanistically, we found that ectopic expression of TAZ markedly promoted chondroprogenitor proliferation, while deletion of TAZ impaired chondrocyte proliferation and differentiation. TAZ associated with Sox5 to regulate the expression and stability of Sox5 and downstream chondrocyte marker genes' expression. In addition, overexpression of TAZ enhanced Col10a1 expression and promoted chondrocyte maturation, which was blocked by deletion of TAZ. Overall, our findings demonstrated that TAZ is required for skeletal development and joint maintenance that provided new insights into therapeutic strategies for fracture healing, heterotopic ossification, osteoarthritis, and other bone diseases.

Liquid-phase ASEM imaging of cellular and structural details in cartilage and bone formed during endochondral ossification: Keap1-deficient osteomalacia

Chondrogenesis and angiogenesis drive endochondral ossification. Using the atmospheric scanning electron microscopy (ASEM) without decalcification and dehydration, we directly imaged angiogenesis-driven ossification at different developmental stages shortly after aldehyde fixation, using aqueous radical scavenger glucose solution to preserve water-rich structures. An embryonic day 15.5 mouse femur was fixed and stained with phosphotungstic acid (PTA), and blood vessel penetration into the hypertrophic chondrocyte zone was visualised. We observed a novel envelope between the perichondrium and proliferating chondrocytes, which was lined with spindle-shaped cells that could be borderline chondrocytes. At postnatal day (P)1, trabecular and cortical bone mineralisation was imaged without staining. Additional PTA staining visualised surrounding soft tissues; filamentous connections between osteoblast-like cells and osteocytes in cortical bone were interpreted as the osteocytic lacunar-canalicular system. By P10, resorption pits had formed on the tibial trabecular bone surface. The applicability of ASEM for pathological analysis was addressed using knockout mice of Keap1, an oxidative-stress sensor. In Keap1-/- femurs, we observed impaired calcification and angiogenesis of epiphyseal cartilage, suggesting impaired bone development. Overall, the quick ASEM method we developed revealed mineralisation and new structures in wet bone tissue at EM resolution and can be used to study mineralisation-associated phenomena of any hydrated tissue.

ScxLin cells directly form a subset of chondrocytes in temporomandibular joint that are sharply increased in Dmp1-null mice

It has been assumed that the secondary cartilage in the temporomandibular joint (TMJ), which is the most complex and mystery joint and expands rapidly after birth, is formed by periochondrium-derived chondrocytes. The TMJ condyle has rich attachment sites of tendon, which is thought to be solely responsible for joint movement with a distinct cell lineage. Here, we used a Scx-Cre ERT2 mouse line (the tracing line for progenitor and mature tendon cells) to track the fate of tendon cells during TMJ postnatal growth. Our data showed a progressive differentiation of Scx lineage cells started at tendon and the fibrous layer, to cells at the prechondroblasts (Sox9 -/Col I +), and then to cells at the chondrocytic layer (Sox9 +/Col I -). Importantly, the Scx + chondrocytes remained as "permanent" chondrocytes to maintain cartilage mass with no further cell trandifferentiation to bone cells. This notion was substantiated in an assessment of these cells in Dmp1 -null mice (a hypophosphatemic rickets model), where there was a significant increase in the number of Scx lineage cells in response to hypophosphatemia. In addition, we showed the origin of disc, which is derived from Scx + cells. Thus, we propose Scx lineage cells play an important role in TMJ postnatal growth by forming the disc and a new subset of Scx + chondrocytes that do not undergo osteogenesis as the Scx - chondrocytes and are sensitive to the level of phosphorous.

SERPINA9 and SERPINB2: Novel Cartilage Lineage Differentiation Markers of Human Mesenchymal Stem Cells with Kartogenin

OBJECTIVE

Human mesenchymal stem cells (hMSCs) are a promising source for regenerative medicine, especially mesodermal lineages. Clinical applications require an understanding of the mechanisms for transcriptional control to maintain the desired cell type. The aim of this study was to identify novel markers for differentiation of hMSCs into bone or cartilage with the use of Kartogenin, by RNA analysis using microarray technology, and explore the role of RhoA-Rho associated protein kinase (ROCK) inhibition in these.

METHODS

Commercial human bone marrow derived primary mesenchymal stem cells were purchased from ATCC. Cells were differentiated in vitro in 2-dimensional cultures using Kartogenin as the main cartilage inducer and bone morphogenetic protein 2 for bone differentiation; cells were cultured with and without ROCK inhibitor Y-27632. After 21 days of culture, whole RNA was extracted and analyzed via Affimetrix microarrays. The most significant hits were validated by quantitative polymerase chain reaction.

RESULTS

We found a total of 1,757 genes that were either up- or downregulated on differentiation, when compared to P1 hMSC (control) at day 0 of differentiation. Two members of the Serpin superfamily, SERPINA9 and SERPINB2, were significantly upregulated in the cartilage groups, whereas they were unchanged in the bone groups with and without ROCK inhibition.

CONCLUSIONS

SERPINA9 and SERPINB2 are novel differentiation markers, and molecular regulator candidates for hMSC lineage commitment toward bone and cartilage, providing a new tool for regenerative medicine. Our study highlights the roles of these 2 genes, with significant upregulation of both in cell cultures stimulated with Kartogenin.

Polymeric Gelatin Scaffolds Affect Mesenchymal Stem Cell Differentiation and Its Diverse Applications in Tissue Engineering

Studies using polymeric scaffolds for various biomedical applications, such as tissue engineering, implants and medical substitutes, and drug delivery systems, have attempted to identify suitable material for tissue regeneration. This study aimed to investigate the biocompatibility and effectiveness of a gelatin scaffold seeded with human adipose stem cells (hASCs), including physical characteristics, multilineage differentiation in vitro, and osteogenic potential, in a rat model of a calvarial bone defect and to optimize its design. This functionalized scaffold comprised gelatin-hASCs layers to improve their efficacy in various biomedical applications. The gelatin scaffold exhibited excellent biocompatibility in vitro after two weeks of implantation. Furthermore, the gelatin scaffold supported and specifically regulated the proliferation and osteogenic and chondrogenic differentiation of hASCs, respectively. After 12 weeks of implantation, upon treatment with the gelatin-hASCs scaffold, the calvarial bone harboring the critical defect regenerated better and displayed greater osteogenic potential without any damage to the surrounding tissues compared to the untreated bone defect. These findings suggest that the present gelatin scaffold is a good potential carrier for stem cells in various tissue engineering applications.

BMPR1A is necessary for chondrogenesis and osteogenesis, whereas BMPR1B prevents hypertrophic differentiation

BMP2 stimulates bone formation and signals preferably through BMP receptor (BMPR) 1A, whereas GDF5 is a cartilage inducer and signals preferably through BMPR1B. Consequently, BMPR1A and BMPR1B are believed to be involved in bone and cartilage formation, respectively. However, their function is not yet fully clarified. In this study, GDF5 mutants with a decreased affinity for BMPR1A were generated. These mutants, and wild-type GDF5 and BMP2, were tested for their ability to induce dimerization of BMPR1A or BMPR1B with BMPR2, and for their chondrogenic, hypertrophic and osteogenic properties in chondrocytes, in the multipotent mesenchymal precursor cell line C3H10T1/2 and the human osteosarcoma cell line Saos-2. Mutants with the lowest potency for inducing BMPR1A-BMPR2 dimerization exhibited minimal chondrogenic and osteogenic activities, indicating that BMPR1A is necessary for chondrogenic and osteogenic differentiation. BMP2, GDF5 and the GDF5 R399E mutant stimulated expression of chondrogenic and hypertrophy markers in C3H10T1/2 cells and chondrocytes. However, GDF5 R399E, which induces the dimerization of BMPR1B and BMPR2 more potently than GDF5 or BMP2, displayed reduced hypertrophic activity. Therefore, we postulate that stronger BMPR1B signaling, compared to BMPR1A signaling, prevents chondrocyte hypertrophy and acts as a cartilage stabilizer during joint morphogenesis.This article has an associated First Person interview with the first author of the paper.

Chondrogenesis Defines Future Skeletal Patterns Via Cell Transdifferentiation from Chondrocytes to Bone Cells

PURPOSE OF REVIEW

The goal of this review is to obtain a better understanding of how chondrogenesis defines skeletal development via cell transdifferentiation from chondrocytes to bone cells.

RECENT FINDINGS

A breakthrough in cell lineage tracing allows bone biologists to trace the cell fate and demonstrate that hypertrophic chondrocytes can directly transdifferentiate into bone cells during endochondral bone formation. However, there is a knowledge gap for the biological significance of this lineage extension and the mechanisms controlling this process. This review first introduces the history of the debate on the cell fate of chondrocytes in endochondral bone formation; then summarizes key findings obtained in recent years, which strongly support a new theory: the direct cell transdifferentiation from chondrocytes to bone cells precisely connects chondrogenesis (for providing a template of the future skeleton, classified as phase I) and osteogenesis (for finishing skeletal construction, or phase II) in a continuous lineage-linked process of endochondral bone formation and limb elongation; and finally outlines nutrition factors and molecules that regulate the cell transdifferentiation process during the relay from chondrogenesis to osteogenesis.

Hypophosphatemic rickets accelerate chondrogenesis and cell trans-differentiation from TMJ chondrocytes into bone cells via a sharp increase in β-catenin

Dentin matrix protein 1 (DMP1) is primarily expressed in osteocytes, although a low level of DMP1 is also detected in chondrocytes. Removing Dmp1 in mice or a mutation in humans leads to hypophosphatemic rickets (identical to X-linked hypophosphatemia). The deformed skeletons were currently thought to be a consequence of an inhibition of chondrogenesis (leading to an accumulation of hypertrophic chondrocytes and a failure in the replacement of cartilage by bone). To precisely study the mechanisms by which DMP1 and phosphorus control temporomandibular condyle formation, we first showed severe malformed condylar phenotypes in Dmp1-null mice (great expansions of deformed cartilage layers and subchondral bone), which worst as aging. Next, we excluded the direct role of DMP1 in condylar hypertrophic-chondrogenesis by conditionally deleting Dmp1 in hypertrophic chondrocytes using Col10a1-Cre and Dmp1 loxP mice (displaying no apparent phosphorous changes and condylar phenotype). To address the mechanism by which the onset of endochondral phenotypes takes place, we generated two sets of tracing lines in the Dmp1 KO background: AggrecanCreERT2-ROSA-tdTomato and Col 10a1-Cre-ROSA-tdTomato, respectively. Both tracing lines displayed an acceleration of chondrogenesis and cell trans-differentiation from chondrocytes into bone cells in the Dmp1 KO. Next, we showed that administrations of neutralizing fibroblast growth factor 23 (FGF23) antibodies in Dmp1-null mice restored hypophosphatemic condylar cartilage phenotypes. In further addressing the rescue mechanism, we generated compound mice containing Col10a1-Cre with ROSA-tdTomato and Dmp1 KO lines with and without a high Pi diet starting at day 10 for 39 days. We demonstrated that hypophosphatemia leads to an acceleration of chondrogenesis and trans-differentiation of chondrocytes to bone cells, which were largely restored under a high Pi diet. Finally, we identified the causative molecule (β-catenin). Together, this study demonstrates that the Dmp1-null caused hypophosphatemia, leading to acceleration (instead of inhibition) of chondrogenesis and bone trans-differentiation from chondrocytes but inhibition of bone cell maturation due to a sharp increase in β-catenin. These findings will aid in the future treatment of hypophosphatemic rickets with FGF23 neutralizing antibodies.

Recent Insights into Long Bone Development: Central Role of Hedgehog Signaling Pathway in Regulating Growth Plate

The longitudinal growth of long bone, regulated by an epiphyseal cartilaginous component known as the “growth plate”, is generated by epiphyseal chondrocytes. The growth plate provides a continuous supply of chondrocytes for endochondral ossification, a sequential bone replacement of cartilaginous tissue, and any failure in this process causes a wide range of skeletal disorders. Therefore, the cellular and molecular characteristics of the growth plate are of interest to many researchers. Hedgehog (Hh), well known as a mitogen and morphogen during development, is one of the best known regulatory signals in the developmental regulation of the growth plate. Numerous animal studies have revealed that signaling through the Hh pathway plays multiple roles in regulating the proliferation, differentiation, and maintenance of growth plate chondrocytes throughout the skeletal growth period. Furthermore, over the past few years, a growing body of evidence has emerged demonstrating that a limited number of growth plate chondrocytes transdifferentiate directly into the full osteogenic and multiple mesenchymal lineages during postnatal bone development and reside in the bone marrow until late adulthood. Current studies with the genetic fate mapping approach have shown that the commitment of growth plate chondrocytes into the skeletal lineage occurs under the influence of epiphyseal chondrocyte-derived Hh signals during endochondral bone formation. Here, we discuss the valuable observations on the role of the Hh signaling pathway in the growth plate based on mouse genetic studies, with some emphasis on recent advances.

The extended chondrocyte lineage: implications for skeletal homeostasis and disorders

Endochondral bone formation relies on a finely controlled sequence of chondrocyte proliferation, maturation and hypertrophy that establishes the growth plate which, combined with the deposition of bone upon the cartilage template, mediates longitudinal skeletal growth. Recent lineage studies support a chondrocyte-osteoblast differentiation continuum and the presence of skeletal stem cells within cartilage. The biological significance of the lineage extension and the mechanisms controlling the process are unclear. In this review, we describe recent work on the extended chondrocyte-osteoblast lineage and its contribution to the development, growth and repair of bone and to bone disorders that provides insight into the process and the molecular controls involved. The implications for skeletal homeostasis are discussed.

Fine-Tunable and Injectable 3D Hydrogel for On-Demand Stem Cell Niche

Stem-cell-based tissue engineering requires increased stem cell retention, viability, and control of differentiation. The use of biocompatible scaffolds encapsulating stem cells typically addresses the first two problems. To achieve control of stem cell fate, fine-tuned biocompatible scaffolds with bioactive molecules are necessary. However, given that the fine-tuning of stem cell scaffolds is associated with UV irradiation and in situ scaffold gelation, this process is in conflict with injectability. Herein, a fine-tunable and injectable 3D hydrogel system is developed with the use of thermosensitive poly(organophosphazene) bearing β-cyclodextrin (β-CD PPZ) and two types of adamantane-peptides (Ad-peptides) that are associated with mesenchymal stem cell (MSC) differentiation and that serve as stoichiometrically controlled pendants for fine-tuning. Given that complexation of hosts and guests subject to strict stoichiometric control is achieved with simple mixing, these fabricated hydrogels exhibit well-aligned, fine-tuning responses, even in living animals. Injection of MSCs in fine-tuned hydrogels also results in various chondrogenic differentiation levels at three weeks postinjection. This is attributed to the differential controls of Ad-peptides, if MSC preconditioning is excluded. Eventually, the fine-tunable and injectable 3D hydrogel could be applied as platform technology by simply switching the types of peptides bearing adamantane and their stoichiometry.

3D hydrogel stem cell niche controlled by host-guest interaction affects stem cell fate and survival rate

Host-guest interaction using β-cyclodextrin (β-CD) and adamantane (Ad) allows facile modulation of guest molecule concentration in 3D hydrogels. Based on this phenomenon, we prepared a thermosensitive poly(organophosphazene) bearing β-CD hydrogel (β-CD PPZ, as host) and Ad-Arg-Gly-Asp (Ad-RGD, as guest). The structures of synthesized thermosensitive β-CD PPZ and Ad-RGD were confirmed by ¹H NMR and FT-IR. The β-CD PPZ/Ad-RGD mixture was prepared by simple mixing and elicited thermosensitive properties with the formation of gelation in all Ad-RGDs mixing proportions at the body temperature. Strong and controlled host-guest interactions between β-CD PPZ and Ad-RGD were observed in 2D-NOESY, DLS, and TEM. Regulated MSC behaviors were elicited based on the use of controlled Ad-RGD amounts at the level of in vitro and in vivo. As the amount of Ad-RGD was increased in the β-CD PPZ hydrogel, MSC survival rate was enhanced and was prone to express osteogenic factors. While Ad-RGD is absent or low in hydrogel, relatively poor MSC survival rate and adipogenesis were exhibited. Altogether, we verified that survival rate and differentiation of MSCs could be controlled by host-guest interaction system with thermosensitive 3D hydrogel. This proposed 3D hydrogel controlling system with host-guest interaction is expected to be a platform technology as changing guest molecules.

DMP1 Ablation in the Rabbit Results in Mineralization Defects and Abnormalities in Haversian Canal/Osteon Microarchitecture

DMP1 (dentin matrix protein 1) is an extracellular matrix protein highly expressed in bones. Studies of Dmp1 knockout (KO) mice led to the discovery of a rare autosomal recessive form of hypophosphatemic rickets (ARHR) caused by DMP1 mutations. However, there are limitations for using this mouse model to study ARHR, including a lack of Haversian canals and osteons (that occurs only in large mammalian bones), high levels of fibroblast growth factor 23 (FGF23), and PTH, in comparison with a moderate elevation of FGF23 and unchanged PTH in human ARHR patients. To better understand this rare disease, we deleted the DMP1 gene in rabbit using CRISPR/Cas9. This rabbit model recapitulated many features of human ARHR, such as the rachitic rosary (expansion of the anterior rib ends at the costochondral junctions), moderately increased FGF23, and normal PTH levels, as well as severe defects in bone mineralization. Unexpectedly, all DMP1 KO rabbits died by postnatal week 8. They developed a severe bone microarchitecture defect: a major increase in the central canal areas of osteons, concurrent with massive accumulation of osteoid throughout all bone matrix (a defect in mineralization), suggesting a new paradigm, where rickets is caused by a combination of a defect in bone microarchitecture and a failure in mineralization. Furthermore, a study of DMP1 KO bones found accelerated chondrogenesis, whereas ARHR has commonly been thought to be involved in reduced chondrogenesis. Our findings with newly developed DMP1 KO rabbits suggest a revised understanding of the mechanism underlying ARHR. © 2019 American Society for Bone and Mineral Research.

A Second Career for Chondrocytes-Transformation into Osteoblasts

PURPOSE OF REVIEW

The goal of the review is to summarize the current knowledge on the process of chondrocyte-to-osteoblast transdifferentiation during endochondral bone formation and its potential implications in fracture healing and disease.

RECENT FINDINGS

Lineage tracing experiments confirmed the transdifferentiation of chondrocytes into osteoblasts. More recent studies lead to the discovery of molecules involved in this process, as well as to the hypothesis that these cells may re-enter a stem cell-like phase prior to their osteoblastic differentiation. This review recapitulates the current knowledge regarding chondrocyte transdifferentiating into osteoblasts, the developmental and postnatal events where transdifferentiation appears to be relevant, and the molecules implicated in this process.

The Emerging Role of Glucose Metabolism in Cartilage Development

PURPOSE OF REVIEW

Proper cartilage development is critical to bone formation during endochondral ossification. This review highlights the current understanding of various aspects of glucose metabolism in chondrocytes during cartilage development.

RECENT FINDINGS

Recent studies indicate that chondrocytes transdifferentiate into osteoblasts and bone marrow stromal cells during endochondral ossification. In cartilage development, signaling molecules, including IGF2 and BMP2, tightly control glucose uptake and utilization in a stage-specific manner. Perturbation of glucose metabolism alters the course of chondrocyte maturation, suggesting a key role for glucose metabolism during endochondral ossification. During prenatal and postnatal growth, chondrocytes experience bursts of nutrient availability and energy expenditure, which demand sophisticated control of the glucose-dependent processes of cartilage matrix production, cell proliferation, and hypertrophy. Investigating the regulation of glucose metabolism may therefore lead to a unifying mechanism for signaling events in cartilage development and provide insight into causes of skeletal growth abnormalities.

Strong binding active constituents of phytochemical to BMPR1A promote bone regeneration: In vitro, in silico docking, and in vivo studies

Two of the most problematic orthopedic and neurosurgeon visits are associated with spine and craniofacial fractures. Therefore, more attention needs to be paid to finding a medicine to repair these fractures. Amongst the most mysterious herbs, Aloe vera stands out. In the present study, the ameliorating function of A. vera on osteogenesis was studied in vitro and in vivo. Osteoblast-like cells were exposed to A. vera, followed by analysis of cell viability, lactate dehydrogenase release, and intracellular reactive oxygen species (ROS) production. The results showed an enhanced cell biocompatibility in a dose-dependent manner due to attenuated intracellular ROS production. Furthermore, a docking study indicated that the strong affinity of A. vera constituents to type I bone morphogenic protein receptor (BMPR1A) without the involvement of the BMPR1A chain B. The induction of osteogenesis prompts extracellular calcium deposition by osteoblasts, which affirms successful in vitro bone regeneration. However, injection of A. vera in rats with critical size calvarial defects induced Runx2, alkaline phosphatase (ALP), OCN, and BMP2 genes overexpression, which led to the formation of victorious bone with enhanced bone density and ALP activity. It is worthy to note that Aloin has the highest affinity to BMPR1A, whereas there are no reports regarding the impact of Aloenin, Aloesin, and γ-sitosterol on osteogenesis. Furthermore, some of them have antitumor potency, and it might be proposed that they are considered as a bone substitute in the osteotomy site of osteosarcoma with the aim of bone recovery and suppression of osteosarcoma. The whole consequences of this investigation manifests the plausibility of using A. vera as an antioxidant and osteoconductive substitute.

Quantitative trait loci for morphometric and mineral composition traits of the tibia bone in a broiler × layer cross

Many economic losses occur in the poultry industry due to leg fragility. Knowing the genomic regions that influence traits associated with the growth and composition of the leg's bone can help to improve the selection process leading to increased leg resistance to fracture. The present study aimed to map quantitative trait loci (QTL) for mineral composition and morphometric traits of the tibia in 478 animals from an F2 broiler × layer cross. The measurement of weight, length and width of Tibia was carried out at 42 days of age. Ash, dry matter, levels of calcium (Ca), phosphorus (P), magnesium (Mg), Zinc (Zn) and Calcium:Phosphorus (Ca:P) ratio were also recorded. The population was genotyped for 128 microsatellite markers and one single nucleotide polymorphism, covering 2630 cM of the chicken genome. A likelihood ratio test was performed to find QTLs. Additive and dominance effects of the QTLs were included in the model. In the chromosomes 2 (GGA2), 6 (GGA6), 8 (GGA8), 24 (GGA24) and 26 (GGA26) some suggestive QTLs (P&lt;0.00276) were mapped for tibia weight (GGA2 and GGA26), ash percentage (GGA2 and GGA6), dry matter percentage (GGA2), Ca (GGA8 and GGA24) and Ca:P ratio (GGA8), many of which are close to genes already identified as good candidates for those traits. The suggestive QTL on GGA2 has a pleiotropic effect on ash percentage, dry matter and bone weight, whereas in the GGA8 there seems to be two QTLs, one for Ca and another for Ca:P ratio. Thus, this study identified at least five genomic regions, in different chromosomes, that can be targeted for further research to identify potential mutations influencing the development and composition of leg bones in Gallus gallus.

Targeting of chondrocyte plasticity via connexin43 modulation attenuates cellular senescence and fosters a pro-regenerative environment in osteoarthritis

Osteoarthritis (OA), a chronic disease characterized by articular cartilage degeneration, is a leading cause of disability and pain worldwide. In OA, chondrocytes in cartilage undergo phenotypic changes and senescence, restricting cartilage regeneration and favouring disease progression. Similar to other wound-healing disorders, chondrocytes from OA patients show a chronic increase in the gap junction channel protein connexin43 (Cx43), which regulates signal transduction through the exchange of elements or recruitment/release of signalling factors. Although immature or stem-like cells are present in cartilage from OA patients, their origin and role in disease progression are unknown. In this study, we found that Cx43 acts as a positive regulator of chondrocyte-mesenchymal transition. Overactive Cx43 largely maintains the immature phenotype by increasing nuclear translocation of Twist-1 and tissue remodelling and proinflammatory agents, such as MMPs and IL-1β, which in turn cause cellular senescence through upregulation of p53, p16^INK4a and NF-κB, contributing to the senescence-associated secretory phenotype (SASP). Downregulation of either Cx43 by CRISPR/Cas9 or Cx43-mediated gap junctional intercellular communication (GJIC) by carbenoxolone treatment triggered rediferentiation of osteoarthritic chondrocytes into a more differentiated state, associated with decreased synthesis of MMPs and proinflammatory factors, and reduced senescence. We have identified causal Cx43-sensitive circuit in chondrocytes that regulates dedifferentiation, redifferentiation and senescence. We propose that chondrocytes undergo chondrocyte-mesenchymal transition where increased Cx43-mediated GJIC during OA facilitates Twist-1 nuclear translocation as a novel mechanism involved in OA progression. These findings support the use of Cx43 as an appropriate therapeutic target to halt OA progression and to promote cartilage regeneration.

The Effects of Indian Hedgehog Deletion on Mesenchyme Cells: Inducing Intermediate Cartilage Scaffold Ossification to Cause Growth Plate and Phalange Joint Absence, Short Limb, and Dwarfish Phenotypes

The endochondral ossification plays a critical role in vertebrate limb development and skeletal homeostasis, where limb mesenchyme cells form an intermediate cartilage scaffold that develops into growth plates and then replaced by bone. Although Indian hedgehog (Ihh) is known to control the hypertrophic differentiation process of chondrocytes, its role from the mesenchyme cells to the early stages of chondrogenesis is unclear. To define the function of Ihh in the mesenchymal cell's early stages of chondrogenesis, we specifically delete Ihh in Prx1-expressed mesenchyme cells at E9.5 using Prx1-Cre;Ihh^fl/fl;Rosa26^-ZsGreen1 mice. We found that deleting Ihh in the mesenchyme cells results in an early and quick ossification of the intermediate cartilage scaffold, causing the growth plate and phalange joint absence, short limbs, and dwarfishness. The green fluorescent protein (GFP)-positive cells derived from deleted Ihh mesenchyme cells overlap with von Kossa- and osteocalcin-positive staining area. These deleted Ihh/GFP-positive cells isolated from Prx1-Cre;Ihh^fl/fl;Rosa26^-ZsGreen1 newborn mice had osteogenic differentiation by showing a positive Alizarin red and von Kossa staining, as well as an enhanced Col1a1, osteocalcin, and Runx2 expression. Our findings demonstrate that deleting Ihh in mesenchyme cells during early limb development promotes intermediate cartilage scaffold ossification, which prevents growth plate formation that causes phalange joint absence, short limb, and dwarfish phenotype.