Dual pathways to endochondral osteoblasts: a novel chondrocyte-derived osteoprogenitor cell identified in hypertrophic cartilage

Transition of endochondral bone formation at the normal and botulinum-treated mandibular condyle of growing juvenile rat

OBJECTIVE

The aim of this study was to understand the temporal and spatial distribution of canonical endochondral ossification (CEO) and non-canonical endochondral ossification (NCEO) of the normal growing rat condyle, and to evaluate their histomorphological changes following the simultaneous hypotrophy of the unilateral masticatory closing muscles with botulinum toxin (BTX).

DESIGN

46 rats at postnatal 4 weeks were used for the experiment and euthanized at postnatal 4, 8, and 16 weeks. The right masticatory muscles of rats in experimental group were injected with BTX, the left being injected with saline as a control. The samples were evaluated using 3D morphometric, histological, and immunohistochemical analysis with three-dimensional regional mapping of endochondral ossifications.

RESULTS

The results showed that condylar endochondral ossification changed from CEO to NCEO at the main articulating surface during the experimental period and that the BTX-treated condyle presented a retroclined smaller condyle with an anteriorly-shifted narrower articulating surface. This articulating region showed a thinner layer of the endochondral cells, and a compact distribution of flattened cells. These were related to the load concentration, decreased cellular proliferation with thin cellular layers, reduced extracellular matrix, increased cellular differentiation toward the osteoblastic bone formation, and accelerated transition of the ossification types from CEO to NCEO.

CONCLUSION

The results suggest that endochondral ossification under loading tended to show more NCEO, and that masticatory muscular hypofunction by BTX had deleterious effects on endochondral bone formation and changed the condylar growth vector, resulting in a retroclined, smaller, asymmetrical, and deformed condyle with thin cartilage.

Skeletal stem and progenitor cells in bone development and repair

Bone development, growth, and repair are complex processes involving various cell types and interactions, with central roles played by skeletal stem and progenitor cells. Recent research brought new insights into the skeletal precursor populations that mediate intramembranous and endochondral bone development. Later in life, many of the cellular and molecular mechanisms determining development are reactivated upon fracture, with powerful trauma-induced signaling cues triggering a variety of postnatal skeletal stem/progenitor cells (SSPCs) residing near the bone defect. Interestingly, in this injury context, the current evidence suggests that the fates of both SSPCs and differentiated skeletal cells can be considerably flexible and dynamic, and that multiple cell sources can be activated to operate as functional progenitors generating chondrocytes and/or osteoblasts. The combined implementation of in vivo lineage tracing, cell surface marker-based cell selection, single-cell molecular analyses, and high-resolution in situ imaging has strongly improved our insights into the diversity and roles of developmental and reparative stem/progenitor subsets, while also unveiling the complexity of their dynamics, hierarchies, and relationships. Albeit incompletely understood at present, findings supporting lineage flexibility and possibly plasticity among sources of osteogenic cells challenge the classical dogma of a single primitive, self-renewing, multipotent stem cell driving bone tissue formation and regeneration from the apex of a hierarchical and strictly unidirectional differentiation tree. We here review the state of the field and the newest discoveries in the origin, identity, and fates of skeletal progenitor cells during bone development and growth, discuss the contributions of adult SSPC populations to fracture repair, and reflect on the dynamism and relationships among skeletal precursors and differentiated cell lineages. Further research directed at unraveling the heterogeneity and capacities of SSPCs, as well as the regulatory cues determining their fate and functioning, will offer vital new options for clinical translation toward compromised fracture healing and bone regenerative medicine.

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.

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.

New Genetic Variants of RUNX2 in Mexican Families Cause Cleidocranial Dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant skeletal dysplasia characterized by persistent open skull sutures with bulging calvaria, hypoplasia, or aplasia of clavicles permitting abnormal opposition of the shoulders; wide public symphysis; short middle phalanx of the fifth fingers; and vertebral, craniofacial, and dental anomalies. It is a rare disease, with a prevalence of 1-9/1,000,000, high penetrance, and variable expression. The gene responsible for CCD is the Runt-related transcription factor 2 (RUNX2) gene. We characterize the clinical, genetic, and bioinformatic results of four CCD cases: two cases within Mexican families with six affected members, nine asymptomatic individuals, and two sporadic cases with CCD, with one hundred healthy controls. Genomic DNA analyses of the RUNX2 gene were performed for Sanger sequencing. Bioinformatics tools were used to predict the function, stability, and structural changes of the mutated RUNX2 proteins. Three novel heterozygous mutations (c.651_652delTA; c.538_539delinsCA; c.662T>A) and a previously reported mutation (c.674G>A) were detected. In silico analysis showed that all mutations had functional, stability-related, and structural alterations in the RUNX2 protein. Our results show novel mutations that enrich the pool of RUNX2 gene mutations with CCD. Moreover, the proband 1 presented clinical data not previously reported that could represent an expanded phenotype of severe expression.

Identification of the metaphyseal skeletal stem cell building trabecular bone

Skeletal stem cells (SSCs) that are capable of self-renewal and multipotent differentiation contribute to bone development and homeostasis. Several populations of SSCs at different skeletal sites have been reported. Here, we identify a metaphyseal SSC (mpSSC) population whose transcriptional landscape is distinct from other bone mesenchymal stromal cells (BMSCs). These mpSSCs are marked by Sstr2 or Pdgfrb+Kitl-, located just underneath the growth plate, and exclusively derived from hypertrophic chondrocytes (HCs). These HC-derived mpSSCs have properties of self-renewal and multipotency in vitro and in vivo, producing most HC offspring postnatally. HC-specific deletion of Hgs, a component of the endosomal sorting complex required for transport, impairs the HC-to-mpSSC conversion and compromises trabecular bone formation. Thus, mpSSC is the major source of BMSCs and osteoblasts in bone marrow, supporting the postnatal trabecular bone formation.

A new perspective on intervertebral disc calcification-from bench to bedside

Disc degeneration primarily contributes to chronic low back and neck pain. Consequently, there is an urgent need to understand the spectrum of disc degeneration phenotypes such as fibrosis, ectopic calcification, herniation, or mixed phenotypes. Amongst these phenotypes, disc calcification is the least studied. Ectopic calcification, by definition, is the pathological mineralization of soft tissues, widely studied in the context of conditions that afflict vasculature, skin, and cartilage. Clinically, disc calcification is associated with poor surgical outcomes and back pain refractory to conservative treatment. It is frequently seen as a consequence of disc aging and progressive degeneration but exhibits unique molecular and morphological characteristics: hypertrophic chondrocyte-like cell differentiation; TNAP, ENPP1, and ANK upregulation; cell death; altered Pi and PPi homeostasis; and local inflammation. Recent studies in mouse models have provided a better understanding of the mechanisms underlying this phenotype. It is essential to recognize that the presentation and nature of mineralization differ between AF, NP, and EP compartments. Moreover, the combination of anatomic location, genetics, and environmental stressors, such as aging or trauma, govern the predisposition to calcification. Lastly, the systemic regulation of calcium and Pi metabolism is less important than the local activity of PPi modulated by the ANK-ENPP1 axis, along with disc cell death and differentiation status. While there is limited understanding of this phenotype, understanding the molecular pathways governing local intervertebral disc calcification may lead to developing disease-modifying drugs and better clinical management of degeneration-related pathologies.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

MicroRNA-138: an emerging regulator of skeletal development, homeostasis, and disease

Noncoding microRNAs are powerful epigenetic regulators of cellular processes by their ability to target and suppress expression of numerous protein-coding mRNAs. This multitargeting function is a unique and complex feature of microRNAs. It is now well-described that microRNAs play important roles in regulating the development and homeostasis of many cell/tissue types, including those that make up the skeletal system. In this review, we focus on microRNA-138 (miR-138) and its effects on regulating bone and cartilage cell differentiation and function. In addition to its reported role as a tumor suppressor, miR-138 appears to function as an inhibitor of osteoblast differentiation. This review provides additional information on studies that have attempted to alter miR-138 expression in vivo as a means to dampen ectopic calcification or alter bone mass. However, a review of the published literature on miR-138 in cartilage reveals a number of contradictory and inconclusive findings with respect to regulating chondrogenesis and chondrocyte catabolism. This highlights the need for more research in understanding the role of miR-138 in cartilage biology and disease. Interestingly, a number of studies in other systems have reported miR-138-mediated effects in dampening inflammation and pain responses. Future studies will reveal if a multifunctional role of miR-138 involving suppression of ectopic bone, inflammation, and pain will be beneficial in skeletal conditions such as osteoarthritis and heterotopic ossification.

Phosphate-induced activation of VEGFR2 leads to caspase-9-mediated apoptosis of hypertrophic chondrocytes

Low circulating phosphate (Pi) leads to rickets, characterized by expansion of the hypertrophic chondrocytes (HCs) in the growth plate due to impaired HC apoptosis. Studies in HCs demonstrate that Pi activates the Raf/MEK/ERK1/2 and mitochondrial apoptotic pathways. To determine how Pi activates these pathways, a small-molecule screen was undertaken to identify inhibitors of Pi-induced ERK1/2 phosphorylation in HCs. Vascular endothelial growth factor receptor 2 (VEGFR2) was identified as a target. In vitro studies in HCs demonstrate that VEGFR2 inhibitors block Pi-induced pERK1/2 and caspase-9 cleavage. Like Pi, rhVEGF activates ERK1/2 and caspase-9 in HCs and induces phosphorylation of VEGFR2, confirming that Pi activates this signaling pathway in HCs. Chondrocyte-specific depletion of VEGFR2 leads to an increase in HCs, impaired vascular invasion, and a decrease in HC apoptosis. Thus, these studies define a role for VEGFR2 in transducing Pi signals and mediating its effects on growth plate maturation.

Distinct differences between calvarial and long bone osteocytes in cell morphologies, gene expression and aging responses

Osteocytes are the terminally differentiated bone cells resulted from bone formation. Although there are two distinct processes of bone formation, intramembranous and endochondral ossifications contributing to the formation of calvarial and long bones, it is not clear whether the distinct pathways determine the differences between calvaria and femoral cortical bone derived osteocytes. In the present study, we employed confocal structured illumination microscopy and mRNA-sequencing analysis to characterize the morphologic and transcriptomic expression of osteocytes from murine calvaria and mid-shaft femoral cortical bone. Structured illumination microscopy and geometric modelling showed round shaped and irregularly scattered calvarial osteocytes compared to spindle shaped and orderly arrayed cortical osteocytes. mRNA-sequencing analysis indicated different transcriptomic profiles between calvarial and cortical osteocytes and provided evidence that mechanical response of osteocytes may contribute to geometrical differences. Furthermore, transcriptomic analysis showed that these two groups of osteocytes come from distinct pathways with 121 ossification-related genes differentially expressed. Analysis of correlation between ossification and osteocyte geometries via a Venn diagram showed that several genes related to ossification, cytoskeleton organization and dendrite development were differentially expressed between calvarial and cortical osteocytes. Finally, we demonstrated that aging disrupted the organization of dendrites and cortical osteocytes but had no significant effects on calvarial osteocytes. Together, we conclude that calvarial and cortical osteocytes are different in various aspects, which is probably the consequence of their distinct pathways of ossification.

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.

Identification of distinct subpopulations of Gli1-lineage cells in the mouse mandible

The Hedgehog pathway gene Gli1 has been proposed to mark a subpopulation of skeletal stem cells (SSCs) in craniofacial bone. Skeletal stem cells (SSCs) are multi-potent cells crucial for the development and homeostasis of bone. Recent studies on long bones have suggested that skeletal stem cells in endochondral or intramembranous ossification sites have different differentiation capacities. However, this has not been well-defined in neural crest derived bones. Generally, the long bones are derived from mesoderm and follow an endochondral ossification model, while most of the cranial bones are neural crest (NC) in origin and follow an intramembranous ossification model. The mandible is unique: It is derived from the neural crest lineage but makes use of both modes of ossification. Early in fetal development, the mandibular body is generated by intramembranous ossification with subsequent endochondral ossification forming the condyle. The identities and properties for SSCs in these two sites remain unknown. Here, we use genetic lineage tracing in mouse to identify cells expressing the Hedgehog responsive gene Gli1, which is thought to mark the tissue resident SSCs. We track the Gli1+ cells, comparing cells within the perichondrium to those in the periosteum covering the mandibular body. In juvenile mice, these have distinct differentiation and proliferative potential. We also assess the presence of Sox10+ cells, thought to mark neural crest stem cells, but find no substantial population associated with the mandibular skeleton, suggesting that Sox10+ cells have limited contribution to maintaining postnatal mandibular bone. All together, our study indicates that the Gli1+ cells display distinct and limited differentiation capacity dependent on their regional associations.

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.

How might we build limbs in vitro informed by the modular aspects and tissue-dependency in limb development?

Building limb morphogenesis in vitro would substantially open up avenues for research and applications of appendage development. Recently, advances in stem cell engineering to differentiate desired cell types and produce multicellular structures in vitro have enabled the derivation of limb-like tissues from pluripotent stem cells. However, in vitro recapitulation of limb morphogenesis is yet to be achieved. To formulate a method of building limbs in vitro, it is critically important to understand developmental mechanisms, especially the modularity and the dependency of limb development on the external tissues, as those would help us to postulate what can be self-organized and what needs to be externally manipulated when reconstructing limb development in vitro. Although limbs are formed on the designated limb field on the flank of embryo in the normal developmental context, limbs can also be regenerated on the amputated stump in some animals and experimentally induced at ectopic locations, which highlights the modular aspects of limb morphogenesis. The forelimb-hindlimb identity and the dorsal-ventral, proximal-distal, and anterior-posterior axes are initially instructed by the body axis of the embryo, and maintained in the limb domain once established. In contrast, the aspects of dependency on the external tissues are especially underscored by the contribution of incoming tissues, such as muscles, blood vessels, and peripheral nerves, to developing limbs. Together, those developmental mechanisms explain how limb-like tissues could be derived from pluripotent stem cells. Prospectively, the higher complexity of limb morphologies is expected to be recapitulated by introducing the morphogen gradient and the incoming tissues in the culture environment. Those technological developments would dramatically enhance experimental accessibility and manipulability for elucidating the mechanisms of limb morphogenesis and interspecies differences. Furthermore, if human limb development can be modeled, drug development would be benefited by in vitro assessment of prenatal toxicity on congenital limb deficiencies. Ultimately, we might even create a future in which the lost appendage would be recovered by transplanting artificially grown human limbs.

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.

Constitutive and conditional gene knockout mice for the study of intervertebral disc degeneration: Current status, decision considerations, and future possibilities

There have been an increasing number of patients with degenerative disc diseases due to the aging population. In light of this, studies on the pathogenesis of intervertebral disc degeneration have become a hot topic, and gene knockout mice have become a valuable tool in this field of research. With the development of science and technology, constitutive gene knockout mice can be constructed using homologous recombination, zinc finger nuclease, transcription activator-like effector nuclease technology and clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9) system, and conditional gene knockout mice can be constructed using the Cre/LoxP system. The gene-edited mice using these techniques have been widely used in the studies on disc degeneration. This paper reviews the development process and principles of these technologies, functions of the edited genes in disc degeneration, advantages, and disadvantages of different methods and possible targets of the specific Cre recombinase in intervertebral discs. Recommendations for the choice of suitable gene-edited model mice are presented. At the same time, possible technological improvements in the future are also discussed.

Look who's TORking: mTOR-mediated integration of cell status and external signals during limb development and endochondral bone growth

The balance of cell proliferation and size is key for the control of organ development and repair. Moreover, this balance has to be coordinated within tissues and between tissues to achieve robustness in the organ's pattern and size. The tetrapod limb has been used to study these topics during development and repair, and several conserved pathways have emerged. Among them, mechanistic target of rapamycin (mTOR) signaling, despite being active in several cell types and developmental stages, is one of the least understood in limb development, perhaps because of its multiple potential roles and interactions with other pathways. In the body of this review, we have collated and integrated what is known about the role of mTOR signaling in three aspects of tetrapod limb development: 1) limb outgrowth; 2) chondrocyte differentiation after mesenchymal condensation and 3) endochondral ossification-driven longitudinal bone growth. We conclude that, given its ability to interact with the most common signaling pathways, its presence in multiple cell types, and its ability to influence cell proliferation, size and differentiation, the mTOR pathway is a critical integrator of external stimuli and internal status, coordinating developmental transitions as complex as those taking place during limb development. This suggests that the study of the signaling pathways and transcription factors involved in limb patterning, morphogenesis and growth could benefit from probing the interaction of these pathways with mTOR components.

A Shifting Paradigm: Transformation of Cartilage to Bone during Bone Repair

While formation and regeneration of the skeleton have been studied for a long period of time, significant scientific advances in this field continue to emerge based on an unmet clinical need to improve options to promote bone repair. In this review, we discuss the relationship between mechanisms of bone formation and bone regeneration. Data clearly show that regeneration is not simply a reinduction of the molecular and cellular programs that were used for development. Instead, the mechanical environment exerts a strong influence on the mode of repair, while during development, cell-intrinsic processes drive the mode of skeletal formation. A major advance in the field has shown that cell fate is flexible, rather than terminal, and that chondrocytes are able to differentiate into osteoblasts and other cell types during development and regeneration. This is discussed in a larger context of regeneration in vertebrates as well as the clinical implication that this shift in understanding presents.

AcanCreER lacks specificity to chondrocytes and targets periosteal progenitors in the fractured callus

Aggrecan (Acan) is a large proteoglycan molecule constituting the extracellular matrix of cartilage, secreted by chondrocytes. To specifically target the chondrocyte lineage, researchers have widely used the AcanCreER mouse model. Evaluation of specificity and efficiency of recombination, requires Cre animals to be crossed with reporter mice. In order to accurately interpret data from Cre models, it is imperative to consider A) the amount of recombination occurring in cells/tissues that are not intended for targeting (i.e., non-specific expression), B) the efficiency of Cre recombination, which can depend on dose and duration of tamoxifen treatment, and C) the activation of CreER without tamoxifen induction, known as "Cre leakage." Using a highly sensitive reporter mouse (Ai9, tdTomato), we performed a comprehensive analysis of the AcanCreER system. Surprisingly, we observed expression in cells within the periosteum. These cells expand at a stage when chondrocytes are not yet present within the forming callus tissue (Acan/Ai9+ cells). In pulse-chase experiments, we confirmed that fibroblastic Acan/Ai9+ cells within the periosteum can directly give rise to osteoblasts. Our results show that Acan/Ai9+ is not specific for the chondrocyte lineage in the fracture callus or with the tibial holes. The expression of AcanCreER in periosteal progenitor cells complicates the interpretation of studies evaluating the transition of chondrocytes to osteoblasts (termed transdifferentiation). Awareness of these issues and the limitations of the system will lead to better data interpretation.

The role of hypertrophic chondrocytes in regulation of the cartilage-to-bone transition in fracture healing

Endochondral bone formation is an important pathway in fracture healing, involving the formation of a cartilaginous soft callus and the process of cartilage-to-bone transition. Failure or delay in the cartilage-to-bone transition causes an impaired bony union such as nonunion or delayed union. During the healing process, multiple types of cells including chondrocytes, osteoprogenitors, osteoblasts, and endothelial cells coexist in the callus, and inevitably crosstalk with each other. Hypertrophic chondrocytes located between soft cartilaginous callus and bony hard callus mediate the crosstalk regulating cell-matrix degradation, vascularization, osteoclast recruitment, and osteoblast differentiation in autocrine and paracrine manners. Furthermore, hypertrophic chondrocytes can become osteoprogenitors and osteoblasts, and directly contribute to woven bone formation. In this review, we focus on the roles of hypertrophic chondrocytes in fracture healing and dissect the intermingled crosstalk in fracture callus during the cartilage-to-bone transition.

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.

Heterozygous pathogenic variants involving CBFB cause a new skeletal disorder resembling cleidocranial dysplasia

BACKGROUND

Cleidocranial dysplasia (CCD) is a rare skeletal dysplasia with significant clinical variability. Patients with CCD typically present with delayed closure of fontanels and cranial sutures, dental anomalies, clavicular hypoplasia or aplasia and short stature. Runt-related transcription factor 2 (RUNX2) is currently the only known disease-causing gene for CCD, but several studies have suggested locus heterogeneity.

METHODS

The cohort consists of eight subjects from five unrelated families partially identified through GeneMatcher. Exome or genome sequencing was applied and in two subjects the effect of the variant was investigated at RNA level.

RESULTS

In each subject a heterozygous pathogenic variant in CBFB was detected, whereas no genomic alteration involving RUNX2 was found. Three CBFB variants (one splice site alteration, one nonsense variant, one 2 bp duplication) were shown to result in a premature stop codon. A large intragenic deletion was found to delete exon 4, without affecting CBFB expression. The effect of a second splice site variant could not be determined but most likely results in a shortened or absent protein. Affected individuals showed similarities with RUNX2-related CCD, including dental and clavicular abnormalities. Normal stature and neurocognitive problems were however distinguishing features. CBFB encodes the core-binding factor β subunit, which can interact with all RUNX proteins (RUNX1, RUNX2, RUNX3) to form heterodimeric transcription factors. This may explain the phenotypic differences between CBFB-related and RUNX2-related CCD.

CONCLUSION

We confirm the previously suggested locus heterogeneity for CCD by identifying five pathogenic variants in CBFB in a cohort of eight individuals with clinical and radiographic features reminiscent of CCD.

Mesenchymal cell-derived Wnt1 signaling regulates subchondral bone remodeling but has no effects on the development of growth plate or articular cartilage in mice

Chondrocyte differentiation is a principal progress in endochondral ossification and in the formation of secondary ossification center (SOC) during the long bone development. We have previously reported that targeted deletion of Wnt1 in mesenchymal progenitors (Wnt1_Prrx^-/-) leads to spontaneous fractures and severe osteopenia in mouse long bones, suggesting that Wnt1 is a key regulator of bone metabolism. However, the effect of Wnt1 on the regulation of cartilage development and chondrocyte differentiation remained unknown. In this study, WNT1 protein expression was observed in lateral superficial cartilage and growth plate pre-hypertrophic chondrocytes in mice. Wnt1 mRNA expression was detected in epiphyseal cartilage from E16.5 to 3 month-old mice. Detailed histological analyses revealed that the average thickness and chondrocyte density of proximal tibial articular cartilage and growth plate were unchanged between Wnt1_Prrx^-/- and control mice. However, μCT analysis of tibial epiphyses showed that the subchondral bone mass was reduced in Wnt1_Prrx^-/- mice compared to control mice, as demonstrated by decreased bone volume, trabecular number, trabecular thickness, and increased trabecular separation in Wnt1_Prrx^-/- mice. Mechanistically, histomorphometric analyses showed that the reduced subchondral bone mass in Wnt1_Prrx^-/- mice was due to impaired bone formation and enhanced bone resorption. In vitro, exogenous Wnt1 inhibited chondrogenesis and chondrocyte hypertrophy in both cell autonomous and juxtacrine manners, while matrix mineralization and the expression of Mmp13, Mmp9 and Opn were induced in a juxtacrine manner. Taken together, mesenchymal cell-derived Wnt1 is an important regulator of subchondral bone remodeling, although it has no effect on the regulation of growth plate or articular cartilage.

Growing Pains: The Need for Engineered Platforms to Study Growth Plate Biology

Growth plates, or physis, are highly specialized cartilage tissues responsible for longitudinal bone growth in children and adolescents. Chondrocytes that reside in growth plates are organized into three distinct zones essential for proper function. Modeling key features of growth plates may provide an avenue to develop advanced tissue engineering strategies and perspectives for cartilage and bone regenerative medicine applications and a platform to study processes linked to disease progression. In this review, a brief introduction of the growth plates and their role in skeletal development is first provided. Injuries and diseases of the growth plates as well as physiological and pathological mechanisms associated with remodeling and disease progression are discussed. Growth plate biology, namely, its architecture and extracellular matrix organization, resident cell types, and growth factor signaling are then focused. Next, opportunities and challenges for developing 3D biomaterial models to study aspects of growth plate biology and disease in vitro are discussed. Finally, opportunities for increasingly sophisticated in vitro biomaterial models of the growth plate to study spatiotemporal aspects of growth plate remodeling, to investigate multicellular signaling underlying growth plate biology, and to develop platforms that address key roadblocks to in vivo musculoskeletal tissue engineering applications are described.

Skeletal stem cells: a game changer of skeletal biology and regenerative medicine?

Skeletal stem cells (SSCs) were originally discovered in the bone marrow stroma. They are capable of self-renewal and multilineage differentiation into osteoblasts, chondrocytes, adipocytes, and stromal cells. Importantly, these bone marrow SSCs localize in the perivascular region and highly express hematopoietic growth factors to create the hematopoietic stem cell (HSC) niche. Thus, bone marrow SSCs play pivotal roles in orchestrating osteogenesis and hematopoiesis. Besides the bone marrow, recent studies have uncovered diverse SSC populations in the growth plate, perichondrium, periosteum, and calvarial suture at different developmental stages, which exhibit distinct differentiation potential under homeostatic and stress conditions. Therefore, the current consensus is that a panel of region-specific SSCs collaborate to regulate skeletal development, maintenance, and regeneration. Here, we will summarize recent advances of SSCs in long bones and calvaria, with a special emphasis on the evolving concept and methodology in the field. We will also look into the future of this fascinating research area that may ultimately lead to effective treatment of skeletal disorders.

Engineering bone-forming callus organoid implants in a xenogeneic-free differentiation medium

The field of tissue engineering aspires to provide clinically relevant solutions for patients through the integration of developmental engineering principles with a bottom-up manufacturing approach. However, the manufacturing of cell-based advanced therapy medicinal products is hampered by protocol complexity, lack of non-invasive critical quality controls, and dependency on animal-derived components for tissue differentiation. We investigate a serum-free, chemically defined, xeno- and lipid-free chondrogenic differentiation medium to generate bone-forming callus organoids. Our results show an increase in microtissue homogeneity during prolonged differentiation and the high quality of in vivo bone-forming organoids. The low protein content of the culture medium potentially allows for the monitoring of relevant secreted biomarkers as (critical) quality attributes. Together, we envisage that this xeno- and lipid-free chondrogenic medium is compatible with industrial scale-up and automation while facilitating the implementation of non-invasive imaging and the use of quality control parameters based on secreted biomarkers.

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.

Whole Aspect of Runx2 Functions in Skeletal Development

Runt-related transcription factor 2 (Runx2) is a fundamental transcription factor for bone development. In endochondral ossification, Runx2 induces chondrocyte maturation, enhances chondrocyte proliferation through Indian hedgehog (Ihh) induction, and induces the expression of vascular endothelial growth factor A (Vegfa), secreted phosphoprotein 1 (Spp1), integrin-binding sialoprotein (Ibsp), and matrix metallopeptidase 13 (Mmp13) in the terminal hypertrophic chondrocytes. Runx2 inhibits the apoptosis of the terminal hypertrophic chondrocytes and induces their transdifferentiation into osteoblasts and osteoblast progenitors. The transdifferentiation is required for trabecular bone formation during embryonic and newborn stages but is dispensable for acquiring normal bone mass in young and adult mice. Runx2 enhances the proliferation of osteoblast progenitors and induces their commitment to osteoblast lineage cells through the direct regulation of the expressions of a hedgehog, fibroblast growth factor (Fgf), Wnt, and parathyroid hormone-like hormone (Pthlh) signaling pathway genes and distal-less homeobox 5 (Dlx5), which all regulate Runx2 expression and/or protein activity. Runx2, Sp7, and Wnt signaling further induce osteoblast differentiation. In immature osteoblasts, Runx2 regulates the expression of bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and bone gamma carboxyglutamate protein (Bglap)/Bglap2, and induces osteoblast maturation. Osteocalcin (Bglap/Bglap2) is required for the alignment of apatite crystals parallel to the collagen fibers; however, it does not physiologically work as a hormone that regulates glucose metabolism, testosterone synthesis, or muscle mass. Thus, Runx2 exerts multiple functions essential for skeletal development.

Dissociation of nanosilicates induces downstream endochondral differentiation gene expression program

Bioactive materials harness the body's innate regenerative potential by directing endogenous progenitor cells to facilitate tissue repair. Dissolution products of inorganic biomaterials provide unique biomolecular signaling for tissue-specific differentiation. Inorganic ions (minerals) are vital to biological processes and play crucial roles in regulating gene expression patterns and directing cellular fate. However, mechanisms by which ionic dissolution products affect cellular differentiation are not well characterized. We demonstrate the role of the inorganic biomaterial synthetic two-dimensional nanosilicates and its ionic dissolution products on human mesenchymal stem cell differentiation. We use whole-transcriptome sequencing (RNA-sequencing) to characterize the contribution of nanosilicates and its ionic dissolution products on endochondral differentiation. Our study highlights the modulatory role of ions in stem cell transcriptome dynamics by regulating lineage-specific gene expression patterns. This work paves the way for leveraging biochemical characteristics of inorganic biomaterials to direct cellular processes and promote in situ tissue regeneration.

Bone Mass and Osteoblast Activity Are Sex-Dependent in Mice Lacking the Estrogen Receptor α in Chondrocytes and Osteoblast Progenitor Cells

While estrogen receptor alpha (ERα) is known to be important for bone development and homeostasis, its exact function during osteoblast differentiation remains unclear. Conditional deletion of ERα during specific stages of osteoblast differentiation revealed different bone phenotypes, which were also shown to be sex-dependent. Since hypertrophic chondrocytes can transdifferentiate into osteoblasts and substantially contribute to long-bone development, we aimed to investigate the effects of ERα deletion in both osteoblast and chondrocytes on bone development and structure. Therefore, we generated mice in which the ERα gene was inactivated via a Runx2-driven cyclic recombinase (ERαfl/fl; Runx2Cre). We analyzed the bones of 3-month-old ERαfl/fl; Runx2Cre mice by biomechanical testing, micro-computed tomography, and cellular parameters by histology. Male ERαfl/fl; Runx2Cre mice displayed a significantly increased cortical bone mass and flexural rigidity of the femurs compared to age-matched controls with no active Cre-transgene (ERαfl/fl). By contrast, female ERαfl/fl; Runx2Cre mice exhibited significant trabecular bone loss, whereas in cortical bone periosteal and endosteal diameters were reduced. Our results indicate that the ERα in osteoblast progenitors and hypertrophic chondrocytes differentially contributes to bone mass regulation in male and female mice and improves our understanding of ERα signaling in bone cells in vivo.

Hypertrophic chondrocytes serve as a reservoir for marrow associated skeletal stem and progenitor cells, osteoblasts, and adipocytes during skeletal development

Hypertrophic chondrocytes give rise to osteoblasts during skeletal development; however, the process by which these non-mitotic cells make this transition is not well understood. Prior studies have also suggested that skeletal stem and progenitor cells (SSPCs) localize to the surrounding periosteum and serve as a major source of marrow-associated SSPCs, osteoblasts, osteocytes, and adipocytes during skeletal development. To further understand the cell transition process by which hypertrophic chondrocytes contribute to osteoblasts or other marrow associated cells, we utilized inducible and constitutive hypertrophic chondrocyte lineage tracing and reporter mouse models (Col10a1CreERT2; Rosa26^fs-tdTomato and Col10a1Cre; Rosa26^fs-tdTomato) in combination with a PDGFRa^H2B-GFP transgenic line, single-cell RNA-sequencing, bulk RNA-sequencing, immunofluorescence staining, and cell transplantation assays. Our data demonstrate that hypertrophic chondrocytes undergo a process of dedifferentiation to generate marrow-associated SSPCs that serve as a primary source of osteoblasts during skeletal development. These hypertrophic chondrocyte-derived SSPCs commit to a CXCL12-abundant reticular (CAR) cell phenotype during skeletal development and demonstrate unique abilities to recruit vasculature and promote bone marrow establishment, while also contributing to the adipogenic lineage.

Uncovering pathways regulating chondrogenic differentiation of CHH fibroblasts

Mutations in the non-coding snoRNA component of mitochondrial RNA processing endoribonuclease (RMRP) are the cause of cartilage-hair hypoplasia (CHH). CHH is a rare form of metaphyseal chondrodysplasia characterized by disproportionate short stature and abnormal growth plate development. The process of chondrogenic differentiation within growth plates of long bones is vital for longitudinal bone growth. However, molecular mechanisms behind impaired skeletal development in CHH patients remain unclear. We employed a transdifferentiation model (FDC) combined with whole transcriptome analysis to investigate the chondrogenic transdifferentiation capacity of CHH fibroblasts and to examine pathway regulation in CHH cells during chondrogenic differentiation. We established that the FDC transdifferentiation model is a relevant in vitro model of chondrogenic differentiation, with an emphasis on the terminal differentiation phase, which is crucial for longitudinal bone growth. We demonstrated that CHH fibroblasts are capable of transdifferentiating into chondrocyte-like cells, and show a reduced commitment to terminal differentiation. We also found a number of key factors of BMP, FGF, and IGF-1 signalling axes to be significantly upregulated in CHH cells during the chondrogenic transdifferentiation. Our results support postulated conclusions that RMRP has pleiotropic functions and profoundly affects multiple aspects of cell fate and signalling. Our findings shed light on the consequences of pathological CHH mutations in snoRNA RMRP during chondrogenic differentiation and the relevance and roles of non-coding RNAs in genetic diseases in general.

Zebrafish endochondral growth zones as they relate to human bone size, shape and disease

Research on the genetic mechanisms underlying human skeletal development and disease have largely relied on studies in mice. However, recently the zebrafish has emerged as a popular model for skeletal research. Despite anatomical differences such as a lack of long bones in their limbs and no hematopoietic bone marrow, both the cell types in cartilage and bone as well as the genetic pathways that regulate their development are remarkably conserved between teleost fish and humans. Here we review recent studies that highlight this conservation, focusing specifically on the cartilaginous growth zones (GZs) of endochondral bones. GZs can be unidirectional such as the growth plates (GPs) of long bones in tetrapod limbs or bidirectional, such as in the synchondroses of the mammalian skull base. In addition to endochondral growth, GZs play key roles in cartilage maturation and replacement by bone. Recent studies in zebrafish suggest key roles for cartilage polarity in GZ function, surprisingly early establishment of signaling systems that regulate cartilage during embryonic development, and important roles for cartilage proliferation rather than hypertrophy in bone size. Despite anatomical differences, there are now many zebrafish models for human skeletal disorders including mutations in genes that cause defects in cartilage associated with endochondral GZs. These point to conserved developmental mechanisms, some of which operate both in cranial GZs and limb GPs, as well as others that act earlier or in parallel to known GP regulators. Experimental advantages of zebrafish for genetic screens, high resolution live imaging and drug screens, set the stage for many novel insights into causes and potential therapies for human endochondral bone diseases.

Properties of Cartilage-Subchondral Bone Junctions: A Narrative Review with Specific Focus on the Growth Plate

OBJECTIVE

The purpose of this narrative review is to summarize what is currently known about the structural, chemical, and mechanical properties of cartilage-bone interfaces, which provide tissue integrity across a bimaterial interface of 2 very different structural materials. Maintaining these mechanical interfaces is a key factor for normal bone growth and articular cartilage function and maintenance.

MATERIALS AND METHODS

A comprehensive search was conducted using Google Scholar and PubMed/Medline with a specific focus on the growth plate cartilage-subchondral bone interface. All original articles, reviews in journals, and book chapters were considered. Following a review of the overall structural and functional characteristics of the physis, the literature on histological studies of both articular and growth plate chondro-osseous junctions is briefly reviewed. Next the literature on biochemical properties of these interfaces is reviewed, specifically the literature on elemental analyses across the cartilage-subchondral bone junctions. The literature on biomechanical studies of these junctions at the articular and physeal interfaces is also reviewed and compared.

RESULTS

Unlike the interface between articular cartilage and bone, growth plate cartilage has 2 chondro-osseous junctions. The reserve zone of the mature growth plate is intimately connected to a plate of subchondral bone on the epiphyseal side. This interface resembles that between the subchondral bone and articular cartilage, although much less is known about its makeup and formation.

CONCLUSION

There is a notably paucity of information available on the structural and mechanical properties of reserve zone-subchondral epiphyseal bone interface. This review reveals that further studies are needed on the microstructural and mechanical properties of chondro-osseous junction with the reserve zone.

Do Nonsteroidal Anti-Inflammatory Drugs Have a Deleterious Effect on Cartilage Repair? A Systematic Review

OBJECTIVES

The purpose of this study was to systematically review the available evidence regarding any plausible deleterious effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on chondrocytes, chondrocyte differentiation, and allograft or autograft incorporation after cartilage repair procedures.

DESIGN

Three databases (PubMed, Science Direct, and Cochrane Library) were screened for eligible studies: investigating the effects of NSAIDs on chondrocytes, chondrogenic differentiation, or allograft/autograft incorporation. This evaluation included studies of any level of evidence, written in English, reporting clinical or preclinical results, published in peer review journals and dealing with our topic. All articles evaluating the effects of NSAIDs on either osteoarthritic (OA) chondrocyte samples or OA chondrocyte models were excluded. Moreover, articles about bone healing in which allograft or autograft incorporation was not investigated were also excluded. Methodologic quality assessment was performed for in vivo animal studies according to ARRIVE guidelines, and risk of bias of each included study was identified using the ROBINS-I tool.

RESULTS

Eighteen studies were included in the review: 4 in vitro studies, 13 animal studies, and 1 human study. According to these studies NSAIDs have no detrimental effect on healthy mature chondrocytes; however, these drugs influence chondrocyte differentiation and graft incorporation and therefore may interfere with chondrogenesis and incorporation after transplantation of chondrocytes or osteochondral grafts.

CONCLUSION

The use of NSAIDs, systemic or local, after cartilage repair procedures should be avoided unless a substantial clinical benefit would otherwise be withheld from the patient. More human studies are needed to analyze the effect of NSAIDs on cartilage repair.

The hypertrophic chondrocyte: To be or not to be

Hypertrophic chondrocytes are the master regulators of endochondral ossification; however, their ultimate cell fates cells remain largely elusive due to their transient nature. Historically, hypertrophic chondrocytes have been considered as the terminal state of growth plate chondrocytes, which are destined to meet their inevitable demise at the primary spongiosa. Chondrocyte hypertrophy is accompanied by increased organelle synthesis and rapid intracellular water uptake, which serve as the major drivers of longitudinal bone growth. This process is delicately regulated by major signaling pathways and their target genes, including growth hormone (GH), insulin growth factor-1 (IGF-1), indian hedgehog (Ihh), parathyroid hormone-related protein (PTHrP), bone morphogenetic proteins (BMPs), sex determining region Y-box 9 (Sox9), runt-related transcription factors (Runx) and fibroblast growth factor receptors (FGFRs). Hypertrophic chondrocytes orchestrate endochondral ossification by regulating osteogenic-angiogenic and osteogenic-osteoclastic coupling through the production of vascular endothelial growth factor (VEGF), receptor activator of nuclear factor kappa-B ligand (RANKL) and matrix metallopeptidases-9/13 (MMP-9/13). Hypertrophic chondrocytes also indirectly regulate resorption of the cartilaginous extracellular matrix, by controlling formation of a special subtype of osteoclasts termed "chondroclasts". Notably, hypertrophic chondrocytes may possess innate potential for plasticity, reentering the cell cycle and differentiating into osteoblasts and other types of mesenchymal cells in the marrow space. We may be able to harness this unique plasticity for therapeutic purposes, for a variety of skeletal abnormalities and injuries. In this review, we discuss the morphological and molecular properties of hypertrophic chondrocytes, which carry out important functions during skeletal growth and regeneration.

Comparative Study of DHA with Different Molecular Forms for Ameliorating Osteoporosis by Promoting Chondrocyte-to-Osteoblast Transdifferentiation in the Growth Plate of Ovariectomized Mice

Osteoblasts play a key role in bone remodeling. Recent studies have reported that some hypertrophic chondrocytes co-expressing collagen I(Col I) and collagen X (ColX) could directly transdifferentiate into osteoblasts during endochondral ossification. However, whether nutrition intervention is beneficial to this transformation to improve osteoporosis (OP) remains unknown. In this study, ovariectomy (OVX)-induced OP mice were orally administered with docosahexaenoic acid (DHA) in different molecular forms for 13 weeks. The results showed that both DHA-triglyceride (DHA-TG) and DHA-phosphatidylcholine (DHA-PC) increased the bone mineral density and bone mineral apposition rate in ovariectomized mice, while DHA-ethyl esters (DHA-EE) had little effect. Interestingly, we found that both DHA-PC and DHA-TG increased the height of the growth plate, mainly increasing the number of hypertrophic chondrocytes. Further investigation by simultaneously labeling ColX and ColI indicated that DHA-PC and DHA-TG promoted the number of chondrocyte-transdifferentiated osteoblasts in the growth plate close to the diaphysis, in which DHA-PC performed better than DHA-TG. Apoptosis was not the only fate of hypertrophic chondrocytes. Western blot results showed that both DHA-TG and DHA-PC downregulated the Bax and cleaved-caspase3 expression and upregulated Bcl-2 expression in the growth plate, suggesting that chondrocyte apoptosis is inhibited. Runx2, the key regulator of chondrocyte-to-osteoblast transdifferentiation, was significantly increased by DHA-TG and DHA-PC, while DHA-EE had no effect on the above indicators. To our best knowledge, this is the first report that both DHA-PC and DHA-TG enhanced bone formation via promoting the chondrocyte-to-osteoblast transdifferentiation in the growth plate, contributing to the amelioration of OP. These activities depend on the molecular forms of DHA and their bioavailabilities. Our results provide guidance for the application of fish oil for bone health.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

The diverse origin of bone-forming osteoblasts

Osteoblasts are the only cells that can give rise to bones in vertebrates. Thus, one of the most important functions of these metabolically active cells is mineralized matrix production. Because osteoblasts have a limited lifespan, they must be constantly replenished by preosteoblasts, their immediate precursors. Because disruption of the regulation of bone-forming osteoblasts results in a variety of bone diseases, a better understanding of the origin of these cells by defining the mechanisms of bone development, remodeling, and regeneration is central to the development of novel therapeutic approaches. In recent years, substantial new insights into the origin of osteoblasts-largely owing to rapid technological advances in murine lineage-tracing approaches and other single-cell technologies-have been obtained. Collectively, these findings indicate that osteoblasts involved in bone formation under various physiological, pathological, and therapeutic conditions can be obtained from numerous sources. The origins of osteoblasts include, but are not limited to, chondrocytes in the growth plate, stromal cells in the bone marrow, quiescent bone-lining cells on the bone surface, and specialized fibroblasts in the craniofacial structures, such as sutures and periodontal ligaments. Because osteoblasts can be generated from local cellular sources, bones can flexibly respond to regenerative and anabolic cues. However, whether osteoblasts derived from different cellular sources have distinct functions remains to be investigated. Currently, we are at the initial stage to aptly unravel the incredible diversity of the origins of bone-forming osteoblasts. © 2021 American Society for Bone and Mineral Research (ASBMR).

Chondrocytes in the resting zone of the growth plate are maintained in a Wnt-inhibitory environment

Chondrocytes in the resting zone of the postnatal growth plate are characterized by slow cell cycle progression, and encompass a population of parathyroid hormone-related protein (PTHrP)-expressing skeletal stem cells that contribute to the formation of columnar chondrocytes. However, how these chondrocytes are maintained in the resting zone remains undefined. We undertook a genetic pulse-chase approach to isolate slow cycling, label-retaining chondrocytes (LRCs) using a chondrocyte-specific doxycycline-controllable Tet-Off system regulating expression of histone 2B-linked GFP. Comparative RNA-seq analysis identified significant enrichment of inhibitors and activators for Wnt signaling in LRCs and non-LRCs, respectively. Activation of Wnt/β-catenin signaling in PTHrP+ resting chondrocytes using Pthlh-creER and Apc-floxed allele impaired their ability to form columnar chondrocytes. Therefore, slow-cycling chondrocytes are maintained in a Wnt-inhibitory environment within the resting zone, unraveling a novel mechanism regulating maintenance and differentiation of PTHrP+ skeletal stem cells of the postnatal growth plate.

Impairment of Cyclo-oxygenase-2 Function Results in Abnormal Growth Plate Development and Bone Microarchitecture but Does Not Affect Longitudinal Growth of the Long Bones in Skeletally Immature Mice

OBJECTIVE

Despite the general awareness that cyclo-oxygenase-2 (COX-2) is crucial for endochondral ossification, the role of COX-2 in skeletal development is largely unknown. We hypothesized that inhibition or genetic loss of COX-2 leads to impaired growth plate development and consequently impaired postnatal development of the long bones.

DESIGN

Skeletally immature (5 weeks old) B6;129S-Ptgs2^tm1Jed/J wildtype mice were treated for 10 weeks with celecoxib (daily oral administration 10 mg/kg) or placebo and compared with B6;129S-Ptgs2^tm1Jed/J homozygous knockout mice (n = 12 per group).

RESULTS

Fifteen weeks postnatally, no significant difference in growth plate (zone) thickness was found between groups. However, significantly higher proteoglycan content and lower expression levels of collagen type II and X staining in the growth plates of celecoxib-treated mice, and to a lesser extent in COX-2 knockout mice. In addition, a significantly decreased cell number and cell size were observed in the hypertrophic zone of the growth plates of both experimental groups. Micro-computed tomography analysis of the subchondral bone region directly beneath the growth plate showed significantly higher bone density and trabecular thickness, following celecoxib treatment. Despite the detected differences in growth plate extracellular matrix composition and subchondral bone morphology, no difference was found in the length of the tibia in celecoxib-treated mice or COX-2 knockout mice.

CONCLUSIONS

Genetic loss of COX-2 or treatment with celecoxib did not result in detectable differences in gross murine formation of the tibia or femur. However, there were notable phenotypic features detected in the maturation of the growth plate (hypertrophic zone and subchondral bone) as a result of the celecoxib treatment.

Enlightenment of Growth Plate Regeneration Based on Cartilage Repair Theory: A Review

The growth plate (GP) is a cartilaginous region situated between the epiphysis and metaphysis at the end of the immature long bone, which is susceptible to mechanical damage because of its vulnerable structure. Due to the limited regeneration ability of the GP, current clinical treatment strategies (e.g., bone bridge resection and fat engraftment) always result in bone bridge formation, which will cause length discrepancy and angular deformity, thus making satisfactory outcomes difficult to achieve. The introduction of cartilage repair theory and cartilage tissue engineering technology may encourage novel therapeutic approaches for GP repair using tissue engineered GPs, including biocompatible scaffolds incorporated with appropriate seed cells and growth factors. In this review, we summarize the physiological structure of GPs, the pathological process, and repair phases of GP injuries, placing greater emphasis on advanced tissue engineering strategies for GP repair. Furthermore, we also propose that three-dimensional printing technology will play a significant role in this field in the future given its advantage of bionic replication of complex structures. We predict that tissue engineering strategies will offer a significant alternative to the management of GP injuries.

Wnt/ß-catenin-mediated p53 suppression is indispensable for osteogenesis of mesenchymal progenitor cells

The developmental origins of mesenchymal progenitor cells (MPCs) and molecular machineries regulating their fate and differentiation are far from defined owing to their complexity. Osteoblasts and adipocytes are descended from common MPCs. Their fates are collectively determined by an orchestra of pathways in response to physiological and external cues. The canonical Wnt pathway signals MPCs to commit to osteogenic differentiation at the expense of adipogenic fate. In contrast to ß-catenin, p53’s anti-osteogenic function is much less understood. Both activities are thought to be achieved through targeting Runx2 and/or Osterix (Osx, Sp7) transcription. Precisely, how Osx activity is dictated by ß-catenin or p53 is not clarified and represents a knowledge gap that, until now, has largely been taken for granted. Using conditional lineage-tracing mice, we demonstrated that chondrocytes gave rise to a sizable fraction of MPCs, which served as progenitors of chondrocyte-derived osteoblasts (Chon-ob). Wnt/ß-catenin activity was only required at the stage of chondrocyte-derived mesenchymal progenitor (C-MPC) to Chon-ob differentiation. ß-catenin^– C-MPCs lost osteogenic ability and favored adipogenesis. Mechanistically, we discovered that p53 activity was elevated in ß-catenin^– MPCs including ß-catenin^– C-MPCs and deleting p53 from the ß-catenin^– MPCs fully restored osteogenesis. While high levels of p53 were present in the nuclei of ß-catenin^– MPCs, Osx was confined to the cytoplasm, implying a mechanism that did not involve direct p53-Osx interaction. Furthermore, we found that p53’s anti-osteogenic activity was dependent on its DNA-binding ability. Our findings identify chondrocytes as an additional source for MPCs and indicate that Wnt/ß-catenin discretely regulates chondrocyte to C-MPC and the subsequent C-MPC to osteoblast developments. Most of all we unveil a previously unrecognized functional link between ß-catenin and p53, placing p53’s negative role in the context of Wnt/ß-catenin signaling-induced MPC osteogenic differentiation.

Regulation and Role of Transcription Factors in Osteogenesis

Bone is a dynamic tissue constantly responding to environmental changes such as nutritional and mechanical stress. Bone homeostasis in adult life is maintained through bone remodeling, a controlled and balanced process between bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoblasts secrete matrix, with some being buried within the newly formed bone, and differentiate to osteocytes. During embryogenesis, bones are formed through intramembraneous or endochondral ossification. The former involves a direct differentiation of mesenchymal progenitor to osteoblasts, and the latter is through a cartilage template that is subsequently converted to bone. Advances in lineage tracing, cell sorting, and single-cell transcriptome studies have enabled new discoveries of gene regulation, and new populations of skeletal stem cells in multiple niches, including the cartilage growth plate, chondro-osseous junction, bone, and bone marrow, in embryonic development and postnatal life. Osteoblast differentiation is regulated by a master transcription factor RUNX2 and other factors such as OSX/SP7 and ATF4. Developmental and environmental cues affect the transcriptional activities of osteoblasts from lineage commitment to differentiation at multiple levels, fine-tuned with the involvement of co-factors, microRNAs, epigenetics, systemic factors, circadian rhythm, and the microenvironments. In this review, we will discuss these topics in relation to transcriptional controls in osteogenesis.

Bone tissue and histological and molecular events during development of the long bones

The bones are of mesenchymal or ectomesenchymal origin, form the skeleton of most vertebrates, and are essential for locomotion and organ protection. As a living tissue they are highly vascularized and remodelled throughout life to maintain intact. Bones consist of osteocytes entrapped in a mineralized extracellular matrix, and via their elaborated network of cytoplasmic processes they do not only communicate with each other but also with the cells on the bone surface (bone lining cells). Bone tissue develops through a series of fine-tuned processes, and there are two modes of bone formation, referred to either as intramembranous or endochondral ossification. In intramembranous ossification, bones develop directly from condensations of mesenchymal cells, and the flat bones of the skull, the clavicles and the perichondral bone cuff develop via this process. The bones of the axial (ribs and vertebrae) and the appendicular skeleton (e.g. upper and lower limbs) form through endochondral ossification where mesenchyme turns into a cartilaginous intermediate with the shape of the future skeletal element that is gradually replaced by bone. Endochondral ossification occurs in all vertebrate taxa and its onset involves differentiation of the chondrocytes, mineralization of the extracellular cartilage matrix and vascularization of the intermediate, followed by disintegration and resorption of the cartilage, bone formation, and finally - after complete ossification of the cartilage model - the establishment of an avascular articular cartilage. The epiphyseal growth plate regulates the longitudinal growth of the bones, achieved by a balanced proliferation and elimination of chondrocytes, and the question whether the late hypertrophic chondrocytes die or transform into osteogenic cells is still being hotly debated. The complex processes leading to endochondral ossification have been studied for over a century, and this review aims to give an overview of the histological and molecular events, arising from the long bones' (e.g. femur, tibia) development. The fate of the hypertrophic chondrocytes will be discussed in the light of new findings obtained from cell tracking studies.

The role of microRNAs in bone development

Epigenetic regulation is critical for proper bone development. Evidence from a large body of published literature informs us that microRNAs (miRNAs) are important epigenetic factors that control many aspects of bone development, homeostasis, and repair processes. These small non-coding RNAs function at the post-transcriptional level to suppress expression of specific target genes. Many target genes may be affected by one miRNA resulting in alteration in cellular pathways and networks. Therefore, changes in levels or activity of a specific miRNA (e.g. via genetic mutations, disease scenarios, or by over-expression or inhibition strategies in vitro or in vivo) can lead to substantial changes in cell processes including proliferation, metabolism, apoptosis and differentiation. In this review, Section 1 briefly covers general background information on processes that control bone development as well as the biogenesis and function of miRNAs. In Section 2, we discuss the importance of miRNAs in skeletal development based on findings from in vivo mouse models and human clinical reports. Section 3 focuses on describing more recent data from the last three years related to miRNA regulation of osteoblast differentiation in vitro. Some of these studies also involve utilization of an in vivo rodent model to study the effects of miRNA modulation in scenarios of osteoporosis, bone repair or ectopic bone formation. In Section 4, we provide some recent information from studies analyzing the potential of miRNA-mediated crosstalk in bone and how exosomes containing miRNAs from one bone cell may affect the differentiation or function of another bone cell type. We then conclude by summarizing where the field currently stands with respect to miRNA-mediated regulation of osteogenesis and how information gained from developmental processes can be instructive in identifying potential therapeutic miRNA targets for the treatment of certain bone conditions.

Overview of Skeletal Development

Development of cartilage and bone, the core components of the mouse skeletal system, depends on coordinated proliferation and differentiation of skeletogenic cells, including chondrocytes and osteoblasts. These cells differentiate from common progenitor cells originating in the mesoderm and neural crest. Multiple signaling pathways and transcription factors tightly regulate differentiation and proliferation of skeletal cells. In this chapter, we overview the process of mouse skeletal development and discuss major regulators of skeletal cells at each developmental stage.

A quantitative serum biomarker of circulating collagen X effectively correlates with endochondral fracture healing

Currently, there are no standardized methods for quantitatively measuring fracture repair. Physicians rely on subjective physical examinations and qualitative evaluation of radiographs to detect mineralized tissue. Since most fractures heal indirectly through a cartilage intermediate, these tools are limited in their diagnostic utility of early repair. Prior to converting to the bone, cartilage undergoes hypertrophic maturation, characterized by the deposition of a provisional collagen X matrix. The objective of this study was to characterize the kinetics of a novel collagen X biomarker relative to other biological measurements of fracture healing using a murine model of endochondral fracture repair in which a closed, mid-shaft tibia fracture was created using the classic drop-weight technique. Serum was collected 5 to 42 days post-fracture in male and female mice and compared to uninjured controls (n = 8-12). Collagen X in the serum was quantified using a recently validated ELISA-based bioassay ("Cxm")1 and compared to genetic and histological markers of fracture healing and inflammation. We found the Cxm biomarker reliably increased from baseline to a statistically unique peak 14 days post-fracture that then resolved to pre-fracture levels by 3 weeks following injury. The shape and timing of the Cxm curve followed the genetic and histological expression of collagen X but did not show a strong correlation with local inflammatory states. Assessment of fracture healing progress is crucial to making correct and timely clinical decisions for patients. This Cxm bioassay represents a minimally invasive, inexpensive technique that could provide reliable information on the biology of the fracture to significantly improve clinical care.

IRX3 and IRX5 Inhibit Adipogenic Differentiation of Hypertrophic Chondrocytes and Promote Osteogenesis

Maintaining the correct proportions of different cell types in the bone marrow is critical for bone function. Hypertrophic chondrocytes (HCs) and osteoblasts are a lineage continuum with a minor contribution to adipocytes, but the regulatory network is unclear. Mutations in transcription factors, IRX3 and IRX5, result in skeletal patterning defects in humans and mice. We found coexpression of Irx3 and Irx5 in late-stage HCs and osteoblasts in cortical and trabecular bone. Irx3 and Irx5 null mutants display severe bone deficiency in newborn and adult stages. Quantitative analyses of bone with different combinations of functional alleles of Irx3 and Irx5 suggest these two factors function in a dosage-dependent manner. In Irx3 and Irx5 nulls, the amount of bone marrow adipocytes was increased. In Irx5 nulls, lineage tracing revealed that removal of Irx3 specifically in HCs exacerbated reduction of HC-derived osteoblasts and increased the frequency of HC-derived marrow adipocytes. β-catenin loss of function and gain of function specifically in HCs affects the expression of Irx3 and Irx5, suggesting IRX3 and IRX5 function downstream of WNT signaling. Our study shows that IRX3 and IRX5 regulate fate decisions in the transition of HCs to osteoblasts and to marrow adipocytes, implicating their potential roles in human skeletal homeostasis and disorders.