Skeletal stem cells in bone development, homeostasis, and disease

Tissue-resident stem cells are essential for development and repair, and in the skeleton, this function is fulfilled by recently identified skeletal stem cells (SSCs). However, recent work has identified that SSCs are not monolithic, with long bones, craniofacial sites, and the spine being formed by distinct stem cells. Recent studies have utilized techniques such as fluorescence-activated cell sorting, lineage tracing, and single-cell sequencing to investigate the involvement of SSCs in bone development, homeostasis, and disease. These investigations have allowed researchers to map the lineage commitment trajectory of SSCs in different parts of the body and at different time points. Furthermore, recent studies have shed light on the characteristics of SSCs in both physiological and pathological conditions. This review focuses on discussing the spatiotemporal distribution of SSCs and enhancing our understanding of the diversity and plasticity of SSCs by summarizing recent discoveries.

Single-cell RNA sequencing uncovers cellular heterogeneity and the progression of heterotopic ossification of the elbow

Heterotopic ossification of the elbow (HOE) is a complicated pathologic process characterized by extra bone formation in the elbow. Bone formation is a complex developmental process involving the differentiation of mesenchymal stem cells into osteoblasts. The aim of this study was to explore the cellular origin and progression of HOE by single-cell RNA sequencing. We identified 13 clusters of cells in HOE and further analyzed the subclusters for 4 of the main cell types. Six subclusters of osteoblasts, nine subclusters of chondrocytes, six subclusters of fibroblasts, and five subclusters of mononuclear phagocytes (MPs) were identified and analyzed. The new findings on osterix (OSX) and SOX9 expression in osteoblast subclusters and chondrocyte subclusters indicate that HOE is mediated through endochondral ossification. Further identification of the corresponding signature gene sets of distinct subclusters indicated that subclusters of osteoblasts_3, osteoblasts_4, osteoblasts_5, and osteoblasts_6 are relatively more mature during the osteoblastic progression of HOE. The trajectory analysis of the osteoblasts demonstrated that some genes were gradually downregulated, such as CRYAB, CCL3, SFRP4, WIF1, and IGFBP3, while other critical genes were upregulated, such as VCAN, IGFBP4, FSTL1, POSTN, MDK, THBS2, and ALPL, suggesting that these factors may participate in HOE progression. Cell-cell communication networks revealed extensive molecular interactions among the 13 HOE clusters. Ligand-receptor pairs for IL6, COL24A1, COL22A1, VWF, FZD6, FGF2, and NOTCH1 were identified, suggesting that multiple signaling pathways may be involved in HOE. In conclusion, this study provided the cellular atlas for HOE. We have established a greater extent of the heterogeneity of HOE cells than previously known through transcriptomic analysis at the single-cell level. We have observed gradual patterns of signature gene expression during the differentiation and maturation progression of osteoblasts from stem cells in HOE with higher resolution. The cell heterogeneity of HOE deserves further investigation to pave the way for identification of potential targets for HOE early diagnosis and therapeutic treatment.

Nano-laponite encapsulated coaxial fiber scaffold promotes endochondral osteogenesis

Osteoinductive supplements without side effects stand out from the growth factors and drugs widely used in bone tissue engineering. Lithium magnesium sodium silicate hydrate (laponite) nanoflake is a promising bioactive component for bone regeneration, attributed to its inherent biosafety and effective osteoinductivity. Up to now, the in vivo osteogenic potential and mechanisms of laponite-encapsulated fibrous membranes remain largely unexplored. This study presents a unique method for homogeneously integrating high concentrations of laponite RDS into a polycaprolactone (PCL) matrix by dispersing laponite RDS sol into the polymer solution. Subsequently, a core-shell fibrous membrane (10RP-PG), embedding laponite-loaded PCL in its core, was crafted using coaxial electrospinning. The PCL core's slow degradation and the shell's gradient degradation enabled the sustained release of bioactive ions (Si and Mg) from laponite. In vivo studies on a critical-sized calvarial bone defect model demonstrated that the 10RP-PG membrane markedly enhanced bone formation and remodeling by accelerating the process of endochondral ossification. Further transcriptome analysis suggested that osteogenesis in the 10RP-PG membrane is driven by Mg and Si from endocytosed laponite, activating pathways related to ossification and endochondral ossification, including Hippo, Wnt and Notch. The fabricated nanocomposite fibrous membranes hold great promise in the fields of critical-sized bone defect repair.

miR-335-5p inhibits endochondral ossification by directly targeting SP1 in TMJ OA

OBJECTIVE

During the development of temporomandibular joint osteoarthritis, endochondral ossification is compromised which leads to condylar degeneration; miR-335-5p in endochondral ossification in osteoarthritic condylar cartilage tissue remains unclear.

METHODS

Up-regulated microRNA and its target gene were searched for endochondral ossification in osteoarthritis articular cartilage. The effect of increased or decreased miR-335-5p on endochondral ossification was evaluated by transfecting miR-335-5p mimics or miR-335-5p inhibitor in vitro in chondrocytes C28/I2. Finally, we injected the temporomandibular joint of rats intra-articularly with agomiR-335 in a unilateral anterior crossbite rat model to determine the in vivo regulation of miR-335.

RESULTS

After the onset of temporomandibular joint osteoarthritis, miR-335-5p levels were gradually up-regulated, whereas endochondral ossification-related genes were down-regulated in condylar cartilage specimens. Our results showed that miR-335 inhibited endochondral ossification after administration of a miR-335 antagonist into the temporomandibular joint articular cavity of a unilateral anterior crossbite rat model. AgomiR-335, a miR-335 agonist, inhibited matrix mineralization in fibrocartilage stem cells in vitro and then miR-335-5p negatively regulated chondrocyte activity by directly targeting SP1 via promoting transforming growth factor-β/Smad signalling.

CONCLUSION

miR-335-5p can significantly inhibit endochondral ossification; therefore, its inhibition may be beneficial for the treatment of temporomandibular joint osteoarthritis.

Biomimetic Hydrogels as the Inductive Endochondral Ossification Template for Promoting Bone Regeneration

Repairing critical size bone defects (CSBD) is a major clinical challenge and requires effective intervention by biomaterial scaffolds. Inspired by the fact that the cartilaginous template-based endochondral ossification (ECO) process is crucial to bone healing and development, developing biomimetic biomaterials to promote ECO is recognized as a promising approach for repairing CSBD. With the unique highly hydrated 3D polymeric network, hydrogels can be designed to closely emulate the physiochemical properties of cartilage matrix to facilitate ECO. In this review, the various preparation methods of hydrogels possessing the specific physiochemical properties required for promoting ECO are introduced. The materiobiological impacts of the physicochemical properties of hydrogels, such as mechanical properties, topographical structures and chemical compositions on ECO, and the associated molecular mechanisms related to the BMP, Wnt, TGF-β, HIF-1α, FGF, and RhoA signaling pathways are further summarized. This review provides a detailed coverage on the materiobiological insights required for the design and preparation of hydrogel-based biomaterials to facilitate bone regeneration.

Investigation of the effectiveness of intermittent electromagnetic field stimulation for early internal cartilaginous ossification in prechondrocytic ATDC5 cells

Pulsed electromagnetic field (PEMF) stimulation has been widely applied clinically to promote bone healing; however, its detailed mechanism of action, particularly in endochondral ossification, remains elusive, and long-term stimulation is required for its satisfactory effect. The aim of this study was to investigate the involvement of the mammalian target of rapamycin (mTOR) pathway in chondrocyte differentiation and proliferation using a mouse prechondroblast cell line (ATDC5), and establish an efficient PEMF stimulation strategy for endochondral ossification. The changes in cell differentiation (gene expression levels of aggrecan, type II collagen, and type X collagen) and proliferation (cellular uptake of bromodeoxyuridine [BrdU]) in ATDC5 cells in the presence or absence of rapamycin, an mTOR inhibitor, was measured. The effects of continuous and intermittent PEMF stimulation on changes in cell differentiation and proliferation were compared. Rapamycin significantly suppressed the induction of cell differentiation markers and the cell proliferation activity. Furthermore, only intermittent PEMF stimulation continuously activated the mTOR pathway in ATDC5 cells, significantly promoting cell proliferation. These results demonstrate the involvement of the mTOR pathway in chondrocyte differentiation and proliferation and suggest that intermittent PEMF stimulation could be effective as a stimulus for endochondral ossification during fracture healing process, thereby reducing stimulation time.

The association and underlying mechanism of the digit ratio (2D:4D) in hypospadias

The second-to-fourth digit (2D:4D) ratio is thought to be associated with prenatal androgen exposure. However, the relationship between the 2D:4D ratio and hypospadias is poorly understood, and its molecular mechanism is not clear. In this study, by analyzing the hand digit length of 142 boys with hypospadias (23 distal, 68 middle, and 51 proximal) and 196 controls enrolled in Shanghai Children's Hospital (Shanghai, China) from December 2020 to December 2021, we found that the 2D:4D ratio was significantly increased in boys with hypospadias ( P < 0.001) and it was positively correlated with the severity of the hypospadias. This was further verified by the comparison of control mice and prenatal low testosterone mice model obtained by knocking out the risk gene (dynein axonemal heavy chain 8 [ DNAH8 ]) associated with hypospadias. Furthermore, the discrepancy was mainly caused by a shift in 4D. Proteomic characterization of a mouse model validated that low testosterone levels during pregnancy can impair the growth and development of 4D. Comprehensive mechanistic explorations revealed that during the androgen-sensitive window, the downregulation of the androgen receptor (AR) caused by low testosterone levels, as well as the suppressed expression of chondrocyte proliferation-related genes such as Wnt family member 5a ( Wnt5a ), Wnt5b , Smad family member 2 ( Smad2 ), and Smad3 ; mitochondrial function-related genes in cartilage such as AMP-activated protein kinase ( AMPK ) and nuclear respiratory factor 1 ( Nrf-1 ); and vascular development-related genes such as myosin light chain ( MLC ), notch receptor 3 ( Notch3 ), and sphingosine kinase 1 ( Sphk1 ), are responsible for the limitation of 4D growth, which results in a higher 2D:4D ratio in boys with hypospadias via decreased endochondral ossification. This study indicates that the ratio of 2D:4D is a risk marker of hypospadias and provides a potential molecular mechanism.

Hierarchically Assembled Nanofiber Scaffold Guides Long Bone Regeneration by Promoting Osteogenic/Chondrogenic Differentiation of Endogenous Mesenchymal Stem Cells

Critical-sized segmental long bone defects represent a challenging clinical dilemma in the management of battlefield and trauma-related injuries. The residual bone marrow cavity of damaged long bones contains many bone marrow mesenchymal stem cells (BMSCs), which provide a substantial source of cells for bone repair. Thus, a three-dimensional (3D) vertically aligned nanofiber scaffold (VAS) is developed with long channels and large pore size. The pore of VAS toward the bone marrow cavity after transplantation, enables the scaffolds to recruit BMSCs from the bone marrow cavity to the defect area. In vivo, it is found that VAS can significantly shorten gap distance and promote new bone formation compared to the control and collagen groups after 4 and 8 weeks of implantation. The single-cell sequencing results discovered that the 3D nanotopography of VAS can promote BMSCs differentiation to chondrocytes and osteoblasts, and up-regulate related gene expression, resulting in enhancing the activities of bone regeneration, endochondral ossification, bone trabecula formation, bone mineralization, maturation, and remodeling. The Alcian blue and bone morphogenetic protein 2 (BMP-2) immunohistochemical staining verified significant cartilage formation and bone formation in the VAS group, corresponding to the single-cell sequencing results. The study can inspire the design of next-generation scaffolds for effective long-bone regeneration is expected by the authors.

Investigation of the Shared Biomarkers in Heterotopic Ossification Between Ossification of the Ligamentum Flavum and Ankylosing Spondylitis

STUDY DESIGN

Bioinformatics analysis of Gene Expression Omnibus (GEO).

OBJECTIVE

Ossification of the ligamentum flavum (OLF) and ankylosing spondylitis (AS) represent intricate conditions marked by the gradual progression of endochondral ossification. This investigation endeavors to unveil common biomarkers associated with heterotopic ossification and explore the potential molecular regulatory mechanisms.

METHODS

Microarray and RNA-sequencing datasets retrieved from the Gene Expression Omnibus (GEO) repository were harnessed to discern differentially expressed genes (DEGs) within the OLF and AS datasets. Subsequently, Weighted Gene Co-expression Network Analysis (WGCNA) was implemented to pinpoint co-expression modules linked to OLF and AS. Common genes were further subjected to an examination of functional pathway enrichment. Moreover, hub intersection genes were identified using the Least Absolute Shrinkage and Selection Operator (LASSO) regression, followed by an evaluation of diagnostic performance in external OLF and AS cohorts. Lastly, an analysis of immune cell infiltration was conducted to scrutinize the correlation of immune cell presence with shared biomarkers in OLF and AS.

RESULTS

A total of 1353 and 91 Differentially Expressed Genes (DEGs) were identified in OLF and AS, respectively. Using the Weighted Gene Co-expression Network Analysis (WGCNA), 2 modules were found to be notably significant for OLF and AS. The integrative bioinformatic analysis revealed 3 hub genes (MAB21L2, MEGF10, ISLR) as shared risk biomarkers, with MAB21L2 being the central focus. Receiver Operating Characteristic (ROC) analysis exhibited a strong diagnostic potential for these hub genes. Gene Ontology (GO) analysis indicated their involvement in the positive regulation of myoblast proliferation. Notably, MAB21L2 was singled out as the optimal common biomarker for OLF and AS. Furthermore, an analysis of immune infiltration demonstrated a correlation between MAB21L2 expression and changes in immune cells. Activated CD8 T cells were identified as shared differential immune infiltrating cells significantly linked to MAB21L2 in both OLF and AS.

CONCLUSION

This study represents the first instance of identifying MAB21L2 as a prospective diagnostic marker for patients contending with OLF associated with AS. The research results indicate that the ECM-receptor interaction and the cell-cell adhesion may play a role in both disease processes. This newfound knowledge not only enhances our understanding of the pathogenesis behind spinal ligament ossification but also uncovers potential targets for therapeutic interventions.

Tolerant and Rapid Endochondral Bone Regeneration Using Framework-Enhanced 3D Biomineralized Matrix Hydrogels

Tissue-engineered bone has emerged as a promising alternative for bone defect repair due to the advantages of regenerative bone healing and physiological functional reconstruction. However, there is very limited breakthrough in achieving favorable bone regeneration due to the harsh osteogenic microenvironment after bone injury, especially the avascular and hypoxic conditions. Inspired by the bone developmental mode of endochondral ossification, a novel strategy is proposed for tolerant and rapid endochondral bone regeneration using framework-enhanced 3D biomineralized matrix hydrogels. First, it is meticulously designed 3D biomimetic hydrogels with both hypoxic and osteoinductive microenvironment, and then integrated 3D-printed polycaprolactone framework to improve their mechanical strength and structural fidelity. The inherent hypoxic 3D matrix microenvironment effectively activates bone marrow mesenchymal stem cells self-regulation for early-stage chondrogenesis via TGFβ/Smad signaling pathway due to the obstacle of aerobic respiration. Meanwhile, the strong biomineralized microenvironment, created by a hybrid formulation of native-constitute osteogenic inorganic salts, can synergistically regulate both bone mineralization and osteoclastic differentiation, and thus accelerate the late-stage bone maturation. Furthermore, both in vivo ectopic osteogenesis and in situ skull defect repair successfully verified the high efficiency and mechanical maintenance of endochondral bone regeneration mode, which offers a promising treatment for craniofacial bone defect repair.

Growth disorders caused by variants in epigenetic regulators: progress and prospects

Epigenetic modifications play an important role in regulation of transcription and gene expression. The molecular machinery governing epigenetic modifications, also known as epigenetic regulators, include non-coding RNA, chromatin remodelers, and enzymes or proteins responsible for binding, reading, writing and erasing DNA and histone modifications. Recent advancement in human genetics and high throughput sequencing technology have allowed the identification of causative variants, many of which are epigenetic regulators, for a wide variety of childhood growth disorders that include skeletal dysplasias, idiopathic short stature, and generalized overgrowth syndromes. In this review, we highlight the connection between epigenetic modifications, genetic variants in epigenetic regulators and childhood growth disorders being established over the past decade, discuss their insights into skeletal biology, and the potential of epidrugs as a new type of therapeutic intervention.

Sex as a Critical Variable in Basic and Pre-Clinical Studies of Fibrodysplasia Ossificans Progressiva

Heterotopic ossification (HO) is most dramatically manifested in the rare and severely debilitating disease, fibrodysplasia ossificans progressiva (FOP), in which heterotopic bone progressively accumulates in skeletal muscles and associated soft tissues. The great majority of FOP cases are caused by a single amino acid substitution in the type 1 bone morphogenetic protein (BMP) receptor ACVR1, a mutation that imparts responsiveness to activin A. Although it is well-established that biological sex is a critical variable in a range of physiological and disease processes, the impact of sex on HO in animal models of FOP has not been explored. We show that female FOP mice exhibit both significantly greater and more variable HO responses after muscle injury. Additionally, the incidence of spontaneous HO was significantly greater in female mice. This sex dimorphism is not dependent on gonadally derived sex hormones, and reciprocal cell transplantations indicate that apparent differences in osteogenic activity are intrinsic to the sex of the transplanted cells. By circumventing the absolute requirement for activin A using an agonist of mutant ACVR1, we show that the female-specific response to muscle injury or BMP2 implantation is dependent on activin A. These data identify sex as a critical variable in basic and pre-clinical studies of FOP.

The cellular landscape of the endochondral bone during the transition to extrauterine life

The cellular complexity of the endochondral bone underlies its essential and pleiotropic roles during organismal life. While the adult bone has received significant attention, we still lack a deep understanding of the perinatal bone cellulome. Here, we have profiled the full composition of the murine endochondral bone at the single-cell level during the transition from fetal to newborn life and in comparison with the adult tissue, with particular emphasis on the mesenchymal compartment. The perinatal bone contains different fibroblastic clusters with blastema-like characteristics in organizing and supporting skeletogenesis, angiogenesis and hematopoiesis. Our data also suggest dynamic inter- and intra-compartment interactions, as well as a bone marrow milieu that seems prone to anti-inflammation, which we hypothesize is necessary to ensure the proper program of lymphopoiesis and the establishment of central and peripheral tolerance in early life. Our study provides an integrative roadmap for the future design of genetic and cellular functional assays to validate cellular interactions and lineage relationships within the perinatal bone.

Engineered phalangeal grafts for children with symbrachydactyly: A proof of concept

Tissue engineering approaches hold great promise in the field of regenerative medicine, especially in the context of pediatric applications, where ideal grafts need to restore the function of the targeted tissue and consider growth. In the present study, we aimed to develop a protocol to engineer autologous phalangeal grafts of relevant size for children suffering from symbrachydactyly. This condition results in hands with short fingers and missing bones. A previously-described, developmentally-inspired strategy based on endochondral ossification (ECO)-the main pathway leading to bone and bone marrow development-and adipose derived-stromal cells (ASCs) as the source of chondroprogenitor was used. First, we demonstrated that pediatric ASCs associated with collagen sponges can generate hypertrophic cartilage tissues (HCTs) in vitro that remodel into bone tissue in vivo via ECO. Second, we developed and optimized an in vitro protocol to generate HCTs in the shape of small phalangeal bones (108-390 mm3) using freshly isolated adult cells from the stromal vascular fraction (SVF) of adipose tissue, associated with two commercially available large collagen scaffolds (Zimmer Plug® and Optimaix 3D®). We showed that after 12 weeks of in vivo implantation in an immunocompromised mouse model such upscaled grafts remodeled into bone organs (including bone marrow tissues) retaining the defined shape and size. Finally, we replicated similar outcome (albeit with a slight reduction in cartilage and bone formation) by using minimally expanded pediatric ASCs (3 × 106 cells per grafts) in the same in vitro and in vivo settings, thereby validating the compatibility of our pediatric phalanx engineering strategy with a clinically relevant scenario. Taken together, these results represent a proof of concept of an autologous approach to generate osteogenic phalangeal grafts of pertinent clinical size, using ASCs in children born with symbrachydactyly, despite a limited amount of tissue available from pediatric patients.

Osteochondrosis in the central and third tarsal bones of young horses

Recently, the central and third tarsal bones of 23 equine fetuses and foals were examined using micro-computed tomography. Radiological changes, including incomplete ossification and focal ossification defects interpreted as osteochondrosis, were detected in 16 of 23 cases. The geometry of the osteochondrosis defects suggested they were the result of vascular failure, but this requires histological confirmation. The study aim was to examine central and third tarsal bones from the 16 cases and to describe the tissues present, cartilage canals, and lesions, including suspected osteochondrosis lesions. Cases included 9 males and 7 females from 0 to 150 days of age, comprising 11 Icelandic horses, 2 standardbred horses, 2 warmblood riding horses, and 1 coldblooded trotting horse. Until 4 days of age, all aspects of the bones were covered by growth cartilage, but from 105 days, the dorsal and plantar aspects were covered by fibrous tissue undergoing intramembranous ossification. Cartilage canal vessels gradually decreased but were present in most cases up to 122 days and were absent in the next available case at 150 days. Radiological osteochondrosis defects were confirmed in histological sections from 3 cases and consisted of necrotic vessels surrounded by ischemic chondronecrosis (articular osteochondrosis) and areas of retained, morphologically viable hypertrophic chondrocytes (physeal osteochondrosis). The central and third tarsal bones formed by both endochondral and intramembranous ossification. The blood supply to the growth cartilage of the central and third tarsal bones regressed between 122 and 150 days of age. Radiological osteochondrosis defects represented vascular failure, with chondrocyte necrosis and retention, or a combination of articular and physeal osteochondrosis.

Endochondral Repair of Jawbone Defects Using Periosteal Cell Spheroids

Recapitulation of the natural healing process is receiving increasing recognition as a strategy to induce robust tissue regeneration. Endochondral ossification has been recognized as an essential reparative approach in natural jawbone defect healing. However, such an approach has been overlooked in the recent development of cell-based therapeutics for jawbone repair. Therefore, this study aimed to explore a bioinspired stem cell-based strategy for jawbone repair by mimicking the mesenchymal condensation of progenitor cells during the early endochondral ossification process. For this purpose, passage 3 of jawbone periosteum-derived cells (jb-PDCs) was cultured in our previously reported nonadherent microwells (200 µm in diameter, 148 µm in depth, and 100 µm space in between) and self-assembled into spheroids with a diameter of 96.4 ± 5.8 µm after 48 h. Compared to monolayer culture, the jb-PDC spheroids showed a significant reduction of stemness marker expression evidenced by flow cytometry. Furthermore, a significant upregulation of chondrogenic transcription factor SOX9 in both gene and protein levels was observed in the jb-PDC spheroids after 48 h of chondrogenic induction. RNA sequencing and Western blotting analysis further suggested that the enhanced SOX9-mediated chondrogenic differentiation in jb-PDC spheroids was attributed to the activation of the p38 MAPK pathway. Impressively, inhibition of p38 kinase activity significantly attenuated chondrogenic differentiation jb-PDC spheroids, evidenced by a significant decline of SOX9 in both gene and protein levels. Strikingly, the jb-PDC spheroids implanted in 6- to 8-wk-old male C57BL/6 mice with critical-size jawbone defects (1.8 mm in diameter) showed an evident contribution to cartilaginous callus formation after 1 wk, evidenced by histological analysis. Furthermore, micro-computed tomography analysis showed that the jb-PDC spheroids significantly accelerated bone healing after 2 wk in the absence of exogenous growth factors. In sum, the presented findings represent the successful development of cell-based therapeutics to reengineer the endochondral bone repair process and illustrate the potential application to improve bone repair and regeneration in the craniofacial skeleton.

The effects of physiological and injurious hydrostatic pressure on murine ex vivo articular and growth plate cartilage explants: an RNAseq study

Introduction

Chondrocytes are continuously exposed to loads placed upon them. Physiological loads are pivotal to the maintenance of articular cartilage health, while abnormal loads contribute to pathological joint degradation. Similarly, the growth plate cartilage is subject to various loads during growth and development. Due to the high-water content of cartilage, hydrostatic pressure is considered one of the main biomechanical influencers on chondrocytes and has been shown to play an important role in the mechano-regulation of cartilage.

Methods

Herein, we conducted RNAseq analysis of ex vivo hip cap (articular), and metatarsal (growth plate) cartilage cultures subjected to physiological (5 MPa) and injurious (50 MPa) hydrostatic pressure, using the Illumina platform (n = 4 replicates).

Results

Several hundreds of genes were shown to be differentially modulated by hydrostatic pressure, with the majority of these changes evidenced in hip cap cartilage cultures (375 significantly upregulated and 322 downregulated in 5 MPa versus control; 1022 upregulated and 724 downregulated in 50 MPa versus control). Conversely, fewer genes were differentially affected by hydrostatic pressure in the metatarsal cultures (5 significantly upregulated and 23 downregulated in 5 MPa versus control; 7 significantly upregulated and 19 downregulated in 50 MPa versus control). Using Gene Ontology annotations for Biological Processes, in the hip cap data we identified a number of pathways that were modulated by both physiological and injurious hydrostatic pressure. Pathways upregulated in response to 50 MPa versus control, included those involved in the generation of precursor metabolites and cellular respiration. Biological processes that were downregulated in this tissue included ossification, connective tissue development, and chondrocyte differentiation.

Discussion

Collectively our data highlights the divergent chondrocyte phenotypes in articular and growth plate cartilage. Further, we show that the magnitude of hydrostatic pressure application has distinct effects on gene expression and biological processes in hip cap cartilage explants. Finally, we identified differential expression of a number of genes that have previously been identified as osteoarthritis risk genes, including Ctsk, and Chadl. Together these data may provide potential genetic targets for future investigations in osteoarthritis research and novel therapeutics.

Curcumin-loaded porous scaffold: an anti-angiogenic approach to inhibit endochondral ossification

Bone marrow stem cells (BMSCs) are recognized for their robust proliferative capabilities and multidirectional differentiation potential. Ectopic endochondral ossification of BMSC-generated cartilage in subcutaneous environments is a concern associated with vascularization. Hence, devising a reliable strategy to inhibit vascularization is crucial. In this study, an anti-angiogenic drug, curcumin (Cur), was encapsulated into gelatin to create a porous Cur/Gelatin scaffold, with the aim of inhibiting vascular invasion and preventing endochondral ossification of BMSC-regenerated cartilage. In vitro wound healing tests demonstrated that a 30 μM Cur solution could inhibit the migration and growth of human umbilical vein endothelial cells without impeding BMSCs migration and growth. Compared to the gelatin scaffold, our findings verified that the Cur/Gelatin scaffold significantly inhibited vascular invasion after being subcutaneously implanted into rabbits for 12 weeks, as evidenced by gross observation and immunofluorescence CD31 staining. Moreover, both the porous gelatin and Cur/Gelatin scaffolds were populated with BMSCs and underwent in vitro chondrogenic cultivation to produce cartilage, followed by subcutaneous implantation in rabbits for 12 weeks. Histological examinations (including HE, Safranin-O/Fast Green, toluidine blue, and immunohistochemical COL II staining) revealed that the BMSC-generated cartilage in the gelatin group exhibited prominent endochondral ossification. In contrast, the BMSC-generated cartilage in the Cur/Gelatin group maintained cartilage features, such as cartilage matrix and lacunar structure. This study suggests that Cur-loaded scaffolds offer a reliable platform to inhibit endochondral ossification of BMSC-generated cartilage.

Loose Bodies Found in the Human Intra-Articular Space Showed Characteristics Similar to Endochondral Bone Formation

OBJECTIVE

Loose bodies are free-floating tissues of cartilage and bone that can cause pain, swelling, the inability to straighten the knee, or intermittent locking of the knee. Loose bodies can arise from degenerative joint disease, flake fractures, osteochondritis dissecans, or chondromatosis. We hypothesized that loose bodies can be classified in stages with tissue characteristics similar to endochondral ossification.

DESIGN

Loose bodies were harvested from patients undergoing joint replacement. Samples were processed for histology, gene expression analysis, and micro-computed tomography (µCT). Cartilage- and bone-related genes and proteins were selected for immunofluorescence stainings (collagen type I, II, and X, SOX9 [SRY-box transcription factor 9], and MMP13 [matrix metalloproteinase 13]) and gene expression analysis (FN [fibronectin], COL1A1, COL2A1, COL10A1, SOX9, MMP13, and aggrecan [ACAN]).

RESULTS

Loose bodies were grouped in 4 stages: fibrous, (mineralized) cartilaginous, cartilage and bone, and bone. Hyaline-like cartilage tissue with Benninghoff arcades was present in stages 2 and 3. A transition from cartilaginous to mineralized tissue and bone trabecula was defined by an increase in COL1A1 and COL10A1 (stage 3 vs. 4: p = 0.047) positive area. Stage 4 showed typical trabecular bone tissue. The relative volume of calcified tissue (mineralized cartilage and bone tissue) decreased with stages (stages 1-2 vs. 3: p = 0.002; stage 1-2 vs. 4: p = 0.012). COL2A1 expression and stained area decreased from stages 1-2 to 4 (p = 0.010 and p = 0.004). ACAN expression decreased from stage 1-2 to stage 3 (p = 0.049) and stage 4 (p = 0.002).

CONCLUSION

Loose bodies show tissue characteristics similar to endochondral ossification. They are probably a relevant substrate for regenerative therapeutic interventions in joint disease.

Costal Cartilage Graft Repair Osteochondral Defect in a Mouse Model

OBJECTIVE

Osteochondral defects develop into osteoarthritis without intervention. Costal cartilage can be utilized as an alternative source for repairing osteochondral defect. Our previous clinical study has shown the successful osteochondral repair by costal cartilage graft with integration into host bone bed. In this study, we investigate how cartilaginous graft adapt to osteochondral environment and the mechanism of bone-cartilage interface formation.

DESIGN

Costal cartilage grafting was performed in C57BL/6J mice and full-thickness osteochondral defect was made as control. 3D optical profiles and micro-CT were applied to evaluate the reconstruction of articular cartilage surface and subchondral bone as well as gait analysis to evaluate articular function. Histological staining was performed at 2, 4, and 8 weeks after surgery. Moreover, costal cartilage from transgenic mice with fluorescent markers were transplanted into wild-type mice to observe the in vivo changes of costal chondrocytes.

RESULTS

At 8 weeks after surgery, 3D optical profiles and micro-CT showed that in the graft group, the articular surface and subchondral bone were well preserved. Gait analysis and International Cartilage Repair Society (ICRS) score evaluation showed a good recovery of joint function and histological repair in the graft group. Safranin O staining showed the gradual integration of graft and host tissue. Costal cartilage from transgenic mice with fluorescent markers showed that donor-derived costal chondrocytes turned into osteocytes in the subchondral area of host femur.

CONCLUSION

Costal cartilage grafting shows both functional and histological repair of osteochondral defect in mice. Graft-derived costal chondrocytes differentiate into osteocytes and contribute to endochondral ossification.

Dysregulation of insulin-like growth factor-1 signaling in postnatal bone elongation

Insulin-like growth factor-1 (IGF-1) is a critical modulator of cell growth and survival, making it a central part of maintaining essentially every biological system in the body. Knowledge of the intricate mechanisms involved in activating IGF-1 signaling is not only key to understanding basic processes of growth and development, but also for addressing diseases, such as cancer and diabetes. This brief review explores how dysregulation of normal IGF-1 signaling can impact growth by examining its role in postnatal bone elongation. IGF-1 actions are dysregulated in autoimmune diseases, such as juvenile idiopathic arthritis and chronic kidney disease, which results in growth stunting. Conversely, childhood obesity results in growth acceleration, premature growth cessation, and ultimately, diminished bone quality, while systemic IGF-1 levels remain normal. Understanding the role of IGF-1 signaling in normal and dysregulated growth can add to other studies that address how this system regulates chronic diseases.

A Bioglass-Poly(lactic-co-glycolic Acid) Scaffold@Fibrin Hydrogel Construct to Support Endochondral Bone Formation

Bone tissue engineering using stem cells to build bone directly on a scaffold matrix often fails due to lack of oxygen at the injury site. This may be avoided by following the endochondral ossification route; herein, a cartilage template is promoted first, which can survive hypoxic environments, followed by its hypertrophy and ossification. However, hypertrophy is so far only achieved using biological factors. This work introduces a Bioglass-Poly(lactic-co-glycolic acid@fibrin (Bg-PLGA@fibrin) construct where a fibrin hydrogel infiltrates and encapsulates a porous Bg-PLGA. The hypothesis is that mesenchymal stem cells (MSCs) loaded in the fibrin gel and induced into chondrogenesis degrade the gel and become hypertrophic upon reaching the stiffer, bioactive Bg-PLGA core, without external induction factors. Results show that Bg-PLGA@fibrin induces hypertrophy, as well as matrix mineralization and osteogenesis; it also promotes a change in morphology of the MSCs at the gel/scaffold interface, possibly a sign of osteoblast-like differentiation of hypertrophic chondrocytes. Thus, the Bg-PLGA@fibrin construct can sequentially support the different phases of endochondral ossification purely based on material cues. This may facilitate clinical translation by decreasing in-vitro cell culture time pre-implantation and the complexity associated with the use of external induction factors.

Expression of calcitonin gene-related peptide induces ligament degeneration through endochondral ossification in osteoarthritis

AIM

Osteoarthritis (OA) is a disease in which degeneration occurs in various tissues such as cartilage and subchondral bone. Degeneration of ligaments also plays an important role in OA progression, resulting in an increase in chondrocytes and ossification, but the factor that causes this is still unclear. It is reported that the expression of calcitonin gene-related peptide (CGRP) increases OA progression, and CGRP might play a role in ligament degeneration because CGRP has a function in endochondral ossification. The purpose of this study is to analyze the mechanism of ligament degeneration and the function of CGRP.

METHODS

To examine the relationship between ligament degeneration and CGRP expression, human posterior cruciate ligaments (PCL) from OA patients, and senescence-accelerated mouse prone 8 (SAMP8) mice were histologically analyzed. The effect of CGRP on human ligament cells on chondrogenesis, osteogenesis, and adipogenesis was also examined.

RESULTS

In human PCL and SAMP8 mice, CGRP expression increased as degeneration progressed, and decreased in severe degeneration. CGRP was expressed in the chondrocyte-like cells with SOX9. CGRP-positive cells expressing type II collagen increased with OA progression. CGRP upregulated the gene expression of VEGF, SOX9, RUNX2, COL10a1, and MMP13 in the human ligament cells. CGRP also promoted chondrogenesis and osteogenesis from the human ligament cells.

CONCLUSION

During OA progression, CGRP plays a role in the transdifferentiation from ligament cells to chondrocytes and promotes endochondral ossification in the ligament. CGRP would be the therapeutic target to prevent ligament degeneration.

A Model of Type II Collagen-Induced Spondylitis and Arthritis in F1 Hybrid Male Mice

In this study, we tested a new model of ankylosing spondylitis in order to determine its histological and radiological features needed to investigate peripheral arthritis, spondylitis, and formation of the new bone tissues. F1 hybrid male mice (BALB/c×DBA/1), a progeny of spondylitis-susceptible BALB/c male mice and rheumatoid arthritis-susceptible DBA/1 female mice, were immunized intraperitoneally with bovine type II collagen (CII) mixed with adjuvant dimethyldioctadecylammonium bromide. Radiological and histological studies were performed at the peak of swelling, redness, and stiffness. The incidence of peripheral arthritis and spondylitis induced by CII in F1 hybrid mice were 66 and 62%, respectively. X-ray examination revealed bone erosion and spondylitis in the peripheral joints, as well as the formation of new bone tissues in the coccygeal vertebrae and between LIII and LIV vertebrae. The histological study showed lymphocyte and plasma cell infiltration, capillary dilation, congestion, and endochondral ossification of the lumbar vertebrae. This novel model of CII-induced spondylitis in F1 hybrid mice provoked axial and peripheral arthritides inducing chronic inflammation. In this model, the formation of new bone tissue in the stiff spine is characterized by endochondral ossification. The advanced model is an additional and valuable tool for investigation of the autoimmune reactions in spondylitis.

Global and Conditional Disruption of the Igf-I Gene in Osteoblasts and/or Chondrocytes Unveils Epiphyseal and Metaphyseal Bone-Specific Effects of IGF-I in Bone

To evaluate the relative importance of IGF-I expression in various cell types for endochondral ossification, we quantified the trabecular bone at the secondary spongiosa and epiphysis of the distal femur in 8-12-week-old male mice with a global knockout of the Igf-I gene, as well as the conditional deletion of Igf-I in osteoblasts, chondrocytes, and osteoblasts/chondrocytes and their corresponding wild-type control littermates. The osteoblast-, chondrocyte-, and osteoblast/chondrocyte-specific Igf-I conditional knockout mice were generated by crossing Igf-I floxed mice with Cre transgenic mice in which Cre expression is under the control of either the Col1α2 or Col2α1 promoter. We found that the global disruption of Igf-I resulted in 80% and 70% reductions in bone size, defined as total volume, at the secondary spongiosa and epiphysis of the distal femur, respectively. The abrogation of Igf-I in Col1α2-producing osteoblasts but not Col2α1-producing chondrocytes decreased bone size by 25% at both the secondary spongiosa and epiphysis. In comparison, the deletion of the Igf-I globally or specifically in osteoblasts or chondrocytes reduced trabecular bone mass by 25%. In contrast, the universal deletion of Igf-I in all cells, but not the conditional disruption of Igf-I in osteoblasts and/or chondrocytes reduced trabecular bone mass in the epiphysis. The reduced trabecular bone mass at the secondary spongiosa in osteoblast- and/or chondrocyte-specific Igf-I conditional knockout mice is caused by the reduced trabecular number and increased trabecular separation. Immunohistochemistry studies found that the expression levels of chondrocyte (COL10, MMP13) and osteoblast (BSP) markers were less in the secondary spongiosa and the epiphyses in the global Igf-I deletion mice. Our data indicate that local and endocrine Igf-I act pleiotropically and in a cell type- and bone compartment-dependent manner in bone.