Interaction of FGF, Ihh/Pthlh, and BMP signaling integrates chondrocyte proliferation and hypertrophic differentiation

Inorganic Biomaterials Shape the Transcriptome Profile to Induce Endochondral Differentiation

Minerals play a vital role, working synergistically with enzymes and other cofactors to regulate physiological functions including tissue healing and regeneration. The bioactive characteristics of mineral-based nanomaterials can be harnessed to facilitate in situ tissue regeneration by attracting endogenous progenitor and stem cells and subsequently directing tissue-specific differentiation. Here, cellular responses of human mesenchymal stem/stromal cells to traditional bioactive mineral-based nanomaterials, such as hydroxyapatite, whitlockite, silicon-dioxide, and the emerging synthetic 2D nanosilicates are investigated. Transcriptome sequencing is utilized to probe the cellular response and determine the significantly affected signaling pathways due to exposure to these inorganic nanomaterials. Transcriptome profiles of stem cells treated with nanosilicates reveals a stabilized skeletal progenitor state suggestive of endochondral differentiation. This observation is bolstered by enhanced deposition of matrix mineralization in nanosilicate treated stem cells compared to control or other treatments. Specifically, use of 2D nanosilicates directs osteogenic differentiation of stem cells via activation of bone morphogenetic proteins and hypoxia-inducible factor 1-alpha signaling pathway. This study provides  insight into impact of nanomaterials on cellular gene expression profile and predicts downstream effects of nanomaterial induction of endochondral differentiation.

KLF2 regulates stemness of human mesenchymal stem cells by targeting FGFR3

Mesenchymal stem cells (MSCs) are an attractive source of pluripotent cells for regenerative therapy; however, maintaining stemness and self-renewal of MSCs during expansion ex vivo is challenging. For future clinical applications, it is essential to define the roles and signaling pathways that regulate the fate of MSCs. Based on our earlier finding that Krüppel-like factor 2 (KLF2) participates in maintaining stemness in MSCs, we examined further the role of this factor in intrinsic signaling pathways. Using a chromatin immunoprecipitation (ChIP)-sequence assay, we found that the FGFR3 gene is a KLF2 binding site. Knockdown of FGFR3 significantly decreased the levels of key pluripotency factors, enhanced the expression of differentiation-related genes and down-regulated colony formation of human bone marrow MSCs (hBMSCs). Using alizarin red S and oil red O staining, we found that knockdown of FGFR3 inhibited the osteogenic and adipogenic ability of MSCs under conditions of differentiation. The ChIP-qPCR assay confirmed that KLF2 interacts with the promoter regions of FGFR3. Our findings suggest that KLF2 promotes hBMSC stemness by direct regulation of FGFR. Our findings may contribute to enhanced MSC stemness by genetic modification of stemness-related genes.

Enhanced BMP signaling leads to enlarged nasal cartilage formation in mice

Bone morphogenetic proteins (BMPs) are required for craniofacial bone development. However, it remains elusive how BMP signaling regulates craniofacial cartilage development. To address this question, we utilized a genetic system to enhance BMP signaling via one of BMP type I receptors ALK2 in a chondrocyte-specific manner (hereafter Ca-Alk2:Col2-Cre) in mice. Ca-Alk2:Col2-Cre mice died shortly after birth due to severe craniofacial abnormalities including cleft palate, defective tongue, and shorter mandible formation. Histological analysis revealed that these phenotypes were attributed to the extensive chondrogenesis. Compared with controls, enhanced SOX9 and RUNX2 production were observed in nasal cartilage of Ca-Alk2:Col2-Cre mice. To reveal the mechanisms responsible for enlarged nasal cartilage, we examined Smad-dependent and Smad-independent BMP signaling pathways. While the Smad-independent BMP signaling pathway including p38, ERK, and JNK remained silent, the Smad1/5/9 was highly phosphorylated in Ca-Alk2:Col2-Cre mice. Interestingly, Ca-Alk2:Col2-Cre mice showed enhanced S6 kinase phosphorylation, a readout of mammalian target of rapamycin complex 1 (mTORC1). These findings may suggest that enhanced Smad-dependent BMP signaling positively regulates the mTOR pathway and stimulates chondrocytes toward hypertrophic differentiation, thereby leading to enlarged nasal cartilage formation in mice.

Advanced Skeletal Ossification Is Associated with Genetic Variants in Chronologically Young Beef Heifers

Osteogenesis is a developmental process critical for structural support and the establishment of a dynamic reservoir for calcium and phosphorus. Changes in livestock breeding over the past 100 years have resulted in earlier bone development and increased physical size of cattle. Advanced skeletal maturity is now commonly observed at harvest, with heifers displaying more mature bone than is expected at 30 months of age (MOA). We surmise that selection for growth traits and earlier reproductive maturity resulted in co-selection for accelerated skeletal ossification. This study examines the relationship of single nucleotide polymorphisms (SNPs) in 793 beef heifers under 30 MOA with USDA-graded skeletal maturity phenotypes (A-, B-, C- skeletal maturity). Further, the estrogen content of FDA-approved hormonal implants provided to heifers prior to harvest was evaluated in association with the identified SNPs and maturities. Association tests were performed, and the impact of the implants were evaluated as covariates against genotypes using a logistic regression model. SNPs from the ESR1, ALPL, PPARGC1B, SORCS1 genes, and SNPs near KLF14, ANKRD61, USP42, H1C1, OVCA2, microRNA mir-29a were determined to be associated with the advanced skeletal ossification phenotype in heifers. Higher dosage estrogen implants increased skeletal maturity in heifers with certain SNP genotypes.

Suppressing Chondrocyte Hypertrophy to Build Better Cartilage

Current clinical strategies for restoring cartilage defects do not adequately consider taking the necessary steps to prevent the formation of hypertrophic tissue at injury sites. Chondrocyte hypertrophy inevitably causes both macroscopic and microscopic level changes in cartilage, resulting in adverse long-term outcomes following attempted restoration. Repairing/restoring articular cartilage while minimizing the risk of hypertrophic neo tissue formation represents an unmet clinical challenge. Previous investigations have extensively identified and characterized the biological mechanisms that regulate cartilage hypertrophy with preclinical studies now beginning to leverage this knowledge to help build better cartilage. In this comprehensive article, we will provide a summary of these biological mechanisms and systematically review the most cutting-edge strategies for circumventing this pathological hallmark of osteoarthritis.

Are serum thyroid hormone, parathormone, calcium, and vitamin D levels associated with lumbar spine degeneration? A cross-sectional observational clinical study

PURPOSE

Low back pain (LBP) impairs the quality of life and rises healthcare costs. The association of spine degeneration and LBP with metabolic disorders have been reported, previously. However, metabolic processes related with spine degeneration remained unclear. We aimed to analyze whether serum thyroid hormones, parathormone, calcium, and vitamin D levels were associated with lumbar intervertebral disc degeneration (IVDD), Modic changes, and fatty infiltration in the paraspinal muscles.

METHODS

We cross-sectionally analyzed a retrospective database. Patients who visited internal medicine outpatient clinics with suspect of endocrine disorders and chronic LBP were searched. Patients with biochemistry results within 1 week before lumbar spine magnetic resonance imaging (MRI) were included. Age- and gender-matched cohorts were made-up and analyzed.

RESULTS

Patients with higher serum free thyroxine levels were more likely to have severe IVDD. They were also more likely to have fattier multifidus and erector spinae at upper lumbar levels, less fatty psoas and less Modic changes at lower lumbar levels. Higher PTH levels were observed in patients with severe IVDD at L4-L5 level. Patients with lower serum vitamin D and calcium levels had more Modic changes and fattier paraspinal muscles at upper lumbar levels.

CONCLUSION

Serum hormone, vitamin D, and calcium levels were associated with not only IVDD and Modic changes but also with fatty infiltration in the paraspinal muscles, mainly at upper lumbar levels in patients with symptomatic backache presenting to a tertiary care center. Complex inflammatory, metabolic, and mechanical factors present in the backstage of spine degeneration.

Roles of Local Soluble Factors in Maintaining the Growth Plate: An Update

The growth plate is a cartilaginous tissue found at the ends of growing long bones, which contributes to the lengthening of bones during development. This unique structure contains at least three distinctive layers, including resting, proliferative, and hypertrophic chondrocyte zones, maintained by a complex regulatory network. Due to its soft tissue nature, the growth plate is the most susceptible tissue of the growing skeleton to injury in childhood. Although most growth plate damage in fractures can heal, some damage can result in growth arrest or disorder, impairing leg length and resulting in deformity. In this review, we re-visit previously established knowledge about the regulatory network that maintains the growth plate and integrate current research displaying the most recent progress. Next, we highlight local secretary factors, such as Wnt, Indian hedgehog (Ihh), and parathyroid hormone-related peptide (PTHrP), and dissect their roles and interactions in maintaining cell function and phenotype in different zones. Lastly, we discuss future research topics that can further our understanding of this unique tissue. Given the unmet need to engineer the growth plate, we also discuss the potential of creating particular patterns of soluble factors and generating them in vitro.

Hopes and opportunities of stem cells from human exfoliated deciduous teeth (SHED) in cartilage tissue regeneration

Cartilage lesions are common conditions, affecting elderly and non-athletic populations. Despite recent advances, cartilage regeneration remains a major challenge today. The absence of an inflammatory response following damage and the inability of stem cells to penetrate into the healing site due to the absence of blood and lymph vessels are assumed to hinder joint repair. Stem cell-based regeneration and tissue engineering have opened new horizons for treatment. With advances in biological sciences, especially stem cell research, the function of various growth factors in the regulation of cell proliferation and differentiation has been established. Mesenchymal stem cells (MSCs) isolated from different tissues have been shown to increase into therapeutically relevant cell numbers and differentiate into mature chondrocytes. As MSCs can differentiate and become engrafted inside the host, they are considered suitable candidates for cartilage regeneration. Stem cells from human exfoliated deciduous teeth (SHED) provide a novel and non-invasive source of MSCs. Due to their simple isolation, chondrogenic differentiation potential, and minimal immunogenicity, they can be an interesting option for cartilage regeneration. Recent studies have reported that SHED-derived secretome contains biomolecules and compounds that efficiently promote regeneration in damaged tissues, including cartilage. Overall, this review highlighted the advances and challenges of cartilage regeneration using stem cell-based therapies by focusing on SHED.

The role of the immune microenvironment in bone, cartilage, and soft tissue regeneration: from mechanism to therapeutic opportunity

Bone, cartilage, and soft tissue regeneration is a complex spatiotemporal process recruiting a variety of cell types, whose activity and interplay must be precisely mediated for effective healing post-injury. Although extensive strides have been made in the understanding of the immune microenvironment processes governing bone, cartilage, and soft tissue regeneration, effective clinical translation of these mechanisms remains a challenge. Regulation of the immune microenvironment is increasingly becoming a favorable target for bone, cartilage, and soft tissue regeneration; therefore, an in-depth understanding of the communication between immune cells and functional tissue cells would be valuable. Herein, we review the regulatory role of the immune microenvironment in the promotion and maintenance of stem cell states in the context of bone, cartilage, and soft tissue repair and regeneration. We discuss the roles of various immune cell subsets in bone, cartilage, and soft tissue repair and regeneration processes and introduce novel strategies, for example, biomaterial-targeting of immune cell activity, aimed at regulating healing. Understanding the mechanisms of the crosstalk between the immune microenvironment and regeneration pathways may shed light on new therapeutic opportunities for enhancing bone, cartilage, and soft tissue regeneration through regulation of the immune microenvironment.

A critical bioenergetic switch is regulated by IGF2 during murine cartilage development

Long bone growth requires the precise control of chondrocyte maturation from proliferation to hypertrophy during endochondral ossification, but the bioenergetic program that ensures normal cartilage development is still largely elusive. We show that chondrocytes have unique glucose metabolism signatures in these stages, and they undergo bioenergetic reprogramming from glycolysis to oxidative phosphorylation during maturation, accompanied by an upregulation of the pentose phosphate pathway. Inhibition of either oxidative phosphorylation or the pentose phosphate pathway in murine chondrocytes and bone organ cultures impaired hypertrophic differentiation, suggesting that the appropriate balance of these pathways is required for cartilage development. Insulin-like growth factor 2 (IGF2) deficiency resulted in a profound increase in oxidative phosphorylation in hypertrophic chondrocytes, suggesting that IGF2 is required to prevent overactive glucose metabolism and maintain a proper balance of metabolic pathways. Our results thus provide critical evidence of preference for a bioenergetic pathway in different stages of chondrocytes and highlight its importance as a fundamental mechanism in skeletal development.

MPSI Manifestations and Treatment Outcome: Skeletal Focus

Mucopolysaccharidosis type I (MPSI) (OMIM #252800) is an autosomal recessive disorder caused by pathogenic variants in the IDUA gene encoding for the lysosomal alpha-L-iduronidase enzyme. The deficiency of this enzyme causes systemic accumulation of glycosaminoglycans (GAGs). Although disease manifestations are typically not apparent at birth, they can present early in life, are progressive, and include a wide spectrum of phenotypic findings. Among these, the storage of GAGs within the lysosomes disrupts cell function and metabolism in the cartilage, thus impairing normal bone development and ossification. Skeletal manifestations of MPSI are often refractory to treatment and severely affect patients’ quality of life. This review discusses the pathological and molecular processes leading to impaired endochondral ossification in MPSI patients and the limitations of current therapeutic approaches. Understanding the underlying mechanisms responsible for the skeletal phenotype in MPSI patients is crucial, as it could lead to the development of new therapeutic strategies targeting the skeletal abnormalities of MPSI in the early stages of the disease.

Identification of COX4I2 as a hypoxia-associated gene acting through FGF1 to promote EMT and angiogenesis in CRC

Background

Current evidence suggests that the hypoxic tumor microenvironment further aggravates tumor progression, leading to poor therapeutic outcomes. There is as yet no biomarker capable of evaluating the hypoxic state of the tumor. The cytochrome c oxidase (COX) subunit is crucial to the mitochondrial respiratory chain.

Methods

We investigated the potential oncogenic role of COX subunit 4 isoform 2 gene (COX4I2) in colorectal cancer (CRC) by least absolute shrinkage and selection operator (LASSO) and COX regression analysis to examine whether COX4I2 overexpression can predict colorectal cancer (CRC) prognosis. The association of COX4I2 levels with clinical features and its biological actions were evaluated both in vitro and in vivo.

Results

Our analysis showed that elevated COX4I2 levels were correlated with poor clinical outcomes. We also observed that that COX4I2 may be involved in epithelial-mesenchymal transition, activation of cancer-related fibroblasts and angiogenesis in relation to fibroblast growth factor 1.

Conclusions

The COX4I2 level may be a predictor of outcome in CRC and may represent a novel target for treatment development.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s11658-022-00380-2.

Cranial Base Synchondrosis: Chondrocytes at the Hub

The cranial base is formed by endochondral ossification and functions as a driver of anteroposterior cranial elongation and overall craniofacial growth. The cranial base contains the synchondroses that are composed of opposite-facing layers of resting, proliferating and hypertrophic chondrocytes with unique developmental origins, both in the neural crest and mesoderm. In humans, premature ossification of the synchondroses causes midfacial hypoplasia, which commonly presents in patients with syndromic craniosynostoses and skeletal Class III malocclusion. Major signaling pathways and transcription factors that regulate the long bone growth plate-PTHrP-Ihh, FGF, Wnt, BMP signaling and Runx2-are also involved in the cranial base synchondrosis. Here, we provide an updated overview of the cranial base synchondrosis and the cell population within, as well as its molecular regulation, and further discuss future research opportunities to understand the unique function of this craniofacial skeletal structure.

Developmental principles informing human pluripotent stem cell differentiation to cartilage and bone

Human pluripotent stem cells can differentiate into any cell type given appropriate signals and hence have been used to research early human development of many tissues and diseases. Here, we review the major biological factors that regulate cartilage and bone development through the three main routes of neural crest, lateral plate mesoderm and paraxial mesoderm. We examine how these routes have been used in differentiation protocols that replicate skeletal development using human pluripotent stem cells and how these methods have been refined and improved over time. Finally, we discuss how pluripotent stem cells can be employed to understand human skeletal genetic diseases with a developmental origin and phenotype, and how developmental protocols have been applied to gain a better understanding of these conditions.

A transcriptome sequencing study on the effect of macro-pores in hydrogel scaffolds on global gene expression of laden human cartilage chondrocytes

The macro-porous hydrogel scaffolds can not only enhance the proliferation of laden chondrocytes but also favor the deposition of hyaline cartilaginous extracellular matrix, however, the underlying molecular mechanism is still unclear. Herein, the global gene expression of human cartilage chondrocytes (HCCs) encapsulated in traditional hydrogel (Gel) constructs and micro-cavitary gel (MCG) constructs are investigated by using high-throughput RNA sequencing (RNA-seq). The differentially expressed genes (DEGs) between the HCCs cultured in Gel and MCG constructs have been identified via bioinformatics analysis. Significantly, the DEGs that promote cell proliferation (e.g. POSTN, MKI67, KIF20A) or neo-cartilage formation (e.g. COL2, ASPN, COMP, FMOD, FN1), are more highly expressed in MCG constructs than in Gel constructs, while the expressions of the DEGs associated with chondrocyte hypertrophy (e.g. EGR1, IBSP) are upregulated in Gel constructs. The expression of representative DEGs is verified at both mRNA and protein levels. Besides, cellular viability and morphology as well as the enriched signaling pathway of DEGs are studied in detail. These results of this work may provide data for functional tissue engineering of cartilage.

Ciliary IFT88 Protects Coordinated Adolescent Growth Plate Ossification From Disruptive Physiological Mechanical Forces

Compared with our understanding of endochondral ossification, much less is known about the coordinated arrest of growth defined by the narrowing and fusion of the cartilaginous growth plate. Throughout the musculoskeletal system, appropriate cell and tissue responses to mechanical force delineate morphogenesis and ensure lifelong health. It remains unclear how mechanical cues are integrated into many biological programs, including those coordinating the ossification of the adolescent growth plate at the cessation of growth. Primary cilia are microtubule-based organelles tuning a range of cell activities, including signaling cascades activated or modulated by extracellular biophysical cues. Cilia have been proposed to directly facilitate cell mechanotransduction. To explore the influence of primary cilia in the mouse adolescent limb, we conditionally targeted the ciliary gene Intraflagellar transport protein 88 (Ift88^fl/fl ) in the juvenile and adolescent skeleton using a cartilage-specific, inducible Cre (AggrecanCreER^T2 Ift88^fl/fl ). Deletion of IFT88 in cartilage, which reduced ciliation in the growth plate, disrupted chondrocyte differentiation, cartilage resorption, and mineralization. These effects were largely restricted to peripheral tibial regions beneath the load-bearing compartments of the knee. These regions were typified by an enlarged population of hypertrophic chondrocytes. Although normal patterns of hedgehog signaling were maintained, targeting IFT88 inhibited hypertrophic chondrocyte VEGF expression and downstream vascular recruitment, osteoclastic activity, and the replacement of cartilage with bone. In control mice, increases to physiological loading also impair ossification in the peripheral growth plate, mimicking the effects of IFT88 deletion. Limb immobilization inhibited changes to VEGF expression and epiphyseal morphology in Ift88cKO mice, indicating the effects of depletion of IFT88 in the adolescent growth plate are mechano-dependent. We propose that during this pivotal phase in adolescent skeletal maturation, ciliary IFT88 protects uniform, coordinated ossification of the growth plate from an otherwise disruptive heterogeneity of physiological mechanical forces. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Two Modulators of Skeletal Development: BMPs and Proteoglycans

During embryogenesis, skeletal development is tightly regulated by locally secreted growth factors that interact with proteoglycans (PGs) in the extracellular matrix (ECM). Bone morphogenetic proteins (BMPs) are multifunctional growth factors that play critical roles in cartilage maturation and bone formation. BMP signals are transduced from plasma membrane receptors to the nucleus through both canonical Smad and noncanonical p38 mitogen-activated protein kinase (MAPK) pathways. BMP signalling is modulated by a variety of endogenous and exogenous molecular mechanisms at different spatiotemporal levels and in both positive and negative manners. As an endogenous example, BMPs undergo extracellular regulation by PGs, which generally regulate the efficiency of ligand-receptor binding. BMP signalling can also be exogenously perturbed by a group of small molecule antagonists, such as dorsomorphin and its derivatives, that selectively bind to and inhibit the intracellular kinase domain of BMP type I receptors. In this review, we present a current understanding of BMPs and PGs functions in cartilage maturation and osteoblast differentiation, highlighting BMP–PG interactions. We also discuss the identification of highly selective small-molecule BMP receptor type I inhibitors. This review aims to shed light on the importance of BMP signalling and PGs in cartilage maturation and bone formation.

Type VI collagen regulates endochondral ossification in the temporomandibular joint

For many years there has been a keen interest in developing regenerative treatment for temporomandibular joint–osteoarthritis (TMJ‐OA). Currently, there is no consensus treatment due to the limited self‐healing ability of articular cartilage and lack of understanding of the complex mechanisms regulating cartilage development in the TMJ. Endochondral ossification, the process of subchondral bone formation through chondrocyte differentiation, is critical for TMJ growth and development, and is tightly regulated by the composition of the extracellular matrix (ECM). Type VI collagen is a highly expressed ECM component in the TMJ cartilage, yet its specific functions are largely unknown. In this study, we investigated α2(VI)‐deficient (Col6a2‐knockout [KO]) mice, which are unable to secret or incorporate type VI collagen into their ECM. Compared with wild‐type (WT) mice, the TMJ condyles of Col6a2‐KO mice exhibit decreased bone volume/tissue volume (BV/TV) and a larger bone marrow space, suggesting the α2(VI)‐deficient condyles have a failure in endochondral ossification. Differentiating chondrocytes are the main source of bone cells during endochondral ossification. Our study shows there is an increased number of chondrocytes in the proliferative zone and decreased Col10‐expressing chondrocytes in Col6a2‐KO cartilage, all pointing to abnormal chondrocyte differentiation and maturation. In addition, RNA sequencing (RNAseq) analysis identified distinct gene expression profiles related to cell cycle and ECM organization that were altered in the mutant condyles. These data also suggest that bone morphogenetic protein 2 (BMP2) activity was deregulated during chondrocyte differentiation. Immunohistochemical analysis indicated an upregulation of Col2 and Acan expression in Col6a2‐KO cartilage. Moreover, the expression of pSmad1/5/8 and Runx2 was decreased in the Col6a2‐KO cartilage compared with WT controls. Taken together, our data indicate that type VI collagen expressed in the TMJ cartilage is important for endochondral ossification, possibly by modulating the ECM and altering/disrupting signaling pathways important for TMJ chondrocyte differentiation. Published 2022. This article is a U.S. Government work and is in the public domain in the USA. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Theobroma cacao improves bone growth by modulating defective ciliogenesis in a mouse model of achondroplasia

A gain-of-function mutation in the fibroblast growth factor receptor 3 gene (FGFR3) results in achondroplasia (ACH), the most frequent form of dwarfism. Constitutive activation of FGFR3 impairs bone formation and elongation and many signal transduction pathways. Identification of new and relevant compounds targeting the FGFR3 signaling pathway is of broad importance for the treatment of ACH, and natural plant compounds are prime drug candidate sources. Here, we found that the phenolic compound (-)-epicatechin, isolated from Theobroma cacao, effectively inhibited FGFR3's downstream signaling pathways. Transcriptomic analysis in an Fgfr3 mouse model showed that ciliary mRNA expression was modified and influenced significantly by the Indian hedgehog and PKA pathways. (-)-Epicatechin is able to rescue mRNA expression impairments that control both the structural organization of the primary cilium and ciliogenesis-related genes. In femurs isolated from a mouse model (Fgfr3^Y367C/+) of ACH, we showed that (-)-epicatechin eliminated bone growth impairment during 6 days of ex vivo culture. In vivo, we confirmed that daily subcutaneous injections of (-)-epicatechin to Fgfr3^Y367C/+ mice increased bone elongation and rescued the primary cilium defects observed in chondrocytes. This modification to the primary cilia promoted the typical columnar arrangement of flat proliferative chondrocytes and thus enhanced bone elongation. The results of the present proof-of-principle study support (-)-epicatechin as a potential drug for the treatment of ACH.

The Regulation of Growth in Developing, Homeostatic, and Regenerating Tetrapod Limbs: A Minireview

The size and shape of the tetrapod limb play central roles in their functionality and the overall physiology of the organism. In this minireview we will discuss observations on mutant animal models and humans, which show that the growth and final size of the limb is most impacted by factors that regulate either limb bud patterning or the elongation of the long bones. We will also apply the lessons that have been learned from embryos to how growth could be regulated in regenerating limb structures and outline the challenges that are unique to regenerating animals.

The role of the protein tyrosine phosphatase SHP2 in ossification

SHP2, encoded by the PTPN11 gene, participates in multiple cell functions including cell proliferation, movement, and differentiation. PTPN11 loss-of-function and gain-of-function mutations are both associated with diseases, such as Noonan syndrome, whose manifestations include bone defects, suggesting a crucial role for SHP2 in the skeleton. However, the exact mechanisms by which SHP2 regulates bone development remain unclear. This review focuses on the current understanding of the regulation of SHP2 and highlights the vital roles of SHP2 in skeletal development, especially its roles in ossification. Overall, a better understanding of the functions of SHP2 in ossification will provide a new avenue to treat-related skeletal diseases.

The Effects of Fluvastatin on Indian Hedgehog Pathway in Endochondral Ossification

Statins have demonstrated to be effective for treating chondrodysplasia and its effects were believed to be associated with the fibroblast growth factor receptor 3 (FGFR3). Statins promoted the degradation of FGFR3 in studies using disease-specific induced pluripotent stem cells and model mice, however, recent studies using normal chondrocytes reported that statins did not degrade FGFR3. In order to further investigate the effects of statins in endochondral ossification, this study examined the influence of statins on Indian hedgehog (Ihh), another important component of endochondral ossification, and its related pathways. The chondrocyte cell line ATDC5 was used to investigate changes in cell proliferation, mRNA, and protein expression levels. In addition, an organ culture of a mouse metatarsal bone was performed followed by hematoxylin-eosin staining and fluorescent immunostaining. Results indicated that expression level of Ihh increased with the addition of statins, which activated the Ihh pathway and altered the localization of Ihh. Changes in cholesterol modification may have affected Ihh diffusibility; however, further experiments are necessary. A reactive increase in parathyroid hormone-related protein (PTHrP) was observed in addition to changes in the Wnt pathway through secreted-related protein 2/3 and low-density lipoprotein 5/6. This led to the promotion of cell proliferation, increase of the hypertrophic chondrocyte layer, inhibition of apoptosis, and decrease in mineralization. This study demonstrated that statins had an influence on Ihh, and that the hyperfunction of Ihh may prevent premature cell death caused by FGFR3-related chondrodysplasia through an indirect increase in the expression of PTHrP.

A single-cell transcriptome of mesenchymal stromal cells to fabricate bioactive hydroxyapatite materials for bone regeneration

The osteogenic microenvironment of bone-repairing materials plays a key role in accelerating bone regeneration but remains incompletely defined, which significantly limits the application of such bioactive materials. Here, the transcriptional landscapes of different osteogenic microenvironments, including three-dimensional (3D) hydroxyapatite (HA) scaffolds and osteogenic medium (OM), for mesenchymal stromal cells (MSCs) in vitro were mapped at single-cell resolution. Our findings suggested that an osteogenic process reminiscent of endochondral ossification occurred in HA scaffolds through sequential activation of osteogenic-related signaling pathways, along with inflammation and angiogenesis, but inhibition of adipogenesis and fibrosis. Moreover, we revealed the mechanism during OM-mediated osteogenesis involves the ZBTB16 and WNT signaling pathways. Heterogeneity of MSCs was also demonstrated. In vitro ossification of LRRC75A⁺ MSCs was shown to have better utilization of WNT-related ossification process, and PCDH10⁺ MSCs with superiority in hydroxyapatite-related osteogenic process. These findings provided further understanding of the cellular activity modulated by OM conditions and HA scaffolds, providing new insights for the improvement of osteogenic biomaterials. This atlas provides a blueprint for research on MSC heterogeneity and the osteogenic microenvironment of HA scaffolds and a database reference for the application of bioactive materials for bone regeneration.

Bone Biomarkers in Mucopolysaccharidoses

The accumulation of glycosaminoglycans (GAGs) in bone and cartilage leads to progressive damage in cartilage that, in turn, reduces bone growth by the destruction of the growth plate, incomplete ossification, and growth imbalance. The mechanisms of pathophysiology related to bone metabolism in mucopolysaccharidoses (MPS) include impaired chondrocyte function and the failure of endochondral ossification, which leads to the release of inflammatory cytokines via the activation of Toll-like receptors by GAGs. Although improvements in the daily living of patients with MPS have been achieved with enzyme replacement, treatment for the bone disorder is limited. There is an increasing need to identify biomarkers related to bone and cartilage to evaluate the progressive status and to monitor the treatment of MPS. Recently, new analysis methods, such as proteomic analysis, have identified new biomarkers in MPS. This review summarizes advances in clinical bone metabolism and bone biomarkers.

Neural control of growth and size in the axolotl limb regenerate

The mechanisms that regulate growth and size of the regenerating limb in tetrapods such as the Mexican axolotl are unknown. Upon the completion of the developmental stages of regeneration, when the regenerative organ known as the blastema completes patterning and differentiation, the limb regenerate is proportionally small in size. It then undergoes a phase of regeneration that we have called the ‘tiny-limb’ stage, which is defined by rapid growth until the regenerate reaches the proportionally appropriate size. In the current study we have characterized this growth and have found that signaling from the limb nerves is required for its maintenance. Using the regenerative assay known as the accessory limb model (ALM), we have found that growth and size of the limb positively correlates with nerve abundance. We have additionally developed a new regenerative assay called the neural modified-ALM (NM-ALM), which decouples the source of the nerves from the regenerating host environment. Using the NM-ALM we discovered that non-neural extrinsic factors from differently sized host animals do not play a prominent role in determining the size of the regenerating limb. We have also discovered that the regulation of limb size is not autonomously regulated by the limb nerves. Together, these observations show that the limb nerves provide essential cues to regulate ontogenetic allometric growth and the final size of the regenerating limb.

Humans’ ability to regrow lost or damaged body parts is relatively limited, but some animals, such as the axolotl (a Mexican salamander), can regenerate complex body parts, like legs, many times over their lives. Studying regeneration in these animals could help researchers enhance humans’ abilities to heal. One way to do this is using the Accessory Limb Model (ALM), where scientists wound an axolotl’s leg, and study the additional leg that grows from the wound.

The first stage of limb regeneration creates a new leg that has the right structure and shape. The new leg is very small so the next phase involves growing the leg until its size matches the rest of the animal. This phase must be controlled so that the limb stops growing when it reaches the right size, but how this regulation works is unclear. Previous research suggests that the number of nerves in the new leg could be important.

Wells et al. used a ALM to study how the size of regenerating limbs is controlled. They found that changing the number of nerves connected to the new leg altered its size, with more nerves leading to a larger leg. Next, Wells et al. created a system that used transplanted nerve bundles of different sizes to grow new legs in different sized axolotls. This showed that the size of the resulting leg is controlled by the number of nerves connecting it to the CNS. Wells et al. also showed that nerves can only control regeneration if they remain connected to the central nervous system.

These results explain how size is controlled during limb regeneration in axolotls, highlighting the fact that regrowth is directly controlled by the number of nerves connected to a regenerating leg. Much more work is needed to reveal the details of this process and the signals nerves use to control growth. It will also be important to determine whether this control system is exclusive to axolotls, or whether other animals also use it.

NF-κB activation impedes the transdifferentiation of hypertrophic chondrocytes at the growth plate of mouse embryos in diabetic pregnancy

BACKGROUND

Diabetes mellitus could cause numerous complications and health problems including abnormality of endochondral bone formation during embryogenesis. However, the underlying mechanisms still remain obscure.

METHODS

Streptozotoci (STZ) was injected to induce pregestational diabetes mellitus (PGDM) mouse model. The femurs of E18.5 mouse embryos from control and PGDM groups were harvested. Morphological staining was implemented to determine the abnormality of the bone development. The expressions of the key genes participating in osteogenesis (e.g., Sox9, Runx2, and Osterix), the NF-κB signaling molecules (e.g., P50, P65, IκBα), and the corresponding regulatory factors (e.g., Bmp2, phospho-p38) were evaluated by immunofluorescence, quantitative PCR and western blot. Finally, in vitro chondrocyte differentiation model was employed to verify the role of NF-κB on the expressions of chondro-osteogenic markers.

RESULTS

Alcian blue/alizarin red double staining and H&E staining demonstrated the restriction of skeletal development and relatively extended hypertrophic zone at growth plate in E18.5 STZ-induced diabetic mouse embryos compared to the control. Immunofluorescent staining and qPCR showed that Sox9 expression increased, while Runx2 and Osterix expressions decreased in the growth plate of the offspring of PGDM mice. Immunofluorescence of P65 manifested the activation of NF-κB signaling in growth plate in PGDM mouse embryos. Furthermore, the relatively extended hypertrophic zone was also observed in the growth plate of the NF-κB-activated transgenic mice, as well as the activated p65 up-regulated the expression of Bmp2 and p-p38. In ATDC5 cells, we could observe the high glucose up-regulated the P50 and P65 expressions and down-regulated IκBα expression, but the high glucose-activated NF-κB signaling could be reversed by addition of Bay (inhibitor of NF-κB signaling). The expression changes of Bmp2, Sox9 and Runx2 in presence of high glucose were resumed too.

CONCLUSION

Our data revealed that NF-κB signaling was involved in mediation effects of dysfunctional trans-differentiation of hypertrophic chondrocytes in the embryonic growth plate induced by maternal diabetic mellitus.

Histone methyltransferase SET domain bifurcated 1 negatively regulates parathyroid hormone/parathyroid hormone-related peptide receptor to control chondrocyte proliferation in Meckel's cartilage

OBJECTIVE

The aim of this study is to show that the proliferation of chondrocytes is regulated by SET domain bifurcated 1 (SETDB1) along with the downregulation of parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor in Meckel's cartilage.

DESIGN

Setdb1 was knocked down or overexpressed in a mouse chondrogenic ATDC5 cells, by transfecting the cells with short interfering RNA against Setdb1 or wild-type Setdb1 expression vector, respectively. Cell proliferation was detected by bromodeoxyuridine incorporation. Setdb1 was conditionally deleted in neural crest cells with Wnt1-Cre (Setdb1 conditional knockout mice). Immunofluorescence staining of paraffin sections of embryonic days 13.5 and 14.5 Setdb1 conditional knockout mice or transfected ATDC5 cells was performed to detect PTH/PTHrP receptor. Protein kinase B (AKT) phosphorylation inhibitor was added to both siRNA-transfected ATDC5 cultures to determine whether AKT activation induces PTH/PTHrP receptor expression after Setdb1 knockdown or vice versa.

RESULTS

Setdb1 knockdown in ATDC5 cells showed increased cell proliferation and parathyroid hormone receptor 1 expression. Contrasting results were observed in the Setdb1-overexpressed wild-type cells. Immunofluorescence staining showed the highly expressed PTH/PTHrP receptor in Setdb1-knocked down ATDC5 cells and in the chondrocytes of Setdb1 conditional knockout embryonic Meckel's cartilage, indicating the negative regulation of SETDB1 on PTH/PTHrP receptor. Strong staining of phosphorylated AKT was observed in Setdb1-knocked down ATDC5 cells. However, the inhibition of AKT phosphorylation significantly reduced both the PTH/PTHrP receptor staining and the Setdb1-knockdown-induced increase in ATDC5 cell proliferation.

CONCLUSIONS

Our findings contribute new insights on SETDB1 function in relation with AKT and PTH/PTHrP receptor during chondrocyte proliferation.

Suppression of Osteoarthritis progression by post-natal Induction of Nkx3.2

Osteoarthritis (OA) is an incurable joint disease affecting 240 million elderly population, and major unmet medical needs exist for better therapeutic options for OA. During skeletal development, Nkx3.2 has been shown to promote chondrocyte differentiation and survival, but to suppress cartilage hypertrophy and blood vessel invasion. Here we show that Nkx3.2 plays a key role in osteoarthritis (OA) pathogenesis. Marked reduction of Nkx3.2 expression was observed in three different murine OA models. Consistent with these findings, analyses of surgery-induced and age-driven OA models revealed that cartilage-specific post-natal induction of Nkx3.2 can suppress OA progression in mice. These results suggest that Nkx3.2 may serve as a promising target for OA drug development.

Molecular Mechanisms of Chondrocyte Proliferation and Differentiation

Cartilage is a kind of connective tissue that buffers pressure and is essential to protect joint movement. It is difficult to self-recover once cartilage is damaged due to the lack of blood vessels, lymph, and nerve tissues. Repair of cartilage injury is mainly achieved by stimulating chondrocyte proliferation and extracellular matrix (ECM) synthesis. Cartilage homeostasis involves the regulation of multiple growth factors and the transduction of cellular signals. It is a very complicated process that has not been elucidated in detail. In this review, we summarized a variety of signaling molecules related to chondrocytes function. Especially, we described the correlation between chondrocyte-specific regulatory factors and cell signaling molecules. It has potential significance for guiding the treatment of cartilage injury.

Bone physiological microenvironment and healing mechanism: Basis for future bone-tissue engineering scaffolds

Bone-tissue defects affect millions of people worldwide. Despite being common treatment approaches, autologous and allogeneic bone grafting have not achieved the ideal therapeutic effect. This has prompted researchers to explore novel bone-regeneration methods. In recent decades, the development of bone tissue engineering (BTE) scaffolds has been leading the forefront of this field. As researchers have provided deep insights into bone physiology and the bone-healing mechanism, various biomimicking and bioinspired BTE scaffolds have been reported. Now it is necessary to review the progress of natural bone physiology and bone healing mechanism, which will provide more valuable enlightenments for researchers in this field. This work details the physiological microenvironment of the natural bone tissue, bone-healing process, and various biomolecules involved therein. Next, according to the bone physiological microenvironment and the delivery of bioactive factors based on the bone-healing mechanism, it elaborates the biomimetic design of a scaffold, highlighting the designing of BTE scaffolds according to bone biology and providing the rationale for designing next-generation BTE scaffolds that conform to natural bone healing and regeneration.

Deficiency of TMEM53 causes a previously unknown sclerosing bone disorder by dysregulation of BMP-SMAD signaling

Bone formation represents a heritable trait regulated by many signals and complex mechanisms. Its abnormalities manifest themselves in various diseases, including sclerosing bone disorder (SBD). Exploration of genes that cause SBD has significantly improved our understanding of the mechanisms that regulate bone formation. Here, we discover a previously unknown type of SBD in four independent families caused by bi-allelic loss-of-function pathogenic variants in TMEM53, which encodes a nuclear envelope transmembrane protein. Tmem53-/- mice recapitulate the human skeletal phenotypes. Analyses of the molecular pathophysiology using the primary cells from the Tmem53-/- mice and the TMEM53 knock-out cell lines indicates that TMEM53 inhibits BMP signaling in osteoblast lineage cells by blocking cytoplasm-nucleus translocation of BMP2-activated Smad proteins. Pathogenic variants in the patients impair the TMEM53-mediated blocking effect, thus leading to overactivated BMP signaling that promotes bone formation and contributes to the SBD phenotype. Our results establish a previously unreported SBD entity (craniotubular dysplasia, Ikegawa type) and contribute to a better understanding of the regulation of BMP signaling and bone formation.

A single day of TGF-β1 exposure activates chondrogenic and hypertrophic differentiation pathways in bone marrow-derived stromal cells

Virtually all bone marrow-derived stromal cell (BMSC) chondrogenic induction cultures include greater than 2 weeks exposure to transforming growth factor-β (TGF-β), but fail to generate cartilage-like tissue suitable for joint repair. Herein we used a micro-pellet model (5 × 103 BMSC each) to determine the duration of TGF-β1 exposure required to initiate differentiation machinery, and to characterize the role of intrinsic programming. We found that a single day of TGF-β1 exposure was sufficient to trigger BMSC chondrogenic differentiation and tissue formation, similar to 21 days of TGF-β1 exposure. Despite cessation of TGF-β1 exposure following 24 hours, intrinsic programming mediated further chondrogenic and hypertrophic BMSC differentiation. These important behaviors are obfuscated by diffusion gradients and heterogeneity in commonly used macro-pellet models (2 × 105 BMSC each). Use of more homogenous micro-pellet models will enable identification of the critical differentiation cues required, likely in the first 24-hours, to generate high quality cartilage-like tissue from BMSC.

Murine Limb Explant Cultures to Assess Cartilage Development

To investigate chondrocyte biology in an organized structure, limb explant cultures have been established that allow for the cultivation of the entire cartilaginous skeletal elements. In these organ cultures, the arrangement of chondrocytes in the cartilage elements and their interaction with the surrounding perichondrium and joint tissue are maintained. Chondrocyte proliferation and differentiation can thus be studied under nearly in vivo conditions. Growth factors and other soluble agents can be administered to the explants and their effect on limb morphogenesis, gene expression and cell-matrix interactions can be studied. Cotreatment with distinct growth factors and their inhibitors as well as the use of transgenic mice will allow one to decipher the epistatic relationship between different signaling systems and other regulators of chondrocyte differentiation. Here we describe the protocol to culture cartilage explants ex vivo and discuss the advantages and disadvantages of the culture system.

Runx1 up-regulates chondrocyte to osteoblast lineage commitment and promotes bone formation by enhancing both chondrogenesis and osteogenesis

One of the fundamental questions in bone biology is where osteoblasts originate and how osteoblast differentiation is regulated. The mechanism underlying which factors regulate chondrocyte to osteoblast lineage commitment remains unknown. Our data showed that Runt-related transcription factor 1 (Runx1) is expressed at different stages of both chondrocyte and osteoblast differentiation. Runx1 chondrocyte-specific knockout (Runx1f/fCol2α1-cre) mice exhibited impaired cartilage formation, decreased bone density, and an osteoporotic phenotype. The expressions of chondrocyte differentiation regulation genes, including Sox9, Ihh, CyclinD1, PTH1R, and hypertrophic chondrocyte marker genes including Col2α1, Runx2, MMP13, Col10α1 in the growth plate were significantly decreased in Runx1f/fCol2α1-cre mice chondrocytes. Importantly, the expression of osteoblast differentiation regulation genes including Osx, Runx2, ATF4, and osteoblast marker genes including osteocalcin (OCN) and osteopontin (OPN) were significantly decreased in the osteoblasts of Runx1f/fCol2α1-cre mice. Notably, our data showed that osteoblast differentiation regulation genes and marker genes are also expressed in chondrocytes and the expressions of these marker genes were significantly decreased in the chondrocytes of Runx1f/fCol2α1-cre mice. Our data showed that chromatin immunoprecipitation (ChIP) and promoter mapping analysis revealed that Runx1 directly binds to the Indian hedgehog homolog (Ihh) promoter to regulate its expression, indicating that Runx1 directly regulates the transcriptional expression of chondrocyte genes. Collectively, we revealed that Runx1 signals chondrocyte to osteoblast lineage commitment and promotes endochondral bone formation through enhancing both chondrogenesis and osteogenesis genes expressions, indicating Runx1 may be a therapeutic target to enhance endochondral bone formation and prevent osteoporosis fractures.

Atlas of Musculoskeletal Stem Cells with the Soft and Hard Tissue Differentiation Architecture

Although being of utmost importance for human health and mobility, stem cell identity and hierarchical organization of musculoskeletal progenitors remain largely unexplored. Here, cells from E10.5, E12.5, and E15.5 murine limbs are analyzed by high throughput single-cell RNA sequencing to illustrate the cellular architecture during limb development. Single-cell transcriptional profiling demonstrates the identity and differentiation architecture of musculoskeletal stem cells (MSSC), soft and hard tissue progenitors through expression pattern of musculoskeletal markers (scleraxis [Scx], Hoxd13, Sox9, and Col1a1). This is confirmed by genetic in vivo lineage tracing. Moreover, single-cell analyses of Scx knockout mice tissues illustrates that Scx regulates MSSC self-renewal and proliferation potential. A high-throughput and low-cost multi-tissues RNA sequencing strategy further provides evidence that musculoskeletal system tissues, including muscle, bone, meniscus, and cartilage, are all abnormally developed in Scx knockout mice. These results establish the presence of an indispensable limb Scx+Hoxd13+ MSSC population and their differentiation into soft tissue progenitors (Scx+Col1a1+) and hard tissue progenitors (Scx+Sox9+). Collectively, this study paves the way for systematically decoding the complex molecular mechanisms and cellular programs of musculoskeletal tissues morphogenesis in limb development and regeneration.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

Human pluripotent stem cell-derived chondroprogenitors for cartilage tissue engineering

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

Mutational spectrum and clinical signatures in 114 families with hereditary multiple osteochondromas: insights into molecular properties of selected exostosin variants

Hereditary multiple osteochondromas (HMO) is a rare autosomal dominant skeletal disorder, caused by heterozygous variants in either EXT1 or EXT2, which encode proteins involved in the biogenesis of heparan sulphate. Pathogenesis and genotype-phenotype correlations remain poorly understood. We studied 114 HMO families (158 affected individuals) with causative EXT1 or EXT2 variants identified by Sanger sequencing, or multiplex ligation-dependent probe amplification and qPCR. Eighty-seven disease-causative variants (55 novel and 32 known) were identified including frameshift (42%), nonsense (32%), missense (11%), splicing (10%) variants and genomic rearrangements (5%). Informative clinical features were available for 42 EXT1 and 27 EXT2 subjects. Osteochondromas were more frequent in EXT1 as compared to EXT2 patients. Anatomical distribution of lesions showed significant differences based on causative gene. Microscopy analysis for selected EXT1 and EXT2 variants verified that EXT1 and EXT2 mutants failed to co-localize each other and loss Golgi localization by surrounding the nucleus and/or assuming a diffuse intracellular distribution. In a cell viability study, cells expressing EXT1 and EXT2 mutants proliferated more slowly than cells expressing wild-type proteins. This confirms the physiological relevance of EXT1 and EXT2 Golgi co-localization and the key role of these proteins in the cell cycle. Taken together, our data expand genotype-phenotype correlations, offer further insights in the pathogenesis of HMO and open the path to future therapies.

Fluoride regulates the expression of extracellular matrix HSPG and related signaling pathways FGFR3 and Ihh/PTHrP feedback loop during endochondral ossification

Skeletal fluorosis causes growth plate impairment and growth retardation during bone development. Longitudinal bone development is accomplished by endochondral ossification in growth plate. However, the mechanism of fluoride impairs growth plate is unclear. To explore the effect of fluoride on various glycosaminoglycans (GAGs) and related signaling pathways in growth plate during endochondral ossification, SD rats and ATDC5 cells were treated with fluoride and carried out a series of experiments. We found that the expression of heparan sulfate (HS), a kind of GAGs in extracellular matrix, was significantly increased in the growth plate of fluoride-treated rats compared with control rats. Furthermore, the expression of HS synthetic enzyme exostosin 1 (EXT1) and glypican 6 (GPC6), a core protein of HS proteoglycan (HSPG), were significantly increased in fluoride-treated ATDC5 cells compared with control cells (P < 0.05). The expression of related molecules including fibroblast growth factor receptor-3 (FGFR3), signal transducer and activator of transcription 1 (STAT1) and parathyroid hormone-related protein (PTHrP) were significantly increased in the fluoride-treated groups compared with control groups (P < 0.05), and there was significantly decreased in the expression of Indian hedgehog (Ihh) in fluoride-treated groups compared with control groups (P < 0.05). Our data suggested that fluoride increased the content of HSPG in extracellular matrix by promoting the expression of EXT1 and GPC6. Fluoride also activated FGFR3 signaling pathway, inhibited Ihh/PTHrP feedback loop and inhibited endochondral ossification. Nevertheless, the regulation of fluoride on HSPG and related pathways FGFR3 and Ihh/PTHrP feedback loop during endochondral ossification needs to be further studied.

Small molecules for mesenchymal stem cell fate determination

Mesenchymal stem cells (MSCs) are adult stem cells harboring self-renewal and multilineage differentiation potential that are capable of differentiating into osteoblasts, adipocytes, or chondrocytes in vitro, and regulating the bone marrow microenvironment and adipose tissue remodeling in vivo. The process of fate determination is initiated by signaling molecules that drive MSCs into a specific lineage. Impairment of MSC fate determination leads to different bone and adipose tissue-related diseases, including aging, osteoporosis, and insulin resistance. Much progress has been made in recent years in discovering small molecules and their underlying mechanisms control the cell fate of MSCs both in vitro and in vivo. In this review, we summarize recent findings in applying small molecules to the trilineage commitment of MSCs, for instance, genistein, medicarpin, and icariin for the osteogenic cell fate commitment; isorhamnetin, risedronate, and arctigenin for pro-adipogenesis; and atractylenolides and dihydroartemisinin for chondrogenic fate determination. We highlight the underlying mechanisms, including direct regulation, epigenetic modification, and post-translational modification of signaling molecules in the AMPK, MAPK, Notch, PI3K/AKT, Hedgehog signaling pathways etc. and discuss the small molecules that are currently being studied in clinical trials. The target-based manipulation of lineage-specific commitment by small molecules offers substantial insights into bone marrow microenvironment regulation, adipose tissue homeostasis, and therapeutic strategies for MSC-related diseases.

Regulation of terminal hypertrophic chondrocyte differentiation in Prmt5 mutant mice modeling infantile idiopathic scoliosis

Idiopathic scoliosis (IS) is the most common type of musculoskeletal defect affecting children worldwide, and is classified by age of onset, location and degree of spine curvature. Although rare, IS with onset during infancy is the more severe and rapidly progressive form of the disease, associated with increased mortality due to significant respiratory compromise. The pathophysiology of IS, in particular for infantile IS, remains elusive. Here, we demonstrate the role of PRMT5 in the infantile IS phenotype in mouse. Conditional genetic ablation of PRMT5 in osteochondral progenitors results in impaired terminal hypertrophic chondrocyte differentiation and asymmetric defects of endochondral bone formation in the perinatal spine. Analysis of these several markers of endochondral ossification revealed increased type X collagen (COLX) and Ihh expression, coupled with a dramatic reduction in Mmp13 and RUNX2 expression, in the vertebral growth plate and in regions of the intervertebral disc in the Prmt5 conditional mutant mice. We also demonstrate that PRMT5 has a continuous role in the intervertebral disc and vertebral growth plate in adult mice. Altogether, our results establish PRMT5 as a critical promoter of terminal hypertrophic chondrocyte differentiation and endochondral bone formation during spine development and homeostasis.

This article has an associated First Person interview with the first author of the paper.

Summary: Loss of Prmt5 in osteochondral progenitors impairs terminal hypertrophic chondrocyte differentiation, leading to defects in endochondral bone formation and models infantile idiopathic scoliosis in mouse.

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

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

Molecular profiling of failed endochondral ossification in mucopolysaccharidosis VII

Mucopolysaccharidosis (MPS) VII is a lysosomal storage disorder characterized by deficient activity of β-glucuronidase, leading to progressive accumulation of incompletely degraded heparan, dermatan, and chondroitin sulfate glycosaminoglycans (GAGs). Patients with MPS VII exhibit progressive skeletal deformity including kyphoscoliosis and joint dysplasia, which decrease quality of life and increase mortality. Previously, using the naturally-occurring canine model, we demonstrated that one of the earliest skeletal abnormalities to manifest in MPS VII is failed initiation of secondary ossification in vertebrae and long bones at the requisite postnatal developmental stage. The objective of this study was to obtain global insights into the molecular mechanisms underlying this failed initiation of secondary ossification. Epiphyseal tissue was isolated postmortem from the vertebrae of control and MPS VII-affected dogs at 9 and 14 days-of-age (n = 5 for each group). Differences in global gene expression across this developmental window for both cohorts were measured using whole-transcriptome sequencing (RNA-Seq). Principal Component Analysis revealed clustering of samples within each group, indicating clear effects of both age and disease state. At 9 days-of-age, 1375 genes were significantly differentially expressed between MPS VII and control, and by 14 days-of-age, this increased to 4719 genes. A targeted analysis focused on signaling pathways important in the regulation of endochondral ossification was performed, and a subset of gene expression differences were validated using qPCR. Osteoactivin (GPNMB) was the top upregulated gene in MPS VII at both ages. In control samples, temporal changes in gene expression from 9 to 14 days-of-age were consistent with chondrocyte maturation, cartilage resorption, and osteogenesis. In MPS VII samples, however, elements of key osteogenic pathways such as Wnt/β-catenin and BMP signaling were not upregulated during this same developmental window suggesting that important bone formation pathways are not activated. In conclusion, this study represents an important step towards identifying therapeutic targets and biomarkers for bone disease in MPS VII patients during postnatal growth.

Extracellular matrix turnover and inflammation in chemically-induced TMJ arthritis mouse models

The temporomandibular joint (TMJ) is a fibrocartilaginous tissue critical for chewing and speaking. In patients with temporomandibular disorders (TMDs), permanent tissue loss can occur. Recapitulating the complexity of TMDs in animal models is difficult, yet critical for the advent of new therapies. Synovial fluid from diseased human samples revealed elevated levels of tumor necrosis factor alpha (TNF-alpha). Here, we propose to recapitulate these findings in mice by subjecting murine TMJs with TNF-alpha or CFA (Complete Freund’s Adjuvant) in mandibular condyle explant cultures and by local delivery in vivo using TMJ intra-articular injections. Both TNF-alpha and CFA delivery to whole mandibular explants and in vivo increased extracellular matrix deposition and increased cartilage thickness, while TNF-alpha treated explants had increased expression of inflammatory cytokines and degradative enzymes. Moreover, the application of TNF-alpha or CFA in both models reduced cell number. CFA delivery in vivo caused soft tissue inflammation, including pannus formation. Our work provides two methods of chemically induced TMJ inflammatory arthritis through a condyle explant model and intra-articular injection model that replicate findings seen in synovial fluid of human patients, which can be used for further studies delineating the mechanisms underlying TMJ pathology.

Experimental Induction of Physeal Injuries by Fracture, Drill, and Ablation Techniques: Analyses of Immunohistochemical Findings

BACKGROUND

Although physeal fractures and physeal bars can result in significant clinical consequences to growth and development of the injured physis, little orthopaedic research has focused upon this topic. Our objective was to extend a previously developed rat model to examine the immunohistochemical features following surgical application of techniques disrupting the physis.

METHODS

Physes were surgically disrupted using fracture (control), epiphyseal scrape (ES), or epiphyseal drill (ED). After 1, 3, 6, 10, or 21 days, animals were euthanized, sites processed for histology and immunohistochemical localization of vascular endothelial growth factor (VEGF), Factor VIII, Sox-9, PTHrP (parathyroid hormone-related protein) and PTHrP-R (parathyroid hormone-related protein receptor) in resting, proliferative, and hypertrophic physeal zones. Incidence of physeal bars, vertical septa and islands within the metaphysis was quantified. Semiquantitative analysis of immunohistochemistry was performed.

RESULTS

Physeal bars, vertical septa, and displaced cartilage islands were present each of the surgical treatments. Fisher's exact test showed a statistically significant increase in the presence of physeal bars (P=0.002) and vertical septa (P=0.012) in the ED group at 10 and 21 days. Analysis of VEGF showed significant differences among the surgical treatments involving the resting zone, and the proliferative zone for days 1, 6, and 21 (P≤0.02) with greater mean scores present in the fracture (control) group, followed by the ED group; the lowest scores were present in the ES group. PTHrP-R immunolocalization showed significant differences among treatments in the hypertrophic zone at days 6 and 21 (P=0.022 and 0.044, respectively).

CONCLUSIONS

On the basis of the type of surgical treatment, results show significant differences in the presence of VEGF (reflecting the vascular bed) in the resting and proliferating zones at days 1, 6, and 21. VEGF localization was less abundant in the ED group (which had more physeal bars), suggesting that lack of vascular ingrowth plays a role in physeal bar formation.

CLINICAL RELEVANCE

Basic science data presented here provide insight into the importance of the various regions of the physis and its repair and continued growth after physeal fracture. We suggest that a better understanding of the cellular basis of physeal arrest following physeal fracture may have future relevance for the development of treatments to prevent or correct arrest.

Mechanisms of synovial joint and articular cartilage development

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

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

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

Metalloprotease inhibitor TIMP proteins control FGF-2 bioavailability and regulate skeletal growth

Saw et al. show via the combinatorial deletion of Timp family members in mice that metalloprotease regulation of FGF-2 is a crucial event in the chondrocyte maturation program, underlying the growth plate development and bone elongation responsible for attaining proper body stature.

Regulated growth plate activity is essential for postnatal bone development and body stature, yet the systems regulating epiphyseal fusion are poorly understood. Here, we show that the tissue inhibitors of metalloprotease (TIMP) gene family is essential for normal bone growth after birth. Whole-body quadruple-knockout mice lacking all four TIMPs have growth plate closure in long bones, precipitating limb shortening, epiphyseal distortion, and widespread chondrodysplasia. We identify TIMP/FGF-2/IHH as a novel nexus underlying bone lengthening where TIMPs negatively regulate the release of FGF-2 from chondrocytes to allow IHH expression. Using a knock-in approach that combines MMP-resistant or ADAMTS-resistant aggrecans with TIMP deficiency, we uncouple growth plate activity in axial and appendicular bones. Thus, natural metalloprotease inhibitors are crucial regulators of chondrocyte maturation program, growth plate integrity, and skeletal proportionality. Furthermore, individual and combinatorial TIMP-deficient mice demonstrate the redundancy of metalloprotease inhibitor function in embryonic and postnatal development.

MiRNA-144-3p inhibits high glucose induced cell proliferation through suppressing FGF16

As a major cause of blindness, diabetic retinopathy (DR) is often found in the developed countries. Our previous study identified a down-regulated miRNA: miR-144-3p in response to hyperglycemia. The present study aims to investigate the role of miR-144-3p in proliferation of microvascular epithelial cells. Endothelial cells were treated with different concentrations of glucose, after which miR-144-3p were detected with real-time PCR assay. MiR-144-3p mimics or inhibitors were used to increase or knockdown the level of this miRNA. Western blotting assay and ELISA assay were used to measure the expression and concentration of VEGF protein. 5-Bromo-2-deoxyUridine (BrdU) labeled cell cycle assay was used to detect cells in S phase. MiRNA targets were predicted by using a TargetScan tool, and were further verified by luciferase reporter assay. In the present study, we focussed on a significantly down-regulated miRNA, miR-144-3p, and investigated its role in high glucose (HG) induced cell proliferation. Our data showed that miR-144-3p mimics significantly inhibited HG induced cell proliferation and reduced the percentage of cells in S phase. HG induced up-regulation of VEGF was also prohibited by miR-144-3p mimics. Through wound-healing assay, we found that miR-144-3p suppressed cell migration after HG treatments. Moreover, we predicted and proved that fibroblast growth factor (FGF)16 is a direct target of miR-144-3p. Finally, miR-144-3p attenuated HG induced MAPK activation. In conclusion, we demonstrated that miR-144-3p inhibited high glucose-induced cell proliferation through suppressing FGF16 and MAPK signaling pathway, suggesting a possible role of miR-144-FGF16 in the development of DR.