ATDC5: an excellent in vitro model cell line for skeletal development

The p.W651fsX666 mutation on COL10A1 results in impaired trimerization of normal collagen X to induce Schmid type Metaphyseal chondrodysplasia

Haploinsufficiency resulting from the degradation of mutant Collagen Type X Alpha 1 Chain (COL10A1) mRNA by nonsense-mediated decay (NMD) has been attributed to the pathogenesis of Schmid-type metaphyseal chondrodysplasia (SMCD) in cases involving nonsense mutations. However, this mechanism does not fully explain the complexity of SMCD. In this study, we identified a c.1951_1952 InsT (p.W651 fsX666) mutation in exon 3 of COL10A1 that is associated with chondrodysplasia phenotypes in a two-generation family with SMCD. The mRNA decay of the mutant COL10A1 (named as affected E666X-COL10A1) is caused by the p.W651fsX666 mutation, which also disrupts the trimerization of normal collagen X. However, the mutant mRNA decay of affected exogenous E666X-COL10A1, as well as the complete degradation of E666X-COL10A1 mRNA in the proband, is not significantly induced by the W651fsX666 mutation. In vitro trimerization analyses results indicate that the trimerization of normal collagen X and wild-type collagen X are disrupted by W651fsX666 and E666X-collagen X mutations, respectively, suggesting that the mutant allele collagen X may impose a dominant-negative effect on the normal collagen X. Our results are the first to reveal that the impaired trimerization of normal collagen X is caused by the W651fsX666 mutation and a dominant-negative effect on the normal allele collagen X exerted by the mutant allele collagen X, causing impaired trimerization of collagen X, which will interpret the phenotype variability among the affected individuals in the pedigree with metaphyseal chondrodysplasia type Schmid (MCDS) studied.

TACC3 facilitates chondrocyte differentiation by attenuating abnormally activated FGFR3 signaling in achondroplasia - An in vitro study

BACKGROUND

Achondroplasia is a common form of dwarfism. It is caused by mutations in the fibroblast growth factor receptor 3 (FGFR3), which inhibits chondrocyte proliferation and differentiation.

AIM

In this study, we intended to investigate the underlying mechanism of FGFR3 mutation-induced chondrocyte differentiation defection.

METHOD

Insulin-transferrin-selenium (ITS-G) stimulated ATDC5 cells was used as an in vitro model. Alcian Blue staining was performed to detect ATDC5 cell differentiation.

RESULTS

TACC3 expression was increased during ATDC5 cell differentiation. ITS-G induced ATDC5 cell differentiation was inhibited by the FGFR3 mutation, as evidenced by the decreased expression of ACAN and COL2A1. The downregulation of TACC3 expression induced by ITS-G stimulation was upregulated by FGFR3 overactivation. The TACC3 inhibitor (KHS101) promoted differentiation in FGFR3 mutant ATDC5 cells. The p38 signaling pathway has been implicated in FGFR3 mutation-induced chondrocyte differentiation defects. KHS101 promoted the expression of p38. KHS101-induced increase in ATDC5 cell differentiation was inhibited by the administration of a p38 inhibitor. These results suggest that TACC3 might play a role through the p38 signaling pathway in chondrocyte differentiation defects caused by FGFR3 mutations.

CONCLUSION

TACC3 might represent a novel target for achondroplasia.

Identification of novel germline mutations in FUT7 and EXT1 linked with hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal dominant skeletal disorder primarily linked with mutations in Exostosin-1 (EXT1) and Exostosin-2 (EXT2) genes. However, not all HME cases can be explained by these mutations, and its pathogenic mechanisms are not fully understood. Herein, utilizing whole-exome sequencing and genetic screening with a family trio design, we identify two novel rare mutations co-segregating with HME in a Chinese family, including a nonsense mutation (c.204G>A, p.Trp68*) in EXT1 and a missense mutation (c.893T>G, p.Phe298Cys) in FUT7. Functional assays reveal that the FUT7 mutation affects the cellular localization of FUT7 protein and regulates cell proliferation. Notably, the simultaneous loss of fut7 and ext1 in a zebrafish model results in severe chondrodysplasia, indicating a functional link between FUT7 and EXT1 in chondrocyte regulation. Additionally, we unveil that FUT7 p.Phe298Cys reduces EXT1 expression through IL6/STAT3/SLUG axis at the transcription level and through ubiquitination-related proteasomal degradation at the protein level. Together, our findings not only identify novel germline mutations in FUT7 and EXT1 genes, but also highlight the critical interaction between these genes, suggesting a potential 'second-hit' mechanism over EXT1 mutations in HME pathogenesis. This insight enhances our understanding of the mechanisms underlying HME and opens new avenues for potential therapeutic interventions.

Deficiency of EXT1 and FGFR3 genes promotes chondrocyte differentiation, leading to the induction of osteochondroma formation

OBJECTIVE

This study aims to investigate the roles of the EXT1 and FGFR3 genes in the development of osteochondromas, focusing specifically on their potential interactions in chondrocyte proliferation, differentiation, and tumor formation.

METHODS

In vitro, the ATDC5 chondroprogenitor cell line was used to examine the effects of inactivation of both EXT1 and FGFR3. In vivo, a mouse model with dual gene knockout of Ext1 and Fgfr3 was constructed to further explore these genes' roles in tumor formation by observing the incidence and distribution patterns of osteochondromas.

RESULTS

The in vitro experiments demonstrated that ATDC5 cells with reduced expression of EXT1 and FGFR3 genes exhibited enhanced chondrogenic differentiation. In vivo, Fgfr3+/-;Ext1+/- mice showed a significant incidence of osteochondromas (72.7 %), primarily located in the humerus, fibula, and tibia, while mice with a single heterozygous deletion did not display notable lesions.

CONCLUSION

The EXT1 and FGFR3 genes play crucial regulatory roles in the development of osteochondromas. Deficiencies in Ext1 and Fgfr3 can induce the formation of osteochondromas.

ANXA2 promotes chondrocyte differentiation and fracture healing by regulating the phosphorylation of STAT3 and PI3K/AKT signaling pathways

Fractures are common and serious skeletal injuries, and accelerating their healing while alleviating patient suffering remains a clinical challenge. Annexin A2 (ANXA2) is a widely distributed, calcium-dependent, phospholipid-binding protein involved in bone remodeling. However, its role in chondrocyte differentiation and endochondral ossification remains unclear. In this study, we found that ANXA2 is expressed in chondrocytes during growth plate development and fracture healing, as well as during chondrocyte differentiation and maturation in vitro, with its highest expression occurring in the most active differentiation phase. Moreover, ANXA2 knockdown inhibited chondrocyte differentiation, while its overexpression significantly promoted it. We also demonstrated that ANXA2 regulates the chondrogenic and hypertrophic differentiation by mediating the phosphorylation and nuclear translocation of STAT3, as well as activating the PI3K/AKT pathway. Finally, recombinant ANXA2 protein was injected into the tibial fracture sites of mice, verifying its role in promoting endochondral ossification during fracture healing. In conclusion, our study shows that ANXA2 promotes chondrocyte differentiation, partially through the STAT3 and PI3K/AKT pathways. These findings provide insights that could aid in developing new therapies to enhance fracture healing.

Pain mediator NGF improves chondrocyte extracellular matrix synthesis via PI3K/AKT pathway

OBJECTIVES

Nerve growth factor (NGF) is a key mediator in osteoarthritis pain signaling. Clinical studies revealed that anti-NGF antibodies are often accompanied by progressively worsening cartilage degeneration, although they exhibit significant analgesic effects. However, the relationship between NGF expression and cartilage destruction remains unclear. Our study aimed to investigate the effects of NGF on chondrocytes and to elucidate the underlying mechanisms involved.

METHODS

The ATDC5 cells were induced to differentiate into chondrocytes and stimulated with NGF at different concentrations (0.5-10 ng/mL). The cell counting kit-8 assay (CCK-8) was used to measure the effects of NGF on chondrocyte proliferation. Chondrocytes were subsequently stimulated with varying doses of NGF to identify the expression levels of the extracellular matrix. Chondrocytes were pretreated with GNF5837 (a tropomyosin receptor kinase A inhibitor) or LY294002 (a phosphoinositide 3-kinase inhibitor) before exposure to 5 ng/mL NGF to analyze associated signaling pathways. Western blotting and immunofluorescence staining were employed to analyze expression of related proteins.

RESULTS

Alcian blue, toluidine blue staining, and type II collagen immunofluorescence staining demonstrated that ATDC5 cells differentiated into functional chondrocytes after 14 days of chondrogenic induction. The CCK-8 assay confirmed that cell proliferation was unaffected. NGF (0.5-5 ng/mL) was found to enhance chondrocyte matrix synthesis in a dose-dependent fashion, particularly in the expression of aggrecan, type II collagen, Sox9, and through the activation of the PI3K/AKT signaling pathway. The highest promoting effects were exhibited at 5 ng/mL of NGF. Further analysis indicated that GNF5837 (TRKA inhibitor) or LY294002 (PI3K inhibitor) could reverse the protective effects of NGF on chondrocyte matrix synthesis.

CONCLUSION

Our study identified a potentially beneficial role of NGF at concentrations of 0.5-5 ng/mL in chondrocytes, enhancing extracellular matrix synthesis, with significant involvement of the PI3K/AKT signaling pathway in this process.

In Vitro Engineered ECM-incorporated Hydrogels for Osteochondral Tissue Repair: A Cell-Free Approach

Prevalence of osteoarthritis has been increasing in aging populations, which has necessitated the use of advanced biomedical treatments. These involve grafts or delivering drug molecules entrapped in scaffolds. However, such treatments often show suboptimal therapeutic effects due to poor half-life and off-target effects of drug molecules. As a countermeasure, a 3D printable robust hydrogel-based tissue-repair platform is developed containing decellularized extracellular matrix (dECM) from differentiated mammalian cells as the therapeutic cargo. Here, pre-osteoblastic and pre-chondrogenic murine cells are differentiated in vitro, decellularized, and incorporated into methacrylated gelatin (GelMA) solutions to form osteogenic (GelO) and chondrogenic (GelC) hydrogels, respectively. Integrating the bioactive dECM from differentiated cell sources allows GelO and GelC to induce differentiation in human adipose-derived stem cells (hASCs) toward osteogenic and chondrogenic lineages. Further, GelO and GelC can be covalently adhered using a carbodiimide coupling reaction, forming a multi-layered hydrogel with potential application as a bioactive osteochondral plug. The designed multi-layered hydrogel can also induce differentiation of hASCs in vitro. In conclusion, the bioactive dECM carrying 3D printed robust hydrogel offers a promising new drug and cell-free therapeutic strategy for bone and cartilage repair and future osteoarthritis management.

Process-Dependent Variations in the Proliferation of Myoblasts, Fibroblasts and Chondrocytes on Laser-Sintered Polypropylene

Additively manufactured polyolefins find broad applications in medical engineering, enabling the manufacturing of patient-specific geometries. For investigating the influence of processing conditions of laser sintered locally macroporous polypropylene substrates, the response of myoblasts, chondrocytes, and fibroblasts has been characterized in this study. An influence of the applied manufacturing parameters on the attachment and viability of the investigated cells is observed, showing the effect of the superficial pore topology on the attachment and the spreading of cells. The viability and attachment of fibroblasts and chondrocytes could be improved by reducing the thermal exposure during the processing step of the dense base part, associated with increased superficial porosity and the corresponding increase of the surface area. The applied additive manufacturing process of macroporous structures influences emerging cell morphologies, leading to an extended morphological expression of chondrocytes and the overgrowth of small pores by fibroblasts. This indicates an improvement in superficial cell adhesion due to larger pores. These findings indicate the significance of the processing conditions in laser sintering of polypropylene on the cell response through the optimization of processing parameters and the attachment of an open-cell pore structure.

Fishroesomes show intrinsic anti-inflammatory bioactivity and ability as celecoxib carriers in vivo

According to the World Health Organization (WHO), chronic inflammatory-related diseases represent the greatest threat to human health. Indeed, failure in the resolution of inflammation leads to serious pathological conditions, such as cardiovascular diseases, arthritis, cancer, diabetes, autoimmune diseases, and neurodegenerative disorders that are often associated with extremely high human suffering and societal and economic burdens. Despite the number and efficacy of available therapeutic agents have been increased, the serious side effects associated with some of them often create a very high risk/benefit ratio for patients. Therefore, herein, a drug delivery system was engineered to overcome important drawbacks of conventional therapies and to have a synergistic action with the incorporated drug. Indeed, it will have an added beneficial role in controlling inflammation. For that, sardine (Sardina pilchardus) roe was used as the lipidic source to produce bioactive liposomes, namely fishroesomes. These spherical vesicles with ≈326 nm in size and a significant negative surface charge (≈-31 mV) were able to encapsulate and control the release of the anti-inflammatory drug celecoxib. Moreover, fishroesomes were cytocompatible for different cell types (chondrocytes and macrophages), at concentrations in which they present anti-inflammatory properties. Importantly, fishroesomes were more effective in reducing pro-inflammatory mediators than the free drug. We also demonstrated that a single intra-articular injection of the fishroesomes encapsulating or not celecoxib in an experimental rat model of inflammatory arthritis was safe and more effective in controlling the pain and reducing the synovial inflammation compared to the free drug. Notably, as the celecoxib concentration in the sardine roe-derived liposomes was less than half of the amount of free drug, this study demonstrates the value of fishroesomes in counteracting inflammation. Therefore, the developed formulations may be considered a promising therapeutic option for inflammatory conditions.

Potential of Liposomal FTY720 for Bone Regeneration: Proliferative, Osteoinductive, Chemoattractive, and Angiogenic Properties Compared to Free Bioactive Lipid

Introduction

FTY720 bioactive lipid has proliferative, osteoinductive, chemo attractive, and angiogenic properties, being thus a potential exogenous administered agent for promotion of bone regeneration. Herein we developed FTY720-loaded liposomes as a potential delivery system that could retain and prolong the bioactivity of the bioactive lipid and at the same time reduce its cytotoxicity (at high doses).

Methods

FTY720 liposomes were prepared by thin-lipid hydration and microfluidic flow focusing, and evaluated for their ability to induce proliferation, osteoinduction, and chemoattraction in three cell types: MC3T3-E1 pre-osteoblast cells, L929 fibroblast cells, and ATDC5 chondrogenic cells. The angiogenic activity of free and liposomal FTY720 was investigated using a chick chorioallantoic membrane assay. NBD-FTY720 cellular uptake was quantitated using flow cytometry and morphologically assessed by confocal microscopy. Implicated cellular signaling mechanisms were investigated by quantifying phosphorylated MAPK and CREB proteins.

Results

FTY720 liposomes (~80-110 nm) with low polydispersity and ~100% loading were prepared using both methods. FTY720 demonstrated the ability to increase cell proliferation at 10-300nM doses but was cytotoxic at doses>400nM while the corresponding liposomal-FTY720 doses were non-cytotoxic, proving its reduced toxicity. In several cases (cells and doses), FTY720 liposomes demonstrated increased osteogenic differentiation of cells, proliferation, and migration compared to free FTY720, whereas both FTY720 forms demonstrated substantial angiogenic activity. Liposomal FTY720 cellular uptake was substantially higher than that of free FTY720 in some cases, a fact that may be connected to its higher bioactivity. Increased phosphorylated MAPK and CREB protein concentrations provided information about the potential cellular signaling mechanisms involved in FTY720-induced osteogenesis.

Discussion

The current results confirm the high potential of FTY720 bioactive lipid, especially in its liposomal form, that demonstrated substantial reduction of cytotoxicity and prolonged preservation of the lipids bioactivity (compared to the free lipid), for accelerated treatment of bone defects. Interestingly, the current studies prove the potential of FTY720, especially in its liposomal form, to promote reprogramming of L929 fibroblasts into osteoblasts, a novel finding deserving future exploitation.

Deficiencies in corin and atrial natriuretic peptide-mediated signaling impair endochondral ossification in bone development

Corin is a protease that activates atrial natriuretic peptide (ANP), a hormone in cardiovascular homeostasis. Structurally, ANP is similar to C-type natriuretic peptide (CNP) crucial in bone development. Here, we examine the role of corin and ANP in chondrocyte differentiation and bone formation. We show that in Corin and Nppa (encoding ANP) knockout (KO) mice, chondrocyte differentiation is impaired, resulting in shortened limb long bones. In adult mice, Corin and Nppa deficiency impairs bone density and microarchitecture. Molecular studies in cartilages from newborn Corin and Nppa KO mice and in cultured chondrocytes indicate that corin and ANP act in chondrocytes via cGMP-dependent protein kinase G signaling to inhibit mitogen-activated protein kinase phosphorylation and stimulate glycogen synthase kinase-3β phosphorylation and β-catenin upregulation. These results indicate that corin and ANP signaling regulates chondrocyte differentiation in bone development and homeostasis, suggesting that enhancing ANP signaling may improve bone quality in patients with osteoporosis.

Development of bioactive solid-foam scaffolds from decellularized cartilage with chondrogenic and osteogenic properties

Full osteochondral regeneration remains a major clinical challenge. Among other experimental cartilage regenerative approaches, decellularized cartilage (DCC) is considered a promising material for generating potentially implantable scaffolds useful as cartilage repair strategy. In this work, we focus on screening and comparing different decellularization methods, aiming to generate DCC potentially useful in biomedical context, and therefore, with biological activity and functional properties in terms of induction of differentiation and regeneration. Data indicates that enzymatic and detergents-based decellularization methods differentially affect ECM components, and that it has consequences in further biological behavior. SDS-treated DCC powder is not useful to be further processed in 2D or 3D structures, because these structures tend to rapidly solubilize, or disaggregate, in physiologic media conditions. Conversely, Trypsin-treated DCC powders can be processed to mechanically stable 2D films and 3D solid-foam scaffolds, presumably due to partial digestion of collagens during decellularization, which would ease crosslinking at DCC during solubilization and processing. In vitro cell culture studies indicate that these structures are biocompatible and induce and potentiate chondrogenic differentiation. In vivo implantation of DCC derived 3D porous scaffolds in rabbit osteochondral defects induce subchondral bone regeneration and fibrocartilage tissue formation after implantation. Therefore, this work defines an optimal cartilage tissue decellularization protocol able to generate DCC powders processable to biocompatible and bioactive 2D and 3D structures. These structures are useful for in vitro cartilage research and in vivo subchondral bone regeneration, while hyaline cartilage regeneration with DCC alone as implantable material remains elusive.

Spatiotemporal analysis of multi-scale cell structure in spheroid culture reveals hypertrophic chondrocyte differentiation

3D cell culture has emerged as a promising approach to replicate the complex behaviors of cells within living organisms. This study aims to analyze spatiotemporal behavior of the morphological characteristics of cell structure at multiscale in 3D scaffold-free spheroids using chondrogenic progenitor ATDC5 cells. Over a 14-day culture period, it exhibited cell hypertrophy in the spheroids regarding cellular and nuclear size as well as changes in morphology. Moreover, biological analysis indicated a signification up-regulation of normal chondrocyte as well as hypertrophic chondrocyte markers, suggesting early hypertrophic chondrocyte differentiation. Cell nuclei underwent changes in volume, sphericity, and distribution in spheroid over time, indicating alterations in chromatin organization. The ratio of chromatin condensation volume to cell nuclear volume decreased as the cell nuclei enlarged, potentially signifying changes in chromatin state during hypertrophic chondrocyte differentiation. Our image analysis techniques in this present study enabled detailed morphological measurement of cell structure at multi-scale, which can be applied to various 3D culture models for in-depth investigation.

A protocol to differentiate the chondrogenic ATDC5 cell-line for the collection of chondrocyte-derived extracellular vesicles

Skeletal growth and fracture healing rely on the mineralization of cartilage in a process called endochondral ossification. Chondrocytes firstly synthesize and then modify cartilage by the release of a wide range of particles into their extracellular space. Extracellular vesicles (EVs) are one type of such particles, but their roles in endochondral ossification are yet to be fully understood. It remains a challenge to obtain representative populations of chondrocyte-derived EVs, owing to difficulties both in preserving the function of primary chondrocytes in culture and in applying the serum-free conditions required for EV production. Here, we used the ATDC5 cell-line to recover chondrocyte-derived EVs from early- and late-differentiation stages, representing chondrocytes before and during cartilage mineralization. After screening different culture conditions, our data indicate that a serum-free Opti-MEM-based culture medium preserves chondrocyte identity and function, matrix mineralization and cell viability. We subsequently scaled-up production and isolated EVs from conditioned medium by size-exclusion chromatography. The obtained chondrocyte-derived EVs had typical ultrastructure and expression of classical EV markers, at quantities suitable for downstream experiments. Importantly, chondrocyte-derived EVs from late-differentiation stages had elevated levels of alkaline phosphatase activity. Hence, we established a method to obtain functional chondrocyte-derived EVs before and during cartilage mineralization that may aid the further understanding of their roles in endochondral bone growth and fracture healing.

Insights into the ANKRD11 variants and short-stature phenotype through literature review and ClinVar database search

Ankyrin repeat domain containing-protein 11 (ANKRD11), a transcriptional factor predominantly localized in the cell nucleus, plays a crucial role in the expression regulation of key genes by recruiting chromatin remodelers and interacting with specific transcriptional repressors or activators during numerous biological processes. Its pathogenic variants are strongly linked to the pathogenesis and progression of multisystem disorder known as KBG syndrome. With the widespread application of high-throughput DNA sequencing technologies in clinical medicine, numerous pathogenic variants in the ANKRD11 gene have been reported. Patients with KBG syndrome usually exhibit a broad phenotypic spectrum with a variable degree of severity, even if having identical variants. In addition to distinctive dental, craniofacial and neurodevelopmental abnormalities, patients often present with skeletal anomalies, particularly postnatal short stature. The relationship between ANKRD11 variants and short stature is not well-understood, with limited knowledge regarding its occurrence rate or underlying biological mechanism involved. This review aims to provide an updated analysis of the molecular spectrum associated with ANKRD11 variants, investigate the prevalence of the short stature among patients harboring these variants, evaluate the efficacy of recombinant human growth hormone in treating children with short stature and ANKRD11 variants, and explore the biological mechanisms underlying short stature from both scientific and clinical perspectives. Our investigation indicated that frameshift and nonsense were the most frequent types in 583 pathogenic or likely pathogenic variants identified in the ANKRD11 gene. Among the 245 KBGS patients with height data, approximately 50% displayed short stature. Most patients showed a positive response to rhGH therapy, although the number of patients receiving treatment was limited. ANKRD11 deficiency potentially disrupts longitudinal bone growth by affecting the orderly differentiation of growth plate chondrocytes. Our review offers crucial insights into the association between ANKRD11 variants and short stature and provides valuable guidance for precise clinical diagnosis and treatment of patients with KBG syndrome.

DDIT3 aggravates TMJOA cartilage degradation via Nrf2/HO-1/NLRP3-mediated autophagy

OBJECTIVE

DNA damage-inducible transcript 3 (DDIT3), as a downstream transcription factor of endoplasmic reticulum stress, is reported to regulate chondrogenic differentiation under physiological and pathological state. However, the specific involvement of DDIT3 in the degradation of condylar cartilage of temporomandibular joint osteoarthritis (TMJOA) is unclarified.

DESIGN

The expression patterns of DDIT3 in condylar cartilage from monosodium iodoacetate-induced TMJOA mice were examined to uncover the potential role of DDIT3 in TMJOA. The Ddit3 knockout (Ddit3-/-) mice and their wildtype littermates (Ddit3+/+) were used to clarify the effect of DDIT3 on cartilage degradation. Primary condylar chondrocytes and ATDC5 cells were applied to explore the mechanisms of DDIT3 on autophagy and extracellular matrix (ECM) degradation in chondrocytes. The autophagy inhibitor chloroquine (CQ) was used to determine the effect of DDIT3-inhibited autophagy in vivo.

RESULTS

DDIT3 were highly expressed in condylar cartilage from TMJOA mice. Ddit3 knockout alleviated condylar cartilage degradation and subchondral bone loss, compared with their wildtype littermates. In vitro study demonstrated that DDIT3 exacerbated ECM degradation in chondrocytes induced by TNF-α through inhibiting autophagy. The intraperitoneal injection of CQ further confirmed that Ddit3 knockout alleviated cartilage degradation in TMJOA through activating autophagy in vivo.

CONCLUSIONS

Our findings identified the crucial role of DDIT3-inhibited autophagy in condylar cartilage degradation during the development of TMJOA.

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.

α-Ketoglutarate alleviates osteoarthritis by inhibiting ferroptosis via the ETV4/SLC7A11/GPX4 signaling pathway

Osteoarthritis (OA) is the most common degenerative joint disorder that causes disability in aged individuals, caused by functional and structural alterations of the knee joint. To investigate whether metabolic drivers might be harnessed to promote cartilage repair, a liquid chromatography-mass spectrometry (LC-MS) untargeted metabolomics approach was carried out to screen serum biomarkers in osteoarthritic rats. Based on the correlation analyses, α-ketoglutarate (α-KG) has been demonstrated to have antioxidant and anti-inflammatory properties in various diseases. These properties make α-KG a prime candidate for further investigation of OA. Experimental results indicate that α-KG significantly inhibited H2O2-induced cartilage cell matrix degradation and apoptosis, reduced levels of reactive oxygen species (ROS) and malondialdehyde (MDA), increased superoxide dismutase (SOD) and glutathione (GSH)/glutathione disulfide (GSSG) levels, and upregulated the expression of ETV4, SLC7A11 and GPX4. Further mechanistic studies observed that α-KG, like Ferrostatin-1 (Fer-1), effectively alleviated Erastin-induced apoptosis and ECM degradation. α-KG and Fer-1 upregulated ETV4, SLC7A11, and GPX4 at the mRNA and protein levels, decreased ferrous ion (Fe2+) accumulation, and preserved mitochondrial membrane potential (MMP) in ATDC5 cells. In vivo, α-KG treatment inhibited ferroptosis in OA rats by activating the ETV4/SLC7A11/GPX4 pathway. Thus, these findings indicate that α-KG inhibits ferroptosis via the ETV4/SLC7A11/GPX4 signaling pathway, thereby alleviating OA. These observations suggest that α-KG exhibits potential therapeutic properties for the treatment and prevention of OA, thereby having potential clinical applications in the future.

Growth deficiency in a mouse model of Kabuki syndrome 2 bears mechanistic similarities to Kabuki syndrome 1

Growth deficiency is a characteristic feature of both Kabuki syndrome 1 (KS1) and Kabuki syndrome 2 (KS2), Mendelian disorders of the epigenetic machinery with similar phenotypes but distinct genetic etiologies. We previously described skeletal growth deficiency in a mouse model of KS1 and further established that a Kmt2d-/- chondrocyte model of KS1 exhibits precocious differentiation. Here we characterized growth deficiency in a mouse model of KS2, Kdm6atm1d/+. We show that Kdm6atm1d/+ mice have decreased femur and tibia length compared to controls and exhibit abnormalities in cortical and trabecular bone structure. Kdm6atm1d/+ growth plates are also shorter, due to decreases in hypertrophic chondrocyte size and hypertrophic zone height. Given these disturbances in the growth plate, we generated Kdm6a-/- chondrogenic cell lines. Similar to our prior in vitro model of KS1, we found that Kdm6a-/- cells undergo premature, enhanced differentiation towards chondrocytes compared to Kdm6a+/+ controls. RNA-seq showed that Kdm6a-/- cells have a distinct transcriptomic profile that indicates dysregulation of cartilage development. Finally, we performed RNA-seq simultaneously on Kmt2d-/-, Kdm6a-/-, and control lines at Days 7 and 14 of differentiation. This revealed surprising resemblance in gene expression between Kmt2d-/- and Kdm6a-/- at both time points and indicates that the similarity in phenotype between KS1 and KS2 also exists at the transcriptional level.

Evaluation of alginate-coated β-tricalcium phosphate fiber scaffold for cell culture

Ex vivo tissue engineering is an effective therapeutic approach for the treatment of severe cartilage diseases that require tissue replenishment or replacement. This strategy demands scaffolds that are durable enough for long-term cell culture to form artificial tissue. Additionally, such scaffolds must be biocompatible to prevent the transplanted matrix from taking a toll on the patient's body. From the viewpoint of structure and bio-absorbability, a β-tricalcium phosphate (β-TCP) fiber scaffold (βTFS) is expected to serve as a good scaffold for tissue engineering. However, the fragility and high solubility of β-TCP fibers make this matrix unsuitable for long-term cell culture. To solve this problem, we developed an alginate-coated β-TCP fiber scaffold (βTFS-Alg). To assess cell proliferation and differentiation in the presence of βTFS-Alg, we characterized ATDC5 cells, a chondrocyte-like cell line, when grown in this matrix. We found that alginate coated the surface of βTFS fiber and suppressed the elution of Ca2+ from β-TCP fibers. Due to the decreased solubility of βTFS-Alg compared with β-TCP, the former provided an improved scaffold for long-term cell culture. Additionally, we observed superior cell proliferation and upregulation of chondrogenesis marker genes in ATDC5 cells cultured in βTFS-Alg. These results suggest that βTFS-Alg is suitable for application in tissue culture.

Gulp1 regulates chondrocyte growth arrest and differentiation via the TGF-β/SMAD2/3 pathway

Chondrocyte differentiation is crucial for cartilage formation. However, the complex processes and mechanisms coordinating chondrocyte proliferation and differentiation remain incompletely understood. Here, we report a novel function of the adaptor protein Gulp1 in chondrocyte differentiation. Gulp1 expression is upregulated during chondrogenic differentiation. Gulp1 knockdown in chondrogenic ATDC5 cells reduces the expression of chondrogenic and hypertrophic marker genes during differentiation. Furthermore, Gulp1 knockdown impairs cell growth arrest during chondrocyte differentiation and reduces the expression of the cyclin-dependent kinase inhibitor p21. The activation of the TGF-β/SMAD2/3 pathway, which is associated with p21 expression in chondrocytes, is impaired in Gulp1 knockdown cells. Collectively, these results demonstrate that Gulp1 contributes to cell growth arrest and chondrocyte differentiation by modulating the TGF-β/SMAD2/3 pathway.

Knockdown of Kmt2d leads to growth impairment by activating the Akt/β-catenin signaling pathway

The KMT2D variant-caused Kabuki syndrome (KS) is characterized by short stature as a prominent clinical characteristic. The initiation and progression of body growth are fundamentally influenced by chondrocyte proliferation. Uncertainty persists regarding the possibility that KMT2D deficiency affects growth by impairing chondrocyte proliferation. In this study, we used the CRISPR/Cas13d technique to knockdown kmt2d in zebrafish embryos and lentivirus to create a stable Kmt2d gene knockdown cell line in chondrocytes (ATDC5 cells). We also used CCK8 and flow cytometric studies, respectively, to determine proliferation and cell cycle state. The relative concentrations of phosphorylated Akt (ser473), phosphorylated β-catenin (ser552), and cyclin D1 proteins in chondrocytes and zebrafish embryos were determined by using western blots. In addition, Akt inhibition was used to rescue the phenotypes caused by kmt2d deficiency in chondrocytes, as well as a zebrafish model that was generated. The results showed that a knockdown of kmt2d significantly decreased body length and resulted in aberrant cartilage development in zebrafish embryos. Furthermore, the knockdown of Kmt2d in ATDC5 cells markedly increased proliferation and accelerated the G1/S transition. In addition, the knockdown of Kmt2d resulted in the activation of the Akt/β-catenin signaling pathway in ATDC5 cells. Finally, Akt inhibition could partly rescue body length and chondrocyte development in the zebrafish model. Our study demonstrated that KMT2D modulates bone growth conceivably via regulation of the Akt/β-catenin pathway.

Preparation of genetically or chemically engineered exosomes and their therapeutic effects in bone regeneration and anti-inflammation

The treatment of bone or cartilage damage and inflammation-related diseases has been a long-standing research hotspot. Traditional treatments such as surgery and cell therapy have only displayed limited efficacy because they can't avoid potential deterioration and ensure cell activity. Recently, exosomes have become a favorable tool for various tissue reconstruction due to their abundant content of proteins, lipids, DNA, RNA and other substances, which can promote bone regeneration through osteogenesis, angiogenesis and inflammation modulation. Besides, exosomes are also promising delivery systems because of stability in the bloodstream, immune stealth capacity, intrinsic cell-targeting property and outstanding intracellular communication. Despite having great potential in therapeutic delivery, exosomes still show some limitations in clinical studies, such as inefficient targeting ability, low yield and unsatisfactory therapeutic effects. In order to overcome the shortcomings, increasing studies have prepared genetically or chemically engineered exosomes to improve their properties. This review focuses on different methods of preparing genetically or chemically engineered exosomes and the therapeutic effects of engineering exosomes in bone regeneration and anti-inflammation, thereby providing some references for future applications of engineering exosomes.

IRE1α regulates the PTHrP-IHH feedback loop to orchestrate chondrocyte hypertrophy and cartilage mineralization

Cartilage development is controlled by the highly synergistic proliferation and differentiation of growth plate chondrocytes, in which the Indian hedgehog (IHH) and parathyroid hormone-related protein-parathyroid hormone-1 receptor (PTHrP-PTH1R) feedback loop is crucial. The inositol-requiring enzyme 1α/X-box-binding protein-1 spliced (IRE1α/XBP1s) branch of the unfolded protein response (UPR) is essential for normal cartilage development. However, the precise role of ER stress effector IRE1α, encoded by endoplasmic reticulum to nucleus signaling 1 (ERN1), in skeletal development remains unknown. Herein, we reported that loss of IRE1α accelerates chondrocyte hypertrophy and promotes endochondral bone growth. ERN1 acts as a negative regulator of chondrocyte proliferation and differentiation in postnatal growth plates. Its deficiency interrupted PTHrP/PTH1R and IHH homeostasis leading to impaired chondrocyte hypertrophy and differentiation. XBP1s, produced by p-IRE1α-mediated splicing, binds and up-regulates PTH1R and IHH, which coordinate cartilage development. Meanwhile, ER stress cannot be activated normally in ERN1-deficient chondrocytes. In conclusion, ERN1 deficiency accelerates chondrocyte hypertrophy and cartilage mineralization by impairing the homeostasis of the IHH and PTHrP/PTH1R feedback loop and ER stress. ERN1 may have a potential role as a new target for cartilage growth and maturation.

From cells to organs: progress and potential in cartilaginous organoids research

While cartilage tissue engineering has significantly improved the speed and quality of cartilage regeneration, the underlying metabolic mechanisms are complex, making research in this area lengthy and challenging. In the past decade, organoids have evolved rapidly as valuable research tools. Methods to create these advanced human cell models range from simple tissue culture techniques to complex bioengineering approaches. Cartilaginous organoids in part mimic the microphysiology of human cartilage and fill a gap in high-fidelity cartilage disease models to a certain extent. They hold great promise to elucidate the pathogenic mechanism of a diversity of cartilage diseases and prove crucial in the development of new drugs. This review will focus on the research progress of cartilaginous organoids and propose strategies for cartilaginous organoid construction, study directions, and future perspectives.

Poly(ethylene Glycol) Methyl Ether Methacrylate-Based Injectable Hydrogels: Swelling, Rheological, and In Vitro Biocompatibility Properties with ATDC5 Chondrogenic Lineage

Here, we present the synthesis of a series of chemical homopolymeric and copolymeric injectable hydrogels based on polyethylene glycol methyl ether methacrylate (PEGMEM) alone or with 2-dimethylamino ethyl methacrylate (DMAEM). The objective of this study was to investigate how the modification of hydrogel components influences the swelling, rheological attributes, and in vitro biocompatibility of the hydrogels. The hydrogels' networks were formed via free radical polymerization, as assured by 1H nuclear magnetic resonance spectroscopy (1H NMR). The swelling of the hydrogels directly correlated with the monomer and the catalyst amounts, in addition to the molecular weight of the monomer. Rheological analysis revealed that most of the synthesized hydrogels had viscoelastic and shear-thinning properties. The storage modulus and the viscosity increased by increasing the monomer and the crosslinker fraction but decreased by increasing the catalyst. MTT analysis showed no potential toxicity of the homopolymeric hydrogels, whereas the copolymeric hydrogels were toxic only at high DMEAM concentrations. The crosslinker polyethylene glycol dimethacrylate (PEGDMA) induced inflammation in ATDC5 cells, as detected by the significant increase in nitric oxide synthase type II activity. The results suggest a range of highly tunable homopolymeric and copolymeric hydrogels as candidates for cartilage regeneration.

Deletion of NADPH oxidase 2 in chondrocytes exacerbates ethanol-mediated growth plate disruption in mice without major effects on bone architecture or gene expression

BACKGROUND

Excess reactive oxygen species generated by NADPH oxidase 2 (Nox2) in response to ethanol exposure mediate aspects of skeletal toxicity including increased osteoclast differentiation and activity. Because perturbation of chondrocyte differentiation in the growth plate by ethanol could be prevented by dietary antioxidants, we hypothesized that Nox2 in the growth plate was involved in ethanol-associated reductions in longitudinal bone growth.

METHODS

Nox2 conditional knockout mice were generated, where the essential catalytic subunit of Nox2, cytochrome B-245 beta chain (Cybb), is deleted in chondrocytes using a Cre-Lox model with Cre expressed from the collagen 2a1 promoter (Col2a1-Cre). Wild-type and Cre-Lox mice were fed an ethanol Lieber-DeCarli-based diet or pair-fed a control diet for 8 weeks.

RESULTS

Ethanol treatment significantly reduced the number of proliferating chondrocytes in the growth plate, enhanced bone marrow adiposity, shortened femurs, reduced body length, reduced cortical bone volume, and decreased mRNA levels of a number of osteoblast and chondrocyte genes. Conditional knockout of Nox2 enzymatic activity in chondrocytes did not consistently prevent any ethanol effects. Rather, knockout mice had fewer proliferating chondrocytes than wild-type mice in both the ethanol- and control-fed animals. Additional analysis of tibia samples from Nox4 knockout mice showed that loss of Nox4 activity also reduced the number of proliferating chondrocytes and altered chondrocyte size in the growth plate.

CONCLUSIONS

Although Nox enzymatic activity regulates growth plate development, ethanol-associated disruption of the growth plate morphology is independent of ethanol-mediated increases in Nox2 activity.

Reinforcement of Hydrogels with a 3D-Printed Polycaprolactone (PCL) Structure Enhances Cell Numbers and Cartilage ECM Production under Compression

Hydrogels show promise in cartilage tissue engineering (CTE) by supporting chondrocytes and maintaining their phenotype and extracellular matrix (ECM) production. Under prolonged mechanical forces, however, hydrogels can be structurally unstable, leading to cell and ECM loss. Furthermore, long periods of mechanical loading might alter the production of cartilage ECM molecules, including glycosaminoglycans (GAGs) and collagen type 2 (Col2), specifically with the negative effect of stimulating fibrocartilage, typified by collagen type 1 (Col1) secretion. Reinforcing hydrogels with 3D-printed Polycaprolactone (PCL) structures offer a solution to enhance the structural integrity and mechanical response of impregnated chondrocytes. This study aimed to assess the impact of compression duration and PCL reinforcement on the performance of chondrocytes impregnated with hydrogel. Results showed that shorter loading periods did not significantly affect cell numbers and ECM production in 3D-bioprinted hydrogels, but longer periods tended to reduce cell numbers and ECM compared to unloaded conditions. PCL reinforcement enhanced cell numbers under mechanical compression compared to unreinforced hydrogels. However, the reinforced constructs seemed to produce more fibrocartilage-like, Col1-positive ECM. These findings suggest that reinforced hydrogel constructs hold potential for in vivo cartilage regeneration and defect treatment by retaining higher cell numbers and ECM content. To further enhance hyaline cartilage ECM formation, future studies should focus on adjusting the mechanical properties of reinforced constructs and exploring mechanotransduction pathways.

Primary cilia support cartilage regeneration after injury

In growing children, growth plate cartilage has limited self-repair ability upon fracture injury always leading to limb growth arrest. Interestingly, one type of fracture injuries within the growth plate achieve amazing self-healing, however, the mechanism is unclear. Using this type of fracture mouse model, we discovered the activation of Hedgehog (Hh) signaling in the injured growth plate, which could activate chondrocytes in growth plate and promote cartilage repair. Primary cilia are the central transduction mediator of Hh signaling. Notably, ciliary Hh-Smo-Gli signaling pathways were enriched in the growth plate during development. Moreover, chondrocytes in resting and proliferating zone were dynamically ciliated during growth plate repair. Furthermore, conditional deletion of the ciliary core gene Ift140 in cartilage disrupted cilia-mediated Hh signaling in growth plate. More importantly, activating ciliary Hh signaling by Smoothened agonist (SAG) significantly accelerated growth plate repair after injury. In sum, primary cilia mediate Hh signaling induced the activation of stem/progenitor chondrocytes and growth plate repair after fracture injury.

Applied Compressive Strain Governs Hyaline-like Cartilage versus Fibrocartilage-like ECM Produced within Hydrogel Constructs

The goal of cartilage tissue engineering (CTE) is to regenerate new hyaline cartilage in joints and treat osteoarthritis (OA) using cell-impregnated hydrogel constructs. However, the production of an extracellular matrix (ECM) made of fibrocartilage is a potential outcome within hydrogel constructs when in vivo. Unfortunately, this fibrocartilage ECM has inferior biological and mechanical properties when compared to native hyaline cartilage. It was hypothesized that compressive forces stimulate fibrocartilage development by increasing production of collagen type 1 (Col1), an ECM protein found in fibrocartilage. To test the hypothesis, 3-dimensional (3D)-bioprinted hydrogel constructs were fabricated from alginate hydrogel impregnated with ATDC5 cells (a chondrogenic cell line). A bioreactor was used to simulate different in vivo joint movements by varying the magnitude of compressive strains and compare them with a control group that was not loaded. Chondrogenic differentiation of the cells in loaded and unloaded conditions was confirmed by deposition of cartilage specific molecules including glycosaminoglycans (GAGs) and collagen type 2 (Col2). By performing biochemical assays, the production of GAGs and total collagen was also confirmed, and their contents were quantitated in unloaded and loaded conditions. Furthermore, Col1 vs. Col2 depositions were assessed at different compressive strains, and hyaline-like cartilage vs. fibrocartilage-like ECM production was analyzed to investigate how applied compressive strain affects the type of cartilage formed. These assessments showed that fibrocartilage-like ECM production tended to reduce with increasing compressive strain, though its production peaked at a higher compressive strain. According to these results, the magnitude of applied compressive strain governs the production of hyaline-like cartilage vs. fibrocartilage-like ECM and a high compressive strain stimulates fibrocartilage-like ECM formation rather than hyaline cartilage, which needs to be addressed by CTE approaches.

YTHDF1 Enhances Chondrogenic Differentiation by Activating the Wnt/β-Catenin Signaling Pathway

Cartilage is derived from the chondrogenic differentiation of stem cells, for which the regulatory mechanism has not been fully elucidated. N6-methyladenosine (m6A) messenger RNA (mRNA) methylation is the most common posttranscriptional modification in eukaryotic mRNAs and is mediated by m6A regulators. However, whether m6A regulators play roles in chondrogenic differentiation is unknown. Herein, we aim to determine the role of a main m6A reader protein, YTH N6-methyladenosine RNA binding protein 1 (YTHDF1), in chondrogenic differentiation regulation. Western blotting (WB) assays found that the expression of YTHDF1 increased during chondrogenic differentiation of human bone marrow mesenchymal stem cells (hBMSCs). The results of quantitative polymerase chain reaction, WB, immunohistochemistry, and Alcian blue staining revealed that overexpression of YTHDF1 increased cartilage matrix synthesis and the expression of chondrogenic markers when hBMSCs, ATDC5 cells, or C3H10T1/2 cells were induced to undergo chondrogenesis. Conversely, chondrogenesis was clearly inhibited when YTHDF1 was knocked down in hBMSCs, ATDC5 cells, or C3H10T1/2 cells. Further RNA sequencing and molecular biology experiments found that YTHDF1 activated the Wnt/β-catenin signaling pathway during chondrogenic differentiation. Finally, the effects of overexpression and knockdown of YTHDF1 on chondrogenic differentiation were reversed by inhibiting or activating β-catenin activity. Therefore, we demonstrated that YTDHF1 promoted chondrogenic differentiation through activation of the Wnt/β-catenin signaling pathway.

Sustained-release losartan from peptide nanofibers promotes chondrogenesis

Introduction: The central pathologic feature of osteoarthritis (OA) is the progressive loss of articular cartilage, which has a limited regenerative capacity. The TGF-β1 inhibitor, losartan, can improve cartilage repair by promoting hyaline rather that fibrous cartilage tissue regeneration. However, there are concerns about side effects associated with oral administration and short retention within the joint following intra-articular injections. To facilitate local and sustained intra-articular losartan delivery we have designed an injectable peptide amphiphile (PA) nanofiber that binds losartan. The aims of this study are to characterize the release kinetics of losartan from two different PA nanofiber compositions followed by testing pro-regenerative bioactivity on chondrocytes. Methods: We tested the impact of electrostatic interactions on nanostructure morphology and release kinetics of the negatively charged losartan molecule from either a positively or negatively charged PA nanofiber. Subsequently, cytotoxicity and bioactivity were evaluated in vitro in both normal and an IL-1β-induced OA chondrocyte model using ATDC5. Results: Both nanofiber systems promoted cell proliferation but that the positively-charged nanofibers also significantly increased glycosaminoglycans production. Furthermore, gene expression analysis suggested that losartan-encapsulated nanofibers had significant anti-inflammatory, anti-degenerative, and cartilage regenerative effects by significantly blocking TGF-β1 in this in vitro system. Discussion: The results of this study demonstrated that positively charged losartan sustained-release nanofibers may be a novel and useful treatment for cartilage regeneration and OA by blocking TGF-β1.

Structural analysis of the interaction between human cytokine BMP-2 and the antagonist Noggin reveals molecular details of cell chondrogenesis inhibition

Bone morphogenetic proteins (BMPs) are secreted cytokines belonging to the transforming growth factor-β superfamily. New therapeutic approaches based on BMP activity, particularly for cartilage and bone repair, have sparked considerable interest; however, a lack of understanding of their interaction pathways and the side effects associated with their use as biopharmaceuticals have dampened initial enthusiasm. Here, we used BMP-2 as a model system to gain further insight into both the relationship between structure and function in BMPs and the principles that govern affinity for their cognate antagonist Noggin. We produced BMP-2 and Noggin as inclusion bodies in Escherichia coli and developed simple and efficient protocols for preparing pure and homogeneous (in terms of size distribution) solutions of the native dimeric forms of the two proteins. The identity and integrity of the proteins were confirmed using mass spectrometry. Additionally, several in vitro cell-based assays, including enzymatic measurements, RT-qPCR, and matrix staining, demonstrated their biological activity during cell chondrogenic and hypertrophic differentiation. Furthermore, we characterized the simple 1:1 noncovalent interaction between the two ligands (K_Dca. 0.4 nM) using bio-layer interferometry and solved the crystal structure of the complex using X-ray diffraction methods. We identified the residues and binding forces involved in the interaction between the two proteins. Finally, results obtained with the BMP-2 N102D mutant suggest that Noggin is remarkably flexible and able to accommodate major structural changes at the BMP-2 level. Altogether, our findings provide insights into BMP-2 activity and reveal the molecular details of its interaction with Noggin.

Neuropeptide Y Promotes mTORC1 to Regulate Chondrocyte Proliferation and Hypertrophy

Peripheral neuropeptide Y (NPY) has been reported to regulate bone metabolism and homeostasis; however, its potential roles in growth plate chondrogenesis remain unclear. Here, we found that NPY expression decreased during chondrocyte differentiation in vitro and in vivo. NPY was required for chondrocyte proliferation; in contrast, knockdown of NPY facilitated chondrocyte hypertrophic differentiation. Administration of recombinant NPY in rat chondrocytes and metatarsal bones uncoupled normal proliferation and hypertrophic differentiation during chondrogenesis and thereby inhibited growth plate chondrogenesis and longitudinal bone growth. Remarkably, NPY activated the mTORC1 pathway in chondrocytes, whereas attenuation of mTORC1 activity by administration of rapamycin in vitro partially abrogated NPY-mediated effects on chondrocyte proliferation and hypertrophic differentiation. In addition, a combination of Y2R antagonist but not Y1R antagonist with NPY abolished NPY-mediated inhibition of metatarsal growth and growth plate chondrogenesis. Mechanistically, NPY activated Erk1/2 by NPY2R, then phosphorylated ERK1/2 activated mTORC1 to initiate PTHrP expression, which in turn promoted chondrocyte proliferation and inhibited chondrocyte hypertrophic differentiation. In conclusion, our data identified NPY as a crucial regulator of chondrogenesis and may provide a promising therapeutic strategy for skeletal diseases.

Emerging structural and pathological analyses on the erectile organ, corpus cavernous containing sinusoids

Background

The corpus cavernosum (CC) containing sinusoids plays fundamental roles for erection. Analysis of pathological changes in the erectile system is studied by recent experimental systems. Various in vitro models utilizing genital mesenchymal-derived cells and explant culture systems are summarized.

Methods

3D reconstruction of section images of murine CC was created. Ectopic chondrogenesis in aged mouse CC was shown by a gene expression study revealing the prominent expression of Sox9. Various experimental strategies utilizing mesenchyme-derived primary cells and tissue explants are introduced.

Main Findings

Possible roles of Sox9 in chondrogenesis and its regulation by several signals are suggested. The unique character of genital mesenchyme is shown by various analyses of external genitalia (ExG) derived cells and explant cultures. Such strategies are also applied to the analysis of erectile contraction/relaxation responses to many signals and aging process.

Conclusion

Erectile dysfunction (ED) is one of the essential topics for the modern aged society. More comprehensive studies are necessary to reveal the nature of the erectile system by combining multiple cell culture strategies.

Calcium signaling of primary chondrocytes and ATDC5 chondrogenic cells under osmotic stress and mechanical stimulation

Calcium signaling plays an essential role in chondrocyte mechanotransduction. Guilak and colleagues have revealed the roles of TRPV4 and Piezo channels in chondrocyte calcium signaling and metabolism. This study compared the calcium responses of primary chondrocytes and ATDC5 cells induced by two different stimuli: osmotic stress and intense mechanical stimulus. Roles of three essential calcium signaling pathways, including extracellular calcium source, intracellular ER calcium store and mechanical-sensitive ion channels, were also investigated and compared between cells. Primary chondrocytes showed more vigorous calcium peaks under osmotic stress than under mechanical stimuli, while an opposite trend was found for ATDC5 cells. Extracellular calcium source, intracellular ER store, and PLC/IP3 pathway each played significant roles in the calcium responses of ATDC5 cells under both osmotic and mechanical stimuli. However, high level shear stress can directly cause ER release in primary cells without the presence of extracellular Ca2+ or involvement of PLC-IP3 pathway. TRPV4 channel is essential for the responses of ATDC5 cells, but not for primary chondrocytes. In contrast, inhibition of mechano-sensitive channels had no significant effects on the ATDC5 cells. Therefore, primary chondrocytes and ATDC5 cells rely on distinct calcium sources and ion channels to initiate intracellular calcium signaling. Together, these results contribute to our understanding of stimulation-induced calcium signaling in primary chondrocytes and ATDC5 cells, and the different roles of three essential pathways between the two cell types.

Adipokines as targets in musculoskeletal immune and inflammatory diseases

Adipokines are the principal mediators in adipose signaling. Nevertheless, besides their role in energy storage, these molecules can be produced by other cells, such as immune cells or chondrocytes. Given their pleiotropic effects, research over the past few years has also focused on musculoskeletal diseases, showing that these adipokines might have relevant roles in worsening the disease or improving the treatment response. In this review, we summarize recent advances in our understanding of adipokines and their role in the most prevalent musculoskeletal immune and inflammatory disorders.

Hyaluronic Acid Oligosaccharide Derivatives Alleviate Lipopolysaccharide-Induced Inflammation in ATDC5 Cells by Multiple Mechanisms

High molecular weight hyaluronic acids (HMW-HAs) have been used for the palliative treatment of osteoarthritis (OA) for decades, but the pharmacological activity of HA fragments has not been fully explored due to the limited availability of structurally defined HA fragments. In this study, we synthesized a series glycosides of oligosaccharides of HA (o-HAs), hereinafter collectively referred to as o-HA derivatives. Their effects on OA progression were examined in a chondrocyte inflammatory model established by the lipopolysaccharide (LPS)-challenged ATDC5 cells. Cell Counting Kit-8 (CCK-8) assays and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) showed that o-HA derivatives (≤100 μg/mL) exhibited no cytotoxicity and pro-inflammatory effects. We found that the o-HA and o-HA derivatives alleviated LPS-induced inflammation, apoptosis, autophagy and proliferation-inhibition of ATDC5 cells, similar to the activities of HMW-HAs. Moreover, Western blot analysis showed that different HA derivatives selectively reversed the effects of LPS on the expression of extracellular matrix (ECM)-related proteins (MMP13, COL2A1 and Aggrecan) in ATDC5 cells. Our study suggested that o-HA derivatives may alleviate LPS-induced chondrocyte injury by reducing the inflammatory response, maintaining cell proliferation, inhibiting apoptosis and autophagy, and decreasing ECM degradation, supporting a potential oligosaccharides-mediated therapy for OA.

DDIT3/CHOP mediates the inhibitory effect of ER stress on chondrocyte differentiation by AMPKα-SIRT1 pathway

Endoplasmic reticulum (ER) stress is an evolutionarily conserved cellular stress response related to multiple diseases, including temporomandibular joint (TMJ) cartilage-related diseases. Recent studies have indicated that DDIT3/CHOP (a downstream transcription factor of ER stress) is an important effector in mediating ER stress to inhibit chondrogenesis. However, the underlying mechanism by which DDIT3 regulates chondrogenesis remains unclear. In this study, tunicamycin (an ER stress agonist)-induced ER stress inhibited chondrocyte differentiation and matrix synthesis in vitro and led to an osteoarthritis-like phenotype in mouse TMJ cartilage. Meanwhile, DDIT3 expression in chondrocytes was robustly upregulated. Loss-of-function experiments validated the inhibiting effect of DDIT3 on chondrocyte differentiation and matrix synthesis. Mechanistically, the inhibiting effect was attributed to the direct and indirect regulatory effect of DDIT3 on SIRT1 (sirtuin1, silent mating type information regulation protein type 1, a member of NAD+ dependent class III histone deacetylases). On one hand, DDIT3 directly promoted the transcription of SIRT1. On the other hand, DDIT3 indirectly increased the expression of SIRT1 by promoting AMPKα phosphorylation and activation. Furthermore, activation of AMPKα or SIRT1 with the corresponding agonist AICAR or resveratrol in the DDIT3-knockdown cells partially restored the inhibiting effect of DDIT3 on chondrocyte differentiation and matrix synthesis. Collectively, these novel findings indicate that DDIT3 regulates the inhibitory effect of ER stress on chondrocyte differentiation and matrix synthesis partially via the AMPKα-SIRT1 pathway. A thorough understanding of ER stress in regulating chondrocyte homeostasis and its role in the onset of osteoarthritis may be promising to develop therapeutic targets and prevent condyle cartilage destruction.

MMP-10 Deficiency Effects Differentiation and Death of Chondrocytes Associated with Endochondral Osteogenesis in an Endemic Osteoarthritis

OBJECTIVE

The objective of this study was to determine the matrix metalloproteinase-10 (MMP-10) expression pattern and to assess how it contributes to endochondral osteogenesis in Kashin-Beck disease (KBD).

DESIGN

The cartilages of KBD patients, Sprague-Dawley rats fed with selenium (Se)-deficient diet and/or T-2 toxin, and ATDC5 cells were used in this study. ATDC5 cells were induced into hypertrophic chondrocytes using a 1% insulin-transferrin-selenium (ITS) culture medium for 21 days. The expressions of MMP-10 in the cartilages were visualized by immunohistochemistry. The messenger RNA (mRNA) and protein expression levels were determined by real-time polymerase chain reaction (RT-PCR) and Western blotting. MMP-10 short hairpin RNA (shRNA) was transfected into hypertrophic chondrocytes to knock down the gene expression of MMP-10. Meanwhile, the cell death of MMP-10-knockdown chondrocyte was detected using flow cytometry.

RESULTS

The expression of MMP-10 was decreased in the growth plates of children with KBD. A decreased expression of MMP-10 also was observed in the growth plates of rats fed with an Se-deficient diet and/or T-2 toxin exposure. The mRNA and protein expression levels of MMP-10 increased during the chondrogenic differentiation of ATDC5 cells. MMP-10 knockdown in hypertrophic chondrocytes significantly decreased the gene and protein expression of collagen type II (Col II), Col X, Runx2, and MMP-13. Besides, the percentage of cell apoptosis was significantly increased after MMP-10 knockdown in hypertrophic chondrocytes.

CONCLUSION

MMP-10 deficiency disrupts chondrocyte terminal differentiation and induces the chondrocyte's death, which impairs endochondral osteogenesis in the pathogenesis of KBD.

Adaptation of the protein translational apparatus during ATDC5 chondrogenic differentiation

Introduction

Ribosome biogenesis is integrated with many cellular processes including proliferation, differentiation and oncogenic events. Chondrogenic proliferation and differentiation require a high cellular translational capacity to facilitate cartilaginous extracellular matrix production. We here investigated the expression dynamics of factors involved in ribosome biogenesis during in vitro chondrogenic differentiation and determined whether protein translation capacity adapts to different phases of chondrogenic differentiation.

Materials

SnoRNA expression during ATDC5 differentiation was analyzed by RNA sequencing of samples acquired from day 0 (progenitor stage), 7 (chondrogenic stage) and day 14 (hypertrophic stage). RT-qPCR was used to determine expression of fibrillarin, dyskerin, UBF-1, Sox9, Col2a1, Runx2, Col10a1 mRNAs and 18S, 5.8S and 28S rRNAs. Protein expression of fibrillarin, dyskerin and UBF-1 was determined by immunoblotting. Ribosomal RNA content per cell was determined by calculating rRNA RT-qPCR signals relative to DNA content (SYBR Green assay). Total protein translational activity was evaluated with a puromycilation assay and polysome profiling.

Results

As a result of initiation of chondrogenic differentiation (Δt0-t7), 21 snoRNAs were differentially expressed (DE). Hypertrophic differentiation caused DE of 23 snoRNAs (Δt7-t14) and 43 when t0 was compared to t14. DE snoRNAs, amongst others, target nucleotide modifications in the 28S rRNA peptidyl transferase center and the 18S rRNA decoding center. UBF-1, fibrillarin and dyskerin expression increased as function of differentiation and displayed highest fold induction at day 5-6 in differentiation. Ribosomal RNA content per cell was significantly increased at day 7, but not at day 14 in differentiation. Similar dynamics in translational capacity and monosomal ribosome fraction were observed during differentiation.

Conclusion

The expression of a great number of ribosome biogenesis factors is altered during chondrogenic differentiation of ATDC5 cells, which is accompanied by significant changes in cellular translational activity. This elucidation of ribosome biogenesis dynamics in chondrogenic differentiation models enables the further understanding of the role of ribosome biogenesis and activity during chondrocyte cell commitment and their roles in human skeletal development diseases.

Galactose Enhances Chondrogenic Differentiation of ATDC5 and Cartilage Matrix Formation by Chondrocytes

Galactose, an important carbohydrate nutrient, is involved in several types of cellular metabolism, participating in physiological activities such as glycosaminoglycan (GAG) synthesis, glycosylation, and intercellular recognition. The regulatory effects of galactose on osteoarthritis have attracted increased attention. In this study, in vitro cell models of ATDC5 and chondrocytes were prepared and cultured with different concentrations of galactose to evaluate its capacity on chondrogenesis and cartilage matrix formation. The cell proliferation assay demonstrated that galactose was nontoxic to both ATDC5 cells and chondrocytes. RT-PCR and immunofluorescence staining indicated that the gene expressions of cartilage matrix type II collagen and aggrecan were significantly upregulated with increasing galactose concentration and the expression and accumulation of the extracellular matrix (ECM) protein. Overall, these results indicated that a galactose concentration below 8 mM exhibited the best effect on promoting chondrogenesis, which entitles galactose as having considerable potential for cartilage repair and regeneration.

Physosmotic Induction of Chondrogenic Maturation Is TGF-β Dependent and Enhanced by Calcineurin Inhibitor FK506

Increasing extracellular osmolarity 100 mOsm/kg above plasma level to the physiological levels for cartilage induces chondrogenic marker expression and the differentiation of chondroprogenitor cells. The calcineurin inhibitor FK506 has been reported to modulate the hypertrophic differentiation of primary chondrocytes under such conditions, but the molecular mechanism has remained unclear. We aimed at clarifying its role. Chondrocyte cell lines and primary cells were cultured under plasma osmolarity and chondrocyte-specific in situ osmolarity (+100 mOsm, physosmolarity) was increased to compare the activation of nuclear factor of activated T-cells 5 (NFAT5). The effects of osmolarity and FK506 on calcineurin activity, cell proliferation, extracellular matrix quality, and BMP- and TGF-β signaling were analyzed using biochemical, gene, and protein expression, as well as reporter and bio-assays. NFAT5 translocation was similar in chondrocyte cell lines and primary cells. High supraphysiological osmolarity compromised cell proliferation, while physosmolarity or FK506 did not, but in combination increased proteoglycan and collagen expression in chondrocytes in vitro and in situ. The expression of the TGF-β-inducible protein TGFBI, as well as chondrogenic (SOX9, Col2) and terminal differentiation markers (e.g., Col10) were affected by osmolarity. Particularly, the expression of minor collagens (e.g., Col9, Col11) was affected. The inhibition of the FK506-binding protein suggests modulation at the TGF-β receptor level, rather than calcineurin-mediated signaling, as a cause. Physiological osmolarity promotes terminal chondrogenic differentiation of progenitor cells through the sensitization of the TGF-β superfamily signaling at the type I receptor. While hyperosmolarity alone facilitates TGF-β superfamily signaling, FK506 further enhances signaling by releasing the FKBP12 break from the type I receptor to improve collagenous marker expression. Our results help explain earlier findings and potentially benefit future cell-based cartilage repair strategies.

ANP32A represses Wnt signaling across tissues thereby protecting against osteoarthritis and heart disease

OBJECTIVES

To investigate how ANP32A, previously linked to the antioxidant response, regulates Wnt signaling as unraveled by transcriptome analysis of Anp32a-deficient mouse articular cartilage, and its implications for osteoarthritis (OA) and diseases beyond the joint.

METHODS

Anp32a knockdown chondrogenic ATDC5 cells were cultured in micromasses. Wnt target genes, differentiation markers and matrix deposition were quantified. Wnt target genes were determined in articular cartilage from Anp32a-deficient mice and primary human articular chondrocytes upon ANP32A silencing, using qPCR, luciferase assays and immunohistochemistry. Co-immunoprecipitation, immunofluorescence and chromatin-immunoprecipitation quantitative PCR probed the molecular mechanism via which ANP32A regulates Wnt signaling. Anp32a-deficient mice were subjected to the destabilization of the medial meniscus (DMM) OA model and treated with a Wnt inhibitor and an antioxidant. Severity of OA was assessed by cartilage damage and osteophyte formation. Human Protein Atlas data analysis identified additional organs where ANP32A may regulate Wnt signaling. Wnt target genes were determined in heart and hippocampus from Anp32a-deficient mice, and cardiac hypertrophy and fibrosis quantified.

RESULTS

Anp32a loss triggered Wnt signaling hyper-activation in articular cartilage. Mechanistically, ANP32A inhibited target gene expression via histone acetylation masking. Wnt antagonist treatment reduced OA severity in Anp32a-deficient mice by preventing osteophyte formation but not cartilage degradation, contrasting with antioxidant treatment. Dual therapy ameliorated more OA features than individual treatments. Anp32a-deficient mice also showed Wnt hyper-activation in the heart, potentially explaining the cardiac hypertrophy phenotype found.

CONCLUSIONS

ANP32A is a novel translationally relevant repressor of Wnt signaling impacting osteoarthritis and cardiac disease.

p38MAPK Signaling Pathway in Osteoarthritis: Pathological and Therapeutic Aspects

Osteoarthritis (OA) is an aging-related joint disease, pathologically featured with degenerated articular cartilage and deformation of subchondral bone. OA has become the fourth major cause of disability in the world, imposing a huge economic burden. At present, the pathogenesis and pathophysiology of OA are still unclear. Complex regulating networks containing different biochemical signaling pathways are involved in OA pathogenesis and progression. The p38MAPK signaling pathway is a member of the MAPK signaling pathway family, which participates in the induction of cellular senescence, the differentiation of chondrocytes, the synthesis of matrix metalloproteinase (MMPs) and the production of pro-inflammatory factors. In recent years, studies on the regulating role of p38MAPK signaling pathway and the application of its inhibitors have attracted growing attention, with an increasing number of in vivo and in vitro studies. One interesting finding is that the inhibition of p38MAPK could suppress chondrocyte inflammation and ameliorate OA, indicating its therapeutic role in OA treatment. Based on this, we reviewed the mechanisms of p38MAPK signaling pathway in the pathogenesis of OA, hoping to provide new ideas for future research and OA treatment.

Nrf2/ARE Signaling Directly Regulates SOX9 to Potentially Alter Age-Dependent Cartilage Degeneration

Oxidative stress is implicated in osteoarthritis, and nuclear factor erythroid 2–related factor 2 (Nrf2)/antioxidant response element (ARE) pathway maintains redox homeostasis. We investigated whether Nrf2/ARE signaling controls SOX9. SOX9 expression in human C-28/I2 chondrocytes was measured by RT–qPCR after shRNA-mediated knockdown of Nrf2 or its antagonist the Kelch-like erythroid cell-derived protein with cap ‘‘n’’ collar homology-associated protein 1 (Keap1). To verify whether Nrf2 transcriptionally regulates SOX9, putative ARE-binding sites in the proximal SOX9 promoter region were inactivated, cloned into pGL3, and co-transfected with phRL–TK for dual-luciferase assays. SOX9 promoter activities without and with Nrf2-inducer methysticin were compared. Sox9 expression in articular chondrocytes was correlated to cartilage thickness and degeneration in wild-type (WT) and Nrf2-knockout mice. Nrf2-specific RNAi significantly decreased SOX9 expression, whereas Keap1-specific RNAi increased it. Putative ARE sites (ARE1, ARE2) were identified in the SOX9 promoter region. ARE2 mutagenesis significantly reduced SOX9 promoter activity, but ARE1 excision did not. Functional ARE2 site was essential for methysticin-mediated induction of SOX9 promoter activity. Young Nrf2-knockout mice revealed significantly lower Sox9-positive chondrocytes, and old Nrf2-knockout animals showed thinner cartilage and more cartilage degeneration. Our results suggest Nrf2 directly regulates SOX9 in articular cartilage, and Nrf2-loss can develop mild osteoarthritis at old age. Pharmacological Nrf2 induction may hold the potential to diminish age-dependent cartilage degeneration through improving SOX9 expression.

Brd4 is required for chondrocyte differentiation and endochondral ossification

Differentiation of multi-potent mesenchymal stromal cells (MSCs) is directed by the activities of lineage-specific transcription factors and co-factors. A subset of these proteins controls the accessibility of chromatin by recruiting histone acetyl transferases or deacetylases that regulate acetylation of the N-termini of H3 and H4 histone proteins. Bromodomain (BRD) proteins recognize these acetylation marks and recruit the RNA pol II containing transcriptional machinery. Our previous studies have shown that Brd4 is required for osteoblast differentiation in vitro. Here, we investigated the role of Brd4 on endochondral ossification in C57BL/6 mice and chondrogenic differentiation in cell culture models. Conditional loss of Brd4 in the mesenchyme (Brd4 cKO, Brd4^fl/fl: Prrx1-Cre) yields smaller mice that exhibit alteration in endochondral ossification. Importantly, abnormal growth plate morphology and delayed long bone formation is observed in juvenile Brd4 cKO mice. One week old Brd4 cKO mice have reduced proliferative and hypertrophic zones within the physis and exhibit a delay in the formation of the secondary ossification center. At the cellular level, Brd4 function is required for chondrogenic differentiation and maturation of both ATDC5 cells and immature mouse articular chondrocytes. Mechanistically, Brd4 loss suppresses Sox9 levels and reduces expression of Sox9 and Runx2 responsive endochondral genes (e.g., Col2a1, Acan, Mmp13 and Sp7/Osx). Collectively, our results indicate that Brd4 is a key epigenetic regulator required for normal chondrogenesis and endochondral ossification.

Jintiange Capsules Ameliorate Osteoarthritis by Modulating Subchondral Bone Remodeling and Protecting Cartilage Against Degradation

Osteoarthritis (OA) is the most prevalent joint disease worldwide, making it a major cause of pain and disability. Identified as a chronic and progressive disease, effective treatment at the early stages of OA has become critical to its management. Jintiange (Jtg) capsules are a traditional Chinese medicine produced from multiple organic components of various animal bones and routinely used to treat osteoporosis in China. However, the effect of Jtg on subchondral bone and cartilage degeneration in OA remains unknown. The purpose of the present study was to investigate the biomolecular role and underlying mechanisms of Jtg in OA progression. Herein, we found that Jtg inhibited receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation and it functions through the NF-κB signaling pathway. Jtg also inhibited chondrocyte apoptosis via reducing the reactive oxygen species concentration in these cells. Moreover, in vivo evaluation revealed that Jtg significantly attenuates subchondral bone remodeling and cartilage destruction in anterior cruciate ligament transection (ACLT) mouse models. Taken together, our data demonstrate that Jtg inhibits osteoclast differentiation in subchondral bone and chondrocyte apoptosis in cartilage, supporting its potential therapeutic value for treating OA.

FoxO3a cooperates with RUNX1 to promote chondrogenesis and terminal hypertrophic of the chondrogenic progenitor cells

FoxO transcription factors (FoxOs) have recently been shown to protect against chondrocyte dysfunction and modulate cartilage homeostasis in osteoarthritis. The mechanism underlying of FoxOs regulate chondrocyte differentiation remains unknown. Runt related transcription factor 1 (RUNX1) mediated both chondrocyte and osteoblast differentiation. Our data showed that FoxO3a and RUNX1 are co-expressed in ATDC5 cells and undifferentiated mesenchyme cells and have similar high levels in chondrocytes undergoing transition from proliferation to hypertrophy. Overexpression of FoxO3a in ATDC5 cells or mouse mesenchymal cells resulted in a potent induction of the chondrocyte differentiation markers. Knockdown FoxO3a or RUNX1 potently inhibits the expressions of chondrocyte differentiation markers, including Sox9, Aggrecan, Col2, and hypertrophic chondrocyte markers including RUNX2, ColX, MMP13 and ADAMTs-5 in ATDC5 cells. Co-immunoprecipitation showed that FoxO3a binds the transcriptional regulator RUNX1. Immunohistochemistry showed that FoxO3a and RUNX1 are highly co-expressed in the proliferative chondrocytes of the growth plates in the hind limbs of newborn mice. Collectively, we revealed that FoxO3a cooperated with RUNX1 promoted chondrocyte differentiation through enhancing both early chondrogenesis and terminal hypertrophic of the chondrogenic progenitor cells, indicating FoxO3a interacting with RUNX1 may be a therapeutic target for the treatment of osteoarthritis and other bone diseases.

Hypertonic Dextrose Stimulates Chondrogenic Cells to Deposit Collagen and Proliferate

OBJECTIVE

Hypertonic dextrose (HD) injections (prolotherapy) for osteoarthritis are reported to reduce pain. Cartilage regeneration is hypothesized as a mechanism. This in vitro study identifies an HD concentration that stimulates chondrogenic cells to increase metabolic activity and assesses whether this concentration affects collagen deposition and proliferation.

DESIGN

ATDC5 chondrogenic cells were cultured in normoglycemic DMEM/F12 medium, treated with concentrations of HD (4-400 mM), and assessed with PrestoBlue. Advanced light microscopy was used to conduct live imaging of collagen deposition through second harmonic generation microscopy (SHG) and proliferation via 2-photon excitation microscopy. Proliferation was additionally assessed with hemocytometer counts.

RESULTS

A linear regression model found that, relative to the 4 mM baseline control, cells treated with 200 mM had a higher mean absorbance (P = 0.023) and cells treated with 250 mM were trending toward a higher mean absorbance (P = 0.076). Polynomial regression interpolated 240 mM as producing the highest average absorbance. Hemocytometer counts validated 250 mM as stimulating proliferation compared with the 4 mM control (P < 0.01). A concentration of 250 mM HD led to an increase in collagen deposition compared with that observed in control (P < 0.05). This HD concentration also led to increases in proliferation of ATDC5 cells relative to that of control (P < 0.001).

CONCLUSIONS

A 250 mM HD solution appears to be associated with increased metabolic activity of chondrocytes, increased collagen deposition, and increased chondrocyte proliferation. These results support clinical prolotherapy research suggesting that intra-articular HD joint injections reduce knee pain. Further study of HD and cellular processes is warranted.