Osteophyte formation and matrix mineralization in a TMJ osteoarthritis mouse model are associated with ectopic hedgehog signaling

CRISPR/CasRx-mediated RNA knockdown targeting β-catenin and Ihh signaling alleviates osteoarthritis

Osteoarthritis (OA) is a chronic degenerative joint disease. Currently, OA is incurable. Abnormal activation of canonical Wnt/β-catenin or Indian hedgehog (Ihh) signaling could lead to OA development and progression. This study aimed to determine if targeting β-catenin and Ihh signaling could yield an effective therapeutic intervention for OA disease. CRISPR/CasRx is a new RNA interference tool that can precisely and efficiently cleave single-strand RNAs. In this study, we screened CRISPR-derived RNA (crRNA) targeting Ctnnb1 and Smo in vitro and selected two optimal crRNAs for each gene. CasRx-mediated Ctnnb1 and Smo knockdown showed high efficiency and specificity with no obvious off-target effects in vitro. We then performed intra-articular injection of selected crRNAs driven by the adeno-associated virus into an OA mouse model. Micro-CT, histological, and histomorphometric analyses were conducted to evaluate the efficacy of CasRx approach on OA treatment. We found that the knockdown of Ctnnb1 and Smo decelerated pathological damage in the keen joint of the experimental OA mouse model. Our findings suggest that CasRx-mediated Ctnnb1 and Smo knockdown could be a potential strategy for OA treatment.

FSP1/S100A4-Expressing Stem/Progenitor Cells Are Essential for Temporomandibular Joint Growth and Homeostasis

The temporomandibular joint (TMJ) is one of the most used joints in the body. Defects and wear in the cartilage of the joint, condyle, and fibrocartilage disc lie at the heart of many common TMJ disorders. During postnatal development, the condyle acts as a growth center for the mandible, with cells moving as a conveyor belt away from the top of the condyle as they differentiate. The superficial layers of the condyle have been proposed to contain stem/progenitor populations to allow growth and maintain homeostasis. Here we have focused on the role of fibroblast-specific protein 1 (FSP1; also known as S100a4) as a key fibroblast stem/progenitor marker for the condyle. Lineage tracing with FSP1-Cre;R26RmTmG mice revealed that FSP1-expressing cells were restricted to the superficial fibroblast zone, giving rise to all layers of the condyle over time. The FSP1-expressing cells overlapped with other putative stem cell markers of the condyle, such as Gli1 and scleraxis. BrdU pulse chase experiments highlighted that a subset of FSP1 fibrocartilage was label retaining, suggesting that FSP1 labels a novel stem/progenitor cell population in the condyle. Destruction of FSP1-expressing cells by conditional diphtheria toxin activity in FSP1-Cre;R26RDTA mice resulted in severe TMJ osteoarthritis with loss of the cartilage structure. Lgr5-expressing cells in the superficial layer of the condyle have been shown to create a Wnt inhibitory niche. FSP1 expression postnatally was associated with a reduction in canonical Wnt activity in the condyle. Importantly, constitutive activation of Wnt/β catenin in FSP1-expressing cells led to a downregulation of FSP1 and progressive postnatal loss of TMJ condylar hyaline cartilage due to loss of the superficial stem/progenitor cells. These data demonstrate a novel role for FSP1-expressing cells in the superficial zone in growth and maintenance of the TMJ condylar cartilage and highlight the importance of regulating Wnt activity in this population.

Stem and progenitor cells in the synovial joint as targets for regenerative therapy

Damage to articular cartilage, tendons, ligaments and entheses as a result of trauma, degeneration or inflammation in rheumatic diseases is prevalent. Regenerative medicine offers promising strategies for repairing damaged tissues, with the aim of restoring both their structure and function. While these strategies have traditionally relied on tissue engineering approaches using exogenous cells, interventions based on the activation of endogenous repair mechanisms are an attractive alternative. Key to advancing such approaches is a comprehensive understanding of the diversity of the stem and progenitor cells that reside in the adult synovial joint and how they function to repair damaged tissues. Advances in developmental biology have provided a lens through which to understand the origins, identities and functions of these cells, and insights into the roles of stem and progenitor cells in joint tissue repair, as well as their complex relationship with fibroblasts, have emerged. Integration of knowledge obtained through studies using advanced single-cell technologies will be crucial to establishing unified models of cell populations, lineage hierarchies and their molecular regulation. Ultimately, a more complete understanding of how cells repair tissues in adult life will guide the development of innovative pro-regenerative drugs, which are poised to enter clinical practice in musculoskeletal medicine.

Unbalanced occlusal loading elicited remodeling responses in growing rat temporomandibular joints and Notch1/Hes1 signaling pathway expression

OBJECTIVE

We aimed to assess the impact of unbalanced occlusal loading on the TMJ and the expression of the Notch1/Hes1 signaling pathway using a unilateral occlusal loss rat model.

METHODS

We established a unilateral occlusal defect animal model. At three different times, all female Wistar rats were randomized into three groups (4 weeks, 6 weeks, and 10 weeks): (1) unilateral occlusal loss experiment group (extraction of the left mandibular molars and trimming of the left mandibular incisors every other day), (2) restoring incisal occlusion experiment group (extraction of the left mandibular molars and trimming of the left mandibular incisors every other day for 4 weeks, and then the incisor trimming was stopped until 10 weeks), (3) control group (normal loading). Micro-CT, histological staining, immunofluorescence, immunohistochemistry, and real-time quantitative polymerase chain reaction were used to identify changes in condylar cartilage, subchondral bone, and articular discs.

RESULTS

Temporomandibular joint condylar cartilage degradation was induced by unbalanced occlusal loading, especially on the extraction side. The dropped cartilage thickness, Cyclin D1 positive chondrocytes, TB staining and collagen II positive areas, the decreased expression levels of Notch1, Jagged1, and Hes1 in condylar cartilage, the loss of TMJ subchondral bone were all reversed in the restoring incisal occlusion experiment group at 6 weeks. The experiment group exhibited catabolic degradative alterations at the molecular level in TMJ discs, showing a notable decrease in the mRNA expression levels of Col I, Col II, and ALP.

CONCLUSIONS

The growing rat TMJ condyle exhibits a significant remodeling capacity in altered occlusal loading, which can be degenerative and recuperative, respectively, in reaction to decreased occlusal loading and restored occlusal loading. Appropriate occlusal loading is essential for transducing the Notch1/Hes1 signaling pathway and normal metabolism of the TMJ disc.

Animal models of temporomandibular joint heterotopic ossification: a scoping review

There are few animal models for heterotopic ossification of the temporomandibular joint (TMJ-HO). This scoping review provides an overview of current knowledge on the induction methods and specific conditions required to produce TMJ-HO in various animal models. Two independent reviewers selected papers from the PubMed, Web of Science, and Cochrane Library databases. The inclusion criteria were articles in English, in vivo studies, and a TMJ-HO induction method. Observational, in vitro, human studies, reviews, and book chapters were excluded. Twenty-four publications were included. HO was surgically, genetically, or chemically induced through single or combined defects in the condyle, articular disc, and temporal bone in animal models (sheep=9 studies, mouse=5, rat=4, rabbit=2, pig=2, goat=1, dog=1, monkey=1) specific for traumatic TMJ-HO (n=4), ankylosis (n=9), osteoarthritis (n=8), experimental disc perforation (n=1), status post-TMJ replacement (n=1), and status post bilateral sagittal split osteotomy (n=1). TMJ-HO remains challenging to study due to its multifactorial etiology and largely unknown pathogenesis, which varies between species. There is a need for more accurate, reproducible animal models that can be extrapolated to human TMJ-HO and a consolidated clinical classification system to allow for meaningful future research.

Equine Models of Temporomandibular Joint Osteoarthritis: A Review of Feasibility, Biomarkers, and Molecular Signaling

Osteoarthritis (OA) of the temporomandibular joint (TMJ) occurs spontaneously in humans and various animal species, including horses. In humans, obtaining tissue samples is challenging and clinical symptoms appear late in the disease progression. Therefore, genetically modified, induced, and naturally occurring animal models play a crucial role in understanding the pathogenesis and evaluating potential therapeutic interventions for TMJ OA. Among the naturally occurring models, the equine TMJ OA model is characterized by slow, age-related progression, a wide range of clinical examinations, and imaging modalities that can be performed on horses, as well as easy tissue and synovial fluid collection. The morphological and functional similarities of TMJ structures in both species make the equine model of TMJ OA an excellent opportunity to track disease progression and response to treatment. However, much work remains to be carried out to determine the utility of human TMJ OA biomarkers in horses. Among the main TMJ OA biomarkers, IL-1, IL-6, TGF-β, TNF-α, and PGE2 have been recently investigated in the equine model. However, the majority of biomarkers for cartilage degradation, chondrocyte hypertrophy, angiogenesis, and TMJ overload-as well as any of the main signaling pathways-have not been studied so far. Therefore, it would be advisable to focus further research on equine specimens, considering both mediators and signaling.

Characterizing hedgehog pathway features in senescence associated osteoarthritis through Integrative multi-omics and machine learning analysis

Purpose: Osteoarthritis (OA) is a disease of senescence and inflammation. Hedgehog's role in OA mechanisms is unclear. This study combines Bulk RNA-seq and scRNA-seq to identify Hedgehog-associated genes in OA, investigating their impact on the pathogenesis of OA. Materials and methods: Download and merge eight bulk-RNA seq datasets from GEO, also obtain a scRNA-seq dataset for validation and analysis. Analyze Hedgehog pathway activity in OA using bulk-RNA seq datasets. Use ten machine learning algorithms to identify important Hedgehog-associated genes, validate predictive models. Perform GSEA to investigate functional implications of identified Hedgehog-associated genes. Assess immune infiltration in OA using Cibersort and MCP-counter algorithms. Utilize ConsensusClusterPlus package to identify Hedgehog-related subgroups. Conduct WGCNA to identify key modules enriched based on Hedgehog-related subgroups. Characterization of genes by methylation and GWAS analysis. Evaluate Hedgehog pathway activity, expression of hub genes, pseudotime, and cell communication, in OA chondrocytes using scRNA-seq dataset. Validate Hedgehog-associated gene expression levels through Real-time PCR analysis. Results: The activity of the Hedgehog pathway is significantly enhanced in OA. Additionally, nine important Hedgehog-associated genes have been identified, and the predictive models built using these genes demonstrate strong predictive capabilities. GSEA analysis indicates a significant positive correlation between all seven important Hedgehog-associated genes and lysosomes. Consensus clustering reveals the presence of two hedgehog-related subgroups. In Cluster 1, Hedgehog pathway activity is significantly upregulated and associated with inflammatory pathways. WGCNA identifies that genes in the blue module are most significantly correlated with Cluster 1 and Cluster 2, as well as being involved in extracellular matrix and collagen-related pathways. Single-cell analysis confirms the significant upregulation of the Hedgehog pathway in OA, along with expression changes observed in 5 genes during putative temporal progression. Cell communication analysis suggests an association between low-scoring chondrocytes and macrophages. Conclusion: The Hedgehog pathway is significantly activated in OA and is associated with the extracellular matrix and collagen proteins. It plays a role in regulating immune cells and immune responses.

Combination of hyperlipidemia and 17β-Estradiol induces TMJOA-like pathological changes in rats

OBJECTIVE

We explored whether hyperlipidemia or combination of hyperlipidemia and E2 could induce TMJOA.

MATERIALS AND METHODS

Four groups of female rats were treated with normal diet, normal diet with E2, high-fat diet, and high-fat diet with E2 (HFD/E2), respectively, to induce TMJOA till 8 weeks. Another three groups were then used for COX2 inhibitor celecoxib to block the induction of TMJOA. Primary condylar chondrocytes were treated with combination of E2, ox-LDL, and corresponding inhibitors for evaluating expressions of related molecules.

RESULTS

Condylar cartilage proliferation with plenty of chondrocyte apoptosis and increased staining for LOX1, nuclear NF-κB, IL-1β, and COX2 at 4 weeks and decreased condylar cartilage and increased subchondral bone density at 8 weeks were observed only in the HFD/E2 group. Celecoxib significantly alleviated the cartilage proliferation and apoptosis in the HFD/E2 group. Serum ox-LDL increased in both high-fat diet groups, while serum IL-1β increased only in the HFD/E2 group. Combination of E2 and ox-LDL synergistically induced expressions of LOX1, phosphorylated NF-κB, IL-1β, and COX2, while LOX1 inhibitor blocked the induction of phosphorylated NF-κB, and NF-κB inhibitor the induction of IL-1β, and IL-1β inhibitor the induction of COX2.

CONCLUSION

Combination of hyperlipidemia and E2-induced TMJOA-like pathological changes through LOX1/NF-κB/IL-1β/COX2-signaling pathway.

Potential pathological and molecular mechanisms of temporomandibular joint osteoarthritis

Temporomandibular joint osteoarthritis (TMJ OA) is a progressive degenerative disease of the temporomandibular joint (TMJ). The unclear etiology and mechanisms of TMJ OA bring great difficulties to early diagnosis and effective treatment, causing enormous burdens to patients' life and social economics. In this narrative review, we summarized the main pathological changes of TMJ OA, including inflammatory responses, degeneration of extracellular matrix (ECM), abnormal cell biological behaviors (apoptosis, autophagy, and differentiation) in TMJ tissue, and aberrant angiogenesis. All pathological features are closely linked to each other, forming a vicious cycle in the process of TMJ OA, which results in prolonged disease duration and makes it difficult to cure. Various molecules and signaling pathways are involved in TMJ OA pathogenesis, including nuclear factor kappa-B (NF-κB), mitogen-activated protein kinases (MAPKs), extracellular regulated protein kinases (ERKs) and transforming growth factor (TGF)-β signaling pathways et al. One molecule or pathway can contribute to several pathological changes, and the crosstalk between different molecules and pathways can further lead to a complicated condition TMJ OA. TMJ OA has miscellaneous etiology, complex clinical status, depressed treatment results, and poor prognosis. Therefore, novel in-vivo and in-vitro models, novel medicine, materials, and approaches for therapeutic procedures might be helpful for further investigation of TMJ OA. Furthermore, the role of genetic factors in TMJ OA needs to be elucidated to establish more reasonable and effective clinical strategies for diagnosing and treating TMJ OA.

A seminal perspective on the role of chondroitin sulfate in biomineralization

Chondroitin sulfate (CS) is an important extracellular matrix component of mineralized tissues. It participates in biomineralization, osteoblast differentiation and promotes bone tissue repair in vitro. However, the mechanism in which CS functions is unclear. Accordingly, an in-depth investigation of how CS participates in mineralization was conducted in the present study. Chondroitin sulfate was found to directly induce intrafibrillar mineralization of the collagen matrix. The mineralization outcome was dependent on whether CS remained free in the extracellular matrix or bound to core proteins; mineralization only occurred when CS existed in a free state. The efficacy of mineralization appeared to increase with ascending CS concentration. This discovery spurred the authors to identify the cause of heterotopic ossification in the Achilles tendon. Chondroitin sulfate appeared to be a therapeutic target for the management of diseases associated with heterotopic calcification. A broader perspective was presented on the applications of CS in tissue engineering.

Temporomandibular Disorders: Surgical Implications and Management

Temporomandibular disorders are a group of conditions that interfere with the daily lives of patients. Pain and limited function are primary concerns, and general dentists are usually the primary clinicians to be made aware of this issue. Evaluation and diagnosis is the key to determining a management plan. Minimally invasive surgical treatment includes arthrocentesis and arthroscopy, which can reduce inflammatory mediators, thereby reducing pain and increasing joint function. Open joint surgery for internal derangements focuses on instrumentation to remove and reshape the diseased condyle and disc as arthroplasty. Patients with severely limited function are candidates for total joint replacement.

The degeneration-pain relationship in the temporomandibular joint: Current understandings and rodent models

Temporomandibular disorders (TMD) represent a group of musculoskeletal conditions involving the temporomandibular joints (TMJ), the masticatory muscles and associated structures. Painful TMD are highly prevalent and conditions afflict 4% of US adults annually. TMD include heterogenous musculoskeletal pain conditions, such as myalgia, arthralgia, and myofascial pain. A subpopulations of TMD patients show structural changes in TMJ, including disc displacement or degenerative joint diseases (DJD). DJD is a slowly progressing, degenerative disease of the TMJ characterized by cartilage degradation and subchondral bone remodeling. Patients with DJD often develop pain (TMJ osteoarthritis; TMJ OA), but do not always have pain (TMJ osteoarthrosis). Therefore, pain symptoms are not always associated with altered TMJ structures, which suggests that a causal relationship between TMJ degeneration and pain is unclear. Multiple animal models have been developed for determining altered joint structure and pain phenotypes in response to various TMJ injuries. Rodent models of TMJOA and pain include injections to induce inflammation or cartilage destruction, sustained opening of the oral cavity, surgical resection of the articular disc, transgenic approaches to knockout or overexpress key genes, and an integrative approach with superimposed emotional stress or comorbidities. In rodents, TMJ pain and degeneration occur during partially overlapping time periods in these models, which suggests that common biological factors may mediate TMJ pain and degeneration over different time courses. While substances such as intra-articular pro-inflammatory cytokines commonly cause pain and joint degeneration, it remains unclear whether pain or nociceptive activities are causally associated with structural degeneration of TMJ and whether structural degeneration of TMJ is necessary for producing persistent pain. A thorough understanding of the determining factors of pain-structure relationships of TMJ during the onset, progression, and chronification by adopting novel approaches and models should improve the ability to simultaneously treat TMJ pain and TMJ degeneration.

Role of the hedgehog signaling pathway in rheumatic diseases: An overview

Hedgehog (Hh) signaling pathway is an evolutionarily conserved signal transduction pathway that plays an important regulatory role during embryonic development, cell proliferation, and differentiation of vertebrates, and it is often inhibited in adult tissues. Recent evidence has shown that Hh signaling also plays a key role in rheumatic diseases, as alterations in their number or function have been identified in rheumatoid arthritis, osteoarthritis, ankylosing spondylitis, systemic sclerosis, and Sjogren's Syndrome. As a result, emerging studies have focused on the blockade of this pathogenic axis as a promising therapeutic target in several autoimmune disorders; nevertheless, a greater understanding of its contribution still requires further investigation. This review aims to elucidate the most recent studies and literature data on the pathogenetic role of Hh signaling in rheumatic diseases.

An updated view on Temporomandibular Joint degeneration: insights from the cell subsets of mandibular condylar cartilage

The high prevalence of temporomandibular joint osteoarthritis (TMJOA), which causes joint dysfunction, indicates the need for more effective methods for treatment and repair. Mandibular condylar cartilage (MCC), a typical fibrocartilage that experiences degenerative changes during the development of TMJOA, has become a research focus and therapeutic target in recent years. MCC is composed of four zones of cells at various stages of differentiation. The cell subsets in MCC exhibit different physiological and pathological characteristics during development and in TMJOA. Most studies of TMJOA are mainly concerned with gene regulation of pathological changes. The corresponding treatment targets with specific cell subsets in MCC may provide more accurate and reliable results for cartilage repair and TMJOA treatment. In this review, we summarized the current research progress on the cell subsets of MCC from the perspective of MCC development and degeneration. We hope to provide a reference for further exploration of the pathological process of TMJOA and improvement of TMJOA treatment.

Molecular signaling in temporomandibular joint osteoarthritis

Objective

Temporomandibular joint (TMJ) osteoarthritis (OA) is a type of TMJ disorders with clinical symptoms of pain, movement limitation, cartilage degeneration and joint dysfunction. This review article is aiming to summarize recent findings on signaling pathways involved in TMJ OA development and progression.

Methods

Most recent findings in TMJ OA studies have been reviewed and cited.

Results

TMJ OA is caused by inflammation, abnormal mechanical loading and genetic abnormalities. The molecular mechanisms related to TMJ OA have been determined using different genetic mouse models. Recent studies demonstrated that several signaling pathways are involved in TMJ OA pathology, including Wnt/β-catenin, TGF-β and BMP, Indian Hedgehog, FGF, NF-κB, and Notch pathways, which are summarized in this review article. Alterations of these signaling pathways lead to the pathological changes in TMJ tissues, affecting cartilage matrix degradation, catabolic metabolism and chondrocyte apoptosis.

Conclusion

Multiple signaling pathways were involved in the pathological process of TMJ OA. New therapeutic strategies, such as stem cell application, gene editing and other techniques may be utilized for TMJ OA treatment.

The translational potential of this article

TMJ OA is a most important subtype of TMJ disorders and may lead to substantial joint pain, dysfunction, dental malocclusion, and reduced health-related quality of life. This review article summarized current findings of signaling pathways involved in TMJ OA, including Wnt/β-catenin, TGF-β and BMP, Indian Hedgehog, FGF, NF-κB, and Notch pathways, to better understand the pathological mechanisms of TMJ OA and define the molecular targets for TMJ OA treatment.

Hedgehog signaling underlying tendon and enthesis development and pathology

Hedgehog (Hh) signaling has been widely acknowledged to play essential roles in many developmental processes, including endochondral ossification and growth plate maintenance. Furthermore, a rising number of studies have shown that Hh signaling is necessary for tendon enthesis development. Specifically, the well-tuned regulation of Hh signaling during development drives the formation of a mineral gradient across the tendon enthesis fibrocartilage. However, aberrant Hh signaling can also lead to pathologic heterotopic ossification in tendon or osteophyte formation at the enthesis. Therefore, the therapeutic potential of Hh signaling modulation for treating tendon and enthesis diseases remains uncertain. For example, increased Hh signaling may enhance tendon-to-bone healing by promoting the formation of mineralized fibrocartilage at the healing interface, but pathologic heterotopic ossification may also be triggered in the adjacent tendon. Further work is needed to elucidate the distinct functions of Hh signaling in the tendon and enthesis to support the development of therapies that target the pathway.

Fibrocartilage Stem Cells in the Temporomandibular Joint: Insights From Animal and Human Studies

Temporomandibular disorders (TMD) are diseases involving the temporomandibular joint (TMJ), masticatory muscles, and osseous components. TMD has a high prevalence, with an estimated 4.8% of the U.S. population experiencing signs and symptoms, and represents a financial burden to both individuals and society. During TMD progression, the most frequently affected site is the condylar cartilage. Comprising both fibrous and cartilaginous tissues, condylar cartilage has restricted cell numbers but lacks a vascular supply and has limited regenerative properties. In 2016, a novel stem cell niche containing a reservoir of fibrocartilage stem cells (FCSCs) was discovered in the condylar cartilage of rats. Subsequently, FCSCs were identified in mouse, rabbit, and human condylar cartilage. Unlike mesenchymal stem cells or other tissue-specific stem/progenitor cells, FCSCs play a unique role in the development and regeneration of fibrocartilage. More importantly, engraftment treatment of FCSCs has been successfully applied in animal models of TMD. In this context, FCSCs play a major role in the regeneration of newly formed cartilage. Furthermore, FCSCs participate in the regeneration of intramembranous bone by interacting with endothelial cells in bone defects. This evidence highlights the potential of FCSCs as an ideal stem cell source for the regeneration of oral maxillofacial tissue. This review is intended to detail the current knowledge of the characteristics and function of FCSCs in the TMJ, as well as the potential therapeutic applications of FCSCs. A deep understanding of the properties of FCSCs can thus inform the development of promising, biologically based strategies for TMD in the future.

The value of the hedgehog signal in osteoblasts in fluoride-induced bone-tissue injury

OBJECTIVE

This study was designed to observe the expression of important hedgehog (Hh) signal factors in the bone tissue of rats with chronic fluorosis and cultured osteoblasts in order to investigate the role and significance of the Hh signal in fluoride-induced bone injury.

METHODS

Healthy Sprague-Dawley (SD) rats were randomly divided into four groups: the control group, the fluorosis group (F Group), the fluoride + blocker group (F + Cycl group: rats were treated with fluoride + cyclopamine), and the fluoride + blocker control group (F + DMSO group). After 6 months of intervention, the urinary fluoride content of rats in each group was detected. The primary osteoblasts of rats were selected for cell experiment, and the experiment was carried out after the cells were passaged from the second to the fourth generation.

RESULTS

The proliferation rate of primary rat osteoblasts presented time-affected and dose-affected relationships in a short time under treatment with a low dose of sodium fluoride (NaF), but the proliferation of osteoblasts was inhibited by long-term and high-dose NaF exposure. In the F group, the alkaline phosphatase (ALP) activity of osteoblasts increased gradually. The ALP activity was lower in the F + Cycl group than in the F group, and there was no significant difference between the F + DMSO group and F group. With the increase in fluoride exposure, the expression of Hh signal factors and osteogenic-related factor proteins increased gradually. The expressions of Indian hedgehog (Ihh), smoothened (Smo), Glioma-associated oncogene homolog (Gli) 2, and Runt-related transcription factor 2 (Runx2)in the F + Cycl group increased with the dose of fluoride but they were significantly inhibited compared with the F group. Compared with the control group, the content of urinary fluoride in the F group was significantly higher (P < 0.05), but there was no significant change in urinary fluoride content in the F + Cycl group and the F + DMSO group. Compared with the control group, the serum bone alkaline phosphatase (BALP) contents of rats in the other groups increased after 6 months' intake of fluoride water (P < 0.05). After drug blocking, the serum BALP content in the F + Cycl group was lower than that in the F + DMSO group (P < 0.05). The BALP content in the F + DMSO group was similar to that in the F group: it did not decrease. The mRNA expressions of Ihh, Smo, Gli2, and Runx2 in bone tissue of the F group were significantly higher than those in the control group (P < 0.05). After cyclopamine blocking, the expressions decreased (P < 0.05), but the differences between the F + DMSO group and F group were not statistically significant.

CONCLUSION

Hh signal plays an important role in fluoride-induced bone injury. The effective inhibition of cyclopamine is expected to be a new target for the treatment of skeletal damage caused by fluorosis.

Immunohistochemical panel of degenerated articular discs from patients with temporomandibular joint osteoarthritis

BACKGROUND

Temporomandibular joint osteoarthritis (TMJOA) is a progressive degenerative disease caused by imbalance between anabolic and catabolic stimuli.

OBJECTIVE

The aim of this study was to evaluate histopathological changes, collagen degeneration and the expression of eleven TMJOA biomarkers in articular discs.

METHODS

Specimens were obtained from eight female patients submitted to discectomy. Discs were divided into anterior band (AB), intermediate zone (IZ) and posterior band (PB) for computerised histomorphometric analyses. Each was assigned a histopathological degeneration score (HDS). Collagen degeneration was assessed with Picrosirius-polarisation method. Biomarkers were evaluated through immunohistochemistry, including IGF-1, OPG, VEGF, TNF-α, FGF-23, IHH, MMP-3, MMP-9, TGF-β₁ , BMP-2 and WNT-3. Image processing software was used to calculate average immature collagen ratios and immunostained areas. Spearman rank tests were applied to verify correlations, with significance level of 0.05.

RESULTS

The HDS showed negative correlation with expression of VEGF in IZ and PB (P < .05) and positive with TNF-α in AB (P < .01). Collagen degeneration correlated with TGF-β₁ (P < .05), BMP-2 (P < .01) and IHH (P < .05) immunostained areas in the IZ; TGF-β_1, BMP-2 and IHH expression correlated among each other in AB and IZ (P < .05).

CONCLUSION

Angiogenesis and tissue fragmentation may result from aberrant physiologic responses mediated by VEGF and TNF-α, compromising TMJ discs during OA progression. The expression of TGF-β_1, BMP-2 and IHH could be related to collagen degeneration in displaced discs and may participate in TMJOA pathogenesis.

Reduction of osteoarthritis severity in the temporomandibular joint of rabbits treated with chondroitin sulfate and glucosamine

Osteoarthritis is a degenerative disease that causes substantial changes in joint tissues, such as cartilage degeneration and subchondral bone sclerosis. Chondroitin sulfate and glucosamine are commonly used products for the symptomatic treatment of osteoarthritis. The aim of the present study was to investigate the effects of these products when used as structure-modifying drugs on the progression of osteoarthritis in the rabbit temporomandibular joint. Thirty-six New Zealand rabbits were divided into 3 groups (n = 12/group): control (no disease); osteoarthritis (disease induction); and treatment (disease induction and administration of chondroitin sulfate and glucosamine). Osteoarthritis was induced by intra-articular injection of monosodium iodoacetate. Animals were killed at 30 and 90 days after initiation of therapy. The treatment was effective in reducing disease severity, with late effects and changes in the concentration of glycosaminoglycans in the articular disc. The results indicate that chondroitin sulfate and glucosamine may have a structure-modifying effect on the tissues of rabbit temporomandibular joints altered by osteoarthritis.

Age-related changes in the cartilage of the temporomandibular joint

Osteoarthritis (OA) of the knee is closely associated with aging; however, little is known about the age-related degeneration in the mandibular condylar cartilage (MCC) of the TMJ. Our objective was to examine whether a correlation exists between aging and degeneration of the MCC of the TMJ. Thirty-two male C57BL/6J wild-type mice were aged to 2, 12, 18, and 25 months old. The mice were euthanized by CO₂ inhalation and were dissected and examined by micro-CT and histology. Sagittal sections of the condyles were stained for tartrate-resistant alkaline phosphatase, alkaline phosphatase, safranin O, picrosirius red, and toluidine blue. In addition, immunostaining for BMP2, BMP4, BMP7, PRG4, and MMP13 was performed. Bone volume fraction and tissue density significantly increased with the age of the animals. There was a significant increase in the Osteoarthritis Research Society International histopathological score and mineralization of the noncalcified cartilage in the aged animals. There was a decrease in cartilage thickness, proteoglycan distribution, and cellularity in the aged animals. Additionally, we noted increased picrosirius red staining with the increase in the age of the animals. Our protein expression showed increased BMP2, BMP4, BMP7, and MMP13, whereas there was a decrease in PRG4 expression in the aged animals. As the animal ages, there is decreased proteoglycan secretion, decreased cellularity, decreased cartilage thickness, increased fibrillation, and increased proteolytic activity. A better understanding of the basic mechanisms underlying the degeneration of the MCC in the older animals could provide novel ways to slow the development of OA.

The Roles of Indian Hedgehog Signaling in TMJ Formation

The temporomandibular joint (TMJ) is an intricate structure composed of the mandibular condyle, articular disc, and glenoid fossa in the temporal bone. Apical condylar cartilage is classified as a secondary cartilage, is fibrocartilaginous in nature, and is structurally distinct from growth plate and articular cartilage in long bones. Condylar cartilage is organized in distinct cellular layers that include a superficial layer that produces lubricants, a polymorphic/progenitor layer that contains stem/progenitor cells, and underlying layers of flattened and hypertrophic chondrocytes. Uniquely, progenitor cells reside near the articular surface, proliferate, undergo chondrogenesis, and mature into hypertrophic chondrocytes. During the past decades, there has been a growing interest in the molecular mechanisms by which the TMJ develops and acquires its unique structural and functional features. Indian hedgehog (Ihh), which regulates skeletal development including synovial joint formation, also plays pivotal roles in TMJ development and postnatal maintenance. This review provides a description of the many important recent advances in Hedgehog (Hh) signaling in TMJ biology. These include studies that used conventional approaches and those that analyzed the phenotype of tissue-specific mouse mutants lacking Ihh or associated molecules. The recent advances in understanding the molecular mechanism regulating TMJ development are impressive and these findings will have major implications for future translational medicine tools to repair and regenerate TMJ congenital anomalies and acquired diseases, such as degenerative damage in TMJ osteoarthritic conditions.

Osteophyte growth in early thumb carpometacarpal osteoarthritis

OBJECTIVE

Osteophyte formation is a critical part of the degeneration of a joint with osteoarthritis (OA). While often qualitatively described, few studies have succeeded in quantifying osteophyte growth over time. Using computed tomography (CT) image data from a longitudinal, observational study of thumb carpometacarpal (CMC) OA, our aim was to quantify osteophyte growth volume and location over a three-year period in men and women.

METHOD

Ninety patients with early thumb OA were recruited and assessed at baseline, 1.5 years, and 3 years with CT imaging. Osteophyte volume and location on the trapezium and first metacarpal were determined using a library of 46 healthy subjects as a nonarthritic reference database.

RESULTS

There was a significant increase in osteophyte volume for women and men over the three-year follow-up in the trapezium (86.8 mm³-120.5 mm³ and 165.1 mm³-235.3 mm³, means respectively) and in the proximal metacarpal (63 mm³-80.4 mm³, and 115.8 mm³-161.7 mm³, respectively). The location of osteophyte initiation and growth was consistent across subjects and was located in non-opposing regions on the trapezium and first metacarpal. Osteophyte growth occurred about the radial and ulnar margins of the trapezial facet, while on the proximal metacarpal, growth occurred principally about the volar and dorsal margins of the facet.

CONCLUSION

Osteophyte growth occurred in early thumb osteoarthritis over three years. Growth was localized in specific, non-opposing regions on the trapezium and metacarpal, raising intriguing questions about the triggers for their formation, whether the mechanisms are mechanical, biological or a combination of both.

Roles of Ihh signaling in chondroprogenitor function in postnatal condylar cartilage

Condylar articular cartilage in mouse temporomandibular joint develops from progenitor cells near the articulating surface that proliferate, undergo chondrogenesis and mature into hypertrophic chondrocytes. However, it remains unclear how these processes are regulated, particularly postnatally. Here we focused on the apical polymorphic layer rich in progenitors and asked whether the phenotype and fate of the cells require signaling by Indian hedgehog (Ihh) previously studied in developing long bones. In condyles in newborn mice, the apical polymorphic/progenitor cell layer was ~10 cell layer-thick and expressed the articular matrix marker Tenascin-C (Tn-C), and the underlying thick cell layer expressed Tn-C as well as the chondrogenic master regulator Sox9. By 1 month, condylar cartilage had gained its full width, but became thinner along its main longitudinal axis and displayed hypertrophic chondrocytes. By 3 months, articular cartilage consisted of a 2-3 cell layer-thick zone of superficial cells and chondroprogenitors expressing both Tn-C and Sox9 and a bottom zone of chondrocytes displaying vertical matrix septa. EdU cell tracing in juvenile mice revealed that conversion of chondroprogenitors into chondrocytes and hypertrophic chondrocytes required about 48 and 72 h, respectively. Notably, EdU injection in 3 month-old mice labeled both progenitors and maturing chondrocytes by 96 h. Conditional ablation of Ihh in juvenile/early adult mice compromised chondroprogenitor organization and function and led to reduced chondroprogenitor and chondrocyte proliferation. The phenotype of mutant condyles worsened over time as indicated by apoptotic chondrocyte incidence, ectopic chondrocyte hypertrophy, chondrocyte column derangement and subchondral bone deterioration. In micromass cultures of condylar apical cells, hedgehog (Hh) treatment stimulated chondrogenesis and alkaline phosphatase (APase) activity, while treatment with HhAntag inhibited both. Our findings indicate that the chondroprogenitor layer is continuously engaged in condylar growth postnatally and its organization and functioning depend on hedgehog signaling.

The Interface of Mechanics and Nociception in Joint Pathophysiology: Insights From the Facet and Temporomandibular Joints

Chronic joint pain is a widespread problem that frequently occurs with aging and trauma. Pain occurs most often in synovial joints, the body's load bearing joints. The mechanical and molecular mechanisms contributing to synovial joint pain are reviewed using two examples, the cervical spinal facet joints and the temporomandibular joint (TMJ). Although much work has focused on the macroscale mechanics of joints in health and disease, the combined influence of tissue mechanics, molecular processes, and nociception in joint pain has only recently become a focus. Trauma and repeated loading can induce structural and biochemical changes in joints, altering their microenvironment and modifying the biomechanics of their constitutive tissues, which themselves are innervated. Peripheral pain sensors can become activated in response to changes in the joint microenvironment and relay pain signals to the spinal cord and brain where pain is processed and perceived. In some cases, pain circuitry is permanently changed, which may be a potential mechanism for sustained joint pain. However, it is most likely that alterations in both the joint microenvironment and the central nervous system (CNS) contribute to chronic pain. As such, the challenge of treating joint pain and degeneration is temporally and spatially complicated. This review summarizes anatomy, physiology, and pathophysiology of these joints and the sensory pain relays. Pain pathways are postulated to be sensitized by many factors, including degeneration and biochemical priming, with effects on thresholds for mechanical injury and/or dysfunction. Initiators of joint pain are discussed in the context of clinical challenges including the diagnosis and treatment of pain.

The inhibitory roles of Ihh downregulation on chondrocyte growth and differentiation

The proliferative rate of chondrocytes affects bone elongation. Chondrocyte hypertrophy is required for endochondral bone formation as chondrocytes secrete factors required for osteoblast differentiation and maturation. Previous studies have demonstrated that the Indian hedgehog (Ihh) signaling pathway is a key regulator of skeletal development and homeostasis. The aim of the present study was to investigate the function of Ihh in chondrocyte proliferation and differentiation, as well as the underlying mechanisms. Ihh was knocked down in mouse chondrocyte cells using short hairpin RNA. Chondrocyte apoptosis and cell cycle arrest were assessed using flow cytometry and the results indicated that knockdown of Ihh significantly inhibited cell growth (P<0.05) and increased apoptosis (P<0.001) compared with negative control cells. Downregulation of Ihh also resulted in cell cycle arrest at G1 to S phase in chondrocytes. It was also observed that knockdown of Ihh decreased alkaline phosphatase activity and mineral deposition of chondrocytes. The inhibitory roles of Ihh downregulation on chondrocyte growth and differentiation may be associated with the transforming growth factor-β/mothers against decapentaplegic and osteoprotegerin/receptor activator of nuclear factor κB ligand signaling pathway. The results of the present study suggest that chondrocyte-derived Ihh is essential for maintaining bone growth plates and that manipulation of Ihh expression or its signaling components may be a novel therapeutic technique for the treatment of skeletal diseases, including achondroplasia.

Significance of new blood vessels in the pathogenesis of temporomandibular joint osteoarthritis

We studied the significance of new blood vessels in the pathogenesis of temporomandibular joint osteoarthritis (TMJOA). Fifteen 8-week-old female Sprague-Dawley rats were selected to establish TMJOA models of gradually induced occlusal disorders. Five rats were sacrificed at 4, 8 and 16 weeks, and histological exam was conducted along with micro-computed tomography observation on the condyle specimen. The distribution and number of new blood vessels breaking were observed through the tidemark through CD34 immunofluorescence staining. The proliferation of chondrocytes were detected through Ki67 immunohistochemical staining, and the differentiation functions of chondrocytes were observed through PTHrP and IHH immunohistochemical staining. The degradation functions of cartilage matrix were observed through matrix metalloproteinase (MMP)-9 immunohistochemical staining to detect the expression of vascular growth promotion and inhibition factors with vascular endothelial growth factor (VEGF), CTGF and CHM-1 immunohistochemical staining and screen differentially expressed genes through gene chip analysis method. It was found that the condyle tissue full thickness, fiber layer thickness and calcified cartilage layer thickness were significantly increased with time (P<0.05). Bone mineral density, trabecular thickness and Tb.Sp were also increased significantly with time, BS/BV and trabecular number were decreased significantly with time (P<0.05). The new blood vessels reached the deep layer of calcified cartilage until the tide line was broken and non-calcified cartilage was invaded. The number of vessels were increased significantly with time (P<0.05). Ki67, PTHrP and IHH-positive rates were increased significantly (P<0.05). MMP-9, VEGF, CTGF and CHM-1 were increased significantly (P<0.05). VEGF, CTGF and CHM-1 mRNA were upregulated differentially with the expressed genes. In conclusion, the new blood vessels may be important in the pathogenesis of TMJOA.

Confocal imaging of mouse mandibular condyle cartilage

Mice are commonly used to study the temporomandibular joint (TMJ) and to model human TMJ disease. However, evaluating TMJ pathology in mice using standard histologic methods is time consuming, labor intensive, and dependent upon investigators’ expertise at consistently orienting and sectioning across tiny specimens. We describe a method that uses confocal microscopy to rapidly and reliably assess indicators of mandibular condyle cartilage pathology in mice. We demonstrate the utility of this method for detecting abnormalities in chondrocyte distribution in mice lacking lubricin (Prg4), the major boundary lubricant of articular cartilage. We further show that the method can provide information about recombination sites and efficiency in mandibular cartilage for Cre-driver strains. Because specimen preparation and data acquisition with confocal microscopy are simple and fast, the method can serve as a primary screening tool for TMJ pathology, before proceeding to complicated, time consuming, secondary analyses.

A Joint Less Ordinary: Intriguing Roles for Hedgehog Signalling in the Development of the Temporomandibular Synovial Joint

This review highlights the essential role of Hedgehog (Hh) signalling in the developmental steps of temporomandibular joint (TMJ) formation. We review evidence for intra- and potentially inter-tissue Hh signaling as well as Glioma-Associated Oncogene Homolog (GLI) dependent and independent functions. Morphogenesis and maturation of the TMJ’s individual components and the general landscape of Hh signalling is also covered. Comparison of the appendicular knee and axial TMJ also reveals interesting differences and similarities in their mechanisms of development, chondrogenesis and reliance on Hh signalling.