A single-cell transcriptional atlas reveals resident progenitor cell niche functions in TMJ disc development and injury

The profiling and analysis of gene expression in rat temporomandibular joint disc tissue and its derived cells

Objective

The temporomandibular joint (TMJ) disc is composed of a fibrocartilaginous connective tissue. Its dysfunction, resulting from excessive jaw movement, can lead to TMJ disorders. In this study, we aimed to investigate the crucial molecular information regarding the extracellular matrix (ECM), which would be necessary for treating such disorders through TMJ disc regeneration. To achieve this, we compared the gene profiles of TMJ disc tissues, their derived cells, and the periodontal ligament (PDL) in our previous study.

Methods

TMJ discs were isolated from male Wistar rats. Cells derived from the TMJ discs were cultured, and mRNA extracted from the disc tissues and derived cells was analyzed for gene profiling via microarray hybridization. Additionally, we compared the ECM expression between the TMJ disc and the PDL.

Results

Collagen (types I, II, III, and VI) and proteoglycans (biglycan and fibromodulin) were highly expressed in the TMJ discs. Significant reduction in decorin, fibromodulin and COL2 were observed in the TMJ-derived cells than in the tissue. Type VI collagen was the third most highly expressed in both the TMJ disc and PDL tissues, following types I and III.

Conclusions

Collagen types VI and II were prominently expressed, followed by collagen types I and III, in TMJ disc tissues, reflecting the unique functions of the disc. Type VI collagen was highly expressed in both TMJ disc and PDL tissues. Overall, type VI collagen might be a key molecule for TMJ disc regeneration, ensuring elasticity and cushioning, and could provide new insights for TMJ regeneration.

Microstructural and Micromechanical Properties of Decellularized Fibrocartilaginous Scaffold

Fibrocartilage decellularized extracellular matrix (dECM) is a promising alternative material for damaged fibrocartilage repair and replacement due to its biomimetic gross morphology and internal microstructure. However, the alterations in the microstructure and micromechanical properties of fibrocartilage after decellularization interfere with the macroscopic functional application of the scaffold. Therefore, this study aims to present an analytical atlas of the microstructure and micromechanics of the fibrocartilaginous dECM scaffold to elucidate the effect of decellularization treatment on the macroscopic function of the scaffold. The fibrocartilage dECM was prepared using the temporomandibular joint (TMJ) disc as the model, and its durability was evaluated under three functional states (physiological, physiological limit, and beyond the limit). The macroscopic function of different fibrocartilage dECM exhibits notable differences, which are attributed to the destruction of the multilevel collagen structure. This process involves unwinding triple-helix tropocollagen molecules, destroying collagen fibril D-periodicity, expanding collagen fiber bundle curling, and loosening of the collagen fiber network. The impairment of multiscale collagen structures degrades the cross-scale mechanical modulus and energy dissipation of dECM from the triple helix molecules to the fibril level to the fiber bundle that extends to the fiber network. This study provides important data for further optimizing decellularized fibrocartilage scaffolds and evaluating their translational potential.

Mapping cell diversity and dynamics in inflammatory temporomandibular joint osteoarthritis with pain at single-cell resolution

Temporomandibular joint (TMJ) osteoarthritis with pain is a highly prevalent disorder affecting patients' quality of life. A comprehensive understanding of cell type diversity and its dynamics in painful TMJ osteoarthritis (TMJOA) is lacking. Here, we utilized an inflammatory TMJOA mouse model via intra-articular injection of CFA. TMJOA mice exhibited cartilage remodeling, bone loss, synovitis, increased osteoarthritis (OA) score, and orofacial pain, recapitulating hallmark symptoms in patients. Single-cell transcriptomic profiling of the TMJ was performed in conjunction with mouse genetic labeling, tissue clearing, light sheet and confocal 3D imaging, multiplex RNAscope, and immunodetection. We visualized, reconstructed, and analyzed the distribution and density of nociceptive innervation of TMJ at single-axon levels. We systematically mapped the heterogeneity and anatomical position of blood endothelial cells, synovial fibroblasts, and immune cells, including Cx3cr1-positive barrier macrophages. Importantly, TMJOA mice exhibited enhanced neurovascular coupling, sublining fibroblast hyperplasia, inflammatory immune cell expansion, disrupted signaling-dependent cell-cell interaction, and a breakdown of the sandwich-like organization consisting of synovial barrier macrophages and fibroblasts. By utilizing a mouse model with combined TMJ pain history and OA, we reveal the cellular diversity, anatomical structure, and cell dynamics of the TMJ at single-cell resolution, which facilitate our understanding and potential targeting of TMJOA.

BMSC Derived Exosomes Attenuate Apoptosis of Temporomandibular Joint Disc Chondrocytes in TMJOA via PI3K/AKT Pathway

Bone marrow mesenchymal stem cell-derived exosomes (BMSC-Exos) are crucial means of intercellular communication and can regulate a range of biological processes by reducing inflammation, decreasing apoptosis and promoting tissue repair. We treated temporomandibular joint (TMJ) disc chondrocytes with TNF-α and performed local injection of sodium iodoacetate (MIA) in the TMJ of rats to establish in vitro and in vivo models of TMJ osteoarthritis (TMJOA). BMSC-Exos were isolated and extracted to evaluate their proliferation and trilineage differentiation abilities, and their antiapoptotic and chondroprotective effects were assessed. This study revealed that BMSC-Exos can be endocytosed by TMJ disc chondrocytes in vitro and that BMSC-Exos pretreatment strongly attenuated the inhibitory effect of TNF-α on the proliferative and chondrogenic potential of TMJ disc chondrocytes. The administration of BMSC-Exos significantly suppressed TNF-α-induced apoptosis in TMJ disc chondrocytes by increasing the phosphorylation level of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB/AKT) pathway-related proteins, whereas the PI3K inhibitor LY294002 neutralized this antiapoptotic effect. Intradiscal injection of BMSC-Exos alleviated the degeneration and inflammation of TMJ discs in a rat model of TMJOA. Our study revealed that BMSC-Exos can attenuate the apoptosis of TMJ disc chondrocytes and destruction of TMJ discs partially by inhibiting the apoptotic pathway and activating the PI3K/AKT pathway, thereby providing a promising treatment strategy for the regeneration of damaged TMJ discs.

Spatial Transcriptomics Unravel the Tissue Complexity of Oral Pathogenesis

Spatial transcriptomics (ST) is a cutting-edge methodology that enables the simultaneous profiling of global gene expression and spatial information within histological tissue sections. Traditional transcriptomic methods lack the spatial resolution required to sufficiently examine the complex interrelationships between cellular regions in diseased and healthy tissue states. We review the general workflows for ST, from specimen processing to ST data analysis and interpretations of the ST dataset using visualizations and cell deconvolution approaches. We show how recent studies used ST to explore the development or pathogenesis of specific craniofacial regions, including the cranium, palate, salivary glands, tongue, floor of mouth, oropharynx, and periodontium. Analyses of cranial suture patency and palatal fusion during development using ST identified spatial patterns of bone morphogenetic protein in sutures and osteogenic differentiation pathways in the palate, in addition to the discovery of several genes expressed at critical locations during craniofacial development. ST of salivary glands from patients with Sjögren's disease revealed co-localization of autoimmune antigens with ductal cells and a subpopulation of acinar cells that was specifically depleted by the dysregulated autoimmune response. ST of head and neck lesions, such as premalignant leukoplakia progressing to established oral squamous cell carcinomas, oral cancers with perineural invasions, and oropharyngeal lesions associated with HPV infection spatially profiled the complex tumor microenvironment, showing functionally important gene signatures of tumor cell differentiation, invasion, and nontumor cell dysregulation within patient biopsies. ST also enabled the localization of periodontal disease-associated gene expression signatures within gingival tissues, including genes involved in inflammation, and the discovery of a fibroblast subtype mediating the transition between innate and adaptive immune responses in periodontitis. The increased use of ST, especially in conjunction with single-cell analyses, promises to improve our understandings of craniofacial development and pathogenesis at unprecedented tissue-level resolution in both space and time.

Type V collagen exhibits distinct regulatory activities in TMJ articular disc versus condylar cartilage during postnatal growth and remodeling

Understanding matrix molecular activities that regulate the postnatal growth and remodeling of the temporomandibular joint (TMJ) articular disc and condylar cartilage will enable the development of effective regenerative strategies targeting TMJ disorders. This study elucidated the distinct roles of type V collagen (collagen V) in regulating these two units. Studying the TMJ of young adult Col5a1+/- mice, we found that loss of collagen V resulted in substantial changes in the proliferation, clustering and density of progenitors in condylar cartilage, but did not have a major impact on disc cells that are more fibroblast-like. Although loss of collagen V led to thickened collagen fibrils with increased heterogeneity in the disc, there were no significant changes in local micromodulus, except for a reduction at the posterior end of the inferior side. Following the induction of aberrant occlusal loading by the unilateral anterior crossbite (UAC) procedure, both wild-type (WT) and Col5a1+/- condylar cartilage exhibited salient remodeling, and Col5a1+/- condyle developed more pronounced degeneration and tissue hypertrophy at the posterior end than the WT. In contrast, neither UAC nor collagen V deficiency induced marked changes in the morphology or biomechanical properties of the disc. Together, our findings highlight the distinct roles of collagen V in regulating these two units during postnatal growth and remodeling, emphasizing its more crucial role in condylar cartilage due to its impact on the highly mechanosensitive progenitors. These results provide the foundation for using collagen V to improve the regeneration of TMJ and the care of patients with TMJ disorders. STATEMENT OF SIGNIFICANCE: Successful regeneration of the temporomandibular joint (TMJ) articular disc and condylar cartilage remains a significant challenge due to the limited understanding of matrix molecular activities that regulate the formation and remodeling of these tissues. This study demonstrates that collagen V plays distinct and critical roles in these processes. In condylar cartilage, collagen V is essential for regulating progenitor cell fate and maintaining matrix integrity. In the disc, collagen V also regulates fibril structure and local micromechanics, but has a limited impact on cell phenotype or its remodeling response. Our findings establish collagen V as a key component in maintaining the integrity of these two units, with a more crucial role in condylar cartilage due to its impact on progenitor cell activities.

Mesenchymal stem cells-derived exosomes alleviate temporomandibular joint disc degeneration in temporomandibular joint disorder

Temporomandibular joint (TMJ) disorder (TMD) is a chronic progressive disease that is commonly seen in clinical settings. TMJ disc degeneration is an important manifestation of TMD, and further aggravates the progression of TMD. However, treatments on TMJ disc degeneration are very limited till now. In this study, we first observed the effects of bone marrow stem cells (BMSC) conditioned medium on functions of TMJ disc fibroblasts. Then BMSC-derived small extracellular vesicles (BMSC-EVs) were isolated and exposed to TMJ disc fibroblasts. RNA-sequencing was used to further investigate the mechanisms. BMSC-EVs were finally injected into a rat model with TMD. Results showed that in the transwell co-culture system, the medium derived from BMSC reduced inflammation and enhanced chondrogenesis in TMJ disc fibroblasts. BMSC-EVs promoted proliferation, migration, and chondrogenic differentiation of TMJ disc fibroblasts, and inhibited apoptosis and inflammatory responses. Local injection of BMSC-EVs into the TMD model alleviated TMJ disc degeneration. Therefore, BMSC-EVs were a potentially effective, sustainable and clinically translational-promising option for TMJ disc degeneration, and further reduce the progression of TMD.

Neural regulation of mesenchymal stem cells in craniofacial bone: development, homeostasis and repair

Mesenchymal stem cells endow various functions, including proliferation, multipotency, migration, etc. Craniofacial bones originate from the cranial neural crest and are developed mainly through intramembranous ossification, which are different from long bones. There are varied mesenchymal stem cells existing in the craniofacial bone, including Gli1 + cells, Axin2 + cells, Prx1 + cells, etc. Nerves distributed in craniofacial area are also derived from the neural crest, and the trigeminal nerve is the major sensory nerve in craniofacial area. The nerves and the skeleton are tightly linked spatially, and the skeleton is broadly innervated by sensory and sympathetic nerves, which also participate in bone development, homeostasis and healing process. In this review, we summarize mesenchymal stem cells located in craniofacial bone or, to be more specific, in jaws, temporomandibular joint and cranial sutures. Then we discuss the research advance concerning neural regulation of mesenchymal stem cells in craniofacial bone, mainly focused on development, homeostasis and repair. Discovery of neural regulation of mesenchymal stem cells may assist in treatment in the craniofacial bone diseases or injuries.

Interaction between immuno-stem dual lineages in jaw bone formation and injury repair

The jawbone, a unique structure in the human body, undergoes faster remodeling than other bones due to the presence of stem cells and its distinct immune microenvironment. Long-term exposure of jawbones to an oral environment rich in microbes results in a complex immune balance, as shown by the higher proportion of activated macrophage in the jaw. Stem cells derived from the jawbone have a higher propensity to differentiate into osteoblasts than those derived from other bones. The unique immune microenvironment of the jaw also promotes osteogenic differentiation of jaw stem cells. Here, we summarize the various types of stem cells and immune cells involved in jawbone reconstruction. We describe the mechanism relationship between immune cells and stem cells, including through the production of inflammatory bodies, secretion of cytokines, activation of signaling pathways, etc. In addition, we also comb out cellular interaction of immune cells and stem cells within the jaw under jaw development, homeostasis maintenance and pathological conditions. This review aims to eclucidate the uniqueness of jawbone in the context of stem cell within immune microenvironment, hopefully advancing clinical regeneration of the jawbone.

Craniofacial developmental biology in the single-cell era

The evolution of a unique craniofacial complex in vertebrates made possible new ways of breathing, eating, communicating and sensing the environment. The head and face develop through interactions of all three germ layers, the endoderm, ectoderm and mesoderm, as well as the so-called fourth germ layer, the cranial neural crest. Over a century of experimental embryology and genetics have revealed an incredible diversity of cell types derived from each germ layer, signaling pathways and genes that coordinate craniofacial development, and how changes to these underlie human disease and vertebrate evolution. Yet for many diseases and congenital anomalies, we have an incomplete picture of the causative genomic changes, in particular how alterations to the non-coding genome might affect craniofacial gene expression. Emerging genomics and single-cell technologies provide an opportunity to obtain a more holistic view of the genes and gene regulatory elements orchestrating craniofacial development across vertebrates. These single-cell studies generate novel hypotheses that can be experimentally validated in vivo. In this Review, we highlight recent advances in single-cell studies of diverse craniofacial structures, as well as potential pitfalls and the need for extensive in vivo validation. We discuss how these studies inform the developmental sources and regulation of head structures, bringing new insights into the etiology of structural birth anomalies that affect the vertebrate head.

Heterogeneous Characteristics of the CD90+ Progenitors in the Fibrocartilage of Different Joints

OBJECTIVE

This study aimed to isolate and compare the mesenchymal stem cell characteristics of CD90+ cells from different fibrocartilage tissues in the temporomandibular joint (TMJ), the knee joint, and the intervertebral joint to further understand the similarities and differences of these 4 fibrocartilage tissues.

METHODS

CD90+ cells were isolated from TMJ disc, condylar cartilage, meniscus, and intervertebral disc by using magnetic-activated cell sorting. Cellular assays including 4.5-ethynyl-2'-deoxyuridine labeling, multilineage differentiation, colony formation, and cell migration were conducted to compare their mesenchymal stem cell characteristics. Immunofluorescent staining was performed for observing the expression of actively proliferating CD90+ cells within the tissues. H&E staining and Safranine O staining were used to compare the histological features.

RESULTS

The CD90+ cells derived from these 4 fibrocartilage tissues exhibited comparable cell proliferation abilities. However, the cells from the TMJ disc displayed limited multilineage differentiation potential, colony formation, and cell migration abilities in comparison with the cells from the other fibrocartilage tissues. In vivo, there was relatively more abundant expression of CD90+ cells in the TMJ disc during the early postnatal stage. The limited EDU+ cell numbers signified a low proliferation capacity of CD90+ cells in the TMJ disc. In addition, we observed a significant decrease in cell density and a restriction in the synthesis of extracellular proteoglycans in the TMJ disc.

CONCLUSION

Our study highlights the spatial heterogeneity of CD90+ cells in the fibrocartilages of different joint tissues, which may contribute to the limited cartilage repair capacity in the TMJ disc.

Regenerative Engineering of a Biphasic Patient-Fitted Temporomandibular Joint Condylar Prosthesis

Regenerative medicine approaches to restore the mandibular condyle of the temporomandibular joint (TMJ) may fill an unmet patient need. In this study, a method to implant an acellular regenerative TMJ prosthesis was developed for orthotopic implantation in a pilot goat study. The scaffold incorporated a porous, polycaprolactone-hydroxyapatite (PCL-HAp, 20wt% HAp) 3D printed condyle with a cartilage-matrix-containing hydrogel. A series of material characterizations was used to determine the structure, fluid transport, and mechanical properties of 3D printed PCL-HAp. To promote marrow uptake for cell seeding, a scaffold pore size of 152 ± 68 μm resulted in a whole blood transport initial velocity of 3.7 ± 1.2 mm·s-1 transported to the full 1 cm height. The Young's modulus of PCL was increased by 67% with the addition of HAp, resulting in a stiffness of 269 ± 20 MPa for etched PCL-HAp. In addition, the bending modulus increased by 2.06-fold with the addition of HAp to 470 MPa for PCL-HAp. The prosthesis design with an integrated hydrogel was compared with unoperated contralateral control and no-hydrogel group in a goat model for 6 months. A guide was used to make the condylectomy cut, and the TMJ disc was preserved. MicroCT assessment of bone suggested variable tissue responses with some regions of bone growth and loss, although more loss may have been exhibited by the hydrogel group than the no-hydrogel group. A benchtop load transmission test suggested that the prosthesis was not shielding load to the underlying bone. Although variable, signs of neocartilage formation were exhibited by Alcian blue and collagen II staining on the anterior, functional surface of the condyle. Overall, this study demonstrated signs of functional TMJ restoration with an acellular prosthesis. There were apparent limitations to continuous, reproducible bone formation, and stratified zonal cartilage regeneration. Future work may refine the prosthesis design for a regenerative TMJ prosthesis amenable to clinical translation.