Synovial joints: from development to homeostasis

Bone and Joint-on-Chip Platforms: Construction Strategies and Applications

Organ-on-a-chip, also known as "tissue chip," is an advanced platform based on microfluidic systems for constructing miniature organ models in vitro. They can replicate the complex physiological and pathological responses of human organs. In recent years, the development of bone and joint-on-chip platforms aims to simulate the complex physiological and pathological processes occurring in human bones and joints, including cell-cell interactions, the interplay of various biochemical factors, the effects of mechanical stimuli, and the intricate connections between multiple organs. In the future, bone and joint-on-chip platforms will integrate the advantages of multiple disciplines, bringing more possibilities for exploring disease mechanisms, drug screening, and personalized medicine. This review explores the construction and application of Organ-on-a-chip technology in bone and joint disease research, proposes a modular construction concept, and discusses the new opportunities and future challenges in the construction and application of bone and joint-on-chip platforms.

Osteoarthritis and chronic pain: Interleukin-6 as a common denominator and therapeutic target

Osteoarthritis (OA) is a common and problematic disorder that is often associated with chronic pain, a combination that renders OA a leading cause of physical disability and an unmet clinical challenge. In this issue of Science Signaling, Liao et al. show that interleukin-6 is a driver of both joint tissue degradation and pain, revealing a common culprit and a possible comprehensive target of therapeutic intervention.

Primary Cilia Direct Murine Articular Cartilage Tidemark Patterning Through Hedgehog Signaling and Ambulatory Load

Articular cartilage (AC) is essential for body movement but is highly susceptible to degenerative diseases and has poor self-repair capacity. To improve current subpar regenerative treatments, developmental mechanisms of AC should be clarified and, specifically, how its postnatal multizone organization is acquired. Primary cilia are cell surface organelles crucial for mammalian tissue morphogenesis. Although their importance for chondrocyte function is appreciated, their specific roles in postnatal AC morphogenesis remain unclear. To explore these mechanisms, we used a murine conditional loss-of-function approach (Ift88-flox) targeting joint-lineage progenitors (Gdf5Cre) and monitored postnatal knee AC development. Joint formation and growth up to juvenile stages were largely unaffected. However, mature AC (aged 2 months) exhibited disorganized extracellular matrix, decreased aggrecan and collagen II due to reduced gene expression (not increased catabolism), and marked reduction of AC modulus by 30%-50%. In addition, and unexpectedly, we discovered that tidemark patterning was severely disrupted, as was hedgehog signaling, and exhibited specificity based on regional load-bearing functions of AC. Interestingly, Prg4 expression was markedly increased in highly loaded sites in mutants. Together, our data provide evidence that primary cilia orchestrate postnatal AC morphogenesis including tidemark topography, zonal matrix composition, and ambulation load responses. © 2022 American Society for Bone and Mineral Research (ASBMR).

Synovial joint cavitation initiates with microcavities in interzone and is coupled to skeletal flexion and elongation in developing mouse embryo limbs

The synovial cavity and its fluid are essential for joint function and lubrication, but their developmental biology remains largely obscure. Here, we analyzed E12.5 to E18.5 mouse embryo hindlimbs and discovered that cavitation initiates around E15.0 with emergence of multiple, discrete, µm-wide tissue discontinuities we term microcavities in interzone, evolving into a single joint-wide cavity within 12 h in knees and within 72-84 h in interphalangeal joints. The microcavities were circumscribed by cells as revealed by mTmG imaging and exhibited a carbohydrate and protein content based on infrared spectral imaging at micro and nanoscale. Accounting for differing cavitation kinetics, we found that the growing femur and tibia anlagen progressively flexed at the knee over time, with peak angulation around E15.5 exactly when the full knee cavity consolidated; however, interphalangeal joint geometry changed minimally over time. Indeed, cavitating knee interzone cells were elongated along the flexion angle axis and displayed oblong nuclei, but these traits were marginal in interphalangeal cells. Conditional Gdf5Cre-driven ablation of Has2 – responsible for production of the joint fluid component hyaluronic acid (HA) – delayed the cavitation process. Our data reveal that cavitation is a stepwise process, brought about by sequential action of microcavities, skeletal flexion and elongation, and HA accumulation.

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

Summary: Synovial joints contain a fluid-filled cavity crucial for skeletal motion and lifelong function, but the developmental biology of cavitation remains largely obscure, hampering basic and translational progress.

Hox11 expression characterizes developing zeugopod synovial joints and is coupled to postnatal articular cartilage morphogenesis into functional zones in mice

Previous studies on mouse embryo limbs have established that interzone mesenchymal progenitor cells emerging at each prescribed joint site give rise to joint tissues over fetal time. These incipient tissues undergo structural maturation and morphogenesis postnatally, but underlying mechanisms of regulation remain unknown. Hox11 genes dictate overall zeugopod musculoskeletal patterning and skeletal element identities during development. Here we asked where these master regulators are expressed in developing limb joints and whether they are maintained during postnatal zeugopod joint morphogenesis. We found that Hoxa11 was predominantly expressed and restricted to incipient wrist and ankle joints in E13.5 mouse embryos, and became apparent in medial and central regions of knees by E14.5, though remaining continuously dormant in elbow joints. Closer examination revealed that Hoxa11 initially characterized interzone and neighboring cells and was then restricted to nascent articular cartilage, intra joint ligaments and structures such as meniscal horns over prenatal time. Postnatally, articular cartilage progresses from a nondescript cell-rich, matrix-poor tissue to a highly structured, thick, zonal and mechanically competent tissue with chondrocyte columns over time, most evident at sites such as the tibial plateau. Indeed, Hox11 expression (primarily Hoxa11) was intimately coupled to such morphogenetic processes and, in particular, to the topographical rearrangement of chondrocytes into columns within the intermediate and deep zones of tibial plateau that normally endures maximal mechanical loads. Revealingly, these expression patterns were maintained even at 6 months of age. In sum, our data indicate that Hox11 genes remain engaged well beyond embryonic synovial joint patterning and are specifically tied to postnatal articular cartilage morphogenesis into a zonal and resilient tissue. The data demonstrate that Hox11 genes characterize adult, terminally differentiated, articular chondrocytes and maintain region-specificity established in the embryo.

Fibroblast growth factor signalling in osteoarthritis and cartilage repair

Regulated fibroblast growth factor (FGF) signalling is a prerequisite for the correct development and homeostasis of articular cartilage, as evidenced by the fact that aberrant FGF signalling contributes to the maldevelopment of joints and to the onset and progression of osteoarthritis. Of the four FGF receptors (FGFRs 1-4), FGFR1 and FGFR3 are strongly implicated in osteoarthritis, and FGFR1 antagonists, as well as agonists of FGFR3, have shown therapeutic efficacy in mouse models of spontaneous and surgically induced osteoarthritis. FGF18, a high affinity ligand for FGFR3, is the only FGF-based drug currently in clinical trials for osteoarthritis. This Review covers the latest advances in our understanding of the molecular mechanisms that regulate FGF signalling during normal joint development and in the pathogenesis of osteoarthritis. Strategies for FGF signalling-based treatment of osteoarthritis and for cartilage repair in animal models and clinical trials are also introduced. An improved understanding of FGF signalling from a structural biology perspective, and of its roles in skeletal development and diseases, could unlock new avenues for discovery of modulators of FGF signalling that can slow or stop the progression of osteoarthritis.

Cell-surface markers identify tissue resident multipotential stem/stromal cell subsets in synovial intimal and sub-intimal compartments with distinct chondrogenic properties

OBJECTIVE

Synovium contains multipotent progenitor/stromal cells (MPCs) with potential to participate in cartilage repair. Understanding the identity of these MPCs will allow their therapeutic potential to be fully exploited. Hence this study aimed to identify primary synovial MPCs and characterize them in the context of cartilage regeneration.

METHODS

Primary MPC/MPC-subset specific markers in synovium were identified by FACS analysis of uncultured cells. MPC-subsets from human synovium obtained from patients undergoing total knee arthroplasty were FACS sorted, cultured, immunophenotyped and chondrogenically differentiated. The anatomical localization of MPCs in synovium was examined using immunohistochemistry. Finally, the presence of these MPC subsets in healthy synovium obtained from human organ donors was examined.

RESULTS

A combination of CD45, CD31, CD73 and CD90 can isolate two distinct MPC-subsets in synovium. These MPC-subsets, freshly isolated from synovium, did not express CD45 or CD31, but expressed CD73. Additionally, a sub-population of CD73⁺ cells also expressed CD90. CD45^-CD31^-CD73⁺CD90^- cells were significantly more chondrogenic than CD45^-CD31^-CD73⁺CD90⁺ cells in the presence of TGFβ1. Interestingly, reduced chondrogenic ability of CD73⁺CD90⁺ cells could be reversed by the addition of BMP2, showing discrete chondrogenic factor requirements by distinct cell-subsets. In addition, these MPCs had distinct anatomical localization; CD73 was expressed both in intimal and sub-intimal region while CD90 was enriched in the sub-intimal region. We further demonstrated that these subsets are also present in healthy synovium.

CONCLUSIONS

We provide indications that primary MPCs in synovial intima and sub-intima are phenotypically and functionally distinct with different chondrogenic properties.

Joints in the appendicular skeleton: Developmental mechanisms and evolutionary influences

The joints are a diverse group of skeletal structures, and their genesis, morphogenesis, and acquisition of specialized tissues have intrigued biologists for decades. Here we review past and recent studies on important aspects of joint development, including the roles of the interzone and morphogenesis of articular cartilage. Studies have documented the requirement of interzone cells in limb joint initiation and formation of most, if not all, joint tissues. We highlight these studies and also report more detailed interzone dissection experiments in chick embryos. Articular cartilage has always received special attention owing to its complex architecture and phenotype and its importance in long-term joint function. We pay particular attention to mechanisms by which neonatal articular cartilage grows and thickens over time and eventually acquires its multi-zone structure and becomes mechanically fit in adults. These and other studies are placed in the context of evolutionary biology, specifically regarding the dramatic changes in limb joint organization during transition from aquatic to land life. We describe previous studies, and include new data, on the knee joints of aquatic axolotls that unlike those in higher vertebrates, are not cavitated, are filled with rigid fibrous tissues and resemble amphiarthroses. We show that when axolotls metamorph to life on land, their intra-knee fibrous tissue becomes sparse and seemingly more flexible and the articular cartilage becomes distinct and acquires a tidemark. In sum, there have been considerable advances toward a better understanding of limb joint development, biological responsiveness, and evolutionary influences, though much remains unclear. Future progress in these fields should also lead to creation of new developmental biology-based tools to repair and regenerate joint tissues in acute and chronic conditions.

Osteoarthritis and intervertebral disc degeneration: Quite different, quite similar

Intervertebral disc degeneration describes the vicious cycle of the deterioration of intervertebral discs and can eventually result in degenerative disc disease (DDD), which is accompanied by low-back pain, the musculoskeletal disorder with the largest socioeconomic impact world-wide. In more severe stages, intervertebral disc degeneration is accompanied by loss of joint space, subchondral sclerosis, and osteophytes, similar to osteoarthritis (OA) in the articular joint. Inspired by this resemblance, we investigated the analogy between human intervertebral discs and articular joints. Although embryonic origin and anatomy suggest substantial differences between the two types of joint, some features of cell physiology and extracellular matrix in the nucleus pulposus and articular cartilage share numerous parallels. Moreover, there are great similarities in the response to mechanical loading and the matrix-degrading factors involved in the cascade of degeneration in both tissues. This suggests that the local environment of the cell is more important to its behavior than embryonic origin. Nevertheless, OA is widely regarded as a true disease, while intervertebral disc degeneration is often regarded as a radiological finding and DDD is undervalued as a cause of chronic low-back pain by clinicians, patients and society. Emphasizing the similarities rather than the differences between the two diseases may create more awareness in the clinic, improve diagnostics in DDD, and provide cross-fertilization of clinicians and scientists involved in both intervertebral disc degeneration and OA.

Infrapatellar Fat Pad Stem Cells: From Developmental Biology to Cell Therapy

The ideal cell type to be used for cartilage therapy should possess a proven chondrogenic capacity, not cause donor-site morbidity, and should be readily expandable in culture without losing their phenotype. There are several cell sources being investigated to promote cartilage regeneration: mature articular chondrocytes, chondrocyte progenitors, and various stem cells. Most recently, stem cells isolated from joint tissue, such as chondrogenic stem/progenitors from cartilage itself, synovial fluid, synovial membrane, and infrapatellar fat pad (IFP) have gained great attention due to their increased chondrogenic capacity over the bone marrow and subcutaneous adipose-derived stem cells. In this review, we first describe the IFP anatomy and compare and contrast it with other adipose tissues, with a particular focus on the embryological and developmental aspects of the tissue. We then discuss the recent advances in IFP stem cells for regenerative medicine. We compare their properties with other stem cell types and discuss an ontogeny relationship with other joint cells and their role on in vivo cartilage repair. We conclude with a perspective for future clinical trials using IFP stem cells.

Role of the C-C chemokine receptor-2 in a murine model of injury-induced osteoarthritis

OBJECTIVE

We previously found in our embryonic studies that proper regulation of the chemokine CCL12 through its sole receptor CCR2, is critical for joint and growth plate development. In the present study, we examined the role of CCR2 in injury-induced-osteoarthritis (OA).

METHOD

We used a murine model of injury-induced-OA (destabilization of medial meniscus, DMM), and systemically blocked CCR2 using a specific antagonist (RS504393) at different times during disease progression. We examined joint degeneration by assessing cartilage (cartilage loss, chondrocyte hypertrophy, MMP-13 expression) and bone lesions (bone sclerosis, osteophytes formation) with or without the CCR2 antagonist. We also performed pain behavioral studies by assessing the weight distribution between the normal and arthritic hind paws using the IITS incapacitance meter.

RESULTS

Testing early vs delayed administration of the CCR2 antagonist demonstrated differential effects on joint damage. We found that OA changes in articular cartilage and bone were ameliorated by pharmacological CCR2 blockade, if given early in OA development: specifically, pharmacological targeting of CCR2 during the first 4 weeks (wks) following injury, reduced OA cartilage and bone damage, with less effectiveness with later treatments. Importantly, our pain-related behavioral studies showed that blockade of CCR2 signaling during early, 1-4 wks post-surgery or moderate, 4-8 wks post-surgery, OA was sufficient to decrease pain measures, with sustained improvement at later stages, after treatment was stopped.

CONCLUSIONS

Our data highlight the potential efficacy of antagonizing CCR2 at early stages to slow the progression of post-injury OA and, in addition, improve pain symptoms.

Building and maintaining joints by exquisite local control of cell fate

We owe the flexibility of our bodies to sophisticated articulations between bones. Establishment of these joints requires the integration of multiple tissue types: permanent cartilage that cushions the articulating bones, synovial membranes that enclose a lubricating fluid-filled cavity, and a fibrous capsule and ligaments that provide structural support. Positioning the prospective joint region involves establishment of an "interzone" region of joint progenitor cells within a nascent cartilage condensation, which is achieved through the interplay of activators and inhibitors of multiple developmental signaling pathways. Within the interzone, tight regulation of BMP and TGFβ signaling prevents the hypertrophic maturation of joint chondrocytes, in part through downstream transcriptional repressors and epigenetic modulators. Synovial cells then acquire further specializations through expression of genes that promote lubrication, as well as the formation of complex structures such as cavities and entheses. Whereas genetic investigations in mice and humans have uncovered a number of regulators of joint development and homeostasis, recent work in zebrafish offers a complementary reductionist approach toward understanding joint positioning and the regulation of chondrocyte fate at joints. The complexity of building and maintaining joints may help explain why there are still few treatments for osteoarthritis, one of the most common diseases in the human population. A major challenge will be to understand how developmental abnormalities in joint structure, as well as postnatal roles for developmental genes in joint homeostasis, contribute to birth defects and degenerative diseases of joints. WIREs Dev Biol 2017, 6:e245. doi: 10.1002/wdev.245 For further resources related to this article, please visit the WIREs website.

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.

Joint Development Involves a Continuous Influx of Gdf5-Positive Cells

Synovial joints comprise several tissue types, including articular cartilage, the capsule, and ligaments. All of these compartments are commonly assumed to originate from an early set of Gdf5-expressing progenitors populating the interzone domain. Here, we provide evidence that joints develop through a continuous influx of cells into the interzone, where they contribute differentially to forming joint tissues. Using a knockin Gdf5-CreER(T2) mouse, we show that early labeling of Gdf5-positive interzone cells failed to mark the entire organ. Conversely, multiple Cre activation steps indicated a contribution of these cells to various joint compartments later in development. Spatiotemporal differences between Gdf5 and tdTomato reporter expression support the notion of a continuous recruitment process. Finally, differential contribution of Gdf5-positive cells to various tissues suggests that the spatiotemporal dynamics of Gdf5 expression may instruct lineage divergence. This work supports the influx model of joint development, which may apply to other organogenic processes.