Rotator Cuff Tenocytes Differentiate into Hypertrophic Chondrocyte-Like Cells to Produce Calcium Deposits in an Alkaline Phosphatase-Dependent Manner

Effect of load reduction on the calcification of rat tibial tuberosity: Focus on calcification factors and chondrocyte mechanosensors

The tibial tuberosity has a superficial patellar tendon-embedded portion and a deep uncalcified cartilage portion. Suppressed calcification of the tibial tuberosity leads to Osgood-Schlatter disease. The tibial tuberosity calcifies with age; load reduction degrades the cartilage matrix and promotes calcification, suggesting that reduced mechanical stimulation of the tibial tuberosity promotes calcification. However, this is yet to be clarified. Therefore, in this study, we aimed to investigate the effects of mechanical stimulation reduction on the tibial tuberosity tissue structure and calcification mechanism. Specifically, we examined the effect of load reduction on tibial tuberosity calcification in 20 male 7-week-old Wistar rats classified into two groups: hind-limb suspension (HS, n = 10) and control (CO, n = 10). We observed superficial and deep tibial tuberosities in both groups. The tibial tuberosity in the HS group had narrower areas of deep portions than did those in the CO group (p = 0.000539), and immature bone tissue and cartilage tissue were observed in the HS group. Enpp1 expression did not significantly differ between the groups (p = 0.804). In contrast, Alpl (p = 0.001) and Mmp3 (p = 0.006) expression increased whereas Timp3 expression decreased (p = 0.002) in the HS group. Thus, these results showed a maturing of bone ossification, and this gene expression trend was similar to that observed in a murine join instability model of osteoarthritis with articular cartilage calcification and ossification. The HS tibial tuberosity also showed immature bone tissue. In conclusion, reduced mechanical stimulation caused tibial tuberosity calcification and pathological changes. These findings highlight the importance of optimal exercise to avoid premature pathological structural changes in bones and joints.

Pathological calcification in canine tendon-derived cells is modulated by extracellular ATP

Tendon calcification is a commonly associated with degenerative tendinopathy of the Achilles tendons in dogs. It is characterised by the formation of calcific deposits and is refractory to treatment, often re-forming after surgical removal. Little is known about its pathogenesis and therefore the aims of this study were to develop an in vitro model of canine tendon calcification and use this model to investigate mechanisms driving calcification. Cells from the canine Achilles tendon were cultured with different calcifying media to establish which conditions were best able to induce specific, cell-mediated calcification. Once optimum calcification conditions had been established, the effect of ATP treatment on calcification was assessed. Results revealed that 2 mM di-sodium phosphate combined with 2 mM calcium chloride provided the optimum calcifying conditions, increasing calcium deposition and expression of osteogenic-related genes similar to those observed in tendon calcification in vivo. ATP treatment inhibited calcification in a dose-dependent manner, reducing calcium deposition and increasing cell viability, while osteogenic-related genes were no longer upregulated. In conclusion, the in vitro model of canine tendon calcification developed in this study provides the ability to study mechanisms driving tendon calcification, demonstrating that ATP plays a role in modulating tendon calcification that should be explored further in future studies.

Tendon-associated gene expression precedes osteogenesis in mid-palatal suture establishment

Orthodontic maxillary expansion relies on intrinsic mid-palatal suture mechanobiology to induce guided osteogenesis, yet establishment of the mid-palatal suture within the continuous secondary palate and causes of maxillary insufficiency remain poorly understood. In contrast, advances in cranial suture research hold promise to improve surgical repair of prematurely fused cranial sutures in craniosynostosis to potentially restore the obliterated signaling environment and ensure continual success of the intervention. We hypothesized that mid-palatal suture establishment is governed by shared principles with calvarial sutures and involves functional linkage between expanding primary ossification centres with the midline mesenchyme. We characterized establishment of the mid-palatal suture from late embryonic to early postnatal timepoints. Suture establishment was visualized using histological techniques and multimodal transcriptomics. We identified that mid-palatal suture formation depends on a spatiotemporally controlled signalling milieu in which tendon-associated genes play a significant role. We mapped relationships between extracellular matrix-encoding gene expression, tenocyte markers, and novel suture patency candidate genes. We identified similar expression patterns in FaceBase-deposited scRNA-seq datasets from cranial sutures. These findings demonstrate shared biological principles for suture establishment, providing further avenues for future development and understanding of maxillofacial interventions.

PPP1R3A inhibits osteogenesis and negatively regulates intracellular calcium levels in calcific tendinopathy

Calcific tendinopathy (CT) is defined by the progressive accumulation of calcium crystals in tendonic regions that results in severe pain in patients. The etiology of CT is not fully elucidated. In this study, we elucidate the role of PPP1R3A in CT. A significant decrease in PPP1R3A expression was observed in CT patient tissues, which was further confirmed in tissues from a CT-induced rat model. Overexpression of PPP1R3A ex vivo reduced the expression of osteo/chondrogenic markers OCN and Sox9, improved tendon tissue architecture, and reduced intracellular Ca2+ levels. Overexpression of SERCA2 and knockdown of Piezo1 decreased expression of osteo/chondrogenic markers and intracellular calcium in PPP1R3A-knockdown tendon cells. Lastly, PPP1R3A expression was regulated at the posttranscriptional level by binding of HuR. Collectively, the present study indicates that PPP1R3A plays an important role in regulating calcium homeostasis in tendon cells via Piezo1/SERCA2, rendering it a promising target for therapeutic interventions of CT.

Heterotopic mineral deposits in intact rat Achilles tendons are characterized by a unique fiber-like structure

Heterotopic mineralization entails pathological mineral formation inside soft tissues. In human tendons mineralization is often associated with tendinopathies, tendon weakness and pain. In Achilles tendons, mineralization is considered to occur through heterotopic ossification (HO) primarily in response to tendon pathologies. However, refined details regarding HO deposition and microstructure are unknown. In this study, we characterize HO in intact rat Achilles tendons through high-resolution phase contrast enhanced synchrotron X-ray tomography. Furthermore, we test the potential of studying local tissue injury by needling intact Achilles tendons and the relation between tissue microdamage and HO. The results show that HO occurs in all intact Achilles tendons at 16 weeks of age. HO deposits are characterized by an elongated ellipsoidal shape and by a fiber-like internal structure which suggests that some collagen fibers have mineralized. The data indicates that deposition along fibers initiates in the pericellular area, and propagates into the intercellular area. Within HO deposits cells are larger and more rounded compared to tenocytes between unmineralized fibers, which are fewer and elongated. The results also indicate that multiple HO deposits may merge into bigger structures with time by accession along unmineralized fibers. Furthermore, the presence of unmineralized regions within the deposits may indicate that HOs are not only growing, but mineral resorption may also occur. Additionally, phase contrast synchrotron X-ray tomography allowed to distinguish microdamage at the fiber level in response to needling. The needle injury protocol could in the future enable to elucidate the relation between local inflammation, microdamage, and HO deposition.

Predicting the formation of different tissue types during Achilles tendon healing using mechanoregulated and oxygen-regulated frameworks

During Achilles tendon healing in rodents, besides the expected tendon tissue, also cartilage-, bone- and fat-like tissue features have been observed during the first twenty weeks of healing. Several studies have hypothesized that mechanical loading may play a key role in the formation of different tissue types during healing. We recently developed a computational mechanobiological framework to predict tendon tissue production, organization and mechanical properties during tendon healing. In the current study, we aimed to explore possible mechanobiological related mechanisms underlying formation of other tissue types than tendon tissue during tendon healing. To achieve this, we further developed our recent framework to predict formation of different tissue types, based on mechanobiological models established in other fields, which have earlier not been applied to study tendon healing. We explored a wide range of biophysical stimuli, i.e., principal strain, hydrostatic stress, pore pressure, octahedral shear strain, fluid flow, angiogenesis and oxygen concentration, that may promote the formation of different tissue types. The numerical framework predicted spatiotemporal formation of tendon-, cartilage-, bone- and to a lesser degree fat-like tissue throughout the first twenty weeks of healing, similar to recent experimental reports. Specific features of experimental data were captured by different biophysical stimuli. Our modeling approach showed that mechanobiology may play a role in governing the formation of different tissue types that have been experimentally observed during tendon healing. This study provides a numerical tool that can contribute to a better understanding of tendon mechanobiology during healing. Developing these tools can ultimately lead to development of better rehabilitation regimens that stimulate tendon healing and prevent unwanted formation of cartilage-, fat- and bone-like tissues.

The role of calcium crystals and their effect on osteoarthritis pathogenesis

Osteoarthritis (OA) is a degenerative joint disease characterized by progressive degeneration of articular cartilage. Due to its high prevalence and limited treatment options, OA has become one of the most disabling diseases in developed countries. In recent years, OA has been recognized as a heterogenic disease with various phenotypes. Calcium crystal-related endotypes, which are defined by either a distinct functional or pathobiological mechanism, are present in approximately 60% of all OA patients. Two different calcium crystals can accumulate in the joint and thereby calcify the cartilage matrix, which are basic calcium phosphate (BCP) and calcium pyrophosphate (CPP) crystals. The formation of these crystals depends mainly on the balance of phosphate and pyrophosphate, which is regulated by specific proteins controlling the pyrophosphate metabolism. Dysregulation of these molecules subsequently leads to preferential formation of either BCP or CPP crystals. BCP crystals, on the one hand, are directly associated with OA severity and cartilage degradation. They are mostly located in the deeper cartilage layers and are associated with chondrocyte hypertrophy. CPP crystal deposition, on the other hand, is a hallmark of chondrocalcinosis and is associated with aging and chondrocyte senescence. Therefore, BCP and CPP crystals are associated with different chondrocyte phenotypes. However, BCP and CPP crystals are not mutually exclusive and can coexist in OA, creating a mixed endotype of OA. Both crystals clearly play a role in the pathogenesis of OA. However, the exact impact of each crystal type on either driving the disease progression or being a result of chondrocyte differentiation is still to be elucidated.

Characterization of Deposits in Calcific Tendinitis of the Shoulder: Deposits Are Composed of Large Aggregates of Highly Crystalline, Rod-Like Crystals

BACKGROUND

In the current literature, deposits in calcific tendinitis are described as amorphous masses of hydroxyapatite with a size in the range of 5 to 20 μm. Theoretically, these are too big to be phagocytized by macrophages and induce an inflammatory reaction.

PURPOSE

To better characterize the deposits seen in calcific tendinitis.

STUDY DESIGN

Case series; Level of evidence, 4.

METHODS

Included in the study were 6 patients with a history of at least 1 year of shoulder pain (range, 1-14 years). Shoulder arthroscopy was performed under general anesthesia, and calcium deposits from the supraspinatus tendon and biopsies from the adjacent subacromial bursa were taken. Samples were analyzed by light microscopy and immunostained for macrophages. Scanning electron microscopy and energy-dispersive x-ray (EDX) analysis were used to assess the morphology and chemical composition of the calcific deposits.

RESULTS

Light microscopy showed round and bulky calcium deposits partially surrounded by activated CD68-positive macrophages within inflammatory tissue. Some hemosiderin positive mononuclear cells, indicative for (micro-) hemorrhage, were seen. Scanning electron microscopy revealed that the large calcific deposits (1-20 μm) were composed of rod-like structures. These highly crystalline rods had a size of approximately 100 nm in length and 20 nm in width. Chemical composition by EDX analysis showed that crystals were composed of mainly calcium, oxygen, and phosphorus, equaling the chemical composition of hydroxyapatite.

CONCLUSION

Deposits in calcific tendinitis of the rotator cuff are not amorphous but composed of highly crystalline structures. Fragmentation of these aggregates and subsequent release of the needle-like nanocrystals might initiate the strong inflammatory reaction often seen in patients with calcifying tendinitis of the rotator cuff.

Pro-inflammatory effects of human apatite crystals extracted from patients suffering from calcific tendinopathy

BACKGROUND

Calcific tendonitis of the rotator cuff is due to carbonated apatite deposits in the shoulder tendons. During the evolution of the disease, an acute inflammatory episode may occur leading to the disappearance of the calcification. Although hydroxyapatite crystal-induced inflammation has been previously studied with synthetic crystals, no data are available with calcifications extracted from patients suffering from calcific tendinopathy. The objective of the study was to explore the inflammatory properties of human calcifications and the pathways involved.

METHODS

Human calcifications and synthetic hydroxyapatite were used in vitro to stimulate human monocytes and macrophages, the human myeloid cell line THP-1, and human tenocytes. The release of IL-1β, IL-6, and IL-8 by cells was quantified by ELISA. The gene expression of pro- and anti-inflammatory cytokines was evaluated by quantitative PCR. NF-kB activation and NLRP3 involvement were assessed in THP-1 cells using a NF-kB inhibitor and a caspase-1 inhibitor. The inflammatory properties were then assessed in vivo using a mouse air pouch model.

RESULTS

Human calcifications were able to induce a significant release of IL-1β when incubated with monocytes, macrophages, and THP-1 only if they were first primed with LPS (monocytes and macrophages) or PMA (THP-1). Stimulation of THP-1 by human calcifications led to similar levels of IL-1β when compared to synthetic hydroxyapatite although these levels were significantly inferior in monocytes and macrophages. The patient's crystals enhanced mRNA expression of pro-IL-1β, as well as IL-18, NF-kB, and TGFβ when IL-6 and TNFα expression were not. IL-1β production was reduced by the inhibition of caspase-1 indicating the role of NLRP3 inflammasome. In vivo, injection of human calcifications or synthetic hydroxyapatite in the air pouch led to a significant increase in membrane thickness although significant overexpression of IL-1β was only observed for synthetic hydroxyapatite.

CONCLUSIONS

As synthetic hydroxyapatite, human calcifications were able to induce an inflammatory response resulting in the production of IL-1β after NF-kB activation and through NLRP3 inflammasome. In some experiments, IL-1β induction was lower with human calcifications compared to synthetic apatite. Differences in size, shape, and protein content may explain this observation.