Tenomodulin knockout mice exhibit worse late healing outcomes with augmented trauma-induced heterotopic ossification of Achilles tendon

Angiogenesis in heterotopic ossification: From mechanisms to clinical significance

Heterotopic ossification (HO) refers to the formation of pathologic bone in nonskeletal tissues (including muscles, tendons or other soft tissues). HO typically occurs after a severe injury and can occur in any part of the body. HO lesions are highly vascularized. Angiogenesis, which is the formation of new blood vessels, plays an important role in the pathophysiology of HO. Surgical resection is considered an effective treatment for HO. However, it is difficult to completely remove new vessels, which can lead to the recurrence of HO and is often accompanied by significant problems such as intraoperative hemorrhage, demonstrating the important role of angiogenesis in HO. Here, we broadly summarize the current understanding of how angiogenesis contributes to HO; in particular, we focus on new insights into the cellular and signaling mechanisms underlying HO angiogenesis. We also review the development and current challenges associated with antiangiogenic therapy for HO.

Disturbed glycolipid metabolism activates CXCL13-CXCR5 axis in senescent TSCs to promote heterotopic ossification

Heterotopic ossification (HO) occurs as a common complication after injury, while its risk factor and mechanism remain unclear, which restricts the development of pharmacological treatment. Clinical research suggests that diabetes mellitus (DM) patients are prone to developing HO in the tendon, but solid evidence and mechanical research are still needed. Here, we combined the clinical samples and the DM mice model to identify that disordered glycolipid metabolism aggravates the senescence of tendon-derived stem cells (TSCs) and promotes osteogenic differentiation. Then, combining the RNA-seq results of the aging tendon, we detected the abnormally activated autocrine CXCL13-CXCR5 axis in TSCs cultured in a high fat, high glucose (HFHG) environment and also in the aged tendon. Genetic inhibition of CXCL13 successfully alleviated HO formation in DM mice, providing a potential therapeutic target for suppressing HO formation in DM patients after trauma or surgery.

NIR-triggered photodynamic therapy of traumatic heterotopic ossification with a type II collagen-targeted photosensitizer

Traumatic heterotopic ossification (HO) represents an intractable sequela following trauma with no currently effective prophylaxis or treatment. Photodynamic therapy (PDT) is a non-invasive treatment for various proliferative diseases. However, the specific effects of PDT on HO development remain unclear. In this study, the therapeutic potential of a near-infrared (NIR) probe-WL-808, composed of type II collagen-binding peptide (WYRGRL) and a PDT photosensitizer (IR-808), was evaluated for the innovative HO-targeted PDT approach. In vitro studies indicated that WL-808 could induce chondrocyte apoptosis and inhibit cell viability through ROS generation under NIR excitation. In vivo, the efficacy of WL-808-mediated PDT was tested on the tenotomy HO model mice. WL-808 specifically targeted the type II collagen cartilaginous template of HO, promoting cell apoptosis and enhancing extracellular matrix (ECM) degradation under 808 nm NIR excitation, which inhibited the final ectopic bone formation. Moreover, no obvious toxicity or side effects were detected after treatment with WL-808. Taken together, WL-808-mediated PDT significantly diminished ectopic cartilage and subsequent bone formation, providing a new perspective for HO prophylaxis and treatment.

Preclinical assessment of IL-1β primed human umbilical cord mesenchymal stem cells for tendon functional repair through TGF-β/IL-10 signaling

Background

Inadequate repair capacity and disturbed immune compartments are the main pathological causes of tendinopathy. Transplantation of mesenchymal stem cells (MSCs) become an effective clinic option to alleviate tendinopathy. Interleukin-1β (IL-1β) could confer on MSCs enhanced immunoregulatory capability to remodel the repair microenvironment favoring tissue repair. Therefore, IL-1β activated UC-MSCs (1βUC-MSCs) may exert favorable efficacy in promoting tendon repair in a preclinical tendinopathy rat model.

Methods

Tendon-derived stem cells (TDSCs) were isolated and characterized. In vitro, the levels of immunoregulatory-related cytokines such as IL-1β, IL-6, IL-10, and TGF-β secreted by 1βUC-MSCs and unprimed UC-MSCs was measured. And tendon-specific markers expressed by TDSCs cultured with primed cultured medium (CM) or unprimed CM were detected. In vivo, Achilles tendinopathy was induced by 30 μL collagenase I injection in Sprague Dawley rats. One week later, the rats were randomly injected with UC-MSCs primed with IL-1β (106 cells per tendon), UC-MSCs, or PBS. After rats were sacrificed, histological evaluation, electron microscopy, biomechanical tests, gait performance were conducted to evaluate the structural and functional recovery of Achilles tendons. The inflammation and metabolic state of the extracellular matrix, and the potential mechanism were assessed by immunohistochemical staining and Western blot.

Results

UC-MSCs were activated by IL-1β to secrete higher levels of IL-10 and TGF-β while the secretion levels of IL-6 and IL-1β were not changed significantly, promoting a higher expression level of COL I and TNMD in TDSCs under proinflammatory environment. In vivo, the transplanted 1βUC-MSCs could survive up to 5 weeks after injection with tenogenic differentiation and improved tendon healing histologically semi-quantified by modified Bonar scores. This structural regeneration was further confirmed by observation of ultrastructural morphology, and led to good functional recovery including improved biomechanical properties and gait performance. During this process, the inflammatory response and metabolism of the extracellular matrix was improved through TGF-β/IL-10 pathway.

Conclusion

This study demonstrated that the transplantation of UC-MSCs activated by IL-1β exhibited satisfactory ability for promoting tendon functional repair in a tendinopathy rat model. During this process, the balance of inflammatory response and extracellular matrix metabolism was remodeled, and the TGF-β/Smad2/3 and IL-10 signaling pathways were activated simultaneously. We cautiously conclude that the IL-1β primed UC-MSCs could be a promising strategy for enhancing the ability of MSCs to treat tendinopathy.

iPSC-derived tenocytes seeded on microgrooved 3D printed scaffolds for Achilles tendon regeneration

Tendons and ligaments have a poor innate healing capacity, yet account for 50% of musculoskeletal injuries in the United States. Full structure and function restoration postinjury remains an unmet clinical need. This study aimed to assess the application of novel three dimensional (3D) printed scaffolds and induced pluripotent stem cell-derived mesenchymal stem cells (iMSCs) overexpressing the transcription factor Scleraxis (SCX, iMSCSCX+ ) as a new strategy for tendon defect repair. The polycaprolactone (PCL) scaffolds were fabricated by extrusion through a patterned nozzle or conventional round nozzle. Scaffolds were seeded with iMSCSCX+ and outcomes were assessed in vitro via gene expression analysis and immunofluorescence. In vivo, rat Achilles tendon defects were repaired with iMSCSCX+ -seeded microgrooved scaffolds, microgrooved scaffolds only, or suture only and assessed via gait, gene expression, biomechanical testing, histology, and immunofluorescence. iMSCSCX+ -seeded on microgrooved scaffolds showed upregulation of tendon markers and increased organization and linearity of cells compared to non-patterned scaffolds in vitro. In vivo gait analysis showed improvement in the Scaffold + iMSCSCX+ -treated group compared to the controls. Tensile testing of the tendons demonstrated improved biomechanical properties of the Scaffold + iMSCSCX+ group compared with the controls. Histology and immunofluorescence demonstrated more regular tissue formation in the Scaffold + iMSCSCX+ group. This study demonstrates the potential of 3D-printed scaffolds with cell-instructive surface topography seeded with iMSCSCX+ as an approach to tendon defect repair. Further studies of cell-scaffold constructs can potentially revolutionize tendon reconstruction by advancing the application of 3D printing-based technologies toward patient-specific therapies that improve healing and functional outcomes at both the cellular and tissue level.

Decellularization of Dense Regular Connective Tissue-Cellular and Molecular Modification with Applications in Regenerative Medicine

Healing of dense regular connective tissue, due to a high fiber-to-cell ratio and low metabolic activity and regeneration potential, frequently requires surgical implantation or reconstruction with high risk of reinjury. An alternative to synthetic implants is using bioscaffolds obtained through decellularization, a process where the aim is to extract cells from the tissue while preserving the tissue-specific native molecular structure of the ECM. Proteins, lipids, nucleic acids and other various extracellular molecules are largely involved in differentiation, proliferation, vascularization and collagen fibers deposit, making them the crucial processes in tissue regeneration. Because of the multiple possible forms of cell extraction, there is no standardized protocol in dense regular connective tissue (DRCT). Many modifications of the structure, shape and composition of the bioscaffold have also been described to improve the therapeutic result following the implantation of decellularized connective tissue. The available data provide a valuable source of crucial information. However, the wide spectrum of decellularization makes it important to understand the key aspects of bioscaffolds relative to their potential use in tissue regeneration.

Chronological Changes in the Expression and Localization of Sox9 between Achilles Tendon Injury and Functional Recovery in Mice

Tendons help transmit forces from the skeletal muscles and bones. However, tendons have inferior regenerative ability compared to muscles. Despite studies on the regeneration of muscles and bone tissue, only a few have focused on tendinous tissue regeneration, especially tendon regeneration. Sex-determining region Y-box transcription factor 9 (Sox9) is an SRY-related transcription factor with a DNA-binding domain and is an important control factor for cartilage formation. Sox9 is critical to the early-to-middle stages of tendon development. However, how Sox9 participates in the healing process after tendon injury is unclear. We hypothesized that Sox9 is expressed in damaged tendons and is crucially involved in restoring tendon functions. We constructed a mouse model of an Achilles tendon injury by performing a 0.3 mm wide partial excision in the Achilles tendon of mice, and chronologically evaluated the function restoration and localization of the Sox9 expressed in the damaged sites. The results reveal that Sox9 was expressed simultaneously with the formation of the pre-structure of the epitenon, an essential part of the tendinous tissue, indicating that its expression is linked to the functional restoration of tendons. Lineage tracing for Sox9 expressed during tendon restoration revealed the tendon restoration involvement of cells that switched into Sox9-expressing cells after tendon injury. The stem cells involved in tendon regeneration may begin to express Sox9 after injury.

Establishing in vivo and ex vivo chick embryo models to investigate fetal tendon healing

Injured adult tendons heal fibrotically and possess high re-injury rates, whereas fetal tendons appear to heal scarlessly. However, knowledge of fetal tendon wound healing is limited due in part to the need for an accessible animal model. Here, we developed and characterized an in vivo and ex vivo chick embryo tendon model to study fetal tendon healing. In both models, injury sites filled rapidly with cells and extracellular matrix during healing, with wound closure occurring faster in vivo. Tendons injured at an earlier embryonic stage improved mechanical properties to levels similar to non-injured controls, whereas tendons injured at a later embryonic stage did not. Expression levels of tendon phenotype markers, collagens, collagen crosslinking regulators, matrix metalloproteinases, and pro-inflammatory mediators exhibited embryonic stage-dependent trends during healing. Apoptosis occurred during healing, but ex vivo tendons exhibited higher levels of apoptosis than tendons in vivo. Future studies will use these in vivo and ex vivo chick embryo tendon injury models to elucidate mechanisms of stage-specific fetal tendon healing to inform the development of therapeutic approaches to regeneratively heal adult tendons.

Young Exosome Bio-Nanoparticles Restore Aging-Impaired Tendon Stem/Progenitor Cell Function and Reparative Capacity

Aging impairs tendon stem/progenitor cell function and tendon homeostasis, however, effective treatments for aging-induced tendon diseases are lacking. Exosomes are naturally derived nanoparticles that contain bioactive molecules, and therefore, have attracted great interest in tissue engineering and regenerative medicine. In this study, it is shown that young exosomes secreted by stem cells from human exfoliated deciduous teeth (SHED-Exos) possess abundant anti-aging signals. These young bio-nanoparticles can alleviate the aging phenotypes of aged tendon stem/progenitor cells (AT-SCs) and maintain their tenogenic capacity. Mechanistically, SHED-Exos modulate histone methylation and inhibit nuclear factor-κB to reverse AT-SC aging. In a naturally aging mouse model, systemic administration of SHED-Exo bio-nanoparticles retards tendon degeneration. Interestingly, local delivery of SHED-Exos-loaded microspheres confers anti-aging phenotypes, including reduced senescent cells and decreased ectopic bone formation, thereby functionally and structurally rescuing endogenous tendon regeneration and repair capacity in aged rats. Overall, SHED-Exos, as natural bioactive nanoparticles, have promising translational and therapeutic potential for aging-related diseases.

Mechanisms of skeletal muscle-tendon development and regeneration/healing as potential therapeutic targets

Skeletal muscle contraction is essential for the movement of our musculoskeletal system. Tendons and ligaments that connect the skeletal muscles to bones in the correct position at the appropriate time during development are also required for movement to occur. Since the musculoskeletal system is essential for maintaining basic bodily functions as well as enabling interactions with the environment, dysfunctions of these tissues due to disease can significantly reduce quality of life. Unfortunately, as people live longer, skeletal muscle and tendon/ligament diseases are becoming more common. Sarcopenia, a disease in which skeletal muscle function declines, and tendinopathy, which involves chronic tendon dysfunction, are particularly troublesome because there have been no significant advances in their treatment. In this review, we will summarize previous reports on the development and regeneration/healing of skeletal muscle and tendon tissues, including a discussion of the molecular and cellular mechanisms involved that may be used as potential therapeutic targets.

Tendon Stem/Progenitor Cell-Laden Nanofiber Hydrogel Enhanced Functional Repair of Patellar Tendon

Functional repair of tendons remains a challenge to be overcome for both clinicians and scientists. We have previously reported a three-dimensional RADA peptide hydrogel that provides a suitable microenvironment for human tendon stem/progenitor cells (TSPCs) survival and tenogenesis. In this study, we explore the potential of in vivo patellar tendon repair by human TSPC-laden RADA hydrogel in rats, which were sacrificed at 4 and 8 weeks after operation. Hind limb function test, macroscopical and histological examination, tendon cell amount and alignment analysis, and radiographic assessments were performed at several time points. Our results demonstrated that human TSPC-laden RADA hydrogel (RADA+TSPC group) boosted in vivo patellar tendon repair with better ambulatory function recovery compared with the control groups, in which tendon defects were untreated (Defect group) or treated with RADA hydrogel alone (RADA group). In addition, better macroscopic appearance and improved matrix organization in the repaired tendon with less cell amount and reduced adipocyte accumulation and blood vessel formation were observed in the RADA+TSPC group. Moreover, tendon defect treated with TSPC-laden RADA hydrogel resulted in diminished heterotopic ossification (HO) at 8 weeks postoperation, which was indicated by both X-ray examination and micro-computed tomography scan. Taken together, the combination of TSPC and nanofiber hydrogel provide an optimistic alternative method to accelerate functional tendon repair with reduced HO. Impact statement Our study clearly demonstrates the combination of tendon stem/progenitor cell and nanofiber hydrogel provide a new and optimistic tissue engineering strategy to treat tendon injury by accelerating functional tendon repair with reduced heterotopic ossification. The clinical translation is also very promising, which can provide a minimally invasive, nonsurgical, or complementary treatment methods to treat human tendon injury.

Mechanical stretch facilitates tenomodulin expression to induce tenocyte migration via MAPK signaling pathway

Tenomodulin (Tnmd) is a type II transmembrane glycoprotein that regulates tendon development and maturation. Our previous study indicated that mechanical stretch could induce Tnmd expression to promote tenocyte migration, associated with reinforcement of fibrous actin (F-actin) stress fibers and chromatin decondensation. However, the detailed molecular mechanisms of this processes are far from clear. Activation of mitogen-activated protein kinase (MAPK) signaling occurs in response to various extracellular stimuli and controls a large number of fundamental cellular processes. The present study we investigated the influence of MAPK signaling on mechanical stretch-induced Tnmd expression and its action way. Expression and activities of extracellular signal-related kinases 1 and 2 (ERK1/2), c-Jun N-terminal kinases (JNK) and p38 MAPK (p38) were determined by Western blot. Cell migration was detected by Transwell assay. Immunofluorescence staining was used to detect F-actin stress fibers. Nuclear chromatin decondensation was detected by in situ DNaseI sensitivity assay. It was found that mechanical stretch promoted Tnmd expression by activating ERK1/2, JNK and p38 signaling. The inhibition of the ERK1/2, JNK or p38 repressed mechanical stretch-promoted tenocyte migration and mechanical stretch-induced reinforcement of F-actin stress fibers. However, only ERK1/2 and p38 inhibitor could repress mechanical stretch-induced chromatin decondensation, and the JNK inhibitor had no significant effect. Moreover, latrunculin (Lat A), the most widely used reagent to depolymerize actin filaments, could inhibit the stretch-induced chromatin decondensation. Taken together, our findings elucidated a molecular pathway by which a mechanical signal is transduced via activation of MAPK signaling to influence reinforcement of F-actin stress fibers and chromatin decondensation, which could further lead Tnmd expression to promote tenocyte migration.

Tendon Aging

Tendons are mechanosensitive connective tissues responsible for the connection between muscles and bones by transmitting forces that allow the movement of the body, yet, with advancing age, tendons become more prone to degeneration followed by injuries. Tendon diseases are one of the main causes of incapacity worldwide, leading to changes in tendon composition, structure, and biomechanical properties, as well as a decline in regenerative potential. There is still a great lack of knowledge regarding tendon cellular and molecular biology, interplay between biochemistry and biomechanics, and the complex pathomechanisms involved in tendon diseases. Consequently, this reflects a huge need for basic and clinical research to better elucidate the nature of healthy tendon tissue and also tendon aging process and associated diseases. This chapter concisely describes the effects that the aging process has on tendons at the tissue, cellular, and molecular levels and briefly reviews potential biological predictors of tendon aging. Recent research findings that are herein reviewed and discussed might contribute to the development of precision tendon therapies targeting the elderly population.

Amelioration of experimental tendinopathy by lentiviral CD44 gene therapy targeting senescence-associated secretory phenotypes

CD44 exerts anti-senescence effects in many disease models. We examined senescence in tendinopathy and the effect of CD44 on senescence-associated secretory phenotypes (SASPs). Senescent markers were determined in human tendinopathic long head of bicep (LHB) and normal hamstring tendons. CD44 gene transfer in rat tendinopathic tenocytes stimulated with interleukin (IL)-1β and a rat Achilles tendinopathy model were performed using lentiviral vectors. Expression levels of p53, p21, and p16 and senescence-associated β-galactosidase (SA-β-gal) activity were positively correlated with the severity of human tendinopathy and were higher in rat and human tendinopathic tenocytes than in normal controls. CD44 overexpressed tenocyte transfectants exhibited reduced levels of IL-6, matrix metalloproteinases (MMPs), cyclooxygenase (COX)-2, p53, p21, p16, SA-β-gal, and phospho-nuclear factor (NF)-κB, whereas their collagen type I alpha 1 (COL1A1) and tenomodulin (tnmd) levels were increased when compared with control transfectants under IL-1β-stimulated conditions. In the animal model, CD44 overexpression lowered the ultrasound and histology scores and expression levels of the senescent and SASP markers COX-2 and phospho-NF-κB. Bromodeoxyuridine (BrdU)- and tnmd-positive cell numbers were increased in the LVCD44-transduced tendinopathic tendons. Senescence is positively correlated with tendinopathic severity, and CD44 overexpression may protect the tendinopathic tendons from SASPs via anti-inflammation and maintenance of extracellular matrix homeostasis.

Tendon healing: a concise review on cellular and molecular mechanisms with a particular focus on the Achilles tendon

Tendon is a bradytrophic and hypovascular tissue, hence, healing remains a major challenge. The molecular key events involved in successful repair have to be unravelled to develop novel strategies that reduce the risk of unfavourable outcomes such as non-healing, adhesion formation, and scarring. This review will consider the diverse pathophysiological features of tendon-derived cells that lead to failed healing, including misrouted differentiation (e.g. de- or transdifferentiation) and premature cell senescence, as well as the loss of functional progenitors. Many of these features can be attributed to disturbed cell-extracellular matrix (ECM) or unbalanced soluble mediators involving not only resident tendon cells, but also the cross-talk with immigrating immune cell populations. Unrestrained post-traumatic inflammation could hinder successful healing. Pro-angiogenic mediators trigger hypervascularization and lead to persistence of an immature repair tissue, which does not provide sufficient mechano-competence. Tendon repair tissue needs to achieve an ECM composition, structure, strength, and stiffness that resembles the undamaged highly hierarchically ordered tendon ECM. Adequate mechano-sensation and -transduction by tendon cells orchestrate ECM synthesis, stabilization by cross-linking, and remodelling as a prerequisite for the adaptation to the increased mechanical challenges during healing. Lastly, this review will discuss, from the cell biological point of view, possible optimization strategies for augmenting Achilles tendon (AT) healing outcomes, including adapted mechanostimulation and novel approaches by restraining neoangiogenesis, modifying stem cell niche parameters, tissue engineering, the modulation of the inflammatory cells, and the application of stimulatory factors.Cite this article: Bone Joint Res 2022;11(8):561-574.

Contemporary perspectives on heterotopic ossification

Heterotopic ossification (HO) is the formation of ectopic bone that is primarily genetically driven (fibrodysplasia ossificans progressiva [FOP]) or acquired in the setting of trauma (tHO). HO has undergone intense investigation, especially over the last 50 years, as awareness has increased around improving clinical technologies and incidence, such as with ongoing wartime conflicts. Current treatments for tHO and FOP remain prophylactic and include NSAIDs and glucocorticoids, respectively, whereas other proposed therapeutic modalities exhibit prohibitive risk profiles. Contemporary studies have elucidated mechanisms behind tHO and FOP and have described new distinct niches independent of inflammation that regulate ectopic bone formation. These investigations have propagated a paradigm shift in the approach to treatment and management of a historically difficult surgical problem, with ongoing clinical trials and promising new targets.

Injection of adipose stem cells in the treatment of rotator cuff disease - a narrative review of current evidence

The purpose of this study is to summarize evidence for the use of adipose stem cell (ASC) injections in the treatment of rotator cuff tears (RCT) and identify future areas of study. A thorough literature search was performed to identify studies investigating the use of ASC injections in the treatment of RCTs. Among animal trials, it is unclear whether ASCs are of benefit for rotator cuff repair. In clinical trials, ASC injection may reduce retear rate with otherwise equivocal clinical outcomes. Although ASC injection may be safe, the literature does not provide a clear consensus as to the efficacy of ASC injections, nor does it delineate which patients would benefit most from this treatment.