Tendon Aging: A Silent Enemy Revealed Strategies for Effective Treatment

Scavenging of reactive oxygen species can adjust the differentiation of tendon stem cells and progenitor cells and prevent ectopic calcification in tendinopathy

BackgroundTendons are traditionally thought to consist of tenocytes only, the resident cells of tendons; however, a recent study has demonstrated that human and mouse tendons also contain stem cells, referred to as tendon stem/progenitor cells (TSCs). However, the differential properties of TSCs and tenocytes remain largely undefined. This study aims to characterize the properties of these tendon cells derived from rabbits.MethodsTSCs and tenocytes were isolated from patellar and Achilles tendons of rabbits. The differentiation potential and cell marker expression of the two types of cells were examined using histochemical, immunohistochemical, and qRT-PCR analysis as well as in vivo implantation. In addition, morphology, colony formation, and proliferation of TSCs and tenocytes were also compared.ResultsIt was found that TSCs were able to differentiate into adipocytes, chondrocytes, and osteocytes in vitro, and form tendon-like, cartilage-like, and bone-like tissues in vivo. In contrast, tenocytes had little such differentiation potential. Moreover, TSCs expressed the stem cell markers Oct-4, SSEA-4, and nucleostemin, whereas tenocytes expressed none of these markers. Morphologically, TSCs possessed smaller cell bodies and larger nuclei than ordinary tenocytes and had cobblestone-like morphology in confluent culture whereas tenocytes were highly elongated. TSCs also proliferated more quickly than tenocytes in culture. Additionally, TSCs from patellar tendons formed more numerous and larger colonies and proliferated more rapidly than TSCs from Achilles tendons.ConclusionsTSCs exhibit distinct properties compared to tenocytes, including differences in cell marker expression, proliferative and differentiation potential, and cell morphology in culture. Future research should investigate the mechanobiology of TSCs and explore the possibility of using TSCs to more effectively repair or regenerate injured tendons.

Rejuvenation of tendon stem/progenitor cells for functional tendon regeneration through platelet-derived exosomes loaded with recombinant Yap1.

Tendon stem/progenitor cells (TSPCs) therapy is a promising strategy for enhancing cell matrix and collagen synthesis, and regulating the metabolism of the tendon microenvironment during tendon injury repair. Nevertheless, the barren microenvironment and gliding shear of tendon cause insufficient nutrition supply, damage, and aggregation of injected TSPCs around tendon tissues, which severely hinders their clinical application in tendinopathy. In this study, a TSPCs delivery system is developed by encapsulating TSPCs within a DNA hydrogel (TSPCs-Gel) as the DNA hydrogel offers an excellent artificial extracellular matrix (ECM) microenvironment by providing nutrition for proliferation and protection against shear forces. This delivery method restricts TSPCs to the tendons, significantly extending their retention time. It is also found that TSPCs-Gel injections can promote the healing of rat tendinopathy in vivo, where cross-sectional area and load to failure of injured tendons in rats are significantly improved compared to the free TSPCs treatment group at 8 weeks. Furthermore, the potential healing mechanism of TSPCs-Gel is investigated by RNA-sequencing to identify a series of potential gene and signaling pathway targets for further clinical treatment strategies. These findings suggest the potential pathways of using DNA hydrogels as artificial ECMs to promote cell proliferation and protect TSPCs in TSPC therapy.

Millions of people suffer from tendon injuries in both occupational and athletic settings. However, the restoration of normal structure and function to injured tendons remains one of the greatest challenges in orthopaedics and sports medicine. In recent years, several advancements have been made in tendon research that suggest the potential for more effective treatment and repair of tendon injuries. First is the discovery of tendon stem/progenitor cells (TSCs). Recent studies have suggested that TSCs may be responsible for the development of degenerative tendinopathy, a chronic tendon injury. Besides, because TSCs are tendon-specific stem cells, they can potentially be used in cell therapy to effectively repair or even regenerate injured tendons. Second, autologous platelet-rich plasma (PRP) has recently been adopted in orthopaedics and sports medicine to treat acute and chronic tendon injuries. Patients treated with PRP injections have reported a significant reduction in injury-induced pain and improvement in joint function. Finally, engineered tendon scaffolds have been shown to promote tenogenesis of TSCs in animal studies in vitro and formation of tendon-like structures in vivo; hence, they may be effectively used to enhance the repair of injured tendons. In this article, a review is provided on the mechanobiology of TSCs, the efficacy of PRP treatment for tendon injuries and the applications of tendon scaffolds to treat tendon-related disorders in clinical settings. Based on the existing data, it is recommended that a multidimensional approach combining all three tissue engineering elements - TSCs, PRP and scaffolds - be used to enhance the healing of injured tendons. Keywords: Cell therapy, mechanical loading, PRP, tendinopathy, tendon injuries, tendon scaffolds, tendon stem cells, tissue engineering.

The repair of injured tendons remains a formidable clinical challenge because of our limited understanding of tendon stem cells and the regulation of tenogenesis. With single-cell analysis to characterize the gene expression profiles of individual cells isolated from tendon tissue, a subpopulation of nestin+ tendon stem/progenitor cells (TSPCs) was identified within the tendon cell population. Using Gene Expression Omnibus datasets and immunofluorescence assays, we found that nestin expression was activated at specific stages of tendon development. Moreover, isolated nestin+ TSPCs exhibited superior tenogenic capacity compared to nestin- TSPCs. Knockdown of nestin expression in TSPCs suppressed their clonogenic capacity and reduced their tenogenic potential significantly both in vitro and in vivo. Hence, these findings provide new insights into the identification of subpopulations of TSPCs and illustrate the crucial roles of nestin in TSPC fate decisions and phenotype maintenance, which may assist in future therapeutic strategies to treat tendon disease.

Our understanding of tendon biology continues to evolve, thus leading to opportunities for developing novel, evidence-based effective therapies for the treatment of tendon disorders. Implementing the knowledge of tendon stem/progenitor cells (TSPCs) and assessing their potential in enhancing tendon repair could fill an important gap in this regard. We described different molecular and phenotypic profiles of TSPCs modulated by culture density, as well as their multipotency and secretory activities. Moreover, in the same experimental setting, we evaluated for different responses to inflammatory stimuli mediated by TNFα and IFNγ. We also preliminarily investigated their immunomodulatory activity and their role in regulating degradation of substance P. Our findings indicated that TSPCs cultured at low density (LD) exhibited cobblestone morphology and a reduced propensity to differentiate. A distinctive immunophenotypic profile was also observed with high secretory and promising immunomodulatory responses when primed with TNFα and IFNγ. In contrast, TSPCs cultured at high density (HD) showed a more elongated fibroblast-like morphology, a greater adipogenic differentiation potential, and a higher expression of tendon-related genes with respect to LD. Finally, HD TSPCs showed immunomodulatory potential when primed with TNFα and IFNγ, which was slightly lower than that shown by LD. A shift from low to high culture density during TSPC expansion demonstrated intermediate features confirming the cellular adaptability of TSPCs. Taken together, these experiments allowed us to identify relevant differences in TSPCs based on culture conditions. This ability of TSPCs to acquire distinguished morphology, phenotype, gene expression profile, and functional response advances our current understanding of tendons at a cellular level and suggests responsivity to cues in their in situ microenvironment.

Background: Tendinopathy is a pervasive clinical problem that afflicts both athletes and the general public. Although the inflammatory changes in tendinopathy are well characterized, how the therapeutic effects of platelet-rich plasma (PRP) on tendinopathy are being modulated by the inflammatory environment is not well defined. Purpose/Hypothesis: In this study, we aimed to compare the therapeutic effects of PRP alone versus a combination of PRP with a primary glucocorticoid (GC) injection at the early stage of tendinopathy. We hypothesized that PRP treatment could promote better tendon regeneration through the suppression of inflammation with GC. Study Design: Controlled laboratory study. Methods: The gene expression profile of tendon stem/progenitor cells (TSPCs) cultured with PRP was analyzed with RNA sequencing. To evaluate the cell viability, senescence, and apoptosis of TSPCs under different conditions, TSPCs were treated with 0.1 mg/mL triamcinolone acetonide (TA) and/or 10% PRP in an IL1B–induced inflammatory environment. To further verify the effects of the sequential therapy of GCs and PRP, an early tendinopathy animal model was established through a local injection of collagenase in the rabbit Achilles tendon. The tendinopathy model was then treated with isopycnic normal saline (NS group), TA (TA group), PRP (PRP group), or TA and PRP successively (TA+PRP group). At 8 weeks after treatment, the tendons were assessed with magnetic resonance imaging (MRI), histological examination, transmission electron microscopy (TEM), and mechanical testing. Results: Gene Ontology enrichment analysis indicated that PRP treatment of TPSCs induced an inflammatory response, regulated cell migration, and remodeled the extracellular matrix. Compared with the sole use of PRP, successive treatment with TA followed by PRP yielded similar results in cell viability and senescence but less cell apoptosis in vitro. In vivo experiments demonstrated that the TA+PRP group achieved significantly better tendon regeneration, as confirmed by MRI, histological examination, TEM, and mechanical testing. Conclusion: This study showed that the primary use of GCs did not exert any obvious deleterious side effects on the treated tendon but instead enhanced the regenerative effects of PRP in early inflammatory tendinopathy. Clinical Relevance: The sequential therapy of GCs followed by PRP provides a promising treatment strategy for tendinopathy in clinical practice. PRP combined with the primary use of GCs appears to promote tendon regeneration in early inflammatory tendinopathy.

Tendon harbors a cell population that possesses stem cell characteristics such as clonogenicity, multipotency and self-renewal capacity, commonly referred to as tendon stem/progenitor cells (TSPCs). Various techniques have been employed to study how TSPCs are implicated in tendon development, homeostasis and healing. Recent advances in single-cell analysis have enabled much progress in identifying and characterizing distinct subpopulations of TSPCs, which provides a more comprehensive view of TSPCs function in tendon biology. Understanding the mechanisms of physiological and pathological processes regulated by TSPCs, especially a particular subpopulation, would greatly benefit treatment of diseased tendons. Here, we summarize the current scientific literature on the various subpopulations of TSPCs, and discuss how TSPCs can contribute to tissue homeostasis and pathogenesis, as well as examine the key modulatory signaling pathways that determine stem/progenitor cell state. A better understanding of the roles that TSPCs play in tendon biology may facilitate the development of novel treatment strategies for tendon diseases.

Millions of people suffer from tendon injuries in both occupational and athletic settings. However, the restoration of normal structure and function to injured tendons still remains as one of the greatest challenges in orthopaedics and sports medicine. In recent years, a remarkable advancement in tendon research field has been the discovery of tendon stem/progenitor cells (TSCs). Unlike tenocytes, the predominant resident cell in tendons, TSCs have the ability to self-renew and multi-differentiate. Because of these distinct properties, TSCs may play a critical role in tendon physiology as well as pathology such as tendinopathy, which is a prevalent chronic tendon injury. Additionally, because TSCs are tendon-specific stem cells, they could potentially be used in tendon tissue engineering in vitro, and serve as a promising cell sourcefor cell-based therapy to effectively repair or even regenerate injured tendons in clinical settings.

BackgroundTendon injury is associated with oxidative stress, leading to reactive oxygen species (ROS) production and inflammation. N-acetyl-L-cysteine (NAC) is a potent antioxidant. However, how NAC affects the biological functions of tendon stem/progenitor cells (TSPCs) and tendon repair has not been clarified. MethodThe impacts of NAC on the viability, ROS production, and differentiation of TSPCs were determined with the cell counting kit-8, fluorescence staining, Western blotting, and immunofluorescence. The effect of NAC on gene transcription in TSPCs was analyzed by transcriptomes and bioinformatics and validated by Western blotting. The potential therapeutic effect of NAC on tendon repair was tested in a rat model of Achilles tendon injury.ResultsCompared with the untreated control, treatment with 500 µM NAC greatly promoted the proliferation of TSPCs and significantly mitigated hydrogen peroxide-induced ROS production and cytotoxicity in vitro. NAC treatment significantly increased the relative protein expression of collagen type 1 alpha 1 (COL1A1), tenascin C (TNC), scleraxis (SCX), and tenomodulin (TNMD) in TPSCs. Bioinformatics analyses revealed that NAC modulated transcriptomes, particularly in the integrin-related phosphoinositide 3-kinase (PI3K)/AKT signaling, and Western blotting revealed that NAC enhanced integrin α5β1 expression and PI3K/AKT activation in TSPCs. Finally, NAC treatment mitigated the tendon injury, but enhanced the protein expression of SCX, TNC, TNMD, and COLIA1 in the injured tissue regions of the rats.ConclusionNAC treatment promoted the survival and differentiation of TSPCs to facilitate tendon repair after tendon injury in rats. Thus, NAC may be valuable for the treatment of tendon injury.

Age-related tendon disorder, a primary motor system disease, is characterized by biological changes in the tendon tissue due to senescence and seriously affects the quality of life of the elderly. The pathogenesis of this disease is not well-understood. Tendon stem/progenitor cells (TSPCs) exhibit multi-differentiation capacity. These cells are important cellular components of the tendon because of their roles in tendon tissue homeostasis, remodeling, and repair. Previous studies revealed alterations in the biological characteristics and tenogenic differentiation potential of TSPCs in senescent tendon tissue, in turn contributing to insufficient differentiation of TSPCs into tenocytes. Poor tendon repair can result in age-related tendinopathies. Therefore, targeting of senescent TSPCs may restore the tenogenic differentiation potential of these cells and achieve homeostasis of the tendon tissue to prevent or treat age-related tendinopathy. In this review, we summarize the biological characteristics of TSPCs and histopathological changes in age-related tendinopathy, as well as the potential mechanisms through which TSPCs contribute to senescence. This information may promote further exploration of innovative treatment strategies to rescue TSPCs from senescence.

Bone marrow mesenchymal stem cell-derived exosomes promote tendon regeneration by facilitating the proliferation and migration of endogenous tendon stem/progenitor cells

An asymmetric chitosan scaffold for tendon tissue engineering: In vitro and in vivo evaluation with rat tendon stem/progenitor cells

Patients with diabetes face an increased risk of developing several tendon disorders, such as tendinopathy, tendon rupture, and impaired tendon healing. Tendon stem/progenitor cells (TSPCs) play a crucial role in maintaining tendon tissue homeostasis and facilitating tendon healing. However, under diabetic conditions, TSPC dysfunction contributes to the development and progression of tendinopathy or tendon injury. Despite this, effective treatments remain limited. This study aims to investigate the potential of prim-O-glucosylcimifugin (POG) in preventing high glucose (HG)-induced senescence and restoring the impaired regenerative phenotype of TSPCs. The results reveal that HG stimulation induces TSPC senescence, characterized by impaired self-renewal capacity, increased expression of senescence markers, and reduced tenogenic differentiation potential. Notably, treatment with POG counteracts HG-induced senescence, restoring the impaired tenogenic differentiation capacity through AMP-activated protein kinase (AMPK) pathway activation. To assess the in vivo effect of POG, mesoporous silica nanoparticles are employed for the local delivery of POG. This approach efficiently promotes tendon healing in diabetic mice with partial-cut-induced tendon injury. Moreover, the combination of POG and biomimetic scaffold transplantation functionally rescues endogenous tendon regeneration and repair capacities in diabetic mice. In conclusion, pharmacological intervention with POG can rescue HG-induced TSPC hypofunction and promote tendon healing under diabetic conditions.