The role of the macrophage in tendinopathy and tendon healing

Berberine alleviates tendinopathy by suppressing the cGAS-STING pathway and Relieving ferroptosis

Berberine, a key bioactive component of Coptis rhizome, has been extensively studied for its therapeutic effects on various diseases. This research aimed to investigate the potential benefits of berberine in treating tendinopathy and to elucidate the underlying mechanisms through animal and laboratory studies. Our findings indicated that berberine effectively treated type I collagenase-induced tendinopathy in rats, confirmed by cellular-level validation. At the molecular level, berberine reduced the activation of the cGAS-STING signaling pathway and decreased the accumulation of malondialdehyde (MDA) and reactive oxygen species (ROS) in both animal models and cell cultures. Additionally, berberine upregulated the expression of glutathione (GSH) and glutathione peroxidase 4 (GPX4) in tissues. These results suggested that berberine alleviated ferroptosis via the cGAS-STING pathway, thus exerting therapeutic effects on tendinopathy. To validate these findings further, we administered the ferroptosis inducer Imidazole Ketone Erastin (IKE) to evaluate the effects of berberine. IKE significantly diminished the therapeutic effects of berberine on tendinopathy, as indicated by the previously mentioned markers. Thus, berberine mitigated ferroptosis by inhibiting the cGAS-STING pathway, highlighting its potential in managing tendinopathy.

Adipose derived stem cell activation by macrophages and tendon fibroblasts

AIMS

Tendon injuries are common, and healing often fails due to an over-exuberant inflammatory response and a lack of regeneration. Inflammatory cells play key roles in these processes, with a balance between classically activated pro-inflammatory M1 macrophages and alternatively activated inflammatory resolving M2 macrophages. Adipose-derived mesenchymal stem cells (ASCs) can dampen the pro-inflammatory effectsof macrophages, promote a regenerative environment, and enhance healing. Therefore, the goal of the study was to understand how ASCs are activated by macrophages in vitro.

METHODS

In vitro co-culture experiments were carried out with ASCs, macrophages, and tendon fibroblasts. RNA-seq and qRT-PCR were performed to determine expression patterns of activated ASCs.

RESULTS

M1 macrophages prompted ASCs to upregulate pro-inflammatory signaling, matrix remodeling, and cytokine production pathways, while downregulating those related to cell adhesion and cell cycle. Conversely, TFs prompted ASCs to upregulate pathways involved in cell cycle and cytoskeleton remodeling, and to downregulate pathways associated with immune cell adhesion, inflammatory mediator production, and protein metabolism.

CONCLUSIONS

The cell-specific activation profiles indicate a possible switch in ASC paracrine signaling depending on the context, from a pro-inflammatory pattern in response to M1 macrophages to a proliferative pattern in response to TFs. Understanding crosstalk between ASCs, TFs, and macrophages is essential for developing stem cell-based therapeutic strategies.

Polylactic acid electrospun membranes coated with chiral hierarchical-structured hydroxyapatite nanoplates promote tendon healing based on a macrophage-homeostatic modulation strategy

Tendon injury is a common and challenging problem in the motor system that lacks an effective treatment, affecting daily activities and lowering the quality of life. Limited tendon regenerative capability and immune microenvironment dyshomeostasis are considered the leading causes hindering tendon repair. The chirality of biomaterials was proved to dictate immune microenvironment and dramatically affect tissue repair. Herein, chiral hierarchical structure hydroxylapatite (CHAP) nanoplates are innovatively synthesized for immunomodulatory purposes and further coated onto polylactic acid electrospinning membranes to achieve long-term release for tendon regeneration adaption. Notably, levorotatory-chiral HAP (L-CHAP) nanoplates rather than dextral-chiral or racemic-chiral exhibit good biocompatibility and bioactivity. In vitro experiments demonstrate that L-CHAP induces macrophage M2 polarization by enhancing macrophage efferocytosis, which alleviates inflammatory damage to tendon stem cells (TDSCs) through downregulated IL-17-NF-κB signaling. Meanwhile, L-CHAP-mediated macrophage efferocytosis also promotes TDSCs proliferation and tenogenic differentiation. By establishing a rat model of Achilles tendon injury, L-CHAP was demonstrated to comprehensively promoting tendon repair by enhancing macrophage efferocytosis and M2 polarization in vivo, finally leading to improvement of tendon ultrastructural and mechanical properties and motor function. This novel strategy highlights the role of L-CHAP in tendon repair and thus provides a promising therapeutic strategy for tendon injury.

Restoring tendon microenvironment in tendinopathy: Macrophage modulation and tendon regeneration with injectable tendon hydrogel and tendon-derived stem cells exosomes

Tendinopathy is a common musculoskeletal disorder in which a significant number of patients do not attain effective therapeutic outcomes. The extent of the inflammatory response and the dynamics of collagen synthesis metabolism are critical factors that influence the intrinsic self-repair capacity of tendons. However, the poor microenvironment within the tendon significantly impedes the self-repair process in tendinopathy. In this study, an injectable tendon-derived decellularized extracellular matrix (tdECM) hydrogel was utilized to treat tendinopathy. This hydrogel provides a more cytocompatible microenvironment while retaining certain bioactive factors of native tendon extracellular matrix (ECM), compared to collagen hydrogel. Notably, it was discovered for the first time that the tdECM hydrogel promotes M2 macrophage polarization, thereby exerting an anti-inflammatory effect in vivo. Furthermore, utilizing tdECM as a carrier for the sustained release of tendon-derived stem cells exosomes (TDSCs-Exos), our findings from both in vitro and in vivo studies indicate that the tdECM hydrogel, in conjunction with exosomes, demonstrated a pronounced synergistic enhancement in modulating inflammation, promoting M2 macrophage polarization, and facilitating tendon regeneration and repair efficacy. These results suggest its potential as a promising therapeutic strategy for tendon disorders.

Pathophysiology and healing of insertional Achilles tendinopathy: Current concepts

Insertional Achilles tendinopathy (IAT) is a challenging condition that significantly impacts athletes and physically active individuals, often leading to chronic pain and impaired performance. IAT is characterized by a complex interplay of mechanical stress, vascular impairment, inflammatory responses, and extracellular matrix (ECM) dysregulation at the Achilles tendon insertion. This review integrates recent advancements in the understanding of IAT pathophysiology, with focus on the effects of tensile and compressive loads, intratendinous pressure changes, tissue hypoxia, and ECM water balance. Emerging evidence indicates that mechanical loading influences tendon homeostasis through mechanotransduction, leading to ECM remodeling and fibrocartilaginous adaptation. Although appropriate compressive loading is necessary to maintain ECM homeostasis and fibrocartilage regeneration, excessive or abnormal loading disrupts tendon repair mechanisms and contributes to degenerative changes. Furthermore, increased intra-tendinous pressure impairs capillary perfusion, thereby promoting a hypoxic microenvironment that exacerbates the inflammatory response. Dysregulated water retention due to glycosaminoglycans (GAGs) and hyaluronic acid affects intra-tendinous pressure, highlighting potential therapeutic strategies targeting ECM hydration. This review also explores the roles of macrophage polarization, cytokine regulation, and growth factors in tendon healing, emphasizing their potential therapeutic implications. By integrating the anatomical, biomechanical, and molecular insights, this review provides a comprehensive perspective of IAT pathophysiology and its healing mechanisms. Understanding these mechanisms is essential to optimizing conservative treatments, refining surgical approaches, and developing novel therapeutic strategies to enhance tendon repair and prevent disease progression.

Obesity: pathophysiology and therapeutic interventions

Over the past few decades, obesity has transitioned from a localized health concern to a pressing global public health crisis affecting over 650 million adults globally, as documented by WHO epidemiological surveys. As a chronic metabolic disorder characterized by pathological adipose tissue expansion, chronic inflammation, and neuroendocrine dysregulation that disrupts systemic homeostasis and impairs physiological functions, obesity is rarely an isolated condition; rather, it is frequently complicated by severe comorbidities that collectively elevate mortality risks. Despite advances in nutritional science and public health initiatives, sustained weight management success rates and prevention in obesity remain limited, underscoring its recognition as a multifactorial disease influenced by genetic, environmental, and behavioral determinants. Notably, the escalating prevalence of obesity and its earlier onset in younger populations have intensified the urgency to develop novel therapeutic agents that simultaneously ensure efficacy and safety. This review aims to elucidate the pathophysiological mechanisms underlying obesity, analyze its major complications-including type 2 diabetes mellitus (T2DM), cardiovascular diseases (CVD), non-alcoholic fatty liver disease (NAFLD), obesity-related respiratory disorders, obesity-related nephropathy (ORN), musculoskeletal impairments, malignancies, and psychological comorbidities-and critically evaluate current anti-obesity strategies. Particular emphasis is placed on emerging pharmacological interventions, exemplified by plant-derived natural compounds such as berberine (BBR), with a focus on their molecular mechanisms, clinical efficacy, and therapeutic advantages. By integrating mechanistic insights with clinical evidence, this review seeks to provide innovative perspectives for developing safe, accessible, and effective obesity treatments.

Functional tendon regeneration is driven by regulatory T cells and IL-33 signaling

Tendon injuries heal by scar, leading to poor function. To date, the role of immune cells remains underexplored. Using a neonatal mouse model of functional tendon healing compared to adult scar-mediated healing, we identified a regenerative immune profile that is associated with type 1 inflammation followed by rapid polarization to type 2, driven by macrophages and regulatory T cells (Treg cells). Single-cell and bulk RNA sequencing also revealed neonatal Treg cells with an immunomodulatory signature distinct from adult. Neonatal Treg cell ablation resulted in a dysregulated immune response, failed tenocyte recruitment, and impaired regeneration. Adoptive transfer further confirmed the unique capacity of neonatal Treg cells to rescue functional regeneration. We showed that neonatal Treg cells mitigate interleukin-33 (IL-33) to enable tenocyte recruitment and structural restoration, and that adult IL-33 deletion improves functional healing. Collectively, these findings demonstrate that Treg cells and IL-33 immune dysfunction are critical components of failed tendon healing and identify potential targets to drive tendon regeneration.

PI3K-Akt signalling regulates Scx-lineage tenocytes and Tppp3-lineage paratenon sheath cells in neonatal tendon regeneration

Tendon injuries are frequently occurring disorders; it is clinically important to enhance tendon regeneration and prevent functional impairment post-injury. While tendon injuries in children heal quickly with minimal scarring, those in adults heal slowly and are accompanied by fibrotic scarring. Therefore, investigating the healing mechanisms after tendon injury, and identifying the factors that regulate the inherent regenerative capacity of tendons are promising approaches to promoting tendon regeneration. Here, we identify that the PI3K-Akt signalling pathway is preferentially upregulated in injured neonatal murine Achilles tendons. Inhibition of PI3K-Akt signalling in a neonatal murine Achilles tendon rupture model decreases cell proliferation and migration in both Scx-lineage intrinsic tenocytes and Tppp3-lineage extrinsic paratenon sheath cells. Moreover, the inhibition of PI3K-Akt signalling decreases stemness and promotes mature tenogenic differentiation in both Scx- and Tppp3-lineage cells. Collectively, these results suggest that PI3K-Akt signalling plays a pivotal role in neonatal tendon regeneration.

Injectable Piezoelectric Hydrogel Promotes Tendon-Bone Healing via Reshaping the Electrophysiological Microenvironment and M2 Macrophage Polarization

Rotator cuff tear (RCT) is a common musculoskeletal disease that poses challenges for functional regeneration of the tendon-bone interface (TBI). The transition of TBI between soft and hard tissues determines its structural and physiological environment complexity. Here, we present an injectable biopiezoelectric material PVA/CNF/BTO@PDA (Piezoelectric) hydrogel based on three-dimensional (3D) printing inspired by the "muscle-electrical coupling". This Piezoelectric hydrogel indicated desirable piezoelectric and mechanical properties, excellent biodegradability, and biosafety. In vitro, electrical stimulation from Piezoelectric hydrogel by the Flexcell Tissue Train system promoted the polarization of macrophages to the M2 phenotype, directing the targeted aggregation and zonal-specific differentiation of bone mesenchymal stem cells (BMSCs) for TBI formation. Also, optimal piezoelectric stimulation of the Piezoelectric hydrogel could alleviate inflammatory factor expression and regulate the osteotendinogenic differentiation of BMSCs under an H2O2/IL-1β inflammation environment. Furthermore, in vivo application of injectable Piezoelectric hydrogel demonstrates its regenerative potential, indicating that physiological repair with Piezoelectric hydrogel significantly accelerates and promotes TBI healing in a chronic RCT model. Therefore, our findings propose a new therapeutic strategy for functional TBI regeneration and enhance the treatment outcomes for RCT.

Reactive oxygen species in tendon injury and repair

Reactive oxygen species (ROS) are chemical moieties that in physiological concentrations serve as fast-acting signaling molecules important for cellular homeostasis. However, their excess either due to overproduction or inability of the antioxidant system to inactivate them results in oxidative stress, contributing to cellular dysfunction and tissue damage. In tendons, which are hypovascular, hypocellular, and composed predominantly of extracellular matrix (ECM), particularly collagen I, ROS likely play a dual role: regulating cellular processes such as inflammation, proliferation, and ECM remodeling under physiological conditions, while contributing to tendinopathy and impaired healing when dysregulated. This review explores the sources of ROS in tendons, including NADPH oxidases and mitochondria, and their role in key processes such as tissue adaptation to mechanical load and injury repair, also in systemic conditions such as diabetes. In addition, we integrate the emerging perspective that calcium signaling-mediated by mechanically activated ion channels-plays a central role in tendon mechanotransduction under daily mechanical loads. We propose that mechanical overuse (overload) may lead to hyperactivation of calcium channels, resulting in chronically elevated intracellular calcium levels that amplify ROS production and oxidative stress. Although direct evidence linking calcium channel hyperactivity, intracellular calcium dysregulation, and ROS generation under overload conditions is currently circumstantial, this review aims to highlight these connections and identify them as critical avenues for future research. By framing ROS within the context of both adaptive and maladaptive responses to mechanical load, this review provides a comprehensive synthesis of redox biology in tendon injury and repair, paving the way for future work, including development of therapeutic strategies targeting ROS and calcium signaling to enhance tendon recovery and resilience.

Procyanidin-crosslinked gradient silk fibroin composite nanofiber scaffold with sandwich structure for rotator cuff repair

Improving the regeneration of the tendon-bone interface (TBI) helps to decrease the risk of rotator cuff retears after repair surgeries. Unfortunately, the lack of inherent healing capacity of the TBI, insufficient mechanical properties, and abnormal and persistent inflammation during repair are the key factors leading to suboptimal healing of the rotator cuff. Therefore, a high-strength rotator cuff repair material capable of regulating the unbalanced immune response and enhancing the regeneration of the TBI is urgently needed. In this study, a novel sandwiched silk fibroin composite nanofiber scaffold with a biomimetic gradient structure was prepared through layer-by-layer continuous electrospinning, and then procyanidin was utilized to further enhance the mechanical properties and biological activities of the scaffold. The physicochemical characterization revealed that the procyanidin-crosslinked sandwiched gradient scaffold (GMPC) possessed an appropriate porosity and pore size and superior mechanical properties. Cytocompatibility assessment and immunofluorescence staining indicated that GMPC allowed rapid adhesion, proliferation, and infiltration of osteoblasts. ELISA and macrophage polarization experiments further confirmed that GMPC could effectively inhibit excessive inflammation in injured tissues and regulate the polarization of macrophages to the beneficial phenotype. Therefore, the procyanidin-crosslinked sandwiched gradient nanofiber scaffold might be a promising candidate for rotator cuff repair.

Extracellular Vesicles from Adipose-Derived Mesenchymal Stem Cells Improve Ligament-Bone Integration After Anterior Cruciate Ligament Primary Repair in Rabbit

BACKGROUNDS

In this research, we want to find out whether extracellular vesicles (EVs) from adipose-derived mesenchymal stem cells (MSCs) can improve ligament-bone integration after primary Anterior Cruciate Ligament (ACL) repair by performing immunological and biomechanical tests.

METHODS

All of the rabbits underwent ACL resection at the proximal attachment to the femur bone, and then were divided into four groups. We performed an ELISA examination from the tissue at the bone-ligament interface of iNOS, CD206, MMP-3, and TIMP-1 to evaluate their levels at the inflammatory stage at the end of the first week. Immunoexpression of type I and III collagen and failure load biomechanical tests were performed at the end of the sixth week.

RESULT

The group that underwent ACL repair with EVs augmentation had significantly higher levels of CD206, significantly lower MMP-3 levels, and significantly higher TIMP-1 levels in the first week. The iNOS levels in the group that underwent ACL repair with EVs augmentation were significantly different compared to the control group that did not receive any. The number of type I collagen fibers and the failure load levels in the group that underwent ACL repair with EVs augmentation were significantly higher.

CONCLUSIONS

EVs from adipose-derived MSCs can improve the outcome of primary ACL repair in rabbits by regulating the inflammatory process during the healing period.

Efficacy of Light-Emitting Diode-Mediated Photobiomodulation in Tendon Healing in a Murine Model

The application of light-emitting diode (LED)-dependent photobiomodulation (PBM) in promoting post-tendon injury healing has been recently reported. Despite establishing a theoretical basis for ligament restoration through PBM, identifying effective LED wavelength combinations and ensuring safety in animal models remain unresolved challenges. In our previous study, we demonstrated that combined irradiation at 630 nm and 880 nm promotes cell proliferation and migration, which are critical processes during the early stage of tendon healing in human-derived tendon fibroblasts. Based on this, we hypothesized that 630/880 nm LED-based PBM might promote rapid healing during the initial phase of tendon healing, and we aimed to analyze the results after PBM treatment in a murine model. Migration kinetics were analyzed at two specific wavelengths: 630 and 880 nm. The Achilles tendon in the hind limbs of Balb/c mice was severed by Achilles tendon transection. Subsequently, the mice were randomized into LED non-irradiation and LED irradiation groups. Mice with intact tendons were employed as healthy controls. The total number of mice was 13 for the healthy and injured groups and 14 for the LED-irradiated injured group, and the data presented in this manuscript were obtained from one representative experiment (n = 4-5 per group). The wounds were LED-irradiated for 20 min daily for two days. Histological properties, tendon healing mediators, and inflammatory mediators were screened on day 14. The roundness of the nuclei and fiber structure, indicating the degree of infiltrated inflammatory cells and severity of fiber fragmentation, respectively, were lower in the LED irradiation group than in the LED non-irradiation group. Immunohistochemical analysis depicted an increase in tenocytes (SCX+ cells) and recovery of wounds with reduced fibrosis (lower collagen 3 and TGF-β1) in the LED irradiation group during healing; conversely, the LED non-irradiation group exhibited tissue fibrosis. Overall, the ratio of M2 macrophages to total macrophages in the LED irradiation group was higher than that in the injured group. LED-based PBM in the Achilles tendon rupture murine model facilitated a rapid restoration of histological and immunochemical outcomes. These findings suggest that LED-based PBM presents remarkable potential as an adjunct therapeutic approach for tendon healing and warrants further research to standardize various parameters to advance and establish it as a reliable treatment regimen.

Tissue Mimetic Membranes for Healing Augmentation of Tendon-Bone Interface in Rotator Cuff Repair

The globally prevalent rotator cuff tear has a high re-rupture rate, attributing to the failure to reproduce the interfacial fibrocartilaginous enthesis. Herein, a hierarchically organized membrane is developed that mimics the heterogeneous anatomy and properties of the natural enthesis and finely facilitates the reconstruction of tendon-bone interface. A biphasic membrane consisting of a microporous layer and a mineralized fibrous layer is constructed through the non-solvent induced phase separation (NIPS) strategy followed by a co-axial electrospinning procedure. Cationic kartogenin (KGN)-conjugated nanogel (nGel-KGN) and osteo-promotive struvite are incorporated within the membranes in a region-specific manner. During in vivo repair, the nGel-KGN-functionalized microporous layer is adjacent to the tendon which intends to suppress scar tissue formation at the lesion and simultaneously heightens chondrogenesis. Meanwhile, the struvite-containing fibrous layer covers the tubercula minus to enhance stem cell aggregation and bony ingrowth. Such tissue-specific features and spatiotemporal release behaviors contribute to effective guidance of specific defect-healing events at the transitional region, further leading to the remarkably promoted regenerative outcome in terms of the fibrocartilaginous tissue formation, collagen fiber alignment, and optimized functional motion of rotator cuff. These findings render a novel biomimetic membrane as a promising material for clinical rotator cuff repair.

Erroneous Differentiation of Tendon Stem/Progenitor Cells in the Pathogenesis of Tendinopathy: Current Evidence and Future Perspectives

Tendinopathy is a condition characterized by persistent tendon pain, structural damage, and compromised functionality. Presently, the treatment for tendinopathy remains a formidable challenge, partly because of its unclear pathogenesis. Tendon stem/progenitor cells (TSPCs) are essential for tendon homeostasis, regeneration, remodeling, and repair. An innovative theory has been previously proposed, with insufficient evidence, that the erroneous differentiation of TSPCs may constitute one of the fundamental mechanisms underpinning tendinopathy. Over the past few years, there has been accumulating evidence for plausibility of this theory. In this review, we delve into alterations in the differentiation potential of TSPCs and the underlying mechanisms in the context of injury-induced tendinopathy, diabetic tendinopathy, and age-related tendinopathy to provide updated evidence on the erroneous differentiation theory. Despite certain limitations inherent in the existing body of evidence, the erroneous differentiation theory emerges as a promising and highly pertinent avenue for understanding tendinopathy. In the future, advanced methodologies will be harnessed to further deepen comprehension of this theory, paving the way for prospective developments in clinical therapies targeting TSPCs for the management of tendinopathy.

Suramin enhances proliferation, migration, and tendon gene expression of human supraspinatus tenocytes

Rotator cuff tendinopathy is a common musculoskeletal disorder with limited pharmacological treatment strategies. This study aimed to investigate tenocytes' functional in vitro response from a ruptured supraspinatus tendon to suramin administration and to elucidate whether suramin can enhance tendon repair and modulate the inflammatory response to injury. Tenocytes were obtained from human supraspinatus tendons (n = 6). We investigated the effect of suramin on LPS-induced inflammatory responses and the underlying molecular mechanisms in THP-1 macrophages. Suramin enhanced the proliferation, cell viability, and migration of tenocytes. It also increased the protein expression of PCNA and Ki-67. Suramin-treated tenocytes exhibited increased expression of COL1A1, COL3A1, TNC, SCX, and VEGF. Suramin significantly reduced LPS-induced iNOS, COX2 synthesis, inflammatory cytokine TNF-α production, and inflammatory signaling by influencing the NF-κB pathways in THP-1 cells. Our results suggest that suramin holds great promise as a therapeutic option for treating rotator cuff tendinopathy.

Pharmacological antagonism of Ccr2+ cell recruitment to facilitate regenerative tendon healing

Successful tendon healing requires sufficient deposition and remodeling of new extracellular matrix at the site of injury, with this process mediating in part through fibroblast activation via communication with macrophages. Moreover, resolution of healing requires clearance or reversion of activated cells, with chronic interactions with persistent macrophages impairing resolution and facilitating the conversion to fibrotic healing. As such, modulation of the macrophage environment represents an important translational target to improve the tendon healing process. Circulating monocytes are recruited to sites of tissue injury, including the tendon, via upregulation of cytokines including Ccl2, which facilitates recruitment of Ccr2+ macrophages to the healing tendon. Our prior work has demonstrated that Ccr2-/- can modulate fibroblast activation and myofibroblast differentiation. However, this approach lacked temporal control and resulted in healing impairments. Thus, in the current study we have leveraged a Ccr2 antagonist to blunt macrophage recruitment to the healing tendon in a time-dependent manner. We first tested the effects of Ccr2 antagonism during the acute inflammatory phase and found that this had no effect on the healing process. In contrast, Ccr2 antagonism during the early proliferative/granulation tissue period resulted in significant improvements in mechanical properties of the healing tendon. Collectively, these data demonstrate the temporally distinct impacts of modulating Ccr2+ cell recruitment and Ccr2 antagonism during tendon healing and highlight the translational potential of transient Ccr2 antagonism to improve the tendon healing process.

Editorial Commentary: Suppression of Inflammatory Macrophages Is a Potential Strategy to Improve Rotator Cuff Healing and Has Shown Promise in Preclinical Models

The pathophysiology of rotator cuff disease is complex, involving intrinsic and extrinsic factors that contribute to mechanical alterations, inflammation, apoptosis, and neovascularization. These changes result in structural and cellular disruptions, including inflammatory cell infiltration and collagen disorganization. Macrophages recently have gained attention as critical mediators of tissue repair and regeneration. M1 macrophages traditionally have been associated with proinflammatory cytokines involved in the acute inflammatory process after injury, whereas M2 macrophages are thought to play a role in resolution of inflammation and tissue healing. Therefore, achieving a balance between M1 and M2 macrophage phenotypes may be crucial in influencing tendon healing outcomes. Strategies have ranged from mediating circulating macrophage recruitment with CCR2 inhibition to promoting M2 macrophage polarization, increasing secretion of transforming growth factor-β1 from M2 macrophages, and subsequently enhancing chondrogenesis of mesenchymal progenitor cells to improve tendon-to-bone healing. Modulating macrophage activity to favor the M2 phenotype also has been hypothesized to not only enhance healing but also to reduce adhesion formation, making it an attractive potential therapeutic strategy for tendon injuries. However, inflammation is complex and multifactorial, and identifying the optimal targets to modulate and at what time points in the healing process can be difficult. In addition, although preclinical models of tendon disorders can be helpful in identifying promising cellular and molecular targets, recapitulating the human disease process, which often consists of chronic, degenerative tendinopathies, remains challenging. Many studies use young, healthy small animal models with acute injuries, which do not fully recreate the chronic degenerative conditions commonly seen in human rotator cuff injuries. In addition, recent studies have used aged mice (∼18 to 20 months), which, although expensive, are likely closer in biological age relative to human patients and thus more representative of the changes in microstructure and composition seen in degenerative rotator cuff pathology.

Primary Human Macrophage and Tenocyte Tendon Healing Phenotypes Changed by Exosomes Per Cell Origin

The high failure rate of surgical repair for tendinopathies has spurred interest in adjunct therapies, including exosomes (EVs). Mesenchymal stromal cell (MSC)-derived EVs (MSCdEVs) have been of particular interest as they improve several metrics of tendon healing in animal models. However, research has shown that EVs derived from tissue-native cells, such as tenocytes, are functionally distinct and may better direct tendon healing. To this end, we investigated the differential regulation of human primary macrophage transcriptomic responses and cytokine secretion by tenocyte-derived EVs (TdEVs) compared with MSCdEVs. Compared with MSCdEVs, TdEVs upregulated TNFa-NFkB and TGFB signaling and pathways associated with osteoclast differentiation in macrophages while decreasing secretion of several pro-inflammatory cytokines. Conditioned media of these TdEV educated macrophages drove increased tenocyte migration and decreased MMP3 and MMP13 expression. In contrast, MSCdEV education of macrophages drove increased gene expression pathways related to INFa, INFg and protection against oxidative stress while increasing cytokine expression of MCP1 and IL6. These data demonstrate that EV cell source differentially impacts the function of key effector cells in tendon healing and that TdEVs, compared with MSCdEVs, promote a more favorable tendon healing phenotype within these cells.

Biologic Adjuvants to Rotator Cuff Repairs Induce Anti-inflammatory Macrophage 2 Polarization and Reduce Inflammatory Macrophage 1 Polarization In Vitro

PURPOSE

To examine the effect of various biologic adjuvants on the polarization of macrophages in an in vitro model for rotator cuff tears.

METHODS

Tissue was harvested from 6 patients undergoing arthroscopic rotator cuff repair. An in vitro model of the supraspinatus and subacromial bursa was created and treated with control, platelet-rich plasma (PRP), autologous activated serum (AAS), or a combination of PRP+AAS. The effect of treatment on macrophage polarization between M1 proinflammatory macrophages or M2 anti-inflammatory macrophages was measured using gene expression, protein expression, flow cytometry, and nitric oxide production.

RESULTS

Tendon and bursa treated with PRP, AAS, and PRP+AAS significantly decreased the gene expression of M1 markers interleukin (IL)-12 and tumor necrosis factor-alpha while significantly increasing the expression of M2 markers arginase, IL-10, and transforming growth factor-β (P < .05) compared with treatment with control. Enzyme-linked immunosorbent assay analysis of protein production demonstrated that, compared with control, coculture treated with PRP, AAS, and PRP+AAS significantly decreased markers of M1-macrophages (IL-6, IL-12, and tumor necrosis factor-alpha) while significantly increasing the expression of markers of M2-macrophages (arginase, IL-10, and transforming growth factor-beta) (P < .05). Flow cytometry analysis of surface markers demonstrated that compared with control, tendon and bursa treated with PRP, AAS, and PRP+AAS significantly decreased markers of M1-macrophages (CD80, CD86, CD64, CD16) while significantly increasing the expression of markers of M2-macrophages (CD163 and CD206) (P < .05). Treatment of the coculture with PRP, AAS, and PRP+AAS consistently demonstrated a decrease in nitric oxide production (P < .05) compared with control. AAS and PRP+AAS demonstrated an increased macrophage shift to M2 compared with PRP alone, whereas there was not as uniform of a shift when comparing PRP+AAS with AAS alone.

CONCLUSIONS

In an in vitro model of rotator cuff tears, the treatment of supraspinatus tendon and subacromial bursa with PRP, AAS, and PRP+AAS demonstrated an increase in markers of anti-inflammatory M2-macrophages and a concomitant decrease in markers of proinflammatory M1-macrophages. AAS and PRP+AAS contributed to a large shift to macrophage polarization to the anti-inflammatory M2 compared with PRP.

CLINICAL RELEVANCE

The mechanism of biologic adjuvant effects on the rotator cuff remains poorly understood. This study suggests that they may contribute to polarization of macrophages for their proinflammatory (M1) state to the anti-inflammatory (M2) state.

Advances focusing on the application of decellularization methods in tendon-bone healing

BACKGROUND

The tendon or ligament is attached to the bone by a triphasic but continuous area of heterogeneous tissue called the tendon-bone interface (TBI). The rapid and functional regeneration of TBI is challenging owing to its complex composition and difficulty in self-healing. The development of new technologies, such as decellularization, has shown promise in the regeneration of TBI. Several ex vivo and in vivo studies have shown that decellularized grafts and decellularized biomaterial scaffolds achieved better efficacy in enhancing TBI healing. However further information on the type of review that is available is needed.

AIM OF THE REVIEW

In this review, we discuss the current application of decellularization biomaterials in promoting TBI healing and the possible mechanisms involved. With this work, we would like to reveal how tissues or biomaterials that have been decellularized can improve tendon-bone healing and to provide a theoretical basis for future related studies.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

Decellularization is an emerging technology that utilizes various chemical, enzymatic and/or physical strategies to remove cellular components from tissues while retaining the structure and composition of the extracellular matrix (ECM). After decellularization, the cellular components of the tissue that cause an immune response are removed, while various biologically active biofactors are retained. This review further explores how tissues or biomaterials that have been decellularized improve TBI healing.

The 'Tree trunk and root' model: key imaging findings may anatomically differentiate axial psoriatic arthritis and DISH from axial spondyloarthropathy

Variable axial skeleton inflammation and axial skeleton tissue remodelling with aberrant ligamentous soft-tissue ossification occurs across the axial spondyloarthritis (ax-SpA) axial psoriatic arthritis (ax-PsA) and the diffuse idiopathic skeletal hyperostosis (DISH) spectrum. In this article, we show how imaging has resulted in an enthesis-centric model for different disease pathology compartmentalisation or a 'root and trunk' model for pathological process development. Whilst ankylosing spondylitis is predominantly characterised by early entheseal bony anchorage-related osteitis (root inflammation) and DISH is characterised by ligamentous soft-tissue ossification, ax-PsA is more heterogenous. Whilst ax-PsA may share an identical osteitis pattern to ax-SpA, a substantial proportion of ax-PsA cases have a soft tissue or tree trunk pathology that manifests as back pain with lack of osteitis but prominent ligamentous trunk ossification at later stages. We illustrate this using different imaging modalities to create a base for imaging research to elucidate this pattern of pathology.

Inhibition of peritendinous adhesion through targeting JAK2-STAT3 signaling pathway: The therapeutic potential of AG490

Peritendinous adhesion is a common complication following tendon injury repair, posing a significant clinical challenge that requires urgent attention. The primary cause of peritendinous adhesion is the excessive deposition of collagen matrix due to the abnormal proliferation of fibroblasts in an inflammatory state. Janus kinase2 (JAK2) and signal transducer and activator of transcription 3 (STAT3) are key signaling molecules involved in cell proliferation and fibrosis development in various organs. However, the role of the JAK-2 and STAT3 signaling pathways in peritendinous adhesion fibrosis remains unclear. In our study, we first observed upregulation of p-JAK2 and p-STAT3 proteins in human peritendinous adhesion specimens and rat peritendinous adhesion models. In vitro, the JAK2/STAT3 pathway inhibitor AG490 effectively inhibited TGF-β1-induced fibroblast proliferation. Wound healing and transwell assays demonstrated that AG490 suppressed TGF-β1-induced fibroblast migration. Furthermore, we found that AG490 decreased the expression of pro-inflammatory factors, including IL-1β and TNF-α, as well as extracellular matrix (ECM) proteins in fibroblasts under TGF-β1 stimulation. In vivo, histological staining showed that AG490 prevented fibrous tissue formation in a rat model of tendon injury. Moreover, AG490 inhibited the overexpression of pro-inflammatory factors IL-1β and TNF-α, as well as ECM in the peritendinous adhesions. In conclusion, AG490 inhibited fibrosis and inflammation in injured tendons by targeting the JAK2-STAT3 signaling pathway, presenting a promising strategy for the prophylaxis of peritendinous adhesions.

CCL4L2 participates in tendinopathy progression by promoting macrophage inflammatory responses: a single-cell analysis

BACKGROUND

Tendinopathy is very common in clinical practice, which is highly prevalent in athletes, sports enthusiasts and other people involved in high-load weight-bearing activities. Common types of tendinopathy include rotator cuff injury, Achilles tendinitis, tennis elbow and so on. Macrophages (Macs) are key immune cells in the pathogenesis of tendinopathy. In this study, CCL4L2+ M1-related signaling pathways were screened by combining single-cell RNA sequencing (scRNA-seq) to explore their significance in tendinopathy treatment.

METHODS

Immune cell populations were screened by Uniform Manifold Approximation and Projection (UMAP) downscaling, and Mac cell subsets were annotated using cell marker genes. The cellular communication mechanism between different cellular subsets such as Macs and tendon stem/progenitor cells (TSPCs) was demonstrated by cellular communication analysis. Based on cell marker genes of CCL4L2 + M1, Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment were performed to compare the expression differences in M1 and M2 between the Disease and Healthy groups. Associations between CCL4L2+ M1 and TSPCs were inferred by cell-cell communication analysis. The effects of CCL4L2 on Mac polarization and TSPCs were verified by enzyme-linked immunosorbent assay (ELISA) and real-time fluorescence quantitative PCR (qPCR).

RESULTS

The proportions of TSPCs, endothelial cells (ECs), smooth muscle cells (SMCs), and immune cells were significantly elevated in the Disease group. The proportion of M1 cells in the Disease group was higher than that in the Healthy group, while the proportion of M2 cells was lower than that in the Healthy group. M1 differentially expressed genes (DEGs) were mainly enriched to disease-related and immunoinflammation-related signaling pathways. Signaling intensities between M1 and TSPCs in pathways related to immunoinflammation and ischemic injury were significantly increased in the Disease group. The proportion of CCL4L2 + M1 in the Disease group was significantly higher than in the Healthy group, and communications between CCL4L2 + M1 and TSPCs varied significantly. Compared with the Control group, the expression levels of inflammatory cytokines were higher in the CCL4L2 group, and the expression levels of tendon differentiation markers (Egr1, Mkx, Scx, Type 1 collagen, Tnmd) were significantly down-regulated.

CONCLUSION

The present study analyzed the heterogeneous alterations in the Healthy and Disease groups by scRNA-seq data and found that there was a significant inflammatory infiltrate in the Disease group with markedly increased Mac activity, which was associated with activation of the CCL4L2 + M1-associated signaling pathways. CCL4L2 promotes M1 polarization and inhibits TSPC differentiation through activating M1-related inflammatory signaling pathways. These findings contribute to a more comprehensive understanding of tendon injury progression and provide potential targets for tendinopathy treatment.

Peritendinous adhesion: Therapeutic targets and progress of drug therapy

Peritendinous adhesion (PA) is one of the most common complications following hand surgery and characterized with abnormal hyperplasia of connective tissue and excessive deposition of extracellular matrix. Subsequently, various clinical symptoms such as chronic pain, limb dyskinesia and even joint stiffness occur and patients are always involved in the vicious cycle of "adhesion - release - re-adhesion", which seriously compromise the quality of life. Until present, the underlying mechanism remains controversial and lack of specific treatment, with symptomatic treatment being the only option to relieve symptoms, but not contributing no more to the fundamentally rehabilitation of basic structure and function. Recently, novel strategies have been proposed to inhibit the formation of adhesion tissues including implantation of anti-adhesion barriers, anti-inflammation, restraint of myofibroblast transformation and regulation of collagen overproduction. Furthermore, gene therapy has also been considered as a promising anti-adhesion treatment. In this review, we provide an overview of anti-adhesion targets and relevant drugs to summarize the potential pharmacological roles and present subsequent challenges and prospects of anti-adhesion drugs.

Challenges in tendon-bone healing: emphasizing inflammatory modulation mechanisms and treatment

Tendons are fibrous connective tissues that transmit force from muscles to bones. Despite their ability to withstand various loads, tendons are susceptible to significant damage. The healing process of tendons and ligaments connected to bone surfaces after injury presents a clinical challenge due to the intricate structure, composition, cellular populations, and mechanics of the interface. Inflammation plays a pivotal role in tendon healing, creating an inflammatory microenvironment through cytokines and immune cells that aid in debris clearance, tendon cell proliferation, and collagen fiber formation. However, uncontrolled inflammation can lead to tissue damage, and adhesions, and impede proper tendon healing, culminating in scar tissue formation. Therefore, precise regulation of inflammation is crucial. This review offers insights into the impact of inflammation on tendon-bone healing and its underlying mechanisms. Understanding the inflammatory microenvironment, cellular interactions, and extracellular matrix dynamics is essential for promoting optimal healing of tendon-bone injuries. The roles of fibroblasts, inflammatory cytokines, chemokines, and growth factors in promoting healing, inhibiting scar formation, and facilitating tissue regeneration are discussed, highlighting the necessity of balancing the suppression of detrimental inflammatory responses with the promotion of beneficial aspects to enhance tendon healing outcomes. Additionally, the review explores the significant implications and translational potential of targeted inflammatory modulation therapies in refining strategies for tendon-bone healing treatments.

Advancements in Therapeutic Approaches for Degenerative Tendinopathy: Evaluating Efficacy and Challenges

Degenerative tendinopathy results from the accumulation of minor injuries following unsuccessful tendon repair during acute tendon injuries. The process of tendon repair is prolonged and varies between individuals, making it susceptible to reinjury. Moreover, treating chronic tendinopathy often requires expensive and extensive rehabilitation, along with a variety of combined therapies to facilitate recovery. This condition significantly affects the quality of life of affected individuals, underscoring the urgent need for more efficient and cost-effective treatment options. Although traditional treatments have improved significantly and are being used as substitutes for surgical interventions, the findings have been inconsistent and conflicting. This review aims to clarify these issues by exploring the strengths and limitations of current treatments as well as recent innovations in managing various forms of degenerative tendinopathy.

Augmentation of Tendon and Ligament Repair with Fiber-Reinforced Hydrogel Composites

This review highlights the promise of fiber-reinforced hydrogel composites (FRHCs) for augmenting tendon and ligament repair and regeneration. Composed of reinforcing fibers embedded in a hydrogel, these scaffolds provide both mechanical strength and a conducive microenvironment for biological processes required for connective tissue regeneration. Typical properties of FRHCs are discussed, highlighting their ability to simultaneously fulfill essential mechanical and biological design criteria for a regenerative scaffold. Furthermore, features of FRHCs are described that improve specific biological aspects of tendon healing including mesenchymal progenitor cell recruitment, early polarization to a pro-regenerative immune response, tenogenic differentiation of recruited progenitor cells, and subsequent production of a mature, aligned collagenous matrix. Finally, the review offers a perspective on clinical translation of tendon FRHCs and outlines key directions for future work.

Molecular mechanisms of mechanosensing and plasticity of tendons and ligaments

Tendons and ligaments, crucial components of the musculoskeletal system, connect muscles to bones. In the realm of sports, tendons and ligaments are vulnerable tissues, with injuries such as Achilles tendon rupture and anterior cruciate ligament tears directly impacting an athlete's career. Furthermore, repetitive trauma and tissue degeneration can lead to conditions like secondary osteoarthritis, ultimately affecting the overall quality of life. Recent research highlights the pivotal role of mechanical stress in maintaining homeostasis within tendons and ligaments. This review delves into the latest insights on the structure of tendons and ligaments and the plasticity of tendon tissue in response to mechanical loads.

Impact of morphological features and chemical composition of tendon biomimetic scaffolds on immune recognition via Toll-like receptors

Tendinopathies are a major worldwide clinical problem. The development of tendon biomimetic scaffolds is considered a promising, therapeutic approach. However, to be clinically effective, scaffolds should avoid immunological recognition. It has been well described that scaffolds composed of aligned fibers lead to a better tenocyte differentiation, vitality, proliferation and motility. However, little has been studied regarding the impact of fiber spatial distribution on the recognition by immune cells. Additionally, it has been suggested that higher hydrophilicity would reduce their immune recognition. Herein, polycaprolactone (PCL)-hyaluronic acid (HA)-based electrospun scaffolds were generated with different fiber diameters (in the nano- and micro-scales) and orientations as well as different grades of wettability and the impact of these properties on immunological recognition has been assessed, by means of Toll-like receptor (TLR) reporter cells. Our results showed that TLR 2/1 and TLR 2/6 were not triggered by the scaffolds. In addition, the TLR 4 signalling pathway seems to be triggered to a greater extent by higher PCL and HA concentrations, but the alignment of the fibers prevents the triggering of this receptor. Taken together, TLR reporter cells were shown to be a useful and effective tool to study the potential of scaffolds to induce immune responses and the results obtained can be used to inform the design of fibrous scaffolds for tendon repair.

Targeted delivery of berberine using bionic nanomaterials for Atherosclerosis therapy

Atherosclerosis (AS) is a prevalent chronic vascular inflammatory disease globally, initiated by injury to vascular endothelial cells (VECs). Macrophages play a pivotal role in disease pathogenesis, involving lipid metabolism and inflammation. The application of nanomaterials has been hindered by their rapid clearance by the immune system. Utilizing macrophage cell membranes can mitigate abnormal immune responses and induce a "homing" effect. Here, M2 macrophage cell membranes (M2) were coated onto berberine polylactic-hydroxylase-polylactide (PLGA) nanoparticles (BBR NPs), employing M2 macrophage immune escape, "homing" ability, and membrane coating nanotechnology, and loaded with mannose (Man) to create bionic nanoparticles (BBR NPs@Man/M2). Subsequently, the physical properties of BBR NPs@Man/M2 were characterized. The biocompatibility and biological function of BBR NPs@Man/M2 were assessed in vitro. Finally, the targeting, therapeutic efficacy, and safety of BBR NPs@M2 were investigated in an AS mouse model. The newly developed BBR NPs@Man/M2 exhibited good biocompatibility. Owing to their M2 coating, the nanoparticles effectively targeted macrophages in vitro, inducing a shift from a pro-inflammatory to an anti-inflammatory state. This transition reduced inflammation in endothelial cells and facilitated the repair of damaged endothelial cells. Moreover, M2-coated nanoparticles efficiently targeted and accumulated in atherosclerotic lesions in vivo. Following four weeks of treatment, BBR NPs@Man/M2 significantly delayed AS progression. Furthermore, BBR NPs@Man/M2 demonstrated a good safety profile after long-term administration. In conclusion, BBR NPs@Man/M2 effectively and safely inhibited AS progression. Biomimetic nanoparticles represent a promising approach for the safe and effective delivery of anti-AS drugs.

Angioembolization in the management of joint pain: current concepts

Joint pain is a common complaint owing to its origin in inflammatory and degenerative joint diseases. Recent research has helped narrow down inadequate angiogenesis as one of the causes. Angioembolization has emerged as a treatment option for this condition when it is refractory to conservative treatment. This review describes angioembolization by elaborating on the mechanism, safety, efficacy, comparative analysis of treatment and the road ahead, in addition to summarizing the existing data on this treatment. The inferences from this review further consolidate transcatheter arterial embolization as one of the prime options for managing joint pain when it is refractory to conservative treatment and label it as one of the most exciting prospects. A limitation of this review is that most of the data were from open label case series or case reports.

Immunomodulatory properties of naïve and inflammation-informed dental pulp stem cell derived extracellular vesicles

Mesenchymal stem cell derived extracellular vesicles (MSC EVs) are paracrine modulators of macrophage function. Scientific research has primarily focused on the immunomodulatory and regenerative properties MSC EVs derived from bone marrow. The dental pulp is also a source for MSCs, and their anatomical location and evolutionary function has primed them to be potent immunomodulators. In this study, we demonstrate that extracellular vesicles derived from dental pulp stem cells (DPSC EVs) have pronounced immunomodulatory effect on primary macrophages by regulating the NFκb pathway. Notably, the anti-inflammatory activity of DPSC-EVs is enhanced following exposure to an inflammatory stimulus (LPS). These inhibitory effects were also observed in vivo. Sequencing of the naïve and LPS preconditioned DPSC-EVs and comparison with our published results from marrow MSC EVs revealed that Naïve and LPS preconditioned DPSC-EVs are enriched with anti-inflammatory miRNAs, particularly miR-320a-3p, which appears to be unique to DPSC-EVs and regulates the NFκb pathway. Overall, our findings highlight the immunomodulatory properties of DPSC-EVs and provide vital clues that can stimulate future research into miRNA-based EV engineering as well as therapeutic approaches to inflammation control and disease treatment.

Electrospinning technology: a promising approach for tendon-bone interface tissue engineering

The regeneration of tendon-bone interface tissue has become a topic of great interest in recent years. However, the complex nature of this interface has posed challenges in finding suitable solutions. Tissue engineering, with its potential to improve clinical outcomes and play a crucial role in musculoskeletal function, has been increasingly explored for tendon-bone interface regeneration. This review focuses on the research advancements of electrospinning technology in interface tissue engineering. By utilizing electrospinning, researchers have been able to fabricate scaffolds with tailored properties to promote the regeneration and integration of tendon and bone tissues. The review discusses the unique structure and function of the tendon-bone interface, the mechanisms involved in its healing, and the limitations currently faced in achieving successful regeneration. Additionally, it highlights the potential of electrospinning technology in scaffold fabrication and its role in facilitating the development of functional and integrated tendon-bone interface tissues. Overall, this review provides valuable insights into the application of electrospinning technology for tendon-bone interface tissue engineering, emphasizing its significance in addressing the challenges associated with regeneration in this complex interface.

Pharmacological Antagonism of Ccr2+ Cell Recruitment to Facilitate Regenerative Tendon Healing

Successful tendon healing requires sufficient deposition and remodeling of new extracellular matrix at the site of injury, with this process mediating in part through fibroblast activation via communication with macrophages. Moreover, resolution of healing requires clearance or reversion of activated cells, with chronic interactions with persistent macrophages impairing resolution and facilitating the conversion the conversion to fibrotic healing. As such, modulation of the macrophage environment represents an important translational target to improve the tendon healing process. Circulating monocytes are recruited to sites of tissue injury, including the tendon, via upregulation of cytokines including Ccl2, which facilitates recruitment of Ccr2+ macrophages to the healing tendon. Our prior work has demonstrated that Ccr2-/- can modulate fibroblast activation and myofibroblast differentiation. However, this approach lacked temporal control and resulted in healing impairments. Thus, in the current study we have leveraged a Ccr2 antagonist to blunt macrophage recruitment to the healing tendon in a time-dependent manner. We first tested the effects of Ccr2 antagonism during the acute inflammatory phase and found that this had no effect on the healing process. In contrast, Ccr2 antagonism during the late inflammatory/ early proliferative period resulted in significant improvements in mechanical properties of the healing tendon. Collectively, these data demonstrate the temporally distinct impacts of modulating Ccr2+ cell recruitment and Ccr2 antagonism during tendon healing and highlight the translational potential of transient Ccr2 antagonism to improve the tendon healing process.

Oxidized Phospholipids Regulate Tenocyte Function via Induction of Amphiregulin in Dendritic Cells

Inflammation is a driving force of tendinopathy. The oxidation of phospholipids by free radicals is a consequence of inflammatory reactions and is an important indicator of tissue damage. Here, we have studied the impact of oxidized phospholipids (OxPAPC) on the function of human tenocytes. We observed that treatment with OxPAPC did not alter the morphology, growth and capacity to produce collagen in healthy or diseased tenocytes. However, since OxPAPC is a known modulator of the function of immune cells, we analyzed whether OxPAPC-treated immune cells might influence the fate of tenocytes. Co-culture of tenocytes with immature, monocyte-derived dendritic cells treated with OxPAPC (Ox-DCs) was found to enhance the proliferation of tenocytes, particularly those from diseased tendons. Using transcriptional profiling of Ox-DCs, we identified amphiregulin (AREG), a ligand for EGFR, as a possible mediator of this proliferation enhancing effect, which we could confirm using recombinant AREG. Of note, diseased tenocytes were found to express higher levels of EGFR compared to tenocytes isolated from healthy donors and show a stronger proliferative response upon co-culture with Ox-DCs, as well as AREG treatment. In summary, we identify an AREG-EGFR axis as a mediator of a DC-tenocyte crosstalk, leading to increased tenocyte proliferation and possibly tendon regeneration.

Widespread Vascularization and Correlation of Glycosaminoglycan Accumulation to Tendon Pain in Human Plantar Fascia Tendinopathy

BACKGROUND

Plantar fasciitis is a painful tendinous condition (tendinopathy) with a high prevalence in athletes. While a healthy tendon has limited blood flow, ultrasound has indicated elevated blood flow in tendinopathy, but it is unknown if this is related to a de facto increase in the tendon vasculature. Likewise, an accumulation of glycosaminoglycans (GAGs) is observed in tendinopathy, but its relationship to clinical pain is unknown.

PURPOSE

To explore to what extent vascularization, inflammation, and fat infiltration were present in patients with plantar fasciitis and if they were related to clinical symptoms.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Biopsy specimens from tendinopathic plantar fascia tissue were obtained per-operatively from both the primary site of tendon pain and tissue swelling ("proximal") and a region that appeared macroscopically healthy at 1 to 2 cm away from the primary site ("distal") in 22 patients. Biopsy specimens were examined with immunofluorescence for markers of blood vessels, tissue cell density, fat infiltration, and macrophage level. In addition, pain during the first step in the morning (registered during an earlier study) was correlated with the content of collagen and GAGs in tissue.

RESULTS

High vascularization (and cellularity) was present in both the proximal (0.89%) and the distal (0.96%) plantar fascia samples, whereas inconsistent but not significantly different fat infiltration and macrophage levels were observed. The collagen content was similar in the 2 plantar fascia regions, whereas the GAG content was higher in the proximal region (3.2% in proximal and 2.8% in distal; P = .027). The GAG content in the proximal region was positively correlated with the subjective morning pain score in the patients with tendinopathy (n = 17).

CONCLUSION

In patients with plantar fasciitis, marked tissue vascularization was present in both the painful focal region and a neighboring nonsymptomatic area. In contrast, the accumulation of hydrophilic GAGs was greater in the symptomatic region and was positively correlated with increased clinical pain levels in daily life.

CLINICAL RELEVANCE

The accumulation of GAGs in tissue rather than the extent of vascularization appears to be linked with the clinical degree of pain symptoms of the disease.

Study of the Synergistic Immunomodulatory and Antifibrotic Effects of Dual-Loaded Budesonide and Serpine1 siRNA Lipid-Polymer Nanoparticles Targeting Macrophage Dysregulation in Tendinopathy

Musculoskeletal diseases involving tissue injury comprise tendon, ligament, and muscle injury. Recently, macrophages have been identified as key players in the tendon repair process, but no therapeutic strategy involving dual drug delivery and gene delivery to macrophages has been developed for targeting the two main dysregulated aspects of macrophages in tendinopathy, i.e., inflammation and fibrosis. Herein, the anti-inflammatory and antifibrotic effects of dual-loaded budesonide and serpine1 siRNA lipid-polymer hybrid nanoparticles (LPNs) are evaluated in murine and human macrophage cells. The modulation of the gene and protein expression of factors associated with inflammation and fibrosis in tendinopathy is demonstrated by real time polymerase chain reaction and Western blot. Macrophage polarization to the M2 phenotype and a decrease in the production of pro-inflammatory cytokines are confirmed in macrophage cell lines and primary cells. The increase in the activity of a matrix metalloproteinase involved in tissue remodelling is proven, and studies evaluating the interactions of LPNs with T cells proved that dual-loaded LPNs act specifically on macrophages and do not induce any collateral effects on T cells. Overall, these dual-loaded LPNs are a promising combinatorial therapeutic strategy with immunomodulatory and antifibrotic effects in dysregulated macrophages in the context of tendinopathy.

Mechanical overload-induced release of extracellular mitochondrial particles from tendon cells leads to inflammation in tendinopathy

Tendinopathy is one of the most common musculoskeletal diseases, and mechanical overload is considered its primary cause. However, the underlying mechanism through which mechanical overload induces tendinopathy has not been determined. In this study, we identified for the first time that tendon cells can release extracellular mitochondria (ExtraMito) particles, a subtype of medium extracellular particles (mEPs), into the environment through a process regulated by mechanical loading. RNA sequencing systematically revealed that oxygen-related reactions, extracellular particles, and inflammation were present in diseased human tendons, suggesting that these factors play a role in the pathogenesis of tendinopathy. We simulated the disease condition by imposing a 9% strain overload on three-dimensional mouse tendon constructs in our cyclic uniaxial stretching bioreactor. The three-dimensional mouse tendon constructs under normal loading with 6% strain exhibited an extended mitochondrial network, as observed through live-cell confocal laser scanning microscopy. In contrast, mechanical overload led to a fragmented mitochondrial network. Our microscopic and immunoblot results demonstrated that mechanical loading induced tendon cells to release ExtraMito particles. Furthermore, we showed that mEPs released from tendon cells overloaded with a 9% strain (mEP9%) induced macrophage chemotaxis and increased the production of proinflammatory cytokines, including IL-6, CXCL1, and IL-18, from macrophages compared to mEP0%, mEP3%, and mEP6%. Partial depletion of the ExtraMito particles from mEP9% by magnetic-activated cell sorting significantly reduced macrophage chemotaxis. N-acetyl-L-cysteine treatment preserved the mitochondrial network in overloaded tendon cells, diminishing overload-induced macrophage chemotaxis toward mEP9%. These findings revealed a novel mechanism of tendinopathy; in an overloaded environment, ExtraMito particles convey mechanical response signals from tendon cells to the immune microenvironment, culminating in tendinopathy.

The role of macrophage polarization in tendon healing and therapeutic strategies: Insights from animal models

Tendon injuries, a common musculoskeletal issue, usually result in adhesions to the surrounding tissue, that will impact functional recovery. Macrophages, particularly through their M1 and M2 polarizations, play a pivotal role in the inflammatory and healing phases of tendon repair. In this review, we explore the role of macrophage polarization in tendon healing, focusing on insights from animal models. The review delves into the complex interplay of macrophages in tendon pathology, detailing how various macrophage phenotypes contribute to both healing and adhesion formation. It also explores the potential of modulating macrophage activity to enhance tendon repair and minimize adhesions. With advancements in understanding macrophage behavior and the development of innovative biomaterials, this review highlights promising therapeutic strategies for tendon injuries.

A Novel Tendon Injury Model, Induced by Collagenase Administration Combined with a Thermo-Responsive Hydrogel in Rats, Reproduces the Pathogenesis of Human Degenerative Tendinopathy

Patellar tendinopathy is a common clinical problem, but its underlying pathophysiology remains poorly understood, primarily due to the absence of a representative experimental model. The most widely used method to generate such a model is collagenase injection, although this method possesses limitations. We developed an optimized rat model of patellar tendinopathy via the ultrasound-guided injection of collagenase mixed with a thermo-responsive Pluronic hydrogel into the patellar tendon of sixty male Wistar rats. All analyses were carried out at 3, 7, 14, 30, and 60 days post-injury. We confirmed that our rat model reproduced the pathophysiology observed in human patients through analyses of ultrasonography, histology, immunofluorescence, and biomechanical parameters. Tendons that were injured by the injection of the collagenase-Pluronic mixture exhibited a significant increase in the cross-sectional area (p < 0.01), a high degree of tissue disorganization and hypercellularity, significantly strong neovascularization (p < 0.01), important changes in the levels of types I and III collagen expression, and the organization and presence of intra-tendinous calcifications. Decreases in the maximum rupture force and stiffness were also observed. These results demonstrate that our model replicates the key features observed in human patellar tendinopathy. Collagenase is evenly distributed, as the Pluronic hydrogel prevents its leakage and thus, damage to surrounding tissues. Therefore, this model is valuable for testing new treatments for patellar tendinopathy.

Carpaine alleviates tendinopathy in mice by promoting the ubiquitin-proteasomal degradation of p65 via targeting the E3 ubiquitin ligase LRSAM1

BACKGROUND

Currently, there are no specific drugs or targets available for the treatment of tendinopathy. However, inflammation has recently been found to play a pivotal role in tendinopathy progression, thereby identifying it as a potential therapeutic target. Carpaine (CA) exhibits potential anti-inflammatory pharmacological properties and may offer a therapeutic option for tendinopathy.

PURPOSE

This study aimed to investigate the effectiveness of CA in addressing tendinopathy and uncovering its underlying mechanisms.

METHODS

Herein, the efficacy of CA by local administration in vivo in comparison to the first-line drug indomethacin was evaluated in a mouse collagenase-induced tendinopathy (CIT) model. Furthermore, IL-1β induced a simulated pathological inflammatory microenvironment in tenocytes to investigate its underlying mechanisms in vitro. Further confirmation experiments were performed by overexpressing or knocking down the selective targets of CA in vivo.

RESULTS

The findings demonstrated that CA was dose-dependent in treating tendinopathy and that the high-dose group outperformed the first-line drug indomethacin. Mechanistically, CA selectively bound to and enhanced the activity of the E3 ubiquitin ligase LRSAM1 in tendinopathy. This effect mediated the ubiquitination of p65 at lysine 93, subsequently promoting its proteasomal degradation. As a result, the NF-κB pathway was inactivated, leading to a reduction in inflammation of tendinopathy. Consequently, CA effectively mitigated the progression of tendinopathy. Moreover, the LRSAM1 overexpression demonstrated effectiveness in mitigating the tendinopathy progression and its knockdown abolished the therapeutic effects of CA.

CONCLUSION

CA attenuates the progression of tendinopathy by promoting the ubiquitin-proteasomal degradation of p65 via increasing the enzyme activity of LRSAM1. The exploration of LRSAM1 has also unveiled a new potential target for treating tendinopathy based on the ubiquitin-proteasomal pathway.

Effect of Platelet-Rich Plasma at Different Initiation Times on Healing of the Bone-Tendon Interface of the Rotator Cuff in a Mouse Model

BACKGROUND

Platelet-rich plasma (PRP) has demonstrated beneficial effects on healing of the bone-tendon interface (BTI).

PURPOSE

To determine the optimal initiation time for PRP application after rotator cuff repair in an animal model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 136 C57BL/6 mice were included; 40 mice were used to prepare PRP, while 96 mice underwent acute supraspinatus tendon (SST) repair. The animals were randomly divided into 4 groups: a control group and 3 groups in which PRP was injected into the injury interface immediately after surgery, on the 7th postoperative day (PRP-7d), and on the 14th postoperative day. At 4 and 8 weeks postoperatively, the animals were sacrificed, blood was collected by eyeball removal, and samples of the SST-humerus complex were collected. Histological, imaging, immunological, and biomechanical data were compared among the groups using 1-way analysis of variance with the Bonferroni post hoc test.

RESULTS

Histological analysis revealed that the fibrocartilage layer at the BTI was larger in the PRP-7d group compared to the other groups at both 4 and 8 weeks postoperatively. Moreover, the PRP-7d group exhibited improved proteoglycan content and distribution compared to the other groups. Enzyme-linked immunosorbent assay results demonstrated that at 4 weeks postoperatively, higher concentrations of transforming growth factor-β1 and platelet-derived growth factor-BB (PDGF-BB) were seen in the PRP-7d group versus the PRP-14d and control gruops (P < .05), and at 8 weeks postoperatively, the concentration of PDGF-BB was higher in the PRP-7d group versus the control group (P < .05). Biomechanical testing at 4 weeks postoperatively revealed that the failure load and ultimate strength of the SST-humerus complex were superior in the PRP-7d group compared to the other groups (P < .05), at 8 weeks, PRP-7d group was superior to the control group (P < .05). Additionally, at 8 weeks postoperatively, the PRP-7d group exhibited a greater trabecular number and trabecular thickness at the BTI compared to the PRP-14d and control gruops (P < .05).

CONCLUSION

PRP promoted healing of the BTI after a rotator cuff injury at an early stage.

CLINICAL RELEVANCE

A PRP injection on the 7th postoperative day demonstrated superior therapeutic effects compared with injections at other time points.

Advances in mesenchymal stem cells therapy for tendinopathies

Tendinopathies are chronic diseases of an unknown etiology and associated with inflammation. Mesenchymal stem cells (MSCs) have emerged as a viable therapeutic option to combat the pathological progression of tendinopathies, not only because of their potential for multidirectional differentiation and self-renewal, but also their excellent immunomodulatory properties. The immunomodulatory effects of MSCs are increasingly being recognized as playing a crucial role in the treatment of tendinopathies, with MSCs being pivotal in regulating the inflammatory microenvironment by modulating the immune response, ultimately contributing to improved tissue repair. This review will discuss the current knowledge regarding the application of MSCs in tendinopathy treatments through the modulation of the immune response.

Pristimerin suppresses AIM2 inflammasome by modulating AIM2-PYCARD/ASC stability via selective autophagy to alleviate tendinopathy

Macroautophagy/autophagy plays an important role in regulating cellular homeostasis and influences the pathogenesis of degenerative diseases. Tendinopathy is characterized by tendon degeneration and inflammation. However, little is known about the role of selective autophagy in tendinopathy. Here, we find that pristimerin (PM), a quinone methide triterpenoid, is more effective in treating tendinopathy than the first-line drug indomethacin. PM inhibits the AIM2 inflammasome and alleviates inflammation during tendinopathy by promoting the autophagic degradation of AIM2 through a PYCARD/ASC-dependent manner. A mechanistic study shows that PM enhances the K63-linked ubiquitin chains of PYCARD/ASC at K158/161, which serves as a recognition signal for SQSTM1/p62-mediated autophagic degradation of the AIM2-PYCARD/ASC complex. We further identify that PM binds the Cys53 site of deubiquitinase USP50 through the Michael-acceptor and blocks the binding of USP50 to PYCARD/ASC, thereby reducing USP50-mediated cleavage of K63-linked ubiquitin chains of PYCARD/ASC. Finally, PM treatment in vivo generates an effect comparable to inflammasome deficiency in alleviating tendinopathy. Taken together, these findings demonstrate that PM alleviates the progression of tendinopathy by modulating AIM2-PYCARD/ASC stability via SQSTM1/p62-mediated selective autophagic degradation, thus providing a promising autophagy-based therapeutic for tendinopathy.Abbreviations: 3-MA: 3-methyladenine; AIM2: absent in melanoma 2; AT: Achilles tenotomy; ATP: adenosine triphosphate; BMDMs: bone marrow-derived macrophages; CHX: cycloheximide; Col3a1: collagen, type III, alpha 1; CQ: chloroquine; Cys: cysteine; DARTS: drug affinity responsive target stability; DTT: dithiothreitol; DUB: deubiquitinase; gDNA: genomic DNA; GSH: glutathione; His: histidine; IL1B/IL-1β: interleukin 1 beta; IND: indomethacin; IP: immunoprecipitation; LPS: lipopolysaccharide; MMP: mitochondrial membrane potential; NLRP3: NLR family, pyrin domain containing 3; PM: pristimerin; PYCARD/ASC: PYD and CARD domain containing; SN: supernatants; SOX9: SRY (sex determining region Y)-box 9; SQSTM1: sequestosome 1; Tgfb: transforming growth factor, beta; TIMP3: tissue inhibitor of metalloproteinase 3; TNMD: tenomodulin; TRAF6: TNF receptor-associated factor 6; Ub: ubiquitin; USP50: ubiquitin specific peptidase 50; WCL: whole cell lysates.

An Update on Orthobiologics: Cautious Optimism

Orthobiologics are rapidly growing in use given their potential to augment healing for multiple musculoskeletal conditions. Orthobiologics consist of a variety of treatments including platelet-rich plasma and stem cells that provide conceptual appeal in providing local delivery of growth factors and inflammation modulation. The lack of standardization in nomenclature and applications within the literature has led to a paucity of high-quality evidence to support their frequent use. The purpose of this review was to describe the current landscape of orthobiologics and the most recent evidence regarding their use.

Exploring polysaccharide and protein-enriched decellularized matrix scaffolds for tendon and ligament repair: A review

Tissue engineering (TE) has become a primary research topic for the treatment of diseased or damaged tendon/ligament (T/L) tissue. T/L injuries pose a severe clinical burden worldwide, necessitating the development of effective strategies for T/L repair and tissue regeneration. TE has emerged as a promising strategy for restoring T/L function using decellularized extracellular matrix (dECM)-based scaffolds. dECM scaffolds have gained significant prominence because of their native structure, relatively high bioactivity, low immunogenicity, and ability to function as scaffolds for cell attachment, proliferation, and differentiation, which are difficult to imitate using synthetic materials. Here, we review the recent advances and possible future prospects for the advancement of dECM scaffolds for T/L tissue regeneration. We focus on crucial scaffold properties and functions, as well as various engineering strategies employed for biomaterial design in T/L regeneration. dECM provides both the physical and mechanical microenvironments required by cells to survive and proliferate. Various decellularization methods and sources of allogeneic and xenogeneic dECM in T/L repair and regeneration are critically discussed. Additionally, dECM hydrogels, bio-inks in 3D bioprinting, and nanofibers are briefly explored. Understanding the opportunities and challenges associated with dECM-based scaffold development is crucial for advancing T/L repairs in tissue engineering and regenerative medicine.

Prospect of Exosome in Ligament Healing: A Systematical Review

Aim

The relationship between ligaments and bone is a complex and heterogeneous junction involving bone, mineralized fibro cartilage, non-mineralized fibro cartilage and ligaments. Mesenchymal stem cells (MSC) can be used in vivo to control inflammation and aid in tissue repair, according to studies. This review focused on using exosomes as an alternative to MSC, as a cell-free therapy for modulating the remodelling process.

Methods

To conduct a systematic review of the literature, the phrases "exosome" and "ligament" or "tendon" and "extracellular vesicle" and "stem cells" were used as the search keywords in PubMed (MEDLINE), OVID, the Cochrane Library, and Science Direct. From the literature, 73 studies in all were found. Six studies were included in this systematic review after full-text evaluation.

Results

Six included studies covered a range of MSC types, isolation techniques, animal models, and interventions. Biomechanical results consistently indicated the beneficial impact of conditioned media, vesicles, and exosomes on treating tendons and ligaments. Noteworthy findings were the reduction of inflammation by iMSC-IEVs, chondrocyte protection by iPSC-EVs (extracellular vesicles generated by inflammation-primed adipose-derived stem cells), osteolysis treatment using DPSC-sEVs (small extracellular vesicles derived from dental pulp stem cells), and the contribution of exosome-educated macrophages to ligament injury wound healing.

Conclusion

Exosomes may serve as a cell-free therapeutic substitute for modulating the remodelling process, particularly in ligament healing.

Multilayer amnion-PCL nanofibrous membrane loaded with celecoxib exerts a therapeutic effect against tendon adhesion by improving the inflammatory microenvironment

Tendon adhesion is a common complication after tendon surgery. The inflammatory phase of tendon healing is characterized by the release of a large number of inflammatory factors, whose mediated excessive inflammatory response is an important cause of tendon adhesion formation. Nonsteroidal anti-inflammatory drugs(NSAIDs) were used to prevent tendon adhesions by reducing the inflammatory response. However, recent studies have shown that the NSAIDs partially impairs tendon healing. Therefore, optimizing the anti-adhesive membrane loaded with NSAIDs to mitigate the effects on tendon healing requires further in-depth study. Amniotic membranes(AM) are natural polymeric semi-permeable membranes from living organisms that are rich in matrix, growth factors, and other active ingredients. In this study, we used electrostatic spinning technology to construct multifunctional nanofiber membranes of the PCL membrane loaded with celecoxib and AM. In vitro cellular assays revealed that celecoxib-loaded PCL membranes significantly inhibited the adhesion and proliferation of fibroblasts with increasing concentrations of celecoxib. In a rabbit tendon repair model, biomechanical tests further confirmed that the PCL membrane loaded with celecoxib had better anti-adhesion effects. Further experimental studies revealed that the PCL/AM membrane improved the inflammatory microenvironment by downregulating the expression of pro-inflammatory factors such as COX-2, IL-1β, and TNF-α proteins; and inhibiting the synthesis of COL I and COL Ⅲ. The PCL/AM membrane can continuously release celecoxib to reduce the inflammatory response and deliver growth factors to the damaged area to build a suitable microenvironment for tendon repair, which provides a new direction to improve the repair efficiency of tendon.

Predominant ligament-centric soft-tissue involvement differentiates axial psoriatic arthritis from ankylosing spondylitis

Since the original description of spondyloarthritis 50 years ago, results have demonstrated similarities and differences between ankylosing spondylitis (AS) and axial psoriatic arthritis (PsA). HLA-B27 gene carriage in axial inflammation is linked to peri-fibrocartilaginous sacroiliac joint osteitis, as well as to spinal peri-entheseal osteitis, which is often extensive and which provides a crucial anatomical and immunological differentiation between the AS and PsA phenotypes. Specifically, HLA-B27-related diffuse bone marrow oedema (histologically an osteitis) and bone marrow fatty corners detected via magnetic resonance imaging, as well as radiographic changes such as sacroiliitis, vertebral squaring, corner erosions and Romanus lesions, all indicate initial bone phenotypes in HLA-B27+ axial disease. However, in much of PsA with axial involvement, enthesitis primarily manifests in ligamentous soft tissue as 'ligamentitis', with characteristic lesions that include para-syndesmophytes and sacroiliac joint bony sparing. Like axial PsA, diffuse idiopathic skeletal hyperostosis phenotypes, which can be indistinguishable from PsA, exhibit a thoracic and cervical spinal ligamentous soft-tissue tropism, clinically manifesting as syndesmophytosis that is soft-tissue-centric, including paravertebral soft-tissue ossification and sacroiliac soft-ligamentous ossification instead of joint-cavity fusion. The enthesis bone and soft tissues have radically different immune cell and stromal compositions, which probably underpins differential responses to immunomodulatory therapy, especially IL-23 inhibition.