Inflamm-aging and arachadonic acid metabolite differences with stage of tendon disease

The roles and mechanisms of the NF-κB signaling pathway in tendon disorders

Both acute and chronic tendon injuries are the most frequently occurring musculoskeletal diseases in human and veterinary medicine, with a limited repertoire of successful and evidenced-based therapeutic strategies. Inflammation has been suggested as a key driver for the formation of scar and adhesion tissue following tendon acute injury, as well as pathological alternations of degenerative tendinopathy. However, prior efforts to completely block this inflammatory process have yet to be largely successful. Recent investigations have indicated that a more precise targeted approach for modulating inflammation is critical to improve outcomes. The nuclear factor-kappaB (NF-κB) is a typical proinflammatory signal transduction pathway identified as a key factor leading to tendon disorders. Therefore, a comprehensive understanding of the mechanism or regulation of NF-κB in tendon disorders will aid in developing targeted therapeutic strategies for human and veterinary tendon disorders. In this review, we discuss what is currently known about molecular components and structures of basal NF-κB proteins and two activation pathways: the canonical activation pathway and the non-canonical activation pathway. Furthermore, we summarize the underlying mechanisms of the NF-κB signaling pathway in fibrosis and adhesion after acute tendon injury, as well as pathological changes of degenerative tendinopathy in all species and highlight the effect of targeting this signaling pathway in tendon disorders. However, to gain a comprehensive understanding of its mechanisms underlying tendon disorders, further investigations are required. In the future, extensive scientific examinations are warranted to full characterize the NF-κB, the exact mechanisms of action, and translate findings into clinical human and veterinary practice.

The Interfascicular Matrix of Energy Storing Tendons Houses Heterogenous Cell Populations Disproportionately Affected by Aging

Energy storing tendons such as the human Achilles and equine superficial digital flexor tendon (SDFT) are prone to injury, with incidence increasing with aging, peaking in the 5th decade of life in the human Achilles tendon. The interfascicular matrix (IFM), which binds tendon fascicles, plays a key role in energy storing tendon mechanics, and aging alterations to the IFM negatively impact tendon function. While the mechanical role of the IFM in tendon function is well-established, the biological role of IFM-resident cell populations remains to be elucidated. Therefore, the aim of this study was to identify IFM-resident cell populations and establish how these populations are affected by aging. Cells from young and old SDFTs were subjected to single cell RNA-sequencing, and immunolabelling for markers of each resulting population used to localise cell clusters. Eleven cell clusters were identified, including tenocytes, endothelial cells, mural cells, and immune cells. One tenocyte cluster localised to the fascicular matrix, whereas nine clusters localised to the IFM. Interfascicular tenocytes and mural cells were preferentially affected by aging, with differential expression of genes related to senescence, dysregulated proteostasis and inflammation. This is the first study to establish heterogeneity in IFM cell populations, and to identify age-related alterations specific to IFM-localised cells.

Tendon and Ligament Genetics: How Do They Contribute to Disease and Injury? A Narrative Review

A significant proportion of patients requiring musculoskeletal management present with tendon and ligament pathology. Our understanding of the intrinsic and extrinsic mechanisms that lead to such disabilities is increasing. However, the complexity underpinning these interactive multifactorial elements is still not fully characterised. Evidence highlighting the genetic components, either reducing or increasing susceptibility to injury, is increasing. This review examines the present understanding of the role genetic variations contribute to tendon and ligament injury risk. It examines the different elements of tendon and ligament structure and considers our knowledge of genetic influence on form, function, ability to withstand load, and undertake repair or regeneration. The role of epigenetic factors in modifying gene expression in these structures is also explored. It considers the challenges to interpreting present knowledge, the requirements, and likely pathways for future research, and whether such information has reached the point of clinical utility.

Changes in Physiological Tendon Substrate Stiffness Have Moderate Effects on Tendon-Derived Cell Growth and Immune Cell Activation

Tendinopathy is characterised by pathological changes in tendon matrix composition, architecture, and stiffness, alterations in tendon resident cell characteristics, and fibrosis, with inflammation also emerging as an important factor in tendinopathy progression. The sequence of pathological changes in tendinopathy and the cellular effects of the deteriorating matrix are largely unknown. This study investigated the effects of substrate stiffness on tendon-derived cells (TDCs) and THP-1 macrophages using PDMS substrates representing physiological tendon stiffness (1.88 MPa), a stiff gel (3.17 MPa) and a soft gel (0.61 MPa). Human TDCs were cultured on the different gel substrates and on tissue culture plastic. Cell growth was determined by alamarBlue™ assay, cell morphology was analysed in f-actin labelled cells, and phenotypic markers were analysed by real-time PCR. We found that in comparison to TDCs growing on gels with physiological stiffness, cell growth increased on soft gels at 48 h (23%, p = 0.003). Cell morphology was similar on all three gels. SCX expression was slightly reduced on the soft gels (1.4-fold lower, p = 0.026) and COL1A1 expression increased on the stiff gels (2.2-fold, p = 0.041). Culturing THP-1 macrophages on soft gels induced increased expression of IL1B (2-fold, p = 0.018), and IL8 expression was inhibited on the stiffer gels (1.9-fold, p = 0.012). We also found that culturing TDCs on plastic increased cell growth, altered cell morphology, and inhibited the expression of SCX, SOX9, MMP3, and COL3. We conclude that TDCs and macrophages respond to changes in matrix stiffness. The magnitude of responses measured in TDCs were minor on the range of substrate stiffness tested by the gels. Changes in THP-1 macrophages suggested a more inflammatory phenotype on substrates with non-physiological stiffness. Although cell response to subtle variations in matrix stiffness was moderate, it is possible that these alterations may contribute to the onset and progression of tendinopathy.

The Effect of Age and Intrinsic Aerobic Exercise Capacity on the Expression of Inflammation and Remodeling Markers in Rat Achilles Tendons

Old age, adiposity, and metabolic disorders are known as risk factors for chronic tendinopathy, which is a common problem in both athletes and the general population. However, the importance of these influencing factors has not yet been well understood. This study investigated alterations in gene expression and histology of Achilles tendons of young (10 weeks) and old (100 weeks) rats bred for low (low capacity runners, LCR) and high (high capacity runners, HCR) intrinsic aerobic exercise capacity. In this rat model, LCR displayed a phenotype of reduced exercise capacity, higher body weight, and metabolic dysfunctions compared to HCR. We hypothesized that the risk factors for tendinopathy in old LCR could lead to more pronounced impairments in Achilles tendon tissue. In quantitative real-time PCR (qPCR), age-related downregulation of tenocyte markers e.g., tenomodulin, genes related to matrix modeling and remodeling (e.g., collagens, elastin, biglycan, fibronectin, tenascin C) as well as transforming growth factor beta 3 (Tgfb3) have been detected. Inflammation marker cyclooxygenase 2 (Cox2) was downregulated in old rats, while microsomal prostaglandin E synthase 2 (Ptges2) was upregulated in old HCR and old LCR. In all groups, interleukin 6 (Il6), interleukin 1 beta (Il1b), and tumor necrosis factor alpha (Tnfa) showed no significant alteration. In histological evaluation, tendons of old rats had fewer and more elongated tenocyte nuclei than young rats. Even though a higher content of glycosaminoglycans, a sign of degeneration, was found in old HCR and LCR, no further signs of tendinopathy were detectable in tendons of old rats by histological evaluation. Low intrinsic aerobic exercise capacity and the associated phenotype did not show significant effects on gene expression and tendon histology. These findings indicate that aging seems to play a prominent role in molecular and structural alterations of Achilles tendon tissue and suggests that other risk factors associated with intrinsic aerobic exercise capacity are less influential in this rat model.

Tendon healing is adversely affected by low-grade inflammation

BACKGROUND

Tendinopathy is common, presents with pain and activity limitation, and is associated with a high risk of recurrence of the injury. Tendinopathy usually occurs as a results of a disrupted healing response to a primary injury where cellular and molecular pathways lead to low grade chronic inflammation.

MAIN FINDINGS

There has been a renewed interest in investigating the role of Inflammation in the pathogenesis of tendinopathy, in particular during the initial phases of the condition where it may not be clinically evident. Understanding the early and late stages of tendon injury pathogenesis would help develop new and effective treatments addressed at targeting the inflammatory pathways.

CONCLUSION

This review outlines the role of low-grade Inflammation in the pathogenesis of tendinopathy, stressing the role of proinflammatory cytokines, proteolytic enzymes and growth factors, and explores how Inflammation exerts a negative influence on the process of tendon healing.

Local shifts in inflammatory and resolving lipid mediators in response to tendon overuse

Tendon inflammation has been implicated in both adaptive connective tissue remodeling and overuse-induced tendinopathy. Lipid mediators control both the initiation and resolution of inflammation, but their roles within tendon are largely unknown. Here, we profiled local shifts in intratendinous lipid mediators via liquid chromatography-tandem mass spectrometry in response to synergist ablation-induced plantaris tendon overuse. Sixty-four individual lipid mediators were detected in homogenates of plantaris tendons from ambulatory control rats. This included many bioactive metabolites of the cyclooxygenase (COX), lipoxygenase (LOX), and epoxygenase (CYP) pathways. Synergist ablation induced a robust inflammatory response at day 3 post-surgery characterized by epitenon infiltration of polymorphonuclear leukocytes and monocytes/macrophages (MΦ), heightened expression of inflammation-related genes, and increased intratendinous concentrations of the pro-inflammatory eicosanoids thromboxane B₂ and prostaglandin E₂ . By day 7, MΦ became the predominant myeloid cell type in tendon and there were further delayed increases in other COX metabolites including prostaglandins D₂ , F_2α , and I₂ . Specialized pro-resolving mediators including protectin D1, resolvin D2 and D6, as well as related pathway markers of D-resolvins (17-hydroxy-docosahexaenoic acid), E-resolvins (18-hydroxy-eicosapentaenoic acid), and lipoxins (15-hydroxy-eicosatetraenoic acid) were also increased locally in response to tendon overuse, as were anti-inflammatory fatty acid epoxides of the CYP pathway (eg, epoxy-eicosatrienoic acids). Nevertheless, intratendinous prostaglandins remained markedly increased even following 28 days of tendon overuse together with a lingering MΦ presence. These data reveal a delayed and prolonged local inflammatory response to tendon overuse characterized by an overwhelming predominance of pro-inflammatory eicosanoids and a relative lack of specialized pro-resolving lipid mediators.

The Lack of a Representative Tendinopathy Model Hampers Fundamental Mesenchymal Stem Cell Research

Overuse tendon injuries are a major cause of musculoskeletal morbidity in both human and equine athletes, due to the cumulative degenerative damage. These injuries present significant challenges as the healing process often results in the formation of inferior scar tissue. The poor success with conventional therapy supports the need to search for novel treatments to restore functionality and regenerate tissue as close to native tendon as possible. Mesenchymal stem cell (MSC)-based strategies represent promising therapeutic tools for tendon repair in both human and veterinary medicine. The translation of tissue engineering strategies from basic research findings, however, into clinical use has been hampered by the limited understanding of the multifaceted MSC mechanisms of action. In vitro models serve as important biological tools to study cell behavior, bypassing the confounding factors associated with in vivo experiments. Controllable and reproducible in vitro conditions should be provided to study the MSC healing mechanisms in tendon injuries. Unfortunately, no physiologically representative tendinopathy models exist to date. A major shortcoming of most currently available in vitro tendon models is the lack of extracellular tendon matrix and vascular supply. These models often make use of synthetic biomaterials, which do not reflect the natural tendon composition. Alternatively, decellularized tendon has been applied, but it is challenging to obtain reproducible results due to its variable composition, less efficient cell seeding approaches and lack of cell encapsulation and vascularization. The current review will overview pros and cons associated with the use of different biomaterials and technologies enabling scaffold production. In addition, the characteristics of the ideal, state-of-the-art tendinopathy model will be discussed. Briefly, a representative in vitro tendinopathy model should be vascularized and mimic the hierarchical structure of the tendon matrix with elongated cells being organized in a parallel fashion and subjected to uniaxial stretching. Incorporation of mechanical stimulation, preferably uniaxial stretching may be a key element in order to obtain appropriate matrix alignment and create a pathophysiological model. Together, a thorough discussion on the current status and future directions for tendon models will enhance fundamental MSC research, accelerating translation of MSC therapies for tendon injuries from bench to bedside.

Inhibition of JAK-STAT Signaling Pathway Alleviates Age-Related Phenotypes in Tendon Stem/Progenitor Cells

Diminished regeneration or healing capacity of tendon occurs during aging. It has been well demonstrated that tendon stem/progenitor cells (TSPCs) play a vital role in tendon maintenance and repair. Here, we identified an accumulation of senescent TSPCs in tendon tissue with aging. In aged TSPCs, the activity of JAK-STAT signaling pathway was increased. Besides, genetic knockdown of JAK2 or STAT3 significantly attenuated TSPC senescence in aged TSPCs. Pharmacological inhibition of JAK-STAT signaling pathway with AG490 similarly attenuated cellular senescence and senescence-associated secretory phenotype (SASP) of aged TSPCs. In addition, inhibition of JAK-STAT signaling pathway also restored the age-related dysfunctions of TSPCs, including self-renewal, migration, actin dynamics, and stemness. Together, our findings reveal the critical role of JAK-STAT signaling pathway in the regulation of TSPC aging and suggest an ideal therapeutic target for the age-related tendon disorders.

"Cell Membrane Theory of Senescence" and the Role of Bioactive Lipids in Aging, and Aging Associated Diseases and Their Therapeutic Implications

Lipids are an essential constituent of the cell membrane of which polyunsaturated fatty acids (PUFAs) are the most important component. Activation of phospholipase A2 (PLA2) induces the release of PUFAs from the cell membrane that form precursors to both pro- and ant-inflammatory bioactive lipids that participate in several cellular processes. PUFAs GLA (gamma-linolenic acid), DGLA (dihomo-GLA), AA (arachidonic acid), EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are derived from dietary linoleic acid (LA) and alpha-linolenic acid (ALA) by the action of desaturases whose activity declines with age. Consequently, aged cells are deficient in GLA, DGLA, AA, AA, EPA and DHA and their metabolites. LA, ALA, AA, EPA and DHA can also be obtained direct from diet and their deficiency (fatty acids) may indicate malnutrition and deficiency of several minerals, trace elements and vitamins some of which are also much needed co-factors for the normal activity of desaturases. In many instances (patients) the plasma and tissue levels of GLA, DGLA, AA, EPA and DHA are low (as seen in patients with hypertension, type 2 diabetes mellitus) but they do not have deficiency of other nutrients. Hence, it is reasonable to consider that the deficiency of GLA, DGLA, AA, EPA and DHA noted in these conditions are due to the decreased activity of desaturases and elongases. PUFAs stimulate SIRT1 through protein kinase A-dependent activation of SIRT1-PGC1α complex and thus, increase rates of fatty acid oxidation and prevent lipid dysregulation associated with aging. SIRT1 activation prevents aging. Of all the SIRTs, SIRT6 is critical for intermediary metabolism and genomic stability. SIRT6-deficient mice show shortened lifespan, defects in DNA repair and have a high incidence of cancer due to oncogene activation. SIRT6 overexpression lowers LDL and triglyceride level, improves glucose tolerance, and increases lifespan of mice in addition to its anti-inflammatory effects at the transcriptional level. PUFAs and their anti-inflammatory metabolites influence the activity of SIRT6 and other SIRTs and thus, bring about their actions on metabolism, inflammation, and genome maintenance. GLA, DGLA, AA, EPA and DHA and prostaglandin E2 (PGE2), lipoxin A4 (LXA4) (pro- and anti-inflammatory metabolites of AA respectively) activate/suppress various SIRTs (SIRt1 SIRT2, SIRT3, SIRT4, SIRT5, SIRT6), PPAR-γ, PARP, p53, SREBP1, intracellular cAMP content, PKA activity and peroxisome proliferator-activated receptor γ coactivator 1-α (PGC1-α). This implies that changes in the metabolism of bioactive lipids as a result of altered activities of desaturases, COX-2 and 5-, 12-, 15-LOX (cyclo-oxygenase and lipoxygenases respectively) may have a critical role in determining cell age and development of several aging associated diseases and genomic stability and gene and oncogene activation. Thus, methods designed to maintain homeostasis of bioactive lipids (GLA, DGLA, AA, EPA, DHA, PGE2, LXA4) may arrest aging process and associated metabolic abnormalities.

Regenerative Medicine for Equine Musculoskeletal Diseases

Simple Summary

Lameness due to musculoskeletal disease is the most common diagnosis in equine veterinary practice. Many of these orthopaedic disorders are chronic problems, for which no clinically satisfactory treatment exists. Thus, high hopes are pinned on regenerative medicine, which aims to replace or regenerate cells, tissues, or organs to restore or establish normal function. Some regenerative medicine therapies have already made their way into equine clinical practice mainly to treat tendon injures, tendinopathies, cartilage injuries and degenerative joint disorders with promising but diverse results. This review summarises the current knowledge of commonly used regenerative medicine treatments and critically discusses their use.

Abstract

Musculoskeletal injuries and chronic degenerative diseases commonly affect both athletic and sedentary horses and can entail the end of their athletic careers. The ensuing repair processes frequently do not yield fully functional regeneration of the injured tissues but biomechanically inferior scar or replacement tissue, causing high reinjury rates, degenerative disease progression and chronic morbidity. Regenerative medicine is an emerging, rapidly evolving branch of translational medicine that aims to replace or regenerate cells, tissues, or organs to restore or establish normal function. It includes tissue engineering but also cell-based and cell-free stimulation of endogenous self-repair mechanisms. Some regenerative medicine therapies have made their way into equine clinical practice mainly to treat tendon injures, tendinopathies, cartilage injuries and degenerative joint disorders with promising results. However, the qualitative and quantitative spatiotemporal requirements for specific bioactive factors to trigger tissue regeneration in the injury response are still unknown, and consequently, therapeutic approaches and treatment results are diverse. To exploit the full potential of this burgeoning field of medicine, further research will be required and is ongoing. This review summarises the current knowledge of commonly used regenerative medicine treatments in equine patients and critically discusses their use.

Expression of alarmins in a murine rotator cuff tendinopathy model

The aim of this study was to investigate the presence of alarmins in a novel murine rotator cuff tendinopathy model. Alarmins have been described as essential early activators of an immune response to tissue damage. Subacromial impingement was induced in both shoulders of 37 male C57Bl/6 mice by placement of a small metal clip in the subacromial space. Animals were allocated to different time points up to 6 weeks. The morphology and cellularity of the supraspinatus tendon were evaluated by hematoxylin-eosin staining, alcian blue, and picrosirius red. The expression and localization of alarmins interleukin-33 (IL-33), c (HMGB1), hypoxia-inducible factor-1 subunit α (HIF1α), and S100A9 were evaluated by immunohistochemical staining and quantitative polymerase chain reaction. The percentage of positively stained cells with HMGB1 and IL-33 was significantly increased in the impingement group at 1w, 4w, and 6w. HIF1α staining was higher in the impingement group at 1w and 6w compared with the control group. HMGB1 gene expression was higher in the 5d impingement group and 6w impingement group. The gene expression of HIF1α was upregulated at all-time points in the impingement group (5d, 2w, 4w, and 6w). The expression of the S100A9 gene was also upregulated in the 5d impingement group. This is the first study to demonstrate the involvement of alarmins in the early phase of tendinopathy using a reproducible animal model. Alarmins may play an important role in the early phases of the development of tendinopathy They may represent potential therapeutic targets for treatment of tendinopathy.

Conjugation with Methylsulfonylmethane Improves Hyaluronic Acid Anti-Inflammatory Activity in a Hydrogen Peroxide-Exposed Tenocyte Culture In Vitro Model

Rotator cuff tears (RCTs) and rotator cuff disease (RCD) are important causes of disability in middle-aged individuals affected by nontraumatic shoulder dysfunctions. Our previous studies have demonstrated that four different hyaluronic acid preparations (HAPs), including Artrosulfur^® hyaluronic acid (HA) (Alfakjn S.r.l., Garlasco, Italy), may exert a protective effect in human RCT-derived tendon cells undergoing oxidative stress damage. Recently, methylsulfonylmethane (MSM) (Barentz, Paderno Dugnano, Italy) has proven to have anti-inflammatory properties and to cause pain relief in patients affected by tendinopathies. This study aims at evaluating three preparations (Artrosulfur^® HA, MSM, and Artrosulfur^® MSM + HA) in the recovery from hydrogen peroxide-induced oxidative stress damage in human tenocyte. Cell proliferation, Lactate Dehydrogenase (LDH) release, and inducible nitric oxide synthases (iNOS) and prostaglandin E2 (PGE2) modulation were investigated. In parallel, expression of metalloproteinases 2 (MMP2) and 14 (MMP14) and collagen types I and III were also examined. Results demonstrate that Artrosulfur^® MSM + HA improves cell escape from oxidative stress by decreasing cytotoxicity and by reducing iNOS and PGE2 secretion. Furthermore, it differentially modulates MMP2 and MMP14 levels and enhances collagen III expression after 24 h, proteins globally related to rapid acceleration of the extracellular matrix (ECM) remodelling and thus tendon healing. By improving the anti-cytotoxic effect of HA, the supplementation of MSM may represent a feasible strategy to ameliorate cuff tendinopathies.

Epigenetic mechanisms in Tendon Ageing

INTRODUCTION

Tendon is a composite material with a well-ordered hierarchical structure exhibiting viscoelastic properties designed to transfer force. It is recognized that the incidence of tendon injury increases with age, suggesting a deterioration in homeostatic mechanisms or reparative processes. This review summarizes epigenetic mechanisms identified in ageing healthy tendon.

SOURCES OF DATA

We searched multiple databases to produce a systematic review on the role of epigenetic mechanisms in tendon ageing.

AREAS OF AGREEMENT

Epigenetic mechanisms are important in predisposing ageing tendon to injury.

AREAS OF CONTROVERSY

The relative importance of epigenetic mechanisms are unknown in terms of promoting healthy ageing. It is also unknown whether these changes represent protective mechanisms to function or predispose to pathology.

GROWING POINT

Epigenetic markers in ageing tendon, which are under-researched including genome-wide chromatin accessibility, should be investigated.

AREAS TIMELY FOR DEVELOPING RESEARCH

Metanalysis through integration of multiple datasets and platforms will enable a holistic understanding of the epigenome in ageing and its relevance to disease.

Sequential inflammation model for Achilles tendinopathy by elastin degradation with treadmill exercise

Background & objective

Tendinopathy is a tendon disease with abnormal mechanical loading to induce chronic repetitive injury. However, lack of a comparable animal model to demonstrate clinical progressions has hindered the understanding of anatomical and pathological changes. The major extracellular matrix (ECM) in the tendon consists of abundant type I collagen (COL) and minimal amount of elastin (ELN).

Methods

To study the ECM breakdown and inflammation, rat Achilles tendon was harvested and ex vivo incubated with specific enzymes of elastase (ELNase) or collagenase (COLase).

Results

The ELNase broke down ELN, loosened the tendon structure, and increased the COL composition. Increases in cyclooxygenase-2 expression levels in tenocytes were revealed to induce inflammation with either ELNase or COLase. However, incubation of COLase for 12 hours severely digested the tendon. To create a proper ELN degradation in rats, the present study used high-frequency ultrasound to guide the injection of ELNase at the paratendon tissue of the Achilles tendon. The effect of mechanically triggered inflammatory responses was investigated by applying treadmill exercise (15 m/min for 20 min per day). After ELNase injection for 14 and 28 days, a significant loss of ELN was observed, and exercise further facilitated the pathological transition of COL. The dynamics of inflammatory cell recruitments was revealed by specific staining of CD-11b (neutrophils) and CD-68 (macrophage) after in vivo injection of ELNase or COLase for 1, 3, 7, 14, and 28 days. The combination of ELNase and exercise caused early recruitment of neutrophil on day 1 and sequential expression of macrophage on day 7 in peritendinous tissue.

Conclusion

These results suggested that ELN degradation with repetitive mechanical loading may present a suitable model for the pathogenesis of tendinopathy.

The Translational potential of this article

This discover the role of elastin degradation in tendinopathy and the interaction of exercise in the histological changes. The established the pathological model mimicking the pathogenesis to the human disease by injecting the elastase using ultrasound guidance and then applying treadmill exercise. The loss of elastin and change of collagen composition in clinical tendinopathy samples were observed in the rats. In addition, the sequential inflammation cascades were observed in the histological outcomes with combination of elastase injection and treadmill exercise. Thus, this model may be used to test the clinical treatment of tendinopathy in different stages.

Comparison of lameness outcomes in horses with acute or chronic digital lameness that underwent magnetic resonance imaging

Aims: To compare the outcome, in terms of lameness score or return to athletic function, of horses with acute vs. chronic digital lameness that underwent magnetic resonance imaging (MRI) of the distal limb and to compare the proportion of horses that received intra-articular therapy of the distal interphalangeal (DIP) joint and pattern of diagnostic analgesia in these groups. Methods: This is a retrospective study of horses (n = 95) with acute (≤12 weeks; n = 46) or chronic (>12 weeks; n = 49) digital lameness that underwent MRI of the distal limb from 2009-2016, at two equine referral centres in the USA. Criteria for inclusion in the study were that a majority of lameness localised distal to the fetlock, and that lameness assessments for ≥12 months following MRI could be obtained from the medical record or the owner could be interviewed regarding their horse's athletic function. Outcome was characterised by an improvement score where 2 = return to work at a previous or higher level or lameness improved by one grade or more, 1 = return to work at a lower level or lameness improved by less than one grade, and 0 = did not return to work or lameness grade worsened. Whether horses had received intra-articular therapy of the DIP joint and the pattern of diagnostic analgesia prior to MRI was also obtained from medical records or by interviewing the owner. Results: There was a difference (p = 0.004) in the proportion of horses assigned to improvement scores of 0, 1 and 2 between horses with acute or chronic lameness. There was no evidence of a difference in the likelihood of having received intra-articular therapy of the DIP joint prior to MRI between horses with chronic or acute lameness (p = 0.085). Similarly, there was no evidence of a difference in the pattern of diagnostic analgesia prior to MRI between the two groups (p = 0.94). Eighty-two percent of owners of horses with acute and 62% of those with horses with chronic lameness had a positive opinion of the utility of MRI as a diagnostic modality. Conclusion: In a population of horses with digital lameness undergoing MRI, a difference in the outcome, in terms of lameness score or return to athletic function was identified between horses with acute lameness compared to those with chronic lameness. Clinical relevance: Horses with digital lameness that undergo MRI when the lameness is acute may have an improved prognosis due to accurate diagnosis and earlier application of appropriate therapy.

The role of the immune system in tendon healing: a systematic review

INTRODUCTION

The role of the immune system in tendon healing relies on polymorphonucleocytes, mast cells, macrophages and lymphocytes, the 'immune cells' and their cytokine production. This systematic review reports how the immune system affects tendon healing.

SOURCES OF DATA

We registered our protocol (registration number: CRD42019141838). After searching PubMed, Embase and Cochrane Library databases, we included studies of any level of evidence published in peer-reviewed journals reporting clinical or preclinical results. The PRISMA guidelines were applied, and risk of bias and the methodological quality of the included studies were assessed. We excluded all the articles with high risk of bias and/or low quality after the assessment. We included 62 articles assessed as medium or high quality.

AREAS OF AGREEMENT

Macrophages are major actors in the promotion of proper wound healing as well as the resolution of inflammation in response to pathogenic challenge or tissue damage. The immune cells secrete cytokines involving both pro-inflammatory and anti-inflammatory factors which could affect both healing and macrophage polarization.

AREAS OF CONTROVERSY

The role of lymphocytes, mast cells and polymorphonucleocytes is still inconclusive.

GROWING POINTS

The immune system is a major actor in the complex mechanism behind the healing response occurring in tendons after an injury. A dysregulation of the immune response can ultimately lead to a failed healing response.

AREAS TIMELY FOR DEVELOPING RESEARCH

Further studies are needed to shed light on therapeutic targets to improve tendon healing and in managing new way to balance immune response.

Alda-1, an activator of ALDH2, ameliorates Achilles tendinopathy in cellular and mouse models

Achilles tendinopathy has a high re-injury rate and poor prognosis. Development of effective therapy for Achilles tendinopathy is important. Excessive accumulation of ROS and resulting oxidative stress are believed to cause tendinopathy. Overproduction of hydrogen peroxide (H₂O₂), the most common ROS, could lead to the tendinopathy by causing oxidative damage, activation of endoplasmic reticulum (ER) stress and apoptotic death of tenocytes. Activation of mitochondrial aldehyde dehydrogenase 2 (ALDH2) is expected to alleviate oxidative stress and ER stress. Alda-1 is a selective and potent activator of ALDH2. In this study, we examined the cytoprotective benefit of Alda-1, an activator of ALDH2, on H₂O₂-induced Achilles tendinopathy in cellular and mouse models. We prepared cellular and mouse models of Achilles tendinopathy by treating cultured Achilles tenocytes and Achilles tendons with oxidative stressor H₂O₂. Subsequently, we studied the protective benefit of Alda-1 on H₂O₂-induced Achilles tendinopathy. Alda-1 pretreatment attenuated H₂O₂-induced cell death of cultured Achilles tenocytes. Treatment of Alda-1 prevented H₂O₂-induced oxidative stress and depolarization of mitochondrial membrane potential in tenocytes. Application of Alda-1 attenuated H₂O₂-triggered mitochondria- and ER stress-mediated apoptotic cascades in cultured tenocytes. Alda-1 treatment ameliorated the severity of H₂O₂-induced Achilles tendinopathy in vivo by preventing H₂O₂-induced pathological histological features of Achilles tendons, apoptotic death of Achilles tenocytes and upregulated expression of inflammatory cytokines IL-1β and TNF-α. Our results provide the evidence that ALDH2 activator Alda-1 ameliorates H₂O₂-induced Achilles tendinopathy. Alda-1 could be used for preventing and treating Achilles tendinopathy.

Avocado/Soybean Unsaponifiables, Glucosamine and Chondroitin Sulfate Combination Inhibits Proinflammatory COX-2 Expression and Prostaglandin E2 Production in Tendon-Derived Cells

Tendinopathy, a common disorder in man and horses, is characterized by pain, dysfunction, and tendon degeneration. Inflammation plays a key role in the pathogenesis of tendinopathy. Tendon cells produce proinflammatory molecules that induce pain and tissue deterioration. Currently used nonsteroidal anti-inflammatory drugs are palliative but have been associated with adverse side effects prompting the search for safe, alternative compounds. This study determined whether tendon-derived cells' expression of proinflammatory cyclooxygenase (COX)-2 and production of prostaglandin E2 (PGE2) could be attenuated by the combination of avocado/soybean unsaponifiables (ASU), glucosamine (GLU), and chondroitin sulfate (CS). ASU, GLU, and CS have been used in the management of osteoarthritis-associated joint inflammation. Tenocytes in monolayer and microcarrier spinner cultures were incubated with media alone, or with the combination of ASU (8.3 μg/mL), GLU (11 μg/mL), and CS (20 μg/mL). Cultures were next incubated with media alone, or stimulated with interleukin-1β (IL-1β; 10 ng/mL) for 1 h to measure COX-2 gene expression, or for 24 h to measure PGE2 production, respectively. Tenocyte phenotype was analyzed by phase-contrast microscopy, immunocytochemistry, and Western blotting. Tendon-derived cells proliferated and produced extracellular matrix component type I collagen in monolayer and microcarrier spinner cultures. IL-1β-induced COX-2 gene expression and PGE2 production were significantly reduced by the combination of (ASU+GLU+CS). The suppression of IL-1β-induced inflammatory response suggests that (ASU+GLU+CS) may help attenuate deleterious inflammation in tendons.

Biology of Tendon Stem Cells and Tendon in Aging

Both tendon injuries and tendinopathies, particularly rotator cuff tears, increase with tendon aging. Tendon stem cells play important roles in promoting tendon growth, maintenance, and repair. Aged tendons show a decline in regenerative potential coupled with a loss of stem cell function. Recent studies draw attention to aging primarily a disorder of stem cells. The micro-environment (“niche”) where stem cells resided in vivo provides signals that direct them to metabolize, self-renew, differentiate, or remain quiescent. These signals include receptors and secreted soluble factors for cell-cell communication, extracellular matrix, oxidative stress, and vascularity. Both intrinsic cellular deficits and aged niche, coupled with age-associated systemic changes of hormonal and metabolic signals can inhibit or alter the functions of tendon stem cells, resulting in reduced fitness of these primitive cells and hence more frequent injuries and poor outcomes of tendon repair. This review aims to summarize the biological changes of aged tendons. The biological changes of tendon stem cells in aging are reviewed after a systematic search of the PubMed. Relevant factors of stem cell aging including cell-intrinsic factors, changes of microenvironment, and age-associated systemic changes of hormonal and metabolic signals are examined, with findings related to tendon stem cells highlighted when literature is available. Future research directions on the aging mechanisms of tendon stem cells are discussed. Better understanding of the molecular mechanisms underlying the functional decline of aged tendon stem cells would provide insight for the rational design of rejuvenating therapies.

Equine Fetal, Adult, and Embryonic Stem Cell-Derived Tenocytes Are All Immune Privileged but Exhibit Different Immune Suppressive Properties In Vitro

In horses and humans, tendon injuries are a significant problem. Not only can they occur in both athletes and nonathletes, they require lengthy periods of recuperation and undergo poor natural regeneration, which leads to high reinjury rates. Embryonic stem cells (ESCs) may provide a renewable source of allogeneic cells to use in clinical applications to aid tissue regeneration. Equine ESCs can undergo tenocyte differentiation in vivo and in vitro, but the immune properties of tenocytes isolated from either ESCs or tissues have not previously been characterized. Here, we demonstrate that equine tenocytes derived from fetal and adult tendon tissue and ESCs express robust levels of major histocompatibility complex (MHC) I but no MHC II in response to inflammatory cytokine interferon gamma (IFNγ). However, MHC expression does not affect their allorecognition by peripheral blood mononuclear cells in vitro. Adult and fetal tenocytes remain immune privileged and strongly immune suppressive in both the presence and absence of exogenously applied IFNγ. In contrast, ESC-derived tenocytes are immune privileged even in the presence of IFNγ, but they are only weakly immune suppressive in the presence but not in the absence of exogenously applied IFNγ. This is despite ESC-tenocytes expressing a number of genes involved in immune modulation at significantly higher levels than those expressed by adult and fetal tenocytes when in standard, nonstimulated monolayer culture. Together, this work suggests that, similar to other fibroblasts, tenocytes have immune modulatory properties, and that culture-expanded tenocytes derived from primary tissues or ESCs may be safe to use in clinical transplantations to injured tendons of unrelated animals.

Pro-resolving lipid mediators: Agents of anti-ageing?

Inflammation is an essential response to injury and its timely and adequate resolution permits tissue repair and avoidance of chronic inflammation. Ageing is associated with increased inflammation, sub-optimal resolution and these act as drivers for a number of ageing-associated pathologies. We describe the role played by specialised proresolving lipid mediators (SPMs) in the resolution of inflammation and how insufficient levels of these mediators, or compromised responsiveness may play a role in the pathogenesis of many ageing-associated pathologies, e.g. Alzheimer's Disease, atherosclerosis, obesity, diabetes and kidney disease. Detailed examination of the resolution phase of inflammation highlights the potential to harness these lipid mediators and or mimetics of their bioactions, in particular, their synthetic analogues to promote effective resolution of inflammation, without compromising the host immune system.

Photochemical tissue bonding promotes the proliferation and migration of injured tenocytes through ROS/RhoA/NF-κB/Dynamin 2 signaling pathway

Photochemical tissue bonding (PTB) has been found to promote the healing of Achilles tendon tissue injury and to reduce postoperative complications. However, the underlying cellular and molecular mechanisms are not clear. In this study, the cell proliferation, ROS generation, migration and the protein expression of DNM2, NF-κB p65, TGF-β1 and VEGF in tenocytes after PTB treatment were measured by CCK-8, flow cytometry, Transwell and western blot assay, respectively. And those in tenocytes after DNM2 silencing or overexpressing or treatment with inhibitors of NF-κB, ROS and RhoA were also measured. Our results showed that 10 mW PTB treatment for 80 and 120 s significantly increased cell proliferation and increased ROS generation in tenocytes. 10 mW PTB treatment for 40 and 80 s significantly activated RhoA and increased the protein expression of DNM2, NF-κB p65, TGF-β1 and VEGF, but 10 mW PTB treatment for 120 s decreased the protein expression of those. DNM2 silencing significantly suppressed cell migration and the expression of DNM2, TGF-β1, and VEGF in tenocytes after PTB treatment (10 mW, 80 s), which was inhibited by DNM2 overexpression. Individual treatment with inhibitor of NF-κB, ROS, and RhoA in tenocytes showed decreased protein expression of DNM2, TGF-β1, and VEGF. Moreover, in vivo experiment found that PTB treatment significantly inhibited cell apoptosis and the expression of DNM2, NF-κB p65, RhoA, TGF-β1, and VEGF in a time-dependent manner. Taken together, our results suggest that PTB promotes the proliferation and migration of injured tenocytes through ROS/RhoA/NF-κB/DNM2 signaling pathway.

MicroRNA29a Treatment Improves Early Tendon Injury

Tendon injuries (tendinopathies) are common in human and equine athletes and characterized by dysregulated collagen matrix, resulting in tendon damage. We have previously demonstrated a functional role for microRNA29a (miR29a) as a post-transcriptional regulator of collagen 3 expression in murine and human tendon injury. Given the translational potential, we designed a randomized, blinded trial to evaluate the potential of a miR29a replacement therapy as a therapeutic option to treat tendinopathy in an equine model that closely mimics human disease. Tendon injury was induced in the superficial digital flexor tendon (SDFT) of 17 horses. Tendon lesions were treated 1 week later with an intralesional injection of miR29a or placebo. miR29a treatment reduced collagen 3 transcript levels at week 2, with no significant changes in collagen 1. The relative lesion cross-sectional area was significantly lower in miR29a tendons compared to control tendons. Histology scores were significantly better for miR29a-treated tendons compared to control tendons. These data support the mechanism of microRNA-mediated modulation of early pathophysiologic events that facilitate tissue remodeling in the tendon after injury and provides a strong proof of principle that a locally delivered miR29a therapy improves early tendon healing.

Differential expression of alarmins-S100A9, IL-33, HMGB1 and HIF-1α in supraspinatus tendinopathy before and after treatment

BACKGROUND

Alarmins, endogenous molecules released on tissue damage have been shown to play an important role in inflammatory musculoskeletal conditions including fracture repair andrheumatoid arthritis. However, the contribution of alarmins to the pathogenesis of tendon disease is not fully understood.

METHODS

We investigated expression of alarmin proteins (S100A9, high-mobility group box 1 (HMGB1) and interleukin-33 (IL-33) and hypoxia-inducible factor 1α (HIF-1α), a subunit of an oxygen sensitive transcription factor, in three cohorts of human supraspinatus tissues: healthy (n=6), painful diseased (n=13) and post-treatment pain-free tendon samples (n=5). Tissue samples were collected during shoulder stabilisation surgery (healthy) or by biopsy needle (diseased/treated). Immunohistochemistry was used to investigate the protein expression of these factors in these healthy, diseased and treated tendons. Kruskal-Wallis with pairwise post hoc Mann-Whitney U tests were used to test for differences in immunopositive staining between these tissue cohorts. Additionally, costaining was performed to identify the cell types expressing HIF-1α, S100A9, IL-33 and HMGB1 in diseased tendons.

RESULTS

Immunostaining showed HIF-1α and S100A9 were increased in diseased compared with healthy and post-treatment pain-free tendons (p<0.05). IL-33 was reduced in diseased compared with healthy tendons (p=0.0006). HMGB1 was increased in post-treatment pain-free compared with healthy and diseased tendons (p<0.01). Costaining of diseased tendon samples revealed that HIF-1α, S100A9 and IL-33 were expressed by CD68+ and CD68- cells, whereas HMGB1 was predominantly expressed by CD68- cells.

CONCLUSIONS

This study provides insight into the pathways contributing to the progressionand resolution of tendon disease. We found potential pro-inflammatory and pathogenic roles for HIF-1α and S100A9, a protective role fornuclear IL-33 and a potentially reparative function for HMGB1 in diseased supraspinatus tendons.

Selenium Deficiency Influences the Expression of Selenoproteins and Inflammatory Cytokines in Chicken Aorta Vessels

Selenium deficiency is known to cause cardiovascular diseases. However, the role of Se deficiency in causing oxidative damage and inflammation injury to the aorta vessels of chickens is not well known. In the present study, 180 1-day-old chickens were randomly divided into two groups, a low-Se group (L group) and a control-Se group (C group). The messenger RNA (mRNA) levels of 25 selenoproteins, the mRNA and protein expression levels of inflammatory cytokines (including NF-κB, TNF-α, COX-2, and PTGES), and the antioxidant levels in chicken aorta vessels were examined. The results showed that the mRNA levels of 25 selenoproteins and the activity of Gpx were decreased, while the mRNA and protein expression levels of inflammatory cytokines and the MDA content were increased by Se deficiency in chicken aorta vessels. The data from the present study indicated that Se deficiency decreases the expression of selenoproteins, reduces antioxidant function, and increases the expression of inflammatory factors in chicken aorta vessels.

Inflammation activation and resolution in human tendon disease

Improved understanding of the role of inflammation in tendon disease is required to facilitate therapeutic target discovery. We studied supraspinatus tendons from patients experiencing pain before and after surgical subacromial decompression treatment. Tendons were classified as having early, intermediate, or advanced disease, and inflammation was characterized through activation of pathways mediated by interferon (IFN), nuclear factor κB (NF-κB), glucocorticoid receptor, and signal transducer and activator of transcription 6 (STAT-6). Inflammation signatures revealed expression of genes and proteins induced by IFN and NF-κB in early-stage disease and genes and proteins induced by STAT-6 and glucocorticoid receptor activation in advanced-stage disease. The proresolving proteins FPR2/ALX and ChemR23 were increased in early-stage disease compared to intermediate- to advanced-stage disease. Patients who were pain-free after treatment had tendons with increased expression of CD206 and ALOX15 mRNA compared to tendons from patients who continued to experience pain after treatment, suggesting that these genes and their pathways may moderate tendon pain. Stromal cells from diseased tendons cultured in vitro showed increased expression of NF-κB and IFN target genes after treatment with lipopolysaccharide or IFNγ compared to stromal cells derived from healthy tendons. We identified 15-epi lipoxin A4, a stable lipoxin isoform derived from aspirin treatment, as potentially beneficial in the resolution of tendon inflammation.

Ageing does not result in a decline in cell synthetic activity in an injury prone tendon

Advancing age is a well-known risk factor for tendon disease. Energy-storing tendons [e.g., human Achilles, equine superficial digital flexor tendon (SDFT)] are particularly vulnerable and it is thought that injury occurs following an accumulation of micro-damage in the extracellular matrix (ECM). Several authors suggest that age-related micro-damage accumulates due to a failure of the aging cell population to maintain the ECM or an imbalance between anabolic and catabolic pathways. We hypothesized that ageing results in a decreased ability of tendon cells to synthesize matrix components and matrix-degrading enzymes, resulting in a reduced turnover of the ECM and a decreased ability to repair micro-damage. The SDFT was collected from horses aged 3-30 years with no signs of tendon injury. Cell synthetic and degradative ability was assessed at the mRNA and protein levels. Telomere length was measured as an additional marker of cell ageing. There was no decrease in cellularity or relative telomere length with increasing age, and no decline in mRNA or protein levels for matrix proteins or degradative enzymes. The results suggest that the mechanism for age-related tendon deterioration is not due to reduced cellularity or a loss of synthetic functionality and that alternative mechanisms should be considered.

Transcriptome analysis of ageing in uninjured human Achilles tendon

INTRODUCTION

The risk of tendon injury and disease increases significantly with increasing age. The aim of the study was to characterise transcriptional changes in human Achilles tendon during the ageing process in order to identify molecular signatures that might contribute to age-related degeneration.

METHODS

RNA for gene expression analysis using RNA-Seq and quantitative real-time polymerase chain reaction analysis was isolated from young and old macroscopically normal human Achilles tendon. RNA sequence libraries were prepared following ribosomal RNA depletion, and sequencing was undertaken by using the Illumina HiSeq 2000 platform. Expression levels among genes were compared by using fragments per kilobase of exon per million fragments mapped. Differentially expressed genes were defined by using Benjamini-Hochberg false discovery rate approach (P<0.05, expression ratios 1.4 log2 fold change). Alternative splicing of exon variants were also examined by using Cufflinks. The functional significance of genes that showed differential expression between young and old tendon was determined by using ingenuity pathway analysis.

RESULTS

In total, the expression of 325 transcribed elements, including protein-coding transcripts and non-coding transcripts (small non-coding RNAs, pseudogenes, long non-coding RNAs and a single microRNA), was significantly different in old compared with young tendon (±1.4 log2 fold change, P<0.05). Of these, 191 were at higher levels in older tendon and 134 were at lower levels in older tendon. The top networks for genes differentially expressed with tendon age were from cellular function, cellular growth, and cellular cycling pathways. Notable differential transcriptome changes were also observed in alternative splicing patterns. Several of the top gene ontology terms identified in downregulated isoforms in old tendon related to collagen and post-translational modification of collagen.

CONCLUSIONS

This study demonstrates dynamic alterations in RNA with age at numerous genomic levels, indicating changes in the regulation of transcriptional networks. The results suggest that ageing is not primarily associated with loss of ability to synthesise matrix proteins and matrix-degrading enzymes. In addition, we have identified non-coding RNA genes and differentially expressed transcript isoforms of known matrix components with ageing which require further investigation.

The equine immune responses to infectious and allergic disease: a model for humans?

The modern horse, Equus caballus has historically made important contributions to the field of immunology, dating back to Emil von Behring's description of curative antibodies in equine serum over a century ago. While the horse continues to play an important role in human serotherapy, the mouse has replaced the horse as the predominant experimental animal in immunology research. Nevertheless, continuing efforts have led to an improved understanding of the equine immune response in a variety of infectious and non-infectious diseases. Based on this information, we can begin to identify specific situations where the horse may provide a unique immunological model for certain human diseases.

Aging delays resolution of acute inflammation in mice: reprogramming the host response with novel nano-proresolving medicines

Aging is associated with an overt inflammatory phenotype and physiological decline. Specialized proresolving lipid mediators (SPMs) are endogenous autacoids that actively promote resolution of inflammation. In this study, we investigated resolution of acute inflammation in aging and the roles of SPMs. Using a self-resolving peritonitis and resolution indices coupled with lipid mediator metabololipidomics, we found that aged mice had both delayed resolution and reduced SPMs. The SPM precursor docosahexaenoic acid accelerated resolution via increased SPMs and promoted human monocyte reprogramming. In aged mice, novel nano-proresolving medicines carrying aspirin-triggered resolvins D1 and D3 reduced inflammation by promoting efferocytosis. These findings provide evidence for age-dependent resolution pathways in acute inflammation and novel means to activate resolution.

Proteomic analysis reveals age-related changes in tendon matrix composition, with age- and injury-specific matrix fragmentation

Energy storing tendons, such as the human Achilles and equine superficial digital flexor tendon (SDFT), are highly prone to injury, the incidence of which increases with aging. The cellular and molecular mechanisms that result in increased injury in aged tendons are not well established but are thought to result in altered matrix turnover. However, little attempt has been made to fully characterize the tendon proteome nor determine how the abundance of specific tendon proteins changes with aging and/or injury. The aim of this study was, therefore, to assess the protein profile of normal SDFTs from young and old horses using label-free relative quantification to identify differentially abundant proteins and peptide fragments between age groups. The protein profile of injured SDFTs from young and old horses was also assessed. The results demonstrate distinct proteomic profiles in young and old tendon, with alterations in the levels of proteins involved in matrix organization and regulation of cell tension. Furthermore, we identified several new peptide fragments (neopeptides) present in aged tendons, suggesting that there are age-specific cleavage patterns within the SDFT. Proteomic profile also differed between young and old injured tendon, with a greater number of neopeptides identified in young injured tendon. This study has increased the knowledge of molecular events associated with tendon aging and injury, suggesting that maintenance and repair of tendon tissue may be reduced in aged individuals and may help to explain why the risk of injury increases with aging.

An overview of structure, mechanical properties, and treatment for age-related tendinopathy

Tendons transfer tensile loads from muscle to bone, which enable joint motions and stabilize joints. Tendons sustain large mechanical loads in vivo and as a result, tendons were frequently injured. Aging has been confirmed as a predisposing factor of tendinopathy and bad recovery quality following tendon repair. Current treatment methods are generally not effective and involve either symptomatic relief with non-steroidal antiinflammatory drugs and physical therapy or surgery when conservative treatments failed. The limitation in treatment options is due to our incomplete knowledge of age-related tendinopathy. Studies over the past decades have uncovered a number of important mechanical and cellular changes of aging tendon. However, the basis of aging as a major risk factor for tendon injury and impaired tendon healing remained poorly understood. The objectives of this review are to provide an overview of the current knowledge about the aging-associated changes of structure, mechanical properties and treatment in tendon and highlight causes and therapies for age-related tendinopathy.

Resolving an inflammatory concept: the importance of inflammation and resolution in tendinopathy

Injuries to the superficial digital flexor tendon (SDFT) are an important cause of morbidity and mortality in equine athletes, but the healing response is poorly understood. One important drive for the healing of connective tissues is the inflammatory cascade, but the role of inflammation in tendinopathy has been contentious in the literature. This article reviews the processes involved in the healing of tendon injuries in natural disease and experimental models. The importance of inflammatory processes known to be active in tendon disease is discussed with particular focus on recent findings related specifically to the horse.

Whilst inflammation is necessary for debridement after injury, persistent inflammation is thought to drive fibrosis, a perceived adverse consequence of tendon healing. Therefore the ability to resolve inflammation by the resident cell populations in tendons at an appropriate time would be crucial for successful outcome. This review summarises new evidence for the importance of resolution of inflammation after tendon injury. Given that many anti-inflammatory drugs suppress both inflammatory and resolving components of the inflammatory response, prolonged use of these drugs may be contraindicated as a therapeutic approach. We propose that these findings have profound implications not only for current treatment strategies but also for the possibility of developing novel therapeutic approaches involving modulation of the inflammatory process.

Proteomic analysis of tendon extracellular matrix reveals disease stage-specific fragmentation and differential cleavage of COMP (cartilage oligomeric matrix protein)

During inflammatory processes the extracellular matrix (ECM) is extensively remodeled, and many of the constituent components are released as proteolytically cleaved fragments. These degradative processes are better documented for inflammatory joint diseases than tendinopathy even though the pathogenesis has many similarities. The aims of this study were to investigate the proteomic composition of injured tendons during early and late disease stages to identify disease-specific cleavage patterns of the ECM protein cartilage oligomeric matrix protein (COMP). In addition to characterizing fragments released in naturally occurring disease, we hypothesized that stimulation of tendon explants with proinflammatory mediators in vitro would induce fragments of COMP analogous to natural disease. Therefore, normal tendon explants were stimulated with IL-1β and prostaglandin E₂, and their effects on the release of COMP and its cleavage patterns were characterized. Analyses of injured tendons identified an altered proteomic composition of the ECM at all stages post injury, showing protein fragments that were specific to disease stage. IL-1β enhanced the proteolytic cleavage and release of COMP from tendon explants, whereas PGE₂ had no catabolic effect. Of the cleavage fragments identified in early stage tendon disease, two fragments were generated by an IL-1-mediated mechanism. These fragments provide a platform for the development of neo-epitope assays specific to injury stage for tendon disease.

Beneficial effects of autologous bone marrow-derived mesenchymal stem cells in naturally occurring tendinopathy

Tendon injuries are a common age-related degenerative condition where current treatment strategies fail to restore functionality and normal quality of life. This disease also occurs naturally in horses, with many similarities to human tendinopathy making it an ideal large animal model for human disease. Regenerative approaches are increasingly used to improve outcome involving mesenchymal stem cells (MSCs), supported by clinical data where injection of autologous bone marrow derived MSCs (BM-MSCs) suspended in marrow supernatant into injured tendons has halved the re-injury rate in racehorses. We hypothesized that stem cell therapy induces a matrix more closely resembling normal tendon than the fibrous scar tissue formed by natural repair. Twelve horses with career-ending naturally-occurring superficial digital flexor tendon injury were allocated randomly to treatment and control groups. 1X10(7) autologous BM-MSCs suspended in 2 ml of marrow supernatant were implanted into the damaged tendon of the treated group. The control group received the same volume of saline. Following a 6 month exercise programme horses were euthanized and tendons assessed for structural stiffness by non-destructive mechanical testing and for morphological and molecular composition. BM-MSC treated tendons exhibited statistically significant improvements in key parameters compared to saline-injected control tendons towards that of normal tendons and those in the contralateral limbs. Specifically, treated tendons had lower structural stiffness (p<0.05) although no significant difference in calculated modulus of elasticity, lower (improved) histological scoring of organisation (p<0.003) and crimp pattern (p<0.05), lower cellularity (p<0.007), DNA content (p<0.05), vascularity (p<0.03), water content (p<0.05), GAG content (p<0.05), and MMP-13 activity (p<0.02). Treatment with autologous MSCs in marrow supernatant therefore provides significant benefits compared to untreated tendon repair in enhancing normalisation of biomechanical, morphological, and compositional parameters. These data in natural disease, with no adverse findings, support the use of this treatment for human tendon injuries.

Tendon cell ciliary length as a biomarker of in situ cytoskeletal tensional homeostasis

To determine if tendon cell ciliary length could be used as a biomarker of cytoskeletal tensional homeostasis, 20 mm long adult rat tail tendons were placed in double artery clamps set 18 mm apart to create a 10% laxity. The tendons were allowed to contract over a 7 day period under culture conditions. At 0, 1, 5, and 7 days the tendon cell cilia were stained and ciliary length measured using confocal imaging. There was a significant (p<0.001) increase in ciliary length at 1 day. At day 5 (when the tendon became visibly taut) there was a significant (p<0.001) decrease in ciliary length compared to day 1. By day 7 the tendon remained taut and ciliary length returned to day zero values (p=0.883). These results suggest that cilia length reflects the local mechanobiological environment of tendon cells and could be used as a potential in situ biomarker of altered cytoskeletal tensional homeostasis.