Genetic Response of Rat Supraspinatus Tendon and Muscle to Exercise

Evaluation of suitable reference genes for qPCR normalisation of gene expression in a Achilles tendon injury model

Tendons are one of the major load-bearing tissues in the body; subjected to enormous peak stresses, and thus vulnerable to injury. Cellular responses to tendon injury are complex, involving inflammatory and repair components, with the latter employing both resident and recruited exogenous cell populations. Gene expression analyses are valuable tools for investigating tendon injury, allowing assessment of repair processes and pathological responses such as fibrosis, and permitting evaluation of therapeutic pharmacological interventions. Quantitative polymerase chain reaction (qPCR) is a commonly used approach for such studies, but data obtained by this method must be normalised to reference genes: genes known to be stably expressed between the experimental conditions investigated. Establishing suitable tendon injury reference genes is thus essential. Accordingly we investigated mRNA expression stability in a rat model of tendon injury, comparing both injured and uninjured tendons, and the effects of rapamycin treatment, at 1 and 3 weeks post injury. We used 11 candidate genes (18S, ACTB, AP3D1, B2M, CSNK2A2, GAPDH, HPRT1, PAK1IP1, RPL13a, SDHA, UBC) and assessed stability via four complementary algorithms (Bestkeeper, deltaCt, geNorm, Normfinder). Our results suggests that ACTB, CSNK2A2, HPRT1 and PAK1IP1 are all stably expressed in tendon, regardless of injury or drug treatment: any three of these would serve as universally suitable reference gene panel for normalizing qPCR expression data in the rat tendon injury model. We also reveal 18S, UBC, GAPDH, and SDHA as consistently poor scoring candidates (with the latter two exhibiting rapamycin- and injury-associated changes, respectively): these genes should be avoided.

Aged Tendons Exhibit Altered Mechanisms of Strain-Dependent Extracellular Matrix Remodeling

Aging is a primary risk factor for degenerative tendon injuries, yet the etiology and progression of this degeneration are poorly understood. While aged tendons have innate cellular differences that support a reduced ability to maintain mechanical tissue homeostasis, the response of aged tendons to altered levels of mechanical loading has not yet been studied. To address this question, we subjected young and aged murine flexor tendon explants to various levels of in vitro tensile strain. We first compared the effect of static and cyclic strain on matrix remodeling in young tendons, finding that cyclic strain is optimal for studying remodeling in vitro. We then investigated the remodeling response of young and aged tendon explants after 7 days of varied mechanical stimulus (stress deprivation, 1%, 3%, 5%, or 7% cyclic strain) via assessment of tissue composition, biosynthetic capacity, and degradation profiles. We hypothesized that aged tendons would show muted adaptive responses to changes in tensile strain and exhibit a shifted mechanical setpoint, at which the remodeling balance is optimal. Interestingly, we found that 1% cyclic strain best maintains native physiology while promoting extracellular matrix (ECM) turnover for both age groups. However, aged tendons display fewer strain-dependent changes, suggesting a reduced ability to adapt to altered levels of mechanical loading. This work has a significant impact on understanding the regulation of tissue homeostasis in aged tendons, which can inform clinical rehabilitation strategies for treating elderly patients.

Aged Tendons Exhibit Altered Mechanisms of Strain-Dependent Extracellular Matrix Remodeling

Aging is a primary risk factor for degenerative tendon injuries, yet the etiology and progression of this degeneration is poorly understood. While aged tendons have innate cellular differences that support a reduced ability to maintain mechanical tissue homeostasis, the response of aged tendons to altered levels of mechanical loading has not yet been studied. To address this question, we subjected young and aged murine flexor tendon explants to various levels of in vitro tensile strain. We first compared the effect of static and cyclic strain on matrix remodeling in young tendons, finding that cyclic strain is optimal for studying remodeling in vitro. We then investigated the remodeling response of young and aged tendon explants after 7 days of varied mechanical stimulus (stress-deprivation, 1%, 3%, 5%, or 7% cyclic strain) via assessment of tissue composition, biosynthetic capacity, and degradation profiles. We hypothesized that aged tendons would show muted adaptive responses to changes in tensile strain and exhibit a shifted mechanical setpoint, at which the remodeling balance is optimal. Interestingly, we found 1% cyclic strain best maintains native physiology while promoting ECM turnover for both age groups. However, aged tendons display fewer strain-dependent changes, suggesting a reduced ability to adapt to altered levels of mechanical loading. This work has significant impact in understanding the regulation of tissue homeostasis in aged tendons, which can inform clinical rehabilitation strategies for treating elderly patients.

Nonsurgical treatment reduces tendon inflammation and elevates tendon markers in early healing

Operative treatment is assumed to provide superior outcomes to nonoperative (conservative) treatment following Achilles tendon rupture, however, this remains controversial. This study explores the effect of surgical repair on Achilles tendon healing. Rat Achilles tendons (n = 101) were bluntly transected and were randomized into groups receiving repair or non-repair treatments. By 1 week after injury, repaired tendons had inferior mechanical properties, which continued to 3- and 6-week post-injury, evidenced by decreased dynamic modulus and failure stress. Transcriptomics analysis revealed >7000 differentially expressed genes between repaired and non-repaired tendons after 1-week post-injury. While repaired tendons showed enriched inflammatory gene signatures, non-repaired tendons showed increased tenogenic, myogenic, and mechanosensitive gene signatures, with >200-fold enrichment in Tnmd expression. Analysis of gastrocnemius muscle revealed elevated MMP activity in tendons receiving repair treatment, despite no differences in muscle fiber morphology. Transcriptional regulation analysis highlighted that the highest expressed transcription factors in repaired tendons were associated with inflammation (Nfκb, SpI1, RelA, and Stat1), whereas non-repaired tendons expressed markers associated with tissue development and mechano-activation (Smarca1, Bnc2, Znf521, Fbn1, and Gli3). Taken together, these data highlight distinct differences in healing mechanism occurring immediately following injury and provide insights for new therapies to further augment tendons receiving repaired and non-repaired treatments.

Advances toward transformative therapies for tendon diseases

Approved therapies for tendon diseases have not yet changed the clinical practice of symptomatic pain treatment and physiotherapy. This review article summarizes advances in the development of novel drugs, biologic products, and biomaterial therapies for tendon diseases with perspectives for translation of integrated therapies. Shifting from targeting symptom relief toward disease modification and prevention of disease progression may open new avenues for therapies. Deep evidence-based clinical, cellular, and molecular characterization of the underlying pathology of tendon diseases, as well as therapeutic delivery optimization and establishment of multidiscipline interorganizational collaboration platforms, may accelerate the discovery and translation of transformative therapies for tendon diseases.

The role of loading in murine models of rotator cuff disease

Rotator cuff disease pathogenesis is associated with intrinsic (e.g., age, joint laxity, muscle weakness) and extrinsic (e.g., mechanical load, fatigue) factors that lead to chronic degeneration of the cuff tissues. However, etiological studies are difficult to perform in patients due to the long duration of disease onset and progression. Therefore, the purpose of this study was to determine the effects of altered joint loading on the rotator cuff. Mice were subjected to one of three load-dependent rotator cuff tendinopathy models: underuse loading, achieved by injecting botulinum toxin-A into the supraspinatus muscle; overuse loading, achieved using downhill treadmill running; destabilization loading, achieved by surgical excision of the infraspinatus tendon. All models were compared to cage activity animals. Whole joint function was assessed longitudinally using gait analysis. Tissue-scale structure and function were determined using microCT, tensile testing, and histology. The molecular response of the supraspinatus tendon and enthesis was determined by measuring the expression of 84 wound healing-associated genes. Underuse and destabilization altered forepaw weight-bearing, decreased tendon-to-bone attachment strength, decreased mineral density of the humeral epiphysis, and reduced tendon strength. Transcriptional activity of the underuse group returned to baseline levels by 4 weeks, while destabilization had significant upregulation of inflammation, growth factors, and extracellular matrix remodeling genes. Surprisingly, overuse activity caused changes in walking patterns, increased tendon stiffness, and primarily suppressed expression of wound healing-related genes. In summary, the tendinopathy models demonstrated how divergent muscle loading can result in clinically relevant alterations in rotator cuff structure, function, and gene expression.

Tendinopathy and tendon material response to load: What we can learn from small animal studies

Tendinopathy is a debilitating disease that causes as much as 30% of all musculoskeletal consultations. Existing treatments for tendinopathy have variable efficacy, possibly due to incomplete characterization of the underlying pathophysiology. Mechanical load can have both beneficial and detrimental effects on tendon, as the overall tendon response depends on the degree, frequency, timing, and magnitude of the load. The clinical continuum model of tendinopathy offers insight into the late stages of tendinopathy, but it does not capture the subclinical tendinopathic changes that begin before pain or loss of function. Small animal models that use high tendon loading to mimic human tendinopathy may be able to fill this knowledge gap. The goal of this review is to summarize the insights from in-vivo animal studies of mechanically-induced tendinopathy and higher loading regimens into the mechanical, microstructural, and biological features that help characterize the continuum between normal tendon and tendinopathy. STATEMENT OF SIGNIFICANCE: This review summarizes the insights gained from in-vivo animal studies of mechanically-induced tendinopathy by evaluating the effect high loading regimens have on the mechanical, structural, and biological features of tendinopathy. A better understanding of the interplay between these realms could lead to improved patient management, especially in the presence of painful tendon.

Tendon Extracellular Matrix Assembly, Maintenance and Dysregulation Throughout Life

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.

Age-associated changes in the response of tendon explants to stress deprivation is sex-dependent

Purpose of the Study: The incidence of tendon injuries increases dramatically with age, which presents a major clinical burden. While previous studies have sought to identify age-related changes in extracellular matrix structure and function, few have been able to explain fully why aged tissues are more prone to degeneration and injury. In addition, recent studies have also demonstrated that age-related processes in humans may be sex-dependent, which could be responsible for muddled conclusions in changes with age. In this study, we investigate short-term responses through an ex vivo explant culture model of stress deprivation that specifically questions how age and sex differentially affect the ability of tendons to respond to altered mechanical stimulus.Materials and Methods: We subjected murine flexor explants from young (4 months of age) and aged (22-24 months of age) male and female mice to stress-deprived culture conditions for up to 1 week and investigated changes in viability, cell metabolism and proliferation, matrix biosynthesis and composition, gene expression, and inflammatory responses throughout the culture period.Results and Conclusions: We found that aging did have a significant influence on the response to stress deprivation, demonstrating that aged explants have a less robust response overall with reduced metabolic activity, viability, proliferation, and biosynthesis. However, age-related changes appeared to be sex-dependent. Together, this work demonstrates that the aging process and the subsequent effect of age on the ability of tendons to respond to stress-deprivation are inherently different based on sex, where male explants favor increased activity, apoptosis, and matrix remodeling while female explants favor reduced activity and tissue preservation.

Doxycycline improves cage activity, but not exercised, supraspinatus tendon and muscle in a rat model

The objective of this study was to investigate the effects of doxycycline, a broad-spectrum MMP inhibitor, on cage activity and exercised supraspinatus tendon and muscle using a Sprague-Dawley rat model of non-injurious exercise. Because exercise may alter muscle and tendon MMP activity and matrix turnover, we hypothesized that doxycycline would abolish the beneficial adaptations found with exercise but have no effect on cage activity muscle and tendon properties. Rats were divided into acute or chronic exercise (EX) or cage activity (CA) groups, and half of the rats received doxycycline orally. Animals in acute EX groups were euthanized 24 h after a single bout of exercise (10 m/min, 1 h) on a flat treadmill. Animals in chronic EX groups walked on a flat treadmill and were euthanized at 2 or 8 week time points. Assays included supraspinatus tendon mechanics and histology and muscle fiber morphologic and type analysis. Doxycycline improved tendon mechanical properties and collagen organization in chronic cage activity groups, which was not consistently evident in exercised groups. Combined with exercise, doxycycline decreased average muscle fiber cross-sectional area. Results of this study suggest that administration of doxycycline at pharmaceutical doses induces beneficial supraspinatus tendon adaptations without negatively affecting the muscle in cage activity animals, supporting the use of doxycycline to combat degenerative processes associated with underuse; however, when combined with exercise, doxycycline does not consistently produce the same beneficial adaptations in rat supraspinatus tendons and reduces muscle fiber cross-sectional area, suggesting that doxycycline is not advantageous when combined with activity.

Muscle does not drive persistent posttraumatic elbow contracture in a rat model

INTRODUCTION

Posttraumatic elbow contracture is clinically challenging because injury often disrupts multiple periarticular soft tissues. Tissue specific contribution to contracture, particularly muscle, remains poorly understood.

METHODS

In this study we used a previously developed animal model of elbow contracture. After surgically inducing a unilateral soft tissue injury, injured limbs were immobilized for 3, 7, 21, and 42 days (IM) or for 42 IM with 42 days of free mobilization (42/42 IM-FM). Biceps brachii active/passive mechanics and morphology were evaluated at 42 IM and 42/42 IM-FM, whereas biceps brachii and brachialis gene expression was evaluated at all time points.

RESULTS

Injured limb muscle exhibited significantly altered active/passive mechanics and decreased fiber area at 42 IM but returned to control levels by 42/42 IM-FM. Gene expression suggested muscle growth rather than a fibrotic response at 42/42 IM-FM.

DISCUSSION

Muscle is a transient contributor to motion loss in our rat model of posttraumatic elbow contracture. Muscle Nerve 58:843-851, 2018.

Rat supraspinatus tendon responds acutely and chronically to exercise

The objective of this study was to identify acute responses and chronic adaptations of supraspinatus tendon to noninjurious exercise. We hypothesized that chronic exercise (EX) increases tendon mechanical properties, and a single exercise bout increases matrix metalloproteinase (MMP) activity acutely. Rats were divided into acute or chronic EX or cage activity groups. Animals in acute EX groups were euthanized, 3, 12, 24, 48, or 72 h upon completion of a single bout of exercise (10 m/min, 1 h) on a flat treadmill. Animals in chronic EX groups walked on a flat treadmill for 3 days or 1, 2, or 8 wk. Tendon histology, MMP activity, and mechanics were measured. A single bout of exercise trended toward reducing tendon mechanical properties, but 2 or 8 wk of chronic EX increased tendon mechanics. Cell density was not affected. Cells became rounder with chronic EX. All tendons were highly organized. MMP activity decreased after a single bout of exercise and returned to baseline by 72 h. MMP activity decreased after 8 wk of chronic EX. Decreased MMP activity may indicate an anabolic instead of catabolic response in contrast to injury. Results suggest that mild, acute decreases in MMP activity and tendon mechanics following a single exercise bout lead to enhanced tendon mechanical adaptations with repeated exercise bouts. This study defines acute and chronic changes of MMP activity, mechanical properties, and histology of the rat supraspinatus tendon in response to beneficial exercise and proposes a mechanism by which acute responses translate to chronic adaptations.NEW & NOTEWORTHY The line between beneficial exercise and overuse has not been elucidated. This study defines the acute and chronic temporal response to exercise of supraspinatus tendon in an in vivo model. We found that decreased matrix metalloproteinase activity and tendon mechanics after a single bout of exercise are followed by beneficial chronic adaptations of the tendon with repeated bouts. How the acute responses to exercise lead to chronic adaptations may distinguish beneficial exercise from overuse.

Ibuprofen Differentially Affects Supraspinatus Muscle and Tendon Adaptations to Exercise in a Rat Model

BACKGROUND

Previous studies have shown that ibuprofen is detrimental to tissue healing after acute injury; however, the effects of ibuprofen when combined with noninjurious exercise are debated.

HYPOTHESIS

Administration of ibuprofen to rats undergoing a noninjurious treadmill exercise protocol will abolish the beneficial adaptations found with exercise but will have no effect on sedentary muscle and tendon properties.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 167 male Sprague-Dawley rats were divided into exercise or cage activity (sedentary) groups and acute (a single bout of exercise followed by 24 hours of rest) and chronic (2 or 8 weeks of repeated exercise) response times. Half of the rats were administered ibuprofen to investigate the effects of this drug over time when combined with different activity levels (exercise and sedentary). Supraspinatus tendons were used for mechanical testing and histologic assessment (organization, cell shape, cellularity), and supraspinatus muscles were used for morphologic (fiber cross-sectional area, centrally nucleated fibers) and fiber type analysis.

RESULTS

Chronic intake of ibuprofen did not impair supraspinatus tendon organization or mechanical adaptations (stiffness, modulus, maximum load, maximum stress, dynamic modulus, or viscoelastic properties) to exercise. Tendon mechanical properties were not diminished and in some instances increased with ibuprofen. In contrast, total supraspinatus muscle fiber cross-sectional area decreased with ibuprofen at chronic response times, and some fiber type-specific changes were detected.

CONCLUSION

Chronic administration of ibuprofen does not impair supraspinatus tendon mechanical properties in a rat model of exercise but does decrease supraspinatus muscle fiber cross-sectional area. This fundamental study adds to the growing literature on the effects of ibuprofen on musculoskeletal tissues and provides a solid foundation on which future work can build.

CLINICAL RELEVANCE

The study findings suggest that ibuprofen does not detrimentally affect regulation of supraspinatus tendon adaptations to exercise but does decrease muscle growth. Individuals should be advised on the risk of decreased muscle hypertrophy when consuming ibuprofen.