Changes in gene expression of individual matrix metalloproteinases differ in response to mechanical unloading of tendon fascicles in explant culture

Macrophage Mechano-Responsiveness Within Three-Dimensional Tissue Matrix upon Mechanotherapy-Associated Strains

Mechano-rehabilitation, also known as mechanotherapy, represents the forefront of noninvasive treatment for musculoskeletal (MSK) tissue disorders, encompassing conditions affecting tendons, cartilage, ligaments, and muscles. Recent emphasis has underscored the significance of macrophage presence in the healing of MSK tissues. However, a considerable gap still exists in comprehending how mechanical strains associated with mechanotherapy impact both the naïve and pro-inflammatory macrophage phenotypes within the three-dimensional (3D) tissue matrix, as well as whether the shift in macrophage phenotype is contingent on the mechanical strains inherent to mechanotherapy. In this study, we delineated alterations in mechano-adaptation and polarization of both naive and M1 macrophages within 3D matrices, elucidating their response to varying degrees of mechanical strain exposure (3%, 6%, and 12%). To evaluate macrophage mechano-adaptation and mechano-sensitivity within 3D collagen matrices under mechanical loading, we employed structural techniques (scanning electron microscopy, histology), quantitative morphological measures for phenotypic assessment, and genotypic methods such as quantitative real-time polymerase chain reaction. Our data reveal that the response of macrophages to mechanical loading is not only contingent on their specific sub-phenotype but also varies with the amplitude of mechanical strain. Notably, although supra-mechanical loading (12% strain) was requisite to induce a phenotypic shift in naive (M0) macrophages, as little as 3% mechanical strain proved sufficient to prompt phenotypic alterations in pro-inflammatory (M1) macrophages. These findings pave the way for leveraging the macrophage mechanome in customized and targeted applications of mechanical strain within the mechano-therapeutic framework. Considering the prevalence of MSK tissue injuries and their profound societal and economic implications, the development of well-informed and effective clinical mechanotherapy modalities for MSK tissue healing becomes an imperative endeavor. Impact statement Mechanotherapy is a primary noninvasive treatment for musculoskeletal (MSK) tissue injuries, but the effect of mechanical strain on macrophage phenotypes is not fully understood. A recent study found that macrophage response to mechanical loading is both sub-phenotype specific and amplitude-dependent, with even small strains enough to induce phenotypic changes in pro-inflammatory macrophages. These findings could pave the way for using macrophage mechanome in targeted mechanotherapy applications for better MSK tissue healing.

Tendon cell and tissue culture: Perspectives and recommendations

The wide variety of cell and tissue culture systems used to study and engineer tendons can make it difficult to choose the best approach and "optimal" culture conditions to test a given hypothesis. Therefore, a breakout session was organized at the 2022 ORS Tendon Section Meeting that focused on establishing a set of guidelines for conducting cell and tissue culture studies of tendon. This paper summarizes the outcomes of that discussion and presents recommendations for future studies. In the case of studying tendon cell behavior, cell and tissue culture systems are reductionist models in which the culture conditions should be strictly defined to approximate the in vivo condition as closely as possible. In contrast, for tissue engineering tendon replacements, the culture conditions do not need to replicate native tendon, but the outcome measures for success should be narrowly defined for the specific clinical application. Common recommendations for both applications are that researchers should perform a baseline phenotypic characterization of the cells that are ultimately used for experimentation. For models of tendon cell behavior, culture conditions should be well justified by existing literature and meticulously reported, tissue explant viability should be assessed, and comparisons to in vivo conditions should be made to determine baseline physiological relevance. For tissue engineering applications, the functional/structural/compositional outcome targets should be defined by the specific tendons they seek to replace, with key biologic and material properties prioritized for construct assessment. Lastly, when engineering tendon replacements, researchers should utilize clinically approved cGMP materials to facilitate clinical translation.

Mechanoresponsive regulation of fibroblast-to-myofibroblast transition in three-dimensional tissue analogues: mechanical strain amplitude dependency of fibrosis

The spatiotemporal interaction and constant iterative feedback between fibroblasts, extracellular matrix, and environmental cues are central for investigating the fibroblast-induced musculoskeletal tissue regeneration and fibroblast-to-myofibroblast transition (FMT). In this study, we created a fibroblast-laden 3D tissue analogue to study (1) how mechanical loading exerted on three-dimensional (3D) tissues affected the residing fibroblast phenotype and (2) to identify the ideal mechanical strain amplitude for promoting tissue regeneration without initiating myofibroblast differentiation. We applied uniaxial tensile strain (0, 4, 8, and 12%) to the cell-laden 3D tissue analogues to understand the interrelation between the degree of applied mechanical loading amplitudes and FMT. Our data demonstrated that 4% mechanical strain created an anabolic effect toward tissue regeneration, but higher strain amplitudes over-stimulated the cells and initiated fibrotic tissue formation. Under increased mechanical strain amplitudes, fibroblasts were activated from a homeostatic state to a proto-myofibroblast state which resulted in increased cellularity accompanied by increased expressions of extracellular matrix (ECM) components, activation stressors (TGF-β1 and TGF-βR1), and profibrotic markers. This further transformed fibroblasts into α-smooth muscle actin expressing myofibroblasts. Understanding the interplay between the applied degree of mechanical loading exerted on 3D tissues and residing fibroblast phenotypic response is important to identify specific mechanomodulatory approaches for tissue regeneration and the informed mechanotherapy-guided tissue healing strategies.

Electrospun, Resorbable, Drug-Eluting, Nanofibrous Membranes Promote Healing of Allograft Tendons

In spite of advances in medical technology, the repair of Achilles tendon ruptures remains challenging. Reconstruction with an autograft tendon provides the advantage of a higher healing rate; nevertheless, the development of donor-site morbidity cannot be ignored. We developed biodegradable, drug-eluting, nanofibrous membranes employing an electrospinning technique and evaluated their effectiveness on the healing of allograft tendons. Poly-D-L-lactide-glycolide was used as the polymeric material for the nanofibers, while doxycycline was selected as the drug for delivery. The in vitro and in vivo drug-release profiles were investigated. The biomechanical properties of allografted Achilles tendons repaired using the nanofibrous membranes were tested in euthanized rabbits at 2-, 4-, and 6-week time intervals. Histological examination was performed for the evaluation of tissue reaction and tendon healing. The level of postoperative animal activity was also monitored using an animal behavior cage. The experimental results showed that the degradable nanofibers used as a vehicle could provide sustained release of doxycycline for 42 days after surgery with very low systemic drug concentration. Allograft Achilles tendon reconstruction assisted by drug-loaded nanofibers was associated with better biomechanical properties at 6 weeks post-surgery. In addition, the animals exhibited a better level of activity after surgery. The use of drug-eluting, nanofibrous membranes could enhance healing in Achilles tendon allograft reconstruction surgery.

Stress-deprivation induces an up-regulation of versican and connexin-43 mRNA and protein synthesis and increased ADAMTS-1 production in tendon cells in situ

Purpose: The proper function of the tenocyte network depends on cell-matrix as well as intercellular communication that is mechanosensitive. Building on the concept that the etiopathogenic stimulus for tendon degeneration is the catabolic response of tendon cells to mechanobiologic under-stimulation, we studied the pericellular matrix rich in versican and its predominant proteolytic enzyme ADAMTS-1, as well as Connexin-43 (Cx43), a major gap junction forming protein in tendons, in stress-deprived rat tail tendon fascicles (RTTfs).Materials and Methods: RTTfs were stress-deprived for up to 7 days under tissue culture conditions. RT-qPCR was used to measure mRNA expression of versican, ADAMTS-1, and Cx43. Protein synthesis was determined using Western blotting and immunohistochemistry.Results: Stress-deprivation (SD) caused a statistically significant up-regulation of versican, ADAMTS-1, and Cx43 mRNA expression that was persistent over the 7-day test period. Western blot analysis and immunohistochemical assessment of protein synthesis revealed a marked increase of the respective proteins with SD. Inhibition of proteolytic enzyme activity with ilomastat prevented the increased versican degradation and Cx43 synthesis in 3 days stress-deprived tendons when compared with non-treated, stress-deprived tendons.Conclusion: In the absence of mechanobiological signaling the immediate pericellular matrix is modulated as tendon cells up-regulate their production of ADAMTS-1, and versican with subsequent proteoglycan degradation potentially leading to cell signaling cues increasing Cx43 gap junctional protein. The results also provide further support for the hypothesis that the cellular changes associated with tendinopathy are a result of decreased mechanobiological signaling and a loss of homeostatic cytoskeletal tension.

Inhibition of ERK 1/2 kinases prevents tendon matrix breakdown

Tendon extracellular matrix (ECM) mechanical unloading results in tissue degradation and breakdown, with niche-dependent cellular stress directing proteolytic degradation of tendon. Here, we show that the extracellular-signal regulated kinase (ERK) pathway is central in tendon degradation of load-deprived tissue explants. We show that ERK 1/2 are highly phosphorylated in mechanically unloaded tendon fascicles in a vascular niche-dependent manner. Pharmacological inhibition of ERK 1/2 abolishes the induction of ECM catabolic gene expression (MMPs) and fully prevents loss of mechanical properties. Moreover, ERK 1/2 inhibition in unloaded tendon fascicles suppresses features of pathological tissue remodeling such as collagen type 3 matrix switch and the induction of the pro-fibrotic cytokine interleukin 11. This work demonstrates ERK signaling as a central checkpoint to trigger tendon matrix degradation and remodeling using load-deprived tissue explants.

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.

Multiscale effects of spaceflight on murine tendon and bone

Despite a wealth of data on the effects of spaceflight on tendons and bones, little is known about its effects on the interfacial tissue between these two structures, the enthesis. Mice were sent to space on three separate missions: STS-131, STS-135, and Bion-M1 to determine how spaceflight affects the composition, structure, mechanics, and gene expression of the humerus-supraspinatus and calcaneus-Achilles entheses. At the nanoscale, spaceflight resulted in decreased carbonate levels in the bone, likely due to increased remodeling, as suggested by increased expression of genes related to osteoclastogenesis (CatK, Tnfsf11) and mature osteoblasts (Col1, Osc). Tendons showed a shift in collagen fibril size towards smaller diameters that may have resulted from increased expression of genes related to collagen degradation (Mmp3, Mmp13). These nanoscale changes did not result in micro- and milliscale changes to the structure and mechanics of the enthesis. There were no changes in bone volume, trabecular structure, failure load, or stiffness with spaceflight. This lack of tissue-level change may be anatomy based, as extremities may be less sensitive to spaceflight than central locations such as vertebrae, yet results highlight that the tendon enthesis may be robust against negative effects of spaceflight.

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.

A multi-chamber tissue culture device for load-dependent parallel evaluation of tendon explants

Background

Injuries in the musculoskeletal system, such as tendon and ligament ruptures, are challenging to manage and often require surgical reconstructions with limited long-term success. Thus, characterizations of these tissues are urgently needed to better understand cellular mechanisms that regulate tissue homeostasis and healing. Explant culturing systems allow for ex vivo analysis of tissues in an environment that mimics the native microenvironment in vivo.

Methods

Collaborative efforts within our institution facilitated the establishment of a novel explant culturing system. Tissue specimens cultured in single wells, with individual applied loading and/or biological environment, allowed characterization of tissue cultured under a variety of biological loading conditions. Quantitative PCR analysis for selected gene markers was our primary outcome.

Results

Data were stratified for analysis by either culture environment or loading condition. Our gene expression results show that specimens clustered by culture condition may differ in molecular markers related to ECM production (e.g., Col1a1, Adamts4) and/or organization (e.g., Tnc, Dnc). In contrast, loading condition did significantly alter the median gene expression levels of tissues in comparison to unloaded control samples, although gene expression values related to ECM degradation (e.g., Mmp1, Mmp10) were altered in tendons cultured under tension in the device.

Conclusion

Our study demonstrates promising utility of a novel explant culturing system for further characterization of musculoskeletal tissues such as native tendons and ligaments, as well as pathologic fibrotic tissues resulting from arthrofibrosis or Dupuytren’s disease.

Tendon tissue microdamage and the limits of intrinsic repair

The transmission of mechanical muscle force to bone for musculoskeletal stability and movement is one of the most important functions of tendon. The load-bearing tendon core is composed of highly aligned collagen-rich fascicles interspersed with stromal cells (tenocytes). Despite being built to bear very high mechanical stresses, supra-physiological/repetitive mechanical overloading leads to tendon microdamage in fascicles, and potentially to tendon disease and rupture. To date, it is unclear to what extent intrinsic healing mechanisms of the tendon core compartment can repair microdamage. In the present study, we investigated the healing capacity of the tendon core compartment in an ex vivo tissue explant model. To do so, we isolated rat tail tendon fascicles, damaged them by applying a single stretch to various degrees of sub-rupture damage and longitudinally assessed downstream functional and structural changes over a period of several days. Functional damage was assessed by changes in the elastic modulus of the material stress-strain curves, and biological viability of the resident tenocytes. Structural damage was quantified using a fluorescent collagen hybridizing peptide (CHP) to label mechanically disrupted collagen structures. While we observed functional mechanical damage for strains above 2% of the initial fascicle length, structural collagen damage was only detectable for 6% strain and beyond. Minimally loaded/damaged fascicles (2-4% strain) progressively lost elastic modulus over the course of tissue culture, despite their collagen structures remaining intact with high degree of maintained cell viability. In contrast, more severely overloaded fascicles (6-8% strain) with damage at the molecular/collagen level showed no further loss of the elastic modulus but markedly decreased cell viability. Surprisingly, in these heavily damaged fascicles the elastic modulus partially recovered, an effect also seen in further experiments on devitalized fascicles, implying the possibility of a non-cellular but matrix-driven mechanism of molecular repair. Overall, our findings indicate that the tendon core has very little capacity for self-repair of microdamage. We conclude that stromal tenocytes likely do not play a major role in anabolic repair of tendon matrix microdamage, but rather mediate catabolic matrix breakdown and communication with extrinsic cells that are able to effect tissue repair.

Migrating Myofibroblastic Iliotibial Band-Derived Fibroblasts Represent a Promising Cell Source for Ligament Reconstruction

The iliotibial band (ITB) is a suitable scaffold for anterior cruciate ligament (ACL) reconstruction, providing a sufficient mechanical resistance to loading. Hence, ITB-derived fibroblasts attract interest for ligament tissue engineering but have so far not been characterized. This present study aimed at characterizing ITB fibroblasts before, during, and after emigration from cadaveric ITB explants to decipher the emigration behavior and to utilize their migratory capacity for seeding biomaterials. ITB and, for comparison, ACL tissues were assessed for the content of alpha smooth muscle actin (αSMA) expressing fibroblasts and degeneration. The cell survival and αSMA expression were monitored in explants used for cell isolation, monolayer, self-assembled ITB spheroids, and spheroids seeded in polyglycolic acid (PGA) scaffolds. The protein expression profile of targets typically expressed by ligamentocytes (collagen types I-III, elastin, lubricin, decorin, aggrecan, fibronectin, tenascin C, CD44, β1-integrins, vimentin, F-actin, αSMA, and vascular endothelial growth factor A [VEGFA]) was compared between ITB and ACL fibroblasts. A donor- and age-dependent differing percentage of αSMA positive cells could be detected, which was similar in ITB and ACL tissues despite the grade of degeneration being significantly higher in the ACL due to harvesting them from OA knees. ITB fibroblasts survived for several months in an explant culture, continuously forming monolayers with VEGFA and an increased αSMA expression. They shared their expression profile with ACL fibroblasts. αSMA decreased during the monolayer to spheroid/scaffold transition. Using self-assembled spheroids, the migratory capacity of reversible myofibroblastic ITB cells can be utilized for colonizing biomaterials for ACL tissue engineering and to support ligament healing.

Molecular Basis for Periodontal Ligament Adaptation to In Vivo Loading

A soft food diet leads to changes in the periodontal ligament (PDL). These changes, which have been recognized for more than a century, are ascribed to alterations in mechanical loading. While these adaptive responses have been well characterized, the molecular, cellular, and mechanical mechanisms underlying the changes have not. Here, we implicate Wnt signaling in the pathoetiology of PDL responses to underloading. We show that Wnt-responsive cells and their progeny in the PDL space exhibit a burst in proliferation in response to mastication. If an animal is fed a soft diet from the time of weaning, then this burst in Wnt-responsive cell proliferation is quelled; as a consequence, both the PDL and the surrounding alveolar bone undergo atrophy. Returning these animals to a hard food diet restores the Wnt signaling in PDL. These data provide, for the first time, a molecular mechanism underlying the adaptive response of the PDL to loading.

Release of pro-inflammatory cytokines from muscle and bone causes tenocyte death in a novel rotator cuff in vitro explant culture model

PURPOSE

Tendinopathy is a significant clinical problem thought to be associated with altered mechanical loading. Explant culture models allow researchers to alter mechanical loading in a controlled in vitro environment while maintaining tenocytes in their native matrix. However, current models do not accurately represent commonly injured tendons, ignoring contributions of associated musculature and bone, as well as regional collagen structure. This study details the characterization of amouse rotator cuff explant culture model, including bone, tendon, and muscle (BTM).

MATERIALS AND METHODS

Following harvest, BTM explants were maintained in stress-deprived culture for one week and tendon was then assessed for changes in cell viability, metabolism, matrix structure and content.

RESULTS

Matrix turnover occurred throughout culture as manifested in both gene expression and biosynthesis, but this did not translate to net changes in total collagen or sulfated glycosaminoglycan content. Furthermore, tendon structure was not significantly altered throughout culture. However, we found significant cell death in BTM tendons after 3 days in culture, which we hypothesize is cytokine-induced. Using a targeted multiplex assay, we found high levels of pro-inflammatory cytokines released to the culture medium from muscle and bone, levels that did cause cell deathin tendon-alone controls.

CONCLUSIONS

Overall, this model presents an innovative approach to understandingrotator cuff injury and tenocyte mechanobiology in a clinically-relevant tendon structure. Our model can be a powerful tool to investigate how mechanical and biological stimuli can alter normal tendon health and lead to tendon degeneration, and may provide a testbed for therapeutics for tendon repair.

Fatigue loading of tendon results in collagen kinking and denaturation but does not change local tissue mechanics

Fatigue loading is a primary cause of tendon degeneration, which is characterized by the disruption of collagen fibers and the appearance of abnormal (e.g., cartilaginous, fatty, calcified) tissue deposits. The formation of such abnormal deposits, which further weakens the tissue, suggests that resident tendon cells acquire an aberrant phenotype in response to fatigue damage and the resulting altered mechanical microenvironment. While fatigue loading produces clear changes in collagen organization and molecular denaturation, no data exist regarding the effect of fatigue on the local tissue mechanical properties. Therefore, the objective of this study was to identify changes in the local tissue stiffness of tendons after fatigue loading. We hypothesized that fatigue damage would reduce local tissue stiffness, particularly in areas with significant structural damage (e.g., collagen denaturation). We tested this hypothesis by identifying regions of local fatigue damage (i.e., collagen fiber kinking and molecular denaturation) via histologic imaging and by measuring the local tissue modulus within these regions via atomic force microscopy (AFM). Counter to our initial hypothesis, we found no change in the local tissue modulus as a consequence of fatigue loading, despite widespread fiber kinking and collagen denaturation. These data suggest that immediate changes in topography and tissue structure - but not local tissue mechanics - initiate the early changes in tendon cell phenotype as a consequence of fatigue loading that ultimately culminate in tendon degeneration.

Tendon injury and repair - A perspective on the basic mechanisms of tendon disease and future clinical therapy

Tendon is an intricately organized connective tissue that efficiently transfers muscle force to the bony skeleton. Its structure, function, and physiology reflect the extreme, repetitive mechanical stresses that tendon tissues bear. These mechanical demands also lie beneath high clinical rates of tendon disorders, and present daunting challenges for clinical treatment of these ailments. This article aims to provide perspective on the most urgent frontiers of tendon research and therapeutic development. We start by broadly introducing essential elements of current understanding about tendon structure, function, physiology, damage, and repair. We then introduce and describe a novel paradigm explaining tendon disease progression from initial accumulation of damage in the tendon core to eventual vascular recruitment from the surrounding synovial tissues. We conclude with a perspective on the important role that biomaterials will play in translating research discoveries to the patient.

STATEMENT OF SIGNIFICANCE

Tendon and ligament problems represent the most frequent musculoskeletal complaints for which patients seek medical attention. Current therapeutic options for addressing tendon disorders are often ineffective, and the need for improved understanding of tendon physiology is urgent. This perspective article summarizes essential elements of our current knowledge on tendon structure, function, physiology, damage, and repair. It also describes a novel framework to understand tendon physiology and pathophysiology that may be useful in pushing the field forward.

The revitalisation of flexor tendon allografts with bone marrow stromal cells and mechanical stimulation: An ex vivo model revitalising flexor tendon allografts

Objectives

The present study describes a novel technique for revitalising allogenic intrasynovial tendons by combining cell-based therapy and mechanical stimulation in an ex vivo canine model.

Methods

Specifically, canine flexor digitorum profundus tendons were used for this study and were divided into the following groups: (1) untreated, unprocessed normal tendon; (2) decellularised tendon; (3) bone marrow stromal cell (BMSC)-seeded tendon; and (4) BMSC-seeded and cyclically stretched tendon. Lateral slits were introduced on the tendon to facilitate cell seeding. Tendons from all four study groups were distracted by a servohydraulic testing machine. Tensile force and displacement data were continuously recorded at a sample rate of 20 Hz until 200 Newton of force was reached. Before testing, the cross-sectional dimensions of each tendon were measured with a digital caliper. Young’s modulus was calculated from the slope of the linear region of the stress-strain curve. The BMSCs were labeled for histological and cell viability evaluation on the decellularized tendon scaffold under a confocal microscope. Gene expression levels of selected extracellular matrix tendon growth factor genes were measured. Results were reported as mean ± SD and data was analyzed with one-way ANOVAs followed by Tukey’s post hoc multiple-comparison test.

Results

We observed no significant difference in cross-sectional area or in Young’s modulus among the four study groups. In addition, histological sections showed that the BMSCs were aligned well and viable on the tendon slices after two-week culture in groups three and four. Expression levels of several extracellular matrix tendon growth factors, including collagen type I, collagen type III, and matrix metalloproteinase were significantly higher in group four than in group three (p < 0.05).

Conclusion

Lateral slits introduced into de-cellularised tendon is a promising method of delivery of BMSCs without compromising cell viability and tendon mechanical properties. In addition, mechanical stimulation of a cell-seeded tendon can promote cell proliferation and enhance expression of collagen types I and III in vitro.

Cite this article: J. H. Wu, A. R. Thoreson, A. Gingery, K. N. An, S. L. Moran, P. C. Amadio, C. Zhao. The revitalisation of flexor tendon allografts with bone marrow stromal cells and mechanical stimulation: An ex vivo model revitalising flexor tendon allografts. Bone Joint Res 2017;6:179–185. DOI: 10.1302/2046-3758.63.BJR-2016-0207.R1.

The Role of Detraining in Tendon Mechanobiology

Introduction: Several conditions such as training, aging, estrogen deficiency and drugs could affect the biological and anatomo-physiological characteristics of the tendon. Additionally, recent preclinical and clinical studies examined the effect of detraining on tendon, showing alterations in its structure and morphology and in tenocyte mechanobiology. However, few data evaluated the importance that cessation of training might have on tendon. Basically, we do not fully understand how tendons react to a phase of training followed by sudden detraining. Therefore, within this review, we summarize the studies where tendon detraining was examined.

Materials and Methods: A descriptive systematic literature review was carried out by searching three databases (PubMed, Scopus and Web of Knowledge) on tendon detraining. Original articles in English from 2000 to 2015 were included. In addition, the search was extended to the reference lists of the selected articles. A public reference manager (www.mendeley.com) was adopted to remove duplicate articles.

Results: An initial literature search yielded 134 references (www.pubmed.org: 53; www.scopus.com: 11; www.webofknowledge.com: 70). Fifteen publications were extracted based on the title for further analysis by two independent reviewers. Abstracts and complete articles were after that reviewed to evaluate if they met inclusion criteria.

Conclusions: The revised literature comprised four clinical studies and an in vitro and three in vivo reports. Overall, the results showed that tendon structure and properties after detraining are compromised, with an alteration in the tissue structural organization and mechanical properties. Clinical studies usually showed a lesser extent of tendon alterations, probably because preclinical studies permit an in-depth evaluation of tendon modifications, which is hard to perform in human subjects. In conclusion, after a period of sudden detraining (e.g., after an injury), physical activity should be taken with caution, following a targeted rehabilitation program. However, further research should be performed to fully understand the effect of sudden detraining on tendons.

Focal experimental injury leads to widespread gene expression and histologic changes in equine flexor tendons

It is not known how extensively a localised flexor tendon injury affects the entire tendon. This study examined the extent of and relationship between histopathologic and gene expression changes in equine superficial digital flexor tendon after a surgical injury. One forelimb tendon was hemi-transected in six horses, and in three other horses, one tendon underwent a sham operation. After euthanasia at six weeks, transected and control (sham and non-operated contralateral) tendons were regionally sampled (medial and lateral halves each divided into six 3 cm regions) for histologic (scoring and immunohistochemistry) and gene expression (real time PCR) analysis of extracellular matrix changes. The histopathology score was significantly higher in transected tendons compared to control tendons in all regions except for the most distal (P ≤ 0.03) with no differences between overstressed (medial) and stress-deprived (lateral) tendon halves. Proteoglycan scores were increased by transection in all but the most proximal region (P < 0.02), with increased immunostaining for aggrecan, biglycan and versican. After correcting for location within the tendon, gene expression for aggrecan, versican, biglycan, lumican, collagen types I, II and III, MMP14 and TIMP1 was increased in transected tendons compared with control tendons (P < 0.02) and decreased for ADAMTS4, MMP3 and TIMP3 (P < 0.001). Aggrecan, biglycan, fibromodulin, and collagen types I and III expression positively correlated with all histopathology scores (P < 0.001), whereas lumican, ADAMTS4 and MMP14 expression positively correlated only with collagen fiber malalignment (P < 0.001). In summary, histologic and associated gene expression changes were significant and widespread six weeks after injury to the equine SDFT, suggesting rapid and active development of tendinopathy throughout the entire length of the tendon. These extensive changes distant to the focal injury may contribute to poor functional outcomes and re-injury in clinical cases. Our data suggest that successful treatments of focal injuries will need to address pathology in the entire tendon, and that better methods to monitor the development and resolution of tendinopathy are required.

Progress in cell-based therapies for tendon repair

The last decade has seen significant developments in cell therapies, based on permanently differentiated, reprogrammed or engineered stem cells, for tendon injuries and degenerative conditions. In vitro studies assess the influence of biophysical, biochemical and biological signals on tenogenic phenotype maintenance and/or differentiation towards tenogenic lineage. However, the ideal culture environment has yet to be identified due to the lack of standardised experimental setup and readout system. Bone marrow mesenchymal stem cells and tenocytes/dermal fibroblasts appear to be the cell populations of choice for clinical translation in equine and human patients respectively based on circumstantial, rather than on hard evidence. Collaborative, inter- and multi-disciplinary efforts are expected to provide clinically relevant and commercially viable cell-based therapies for tendon repair and regeneration in the years to come.

Metabolic and cytoprotective effects of in vivo peri-patellar hyaluronic acid injections in cultured tenocytes

The purpose of this study was to investigate tenocyte mechanobiology after sudden-detraining and to examine the hypothesis that repeated peri-patellar injections of hyaluronic acid (HA) on detrained patellar tendon (PT) may reduce and limit detrained-associated damage in tenocytes. Twenty-four male Sprague-Dawley rats were divided into three groups: Untrained, Trained and Detrained. In the Detrained rats, the left tendon was untreated while the right tendon received repeated peri-patellar injections of either HA or saline (NaCl). Tenocyte morphology, metabolism and synthesis of C-terminal-propeptide of type I collagen, collagen-III, fibronectin, aggrecan, tenascin-c, interleukin-1β, matrix-metalloproteinase-1 and-3 were evaluated after 1, 3, 7 and 10 days of culture. Transmission-electronic-microscopy showed a significant increase in mitochondria and rough endoplasmic reticulum in cultured tenocytes from Detrained-HA with respect to those from Detrained-NaCl. Additionally, Detrained-HA cultures showed a significantly higher proliferation rate and viability, and increased synthesis of C-terminal-Propeptide of type I collagen, fibronectin, aggrecan, tenascin-c and matrix-metalloproteinase-3 with respect to Detrained-NaCl ones, whereas synthesis of matrix-metalloproteinase-1 and interleukin-1β was decreased. Our study demonstrates that discontinuing training activity in the short-term alters tenocyte synthetic and metabolic activity and that repeated peri-patellar infiltrations of HA during detraining allow the maintenance of tenocyte anabolic activity.

A gene expression study of normal and damaged cartilage in anteromedial gonarthrosis, a phenotype of osteoarthritis

OBJECTIVE

To identify osteoarthritis (OA) relevant genes and pathways in damaged and undamaged cartilage isolated from the knees of patients with anteromedial gonarthrosis (AMG) - a specific form of knee OA.

DESIGN

Cartilage was obtained from nine patients undergoing unicompartmental knee replacement (UKR) for AMG. AMG provides a spatial representation of OA progression; showing a reproducible and histologically validated pattern of cartilage destruction such that damaged and undamaged cartilage from within the same knee can be consistently isolated and examined. Gene expression was analysed by microarray and validated using real-time PCR.

RESULTS

Damaged and undamaged cartilage showed distinct gene expression profiles. 754 genes showed significant up- or down-regulation (non-False discovery rate (FDR) P < 0.05) with enrichment for genes involved in cell signalling, Extracellular Matrix (ECM) and inflammatory response. A number of genes previously unreported in OA showed strongly altered expression including RARRES3, ADAMTSL2 and DUSP10. Confirmation of genes previously identified as modulated in OA was also obtained e.g., SFRP3, MMP3 and IGF1.

CONCLUSIONS

This is the first study to examine a common and consistent phenotype of OA to allow direct comparison of damaged and undamaged cartilage from within the same joint compartment. We have identified specific gene expression profiles in damaged and undamaged cartilage and have determined relevant genes and pathways in OA progression. Importantly this work also highlights the necessity for phenotypic and microanatomical characterization of cartilage in future studies of OA pathogenesis and therapeutic development.

Cytoskeletal tension modulates MMP-1 gene expression from tenocytes on micropillar substrates

Actin cytoskeletons, aggregated with myosin II, generate intracellular cytoskeletal tension, which is induced to cell attaching substrate as cell traction forces. It is thought that cytoskeletal tension links closely to cell functions. The present study examined quantitative relationships between cytoskeleton tension and the balance of cell metabolism of tenocytes. Using micromachining techniques, micropillar substrates were prepared with polydimethylsiloxane, having three different values of substrate elasticity (6, 18 and 33 kPa) by changing the micropillar height. After 24h incubation of bovine tenocytes on these micropillar substrates, cell traction forces were determined. Gene expressions for type I collagen (anabolic marker) and MMP-1 (catabolic marker) from tenocytes on micropillars were also analyzed with qPCR. In addition, effects of an inhibition of myosin II activity on tenocyte cytoskeletal tension and metabolism were examined using the inhibitor, blebbistatin. It was exhibited that cell traction forces were significantly larger in tenocytes on 33 kPa substrates compared to those on 6 kPa substrates. This was associated with significant lower expression of MMP-1 mRNA on 33 kPa substrates. Cell traction forces were decreased significantly by the supplementation of blebbistatin in a dose-dependent manner. Indeed, there were significant smaller traction forces and higher expression for MMP-1 mRNA from tenocytes treated with 10 μM blebbistatin compared to their corresponding controls. Accordingly, tenocyte responses to substrate stiffness are associated with alterations in intracellular tension and catabolic gene expression. On the other hand, tenocyte anabolism, as measured by type I collagen mRNA expression, was not altered with substrate stiffness.

Aging is associated with increased activities of matrix metalloproteinase-2 and -9 in tenocytes

Background

Most tendon pathology is associated with degeneration, which is thought to involve cyclic loading and cumulative age-related changes in tissue architecture. However, the association between aging and degeneration of the extracellular matrix (ECM) in tendons has not been investigated extensively.

Methods

We examined tenocytes from Achilles tendons taken from rats of three different ages (2, 12, and 24 months). Tenocyte viability was assessed using the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Quantitative real-time polymerase chain reaction (PCR) was used to determine the levels of mRNAs that encode type-I collagen, matrix metalloproteinase (MMP)-2 and −9, tissue inhibitor of metalloproteinase (TIMP)-1 and −2 and transforming growth factor (TGF)-β1. Gelatin zymography was used to evaluate the enzymatic activities of MMP-2 and −9. Furthermore, the concentration of TGF-β1 in conditioned medium was evaluated using enzyme-linked immunosorbent assay (ELISA).

Results

The results of the MTT assay showed that the number of viable tenocytes decreased with age. No differences were observed in the levels of mRNAs that encode type-I collagen and TGF-β1 among the three age groups, and the TGF-β1 concentration did not change with age. However, mRNAs that encode MMP-2 and −9 were significantly more abundant in tenocytes from the aging group, and gelatin zymography revealed that the enzymatic activities of MMP-2 and −9 also increased significantly with age. Furthermore, as compared with young group, mRNAs that encode TIMP-1 and −2 were significantly decreased in tenocytes from the aging group.

Conclusions

Activities of MMP-2 and MMP-9 in tenocytes increase with age. This might provide a mechanistic explanation of how aging contributes to tendinopathy or tendon rupture with age.

Stochastic amplitude-modulated stretching of rabbit flexor digitorum profundus tendons reduces stiffness compared to cyclic loading but does not affect tenocyte metabolism

Background

It has been demonstrated that frequency modulation of loading influences cellular response and metabolism in 3D tissues such as cartilage, bone and intervertebral disc. However, the mechano-sensitivity of cells in linear tissues such as tendons or ligaments might be more sensitive to changes in strain amplitude than frequency. Here, we hypothesized that tenocytes in situ are mechano-responsive to random amplitude modulation of strain.

Methods

We compared stochastic amplitude-modulated versus sinusoidal cyclic stretching. Rabbit tendon were kept in tissue-culture medium for twelve days and were loaded for 1h/day for six of the total twelve culture days. The tendons were randomly subjected to one of three different loading regimes: i) stochastic (2 – 7% random strain amplitudes), ii) cyclic_RMS (2–4.42% strain) and iii) cyclic_high (2 - 7% strain), all at 1 Hz and for 3,600 cycles, and one unloaded control.

Results

At the end of the culture period, the stiffness of the “stochastic” group was significantly lower than that of the cyclic_RMS and cyclic_high groups (both, p < 0.0001). Gene expression of eleven anabolic, catabolic and inflammatory genes revealed no significant differences between the loading groups.

Conclusions

We conclude that, despite an equivalent metabolic response, stochastically stretched tendons suffer most likely from increased mechanical microdamage, relative to cyclically loaded ones, which is relevant for tendon regeneration therapies in clinical practice.

Host response to xenograft ECM implantation is not different between the shoulder and body wall sites in the rat model

In the context of tendon and ligament repair, mechanical loading and the presence of joint synovial fluid are known to profoundly influence the form and function of the repair tissue and potentially the host response to biomaterials. Previously, we demonstrated that a xenograft extra cellular matrix (ECM) scaffold implanted in the rat shoulder elicited a unique host response from that seen in the body wall. However, the host response to xenografts implanted in shoulders with a tendon/capsule injury was not different from xenografts implanted in shoulders with no injury. In the current study, we hypothesized that varying clinically relevant surgical and environmental factors would introduce significant differences in host response to xenograft implantation at the shoulder. Contrary to our hypothesis, we found no significant differences in host response between any shoulder implantation conditions or between shoulder and body wall implantation in the rat model. These findings suggest that there is no advantage to using an orthotopic shoulder model to investigate the host response to rotator cuff scaffold materials in the rat model, and due to the insensitivity of its host response to various clinically relevant surgical conditions, may suggest that the rat does not provide a surrogate for directly translating the host response to biomaterials to the human application.

Managing the injured tendon: current concepts

Despite advances in understanding of the mechanical aspects of tendon management with improved suture technique and early stress application with postoperative therapy, clinical results remain inconsistent after repair, especially within the synovial regions. Complementary research to enhance the intrinsic pathway of healing, suppress the extrinsic pathway of healing, and manipulate frictional resistance to tendon gliding is now the focus of current basic science research on tendons. In the future, application of these new biologic therapies may increase the "safety zone" (or tolerance for load and excursion without dysfunctional gapping) as therapists apply stress to healing tendons and may alter future rehabilitation protocols by allowing greater angles of motion (and thus tendon excursion), increased external load, and decreased time in protective orthoses (splints). However, at this time, the stronger repair techniques and the application of controlled stress remain the best and most well-supported intervention after tendon injury and repair in the recovery of functional tendon excursion and joint range of motion. The hand therapist's role in this process remains a critical component contributing to satisfactory outcomes.

Low-level laser therapy in experimental model of collagenase-induced tendinitis in rats: effects in acute and chronic inflammatory phases

A variety of treatments for tendinopathies is currently used or has been trialed. However, in fact, there is a remarkably little evidence that any conventional therapies are effective. In the last years, low-level laser therapy (LLLT) has been showing interesting results in inflammatory modulation in different musculoskeletal disorders, but the optimal parameters and mechanisms behind these effects are not fully understood. The aim of this study is to investigate if the LLLT modulates the acute and chronic phase of collagenase-induced tendinitis in rat by interfering in mRNA expression for matrix metalloproteinases (MMP13 and MMP1), vascular endothelial growth factor (VEGF), and anti-inflammatory mediator (interleukin (IL)-10). For such, tendinitis was induced by collagenase injection in male Wistar rats. Animals were treated with LLLT (780 nm, potency of 22 mW, 107 mW/cm(2), energy density of 7.5 J/cm(2), and energy delivered of 1.54 J) with different number of treatments in accordance with the inflammatory phase analyzed. LLLT was able to modulate mRNA gene expression of IL-10, VGEF, MMP1, and MMP13 both in acute than in chronic inflammatory phase (p<0.05). Our results suggest that LLLT with parameters employed in the present study was able to modulate IL-10, VEGF, MMP1, and MMP13 mRNA gene expression both in acute than in chronic tendon inflammation. However, further studies are needed to establish optimal parameters for LLLT.

Effect of implantation site and injury condition on host response to human-derived fascia lata ECM in a rat model

The host response and remodeling of ECM scaffolds are believed to be critical determinants of success or failure in repair or reconstructive procedures. Host response has been investigated in subcutaneous or abdominal wall implantation models. The extent to which evaluation of the host response to ECM intended for tendon or ligament repair should be performed in an orthotopic site is not known. This study compared the host response to human-derived fascia lata ECM among various implantation sites in the rat model. Results showed that a xenograft in the rat shoulder does not exhibit a different host response at 7 days from xenograft in the body wall, suggesting that either site may be appropriate to study the early host response to biologic grafts as well as the effect of various treatments aimed to modify the early host response. By 28 days, a xenograft in the rat shoulder does elicit a unique host response from that seen in the body wall. Therefore, it may be more appropriate to use an orthotopic shoulder model for investigating the long-term host response and remodeling of biologic grafts to be used for rotator cuff repair.

Metalloproteases and rotator cuff disease

The molecular changes occurring in rotator cuff tears are still unknown, but much attention has been paid to better understand the role of matrix metalloproteinases (MMP) in the development of tendinopathy. These are potent enzymes that, once activated, can completely degrade all components of the connective tissue, modify the extracellular matrix (ECM), and mediatethe development of painful tendinopathy and tendon rupture. To control the local activity of activated proteinases, the same cells produce tissue inhibitors of metalloproteinases (TIMP) that bind to the enzymes and prevent degradation. The balance between the activities of MMPs and TIMPs regulates tendon remodeling, whereas an imbalance produces a collagen dis-regulation and disturbances intendons. ADAMs (a disintegrin and metalloproteinase) are cell membrane-linked enzymes with proteolytic and cell signaling functions. ADAMTSs (ADAM with thrombospondin motifs) are secreted into the circulation, and constitute a heterogenous family of proteases with both anabolic and catabolic functions. Biologic modulation of endogenous MMP activity to basal levels may reduce pathologic tissue degradation and favorably influence healing after rotator cuff repair. Further studies are needed to better define the mechanism of action, and whether these new strategies are safe and effective in larger models.

Cyclic loading of tendon fascicles using a novel fatigue loading system increases interleukin-6 expression by tenocytes

Repetitive strain or 'overuse' is thought to be a major factor contributing to the development of tendinopathy. The aims of our study were to develop a novel cyclic loading system, and use it to investigate the effect of defined loading conditions on the mechanical properties and gene expression of isolated tendon fascicles. Tendon fascicles were dissected from bovine-foot extensors and subjected to cyclic tensile strain (1 Hz) at 30% or 60% of the strain at failure, for 0 h (control), 15 min, 30 min, 1 h, or 5 h. Post loading, a quasi-static test to failure assessed damage. Gene expression at a selected loading regime (1 h at 30% failure strain) was analyzed 6 h post loading by quantitative real-time polymerase chain reaction. Compared with unloaded controls, loading at 30% failure strain took 5 h to lead to a significant decrease in failure stress, whereas loading to 60% led to a significant reduction after 15 min. Loading for 1 h at 30% failure strain did not create significant structural damage, but increased Collagen-1-alpha-chain-1 and interleukin-6 (IL6) expression, suggesting a role of IL6 in tendon adaptation to exercise. Correlating failure properties with fatigue damage provides a method by which changes in gene expression can be associated with different degrees of fatigue damage.

Analysis of the deep digital flexor tendon in rats submitted to stretching after immobilization

Few studies have analyzed the effect of stretching after immobilization on the structural and biochemical properties of tendons. Here, the effect of stretching and immobilization on the proximal (p), intermediate (i), and distal (d) regions of the deep digital flexor tendon in rats was analyzed. The d region was subjected to compression and tension forces, the i region was subjected to compressive forces and the p region received tension forces. Rats were separated into five groups: GI--control for GII; GII--immobilized rats; GIII--control for GIV and GV groups; GIV--immobilized and stretched rats; and GV--immobilized rats which were allowed free cage activity. GII showed a higher molecular organization in the d and p regions as detected by measuring optical retardation, a lower concentration of hydroxyproline in the i region and a significant decrease in noncollagenous proteins found in the three regions of the tendon. Regarding the glycosaminoglycans, diminishing dermatan sulfate and the absence of chondroitin sulfate in the i region were observed in GII when compared to GI. However, in the same region of GIV, higher concentrations of chondroitin and dermatan sulfate were observed along with a strong metachromasy. An increase in hydroxyproline content in the i region and a higher molecular organization in the d and p regions were observed in GIV. Apparently, the active isoforms of metalloproteinase-2 also increased after stretching in all regions. These results suggest that stretching after immobilization contributed to the increase in molecular organization and to the synthesis of extracellular matrix components.

Functional analysis of tenocytes gene expression in tendon fascicles subjected to cyclic tensile strain

Tenocytes are known to be mechanoresponsive and the present study tests the hypothesis that distinct mechanical stimulation regimes, associated with the short-term and extended application of cyclic tensile strain, alters the balance between anabolic and catabolic processes. Microarray technology has been used to provide a comprehensive analysis of alterations in gene expression within isolated tendon fascicles in response to cyclic tensile strain using a well-established model system. Isolated rat tail tendon fascicles were subjected to cyclic tensile strain (3% amplitude superimposed on a 2% static strain) for 1 or 24 hr. Messenger RNA expression level was assessed using Illumina microarray. The number of genes significantly altered in strained fascicles from the level of unstrained control fascicles was greater at 24 hr than 1 hr. The expression levels of many extracellular matrix components remained unchanged at both time points; however, a number of members of the matrix metalloproteinase (MMP) and a disintegrin and metalloproteinase with a thrombospondin (ADAMTS) families were significantly downregulated at 24 hr. Functional annotation revealed that upregulated genes were significantly associated with the regulation of transcription at 1 hr and translation at 24 hr. Downregulated genes were associated with inflammatory responses at 1 hr, and genes inhibited at 24 hr were significantly associated with cell apoptosis and a variety of metabolic functions. The present results suggest that the metabolic balance was shifted in favor of catabolism by the application of a small number of tensile strain cycles, whereas an extended number stimulates strong anti-catabolic effects.

Remodeling and repair of orthopedic tissue: role of mechanical loading and biologics

Orthopedic tissues respond to mechanical loads to maintain normal homeostasis and in response to injury. As the body of work on this continues to grow, it is important to synthesize the recent studies across tissues and specialties with one another and with past studies. Hence, this review highlights the knowledge gained since 2000, with only few exceptions, concerning the effects of mechanical load and biologics on remodeling and repair of orthopedic tissue.

The cell biology of suturing tendons

Trauma by suturing tendon form areas devoid of cells termed "acellular zones" in the matrix. This study aimed to characterise the cellular insult of suturing and acellular zone formation in mouse tendon. Acellular zone formation was evaluated using single grasping sutures placed using flexor tendons with time lapse cell viability imaging for a period of 12h. Both tension and injury were required to induce cell death and cell movement in the formation of the acellular zone. DNA fragmentation studies and transmission electron microscopy indicated that cells necrosed. Parallel in vivo studies showed that cell-to-cell contacts were disrupted following grasping by the suture in tensioned tendon. Without tension, cell death was lessened and cell-to-cell contacts remained intact. Quantitative immunohistochemistry and 3D cellular profile mapping of wound healing markers over a one year time course showed that acellular zones arise rapidly and showed no evidence of healing whilst the wound healing response occurred in the surrounding tissues. The acellular zones were also evident in a standard modified "Kessler" clinical repair. In conclusion, the suture repair of injured tendons produces acellular zones, which may potentially cause early tendon failure.

The effect of matrix metalloproteinase inhibition on tendon-to-bone healing in a rotator cuff repair model

HYPOTHESIS

Recent studies have demonstrated a potentially critical role of matrix metalloproteinases (MMPs) and tissue inhibitors of matrix metalloproteinases (TIMPs) in the pathophysiology of rotator cuff tears. We hypothesize that local delivery of a MMP inhibitor after surgical repair of the rotator cuff will improve healing at the tendon-to-bone surface interface.

MATERIALS AND METHODS

Sixty-two male Sprague-Dawley rats underwent acute supraspinatus detachment and repair. In the control group (n=31), the supraspinatus was repaired to its anatomic footprint. In the experimental group (n=31), recombinant alpha-2-macroglobulin (A2M) protein, a universal MMP inhibitor, was applied at the tendon-bone interface with an identical surgical repair. Animals were sacrificed at 2 and 4 weeks for histomorphometry, immunohistochemistry, and biomechanical testing. Statistical comparisons were performed using unpaired t tests. Significance was set at P < .05.

RESULTS

Significantly greater fibrocartilage was seen at the healing enthesis in the A2M-treated specimens compared with controls at 2 weeks (P < .05). Significantly greater collagen organization was observed in the A2M-treated animals compared with controls at 4 weeks (P < .01). A significant reduction in collagen degradation was observed at both 2 and 4 weeks in the experimental group (P < .05). Biomechanical testing revealed no significant differences in stiffness or ultimate load-to-failure.

CONCLUSION

Local delivery of an MMP inhibitor is associated with distinct histologic differences at the tendon-to-bone interface after rotator cuff repair. Modulation of MMP activity after rotator cuff repair may offer a novel biologic pathway to augment tendon-to-bone healing after rotator cuff repair.

Doxycycline-mediated inhibition of matrix metalloproteinases improves healing after rotator cuff repair

BACKGROUND

Recent studies demonstrate a potentially critical role of matrix metalloproteinases (MMPs) and their inhibitors in the pathophysiology of rotator cuff tears.

HYPOTHESIS

Doxycycline-mediated MMP inhibition after rotator cuff repair will improve tendon-to-bone healing.

STUDY DESIGN

Controlled laboratory study.

METHODS

Rats (n = 183) underwent acute detachment and repair of the supraspinatus tendon and the animals were divided into 4 groups: In controls (n = 66), the supraspinatus was repaired to its anatomical footprint. In experimental groups, an identical surgery was performed with doxycycline (130 mg/kg/d) administered orally at (1) preoperative day 1 (n = 66), (2) postoperative day (POD) 5 (n = 28), or (3) POD 14 (n = 23). Animals were sacrificed at 5 days, 8 days, 2 weeks, and 4 weeks. Tendon-bone interface was evaluated with histomorphometry. Enzyme-linked immunosorbent assay for local MMP-13 activity was performed at 8 days and 4 weeks. Biomechanical testing of the healing enthesis was performed at 8 days, 2 weeks, and 4 weeks. Serum doxycycline levels were measured at sacrifice. Statistical analysis was performed using unpaired t tests and 2-way analysis of variance (P < .05).

RESULTS

Serum doxycycline levels were significantly higher in all treated groups compared with controls (1830 +/- 835 vs 3 +/- 3 ng/mL, respectively; P < .001). Doxycycline-treated animals demonstrated greater metachromasia and improved collagen organization at the healing enthesis at POD 5 (P < .06), POD 8 (P < .03), and 2 weeks (P < .04). The MMP-13 activity was significantly reduced in doxycycline-treated compared with control animals at POD 8 (6740 +/- 2770 vs 10400 +/- 2930 relative fluorescent units [RFU], respectively; P < .02) but not at 4 weeks (3600 +/- 3280 vs 4530 +/- 2720 RFU, respectively). The healing enthesis of animals started on doxycycline preoperatively or at POD 5 had an increased load to failure compared to controls at 2 weeks (13.6 +/- 1.8 and 13.2 +/- 1.94 N vs 9.1 +/- 2.5 N, respectively; P < .01).

CONCLUSION/CLINICAL RELEVANCE

Modulation of MMP-13 activity after rotator cuff repair may offer a novel biological pathway to augment tendon-to-bone healing.

Primary cilia are highly oriented with respect to collagen direction and long axis of extensor tendon

Skeletal tissues adapt to their mechanical environments by modulating gene expression, cell metabolism, and extracellular matrix (ECM) architecture; however, the mechanosensory mechanisms for these processes are incompletely understood. Primary cilia have emerged as critical components of the cellular mechanosensory apparatus and have been hypothesized to participate in establishment of cellular and ECM orientation, but their function in skeletal tissues is just beginning to be examined. Here we focused on tendon, a tissue with an oriented matrix that is ideal for analysis of spatial relationships between primary cilia and the ECM. The objective of this study was to characterize the incidence and orientation of tenocyte primary cilia in their native ECM. Primary cilia, nuclei, and collagen were analyzed three-dimensionally in immunofluorescently labeled rat extensor tendon using multiphoton microscopy and semiautomated morphometry. Primary cilia were observed in 64% of tenocytes. The cilia were highly oriented with respect to the ECM: cilia were aligned parallel to the collagen fibers and the long axis of the tendon. This study represents the first quantification of the in situ incidence and orientation of primary cilia in tendon.

Differential regulation of gene expression in isolated tendon fascicles exposed to cyclic tensile strain in vitro

Mechanical stimulus is a regulator of tenocyte metabolism. The present study investigated temporal regulation of the expression of selected genes by tenocytes in isolated fascicles subjected to tensile strain in vitro. Cyclic tensile strain with a 3% amplitude superimposed on a 2% static strain was provided for 10 min, followed by either an unstrained period or continuous cyclic strain until the end of a 24-h incubation period. mRNA expression of selected anabolic and catabolic genes were evaluated with quantitative PCR at 10 min, 1 h, 6 h, and 24 h. The application of 6-h cyclic strain significantly upregulated type III collagen mRNA expression in strained fascicles compared with unstrained controls, but no alterations were observed in mRNA expression of type I collagen and biglycan. Significant downregulation in the expression of the decorin core protein was observed in fascicles subjected to 24-h cyclic strain. MMP3 and MMP13 expression levels were upregulated by the application of 10 min of cyclic strain, followed by a progressive downregulation until the end of the incubation period in both the absence and the presence of the continuing cyclic strain. Accordingly, alterations in the expression of anabolic genes were limited to the upregulation of type III collagen by prolonged exposure to cyclic strain, whereas catabolic genes were upregulated by a small number of strain cycles and downregulated by a prolonged cyclic strain. These findings demonstrate distinctive patterns of mechanoregulation for anabolic and catabolic genes and help our understanding of tenocyte response to mechanical stimulation.