Lubricin distribution in the goat infraspinatus tendon: a basis for interfascicular lubrication

The role of mechanotransduction in tendon

Tendons play an important role in the maintenance of motor function by connecting muscles and bones and transmitting forces. Particularly, the role of mechanical stress has primarily focused on the key mechanism of tendon homeostasis, with much research on this topic. With the recent development of molecular biological techniques, the mechanisms of mechanical stress sensing and signal transduction have been gradually elucidated with the identification of mechanosensor in tendon cells and the master regulator in tendon development. This review provides a comprehensive overview of the structure and function of tendon tissue, including the role for physical performance and the detailed mechanism of mechanotransduction in its regulation. An important lesson is that the role of mechanotransduction in tendon tissue is only partially clarified, indicating the complexity of the mechanisms of motor function and fueling increasing interest in uncovering these mechanisms.

Aging-Related Rotator Cuff Tears: Molecular Mechanisms and Implications for Clinical Management

Shoulder pain and disabilities are prevalent issues among the elderly population, with rotator cuff tear (RCT) being one of the leading causes. Although surgical treatment has shown some success, high postoperative retear rates remain a great challenge, particularly in elderly patients. Aging-related degeneration of muscle, tendon, tendon-to-bone enthesis, and bone plays a critical role in the development and prognosis of RCT. Studies have demonstrated that aging worsens muscle atrophy and fatty infiltration, alters tendon structure and biomechanical properties, exacerbates enthesis degeneration, and reduces bone density. Although recent researches have contributed to understanding the pathophysiological mechanisms of aging-related RCT, a comprehensive systematic review of this topic is still lacking. Therefore, this article aims to present a review of the pathophysiological changes and their clinical significance, as well as the molecular mechanisms underlying aging-related RCT, with the goal of shedding light on new therapeutic approaches to reduce the occurrence of aging-related RCT and improve postoperative prognosis in elderly patients.

The mechanosensitive ion channel PIEZO1 is expressed in tendons and regulates physical performance

How mechanical stress affects physical performance via tendons is not fully understood. Piezo1 is a mechanosensitive ion channel, and E756del PIEZO1 was recently found as a gain-of-function variant that is common in individuals of African descent. We generated tendon-specific knock-in mice using R2482H Piezo1, a mouse gain-of-function variant, and found that they had higher jumping abilities and faster running speeds than wild-type or muscle-specific knock-in mice. These phenotypes were associated with enhanced tendon anabolism via an increase in tendon-specific transcription factors, Mohawk and Scleraxis, but there was no evidence of changes in muscle. Biomechanical analysis showed that the tendons of R2482H Piezo1 mice were more compliant and stored more elastic energy, consistent with the enhancement of jumping ability. These phenotypes were replicated in mice with tendon-specific R2482H Piezo1 replacement after tendon maturation, indicating that PIEZO1 could be a target for promoting physical performance by enhancing function in mature tendon. The frequency of E756del PIEZO1 was higher in sprinters than in population-matched nonathletic controls in a small Jamaican cohort, suggesting a similar function in humans. Together, this human and mouse genetic and physiological evidence revealed a critical function of tendons in physical performance, which is tightly and robustly regulated by PIEZO1 in tenocytes.

The Role of the Non-Collagenous Extracellular Matrix in Tendon and Ligament Mechanical Behavior: A Review

Tendon is a connective tissue that transmits loads from muscle to bone, while ligament is a similar tissue that stabilizes joint articulation by connecting bone to bone. The 70-90% of tendon and ligament's extracellular matrix (ECM) is composed of a hierarchical collagen structure that provides resistance to deformation primarily in the fiber direction, and the remaining fraction consists of a variety of non-collagenous proteins, proteoglycans, and glycosaminoglycans (GAGs) whose mechanical roles are not well characterized. ECM constituents such as elastin, the proteoglycans decorin, biglycan, lumican, fibromodulin, lubricin, and aggrecan and their associated GAGs, and cartilage oligomeric matrix protein (COMP) have been suggested to contribute to tendon and ligament's characteristic quasi-static and viscoelastic mechanical behavior in tension, shear, and compression. The purpose of this review is to summarize existing literature regarding the contribution of the non-collagenous ECM to tendon and ligament mechanics, and to highlight key gaps in knowledge that future studies may address. Using insights from theoretical mechanics and biology, we discuss the role of the non-collagenous ECM in quasi-static and viscoelastic tensile, compressive, and shear behavior in the fiber direction and orthogonal to the fiber direction. We also address the efficacy of tools that are commonly used to assess these relationships, including enzymatic degradation, mouse knockout models, and computational models. Further work in this field will foster a better understanding of tendon and ligament damage and healing as well as inform strategies for tissue repair and regeneration.

Structure-function specialisation of the interfascicular matrix in the human achilles tendon

Tendon consists of highly aligned collagen-rich fascicles surrounded by interfascicular matrix (IFM). Some tendons act as energy stores to improve locomotion efficiency, but such tendons commonly obtain debilitating injuries. In equine tendons, energy storing is achieved primarily through specialisation of the IFM. However, no studies have investigated IFM structure-function specialisation in human tendons. Here, we compare the human positional anterior tibial tendon and energy storing Achilles tendons, testing the hypothesis that the Achilles tendon IFM has specialised composition and mechanical properties, which are lost with ageing. Data demonstrate IFM specialisation in the energy storing Achilles, with greater elasticity and fatigue resistance than in the positional anterior tibial tendon. With ageing, alterations occur predominantly to the proteome of the Achilles IFM, which are likely responsible for the observed trends towards decreased fatigue resistance. Knowledge of these key energy storing specialisations and their changes with ageing offers crucial insight towards developing treatments for tendinopathy.

Statement of significance

Developing effective therapeutics or preventative measures for tendon injury necessitates the understanding of healthy tendon function and mechanics. By establishing structure-function relationships in human tendon and determining how these are affected by ageing, potential targets for therapeutics can be identified. In this study, we have used a combination of mechanical testing, immunolabelling and proteomics analysis to study structure-function specialisations in human tendon. We demonstrate that the interfascicular matrix is specialised for energy storing in the Achilles tendon, and that its proteome is altered with ageing, which is likely responsible for the observed trends towards decreased fatigue resistance. Knowledge of these key energy storing specialisations and their changes with ageing offers crucial insight towards developing treatments and preventative approaches for tendinopathy.

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.

Force Transmission Between the Gastrocnemius and Soleus Sub-Tendons of the Achilles Tendon in Rat

The Achilles tendon (AT) is comprised of three distinct sub-tendons bound together by the inter-subtendon matrix (ISTM). The interactions between sub-tendons will have important implications for AT function. The aim of this study was to investigate the extent to which the ISTM facilitates relative sliding between sub-tendons, and serves as a pathway for force transmission between the gastrocnemius (GAS) and soleus (SOL) sub-tendons of the rat AT. In this study, ATs were harvested from Wistar rats, and the mechanical behavior and composition of the ISTM were explored. To determine force transmission between sub-tendons, the proximal and distal ends of the GAS and SOL sub-tendons were secured, and the forces at each of these locations were measured during proximal loading of the GAS. To determine the ISTM mechanical behavior, only the proximal GAS and distal SOL were secured, and the ISTM was loaded in shear. Finally, for compositional analysis, histological examination assessed the distribution of matrix proteins throughout sub-tendons and the ISTM. The results revealed distinct differences between the forces at the proximal and distal ends of both sub-tendons when proximal loading was applied to the GAS, indicating force transmission between GAS and SOL sub-tendons. Inter-subtendon matrix tests demonstrated an extended initial low stiffness toe region to enable some sub-tendon sliding, coupled with high stiffness linear region such that force transmission between sub-tendons is ensured. Histological data demonstrate an enrichment of collagen III, elastin, lubricin and hyaluronic acid in the ISTM. We conclude that ISTM composition and mechanical behavior are specialized to allow some independent sub-tendon movement, whilst still ensuring capacity for force transmission between the sub-tendons of the AT.

Lubricin expression in the lumbar endplate and its association with Modic changes

Objective

To explore the expression of lubricin in the lumbar endplate and its association with Modic changes (MCs).

Methods

Human endplate specimens harvested from patients undergoing surgery for thoracolumbar spine fractures ​or lumbar interbody fusion were divided into two groups: MCs group and normal group. Lubricin expression was examined by immunohistochemistry, and differences between the groups were analysed using quantitative polymerase chain reaction (qPCR). Lubricin expression and differences between endplates with MCs and normal endplates were confirmed using a rabbit model. In a final experiment, rabbit endplate chondrocytes were cocultured with Propionibacteria acnes (P. acnes) supernatant, and the expression of lubricin and endplate degeneration related genes were evaluated. In addition, the expression of matrix metalloproteinase 1(MMP-1), A disintegrin-like and metalloproteinase with thrombospondin type 5 motif (ADAMTS5) and inflammatory factors (Interleukin- 1β (IL-1β) and Interleukin-6 (IL-6)) were evaluated after lubricin overexpression.

Results

Lubricin was found in human lumbar endplates and its expression was lower in the MCs group compared to the normal group. In the rabbit model, lubricin was also found in the endplate. In rabbits injected with P. acnes (the MCs group), lubricin expression of endplate decreased compared to the normal group. In the culture of rabbit endplate chondrocytes with P. acnes supernatant, the expression of lubricin, aggrecan, sox9 and collagen type-II decreased significantly, while that of MMP-1 and ADAMTS5 increased significantly. Moreover, lubricin overexpression could downregulate the expression of MMP-1, ADAMTS5 and inflammatory factors (IL-1β and IL-6) compared to negative control.

Conclusion

Lubricin is present in the lumbar endplate where it may have an anti-inflammatory role. P. acnes infection inhibits lubricin expression by cartilage endplate cells and this may facilitate the progression of MCs and endplate degeneration.

The translational potential of this article

Lubricin may have an anti-inflammatory role. P. acnes infection inhibits lubricin expression by cartilage endplate cells and this may facilitate the progression of MCs and endplate degeneration.

Gene-expression changes in knee-joint tissues with aging and menopause: implications for the joint as an organ

Background

When considering the “joint as an organ”, the tissues in a joint act as complementary components of an organ, and the “set point” is the cellular activity for homeostasis of the joint tissues. Even in the absence of injury, joint tissues have adaptive responses to processes, like aging and menopause, which result in changes to the set point.

Purpose

The purpose of this study in a preclinical model was to investigate age-related and menopause-related changes in knee-joint tissues with the hypothesis that tissues will change in unique ways that reflect their differing contributions to maintaining joint function (as measured by joint laxity) and the differing processes of aging and menopause.

Methods

Rabbit knee-joint tissues from three groups were evaluated: young adult (gene expression, n=8; joint laxity, n=7; water content, n=8), aging adult (gene expression, n=6; joint laxity, n=7; water content, n=5), and menopausal adult (gene expression, n=8; joint laxity, n=7; water content, n=8). Surgical menopause was induced with ovariohysterectomy surgery and gene expression was assessed using reverse-transcription quantitative polymerase chain reaction.

Results

Aging resulted in changes to 37 of the 150 gene–tissue combinations evaluated, and menopause resulted in changes to 39 of the 150. Despite the similar number of changes, only eleven changes were the same in both aging and menopause. No differences in joint laxity were detected comparing young adult rabbits with aging adult rabbits or with menopausal adult rabbits.

Conclusion

Aging and menopause affected the gene-expression patterns of the tissues of the knee joint differently, suggesting unique changes to the set point of the knee. Interestingly, aging and menopause did not affect knee-joint laxity, suggesting that joint function was maintained, despite changes in gene expression. Taken together, these findings support the theory of the joint as an organ where the tissues of the joint adapt to maintain joint function.

In vitro loading models for tendon mechanobiology

Tendons are the connective tissue responsible for transferring force from muscles to bones. A key factor in tendon development, maturation, repair, and degradation is its biomechanical environment. Understanding tendon mechanobiology is essential for the development of injury prevention strategies, rehabilitation protocols and potentially novel treatments in tendon injury and degeneration. Despite the simple overall loading on tendon tissue, cells within the tissue in vivo experience a much more complex mechanical environment including tension, compression and shear forces. This creates a substantial challenge in the establishment of in vitro loading models of the tendon. This article reviews multiple loading models used for the study of tendon mechanobiology and summarizes the main findings. Although impressive progress has been achieved in the functionality and mimicry of in vitro loading models, an ideal platform is yet to be developed. Multidisciplinary approaches and collaborations will be the key to unveiling the tendon mechanobiology. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:566-575, 2018.

Engineered cartilage regeneration from adipose tissue derived-mesenchymal stem cells: A morphomolecular study on osteoblast, chondrocyte and apoptosis evaluation

The poor self-repair capacity of cartilage tissue in degenerative conditions, such as osteoarthritis (OA), has prompted the development of a variety of therapeutic approaches, such as cellular therapies and tissue engineering based on the use of mesenchymal stem cells (MSCs). The aim of this study is to demonstrate, for the first time, that the chondrocytes differentiated from rat adipose tissue derived-MSCs (AMSCs), are able to constitute a morphologically and biochemically healthy hyaline cartilage after 6 weeks of culture on a Collagen Cell Carrier (CCC) scaffold. In this study we evaluated the expression of some osteoblasts (Runt-related transcription factor 2 (RUNX2) and osteocalcin), chondrocytes (collagen I, II and lubricin) and apoptosis (caspase-3) biomarkers in undifferentiated AMSCs, differentiated AMSCs in chondrocytes cultured in monolayer and AMSCs-derived chondrocytes seeded on CCC scaffolds, by different techniques such as immunohistochemistry, ELISA, Western blot and gene expression analyses. Our results showed the increased expression of collagen II and lubricin in AMSCs-derived chondrocytes cultured on CCC scaffolds, whereas the expression of collagen I, RUNX2, osteocalcin and caspase-3 resulted decreased, when compared to the controls. In conclusion, this innovative basic study could be a possible key for future therapeutic strategies for articular cartilage restoration through the use of CCC scaffolds, to reduce the morbidity from acute cartilage injuries and degenerative joint diseases.

Livestock in biomedical research: history, current status and future prospective

Livestock models have contributed significantly to biomedical and surgical advances. Their contribution is particularly prominent in the areas of physiology and assisted reproductive technologies, including understanding developmental processes and disorders, from ancient to modern times. Over the past 25 years, biomedical research that traditionally embraced a diverse species approach shifted to a small number of model species (e.g. mice and rats). The initial reasons for focusing the main efforts on the mouse were the availability of murine embryonic stem cells (ESCs) and genome sequence data. This powerful combination allowed for precise manipulation of the mouse genome (knockouts, knockins, transcriptional switches etc.) leading to ground-breaking discoveries on gene functions and regulation, and their role in health and disease. Despite the enormous contribution to biomedical research, mouse models have some major limitations. Their substantial differences compared with humans in body and organ size, lifespan and inbreeding result in pronounced metabolic, physiological and behavioural differences. Comparative studies of strategically chosen domestic species can complement mouse research and yield more rigorous findings. Because genome sequence and gene manipulation tools are now available for farm animals (cattle, pigs, sheep and goats), a larger number of livestock genetically engineered (GE) models will be accessible for biomedical research. This paper discusses the use of cattle, goats, sheep and pigs in biomedical research, provides an overview of transgenic technology in farm animals and highlights some of the beneficial characteristics of large animal models of human disease compared with the mouse. In addition, status and origin of current regulation of GE biomedical models is also reviewed.

Distribution of proteins within different compartments of tendon varies according to tendon type

Although the predominant function of all tendons is to transfer force from muscle to bone and position the limbs, some tendons additionally function as energy stores, reducing the energetic cost of locomotion. To maximise energy storage and return, energy‐storing tendons need to be more extensible and elastic than tendons with a purely positional function. These properties are conferred in part by a specialisation of a specific compartment of the tendon, the interfascicular matrix, which enables sliding and recoil between adjacent fascicles. However, the composition of the interfascicular matrix is poorly characterised and we therefore tested the hypothesis that the distribution of elastin and proteoglycans differs between energy‐storing and positional tendons, and that protein distribution varies between the fascicular matrix and the interfascicular matrix, with localisation of elastin and lubricin to the interfascicular matrix. Protein distribution in the energy‐storing equine superficial digital flexor tendon and positional common digital extensor tendon was assessed using histology and immunohistochemistry. The results support the hypothesis, demonstrating enrichment of lubricin in the interfascicular matrix in both tendon types, where it is likely to facilitate interfascicular sliding. Elastin was also localised to the interfascicular matrix, specifically in the energy‐storing superficial digital flexor tendon, which may account for the greater elasticity of the interfascicular matrix in this tendon. A differential distribution of proteoglycans was identified between tendon types and regions, which may indicate a distinct role for each of these proteins in tendon. These data provide important advances into fully characterising structure–function relationships within tendon.

Tendon Structure and Composition

Tendons are soft, fibrous tissues that connect muscle to bone. Their main function is to transfer muscle generated force to the bony skeleton, facilitating movement around a joint, and as such they are relatively passive, inelastic structures, able to resist high forces. Tendons are predominantly composed of collagen, which is arranged in a hierarchical manner parallel to the long axis of the tendon, resulting in high tensile strength. Tendon also contains a range of non-collagenous proteins, present in low amounts, which nevertheless have important functional roles. In this chapter, we describe general tendon composition and structure, and discuss how variations in composition and structure at different levels of the tendon hierarchy confer specific mechanical properties, which are related to tendon function.

Increased lubricin/proteoglycan 4 gene expression and decreased modulus in medial collateral ligaments following ovariohysterectomy in the adult rabbit: Evidence consistent with aging

This study investigated whether ovariohysterectomy (OVH) surgery to induce menopause resulted in changes to modulus, failure strain and lubricin/proteoglycan 4 (PRG4) gene expression in rabbit medial collateral ligaments (MCLs), similar to aging (Thornton et al., 2015a). The MCLs from adult rabbits that underwent OVH surgery as adolescents (15-week-old) and adults (1-year-old) were compared by evaluating mechanical behaviour (adolescent OVH, n=8; adult OVH, n=7; normal, n=7), gene expression (adolescent OVH, n=9; adult OVH, n=8; normal, n=8), and collagen and glycosaminoglycan (adolescent OVH, n=9; adult OVH, n=8; normal, n=8) and water (adolescent OVH, n=9; adult OVH, n=8; normal, n=8) content. Mechanical behaviour evaluated cyclic, static and total creep strain, and ultimate tensile strength, modulus and failure strain. The RT-qPCR assessed mRNA levels for matrix regulatory genes. Adult OVH MCLs exhibited increased cyclic creep and failure strain, and decreased modulus with increased mRNA levels for lubricin/PRG4 and collagen I compared with normal MCLs. Adolescent OVH MCLs exhibited increased cyclic, static and total creep strain with decreased mRNA levels for the progesterone receptor. Lubricin/PRG4 plays a role in the lubrication of collagen fascicles which is likely related to the decreased modulus and increased failure strain observed in ligaments from adult OVH rabbits. Progesterone and its receptor are thought to play a role in the stretching of ligaments in pelvic organ prolapse and pregnancy which is likely related to the increase in creep strain observed in ligaments from adolescent OVH rabbits. Ovariohysterectomy in adult rabbits resulted in changes that were consistent with the aging MCL.

Tendon functional extracellular matrix

This article is one of a series, summarizing views expressed at the Orthopaedic Research Society New Frontiers in Tendon Research Conference. This particular article reviews the three workshops held under the "Functional Extracellular Matrix" stream. The workshops focused on the roles of the tendon extracellular matrix, such as performing the mechanical functions of tendon, creating the local cell environment, and providing cellular cues. Tendon is a complex network of matrix and cells, and its biological functions are influenced by widely varying extrinsic and intrinsic factors such as age, nutrition, exercise levels, and biomechanics. Consequently, tendon adapts dynamically during development, aging, and injury. The workshop discussions identified research directions associated with understanding cell-matrix interactions to be of prime importance for developing novel strategies to target tendon healing or repair.

Lubricin in human achilles tendon: The evidence of intratendinous sliding motion and shear force in achilles tendon

Achilles tendon is one of the most commonly injured tendons. Mechanical force is regarded as a major causative factor. However, the biomechanics of Achilles tendon and mechanical mechanism of the injuries are unclear. Lubricin expresses at regions exposed to sliding motion and shear force in a number of tissues. This study investigated the distribution and concentration of lubricin in human Achilles tendons for better understanding the biomechanics of Achilles tendon. Achilles tendons were harvested from nine cadavers. Lubricin was extracted from various locations proximal to the calcaneal insertion and quantified with ELISA. The distribution of lubricin was investigated with immunohistochemistry. Lubricin was mainly identified at the interfaces of tendon fascicles, especially in the mid-portion of the tendon. The concentration of lubricin in Achilles tendons varied by individual and the distance from its calcaneal insertion. The distal portion of the tendon had a higher concentration of lubricin than the proximal regions of the tendon. This study suggests the presence of intratendinous sliding motion of fascicles and shear force at interfaces of fascicles in human Achilles tendon. Shear force could be an important mechanical factor for the development of Achilles tendinopathy and rupture.

The interfascicular matrix enables fascicle sliding and recovery in tendon, and behaves more elastically in energy storing tendons

While the predominant function of all tendons is to transfer force from muscle to bone and position the limbs, some tendons additionally function as energy stores, reducing the cost of locomotion. Energy storing tendons experience extremely high strains and need to be able to recoil efficiently for maximum energy storage and return. In the equine forelimb, the energy storing superficial digital flexor tendon (SDFT) has much higher failure strains than the positional common digital extensor tendon (CDET). However, we have previously shown that this is not due to differences in the properties of the SDFT and CDET fascicles (the largest tendon subunits). Instead, there is a greater capacity for interfascicular sliding in the SDFT which facilitates the greater extensions in this particular tendon (Thorpe et al., 2012). In the current study, we exposed fascicles and interfascicular matrix (IFM) from the SDFT and CDET to cyclic loading followed by a test to failure. The results show that IFM mechanical behaviour is not a result of irreversible deformation, but the IFM is able to withstand cyclic loading, and is more elastic in the SDFT than in the CDET. We also assessed the effect of ageing on IFM properties, demonstrating that the IFM is less able to resist repetitive loading as it ages, becoming stiffer with increasing age in the SDFT. These results provide further indications that the IFM is important for efficient function in energy storing tendons, and age-related alterations to the IFM may compromise function and predispose older tendons to injury.

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Fascicle sliding enables high levels of extension in energy storing tendons.

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Sliding mechanics are governed by the interfascicular matrix (IFM).

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We assessed IFM extension and recovery.

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IFM elasticity and recovery are greater in energy storing tendons.

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The IFM plays an important role in the function of energy storing tendons.

Biosynthesis of collagen I, II, RUNX2 and lubricin at different time points of chondrogenic differentiation in a 3D in vitro model of human mesenchymal stem cells derived from adipose tissue

The first aim of the study was to identify the most appropriate time for differentiation of adipose tissue derived mesenchymal stem cells (MSCs) to chondrocytes, through the self-assembly process. For this purpose, the expression of some chondrocyte markers, such as collagen type I, collagen type II, RUNX2 and lubricin was investigated at different times (7, 14, 21 and 28 days) of chondrogenic differentiation of MSCs, by using immunohistochemistry and Western blot analysis. The second aim of the study was to demonstrate that the expression of lubricin, such as the expression of collagen type II, could be a possible biomarker for the detection of chondrocytes well-being and viability in the natural self-assembling constructs, called 'cell pellets'. Histology (hematoxylin and eosin) and histochemistry (alcian blue staining) methods were used to assess the chondrogenic differentiation of MSCs. The results showed that after 21 days the differentiated chondrocytes, when compared with MSCs cultured without chondrogenic medium (CD44, CD90 and CD105 positive; CD45, CD14 and CD34 negative), were able to produce significant quantities of collagen type I, collagen type II, and lubricin, suggesting hyaline cartilage formation. During the differentiation phase, the cells showed a reduced expression of RUNX2, a protein expressed by osteoblasts. Our studies demonstrated that 21 days is the optimum time for the implantation of chondrocytes differentiated from adipose tissue-derived MSCs. This information could be useful for the future development of cell-based repair therapies for degenerative diseases of articular cartilage.

Age-related changes in structure and extracellular matrix protein expression levels in rat tendons

The musculoskeletal system (muscle-tendon-bone) demonstrates numerous age-related changes, with modifications in tendons the least well studied, although increased predisposition to tendinopathy and rupture have been reported. In order to gain insights into the basis of age-associated increase in tendon injuries, we compared Achilles and tibialis anterior tendons and myotendinous junctions (MTJs) from 3- to 5- and 22- to 25-month-old rats for underlying structure and composition. Significant decreases were observed by qRT-PCR for collagen I, III, and V mRNA expression in tendons of old rats, but immunostaining detected no apparent differences in collagen I and V expression on the protein level. Tendons of old compared with young rats had decreased mRNA expression levels of proteoglycan 4 (PRG4) and elastin (Eln), but no differences in the mRNA expression of connective tissue growth factor, TGF-beta 1, or stromal cell-derived factor 1. For PRG4, immunostaining showed good correlation with qRT-PCR results. This is the first study to show reductions in PRG4 in tendons and MTJs of old rats. Decreased PRG4 expression in tendons could result in increased tendon stiffness and may be associated with decreased activity in the elderly. The diminished collagen mRNA expression in combination with decreased PRG4 and Eln mRNA expression may be associated with increased risk of tendon injury with aging.

Direct radiofrequency application improves pain and gait in collagenase-induced acute achilles tendon injury

Radiofrequency (RF) is often used as a supplementary and alternative method to alleviate pain for chronic tendinopathy. Whether or how it would work for acute tendon injury is not addressed in the literatures. Through detailed pain and gait monitoring, we hypothesized that collagenase-induce acute tendinopathy model may be able to answer these questions. Gait parameters, including time, distance, and range of motion, were recorded and analyzed using a walking track equipped with a video-based system. Expression of substance P (SP), calcitonin gene related peptide (CGRP), and galanin were used as pain markers. Beta-III tubulin and Masson trichrome staining were used as to evaluate nerve sprouting, matrix tension, and degeneration in the tendon. Of fourteen analyzed parameters, RF significantly improved stance phase, step length, preswing, and intermediary toe-spread of gait. Improved gait related to the expression of substance P, CGRP, and reduced nerve fiber sprouting and matrix tension, but not galanin. The study indicates that direct RF application may be a valuable approach to improve gait and pain in acute tendon injury. Altered gait parameters may be used as references to evaluate therapeutic outcomes of RF or other treatment plan for tendinopathy.

Defects in tendon, ligament, and enthesis in response to genetic alterations in key proteoglycans and glycoproteins: a review

This review summarizes the genetic alterations and knockdown approaches published in the literature to assess the role of key proteoglycans and glycoproteins in the structural development, function, and repair of tendon, ligament, and enthesis. The information was collected from (i) genetically altered mice, (ii) in vitro knockdown studies, (iii) genetic variants predisposition to injury, and (iv) human genetic diseases. The genes reviewed are for small leucine-rich proteoglycans (lumican, fibromodulin, biglycan, decorin, and asporin); dermatan sulfate epimerase (Dse) that alters structure of glycosaminoglycan and hence the function of small leucine-rich proteoglycans by converting glucuronic to iduronic acid; matricellular proteins (thrombospondin 2, secreted phosphoprotein 1 (Spp1), secreted protein acidic and rich in cysteine (Sparc), periostin, and tenascin X) including human tenascin C variants; and others, such as tenomodulin, leukocyte cell derived chemotaxin 1 (chondromodulin-I, ChM-I), CD44 antigen (Cd44), lubricin (Prg4), and aggrecan degrading gene, a disintegrin-like and metallopeptidase (reprolysin type) with thrombospondin type 1 motif, 5 (Adamts5). Understanding these genes represents drug targets for disrupting pathological mechanisms that lead to tendinopathy, ligamentopathy, enthesopathy, enthesitis and tendon/ligament injury, that is, osteoarthritis and ankylosing spondylitis.

Resurfacing with chemically modified hyaluronic acid and lubricin for flexor tendon reconstruction

We assessed surface coating with carbodiimide derivatized hyaluronic acid combined with lubricin (cd-HA-Lubricin) as a way to improve extrasynovial tendon surface quality and, consequently, the functional results in flexor tendon reconstruction, using a canine in vivo model. The second and fifth flexor digitorum profundus tendons from 14 dogs were reconstructed with autologs peroneus longus (PL) tendons 6 weeks after a failed primary repair. One digit was treated with cd-HA-Lubricin, and the other was treated with saline as the control. Six weeks following grafting, the digits and graft tendons were functionally and histologically evaluated. Adhesion score, normalized work of flexion, graft friction in zone II, and adhesion breaking strength at the proximal repair site in zone III were all lower in the cd-HA-Lubricin treated group compared to the control group. The strength at the distal tendon/bone interface was decreased in the cd-HA-Lubricin treated grafts compared to the control grafts. Histology showed inferior healing in the cd-HA-Lubricin group at both proximal and distal repair sites. However, cd-HA-Lubricin treatment did not result in any gap or rupture at either the proximal or distal repair sites. These results demonstrate that cd-HA-Lubricin can eliminate graft adhesions and improve digit function, but that treatment may have an adverse effect on tendon healing.

The role of the non-collagenous matrix in tendon function

Tendon consists of highly ordered type I collagen molecules that are grouped together to form subunits of increasing diameter. At each hierarchical level, the type I collagen is interspersed with a predominantly non-collagenous matrix (NCM) (Connect. Tissue Res., 6, 1978, 11). Whilst many studies have investigated the structure, organization and function of the collagenous matrix within tendon, relatively few have studied the non-collagenous components. However, there is a growing body of research suggesting the NCM plays an important role within tendon; adaptations to this matrix may confer the specific properties required by tendons with different functions. Furthermore, age-related alterations to non-collagenous proteins have been identified, which may affect tendon resistance to injury. This review focuses on the NCM within the tensional region of developing and mature tendon, discussing the current knowledge and identifying areas that require further study to fully understand structure-function relationships within tendon. This information will aid in the development of appropriate techniques for tendon injury prevention and treatment.

Lubricin and smooth muscle α-actin-containing myofibroblasts in the pseudomembranes around loose hip and knee prostheses

The objective was to evaluate the presence and distribution of the lubricating and anti-adhesion glycoprotein lubricin and cells containing the contractile isoform smooth muscle α-actin (SMA) in pseudomembranes around loose hip prostheses. Periprosthetic tissue was obtained at revision arthroplasty of eight aseptic, loose hip implants, and for comparison three loose knee prostheses. Immunohistochemical analysis was performed in 3 zones: zone 1, within 300μm of the edge of the implant-tissue interface; zone 2, between zones 1 and 3; zone 3, within 300μm of the resected/trimmed edge. The presence of lubricin was extensive in all samples: (1) as a discrete layer at the implant-tissue interface; (2) within the extracellular matrix (ECM); (3) intracellularly. There was significantly more high grade (>50%) lubricin surface staining at the implant-tissue interface compared with the resected edge. While there was also a significant effect of location of high grade ECM lubricin staining, there was no significant effect of implant type (i.e. hip versus knee). All but two hip pseudomembrane samples showed the presence of many SMA-containing cells. There was a significant effect of location on the number of SMA-expressing cells, but not of implant type. These findings might explain why the management of loose prosthesis is so challenging.

Effects of stress deprivation on lubricin synthesis and gliding of flexor tendons in a canine model in vivo

BACKGROUND

Lubricin facilitates boundary lubrication of cartilage. The synthesis of lubricin in cartilage is regulated by mechanical stimuli, especially shear force. Lubricin is also found in flexor tendons. However, little is known about the effect of mechanical loading on lubricin synthesis in tendons or about the function of lubricin in flexor tendons. The purpose of this study was to investigate the relationship of mechanical loading to lubricin expression and gliding resistance of flexor tendons.

METHODS

Flexor tendons were harvested from canine forepaws that had been suspended without weight-bearing for twenty-one days and from the contralateral forepaws that had been allowed free motion. Lubricin expression in each flexor tendon was investigated with real-time RT-PCR (reverse transcription polymerase chain reaction) and immunohistochemistry. Lubricin in the flexor tendon was extracted and quantified with ELISA (enzyme-linked immunosorbent assay). The friction between the flexor tendon and the proximal pulley was measured.

RESULTS

The non-weight-bearing flexor tendons had a 40% reduction of lubricin expression (p < 0.01) and content (p < 0.01) compared with the flexor tendons in the contralateral limb. However, the gliding resistance of the tendons in the non-weight-bearing limb was the same as that of the tendons on the contralateral, weight-bearing side.

CONCLUSIONS

Mechanical loading affected lubricin expression in flexor tendons, resulting in a 40% reduction of lubricin content, but these changes did not affect the gliding resistance of the flexor tendons.

Domestic animal models for biomedical research

Experimental animals in biomedical research provide insights into disease mechanisms and models for determining the efficacy and safety of new therapies and for discovery of corresponding biomarkers. Although mouse and rat models are most widely used, observations in these species cannot always be faithfully extrapolated to human patients. Thus, a number of domestic species are additionally used in specific disease areas. This review summarizes the most important applications of domestic animal models and emphasizes the new possibilities genetic tailoring of disease models, specifically in pigs, provides.

Specialization of tendon mechanical properties results from interfascicular differences

Tendons transfer force from muscle to bone. Specific tendons, including the equine superficial digital flexor tendon (SDFT), also store and return energy. For efficient function, energy-storing tendons need to be more extensible than positional tendons such as the common digital extensor tendon (CDET), and when tested in vitro have a lower modulus and failure stress, but a higher failure strain. It is not known how differences in matrix organization contribute to distinct mechanical properties in functionally different tendons. We investigated the properties of whole tendons, tendon fascicles and the fascicular interface in the high-strain energy-storing SDFT and low-strain positional CDET. Fascicles failed at lower stresses and strains than tendons. The SDFT was more extensible than the CDET, but SDFT fascicles failed at lower strains than CDET fascicles, resulting in large differences between tendon and fascicle failure strain in the SDFT. At physiological loads, the stiffness at the fascicular interface was lower in the SDFT samples, enabling a greater fascicle sliding that could account for differences in tendon and fascicle failure strain. Sliding between fascicles prior to fascicle extension in the SDFT may allow the large extensions required in energy-storing tendons while protecting fascicles from damage.

The aging athlete: part 1, "boomeritis" of the lower extremity

OBJECTIVE

The purpose of this review is to describe the physiologic changes that occur in the musculoskeletal system during aging and the common injuries that occur in the lower extremity as a consequence of these changes. Several clinical presentations are addressed, and their differential diagnoses are discussed with an emphasis on the most likely injury for each presentation.

CONCLUSION

A unique quality of the newly aging group of people referred to as baby boomers is their expectation to continue exercising as they grow older, thus the incidence of exercise-induced injuries among older people is increasing. The concepts behind factors that predispose older athletes to certain pathologic conditions that affect the muscles, tendons, and bones of the lower extremity must be understood.

Immunolocalization of lubricin in the rat periodontal ligament during experimental tooth movement

Lubricin is a protein which contributes to the boundary lubrication, facilitating low friction levels at the interfacing surfaces of joints. In tendons and ligaments it facilitates the relative movement of collagen bundles. Its expression is affected by mechanical signals and cytokines. During application of orthodontic forces to teeth, there is a transduction of mechanical forces to the cells of the periodontal ligament (PDL), which triggers several biological reactions causing the synthesis of prostaglandins, cytokines and growth factors. The aim of the present study was to examine the immunolocalization of lubricin and to evaluate if it is time-dependently and differentially detected within the PDL following the application of orthodontic forces to create areas of compression and tension. This was achieved by placing elastic bands between the maxillary first and second molars of 16 male Sprague-Dawley rats (each weighing 120-200g) for 12 and 24h. The molar-bearing segments were dissected and processed for histological and immunohistochemical examination. Binding of a monoclonal antibody was used to evaluate lubricin localization using an indirect streptavidin/biotin immunperoxidase technique. Lubricin, was constitutively expressed in the PDL of rat molars. After the experimental force was applied to the tooth, lubricin was down-regulated, on both sides (compression and tension) of the PDL, in a time-dependent fashion, although to a different extent, being at any time more expressed on the tension side. Furthermore, in every sample, almost all PDL cells in the adjacent tooth cementum and alveolar bone, were more heavily immunolabeled by lubricin antibody, contrary to those located in the central portion of the PDL. Lubricin expression therefore seems related to PDL remodeling and tooth displacement following the application of an orthodontic force, and it appears that lubricin may play an important role during tooth movement.

Acute injury affects lubricin expression in knee menisci: an immunohistochemical study

The aim of this study was to investigate for the first time lubricin expression in intact menisci and in menisci from patients with recent knee joint injury using histology, immunohistochemistry, Western blotting and gene expression analysis, to provide insights into pathological processes affecting meniscal tissue. Lubricin expression was studied in vivo in 20 patients (14 males and 6 females) with recent joint injury subjected to arthroscopic partial meniscectomy and in vitro in fibroblast-like cells from meniscus tissue to establish whether it is down-regulated following acute traumatic knee injury. The control group consisted of cadaver donors with normal menisci. Histology demonstrated a normal tissue without structural changes in control samples and structural alterations and clefts in injured menisci. Very strong lubricin immunohistochemical staining was observed in intact menisci; in contrast weak staining was seen in injured menisci. Western blot and mRNA expression analysis also demonstrated strong lubricin expression in control cells and a negligible amount of lubricin in injured fibroblast-like cells. Our data provide information concerning the immediate in vivo response to injury of human knee menisci by documenting early changes in the boundary-lubricating ability of synovial fluid and articular cartilage integrity. These findings may provide the biological basis for developing novel medical therapies to be applied before surgical treatment to preserve tissue function and prevent cartilage damage.

The aging athlete: part 2, "boomeritis" of the upper extremity

OBJECTIVE

The purpose of this review is to describe the upper extremity injuries that frequently accompany aging, the typical clinical presentations, and the differential diagnoses with an emphasis on the injury most likely encountered with each presentation.

CONCLUSION

Expectation of continued participation in exercise and sports activities by the baby boomer population has presented new challenges to the medical field. The concepts behind factors that predispose older athletes to certain pathologic conditions that affect the muscles, tendons, and bones of the upper extremity must be understood.

Lubricin in human breast tissue expander capsules

Capsular contraction is the most common complication of breast reconstruction surgery. While presence of the contractile protein alpha smooth muscle actin (α-SMA) is considered among the causes of capsular contraction, the exact etiology and pathophysiology is not fully understood. The objective of this study was to investigate the possible role of lubricin in capsular formation and contraction by determining the presence and distribution of the lubricating protein lubricin in human breast tissue expander capsules. Related aims were to evaluate select histopathologic features of the capsules, and the percentage of cells expressing α-SMA, which reflects the myofibroblast phenotype. Capsules from tissue expanders were obtained from eight patients. Lubricin, at the tissue-implant interface, in the extracellular matrix, and in cells, and α-SMA-containing cells were evaluated immunohistochemically. The notable finding was that lubricin was identified in all tissue expander capsules: as a discrete layer at the tissue-implant interface, extracellular, and intracellular. There was a greater amount of lubricin in the extracellular matrix in the intimal-subintimal zone when compared with the tissue away from the implant. Varying degrees of synovial metaplasia were seen at the tissue-implant interface. α-SMA-containing cells were also seen in all but one patient. The findings might help us better understand factors involved in capsule formation.

Lubricin Distribution in the Menisci and Labra of Human Osteoarthritic Joints

OBJECTIVE

Lubricin is the principal boundary lubricant on articular cartilage. We aimed to describe the distribution of lubricin in the other articulating structures in the human knee and hip-menisci and labra-and to relate this distribution to the degree of tissue degeneration.

METHODS

Eighteen menisci and 6 labra were obtained from patients with osteoarthritis undergoing total knee and total hip replacements, respectively. Macroscopically intact specimens were fixed in formalin and processed for H&E staining and immunohistochemical evaluation with an antilubricin monoclonal antibody.

RESULTS

Lubricin was found in all tissues as a discrete layer on the tissue surface, within the extracellular matrix, and intracellularly, indicating that it plays a role in the tribology of these tissues in human subjects, and can be synthesized by cells within the tissues. While none of the samples displayed macroscopic tears, approximately 40% of the surface of the menisci and 80% of the surface of the labra displayed microscopic fibrillations and slight fraying. There was no effect of the degenerative changes on the distribution of lubricin.

CONCLUSIONS

Lubricin coats nearly the entirety of the surfaces of menisci and labra, including microfibrillations and tears, with possible implications towards the tribology of the tissues and healing of tissue damage.

Effect of functional shift of the mandible on lubrication of the temporomandibular joint

Lubrication of synovial joints reduces the coefficient of friction of the articular cartilage surface. To investigate the effect of malocclusion on the lubrication of the temporomandibular joint (TMJ), we evaluated lubricin expression in the rat TMJ immunohistochemically, under conditions of functional lateral shift of the mandible, during period of growth. Thirty 5-week-old male Wistar rats were divided into experimental, recovery, and control groups. Each rt in the experimental and recovery groups was fitted with an acrylic-plate guiding appliance. The rats in the experimental and control groups were killed at 14 and 28 days after the appliance was attached. Each rat in the recovery group was detached from the appliance at 14 days, and was killed 14 days after the appliance was removed. In the experimental group, the expression of lubricin staining in TMJ cartilage was significantly decreased during the experimental period. In the recovery group, the expression of lubricin staining in TMJ cartilage was significantly greater than in the experimental group, and there was no significant difference at 28 days between the control and recovery groups. Analysis of these data suggests that a functional lateral shift of the mandible during the growth period influences lubrication of the TMJ.

Lubricin in human temporomandibular joint disc: an immunohistochemical study

AIMS

To evaluate, immunohistochemically, the presence and distribution of lubricin in human temporomandibular joint (TMJ) discs without any degenerative changes, obtained from autopsies, in order to elucidate the TMJ lubrication system and disc tribology.

METHODS

Immunohistochemistry for lubricin detection was carried out on 34 TMJ discs. Any disc had signs of degenerative or inflammatory joint disease nor disc were displaced. Sections were incubated with diluted rabbit polyclonal anti-lubricin antibody and scored according to the percentage of lubricin immunopositive cells. Three different TMJ disc tissue compartments taken from the intermediate zone were analysed, namely: the central region as well as the temporal (superior) and condylar (inferior) disc surfaces. The Friedman test, was used to compare lubricin at a protein level expression, amongst the regions of disc specimens.

RESULTS

Staining was noted within the TMJ disc cell populations in every disc tissue sample, however, the number of disc cells immunolabelled varied according to disc tissue regions. The percentage of immunostained cells, was statistically significant lower in the central region than in each disc surface (p<0.0001), whilst any statistically significant difference was found when comparing the two surfaces one another.

CONCLUSIONS

Lubricin is present in several location of TMJ disc being significantly more expressed at disc surfaces than in the central part.

Immunolocalization and expression of lubricin in the bilaminar zone of the human temporomandibular joint disc

Lubricin, which is a boundary joint lubricant, was investigated immunohistochemically in the bilaminar zone (BZ) of the human temporomandibular joint (TMJ), without any degenerative changes. Immunohistochemistry for lubricin detection was carried out on 33 TMJ discs obtained from 17 cadavers. Sections were incubated with diluted rabbit polyclonal anti-lubricin antibody and scored according to the percentage of lubricin immunopositive cells. Three different TMJ disc tissue compartments were analyzed, namely: the upper lamina, the inferior lamina and the loose connective tissue in the space between the laminae. The Mann-Whitney U test was used to compare protein expression (lubricin) among disc specimens' regions. Staining was noted within the TMJ disc cell populations in every disc tissue sample, with almost every cell immunolabeled by the lubricin antibody. The number of disc cells immunolabeled was almost the same in the 3 bilaminar zone regions. Positive extracellular matrix staining was also seen. The results of the present study suggest that lubricin is expressed in the TMJ disc bilaminar zone. Lubricin may have a role in normal disc posterior attachment physiology through the prevention of cellular adhesion as well as providing lubrication during normal bilaminar zone function.

Animal models for translational research on shoulder pathologies: from bench to bedside

Several animal models have been used for in vivo and in vitro shoulder research. In vitro models, consisting of cadaveric specimens, are useful in providing basic understanding of the functioning of the shoulder and for biomechanical experiments. In vivo models provide the means to model living phenomena, such as tendon healing process, tendinopathy, instability, and adaptive responses to surgery. However, intrinsic differences among different species make translation to human shoulder pathologies difficult. Most of the animals used in experimental settings are quadrupeds, using the forelimbs for weight-bearing during locomotion, with no or minimal overhead activity. The various animal models already used to study shoulder pathologies are presented in this article. However, there is a lack of validation for these animal models, which provides challenge to the further research in this field.

Lubricin distribution in the torn human anterior cruciate ligament and meniscus

The objective of this study was to: (1) determine the distribution of lubricin in the human torn anterior cruciate ligament (ACL) and meniscus; (2) determine the distribution of lubricin in the human intact ACL and meniscus; (3) and identify potential cellular sources of lubricin in these tissues. Ten torn ACLs and six torn menisci were obtained from surgeries; for comparison, 11 intact ACLs and 13 intact menisci were obtained from total knee replacements. Samples were formalin fixed and processed for immunohistochemical staining with a monoclonal antibody for lubricin. In torn ACLs and menisci, lubricin was generally found as a discrete layer covering the torn surface. No surface lubricin staining was found on the transected edges produced during excision. Lubricin was also found on the native surfaces of intact ACLs and menisci. In all tissues, lubricin was found in the matrix and intracellularly. The surface layer of lubricin coating torn edges of ACLs and menisci may interfere with the integrative healing process needed for repair.

A novel deletion mutation in proteoglycan-4 underlies camptodactyly-arthropathy-coxa-vara-pericarditis syndrome in a consanguineous pakistani family

BACKGROUND AND AIMS

Camptodactyly-arthropathy-coxa-vara-pericarditis (CACP) syndrome is an autosomal recessive condition that mostly affects joints and tendons but can also affect the pericardium, which is a surface surrounding the heart. CACP syndrome is caused by mutations in a secreted proteoglycan 4 (PRG4) gene, which expresses in skeletal as well as nonskeletal tissues. We undertook this study to genetically screen a large consanguineous Pakistani family segregating CACP in an autosomal recessive manner.

METHODS

Genome-wide homozygosity mapping of 10 members of a Pakistani family including six affected and four normal individuals was carried out using 250K SNP genotyping array. To screen for mutation in PRG4 gene, all coding exons and exon-intron junctions were sequenced using ABI prism 3730 automated DNA sequencer.

RESULTS

Genome-wide homozygosity mapping revealed a large homozygous region on chromosome 1 carried by all the affected individuals. This region contains the previously described PRG4 gene involved in CACP syndrome. Sequence analysis of PRG4 gene in affected individuals of the family presented here revealed a 2 base-pair (bp) deletion (c.2816_2817delAA) predicting a frame shift mutation (p.Lys939fsX38). To our knowledge, this is probably the first mutation identified in PRG4 gene in a Pakistani family.

CONCLUSIONS

We described a 2-bp novel deletion mutation in PRG4 gene in a Pakistani family with CACP. Our findings extend the body of evidence that only nonsense mutation in PRG4 gene triggers the phenotype.

Extracorporeal shockwave-induced expression of lubricin in tendons and septa

Lubricin, a lubricating glycoprotein that facilitates tendon gliding, is upregulated by mechanical as well as biochemical stimuli, prompting this study of its induction by extracorporeal shockwave therapy (ESWT). The objective of this study was to characterize and quantify the effect of ESWT on lubricin expression in tendons and septa in a rat model. Hindlimbs of six rats were treated with low-dose ESWT and those of another six with high-dose ESWT, using contralateral limbs as controls. After 4 days, resected samples were processed for immunolocalization of lubricin using a purified monoclonal antibody. ESWT was found to increase lubricin expression in both low-dose and high-dose ESWT-treated tendons and also in septa. Lubricin expression generally increased with increasing dose of ESWT. Increased lubricin expression may contribute to the beneficial effects of ESWT in providing pain and symptom relief in musculoskeletal disorders by decreasing erosive wear.

Lubricin is expressed in chondrocytes derived from osteoarthritic cartilage encapsulated in poly (ethylene glycol) diacrylate scaffold

Osteoarthritis (OA) is characterized by degenerative changes within joints that involved quantitative and/or qualitative alterations of cartilage and synovial fluid lubricin, a mucinous glycoprotein secreted by synovial fibroblasts and chondrocytes. Modern therapeutic methods, including tissue-engineering techniques, have been used to treat mechanical damage of the articular cartilage but to date there is no specific and effective treatment. This study aimed at investigating lubricin immunohistochemical expression in cartilage explant from normal and OA patients and in cartilage constructions formed by Poly (ethylene glycol) (PEG) based hydrogels (PEG-DA) encapsulated OA chondrocytes. The expression levels of lubricin were studied by immunohistochemistry: i) in tissue explanted from OA and normal human cartilage; ii) in chondrocytes encapsulated in hydrogel PEGDA from OA and normal human cartilage. Moreover, immunocytochemical and western blot analysis were performed in monolayer cells from OA and normal cartilage. The results showed an increased expression of lubricin in explanted tissue and in monolayer cells from normal cartilage, and a decreased expression of lubricin in OA cartilage. The chondrocytes from OA cartilage after 5 weeks of culture in hydrogels (PEGDA) showed an increased expression of lubricin compared with the control cartilage. The present study demonstrated that OA chondrocytes encapsulated in PEGDA, grown in the scaffold and were able to restore lubricin biosynthesis. Thus our results suggest the possibility of applying autologous cell transplantation in conjunction with scaffold materials for repairing cartilage lesions in patients with OA to reduce at least the progression of the disease.

Tendon fascicle gliding in wild type, heterozygous, and lubricin knockout mice

The objective of this study was to investigate the role of lubricin in the lubrication of tendon fascicles. Lubricin, a glycoprotein, lubricates cartilage and tendon surfaces, but the function of lubricin within the tendon fascicle is unclear. We developed a novel method to assess the gliding resistance of a single fascicle in a mouse tail model and used it to test the hypothesis that gliding resistance would be increased in lubricin knockout mice. Thirty-six mouse tails were used from 12 wild type, 12 heterozygous, and 12 lubricin knockout mice. A 15 mm long fascicle segment was pulled proximally after being divided distally. The peak resistance during fascicle pullout and the fascicle perimeter were measured. Lubricin expression was evaluated by immunohistochemistry. The peak gliding resistance in the lubricin knockout mice was significantly higher than in the wild type (p < 0.05). Fascicles from heterozygous mice were intermediate in value, but not significantly different from either wild type or lubricin knockout fascicles in peak gliding resistance. No significant difference was found in fascicle perimeter among the three groups. No correlation was observed between fascicle perimeter and gliding resistance. While lubricin was detected by immunostaining on the fascicle surface in wild type and heterozygous mice, lubricin was not detectable in the tendons of knockout mice. We conclude that the absence of lubricin is associated with increased interfascicular friction and that lubricin may play an important role in interfascicular lubrication.

Lubricin immunohistochemical expression in human temporomandibular joint disc with internal derangement

Lubricin is a chondroprotective, mucinous glycoprotein which contribute to joint lubrication, especially to boundary lubrication and maintains joint integrity. The present investigation aimed to study the immunolocalization of lubricin in TMJ discs from patients affected by anterior disc displacement with reduction (ADDwR) ADDwoR. Eighteen TMJ displaced disc affected by ADDwoR were processed immunohistochemically, with a polyclonal anti-lubricin antibody, used at 1:50 working dilution. The percentage of lubricin immunopositive cells (extent score = ES) and the extent of lubricin staining of the disc extracellular matrix (ECM), were evaluated. Each sample was scored for histopathological changes. Percentage of immunostained surface disc cells was the same (ES = 4) in both control and ADDwOR cells, being this data not statistically significant (P < 0.05). In pathological specimens the percentages of lubricin-stained cells was very high with an ES of 4 respect to control specimen, and this difference was statistically significant different (P > 0.05). The extracellular matrix (ECM) of discs at the disc surfaces of both pathological and normal specimens was very heavily stained (++++). Both the ES and ECM staining were not statistically correlated to the TMJ degeneration score according to the Spearman's rank correlation coefficient. According to our findings, a longstanding TMJ disc injury, affects lubricin expression in the TMJ disc tissue and not its surfaces, moreover, lubricin immunostaining is not correlated to TMJ disc histopathological changes.

Loss of cartilage structure, stiffness, and frictional properties in mice lacking PRG4

OBJECTIVE

To assess the role of the glycoprotein PRG4 in joint lubrication and chondroprotection by measuring friction, stiffness, surface topography, and subsurface histology of the hip joints of Prg4(-/-) and wild-type (WT) mice.

METHODS

Friction and elastic modulus were measured in cartilage from the femoral heads of Prg4(-/-) and WT mice ages 2, 4, 10, and 16 weeks using atomic force microscopy, and the surface microstructure was imaged. Histologic sections of each femoral head were stained and graded.

RESULTS

Histologic analysis of the joints of Prg4(-/-) mice showed an enlarged, fragmented surface layer of variable thickness with Safranin O-positive formations sometimes present, a roughened underlying articular cartilage surface, and a progressive loss of pericellular proteoglycans. Friction was significantly higher on cartilage of Prg4(-/-) mice at age 16 weeks, but statistically significant differences in friction were not detected at younger ages. The elastic modulus of the cartilage was similar between cartilage surfaces of Prg4(-/-) and WT mice at young ages, but cartilage of WT mice showed increasing stiffness with age, with significantly higher moduli than cartilage of Prg4(-/-) mice at older ages.

CONCLUSION

Deletion of the gene Prg4 results in significant structural and biomechanical changes in the articular cartilage with age, some of which are consistent with osteoarthritic degeneration. These findings suggest that PRG4 plays a significant role in preserving normal joint structure and function.

Chondrogenic differentiation and lubricin expression of caprine infraspinatus tendon cells

Reparative strategies for the treatment of injuries to tendons, including those of the rotator cuff of the shoulder, need to address the formation of the cartilage which serves as the attachment apparatus to bone and which forms at regions undergoing compressive loading. Moreover, recent work indicates that cells employed for rotator cuff repair may need to synthesize a lubricating glycoprotein, lubricin, which has recently been found to play a role in tendon tribology. The objective of the present study was to investigate the chondrogenic differentiation and lubricin expression of caprine infraspinatus tendon cells in monolayer and three-dimensional culture, and to compare the behavior with bone marrow-derived mesenchymal stem cells (MSCs). The results demonstrated that while tendon cells in various media, including chondrogenic medium, expressed lubricin, virtually none of the MSCs synthesized this important lubricating molecule. Also of interest was that the cartilage formation capacity of the tendon cells grown in pellet culture in chondrogenic medium was comparable with MSCs. These data inform the use of tendon cells for rotator cuff repair, including for fibrocartilaginous zones.

Distribution of lubricin in the ruptured human rotator cuff and biceps tendon: a pilot study

BACKGROUND

Lubricin is a lubricant for diarthrodial joint tissues and has antiadhesion properties; its presence in the (caprine) rotator cuff suggests it may have a role in intrafascicular lubrication.

QUESTIONS/PURPOSES

To preliminarily address this role, we asked: (1) What is the distribution of lubricin in human ruptured supraspinatus and biceps tendons? (2) What are the potential cellular sources of lubricin?

METHODS

We obtained seven torn rotator cuff samples and four torn biceps tendon samples from 10 patients; as control tissues, we obtained the right and left supraspinatus tendons from each of six cadavers. Specimens were fixed in formalin and processed for immunohistochemical evaluation using a monoclonal antibody for lubricin.

RESULTS

We found lubricin as a discrete layer on the torn edges of all of the ruptured supraspinatus and biceps tendon samples. None of the transected edges of the tissues produced during excision of the tissues showed the presence of lubricin. Lubricin was found in 3% to 10% of the tendon cells in the cadaveric controls and in 1% to 29% of the tendon cells in the torn supraspinatus and biceps tendon samples.

CLINICAL RELEVANCE

The lubricin layer on the torn edges of ruptured human supraspinatus and biceps tendons may interfere with the integrative bonding of the torn edges necessary for repair.