Summary While there is a significant amount of information available on the clinical presentation(s) and pathological changes associated with tendinopathy, the precise aetiopathogenesis of this condition remains a topic of debate. Classically, the aetiology of tendinopathy has been linked to the performance of repetitive activities (so‐called overuse injuries). This has led many investigators to suggest that it is the mechanobiologic over‐stimulation of tendon cells that is the initial stimulus for the degradative processes which have been shown to accompany tendinopathy. Although several studies have been able to demonstrate that the in vitro over‐stimulation of tendon cells in monolayer can result in a pattern(s) of gene expression seen in clinical cases of tendinopathy, the strain magnitudes and durations used in these in vitro studies, as well as the model systems, may not be clinically relevant. Using a rat tail tendon model, we have studied the in vitro mechanobiologic response of tendon cells in situ to various tensile loading regimes. These studies have led to the hypothesis that the aetiopathogenic stimulus for the degenerative cascade which precedes the overt pathologic development of tendinopathy is the catabolic response of tendon cells to mechanobiologic under‐stimulation as a result of microscopic damage to the collagen fibres of the tendon. In this review, we examine the rationale for this hypothesis and provide evidence in support of this theory.

To determine the effect of various degrees of ex vivo static tensile loading on the expression of collagenase (MMP-1) in tendon cells, rat tail tendons were statically loaded in tension at 0.16, 0.77, 1.38 or 2.6 MPa for 24 h. Northern blot analysis was used to assay for mRNA expression of MMP-1 in freshly harvested, 24 h load deprived, and 24 h statically loaded tendons. Western blot analysis was used to assay for pro-MMP-1 and MMP-1 protein expression in fresh and 24 h load deprived tendons. Freshly harvested rat tail tendons demonstrated no evidence of MMP-1 mRNA expression and no evidence of the pro-MMP-l or MMP-1 protein. Ex vivo load deprivation for 24 h resulted in a marked increase in the mRNA expression of MMP-1 which coincided with a marked increase of both pro-MMP-1 and MMP-1 protein expression. When tendons were subjected to ex vivo static tensile loading during the 24 h culture period, a significant inhibition of this upregulation of MMP-1 mRNA expression was found with increasing load (p < 0.05). A strong (r' = 0.78) and significant (p < 0.001) inverse correlation existed between the level of static tensile load and the expression of MMP-1. Disruption of the actin cytoskeleton with cytochalasin D abolished the inhibitory effect of ex vivo static tensile loading on MMP-I expression. The results of this study suggest that up-regulation of MMP-1 expression in tendon cells ex vivo can be inhibited by static tensile loading, presumably through a cytoskeletally based mechanotransduction pathway.

Cyclic strain has been shown to benefit tendon health. However, repetitive loading has also been implicated in the etiology of tendon overuse injuries. Recent studies demonstrated that in several cell lines cyclic strain was associated with an activation of stress-activated protein kinases (SAPKs). These SAPKs, in turn, were shown to be important upstream regulators of a variety of cell processes including apoptosis. To examine the effect of repetitive loading on SAPK activation in tendon cells in vitro, canine patellar tendon cells were cyclically strained, and the cellular stress response evaluated by measuring c-Jun N-terminal kinase (JNK) activation. The effects of strain frequency and strain magnitude as well as the role of calcium signaling in this mechanotransduction mechanism were also examined. Cyclic strain resulted in an immediate activation of JNK, which peaked at 30 min and returned to resting levels by 2 h. This activation was regulated by a magnitude-dependent but not frequency-dependent response and appeared to be mediated through a calcium-dependent mechanotransduction pathway. While transient JNK activation is associated with normal cell processes, persistent JNK activation has been linked to the initiation of the apoptotic cascade. A similar mechanism could be responsible for initiating the pathological events (localized cell death) seen in tendon overuse injury.

An in vitro collagen gel system was used to determine the effect of alterations in cytoskeletal tensional homeostasis on gene expression in tendon cells. Collagen gel matrices, seeded with rat tail tendon cells, underwent cytochalasin D and gel contraction treatments designed to alter the internal cytoskeletal homeostasis of the cells. Gels were examined for cytoskeletal organization using a rhodamine phalloidin stain for actin. The effect of altered cytoskeletal organization on mRNA expression of a catabolic (interstitial collagenase) and anabolic (ul (I) collagen) gene was examined using northern blot analysis. Tendon cells in adhered gels demonstrated a highly organized cytoskeleton and showed evidence of ul (I) collagen mRNA expression but no evidence of collagenase mRNA expression. Treatment of the attached gel with cytochalasin D disrupted the cytoskeletal organization and resulted in the upregulation of collagenase mRNA and the inhibition of ul(1) collagen mRNA expression. Release of the gels resulted in a cell mediated gel contraction, an immediate loss of cytoskeletal organization, and an mRNA expression pattern similar to that seen with cytochalasin D treatment. Isometric contraction of the gel on itself or around a 3-point traction device resulted in an mRNA expression pattern similar to the adhered gel. Gene expression in the contracted gels could be reversed through chemical cytoskeletal disruption or removal of the traction device which permitted further gel contraction. The results of the study suggest that tendon cells can establish an internal cytoskeletal tension through interactions with their local extracellular environment. Alterations in this tension appear to control the expression of both catabolic and anabolic genes in a reciprocal manner.

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Cell and cell nucleus deformations have been implicated in the mechanotransduction of mechanical loads acting on tissues. While in situ cell nucleus deformation in response to increasing tissue strains has been examined in articular cartilage this phenomenon has not been investigated in tendons. To examine in situ cell nuclei deformation in tendons undergoing tensile strain rat tail tendons were harvested from adult Sprague-Dawley rats and stained with acridine orange to highlight the cell nuclei. The tendons were mounted on a custom-designed, low-load, tensile testing device affixed to the mechanical stage of a confocal laser microscope. Cells within the tendons were isolated for analysis. Images of individual cells were captured at 0'%1 strain as well as sequentially at 2'%,, 4% and 6'%1 gripto-grip tendon strain. Digital images of the cell nuclei were then measured in the s (length) and y (height) axis and deformation expressed as a percentage of cell nuclei strain. In addition, centroid-to-centroid distances of adjacent cell nuclei within each image were measured and used to calculate local tissue strain. There was a weak (r2 = 0.34) but significant (p < 0.01) correlation between local tissue strain and cell nucleus strain in the x axis. The results of this study support the hypothesis that in situ cell nucleus deformation does occur during tensile loading of tendons. This deformation may play a significant role in the mechanical signal transduction pathway of this tissue.

Matrix metalloproteinase inhibitors may play a supportive role in the treatment of tendinopathy by limiting the MMP-mediated degradation of the extracellular matrix.

Tendons mainly function as load-bearing tissues in the muscloskeletal system, transmitting loads from muscle to bone. Tendons are dynamic structures that respond to the magnitude, direction, frequency, and duration of physiologic as well as pathologic mechanical loads via complex interactions between cellular pathways and the highly specialized extracellular matrix. This paper reviews the evolution and current knowledge of mechanobiology in tendon development, homeostasis, disease, and repair. In addition, we review several novel mechanotransduction pathways that have been identified recently in other tissues and cell types, providing potential research opportunities in the field of tendon mechanobiology. We also highlight current methods, models, and technologies being used in a wide variety of mechanobiology research that could be investigated in the context of their potential applicability for answering some of the fundamental unanswered questions in this field. The article concludes with a review of the major questions and future goals discussed during the recent ORS/ISMMS New Frontiers in Tendon Research Conference held September 10–11, 2014 in New York City.