Tendon injuries are common and can dramatically impair patient mobility and productivity, resulting in a significant socioeconomic burden and reduced quality of life. Because the tendon healing process results in the formation of a fibrotic scar, injured tendons never regain the mechanical strength of the uninjured tendon, leading to frequent re-injury. Many tendons are also prone to the development of peritendinous adhesions and excess scar formation, which further reduce tendon function and lead to chronic complications. Despite this, there are currently no treatments that adequately improve the tendon healing process due in part to a lack of information regarding the contributions of various cell types to tendon healing and how their activity may be modulated for therapeutic value. In this review, we summarize recent efforts to identify and characterize the distinct cell populations involved at each stage of tendon healing. In addition, we examine the mechanisms through which different cell populations contribute to the fibrotic response to tendon injury, and how these responses can be affected by systemic factors and comorbidities. We then discuss gaps in our current understanding of tendon fibrosis and highlight how new technologies and research areas are shedding light on this clinically important and intractable challenge. A better understanding of the complex cellular environment during tendon healing is crucial to the development of new therapies to prevent fibrosis and promote tissue regeneration.

Identification of pro-regenerative approaches to improve tendon healing is critically important as the fibrotic healing response impairs physical function. In the present study we tested the hypothesis that S100a4 haploinsufficiency or inhibition of S100a4 signaling improves tendon function following acute injury and surgical repair in a murine model. We demonstrate that S100a4 drives fibrotic tendon healing primarily through a cell non-autonomous process, with S100a4 haploinsufficiency promoting regenerative tendon healing. Moreover, inhibition of S100a4 signaling via antagonism of its putative receptor, RAGE, also decreases scar formation. Mechanistically, S100a4 haploinsufficiency decreases myofibroblast and macrophage content at the site of injury, with both cell populations being key drivers of fibrotic progression. Moreover, S100a4-lineage cells become α-SMA+ myofibroblasts, via loss of S100a4 expression. Using a combination of genetic mouse models, small molecule inhibitors and in vitro studies we have defined S100a4 as a novel, promising therapeutic candidate to improve tendon function after acute injury.

Despite the requirement for Scleraxis-lineage (ScxLin) cells during tendon development, the function of ScxLin cells during adult tendon repair, post-natal growth, and adult homeostasis have not been defined. Therefore, we inducibly depleted ScxLin cells (ScxLinDTR) prior to tendon injury and repair surgery and hypothesized that ScxLinDTR mice would exhibit functionally deficient healing compared to wild-type littermates. Surprisingly, depletion of ScxLin cells resulted in increased biomechanical properties without impairments in gliding function at 28 days post-repair, indicative of regeneration. RNA sequencing of day 28 post-repair tendons highlighted differences in matrix-related genes, cell motility, cytoskeletal organization, and metabolism. We also utilized ScxLinDTR mice to define the effects on post-natal tendon growth and adult tendon homeostasis and discovered that adult ScxLin cell depletion resulted in altered tendon collagen fibril diameter, density, and dispersion. Collectively, these findings enhance our fundamental understanding of tendon cell localization, function, and fate during healing, growth, and homeostasis.

Although inflammation is necessary during the early phases of tissue repair, persistent inflammation contributes to fibrosis. Acute tendon injuries often heal through a fibrotic mechanism, which impedes regeneration and functional recovery. Because inflammation mediated by nuclear factor κB (NF-κB) signaling is implicated in this process, we examined the spatial, temporal, and cell type–specific activation profile of canonical NF-κB signaling during tendon healing. NF-κB signaling was maintained through all phases of tendon healing in mice, including the remodeling phase, and tenocytes and myofibroblasts from the Scleraxis (Scx) lineage were the predominant populations that retained NF-κB activation into the late stages of repair. We confirmed persistent NF-κB activation in myofibroblasts in human tendon scar tissue. Deleting the canonical NF-κB kinase, IKKβ, in Scx-lineage cells in mice increased apoptosis and the deposition of the matrix protein periostin during the late stages of tendon repair, suggesting that persistent NF-κB signaling may facilitate myofibroblast survival and fibrotic progression. Consistent with this, myofibroblasts in human tendon scar samples displayed enhanced prosurvival signaling compared to control tissue. Together, these data suggest that NF-κB may contribute to fibrotic tendon healing through both inflammation-dependent and inflammation-independent functions, such as NF-κB–mediated cell survival.

BackgroundTendinopathies are common and difficult to resolve due to the formation of scar tissue that reduces the mechanical integrity of the tissue, leading to frequent reinjury. Tenocytes respond to both excessive loading and unloading by producing pro-inflammatory mediators, suggesting that these cells are actively involved in the development of tendon degeneration. The transcription factor scleraxis (Scx) is required for the development of force-transmitting tendon during development and for mechanically stimulated tenogenesis of stem cells, but its function in adult tenocytes is less well-defined. The aim of this study was to further define the role of Scx in mediating the adult tenocyte mechanoresponse.ResultsEquine tenocytes exposed to siRNA targeting Scx or a control siRNA were maintained under cyclic mechanical strain before being submitted for RNA-seq analysis. Focal adhesions and extracellular matrix-receptor interaction were among the top gene networks downregulated in Scx knockdown tenocytes. Correspondingly, tenocytes exposed to Scx siRNA were significantly softer, with longer vinculin-containing focal adhesions, and an impaired ability to migrate on soft surfaces. Other pathways affected by Scx knockdown included increased oxidative phosphorylation and diseases caused by endoplasmic reticular stress, pointing to a larger role for Scx in maintaining tenocyte homeostasis.ConclusionsOur study identifies several novel roles for Scx in adult tenocytes, which suggest that Scx facilitates mechanosensing by regulating the expression of several mechanosensitive focal adhesion proteins. Furthermore, we identified a number of other pathways and targets affected by Scx knockdown that have the potential to elucidate the role that tenocytes may play in the development of degenerative tendinopathy.Electronic supplementary materialThe online version of this article (10.1186/s12860-018-0166-z) contains supplementary material, which is available to authorized users.