Recruitment and maintenance of tendon progenitors by TGFbeta signaling are essential for tendon formation

Lysyl Oxidase Production by Murine C3H10T1/2 Mesenchymal Stem Cells Is Increased by TGFβs and Differentially Modulated by Mechanical Stimuli

Tendons are frequently injured and have limited regenerative capacity. This motivates tissue engineering efforts aimed at restoring tendon function through strategies to direct functional tendon formation. Generation of a crosslinked collagen matrix is paramount to forming mechanically functional tendon. However, it is unknown how lysyl oxidase (LOX), the primary mediator of enzymatic collagen crosslinking, is regulated by stem cells. This study investigates how multiple factors previously identified to promote tendon formation and healing (transforming growth factor [TGF]β1 and TGFβ2, mechanical stimuli, and hypoxia-inducible factor [HIF]-1α) regulate LOX production in the murine C3H10T1/2 mesenchymal stem cell (MSC) line. We hypothesized that TGFβ signaling promotes LOX activity in C3H10T1/2 MSCs, which is regulated by both mechanical stimuli and HIF-1α activation. TGFβ1 and TGFβ2 increased LOX levels as a function of concentration and time. Inhibiting the TGFβ type I receptor (TGFβRI) decreased TGFβ2-induced LOX production by C3H10T1/2 MSCs. Low (5 mPa) and high (150 mPa) magnitudes of fluid shear stress were applied to test impacts of mechanical stimuli, but without TGFβ2, loading alone did not alter LOX levels. Low loading (5 mPa) with TGFβ2 increased LOX at 7 days greater than TGFβ2 treatment alone. Neither HIF-1α knockdown (siRNA) nor activation (CoCl2) affected LOX levels. Ultimately, results suggest that TGFβ2 and appropriate loading magnitudes contribute to LOX production by C3H10T1/2 MSCs. Potential application of these findings includes treatment with TGFβ2 and appropriate mechanical stimuli to modulate LOX production by stem cells to ultimately control collagen matrix stiffening and support functional tendon formation.

Characterization of TGFβ1-induced tendon-like structure in the scaffold-free three-dimensional tendon cell culture system

The biological mechanisms regulating tenocyte differentiation and morphological maturation have not been well-established, partly due to the lack of reliable in vitro systems that produce highly aligned collagenous tissues. In this study, we developed a scaffold-free, three-dimensional (3D) tendon culture system using mouse tendon cells in a differentially adherent growth channel. Transforming Growth Factor-β (TGFβ) signaling is involved in various biological processes in the tendon, regulating tendon cell fate, recruitment and maintenance of tenocytes, and matrix organization. This known function of TGFβ signaling in tendon prompted us to utilize TGFβ1 to induce tendon-like structures in 3D tendon constructs. TGFβ1 treatment promoted a tendon-like structure in the peripheral layer of the constructs characterized by increased thickness with a gradual decrease in cell density and highly aligned collagen matrix. TGFβ1 also enhanced cell proliferation, matrix production, and morphological maturation of cells in the peripheral layer compared to vehicle treatment. TGFβ1 treatment also induced early tenogenic differentiation and resulted in sufficient mechanical integrity, allowing biomechanical testing. The current study suggests that this scaffold-free 3D tendon cell culture system could be an in vitro platform to investigate underlying biological mechanisms that regulate tenogenic cell differentiation and matrix organization.

Recent Advances in the Use of Stem Cells in Tissue Engineering and Adjunct Therapies for Tendon Reconstruction and Future Perspectives

Due to their function, tendons are exposed to acute injuries. This type of damage to the musculoskeletal system represents a challenge for clinicians when natural regeneration and treatment methods do not produce the expected results. Currently, treatment is long and associated with long-term complications. In this review, we discuss the use of stem cells in the treatment of tendons, including how to induce appropriate cell differentiation based on gene therapy, growth factors, tissue engineering, proteins involved in regenerative process, drugs and three-dimensional (3D) structures. A multidirectional approach as well as the incorporation of novel components of the therapy will improve the techniques used and benefit patients with tendon injuries in the future.

FGF-stimulated tendon cells embrace a chondrogenic fate with BMP7 in newt tissue culture

Newts can regenerate functional elbow joints after amputation at the joint level. Previous studies have suggested the potential contribution of cells from residual tendon tissues to joint cartilage regeneration. A serum-free tissue culture system for tendons was established to explore cell dynamics during joint regeneration. Culturing isolated tendons in this system, stimulated by regeneration-related factors, such as fibroblast growth factor (FGF) and platelet-derived growth factor, led to robust cell migration and proliferation. Moreover, cells proliferating in an FGF-rich environment differentiated into Sox9-positive chondrocytes upon BMP7 introduction. These findings suggest that FGF-stimulated cells from tendons may aid in joint cartilage regeneration during functional elbow joint regeneration in newts.

Assessing the functional potential of conditioned media derived from amniotic epithelial stem cells engineered on 3D biomimetic scaffolds: An in vitro model for tendon regeneration

Tendon diseases pose a significant challenge in regenerative medicine due to the limited healing capacity of this tissue. Successful tendon regeneration requires a combination of angiogenesis, immune response, and tenogenesis processes. An effective tendon engineering (TE) strategy must finely tune this systems' interplay toward homeostasis. This study explores in vitro the paracrine influence of amniotic epithelial stem cells (AECs) engineered on a validated 3D electrospun PLGA scaffolds on HUVECs (angiogenesis), PBMCs/Jurkat (immune response), and AECs (tenogenic stem cell activation). The results revealed the role of scaffold's topology and topography in significantly modulating the paracrine profile of the cells. In detail, AECs basal release of bioactive molecules was boosted in the cells engineered on 3D scaffolds, in particular VEGF-D, b-FGF, RANTES, and PDGF-BB (p < 0.0001 vs. CMCTR). Moreover, biological tests demonstrated 3D scaffolds' proactive role in potentiating AECs' paracrine inhibition on PBMCs proliferation (CM3Dvs. CTR, p < 0.001) and LPS-mediated Jurkat activation with respect to controls (CM3D and CM2Dvs. CTR, p < 0.01 and p < 0.05, respectively), without exerting any in vitro pro-angiogenic role in promoting HUVECs proliferation and tubule formation. Teno-inductive paracrine ability of AECs engineered on 3D scaffolds was assessed on co-cultured ones, which formed tendon-like structures. These latter demonstrated an upregulation of tendon-related genes (SCX, THBS4, COL1, and TNMD) and the expression TNMD and COL1 proteins. Overall, this research underscores the pivotal role of the 3D topology and topography of PLGA tendon mimetic scaffolds in orchestrating effective tendon regeneration through modulating cell behavior and crosstalk between engineered stem cells and different subpopulations in the damaged tendon.

Macromolecular crowding in human tenocyte and skin fibroblast cultures: A comparative analysis

Although human tenocytes and dermal fibroblasts have shown promise in tendon engineering, no tissue engineered medicine has been developed due to the prolonged ex vivo time required to develop an implantable device. Considering that macromolecular crowding has the potential to substantially accelerate the development of functional tissue facsimiles, herein we compared human tenocyte and dermal fibroblast behaviour under standard and macromolecular crowding conditions to inform future studies in tendon engineering. Basic cell function analysis made apparent the innocuousness of macromolecular crowding for both cell types. Gene expression analysis of the without macromolecular crowding groups revealed expression of tendon related molecules in human dermal fibroblasts and tenocytes. Protein electrophoresis and immunocytochemistry analyses showed significantly increased and similar deposition of collagen fibres by macromolecular crowding in the two cell types. Proteomics analysis demonstrated great similarities between human tenocyte and dermal fibroblast cultures, as well as the induction of haemostatic, anti-microbial and tissue-protective proteins by macromolecular crowding in both cell populations. Collectively, these data rationalise the use of either human dermal fibroblasts or tenocytes in combination with macromolecular crowding in tendon engineering.

A role for TGFβ signaling in Gli1+ tendon and enthesis cells

The development of musculoskeletal tissues such as tendon, enthesis, and bone relies on proliferation and differentiation of mesenchymal progenitor cells. Gli1+ cells have been described as putative stem cells in several tissues and are presumed to play critical roles in tissue formation and maintenance. For example, the enthesis, a fibrocartilage tissue that connects tendon to bone, is mineralized postnatally by a pool of Gli1+ progenitor cells. These cells are regulated by hedgehog signaling, but it is unclear if TGFβ signaling, necessary for tenogenesis, also plays a role in their behavior. To examine the role of TGFβ signaling in Gli1+ cell function, the receptor for TGFβ, TbR2, was deleted in Gli1-lineage cells in mice at P5. Decreased TGFβ signaling in these cells led to defects in tendon enthesis formation by P56, including defective bone morphometry underlying the enthesis and decreased mechanical properties. Immunohistochemical staining of these Gli1+ cells showed that loss of TGFβ signaling reduced proliferation and increased apoptosis. In vitro experiments using Gli1+ cells isolated from mouse tail tendons demonstrated that TGFβ controls cell proliferation and differentiation through canonical and non-canonical pathways and that TGFβ directly controls the tendon transcription factor scleraxis by binding to its distant enhancer. These results have implications in the development of treatments for tendon and enthesis pathologies.

Building a vertebra: Development of the amniote sclerotome

In embryonic development, the vertebral column arises from the sclerotomal compartment of the somites. The sclerotome is a mesenchymal cell mass which can be subdivided into several subpopulations specified by different regulatory mechanisms and giving rise to different parts of the vertebrae like vertebral body, vertebral arch, ribs, and vertebral joints. This review gives a short overview on the molecular and cellular basis of the formation of sclerotomal subdomains and the morphogenesis of their vertebral derivatives.

A systematic investigation of the effects of TGF-β3 and mechanical stimulation on tenogenic differentiation of mesenchymal stromal cells in a poly(ethylene glycol)/gelatin-based hydrogel

Background

High post-surgical failure rates following tendon injury generate high medical costs and poor patient recovery. Cell-based tendon tissue engineering has the potential to produce fully functional replacement tissue and provide new strategies to restore tendon function and healing. In this endeavour, the application of mesenchymal stromal cells (MSCs) encapsulated in biomaterial scaffolds has shown great promise. However, a consensus on optimal promotion of tenogenic differentiation of MSCs has yet to be reached, although growth factors and mechanical cues are generally acknowledged as important factors.

Methods

In this study, we prepared a hydrogel cell culture system consisting of methacrylated poly(d,l-lactic acid-ethylene glycol-d,l-lactic acid) (P(LA-EG-LA)) and gelatin methacrylate (GelMA) to encapsulate human bone marrow-derived MSCs (hBMSCs). We further systematically investigated the influence of static and intermittent cyclic uniaxial strain mechanical stimulation, in combination with transforming growth factor-β3 (TGF-β3) supplementation, on tenogenic differentiation of hBMSCs.

Results

Increased TGF-β3 concentration upregulated the tenogenic genes Scleraxis (SCX) and collagen type I (COL1A1) but showed no effects on tenascin-c (TNC) and collagen type III (COL3A1) expression. Mechanical stimulation had no observable effect on gene expression, but intermittent cyclic uniaxial strain stimulation improved matrix deposition. Together, these data provide new insights into how TGF-β3 and mechanical stimulation regulate MSC tenogenesis, with TGF-β3 promoting the expression of key tenogenic genes whilst mechanical stimulation aided matrix deposition in the engineered tissue. Furthermore, intermittent cyclic uniaxial strain at 3% elongation and 0.33 ​Hz for 1 ​h/day showed improved matrix effects compared to static strain.

Conclusion

Together, the most promising result for tenogenic differentiation of hBMSCs was identified as treatment with 5 ​ng/ml TGF-β3 under intermittent cyclic uniaxial strain (3% strain; 0.33 ​Hz; 1 ​h/day). This knowledge is of importance for the development of an improved protocol for tenogenic differentiation of MSCs and thereby for tendon tissue engineering.

The translational potential of this article

Tissue-engineered strategies for tendon repair require a consensus on the differentiation of mesenchymal stromal cells to tenocytes, which is currently lacking. This article provides a systematic investigation of two main tenogenic differentiation conditions to further development of a tenogenic differentiation protocol.

Current progress in growth factors and extracellular vesicles in tendon healing

Tendon injury healing is a complex process that involves the participation of a significant number of molecules and cells, including growth factors molecules in a key role. Numerous studies have demonstrated the function of growth factors in tendon healing, and the recent emergence of EV has also provided a new visual field for promoting tendon healing. This review examines the tendon structure, growth, and development, as well as the physiological process of its healing after injury. The review assesses the role of six substances in tendon healing: insulin-like growth factor-I (IGF-I), transforming growth factor β (TGFβ), vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), and EV. Different growth factors are active at various stages of healing and exhibit separate physiological activities. IGF-1 is expressed immediately after injury and stimulates the mitosis of various cells while suppressing the response to inflammation. VEGF, which is also active immediately after injury, accelerates local metabolism by promoting vascular network formation and positively impacts the activities of other growth factors. However, VEGF's protracted action could be harmful to tendon healing. PDGF, the earliest discovered cytokine to influence tendon healing, has a powerful cell chemotaxis and promotes cell proliferation, but it can equally accelerate the response to inflammation and relieve local adhesions. Also useful for relieving tendon adhesion is TGF- β, which is active almost during the entire phase of tendon healing. As a powerful active substance, in addition to its participation in the field of cardiovascular and cerebrovascular vessels, tumour and chronic wounds, TGF- β reportedly plays a role in promoting cell proliferation, activating growth factors, and inhibiting inflammatory response during tendon healing.

From fetal tendon regeneration to adult therapeutic modalities: TGF-β3 in scarless healing

Tendon injuries are common disorders that can significantly impact people's lives. Unfortunately, the limited regenerative ability of tendons results in tissue healing in a scar-mediated manner. The current therapeutic strategies fail to fully recover the functions of the injured tendons, and as such, the conception of 'scarless healing' has gained prominent attention in the field of regenerative medicine. Interestingly, injured fetal tendons possess the capability to heal through regeneration, which builds an ideal blueprint for adult tendon regeneration. Studies have shown that fetal biochemical cues have the potential to improve adult tendon healing. Here we review the biological factors that contribute to fetal tendon regeneration and how manipulation of these biochemical cues in the adult tendon healing process could achieve regeneration.

PAI-1 mediates TGF-β1-induced myofibroblast activation in tenocytes via mTOR signaling

Transforming growth factor-beta (TGF-β1) induces plasminogen activator inhibitor 1 (PAI-1) to effect fibrotic pathologies in several organs including tendon. Recent data implicated PAI-1 with inhibition of phosphatase and tensin homolog (PTEN) suggesting that PAI-1-induced adhesions involves phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin (mTOR) signaling. Ergo, we investigated effects of TGF-β1, PAI-1, and mTOR signaling crosstalk on myofibroblast activation, senescence, and proliferation in primary flexor tenocytes from wild-type (WT) and PAI-1 knockout (KO) mice. PAI-1 deletion blunted TGF-β1-induced myofibroblast activation in murine flexor tenocytes and increased the gene expression of Mmp-2 to confer protective effects against fibrosis. While TGF-β1 significantly reduced phosphorylation of PTEN in WT cells, PAI-1 deletion rescued the activation of PTEN. Despite that, there were no differences in TGF-β1-induced activation of mTOR signaling (AKT, 4EBP1, and P70S6K) in WT or KO tenocytes. Phenotypic changes in distinct populations of WT or KO tenocytes exhibiting high or low mTOR activity were then examined. TGF-β1 increased alpha-smooth muscle actin abundance in WT cells exhibiting high mTOR activity, but this increase was blunted in KO cells exhibiting high 4EBP1 activity but not in cells exhibiting high S6 activity. DNA damage (γH2AX) was increased with TGF-β1 treatment in WT tenocytes but was blunted in KO cells exhibiting high mTOR activity. Increased mTOR activity enhanced proliferation (Ki67) in both WT and KO tenocytes. These findings point to a complex nexus of TGF-β1, PAI-1, and mTOR signaling in regulating proliferation, myofibroblast differentiation, and senescence in tenocytes, which could define therapeutic targets for chronic tendon adhesions and other fibrotic pathologies.

Endogenous tenocyte activation underlies the regenerative capacity of the adult zebrafish tendon

Tendons are essential, frequently injured connective tissues that transmit forces from muscle to bone. Their unique highly ordered, matrix-rich structure is critical for proper function. While adult mammalian tendons heal after acute injuries, endogenous tendon cells, or tenocytes, fail to respond appropriately, resulting in the formation of disorganized fibrovascular scar tissue with impaired function and increased propensity for re-injury. Here, we show that, unlike mammals, adult zebrafish tenocytes activate upon injury and fully regenerate the tendon. Using a full tear injury model in the adult zebrafish craniofacial tendon, we defined the hallmark stages and cellular basis of tendon regeneration through multiphoton imaging, lineage tracing, and transmission electron microscopy approaches. Remarkably, we observe that zebrafish tendons regenerate and restore normal collagen matrix ultrastructure by 6 months post-injury (mpi). Tendon regeneration progresses in three main phases: inflammation within 24 h post-injury (hpi), cellular proliferation and formation of a cellular bridge between the severed tendon ends at 3-5 days post-injury (dpi), and re-differentiation and matrix remodeling beginning from 5 dpi to 6 mpi. Importantly, we demonstrate that pre-existing tenocytes are the main cellular source of regeneration, proliferating and migrating upon injury to ultimately bridge the tendon ends. Finally, we show that TGF-β signaling is required for tenocyte recruitment and bridge formation. Collectively, our work debuts and aptly positions the adult zebrafish tendon as an invaluable comparative system to elucidate regenerative mechanisms that may inspire new therapeutic strategies.

The rat Achilles and patellar tendons have similar increases in mechanical properties but become transcriptionally divergent during postnatal development

The molecular events that drive post-natal tendon development are poorly characterized. In this study, we profiled morphological, mechanical, and transcriptional changes in the rat Achilles and patellar tendon before walking (P7), shortly after onset of walking (P14), and at motor maturity (P28). The Achilles and patellar tendons increased collagen content and mechanical strength similarly throughout post-natal development. However, at P28 the patellar tendon tended to display a higher maximal tensile load (MTL) (P = 0.0524) than the Achilles tendon, but a similar ultimate tensile strength (UTS; load relative to cross-sectional area) probably due to its increased cross-sectional area during development. The tendons started transcriptionally similar, with overlapping PCA clusters at P7 and P14, before becoming transcriptionally distinct at P28. In both tendons, there was an increase in extracellular matrix (ECM) gene expression and a concomitant decrease in cell cycle and mitochondrial gene expression. The transcriptional divergence at P28 suggested that STAT signalling was lower in the patellar tendon where MTL increased the most. Treating engineered human ligaments with the STAT inhibitor itacitinib increased collagen content and MTL. Our results suggest that during post-natal development, cellular resources are initially allocated towards cell proliferation before shifting towards extracellular matrix development following the onset of mechanical load and provide potential targets for improving tendon function. KEY POINTS: Little is known about mechanisms of post-natal tendon growth. We characterized morphological, mechanical, and transcriptional changes that occur before (P7), and early (P14) and late after (P28) rats begin to walk. From P7 to P28, the Achilles tendon increased in length, whereas the patellar tendon increased in cross-sectional area. Mechanical and material properties of the Achilles and patellar tendon increased from P7 to P28. From P7 to P28, the Achilles and patellar tendons increased expression of ECM genes and decreased mitochondrial and cell cycle gene expression. Ribosomal gene expression also significantly decreased in the Achilles and tended to decrease in the patellar tendon. At P28, STAT1 signalling tended to be lower in the patellar tendon which had grown by increasing cross-sectional area and inhibiting STAT activation in vitro improved mechanical properties in engineered human ligaments.

Mechanical force regulates Sox9 expression at the developing enthesis

Entheses transmit force from tendons and ligaments to the skeleton. Regional organization of enthesis extracellular matrix (ECM) generates differences in stiffness required for force transmission. Two key transcription factors co-expressed in entheseal tenocytes, scleraxis (Scx) and Sox9, directly control production of enthesis ECM components. Formation of embryonic craniofacial entheses in zebrafish coincides with onset of jaw movements, possibly in response to the force of muscle contraction. We show dynamic changes in scxa and sox9a mRNA levels in subsets of entheseal tenocytes that correlate with their roles in force transmission. We also show that transcription of a direct target of Scxa, Col1a, in enthesis ECM is regulated by the ratio of scxa to sox9a expression. Eliminating muscle contraction by paralyzing embryos during early stages of musculoskeletal differentiation alters relative levels of scxa and sox9a in entheses, primarily owing to increased sox9a expression. Force-dependent TGF-β (TGFβ) signaling is required to maintain this balance of scxa and sox9a expression. Thus, force from muscle contraction helps establish a balance of transcription factor expression that controls specialized ECM organization at the tendon enthesis and its ability to transmit force.

Combined Growth Factor Hydrogel Enhances Rotator Cuff Enthesis Healing in Rat But Not Sheep Model

We hypothesized that a combined growth factor hydrogel would improve chronic rotator cuff tear healing in a rat and sheep model. Insulin-like growth factor 1, transforming growth factor β1, and parathyroid hormone were combined into a tyraminated poly-vinyl-alcohol (PVA-Tyr) hydrogel and applied directly at the enthesis. In total, 30 Sprague-Dawley rats and 16 Romney ewes underwent unilateral rotator cuff tenotomy and then delayed repairs were performed after 3-4 weeks. The animals were divided into a control group (repair alone) and treatment group. The rotator cuffs were harvested at 12 weeks after surgery for biomechanical and histological analyses of the repair site. In the rat model, the stress at failure and Young's modulus were higher in the treatment group in comparison with the control group (73% improvement, p = 0.010 and 56% improvement, p = 0.028, respectively). Histologically, the repaired entheses in the treatment group demonstrated improved healing with higher semi-quantitative scores (10.1 vs. 6.55 of 15, p = 0.032). In the large animal model, there was no observable treatment effect. This PVA-Tyr bound growth factor system holds promise for improving rotator cuff healing. However, our approach was not scalable from a small to a large animal model. Further tailoring of this growth factor delivery system is still required. Level of Evidence: Basic Science Study; Biomechanics and Histology; Animal Model Impact Statement Previous studies using single-growth factor treatment to improve enthesis healing after rotator cuff repair have reported promising, but inconsistent results. A novel approach is to combine multiple growth factors using controlled-release hydrogels that mimic the normal healing process. In this study, we report that a combined growth factor hydrogel can improve the histological quality and strength of rotator cuff repair in a rat chronic tear model. This novel hydrogel growth factor treatment has the potential to be used in human clinical applications to improve healing after rotator cuff repair.

Lineage-specific differences and regulatory networks governing human chondrocyte development

To address large gaps in our understanding of the molecular regulation of articular and growth plate cartilage development in humans, we used our directed differentiation approach to generate these distinct cartilage tissues from human embryonic stem cells. The resulting transcriptomic profiles of hESC-derived articular and growth plate chondrocytes were similar to fetal epiphyseal and growth plate chondrocytes, with respect to genes both known and previously unknown to cartilage biology. With the goal to characterize the regulatory landscapes accompanying these respective transcriptomes, we mapped chromatin accessibility in hESC-derived chondrocyte lineages, and mouse embryonic chondrocytes, using ATAC-sequencing. Integration of the expression dataset with the differentially accessible genomic regions revealed lineage-specific gene regulatory networks. We validated functional interactions of two transcription factors (TFs) (RUNX2 in growth plate chondrocytes and RELA in articular chondrocytes) with their predicted genomic targets. The maps we provide thus represent a framework for probing regulatory interactions governing chondrocyte differentiation. This work constitutes a substantial step towards comprehensive and comparative molecular characterizations of distinct chondrogenic lineages and sheds new light on human cartilage development and biology.

Stem Cell Applications and Tenogenic Differentiation Strategies for Tendon Repair

Tendons are associated with a high injury risk because of their overuse and age-related tissue degeneration. Thus, tendon injuries pose great clinical and economic challenges to the society. Unfortunately, the natural healing capacity of tendons is far from perfect, and they respond poorly to conventional treatments when injured. Consequently, tendons require a long period of healing and recovery, and the initial strength and function of a repaired tendon cannot be completely restored as it is prone to a high rate of rerupture. Nowadays, the application of various stem cell sources, including mesenchymal stem cells (MSCs) and embryonic stem cells (ESCs), for tendon repair has shown great potential, because these cells can differentiate into a tendon lineage and promote functional tendon repair. However, the mechanism underlying tenogenic differentiation remains unclear. Moreover, no widely adopted protocol has been established for effective and reproducible tenogenic differentiation because of the lack of definitive biomarkers for identifying the tendon differentiation cascades. This work is aimed at reviewing the literature over the past decade and providing an overview of background information on the clinical relevance of tendons and the urgent need to improve tendon repair; the advantages and disadvantages of different stem cell types used for boosting tendon repair; and the unique advantages of reported strategies for tenogenic differentiation, including growth factors, gene modification, biomaterials, and mechanical stimulation.

Mechanisms of skeletal muscle-tendon development and regeneration/healing as potential therapeutic targets

Skeletal muscle contraction is essential for the movement of our musculoskeletal system. Tendons and ligaments that connect the skeletal muscles to bones in the correct position at the appropriate time during development are also required for movement to occur. Since the musculoskeletal system is essential for maintaining basic bodily functions as well as enabling interactions with the environment, dysfunctions of these tissues due to disease can significantly reduce quality of life. Unfortunately, as people live longer, skeletal muscle and tendon/ligament diseases are becoming more common. Sarcopenia, a disease in which skeletal muscle function declines, and tendinopathy, which involves chronic tendon dysfunction, are particularly troublesome because there have been no significant advances in their treatment. In this review, we will summarize previous reports on the development and regeneration/healing of skeletal muscle and tendon tissues, including a discussion of the molecular and cellular mechanisms involved that may be used as potential therapeutic targets.

Synergistic effects of growth factor-based serum-free medium and tendon-like substrate topography on tenogenesis of mesenchymal stem cells

Addressing clinical challenges for tendon injuries requires a deeper understanding of the effects that biological and biophysical cues have on tenogenesis. Although prior studies have identified tenogenic growth factors (GFs) or elucidated the effects of substrate topography on tenocyte behavior, few have characterized their combined effect in the presence of a tendon-like substrate. In this study, we assessed the effect of biological (GFs) and biophysical (substrate topography) cues on tenogenic proliferation and differentiation under defined, serum-free conditions. Specifically, human bone marrow-derived mesenchymal stem cells (hMSCs) were cultured in a serum-free culture medium containing a GF cocktail comprised of fibroblast growth factor-2 (FGF-2), transforming growth factor-beta 3 (TGF-β3), and insulin-like growth factor-1 (IGF-1), either alone or in combination with tendon-like substrate topography produced by replica casting of tendon tissue sections. Our data demonstrated that the use of serum-free GF cocktail medium alone promoted hMSC proliferation, as shown via DNA staining as well as Ki67 protein levels and gene expression. In particular, gene expression of Ki67 was increased by 8.46-fold in all three donors relative to serum-free medium control. Also, serum-free GF cocktail promoted tenogenic differentiation, on the basis of expression of tendon-associated gene and protein markers, scleraxis (SCX), tenascin C (TNC), and collagen type I (COL1A1) including increased normalized collagen production by 1.4-fold in two donors relative to serum-free medium control. Interestingly, hMSCs cultured on a tendon-like substrate exhibited highly oriented cell morphology and extracellular matrix (ECM) alignment reminiscent of tendon. In particular, when this GF cocktail was combined with tendon-like topography, they showed a synergistically increased expression of tendon-related markers and anisotropic organization of ECM proteins with moderate-to-large effect sizes. Together, in addition to showing the utility of a GF cocktail for expansion and differentiation of tenocyte-like cells, our findings clearly demonstrate the synergistic relationship between GF-mediated and substrate topography-related effects on hMSC tenogenic differentiation. This information provides insights into the design of strategies that combine biological and biophysical cues for ex vivo tenocyte production and tendon tissue engineering.

Human Muscle-Derived Cells Are Capable of Tenogenic Differentiation and Contribution to Tendon Repair

BACKGROUND

It has been reported that the harvested hamstring tendon for autograft could be regenerated with well-oriented fibers and uniformly distributed spindle-shaped cells after removal. However, which cell type might participate in the repair process remains unknown.

PURPOSE

To investigate the tenogenic differentiation potential of human muscle-derived cells (MDCs) both in vitro and in vivo.

STUDY DESIGN

Controlled laboratory study.

METHODS

Primary human MDCs and tenocytes were isolated from discarded materials during a peroneus longus tendon-harvesting procedure. Expression of tenogenic genes were evaluated and compared among MDCs, MDCs with tenogenic induction, and tenocytes. RNA sequencing was performed to evaluate the expression profile of differentiated MDCs. Human MDCs were implanted in a tendon injury model to investigate the in vivo tenogenic differentiation potential. Histologic and functional analyses were performed to evaluate the function of MDCs for tendon repair.

RESULTS

The relative expression levels (in fold change) of tenogenic genes Col I, MKX, SCX, THBS4, and TNC in MDCs were significantly upregulated 11.5 ± 1.3, 957.1 ± 63.7, 19.1 ± 2.8, 61.9 ± 4.8, and 10.2 ± 2.8 after tenogenic induction, respectively. The expression profile of tenogenically differentiated MDCs was much closer to primary tenocytes. Activation of TGF-β/Smad3 signaling significantly promoted the tenogenic differentiation ability of MDCs. Transplanted human MDCs were identified in regenerated tendon and expressed tenogenic genes. As for biomechanical properties, the failure loads in the Matrigel, transplantation, and uninjured groups were 7.2 ± 0.5, 11.6 ± 0.3, and 13.9 ± 0.7 N, while the stiffness values were 4.4 ± 1.3 × 10³, 7.6 ± 0.8 × 10³, and 10.9 ± 1.1 × 10³ N/m. Plantarflexion force, histologic morphology, and motor function were also significantly improved after MDC transplantation in a tendon injury model.

CONCLUSION

There exist cells with tenogenic differentiation potential in human skeletal muscles. Activation of TGF-β/Smad3 signaling plays an important role in tenogenic differentiation for human MDCs. Human MDCs contribute to structural and functional repair for the injured tendon. MDCs are a potential cell source to participate in the repair process after tendon injury.

CLINICAL RELEVANCE

The MDCs could be a promising cell source to repair tendon injury.

Eccentric contraction-dominant exercise leads to molecular biological changes in enthesis and enthesopathy-like morphological changes

Entheses, which are tendon-to-bone attachment sites in the musculoskeletal system, play important roles in optimizing the mechanical stress and force transmitted from the muscle to the bone. Sports-related enthesopathy shows pathological features, including hyperplasia of the fibrocartilage (FC) region in the enthesis. The amount of exercise and type of muscle contraction during movement is involved in the pathogenesis of sports-related enthesopathy; however, the details of this condition are unclear. Here we examined the molecular pathways involved in the morphological changes of the muscle-tendon-enthesis complex and enthesis FC region in the supraspinatus muscle enthesis of mice under different exercise conditions. Following intervention, morphological changes in the muscle-tendon-enthesis complex were initiated in the eccentric contraction-dominant exercise group at 2 weeks, with activation of the transforming growth factor-β (TGFβ) superfamily pathway predicted by proteome and ingenuity pathway analyses. Histological and molecular biological analyses confirmed the activation of the TGFβ/bone morphogenetic protein (BMP)-Smad pathway. The concentric contraction-dominant exercise group showed no change in the morphology of the muscle-tendon-enthesis complex or activation of the TGFβ/BMP-Smad pathway, despite overuse exercise. Statement of Clinical Significance: These results suggest that eccentric contraction-dominant exercise induces sports-related enthesopathy-like morphological changes in the early stages as well as molecular biological changes, mainly in the transforming growth factor-β superfamily pathway in enthesis. Statement of Clinical Significance: These results suggest that eccentric contraction-dominant exercise induces sports-related enthesopathy-like morphological changes in the early stages as well as molecular biological changes, mainly in the transforming growth factor-β superfamily pathway in enthesis.

Scleraxis-lineage cells are required for tendon homeostasis and their depletion induces an accelerated extracellular matrix aging phenotype

Aged tendons have disrupted homeostasis, increased injury risk, and impaired healing capacity. Understanding mechanisms of homeostatic disruption is crucial for developing therapeutics to retain tendon health through the lifespan. Here, we developed a novel model of accelerated tendon extracellular matrix (ECM) aging via depletion of Scleraxis-lineage cells in young mice (Scx-DTR). Scx-DTR recapitulates many aspects of tendon aging including comparable declines in cellularity, alterations in ECM structure, organization, and composition. Single-cell RNA sequencing demonstrated a conserved decline in tenocytes associated with ECM biosynthesis in aged and Scx-DTR tendons, identifying the requirement for Scleraxis-lineage cells during homeostasis. However, the remaining cells in aged and Scx-DTR tendons demonstrate functional divergence. Aged tenocytes become pro-inflammatory and lose proteostasis. In contrast, tenocytes from Scx-DTR tendons demonstrate enhanced remodeling capacity. Collectively, this study defines Scx-DTR as a novel model of accelerated tendon ECM aging and identifies novel biological intervention points to maintain tendon function through the lifespan.

Biological and Mechanical Factors and Epigenetic Regulation Involved in Tendon Healing

Tendons are an important part of the musculoskeletal system. Connecting muscles to bones, tendons convert force into movement. Tendon injury can be acute or chronic. Noticeably, tendon healing requires a long time span and includes inflammation, proliferation, and remodeling processes. The mismatch between endogenous and exogenous healing may lead to adhesion causing further negative effects. Management of tendon injuries and complications such as subsequent adhesion formation are still challenges for clinicians. Due to numerous factors, tendon healing is a complex process. This review introduces the role of various biological and mechanical factors and epigenetic regulation processes involved in tendon healing.

RAB23 regulates musculoskeletal development and patterning

RAB23 is a small GTPase which functions at the plasma membrane to regulate growth factor signaling. Mutations in RAB23 cause Carpenter syndrome, a condition that affects normal organogenesis and patterning. In this study, we investigate the role of RAB23 in musculoskeletal development and show that it is required for patella bone formation and for the maintenance of tendon progenitors. The patella is the largest sesamoid bone in mammals and plays a critical role during movement by providing structural and mechanical support to the knee. Rab23 ^-/- mice fail to form a patella and normal knee joint. The patella is formed from Sox9 and scleraxis (Scx) double-positive chondroprogenitor cells. We show that RAB23 is required for the specification of SOX9 and scleraxis double-positive patella chondroprogenitors during the formation of patella anlagen and the subsequent establishment of patellofemoral joint. We find that scleraxis and SOX9 expression are disrupted in Rab23 ^-/- mice, and as a result, development of the quadriceps tendons, cruciate ligaments, patella tendons, and entheses is either abnormal or lost. TGFβ-BMP signaling is known to regulate patella initiation and patella progenitor differentiation and growth. We find that the expression of TGFβR2, BMPR1, BMP4, and pSmad are barely detectable in the future patella site and in the rudimentary tendons and ligaments around the patellofemoral joint in Rab23 ^-/- mice. Also, we show that GLI1, SOX9, and scleraxis, which regulate entheses establishment and maturation, are weakly expressed in Rab23 ^-/- mice. Further analysis of the skeletal phenotype of Rab23 ^-/- mice showed a close resemblance to that of Tgfβ2 ^-/- mice, highlighting a possible role for RAB23 in regulating TGFβ superfamily signaling.

Mammal comparative tendon biology: advances in regulatory mechanisms through a computational modeling

There is high clinical demand for the resolution of tendinopathies, which affect mainly adult individuals and animals. Tendon damage resolution during the adult lifetime is not as effective as in earlier stages where complete restoration of tendon structure and property occurs. However, the molecular mechanisms underlying tendon regeneration remain unknown, limiting the development of targeted therapies. The research aim was to draw a comparative map of molecules that control tenogenesis and to exploit systems biology to model their signaling cascades and physiological paths. Using current literature data on molecular interactions in early tendon development, species-specific data collections were created. Then, computational analysis was used to construct Tendon NETworks in which information flow and molecular links were traced, prioritized, and enriched. Species-specific Tendon NETworks generated a data-driven computational framework based on three operative levels and a stage-dependent set of molecules and interactions (embryo-fetal or prepubertal) responsible, respectively, for signaling differentiation and morphogenesis, shaping tendon transcriptional program and downstream modeling of its fibrillogenesis toward a mature tissue. The computational network enrichment unveiled a more complex hierarchical organization of molecule interactions assigning a central role to neuro and endocrine axes which are novel and only partially explored systems for tenogenesis. Overall, this study emphasizes the value of system biology in linking the currently available disjointed molecular data, by establishing the direction and priority of signaling flows. Simultaneously, computational enrichment was critical in revealing new nodes and pathways to watch out for in promoting biomedical advances in tendon healing and developing targeted therapeutic strategies to improve current clinical interventions.

Establishment of a human pluripotent stem cell-derived MKX-td Tomato reporter system

Tendon regeneration is difficult because detailed knowledge about tendon progenitor cells (TPCs), which produce tenocytes to repair tendon tissue, has not been revealed. Mohawk homeobox (MKX) is a marker of TPCs or tenocytes, but a human pluripotent stem cell (hPSC)-based reporter system that visualizes MKX+ cells has not been developed. Here, we established an hPSC-derived MKX-tdTomato reporter cell line and tested the induction ratio of MKX-tdTomato+ cells using our stepwise/xeno-free differentiation protocol. MKX-tdTomato+ cells were generated with high efficiency and expressed tendon-specific markers, including MKX, SCX, TNMD, and COL1A1. Our MKX-tdTomato hPSC line would be a useful tool for studying the development or regeneration of tendon tissue.

The transcription factors Foxf1 and Foxf2 integrate the SHH, HGF and TGFβ signaling pathways to drive tongue organogenesis

The tongue is a highly specialized muscular organ with diverse cellular origins, which provides an excellent model for understanding mechanisms controlling tissue-tissue interactions during organogenesis. Previous studies showed that SHH signaling is required for tongue morphogenesis and tongue muscle organization, but little is known about the underlying mechanisms. Here we demonstrate that the Foxf1/Foxf2 transcription factors act in the cranial neural crest cell (CNCC)-derived mandibular mesenchyme to control myoblast migration into the tongue primordium during tongue initiation, and thereafter continue to regulate intrinsic tongue muscle assembly and lingual tendon formation. We performed chromatin immunoprecipitation sequencing analysis and identified Hgf, Tgfb2 and Tgfb3 among the target genes of Foxf2 in the embryonic tongue. Through genetic analyses of mice with CNCC-specific inactivation of Smo or both Foxf1 and Foxf2, we show that Foxf1 and Foxf2 mediate hedgehog signaling-mediated regulation of myoblast migration during tongue initiation and intrinsic tongue muscle formation by regulating the activation of the HGF and TGFβ signaling pathways. These data uncover the molecular network integrating the SHH, HGF and TGFβ signaling pathways in regulating tongue organogenesis.

The Influence of Different Modes of Exercise on Healthy and Injured Tendons

Tendons are essential components of the musculoskeletal system that links the skeletal muscle to the skeleton. This dense connective tissue exhibits great plasticity. Therefore, research on the influence of types of exercise, including acute and long-term training, on the structural and mechanical properties of tendons in athletic and sedentary populations is of critical importance in the design of scientific-based exercise plans and effective tendinopathy treatment. Here, we review recent studies on the relationship between exercise and tendon health and tendinopathy repair to provide a general understanding of how exercise may reshape tendons.

Axin2-lineage cells contribute to neonatal tendon regeneration

PURPOSE

Tendon injuries are a challenging clinical problem with few treatment options. Identifying the molecular regulators of tendon is required for the development of new therapies. While the Wnt pathway is critical for the maintenance and differentiation of many tissues, the role of Wnt signaling in tendon cell biology remains largely unexplored.

METHODS

The effects of Wnt activation were tested in vitro using neonatal tendon-derived cells cultured in 2D and 3D conditions. The inducible Axin2CreERT2 was then used to label Axin2+ cells in vivo and cells were traced during neonatal tendon regeneration.

RESULTS

We showed that activation of Wnt signaling results in proliferation of neonatal tendon cells. While tendon marker expression was inhibited by Wnt activation under 2D conditions, Scx expression was not affected under 3D uniaxial tension, suggesting that the microenvironment contextualizes tendon cell response to Wnt signaling. Using an in vivo model of neonatal tendon regeneration, we further showed that Wnt signaling cells comprise a subpopulation of tenocyte and epitenon cells that proliferate after injury and are recruited during regeneration.

DISCUSSION

Collectively, these studies suggest that Wnt signaling may play a role in tendon cell proliferation, differentiation, and regeneration.

TGFβ signaling is required for sclerotome resegmentation during development of the spinal column in Gallus gallus

We previously showed the importance of TGFβ signaling in development of the mouse axial skeleton. Here, we provide the first direct evidence that TGFβ signaling is required for resegmentation of the sclerotome using chick embryos. Lipophilic fluorescent tracers, DiO and DiD, were microinjected into adjacent somites of embryos treated with or without TGFβRI inhibitors, SB431542, SB525334 or SD208, at developmental day E2.5 (HH16). Lineage tracing of labeled cells was observed over the course of 4 days until the completion of resegmentation at E6.5 (HH32). Vertebrae were malformed and intervertebral discs were small and misshapen in inhibitor injected embryos. Hypaxial myofibers were also increased in thickness after treatment with the inhibitor. Inhibition of TGFβ signaling resulted in alterations in resegmentation that ranged between full, partial, and slanted shifts in distribution of DiO or DiD labeled cells within vertebrae. Patterning of rostro-caudal markers within sclerotome was disrupted at E3.5 after treatment with TGFβRI inhibitor with rostral domains expressing both rostral and caudal markers. We propose that TGFβ signaling regulates rostro-caudal polarity and subsequent resegmentation in sclerotome during spinal column development.

GDF5+ chondroprogenitors derived from human pluripotent stem cells preferentially form permanent chondrocytes

It has been established in the mouse model that during embryogenesis joint cartilage is generated from a specialized progenitor cell type, distinct from that responsible for the formation of growth plate cartilage. We recently found that mesodermal progeny of human pluripotent stem cells gave rise to two types of chondrogenic mesenchymal cells in culture: SOX9+ and GDF5+ cells. The fast-growing SOX9+ cells formed in vitro cartilage that expressed chondrocyte hypertrophy markers and readily underwent mineralization after ectopic transplantation. In contrast, the slowly growing GDF5+ cells derived from SOX9+ cells formed cartilage that tended to express low to undetectable levels of chondrocyte hypertrophy markers, but expressed PRG4, a marker of embryonic articular chondrocytes. The GDF5+-derived cartilage remained largely unmineralized in vivo. Interestingly, chondrocytes derived from the GDF5+ cells seemed to elicit these activities via non-cell-autonomous mechanisms. Genome-wide transcriptomic analyses suggested that GDF5+ cells might contain a teno/ligamento-genic potential, whereas SOX9+ cells resembled neural crest-like progeny-derived chondroprogenitors. Thus, human pluripotent stem cell-derived GDF5+ cells specified to generate permanent-like cartilage seem to emerge coincidentally with the commitment of the SOX9+ progeny to the tendon/ligament lineage.

Tendon tissue engineering: Current progress towards an optimized tenogenic differentiation protocol for human stem cells

Tendons are integral to our daily lives by allowing movement and locomotion but are frequently injured, leading to patient discomfort and impaired mobility. Current clinical procedures are unable to fully restore the native structure of the tendon, resulting in loss of full functionality, and the weakened tissue following repair often re-ruptures. Tendon tissue engineering, involving the combination of cells with biomaterial scaffolds to form new tendon tissue, holds promise to improve patient outcomes. A key requirement for efficacy in promoting tendon tissue formation is the optimal differentiation of the starting cell populations, most commonly adult tissue-derived mesenchymal stem/stromal cells (MSCs), into tenocytes, the predominant cellular component of tendon tissue. Currently, a lack of consensus on the protocols for effective tenogenic differentiation is hampering progress in tendon tissue engineering. In this review, we discuss the current state of knowledge regarding human stem cell differentiation towards tenocytes and tendon tissue formation. Tendon development and healing mechanisms are described, followed by a comprehensive overview of the current protocols for tenogenic differentiation, including the effects of biochemical and biophysical cues, and their combination, on tenogenesis. Lastly, a synthesis of the key features of these protocols is used to design future approaches. The holistic evaluation of current knowledge should facilitate and expedite the development of efficacious stem cell tenogenic differentiation protocols with future impact in tendon tissue engineering. STATEMENT OF SIGNIFICANCE: The lack of a widely-adopted tenogenic differentiation protocol has been a major hurdle in the tendon tissue engineering field. Building on current knowledge on tendon development and tendon healing, this review surveys peer-reviewed protocols to present a holistic evaluation and propose a pathway to facilitate and expedite the development of a consensus protocol for stem cell tenogenic differentiation and tendon tissue engineering.

Tendon Healing Response Is Dependent on Epithelial-Mesenchymal-Tendon Transition State of Amniotic Epithelial Stem Cells

Tendinopathies are at the frontier of advanced responses to health challenges and sectoral policy targets. Cell-based therapy holds great promise for tendon disorder resolution. To verify the role of stepwise trans-differentiation of amniotic epithelial stem cells (AECs) in tendon regeneration, in the present research three different AEC subsets displaying an epithelial (eAECs), mesenchymal (mAECs), and tendon-like (tdAECs) phenotype were allotransplanted in a validated experimental sheep Achilles tendon injury model. Tissue healing was analyzed adopting a comparative approach at two early healing endpoints (14 and 28 days). All three subsets of transplanted cells were able to accelerate regeneration: mAECs with a lesser extent than eAECs and tdAECs as indicated in the summary of the total histological scores (TSH), where at day 28 eAECs and tdAECs had better significant scores with respect to mAEC-treated tendons (p < 0.0001). In addition, the immunomodulatory response at day 14 showed in eAEC-transplanted tendons an upregulation of pro-regenerative M2 macrophages with respect to mAECs and tdAECs (p < 0.0001). In addition, in all allotransplanted tendons there was a favorable IL10/IL12 compared to CTR (p < 0.001). The eAECs and tdAECs displayed two different underlying regenerative mechanisms in the tendon. The eAECs positively influenced regeneration mainly through their greater ability to convey in the host tissue the shift from pro-inflammatory to pro-regenerative responses, leading to an ordered extracellular matrix (ECM) deposition and blood vessel remodeling. On the other hand, the transplantation of tdAECs acted mainly on the proliferative phase by impacting the density of ECM and by supporting a prompt recovery, inducing a low cellularity and angle alignment of the host cell compartment. These results support the idea that AECs lay the groundwork for production of different cell phenotypes that can orient tendon regeneration through a crosstalk with the host tissue. In particular, the obtained evidence suggests that eAECs are a practicable and efficient strategy for the treatment of acute tendinopathies, thus reinforcing the grounds to move their use towards clinical practice.

The loop of phenotype: Dynamic reciprocity links tenocyte morphology to tendon tissue homeostasis

Cells and their surrounding extracellular matrix (ECM) are engaged in dynamic reciprocity to maintain tissue homeostasis: cells deposit ECM, which in turn presents the signals that define cell identity. This loop of phenotype is obvious for biochemical signals, such as collagens, which are produced by and presented to cells, but the role of biomechanical signals is also increasingly recognised. In addition, cell shape goes hand in hand with cell function and tissue homeostasis. Aberrant cell shape and ECM is seen in pathological conditions, and control of cell shape in micro-fabricated platforms disclose the causal relationship between cell shape and cell function, often mediated by mechanotransduction. In this manuscript, we discuss the loop of phenotype for tendon tissue homeostasis. We describe cell shape and ECM organization in normal and diseased tissue, how ECM composition influences tenocyte shape, and how that leads to the activation of signal transduction pathways and ECM deposition. We further describe the use of technologies to control cell shape to elucidate the link between cell shape and its phenotypical markers and focus on the causal role of cell shape in the loop of phenotype. STATEMENT OF SIGNIFICANCE: The dynamic reciprocity between cells and their surrounding extracellular matrix (ECM) influences biomechanical and biochemical properties of ECM as well as cell function through activation of signal transduction pathways that regulate gene and protein expression. We refer to this reciprocity as Loop of Phenotype and it has been studied and demonstrated extensively by using micro-fabricated platforms to manipulate cell shape and cell fate. In this manuscript, we discuss this concept in tendon tissue homeostasis by giving examples in healthy and pathological tenson tissue. Furthermore, we elaborate this by showing how biomaterials are used to feed this reciprocity to manipulate cell shape and function. Finally, we elucidate the link between cell shape and its phenotypical markers and focus on the activation of signal transduction pathways and ECM deposition.

Tendon and Ligament Genetics: How Do They Contribute to Disease and Injury? A Narrative Review

A significant proportion of patients requiring musculoskeletal management present with tendon and ligament pathology. Our understanding of the intrinsic and extrinsic mechanisms that lead to such disabilities is increasing. However, the complexity underpinning these interactive multifactorial elements is still not fully characterised. Evidence highlighting the genetic components, either reducing or increasing susceptibility to injury, is increasing. This review examines the present understanding of the role genetic variations contribute to tendon and ligament injury risk. It examines the different elements of tendon and ligament structure and considers our knowledge of genetic influence on form, function, ability to withstand load, and undertake repair or regeneration. The role of epigenetic factors in modifying gene expression in these structures is also explored. It considers the challenges to interpreting present knowledge, the requirements, and likely pathways for future research, and whether such information has reached the point of clinical utility.

Mechanical feedback defines organizing centers to drive digit emergence

During embryonic development, digits gradually emerge in a periodic pattern. Although genetic evidence indicates that digit formation results from a self-organizing process, the underlying mechanisms are still unclear. Here, we find that convergent-extension tissue flows driven by active stresses underlie digit formation. These active stresses simultaneously shape cartilage condensations and lead to the emergence of a compressive stress region that promotes high activin/p-SMAD/SOX9 expression, thereby defining digit-organizing centers via a mechanical feedback. In Wnt5a mutants, such mechanical feedback is disrupted due to the loss of active stresses, organizing centers do not emerge, and digit formation is precluded. Thus, digit emergence does not result solely from molecular interactions, as was previously thought, but requires a mechanical feedback that ensures continuous coupling between phalanx specification and elongation. Our work, which links mechanical and molecular signals, provides a mechanistic context for the emergence of organizing centers that may underlie various developmental processes.

Regulators of collagen crosslinking in developing and adult tendons

Tendons are collagen-rich musculoskeletal tissues that possess the mechanical strength needed to transfer forces between muscles and bones. The mechanical development and function of tendons are impacted by collagen crosslinks. However, there is a limited understanding of how collagen crosslinking is regulated in tendon during development and aging. Therefore, the objective of the present review was to highlight potential regulators of enzymatic and non-enzymatic collagen crosslinking and how they impact tendon function. The main collagen crosslinking enzymes include lysyl oxidase (LOX) and the lysyl oxidase-like isoforms (LOXL), whereas non-enzymatic crosslinking is mainly mediated by the formation of advanced glycation end products (AGEs). Regulators of the LOX and LOXL enzymes may include mechanical stimuli, mechanotransducive cell signaling pathways, sex hormones, transforming growth factor (TGF)β family, hypoxia, and interactions with intracellular or extracellular proteins. AGE accumulation in tendon is due to diabetic conditions and aging, and can be mediated by diet and mechanical stimuli. The formation of these enzymatic and non-enzymatic collagen crosslinks plays a major role in tendon biomechanics and in the mechanisms of force transfer. A more complete understanding of how enzymatic and non-enzymatic collagen crosslinking is regulated in tendon will better inform tissue engineering and regenerative therapies aimed at restoring the mechanical function of damaged tendons.

Inhibition of Integrin αvβ6 Activation of TGF-β Attenuates Tendinopathy

Tendinopathy is a common tendon disorder that causes pain and impairs function. It is the most common reason for consultation with musculoskeletal specialists. The available therapies for tendinopathy are limited in number and efficacy and have unclear cellular and molecular mechanisms. Here it is shown that transforming growth factor-beta (TGF-β) activated by integrin αvβ6 promotes tendinopathy in mice. Excessive active TGF-β is found during tendinopathy progression, which led to tenocytes' phenotype transition to chondrocytes. Transgenic expression of active TGF-β in tendons induced spontaneous tendinopathy, whereas systemic injection of a TGF-β neutralizing antibody attenuated tendinopathy. Inducible knockout of the TGF-β type 2 receptor gene (Tgfbr2) in tenocytes inhibited tendinopathy progression in mice. Moreover, it is found that integrin αvβ6 induces TGF-β activation in response to mechanical load in tendons. Conditional knockout of the integrin αv gene in tendons prevented tendinopathy in mice. The study suggests that integrin αvβ6 activation of TGF-β is the mechanism of tendinopathy, and that integrin αvβ6 may be a therapeutic target in tendinopathy.

Tenogenic Induction From Induced Pluripotent Stem Cells Unveils the Trajectory Towards Tenocyte Differentiation

The musculoskeletal system is integrated by tendons that are characterized by the expression of scleraxis (Scx), a functionally important transcription factor. Here, we newly developed a tenocyte induction method using induced pluripotent stem cells established from ScxGFP transgenic mice by monitoring fluorescence, which reflects a dynamic differentiation process. Among several developmentally relevant factors, transforming growth factor-beta 2 (TGF-β2) was the most potent inducer for differentiation of tenomodulin-expressing mature tenocytes. Single-cell RNA sequencing (scRNA-seq) revealed 11 distinct clusters, including mature tenocyte population and tenogenic differentiation trajectory, which recapitulated the in vivo developmental process. Analysis of the scRNA-seq dataset highlighted the importance of retinoic acid (RA) as a regulatory pathway of tenogenic differentiation. RA signaling was shown to have inhibitory effects on entheseal chondrogenic differentiation as well as TGF-β2-dependent tenogenic/fibrochondrogenic differentiation. The collective findings provide a new opportunity for tendon research and further insight into the mechanistic understanding of the differentiation pathway to a tenogenic fate.

Growth and mechanobiology of the tendon-bone enthesis

Tendons are cable-like connective tissues that transfer both active and passive forces generated by skeletal muscle to bone. In the mature skeleton, the tendon-bone enthesis is an interfacial zone of transitional tissue located between two mechanically dissimilar tissues: compliant, fibrous tendon to rigid, dense mineralized bone. In this review, we focus on emerging areas in enthesis development related to its structure, function, and mechanobiology, as well as highlight established and emerging signaling pathways and physiological processes that influence the formation and adaptation of this important transitional tissue.

Evaluation of the Effects of Transforming Growth Factor-Beta 3 (TGF-β3) Loaded Nanoparticles on Healing in a Rat Achilles Tendon Injury Model

BACKGROUND

Achilles tendon (AT) midsubstance injuries may heal suboptimally, especially in athletes. Transforming growth factor-beta 3 (TGF-β3) shows promise because of its recently discovered tendinogenic effects. Using poly(lactic-co-glycolic acid)-b-poly(ethylene glycol) (PLGA-b-PEG) nanoparticles (NPs) may enhance the results by a sustained-release effect.

HYPOTHESIS

The application of TGF-β3 will enhance AT midsubstance healing, and the NP form will achieve better outcomes.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 80 rats underwent unilateral AT transection and were divided into 4 groups: (1) control (C); (2) empty chitosan film (Ch); (3) chitosan film containing free TGF-β3 (ChT); and (4) chitosan film containing TGF-β3-loaded NPs (ChN). The animals were sacrificed at 3 and 6 weeks. Tendons were evaluated for morphology (length and cross-sectional area [CSA]), biomechanics (maximum load, stress, stiffness, and elastic modulus), histology, immunohistochemical quantification (types I and III collagen [COL1 and COL3]), and gene expression (COL1A1, COL3A1, scleraxis, and tenomodulin).

RESULTS

Morphologically, at 3 weeks, ChT (15 ± 2.7 mm) and ChN (15.6 ± 1.6 mm) were shorter than C (17.6 ± 1.8 mm) (P = .019 and = .004, respectively). At 6 weeks, the mean CSA of ChN (10.4 ± 1.9 mm²) was similar to that of intact tendons (6.4 ± 1.1 mm²) (P = .230), while the other groups were larger. Biomechanically, at 3 weeks, ChT (42.8 ± 4.9 N) had a higher maximum load than C (27 ± 9.1 N; P = .004) and Ch (29.2 ± 5.7 N; P = .005). At 6 weeks, ChN (26.9 ± 3.9 MPa) had similar maximum stress when compared with intact tendons (34.1 ± 7.8 MPa) (P = .121); the other groups were significantly lower. Histologically, at 6 weeks, the mean Movin score of ChN (4.5 ± 1.5) was lower than that of ChT (6.3 ± 1.8). Immunohistochemically, ChN had higher COL3 (1.469 ± 0.514) at 3 weeks and lower COL1 (1.129 ± 0.368) at 6 weeks. COL1A1 gene expression was higher in ChT and ChN at 3 weeks, but COL3A1 gene expression was higher in ChN.

CONCLUSION

The application of TGF-β3 had a positive effect on AT midsubstance healing, and the sustained-release NP form improved the outcomes, more specifically accelerating the remodeling process.

CLINICAL RELEVANCE

This study demonstrated the effectiveness of TGF-β3 on tendon healing on a rat model, which is an important step toward clinical use. The novel method of using PLGA-b-PEG NPs as a drug-delivery system with sustained-release properties had promising results.

Altered TGFB1 regulated pathways promote accelerated tendon healing in the superhealer MRL/MpJ mouse

To better understand the molecular mechanisms of tendon healing, we investigated the Murphy Roth's Large (MRL) mouse, which is considered a model of mammalian tissue regeneration. We show that compared to C57Bl/6J (C57) mice, injured MRL tendons have reduced fibrotic adhesions and cellular proliferation, with accelerated improvements in biomechanical properties. RNA-seq analysis revealed that differentially expressed genes in the C57 healing tendon at 7 days post injury were functionally linked to fibrosis, immune system signaling and extracellular matrix (ECM) organization, while the differentially expressed genes in the MRL injured tendon were dominated by cell cycle pathways. These gene expression changes were associated with increased α-SMA+ myofibroblast and F4/80+ macrophage activation and abundant BCL-2 expression in the C57 injured tendons. Transcriptional analysis of upstream regulators using Ingenuity Pathway Analysis showed positive enrichment of TGFB1 in both C57 and MRL healing tendons, but with different downstream transcriptional effects. MRL tendons exhibited of cell cycle regulatory genes, with negative enrichment of the cell senescence-related regulators, compared to the positively-enriched inflammatory and fibrotic (ECM organization) pathways in the C57 tendons. Serum cytokine analysis revealed decreased levels of circulating senescence-associated circulatory proteins in response to injury in the MRL mice compared to the C57 mice. These data collectively demonstrate altered TGFB1 regulated inflammatory, fibrosis, and cell cycle pathways in flexor tendon repair in MRL mice, and could give cues to improved tendon healing.

Histological Changes of Cervical Disc Tissue in Patients with Degenerative Ossification

Objective

To explore the histological feature of the cervical disc degeneration in patients with degenerative ossification (DO) and its potential mechanisms.

Methods

A total of 96 surgical segments, from cervical disc degenerative disease patients with surgical treatment, were divided into ossification group (group O, n=46) and non-ossification group (group NO, n=50) based on preoperative radiological exams. Samples of disc tissues and osteophytes were harvested during the decompression operation. The hematoxylin-eosin staining, Masson trichrome staining and Safranin O-fast green staining were used to compare the histological differences between the two groups. And the distribution and content of transforming growth factor (TGF)-β1, p-Smad2 and p-Smad3 between the two groups were compared by a semi-quantitative immunohistochemistry (IHC) method.

Results

For all the disc tissues, the content of disc cells and collagen fibers decreased gradually from the outer annulus fibrosus (OAF) to the central nucleus pulposus (NP). Compared with group NO, the number of disc cells in group O increased significantly. But for proteoglycan in the inner annulus fibrosus (IAF) and NP, the content in group O decreased significantly. IHC analysis showed that TGF-β1, p-Smad2, and p-Smad3 were detected in all tissues. For group O, the content of TGF-β1 in the OAF and NP was significantly higher than that in group NO. For p-Smad2 in IAF and p-Smad3 in OAF, the content in group O were significantly higher than group NO.

Conclusion

Histologically, cervical disc degeneration in patients with DO is more severe than that without DO. Local higher content of TGF-β1, p-Smad2, and p-Smad3 are involved in the disc degeneration with DO. Further studies with multi-approach analyses are needed to better understand the role of TGF-β/Smads signaling pathway in the disc degeneration with DO.

The Role of Indian Hedgehog Signaling in Tendon Response to Subacromial Impingement: Evaluation Using a Mouse Model

BACKGROUND

The underlying cellular and molecular mechanisms involved in the development of tendinopathy due to subacromial supraspinatus tendon (SST) impingement and the response to subsequent removal of impingement remain unknown.

PURPOSE

To investigate the involvement of Indian hedgehog (IHH) signaling in the development of SST tendinopathy and the subsequent healing process after the relief of subacromial impingement in a novel mouse shoulder impingement model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 48 male wild-type C57BL/6 mice were used in this study. Supraspinatus tendinopathy was induced by inserting a microsurgical clip into the subacromial space bilaterally. Eleven mice were sacrificed at 4 weeks after surgery to establish impingement baseline; 24 mice underwent clip removal at 4 weeks after surgery and then were euthanized at 2 or 4 weeks after clip removal. Thirteen mice without surgical intervention were utilized as the control group. All SSTs were evaluated with biomechanical testing; quantitative histomorphometry after staining with hematoxylin and eosin, Alcian blue, and picrosirius red; and immunohistochemical staining (factor VIII, IHH, Patched1 [PTCH1], and glioma-associated oncogene homolog 1 [GLI1]).

RESULTS

The mean failure force and stiffness in the 4-week impingement group decreased significantly compared with the control group (P < .001) and gradually increased at 2 and 4 weeks after clip removal. Histological analysis demonstrated increased cellularity and disorganized collagen fibers in the SST, with higher modified Bonar scores at 4 weeks, followed by gradual improvement after clip removal. The IHH-positive area and PTCH1- and GLI1-positive cell percentages significantly increased after 4 weeks of clip impingement (20.64% vs 2.06%, P < .001; 53.9% vs 28.03%, P = .016; and 30% vs 12.19%, P = .036, respectively) and continuously increased after clip removal.

CONCLUSION

The authors' findings suggest that the hedgehog signaling pathway and its downstream signaling mediator and target GLI1 may play a role in the development and healing process of rotator cuff tendinopathy due to extrinsic rotator cuff impingement.

CLINICAL RELEVANCE

This study suggests the potential for the hedgehog pathway, together with its downstream targets, as candidates for further study as potential therapeutic targets in the treatment of supraspinatus tendinopathy.

Stem cells in tendon regeneration and factors governing tenogenesis

Tendons are connective tissue structures of paramount importance to the human ability of locomotion. Tendinopathy and tendon rupture can be resistant to treatment and often recurs, thus resulting in a significant health problem with a relevant social impact worldwide. Unfortunately, existing treatment approaches are suboptimal. A better understanding of the basic biology of tendons may provide a better way to solve these problems and promote tendon regeneration. Stem cells, either obtained from tendons or non-tendon sources, such as bone marrow (BMSCs), adipose tissue (AMSCs), as well as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), have received increasing attention toward enhancing tendon healing. There are many studies showing that stem cells can contribute to improving tendon healing. Hence, in this review, the current knowledge of BMSCs, AMSCs, TSPCs, ESCs and iPSCs for tendon regeneration, as well as the advantages and limitations among them, has been highlighted. Moreover, the transcriptional and bioactive factors governing tendon healing processes have been discussed.

Tendon Immune Regeneration: Insights on the Synergetic Role of Stem and Immune Cells during Tendon Regeneration

Tendon disorders represent a very common pathology in today's population, and tendinopathies that account 30% of tendon-related injuries, affect yearly millions of people which in turn cause huge socioeconomic and health repercussions worldwide. Inflammation plays a prominent role in the development of tendon pathologies, and advances in understanding the underlying mechanisms during the inflammatory state have provided additional insights into its potential role in tendon disorders. Different cell compartments, in combination with secreted immune modulators, have shown to control and modulate the inflammatory response during tendinopathies. Stromal compartment represented by tenocytes has shown to display an important role in orchestrating the inflammatory response during tendon injuries due to the interplay they exhibit with the immune-sensing and infiltrating compartments, which belong to resident and recruited immune cells. The use of stem cells or their derived secretomes within the regenerative medicine field might represent synergic new therapeutical approaches that can be used to tune the reaction of immune cells within the damaged tissues. To this end, promising opportunities are headed to the stimulation of macrophages polarization towards anti-inflammatory phenotype together with the recruitment of stem cells, that possess immunomodulatory properties, able to infiltrate within the damaged tissues and improve tendinopathies resolution. Indeed, the comprehension of the interactions between tenocytes or stem cells with the immune cells might considerably modulate the immune reaction solving hence the inflammatory response and preventing fibrotic tissue formation. The purpose of this review is to compare the roles of distinct cell compartments during tendon homeostasis and injury. Furthermore, the role of immune cells in this field, as well as their interactions with stem cells and tenocytes during tendon regeneration, will be discussed to gain insights into new ways for dealing with tendinopathies.

Comparison of Tendon Development Versus Tendon Healing and Regeneration

Tendon is a vital connective tissue in human skeletal muscle system, and tendon injury is very common and intractable in clinic. Tendon development and repair are two closely related but still not fully understood processes. Tendon development involves multiple germ layer, as well as the regulation of diversity transcription factors (Scx et al.), proteins (Tnmd et al.) and signaling pathways (TGFβ et al.). The nature process of tendon repair is roughly divided in three stages, which are dominated by various cells and cell factors. This review will describe the whole process of tendon development and compare it with the process of tendon repair, focusing on the understanding and recent advances in the regulation of tendon development and repair. The study and comparison of tendon development and repair process can thus provide references and guidelines for treatment of tendon injuries.

Autograft Long Head Biceps Tendon Can Be Used as a Scaffold for Biologically Augmenting Rotator Cuff Repairs

PURPOSE

We create a viable, mechanically expanded autograft long head biceps tendon (LHBT) scaffold for biologically augmenting the repair of torn rotator cuffs.

METHODS

The proximal aspect of the tenotomized LHBTs was harvested from patients during rotator cuff repair surgery and was mechanically formed into porous scaffolds using a surgical graft expander. LHBT scaffolds were evaluated for change in area, tensile properties, and tenocyte viability before and after expansion. The ability of endogenous tenocytes derived from the LHBT scaffold to promote tenogenic differentiation of human adipose-derived mesenchymal stromal cells (ADMSCs) was also determined.

RESULTS

Autograft LHBTs were successfully expanded using a modified surgical graft expander to create a porous scaffold containing viable resident tenoctyes from patients undergoing rotator cuff repair. LHBT scaffolds had significantly increased area (length: 24.91 mm [13.91, 35.90] × width: 22.69 mm [1.87, 34.50]; P = .011) compared with the native LHBT tendon (length: 27.16 mm [2.70, 33.62] × width: 6.68 mm [5.62, 7.74]). The structural properties of the autograft were altered, including the ultimate tensile strength (LHBT scaffold: .56 MPa [.06, 1.06] vs. native LHBT: 2.35 MPa [1.36, 3.33]; P = .002) and tensile modulus (LHBT scaffold: 4.72 MPa [-.80, 1.24] versus native LHBT: 37.17 MPa [24.56, 49.78]; P = .001). There was also a reduction in resident tenocyte percent viability (LHBT scaffold: 38.52% [17.94, 59.09] vs. native LHBT: 68.87% [63.67, 74.37]; P =.004). Tenocytes derived from the LHBT scaffold produced soluble signals that initiated ADMSC differentiation into an immature tenocyte-like phenotype, as indicated by an 8.7× increase in scleraxis (P = .040) and a 3.6× increase in collagen type III mRNA expression (P = .050) compared with undifferentiated ADMSC controls.

CONCLUSIONS

The ability to produce a viable autologous scaffold from the proximal biceps tendon having dimensions, porosity, mechanical characteristics, native ECM components, and viable tenocytes that produce bioactive signals conducive to supporting the biologic augmentation of rotator cuff repair surgery has been demonstrated.

CLINICAL RELEVANCE

This biologically active construct may help to improve the quality of healing and regeneration at the repair site of rotator cuff tears, especially those at high risk for retear.