Tendon repair through stem cell intervention: cellular and molecular approaches

Therapeutic potential of GDF-5 for enhancing tendon regenerative healing

Tendon injury is a common disorder of the musculoskeletal system, with a higher possibility of occurrence in elderly individuals and athletes. After a tendon injury, the tendon suffers from inadequate and slow healing, resulting in the formation of fibrotic scar tissue, ending up with inferior functional properties. Therapeutic strategies involving the application of growth factors have been advocated to promote tendon healing. Growth and differentiation-5 (GDF-5) represents one such factor that has shown promising effect on tendon healing in animal models and in vitro cultures. Although promising, these studies are limited as the molecular mechanisms by which GDF-5 exerts its effect remain incompletely understood. Starting from broadly introducing essential elements of current understanding about GDF-5, the present review aims to define the effect of GDF-5 and its possible mechanisms of action in tendon healing. Nevertheless, we still need more in vivo studies to explore dosage, application time and delivery strategy of GDF-5, so as to pave the way for future clinical translation.

Cerium oxide nanoparticles-carrying human umbilical cord mesenchymal stem cells counteract oxidative damage and facilitate tendon regeneration

BACKGROUND

Tendon injuries have a high incidence and limited treatment options. Stem cell transplantation is essential for several medical conditions like tendon injuries. However, high local concentrations of reactive oxygen species (ROS) inhibit the activity of transplanted stem cells and hinder tendon repair. Cerium oxide nanoparticles (CeONPs) have emerged as antioxidant agents with reproducible reducibility.

RESULTS

In this study, we synthesized polyethylene glycol-packed CeONPs (PEG-CeONPs), which were loaded into the human umbilical cord mesenchymal stem cells (hUCMSCs) to counteract oxidative damage. H2O2 treatment was performed to evaluate the ROS scavenging ability of PEG-CeONPs in hUCMSCs. A rat model of patellar tendon defect was established to assess the effect of PEG-CeONPs-carrying hUCMSCs in vivo. The results showed that PEG-CeONPs exhibited excellent antioxidant activity both inside and outside the hUCMSCs. PEG-CeONPs protect hUCMSCs from senescence and apoptosis under excessive oxidative stress. Transplantation of hUCMSCs loaded with PEG-CeONPs reduced ROS levels in the tendon injury area and facilitated tendon healing. Mechanistically, NFκB activator tumor necrosis factor α and MAPK activator dehydrocrenatine, reversed the therapeutic effect of PEG-CeONPs in hUCMSCs, indicating that PEG-CeONPs act by inhibiting the NFκB and MAPK signaling pathways.

CONCLUSIONS

The carriage of the metal antioxidant oxidase PEG-CeONPs maintained the ability of hUCMSCs in the injured area, reduced the ROS levels in the microenvironment, and facilitated tendon regeneration. The data presented herein provide a novel therapeutic strategy for tendon healing and new insights into the use of stem cells for disease treatment.

Tendon tissue engineering: An overview of biologics to promote tendon healing and repair

Tendons are dense connective tissues with a hierarchical polarized structure that respond to and adapt to the transmission of muscle contraction forces to the skeleton, enabling motion and maintaining posture. Tendon injuries, also known as tendinopathies, are becoming more common as populations age and participation in sports/leisure activities increases. The tendon has a poor ability to self-heal and regenerate given its intrinsic, constrained vascular supply and exposure to frequent, severe loading. There is a lack of understanding of the underlying pathophysiology, and it is not surprising that disorder-targeted medicines have only been partially effective at best. Recent tissue engineering approaches have emerged as a potential tool to drive tendon regeneration and healing. In this review, we investigated the physiochemical factors involved in tendon ontogeny and discussed their potential application in vitro to reproduce functional and self-renewing tendon tissue. We sought to understand whether stem cells are capable of forming tendons, how they can be directed towards the tenogenic lineage, and how their growth is regulated and monitored during the entire differentiation path. Finally, we showed recent developments in tendon tissue engineering, specifically the use of mesenchymal stem cells (MSCs), which can differentiate into tendon cells, as well as the potential role of extracellular vesicles (EVs) in tendon regeneration and their potential for use in accelerating the healing response after injury.

Leukocyte-Rich Platelet-Rich Plasma Has Better Stimulating Effects on Tenocyte Proliferation Compared With Leukocyte-Poor Platelet-Rich Plasma

Background:

Rotator cuff (RC) tendinopathy is one of the most common causes of shoulder pain. Platelet-rich plasma (PRP) has been frequently used in clinical scenarios, but its efficacy remains inconsistent.

Purpose:

To investigate the different responses of human tenocytes from torn RCs to leukocyte-rich PRP (LR-PRP) and leukocyte-poor PRP (LP-PRP) in a 2-chamber coculture device.

Study Design:

Controlled laboratory study.

Methods:

PRP was prepared using different platelet and leukocyte concentrations according to 5 groups: (1) LR-PRP with 5000 platelets/µL, (2) LR-PRP with 10,000 platelets/µL, (3) LP-PRP with 5000 platelets/µL, (4) LP-PRP with 10,000 platelets/µL, and (5) control with only culture medium supplementation and without PRP stimulation. Platelet-derived growth factor–AB (PDGF-AB) and transforming growth factor–β1 (TGF-β1) were measured in LR-PRP and LP-PRP via enzyme-linked immunosorbent assay. Microscopy, water-soluble tetrazolium salt assay, and quantitative real-time polymerase chain reaction were used to investigate the morphology, proliferation, and gene expression of RC tenocytes exposed to different PRP formulations. Data were collected from at least 3 independent measurements. The results were analyzed via 1-way analysis of variance, followed by the post hoc Bonferroni test.

Results:

The ratio of leukocytes to 5000 platelets/µL was 29.5 times higher in LR-PRP than in LP-PRP (P < .05). In the 5000 platelets/µL groups, the levels of TGF-β1 and PDGF-AB were both significantly higher in LR-PRP versus LP-PRP (TGF-β1: 367.0 ± 16.5 vs 308.6 ± 30.3 pg/mL, respectively [P = .043]; PDGF-AB: 172.1 ± 1.8 vs 94.1 ± 4.2 pg/mL, respectively [P < .001]). Compared with the control group, RC tenocyte proliferation was 1.42 ± 0.01 and 1.41 ± 0.03 times higher in the LR-PRP groups with 5000 platelets/µL and 10,000 platelets/µL, respectively (P < .05). The expression of tenocyte-related genes was higher in tenocytes cultured in LR-PRP.

Conclusion:

Both the LR-PRP groups with 5000 platelets/µL and 10,000 platelets/µL induced more growth factor release and increased RC tenocyte proliferation than did the LP-PRP groups.

Clinical Relevance:

In RC repair, LR-PRP may be better than LP-PRP for increasing the proliferation of tenocytes.

Amnion-Derived Teno-Inductive Secretomes: A Novel Approach to Foster Tendon Differentiation and Regeneration in an Ovine Model

Regenerative medicine has greatly progressed, but tendon regeneration mechanisms and robust in vitro tendon differentiation protocols remain to be elucidated. Recently, tendon explant co-culture (CO) has been proposed as an in vitro model to recapitulate the microenvironment driving tendon development and regeneration. Here, we explored standardized protocols for production and storage of bioactive tendon-derived secretomes with an evaluation of their teno-inductive effects on ovine amniotic epithelial cells (AECs). Teno-inductive soluble factors were released in culture-conditioned media (CM) only in response to active communication between tendon explants and stem cells (CMCO). Unsuccessful tenogenic differentiation in AECs was noted when exposed to CM collected from tendon explants (CMFT) only, whereas CMCO upregulated SCXB, COL I and TNMD transcripts, in AECs, alongside stimulation of the development of mature 3D tendon-like structures enriched in TNMD and COL I extracellular matrix proteins. Furthermore, although the tenogenic effect on AECs was partially inhibited by freezing CMCO, this effect could be recovered by application of an in vivo-like physiological oxygen (2% O2) environment during AECs tenogenesis. Therefore, CMCO can be considered as a waste tissue product with the potential to be used for the development of regenerative bio-inspired devices to innovate tissue engineering application to tendon differentiation and healing.

Tendon and Cytokine Marker Expression by Human Bone Marrow Mesenchymal Stem Cells in a Hyaluronate/Poly-Lactic-Co-Glycolic Acid (PLGA)/Fibrin Three-Dimensional (3D) Scaffold

We developed a (three-dimensional) 3D scaffold, we named HY-FIB, incorporating a force-transmission band of braided hyaluronate embedded in a cell localizing fibrin hydrogel and poly-lactic-co-glycolic acid (PLGA) nanocarriers as transient components for growth factor controlled delivery. The tenogenic supporting capacity of HY-FIB on human-Bone Marrow Mesenchymal Stem Cells (hBM-MSCs) was explored under static conditions and under bioreactor-induced cyclic strain conditions. HY-FIB elasticity enabled to deliver a mean shear stress of 0.09 Pa for 4 h/day. Tendon and cytokine marker expression by hBM-MSCs were studied. Results: hBM-MSCs embedded in HY-FIB and subjected to mechanical stimulation, resulted in a typical tenogenic phenotype, as indicated by type 1 Collagen fiber immunofluorescence. RT-qPCR showed an increase of type 1 Collagen, scleraxis, and decorin gene expression (3-fold, 1600-fold, and 3-fold, respectively, at day 11) in dynamic conditions. Cells also showed pro-inflammatory (IL-6, TNF, IL-12A, IL-1β) and anti-inflammatory (IL-10, TGF-β1) cytokine gene expressions, with a significant increase of anti-inflammatory cytokines in dynamic conditions (IL-10 and TGF-β1 300-fold and 4-fold, respectively, at day 11). Mechanical signaling, conveyed by HY-FIB to hBM-MSCs, promoted tenogenic gene markers expression and a pro-repair cytokine balance. The results provide strong evidence in support of the HY-FIB system and its interaction with cells and its potential for use as a predictive in vitro model.

Optimization of tenocyte lineage-related factors from tonsil-derived mesenchymal stem cells using response surface methodology

BACKGROUND

In order to optimize the tenogenic differentiation of mesenchymal stem cells (MSCs), researchers should consider various factors. However, this requires testing numerous experimental settings, which is costly and time-consuming. We aimed to assess the differential effects of transforming growth factor beta-3 (TGF-β3) on the tenogenesis of tonsil-derived MSCs (T-MSCs) and bone marrow-derived MSCs (BM-MSCs) using response surface methodology (RSM).

METHODS

Bone marrow and tonsillar tissue were collected from four patients; mononuclear cells were separated and treated with 5 or 10 ng/mL of TGF-β3. A full factorial experimental design with a categorical factor of 0 was employed to study the effect of tension based on T-MSCs. Eighty-four trials were fitted with RSM and then used to obtain mathematical prediction models.

RESULTS

Exposure of T-MSCs and BM-MSCs to TGF-β3 increased the expression of scleraxis (SCX), tenomodulin (TNMD), decorin, collagen I, and tenascin C. Expression of most of these factors reached a maximum after 2-3 days of treatment. The model predicted that the values of the tenocyte lineage-related factors assessed would be significantly increased at 2.5 days of culture with 2.7 ng/mL of TGF-β3 for T-MSCs and at 2.3 days of culture regardless of TGF-β3 concentration for BM-MSCs.

CONCLUSIONS

This study demonstrated that the RSM prediction of the culture time necessary for the tenogenic differentiation of T-MSCs and BM-MSCs under TGF-β3 stimulation was similar to the experimentally determined time of peak expression of tenocyte-related mRNAs, suggesting the potential of using the RSM approach for optimization of the culture protocol for tenogenesis of MSCs.

Fate of tenogenic differentiation potential of human bone marrow stromal cells by uniaxial stretching affected by stretch-activated calcium channel agonist gadolinium

The role for mechanical stimulation in the control of cell fate has been previously proposed, suggesting that there may be a role of mechanical conditioning in directing mesenchymal stromal cells (MSCs) towards specific lineage for tissue engineering applications. Although previous studies have reported that calcium signalling is involved in regulating many cellular processes in many cell types, its role in managing cellular responses to tensile loading (mechanotransduction) of MSCs has not been fully elucidated. In order to establish this, we disrupted calcium signalling by blocking stretch-activated calcium channel (SACC) in human MSCs (hMSCs) in vitro. Passaged-2 hMSCs were exposed to cyclic tensile loading (1 Hz + 8% for 6, 24, 48, and 72 hours) in the presence of the SACC blocker, gadolinium. Analyses include image observations of immunochemistry and immunofluorescence staining from extracellular matrix (ECM) production, and measuring related tenogenic and apoptosis gene marker expression. Uniaxial tensile loading increased the expression of tenogenic markers and ECM production. However, exposure to strain in the presence of 20 μM gadolinium reduced the induction of almost all tenogenic markers and ECM staining, suggesting that SACC acts as a mechanosensor in strain-induced hMSC tenogenic differentiation process. Although cell death was observed in prolonged stretching, it did not appear to be apoptosis mediated. In conclusion, the knowledge gained in this study by elucidating the role of calcium in MSC mechanotransduction processes, and that in prolonged stretching results in non-apoptosis mediated cell death may be potential useful for regenerative medicine applications.

Identification of Novel Equine (Equus caballus) Tendon Markers Using RNA Sequencing

Although several tendon-selective genes exist, they are also expressed in other musculoskeletal tissues. As cell and tissue engineering is reliant on specific molecular markers to discriminate between cell types, tendon-specific genes need to be identified. In order to accomplish this, we have used RNA sequencing (RNA-seq) to compare gene expression between tendon, bone, cartilage and ligament from horses. We identified several tendon-selective gene markers, and established eyes absent homolog 2 (EYA2) and a G-protein regulated inducer of neurite outgrowth 3 (GPRIN3) as specific tendon markers using RT-qPCR. Equine tendon cells cultured as three-dimensional spheroids expressed significantly greater levels of EYA2 than GPRIN3, and stained positively for EYA2 using immunohistochemistry. EYA2 was also found in fibroblast-like cells within the tendon tissue matrix and in cells localized to the vascular endothelium. In summary, we have identified EYA2 and GPRIN3 as specific molecular markers of equine tendon as compared to bone, cartilage and ligament, and provide evidence for the use of EYA2 as an additional marker for tendon cells in vitro.

Directing the Differentiation of Parthenogenetic Stem Cells into Tenocytes for Tissue-Engineered Tendon Regeneration

Uniparental parthenogenesis yields pluripotent stem cells without the political and ethical concerns surrounding the use of embryonic stem cells (ESCs) for biomedical applications. In the current study, we hypothesized that parthenogenetic stem cells (pSCs) could be directed to differentiate into tenocytes and applied for tissue‐engineered tendon. We showed that pSCs displayed fundamental properties similar to those of ESCs, including pluripotency, clonogenicity, and self‐renewal capacity. pSCs spontaneously differentiated into parthenogenetic mesenchymal stem cells (pMSCs), which were positive for mesenchymal stem cell surface markers and possessed osteogenic, chondrogenic, and adipogenic potential. Then, mechanical stretch was applied to improve the tenogenic differentiation of pMSCs, as indicated by the expression of tenogenic‐specific markers and an increasing COL1A1:3A1 ratio. The pSC‐derived tenocytes could proliferate and secrete extracellular matrix on the surface of poly(lactic‐co‐glycolic) acid scaffolds. Finally, engineered tendon‐like tissue was successfully generated after in vivo heterotopic implantation of a tenocyte‐scaffold composite. In conclusion, our experiment introduced an effective and practical strategy for applying pSCs for tendon regeneration. Stem Cells Translational Medicine2017;6:196–208

Effect of platelet mediator concentrate (PMC) on Achilles tenocytes: an in vitro study

Background

Although there are many studies discussing the etiological and pathological factors leading to both, acute and chronic tendon injuries, the pathophysiology of tendon injuries is still not clearly understood. Although most lesions are uncomplicated, treatment is long and unsatisfactory due to the poor vascularity of tendon tissue. Platelet mediator concentrate (PMC) contains many growth factors derived from platelets, which can promote wound healing. In this study we investigate the effects of PMC on tenocyte proliferation and differentiation in order to provide an experimental basis for tissue regeneration strategies and to develop new treatment concepts.

Methods

Using enzyme linked immunosorbent assay (ELISA) we were able to quantify the several growth factors and cytokines found in PMC. Tenocytes were isolated both from human and from mouse Achilles tendons and stimulated with PMC. CyQuant® and Cell Titer Blue® assays were carried out to analyze tendon growth and viability at different concentrations of PMC. Real time RT-PCR was used to analyze tenocyte gene expression with or without PMC treatment. Immunohistochemistry was carried out to detect the tenocyte-specific antibody tenomodulin (TNMD) and scleraxis (SCX).

Results

We were able to detect numerous mediators such as platelet derived growth factor BB (PDGF-BB), interleukin 6 (IL-6), vascular endothelial growth factor (VEGF), tumor necrosis factor (TNF-α), transforming growth factor beta 1 (TGF-ß1), and bone morphogenetic proteins 2, 4 and 7 (BMP-4, BMP-2, BMP-7) in PMC. It was possible to show a positive effect of PMC on human tendon cell growth and viability in a dose-dependent manner. Furthermore, PMC treatment led to induction of gene expression of scleraxis (SCX), type I collagen A 1 (Col1A1) and TNMD by tenocytes.

Conclusions

We suggest that the use of autologous PMC may be a suitable addition to conventional tendon therapy that is capable of increasing and optimizing tendon healing and reducing the risk of recurrence.

Expression of tenocyte lineage-related factors from tonsil-derived mesenchymal stem cells

Human palatine tonsil-derived mesenchymal stem cells (TMSCs) are known to be a new source of progenitor cells. Using waste tissue after tonsillectomy as a cell provider can be the biggest benefit of TMSCs, compared with other stem cells. The purpose of this study was to investigate tenogenic differentiation of TMSCs and to access the differential effects of transforming growth factor beta 3 (TGF-β3) on the tenogenesis of TMSCs. Human tonsil was obtained after tonsillectomy. Using a cytometric analysis, we were able to find that the TMSCs had typical mesenchymal stem cell markers: positive for CD73, CD90, and CD105, and negative for CD14, CD34, and CD45. Using TGF-β3, the expressions of tenocyte-specific genes and proteins, such as collagen type 1 (COL1), tenomodulin (TNMD), and scleraxis (SCX), were measured by a quantitative polymerase chain reaction (PCR), immunofluorescence staining, immunohistochemistry and Western blot analyses. Quantitative PCR assay showed that TGF-β3 significantly increased the expressions of tenocyte lineage marker genes, including COL1, TNMD, and SCX, at a 3-day treatment, compared with control. However, these increases were not found at long-term exposures (7 or 10 days), except that TNMD expression was maintained at 50 ng/mL at a 7-day exposure to TGF-β3. Like genes, the protein expression levels of COL1, TNMD, and SCX were also induced in TGF-β3-treated TMSCs in a 3-day treatment, which were maintained for 10 days, as evidenced by immunofluorescence staining, immunohistochemistry and Western blot analyses. This study demonstrated that TMSCs in tenogenic stimulation with TGF-β3 have a high tenogenic differentiation potential.

Treatment of Lateral Epicondylosis by Using Allogeneic Adipose-Derived Mesenchymal Stem Cells: A Pilot Study

Mesenchymal stem cell therapy is a novel regenerative approach for treating tendinopathy. Here, we evaluated the safety and efficacy of allogeneic adipose-derived mesenchymal stem cells (allo-ASC) in treating lateral epicondylosis (LE). Under ultrasound guidance, allo-ASCs mixed with fibrin glue were injected into the hypoechoic common extensor tendon lesions of 12 participants with chronic LE; 6 subjects each were administered 10(6) or 10(7) cells in 1 ml. Safety was evaluated at day 3 and weeks 2, 6, 12, 26, and 52 post-injection. Efficacy was assessed by measuring patients' visual analog scale (VAS) score for elbow pain, modified Mayo clinic performance index for the elbow, and by evaluating longitudinal and transverse ultrasound images of tendon defect areas after 6, 12, 26, and 52 weeks. No significant adverse effects of allo-ASC injection were observed through 52 weeks of follow-up. From baseline through 52 weeks of periodic follow-up, VAS scores progressively decreased from 66.8 ± 14.5 mm to 14.8 ± 13.1 mm and elbow performance scores improved from 64.0 ± 13.5 to 90.6 ± 5.8. Tendon defects also significantly decreased through this period. Allo-ASC therapy was thus safe and effective in improving elbow pain, performance, and structural defects for 52 weeks. This clinical study is the first to reveal therapeutic value of mesenchymal stem cell injection for treating chronic tendinopathy.

Harnessing endogenous stem/progenitor cells for tendon regeneration

Current stem cell-based strategies for tissue regeneration involve ex vivo manipulation of these cells to confer features of the desired progenitor population. Recently, the concept that endogenous stem/progenitor cells could be used for regenerating tissues has emerged as a promising approach that potentially overcomes the obstacles related to cell transplantation. Here we applied this strategy for the regeneration of injured tendons in a rat model. First, we identified a rare fraction of tendon cells that was positive for the known tendon stem cell marker CD146 and exhibited clonogenic capacity, as well as multilineage differentiation ability. These tendon-resident CD146+ stem/progenitor cells were selectively enriched by connective tissue growth factor delivery (CTGF delivery) in the early phase of tendon healing, followed by tenogenic differentiation in the later phase. The time-controlled proliferation and differentiation of CD146+ stem/progenitor cells by CTGF delivery successfully led to tendon regeneration with densely aligned collagen fibers, normal level of cellularity, and functional restoration. Using siRNA knockdown to evaluate factors involved in tendon generation, we demonstrated that the FAK/ERK1/2 signaling pathway regulates CTGF-induced proliferation and differentiation of CD146+ stem/progenitor cells. Together, our findings support the use of endogenous stem/progenitor cells as a strategy for tendon regeneration without cell transplantation and suggest this approach warrants exploration in other tissues.

Superparamagnetic iron oxide nanoparticles as a means to track mesenchymal stem cells in a large animal model of tendon injury

The goal of this study was to establish an SPIO-based cell-tracking method in an ovine model of tendonitis and to determine if this method may be useful for further study of cellular therapies in tendonitis in vivo. Functional assays were performed on labeled and unlabeled cells to ensure that no significant changes were induced by intracellular SPIOs. Following biosafety validation, tendon lesions were mechanically (n = 4) or chemically (n = 4) induced in four sheep and scanned ex vivo at 7 and 14 days to determine the presence and distribution of intralesional cells. Ovine MSCs labeled with 50 µg SPIOs/mL remained viable, proliferate, and undergo tri-lineage differentiation (p < 0.05). Labeled ovine MSCs remained detectable in vitro in concentrated cell numbers as low as 10 000 and in volumetric distributions as low as 100 000 cells/mL. Cells remained detectable by MRI at 7 days, as confirmed by correlative histology for dually labeled SPIO+/GFP+ cells. Histological evidence at 14 days suggested that SPIO particles remained embedded in tissue, providing MRI signal, although cells were no longer present. SPIO labeling has proven to be an effective method for cell tracking for a large animal model of tendon injury for up to 7 days post-injection. The data obtained in this study justify further investigation into the effects of MSC survival and migration on overall tendon healing and tissue regeneration.

Assessment of stem cell carriers for tendon tissue engineering in pre-clinical models

Tendon injuries are prevalent and problematic, especially among young and otherwise healthy individuals. The inherently slow innate healing process combined with the inevitable scar tissue formation compromise functional recovery, imposing the need for the development of therapeutic strategies. The limited number of low activity/reparative capacity tendon-resident cells has directed substantial research efforts towards the exploration of the therapeutic potential of various stem cells in tendon injuries and pathophysiologies. Severe injuries require the use of a stem cell carrier to enable cell localisation at the defect site. The present study describes advancements that injectable carriers, tissue grafts, anisotropically orientated biomaterials, and cell-sheets have achieved in preclinical models as stem cell carriers for tendon repair.

Progress in cell-based therapies for tendon repair

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

The past, present and future in scaffold-based tendon treatments

Tendon injuries represent a significant clinical burden on healthcare systems worldwide. As the human population ages and the life expectancy increases, tendon injuries will become more prevalent, especially among young individuals with long life ahead of them. Advancements in engineering, chemistry and biology have made available an array of three-dimensional scaffold-based intervention strategies, natural or synthetic in origin. Further, functionalisation strategies, based on biophysical, biochemical and biological cues, offer control over cellular functions; localisation and sustained release of therapeutics/biologics; and the ability to positively interact with the host to promote repair and regeneration. Herein, we critically discuss current therapies and emerging technologies that aim to transform tendon treatments in the years to come.

Reduction of tendon adhesions following administration of Adaprev, a hypertonic solution of mannose-6-phosphate: mechanism of action studies

Repaired tendons may be complicated by progressive fibrosis, causing adhesion formation or tendon softening leading to tendon rupture and subsequent reduced range of motion. There are few therapies available which improve the gliding of damaged tendons in the hand. We investigate the role of Mannose 6-phosphate (M6P) in a 600 mM hypertonic solution (Adaprev) on tendon adhesion formation in vivo using a mouse model of severed tendon in conjunction with analysis of collagen synthesis, cellular proliferation and receptors involved in TGF beta signalling. Cytotoxicity was assessed by measuring tissue residency, mechanical strength and cell viability of tendons after treatment with Adaprev. To elicit potential modes of action, in vitro and ex vivo studies were performed investigating phosphorylation of p38, cell migration and proliferation. Adaprev treatment significantly (p<0.05) reduced the development of adhesions and improved collagen organisation without reducing overall collagen synthesis following tendon injury in vivo. The bioavailability of Adaprev saw a 40% reduction at the site of administration over 45 minutes and tendon fibroblasts tolerated up to 120 minutes of exposure without significant loss of cell viability or tensile strength. These favourable effects were independent of CI-MPR and TGF-β signalling and possibly highlight a novel mechanism of action related to cellular stress demonstrated by phosphorylation of p38. The effect of treatment reduced tendon fibroblast migration and transiently halted tendon fibroblast proliferation in vitro and ex vivo. Our studies demonstrate that the primary mode of action for Adaprev is potentially via a physical, non-chemical, hyperosmotic effect.

DIFFERENCES BETWEEN PROTEIN EXPRESSION AND EXTRACELLULAR MATRIX STATE ON UNIAXIAL STRETCHING FOR TENOGENIC DIFFERENTIATION

There have been no previous reports of tendon tissue engineering using mesenchymal stem cells (MSCs) with regard to quantitative evaluation of protein expression levels and observation of derived extracellular matrix (ECM) state. Therefore, we approached tendon tissue engineering from both perspectives. Human bone marrow MSCs (hBMSCs) were subjected to 8% or 10% cyclic stretching at 1 Hz to promote differentiation into tenocytes and ECM production. The type I collagen (Col I) and Tenascin-C (Tnc) protein expression levels were evaluated quantitatively by enzyme-linked immunosorbent assay (ELISA). Confocal fluorescence microscopy was employed to observe the derived ECM state. Col I state derived from 10%-stretched cells as ECM was elongated like actual tendon ECM, although the quantitative protein expression levels were slightly higher in 8%-stretched cells. The results suggested that the optimal uniaxial stretching ratio was different between protein expression levels and derived ECM state. Therefore, it is important to pay attention to both protein expression levels and ECM state in tendon tissue engineering.

Preferential tendon stem cell response to growth factor supplementation

Tendon injuries are increasingly prevalent around the world, accounting for more than 100 000 new clinical cases/year in the USA alone. Cell-based therapies have been proposed as a therapeutic strategy, with recent data advocating the use of tendon stem cells (TSCs) as a potential cell source with clinical relevance for tendon regeneration. However, their in vitro expansion is problematic, as they lose their multipotency and change their protein expression profile in culture. Herein, we ventured to assess the influence of insulin-like growth factor 1 (IGF-1), growth and differentiation factor-5 (GDF-5) and transforming growth factor-β1 (TGFβ1) supplementation in TSC culture. IGF-1 preserved multipotency for up to 28 days. Upregulation of decorin and scleraxis expression was observed as compared to freshly isolated cells. GDF-5 treated cells exhibited reduced differentiation along adipogenic and chondrogenic pathways after 28 days, and decorin, scleraxis and collagen type I expression was increased. After 28 days, TGFβ1 supplementation led to increased scleraxis, osteonectin and collagen type II expression. The varied responses to each growth factor may reflect their role in tendon repair, suggesting that: GDF-5 promotes the transition of tendon stem cells towards tenocytes; TGFβ1 induces differentiation along several pathways, including a phenotype indicative of fibrocartilage or calcified tendon, common problems in tendon healing; and IGF-1 promotes proliferation and maintenance of TSC phenotypes, thereby creating a population sufficient to have a beneficial effect. Copyright © 2014 John Wiley & Sons, Ltd.

Evaluation of stem cell-to-tenocyte differentiation by atomic force microscopy to measure cellular elastic moduli

In the present study, we evaluated whether stem cell-to-tenocyte differentiation could be evaluated via measurement of the mechanical properties of the cell. We used mechanical uniaxial cyclic stretching to induce the differentiation of human bone marrow mesenchymal stem cells into tenocytes. The cells were subjected to cyclic elongation of 10 or 15 % at a cyclic frequency of 1 Hz for 24 or 48 h, and differentiation was assessed by real-time PCR (rtPCR) determination of messenger RNA expression levels for four commonly used markers of stem cell-to-tenocyte differentiation: type I collagen, type III collagen, tenascin-C, and scleraxis. The rtPCR results showed that cells subjected to 10 % cyclic elongation for 24 or 48 h differentiated into tenocytes. Atomic force microscopy (AFM) was then used to measure the force curves around the cell nuclei, and the AFM data were used to calculate the elastic moduli of the cell surfaces. The elastic modulus values of the control (non-stretched) cells differed significantly from those of cells stretched at 10 % for 24 or 48 h (P < 0.01). Confocal fluorescence microscopic observations of actin stress fibers suggested that the change in elastic modulus was ascribable to the development of the cellular cytoskeleton during the differentiation process. Therefore, we conclude that the atomic force microscopic measurement of the elastic modulus of the cell surface can be used to evaluate stem cell-to-tenocyte differentiation.

Development of a surgically optimized graft insertion suture technique to accommodate a tissue-engineered tendon in vivo

The traumatic rupture of tendons is a common clinical problem. Tendon repair is surgically challenging because the tendon often retracts, resulting in a gap between the torn end and its bony insertion. Tendon grafts are currently used to fill this deficit but are associated with potential complications relating to donor site morbidity and graft necrosis. We have developed a highly reproducible, rapid process technique to manufacture compressed cell-seeded type I collagen constructs to replace tendon grafts. However, the material properties of the engineered constructs are currently unsuitable to withstand complete load bearing in vivo. A modified suture technique has been developed to withstand physiological loading and off load the artificial construct while integration occurs. Lapine tendons were used ex vivo to test the strength of different suture techniques with different sizes of Prolene sutures and tissue-engineered collagen constructs in situ. The data were compared to standard modified Kessler suture using a standard tendon graft. Mechanical testing was carried out and a finite element analysis stress distribution model constructed using COMSOL 3.5 software. The break point for modified suture technique with a tissue-engineered scaffold was significantly higher (50.62 N) compared to a standard modified Kessler suture (12.49 N, p<0.05). Distributing suture tension further proximally and distally from the tendon ends increased the mechanical strength of the repairs. We now have ex vivo proof of concept that this suture technique is suitable for testing in vivo, and this will be the next stage of our research.

The application of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) scaffolds for tendon repair in the rat model

Tendon injuries and defects present a substantial burden to global healthcare economies. There are no synthetic/biosynthesised implants available which can restore full function or match the mechanical properties of native tendon. Therefore, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) was investigated for its utility as a scaffold in a rat Achilles tendon repair model. Porous PHBHHx tubes and fibres were prepared with particle leaching and extrusion methods, respectively. Collagen gels reinforced by polymer fibres were inserted into the lumen of scaffold tubes to create the operational scaffold unit. Mechanical testing demonstrated that PHBHHx scaffolds had comparable mechanical properties to rat tendon, with maximal loads of 23.73 ± 1.08 N, compared to 17.35 ± 1.76 N in undamaged rat Achilles tendon. Sprague-Dawley (SD) rats were split into four experimental groups: control, PHBHHx scaffold only, PHBHHx scaffold and collagen, PHBHHx scaffold, collagen and tenocyte compositions for implantation to repair an induced Achilles tendon defect. No secondary immune response to PHBHHx was observed over a 40 days period of implantation. Movement was restored in PHBHHx scaffold-collagen-tenocyte recipient rats at an earlier time point than in other experimental groups, with complete load-bearing and function returning 20 days post-surgery as determined by the Achilles Functional Index. In vitro testing of tendon constructs after 40 days demonstrated reductions in PHBHHx molecular weight and polydispersity index accompanied by an increase in mean chain length indicating degradation of smaller polymer chain subunits. Similarly a reduction in PHBHHx tube ultimate tensile strength and elastic modulus was observed. Histological analysis provided evidence of tissue remodelling and cell alignment. In summary, PHBHHx scaffolds have been successfully applied in an in vivo tendon repair model raising promise for future utility in tissue engineering applications.

Poly (3-hydroxybutyrate-co-3-hydroxyhexanoate)/collagen hybrid scaffolds for tissue engineering applications

The benefits associated with polyhydroxyalkanoates (PHA) in tissue engineering include high immunotolerance, low toxicity, and biodegradability. Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), a molecule from the PHA family of biopolymers, shares these features. In this study, the applicability of human embryonic stem cells (hESCs), spontaneously differentiated hESCs (SDhESCs), and mesenchymal stem cells (hMSCs) in conjunction with PHBHHx and collagen as a biocompatible replacement strategy for damaged tissues was exploited. Collagen gel contraction was monitored by seeding cells at controlled densities (0, 10(3), 10(4), and 10(5) cells/mL) and measuring length and diameter at regular time intervals thereafter when cultured in a complete medium. Cell viability was measured by trypan blue exclusion assay. Porous PHBHHx tube scaffolds were prepared using a dipping method followed by salt leaching. PHBHHx/collagen composites were generated via syringe injection of collagen/cell mixtures into sterile PHBHHx porous tubes. Reverse transcription polymerase chain reaction was used to determine the fate of cells within PHBHHx/collagen scaffolds with tendon, bone, cartilage, and fat-linked transcript expression being explored at days 0, 5 10, and 20. The capacity of PHBHHx/collagen scaffolds to support differentiation was explored using a medium specific for osteogenic, chondrogenic, and adipogenic lineage generation. Collagen gel tube contraction required initial seeding densities of ≥10(5) hMSCs or SDhESCs in 1.5 mg/mL collagen gel tubes. Gels with a collagen concentration of 3 mg/mL did not display contraction across the examined cell seeding densities. Cell viability was ∼50% for SDhESC and 90% for hMSCs at all cell densities tested in porous PHBHHx tube/3 mg/mL collagen hybrid scaffolds after 20 days in vitro culture. Undifferentiated hESCs did not contract collagen gel tubes and were unviable after 20 days culture. In the absence of additional stimuli, SOX9 was sporadically found, while RUNX2 was not present in both hMSC and SDhESC. Hybrid scaffolds were shown to promote retention of osteogenic, chondrogenic, and adipogenic differentiation by expression of RUNX2, SOX9, and PPARγ genes, respectively, following exposure to the appropriate induction medium. PHBHHx/collagen scaffolds have been successfully used to culture hMSC and SDhESC over an extended period supporting the potential of this scaffold combination in future tissue engineering applications.

Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) supports adhesion and migration of mesenchymal stem cells and tenocytes

AIM

To establish the potential of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) as a material for tendon repair.

METHODS

The biocompatibility of PHBHHx with both rat tenocytes (rT) and human mesenchymal stem cells (hMSC) was explored by monitoring adhesive characteristics on films of varying weight/volume ratios coupled to a culture atmosphere of either 21% O(2) (air) or 2% O(2) (physiological normoxia). The diameter and stiffness of PHBHHx films was established using optical coherence tomography and mechanical testing, respectively.

RESULTS

Film thickness correlated directly with weight/volume PHBHHx (r(2) = 0.9473) ranging from 0.1 mm (0.8% weight/volume) to 0.19 mm (2.4% weight/volume). Film stiffness on the other hand displayed a biphasic response which increased rapidly at values > 1.6% weight/volume. Optimal cell attachment of rT required films of ≥ 1.6% and ≥ 2.0% weight/volume PHBHHx in 2% O(2) and 21% O(2) respectively. A qualitative adhesion increase was noted for hMSC in films ≥ 1.2% weight/volume, becoming significant at 2% weight/volume in 2% O(2). An increase in cell adhesion was also noted with ≥ 2% weight/volume PHBHHx in 21% O(2). Cell migration into films was not observed.

CONCLUSION

This evaluation demonstrates that PHBHHx is a suitable polymer for future cell/polymer replacement strategies in tendon repair.

Effect of growth differentiation factor 5 on the proliferation and tenogenic differentiation potential of human mesenchymal stem cells in vitro

The use of growth differentiation factor 5 (GDF-5) in damaged tendons has been shown to improve tendon repair. It has been hypothesized that further improvements may be achieved when GDF-5 is used to promote cell proliferation and induce tenogenic differentiation in human bone marrow-derived mesenchymal stem cells (hMSCs). However, the optimal conditions required to produce these effects on hMSCs have not been demonstrated in previous studies. A study to determine cell proliferation and tenogenic differentiation in hMSCs exposed to different concentrations of GDF-5 (0, 5, 25, 50, 100 and 500 ng/ml) was thus conducted. No significant changes were observed in the cell proliferation rate in hMSCs treated at different concentrations of GDF-5. GDF-5 appeared to induce tenogenic differentiation at 100 ng/ml, as reflected by (1) a significant increase in total collagen expression, similar to that of the primary native human tenocyte culture; (2) a significant upregulation in candidate tenogenic marker gene expression, i.e. scleraxis, tenascin-C and type-I collagen; (3) the ratio of type-I collagen to type-III collagen expression was elevated to levels similar to that of human tenocyte cultures, and (4) a significant downregulation of the non-tenogenic marker genes runt-related transcription factor 2 and sex determining region Y (SRY)-box 9 at day 7 of GDF-5 induction, further excluding hMSC differentiation into other lineages. In conclusion, GDF-5 does not alter the proliferation rates of hMSCs, but, instead, induces an optimal tenogenic differentiation response at 100 ng/ml.

Paracrine interactions between mesenchymal stem cells affect substrate driven differentiation toward tendon and bone phenotypes

We investigated substrate dependent paracrine signaling between subpopulations of bone marrow stromal cells (BMSCs) that may affect the formation, or perhaps malformation, of the regenerating tendon to bone enthesis. Polyacrylamide substrates approximating the elastic modulus of tendon granulation tissue and the osteoid of healing bone (10–90 kPa) were functionalized with whole length fibronectin (Fn), type-I collagen (Col), or a mixed ligand solution (Fn/Col), and BMSCs were cultured in growth media alone or media supplemented with soluble Col or Fn. More rigid substrates with a narrow mechanical gradient (70–90 kPa) robustly induced osteogenic cell differentiation when functionalized with either Col or Fn. On broader mechanical gradient substrates (with a linear elastic modulus gradient from 10–90 kPa), cell differentiation was markedly osteogenic on subregions of Fn functionalized substrates above 20 kPa, but osteogenic activity was inhibited on all subregions of Col substrates. Osteogenic behavior was not observed when cells were cultured on Fn substrates if Col was present either in the media or on the substrate (Fn/Col). Tenogenic differentiation markers were observed only on Col substrates with moderate rigidity (∼30–50 kPa). Tenogenic differentiation was unaltered by soluble or substrate bound Fn. Co-culture of narrow gradient subsections revealed that any inclusion of tenogenic substrates (30–50 kPa, Col), caused otherwise osteogenic substrates to not develop markers of osteogenic differentiation, while increasing cell proliferation. These apparently paracrine effects could be mediated by bone morphogenetic protein-2 (BMP-2), as first confirmed by gene-level expression of BMP-2 and the transcription factor Smad8, and verified by BMP-2 media supplementation at levels similar to observed cell-secreted concentrations, which arrested osteogenic differentiation in 14 day cultures. Thus, cell instructive biomaterials with engineered mechanical and biochemical properties represent potentially powerful tools for directing BMSC differentiation to tendon and bone, however paracrine signals from tenogenic cells may delay osteogenesis at the healing enthesis.

Stem cell applications in tendon disorders: a clinical perspective

Tendon injuries are a common cause of morbidity and a significant health burden on society. Tendons are structural tissues connecting muscle to bone and are prone to tearing and tendinopathy, an overuse or degenerative condition that is characterized by failed healing and cellular depletion. Current treatments, for tendon tear are conservative, surgical repair or surgical scaffold reconstruction. Tendinopathy is treated by exercises, injection therapies, shock wave treatments or surgical tendon debridement. However, tendons usually heal with fibrosis and scar tissue, which has suboptimal tensile strength and is prone to reinjury, resulting in lifestyle changes with activity restriction. Preclinical studies show that cell therapies have the potential to regenerate rather than repair tendon tissue, a process termed tenogenesis. A number of different cell lines, with varying degrees of differentiation, have being evaluated including stem cells, tendon derived cells and dermal fibroblasts. Even though cellular therapies offer some potential in treating tendon disorders, there have been few published clinical trials to determine the ideal cell source, the number of cells to administer, or the optimal bioscaffold for clinical use.

Bupivacaine, ropivacaine, and morphine: comparison of toxicity on human hamstring-derived stem/progenitor cells

PURPOSE

Bupivacaine, ropivacaine, and morphine are commonly administered intraarticularly after anterior cruciate ligament (ACL) reconstruction. However, their effects on human tendon stem/progenitor cells (TSPC) have not been studied. Therefore, this study investigates the cytotoxicity of these analgetics on TSPC.

METHODS

Cells were isolated from human hamstring grafts of 3 female (age 15, 16 and 59) and 2 male patients (age 16 and 47). Cells were incubated using 0.5% bupivacaine, 0.5/0.75% ropivacaine, and 0.025% morphine. Cell viability was assessed after 0.5, 2, and 6 h using live/dead assay. Metabolic activity and apoptosis were measured by WST- and Annexin-V-FACS-assay after 2 h.

RESULTS

Cell viability remained unchanged after 0.5 h in all groups, while treatment with bupivacaine and 0.5/0.75% ropivacaine resulted in a complete cell loss after 6 h. Contrarily, morphine showed no cytotoxic effect. Cell viability and metabolism were significantly reduced after treatment with bupivacaine (22.1; 8.3%) and 0.75% ropivacaine (56.5; 23.8%), while 0.5% ropivacaine and morphine showed no significant difference compared with controls. Apoptosis was significantly induced after incubation with bupivacaine (58.1%) and 0.75% ropivacaine (26.2%), whereas 0.5% ropivacaine only led to a slight induction compared with morphine and controls.

CONCLUSIONS

Clinically administered concentrations of bupivacaine (0.5%) and ropivacaine (0.75%) have a significant cytotoxic effect on human TSPC in vitro, while ropivacaine in a concentration of 0.5% has a mild but not significant effect on apoptosis and cell metabolism. In contrast, morphine does not affect cell survival, metabolism, or apoptosis. Knowing that morphine provides comparable to even prolonged pain reduction after ACL reconstruction, the presented in vitro study suggests morphine as a potentially less toxic analgetic drug for intraarticular application in clinical practice.

Tissue engineered biological augmentation for tendon healing: a systematic review

INTRODUCTION

Tendon injuries give rise to significant morbidity. In the last few decades, several techniques have been increasingly used to optimize tendon healing.

SOURCES OF DATA

We performed a comprehensive search of PubMed, Medline, Cochrane, CINAHL and Embase databases using various combinations of the commercial names of each scaffold and the keywords 'tendon', 'rotator cuff', 'supraspinatus tendon', 'Achilles tendon', 'growth factors', 'cytokines', 'gene therapy', 'tissue engineering', 'mesenchymal' and 'stem cells' over the years 1966-2009. All articles relevant to the subject were retrieved, and their bibliographies were hand searched for further references in the context to tissue-engineered biological augmentation for tendon healing.

AREAS OF AGREEMENT

Several new techniques are available for tissue-engineered biological augmentation for tendon healing, growth factors, gene therapy and mesenchimal stem cells.

AREAS OF CONTROVERSY

Data are lacking to allow definitive conclusions on the use of these techniques for routine management of tendon ailments.

GROWING POINTS

The emerging field of tissue engineering holds the promise to use new techniques for tendon augmentation and repair. Preliminary studies support the idea that these techniques can provide an alternative for tendon augmentation with great therapeutic potential.

AREAS TIMELY FOR DEVELOPING RESEARCH

The optimization strategies discussed in this article are currently at an early stage of development. Although these emerging technologies may develop into substantial clinical treatment options, their full impact needs to be critically evaluated in a scientific fashion.

Biochemical and biomechanical gradients for directed bone marrow stromal cell differentiation toward tendon and bone

Substrates with mechanical property gradients and various extracellular matrix ligand loadings were evaluated for their ability to direct bone marrow stromal cell differentiation along osteogenic and tenogenic lineages. After verifying reproducible mechanical compliance characteristics of commercial hydrogel gradient substrates, substrates were functionalized with whole length fibronectin or collagen, both of which are found in skeletal structures and are relevant to cell-matrix signalling. Bone marrow stromal cells were seeded onto the substrates in growth media and cultured first to examine cell attachment and morphology, indicating higher levels of attachment on collagen substrates after 1h, and increased spreading and organization trends after 24h. Differentiation studies showed an increase in osteoblast differentiation on fibronectin substrates while collagen substrates lacked osteogenic differentiation. Osteogenic differentiation decreased on substrates of lower stiffness and lower ligand density. Molecular investigations revealed an increase in relevant signalling molecules for osteoblasts that were consistent with differentiation studies, but detected the presence of tenoblast markers on collagen substrates within a narrow range of stiffness. Our results indicate that mechanovariant substrates do hold promise as a culture platform for directed differentiation to tendon and bone by altering gene level expression of relevant signalling molecules. This study aids in understanding the molecular mechanisms that drive differentiation from substrate based cues, and could aid the design of therapeutic biomaterials at the transition from tendon to bone.

Pathogenesis of tendinopathies: inflammation or degeneration?

The intrinsic pathogenetic mechanisms of tendinopathies are largely unknown and whether inflammation or degeneration has the prominent role is still a matter of debate. Assuming that there is a continuum from physiology to pathology, overuse may be considered as the initial disease factor; in this context, microruptures of tendon fibers occur and several molecules are expressed, some of which promote the healing process, while others, including inflammatory cytokines, act as disease mediators. Neural in-growth that accompanies the neovessels explains the occurrence of pain and triggers neurogenic-mediated inflammation. It is conceivable that inflammation and degeneration are not mutually exclusive, but work together in the pathogenesis of tendinopathies.