TGF-β1 Suppresses Plasmin and MMP Activity in Flexor Tendon Cells via PAI-1: Implications for Scarless Flexor Tendon Repair

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.

Study of the Synergistic Immunomodulatory and Antifibrotic Effects of Dual-Loaded Budesonide and Serpine1 siRNA Lipid-Polymer Nanoparticles Targeting Macrophage Dysregulation in Tendinopathy

Musculoskeletal diseases involving tissue injury comprise tendon, ligament, and muscle injury. Recently, macrophages have been identified as key players in the tendon repair process, but no therapeutic strategy involving dual drug delivery and gene delivery to macrophages has been developed for targeting the two main dysregulated aspects of macrophages in tendinopathy, i.e., inflammation and fibrosis. Herein, the anti-inflammatory and antifibrotic effects of dual-loaded budesonide and serpine1 siRNA lipid-polymer hybrid nanoparticles (LPNs) are evaluated in murine and human macrophage cells. The modulation of the gene and protein expression of factors associated with inflammation and fibrosis in tendinopathy is demonstrated by real time polymerase chain reaction and Western blot. Macrophage polarization to the M2 phenotype and a decrease in the production of pro-inflammatory cytokines are confirmed in macrophage cell lines and primary cells. The increase in the activity of a matrix metalloproteinase involved in tissue remodelling is proven, and studies evaluating the interactions of LPNs with T cells proved that dual-loaded LPNs act specifically on macrophages and do not induce any collateral effects on T cells. Overall, these dual-loaded LPNs are a promising combinatorial therapeutic strategy with immunomodulatory and antifibrotic effects in dysregulated macrophages in the context of tendinopathy.

Targeting the TGF-β signaling pathway: an updated patent review (2021-present)

INTRODUCTION

The TGF-β signaling pathway is a complex network that plays a crucial role in regulating essential biological functions and is implicated in the onset and progression of multiple diseases. This review highlights the recent advancements in developing inhibitors targeting the TGF-β signaling pathway and their potential therapeutic applications in various diseases.

AREA COVERED

The review discusses patents on active molecules related to the TGF-β signaling pathway, focusing on three strategies: TGF-β activity inhibition, blocking TGF-β receptor binding, and disruption of the signaling pathway using small molecule inhibitors. Combination therapies and the development of fusion proteins targeting multiple pathways are also explored. The literature search was conducted using the Cortellis Drug Discovery Intelligence database, covering patents from 2021 onwards.

EXPERT OPINION

The development of drugs targeting the TGF-β signaling pathway has made significant progress in recent years. However, addressing challenges such as specificity, systemic toxicity, and patient selection is crucial for their successful clinical application. Targeting the TGF-β signaling pathway holds promise as a promising approach for the treatment of various diseases.

The mechanisms and functions of TGF-β1 in tendon healing

Tendon injury accounts for 30% of musculoskeletal diseases and often leads to disability, pain, healthcare cost, and lost productivity. Following injury to tendon, tendon healing proceeds via three overlapping healing processes. However, due to the structural defects of the tendon itself, the tendon healing process is characterized by the formation of excessive fibrotic scar tissue, and injured tendons rarely return to native tendons, which can easily contribute to tendon reinjury. Moreover, the resulting fibrous scar is considered to be a precipitating factor for subsequent degenerative tendinopathy. Despite this, therapies are almost limited because underlying molecular mechanisms during tendon healing are still unknown. Transforming Growth Factor-β1 (TGF-β1) is known as one of most potent profibrogenic factors during tendon healing process. However, blockage TGF-β1 fails to effectively enhance tendon healing. A detailed understanding of real abilities of TGF-β1 involved in tendon healing can bring promising perspectives for therapeutic value that improve the tendon healing process. Thus, in this review, we describe recent efforts to identify and characterize the roles and mechanisms of TGF-β1 involved at each stage of the tendon healing and highlight potential roles of TGF-β1 leading to the fibrotic response to tendon injury.

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.

Targeting TGF-β signal transduction for fibrosis and cancer therapy

Transforming growth factor β (TGF-β) has long been identified with its intensive involvement in early embryonic development and organogenesis, immune supervision, tissue repair, and adult homeostasis. The role of TGF-β in fibrosis and cancer is complex and sometimes even contradictory, exhibiting either inhibitory or promoting effects depending on the stage of the disease. Under pathological conditions, overexpressed TGF-β causes epithelial-mesenchymal transition (EMT), extracellular matrix (ECM) deposition, cancer-associated fibroblast (CAF) formation, which leads to fibrotic disease, and cancer. Given the critical role of TGF-β and its downstream molecules in the progression of fibrosis and cancers, therapeutics targeting TGF-β signaling appears to be a promising strategy. However, due to potential systemic cytotoxicity, the development of TGF-β therapeutics has lagged. In this review, we summarized the biological process of TGF-β, with its dual role in fibrosis and tumorigenesis, and the clinical application of TGF-β-targeting therapies.

Three-Dimensional Tendon Scaffold Loaded with TGF-β1 Gene Silencing Plasmid Prevents Tendon Adhesion and Promotes Tendon Repair

Tendon adhesion formation is associated with the aberrant expression of many genes, and interfering with the expression of these genes can prevent adhesion and promote tendon repair. Recent studies have found that silencing the transforming growth factor β-1 (TGF-β1) gene can reduce the occurrence of tendon adhesions. The development of tissue engineering and three-dimensional (3D) printing technology have provided new solutions for tendon repair. In this study, TGF-β1 gene silencing microRNA (miRNA) based RNAi plasmid was loaded on a 3D tendon scaffold using 3D printing technology. In vitro experiments confirmed the sustained release of plasmid and the good biocompatibility of the printed tendon scaffold. Subsequently, the TGF-β1 gene silencing plasmid loaded tendon scaffold was implanted in a chicken tendon defect model to evaluate the effect of the scaffold in vivo. The results from biomechanical tests and histological examinations showed that the scaffold not only promoted tendon regeneration but also prevented tendon adhesion, which was conducive to the recovery of biofunction. Evaluation of protein expression showed that the loaded plasmids prevented tendon adhesion and promoted tendon functional repair via silencing of the TGF-β1 gene.

Mesenchymal Stem Cells Seeded Decellularized Tendon Scaffold for Tissue Engineering

Tendon is a collagenous tissue to connect bone and muscle. Healing of damaged/injured tendon is the primary clinical challenge in musculoskeletal regeneration because they often react poorly to treatment. Tissue engineering (a triad strategy of scaffolds, cells and growth factors) may have the potential to improve the quality of tendon tissue healing under such impaired situations. Tendon tissue engineering aims to synthesize graft alternatives to repair the injured tendon. Biological scaffolds derived from decellularized tissue may be a better option as their biomechanical properties are similar to the native tissue. This review is designed to provide background information on the current challenges in curing torn/worn out the tendon and the clinical relevance of decellularized scaffolds for such applications.

The effect of intravenous human adipose-derived stem cells (hADSC) on transforming growth factor β1 (TGF-β1), collagen type 1, and kidney histopathological features in the unilateral ureteropelvic junction obstruction model of wistar rats

OBJECTIVE

The fibrotic process of kidney resulting in glomerulosclerosis was found in patients with ureteropelvic junction obstruction (UPJO) who underwent renal biopsy during pyeloplasty. Transforming growth factor β1 (TGF-β1) plays a role in collagen accumulation, resulting in fibrosis. Adipose tissue-derived stem cells (ADSCs) have an anti-apoptotic effect on target cells and enhance the kidney function recovery. We will further investigate the use of ADSC in the prevention of kidney fibrosis in the unilateral UPJO model of Wistar rats.

MATERIAL AND METHODS

A total of twenty-two 12-week-old Wistar rats were divided into three groups. We made the UPJO models using nylon 6-0 inside the left ureter and tied the ureter with nylon 6-0, creating partial ureteral obstruction. The treatment group was then injected with 1.0 × 10⁶ cells of human ADSC via the tail vein of rats. All rats were euthanized after 2 and 4 weeks of treatment. The left kidney used hematoxylin-eosin for histopathological examination. Statistical analysis using one-way analysis of variance (ANOVA) was done with SPSS version 21.0.

RESULTS

TGF-β1 concentration in the treatment group was significantly lower in the 4^th week of observation (p4=0.0001), as well as collagen type 1, which was also significantly lower in the 4^th week (p4=0.0001). There was a significant difference in the glomerulus count between the control group and the human ADSC (hADSC) group therapy in week 2 and week 4 (p2=0.0001 and p4=0.026).

CONCLUSION

Administration of hADSC therapy reduces TGF-β1 and collagen type 1 levels and then improves the histopathological features in the process of renal fibrosis in the UPJO model.

Transcriptomic Analysis of Cellular Pathways in Healing Flexor Tendons of Plasminogen Activator Inhibitor 1 (PAI-1/Serpine1) Null Mice

Injuries to flexor tendons can be complicated by fibrotic adhesions, which severely impair the function of the hand. Plasminogen activator inhibitor 1 (PAI-1/SERPINE1), a master suppressor of fibrinolysis and protease activity, is associated with adhesions. Here, we used next-generation RNA sequencing (RNA-Seq) to assess genome-wide differences in messenger RNA expression due to PAI-1 deficiency after zone II flexor tendon injury. We used the ingenuity pathway analysis to characterize molecular pathways and biological drivers associated with differentially expressed genes (DEG). Analysis of hundreds of overlapping and DEG in PAI-1 knockout (KO) and wild-type mice (C57Bl/6J) during tendon healing revealed common and distinct biological processes. Pathway analysis identified cell proliferation, survival, and senescence, as well as chronic inflammation as potential drivers of fibrotic healing and adhesions in injured tendons. Importantly, we identified the activation of PTEN signaling and the inhibition of FOXO1-associated biological processes as unique transcriptional signatures of the healing tendon in the PAI-1/Serpine1 KO mice. Further, transcriptomic differences due to the genetic deletion of PAI-1 were mechanistically linked to PI3K/Akt/mTOR, PKC, and MAPK signaling cascades. These transcriptional observations provide novel insights into the biological roles of PAI-1 in tendon healing and could identify therapeutic targets to achieve scar-free regenerative healing of tendons. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:43-58, 2020.

Tendon stem progenitor cells: Understanding the biology to inform therapeutic strategies for tendon repair

Tendon and ligament injuries are a leading cause of healthcare visits with significant impact in terms of economic cost and reduced quality of life. To date, reparative strategies remain largely restricted to conservative treatment or surgical repair. However, these therapies fail to restore native tendon structure and function; thus, the tissue may re-rupture or degenerate with time. To improve tendon healing, one promising strategy may be harnessing the innate potential of resident tendon stem/progenitor cells (TSPCs) to guide tenogenic regeneration. In this review, we outline recent advances in the identification and characterization of putative TSPC populations, and discuss biochemical, biomechanical, and biomaterial methods employed for their culture and differentiation. Finally, we identify limitations in our current understanding of TSPC biology, key challenges for their use, and potential therapeutic strategies to inform cell-based tendon repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1270-1280, 2019.

Evaluation of two distinct placental-derived membranes and their effect on tenocyte responses in vitro

Tendon healing is a complex, multiphase process that results in increased scar tissue formation, leading to weaker tendons. The purpose of this study was to evaluate the response of tenocytes to both hypothermically stored amniotic membrane (HSAM) and dehydrated amnion/chorion membrane (dACM). Composition and growth factor release from HSAM and dACM were evaluated using proteomics microarrays. HSAM and dACM releasate was used to assess tenocyte proliferation, migration, gene expression, extracellular matrix (ECM) protein deposition, and response to inflammation. Additionally, tenocyte-ECM interactions were evaluated. HSAM and dACM contain and release growth factors relevant to tendon healing, including insulin-like growth factor I, platelet-derived growth factor, and basic fibroblast growth factor. Both dACM and HSAM promoted increased tenocyte proliferation and migration; tenocytes treated with dACM proliferated more robustly, whereas treatment with HSAM resulted in higher migration. Both dACM and HSAM resulted in altered ECM gene expression; dACM grafts alone resulted in increases in collagen deposition. Furthermore, both allografts resulted in altered tenocyte responses to inflammation with reduced transforming growth factor beta levels. Additionally, dACM treatment resulted in increased expression and production of matrix metalloprotease-1 (MMP-1), whereas HSAM treatment resulted in decreased production of MMP-1. Tenocytes migrated into and remodeled HSAM only. These results indicate that both grafts have properties that support tendon healing; however, the results presented here suggest that the responses to each type of graft may be different. Due to the complex environment during tendon repair, additional work is needed to evaluate these effects using in vivo models.

Construction of tendon replacement tissue based on collagen sponge and mesenchymal stem cells by coupled mechano-chemical induction and evaluation of its tendon repair abilities

Tissue engineering is an ideal therapeutic strategy for the development of functional tendon replacement tissue for tendon repair in the clinic. Currently, the synergistic roles of mechano-chemical factors and the mechanisms involved in tendon repair and regeneration are not fully understood. In this study, we developed a three-dimensional (3D) culture system based on a silicone chamber and collagen sponge scaffold that can deliver cyclic mechanical stretch and biochemical stimulation to bone marrow-derived mesenchymal stem cells (BMSCs) seeded on the scaffold. We found that the combined stimulation of cyclic stretch and transforming growth factor beta 1 (TGF-β1) treatment not only increased cell viability but also synergistically promoted the differentiation of BMSCs into tenocytes in a 3D culture environment. Meanwhile, the combined stimulation increased the Young's modulus of the BMSC-collagen sponge constructs by reducing the porosity of the scaffold compared to the non-treated constructs. Furthermore, a rat Achilles tendon in situ repair experiment showed that enhanced tendon regeneration was achieved using the BMSC-collagen sponge construct combined with cyclic stretch and TGF-β1, as confirmed by Achilles functional index (AFI) measurement, morphological observation, histological analysis, and mechanical testing. These results suggest that this approach could offer a practical benefit in tendon healing and future tendon tissue engineering.

STATEMENT OF SIGNIFICANCE

This study aims to disclose the crucial roles of the coupled induction by mechano-chemical stimulation in tendon tissue engineering and clarifies their collaborative control mechanisms. We developed a three-dimensional (3D) culture system based on a silicone chamber and collagen sponge scaffold that could deliver cyclic mechanical stretch and biochemical stimulation to bone marrow-derived mesenchymal stem cells (BMSCs). We found that the combined stimulation of cyclic stretch and transforming growth factor beta 1 (TGF-β1) could result in an improvement of tissue-engineered construct for enhancing tendon healing. These results suggest that this approach could offer a practical benefit in tendon healing and future tendon tissue engineering.

Serpine1 Knockdown Enhances MMP Activity after Flexor Tendon Injury in Mice: Implications for Adhesions Therapy

Injuries to flexor tendons can be complicated by fibrotic adhesions, which severely impair the function of the hand. Adhesions have been associated with TGF-β1, which causes upregulation of PAI-1, a master suppressor of protease activity, including matrix metalloproteinases (MMP). In the present study, the effects of inhibiting PAI-1 in murine zone II flexor tendon injury were evaluated utilizing knockout (KO) mice and local nanoparticle-mediated siRNA delivery. In the PAI-1 KO murine model, reduced adherence of injured tendon to surrounding subcutaneous tissue and accelerated recovery of normal biomechanical properties compared to wild type controls were observed. Furthermore, MMP activity was significantly increased in the injured tendons of the PAI-1 KO mice, which could explain their reduced adhesions and accelerated remodeling. These data demonstrate that PAI-1 mediates fibrotic adhesions in injured flexor tendons by suppressing MMP activity. In vitro siRNA delivery to silence Serpine1 expression after treatment with TGF-β1 increased MMP activity. Nanoparticle-mediated delivery of siRNA targeting Serpine1 in injured flexor tendons significantly reduced target gene expression and subsequently increased MMP activity. Collectively, the data demonstrate that PAI-1 can be a druggable target for treating adhesions and accelerating the remodeling of flexor tendon injuries.

Growth factor delivery strategies for rotator cuff repair and regeneration

The high incidence of degenerative tears and prevalence of retears (20-95%) after surgical repair makes rotator cuff injuries a significant health problem. This high retear rate is attributed to the failure of the repaired tissue to regenerate the native tendon-to-bone insertion (enthesis). Biological augmentation of surgical repair such as autografts, allografts, and xenografts are confounded by donor site morbidity, immunogenicity, and disease transmission, respectively. In contrast, these risks may be alleviated via growth factor therapy, which can actively influence the healing environment to promote functional repair. Several challenges have to be overcome before growth factor delivery can translate into clinical practice such as the selection of optimal growth factor(s) or combination, identification of the most efficient stage and duration of delivery, and the design considerations for the delivery device. Emerging insight into the injury-repair microenvironment and our understanding of growth factor mechanisms in healing are informing the design of advanced delivery scaffolds to effectively treat rotator cuff tears. Here, we review potential growth factor candidates, design parameters and material selection for growth factor delivery, innovative and dynamic delivery scaffolds, and novel therapeutic targets from tendon and developmental biology for the structural and functional healing of rotator cuff repair.

[In vitrodifferentiation of human amniotic mesenchymal stem cells into ligament fibroblasts after induced by transforming growth factor β 1 and vascular endothelial growth factor]

Objective

To investigate whether human amniotic mesenchymal stem cells (hAMSCs) have the characteristics of mesenchymal stem cells (MSCs) and the differentiation capacity into ligament fibroblasts in vitro.

Methods

The hAMSCs were separated through trypsin and collagenase digestion from placenta, the phenotypic characteristics of hAMSCs were detected by flow cytometry, the cytokeratin-19 (CK-19) and vimentin expression of hAMSCs were tested through immunofluorescence staining. The hAMSCs at the 3rd passage were cultured with L-DMEM/F12 medium containing transforming growth factor β ₁ (TGF-β ₁) and vascular endothelial growth factor (VEGF) as the experimental group and with single L-DMEM/F12 medium as the control group. The morphology of hAMSCs was observed by inverted phase contrast microscope; the cellular activities and ability of proliferation were examined by cell counting kit-8 (CCK-8) method; the ligament fibroblasts related protein expressions including collagen type I, collagen type III, Fibronectin, and Tenascin-C were detected by immunofluorescence staining; specific mRNA expressions of ligament fibroblasts and angiogenesis including collagen type I, collagen type III, Fibronectin, α-smooth muscle actin (α-SMA), and VEGF were measured by real-time fluorescence quantitative PCR.

Results

The hAMSCs presented monolayer and adherent growth under inverted phase contrast microscope; the flow cytometry results demonstrated that hAMSCs expressed the MSCs phenotypes; the immunofluorescence staining results indicated the hAMSCs had high expression of the vimentin and low expression of CK-19; the hAMSCs possessed the differentiation ability into the osteoblasts, chondroblasts, and lipoblasts. The CCK-8 results displayed that cells reached the peak of growth curve at 7 days in each group, and the proliferation ability in the experimental group was significantly higher than that in the control group at 7 days ( P<0.05). The immunofluorescence staining results showed that the expressions of collagen type I, collagen type III, Fibronectin, and Tenascin-C in the experimental group were significantly higher than those in the control group at 5, 10, and15 days after culture ( P<0.05). The real-time fluorescence quantitative PCR results revealed that the mRNA relative expressions had an increasing tendency at varying degrees with time in the experimental group ( P<0.05). The relative mRNA expressions of collagen type I, collagen type III, Fibronectin, α-SMA, and VEGF in the experimental group were significantly higher than those in the control group at the other time points ( P<0.05), but no significant difference was found in the relative mRNA expressions of collagen type I, collagen type III, and VEGF between 2 groups at 5 days ( P>0.05).

Conclusion

The hAMSCs possesses the characteristics of MSCs and good proliferation ability which could be chosen as seed cell source in tissue engineering. The expressions of ligament fibroblasts and angiogenesis related genes could be up-regulated, after induction in vitro, and the synthesis of ligament fibroblasts related proteins could be strengthened. In addition, the application of TGF-β ₁ and VEGF could be used as growth factors sources in constructing tissue engineered ligament.

The Biology of Bone and Ligament Healing

This review describes the normal healing process for bone, ligaments, and tendons, including primary and secondary healing as well as bone-to-bone fusion. It depicts the important mediators and cell types involved in the inflammatory, reparative, and remodeling stages of each healing process. It also describes the main challenges for clinicians when trying to repair bone, ligaments, and tendons with a specific emphasis on Charcot neuropathy, fifth metatarsal fractures, arthrodesis, and tendon sheath and adhesions. Current treatment options and research areas are also reviewed.

Cell and Biologic-Based Treatment of Flexor Tendon Injuries

The two primary factors leading to poor clinical results after intrasynovial tendon repair are adhesion formation within the digital sheath and repair-site elongation and rupture. As the outcomes following modern tendon multi-strand repair and controlled rehabilitation techniques are often unsatisfactory, alternative approaches, such as the application of growth factors and mesenchymal stem cells (MSCs), have become increasingly attractive treatment options. Successful biological therapies require carefully controlled spatiotemporal delivery of cells, growth factors, and biocompatible scaffold matrices in order to simultaneously (1) promote matrix synthesis at the tendon repair site leading to increased biomechanical strength and stiffness and (2) suppress matrix synthesis along the tendon surface and synovial sheath preventing adhesion formation. This review summarizes recent cell and biologic-based experimental treatments for flexor tendon injury, with an emphasis on large animal translational studies.

Tendon development and musculoskeletal assembly: emerging roles for the extracellular matrix

Tendons and ligaments are extracellular matrix (ECM)-rich structures that interconnect muscles and bones. Recent work has shown how tendon fibroblasts (tenocytes) interact with muscles via the ECM to establish connectivity and strengthen attachments under tension. Similarly, ECM-dependent interactions between tenocytes and cartilage/bone ensure that tendon-bone attachments form with the appropriate strength for the force required. Recent studies have also established a close lineal relationship between tenocytes and skeletal progenitors, highlighting the fact that defects in signals modulated by the ECM can alter the balance between these fates, as occurs in calcifying tendinopathies associated with aging. The dynamic fine-tuning of tendon ECM composition and assembly thus gives rise to the remarkable characteristics of this unique tissue type. Here, we provide an overview of the functions of the ECM in tendon formation and maturation that attempts to integrate findings from developmental genetics with those of matrix biology.

Inflammation as a Keystone of Bone Marrow Stroma Alterations in Primary Myelofibrosis

Primary myelofibrosis (PMF) is a clonal myeloproliferative neoplasm where severity as well as treatment complexity is mainly attributed to a long lasting disease and presence of bone marrow stroma alterations as evidenced by myelofibrosis, neoangiogenesis, and osteosclerosis. While recent understanding of mutations role in hematopoietic cells provides an explanation for pathological myeloproliferation, functional involvement of stromal cells in the disease pathogenesis remains poorly understood. The current dogma is that stromal changes are secondary to the cytokine “storm” produced by the hematopoietic clone cells. However, despite therapies targeting the myeloproliferation-sustaining clones, PMF is still regarded as an incurable disease except for patients, who are successful recipients of allogeneic stem cell transplantation. Although the clinical benefits of these inhibitors have been correlated with a marked reduction in serum proinflammatory cytokines produced by the hematopoietic clones, further demonstrating the importance of inflammation in the pathological process, these treatments do not address the role of the altered bone marrow stroma in the pathological process. In this review, we propose hypotheses suggesting that the stroma is inflammatory-imprinted by clonal hematopoietic cells up to a point where it becomes “independent” of hematopoietic cell stimulation, resulting in an inflammatory vicious circle requiring combined stroma targeted therapies.

Biologics for tendon repair

Tendon injuries are common and present a clinical challenge to orthopedic surgery mainly because these injuries often respond poorly to treatment and require prolonged rehabilitation. Therapeutic options used to repair ruptured tendons have consisted of suture, autografts, allografts, and synthetic prostheses. To date, none of these alternatives has provided a successful long-term solution, and often the restored tendons do not recover their complete strength and functionality. Unfortunately, our understanding of tendon biology lags far behind that of other musculoskeletal tissues, thus impeding the development of new treatment options for tendon conditions. Hence, in this review, after introducing the clinical significance of tendon diseases and the present understanding of tendon biology, we describe and critically assess the current strategies for enhancing tendon repair by biological means. These consist mainly of applying growth factors, stem cells, natural biomaterials and genes, alone or in combination, to the site of tendon damage. A deeper understanding of how tendon tissue and cells operate, combined with practical applications of modern molecular and cellular tools could provide the long awaited breakthrough in designing effective tendon-specific therapeutics and overall improvement of tendon disease management.