Loss of tenomodulin results in reduced self-renewal and augmented senescence of tendon stem/progenitor cells

Stem cell derived exosomes: microRNA therapy for age-related musculoskeletal disorders

Age-associated musculoskeletal disorders (MSDs) have been historically overlooked by mainstream biopharmaceutical researchers. However, it has now been recognized that stem and progenitor cells confer innate healing capacity for the musculoskeletal system. Current evidence indicates that exosomes are particularly important in this process as they can mediate sequential and reciprocal interactions between cells to initiate and enhance healing. The present review focuses on stem cells (SCs) derived exosomes as a regenerative therapy for treatment of musculoskeletal disorders. We discuss mechanisms involving exosome-mediated transfer of RNAs and how these have been demonstrated in vitro and in vivo to affect signal transduction pathways in target cells. We envision that standardized protocols for stem cell culture as well as for the isolation and characterization of exosomes enable GMP-compliant large-scale production of SCs-derived exosomes. Hence, potential new treatment for age-related degenerative diseases can be seen in the horizon.

Tenomodulin regulates matrix remodeling of mouse tendon stem/progenitor cells in an ex vivo collagen I gel model

Tenomodulin (Tnmd) is predominantly expressed in tendon and ligament tissues. Loss of Tnmd in mice leads to a profound phenotype in vitro, characterized by reduced self-renewal but increased senescence of mouse tendon stem/progenitor cells (mTSPCs), as well as in vivo, by significantly impaired early tendon healing. Interestingly, injuried Achilles tendons from Tnmd-deficient mice showed inferior tendon repair, which was characterized by less contracted fibrovascular scars with disorganized matrix composition in comparison to wild type (WT) mice at day 8 after injury. To better understand Tnmd role in tendon repair, here we implemented an ex vivo three-dimensional (3D) collagen gel model and investigated whether Tnmd knockout affects the collagen contraction of mTSPCs. TSPCs were isolated from WT and Tnmd knockout (KO) tendons at 6, 9, 12, and 18 months of age. Adhesion assay demonstrated that loss of Tnmd in mTSPCs resulted in reduced adhesion to collagen type I. Quantitative time-dependent analysis revealed that Tnmd-deficient mTSPCs of all ages have significantly reduced capacity to contract collagen matrix in comparison to WT cells. Furthermore, 18 months old mTSPCs of both genotypes showed lower collagen contractility than cells obtained from 6, 9, and 12 months old animals, demonstrating an overall effect of organismal aging on matrix remodeling. Nevertheless, both cell types had a similar survival rate for the 5 days of cultivation within the gels. Lastly, quantitative PCR for 48 different genes revealed that the knockout of Tnmd majorly affected the gene expression profile of mTSPCs, as several transcription factors, tendon matrix, collagen cross-linking, and lineage maker genes were down-regulated. Taken together, our results clearly demonstrated that loss of Tnmd in mTSPCs led to profoundly altered gene expression profile, insufficient adhesion to collagen type I, and impaired ability to contract the extracellular matrix.

Development, repair, and regeneration of the limb musculoskeletal system

The limb musculoskeletal system provides a primary means for locomotion, manipulation of objects and protection for most vertebrate organisms. Intricate integration of the bone, tendon and muscle tissues are required for function. These three tissues arise largely independent of one another, but the connections formed during later development are maintained throughout life and are re-established following injury. Each of these tissues also have mesenchymal stem/progenitor cells that function in maintenance and repair. Here in, we will review the major events in the development of limb skeleton, tendon, and muscle tissues, their response to injury, and discuss current knowledge regarding resident progenitor/stem cells within each tissue that participate in development, repair, and regeneration in vivo.

Age related changes in cell stiffness of tendon stem/progenitor cells and a rejuvenating effect of ROCK-inhibition

Tendon stem/progenitor cells (TSPC) are potential targets for regenerative medicine and the treatment of tendon injuries. The frequency of such injuries increases in elderly patients while the proportion of functional TSPCs in tendon tissue decreases, protracting tendon repair. Using atomic force microscopy (AFM), we show that cell stiffness and size increase in TSPCs isolated from elderly patients (A-TSPC) compared to TSPCs from younger patients (Y-TSPC). Additionally, two-photon excited fluorescence (TPEF) microscopy revealed a denser, well-structured actin cytoskeleton in A-TSPC, which correlates with the augmented cell stiffness. Treating A-TSPC with ROCK-inhibitor, reverses these age-related changes, and has rejuvenating effect on cell morphology and stiffness. We assume that cellular stiffness is a suitable marker for cell aging and ROCK a potential target for therapeutic applications of cell rejuvenation.

Enhanced Effect of Tendon Stem/Progenitor Cells Combined With Tendon-Derived Decellularized Extracellular Matrix on Tendon Regeneration

Decellularized extracellular matrices have been clinically used for tendon regeneration. However, only a few systematic studies have compared tendon stem/progenitor cells to mesenchymal stromal cells on the tendon-derived decellularized matrix. In the present study, we prepared extracellular matrix derived from porcine tendons and seeded with tendon stem/progenitor cells, embryonic stem cell-derived mesenchymal stromal cells or without stem cells. Then we implanted the mixture (composed of stem cells and scaffold) into the defect of a rat Achilles tendon. Next, 4 weeks post-surgery the regenerated tendon tissue was collected. Histological staining, immunohistochemistry, determination of collagen content, transmission electron microscopy, and biomechanical testing were performed to evaluate the tendon structure and biomechanical properties. Our study collectively demonstrated that decellularized extracellular matrix derived from porcine tendons significantly promoted the regeneration of injured tendons when combined with tendon stem/progenitor cells or embryonic stem cell-mesenchymal stromal cells. Compared to embryonic stem cell-mesenchymal stromal cells, tendon stem/progenitor cells combined with decellularized matrix showed more improvement in the structural and biomechanical properties of regenerated tendons in vivo. These findings suggest a promising strategy for functional tendon tissue regeneration and further studies are warranted to develop a functional tendon tissue regeneration utilizing tendon stem/progenitor cells integrated with a tendon-derived decellularized matrix.

Streptozotocin-induced diabetes alters transcription of multiple genes necessary for extracellular matrix remodeling in rat patellar tendon

OVERVIEW

Tendon collagen fibril degradation is commonly seen in tendons of diabetics, but the mechanisms responsible for these changes remain to be elucidated. We have demonstrated that streptozotocin (STZ)-induced diabetes increases tendon cell proliferation and collagen content. In the present study, we evaluated that impact of STZ-induced diabetes on mRNA transcripts involved with collagen fibril organization, extracellular matrix (ECM) remodeling, apoptosis, and proliferation.

MATERIALS AND METHODS

Rats were divided into four groups: nondiabetic (control, n = 9), 1 week (acute, n = 8) or 10 weeks of diabetes (chronic, n = 7), and 10 weeks of diabetes with insulin (insulin, n = 8). RNA was isolated from the patellar tendon for determination of mRNA transcripts using droplet digital PCR (ddPCR).

RESULTS

Transcripts for Col1a1, Col3a1, Mmp2, Timp1, Scx, Tnmd, Casp3, Casp8, and Ager were lower in acute relative to control and insulin rats (p ≤ 0.05). With the exception of Scx, transcripts for Col1a1, Col3a1, Mmp2, Timp1, Tnmd, Casp3, Casp8, and Ager were also lower in chronic when compared to control (p < 0.05). Transcripts for Col1a1, Col3a1, Mmp2, Timp1, Tnmd, Casp3, Casp8, and Ager were not different between control and insulin (p > 0.05). Transcripts for Dcn, Mmp1a, Mmp9, Pcna, Tgfbr3, Ptgs2, Ptger2, Ptges, and iNos were not altered by diabetes or insulin (p > 0.05).

CONCLUSION

Our findings indicated that STZ-induced diabetes results in rapid and large changes in the expression of several genes that are key to ECM remodeling, maintenance, and maturation.

Understanding Tendons: Lessons from Transgenic Mouse Models

Tendons and ligaments are connective tissues that have been comparatively less studied than muscle and cartilage/bone, even though they are crucial for proper function of the musculoskeletal system. In tendon biology, considerable progress has been made in identifying tendon-specific genes (Scleraxis, Mohawk, and Tenomodulin) in the past decade. However, besides tendon function and the knowledge of a small number of important players in tendon biology, neither the ontogeny of the tenogenic lineage nor signaling cascades have been fully understood. This results in major drawbacks in treatment and repair options following tendon degeneration. In this review, we have systematically evaluated publications describing tendon-related genes, which were studied in depth and characterized by using knockout technologies and the subsequently generated transgenic mouse models (Tg) (knockout mice, KO). We report in a tabular manner, that from a total of 24 tendon-related genes, in 22 of the respective knockout mouse models, phenotypic changes were detected. Additionally, in some of the models it was described at which developmental stages these changes appeared and progressed. To summarize, only loss of Scleraxis and TGFβ signaling led to severe tendon developmental phenotypes, while mice deficient for various proteoglycans, Mohawk, EGR1 and 2, and Tenomodulin presented mild phenotypes. These data suggest that the tendon developmental system is well organized, orchestrated, and backed up; this is even more evident among the members of the proteoglycan family, where the compensatory effects are much clearer. In future, it will be of great importance to discover additional master tendon transcription factors and the genes that play crucial roles in tendon development. This would improve our understanding of the genetic makeup of tendons, and will increase the chances of generating tendon-specific drugs to advance overall treatment strategies.

Downregulation of FOXP1 correlates with tendon stem/progenitor cells aging

Aging is known as a major risk factor for tendon disorders whereas the molecular mechanisms of age-related tendon disorders still remains unclear. Since tendon-derived stem/progenitor cells (TSPCs) play a vital role in tendon maintenance and healing, in this study we aimed to investigate the role of Forkhead box P1 (FOXP1) in aged TSPCs, we found that FOXP1 mRNA and protein levels were markedly decreased in the aged TSPCs. Moreover, overexpression of FOXP1 attenuates TSPCs aging, as confirmed by decreased of senescence-associated β-gal staining, as well as the senescence marker, p16^INK4A. Conversely, FOXP1 depletion by siRNA promoted senescence in young TSPCs. Meanwhile, FOXP1 overexpression also restores the age-associated reduction of self-renewal, migration and differentiation of TSPCs. In addition, FOXP1 overexpression rescued decreased levels of E2F1, pRb and cyclin D1 in aged TSPCs, which suggested that FOXP1 regulates TSPCs aging through cellular senescence. These results indicate that FOXP1 plays a crucial role in TSPCs aging.

Proliferative and osteogenic differentiation capacity of mesenchymal stromal cells: Influence of harvesting site and donor age

Human mesenchymal stromal cells (hMSCs) are the cellular source of new bone formation and an essential component of autologous bone grafts. Autologous bone graft harvesting is routinely conducted at the iliac crest, although alternative donor sites with lower complication rates are available. Thus, the aim of this study was to compare hMSCs harvested from the iliac crest and the proximal tibia regarding their proliferative and osteogenic differentiation capacity. Furthermore, we investigated the influence of donor age on these biological properties. HMSCs were isolated from iliac crest or proximal tibia bone grafts of 46 patients. Proliferative capacity was assessed by cumulative population doublings, population doubling time, colony forming units and cell proliferation assays. Osteogenic capacity was assessed by quantification of extracellular calcium deposition and marker gene expression levels. The number of hMSCs per gram harvested tissue was determined. Furthermore, the adipogenic and chondrogenic differentiation capacity were quantified using BODIPY and Safranin Orange staining, respectively. Additional analyses were carried out after grouping young (18-49 years) and aged (≥50 years) donors. HMSCs derived from the proximal tibia featured a comparable proliferative and osteogenic differentiation capacity. No significant differences were found for any analysis conducted, when compared to hMSCs obtained from the iliac crest. Furthermore, no significant differences could be revealed when comparing young and aged donors. This was equally true for hMSCs from both donor sites after comparison within the same age group. Our study demonstrates comparable biological properties of hMSCs derived from both donor sites, the iliac crest and the proximal tibia. Furthermore, aging does not alter proliferative and osteogenic differentiation capacity. Consequently, the proximal tibia should be considered more closely as an alternative donor site in patients of all age groups.

A comparison of the stem cell characteristics of murine tenocytes and tendon-derived stem cells

Tendon is a commonly injured soft musculoskeletal tissue, however, poor healing potential and ineffective treatment strategies result in persistent injuries and tissue that is unable to perform its normal physiological function. The identification of a stem cell population within tendon tissue holds therapeutic potential for treatment of tendon injuries. This study aimed, for the first time, to characterise and compare tenocyte and tendon-derived stem cell (TDSC) populations in murine tendon. Tenocytes and TDSCs were isolated from murine tail tendon. The cells were characterised for morphology, clonogenicity, proliferation, stem cell and tenogenic marker expression and multipotency. TDSCs demonstrated a rounded morphology, compared with a more fibroblastic morphology for tenocytes. Tenocytes had greater clonogenic potential and a smaller population doubling time compared with TDSCs. Stem cell and early tenogenic markers were more highly expressed in TDSCs, whereas late tenogenic markers were more highly expressed in tenocytes. Multipotency was increased in TDSCs with the presence of adipogenic differentiation which was absent in tenocytes. The differences in morphology, clonogenicity, stem cell marker expression and multipotency observed between tenocytes and TDSCs indicate that at least two cell populations are present in murine tail tendon. Determination of the most effective cell population for tendon repair is required in future studies, which in turn may aid in tendon repair strategies.

Scleraxis is a transcriptional activator that regulates the expression of Tenomodulin, a marker of mature tenocytes and ligamentocytes

Tenomodulin (Tnmd) is a type II transmembrane glycoprotein predominantly expressed in tendons and ligaments. We found that scleraxis (Scx), a member of the Twist-family of basic helix-loop-helix transcription factors, is a transcriptional activator of Tnmd expression in tenocytes. During embryonic development, Scx expression preceded that of Tnmd. Tnmd expression was nearly absent in tendons and ligaments of Scx-deficient mice generated by transcription activator-like effector nucleases-mediated gene disruption. Tnmd mRNA levels were dramatically decreased during serial passages of rat tenocytes. Scx silencing by small interfering RNA significantly suppressed endogenous Tnmd mRNA levels in tenocytes. Mouse Tnmd contains five E-box sites in the ~1-kb 5′-flanking region. A 174-base pair genomic fragment containing a TATA box drives transcription in tenocytes. Enhancer activity was increased in the upstream region (−1030 to −295) of Tnmd in tenocytes, but not in NIH3T3 and C3H10T1/2 cells. Preferential binding of both Scx and Twist1 as a heterodimer with E12 or E47 to CAGATG or CATCTG and transactivation of the 5′-flanking region were confirmed by electrophoresis mobility shift and dual luciferase assays, respectively. Scx directly transactivates Tnmd via these E-boxes to positively regulate tenocyte differentiation and maturation.

Tenomodulin is essential for prevention of adipocyte accumulation and fibrovascular scar formation during early tendon healing

Tenomodulin (Tnmd) is the best-known mature marker for tendon and ligament lineage cells. It is important for tendon maturation, running performance and has key implications for the resident tendon stem/progenitor cells (TSPCs). However, its exact functions during the tendon repair process still remain elusive. Here, we established an Achilles tendon injury model in a Tnmd knockout (Tnmd^−/−) mouse line. Detailed analyses showed not only a very different scar organization with a clearly reduced cell proliferation and expression of certain tendon-related genes, but also increased cell apoptosis, adipocyte and blood vessel accumulation in the early phase of tendon healing compared with their wild-type (WT) littermates. In addition, Tnmd^−/− tendon scar tissue contained augmented matrix deposition of biglycan, cartilage oligomeric matrix protein (Comp) and fibronectin, altered macrophage profile and reduced numbers of CD146-positive cells. In vitro analysis revealed that Tnmd^−/− TSPCs exhibited significantly reduced migration and proliferation potential compared with that of WT TSPCs. Furthermore, Tnmd^−/− TSPCs had accelerated adipogenic differentiation accompanied with significantly increased peroxisome proliferator-activated receptor gamma (Pparγ) and lipoprotein lipase (Lpl) mRNA levels. Thus, our results demonstrate that Tnmd is required for prevention of adipocyte accumulation and fibrovascular scar formation during early tendon healing.

Tenomodulin highly expressing MSCs as a better cell source for tendon injury healing

Tendon injuries are common orthopedic problems which may cause severe morbidity. MSCs (mesenchymal stem cells) have shown promising effect on tissue engineering and have been used for the treatment of tendon injury. But the low tenogenic differentiation capacity of MSCs have hindered their application. In the present study, we have constructed the Tenomodulin (Tnmd) promoter-driven GFP expression lentiviral plasmid. After transduced into BMSCs, the expression of GFP was used to select BMSCs highly expressing Tnmd by flow cytometry. We found that MSCs with higher level of Tnmd expression had stronger tenogenic differentiation ability. Furthermore, RNA sequencing was performed to identify the molecular difference between BMSCs expressing higher and lower levels of Tnmd. And finally we demonstrated that GDF7 was upregulated in BMSCs highly expressing Tnmd and played an vital role in promoting tenogenic differentiation of BMSCs. GDF7 was mainly accounted for the elevated tenogenic differentiation ability of BMSCs with higher Tnmd expression as silencing the endogenous GDF7 significantly inhibited tenogenesis in BMSCs. In addition, the effect of BMSCs with higher Tnmd level on tendon healing was evaluated by a rat patellar tendon injury model. Taken together, our study showed that Tnmd could be used as an ideal cell surface marker to select cells with higher tenogenic differentiation ability from BMSCs, and GDF7 was indispensable for tenogenesis of MSCs.

Glutamate signaling through the NMDA receptor reduces the expression of scleraxis in plantaris tendon derived cells

Background

A body of evidence demonstrating changes to the glutaminergic system in tendinopathy has recently emerged. This hypothesis was further tested by studying the effects of glutamate on the tenocyte phenotype, and the impact of loading and exposure to glucocorticoids on the glutamate signaling machinery.

Methods

Plantaris tendon tissue and cultured plantaris tendon derived cells were immunohisto-/cytochemically stained for glutamate, N-Methyl-D-Aspartate receptor 1 (NMDAR1) and vesicular glutamate transporter 2 (VGluT2). Primary cells were exposed to glutamate or receptor agonist NMDA. Cell death/viability was measured via LDH/MTS assays, and Western blot for cleaved caspase 3 (c-caspase 3) and cleaved poly (ADP-ribose) polymerase (c-PARP). Scleraxis mRNA (Scx)/protein(SCX) were analyzed by qPCR and Western blot, respectively. A FlexCell system was used to apply cyclic strain. The effect of glucocorticoids was studies by adding dexamethasone (Dex). The mRNA of the glutamate synthesizing enzymes Got1 and Gls, and NMDAR1 protein were measured. Levels of free glutamate were determined by a colorimetric assay.

Results

Immunoreactions for glutamate, VGluT2, and NMDAR1 were found in tenocytes and peritendinous cells in tissue sections and in cultured cells. Cell death was induced by high concentrations of glutamate but not by NMDA. Scleraxis mRNA/protein was down-regulated in response to NMDA/glutamate stimulation. Cyclic strain increased, and Dex decreased, Gls and Got1 mRNA expression. Free glutamate levels were lower after Dex exposure.

Conclusions

In conclusion, NMDA receptor stimulation leads to a reduction of scleraxis expression that may be involved in a change of phenotype in tendon cells. Glutamate synthesis is increased in tendon cells in response to strain and decreased by glucocorticoid stimulation. This implies that locally produced glutamate could be involved in the tissue changes observed in tendinopathy.

Tenomodulin is Required for Tendon Endurance Running and Collagen I Fibril Adaptation to Mechanical Load

Tendons are dense connective tissues that attach muscles to bone with an indispensable role in locomotion because of their intrinsic properties of storing and releasing muscle- generated elastic energy. Tenomodulin (Tnmd) is a well-accepted gene marker for the mature tendon/ligament lineage and its loss-of -function in mice leads to a phenotype with distinct signs of premature aging on tissue and stem/progenitor cell levels. Based on these findings, we hypothesized that Tnmd might be an important factor in the functional performance of tendons. Firstly, we revealed that Tnmd is a mechanosensitive gene and that the C-terminus of the protein co-localize with collagen I-type fibers in the extracellular matrix. Secondly, using an endurance training protocol, we compared Tnmd knockout mice with wild types and showed that Tnmd deficiency leads to significantly inferior running performance that further worsens with training. In these mice, endurance running was hindered due to abnormal response of collagen I cross-linking and proteoglycan genes leading to an inadequate collagen I fiber thickness and elasticity. In sum, our study demonstrates that Tnmd is required for proper tendon tissue adaptation to endurance running and aids in better understanding of the structural-functional relationships of tendon tissues.

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Tnmd is a mechanosensitive gene and its protein is co-localized with collagen I fibers in the ECM of tendons.

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Tnmd knockout mice fail in endurance running tests, a phenotype that worsens with training.

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Tnmd knockout tendons had significantly thicker and stiffer collagen I fibers and altered crosslinking gene expression.

We performed a multidisciplinary approach to decipher the role of tenomodulin, a gene marker for the mature tendon lineage, in tendon functional performance. Loss-of-function in mice led to significantly inferior endurance running and detailed analyses revealed that tenomodulin is involved in the regulation of collagen I fiber structural and biomechanical properties in response to exercise. Our study expands the current view on the complex structural-functional relationships of tendon tissues, and tenomodulin expression levels may indicate whether an individual is suitable for a certain sport.

Scleraxis is required for maturation of tissue domains for proper integration of the musculoskeletal system

Scleraxis (Scx) is a basic helix-loop-helix transcription factor that is expressed persistently in tendons/ligaments, but transiently in entheseal cartilage. In this study, we generated a novel Scx^Cre knock-in (KI) allele, by in-frame replacement of most of Scx exon 1 with Cre recombinase (Cre), to drive Cre expression using Scx promoter and to inactivate the endogenous Scx. Reflecting the intensity and duration of endogenous expression, Cre-mediated excision occurs in tendinous and ligamentous tissues persistently expressing Scx. Expression of tenomodulin, a marker of mature tenocytes and ligamentocytes, was almost absent in tendons and ligaments of Scx^Cre/Cre KI mice lacking Scx to indicate defective maturation. In homozygotes, the transiently Scx-expressing entheseal regions such as the rib cage, patella cartilage, and calcaneus were small and defective and cartilaginous tuberosity was missing. Decreased Sox9 expression and phosphorylation of Smad1/5 and Smad3 were also observed in the developing entheseal cartilage, patella, and deltoid tuberosity of Scx^Cre/Cre KI mice. These results highlighted the functional importance of both transient and persistent expression domains of Scx for proper integration of the musculoskeletal components.

EGR1 Regulates Transcription Downstream of Mechanical Signals during Tendon Formation and Healing

BACKGROUND

Tendon is a mechanical tissue that transmits forces generated by muscle to bone in order to allow body motion. The molecular pathways that sense mechanical forces during tendon formation, homeostasis and repair are not known. EGR1 is a mechanosensitive transcription factor involved in tendon formation, homeostasis and repair. We hypothesized that EGR1 senses mechanical signals to promote tendon gene expression.

METHODOLOGY/PRINCIPAL FINDINGS

Using in vitro and in vivo models, we show that the expression of Egr1 and tendon genes is downregulated in 3D-engineered tendons made of mesenchymal stem cells when tension is released as well as in tendon homeostasis and healing when mechanical signals are reduced. We further demonstrate that EGR1 overexpression prevents tendon gene downregulation in 3D-engineered tendons when tension is released. Lastly, ultrasound and microbubbles mediated EGR1 overexpression prevents the downregulation of tendon gene expression during tendon healing in reduced load conditions.

CONCLUSION/SIGNIFICANCE

These results show that Egr1 expression is sensitive to mechanical signals in tendon cells. Moreover, EGR1 overexpression prevents the downregulation of tendon gene expression in the absence of mechanical signals in 3D-engineered tendons and tendon healing. These results show that EGR1 induces a transcriptional response downstream of mechanical signals in tendon cells and open new avenues to use EGR1 to promote tendon healing in reduced load conditions.

TGFβ and FGF promote tendon progenitor fate and act downstream of muscle contraction to regulate tendon differentiation during chick limb development

The molecular programme underlying tendon development has not been fully identified. Interactions with components of the musculoskeletal system are important for limb tendon formation. Limb tendons initiate their development independently of muscles; however, muscles are required for further tendon differentiation. We show that both FGF/ERK MAPK and TGFβ/SMAD2/3 signalling pathways are required and sufficient for SCX expression in chick undifferentiated limb cells, whereas the FGF/ERK MAPK pathway inhibits Scx expression in mouse undifferentiated limb mesodermal cells. During differentiation, muscle contraction is required to maintain SCX, TNMD and THBS2 expression in chick limbs. The activities of FGF/ERK MAPK and TGFβ/SMAD2/3 signalling pathways are decreased in tendons under immobilisation conditions. Application of FGF4 or TGFβ2 ligands prevents SCX downregulation in immobilised limbs. TGFβ2 but not FGF4 prevent TNMD and THBS2 downregulation under immobilisation conditions. We did not identify any intracellular crosstalk between both signalling pathways in their positive effect on SCX expression. Independently of each other, both FGF and TGFβ promote tendon commitment of limb mesodermal cells and act downstream of mechanical forces to regulate tendon differentiation during chick limb development.

Stem Cells in Skeletal Tissue Engineering: Technologies and Models

This review surveys the use of pluripotent and multipotent stem cells in skeletal tissue engineering. Specific emphasis is focused on evaluating the function and activities of these cells in the context of development in vivo, and how technologies and methods of stem cell-based tissue engineering for stem cells must draw inspiration from developmental biology. Information on the embryonic origin and in vivo differentiation of skeletal tissues is first reviewed, to shed light on the persistence and activities of adult stem cells that remain in skeletal tissues after embryogenesis. Next, the development and differentiation of pluripotent stem cells is discussed, and some of their advantages and disadvantages in the context of tissue engineering are presented. The final section highlights current use of multipotent adult mesenchymal stem cells, reviewing their origin, differentiation capacity, and potential applications to tissue engineering.

Molecular characterization and function of tenomodulin, a marker of tendons and ligaments that integrate musculoskeletal components

Tendons and ligaments are dense fibrous bands of connective tissue that integrate musculoskeletal components in vertebrates. Tendons connect skeletal muscles to the bone and function as mechanical force transmitters, whereas ligaments bind adjacent bones together to stabilize joints and restrict unwanted joint movement. Fibroblasts residing in tendons and ligaments are called tenocytes and ligamentocytes, respectively. Tenomodulin (Tnmd) is a type II transmembrane glycoprotein that is expressed at high levels in tenocytes and ligamentocytes, and is also present in periodontal ligament cells and tendon stem/progenitor cells. Tnmd is related to chondromodulin-1 (Chm1), a cartilage-derived angiogenesis inhibitor, and both Tnmd and Chm1 are expressed in the CD31⁻ avascular mesenchyme. The conserved C-terminal hydrophobic domain of these proteins, which is characterized by the eight Cys residues to form four disulfide bonds, may have an anti-angiogenic function. This review highlights the molecular characterization and function of Tnmd, a specific marker of tendons and ligaments.

Tenogenic modulating insider factor: Systematic assessment on the functions of tenomodulin gene

Tenomodulin (TNMD, Tnmd) is a gene highly expressed in tendon known to be important for tendon maturation with key implications for the residing tendon stem/progenitor cells as well as for the regulation of endothelial cell migration in chordae tendineae cordis in the heart and in experimental tumour models. This review aims at providing an encompassing overview of this gene and its protein. In addition, its known expression pattern as well as putative signalling pathways will be described. A chronological overview of the discovered functions of this gene in tendon and other tissues and cells is provided as well as its use as a tendon and ligament lineage marker is assessed in detail and discussed. Last, information about the possible connections between TNMD genomic mutations and mRNA expression to various diseases is delivered. Taken together this review offers a solid synopsis on the up-to-date information available about TNMD and aids at directing and focusing the future research to fully uncover the roles and implications of this interesting gene.

A Prospectus of Tenomodulin

Tenomodulin is a type II transmembrane glycoprotein that can regulate growth of tendon. The human tenomodulin encoding gene is mapped to Х chromosome and encodes a polypeptide consisting of 317 alpha amino acids. The protein is located on the cell surface as N-glycosylated or non-N-glycosylated polypeptide with molecular mass of 45 and 40 kDa, respectively. The molecule consists of three domains and a short cytoplasmic tail at N-terminus. Tenomodulin is predominantly expressed in dense connective hypovascularized tissues such as tendons, skeletal muscle epimysium, and ligaments. Furthermore, tenomodulin is an efficient marker of tenocyte differentiation and plays an important role in the regulation of tenocyte proliferation, tendon development, and angiogenesis inhibition. A number of tenomodulin gene polymorphisms have been recently associated with a risk of obesity, diabetes, systemic immune mediators, cholesterol levels, Alzheimer disease, and age-related macular degeneration. Tenomodulin is involved in cell adhesion, determination of cell morphology, cell aging and bone mineral density. It is expected to play an important role in tendon recovery, tendon tissue engineering, anti-tumor therapy, treatment of chordal disruption, and remodeling of extracellular matrix.

Tendon-derived stem cells undergo spontaneous tenogenic differentiation

Tendon-derived stem cell (TDSC) is a subpopulation of residing stem cells within the intact tendon tissues, with the capacities of self-renewal, clonogenicity, and three-lineage differentiation. Compared with bone marrow derived mesenchymal stem cells (BMSCs), TDSCs are superior for tendon injuries repair as they remain some tendon tissue-specific differentiation properties. In the present study, TDSC was found to undergo spontaneous tenogenic differentiation in which the expression of tenogenic markers were increased while the expression of stemness markers decreased with time in TDSCs culture (without tenogenic induction medium). The further collagen synthesis ability was correspondingly increased during this process. After a longer period of culture, the monolayer of TDSCs formed a "3D" layers with rich extracellular matrices of typical tendon tissues. In addition, the key tenogenic transcription factors, such as Scx, Mkx, Egr1 and Eya1 were all up-regulated in this process. Finally, we compared the spontaneous tenogenic differentiation with TGF-β1-induced tenogenic differentiation of TDSCs, and the results showed that the spontaneous tenogenic differentiation of TDSCs was general character of TDSCs, similar to but weaker than the effect of TDSCs under tenogenic induction. Taken together, the present study identified that TDSCs had the potential of spontaneous tenogenic differentiation, which may be a better cell source for the treatment of tendon injury.

Use of biomimetic microtissue spheroids and specific growth factor supplementation to improve tenocyte differentiation and adaptation to a collagen-based scaffold in vitro

Tenocytes represent a valuable source of cells for the purposes of tendon tissue engineering and regenerative medicine and as such, should possess a high degree of tenogenic differentiation prior to their use in vivo in order to achieve maximal efficacy. In the current report, we identify an efficient means by which to maintain differentiated tenocytes in vitro by employing the hanging drop technique in combination with defined growth media supplements. Equine tenocytes retained a more differentiated state when cultured as scaffold-free microtissue spheroids in low serum-containing medium supplemented with L-ascorbic acid 2-phosphate, insulin and transforming growth factor (TGF)-β1. This was made evident by significant increases in the expression levels of pro-tenogenic markers collagen type I (COL1A2), collagen type III (COL3A1), scleraxis (SCX) and tenomodulin (TNMD), as well as by enhanced levels of collagen type I and tenomodulin protein. Furthermore, tenocytes cultured under these conditions demonstrated a typical spindle-like morphology and when embedded in collagen gels, became highly aligned with respect to the orientation of the collagen structure following their migration out from the microtissue spheroids. Our findings therefore provide evidence to support the use of a biomimetic microtissue approach to culturing tenocytes and that in combination with the defined growth media described, can improve their differentiation status and functional repopulation of collagen matrix.

Tendon development and diseases

Tendon is a uniaxial connective tissue component of the musculoskeletal system. Tendon is involved in force transmission between muscle and bone. Tendon injury is very common and debilitating but tendon repair remains a clinical challenge for orthopedic medicine. In vertebrates, tendon is mainly composed of type I collagen fibrils, displaying a parallel organization along the tendon axis. The tendon-specific spatial organization of type I collagen provides the mechanical properties for tendon function. In contrast to other components of the musculoskeletal system, tendon biology is poorly understood. An important goal in tendon biology is to understand the mechanisms involved in the production and assembly of type I collagen fibrils during development, postnatal formation, and healing processes in order to design new therapies for tendon repair. In this review we highlight the current understanding of the molecular and mechanical signals known to be involved in tenogenesis during development, and how development provides insights into tendon healing processes. WIREs Dev Biol 2016, 5:5-23. doi: 10.1002/wdev.201 For further resources related to this article, please visit the WIREs website.

Markers for the identification of tendon-derived stem cells in vitro and tendon stem cells in situ - update and future development

The efficacy of tendon-derived stem cells (TDSCs) for the promotion of tendon and tendon-bone junction repair has been reported in animal studies. Modulation of the tendon stem cell niche in vivo has also been reported to influence tendon structure. There is a need to have specific and reliable markers that can define TDSCs in vitro and tendon stem cells in situ for several reasons: to understand the basic biology of TDSCs and their subpopulations in vitro; to understand the identity, niches and functions of tendon/progenitor stem cells in vivo; to meet the governmental regulatory requirements for quality of TDSCs when translating the exciting preclinical findings into clinical trial/practice; and to develop new treatment strategies for mobilizing endogenous stem/progenitor cells in tendon. TDSCs were reported to express the common mesenchymal stem cell (MSC) markers and some embryonic stem cell (ESC) markers, and there were attempts to use these markers to label tendon stem cells in situ. Are these stem cell markers useful for the identification of TDSCs in vitro and tracking of tendon stem cells in situ? This review aims to discuss the values of the panel of MSC, ESC and tendon-related markers for the identification of TDSCs in vitro. Important factors influencing marker expression by TDSCs are discussed. The usefulness and limitations of the panel of MSC, ESC and tendon-related markers for tracking stem cells in tendon, especially tendon stem cells, in situ are then reviewed. Future research directions are proposed.

Biology and mechano-response of tendon cells: Progress overview and perspectives

In this review, we summarize the group discussions on Cell Biology & Mechanics from the 2014 ORS/ISMMS New Frontiers in Tendon Research Conference. The major discussion topics included: (1) the biology of tendon stem/progenitor cells (TSPCs) and the potential of stem cell-based tendon therapy using TSPCs and other types of stem cells, namely, embryonic and/or induced pluripotent stem cells (iPSCs), (2) the biological concept and potential impact of cellular senescence on tendon aging, tendon injury repair and the development of degenerative disease, and (3) the effects of tendon cells' mechano-response on tendon cell fate and metabolism. For each topic, a brief overview is presented which summarizes the major points discussed by the group participants. The focus of the discussions ranged from current research progress, challenges and opportunities, to future directions on these topics. In the preparation of this manuscript, authors consulted relevant references as a part of their efforts to present an accurate view on the topics discussed.

Tendon functional extracellular matrix

This article is one of a series, summarizing views expressed at the Orthopaedic Research Society New Frontiers in Tendon Research Conference. This particular article reviews the three workshops held under the "Functional Extracellular Matrix" stream. The workshops focused on the roles of the tendon extracellular matrix, such as performing the mechanical functions of tendon, creating the local cell environment, and providing cellular cues. Tendon is a complex network of matrix and cells, and its biological functions are influenced by widely varying extrinsic and intrinsic factors such as age, nutrition, exercise levels, and biomechanics. Consequently, tendon adapts dynamically during development, aging, and injury. The workshop discussions identified research directions associated with understanding cell-matrix interactions to be of prime importance for developing novel strategies to target tendon healing or repair.