Identification of a tendon phenotype in GDF6 deficient mice

Nonwoven-based gelatin/polycaprolactone membrane loaded with ERK inhibitor U0126 for treatment of tendon defects

BACKGROUND

Tendon is a major component of musculoskeletal system connecting the muscles to the bone. Tendon injuries are very common orthopedics problems leading to impeded motion. Up to now, there still lacks effective treatments for tendon diseases.

METHODS

Tendon stem/progenitor cells (TSPCs) were isolated from the patellar tendons of SD rats. The expression levels of genes were evaluated by quantitative RT-PCR. Immunohistochemistry staining was performed to confirm the presence of tendon markers in tendon tissues. Bioinformatics analysis of data acquired by RNA-seq was used to find out the differentially expressed genes. Rat patellar tendon injury model was used to evaluate the effect of U0126 on tendon injury healing. Biomechanical testing was applied to evaluate the mechanical properties of newly formed tendon tissues.

RESULTS

In this study, we have shown that ERK inhibitor U0126 rather PD98059 could effectively increase the expression of tendon-related genes and promote the tenogenesis of TSPCs in vitro. To explore the underlying mechanisms, RNA sequencing was performed to identify the molecular difference between U0126-treated and control TSPCs. The result showed that GDF6 was significantly increased by U0126, which is an important factor of the TGFβ superfamily regulating tendon development and tenogenesis. In addition, NBM (nonwoven-based gelatin/polycaprolactone membrane) which mimics the native microenvironment of the tendon tissue was used as an acellular scaffold to carry U0126. The results demonstrated that when NBM was used in combination with U0126, tendon healing was significantly promoted with better histological staining outcomes and mechanical properties.

CONCLUSION

Taken together, we have found U0126 promoted tenogenesis in TSPCs through activating GDF6, and NBM loaded with U0126 significantly promoted tendon defect healing, which provides a new treatment for tendon injury.

Transcriptional profiling of mESC-derived tendon and fibrocartilage cell fate switch

The transcriptional regulators underlying induction and differentiation of dense connective tissues such as tendon and related fibrocartilaginous tissues (meniscus and annulus fibrosus) remain largely unknown. Using an iterative approach informed by developmental cues and single cell RNA sequencing (scRNA-seq), we establish directed differentiation models to generate tendon and fibrocartilage cells from mouse embryonic stem cells (mESCs) by activation of TGFβ and hedgehog pathways, achieving 90% induction efficiency. Transcriptional signatures of the mESC-derived cells recapitulate embryonic tendon and fibrocartilage signatures from the mouse tail. scRNA-seq further identify retinoic acid signaling as a critical regulator of cell fate switch between TGFβ-induced tendon and fibrocartilage lineages. Trajectory analysis by RNA sequencing define transcriptional modules underlying tendon and fibrocartilage fate induction and identify molecules associated with lineage-specific differentiation. Finally, we successfully generate 3-dimensional engineered tissues using these differentiation protocols and show activation of mechanotransduction markers with dynamic tensile loading. These findings provide a serum-free approach to generate tendon and fibrocartilage cells and tissues at high efficiency for modeling development and disease.

Skeletal Muscle Transcriptome Analysis of Hanzhong Ma Duck at Different Growth Stages Using RNA-Seq

As one of the most important poultry worldwide, ducks (Anas platyrhynchos) are raised mainly for meat and egg products, and muscle development in ducks is important for meat production. Therefore, an investigation of gene expression in duck skeletal muscle would significantly contribute to our understanding of muscle development. In this study, twenty-four cDNA libraries were constructed from breast and leg muscles of Hanzhong Ma ducks at day 17, 21, 27 of the embryo and postnatal at 6-month-old. High-throughput sequencing and bioinformatics were used to determine the abundances and characteristics of transcripts. A total of 632,172,628 (average 52,681,052) and 637,213,938 (average 53,101,162) reads were obtained from the sequencing data of breast and leg muscles, respectively. Over 71.63% and 77.36% of the reads could be mapped to the Anas platyrhynchos genome. In the skeletal muscle of Hanzhong duck, intron variant (INTRON), synonymous variant (SYNONYMOUS_CODING), and prime 3' UTR variant (UTR_3_PRIME) were the main single nucleotide polymorphisms (SNP) annotation information, and "INTRON", "UTR_3_PRIME", and downstream-gene variant (DOWNSTREAM) were the main insertion-deletion (InDel) annotation information. The predicted number of alternative splicing (AS) in all samples were mainly alternative 5' first exon (transcription start site)-the first exon splicing (TSS) and alternative 3' last exon (transcription terminal site)-the last exon splicing (TTS). Besides, there were 292 to 2801 annotated differentially expressed genes (DEGs) in breast muscle and 304 to 1950 annotated DEGs in leg muscle from different databases. It is worth noting that 75 DEGs in breast muscle and 49 DEGs in leg muscle were co-expressed at all developmental points of comparison, respectively. The RNA-Seq data were confirmed to be reliable by qPCR. The identified DEGs, such as CREBL2, RHEB, GDF6, SHISA2, MYLK2, ACTN3, RYR3, and STMN1, were specially highlighted, indicating their strong associations with muscle development in the Hanzhong Ma duck. KEGG pathway analysis suggested that regulation of actin cytoskeleton, oxidative phosphorylation, and focal adhesion were involved in the development of skeletal muscle. The findings from this study can contribute to future investigations of the growth and development mechanism in duck skeletal muscle.

Regulation of tendon and ligament differentiation

Tendons transmit power from muscles to bones, and ligaments maintain the stability of joints, thus producing smooth and flexible movements of articular joints. However, tendons have poor self-healing ability upon damage due to injuries, diseases, or aging. To maintain homeostasis or promote regeneration of the tendon/ligament, it is critical to understand the mechanism responsible for the coordination of tendon/ligament-specific gene expression and subsequent cell differentiation. In this review, we have discussed the core molecular mechanisms involved in the development and homeostasis of tendons and ligaments, with particular focus on transcription factors, signaling, and mechanical stress.

Growth differentiation factor 6, a repressive target of EZH2, promotes the commitment of human embryonic stem cells to mesenchymal stem cells

Mesenchymal stem cells (MSCs) derived from human embryonic stem cells (hESCs) have significant potential for cell-mediated bone regeneration. Our recent study revealed that inhibiting the epigenetic regulator EZH2 plays a key role in promoting the mesodermal differentiation of hESCs. In this study, an epigenome-wide analysis of hESCs and MSCs revealed that growth differentiation factor 6 (GDF6), which is involved in bone formation, was the most upregulated gene associated with MSCs compared to hESCs. Furthermore, we identified GDF6 as a repressive target of EZH2 and found that ectopic GDF6 selectively promoted hESC differentiation towards the mesodermal lineage and enriched the MSC population. Our results provide molecular insights governing the mesenchymal commitment of hESCs and identify an inducing factor that offers strong promise for the future of regenerative medicine.

Rare heterozygous GDF6 variants in patients with renal anomalies

Although over 50 genes are known to cause renal malformation if mutated, the underlying genetic basis, most easily identified in syndromic cases, remains unsolved in most patients. In search of novel causative genes, whole-exome sequencing in a patient with renal, i.e., crossed fused renal ectopia, and extrarenal, i.e., skeletal, eye, and ear, malformations yielded a rare heterozygous variant in the GDF6 gene encoding growth differentiation factor 6, a member of the BMP family of ligands. Previously, GDF6 variants were reported to cause pleiotropic defects including skeletal, e.g., vertebral, carpal, tarsal fusions, and ocular, e.g., microphthalmia and coloboma, phenotypes. To assess the role of GDF6 in the pathogenesis of renal malformation, we performed targeted sequencing in 193 further patients identifying rare GDF6 variants in two cases with kidney hypodysplasia and extrarenal manifestations. During development, gdf6 was expressed in the pronephric tubule of Xenopus laevis, and Gdf6 expression was observed in the ureteric tree of the murine kidney by RNA in situ hybridization. CRISPR/Cas9-derived knockout of Gdf6 attenuated migration of murine IMCD3 cells, an effect rescued by expression of wild-type but not mutant GDF6, indicating affected variant function regarding a fundamental developmental process. Knockdown of gdf6 in Xenopus laevis resulted in impaired pronephros development. Altogether, we identified rare heterozygous GDF6 variants in 1.6% of all renal anomaly patients and 5.4% of renal anomaly patients additionally manifesting skeletal, ocular, or auricular abnormalities, adding renal hypodysplasia and fusion to the phenotype spectrum of GDF6 variant carriers and suggesting an involvement of GDF6 in nephrogenesis.

Atypical protein kinase C iota (PKCλ/ι) ensures mammalian development by establishing the maternal-fetal exchange interface

In utero mammalian development relies on the establishment of the maternal-fetal exchange interface, which ensures transportation of nutrients and gases between the mother and the fetus. This exchange interface is established via development of multinucleated syncytiotrophoblast cells (SynTs) during placentation. In mice, SynTs develop via differentiation of the trophoblast stem cell-like progenitor cells (TSPCs) of the placenta primordium, and in humans, SynTs are developed via differentiation of villous cytotrophoblast (CTB) progenitors. Despite the critical need in pregnancy progression, conserved signaling mechanisms that ensure SynT development are poorly understood. Herein, we show that atypical protein kinase C iota (PKCλ/ι) plays an essential role in establishing the SynT differentiation program in trophoblast progenitors. Loss of PKCλ/ι in the mouse TSPCs abrogates SynT development, leading to embryonic death at approximately embryonic day 9.0 (E9.0). We also show that PKCλ/ι-mediated priming of trophoblast progenitors for SynT differentiation is a conserved event during human placentation. PKCλ/ι is selectively expressed in the first-trimester CTBs of a developing human placenta. Furthermore, loss of PKCλ/ι in CTB-derived human trophoblast stem cells (human TSCs) impairs their SynT differentiation potential both in vitro and after transplantation in immunocompromised mice. Our mechanistic analyses indicate that PKCλ/ι signaling maintains expression of GCM1, GATA2, and PPARγ, which are key transcription factors to instigate SynT differentiation programs in both mouse and human trophoblast progenitors. Our study uncovers a conserved molecular mechanism, in which PKCλ/ι signaling regulates establishment of the maternal-fetal exchange surface by promoting trophoblast progenitor-to-SynT transition during placentation.

Aspirin promotes tenogenic differentiation of tendon stem cells and facilitates tendinopathy healing through regulating the GDF7/Smad1/5 signaling pathway

Tendinopathy is a common musculoskeletal system disorder in sports medicine, but regeneration ability of injury tendon is limited. Tendon stem cells (TSCs) have shown the definitive treatment evidence for tendinopathy and tendon injuries due to their tenogenesis capacity. Aspirin, as the representative of nonsteroidal anti-inflammatory drugs for its anti-inflammatory and analgestic actions, has been commonly used in treating tendinopathy in clinical, but the effect of aspirin on tenogenesis of TSCs is unclear. We hypothesized that aspirin could promote injury tendon healing through inducing TSCs tenogenesis. The aim of the present study is to make clear the effect of aspirin on TSC tenogenesis and tendon healing in tendinopathy, and thus provide new treatment evidence and strategy of aspirin for clinical practice. First, TSCs were treated with aspirin under tenogenic medium for 3, 7, and 14 days. Sirius Red staining was performed to observe the TSC differentiation. Furthermore, RNA sequencing was utilized to screen out different genes between the induction group and aspirin treatment group. Then, we identified the filtrated molecules and compared their effect on tenogenesis and related signaling pathway. At last, we constructed the tendinopathy model and compared biomechanical changes after aspirin intake. From the results, we found that aspirin promoted tenogenesis of TSCs. RNA sequencing showed that growth differentiation factor 6 (GDF6), GDF7, and GDF11 were upregulated in induction medium with the aspirin group compared with the induction medium group. GDF7 increased tenogenesis and activated Smad1/5 signaling. In addition, aspirin increased the expression of TNC, TNMD, and Scx and biomechanical properties of the injured tendon. In conclusion, aspirin promoted TSC tenogenesis and tendinopathy healing through GDF7/Smad1/5 signaling, and this provided new treatment evidence of aspirin for tendinopathy and tendon injuries.

Widespread cis-regulatory convergence between the extinct Tasmanian tiger and gray wolf

The extinct marsupial Tasmanian tiger, or thylacine, and the eutherian gray wolf are among the most widely recognized examples of convergent evolution in mammals. Despite being distantly related, these large predators independently evolved extremely similar craniofacial morphologies, and evidence suggests that they filled similar ecological niches. Previous analyses revealed little evidence of adaptive convergence between their protein-coding genes. Thus, the genetic basis of their convergence is still unclear. Here, we identified candidate craniofacial cis-regulatory elements across vertebrates and compared their evolutionary rates in the thylacine and wolf, revealing abundant signatures of convergent positive selection. Craniofacial thylacine-wolf accelerated regions were enriched near genes involved in TGF beta (TGFB) and BMP signaling, both of which are key morphological signaling pathways with critical roles in establishing the identities and boundaries between craniofacial tissues. Similarly, enhancers of genes involved in craniofacial nerve development showed convergent selection and involvement in these pathways. Taken together, these results suggest that adaptation in cis-regulators of TGF beta and BMP signaling may provide a mechanism to explain the coevolution of developmentally and functionally integrated craniofacial structures in these species. We also found that despite major structural differences in marsupial and eutherian brains, accelerated regions in both species were common near genes with roles in brain development. Our findings support the hypothesis that, relative to protein-coding genes, positive selection on cis-regulatory elements is likely to be an essential driver of adaptive convergent evolution and may underpin thylacine-wolf phenotypic similarities.

Stem Cell Therapy for Tendon Regeneration: Current Status and Future Directions

In adults the healing tendon generates fibrovascular scar tissue and recovers never histologically, mechanically, and functionally which leads to chronic and to degenerative diseases. In this review, the processes and mechanisms of tendon development and fetal regeneration in comparison to adult defect repair and degeneration are discussed in relation to regenerative therapeutic options. We focused on the application of stem cells, growth factors, transcription factors, and gene therapy in tendon injury therapies in order to intervene the scarring process and to induce functional regeneration of the lesioned tissue. Outlines for future therapeutic approaches for tendon injuries will be provided.

Fibroblast Growth Factor 2 Enhances Tendon-to-Bone Healing in a Rat Rotator Cuff Repair of Chronic Tears

BACKGROUND

The effects of fibroblast growth factor 2 (FGF-2) on healing after surgical repair of chronic rotator cuff (RC) tears remain unclear.

HYPOTHESIS

FGF-2 enhances tenogenic healing response, leading to biomechanical and histological improvement of repaired chronic RC tears in rats.

STUDY DESIGN

Controlled laboratory study.

METHODS

Adult male Sprague-Dawley rats (n = 117) underwent unilateral surgery to refix the supraspinatus tendon to its insertion site 3 weeks after detachment. Animals were assigned to either the FGF-2 group or a control group. The effects of FGF-2 were assessed via biomechanical tests at 3 weeks after detachment and at 6 and 12 weeks postoperatively and were assessed histologically and immunohistochemically for proliferating cell nuclear antigen and mesenchymal stem cell (MSC)-related markers at 2, 6, and 12 weeks postoperatively. The expression of tendon/enthesis-related markers, including SRY-box 9 (Sox9), scleraxis (Scx), and tenomodulin (Tnmd), were assessed by real-time reverse transcription polymerase chain reaction, in situ hybridization, and immunohistochemistry. The effect of FGF-2 on comprehensive gene expressions at the healing site was evaluated by microarray analysis.

RESULTS

The FGF-2 group showed a significant increase in mechanical strength at 6 and 12 weeks compared with control; the FGF-2 group also showed significantly higher histological scores at 12 weeks than control, indicating the presence of more mature tendon-like tissue. At 12 weeks, Scx and Tnmd expression increased significantly in the FGF-2 group, whereas no significant differences in Sox9 were found between groups over time. At 2 weeks, the percentage of positive cells expressing MSC-related markers increased in the FGF-2 group. Microarray analysis at 2 weeks after surgery showed that the expression of several growth factor genes and extracellular matrix-related genes was influenced by FGF-2 treatment.

CONCLUSION

FGF-2 enhanced the formation of tough tendon-like tissues including an increase in Scx- or Tnmd-expressing cells at 12 weeks after surgical repair of chronic RC tears. The increase in mesenchymal progenitors and the changes in gene expression upon FGF-2 treatment in the early phase of healing appear to be related to a certain favorable microenvironment for tenogenic healing response of chronic RC tears.

CLINICAL RELEVANCE

These findings may provide advantages in therapeutic strategies for patients with RC tears.

Fos Promotes Early Stage Teno-Lineage Differentiation of Tendon Stem/Progenitor Cells in Tendon

Stem cells have been widely used in tendon tissue engineering. The lack of refined and controlled differentiation strategy hampers the tendon repair and regeneration. This study aimed to find new effective differentiation factors for stepwise tenogenic differentiation. By microarray screening, the transcript factor Fos was found to be expressed in significantly higher amounts in postnatal Achilles tendon tissue derived from 1 day as compared with 7‐days‐old rats. It was further confirmed that expression of Fos decreased with time in postnatal rat Achilles tendon, which was accompanied with the decreased expression of multiply tendon markers. The expression of Fos also declined during regular in vitro cell culture, which corresponded to the loss of tendon phenotype. In a cell‐sheet and a three‐dimensional cell culture model, the expression of Fos was upregulated as compared with in regular cell culture, together with the recovery of tendon phenotype. In addition, significant higher expression of tendon markers was found in Fos‐overexpressed tendon stem/progenitor cells (TSPCs), and Fos knock‐down gave opposite results. In situ rat tendon repair experiments found more normal tendon‐like tissue formed and higher tendon markers expression at 4 weeks postimplantation of Fos‐overexpressed TSPCs derived nonscaffold engineering tendon (cell‐sheet), as compared with the control group. This study identifies Fos as a new marker and functional driver in the early stage teno‐lineage differentiation of tendon, which paves the way for effective stepwise tendon differentiation and future tendon regeneration. Stem Cells Translational Medicine2017;6:2009–2019

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.

Novel Model of Tendon Regeneration Reveals Distinct Cell Mechanisms Underlying Regenerative and Fibrotic Tendon Healing

To date, the cell and molecular mechanisms regulating tendon healing are poorly understood. Here, we establish a novel model of tendon regeneration using neonatal mice and show that neonates heal via formation of a ‘neo-tendon’ that differentiates along the tendon specific lineage with functional restoration of gait and mechanical properties. In contrast, adults heal via fibrovascular scar, aberrant differentiation toward cartilage and bone, with persistently impaired function. Lineage tracing identified intrinsic recruitment of Scx-lineage cells as a key cellular mechanism of neonatal healing that is absent in adults. Instead, adult Scx-lineage tenocytes are not recruited into the defect but transdifferentiate into ectopic cartilage; in the absence of tenogenic cells, extrinsic αSMA-expressing cells persist to form a permanent scar. Collectively, these results establish an exciting model of tendon regeneration and uncover a novel cellular mechanism underlying regenerative vs non-regenerative tendon healing.

Current concepts on tenogenic differentiation and clinical applications

Tendon is a tissue that transmits force from muscle to bone. Chronic or acute tendon injuries are very common, and are always accompanied by pain and a limited range of motion in patients. In clinical settings, management of tendon injuries still remains a big challenge. Cell therapies, such as the application of stem cells for tenogenic differentiation, were suggested to be an ideal strategy for clinical translation. However, there is still a lack of specific methods for tenogenic differentiation due to the limited understanding of tendon biology currently. This review focuses on the summary of current published strategies for tenogenic differentiation, such as the application of growth factors, mechanical stimulation, biomaterials, coculture, or induced pluripotent stem cells. Current clinical applications of stem cells for treatment of tendon injuries and their limitations have also been discussed in this review.

Myostatin promotes tenogenic differentiation of C2C12 myoblast cells through Smad3

Myostatin, a member of the transforming growth factor-β (TGF-β) superfamily, is expressed in developing and adult skeletal muscle and negatively regulates skeletal muscle growth. Recently, myostatin has been found to be expressed in tendons and increases tendon fibroblast proliferation and the expression of tenocyte markers. C2C12 is a mouse myoblast cell line, which has the ability to transdifferentiate into osteoblast and adipocyte lineages. We hypothesized that myostatin is capable of inducing tenogenic differentiation of C2C12 cells. We found that the expression of scleraxis, a tendon progenitor cell marker, is much higher in C2C12 than in the multipotent mouse mesenchymal fibroblast cell line C3H10T1/2. In comparison with other growth factors, myostatin significantly up-regulated the expression of the tenogenic marker in C2C12 cells under serum-free culture conditions. Immunohistochemistry showed that myostatin inhibited myotube formation and promoted the formation of spindle-shaped cells expressing tenomodulin. We examined signaling pathways essential for tenogenic differentiation to clarify the mechanism of myostatin-induced differentiation of C2C12 into tenocytes. The expression of tenomodulin was significantly suppressed by treatment with the ALK inhibitor SB341542, in contrast to p38MAPK (SB203580) and MEK1 (PD98059) inhibitors. RNAi silencing of Smad3 significantly suppressed myostatin-induced tenomodulin expression. These results indicate that myostatin has a potential role in the induction of tenogenic differentiation of C2C12 cells, which have tendon progenitor cell characteristics, through activation of Smad3-mediated signaling.

Therapeutic Roles of Tendon Stem/Progenitor Cells in Tendinopathy

Tendinopathy is a tendon disorder characterized by activity-related pain, local edema, focal tenderness to palpation, and decreased strength in the affected area. Tendinopathy is prevalent in both athletes and the general population, highlighting the need to elucidate the pathogenesis of this disorder. Current treatments of tendinopathy are both conservative and symptomatic. The discovery of tendon stem/progenitor cells (TSPCs) and erroneous differentiation of TSPCs have provided new insights into the pathogenesis of tendinopathy. In this review, we firstly present the histopathological characteristics of tendinopathy and explore the cellular and molecular cues in the pathogenesis of tendinopathy. Current evidence of the depletion of the stem cell pool and altered TSPCs fate in the pathogenesis of tendinopathy has been presented. The potential regulatory factors for either tenogenic or nontenogenic differentiation of TSPCs are also summarized. The regulation of endogenous TSPCs or supplementation with exogenous TSPCs as therapeutic targets for the treatment of tendinopathy is proposed. Therefore, inhibiting the erroneous differentiation of TSPCs and regulating the differentiation of TSPCs into tendon cells might be important areas of future research and could provide new clinical treatments for tendinopathy. The current evidence suggests that TSPCs are promising therapeutic targets for the management of tendinopathy.

Tenogenic differentiation of mesenchymal stem cells and noncoding RNA: From bench to bedside

Tendon is a critical unit of musculoskeletal system that connects muscle to bone to control bone movement. More population participate in physical activities, tendon injuries, such as acute tendon rupture and tendinopathy due to overuse, are common causing unbearable pain and disability. However, the process of tendon development and the pathogenesis of tendinopathy are not well defined, limiting the development of clinical therapy for tendon injuries. Studying the tendon differentiation control pathways may help to develop novel therapeutic strategies. This review summarized the novel molecular and cellular events in tendon development and highlighted the clinical application potential of non-coding RNAs and tendon-derived stem cells in gene and cell therapy for tendon injuries, which may bring insights into research and new therapy for tendon disorders.

Structural insights into BMP receptors: Specificity, activation and inhibition

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-β family (TGFβ), which signal through hetero-tetrameric complexes of type I and type II receptors. In humans there are many more TGFβ ligands than receptors, leading to the question of how particular ligands can initiate specific signaling responses. Here we review structural features of the ligands and receptors that contribute to this specificity. Ligand activity is determined by receptor-ligand interactions, growth factor prodomains, extracellular modulator proteins, receptor assembly and phosphorylation of intracellular signaling proteins, including Smad transcription factors. Detailed knowledge about the receptors has enabled the development of BMP-specific type I receptor kinase inhibitors. In future these may help to treat human diseases such as fibrodysplasia ossificans progressiva.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

Molecular regulation of tendon cell fate during development

Although there have been several advances identifying novel mediators of tendon induction, differentiation, and patterning, much of the basic landscape of tendon biology from developmental stages onward remain almost completely undefined. During the New Frontiers in Tendon Research meeting, a group of developmental biologists with expertise across musculoskeletal disciplines identified key challenges for the tendon development field. The tools generated and the molecular regulators identified in developmental research have enhanced mechanistic studies in tendon injury and repair, both by defining benchmarks for success, as well as informing regenerative strategies. To address the needs of the orthopedic research community, this review will therefore focus on three key areas in tendon development that may have critical implications for the fields of tendon repair/regeneration and tendon tissue engineering, including functional markers of tendon cell identity, signaling regulators of tendon induction and differentiation, and in vitro culture models for tendon cell differentiation. Our goal is to provide a useful list of the currently known molecular players and their function in tendon differentiation within the context of development.

Induction of transient tenogenic phenotype of high-density cultured human dermal fibroblasts

Previous study showed that high-density culture supported phenotype maintenance of in vitro expanded tenocytes. This study explored the possibility of inducing the tenogenic phenotype of dermal fibroblasts by high-density monolayer culture. Human fibroblasts were seeded either in high-density (2.5 × 10(6) per 10 cm dish) or at low-density (0.36 × 10(6) per 10 cm dish). A preliminary tenogenic phenotype was observed in high-density cultured cells after one passage with significantly enhanced tenogenic gene expression. With continued cultivation to passage 3, scleraxis (SCX), tenomodulin (TNMD), collagen I, III, VI, decorin and tenascin-c were all significantly upregulated in high-density cultured dermal fibroblasts as opposed to low-density cells. High-density culture also led to relatively elongated cell shape, whereas cells appeared in spread shape in low-density culture. In addition, cytochalasin D treatment disrupted the cellular cytoskeleton and resulted in inhibition of density-induced tenogenic gene expression. However, high-density cultured fibroblasts failed to induce other lineage differentiations (osteogenic, chondrogenic and adipogenic). It also failed to induce tenogenic phenotype in high-density cultured chondrocytes. Mechanism studies revealed enhanced gene expression of growth and differentiation factors (GDF) 5, 6, 7 and 8 and transforming growth factor-β (TGF-β)1 in the high-density group and enhanced protein production of both GDF8 and TGF-β1. Moreover, BMP/GDF signaling inhibitor (LDN193189) and TGF-β signaling inhibitor (LY2109761) could both abrogate the density induced phenotype. In conclusion, high-density culture was able to induce transient tenogenic phenotype of dermal fibroblasts likely via cell morphology change and production of pro-tenogenic factors.

Bone Morphogenetic Protein (BMP) signaling in development and human diseases

Bone Morphogenetic Proteins (BMPs) are a group of signaling molecules that belongs to the Transforming Growth Factor-β (TGF-β) superfamily of proteins. Initially discovered for their ability to induce bone formation, BMPs are now known to play crucial roles in all organ systems. BMPs are important in embryogenesis and development, and also in maintenance of adult tissue homeostasis. Mouse knockout models of various components of the BMP signaling pathway result in embryonic lethality or marked defects, highlighting the essential functions of BMPs. In this review, we first outline the basic aspects of BMP signaling and then focus on genetically manipulated mouse knockout models that have helped elucidate the role of BMPs in development. A significant portion of this review is devoted to the prominent human pathologies associated with dysregulated BMP signaling.

Tendon and ligament regeneration and repair: clinical relevance and developmental paradigm

As dense connective tissues connecting bone to muscle and bone to bone, respectively, tendon and ligament (T/L) arise from the somitic mesoderm, originating in a recently discovered somitic compartment, the syndetome. Inductive signals from the adjacent sclerotome and myotome upregulate expression of Scleraxis, a key transcription factor for tenogenic and ligamentogenic differentiation. Understanding T/L development is critical to establishing a knowledge base for improving the healing and repair of T/L injuries, a high-burden disease due to the intrinsically poor natural healing response. Current treatment of the three most common tendon injuries-tearing of the rotator cuff of the shoulder, flexor tendon of the hand, and Achilles tendon-include mostly surgical repair and/or conservative approaches, including biophysical modalities such as rehabilitation and cryotherapy. Unfortunately, the fibrovascular scar formed during healing possesses inferior mechanical and biochemical properties, resulting in compromised tissue functionality. Regenerative approaches have sought to augment the injured tissue with cells, scaffolds, bioactive agents, and mechanical stimulation to improve the natural healing response. The key challenges in restoring full T/L function following injury include optimal combination of these biological agents as well as their delivery to the injury site. A greater understanding of the molecular mechanisms involved in T/L development and natural healing, coupled with the capability of producing complex biomaterials to deliver multiple biofactors with high spatiotemporal resolution and specificity, should lead to regenerative procedures that more closely recapitulate T/L morphogenesis, thereby offering future patients the prospect of T/L regeneration, as opposed to simple tissue repair.

Divergent differentiation of skeletal progenitors into cartilage and tendon: lessons from the embryonic limb

Repairing damaged cartilage and tendons is a major challenge of regenerative medicine. There has been great progress in the past decade toward obtaining stem cells for regenerative purposes from a variety of sources. However, the development of procedures to direct and maintain the differentiation of progenitors into cartilage or tendon is still a hurdle to overcome in regenerative medicine of the musculoskeletal system. This is because connective tissues often lack stable phenotypes and retain plasticity to return to the initial stages of differentiation or to transdifferentiate into another connective tissue cell lineage. This makes it necessary to unravel the molecular basis that is responsible for the differentiation of connective tissue cell lineages. In this review, we summarize the investigations performed in the past two decades to unravel the signals that regulate the differentiation of skeletal cell progenitors into cartilage and tendons during embryonic limb development. The data obtained in those studies demonstrate that Tgfβ, BMP, FGF, and Wnt establish a complex signaling network that directs the differentiation of skeletal cell progenitors. Remarkably, in the embryonic digit model, the divergent differentiation of progenitors depends on the temporal coordination of those signals, rather than being specified by an individual signaling pathway. Due to its potential medical relevance, we highlight the importance of the coordinate influence of the Tgfβ and BMP pathways in the differentiation of cell progenitors into tendon or cartilage.

BMPs are mediators in tissue crosstalk of the regenerating musculoskeletal system

The musculoskeletal system is a tight network of many tissues. Coordinated interplay at a biochemical level between tissues is essential for development and repair. Traumatic injury usually affects several tissues and represents a large challenge in clinical settings. The current demand for potent growth factors in such applications thus accompanies the keen interest in molecular mechanisms and orchestration of tissue formation. Of special interest are multitasking growth factors that act as signals in a variety of cell types, both in a paracrine and in an autocrine manner, thereby inducing cell differentiation and coordinating not only tissue assembly at specific sites but also maturation and homeostasis. We concentrate here on bone morphogenetic proteins (BMPs), which are important crosstalk mediators known for their irreplaceable roles in vertebrate development. The molecular crosstalk during embryonic musculoskeletal tissue formation is recapitulated in adult repair. BMPs act at different levels from the initiation to maturation of newly formed tissue. Interestingly, this is influenced by the spatiotemporal expression of different BMPs, their receptors and co-factors at the site of repair. Thus, the regenerative potential of BMPs needs to be evaluated in the context of highly connected tissues such as muscle and bone and might indeed be different in more poorly connected tissues such as cartilage. This highlights the need for an understanding of BMP signaling across tissues in order to eventually improve BMP regenerative potential in clinical applications. In this review, the distinct members of the BMP family and their individual contribution to musculoskeletal tissue repair are summarized by focusing on their paracrine and autocrine functions.

Tenogenic differentiation of stem cells for tendon repair-what is the current evidence?

Tendon/ligament injuries are very common in sports and other rigorous activities. Tendons regenerate and repair slowly and inefficiently in vivo after injury. The limited ability of tendon to self-repair and the general inefficiencies of current treatment regimes have hastened the motivation to develop tissue-engineering strategies for tissue repair. Of particular interest in recent years has been the use of adult mesenchymal stem cells (MSCs) to regenerate functional tendons and ligaments. Different sources of MSCs have been studied for their effects on tendon repair. However, ectopic bone and tumour formation has been reported in some special circumstances after transplantation of MSCs. The induction of MSCs to differentiate into tendon-forming cells in vitro prior to transplantation is a possible approach to avoid ectopic bone and tumour formation while promoting tendon repair. While there are reports about the factors that might promote tenogenic differentiation, the study of tenogenic differentiation is hampered by the lack of definitive biomarkers for tendons. This review aims to summarize the cell sources currently used for tendon repair as well as their advantages and limitations. Factors affecting tenogenic differentiation were summarized. Molecular markers currently used for assessing tenogenic differentiation or neotendon formation are summarized and their advantages and limitations are commented upon. Finally, further directions for promoting and assessing tenogenic differentiation of stem cells for tendon repair are discussed.

Tendon tissue engineering: adipose-derived stem cell and GDF-5 mediated regeneration using electrospun matrix systems

Tendon tissue engineering with a biomaterial scaffold that mimics the tendon extracellular matrix (ECM) and is biomechanically suitable, and when combined with readily available autologous cells, may provide successful regeneration of defects in tendon. Current repair strategies using suitable autografts and freeze-dried allografts lead to a slow repair process that is sub-optimal and fails to restore function, particularly in difficult clinical situations such as zone II flexor tendon injuries of the hand. We have investigated the effect of GDF-5 on cell proliferation and gene expression by primary rat adipose-derived stem cells (ADSCs) that were cultured on a poly(DL-lactide-co-glycolide) PLAGA fiber scaffold and compared to a PLAGA 2D film scaffold. The electrospun scaffold mimics the collagen fiber bundles present in native tendon tissue, and supports the adhesion and proliferation of multipotent ADSCs. Gene expression of scleraxis, the neotendon marker, was upregulated seven- to eightfold at 1 week with GDF-5 treatment when cultured on a 3D electrospun scaffold, and was significantly higher at 2 weeks compared to 2D films with or without GDF-5 treatment. Expression of the genes that encode the major tendon ECM protein, collagen type I, was increased by fourfold starting at 1 week on treatment with 100 ng mL(-1) GDF-5, and at all time points the expression was significantly higher compared to 2D films irrespective of GDF-5 treatment. Thus stimulation with GDF-5 can modulate primary ADSCs on a PLAGA fiber scaffold to produce a soft, collagenous musculoskeletal tissue that fulfills the need for tendon regeneration.

Sexual dimorphism in the effect of GDF-6 deficiency on murine tendon

Three members of the growth/differentiation factor (GDF) subfamily of bone morphogenetic proteins (BMPs), GDFs-5, -6, and -7, have demonstrated the potential to augment tendon and ligament repair. To gain further insight into the in vivo role of these molecules, previous studies have characterized intact and healing tendons in mice with functional null mutations in GDF-5 and -7. The primary goal of the present study was to perform a detailed characterization of the intact tendon phenotype in 4- and 16-week-old male and female GDF6-/- mice and their +/+ littermates. The results demonstrate that GDF6 deficiency was associated with an altered tendon phenotype that persisted into adulthood. Among males, GDF6-/- tail tendon fascicles had significantly less collagen and glycosaminoglycan content, and these compositional differences were associated with compromised material properties. The effect of GDF6 deficiency on tendon was sexually dimorphic, however, for among female GDF6-/- mice, neither differences in tendon composition nor in material properties were detected. The tendon phenotype that was observed in males appeared to be stronger in the tail site than in the Achilles tendon site, where some compositional differences were present, but no material property differences were detected. These data support existing in vitro studies, which suggest a potential role for BMP-13 (the human homologue to GDF-6) in tendon matrix modeling and/or remodeling.