GDF-5 deficiency in mice delays Achilles tendon healing

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.

What we should know before using tissue engineering techniques to repair injured tendons: a developmental biology perspective

Tendons connect muscles to bones, and serve as the transmitters of force that allow all the movements of the body. Tenocytes are the basic cellular units of tendons, and produce the collagens that form the hierarchical fiber system of the tendon. Tendon injuries are common, and difficult to repair, particularly in the case of the insertion of tendon into bone. Successful attempts at cell-based repair therapies will require an understanding of the normal development of tendon tissues, including their differentiated regions such as the fibrous mid-section and fibrocartilaginous insertion site. Many genes are known to be involved in the formation of tendon. However, their functional roles in tendon development have not been fully characterized. Tissue engineers have attempted to generate functional tendon tissue in vitro. However, a lack of knowledge of normal tendon development has hampered these efforts. Here we review studies focusing on the developmental mechanisms of tendon development, and discuss the potential applications of a molecular understanding of tendon development to the treatment of tendon injuries.

Growth/differentiation factor-5 modulates the synthesis and expression of extracellular matrix and cell-adhesion-related molecules of rat Achilles tendon fibroblasts

This study was designed to examine the cellular and molecular response of tendon fibroblasts to growth/differentiation factor-5 (GDF-5). Rat Achilles tendon fibroblasts (ATFs) were treated in culture with varying concentrations of GDF-5 (0-1000 ng/ml) over varying periods of time (0-12 days). Cell proliferation, evaluated through use of a standard MTT colorimetric assay, confirmed that GDF-5 stimulates ATF proliferation in a concentration- and time-dependent fashion. Temporal and concentration analysis revealed that GDF-5 increases total DNA, glycosaminoglycan (GAG), and hydroxyproline (HYP) content. Ratios of HYP/DNA and GAG/DNA increased with increasing concentrations of GDF-5 (0-1000 ng/ml). Expression of the following 12 extracellular matrix (ECM) and cell-adhesion-related genes was assessed using real-time reverse transcriptase polymerase chain reaction (RT-PCR): collagen I (col I), collagen III (col III), matrix metalloproteinases (MMP)-3 and -13, aggrecan, tissue inhibitor of matrix metalloproteinase (TIMP)-2, syndecan-4, N-cadherin, tenascin-C, biglycan, versican, and decorin. RT-PCR data revealed an increase in the expression of col I, col III, MMP-3, MMP-13, TIMP-2, syndecan-4, N-cadherin, tenascin-C, and aggrecan genes by day 6. A statistically significant decrease in TIMP-2 and MMP-13 was observed on day 12. Decorin expression was depressed at all time points in cells treated with GDF-5. There was no significant change in biglycan expression in ATFs supplemented with GDF-5. These findings suggest that GDF-5 induces cellular proliferation and ECM synthesis as well as expression of ECM and cell-adhesion-related genes in ATFs. This study further defines the influence of GDF-5 on rat ATFs through its action on the expression of genes that are associated with tendon ECM.

Adipose-derived mesenchymal stem cells treated with growth differentiation factor-5 express tendon-specific markers

OBJECTIVES

Adipose-derived mesenchymal stem cells (ADMSCs) are a unique population of stem cells with therapeutic potential in the treatment of connective tissue injuries. Growth differentiation factor-5 (GDF)-5 is known to play a role in tendon repair and maintenance. The aim of this study was to investigate the effects of GDF-5 on proliferation and tendonogenic gene expression of rat ADMSCs.

METHODS

ADMSCs were treated in culture with different concentrations of GDF-5 (0-1000 ng/mL) for 12 days. Biochemical, temporal, and concentration kinetic studies were done. Extracellular matrix (ECM) synthesis, tendonogenic differentiation, and matrix remodeling gene and protein expression were analyzed.

RESULTS

GDF-5 led to increased ADMSC proliferation in a dose- and time-dependent manner. ADMSCs demonstrated enhanced ECM (collagen type I, decorin, and aggrecan) and tendonogenic marker (scleraxis, tenomodulin, and tenascin-C) gene expression with 100 ng/mL of GDF-5 (p < 0.05). ECM and tendon-specific markers showed time-dependent increases at various time points (p < 0.05), although decorin decreased at day 9 (p < 0.05). GDF-5 did alter expression of matrix remodeling genes, with no specific trends observed. Western blot analysis confirmed dose- and time-dependent increases in protein expression of tenomodulin, tenascin-C, Smad-8, and matrix metalloproteinase-13.

CONCLUSION

In vitro GDF-5 treatment can induce cellular events leading to the tendonogenic differentiation of ADMSCs. The use of combined GDF-5 and ADMSCs tissue-engineered therapies may have a role in the future of tendon repair.

Augmentation of zone II flexor tendon repair using growth differentiation factor 5 in a rabbit model

PURPOSE

Modulation of zone II flexor tendon repair healing using growth factors may reduce the incidence of complications, such as rupture and fibrosis. We hypothesized that sutures coated with growth differentiation factor 5 (GDF5) will stimulate the healing of zone II flexor tendon repairs.

METHODS

We created and immediately repaired zone II flexor tendon lacerations in the second and fourth toe of the right forepaw of 44 New Zealand White rabbits. One tendon was repaired with suture coated with GDF5, whereas the other tendon was repaired with suture without GDF5 (control). We randomized the allocation of GDF5 and control suture to either toe. A proximal tenotomy of the flexor digitorum profundus at the level of the wrist was performed to relieve tension on the more distal repairs. Rabbits were euthanized at 21 or 42 days after repair. Four rabbits (8 tendons) underwent histological analysis at each time point; the remaining repairs were tested biomechanically in a blinded fashion.

RESULTS

Control tendons demonstrated distinct borders at the transection site and less endogenous repair at 3 weeks. The Soslowsky histological score for collagen was better in the GDF5 group at both time points (p≤.003). All tendons failed at the repair site. The maximum load was significantly greater (p=.04) in the GDF5 group (11.6 ± 3.5 N) compared with control tendons (8.6 ± 3.0 N) at 3 weeks. The maximum load was not significantly different (p=.12) at 6 weeks. We observed no significant differences in stiffness at either time point (p>.11).

CONCLUSIONS

The results demonstrate that GDF5 has an early beneficial effect on tendon healing in zone II flexor tendon repairs in a rabbit flexor tendon injury model.

Histologic stages of healing correlate with restoration of tensile strength in a model of experimental tendon repair

Much current research is focused on biologic enhancement of the tendon repair process. To evaluate the different methods, which include a variety of gene therapy and tissue engineering techniques, histological and biomechanical testing is often employed. Both modalities offer information on the progress and quality of repair; however, they have been historically considered as two separate entities. Histological evaluation is a less costly undertaking; however, there is no validated scoring scale to compare the results of different studies or even the results within a given study. Biomechanical testing can provide validated outcome measures; however, it is associated with increased cost and is more labor intensive. We hypothesized that a properly developed, objective histological scoring system would provide a validated outcome measure to compare histological results and correlate with biomechanics. In an Achilles tendon model, we have developed a histological scoring scale to assess tendon repair. The system grades collagen orientation, angiogenesis, and cartilage induction. In this study, histology scores were plotted against biomechanical testing results of healing tendons which indicated that a strong linear correlation exists between the histological properties of repaired tendons and their biomechanical characteristics. Concordantly, this study provides a pragmatic and financially feasible means of evaluating repair while accounting for both the histology and biomechanical properties observed in surgically repaired, healing tendon.

Novel strategies in tendon and ligament tissue engineering: Advanced biomaterials and regeneration motifs

Tendon and ligaments have poor healing capacity and when injured often require surgical intervention. Tissue replacement via autografts and allografts are non-ideal strategies that can lead to future problems. As an alternative, scaffold-based tissue engineering strategies are being pursued. In this review, we describe design considerations and major recent advancements of scaffolds for tendon/ligament engineering. Specifically, we outline native tendon/ligament characteristics critical for design parameters and outcome measures, and introduce synthetic and naturally-derived biomaterials used in tendon/ligament scaffolds. We will describe applications of these biomaterials in advanced tendon/ligament engineering strategies including the utility of scaffold functionalization, cyclic strain, growth factors, and interface considerations. The goal of this review is to compile and interpret the important findings of recent tendon/ligament engineering research in an effort towards the advancement of regenerative strategies.

Role of myostatin (GDF-8) signaling in the human anterior cruciate ligament

Myostatin, also referred to as growth and differentiation factor-8 (GDF-8), is expressed in muscle tissue where it functions to suppress myoblast proliferation and myofiber hypertrophy. Recently, myostatin and its receptor, the type IIB activin receptor (ActRIIB), were detected in the leg tendons of mice, and recombinant myostatin was shown to increase cellular proliferation and the expression of type 1 collagen in primary fibroblasts from mouse tendons. We sought to determine whether myostatin and its receptor were present in human anterior cruciate ligament (ACL) tissue, and if myostatin treatment had any effect on primary ACL fibroblasts. ACL tissue samples were obtained from material discarded during ACL reconstruction surgery. Real-time PCR and immunohistochemistry demonstrate that both myostatin and its receptor are abundant in the human ACL. Primary fibroblasts isolated from human ACL specimens were treated with recombinant myostatin, and myostatin treatment increased fibroblast proliferation as well as the expression of tenascin C (TNC), type 1 collagen, and transforming growth factor-beta1. Real-time PCR analysis of TNC and type 1 collagen expression in ACL specimens from normal mice and mice lacking myostatin supported these findings by showing that both TNC and type 1 collagen were downregulated in ACL tissue from myostatin-deficient mice. Together, these data suggest that myostatin is a pro-fibrogenic factor that enhances cellular proliferation and extracellular matrix synthesis by ACL fibroblasts. Recombinant myostatin may therefore have therapeutic applications in the area of tendon and ligament engineering and regeneration.

Growth differentiation factor 5 regulates cardiac repair after myocardial infarction

OBJECTIVES

The aim of this study was to examine the function of the bone morphogenic protein growth differentiation factor 5 (Gdf5) in a mouse model of myocardial infarction (MI).

BACKGROUND

The Gdf5 has been implicated in skeletal development, but a potential role in the heart had not been studied.

METHODS

The Gdf5-knockout (KO) and wild-type (WT) mice were subjected to permanent left anterior descending coronary artery (LAD) ligation. Cardiac pathology, function, gene expression levels, and signaling pathways downstream of Gdf5 were examined. Effects of recombinant Gdf5 (rGdf5) were tested in primary cardiac cell cultures.

RESULTS

The WT mice showed increased cardiac Gdf5 levels after MI, with increased expression in peri-infarct cardiomyocytes and myofibroblasts. At 1 and 7 days after MI, no differences were observed in ischemic or infarct areas between WT and Gdf5-KO mice. However, by 28 days after MI, Gdf5-KO mice exhibited increased infarct scar expansion and thinning with decreased arteriolar density compared with WT. The Gdf5-KO hearts also displayed increased left ventricular dilation, with decreased contractility after MI. At 4 days after MI, Gdf5-KO mice exhibited increased cardiomyocyte apoptosis and decreased expression of anti-apoptotic genes Bcl2 and Bcl-xL compared with WT. Unexpectedly, Gdf5-KO hearts displayed increased Smad 1/5/8 phosphorylation but decreased p38-mitogen-activated protein kinase (MAPK) phosphorylation versus WT. The latter was associated with increased collagen gene (Col1a1, Col3a1) expression and fibrosis. In cultures, rGdf5 induced p38-MAPK phosphorylation in cardiac fibroblasts and Smad-dependent increases in Bcl2 and Bcl-xL in cardiomyocytes.

CONCLUSIONS

Increased expression of Gdf5 after MI limits infarct scar expansion in vivo. These effects might be mediated by Gdf5-induced p38-MAPK signaling in fibroblasts and Gdf5-driven Smad-dependent pro-survival signaling in cardiomyocytes.

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.

Synergistic effects of growth and differentiation factor-5 (GDF-5) and insulin on expanded chondrocytes in a 3-D environment

OBJECTIVE

To investigate the effects of growth and differentiation factor-5 (GDF-5) alone or in combination with insulin on engineered cartilage from primary or expanded chondrocytes during 3-dimensional in vitro culture.

DESIGN

Juvenile bovine chondrocytes were seeded either as primary or as expanded (passage 2) cells onto polyglycolic acid fiber meshes and cultured for 3 weeks in vitro. Additionally, adult human chondrocytes were grown in pellet culture after expansion (passage 2). The culture medium was supplemented either with GDF-5 in varying concentrations or insulin alone, or with combinations thereof.

RESULTS

For primary chondrocytes, the combination of GDF-5 and insulin led to increased proliferation and construct weight, as compared to either factor alone, however, the production of glycosaminoglycans (GAG) and collagen per cell were not affected. With expanded bovine chondrocytes, the use of GDF-5 or insulin alone led to only very small constructs with no type II collagen detectable. However, the combination of GDF-5 (0.01 or 0.1 microg/ml) and insulin (2.5 microg/ml) yielded cartilaginous constructs and, in contrast to the primary cells, the observed redifferentiating effects were elicited on the cellular level independent of proliferation (increased production of GAG and collagen per cell, clear shift in collagen subtype expression with type II collagen observed throughout the construct). The synergistic redifferentiating effects of the GDF-5/insulin combination were confirmed with expanded adult human cells, also exhibiting a clear shift in collagen subtype expression on the mRNA and protein level.

CONCLUSIONS

In combination with insulin, GDF-5 appears to enable the redifferentiation of expanded chondrocytes and the concurrent generation of cartilaginous constructs. The demonstration of these synergistic effects also for adult human chondrocytes supports the clinical relevance of the findings.

Remodeling of murine intrasynovial tendon adhesions following injury: MMP and neotendon gene expression

Tendon injury frequently results in the formation of adhesions that reduce joint range of motion. To study the cellular, molecular, and biomechanical events involved in intrasynovial tendon healing and adhesion formation, we developed a murine flexor tendon healing model in which the flexor digitorum longus (FDL) tendon of C57BL/6 mice was transected and repaired using suture. This model was used to test the hypothesis that murine flexor tendons heal with differential expression of matrix metalloproteases (MMPs), resulting in the formation of scar tissue as well as the subsequent remodeling of scar and adhesions. Healing tendons were evaluated by histology, gene expression via real-time RT-PCR, and in situ hybridization, as well as biomechanical testing to assess the metatarsophalangeal (MTP) joint flexion range of motion (ROM) and the tensile failure properties. Tendons healed with a highly disorganized fibroblastic tissue response that was progressively remodeled through day 35 resulting in a more organized pattern of collagen fibers. Initial repair involved elevated levels of Mmp-9 at day 7, which is associated with catabolism of damaged collagen fibers. High levels of Col3 are consistent with scar tissue, and gradually transition to the expression of Col1. Scleraxis expression peaked at day 7, but the expression was limited to the original tendon adjacent to the injury site, and no expression was present in granulation tissue involved in the repair response. The MTP joint ROM with standardized force on the tendon was decreased on days 14 and 21 compared to day 0, indicating the presence of adhesions. Peak expressions of Mmp-2 and Mmp-14 were observed at day 21, associated with tendon remodeling. At day 28, two genes associated with neotendon formation, Smad8 and Gdf-5, were elevated and an improvement in MTP ROM occurred. Tensile strength of the tendon progressively increased, but by 63 days the repaired tendons had not reached the tensile strength of normal tendon. The murine model of primary tendon repair, described here, provides a novel mechanism to study the tendon healing process, and further enhances the understanding of this process at the molecular, cellular, and biomechanical level.

Identification of a tendon phenotype in GDF6 deficient mice

Increasing evidence suggests that the growth/differentiation factors, GDFs 5, 6, and 7 in particular, may play a role in tendon and ligament biology. Mice with genetic mutations in Gdf5 have altered tendon composition and mechanical behavior, whereas animals with functional null mutations in Gdf7 have a more subtle tendon phenotype. The present study demonstrates for the first time that a null mutation in Gdf6 is associated with substantially lower levels of tail tendon collagen content (-33%) in 4-week-old male mice, which has direct functional consequences for the mechanical integrity of the tissue (45-50% reduction in material properties). These data support a role for GDF6 in tendon matrix modeling.

Assessment of the potential of growth factors for localized alveolar ridge augmentation: a systematic review

OBJECTIVE

To systematically assess the literature regarding the clinical, histological, and radiographic outcome of bone morphogenetic proteins (BMP-2, BMP-7), growth/differentiation factor-5 (GDF-5), platelet-derived growth factor (PDGF), and parathyroid hormone (PTH) for localized alveolar ridge augmentation.

MATERIAL AND METHODS

Five separate Medline searches were performed in duplicate for human and animal studies, respectively. The primary outcome of the included studies was bone regeneration of localized alveolar ridge defects or craniofacial defects.

RESULTS

In six human studies, BMP-2 affected local bone augmentation with increasing volume for higher doses. A majority (43 of 45) of animal studies using BMP-2 showed a positive effect in favour of the growth factor (GF). In six of eight studies, a positive effect was associated with the use of BMP-7. Only one animal study was included for GDF-5 revealing statistically significantly higher bone volume. Regarding PDGF, statistically significantly higher bone volume was observed in five of 10 included studies. Four animal studies using PTH revealed statistically significantly more bone regeneration compared with controls.

CONCLUSIONS

Differing levels and quantity of evidence were noted to be available for the GFs evaluated, revealing that BMP-2, BMP-7, GDF-5, PDGF, and PTH may stimulate local bone augmentation to various degrees. Human data for the potential of rhBMP-2 are supportive.

Bone morphogenetic protein-7 regulates differentially the mRNA expression of bone morphogenetic proteins and their receptors in rat achilles and patellar tendon cell cultures

Previous animal studies have suggested that certain bone morphogenetic proteins (BMPs) may be useful therapeutically in treating tendon healing. To better understand the relationship among the different BMPs in the healing process, we initiated the present study to examine the effects of a member of the BMP family, BMP-7 (also called Osteogenic Protein-1) on the temporal and spatial expression patterns of other BMPs and the BMP receptors in cell cultures of adult rat Achilles and Patellar tendons. Cultures from both tendon types expressed detectable but variable levels of biochemical markers characteristics of tendons. RNAs coding for type II collagen and transcription factors Six1, Scleraxis, and Tendin were detected in both types of cultures. Distinct patterns of expression of several BMP members and their receptors were observed in these cultured cells and BMP-7 exerted differential effects on their expression. The findings may have implications in the treatment of different tendon injuries with BMPs.

The effects of GDF-5 and uniaxial strain on mesenchymal stem cells in 3-D culture

Recent endeavors in tissue engineering have attempted to identify the optimal parameters to create an artificial ligament. Both mechanical and biochemical stimulation have been used by others to independently modulate growth and differentiation, although few studies have explored their interactions. We applied previously described fabrication techniques to create a highly porous (90%-95% porosity, 212-300 microm), 3-D, bioabsorbable polymer scaffold (polycaprolactone). Scaffolds were coated with bovine collagen, and growth and differentiation factor 5 (GDF-5) was added to half of the scaffolds. Scaffolds were seeded with mesenchymal stem cells and cultured in a custom bioreactor under static or cyclic strain (10% strain, 0.33 Hz) conditions. After 48 hours, both mechanical stimulation and GDF-5 increased mRNA production of collagen I, II, and scleraxis compared to control; tenascin C production was not increased. Combining stimuli did not change gene expression; however, cellular metabolism was 1.7 times higher in scaffolds treated with both stimuli. We successfully grew a line of mesenchymal stem cells in 3-D culture, and our initial data indicate mechanical stimulation and GDF-5 influenced cellular activity and mRNA production; we did not, however, observe additive synergism with the mechanical and biological stimuli.

Mechanical load and BMP signaling during tendon repair: a role for follistatin?

Healing of the rat Achilles tendon is sensitive to mechanical loading, and the callus strength is reduced by 3/4 after 14 days, if loading is prevented. Exogenous GDFs stimulate tendon healing. This response is influenced by loading: without loading, cartilage and bone formation is initiated. This implies BMP signaling is crucial during tendon healing and influenced by mechanical loading. We therefore asked if mechanical loading influences the gene expression of the BMP signaling system in intact and healing tendons, and how the BMP signaling system changes during healing. The genes were four BMPs (OP-1/BMP-7, GDF-5/CDMP-1/BMP-14, GDF-6/CDMP-2/BMP-13, and GDF-7/CDMP-3/BMP-12), two receptors (BMPR1b and BMPR2), and the antagonists follistatin and noggin. The Achilles tendon was transected in rats and left to heal. Half of the rats had one Achilles tendon unloaded by injection of Botox in the calf muscles. Ten tendons were analyzed before transection and for each of four time points. All genes except noggin were expressed at all time points, but followed different patterns during healing. Loading strongly decreased the expression of follistatin, which could lead to increased signaling. The BMP system appears involved in tendon maintenance and healing, and may respond to mechanical loading.

Effect of GDF-7 deficiency on tail tendon phenotype in mice

The subfamily of growth/differentiation factors (GDFs) known as GDFs 5, 6, and 7 appears to be involved in tendon maintenance and repair, although the precise nature of this role has yet to be elucidated. The aim of the present study was to examine the role of GDF-7 in tendon maintenance by studying tail tendon fascicle gene expression, composition, and material property strain rate dependency in 16-week-old male and female GDF-7 deficient mice. GDF-7 deficiency did not affect the biochemical composition of tail tendon fascicles, nor did it significantly affect the tensile material properties obtained at either slow (5%/s) or fast (50%/s) strain rates. Further, no difference was found between genotypes in the strain rate sensitivity of any tensile material property. Consistent with the compositional analyses, QRT-PCR data did not reveal any differences of twofold or greater in the gene expression levels of collagens I, III, V, nor in the proteoglycans decorin, fibromodulin, lumican, biglycan, versican, or aggrecan. Gdf5 expression was upregulated twofold in GDF-7 deficient tail tendons, and Bmp7 expression was downregulated twofold. No notable differences in expression levels for Bmp1-6 or Gdf6 were detected. GDF-5 protein levels were 50% higher in GDF-7 deficient tail tendon compared to wild type tail tendon. The results of this study support the intriguing possibility that compensation by Gdf-5 may be at least in part responsible for the absence of a strong phenotype in GDF-7 deficient mice.

Biological augmentation of rotator cuff tendon repair

A histologically normal insertion site does not regenerate following rotator cuff tendon-to-bone repair, which is likely due to abnormal or insufficient gene expression and/or cell differentiation at the repair site. Techniques to manipulate the biologic events following tendon repair may improve healing. We used a sheep infraspinatus repair model to evaluate the effect of osteoinductive growth factors and BMP-12 on tendon-to-bone healing. Magnetic resonance imaging and histology showed increased formation of new bone and fibrocartilage at the healing tendon attachment site in the treated animals, and biomechanical testing showed improved load-to-failure. Other techniques with potential to augment repair site biology include use of platelets isolated from autologous blood to deliver growth factors to a tendon repair site. Modalities that improve local vascularity, such as pulsed ultrasound, have the potential to augment rotator cuff healing. Important information about the biology of tendon healing can also be gained from studies of substances that inhibit healing, such as nicotine and antiinflammatory medications. Future approaches may include the use of stem cells and transcription factors to induce formation of the native tendon-bone insertion site after rotator cuff repair surgery.

Freeze-dried tendon allografts as tissue-engineering scaffolds for Gdf5 gene delivery

Tendon reconstruction using grafts often results in adhesions that limit joint flexion. These adhesions are precipitated by inflammation, fibrosis, and the paucity of tendon differentiation signals during healing. In order to study this problem, we developed a mouse model in which the flexor digitorum longus (FDL) tendon is reconstructed using a live autograft or a freeze-dried allograft, and identified growth and differentiation factor 5 (Gdf5) as a therapeutic target. In this study we have investigated the potential of rAAV-Gdf5 -loaded freeze-dried tendon allografts as "therapeutically endowed" tissue-engineering scaffolds to reduce adhesions. In reporter gene studies we have demonstrated that recombinant adeno-associated virus (rAAV)-loaded tendon allografts mediate efficient transduction of adjacent soft tissues, with expression peaking at 7 days. We have also demonstrated that the rAAV-Gdf5 vector significantly accelerates wound healing in an in vitro fibroblast scratch model and, when loaded onto freeze-dried FDL tendon allografts, improves the metatarsophalangeal (MTP) joint flexion to a significantly greater extent than the rAAV-lacZ controls do. Collectively, our data demonstrate the feasibility and efficacy of therapeutic tendon allograft processing as a novel paradigm in tissue engineering in order to address difficult clinical problems such as tendon adhesions.

Tendon: biology, biomechanics, repair, growth factors, and evolving treatment options

Surgical treatment of tendon ruptures and lacerations is currently the most common therapeutic modality. Tendon repair in the hand involves a slow repair process, which results in inferior repair tissue and often a failure to obtain full active range of motion. The initial stages of repair include the formation of functionally weak tissue that is not capable of supporting tensile forces that allow early active range of motion. Immobilization of the digit or limb will promote faster healing but inevitably results in the formation of adhesions between the tendon and tendon sheath, which leads to friction and reduced gliding. Loading during the healing phase is critical to avoid these adhesions but involves increased risk of rupture of the repaired tendon. Understanding the biology and organization of the native tendon and the process of morphogenesis of tendon tissue is necessary to improve current treatment modalities. Screening the genes expressed during tendon morphogenesis and determining the growth factors most crucial for tendon development will likely lead to treatment options that result in superior repair tissue and ultimately improved functional outcomes.

Tendon and ligament engineering in the adult organism: mesenchymal stem cells and gene-therapeutic approaches

Tendons and ligaments are elastic collagenous tissues with similar composition and hierarchical structure, contributing to motion. Their strength is related to the number and size of the collagen fibrils. Collagen fibrils increase in size during development and in response to increased physical demands or training. Tendon disorders are commonly seen in clinical practice and give rise to significant morbidity. Treatment is difficult and patients often suffer from the symptoms for quite a long time. Despite remodelling, the biochemical and mechanical properties of healed tendon tissue never match those of intact tendon. The prerequisite for focussed treatment strategies in the future will be an improved understanding of the molecular events both in the embryo and contributing to regeneration in the adult organism. Novel approaches include the local delivery of growth factors, stem- and tendon-cell-derived therapy, the application of mechanical load and gene-therapeutic approaches based on vehicles encoding selected factors, or combinations of these. Important factors are proteins of the extracellular matrix like the metalloproteinases, growth factors like the bone morphogenetic proteins but also intracellular signalling mediator proteins, such as the Smads and transcription factors from the helix-loop-helix and other families. In this review, we focus specifically on such molecular approaches based on mesenchymal stem cells.

Tendon and ligament engineering: from cell biology to in vivo application

Tendons and ligaments are related connective tissues that join muscle to bone and bone to bone, respectively. Tendon and ligament injuries are widely distributed clinical problems in society and while healing of such disorders can occur, the original biological properties of the tissue do not return to normal. In this review, recent work on tendon and ligament development and the use of growth factors for successful cellular therapy of tendon and ligament disorders are discussed. In addition, anti-inflammatory concepts for the treatment of tendon and ligament injuries and recent developments in stem cell engineering for tendon and ligament tissues are examined. Lastly, gene transfer strategies for therapeutic applications to heal tendon and ligament disorders are reviewed.

Microscopic and histological examination of the mouse hindpaw digit and flexor tendon arrangement with 3D reconstruction

Mice are currently the species of choice for the in vivo study of injury, but few detailed anatomical descriptions have been made of rodent digits, limiting their use for the investigation of intrasynovial tendon healing. In this study a detailed microscopic and histological investigation was performed using C57/BL6 and Tie2 LacZ reporter gene transgenic mice. Serial-sectioned mouse hindpaw digits were characterized using haematoxylin and eosin, Masson's trichrome (collagen), Alcian blue (fibrocartilage), Miller's stain (elastin) and TRITC-phalloidin (cellular cytoskeleton) staining. Digital vasculature was demonstrated using FITC-labelled dextran perfusion studies supplemented with LacZ expression in Tie2 LacZ transgenic mice digits. Imaging of the digit used a combination of brightfield and confocal microscopy with three-dimensional reconstruction. Our findings demonstrated that the mouse hindpaw possesses deep and superficial flexor tendons within a synovial sheath comparable with that found in other mammalian species. The intrasynovial tendons were avascular and had regions of fibrocartilaginous specialization relating to areas of compression. Corresponding vascular networks were demonstrated around the sheath using Tie2 LacZ mice and FITC-perfused hindpaws. Furthermore, there is an area of digit where both deep and superficial tendons reside between two pulleys, similar to zone 2 in the human hand where it would be possible to study intrasynovial tendon injury and adhesion formation. In conclusion, although the dimensions of the mouse digit pose technical challenges for surgical intervention, we have identified a model for the study of flexor tendon injury that will permit future genetic manipulation studies.

The effect of growth differentiation factor-5-coated sutures on tendon repair in a rat model

Tendon ruptures are common injuries that are often treated surgically. Growth Differentiation Factor-5 (GDF-5) has been shown to accelerate tendon healing with varying degrees of success. We used a novel technique to apply recombinant human GDF-5 (rhGDF-5) to suture and hypothesized that controlled, local delivery of rhGDF-5 can be used to enhance tendon repair. Tendons of 92 rats were transected and repaired with sutures. All researchers were blinded to the following treatment groups (24 rats in each group): 0 rhGDF (control), 24 ng/cm rhGDF, 55 ng/cm rhGDF, and 556 ng/cm rhGDF. Rats were euthanized at 3 weeks (n = 48) and at 6 weeks (n = 48). Sutures were coated with rhGDF-5 using a novel dip-coat technique. Enzyme-linked immunosorbent assay confirmed consistent and reproducible delivery of rhGDF-5. Within each group, 8 were tested biomechanically, and 4 were assessed histologically. Histologic grading at 3 weeks showed improved healing in tendons repaired with coated suture versus controls. By 6 weeks, there were no significant differences. At 3 weeks, minimal isolated cartilage formation was observed; 6-week samples showed more extensive presence, typically surrounding suture fibers. At 3 weeks, tendons repaired with rhGDF-5-coated sutures resulted in significantly higher ultimate tensile load and stiffness compared with control sutures (P < .05) At 6 weeks, there were no significant differences in the mechanical properties of repaired tendons. At 3 weeks, rhGDF-5 induced significant tendon hypertrophy that was more pronounced than at 6 weeks. In addition, tendons repaired with rhGDF-5 showed an increased rate of healing versus control repairs at 3 weeks. This study showed that a novel dip-coating technique can be used to deliver growth factors in varying concentrations to local repair sites to accelerate tendon healing.

Clinical Applications of Bioactive Factors in Sports Medicine

The ability to biologically manipulate musculoskeletal healing and augment bone and soft tissue repair and regeneration holds great promise. Advances in the basic science study and clinical application of bioactive proteins and growth factors continues to evolve. Improvement in the surgical resurfacing of articular cartilage defects and tendon and ligament repair through the addition of bioactive polypeptides is currently underway. The purpose of this article is to review the present array of biologically active materials that may be clinically applicable in sports medicine and arthroscopy. Mechanisms for biologic augmentation of tissue repair and regeneration will be discussed. Current limitations and future considerations will be reviewed particularly as they relate to practical clinical approaches.

Achilles tendon characterization in GDF-7 deficient mice

Growth/differentiation factors (GDFs) play a significant role in numerous skeletal tissues and processes. Previous work using the brachypod mouse has suggested that GDF-5 affects Achilles tendon composition, ultrastructure, and material behavior, as well as tendon repair. The aim of the present study was to examine the role of a related GDF family member, GDF-7 (BMP-12), in intact tendon by studying the Achilles tendon of genetically engineered knockout mice. Achilles tendons from 16-week-old GDF-7 -/- mice contained 14% less GAG/DNA than did wild type littermates (p = 0.0481), although collagen content was comparable to controls. Quantitative reverse transcriptase-polymerase chain reaction (QRT-PCR) results show that GDF-5 was upregulated two-threefold in response to the absence of GDF-7 protein. GDF-6 was also upregulated in knockouts, but to a lesser extent (twofold, p = 0.0013). On an ultrastructural level, GDF-7 deficient Achilles tendons exhibited a shift towards smaller diameter fibrils which resulted in a small but significant reduction in mean fibril diameter (-8%, p = 0.05). GDF-7 deficiency did not noticeably affect the expression of fibrillar collagens (I, III, V) or tendon proteoglycans (decorin, fibromodulin, lumican, biglycan, versican, aggrecan). Differences in tendon composition and ultrastructure were not biologically significant enough to have a noticeable effect on the structural or material behavior of the tendons. These results demonstrate that GDF-7 deficiency has a subtle effect on the composition and ultrastructure of murine Achilles tendon. The small magnitude of the observed differences may be due to overcompensation by related GDF family members.

Tendon and ligament: development, repair and disease

Tendons and ligaments (T/L) are very similar fibrous tissues that respectively connect muscle to bone and bone to bone. They are comprised of fibroblasts that produce large amounts of extra-cellular matrix, resulting in a dense and hypocellular structure. The complex molecular organization of T/L, together with high water content, are responsible for their viscoelastic properties, hence insuring their mechanical function. We will first review recent work on tendon embryology and discuss ligament formation, which has been less documented. We will next summarize our current knowledge of T/L molecular architecture, alterations of which are a major cause for disease. We will finally focus on T/L repair after injury and on genetic diseases responsible for T/L defects.

CDMP-2 injection improves early tendon healing in a rabbit model for surgical repair

This study examines the hypothesis that cartilage-derived morphogenic protein-2 (CDMP-2) can improve tendon healing after surgical repair. We have previously found improved tendon healing by applying CDMP-2 in models for conservative treatment with mechanically loaded Achilles tendon defects in rats and rabbits. In this study, the patellar tendon was unloaded by patello- tibial cerclage and cut transversely in 40 rabbits. Two hours post-operative, the rabbits received a dose of 20 microg of CDMP-2 or vehicle injected into the hematoma. Specimens were harvested after 14 and 28 days and evaluated by biomechanical testing, radiography and histology. At 14 days, CDMP-2 caused a 65% increase in force at failure, a 50% increase in ultimate stress and a 57% increase in stiffness, as compared with controls. There was no effect on callus size. At 28 days, no differences between the treatment groups could be demonstrated. No bone or cartilage was found in any tendon or regenerated tissue at any time point. Thus, early tendon repair can be stimulated by CDMP-2 in an unloaded model. These results suggest that CDMP-2 might be of interest for clinical use as a complement to surgical treatment of tendon ruptures.

Tendon properties in interleukin-4 and interleukin-6 knockout mice

Cytokines are known to play an important role in normal tendon development, function, and maintenance through interactions with fibroblasts and extracellular matrix proteins. However, the role of interleukins on normal tendon activity remains poorly understood. Previous studies that have researched the role of specific cytokines by exogenously applying them have often reported conflicting results. Therefore, a knockout mouse model was used to investigate the role of interleukins 4 and 6 on normal tendon organizational and biomechanical properties. It was hypothesized that interleukin-6 knockout (IL6 -/-) mice will display more organized collagen orientation and greater cross-sectional area and mechanical properties when compared to that of control mice. In addition, interleukin-4 knockout (IL4 -/-) mice will display the most disorganized collagen orientation and lowest cross-sectional area and mechanical properties. As hypothesized, IL6 -/- mice show a trend towards lower angular deviation (more organized) (p<0.1) when compared to IL4 -/- mice. In addition, the IL6 -/- mice show a trend towards a higher percent relaxation (p<0.1) and a significantly higher modulus (p<0.01) when compared to CTL and IL4 -/- mice. Unexpectedly, the IL6 -/- mice exhibited no significant differences in collagen fiber distribution and maximum stress from the other groups and actually had a smaller cross-sectional area than CTL mice (p<0.1). This study supports transgenic mice as an animal model for investigating how cytokines affect normal tendon properties. In addition, this study demonstrates that interleukins may play an important role in tendon development, function, and maintenance.

Effects of increased muscle mass on mouse sagittal suture morphology and mechanics

The purpose of this study is to test predicted form-function relationships between cranial suture complexity and masticatory muscle mass and biomechanics in a mouse model. Specifically, to test the hypothesis that increased masticatory muscle mass increases sagittal suture complexity, we measured the fractal dimension (FD), temporalis mass, and temporalis bite force in myostatin-deficient (GDF8(-/-)) mice and wild-type CD-1 mice (all male, 6 months old). Myostatin is a negative regulator of muscle mass, and myostatin-deficient mice show a marked increase in muscle mass compared to normal mice. We predicted that increased sagittal suture complexity would decrease suture stiffness. The data presented here demonstrate that increased suture complexity (measured as FD) was observed in a hypermuscular mouse model (GDF8(-/-)) with significantly increased temporalis muscle mass and bite forces. Hypermuscular mice were also found to possess suture connective tissue that was less stiff (i.e., underwent more displacement before failure occurred) when loaded in tension. By decreasing stiffness, suture complexity apparently helps to dissipate mechanical loads within the cranium that are related to chewing. These results suggest that cranial suture connective tissue locally adapts to functional demands of the biomechanical suture environment. As such, cranial sutures provide a novel model for studies in connective tissue mechanotransduction.