GDF-5 deficiency in mice leads to disruption of tail tendon form and function

Therapeutic potential of GDF-5 for enhancing tendon regenerative healing

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

Mechanisms of skeletal muscle-tendon development and regeneration/healing as potential therapeutic targets

Skeletal muscle contraction is essential for the movement of our musculoskeletal system. Tendons and ligaments that connect the skeletal muscles to bones in the correct position at the appropriate time during development are also required for movement to occur. Since the musculoskeletal system is essential for maintaining basic bodily functions as well as enabling interactions with the environment, dysfunctions of these tissues due to disease can significantly reduce quality of life. Unfortunately, as people live longer, skeletal muscle and tendon/ligament diseases are becoming more common. Sarcopenia, a disease in which skeletal muscle function declines, and tendinopathy, which involves chronic tendon dysfunction, are particularly troublesome because there have been no significant advances in their treatment. In this review, we will summarize previous reports on the development and regeneration/healing of skeletal muscle and tendon tissues, including a discussion of the molecular and cellular mechanisms involved that may be used as potential therapeutic targets.

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.

Injectable hydrogels for tendon and ligament tissue engineering

The problem of tendon and ligament (T/L) regeneration in musculoskeletal diseases has long constituted a major challenge. In situ injection of formable biodegradable hydrogels, however, has been demonstrated to treat T/L injury and reduce patient suffering in a minimally invasive manner. An injectable hydrogel is more suitable than other biological materials due to the special physiological structure of T/L. Most other materials utilized to repair T/L are cell-based, growth factor-based materials, with few material properties. In addition, the mechanical property of the gel cannot reach the normal T/L level. This review summarizes advances in natural and synthetic polymeric injectable hydrogels for tissue engineering in T/L and presents prospects for injectable and biodegradable hydrogels for its treatment. In future T/L applications, it is necessary develop an injectable hydrogel with mechanics, tissue damage-specific binding, and disease response. Simultaneously, the advantages of various biological materials must be combined in order to achieve personalized precision therapy.

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.

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

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

Growth factor delivery strategies for rotator cuff repair and regeneration

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

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.

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.

Tendon injury: from biology to tendon repair

Tendon is a crucial component of the musculoskeletal system. Tendons connect muscle to bone and transmit forces to produce motion. Chronic and acute tendon injuries are very common and result in considerable pain and disability. The management of tendon injuries remains a challenge for clinicians. Effective treatments for tendon injuries are lacking because the understanding of tendon biology lags behind that of the other components of the musculoskeletal system. Animal and cellular models have been developed to study tendon-cell differentiation and tendon repair following injury. These studies have highlighted specific growth factors and transcription factors involved in tenogenesis during developmental and repair processes. Mechanical factors also seem to be essential for tendon development, homeostasis and repair. Mechanical signals are transduced via molecular signalling pathways that trigger adaptive responses in the tendon. Understanding the links between the mechanical and biological parameters involved in tendon development, homeostasis and repair is prerequisite for the identification of effective treatments for chronic and acute tendon injuries.

Freeze-dried allograft-mediated gene or protein delivery of growth and differentiation factor 5 reduces reconstructed murine flexor tendon adhesions

Advances in allograft processing have opened new horizons for clinical adaptation of flexor tendon allografts as delivery scaffolds for antifibrotic therapeutics. Recombinant adeno-associated-virus (rAAV) gene delivery of the growth and differentiation factor 5 (GDF-5) has been previously associated with antifibrotic effects in a mouse model of flexor tendoplasty. In this study, we compared the effects of loading freeze-dried allografts with different doses of GDF-5 protein or rAAV-Gdf5 on flexor tendon healing and adhesions. We first optimized the protein and viral loading parameters using reverse transcription polymerase chain reaction (RT-PCR), enzyme-linked immunosorbent assay (ELISA), and in vivo bioluminescent imaging. We then reconstructed flexor digitorum longus (FDL) tendons of the mouse hindlimb with allografts loaded with low and high doses of recombinant GDF-5 protein and rAAV-Gdf5 and evaluated joint flexion and biomechanical properties of the reconstructed tendon. In vitro optimization studies determined that both the loading time and concentration of the growth factor and viral vector had dose-dependent effects on their retention on the freeze-dried allograft. In vivo data suggest that protein and gene delivery of GDF-5 had equivalent effects on improving joint flexion function, in the range of doses used. Within the doses tested, the lower doses of GDF-5 had more potent effects on suppressing adhesions without adversely affecting the strength of the repair. These findings indicate equivalent antifibrotic effects of Gdf5 gene and protein delivery, but suggest that localized delivery of this potent factor should also carefully consider the dosage used to eliminate untoward effects, regardless of the delivery mode.

Preferential tendon stem cell response to growth factor supplementation

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

The growth factors involved in flexor tendon repair and adhesion formation

Flexor tendon injuries remain a significant clinical problem, owing to the formation of adhesions or tendon rupture. A number of strategies have been tried to improve outcomes, but as yet none are routinely used in clinical practice. Understanding the role that growth factors play in tendon repair should enable a more targeted approach to be developed to improve the results of flexor tendon repair. This review describes the main growth factors in tendon wound healing, and the role they play in both repair and adhesion formation.

Bioreactor design for tendon/ligament engineering

Tendon and ligament injury is a worldwide health problem, but the treatment options remain limited. Tendon and ligament engineering might provide an alternative tissue source for the surgical replacement of injured tendon. A bioreactor provides a controllable environment enabling the systematic study of specific biological, biochemical, and biomechanical requirements to design and manufacture engineered tendon/ligament tissue. Furthermore, the tendon/ligament bioreactor system can provide a suitable culture environment, which mimics the dynamics of the in vivo environment for tendon/ligament maturation. For clinical settings, bioreactors also have the advantages of less-contamination risk, high reproducibility of cell propagation by minimizing manual operation, and a consistent end product. In this review, we identify the key components, design preferences, and criteria that are required for the development of an ideal bioreactor for engineering tendons and ligaments.

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.

Treatment with rhBMP12 or rhBMP13 increase the rate and the quality of rat Achilles tendon repair

Tendon injuries that result in partial or complete tears often come from chronic, repetitive use, or from sudden trauma. In some cases, torn tendons can be repaired, but such repairs often fail to completely restore tendon function. We used global gene expression profiling and histological examination to study tendon repair to elucidate key molecular processes that regulate the rate and quality of tissue restoration. Using a rat Achilles tendon transection model, tissue was collected at 3, 7, 10, and 15 days postinjury. The pattern of gene expression in the repairing tissue paralleled the healing phases of inflammation, matrix formation, and matrix reorganization. Newly formed repaired tissue is characterized by cells expressing many genes associated with tendon formation, thereby potentially distinguishing this repair tissue from other types of repair or scar tissue. Addition of recombinant human bone morphogenic protein (rhBMP)12 or rhBMP13, also known as growth and differentiation factors (GDFs) 6 and 7, 1 day after injury yielded increases in tissue volume, rate of cellular infiltration, and in changes in levels of key mRNAs involved in tendon repair. Altogether, our results indicate that rhBMP12 or rhBMP13 enhance the rate of tendon repair. A better understanding of the key molecular regulators of tendon repair could lead to the development of new therapies for tendon injuries and the identification of diagnostic markers that indicate the status of tendon repair after injury.

Growth differentiation factor-5 regulation of extracellular matrix gene expression in murine tendon fibroblasts

The synthesis and organization of extracellular matrix (ECM) of tendon, in resting and states of repair, are governed by fibroblasts. Growth differentiation factor-5 (GDF-5) may enhance the cellular response to tendon injury, thus improving the structural outcome of the regenerative tissue. This study was an attempt to identify potential mechanisms controlling the response of fibroblasts to injury and GDF-5, in the pursuit of improved tissue regeneration. There were two sets of experiments. Isolated mice Achilles tendon fibroblasts were treated with different concentrations of rGDF-5 (0-100 ng/ml) for 0-12 days in cell culture. The temporal effect of rGDF-5 on ECM gene expression was analysed for type I collagen and aggrecan expression. Microarray and gene expression analysis were performed on cells treated with 100 ng/ml for 4 days. Forty-five mice underwent bilateral mid-substance Achilles tendon tenotomy and suture repair. Repair sites were injected with 10 µg rGDF-5 or saline. Tendons were assessed histologically at 2, 4 and 6 weeks. Expression of ECM genes procollagen IX, aggrecan, matrix metalloproteinase 9 and fibromodulin were upregulated. Proinflammatory reaction genes were downregulated. rGDF-5 led to an increase in total DNA, glycosaminoglycan (GAG) and hydroxyproline (OHP). The OHP:DNA ratio of fibroblast cultures was increased over all time points, with increased GAG:DNA at day 12. rGDF-5 treatment showed improved collagen organization over controls. The results delineate the mode of action of rGDF-5 at the cellular and gene level. rGDF-5 could play a role in tendon repair and be used for future therapies that promote tendon healing.

Quantitative trait loci analysis of tail tendon break time in mice of C57BL/6J and DBA/2J lineage

Tail tendon break time (TTBT), a measure of collagen cross-linking, shown to increase with age differs significantly among inbred strains of mice, indicating underlying genetic influences. This study was aimed to identify quantitative trait loci (QTLs) associated with tail tendon break time at three ages (200, 500, and 800 days of age) for 23 BxD recombinant inbred strains of mice and B6D2F(2) mice derived from C57BL/6J and DBA/2J strains. Heritability estimates were calculated, and QTL analyses were conducted using interval-mapping methods. Mean tail tendon break time values were higher in males and increased nonlinearly with age. Eight total QTLs were nominated in the B6D2F(2) mice at the three measured ages, with the QTL at 800 days confirmed in the recombinant inbred strains. Allelic effect modeling for the identified QTLs suggests differences in gene action between sexes. Candidate genes in the QTL regions include collagen genes and an advanced glycation end-product receptor. The QTLs identified demonstrate influence at some but not all ages.

Effects of cartilage-derived morphogenetic protein-3 on the expression of chondrogenic and osteoblastic markers in the pluripotent mesenchymal C3H10T1/2 cell line

CDMP-3/GDF-7/BMP-12 treatment of pluripotent mesenchymal C3H10T1/2 cells resulted in a dose- and time-dependent change in cell morphology and in the expression of alkaline phosphatase, mRNA expression of osteocalcin, and bone sialoprotein, as well as mineralized bone nodule formation. CDMP-3 also stimulated Alcian Blue staining indicative of extracellular matrix formation without affecting aggrecan expression. CDMP-3 downregulated mRNA expression of BMP-4 and BMP-8A. CDMP-3 stimulated mRNA expression of ALK-1, ALK-2(ActR-IA), ALK-3(BMPR-IA), and ALK-4 without affecting that of ALK-6(BMPR-IB), ALK-7, and BMPR-II. These findings suggest that, under the experimental conditions studied, CDMP-3 induces the pluripotent mesenchymal C3H10T1/2 cells to express both chondrocytic and osteoblastic markers. The results further reveal potential complex interplay between the different bone morphogenetic proteins and their receptors in these processes.

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.

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.

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.

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.

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.

Variability in tendon and knee joint biomechanics among inbred mouse strains

Hereditary factors are thought to be responsible for impaired tendon function and joint laxity. The present study investigated the genotypic variability of knee laxity and stiffness and tendon mechanical and geometric properties among 16-week-old female A/J, C57BL/6J (B6), and C3H/HeJ (C3H) inbred mice. In one group of mice, knee mechanics were quantified using a custom loading apparatus enabling translation of the tibia against a stationary femur. In a second group, flexor digitorum longus and Achilles tendons from the left hind limb underwent biomechanical testing, while those of the contralateral limb were analyzed histologically for determination of cross-sectional area. Our results demonstrate that tendon and joint mechanics varied significantly among the inbred mouse strains, indicating that biomechanical properties are genetically determined. A/J mouse knees exhibited greater laxity (p < 0.001) and lower stiffness (p < 0.001) compared to those of the B6 and C3H mice. The genotypic differences in whole joint properties were similar to those of the tendons' structural biomechanical traits. Although body mass did not differ (p > 0.2) among the three strains, significant genotypic differences were found at the whole tendon, material quality, and morphological levels of the tissue hierarchy. Furthermore, genetic regulation of tendon mechanical properties varied with anatomic site. Patterns of genotypic differences in tendon size were not consistent with those of biomechanical properties, suggesting that unique combinations of structural and compositional factors contribute to tendon growth, adaptation, and development. Therefore, the three inbred strains constitute a useful experimental model to elucidate genetic control of structure-function relationships in normal and healing tendons and ligaments.

Neotendon formation induced by manipulation of the Smad8 signalling pathway in mesenchymal stem cells

Tissue regeneration requires the recruitment of adult stem cells and their differentiation into mature committed cells. In this study we describe what we believe to be a novel approach for tendon regeneration based on a specific signalling molecule, Smad8, which mediates the differentiation of mesenchymal stem cells (MSCs) into tendon-like cells. A biologically active Smad8 variant was transfected into an MSC line that coexpressed the osteogenic gene bone morphogenetic protein 2 (BMP2). The engineered cells demonstrated the morphological characteristics and gene expression profile of tendon cells both in vitro and in vivo. In addition, following implantation in an Achilles tendon partial defect, the engineered cells were capable of inducing tendon regeneration demonstrated by double quantum filtered MRI. The results indicate what we believe to be a novel mechanism in which Smad8 inhibits the osteogenic pathway in MSCs known to be induced by BMP2 while promoting tendon differentiation. These findings may have considerable importance for the therapeutic replacement of tendons or ligaments and for engineering other tissues in which BMP plays a pivotal developmental role.

Recombinant human growth/differentiation factor-5 (rhGDF-5) induced bone formation in murine calvariae

OBJECTIVES

Growth/differentiation factor-5 (GDF-5), a member of the transforming growth factor-beta superfamily, shows a close structural relationship to bone morphogenetic proteins and plays crucial roles in skeletal morphogenesis. Recombinant human (rh) GDF-5 was reported as a suitable factor for enhancing healing in bone defect and inducing ectopic bone formation. The purpose of the present study was to investigate the mechanism of bone formation induced by rhGDF-5 in murine calvariae by radiological, histological and immunohistochemical methods. Cell proliferation was also examined in vitro.

MATERIAL AND METHODS

Cells including primary osteoblasts, periosteum cells and connective tissue fibroblasts were isolated enzymatically from neonatal murine calvariae or head skin. In the presence or absence of rhGDF-5, cell proliferation was estimated by tetrazolium reduction assay. To examine the mechanism of osteoinduction, rhGDF-5/atelocollagen (AC) composite or 0.01 N HCl/AC composite were injected into murine calvariae subcutaneously. Tissue was examined radiologically, histologically and immunohistochemically.

RESULTS

In the presence of rhGDF-5, proliferation of primary osteoblasts, periosteum cells, and connective tissue fibroblasts was increased significantly in culture. Immunohistochemical observations showed cells at the site injected with rhGDF-5/AC displayed immunoreactivity for proliferating cell nuclear antigen (PCNA). Newly formed bone- and cartilage-like tissue contained chondrocyte osteocyte and osteoclastic cells, and were immunoreactive for both type I and II collagen.

CONCLUSION

Exposure to GDF-5 promotes proliferation and differentiation of calvarial cells, which give rise to ectopic bone formation.

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.

Effect of GDF-5 on ligament healing

The effects of growth and differentiation factor-5 (GDF-5) on ligament healing were studied using a gap injury model of the medial collateral ligament in rat knee joints. The administration of GDF-5 once at the time of surgery significantly improved the mechanical properties of the femur-ligament-tibia complex. At 3 weeks after surgery, 30 microg of GDF-5 improved the ultimate tensile strength of the complex by 41%, and the stiffness by 60%, compared with the vehicle control (p < 0.05 for both; Fisher's PLSD test). The observation with a transmission electron microscopy revealed that GDF-5 increased the diameter of collagen fibrils in the repair tissue, which was considered to be a possible mechanism for the positive result in the biomechanical testing. Quantitative PCR and in situ hybridization revealed enhanced type I procollagen expression by GDF-5, and the PCR analysis also revealed that the GDF-5 treatment reduced the expression of type III procollagen relative to type I procollagen. The PCR analysis further showed that the expression of decorin and fibromodulin was relatively reduced against type I procollagen by the growth factor, which was considered to be responsible for the increase of collagen fibril diameter in the repair tissue. No adverse effects were observed, and the use of GDF-5 was considered a promising approach to facilitate ligament healing.

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.

Viscoelastic properties of skin in Mov-13 and Tsk mice

Viscoelastic properties of skin samples were measured in three types of mice (tight skin, Tsk, control and Mov-13), that are known to differ with regard to content of type I collagen. The experimental design used uniaxial stretching and measured the creep response and the complex compliance. The creep response was measured directly. The complex compliance was determined using a Wiener-Volterra constitutive model for each sample. The models were calculated from data obtained by applying a stress input having a pseudo-Gaussian waveform and measuring the strain response. The storage compliance of Mov-13 and control skin were similar and were greater than Tsk (p<0.001). The loss compliance of each group was significantly different (p<0.001) from each other group; Tsk had the lowest and control had the highest loss compliance. The phase angle of the Mov-13 and Tsk were similar and were less than the controls (p<0.001). The creep response was fit with a linear viscoelastic model. None of the parameters in the creep model differed between groups. The results indicate that gene-targeted and mutant animals have soft tissue mechanical phenotypes that differ in complex ways. Caution should be exercised when using such animals as models to explore the role of specific constituents on tissue properties.

Cartilage-derived morphogenetic proteins induce osteogenic gene expression in the C2C12 mesenchymal cell line

Cartilage-derived morphogenetic protein-1, -2, and -3 (CDMP-1, -2, and -3) are members of the bone morphogenetic protein (BMP) family and have been shown to exhibit a variety of biological activities. In the present study, effects of these CDMPs on the temporal and spatial expression of genes in the pluripotent mesenchymal cell line C2C12 were examined. Cells cultured in the presence of CDMPs lost the characteristic elongated shape of myoblasts. At the molecular level, CDMP treatment did not change the mRNA expression of MyoD, aggrecan, Six1, and tendin. Scleraxis mRNA level was reduced by CDMP treatment. CDMP-1 and -3, but not CDMP-2, stimulated expression of osteogenic markers, such as alkaline phosphatase (AP), osteocalcin (OC), BSP, and type I collagen, in a dose- and time-dependent manner. With few exceptions, the three CDMPs changed, with different potencies, the expression profile of different members of the BMP family in a similar temporal pattern. Except at the late phase of treatment, CDMP treatment did not change the expression of ActR-IA, BMPR-IA, BMPR-IB, BMPR-II, and ALK-7 mRNAs. Based on the current data, the CDMPs appear to be able to stimulate the C2C12 cells to differentiate into the osteoblast pathway.

Differential expression of heparan sulfate 6-O-sulfotransferase isoforms in the mouse embryo suggests distinctive roles during organogenesis

Heparan sulfate (HS) interactions with secreted morphogens such as fibroblast growth factors, hedgehogs, and Wnts are essential for embryonic development. Formation of biologically relevant HS structures is a result of the coordinated action of various biosynthetic enzymes. HS 6-O-sulfotransferases (6OST) catalyze the transfer of sulfate groups to the 6-O position of glucosamine residues in HS. Three 6OST isoforms have been described in the mouse; however, little is known about their role in generating specific HS protein-binding sequences, expression pattern, and function in vivo. To gain insights into the distribution of these isoforms and their potential role in development, we mapped 6OST1-3 gene expression during mouse organogenesis. We report dynamic expression of these isoforms with striking differences in tissue distribution in many developing organs. We show that 6OST transcripts are differentially expressed in several sites where heparin-binding growth factors are critical for development. 6OST1 is predominantly transcribed in epithelial and neural-derived tissues, whereas 6OST2 is more mesenchymal. 6OST3 appears at later stages and in a more restricted manner. The patterns reported here strongly suggest that the HS structures modified by these enzymes have different roles in growth factor-induced developmental processes.

Cartilage-derived morphogenetic protein-1 promotes the differentiation of mesenchymal stem cells into chondrocytes

Mesenchymal stem cells (MSCs) are able to differentiate into many types of cells including chondrocytes. Transforming growth factor beta1 (TGF-beta1) is very important in the regulation of chondrogenesis. Since cartilage-derived morphogenetic protein-1 (CDMP-1) belongs to the TGF-beta superfamily, we tested whether CDMP-1 plays any role in the regulation of the differentiation of MSCs into chondrocytes using a high density pellet culture system. Based on the histological staining of glycosaminoglycan using toluidine blue dye-binding method we found that CDMP-1 could initiate chondrogenic differentiation of MSCs as did TGF-beta1. However, CDMP-1 was less stimulatory than TGF-beta1. The combination of CDMP-1 and TGF-beta1 synergically induced chondrogenesis of MSCs. This synergic chondrogenic effect of CDMP-1 together with TGF-beta1 was further confirmed by quantification of GAG using dimethylmethylene blue dye-binding assay and immunohistochemical analysis of the expression of cartilage-specific protein collagen II. This study may provide an improved induction approach using MSCs for repairing damaged cartilage.

Collagen fibril diameter distribution does not reflect changes in the mechanical properties of in vitro stress-deprived tendons

The purpose of this study was to determine if an association exists between the tensile properties and the collagen fibril diameter distribution in in vitro stress-deprived rat tail tendons. Rat tail tendons were paired into two groups of 21 day stress-deprived and 0 time controls and compared using transmission electron microscopy (n = 6) to measure collagen fibril diameter distribution and density, and mechanical testing (n =6) to determine ultimate stress and tensile modulus. There was a statistically significant decrease in both ultimate tensile strength (control: 17.95+/-3.99 MPa, stress-deprived: 6.79+/-3.91 MPa) and tensile modulus (control: 312.8+/-89.5 MPa, stress-deprived: 176.0+/-52.7 MPa) in the in vitro stress-deprived tendons compared to controls. However, there was no significant difference between control and stress-deprived tendons in the number of fibrils per tendon counted, mean fibril diameter, mean fibril density, or fibril size distribution. The results of this study demonstrate that the decrease in mechanical properties observed in in vitro stress-deprived rat tail tendons is not correlated with the collagen fibril diameter distribution and, therefore, the collagen fibril diameter distribution does not, by itself, dictate the decrease in mechanical properties observed in in vitro stress-deprived rat tail tendons.

Evaluation of HealosMP52 osteoinductive bone graft for instrumented lumbar intertransverse process fusion in sheep

STUDY DESIGN

HealosMP52 was evaluated in a sheep model of instrumented lumbar intertransverse process spine fusion and compared to autogenous bone graft.

OBJECTIVES

To determine the long-term efficacy and safety of HealosMP52 as a bone graft substitute in posterolateral instrumented spinal fusion.

SUMMARY OF BACKGROUND DATA

Although the standard intertransverse fusion method employs autogenous iliac crest bone, autograft has certain limitations. HealosMP52, an osteoinductive bone graft material, can facilitate noninstrumented posterolateral spine fusion in rabbits and nonhuman primates, but the long-term outcome of such fusions has not been evaluated.

METHODS

Eleven skeletally mature, female sheep were instrumented with pedicle screws and rods at L2-L3 and L5-L6. Each animal was treated with autograft bone at one fusion level and HealosMP52 at the other. At 6 and 12 months after surgery, bone formation was measured on contact microradiographs and by backscattered electron imaging. Bone core biopsies taken from 6-month and 12-month specimens were evaluated histologically for pathology indicative of osteosarcoma.

RESULTS

Grossly, all autograft- and HealosMP52-treated levels showed stable fusions at 6 and 12 months. HealosMP52 and autograft treatments resulted in equivalent mean percent bone volumes within fusion bodies; similar values were observed at 6 and 12 months. Fusion bodies contained cortical and trabecular bone with osteoid seams and fatty marrow, and fusion masses showed maturation from 6 to 12 months. HealosMP52 treatment was not associated with implant migration, ectopic bone formation, or pathologic abnormality. No histologic evidence of osteosarcoma was seen on bone core biopsies.

CONCLUSIONS

This long-term assessment of the use of HealosMP52 in posterolateral instrumented spine fusion indicates that HealosMP52 possesses safe and efficacious bone grafting properties and can potentially serve as anosteoinductive alternative to autograft bone.

Collagen and proteoglycan abnormalities in the GDF-5-deficient mice and molecular changes when treating disk cells with recombinant growth factor

STUDY DESIGN

A magnetic resonance image, histologic, biochemical, and gene expression study was conducted to characterize the effects of growth and development factor-5 (GDF-5) deficiency on the health of the intervertebral disc.

OBJECTIVE

To determine the effect of GDF-5 deficiency on extracellular matrix and gene expression on the intervertebral disc.

SUMMARY OF BACKGROUND DATA

Developmental and degenerative changes in intervertebral disc are not fully understood. Molecular abnormalities and spontaneous mutations that lead to the deficiency in a normal protein have been useful in understanding the function of certain molecules and the role they play in the structure and health of certain tissues. Although the role of GDF-5 in the disc has not been elucidated, this factor may have an important role in the disc as a result of the well-documented effect of GDF-5 in other chondrogenic tissues. METHODS.: Intervertebral discs of 20-week-old GDF-5 (-/-) and (+/+) mice were examined radiographically, histologically, biochemically, and with gene expression studies. Cells isolated from GDF-5-deficient mouse discs were treated with recombinant GDF-5 and gene expression was subsequently analyzed.

RESULTS

GDF-5 (-/-) mice demonstrated significantly lower T2-weighted signal intensity in the central region of their lumbar discs, and disc histology revealed loss of the normal lamellar architecture of the anulus fibrosus and a shrunken, disorganized nucleus pulposus. Biochemical analysis revealed decreased proteoglycan content but no appreciable change in total collagen content of the discs. Significant downregulation of both aggrecan and type II collagen mRNA, without an appreciable change in type I collagen expression, was noted on gene expression studies. Recombinant GDF-5 treatment of disc cells from the GDF-5-deficient mice resulted in a dose-dependent upregulation of the aggrecan and type II collagen genes.

CONCLUSION

The intervertebral disc is markedly affected by GDF-5 deficiency. This relatively simple (single gene) system with a known molecular defect may be useful in studies designed to define the response of the intervertebral disc to treatment with growth factor in vivo.

Signals regulating tendon formation during chick embryonic development

Tendons are collagen-rich structures that link muscle to cartilage. By using quail-chick chimeras, it has been shown that tendon and cartilage cells originate from the same mesodermic compartment, which is distinct from that giving rise to muscle cells. Axial tendons originate from the sclerotomal compartment, and limb tendons originate from the lateral plate, whereas axial and limb muscles derive from dermomyotomes. Despite these different embryologic origins, muscle and tendon morphogenesis occurs in close spatial and temporal association. Facilitated by the distinct embryologic origin of myogenic and tendon cells, surgical studies in the avian embryo have highlighted interactions between tendons and muscles, during embryonic development. However, these interactions seem to differ between axial and limb levels. The molecular mechanisms underlying muscle and tendon interactions have been shown recently to involve different members of the fibroblast growth factor family. This review covers the available data on the early steps of tendon formation in the limb and along the primary axis. The relationship with muscle morphogenesis will be highlighted.

Strain-Rate Sensitive Mechanical Properties of Tendon Fascicles From Mice With Genetically Engineered Alterations in Collagen and Decorin

Tendons have complex mechanical behaviors that are nonlinear and time dependent. It is widely held that these behaviors are provided by the tissue’s composition and structure. It is generally thought that type I collagen provides the primary elastic strength to tendon while proteoglycans, such as decorin, play a role in failure and viscoelastic properties. This study sought to quantify such structure-function relationships by comparing tendon mechanical properties between normal mice and mice genetically engineered for altered type I collagen content and absence of decorin. Uniaxial tensile ramp to failure experiments were performed on tail tendon fascicles at two strain rates, 0.5%/s and 50%/s. Mutations in type I collagen led to reduced failure load and stiffness with no changes in failure stress, modulus or strain rate sensitivity. Fascicles without decorin had similar elastic properties to normal fascicles, but reduced strain rate sensitivity. Fascicles from immature mice, with increased decorin content compared to adult fascicles, had inferior elastic properties but higher strain rate sensitivity. These results showed that tendon viscoelasticity is affected by decorin content but not by collagen alterations. This study provides quantitative evidence for structure-function relationships in tendon, including the role of proteoglycan in viscoelasticity.

Cartilage-derived morphogenetic proteins enhance the osteogenic protein-1-induced osteoblastic cell differentiation of C2C12 cells

Previous studies have shown that osteogenic protein-1 (OP-1; also known as BMP-7) induces differentiation of the pluripotent mesenchymal cell line C2C12 into osteoblastic cells. OP-1 also alters the steady-state levels of messenger RNA (mRNA) encoding for the cartilage-derived morphogenetic proteins (CDMPs) in C2C12 cells. In the present study, the effects of exogenous CDMPs on bone cell differentiation induced by OP-1 in C2C12 cells were examined. Exogenous CDMP-1, -2, and -3 synergistically and dose-dependently enhanced OP-1 action in stimulating alkaline phosphatase (AP) activity and osteocalcin (OC) mRNA expression. AP staining studies revealed that the combination of OP-1 and CDMP enhanced OP-1 action by stimulating those cells that had responded to OP-1 and not by activating additional cells. The combination did not change the mRNA expression of the BMPs and their receptors. CDMP-1 enhanced the suppression of the OP-1-induced expression of the myogeneic differentiation regulator MyoD. CDMP-1 and OP-1 alone stimulated Smad5 protein expression, but the combination of OP-1 and CDMP-1 stimulated synergistically Smad5 protein expression. Thus, one mechanism of the observed synergy involved enhancement of the induced Smad5 protein expression. At the same protein concentration, CDMP-1 is most potent in enhancing OP-1 activity in inducing osteoblastic cell differentiation of C2C12 cells. CDMP-3 is about 80% as potent as CDMP-1, and CDMP-2 is the least potent (about 50% of CDMP-1).

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.