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

Association of myostatin deficiency with collagen related disease-umbilical hernia and tippy toe standing in pigs

Herein, we investigate the high incidence of umbilical hernia and tippy-toe standing and their underlying changes in gene expression and proliferation in myostatin knockout (MSTN^-/-) pigs. Thirty-six male MSTN^-/- pigs were generated by somatic cell nuclear transfer (SCNT). These pigs presented a considerably high incidence of tippy-toe standing and umbilical hernia (69.4% and 61.1%, respectively). The tendon to body weight ratio was significantly lower than wild-type pigs (0.202 ± 0.017 vs 0.250 ± 0.004, respectively). The crimp length of the MSTN^-/- tendon was significantly longer than that of wild-type pigs. The expression of MSTN and the activin type IIB (ACVR2B) was detected in the tendon and linea alba of MSTN^-/- pigs. MSTN treatment significantly increased the phosphorylation of Smad2/3 in both tendon and linea alba fibroblasts. Type I collagen (Col1A) and Scleraxis (Scx) expression levels in the tendon and linea alba of MSTN^-/- pigs were significantly lower than those in wild-type in vivo, whereas and cyclin-dependent kinase inhibitor 1 (p21) expression levels were higher. Treatment of tendon and linea alba fibroblasts with recombinant MSTN increased Col1A and Scx and decreased p21 expression in vivo. Moreover, there was a significant increase in fibroblast proliferation after treatment. The results indicated that MSTN regulates collagen expression and proliferation in tendon and linea alba fibroblasts; thus, MSTN deficiency causes collagen-related pathological features in MSTN^-/- pigs. Hence, MSTN could be used as a therapeutic target for treating UH and tendon abnormalities.

Similar sequences but dissimilar biological functions of GDF11 and myostatin

Growth differentiation factor 11 (GDF11) and myostatin (MSTN) are closely related TGFβ family members that are often believed to serve similar functions due to their high homology. However, genetic studies in animals provide clear evidence that they perform distinct roles. While the loss of Mstn leads to hypermuscularity, the deletion of Gdf11 results in abnormal skeletal patterning and organ development. The perinatal lethality of Gdf11-null mice, which contrasts with the long-term viability of Mstn-null mice, has led most research to focus on utilizing recombinant GDF11 proteins to investigate the postnatal functions of GDF11. However, the reported outcomes of the exogenous application of recombinant GDF11 proteins are controversial partly because of the different sources and qualities of recombinant GDF11 used and because recombinant GDF11 and MSTN proteins are nearly indistinguishable due to their similar structural and biochemical properties. Here, we analyze the similarities and differences between GDF11 and MSTN from an evolutionary point of view and summarize the current understanding of the biological processing, signaling, and physiological functions of GDF11 and MSTN. Finally, we discuss the potential use of recombinant GDF11 as a therapeutic option for a wide range of medical conditions and the possible adverse effects of GDF11 inhibition mediated by MSTN inhibitors.

The Effect of Myostatin (GDF-8) on Proliferation and Tenocyte Differentiation of Rat Bone Marrow-Derived Mesenchymal Stem Cells

BACKGROUND

The future in flexor tendon surgery involves tissue engineering approaches directed toward increasing early repair strength to accelerate tendon healing and to allow for earlier onset of rehabilitation. Previous work has shown that pluripotential mesenchymal stem cells may be successfully delivered to a tendon repair site using a suture carrier. The current work describes the use of Myostatin (GDF-8) to help guide these delivered pluripotential stem cells to differentiate down a tenocyte lineage to potentially maximize the reparative effects of these cells at the tendon repair site.

METHODS

Primary rat bone marrow mesenchymal stem cells isolated from the long bones of male Sprague-Dawley rats were treated with 500 ng/ml myostatin for 24 h, 48 h, and 72 h. Collagen 1 A, scleraxis (Scx), and tenomodulin (Tnmd) expression, indicative of tenogenesis, was analyzed using real time PCR and immunohistochemistry staining. A migration assay was performed to assess the functional activity of BMSCs after they were treated with myostatin.

RESULTS

Compared to the control cells (without treatment), the cells treated with 500 ng/ml myostatin for 72 h exhibited higher expression of Col 1A, Scx, and Tnmd. The mRNA expression of Col1A, Scx, Tnmd increased 15.3, 13 and 7 times respectively. Immunohistochemistry staining showed Scx and Tnmd were expressed in the cellular cytoplasm. In response to myostatin, the cells also showed a tendency to proliferate and migrate more than the control cells.

CONCLUSIONS

Myostatin (GDF-8) has the ability to increase rat bone marrow mesenchymal stem cell growth and differentiation toward a tenocyte lineage. This information could be useful for future studies regarding tendon repair.

Age-Dependent Oxidative Stress Elevates Arginase 1 and Uncoupled Nitric Oxide Synthesis in Skeletal Muscle of Aged Mice

Aging is associated with reduced muscle mass (sarcopenia) and poor bone quality (osteoporosis), which together increase the incidence of falls and bone fractures. It is widely appreciated that aging triggers systemic oxidative stress, which can impair myoblast cell survival and differentiation. We previously reported that arginase plays an important role in oxidative stress-dependent bone loss. We hypothesized that arginase activity is dysregulated with aging in muscles and may be involved in muscle pathophysiology. To investigate this, we analyzed arginase activity and its expression in skeletal muscles of young and aged mice. We found that arginase activity and arginase 1 expression were significantly elevated in aged muscles. We also demonstrated that SOD2, GPx1, and NOX2 increased with age in skeletal muscle. Most importantly, we also demonstrated elevated levels of peroxynitrite formation and uncoupling of eNOS in aged muscles. Our in vitro studies using C2C12 myoblasts showed that the oxidative stress treatment increased arginase activity, decreased cell survival, and increased apoptotic markers. These effects were reversed by treatment with an arginase inhibitor, 2(S)-amino-6-boronohexanoic acid (ABH). Our study provides strong evidence that L-arginine metabolism is altered in aged muscle and that arginase inhibition could be used as a novel therapeutic target for age-related muscle complications.

Anterior Cruciate Ligament Tear Promotes Skeletal Muscle Myostatin Expression, Fibrogenic Cell Expansion, and a Decline in Muscle Quality

BACKGROUND

Anterior cruciate ligament (ACL) tears result in significant quadriceps muscle atrophy that is resistant to recovery despite extensive rehabilitation. Recent work suggests an elevated fibrotic burden in the quadriceps muscle after the injury, which may limit recovery. Elucidating the mechanisms and cell types involved in the progression of fibrosis is critical for developing new treatment strategies.

PURPOSE

To identify factors contributing to the elevated fibrotic burden found after the injury.

STUDY DESIGN

Descriptive laboratory study.

METHODS

After an ACL injury, muscle biopsy specimens were obtained from the injured and noninjured vastus lateralis of young adults (n = 14, mean ± SD: 23 ± 4 years). The expression of myostatin, transforming growth factor β, and other regulatory factors was measured, and immunohistochemical analyses were performed to assess turnover of extracellular matrix components.

RESULTS

Injured limb skeletal muscle demonstrated elevated myostatin gene ( P < .005) and protein ( P < .0005) expression, which correlated ( R² = 0.38, P < .05) with fibroblast cell abundance. Immunohistochemical analysis showed that human fibroblasts express the activin type IIB receptor and that isolated primary human muscle-derived fibroblasts increased proliferation after myostatin treatment in vitro ( P < .05). Collagen 1 and fibronectin, primary components of the muscle extracellular matrix, were significantly higher in the injured limb ( P < .05). The abundance of procollagen 1-expressing cells as well as a novel index of collagen remodeling was also elevated in the injured limb ( P < .05).

CONCLUSION

These findings support a role for myostatin in promoting fibrogenic alterations within skeletal muscle after an ACL injury.

CLINICAL RELEVANCE

The current work shows that the cause of muscle quality decline after ACL injury likely involves elevated myostatin expression, and future studies should explore therapeutic inhibition of myostatin to facilitate improvements in muscle recovery and return to sport.

Ex-vivo Tendon Repair Augmented with Bone Marrow Derived Mesenchymal Stem Cells Stimulated with Myostatin for Tenogenesis

BACKGROUND

To investigate the effect of myostatin (GDF-8) stimulation of bone marrow derived mesenchymal stem cells (BMSCs) on tenogenesis in the setting of tendon repair. GDF-8 has demonstrated the ability to augment tenogenesis and we sought to identify if this effect could lead to the focused differentiation of pluripotential stem cells down a tenocyte lineage ex vivo.

METHODS

Cadaveric upper limb flexor tendons were harvested, decellularized and divided into 1 cm segments. Sutures seeded with stem cells were passed through tendon segments to simulate repair. The repaired tendons were then cultured either with or without myostatin for 3, 5, and 7 days. The experiment was also repeated with non-decellularized tendons for a total of 4 groups. The tendons were then evaluated for the expression of scleraxis and tenomodulin, two biomarkers for tendon.

RESULTS

Myostatin stimulation led to an increase in expression of tenomodulin and scleraxis at 5 and 7 days in both the decellularized and non-decellularized tendons. Myostatin increased the differentiation of BMSCs into tenocytes and/or led to the upregulation of tenomodulin and scleraxis production by the native tenocytes present within the non-decellularized tendons.

CONCLUSIONS

The addition of myostatin to BMSCs leads to tenocyte differentiation as evidenced by the expression of tenocyte biomarkers, scleraxis and tenomodulin. This effect is maintained in an ex vivo tendon repair model suggestive that these cells survive the passage through tendon tissue and remain metabolically active.

Spheroid formation and modulation of tenocyte-specific gene expression under simulated microgravity

Background

For tendon tissue engineering, tenocyte-seeded scaffolds are a promising approach. Under conventional 2D culture however, tenocytes show rapid senescene and phenotype loss. We hypothesized that phenotype loss could be counteracted by simulated microgravity conditions.

Methods

Human tenocytes were exposed to microgravity for 9 days on a Random Positioning Machine (RPM). Formation of 3D-structures (spheroids) was observed under light microscopy, gene expression was measured by real-time PCR. Cells under conventional 2D-culture served as control group.

Results

Simulated microgravity reached a value of as low as 0.003g. Spheroid formation was observed after 4 days, and spheroids showed stable existance to the end of the observation period. After 9 days, spheroids showed a significantly higher gene expression of collagen 1 (Col1A1) compared to adherent cells under microgravity (4.4x, p=0.04) and compared to the control group (5.6x, p=0.02). Gene expression of collagen 3 (COL3A1) was significantly increased in spheroids compared to the control group (2.3x, p=0.03). Gene expressions of the extracellular matrix genes Tenascin C und Fibronectin (TNC and FN) were increased in adherent cells under microgravity compared to the 1g-control group, not reaching statistical significance (p=0.1 and p=0.3). For the gene expression of vimentin, no significant alteration was observed both in the adherent cells and in the spheroids compared to the 1g control group. Gene expression of the tenocyte-specific transcription factor scleraxis (SCX) was significantly increased in spheroids compared to the control group (3.7x, p=0.03).

Conclusion

Simulated microgravity could counteract tenocyte senescence in vitro and serve as a promising model for scaffold-free 3D cell culturing and tissue engineering.

Level of evidence

V (laboratory study).

Tissue Engineering of Tendons: A Comparison of Muscle-Derived Cells, Tenocytes, and Dermal Fibroblasts as Cell Sources

BACKGROUND

The rapid development of tendon tissue-engineering technology may offer an alternative graft for reconstruction of severe tendon losses. One critical factor for tendon tissue engineering is the optimization of seed cells. Little is known about the optimal cell source for engineered tendons. The aim of this study was to compare mouse muscle-derived cells, dermal fibroblasts, and tenocytes and determine the optimal cell source for tendon tissue engineering.

METHODS

Mouse muscle-derived cells, dermal fibroblasts, and tenocytes were isolated and cultured in vitro. At passage 1, cellular morphology, cell proliferation, and tenogenic marker expression were evaluated. After seeding on the polyglycolic acid scaffolds for 2 weeks in vitro and 12 weeks in vivo, histologic qualities, ultrastructure, and biomechanical characteristics were evaluated.

RESULTS

Proliferation and cellular morphology were similar for dermal fibroblasts and tenocytes, whereas muscle-derived cells proliferated faster than the other two groups. With regard to the phenotype difference between them, muscle-derived cells and tenocytes shared the gene expression of SCX, TNMD, GDF-8, and Col-I, but with MyoD gene expression only in muscle-derived cells. In contrast to dermal fibroblast and tenocyte constructed tendons, neotendon with muscle-derived cells exhibited better aligned collagen fibers, more mature collagen fibril structure, and stronger mechanical properties, whereas no significant difference in the dermal fibroblast and tenocyte groups was observed.

CONCLUSION

Although dermal fibroblasts are candidates for tendon tissue engineering because they are similar to tenocytes in proliferation and neotendon formation, muscle-derived cells appear to be the most suitable cells for further study and development of engineered tendon.

Joint dysfunction and functional decline in middle age myostatin null mice

Since its discovery as a potent inhibitor for muscle development, myostatin has been actively pursued as a drug target for age- and disease-related muscle loss. However, potential adverse effects of long-term myostatin deficiency have not been thoroughly investigated. We report herein that male myostatin null mice (mstn(-/-)), in spite of their greater muscle mass compared to wild-type (wt) mice, displayed more significant functional decline from young (3-6months) to middle age (12-15months) than age-matched wt mice, measured as gripping strength and treadmill endurance. Mstn(-/-) mice displayed markedly restricted ankle mobility and degenerative changes of the ankle joints, including disorganization of bone, tendon and peri-articular connective tissue, as well as synovial thickening with inflammatory cell infiltration. Messenger RNA expression of several pro-osteogenic genes was higher in the Achilles tendon-bone insertion in mstn(-/-) mice than wt mice, even at the neonatal age. At middle age, higher plasma concentrations of growth factors characteristic of excessive bone remodeling were found in mstn(-/-) mice than wt controls. These data collectively indicate that myostatin may play an important role in maintaining ankle and wrist joint health, possibly through negative regulation of the pro-osteogenic WNT/BMP pathway.

The crucial role of vitamin C and its transporter (SVCT2) in bone marrow stromal cell autophagy and apoptosis

Vitamin C is an antioxidant that plays a vital role in various biological processes including bone formation. Previously, we reported that vitamin C is transported into bone marrow stromal cells (BMSCs) through the sodium dependent Vitamin C Transporter 2 (SVCT2) and this transporter plays an important role in osteogenic differentiation. Furthermore, this transporter is regulated by oxidative stress. To date, however, the exact role of vitamin C and its transporter (SVCT2) in ROS regulated autophagy and apoptosis in BMSCs is poorly understood. In the present study, we observed that oxidative stress decreased survival of BMSCs in a dose-dependent manner and induced growth arrest in the G1 phase of the cell cycle. These effects were accompanied by the induction of autophagy, confirmed by P62 and LC3B protein level and punctate GFP-LC3B distribution. The supplementation of vitamin C significantly rescued the BMSCs from oxidative stress by regulating autophagy. Knockdown of the SVCT2 transporter in BMSCs synergistically decreased cell survival even under low oxidative stress conditions. Also, supplementing vitamin C failed to rescue cells from stress. Our results reveal that the SVCT2 transporter plays a vital role in the mechanism of BMSC survival under stress conditions. Altogether, this study has given new insight into the role of the SVCT2 transporter in oxidative stress related autophagy and apoptosis in BMSCs.

Progress in cell-based therapies for tendon repair

The last decade has seen significant developments in cell therapies, based on permanently differentiated, reprogrammed or engineered stem cells, for tendon injuries and degenerative conditions. In vitro studies assess the influence of biophysical, biochemical and biological signals on tenogenic phenotype maintenance and/or differentiation towards tenogenic lineage. However, the ideal culture environment has yet to be identified due to the lack of standardised experimental setup and readout system. Bone marrow mesenchymal stem cells and tenocytes/dermal fibroblasts appear to be the cell populations of choice for clinical translation in equine and human patients respectively based on circumstantial, rather than on hard evidence. Collaborative, inter- and multi-disciplinary efforts are expected to provide clinically relevant and commercially viable cell-based therapies for tendon repair and regeneration in the years to come.

Co-culture of human adipose-derived stem cells with tenocytes increases proliferation and induces differentiation into a tenogenic lineage

BACKGROUND

Seeding acellularized tendons with cells is an approach for creating tissue-engineered tendon grafts with favorable biomechanical properties. It was the authors' aim to evaluate whether human adipose-derived stem cells could replace tenocytes for scaffold seeding.

METHODS

Adipose-derived stem cells and tenocytes were co-cultured in different ratios (3:1, 1:1, and 1:3) and with three different methods: (1) direct co-culture, (2) tenocyte-conditioned media on adipose-derived stem cells, and (3) an insert system to keep both cell types in the same media without contact. Proliferation, collagen production, and tenogenic marker expression were measured by hematocytometry, immunocytochemistry, enzyme-linked immunosorbent assay, and real-time polymerase chain reaction.

RESULTS

Proliferation and collagen production were similar for tenocytes and adipose-derived stem cells alone. Phenotype difference between adipose-derived stem cells and tenocytes was indicated by higher tenascin C and scleraxis expression in tenocytes. Proliferation was increased in direct co-cultures, especially at an adipose-derived stem cells-to-tenocyte ratio of 3:1, and for tenocytes in adipose-derived stem cell-conditioned media. Direct co-culture caused significant up-regulation in tenascin C expression in adipose-derived stem cells (4.0-fold; p<005). In tenocyte-conditioned media, tenascin C expression was up-regulated 2.5-fold (p<0.05). In the insert system, tenascin C expression was up-regulated 2.3-fold (p<0.05).

CONCLUSIONS

Adipose-derived stem cells are good candidates for tendon tissue engineering because they are similar to tenocytes in proliferation and collagen production. With an optimal ratio of 3:1, they increase proliferation in co-culture and change their phenotype toward a tenogenic direction.

Changes in circulating biomarkers of muscle atrophy, inflammation, and cartilage turnover in patients undergoing anterior cruciate ligament reconstruction and rehabilitation

BACKGROUND

After anterior cruciate ligament (ACL) reconstruction, there is significant atrophy of the quadriceps muscles that can limit full recovery and place athletes at risk for recurrent injuries with return to play. The cause of this muscle atrophy is not fully understood.

HYPOTHESIS

Circulating levels of proatrophy, proinflammatory, and cartilage turnover cytokines and biomarkers would increase after ACL reconstruction.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Patients (N = 18; mean age, 28 ± 2.4 years) underwent surgical reconstruction of the ACL after a noncontact athletic injury. Circulating levels of biomarkers were measured along with Short Form-12, International Knee Documentation Committee, and objective knee strength measures preoperatively and at 6 postoperative visits. Differences were tested using repeated-measures 1-way analysis of variance.

RESULTS

Myostatin, TGF-β, and C-reactive protein levels were significantly increased in the early postoperative period and returned to baseline. Cartilage oligomeric matrix protein levels decreased immediately after surgery and then returned to baseline. CCL2, CCL3, CCL4, CCL5, EGF, FGF-2, IGF-1, IL-10, IL-1α, IL-1β, IL-1ra, IL-6, myoglobin, and TNF-α were not different over the course of the study.

CONCLUSION

An increase in potent atrophy-inducing cytokines and corresponding changes in knee strength and functional scores were observed after ACL reconstruction.

CLINICAL RELEVANCE

Although further studies are necessary, the therapeutic inhibition of myostatin may help prevent the muscle atrophy that occurs after ACL reconstruction and provide an accelerated return of patients to sport.

Characterisation of connective tissue from the hypertrophic skeletal muscle of myostatin null mice

Myostatin is a potent inhibitor of muscle development. Genetic deletion of myostatin in mice results in muscle mass increase, with muscles often weighing three times their normal values. Contracting muscle transfers tension to skeletal elements through an elaborate connective tissue network. Therefore, the connective tissue of skeletal muscle is an integral component of the contractile apparatus. Here we examine the connective tissue architecture in myostatin null muscle. We show that the hypertrophic muscle has decreased connective tissue content compared with wild-type muscle. Secondly, we show that the hypertrophic muscle fails to show the normal increase in muscle connective tissue content during ageing. Therefore, genetic deletion of myostatin results in an increase in contractile elements but a decrease in connective tissue content. We propose a model based on the contractile profile of muscle fibres that reconciles this apparent incompatible tissue composition phenotype.

Immunolocalization of myostatin (GDF-8) following musculoskeletal injury and the effects of exogenous myostatin on muscle and bone healing

The time course and cellular localization of myostatin expression following musculoskeletal injury are not well understood; therefore, the authors evaluated the temporal and spatial localization of myostatin during muscle and bone repair following deep penetrant injury in a mouse model. They then used hydrogel delivery of exogenous myostatin in the same injury model to determine the effects of myostatin exposure on muscle and bone healing. Results showed that a "pool" of intense myostatin staining was observed among injured skeletal muscle fibers 12-24 hr postsurgery and that myostatin was also expressed in the soft callus chondrocytes 4 days following osteotomy. Hydrogel delivery of 10 or 100 µg/ml recombinant myostatin decreased fracture callus cartilage area relative to total callus area in a dose-dependent manner by 41% and 80% (p<0.05), respectively, compared to vehicle treatment. Myostatin treatment also decreased fracture callus total bone volume by 30.6% and 38.8% (p<0.05), with the higher dose of recombinant myostatin yielding the greatest decrease in callus bone volume. Finally, exogenous myostatin treatment caused a significant dose-dependent increase in fibrous tissue formation in skeletal muscle. Together, these findings suggest that early pharmacological inhibition of myostatin is likely to improve the regenerative potential of both muscle and bone following deep penetrant musculoskeletal injury.

Myostatin (GDF-8) inhibits chondrogenesis and chondrocyte proliferation in vitro by suppressing Sox-9 expression

Here, we investigate a possible direct role for myostatin in chondrogenesis. First, we examined the effects of myostatin on the proliferation of bone marrow stromal cells (BMSCs) and epiphyseal growth plate (EGP) chondrocytes (EGPCs) isolated from myostatin-deficient mice. Results show that myostatin deficiency is associated with a significant (P < 0.001) increase in proliferation of both BMSCs (+25%) and EGPCs (+35%) compared with wild-type cells. Next, we examined the effects of myostatin treatment on chondrogenic differentiation of BMSCs. These experiments show that myostatin treatment starting at either 0 or 48 h induces a significant decrease in collagen type II protein synthesis by 31% (P < 0.001) and 25% (P < 0.05), respectively. Real-time PCR reveals significant (P < 0.01) down regulation of Sox9 mRNA expression with 10 and 100 ng/ml treatments. Together, these findings suggest that myostatin has direct effects on chondrogenesis, and may, therefore, represent a potential therapeutic target for improving bone repair.