Proliferating and differentiating effects of three different growth factors on pluripotent mesenchymal cells and osteoblast like cells

A rhPDGF-BB/bovine type I collagen/β-TCP mixture for the treatment of critically sized non-union tibial defects: An in vivo study in rabbits

Non-union during healing of bone fractures affects up to ~5% of patients worldwide. Given the success of recombinant human platelet-derived growth factor-B chain homodimer (rhPDGF-BB) in promoting angiogenesis and bone fusion in the hindfoot and ankle, rhPDGF-BB combined with bovine type I collagen/β-TCP matrix (AIBG) could serve as a viable alternative to autografts in the treatment of non-unions. Defects (~2 mm gaps) were surgically induced in tibiae of skeletally mature New Zealand white rabbits. Animals were allocated to one of four groups-(1) negative control (empty defect, healing for 8 weeks), (2 and 3) acute treatment with AIBG (healing for 4 or 8 weeks), and (4) chronic treatment with AIBG (injection 4 weeks post defect creation and then healing for 8 weeks). Bone formation was analyzed qualitatively and semi-quantitatively through histology. Samples were imaged using dual-energy X-ray absorptiometry and computed tomography for defect visualization and volumetric reconstruction, respectively. Delayed healing or non-healing was observed in the negative control group, whereas defects treated with AIBG in an acute setting yielded bone formation as early as 4 weeks with bone growth appearing discontinuous. At 8 weeks (acute setting), substantial remodeling was observed with higher degrees of bone organization characterized by appositional bone growth. The chronic healing, experimental, group yielded bone formation and remodeling, with no indication of non-union after treatment with AIBG. Furthermore, bone growth in the chronic healing group was accompanied by an increased presence of osteons, osteonal canals, and interstitial lamellae. Qualitatively and semiquantitatively, chronic application of AI facilitated complete bridging of the induced non-union defects, while untreated defects or defects treated acutely with AIBG demonstrated a lack of complete bridging at 8 weeks.

Osteoimmunology of Spondyloarthritis

The mechanisms underlying the development of bone damage in the context of spondyloarthritis (SpA) are not completely understood. To date, a considerable amount of evidence indicates that several developmental pathways are crucially involved in osteoimmunology. The present review explores the biological mechanisms underlying the relationship between inflammatory dysregulation, structural progression, and osteoporosis in this diverse family of conditions. We summarize the current knowledge of bone biology and balance and the foundations of bone regulation, including bone morphogenetic protein, the Wnt pathway, and Hedgehog signaling, as well as the role of cytokines in the development of bone damage in SpA. Other areas surveyed include the pathobiology of bone damage and systemic bone loss (osteoporosis) in SpA and the effects of pharmacological treatment on focal bone damage. Lastly, we present data relative to a survey of bone metabolic assessment in SpA from Italian bone specialist rheumatology centers. The results confirm that most of the attention to bone health is given to postmenopausal subjects and that the aspect of metabolic bone health may still be underrepresented. In our opinion, it may be the time for a call to action to increase the interest in and focus on the diagnosis and management of SpA.

Three-dimensional bioactive hydrogel-based scaffolds for bone regeneration in implant dentistry

Bone tissue requires a range of complex mechanisms to allow the restoration of its structure and function. Bone healing is a signaling cascade process, involving cells secreting cytokines, growth factors, and pro-inflammatory factors in the defect site that will, subsequently, recruit surrounding stem cells to migrate, proliferate, and differentiate into bone-forming cells. Bioactive functional scaffolds could be applied to improve the bone healing processes where the organism is not able to fully regenerate the lost tissue. However, to be optimal, such scaffolds should act as osteoconductors - supporting bone-forming cells, providing nutrients, and sustaining the arrival of new blood vessels, and act as osteoinducers - slowly releasing signaling molecules that stimulate mesenchymal stem cells to differentiate and deposit mineralized bone matrix. Different compositions and shapes of scaffolds, cutting-edge technologies, application of signaling molecules to promote cell differentiation, and high-quality biomaterials are reaching favorable outcomes towards osteoblastic differentiation of stem cells in in vitro and in vivo researches for bone regeneration. Hydrogel-based biomaterials are being pointed as promising for bone tissue regeneration; however, despite all the research and high-impact scientific publications, there are still several challenges that prevent the use of hydrogel-based scaffolds for bone regeneration being feasible for their clinical application. Hence, the objective of this review is to consolidate and report, based on the current scientific literature, the approaches for bone tissue regeneration using bioactive hydrogel-based scaffolds, cell-based therapies, and three-dimensional bioprinting to define the key challenges preventing their use in clinical applications.

Oral delivery of novel human IGF-1 bioencapsulated in lettuce cells promotes musculoskeletal cell proliferation, differentiation and diabetic fracture healing

Human insulin-like growth factor-1 (IGF-1) plays important roles in development and regeneration of skeletal muscles and bones but requires daily injections or surgical implantation. Current clinical IGF-1 lacks e-peptide and is glycosylated, reducing functional efficacy. In this study, codon-optimized Pro-IGF-1 with e-peptide (fused to GM1 receptor binding protein CTB or cell penetrating peptide PTD) was expressed in lettuce chloroplasts to facilitate oral delivery. Pro-IGF-1 was expressed at high levels in the absence of the antibiotic resistance gene in lettuce chloroplasts and was maintained in subsequent generations. In lyophilized plant cells, Pro-IGF-1 maintained folding, assembly, stability and functionality up to 31 months, when stored at ambient temperature. CTB-Pro-IGF-1 stimulated proliferation of human oral keratinocytes, gingiva-derived mesenchymal stromal cells and mouse osteoblasts in a dose-dependent manner and promoted osteoblast differentiation through upregulation of ALP, OSX and RUNX2 genes. Mice orally gavaged with the lyophilized plant cells significantly increased IGF-1 levels in sera, skeletal muscles and was stable for several hours. When bioencapsulated CTB-Pro-IGF-1 was gavaged to femoral fractured diabetic mice, bone regeneration was significantly promoted with increase in bone volume, density and area. This novel delivery system should increase affordability and patient compliance, especially for treatment of musculoskeletal diseases.

Current and Future Concepts for the Treatment of Impaired Fracture Healing

Bone regeneration represents a complex process, of which basic biologic principles have been evolutionarily conserved over a broad range of different species. Bone represents one of few tissues that can heal without forming a fibrous scar and, as such, resembles a unique form of tissue regeneration. Despite a tremendous improvement in surgical techniques in the past decades, impaired bone regeneration including non-unions still affect a significant number of patients with fractures. As impaired bone regeneration is associated with high socio-economic implications, it is an essential clinical need to gain a full understanding of the pathophysiology and identify novel treatment approaches. This review focuses on the clinical implications of impaired bone regeneration, including currently available treatment options. Moreover, recent advances in the understanding of fracture healing are discussed, which have resulted in the identification and development of novel therapeutic approaches for affected patients.

Serum Sclerostin and Bone Morphogenetic Protein-2 Levels in Patients with Ankylosing Spondylitis: A Meta-Analysis

Various studies have investigated the serum sclerostin and bone morphogenetic protein-2 (BMP-2) levels in patients with ankylosing spondylitis (AS), but the results were inconsistent. The aim of this meta-analysis was to synthetically assess the associations of serum levels of sclerostin and BMP-2 with AS. Multiple electronic databases were searched to locate relevant articles published before November 2018. Pooled standard mean difference (SMD) with 95% confidence interval (CI) was calculated by the random-effect model. Totally, 21 studies were included. Meta-analysis results showed no significant difference between AS group and control group in serum sclerostin levels (SMD = 0.098, 95% CI - 0.395 to 0.591, p = 0.697). Nevertheless, serum BMP-2 levels in AS patients were higher than that in controls (SMD = 1.184, 95% CI 0.209 to 2.159, p = 0.017). Subgroup analysis demonstrated that European and South American AS patients had lower serum levels of sclerostin than controls. AS patients with age ≥ 40 years, erythrocyte sedimentation rate (ESR) ≤ 20 mm/h and Bath Ankylosing Spondylitis Functional Index (BASFI) < 4 had statistically significant lower serum sclerostin concentrations compared to controls. Chinese and Korean AS patients as well as patients with lower CRP had higher serum BMP-2 levels than controls, and country may be a source of heterogeneity across the studies. No publication bias existed and sensitivity analysis confirmed the stability of results. Serum BMP-2, but not sclerostin levels may be closely related to the development of AS.

Insulin-Like Growth Factor-1 as a Possible Alternative to Bone Morphogenetic Protein-7 to Induce Osteogenic Differentiation of Human Mesenchymal Stem Cells in Vitro

Growth factors and mesenchymal stem cells (MSC) support consolidation of bone defects. Bone Morphogenetic Protein-7 (BMP-7) has been used clinically and experimentally, but the outcomes remain controversial. Increased systemic expression of Insulin-like Growth Factor-1 (IGF-1) significantly correlates with successful regeneration of bone healing disorders, making IGF-1 a promising alternative to BMP-7. There is no experimental data comparing the osteoinductive potential of IGF-1 and BMP-7. Therefore, in this study, the influence of IGF-1 and BMP-7 in different concentrations on the osteogenic differentiation of two human MSC-subtypes, isolated from reaming debris (RMSC) and iliac crest bone marrow (BMSC) has been assessed. A more sensitive reaction of BMSC towards stimulation with IGF-1 in concentrations of 400–800 ng/mL was found, leading to a significantly higher degree of osteogenic differentiation compared to stimulation with BMP-7. RMSC react more sensitively to stimulation with BMP-7 compared to BMSC. Lower concentrations of IGF-1 were necessary to significantly increase osteogenic differentiation of RMSC and BMSC compared to BMP-7. Therefore, IGF-1 should be considered as a valuable option to improve osteogenic differentiation of MSC and merits further experimental consideration. The MSC subtype and method of differentiation factor application also have to be considered, as they affect the outcome of osteogenic differentiation.

Prospective Randomized Controlled Trial of Hindfoot and Ankle Fusions Treated With rhPDGF-BB in Combination With a β-TCP-Collagen Matrix

BACKGROUND

Ankle and hindfoot arthrodesis is often supplemented with autograft to promote bony union. Autograft harvest can lead to increased perioperative morbidity. Purified recombinant human platelet-derived growth factor BB homodimer (rhPDGF-BB) has stimulated bone formation in mandibular defects and hindfoot fusion. This randomized controlled trial evaluated the efficacy and safety of rhPDGF-BB combined with an injectable, osteoconductive beta-tricalcium phosphate (β-TCP)-collagen matrix versus autograft in ankle and hindfoot fusions.

METHODS

Seventy-five patients requiring ankle or hindfoot fusion were randomized 5:1 for rhPDGF-BB/β-TCP-collagen (treatment, n = 63) or autograft (control, n = 12). Prospective analysis included 142 autograft control subjects from another clinical trial with identical study protocols. Standardized operative and postoperative protocols were used. Patients underwent standard internal fixation augmented with autograft or 0.3 mg/mL rhPDGF-BB/β-TCP-collagen. Radiologic, clinical, and quality-of-life outcomes were assessed over 52 weeks. Primary outcome was joint fusion (50% or more osseous bridging on computed tomography) at 24 weeks. Secondary outcomes included radiographs, clinical healing status, visual analog scale pain score, American Orthopaedic Foot & Ankle Society Ankle-Hindfoot Scale score, Foot Function Index score, and Short Form-12 score. Noninferiority P values were calculated.

RESULTS

Complete fusion of all involved joints at 24 weeks as indicated by computed tomography was achieved in 53 of 63 (84%) rhPDGF-BB/β-TCP-collagen-treated patients and 100 of 154 (65%) autograft-treated patients (P < .001). Mean time to fusion was 14.3 ± 8.9 weeks for rhPDGF-BB/β-TCP-collagen patients versus 19.7 ± 11.5 weeks for autograft patients (P < .01). Clinical success at 52 weeks was achieved in 57 of 63 (91%) rhPDGF-BB/β-TCP-collagen patients and 120 of 154 (78%) autograft patients (P < .001). Safety-related outcomes were equivalent. Autograft controls had 2 bone graft harvest infections.

CONCLUSIONS

Application of rhPDGF-BB/β-TCP-collagen was a safe, effective alternative to autograft for ankle and hindfoot fusions, eliminating the pain and morbidity associated with autograft harvesting.

LEVEL OF EVIDENCE

Level I, prospective randomized study.

MicroRNA-214 suppresses osteogenic differentiation of C2C12 myoblast cells by targeting Osterix

Osterix (Osx) is an osteoblast-specific transcription factor that is essential for osteoblast differentiation and bone formation. Osx-null mice, which exhibit a complete absence of bone formation and arrested osteoblast differentiation, die immediately after birth. However, our understanding of the regulatory mechanism of Osx expression remains poor. MicroRNAs (miRNAs) are a class of small non-coding RNAs that play pivotal roles in diverse biological processes, including the development, differentiation, proliferation, survival, and oncogenesis of cells and organisms. In this study, we aimed to investigate the impact of miRNAs on Osx expression. Bioinformatic analyses predicted that miR-214 would be a potential regulator of Osx. The direct binding of miR-214 to the Osx 3' untranslated region (3' UTR) was demonstrated by a luciferase reporter assay using a construct containing the Osx 3' UTR. Deletion mutant construction revealed that the Osx 3' UTR contained two miR-214 binding sites. MiR-214 expression was inversely correlated with Osx expression in Saos-2 and U2OS cells. The forced expression of miR-214 in Saos-2 cells led to a reduction in the level of Osx protein. Moreover, the role of miR-214 in the osteogenic differentiation of C2C12 cells was investigated. We found that the osteogenic differentiation of C2C12 cells was enhanced by the downregulation of miR-214 expression, as measured by increased alkaline phosphatase activity and matrix mineralization. Taken together, these results indicate that miR-214 is a novel regulator of Osx, and that it plays an important role in the osteogenic differentiation of C2C12 cells as a suppressor.

Recombinant human platelet-derived growth factor-BB and beta-tricalcium phosphate (rhPDGF-BB/β-TCP): an alternative to autogenous bone graft

BACKGROUND

Joint arthrodesis employing autogenous bone graft (autograft) remains a mainstay in the treatment of many foot and ankle problems. However, graft harvest can lead to perioperative morbidity and increased cost. We tested the hypothesis that purified recombinant human platelet-derived growth factor-BB (rhPDGF-BB) homodimer combined with an osteoconductive matrix (beta-tricalcium phosphate [β-TCP]) would be a safe and effective alternative to autograft.

METHODS

A total of 434 patients were enrolled in thirty-seven clinical sites across North America in a prospective, randomized (2:1), controlled, non-inferiority clinical trial to compare the safety and efficacy of the combination rhPDGF-BB and β-TCP with those of autograft in patients requiring hindfoot or ankle arthrodesis. Radiographic, clinical, functional, and quality-of-life end points were assessed through fifty-two weeks postoperatively.

RESULTS

Two hundred and sixty patients (394 joints) underwent arthrodesis with use of rhPDGF-BB/β-TCP. One hundred and thirty-seven patients (203 joints) underwent arthrodesis with use of autograft. With regard to the primary end point, 159 patients (61.2% [262 joints (66.5%)]) in the rhPDGF-BB/β-TCP group and eighty-five patients (62.0% [127 joints (62.6%)]) in the autograft group were fused as determined by computed tomography at six months (p < 0.05). Clinically, 224 patients (86.2%) [348 joints (88.3%)]) in the rhPDGF-BB/β-TCP group were considered healed at fifty-two weeks, compared with 120 patients (87.6% [177 joints (87.2%)] in the autograft group (p = 0.008). Overall, fourteen of sixteen secondary end points at twenty-four weeks and fifteen of sixteen secondary end points at fifty-two weeks demonstrated statistical non-inferiority between the groups, and patients in the rhPDGF-BB/β-TCP group were found to have less pain and an improved safety profile.

CONCLUSIONS

In patients requiring hindfoot or ankle arthrodesis, treatment with rhPDGF-BB/β-TCP resulted in comparable fusion rates, less pain, and fewer side effects as compared with treatment with autograft.

Insulin-like growth factor-I enhances proliferation and differentiation of human mesenchymal stromal cells in vitro

Human mesenchymal stromal cells (hMSCs) offer great potential for bone tissue engineering applications, but their in vivo performance remains limited. Preconditioning of these cells with small molecules to improve their differentiation before implantation, or incorporation of growth factors are possible solutions. Insulin-like growth factor-1 (IGF-1) is one of the most abundant growth factors in bone, involved in growth, development, and metabolism, but its effects on hMSCs are still subject of debate. Here we examined the effects of IGF-1 on proliferation and differentiation of hMSCs in vitro and we found that serum abolished the effects of IGF-1. Only in the absence of serum, IGF-1 increased proliferation, alkaline phosphatase expression, and osteogenic gene expression of hMSCs. Furthermore, we examined synergistic effects of bone morphogenetic protein-2 (BMP-2) and IGF-1 and, although IGF-1 enhanced BMP-2-induced mineralization, IGF-1 only slightly affected in vivo bone formation.

Vascular calcifying progenitor cells possess bidirectional differentiation potentials

Calcifying progenitor cells in blood vessels have the potential to differentiate into cells that either promote calcium accumulation or reverse accumulation, and treatment with PPAR? can shift the direction of this differentiation.

Vascular calcification is an advanced feature of atherosclerosis for which no effective therapy is available. To investigate the modulation or reversal of calcification, we identified calcifying progenitor cells and investigated their calcifying/decalcifying potentials. Cells from the aortas of mice were sorted into four groups using Sca-1 and PDGFRα markers. Sca-1⁺ (Sca-1⁺/PDGFRα⁺ and Sca-1⁺/PDGFRα⁻) progenitor cells exhibited greater osteoblastic differentiation potentials than Sca-1⁻ (Sca-1⁻/PDGFRα⁺ and Sca-1⁻/PDGFRα⁻) progenitor cells. Among Sca-1⁺ progenitor populations, Sca-1⁺/PDGFRα⁻ cells possessed bidirectional differentiation potentials towards both osteoblastic and osteoclastic lineages, whereas Sca-1⁺/PDGFRα⁺ cells differentiated into an osteoblastic lineage unidirectionally. When treated with a peroxisome proliferator activated receptor γ (PPARγ) agonist, Sca-1⁺/PDGFRα⁻ cells preferentially differentiated into osteoclast-like cells. Sca-1⁺ progenitor cells in the artery originated from the bone marrow (BM) and could be clonally expanded. Vessel-resident BM-derived Sca-1⁺ calcifying progenitor cells displayed nonhematopoietic, mesenchymal characteristics. To evaluate the modulation of in vivo calcification, we established models of ectopic and atherosclerotic calcification. Computed tomography indicated that Sca-1⁺ progenitor cells increased the volume and calcium scores of ectopic calcification. However, Sca-1⁺/PDGFRα⁻ cells treated with a PPARγ agonist decreased bone formation 2-fold compared with untreated cells. Systemic infusion of Sca-1⁺/PDGFRα⁻ cells into Apoe^−/− mice increased the severity of calcified atherosclerotic plaques. However, Sca-1⁺/PDGFRα⁻ cells in which PPARγ was activated displayed markedly decreased plaque severity. Immunofluorescent staining indicated that Sca-1⁺/PDGFRα⁻ cells mainly expressed osteocalcin; however, activation of PPARγ triggered receptor activator for nuclear factor-κB (RANK) expression, indicating their bidirectional fate in vivo. These findings suggest that a subtype of BM-derived and vessel-resident progenitor cells offer a therapeutic target for the prevention of vascular calcification and that PPARγ activation may be an option to reverse calcification.

Atherosclerosis involves hardening of the arteries and can lead to heart disease. Calcium accumulation in blood vessels contributes to this process, and this process is regulated by cells that promote calcium accumulation (osteoblasts) and cells that reverse the accumulation (osteoclasts). In this study, we show that vascular calcifying progenitor cells in the blood vessel have the potential to become either osteoblasts or osteoclasts, and that a drug can push these cells towards becoming osteoclasts instead of osteoblasts. Progenitor cells that express both Sca-1 and PDGFRα cell surface proteins were more committed to differentiate into osteoblasts, while cells that only expressed Sca-1 could differentiate into osteoblasts or osteoclasts in a bidirectional manner. Moreover, treatment with a PPARγ agonist could shift the direction of differentiation of Sca-1⁺/PDGFRα⁻ progenitor cells toward osteoclast-like cells, whereas it cannot influence the fates of Sca-1⁺/PDGFRα⁺ progenitors. These results offer new therapeutic targets for reversing calcium accumulation in blood vessels.

Osteoinduction: a review of old concepts with new standards

Since the discovery of osteoinduction in the early 20th century, innovative biomaterials with osteoinductive potential have emerged. Over the last 50 years, however, our ability to describe biological phenomena accurately has been improved dramatically by advancements in cell and molecular biology. The aim of this review is to divide the osteoinduction phenomenon into 3 principles: (1) mesenchymal cell recruitment, (2) mesenchymal differentiation to bone-forming osteoblasts, and (3) ectopic bone formation in vivo. Furthermore, this review formulates guidelines for in vitro and in vivo experimental testing for accurately defining new biomaterials as osteoinductive. The use of growth factors with osteoinductive potential in periodontal and oral surgery is discussed. These concepts and guidelines aim to guide the future direction of emerging biomaterials in bone regeneration.

Analysis of hydrolyzable polyethylene glycol hydrogels and deproteinized bone mineral as delivery systems for glycosylated and non-glycosylated bone morphogenetic protein-2

Bone morphogenetic proteins (BMP), in particular BMP-2, are the growth factors primarily responsible for osteoinduction. A knowledge of interactions between bone substitute materials and growth factor variants is crucial to designing bone substitutes with an ideal release profile. Here we compare glycosylated and non-glycosylated recombinant human bone morphogenetic protein-2 (rhBMP-2) either incorporated into a hydrolyzable polyethylene glycol (PEG) hydrogel developed as a slow release system or adsorbed to a deproteinized bovine bone matrix (DBBM), a clinically well-established bone substitute material. rhBMP-2 loaded materials were immersed in cell culture medium and rhBMP-2 concentration profiles in the supernatant were determined by an enzyme-linked immunosorbent assay. The corresponding biological activities were assessed in vitro by alkaline phosphatase activity assay. We show a strong affinity of rhBMP-2 for DBBM and reduced biological activity after its release from PEG hydrogels. Glycosylated rhBMP-2 was significantly less affected by the hydrogel and interacted significantly more strongly with DBBM than non-glycosylated rhBMP-2. We therefore question the combination of PEG hydrogels with DBBM as a rhBMP-2 delivery system over DBBM alone, since rhBMP-2 released from the hydrogel will be trapped by DBBM. Moreover, our results suggest that glycosylated rhBMP-2 is favorable in combination with PEG hydrogels, since its activity is better preserved, whereas in combination with DBBM non-glycosylated rhBMP-2 is favorable, benefiting from an initially higher concentration of free rhBMP-2.

Cooperative effects in differentiation and proliferation between PDGF-BB and matrix derived synthetic peptides in human osteoblasts

Background

Enhancing osteogenic capabilities of bone matrix for the treatment of fractures and segmental defects using growth factors is an active area of research. Recently, synthetic peptides like AC- 100, TP508 or p-15 corresponding to biologically active sequences of matrix proteins have been proven to stimulate bone formation. The platelet-derived growth factor (PDGF) BB has been identified as an important paracrine factor in early bone healing. We hypothesized that the combined use of PDGF-BB with synthetic peptides could result in an increase in proliferation and calcification of osteoblast-like cells.

Methods

Osteoblast-like cell cultures were treated with PDGF and synthetic peptides, singly and as combinations, and compared to non-treated control cell cultures. The cultures were evaluated at days 2, 5, and 10 in terms of cell proliferation, calcification and gene expression of alkaline phosphate, collagen I and osteocalcin.

Results

Experimental findings revealed that the addition of PDGF, p-15 and TP508 and combinations of PDGF/AC-100, PDGF/p-15 and PDGF/TP508 resulted in an increase in proliferating osteoblasts, especially in the first 5 days of cultivation. Proliferation did not significantly differ between single factors and factor combinations (p > 0.05). The onset of calcification in osteoblasts occurred earlier and was more distinct compared to the corresponding control or PDGF stimulation alone. Significant difference was found for the combined use of PDGF/p-15 and PDGF/AC-100 (p < 0.05).

Conclusions

Our findings indicate that PDGF exhibits cooperative effects with synthetic peptides in differentiation and proliferation. These cooperative effects cause a significant early calcification of osteoblast-like cells (p < 0.05). We suggest the combination of synthetic peptides and PDGF as a potential clinical approach for accelerating bone healing or coating osteosynthesis materials.

Evaluation of biological activity of bone morphogenetic proteins on exposure to commonly used electrospinning solvents

Bone tissue engineering is one of the emerging strategies for developing functionally viable bone substitutes. The recent trend in bone tissue engineering is to combine the benefits of a three-dimensional nanofibrous scaffold with biologically active molecules and responsive stem cells. Electrospinning is the most versatile of the scaffold fabrication strategies and may involve the use of an organic solvent at one stage or another. In spite of all distinct advantages of electrospinning, valid concerns about potentially denaturing interactions between the organic solvent and the biomolecules exist. Efforts are ongoing to incorporate osteoinductive molecules, such as bone morphogenetic proteins (BMPs), during the electrospinning process. The challenge lies in ensuring that the biological activity of these incorporated molecules survives the process. This study was specifically designed to investigate the effects of exposure to commonly used organic solvents on heterodimeric BMP-2/7 using slot-blot assay quantified by infrared imaging and on embryonic myoblasts stably transfected with BMP-specific response element linked to a luciferase reporter – C2C12BRA. Overall, the biological activity of these molecules significantly decreased when exposed to organic solvents but can be restored to their original values by increasing the polarity of the solvent. It was found that an aqueous buffer can effectively overcome the deleterious effects of organic solvents on BMPs, thus generating osteoinductive bone scaffolds.

BMP signaling balances proliferation and differentiation of muscle satellite cell descendants

BACKGROUND

The capacity of muscle to grow or to regenerate after damage is provided by adult stem cells, so called satellite cells, which are located under the basement lamina of each myofiber. Upon activation satellite cells enter the cell cycle, proliferate and differentiate into myoblasts, which fuse to injured myofibers or form new fibers. These processes are tightly controlled by many growth factors.

RESULTS

Here we investigate the role of bone morphogenetic proteins (BMPs) during satellite cell differentiation. Unlike the myogenic C2C12 cell line, primary satellite cells do not differentiate into osteoblasts upon BMP signaling. Instead BMP signaling inhibits myogenic differentiation of primary satellite cells ex vivo. In contrast, inhibition of BMP signaling results in cell cycle exit, followed by enhanced myoblast differentiation and myotube formation. Using an in vivo trauma model we demonstrate that satellite cells respond to BMP signals during the regeneration process. Interestingly, we found the BMP inhibitor Chordin upregulated in primary satellite cell cultures and in regenerating muscles. In both systems Chordin expression follows that of Myogenin, a marker for cells committed to differentiation.

CONCLUSION

Our data indicate that BMP signaling plays a critical role in balancing proliferation and differentiation of activated satellite cells and their descendants. Initially, BMP signals maintain satellite cells descendants in a proliferating state thereby expanding cell numbers. After cells are committed to differentiate they upregulate the expression of the BMP inhibitor Chordin thereby supporting terminal differentiation and myotube formation in a negative feedback mechanism.

Bone morphogenetic proteins and tissue engineering: future directions

As long as bone repair and regeneration is considered as a complex clinical condition, the administration of more than one factor involved in fracture healing might be necessary. The effectiveness or not of bone morphogenetic proteins (BMPs) in association with other growth factors and with mesenchymal stem cells in bone regeneration for fracture healing and bone allograft integration is of great interest to the scientific community. In this study we point out possible future developments in BMPs, concerning research and clinical applications.