Transplantation of skin fibroblasts expressing BMP-2 promotes bone repair more effectively than those expressing Runx2

Dietary calcium requirements of broilers fed a conventional corn-soybean meal diet from 1 to 21 days of age

Background

The current calcium (Ca) recommendation for broilers is primarily based on studies conducted more than 30 years ago with birds of markedly different productive potentials from those which exist today. And the response indicators in these studies are mainly growth performance and bone ash percentage. Therefore, the present study was carried out to investigate the effect of dietary Ca level on growth performance, serum parameters, bone characteristics and Ca metabolism-related gene expressions, so as to estimate dietary Ca requirements of broilers fed a conventional corn-soybean meal diet from 1 to 21 days of age.

Methods

A total of 420 1-day-old Arbor Acres male broilers were randomly assigned to 1 of 7 treatments with 6 replicates (10 birds per cage) and fed the corn-soybean meal diets containing 0.60%, 0.70%, 0.80%, 0.90%, 1.00%, 1.10% or 1.20% Ca for 21 days. Each diet contained the constant non-phytate phosphorus content of about 0.39%.

Results

The average daily gain decreased linearly (P < 0.001) as dietary Ca level increased. The serum and tibia alkaline phosphatase (ALP) activities, tibia bone mineral density (BMD), middle toe BMD, tibia ash percentage, tibia breaking strength, and tibia ALP protein expression level were affected (P < 0.05) by dietary Ca level, and showed significant quadratic responses (P < 0.02) to dietary Ca levels. The estimates of dietary Ca requirements were 0.80 to 1.00% based on the best fitted broken-line or quadratic models (P < 0.03) of the above serum and bone parameters, respectively.

Conclusions

The results from the present study indicate that the Ca requirements would be about 0.60% to obtain the best growth rate, and 1.00% to meet all of the Ca metabolisms and bone development of broilers fed a conventional corn-soybean meal diet from 1 to 21 days of age.

A Narrative Review of Cell-Based Approaches for Cranial Bone Regeneration

Current cranial repair techniques combine the use of autologous bone grafts and biomaterials. In addition to their association with harvesting morbidity, autografts are often limited by insufficient quantity of bone stock. Biomaterials lead to better outcomes, but their effectiveness is often compromised by the unpredictable lack of integration and structural failure. Bone tissue engineering offers the promising alternative of generating constructs composed of instructive biomaterials including cells or cell-secreted products, which could enhance the outcome of reconstructive treatments. This review focuses on cell-based approaches with potential to regenerate calvarial bone defects, including human studies and preclinical research. Further, we discuss strategies to deliver extracellular matrix, conditioned media and extracellular vesicles derived from cell cultures. Recent advances in 3D printing and bioprinting techniques that appear to be promising for cranial reconstruction are also discussed. Finally, we review cell-based gene therapy approaches, covering both unregulated and regulated gene switches that can create spatiotemporal patterns of transgenic therapeutic molecules. In summary, this review provides an overview of the current developments in cell-based strategies with potential to enhance the surgical armamentarium for regenerating cranial vault defects.

Formation, structure, and function of extra-skeletal bones in mammals

This review describes the formation, structure, and function of bony compartments in antlers, horns, ossicones, osteoderm and the os penis/os clitoris (collectively referred to herein as AHOOO structures) in extant mammals. AHOOOs are extra-skeletal bones that originate from subcutaneous (dermal) tissues in a wide variety of mammals, and this review elaborates on the co-development of the bone and skin in these structures. During foetal stages, primordial cells for the bony compartments arise in subcutaneous tissues. The epithelial-mesenchymal transition is assumed to play a key role in the differentiation of bone, cartilage, skin and other tissues in AHOOO structures. AHOOO ossification takes place after skeletal bone formation, and may depend on sexual maturity. Skin keratinization occurs in tandem with ossification and may be under the control of androgens. Both endochondral and intramembranous ossification participate in bony compartment formation. There is variation in gradients of density in different AHOOO structures. These gradients, which vary according to function and species, primarily reduce mechanical stress. Anchorage of AHOOOs to their surrounding tissues fortifies these structures and is accomplished by bone-bone fusion and Sharpey fibres. The presence of the integument is essential for the protection and function of the bony compartments. Three major functions can be attributed to AHOOOs: mechanical, visual, and thermoregulatory. This review provides the first extensive comparative description of the skeletal and integumentary systems of AHOOOs in a variety of mammals.

BMP-2 Gene Delivery-Based Bone Regeneration in Dentistry

Bone morphogenetic protein-2 (BMP-2) is a potent growth factor affecting bone formation. While recombinant human BMP-2 (rhBMP-2) has been commercially available in cases of non-union fracture and spinal fusion in orthopaedics, it has also been applied to improve bone regeneration in challenging cases requiring dental implant treatment. However, complications related to an initially high dosage for maintaining an effective physiological concentration at the defect site have been reported, although an effective and safe rhBMP-2 dosage for bone regeneration has not yet been determined. In contrast to protein delivery, BMP-2 gene transfer into the defect site induces BMP-2 synthesis in vivo and leads to secretion for weeks to months, depending on the vector, at a concentration of nanograms per milliliter. BMP-2 gene delivery is advantageous for bone wound healing process in terms of dosage and duration. However, safety concerns related to viral vectors are one of the hurdles that need to be overcome for gene delivery to be used in clinical practice. Recently, commercially available gene therapy has been introduced in orthopedics, and clinical trials in dentistry have been ongoing. This review examines the application of BMP-2 gene therapy for bone regeneration in the oral and maxillofacial regions and discusses future perspectives of BMP-2 gene therapy in dentistry.

Bone morphogenetic protein 7 enhances the osteogenic differentiation of human dermal-derived CD105+ fibroblast cells through the Smad and MAPK pathways

The skin, as the largest organ of the human body, is an important source of stromal stem cells with multipotent differentiation potential. CD105⁺ mesenchymal stem cells exhibit a higher level of stemness than CD105⁻ cells. In the present study, human dermal-derived CD105⁺ fibroblast cells (CD105⁺ hDDFCs) were isolated from human foreskin specimens using immunomagnetic isolation methods to examine the role of bone morphogenetic protein (BMP)-7 in osteogenic differentiation. Adenovirus-mediated recombinant BMP7 expression enhanced osteogenesis-associated gene expression, calcium deposition, and alkaline phosphatase activity. Investigation of the underlying mechanisms showed that BMP7 activated small mothers against decapentaplegic (Smad) and p38/mitogen-activated protein kinase signaling in CD105⁺ hDDFCs. The small interfering RNA-mediated knockdown of Smad4 or inhibition of p38 attenuated the BMP7-induced enhancement of osteogenic differentiation. In an in vivo ectopic bone formation model, the adenovirus-mediated overexpression of BMP7 enhanced bone formation from CD105⁺ hDDFCs. Taken together, these data indicated that adenoviral BMP7 gene transfer in CD105⁺ hDDFCs may be developed as an effective tool for bone tissue engineering.

Gene Therapy for Bone Repair Using Human Cells: Superior Osteogenic Potential of Bone Morphogenetic Protein 2-Transduced Mesenchymal Stem Cells Derived from Adipose Tissue Compared to Bone Marrow

Ex vivo regional gene therapy strategies using animal mesenchymal stem cells genetically modified to overexpress osteoinductive growth factors have been successfully used in a variety of animal models to induce both heterotopic and orthotopic bone formation. However, in order to adapt regional gene therapy for clinical applications, it is essential to assess the osteogenic capacity of transduced human cells and choose the cell type that demonstrates the best clinical potential. Bone-marrow stem cells (BMSC) and adipose-derived stem cells (ASC) were selected in this study for in vitro evaluation, before and after transduction with a lentiviral two-step transcriptional amplification system (TSTA) overexpressing bone morphogenetic protein 2 (BMP-2; LV-TSTA-BMP-2) or green fluorescent protein (GFP; LV-TSTA-GFP). Cell growth, transduction efficiency, BMP-2 production, and osteogenic capacity were assessed. The study demonstrated that BMSC were characterized by a slower cell growth compared to ASC. Fluorescence-activated cell sorting analysis of GFP-transduced cells confirmed successful transduction with the vector and revealed an overall higher but not statistically significant transduction efficiency in ASC versus BMSC (90.2 ± 4.06% vs. 80.4 ± 8.51%, respectively; p = 0.146). Enzyme-linked immunosorbent assay confirmed abundant BMP-2 production by both cell types transduced with LV-TSTA-BMP-2, with BMP-2 production being significantly higher in ASC versus BMSC (239.5 ± 116.55 ng vs. 70.86 ± 24.7 ng; p = 0.001). Quantitative analysis of extracellular deposition of calcium (Alizarin red) and alkaline phosphatase activity showed that BMP-2-transduced cells had a higher osteogenic differentiation capacity compared to non-transduced cells. When comparing the two cell types, ASC/LV-TSTA-BMP-2 demonstrated a significantly higher mineralization potential compared to BMSC/LV-TSTA-BMP-2 7 days post transduction (p = 0.014). In conclusion, this study demonstrates that transduction with LV-TSTA-BMP-2 can significantly enhance the osteogenic potential of both human BMSC and ASC. BMP-2-treated ASC exhibited higher BMP-2 production and greater osteogenic differentiation capacity compared to BMP-2-treated BMSC. These results, along with the fact that liposuction is an easy procedure with lower donor-site morbidity compared to BM aspiration, indicate that adipose tissue might be a preferable source of MSCs to develop a regional gene therapy approach to treat difficult bone-repair scenarios.

Insight into skin cell-based osteogenesis: a review

For decades, researchers have been fascinated by the strategy of using cell therapy for bone defects; some progress in the field has been made. Owing to its ample supply and easy access, skin, the largest organ in the body, has gained attention as a potential source of stem cells. Despite extensive applications in skin and nerve regeneration, an increasing number of reports indicate its potential use in bone tissue engineering and regeneration. Unfortunately, few review articles are available to outline current research efforts in skin-based osteogenesis. This review first summarizes the latest findings on stem cells or progenitors in skin and their niches and then discusses the strategies of skin cell-based osteogenesis. We hope this article elucidates this topic and generates new ideas for future studies.

Cationized gelatin hydrogels mixed with plasmid DNA induce stronger and more sustained gene expression than atelocollagen at calvarial bone defects in vivo

Gene transduction of exogenous factors at local sites in vivo is a promising approach to promote regeneration of tissue defects owing to its simplicity and capacity for expression of a variety of genes. Gene transduction by viral vectors is highly efficient; however, there are safety concerns associated with viruses. As a method for nonviral gene transduction, plasmid DNA delivery is safer and simpler, but requires an efficient carrier substance. Here, we aimed to develop a simple, efficient method for bone regeneration by gene transduction and to identify optimal conditions for plasmid DNA delivery at bone defect sites. We focused on carrier substances and compared the efficiencies of two collagen derivatives, atelocollagen, and gelatin hydrogel, as substrates for plasmid DNA delivery in vivo. To assess the efficiencies of these substrates, we examined exogenous expression of green fluorescence protein (GFP) by fluorescence microscopy, polymerase chain reaction, and immunohistochemistry. GFP expression at the bone defect site was higher when gelatin hydrogel was used as a substrate to deliver plasmids than when atelocollagen was used. Moreover, the gelatin hydrogel was almost completely absorbed at the defect site, whereas some atelocollagen remained. When a plasmid harboring bone morphogenic protein 2 was delivered with the substrate to bony defect sites, more new bone formation was observed in the gelatin group than in the atelocollagen group. These results suggested that the gelatin hydrogel was more efficient than atelocollagen as a substrate for local gene delivery and may be a superior material for induction of bone regeneration.

Gene-Activated Matrix Comprised of Atelocollagen and Plasmid DNA Encoding BMP4 or Runx2 Promotes Rat Cranial Bone Augmentation

To date, therapeutic method for in vivo gene delivery has not been established on bone engineering though its potential usefulness has been suggested. For clinical applications, an effective condition should be developed to transfer the genes in vivo without any transfection reagents or virus vectors. In this study, to facilitate the clinical setting of this strategy, particularly aimed at atrophic bone repair, we simply investigated whether manufactured gene-activated matrix (GAM) with atelocollagen containing a certain amount of plasmid (p) DNA encoding osteogenic proteins could augment the cranial bone in rat. GAMs were manufactured by mixing 0.02, 0.1, or 1 mg of AcGFP plasmid vectors harboring cDNA of BMP4 (pBMP4) or Runx2 (pRunx2) with 2% bovine atelocollagen and β-tricalcium phosphate granules. Before manufacturing GAMs, to determine the biological activity of generated pDNAs, we confirmed GFP expression and increased level of alkaline phosphatase activities in MC3T3-E1 cells transfected with pBMP4 or pRunx2 during culture. Then, GAMs were lyophilized and transplanted to onlay placement on the cranium. At 2 weeks of transplantation, GFP-expressing cells could be detectable in only GAMs containing 1 mg of AcGFP plasmid vectors. Then, at 4 weeks, significant bone formation was recognized in GAMs containing 1 mg of pDNAs encoding BMP4 or Runx2 but not in 0.02 or 0.1 mg of GAMs. These newly formed bone tissues surrounded by osteocalcin-stained area were augmented markedly until 8 weeks after transplantation. In contrast, minimal bone formation was observed in GAMs without harboring cDNA of osteogenic proteins. Meanwhile, when GAMs were transplanted to the cranial bone defect, bone formation was detectable in specimens containing 1 mg of pBMP4 or pRunx2 at 8 weeks as well. Thus, atelocollagen-based GAM reliably could form the engineered bone even for the vertical augmentation when containing a certain amount of plasmid vectors encoding osteogenic proteins. This study supports facilitating the clinical application of GAM for bone engineering.

Skin fibroblasts from patients with type 1 diabetes (T1D) can be chemically transdifferentiated into insulin-expressing clusters: a transgene-free approach

The conversion of differentiated cells into insulin-producing cells is a promising approach for the autologous replacement of pancreatic cells in patients with type 1 diabetes (T1D). At present, cellular reprogramming strategies encompass ethical problems, epigenetic failure or teratoma formation, which has prompted the development of new approaches. Here, we report a novel technique for the conversion of skin fibroblasts from T1D patients into insulin-expressing clusters using only drug-based induction. Our results demonstrate that skin fibroblasts from diabetic patients have pancreatic differentiation capacities and avoid the necessity of using transgenic strategies, stem cell sources or global demethylation steps. These findings open new possibilities for studying diabetes mechanisms, drug screenings and ultimately autologous transgenic-free regenerative medicine therapies in patients with T1D.

Gene delivery from polymer scaffolds for tissue engineering

The combination of gene therapy with tissue engineering offers the potential to direct progenitor cell proliferation and differentiation into functional tissue replacements. Many approaches to engineering tissue replacements feature a polymer scaffold to create and maintain a space, support cell adhesion, and organize tissue formation. Polymer scaffolds, either natural, synthetic, or a combination of the two, have also been adapted to serve as delivery vehicles for viral and nonviral vectors, which can induce the expression of tissue inductive factors. Gene delivery is a versatile approach, capable of targeting any cellular process through localized expression of tissue inductive factors. The design and application of tissue engineering scaffolds for localized gene transfer are reviewed. Scaffolds are designed either to release the vector into the local tissue environment or maintain the vector at the polymer surface, which is regulated by the effective affinity of the vector for the polymer. Polymeric delivery can enhance gene transfer locally, promote and extend transgene expression, avoid vector distribution to distant tissues, and reduce the immune response to the vector. Scaffolds capable of controlled DNA delivery can provide a fundamental tool for directing progenitor cell function, which has applications with the engineering of numerous types of tissue. The utility of this approach will increase with the development of design parameters that correlate release and transgene expression, and with continued research into the biology of tissue formation.

Bone formation in rabbit's leg muscle after autologous transplantation of bone marrow-derived mesenchymal stem cells expressing human bone morphogenic protein-2

BACKGROUND

To test whether autologous transplantation of bone marrow-derived mesenchymal stem cells (BM-MSCs) expressing human bone morphogenic protein-2 (hBMP-2) can produce bone in rabbit leg muscles.

MATERIALS AND METHODS

MSCs were isolated from BM of the iliac crest of rabbits and then infected with lentiviral vectors (LVs) bearing hBMP-2 and green fluorescent protein under the control of the cytomegalovirus (immediate early promoter). Differentiation of transduced MSCs to osteoblasts in vitro was evaluated with an alkaline phosphatase activity assay and immuohistochemistry against osteoblast specific markers. MSCs expressing hBMP-2 were placed in an absorbable gelatin sponge, which was then transplanted into the gastrocnemius of rabbits from which MSCs were isolated. Bone formation was examined by X-ray and histological analysis.

RESULTS

LVs efficiently mediated hBMP-2 gene expression in rabbit BM-MSCs. Ectopic expression of hBMP in these MSCs induced osteoblastic differentiation in vitro. Bone was formed after the MSCs expressing hBMP-2 were transplanted into rabbit muscles.

CONCLUSION

Ectopic expression of hBMP-2 in rabbit MSCs induces them to differentiate into osteoblasts in vitro and to form a bone in vivo.

Periodontal tissue regeneration by combined implantation of adipose tissue-derived stem cells and platelet-rich plasma in a canine model

BACKGROUND AIMS

One goal of periodontal therapy is to regenerate periodontal tissues. Stem cells, growth factors and scaffolds and biomaterials are vital for the restoration of the architecture and function of complex tissues. Adipose tissue-derived stem cells (ASCs) are an ideal population of stem cells for practical regenerative medicine. In addition, platelet-rich plasma (PRP) can be useful for its ability to stimulate tissue regeneration. PRP contains various growth factors and may be useful as a cell carrier in stem cell therapies. The purpose of this study was to determine whether a mixture of ASCs and PRP promoted periodontal tissue regeneration in a canine model.

METHODS

Autologous ASCs and PRP were implanted into areas with periodontal tissue defects. Periodontal tissue defects that received PRP alone or non-implantation were also examined. Histologic, immunohistologic and x-ray studies were performed 1 or 2 months after implantation. The amount of newly formed bone and the scale of newly formed cementum in the region of the periodontal tissue defect were analyzed on tissue sections.

RESULTS

The areas of newly formed bone and cementum were greater 2 months after implantation of ASCs and PRP than at 1 month after implantation, and the radiopacity in the region of the periodontal tissue defect increased markedly by 2 months after implantation. The ASCs and PRP group exhibited periodontal tissue with the correct architecture, including alveolar bone, cementum-like structures and periodontal ligament-like structures, by 2 months after implantation.

CONCLUSIONS

These findings suggest that a combination of autologous ASCs and PRP promotes periodontal tissue regeneration that develops the appropriate architecture for this complex tissue.

Bone marrow stromal cells can cause subcutaneous fibroblasts to differentiate into osteocytes in a physically stable spatial microenvironment in rats

In this study, we investigated how rat bone marrow stromal cells (BMSCs) under a physically stable microenvironment influenced the subcutaneous fibroblasts. The model for this study involved setting up a space made up of a titanium mesh cage inserted into the subcutaneous region in rats and filled with a collagen matrix seeded with (1) BMSCs, (2) fibroblasts or (3) a combination of BMSCs and fibroblasts. Fibroblasts for transplantations were taken from enhanced green fluorescence protein (EGFP) transgenic "green rats" which enabled us to trace the fate of the cells in vivo. A series of X-ray computed tomographic (CT) images were taken of each implant over a period of 8 weeks, and the implants were then removed and examined histologically. As a result, while generated bone was observed in each case that included BMSCs (the BMSCs and combination group), there was no generated bone observed in the group using fibroblasts only. Interestingly, EGFP-positive osteocytes were observed in the generated bone of the combination group, indicating that the transplanted fibroblasts differentiated into osteocytes during the bone formation. Thus, we demonstrated that genuine intrinsic fibroblasts are able to become osteocytes as a result of the influence of BMSCs.

Cholesteryl group- and acryloyl group-bearing pullulan nanogel to deliver BMP2 and FGF18 for bone tissue engineering

To create a drug delivery system that allows the controlled release of proteins, such as growth factors, over a long-term period, cholesteryl group- and acryloyl group-bearing pullulan (CHPOA) nanogels were aggregated to form fast-degradable hydrogels (CHPOA/hydrogels) by cross-linking with thiol-bearing polyethylene glycol. The gold standard of clinical bone reconstruction therapy with a physiologically active material is treatment with recombinant human bone morphogenetic protein 2 (BMP2); however, this approach has limitations, such as inflammation, poor cost-efficiency, and varying interindividual susceptibility. In this study, two distinct growth factors, BMP2 and recombinant human fibroblast growth factor 18 (FGF18), were applied to a critical-size skull bone defect for bone repair by the CHPOA/hydrogel system. The CHPOA-FGF18/hydrogel displayed identical results to the control CHPOA-PBS/hydrogel, and the CHPOA-BMP2/hydrogel treatment imperfectly induced bone repair. By contrast, the CHPOA-FGF18 + BMP2/hydrogel treatment strongly enhanced and stabilized the BMP2-dependent bone repair, inducing osteoprogenitor cell infiltration inside and around the hydrogel. This report indicates that the CHPOA/hydrogel system can successfully deliver two different proteins to the bone defect to induce effective bone repair. The combination of the CHPOA/hydrogel system with the growth factors FGF18 and BMP2 might be a step towards efficient bone tissue engineering.

Canine oral mucosal fibroblasts differentiate into osteoblastic cells in response to BMP-2

Several lines of evidence show that transplantation of osteoblastic cells or genetically engineered nonosteogenic cells expressing osteoblast-related genes into bone defects effectively promotes bone regeneration. To extend this possibility, we investigated whether oral mucosal fibroblasts are capable of differentiating into osteoblastic cells by conducting in vitro and in vivo experiments. We investigated the effects of bone morphogenetic protein-2 (BMP-2) on osteoblast differentiation of cultured fibroblasts isolated from canine buccal mucosa. We also transplanted green fluorescence protein (GFP)-expressing fibroblasts with gelatin/BMP-2 complexes into the subfascial regions of athymic mice, and investigated the localization of GFP-positive cells in the ectopically formed bones. The cultured canine buccal mucosal fibroblasts differentiated into osteoblastic cells by increasing their alkaline phosphatase (ALP) activity and Osteocalcin, Runx2, and Osterix mRNA expression levels in response to BMP-2. Transplantation experiments of GFP-expressing oral mucosal fibroblasts with gelatin/BMP-2 complexes revealed that 17.1% of the GFP-positive fibroblasts differentiated into ALP-positive cells, and these cells accounted for 6.2% of total ALP-positive cells in the ectopically formed bone. This study suggests that oral mucosal fibroblasts can differentiate into osteogenic cells in response to BMP-2. Thus, these cells are potential candidates for cell-mediated bone regeneration therapy in dentistry.

Cell-mediated and direct gene therapy for bone regeneration

INTRODUCTION

Bone regeneration is required for the treatment of fracture non/delayed-unions and bone defects. However, most current treatment modalities have limited efficacy, and newer therapeutic strategies, such as gene therapy, have substantial benefit for bone repair and regeneration.

AREAS COVERED

This review discusses experimental and clinical applications of cell-mediated and direct gene therapy for bone regeneration. The review covers literature on this subject from 2000 to February 2012.

EXPERT OPINION

Direct gene therapy using various viral and non-viral vectors of cell-mediated genes has been demonstrated to induce bone regeneration, although use of such vectors has shown some risk in human application. Osteoinductive capability of a number of progenitor cells isolated from bone marrow, fat, muscle and skin tissues, has been demonstrated by genetic modification with osteogenic genes. Cell-mediated gene therapy using such osteogenic gene-expressing progenitor cells has shown promising results in promoting bone regeneration in extensive animal work in recent years.

LIM domain protein-3 (LMP3) cooperates with BMP7 to promote tissue regeneration by ligament progenitor cells

Gene transfer of key regulators of osteogenesis for mesenchymal stem cells represents a promising strategy to regenerate bone. It has been reported that LMP3, a transcription variant of LIM domain mineralization protein (LMP) lacking LIM domains, can induce osteogenesis in vitro and in vivo. As little is known about the effects of LMP3 gene therapy on periodontal ligament (PDL) cell osteogenic differentiation, this study sought to explore whether gene delivery of LMP3 can promote PDL cell mineralization and bone formation. Our results showed that adenoviral mediated gene transfer of LMP3 (AdLMP3) significantly upregulated ALP (Alkaline Phosphatase), BSP (Bone Sialoprotein) and BMP2 gene expression and increased in vitro matrix mineralization in human PDL. Although AdLMP3 gene delivery to PDL cells did not induce ectopic bone formation in vivo, we found that AdLMP3 augments new bone formation, which co-delivered with AdBMP7 gene transfer. Our study provides the evidence that there is a synergistic effect between LMP3 and BMP-7 in vivo, suggesting that LMP3 delivery may be used to augment BMP-mediated osteogenesis. LMP3 and BMP-7 combinatory gene therapy may also have specific applications for oral and periodontal regenerative medicine.

Effects of designed PLLA and 50:50 PLGA scaffold architectures on bone formation in vivo

Biodegradable porous scaffolds have been investigated as an alternative approach to current metal, ceramic, and polymer bone graft substitutes for lost or damaged bone tissues. Although there have been many studies investigating the effects of scaffold architecture on bone formation, many of these scaffolds were fabricated using conventional methods such as salt leaching and phase separation, and were constructed without designed architecture. To study the effects of both designed architecture and material on bone formation, this study designed and fabricated three types of porous scaffold architecture from two biodegradable materials, poly (L-lactic acid) (PLLA) and 50:50 Poly(lactic-co-glycolic acid) (PLGA), using image based design and indirect solid freeform fabrication techniques, seeded them with bone morphogenetic protein-7 transduced human gingival fibroblasts, and implanted them subcutaneously into mice for 4 and 8 weeks. Micro-computed tomography data confirmed that the fabricated porous scaffolds replicated the designed architectures. Histological analysis revealed that the 50:50 PLGA scaffolds degraded but did not maintain their architecture after 4 weeks implantation. However, PLLA scaffolds maintained their architecture at both time points and showed improved bone ingrowth, which followed the internal architecture of the scaffolds. Mechanical properties of both PLLA and 50:50 PLGA scaffolds decreased but PLLA scaffolds maintained greater mechanical properties than 50:50 PLGA after implantation. The increase of mineralized tissue helped support the mechanical properties of bone tissue and scaffold constructs between 4-8 weeks. The results indicate the importance of choice of scaffold materials and computationally designed scaffolds to control tissue formation and mechanical properties for desired bone tissue regeneration.

Future of local bone regeneration - Protein versus gene therapy

The most promising attempts to achieve bone regeneration artificially are based on the application of mediators such as bone morphogenetic proteins (BMPs) directly to the deficient tissue site. BMPs, as promoters of the regenerative process, have the ability to induce de novo bone formation in various tissues, and many animal models have demonstrated their high potential for ectopic and orthotopic bone formation. However, the biological activity of the soluble factors that promote bone formation in vivo is limited by diffusion and degradation, leading to a short half-life. Local delivery remains a problem in clinical applications. Several materials, including hydroxyapatite, tricalcium phosphate, demineralised bone matrices, poly-lactic acid homo- and heterodimers, and collagen have been tested as carriers and delivery systems for these factors in a sustained and appropriate manner. Unfortunately these delivery vehicles often have limitations in terms of biodegradability, inflammatory and immunological rejection, disease transmission, and most importantly, an inability to provide a sustained, continuous release of these factors at the region of interest. In coping with these problems, new approaches have been established: genes encoding these growth factor proteins can be delivered to the target cells. In this way the transfected cells serve as local "bioreactors", as they express the exogenous genes and secrete the synthesised proteins into their vicinity. The purpose of this review is to present the different methods of gene versus growth factor delivery in tissue engineering. Our review focuses on these promising and innovative methods that are defined as regional gene therapy and provide an alternative to the direct application of growth factors. Various advantages and disadvantages of non-viral and viral vectors are discussed. This review identifies potential candidate genes and target cells, and in vivo as well as ex vivo approaches for cell transduction and transfection. In explaining the biological basis, this paper also refers to current experimental and clinical applications.

Transcription factor osterix modified bone marrow mesenchymal stem cells enhance callus formation during distraction osteogenesis

This study was designed to investigate the effects of local delivery of bone marrow mesenchymal stem cells (BMMSCs) with or without osterix (OSX) gene transfected on bone regeneration in the distracted zone using a rabbit model of mandibular lengthening. Fifty-four New Zealand white rabbits underwent osteodistraction of the left mandible and were then randomly divided into group A, group B, and group C (n = 18 for each group). At the end of distraction BMMSCs transfected with OSX, autologous BMMSCs and physiological saline were injected into the distraction gaps in groups A, B, and C, respectively. Nine animals from each group were humanely killed at 2 and 6 weeks after completion of distraction. The distracted mandibles were harvested and processed for radiographic, histological, and immunohistochemical examination. Excellent bone formation in the distracted callus was observed in group A and group B; the former showed better bone formation and highest bone mineral density (BMD), thickness of new trabeculae (TNT, mm) and volumes of the newly formed bone area (NBV) in the distraction zones. Group C animals showed poor bone formation in the distracted callus when compared with groups A and B. Positive immunostaining of bone sialoprotein (BSP) was observed in the distracted callus in all groups; however, BSP expression was much stronger in group A than in groups B and C. The results of this study suggest transplantation of BMMSCs can promote bone formation in DO; OSX-mediated ex vivo gene therapy was more effective during bone deposition and callus formation in distraction osteogenesis.

Suppression of NF-kappaB increases bone formation and ameliorates osteopenia in ovariectomized mice

Bone degenerative diseases, including osteoporosis, impair the fine balance between osteoclast bone resorption and osteoblast bone formation. Single-agent therapy for anabolic and anticatabolic effects is attractive as a drug target to ameliorate such conditions. Inhibition of nuclear factor (NF)-κB reduces the osteoclast bone resorption. The role of NF-κB inhibitors on osteoblasts and bone formation, however, is minimal and not well investigated. Using an established NF-κB inhibitor named S1627, we demonstrated that inhibition of NF-κB increases osteoblast differentiation and bone formation in vitro by up-regulating the mRNAs of osteoblast-specific genes like type I collagen, alkaline phosphatase, and osteopontin. In addition, S1627 was able to increase bone formation and repair bone defect in a murine calvarial defect model. To determine the effect of NF-κB on a model of osteoporosis, we injected two doses of inhibitor (25 and 50 mg/kg·d) twice a day in sham-operated or ovariectomized 12-wk-old mice and killed them after 4 wk. The anabolic effect of S1627 on trabecular bone was determined by micro focal computed tomography and histomorphometry. Bone mineral density of inhibitor-treated ovariectomized animals was significantly increased compared with sham-operated mice. Osteoblast-related indices like osteoblast surface, mineral apposition rate, and bone formation rate were increased in S1627-treated animals in a dose-dependent manner. NF-κB inhibition by S1627 increased the trabecular bone volume in ovariectomized mice. Furthermore, S1627 could inhibit the osteoclast number, and osteoclast surface to bone surface. In vitro osteoclastogenesis and bone resorbing activity were dose-dependently reduced by NF-κB inhibitor S1627. Taken collectively, our results suggest that NF-κB inhibitors are effective in treating bone-related diseases due to their dual anabolic and antiresorptive activities.

Enhancement of bone regeneration by gene delivery of BMP2/Runx2 bicistronic vector into adipose-derived stromal cells

Adipose tissue contains multipotent mesenchymal stem cells (MSCs) that are able to differentiate into various tissues. Bone morphogenetic protein 2 (BMP2) is known as one of the key osteogenesis induction factors in MSCs. Recently, several new transcription factors that contribute to osteogenic differentiation have been reported, among them Runx2, Osterix, and Dlx5. We hypothesized that adipose-derived stromal cells (ASCs) could be induced to efficiently differentiate into osteocytes by the co-expression of the BMP2 and Runx2 genes. To prove this hypothesis, we constructed a bicistronic vector encoding the BMP2 and Runx2 genes linked to the 'self-cleaving' 2A peptide sequence. BMP2/Runx2-ASCs showed a gradual increase in alkaline phosphatase activity for two weeks. RT-PCR analysis and alizarin red staining revealed a high expression of osteogenesis-related markers (osteopontin, osteocalcin and collagen type I) and increased mineralization in BMP2/Runx2-ASCs compared to BMP2-ASCs. Six weeks after in vivo transplantation, BMP2/Runx2-ASCs also showed a significant increase in bone formation compared to ASCs and BMP2-ASCs. These findings demonstrate that the co-transfection of two osteogenic lineage-determining genes can enhance osteogenic differentiation of ASCs.

Enhancement of bone formation ex vivo and in vivo by a helioxanthin-derivative

To effectively treat serious bone defects using bone-regenerative medicine, a small chemical compound that potently induces bone formation must be developed. We previously reported on the osteogenic effect of 4-(4-methoxyphenyl)pyrido[40,30:4,5]thieno[2,3-b]pyridine-2-carboxamide (TH), a helioxanthin-derivative, in vitro. Here, we report on TH's osteogenic effects ex vivo and in vivo. TH-induced new bone formation in both calvarial and metatarsal organ cultures. A novel monitoring system of osteoblastic differentiation using MC3T3-E1 cells revealed that TH was released from alpha-TCP bone cement and this release continued for more than one month. Lastly, the implantation of the alpha-TCP carrier containing TH into defects in mouse skull resulted in increased new bone areas within the defects after 4 weeks. A TH-containing scaffold may help establish a more efficient bone regeneration system.

Dermal fibroblast-mediated BMP2 therapy to accelerate bone healing in an equine osteotomy model

This study evaluated healing of equine metacarpal/metatarsal osteotomies in response to percutaneous injection of autologous dermal fibroblasts (DFbs) genetically engineered to secrete bone morphogenetic protein-2 (BMP2) or demonstrate green fluorescent protein (GFP) gene expression administered 14 days after surgery. Radiographic assessment of bone formation indicated greater and earlier healing of bone defects treated with DFb with BMP2 gene augmentation. Quantitative computed tomography and biomechanical testing revealed greater mineralized callus and torsional strength of DFb-BMP2-treated bone defects. On the histologic evaluation, the bone defects with DFb-BMP2 implantation had greater formation of mature cartilage and bone nodules within the osteotomy gap and greater mineralization activity on osteotomy edges. Autologous DFbs were successfully isolated in high numbers by a skin biopsy, rapidly expanded without fastidious culture techniques, permissive to adenoviral vectors, and efficient at in vitro BMP2 protein production and BMP2-induced osteogenic differentiation. This study demonstrated an efficacy and feasibility of DFb-mediated BMP2 therapy to accelerate the healing of osteotomies. Skin cell-mediated BMP2 therapy may be considered as a potential treatment for various types of fractures and bone defects.

Design and fabrication of 3D porous scaffolds to facilitate cell-based gene therapy

Biomaterials capable of efficient gene delivery by embedded cells provide a fundamental tool for the treatment of acquired or hereditary diseases. A major obstacle is maintaining adequate nutrient and oxygen diffusion to cells within the biomaterial. In this study, we combined the solid free-form fabrication and porogen leaching techniques to fabricate three-dimensional scaffolds, with bimodal pore size distribution, for cell-based gene delivery. The objective of this study was to design micro-/macroporous scaffolds to improve cell viability and drug delivery. Murine bone marrow-derived mesenchymal stromal cells (MSCs) genetically engineered to secrete erythropoietin (EPO) were seeded onto poly-L-lactide (PLLA) scaffolds with different microporosities. Over a period of 2 weeks in culture, an increase in cell proliferation and metabolic activity was observed with increasing scaffold microporosity. The concentration of EPO detected in supernatants also increased with increasing microporosity level. Our study shows that these constructs can promote cell viability and release of therapeutic proteins, and clearly demonstrates their capacity for a dual role as scaffolds for tissue regeneration and as delivery systems for soluble gene products.

Endogenous bone morphogenetic proteins mediate 1alpha, 25-dihydroxyvitamin D(3)-induced expression of osteoblast differentiation markers in human dermal fibroblasts

Human dermal fibroblasts are generally considered to be restricted to a fibroblastic lineage. Although dermal fibroblasts do not typically express markers of osteoblastic differentiation, they have previously been shown to undergo osteoinduction when stimulated with bone morphogenetic proteins (BMPs) or vitamin D(3). However, involvement of BMP signaling in vitamin D(3)-mediated osteoinduction has not been reported. In this study, human dermal fibroblasts were cultured in chemically defined medium containing vitamin D(3), in the presence of the BMP antagonist noggin or neutralizing antibodies specific for BMP-4 or BMP-6, and characterized for markers of osteoblastic differentiation. Treatment of dermal fibroblasts with vitamin D(3) induced expression of BMP-4 (1.2 +/- 0.2, 1.7 +/- 0.2, and 1.8 +/- 0.2 relative fold increase) and BMP-6 (9.1 +/- 0.3, 23.3 +/- 2.1, and 30.4 +/- 3.0 relative fold increase) at 3, 14, and 21 days, respectively. Vitamin D(3) was also shown to induce the expression of the osteoblast-specific markers, alkaline phosphatase and osteocalcin, in a dose-dependent manner in human dermal fibroblasts. Addition of noggin, BMP-4 antibodies, and BMP-6 antibodies resulted in a downregulation of alkaline phosphatase activity (by 42%, 22%, and 20%, respectively) and secreted osteocalcin (by 20%, 31%, and 49%, respectively) after 21 days in culture. However, blocking BMP signaling did not result in complete recovery of a fibroblastic phenotype. Taken together, these results suggest that BMP signaling plays a role in the induction of an osteoblastic phenotype in human dermal fibroblasts in response to vitamin D(3) stimulation.

Ex vivo-transduced autologous skin fibroblasts expressing human Lim mineralization protein-3 efficiently form new bone in animal models

Local gene transfer of the human Lim mineralization protein (LMP), a novel intracellular positive regulator of the osteoblast differentiation program, can induce efficient bone formation in rodents. To develop a clinically relevant gene therapy approach to facilitate bone healing, we have used primary dermal fibroblasts transduced ex vivo with Ad.LMP-3 and seeded on a hydroxyapatite/collagen matrix prior to autologous implantation. Here, we demonstrate that genetically modified autologous dermal fibroblasts expressing Ad.LMP-3 are able to induce ectopic bone formation following implantation of the matrix into mouse triceps and paravertebral muscles. Moreover, implantation of the Ad.LMP-3-modified dermal fibroblasts into a rat mandibular bone critical size defect model results in efficient healing, as determined by X-rays, histology and three-dimensional microcomputed tomography (3DmuCT). These results demonstrate the effectiveness of the non-secreted intracellular osteogenic factor LMP-3 in inducing bone formation in vivo. Moreover, the utilization of autologous dermal fibroblasts implanted on a biomaterial represents a promising approach for possible future clinical applications aimed at inducing new bone formation.

Microporous nanofibrous fibrin-based scaffolds for bone tissue engineering

The fibrotic response of the body to synthetic polymers limits their success in tissue engineering and other applications. Though porous polymers have demonstrated improved healing, difficulty in controlling their pore sizes and pore interconnections has clouded the understanding of this phenomenon. In this study, a novel method to fabricate natural polymer/calcium phosphate composite scaffolds with tightly controllable pore size, pore interconnection, and calcium phosphate deposition was developed. Microporous, nanofibrous fibrin scaffolds were fabricated using sphere-templating methods. Composite scaffolds were created by solution deposition of calcium phosphate on fibrin surfaces or by direct incorporation of nanocrystalline hydroxyapatite (nHA). The SEM results showed that fibrin scaffolds exhibited a highly porous and interconnected structure. Osteoblast-like cells, obtained from murine calvaria, attached, spread and showed a polygonal morphology on the surface of the biomaterial. Multiple cell layers and fibrillar matrix deposition were observed. Moreover, cells seeded on mineralized fibrin scaffolds exhibited significantly higher alkaline phosphatase activity as well as osteoblast marker gene expression compared to fibrin scaffolds and nHA incorporated fibrin scaffolds (0.25 and 0.5g). All types of scaffolds were degraded both in vitro and in vivo. Furthermore, these scaffolds promoted bone formation in a mouse calvarial defect model and the bone formation was enhanced by addition of rhBMP-2.

Retroviral-mediated gene therapy for the differentiation of primary cells into a mineralizing osteoblastic phenotype

Bone tissue engineering has emerged as a promising strategy for the repair of critical-sized skeletal fractures. However, the clinical application of this approach has been limited by the availability of a robust mineralizing cell source. Non-osteogenic cells, such as skin fibroblasts, are an attractive cell-source alternative because they are easy to harvest from autologous donor skin biopsies and display a high capacity for in vitro expansion. We have recently demonstrated that retroviral gene delivery of the osteoblastic transcription factor Runx2/Cbfa1 promotes osteogenic differentiation in primary dermal fibroblasts cultured in monolayer. Notably, sustained expression of Runx2 was not sufficient to promote functional osteogenesis in these cells, and co-treatment with the steroid hormone dexamethasone was required to induce deposition of biologically-equivalent matrix mineralization. On the basis of these results, we then investigated the osteogenic capacity of these genetically engineered fibroblasts when seeded on polymeric scaffolds in vitro and in vivo. These experiments demonstrated that Runx2-expressing fibroblasts seeded on collagen scaffolds produce significant levels of matrix mineralization after 28 days in vivo implantation in a subcutaneous, heterotopic site. Overall, these results offer evidence that transcription factor-based gene therapy may be a powerful strategy for the conversion of a non-osteogenic cellular phenotype into a mineralizing cell source for bone repair applications. This concept may also be applied to control functional differentiation in a broad range of cell types and tissue engineering applications. The chapter below outlines detailed methods for the isolation and ex vivo genetic modification of primary dermal fibroblasts using retroviral-mediated delivery of the Runx2 transgene in both monolayer culture and three-dimensional scaffolds.

A perspective: engineering periosteum for structural bone graft healing

Autograft is superior to both allograft and synthetic bone graft in repair of large structural bone defect largely due to the presence of multipotent mesenchymal stem cells in periosteum. Recent studies have provided further evidence that activation, expansion and differentiation of the donor periosteal progenitor cells are essential for the initiation of osteogenesis and angiogenesis of donor bone graft healing. The formation of donor cell-derived periosteal callus enables efficient host-dependent graft repair and remodeling at the later stage of healing. Removal of periosteum from bone autograft markedly impairs healing whereas engraftment of multipotent mesenchymal stem cells on bone allograft improves healing and graft incorporation. These studies provide rationale for fabrication of a biomimetic periosteum substitute that could fit bone of any size and shape for enhanced allograft healing and repair. The success of such an approach will depend on further understanding of the molecular signals that control inflammation, cellular recruitment as well as mesenchymal stem cell differentiation and expansion during the early phase of the repair process. It will also depend on multidisciplinary collaborations between biologists, material scientists and bioengineers to address issues of material selection and modification, biological and biomechanical parameters for functional evaluation of bone allograft healing.

Multilineage differentiation potential of human dermal skin-derived fibroblasts

Dermal skin-derived fibroblasts from rodent and human have been found to exhibit mesenchymal surface antigen immunophenotype and differentiation potential along the three main mesenchymal-derived tissues: bone, cartilage and fat. Human dermal skin-derived mesenchymal stem cells constitute a promising cell source in clinical applications. Therefore, we isolated fibroblastic mesenchymal stem-cell-like cells from human dermis derived from juvenile foreskins, which share a mesenchymal stem cell phenotype and multi-lineage differentiation potential. We could show similar expression patterns for CD14(-), CD29(+), CD31(-), CD34(-), CD44(+), CD45(-), CD71(+), CD73/SH3-SH4(+), CD90/Thy-1(+), CD105/SH2(+), CD133(-) and CD166/ALCAM(+) in well-established adipose tissue derived-stem cells and fibroblastic mesenchymal stem-cell-like cells by flow cytometry. Immunostainings showed that fibroblastic mesenchymal stem-cell-like cells expressed vimentin, fibronectin and collagen; they were less positive for alpha-smooth muscle actin and nestin, while they were negative for epithelial cytokeratins. When cultured under appropriate inducible conditions, both cell types could differentiate along the adipogenic and osteogenic lineages. Additionally, fibroblastic mesenchymal stem-cell-like cells demonstrated a high proliferation potential. These findings are of particular importance, because skin or adipose tissues are easily accessible for autologous cell transplantations in regenerative medicine. In summary, these data indicate that dermal fibroblasts with multilineage differentiation potential are present in human dermis and they might play a key role in cutaneous wound healing.

Zfp64 participates in Notch signaling and regulates differentiation in mesenchymal cells

Notch signaling is required for multiple aspects of tissue and cell differentiation. In this study, we identified zinc finger protein 64 (Zfp64) as a novel coactivator of Notch1. Zfp64 is associated with the intracellular domain of Notch1, recruited to the promoters of the Notch target genes Hes1 and Hey1, and transactivates them. Zfp64 expression is under the control of Runx2, and is upregulated by direct transactivation of its promoter. Zfp64 suppresses the myogenic differentiation of C2C12 cells and promotes their osteoblastic differentiation. Our data demonstrate two functions of Zfp64: (1) it is a downstream target of Runx2 and, (2) its cognate protein acts as a coactivator of Notch1, which suggests that Zfp64 mediates mesenchymal cell differentiation by modulating Notch signaling.

Proliferative capacity and osteogenic potential of novel dura mater stem cells on poly-lactic-co-glycolic acid

The rational design of biomimetic structures for the regeneration of damaged or missing tissue is a fundamental principle of tissue engineering. Multiple variables must be optimized, ranging from the scaffold type to the selection and properties of implanted cell(s). In this study, the osteogenic potential of a novel stem cell was analyzed on biodegradable poly(lactic-co-glycolic acid) (PLGA) biomaterials as a step toward creating new cell-materials constructs for bony regeneration. Dura mater stem cells (DSCs), isolated from rat dura mater, were evaluated and compared to bone marrow stem cells (BMSCs) for proliferative and differentiative properties in vitro. Experiments were carried out on both tissue culture plastic (TCP) and 2D planar films of PLGA. Proliferation of DSCs on both TCP and PLGA films increased over 21 days. Positive fold inductions in all five bone marker genes were observed at days 7, 14, 21 in all experimental samples compared with day 0 controls. DSCs demonstrated greater cell coverage and enhanced matrix staining on 2D PLGA films when compared with BMSCs. These cells can be isolated and expanded in culture and can subsequently attach, proliferate, and differentiate on both TCP and PLGA films to a greater extent than BMSCs. This suggests that DSCs are promising for cell-based bone tissue engineering therapies, particularly those applications involving regeneration of cranial bones.

Gene therapy applications for fracture-healing

Biologic therapies to promote fracture-healing such as use of bone morphogenetic proteins (BMPs) are being increasingly employed in multiple clinical scenarios. However, it has been challenging to design therapies that deliver sufficient quantities of protein over a sustained time period. A potential solution is the application of gene therapy that transfers genetic information to host cells at the fracture site, resulting in the continuous and localized production of the desired proteins. This approach has demonstrated tremendous potential in preclinical animal models of fracture-healing. This article will review the current state of gene therapy approaches to fracture-healing with an emphasis on potential clinical applications.

Adult neural stem and progenitor cells modified to secrete GDNF can protect, migrate and integrate after intracerebral transplantation in rats with transient forebrain ischemia

Adult neural stem and progenitor cells (NSPCs) are important autologous transplantation tools in regenerative medicine, as they can secrete factors that protect the ischemic brain. We investigated whether adult NSPCs genetically modified to secrete more glial cell line-derived neurotrophic factor (GDNF) could protect against transient ischemia in rats. NSPCs were harvested from the subventricular zone of adult Wistar rats and cultured for 3 weeks in the presence of epidermal growth factor. The NSPCs were treated with fibre-mutant Arg-Gly-Asp adenovirus containing the GDNF gene (NSPC-GDNF) or enhanced green fluorescent protein (EGFP) gene (NSPC-EGFP; control group). In one experiment, cultured cells were transplanted into the right ischemic boundary zone of Wistar rat brains. One week later, animals underwent 90 min of intraluminal right middle cerebral artery occlusion followed by magnetic resonance imaging and behavioural tests. The NSPC-GDNF group had higher behavioural scores and lesser infarct volume than did controls at 1, 7 and 28 days postocclusion. In the second experiment, we transplanted NSPCs 3 h after ischemic insult. Compared to controls, rats receiving NSPC-GDNF had decreased infarct volume and better behavioural assessments at 7 days post-transplant. Animals were killed on day 7 and brains were collected for GDNF ELISA and morphological assessment. Compared to controls, more GDNF was secreted, more NSPC-GDNF cells migrated toward the ischemic core and more NSPC-GDNF cells expressed immature neuronal marker. Moreover, the NSPC-GDNF group showed more effective inhibition of microglial invasion and apoptosis. These findings suggest that NSPC-GDNF may be useful in treatment of cerebral ischemia.

Human skin fibroblasts: From mesodermal to hepatocyte-like differentiation

The phenotypic homology of fibroblasts and mesenchymal stem cells (MSCs) has been recently described. Our study investigated the in vitro potential of human skin fibroblasts to differentiate into mesodermal (osteocyte and adipocyte) and endodermal (hepatocyte) cell lineages by comparison with human bone marrow (hBM) MSCs. The endodermal potential of fibroblasts was then explored in vivo in a mouse model of liver injury. Fibroblasts were able to acquire osteocyte and adipocyte phenotypes as assessed by cytochemistry and gene expression analyses. After exposure to a specific differentiation cocktail, these cells presented hepatocyte-like morphology and acquired liver-specific markers on protein and gene expression levels. Furthermore, these fibroblast-derived hepatocyte-like cells (FDHLCs) displayed the ability to store glycogen and synthesize small amounts of urea. By gene expression analysis, we observed that fibroblasts remained in a mesenchymal-epithelial transition state after hepatocyte differentiation. Moreover, FDHLCs lost their hepatocyte-like phenotype after dedifferentiation. In vivo, human fibroblasts infused directly into the liver of hepatectomized severe combined immunodeficient (SCID) mice engrafted in situ and expressed hepatocyte markers (albumin, alpha-fetoprotein, and cytokeratin 18) together with the mesodermal marker fibronectin. Despite lower liver-specific marker expression, the in vitro and in vivo differentiation profile of fibroblasts was comparable to that of mesenchymal-derived hepatocyte-like cells (MDHLCs). In conclusion, our work demonstrates that human skin fibroblasts are able to display mesodermal and endodermal differentiation capacities and provides arguments that these cells share MSCs features both on the phenotypic and functional levels.