Chondrogenic differentiation of cultured human mesenchymal stem cells from marrow

Isolation, characterization, gene modification, and nuclear reprogramming of porcine mesenchymal stem cells

Bone marrow mesenchymal stem cells (MSCs) are adult pluripotent cells that are considered to be an important resource for human cell-based therapies. Understanding the clinical potential of MSCs may require their use in preclinical large-animal models, such as pigs. The objectives of the present study were 1) to establish porcine MSC (pMSC) cultures; 2) to optimize in vitro pMSC culture conditions, 3) to investigate whether pMSCs are amenable to genetic manipulation, and 4) to determine pMSC reprogramming potential using somatic cell nuclear transfer (SCNT). The pMSCs isolated from bone marrow grew, attached to plastic with a fibroblast-like morphology, and expressed the mesenchymal surface marker THY1 but not the hematopoietic marker ITGAM. Furthermore, pMSCs underwent lipogenic, chondrogenic, and osteogenic differentiation when exposed to specific inducing conditions. The pMSCs grew well in a variety of media, and proliferative capacity was enhanced by culture under low oxygen atmosphere. Transient transduction of pMSCs and isogenic skin fibroblasts (SFs) with a human adenovirus carrying the gene for green fluorescent protein (GFP; Ad5-F35eGFP) resulted in more pMSCs expressing GFP compared with SFs. Cell lines with stable genetic modifications and extended expression of transgene were obtained when pMSCs were transfected with a plasmid containing the GFP gene. Infection of pMSC and SF cell lines by an adeno-associated virus resulted in approximately 12% transgenic cells, which formed transgenic clonal lines after propagation as single cells. The pMSCs can be expanded in vitro and used as nuclear donors to produce SCNT embryos. Thus, pMSCs are an attractive cell type for large-animal autologous and allogenic cell therapy models and for SCNT transgenesis.

Multilineage differentiation of human mesenchymal stem cells in a three-dimensional nanofibrous scaffold

Functional engineering of musculoskeletal tissues generally involves the use of differentiated or progenitor cells seeded with specific growth factors in biomaterial scaffolds. Ideally, the scaffold should be a functional and structural biomimetic of the native extracellular matrix and support multiple tissue morphogenesis. We have previously shown that electrospun, three-dimensional nanofibrous scaffolds that morphologically resemble collagen fibrils are capable of promoting favorable biological responses from seeded cells, indicative of their potential application for tissue engineering. In this study, we tested a three-dimensional nanofibrous scaffold fabricated from poly(epsilon-caprolactone) (PCL) for its ability to support and maintain multilineage differentiation of bone marrow-derived human mesenchymal stem cells (hMSCs) in vitro. hMSCs were seeded onto pre-fabricated nanofibrous scaffolds, and were induced to differentiate along adipogenic, chondrogenic, or osteogenic lineages by culturing in specific differentiation media. Histological and scanning electron microscopy observations, gene expression analysis, and immunohistochemical detection of lineage-specific marker molecules confirmed the formation of three-dimensional constructs containing cells differentiated into the specified cell types. These results suggest that the PCL-based nanofibrous scaffold is a promising candidate scaffold for cell-based, multiphasic tissue engineering.

Hypoxia induces chondrocyte-specific gene expression in mesenchymal cells in association with transcriptional activation of Sox9

Endochondral bone is formed during an avascular period in an environment of low oxygen. Under these conditions, pluripotential mesenchymal stromal cells preferentially differentiate into chondrocytes and form cartilage. In this study, we investigated the hypothesis that oxygen tension modulates bone mesenchymal cell fate by altering the expression of genes that function to promote chondrogenesis. Microarray of RNA samples from ST2 cells revealed significant changes in 728 array elements (P < 0.01) in response to hypoxia. Real-time PCR on these RNA samples, and separate samples from C3H10T1/2 cells, revealed hypoxia-induced changes in the expression of additional genes known to be expressed by chondrocytes including Sox9 and its downstream targets aggrecan and Col2a. These changes were accompanied by the accumulation of mucopolysacharide as detected by alcian blue staining. To investigate the mechanisms responsible for upregulation of Sox9 by hypoxia, we determined the effect of hypoxia on HIF-1alpha levels and Sox9 promoter activity in ST2 cells. Hypoxia increased nuclear accumulation of HIF-1alpha and activated the Sox9 promoter. The ability of hypoxia to transactivate the Sox9 promoter was virtually abolished by deletion of HIF-1alpha consensus sites within the proximal promoter. These findings suggest that hypoxia promotes the differentiation of mesenchymal cells along a chondrocyte pathway in part by activating Sox-9 via a HIF-1alpha-dependent mechanism.

TGF-beta3 induces ectopic mineralization in fetal mouse dental pulp during tooth germ development

Several members of the transforming growth factor (TGF)-beta superfamily are expressed in developing teeth from the initiation stage through adulthood. Of those, TGF-beta1 regulates odontoblast differentiation and dentin extracellular matrix synthesis. However, the molecular mechanism of TGF-beta3 in dental pulp cells is not clearly understood. In the present study, beads soaked with human recombinant TGF-beta3 induced ectopic mineralization in dental pulp from fetal mouse tooth germ samples, which increased in a dose-dependent manner. Further, TGF-beta3 promoted mRNA expression, and increased protein levels of osteocalcin (OCN) and type I collagen (COL I) in dental pulp cells. We also observed that the expression of dentin sialophosphoprotein and dentin matrix protein 1 was induced by TGF-beta3 in primary cultured dental pulp cells, however, not in calvaria osteoblasts, whereas OCN, osteopontin and osteonectin expression was increased after treatment with TGF-beta3 in both dental pulp cells and calvaria osteoblasts. Dentin sialoprotein was also partially detected in the vicinity of TGF-beta3 soaked beads in vivo. These results indicate for the first time that TGF-beta3 induces ectopic mineralization through upregulation of OCN and COL I expression in dental pulp cells, and may regulate the differentiation of dental pulp stem cells to odontoblasts.

Morphological examination during in vitro cartilage formation by human mesenchymal stem cells

The formation of the skeleton through endochondral ossification is one of the most complex processes in development. One approach to resolving this complexity is to examine simplified systems. In vitro cartilage formation by mesenchymal stem cells (MSCs) is observed when the cells are cultured as a micromass. Several studies have confirmed the molecular events, showing the usefulness of these cells as a differentiation model. We have elucidated the process of cartilage formation in MSCs from the morphological point of view by light and transmission electron microscopy and immunohistochemical examination. The morphology of the MSCs changed from spherical to spindle-shaped, and the cells aggregated and formed junctional complexes during Day 1. At Day 7, three layers were observed. The superficial zone consisted of several layers of elongated cells with junctional complexes. The middle zone was composed of apoptotic bodies, and the deep zone was occupied by chondrocyte-like cells excreting extracellular matrices. At Day 14, the middle zone had disappeared, and the chondrocyte-like cells in the deep zone were detected within cartilage lacuna. They were covered by cartilage matrices containing collagen types I, II, and X and chondroitin sulfate. By Day 21, the outer layer consisting of spindle-shaped cells had disappeared in places. As the pellet grew, the outer layer seemed to be unable to stretch to maintain a constant covering around the pellet. Our findings have thus revealed that MSCs change their morphology depending upon their microenvironment during differentiation. In vitro cartilage formation by MSCs makes it possible to clarify the detailed morphological events that occur during chondrogenesis.

High-potential human mesenchymal stem cells

Bone marrow-derived stromal mesenchymal stem cells (MSCs) have been characterized in vitro by their growth characteristics, the expression of a panel of surface antigens, and their potential to differentiate into mesenchymal lineages. They can be separated by physical methods as well as by immunological or chemical separation or cultivation. Different protocols are used in different laboratories, making the comparison of various reported MSC populations difficult. Here we describe a population of bone marrow-derived adult stem cells that has been separated on a Percoll gradient with low density. It is characterized by an extraordinary high proliferative potential and a conserved phenotype characteristic of MSCs that retain their plutipotentiality in culture, as evidenced by their ability to differentiate into osteo-, chondro-, and adipogenic lineages. Separation of these cells provide an effective and convenient method for rapid expansion of pluripotential human MSCs for clinical use where large amounts of stem cells are needed.

In vitro engineering of cartilage: effects of serum substitutes, TGF-beta, and IL-1alpha

OBJECTIVES

Cartilage is avascular and relatively homogeneous, making it an attractive tissue for in vitro histogenesis and surgical use in patients. We developed novel platform technologies in order to define the requirements for optimal in vitro chondrogenesis by isolated cells. In this series of studies, we tested alternatives to fetal bovine serum (FBS) and the effects of growth factors on formation of cartilage in 3D porous collagen sponges.

DESIGN

We used porous collagen sponges to assess the effects of serum substitutes and exogenous TGF-beta1 and IL-1alpha on chondrocytes (bovine articular chondrocytes, bACs) and on chondroinduced human dermal fibroblasts (hDFs). We determined the effects of low concentrations of FBS and two serum substitutes, Nutridoma and ITS(+3), on cellularity and matrix production. After culture for intervals, sponges were harvested for histological and biochemical measurement of cartilage-specific chondroitin 4-sulfate proteoglycan (C 4-S PG).

RESULTS

Cultured bACs showed equivalent growth in Nutridoma (1%) and 10% FBS. Both TGF-beta1 and IL-1alpha significantly stimulated accumulation of C 4-S PG by bACs in 3D porous collagen sponges. Many endogenous growth factors were upregulated in hDFs cultured with chondroinductive DBP. Addition of TGF-beta1 and IL-1alpha for 11 days significantly stimulated accumulation of C 4-S PG by hDFs cultured in DMEM with 1% Nutridoma.

CONCLUSION

Porous collagen sponges are supportive of chondrogenesis and of chondroinduction by DBP. Optimization of serum-free culture conditions, including growth factors, matrix components, and mechanical stimuli will expedite translation to wider clinical applications. Use of autogenous dermal fibroblasts pre-cultured with DBP and induced to chondrocytes offers an alternative to autogenous chondrocytes.

Adenoviral-mediated transfer of TGF-beta1 but not IGF-1 induces chondrogenic differentiation of human mesenchymal stem cells in pellet cultures

OBJECTIVE

The objective of the present study was to investigate the potential of application of growth factor genes to induce chondrogenic differentiation of human-derived mesenchymal stem cells (MSCs). The growth factor genes evaluated in the present study were transforming growth factor 1 (TGF-beta1) and insulin-like growth factor 1 (IGF-1).

METHODS

Human MSCs were transduced with the adenoviral vectors carrying either TGF-beta1 or IGF-1 (AdTGF-beta1 and AdIGF-1 respectively) or a combination of both growth factor genes at different multiplicities of infection (MOI) and were then made into pellets. Pellets were also made from nontransduced cells and maintained in culture medium supplemented with 10 ng/mL of TGF-beta1. At specified time points, histological analysis, cartilage matrix gene expression, and immunofluorescence were performed to determine the extent of chondrogenic differentiation.

RESULTS

MSCs transduced with the AdTGF-beta1 demonstrated robust chondrogenic differentiation, while those made from AdIGF-1 did not. AdTGF-beta1 pellets demonstrated aggrecan gene expression as early as day 3 of pellet culture, while type II collagen gene expression was detected by day 10 of culture. The AdIGF-1, alone or in combination with TGF-beta1 pellets, did not show any type II collagen gene expression at any time point. By immunofluoresecence, type X collagen was distributed throughout the matrix in TGF-beta1 protein pellets while the growth factor gene pellets displayed scant staining.

CONCLUSION

The results suggest that sustained administration of TGF-beta1 may be more effective in suppressing terminal differentiation than intermittent dosing and thus effective for cartilage repair.

Adult bone marrow stem/progenitor cells (MSCs) are preconditioned by microenvironmental "niches" in culture: a two-stage hypothesis for regulation of MSC fate

Mesenchymal stem cells (MSCs) are clonal, plastic adherent cells from bone marrow that can differentiate into various tissue lineages, including osteoblasts, adipocytes, chondrocytes, myoblasts, hepatocytes, and possibly even neural cells. Because MSCs are multipotent and their numbers are easily expanded in culture, there has been much interest in their clinical potential for tissue repair and gene therapy. Consequently, numerous studies have been carried out demonstrating the migration and multiorgan engraftment potential of MSCs in animal models and in human clinical trials. Understanding the mechanisms behind MSC cell fate determination is not easy, because the molecular processes that drive engraftment and differentiation are complex. Even in an in vitro system, the molecular cues necessary to induce differentiation are not easily identified or reproduced. In this Perspective, we emphasize the importance of microenvironmental factors in culture and suggest that MSC differentiation in vitro is regulated by a two-stage mechanism involving preconditioning by factors in the culture microenvironment followed by response to soluble differentiating factors.

Mesenchymal stem cells avoid allogeneic rejection

Adult bone marrow derived mesenchymal stem cells offer the potential to open a new frontier in medicine. Regenerative medicine aims to replace effete cells in a broad range of conditions associated with damaged cartilage, bone, muscle, tendon and ligament. However the normal process of immune rejection of mismatched allogeneic tissue would appear to prevent the realisation of such ambitions. In fact mesenchymal stem cells avoid allogeneic rejection in humans and in animal models. These finding are supported by in vitro co-culture studies. Three broad mechanisms contribute to this effect. Firstly, mesenchymal stem cells are hypoimmunogenic, often lacking MHC-II and costimulatory molecule expression. Secondly, these stem cells prevent T cell responses indirectly through modulation of dendritic cells and directly by disrupting NK as well as CD8+ and CD4+ T cell function. Thirdly, mesenchymal stem cells induce a suppressive local microenvironment through the production of prostaglandins and interleukin-10 as well as by the expression of indoleamine 2,3,-dioxygenase, which depletes the local milieu of tryptophan. Comparison is made to maternal tolerance of the fetal allograft, and contrasted with the immune evasion mechanisms of tumor cells. Mesenchymal stem cells are a highly regulated self-renewing population of cells with potent mechanisms to avoid allogeneic rejection.

Transforming growth factor-beta2 enhances differentiation of cardiac myocytes from embryonic stem cells

Stem cell therapy holds great promise for the treatment of injured myocardium, but is challenged by a limited supply of appropriate cells. Three different isoforms of transforming growth factor-beta (TGF-beta) -beta1, -beta2, and -beta3 exhibit distinct regulatory effects on cell growth, differentiation, and migration during embryonic development. We compared the effects of these three different isoforms on cardiomyocyte differentiation from embryonic stem (ES) cells. In contrast to TGF-beta1, or -beta3, treatment of mouse ES cells with TGF-beta2 isoform significantly increased embryoid body (EB) proliferation as well as the extent of the EB outgrowth that beat rhythmically. At 17 days, 49% of the EBs treated with TGF-beta2 exhibited spontaneous beating compared with 15% in controls. Cardiac myocyte specific protein markers sarcomeric myosin and alpha-actin were demonstrated in beating EBs and cells isolated from EBs. In conclusion, TGF-beta2 but not TGF-beta1, or -beta3 promotes cardiac myocyte differentiation from ES cells.

Tissue engineering strategies for cartilage generation—Micromass and three dimensional cultures using human chondrocytes and a continuous cell line

Utilizing ATDC5 murine chondrogenic cells and human articular chondrocytes, this study sought to develop facile, reproducible three-dimensional models of cartilage generation with the application of tissue engineering strategies, involving biodegradable poly(glycolic acid) scaffolds and rotating wall bioreactors, and micromass pellet cultures. Chondrogenic differentiation, assessed by histology, immunohistochemistry, and gene expression analysis, in ATDC5 and articular chondrocyte pellets was evident by the presence of distinct chondrocytes, expressing Sox-9, aggrecan, and type II collagen, in lacunae embedded in a cartilaginous matrix of type II collagen and proteoglycans. Tissue engineered explants of ATDC5 cells were reminiscent of cartilaginous structures composed of numerous chondrocytes, staining for typical chondrocytic proteins, in lacunae embedded in a matrix of type II collagen and proteoglycans. In comparison, articular chondrocyte explants exhibited areas of Sox-9, aggrecan, and type II collagen-expressing cells growing on fleece, and discrete islands of chondrocytic cells embedded in a cartilaginous matrix.

Temporal expression patterns and corresponding protein inductions of early responsive genes in rabbit bone marrow-derived mesenchymal stem cells under cyclic compressive loading

Our recent study suggested that cyclic compressive loading may promote chondrogenesis of rabbit bone-marrow mesenchymal stem cells (BM-MSCs) in agarose cultures through the transforming growth factor (TGF)-beta signaling pathway. It has been shown that the activating protein 1 (AP-1) (Jun-Fos) complex mediated autoinduction of TGF-beta1 and its binding activity was essential for promoting chondrogenesis of mesenchymal cells, whereas Sox9 was identified as an essential transcription factor for chondrogenesis of embryonic mesenchymal cells. The objective of this study was to examine temporal expression patterns of early responsive genes (Sox9, c-Fos, c-Jun, and TGF-beta type I and II receptors) and induction of their corresponding proteins in agarose culture of rabbit BM-MSCs subjected to cyclic compressive loading. The rabbit BM-MSCs were obtained from the tibias and femurs of New Zealand White rabbits. Cell-agarose constructs were made by suspending BM-MSCs in 2% agarose gel (10(7) cells/ml) for cyclic, unconfined compression tests performed in a custom-made bioreactor. In the loading experiment, specimens were subjected to sinusoidal loading with a magnitude of 15% strain at a frequency of 1 hertz for 4 hours per day. Experiments were conducted for 2 consecutive days. This study showed that cyclic compressive loading promoted gene expressions of Sox9, c-Jun, and both TGF-beta receptors and productions of their corresponding proteins, whereas those gene expressions exhibited different temporal expression patterns among genes and between 2 days of testing. The gene expression of c-Fos was detected only in the samples subjected to1-hour dynamic compressive loading. These findings suggest that the TGF-beta signal transduction and activities of AP-1 and Sox9 are involved in the early stage of BM-MSC chondrogenesis promoted by dynamic compressive loading.

Milieu-Adopted in vitro and in vivo Differentiation of Mesenchymal Tissues Derived from Different Adult Human CD34-Negative Progenitor Cell Clones

Adult mesenchymal stem cells with multilineage differentiation potentially exist in the bone marrow, but have also been isolated from the peripheral blood. The differentiation of stem cells after leaving their niches depends predominately on the local milieu and its new microenvironment, and is facilitated by soluble factors but also by the close cell-cell interaction in a three-dimensional tissue or organ system. We have isolated CD34-negative, mesenchymal stem cell lines from human bone marrow and peripheral blood and generated monoclonal cell populations after immortalization with the SV40 large T-antigen. The cultivation of those adult stem cell clones in an especially designed in vitro environment, including self-constructed glass capillaries with defined growth conditions, leads to the spontaneous establishment of pleomorphic three-dimensional cell aggregates (spheroids) from the monoclonal cell population, which consist of cells with an osteoblast phenotype and areas of mineralization along with well-vascularized tissue areas. Modifications of the culture conditions favored areas of bone-like calcifications. After the transplantation of the at least partly mineralized human spheroids into different murine soft tissue sites but also a dorsal skinfold chamber, no further bone formation could be observed, but angiogenesis and neovessel formation prevailed instead, enabling the transplanted cells and cell aggregates to survive. This study provides evidence that even monoclonal adult human CD34-negative stem cells from the bone marrow as well as peripheral blood can potentially differentiate into different mesenchymal tissues depending on the local milieu and responding to the needs within the microenvironment.

FGF-2 enhances the mitotic and chondrogenic potentials of human adult bone marrow-derived mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) expanded with and without fibroblast growth factor (FGF) supplementation were compared with respect to their proliferation rate, ability to differentiate along the chondrogenic pathway in vitro, and their gene expression profiles. hMSCs expanded in FGF-supplemented medium were smaller and proliferated more rapidly than hMSCs expanded in control conditions. Chondrogenic cultures made with FGF-treated cells were larger and contain more proteoglycan than those made with control cells. Furthermore, aggregates of FGF-treated cells lacked the collagen type I-positive and collagen type II-negative outer layer characteristic of aggregates of control cells. A total of 358 unique transcripts were differentially expressed in FGF-treated hMSCs. Of these, 150 were upregulated and 208 downregulated. Seventeen percent of these genes affect proliferation. Known genes associated with cellular signaling functions comprised the largest percentage ( approximately 20%) of differentially expressed transcripts. Eighty percent of differentially expressed extracellular matrix-related genes were downregulated. The present findings that FGF-2 enhances proliferation and differentiation of hMSCs adds to a growing body of evidence that cytokines modulate the differentiation potential and, perhaps, the multipotentiality of adult stem cells. With the generation of gene expression profiles of FGF-treated and control cells we have taken the first steps in the elucidation of the molecular mechanism(s) behind these phenomena.

Directing stem cell differentiation into the chondrogenic lineage in vitro

A major area in regenerative medicine is the application of stem cells in cartilage tissue engineering and reconstructive surgery. This requires well-defined and efficient protocols for directing the differentiation of stem cells into the chondrogenic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying chondrogenesis and cartilaginous tissue biology. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for cartilage-related biomaterials and drugs could also utilize protocols developed for the chondrogenic differentiation of stem cells. Hence, this review critically examines the various strategies that could be used to direct the differentiation of stem cells into the chondrogenic lineage in vitro.

Hyaluronic acid and autologous synovial fluid induce chondrogenic differentiation of equine mesenchymal stem cells: a preliminary study

Mesenchymal stem cells (MSC) have the potential to differentiate into distinct mesenchymal tissues including cartilage, which suggest these cells as an attractive cell source for cartilage tissue engineering approaches. Our objective was to study the effects of TGF-beta1, hyaluronic acid and synovial fluid on chondrogenic differentiation of equine MSC. For that, bone marrow was aspirated from the tibia of one 18-month-old horse (Haflinger) and MSC were isolated using percoll-density centrifugation. To promote chondrogenesis, MSC were centrifuged to form a micromass and were cultured in a medium containing 10 ng/ml TGF-beta1 or 0.1mg/ml hyaluronic acid (Hylartil, Ostenil) or either 5%, 10% or 50% autologous synovial fluid as the chondrogenesis inducing factor. Differentiation along the chondrogenic lineage was documented by type II collagen and proteoglycan expression. MSC induced by TGF-beta1 alone showed the highest proteoglycan expression. Combining TGF-beta1 with hyaluronic acid could not increase the proteoglycan expression. Cultures stimulated by autologous synovial fluid (independent of concentration) and hyaluronic acid demonstrated a pronounced, but lower proteoglycan expression than cultures stimulated by TGF-beta1. The expression of cartilage-specific type II collagen was high and about the same in all stimulated cultures. In summary, hyaluronic acid and autologous synovial fluid induces chondrogenesis of equine mesenchymal stem cells, which encourage tissue engineering applications of MSC in chondral defects, as the natural environment in the joint is favorable for chondrogenic differentiation.

Mesenchymal stem cells from the outer ear: a novel adult stem cell model system for the study of adipogenesis

Adipocytes arise from multipotent stem cells of mesodermal origin, which also give rise to the muscle, bone, and cartilage lineages. However, signals and early molecular events that commit multipotent stem cells into the adipocyte lineage are not well established mainly due to lack of an adequate model system. We have identified a novel source of adult stem cells from the external murine ears referred to here as an ear mesenchymal stem cells (EMSC). EMSC have been isolated from several standard and mutant strains of mice. They are self-renewing, clonogenic, and multipotent, since they give rise to osteocytes, chondrocytes, and adipocytes. The in vitro characterization of EMSC indicates very facile adipogenic differentiation. Morphological, histochemical, and molecular analysis after the induction of differentiation showed that EMSC maintain adipogenic potentials up to fifth passage. A comparison of EMSC to the stromal-vascular (S-V) fraction of fat depots, under identical culture conditions (isobutyl-methylxanthine, dexamethasone, and insulin), revealed much more robust and consistent adipogenesis in EMSC than in the S-V fraction. In summary, we show that EMSC can provide a novel, easily obtainable, primary culture model for the study of adipogenesis.

Non-hematopoietic bone marrow stem cells: molecular control of expansion and differentiation

The first non-hematopoietic mesenchymal stem cells (MSCs) were discovered by Friedenstein in 1976, who described clonal, plastic adherent cells from bone marrow capable of differentiating into osteoblasts, adipocytes, and chondrocytes. More recently, investigators have now demonstrated that multi-potent MSCs can be recovered from a variety of other adult tissues and differentiate into numerous tissue lineages including myoblasts, hepatocytes and possibly even neural tissue. Because MSCs are multipotent and easily expanded in culture, there has been much interest in their clinical potential for tissue repair and gene therapy and as a result, numerous studies have been carried out demonstrating the migration and multi-organ engraftment potential of MSCs in animal models and in human clinical trials. This review describes the recent advances in the understanding of MSC biology.

Characterization of nonexpanded mesenchymal progenitor cells from normal adult human bone marrow

OBJECTIVE

Adult bone marrow (BM) mesenchymal stem/progenitor cells (MS/PC) are a potentially useful tool for cell therapy and tissue repair. However, the identification of cell subsets rich in MS/PC from fresh BM has not been described. We have developed a means of identifying such subsets from untouched bone marrow.

MATERIAL AND METHODS

First, MS/PC were enriched by short-time adherence (D(1-3)) before any cell division to evaluate the efficiency of CD73, CD105, CDw90, and CD49a antigens to select highly purified CD45(-)CD14(-) fluorescence-activated sorted subsets enriched in clonogenic mesenchymal cells. Then, we adapted this method to unmanipulated BM mononuclear cells (MNC).

RESULTS

Short-time (D(1-3)) adherent CD45(-)CD14(-) cells expressing CD73 or CD49a antigens contained all the CFU-F, even though the CD105(+) and CDw90(+) subsets comprised less than half the total. In fresh unmanipulated BM MNC, CD73 and CD49a were also highly discriminative and allowed up to a 3 log enrichment of CFU-F when compared to BM MNC. Normal culture conditions upregulated most of the tested antigens.

CONCLUSION

The CD45(-)CD14(-)/CD73(+) and CD45(-)CD14(-)/CD49a(+) phenotypes identified subsets containing all the CFU-F and sufficiently enriched to detect them in fresh BM, enabling evaluation of mesenchymal content of BM collections for cell therapy.

Expression profiling and functional analysis of wnt signaling mechanisms in mesenchymal stem cells

Through their broad differentiation potential, mesenchymal stem cells (MSCs) are candidates for a range of therapeutic applications, but the precise signaling pathways that determine their differentiated fate are not fully understood. Evidence is emerging that developmental signaling cues may be important in regulating stem cell self-renewal and differentiation programs. Here we have identified a consistent expression profile of Wnt signaling molecules in MSCs and provide evidence that an endogenous canonical Wnt pathway functions in these cells. Wnts bind to Frizzled (Fz) receptors and subsequent canonical signaling inhibits glycogen synthase kinase-3beta (GSK-3beta), causing beta-catenin translocation into the nucleus to induce target gene expression. In human MSCs isolated from bone marrow of different donors, we appear to have identified a common Wnt/Fz expression profile using reverse transcriptase polymerase chain reaction (RT-PCR). Associated Wnt signaling components, including low-density lipoprotein receptor-related protein-5 (LRP-5), kremen-1, dickkopf-1 (Dkk-1), secreted Frizzled-related peptide (sFRP)-2, sFRP3, sFRP4, Disheveled (Dvl), GSK-3beta, adenomatous polyposis coli (APC), beta-catenin,T-cell factor (TCF)-1, and TCF-4, were also identified. Nuclear beta-catenin was observed in 30%-40% of MSCs, indicative of endogenous Wnt signaling. Exposure to both Wnt3a and Li+ ions, which promotes canonical Wnt signaling by inhibiting GSK-3beta, reduced phosphorylation of beta-catenin in MSCs and increased beta-catenin nuclear translocation approximately threefold over that of the controls. Our findings indicate that autocrine Wnt signaling operates in primitive MSC populations and supports previous evidence that Wnt signaling regulates mesenchymal lineage specification. The identification of a putative common Wnt/Fz molecular signature in MSCs will contribute to our understanding of the molecular mechanisms that regulate self-renewal and lineage-specific differentiation.

Human mesenchymal progenitor cell-based tissue engineering of a single-unit osteochondral construct

A desirable strategy for articular cartilage repair is to surgically replace the damaged area with an in vitro-engineered osteochondral plug. We report here the development of a novel osteochondral construct using human trabecular bone-derived mesenchymal progenitor cells and a biodegradable poly-D,L-lactic acid scaffold. The cartilage layer was fabricated by press-coating a chondrifying high-density cell pellet onto the scaffold, which was then loaded with cells previously initiated to undergo osteogenesis. The composite was then cultured in a cocktail medium formulated to maintain both chondrogenesis and osteogenesis. Macroscopically, the construct consisted of a cartilage-like layer adherent to, and overlying, a dense bone-like component. RT-PCR, immunohistochemistry, and histology revealed hyaline-like cartilage and bone with an interface resembling the native osteochondral junction. All parameters, including mechanical properties, improved with increased culture time. The single-cell source nature of the construct, which minimizes handling while maximizing biocompatibility, suggests applicability for articular cartilage repair.

Regeneration in the ears of immunodeficient mice: identification and lineage analysis of mesenchymal stem cells

Wound healing in the ears of Athymic Nude-nu mice resembles regeneration. Histological analysis of the ear-punched tissues revealed the initial formation of a blastema-like structure followed by dermal, vascular, cartilage, and muscle regrowth exclusively in Athymic Nude-nu mice but not in wild-type controls (C57BL/6J). A subset of stem cells referred to here as ear mesenchymal stem cells (EMSCs) has been isolated from the external ears of regenerative (Athymic Nude-nu) and nonregenerative strains of mice. Morphological, histochemical, and molecular analysis after the induction of EMSC differentiation revealed multiple mesenchymal cell lineages (adipocytes, chondrocytes, and osteocytes) in all murine strains independent of their ability for regeneration. Thus, the absence of regeneration in wounded ears of C57BL/6J wild-type mice is not related to the absence of mesenchymal stem cell differentiation in tissue culture. Because nude mice lack T lymphocytes, it appears that in this model the absence of T lymphocytes in the wounded ears provides a microenvironment conducive to regeneration of mesenchymal tissues. These findings provide a new model to study the influence of the immune system on tissue regeneration.

Critical parameters for the isolation of mesenchymal stem cells from umbilical cord blood

Evidence has emerged that mesenchymal stem cells (MSCs) represent a promising population for supporting new clinical concepts in cellular therapy. However, attempts to isolate MSCs from umbilical cord blood (UCB) of full-term deliveries have previously either failed or been characterized by a low yield. We investigated whether cells with MSC characteristics and multi-lineage differentiation potential can be cultivated from UCB of healthy newborns and whether yields might be maximized by optimal culture conditions or by defining UCB quality criteria. Using optimized isolation and culture conditions, in up to 63% of 59 low-volume UCB units, cells showing a characteristic mesenchymal morphology and immune phenotype (MSC-like cells) were isolated. These were similar to control MSCs from adult bone marrow (BM). The frequency of MSC-like cells ranged from 0 to 2.3 clones per 1 x 10(8) mononuclear cells (MNCs). The cell clones proliferated extensively with at least 20 population doublings within eight passages. In addition, osteogenic and chondrogenic differentiation demonstrated a multi-lineage capacity comparable with BM MSCs. However, in contrast to MSCs, MSC-like cells showed a reduced sensitivity to undergo adipogenic differentiation. Crucial points to isolate MSC-like cells from UCB were a time from collection to isolation of less than 15 hours, a net volume of more than 33 ml, and an MNC count of more than 1 x 10(8) MNCs. Because MSC-like cells can be isolated at high efficacy from full-term UCB donations, we regard UCB as an additional stem cell source for experimental and potentially clinical purposes.

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

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

A three-dimensional nanofibrous scaffold for cartilage tissue engineering using human mesenchymal stem cells

The utilization of adult stem cells in tissue engineering is a promising solution to the problem of tissue or organ shortage. Adult bone marrow derived mesenchymal stem cells (MSCs) are undifferentiated, multipotential cells which are capable of giving rise to chondrocytes when maintained in a three-dimensional culture and treated with members of the transforming growth factor-beta (TGF-beta) family of growth factors. In this study, we fabricated a nanofibrous scaffold (NFS) made of a synthetic biodegradable polymer, poly(-caprolactone) (PCL), and examined its ability to support in vitro chondrogenesis of MSCs. The electrospun PCL porous scaffold was constructed of uniform, randomly oriented nanofibers with a diameter of 700 nm, and structural integrity of this scaffold was maintained over a 21-day culture period. MSCs cultured in NFSs in the presence of TGF-beta1 differentiated to a chondrocytic phenotype, as evidenced by chondrocyte-specific gene expression and synthesis of cartilage-associated extracellular matrix (ECM) proteins. The level of chondrogenesis observed in MSCs seeded within NFSs was comparable to that observed for MSCs maintained as cell aggregates or pellets, a widely used culture protocol for studying chondrogenesis of MSCs in vitro. Due to the physical nature and improved mechanical properties of NFSs, particularly in comparison to cell pellets, the findings reported here suggest that the PCL NFS is a practical carrier for MSC transplantation, and represents a candidate scaffold for cell-based tissue engineering approaches to cartilage repair.

In vivo chondrogenesis of adult bone-marrow-derived autologous mesenchymal stem cells

The purpose of this study has been to investigate the possible effects of the normal joint cavity environment on chondrocytic differentiation of bone-marrow-derived mesenchymal stem cells (MSCs). Autologous bone marrow was aspirated from the iliac crest of male sheep. MSCs were purified, expanded, and labeled with the fluorescent dye PKH26. Labeled MSCs were then grown on a three-dimensional porous scaffold of poly (L-lactic-co-glycolic acid) in vitro and implanted into the joint cavity by a surgical procedure. At 4 or 8 weeks after implantation, the implants were removed for histochemical and immunohistochemical analysis. The cells labeled with red fluorescent PKH26 in the implants expressed type II collagen and synthesized sulfated proteoglycans. However, the osteoblast-specific marker, osteocalcin, was not detected by immunohistochemistry indicating that the implanted MSCs had not differentiated into osteoblasts by being directly exposed to the normal joint cavity. To investigate the possible factors involved in chondrocytic differentiation of MSCs further, we co-cultured sheep MSCs with the main components of the normal joint cavity, viz., synovial fluid or synovial cells, in vitro. After 1 or 2 weeks of co-culture, the MSCs in both co-culture systems expressed markers of chondrogenesis. These results suggest that synovial fluid and synovium from normal joint cavity are important for the chondrocytic differentiation of adult bone-marrow-derived MSCs.

Quantitative Study of Tissue-Engineered Cartilage With Human Bone Marrow Mesenchymal Stem Cells

OBJECTIVES

To assess the possibility of cartilage tissue engineering using human mesenchymal stem cells (hMSCs) and to investigate the quantitative relationship between hMSCs and engineered cartilage.

DESIGN

Human mesenchymal stem cells were cultured, cryopreserved, and expanded in vitro. Surface antigens were detected by flow cytometry. In vitro chondrogenesis of hMSCs and cryopreserved hMSCs was performed. The chondrogenesis-induced hMSCs were seeded onto polyglycolic acid scaffolds, cultured in vitro for 3 weeks in chondrogenic medium, and then implanted into nude mice. The implants were harvested after 10 weeks and examined with histologic and immunochemical staining.

RESULTS

The construction of cartilages was identified grossly and histologically: 1.9 to 2.5 x 10(7) nucleated cells were obtained from 1 mL of bone marrow, and about 1 to 2 x 10(6) hMSCs were obtained from the primary culture. The number of hMSCs tripled at every passage and reached 1.4 to 2.8 x 10(12) at passage 15. The purity of hMSCs was 95% and 98% at the primary and the fourth passages, respectively. Twenty-one days was the optimal (induction rate, 95%) induction time, with no apparent differences in induction rates among different passages. Based on our findings, hMSCs from 0.07 to 0.14 mL of bone marrow, expanded during 4 passages and induced for 21 days, would be sufficient to engineer 1 cm(2) of cartilage, 3-mm thick.

CONCLUSION

Quantitative standards of hMSCs as seed cells for cartilage tissue engineering were established and may have value for later clinical work.

Maintenance of differentiation potential of human bone marrow mesenchymal stem cells immortalized by human telomerase reverse transcriptase gene despite of extensive proliferation

Human bone marrow mesenchymal stem cells (hMSC) represent a population of stem cells that are capable of differentiation into multiple lineages. However, these cells exhibit senescence-associated growth arrest and phenotypic changes during long-term in vitro culture. We have recently demonstrated that overexpression of human telomerase reverse transcriptase (hTERT) in hMSC reconstitutes telomerase activity and extends life span of the cells [Nat. Biotechnol. 20 (2002) 592]. In the present study, we have performed extensive characterization of three independent cell lines derived from the parental hMSC-TERT cell line based on different plating densities during expansion in culture: 1:2 (hMSC-TERT2), 1:4 (hMSC-TERT4), and 1:20 (hMSC-TERT20). The 3 cell lines exhibited differences in morphology and growth rates but they all maintained the characteristics of self-renewing stem cells and the ability to differentiate into multiple mesoderm-type cell lineages: osteoblasts, adipocytes, chondrocytes, and endothelial-like cells over a 3-year period in culture. Also, surface marker studies using flow cytometry showed a pattern similar to that known from normal hMSC. Thus, telomerization of hMSC by hTERT overexpression maintains the stem cell phenotype of hMSC and it may be a useful tool for obtaining enough number of cells with a stable phenotype for mechanistic studies of cell differentiation and for tissue engineering protocols.

Feridex labeling of mesenchymal stem cells inhibits chondrogenesis but not adipogenesis or osteogenesis

Magnetic resonance (MR) tracking of superparamagnetic iron oxide (SPIO)-labeled cells is a relatively new technique to non-invasively determine the biodistribution and migration of transplanted stem cells. A number of studies have recently reported encouraging results in the use of bone marrow-derived mesenchymal stem cells (MSCs) for repair of a variety of tissues. For MR tracking of SPIO-labeled MSCs, it is important to determine the effect that the magnetic labeling procedure may have on the differentiation capacity of labeled MSCs. Human MSCs were labeled with poly-L-lysine (PLL)-coated Feridex, with Feridex being an FDA-approved SPIO formulation in an off-label application, and assayed for cellular differentiation using five different assays. As compared with unlabeled controls, labeled MSCs exhibited an unaltered viability, proliferated similarly, and underwent normal adipogenic and osteogenic differentiation. However, there was a marked inhibition of chondrogenesis. The blocking of chondrogenic activity was mediated by the Feridex, rather than by the transfection agent (PLL). This is the first report showing Feridex blocking of cellular differentiation down a specific pathway (while not affecting viability and proliferation), and caution should thus be exercised when using Feridex-labeled MSCs for chondrogenic MR tracking studies. On the other hand, no detrimental effects of Feridex-labeling are anticipated for MR-guided osteogenic or adipogenic transplantation studies.

Engineering cartilage-like tissue using human mesenchymal stem cells and silk protein scaffolds

Human mesenchymal stem cells (hMSC) derived from bone marrow aspirates can form the basis for the in vitro cultivation of autologous tissue grafts and help alleviate the problems of immunorejection and disease transmission associated with the use of allografts. We explored the utility of hMSC cultured on protein scaffolds for tissue engineering of cartilage. hMSC were isolated, expanded in culture, characterized with respect to the expression of surface markers and ability for chondrogenic and osteogenic differentiation, and seeded on scaffolds. Four different scaffolds were tested, formed as a highly porous sponge made of: 1) collagen, 2) cross-linked collagen, 3) silk, and 4) RGD-coupled silk. Cell-seeded scaffolds were cultured for up to 4 weeks in either control medium (DMEM supplemented with 10% fetal bovine serum) or chondrogenic medium (control medium supplemented with chondrogenic factors). hMSC attachment, proliferation, and metabolic activity were markedly better on slowly degrading silk than on fast-degrading collagen scaffolds. In chondrogenic medium, hMSC formed cartilaginous tissues on all scaffolds, but the extent of chondrogenesis was substantially higher for hMSC cultured on silk as compared to collagen scaffolds. The deposition of glycosaminoglycan (GAG) and type II collagen and the expression of type II collagen mRNA were all higher for hMSC cultured on silk than on collagen scaffolds. Taken together, these results suggest that silk scaffolds are particularly suitable for tissue engineering of cartilage starting from hMSC, presumably due to their high porosity, slow biodegradation, and structural integrity.

Reevaluation of in vitro differentiation protocols for bone marrow stromal cells: disruption of actin cytoskeleton induces rapid morphological changes and mimics neuronal phenotype

Bone marrow stromal cells (MSC), which represent a population of multipotential mesenchymal stem cells, have been reported to undergo rapid and robust transformation into neuron-like phenotypes in vitro following treatment with chemical induction medium including dimethyl sulfoxide (DMSO; Woodbury et al. [2002] J. Neurosci. Res. 96:908). In this study, we confirmed the ability of cultured rat MSC to undergo in vitro osteogenesis, chondrogenesis, and adipogenesis, demonstrating differentiation of these cells to three mesenchymal cell fates. We then evaluated the potential for in vitro neuronal differentiation of these MSC, finding that changes in morphology upon addition of the chemical induction medium were caused by rapid disruption of the actin cytoskeleton. Retraction of the cytoplasm left behind long processes, which, although strikingly resembling neurites, showed essentially no motility and no further elaboration during time-lapse studies. Similar neurite-like processes were induced by treating MSC with DMSO only or with actin filament-depolymerizing agents. Although process formation was accompanied by rapid expression of some neuronal and glial markers, the absence of other essential neuronal proteins pointed toward aberrantly induced gene expression rather than toward a sequence of gene expression as is required for neurogenesis. Moreover, rat dermal fibroblasts responded to neuronal induction by forming similar processes and expressing similar markers. These studies do not rule out the possibility that MSC can differentiate into neurons; however, we do want to caution that in vitro differentiation protocols may have unexpected, misleading effects. A dissection of molecular signaling and commitment events may be necessary to verify the ability of MSC transdifferentiation to neuronal lineages.

Chondrogenic and adipogenic potential of microvascular pericytes

BACKGROUND

Previous studies have shown that pericytes can differentiate into osteoblasts and form bone. This study investigated whether pericytes can also differentiate into chondrocytes and adipocytes.

METHODS AND RESULTS

Reverse transcription-polymerase chain reaction demonstrated that pericytes express mRNA for the chondrocyte markers Sox9, aggrecan, and type II collagen. Furthermore, when cultured at high density in the presence of a defined chondrogenic medium, pericytes formed well-defined pellets comprising cells embedded in an extracellular matrix rich in sulfated proteoglycans and type II collagen. In contrast, when endothelial cells were cultured under the same conditions, the pellets disintegrated after 48 hours. In the presence of adipogenic medium, pericytes but not endothelial cells expressed mRNA for peroxisome proliferator-activated receptor-gamma2 (an adipocyte-specific transcription factor) and incorporated lipid droplets that stained with oil red O. To confirm that pericytes can differentiate along the chondrocytic and adipocytic lineages in vivo, these cells were inoculated into diffusion chambers and implanted into athymic mice for 56 days. Accordingly, mineralized cartilage, fibrocartilage, and a nonmineralized cartilaginous matrix with lacunae containing chondrocytes were observed within these chambers. Small clusters of cells that morphologically resembled adipocytes were also identified.

CONCLUSIONS

These data demonstrate that pericytes are multipotent cells that may contribute to growth, wound healing, repair, and/or the development and progression of various pathological states.

Primary mouse embryonic fibroblasts: a model of mesenchymal cartilage formation

Cartilage formation is an intricate process that requires temporal and spatial organization of regulatory factors in order for a mesenchymal progenitor cell to differentiate through the distinct stages of chondrogenesis. Gene function during this process has best been studied by analysis of in vivo cartilage formation in genetically altered mouse models. Mouse embryonic fibroblasts (MEFs) isolated from such mouse models have been widely used for the study of growth control and DNA damage response. Here, we address the potential of MEFs to undergo chondrogenic differentiation. We demonstrate for the first time that MEFs can enter and complete the program of chondrogenic differentiation ex vivo, from undifferentiated progenitor cells to mature, hypertrophic chondrocytes. We show that chondrogenic differentiation can be induced by cell-cell contact or BMP-2 treatment, while in combination, these conditions synergistically enhance chondrocyte differentiation resulting in the formation of 3-dimensional (3-D) cartilaginous tissue ex vivo. Temporal expression profiles of pro-chondrogenic transcription factors Bapx1 and Sox9 and cartilaginous extracellular matrix (ECM) proteins Collagen Type II and X (Coll II and Coll X) demonstrate that the in vivo progression of chondrocyte maturation is recapitulated in the MEF model system. Our findings establish the MEF as a powerful tool for the generation of cartilaginous tissue ex vivo and for the study of gene function during chondrogenesis.

Chondrogenic differentiation of human mesenchymal stem cells cultured in a cobweb-like biodegradable scaffold

Human mesenchymal stem cells (MSCs) were cultured in vitro in a cobweb-like biodegradable polymer scaffold: a poly(dl-lactic-co-glycolic acid)-collagen hybrid mesh in serum-free DMEM containing TGF-beta3 for 1-10 weeks. The cells adhered to the hybrid mesh, distributed evenly, and proliferated to fill the spaces in the scaffold. The ability of the cells to express gene encoding type I collagen decreased, whereas its ability to express type II collagen and aggrecan increased. Histological examination by HE staining indicated that the cells showed fibroblast morphology at the early stage and became round after culture for 4 weeks. The cartilaginous matrices were positively stained by safranin O and toluidine blue. Immunostaining with anti-type II collagen and anti-cartilage proteoglycan showed that type II collagen and cartilage proteoglycan were detected around the cells. In addition, a homogeneous distribution of cartilaginous extracellular matrices was detected around the cells. These results suggest the chondrogenic differentiation of the mesenchymal stem cells in the hybrid mesh. The PLGA-collagen hybrid mesh enabled the aggregation of mesenchymal stem cells and provided a promotive microenvironment for the chondrogenic differentiation of the MSCs.

Coculture of Mesenchymal Stem Cells and Respiratory Epithelial Cells to Engineer a Human Composite Respiratory Mucosa

In this study, we describe a novel in vitro reconstitution system for tracheal epithelium that could be useful for investigating the cellular and molecular interaction of epithelial and mesenchymal cells. In this system, a Transwell insert was used as a basement membrane on which adult bone marrow mesenchymal stem cells (MSCs) were cultured on the lower side whereas normal human bronchial epithelial (NHBE) cells were cultured on the opposite upper side. Under air-liquid interface conditions, the epithelial cells maintained their capacity to progressively differentiate and form a functional epithelium, leading to the differentiation of mucin-producing cells between days 14 and 21. Analysis of apical secretions showed that mucin production increased over time, with peak secretion on day 21 for NHBE cells alone, whereas mucin secretion by NHBE cells cocultured with MSCs remained constant between days 18 and day 25. This in vitro model of respiratory epithelium, which exhibited morphologic, histologic, and functional features of a tracheal mucosa, could help to understand interactions between mesenchymal and epithelial cells and mechanisms involved in mucus production, inflammation, and airway repair. It might also play an important role in the design of an composite prosthesis for tracheal replacement.

BMP2 initiates chondrogenic lineage development of adult human mesenchymal stem cells in high-density culture

Human bone marrow-derived mesenchymal stem cells (MSCs) have been shown to differentiate into distinct mesenchymal tissues including bone and cartilage. The capacity of MSCs to replicate undifferentiated and to mature into cartilaginous tissues suggests these cells as an attractive cell source for cartilage tissue engineering. Here we show that the stimulation of human bone marrow-derived MSCs with recombinant bone morphogenetic protein-2 (BMP2) results in chondrogenic lineage development under serum-free conditions. Histological staining of proteoglycan with Alcian blue and immunohistochemical staining of cartilage-specific type II collagen revealed the deposition of typical cartilage extracellular matrix components. Semi-quantitative real-time gene expression analysis of characteristic chondrocytic matrix genes, such as cartilage link protein, cartilage oligomeric matrix protein, aggrecan, and types I, II, and IX collagen, confirmed the induction of the chondrocytic phenotype in high-density culture upon stimulation with BMP2 and transforming growth factor-beta3 (TGFbeta3). Histologic staining of mineralized extracellular matrix with von Kossa, immunostaining of type X collagen (typical for hypertrophic chondrocytes), and gene expression analysis of osteocalcin and adipocyte-specific fatty acid binding protein (aP2) further documented that BMP2 induced chondrogenic lineage development and not osteogenesis and/or adipogenesis in human MSCs. These results suggest BMP2 as a promising candidate for tissue engineering approaches regenerating articular cartilage on the basis of mesenchymal progenitors from bone marrow.

What is regenerative medicine? Emergence of applied stem cell and developmental biology

Regenerative medicine is an emerging, but still poorly defined, field of biomedicine. The ongoing 'regenerative medicine revolution' is based on a series of new exciting breakthrough discoveries in the field of stem cell biology and developmental biology. The main problem of regenerative medicine is not so much stem cell differentiation, isolation and lineage diversity, although these are very important issues, but rather stem cell mobilisation, recruitment and integration into functional tissues. The key issue in enhancing tissue and organ regeneration is how to mobilise circulating stem and progenitor cells and how to provide an appropriate environment ('niche') for their tissue and organo-specific recruitment, 'homing' and complete functional integration. We need to know more about basic tissue biology, tissue regeneration and the cellular and molecular mechanisms of tissue turnover (both cellular and extracellular components) at different periods of human life and in different diseases. Systematic in silico, in vitro and in vivo research is a foundation for further progress in regenerative medicine. Regenerative medicine is a rapidly advancing field that opens new and exciting opportunities for completely revolutionary therapeutic modalities and technologies. Regenerative medicine is, at its essence, an emergence of applied stem cell and developmental biology.

Proteomic profiling of bone marrow mesenchymal stem cells upon transforming growth factor beta1 stimulation

Bone marrow mesenchymal stem cells (MSCs) can differentiate into different types of cells and have tremendous potential for cell therapy and tissue engineering. Transforming growth factor beta1 (TGF-beta) plays an important role in cell differentiation and vascular remodeling. We showed that TGF-beta induced cell morphology change and an increase in actin fibers in MSCs. To determine the global effects of TGF-beta on MSCs, we employed a proteomic strategy to analyze the effect of TGF-beta on the human MSC proteome. By using two-dimensional gel electrophoresis and electrospray ionization coupled to quadrupole/time-of-flight tandem mass spectrometers, we have generated a proteome reference map of MSCs, and we identified approximately 30 proteins with an increase or decrease in expression or phosphorylation in response to TGF-beta. The proteins regulated by TGF-beta included cytoskeletal proteins, matrix synthesis proteins, membrane proteins, metabolic enzymes, etc. TGF-beta increased the expression of smooth muscle alpha-actin and decreased the expression of gelsolin. Overexpression of gelsolin inhibited TGF-beta-induced assembly of smooth muscle alpha-actin; on the other hand, knocking down gelsolin expression enhanced the assembly of alpha-actin and actin filaments without significantly affecting alpha-actin expression. These results suggest that TGF-beta coordinates the increase of alpha-actin and the decrease of gelsolin to promote MSC differentiation. This study demonstrates that proteomic tools are valuable in studying stem cell differentiation and elucidating the underlying molecular mechanisms.

Effects of transforming growth factor beta1 and dexamethasone on the growth and chondrogenic differentiation of adipose-derived stromal cells

The effects of soluble mediators and medium supplements commonly used to induce chondrogenic differentiation in different cell culture systems were investigated to define their dose-response profiles and potentially synergistic effects on the chondrogenic differentiation of adipose-derived adult stromal (ADAS) cells. Human ADAS cells were suspended within alginate beads and cultured in basal medium with insulin, transferrin, and selenious acid (ITS+) or fetal bovine serum (FBS) and treated with different doses and combinations of TGF-beta1 (0, 1, and 10 ng/mL) and dexamethasone (0, 10, and 100 nM). Cell growth and chondrogenic differentiation were assessed by measuring DNA content, protein and proteoglycan synthesis rates, and proteoglycan accumulation. The combination of ITS+ and TGF-beta1 significantly increased cell proliferation. Protein synthesis rates were increased by TGF-beta1 and dexamethasone in the presence of ITS+ or FBS. While TGF-beta1 significantly increased proteoglycan synthesis and accumulation by 1.5- to 2-fold in the presence of FBS, such effects were suppressed by dexamethasone. In summary, the combination of TGF-beta1 and ITS+ stimulated cell growth and synthesis of proteins and proteoglycans by human ADAS cells. The addition of dexamethasone appeared to amplify protein synthesis but had suppressive effects on proteoglycan synthesis and accumulation.

Mesenchymal stem cells: paradoxes of passaging

The field of stem cell biology continues to evolve with the ongoing characterization of multiple types of stem cells with their inherent potential for experimental and clinical application. Mesenchymal stem cells (MSC) are one of the most promising stem cell types due to their availability and the relatively simple requirements for in vitro expansion and genetic manipulation. Multiple populations described as "MSCs" have now been isolated from various tissues in humans and other species using a variety of culture techniques. Despite extensive in vitro characterization, relatively little has been demonstrated regarding their in vivo biology and therapeutic potential. Nevertheless, clinical trials utilizing MSCs are currently underway. The aim of this review is to critically analyze the field of MSC biology, particularly with respect to the current paradox between in vitro promise and in vivo efficacy. It is the authors' opinion that until this paradox is better understood, therapeutic applications will remain limited.

Identification of mesenchymal progenitor cells in normal and osteoarthritic human articular cartilage

OBJECTIVE

To determine the presence of mesenchymal progenitor cells (MPCs) in human articular cartilage.

METHODS

Primary cell cultures established from normal and osteoarthritic (OA) human knee articular cartilage were analyzed for the expression of CD105 and CD166, cell surface markers whose coexpression defines mesenchymal stem cells (MSCs) in bone marrow and perichondrium. The potential of cartilage cells to differentiate to adipogenic, osteogenic, and chondrogenic lineages was analyzed after immunomagnetic selection for CD105+/CD166+ cells and was compared with bone marrow-derived MSCs (BM-MSCs).

RESULTS

Up to 95% of isolated cartilage cells were CD105+ and approximately 5% were CD166+. The mean +/- SEM percentage of CD105+/CD166+ cells in normal cartilage was 3.49 +/- 1.93%. Primary cell cultures from OA cartilage contained significantly increased numbers of CD105+/CD166+ cells. Confocal microscopy confirmed the coexpression of both markers in the majority of BM-MSCs and a subpopulation of cartilage cells. Differentiation to adipocytes occurred in cartilage-derived cell cultures, as indicated by characteristic cell morphology and oil red O staining of lipid vacuoles. Osteogenesis was observed in isolated CD105+/CD166+ cells as well as in primary chondrocytes cultured in the presence of osteogenic supplements. Purified cartilage-derived CD105+/CD166+ cells did not express markers of differentiated chondrocytes. However, the cells were capable of chondrocytic differentiation and formed cartilage tissue in micromass pellet cultures.

CONCLUSION

These findings indicate that multipotential MPCs are present in adult human articular cartilage and that their frequency is increased in OA cartilage. This observation has implications for understanding the intrinsic repair capacity of articular cartilage and raises the possibility that these progenitor cells might be involved in the pathogenesis of arthritis.

Mechanical signals as regulators of stem cell fate

In recent years, stem cells have shown significant promise for their potential to provide a source of undifferentiated progenitor cells for therapeutic applications in tissue or organ repair. Significant questions still remain, however, as to the genetic and epigenetic signals that regulate the fate of stem cells. It is now well accepted that the micro-environment of the stem cell can have a significant influence on its differentiation and phenotypic expression. Although emphasis has been placed in previous work on the role of soluble mediators such as growth factors and cytokines on stem cell differentiation, there is now significant evidence, both direct and indirect, that mechanical signals may also regulate stem cell fate. We review a number of in vivo and in vitro studies that have provided evidence that mechanical factors have the ability to influence the differentiation of a number of cells that have been classified as either precursor, progenitor, or stem cells. Taken together, these studies show that specific mechanical signals may promote cell differentiation into a particular phenotype, potentially having an effect on embryonic development. The use of such mechanical signals in vitro in specially designed "bioreactors" may provide important adjuncts to standard biochemical signaling pathways for promoting engineered tissue growth. A further understanding of the biomechanical and biochemical pathways involved in mechanical signal transduction by stem cells will hopefully provide new insight for the improvement of stem-cell based therapies.

Marrow-isolated adult multilineage inducible (MIAMI) cells, a unique population of postnatal young and old human cells with extensive expansion and differentiation potential

We report here the isolation of a population of non-transformed pluripotent human cells from bone marrow after a unique expansion/selection procedure. This procedure was designed to provide conditions resembling the in vivo microenvironment that is home for the most-primitive stem cells. Marrow-adherent and -nonadherent cells were co-cultured on fibronectin, at low oxygen tension, for 14 days. Colonies of small adherent cells were isolated and further expanded on fibronectin at low density, low oxygen tension with 2% fetal bovine serum. They expressed high levels of CD29, CD63, CD81, CD122, CD164, hepatocyte growth factor receptor (cMet), bone morphogenetic protein receptor 1B (BMPR1B), and neurotrophic tyrosine kinase receptor 3 (NTRK3) and were negative for CD34, CD36, CD45, CD117 (cKit) and HLADR. The embryonic stem cell markers Oct-4 and Rex-1, and telomerase were expressed in all cultures examined. Cell-doubling time was 36 to 72 hours, and cells have been expanded in culture for more than 50 population doublings. This population of cells was consistently isolated from men and women of ages ranging from 3- to 72-years old. Colonies of cells expressed numerous markers found among embryonic stem cells as well as mesodermal-, endodermal- and ectodermal-derived lineages. They have been differentiated to bone-forming osteoblasts, cartilage-forming chondrocytes, fat-forming adipocytes and neural cells and to attachment-independent spherical clusters expressing genes associated with pancreatic islets. Based on their unique characteristics and properties, we refer to them as human marrow-isolated adult multilineage inducible cells, or MIAMI cells. MIAMI cells proliferate extensively without evidence of senescence or loss of differentiation potential and thus may represent an ideal candidate for cellular therapies of inherited or degenerative diseases.

Inhibitory effect of alcohol on osteogenic differentiation in human bone marrow-derived mesenchymal stem cells

BACKGROUND

Alcohol-induced osteoporosis is characterized by a considerable suppression of osteogenesis. The objective of this investigation was to determine the effect of alcohol on gene expression, protein synthesis, and mineralization in human bone marrow-derived mesenchymal stem cells induced toward osteogenic differentiation in vitro.

METHODS

Human bone marrow-derived mesenchymal stem cells induced toward osteogenesis were cultured in the presence or absence of 50 mM alcohol. Stem cells were characterized by using SH2 antibody to the cell-surface antigen CD105/endoglin, and their proliferation in the presence of alcohol was quantified. The expression of genes for early, middle, and late markers of the osteogenic lineage was quantified by Northern analysis, and bone matrix protein synthesis was assayed. The effect of alcohol on cell-mediated matrix mineralization in terminally differentiated cultures was determined by von Kossa staining.

RESULTS

Fluorescence-activated cell sorting analysis of human mesenchymal stem cells separated with a Percoll gradient proved 99% homogeneity by using SH2 antibody to the surface antigen CD105. Dose-dependent inhibition of proliferation of these stem cells occurred at concentrations greater than 50 mM alcohol. Gene expression of osteoblast-specific factor 2/core binding factor a1 (Osf2/Cbfa1), type I collagen, alkaline phosphatase, and osteocalcin (early, middle, and late markers for osteogenesis, respectively) was analyzed with and without osteogenic induction and treatment with 50 mM alcohol. After induction, Osf2/Cbfa1 levels were unresponsive to alcohol. To determine the effect of alcohol on human mesenchymal stem cell progression along the osteogenic pathway, messenger RNA (mRNA) levels for type I collagen, alkaline phosphatase, and osteocalcin were examined after osteogenic induction. After osteogenic induction, alcohol down-regulated the gene expression of type I collagen and significantly reduced its synthesis. Alcohol did not alter mRNA expression of alkaline phosphatase, a midstage marker for osteogenesis, but significantly decreased its activity compared with osteogenic induction alone. After induction, osteocalcin remained unchanged by alcohol at both the mRNA and protein levels. Histochemistry revealed decreased alkaline phosphatase staining and fewer alkaline phosphatase-positive cells in alcohol-treated human mesenchymal stem cell cultures. von Kossa staining revealed a reduction in the number of mineralizing nodules in stem cell cultures after alcohol treatment.

CONCLUSIONS

Collectively, the data suggest that alcohol alters osteogenic differentiation in human bone marrow-derived mesenchymal stem cell cultures during lineage progression and provide further insight into alcohol-induced reduced bone formation.

Isolation of mouse marrow mesenchymal progenitors by a novel and reliable method

Bone marrow contains a population of rare progenitor cells capable of differentiating into osteoblasts, chondrocytes, adipocytes, myoblasts, and hematopoiesis-supporting stromal cells. These cells, referred to as mesenchymal progenitor cells (MPCs), can be purified and culture-expanded from animals and humans. Using bone-marrow-conditioned medium combined with basic fibroblast growth factor, we cultured a relatively homogeneous population of MPCs from murine bone marrow, which uniformly expressed stem cell antigen-1, CD29, CD44, c-kit, and CD105, while being negative for expression of CD45, CD31, and CD34. In vitro differentiation assays showed the tripotential differentiation capacities of these cells toward adipogenic, osteogenic, and chondrogenic lineages. Most importantly, immunophenotypic analyses demonstrated that MPCs did not express major histocompatibility complex class II molecules or the T-cell costimulatory molecules CD80 and CD86, consistent with further investigation showing that MPCs failed to elicit a proliferative response from allogeneic lymphocytes. Moreover, when allogeneic or third-party MPCs were added to T cells stimulated by allogeneic lymphocytes or the potent T-cell mitogen concanavalin-A, a significant reduction in T-cell proliferation was observed. In conclusion, our data demonstrate that we successfully isolated and culture-expanded a relatively homogeneous population of MPCs from adult murine bone marrow. Additionally, these primary cells could suppress T-lymphocyte proliferation induced by cellular or nonspecific mitogenic stimuli. This immunoregulatory feature of MPCs strongly implies that they may have potential applications in allograft transplantation.

Enumeration and phenotypic characterization of synovial fluid multipotential mesenchymal progenitor cells in inflammatory and degenerative arthritis

OBJECTIVE

To evaluate synovial fluid (SF) for the presence of mesenchymal progenitor cells (MPCs), to compare SF MPCs with bone marrow (BM) MPCs, and to enumerate these cells in both inflammatory arthritis and osteoarthritis (OA).

METHODS

SF from 100 patients with arthritis (53 rheumatoid arthritis [RA], 20 OA, and 27 other arthropathies) was evaluated. To establish multipotentiality, polyclonal and single cell-derived cultures of SF fibroblasts were examined by standard and quantitative differentiation assays. Their phenotype before and after expansion was determined by multiparameter flow cytometry. A colony-forming unit-fibroblast assay was used for SF MPC enumeration.

RESULTS

Regardless of the nature of the arthritis, both polyclonal and single cell-derived cultures of SF fibroblasts possessed trilineage mesenchymal differentiation potentials. The number of MPCs in a milliliter of SF was higher in OA (median 37) than in RA (median 2) (P < 0.00001). No significant differences in MPC numbers were found between early and established RA (median 3 and 2 cells/ml, respectively). Culture-expanded SF and BM MPCs had the same phenotype (negative for CD45 and positive for D7-FIB, CD13, CD105, CD55, and CD10). Rare, uncultured SF fibroblasts were CD45(low) and expressed low-affinity nerve growth factor receptor, similar to in vivo BM MPCs.

CONCLUSION

Our findings prove the presence of rare tripotential MPCs, at the single-cell level, in the SF of patients with arthritis. SF MPCs are clonogenic and multipotential fibroblasts that, despite the pathologic environment within a diseased joint, have a phenotype similar to that of uncultured BM MPCs. The higher prevalence of MPCs in OA SF suggests their likely origin from disrupted joint structures. These findings could determine the role of MPCs in the pathogenesis of inflammatory arthritis, together with their role in attempted joint regeneration in degenerative arthritis, which has yet to be established.