Hypoxia affects mesenchymal stromal cell osteogenic differentiation and angiogenic factor expression

Unsaturated fatty acids induce mesenchymal stem cells to increase secretion of angiogenic mediators

Mesenchymal stem cells (MSC) represent emerging cell-based therapies for diabetes and associated complications. Ongoing clinical trials are using exogenous MSC to treat type 1 and 2 diabetes, cardiovascular disease and non-healing wounds due to diabetes. The majority of these trials are aimed at exploiting the ability of these multipotent mesenchymal stromal cells to release soluble mediators that reduce inflammation and promote both angiogenesis and cell survival at sites of tissue damage. Growing evidence suggests that MSC secretion of soluble factors is dependent on tissue microenvironment. Despite the contribution of fatty acids to the metabolic environment of type 2 diabetes, almost nothing is known about their effects on MSC secretion of growth factors and cytokines. In this study, human bone marrow-derived MSC were exposed to linoleic acid, an omega-6 polyunsaturated fatty acid, or oleic acid, a monounsaturated fatty acid, for seven days in the presence of 5.38 mM glucose. Outcomes measured included MSC proliferation, gene expression, protein secretion and chemotaxis. Linoleic and oleic acids inhibited MSC proliferation and altered MSC expression and secretion of known mediators of angiogenesis. Both unsaturated fatty acids induced MSC to increase secretion of interleukin-6, VEGF and nitric oxide. In addition, linoleic acid but not oleic acid induced MSC to increase production of interleukin-8. Collectively these data suggest that exposure to fatty acids may have functional consequences for MSC therapy. Fatty acids may affect MSC engraftment to injured tissue and MSC secretion of cytokines and growth factors that regulate local cellular responses to injury.

The subcutaneous adipose tissue reservoir of functionally active stem cells is reduced in obese patients

It has been demonstrated that the adipose tissue, a highly functional metabolic tissue, is a reservoir of mesenchymal stem cells. The potential use of adipose-derived stem cells (ADSCs) from white adipose tissue (WAT) for organ repair and regeneration has been considered because of their obvious benefits in terms of accessibility and quantity of available sample. However, the functional capability of ADSCs from subjects with different adiposity has not been investigated. It has been our hypothesis that ADSCs from adipose tissue of patients with metabolic syndrome and high adiposity may be functionally impaired. We report that subcutaneous WAT stromal vascular fraction (SVF) from nonobese individuals had a significantly higher number of CD90+ cells than SVF from obese patients. The isolated ADSCs from WAT of obese patients had reduced differentiation potential and were less proangiogenic. Therefore, ADSCs in adipose tissue of obese patients have lower capacity for spontaneous or therapeutic repair than ADSCs from nonobese metabolically normal individuals.

Hypoxia impedes hypertrophic chondrogenesis of human multipotent stromal cells

Within the field of bone tissue engineering, the endochondral approach to forming bone substitutes represents a novel concept, where cartilage will undergo hypertrophic differentiation before its conversion into bone. For this purpose, clinically relevant multipotent stromal cells (MSCs), MSCs, can be differentiated into the chondrogenic lineage before stimulating hypertrophy. Controversy exists in literature on the oxygen tensions naturally present during this transition in, for example, the growth plate. Therefore, the present study focused on the effects of different oxygen tensions on the progression of the hypertrophic differentiation of MSCs. Bone marrow-derived MSCs of four human donors were expanded, and differentiation was induced in aggregate cultures. Normoxic (20% oxygen) and hypoxic (5%) conditions were imposed on the cultures in chondrogenic or hypertrophic differentiation media. After 4 weeks, the cultures were histologically examined and by real-time polymerase chain reaction. Morphological assessment showed the chondrogenic differentiation of cultures from all donors under normoxic chondrogenic conditions. In addition, hypertrophic differentiation was observed in cultures derived from all but one donor. The deposition of collagen type X was evidenced in both chondrogenically and hypertrophically stimulated cultures. However, mineralization was exclusively observed in hypertrophically stimulated, normoxic cultures. Overall, the progression of hypertrophy was delayed in hypoxic compared with normoxic groups. The observed delay was supported by the gene expression patterns, especially showing the up-regulation of the late hypertrophic markers osteopontin and osteocalcin under normoxic hypertrophic conditions. Concluding, normoxic conditions are more beneficial for hypertrophic differentiation of MSCs than are hypoxic conditions, as long as the MSCs possess hypertrophic potential. This finding has implications for cartilage tissue engineering as well as for endochondral bone tissue engineering, as these approaches deal with, respectively, the inhibition or enhancement of hypertrophic chondrogenesis.

Bioreactors to influence stem cell fate: augmentation of mesenchymal stem cell signaling pathways via dynamic culture systems

BACKGROUND

Mesenchymal stem cells (MSCs) are a promising cell source for bone and cartilage tissue engineering as they can be easily isolated from the body and differentiated into osteoblasts and chondrocytes. A cell based tissue engineering strategy using MSCs often involves the culture of these cells on three-dimensional scaffolds; however the size of these scaffolds and the cell population they can support can be restricted in traditional static culture. Thus dynamic culture in bioreactor systems provides a promising means to culture and differentiate MSCs in vitro.

SCOPE OF REVIEW

This review seeks to characterize key MSC differentiation signaling pathways and provides evidence as to how dynamic culture is augmenting these pathways. Following an overview of dynamic culture systems, discussion will be provided on how these systems can effectively modify and maintain important culture parameters including oxygen content and shear stress. Literature is reviewed for both a highlight of key signaling pathways and evidence for regulation of these signaling pathways via dynamic culture systems.

MAJOR CONCLUSIONS

The ability to understand how these culture systems are affecting MSC signaling pathways could lead to a shear or oxygen regime to direct stem cell differentiation. In this way the efficacy of in vitro culture and differentiation of MSCs on three-dimensional scaffolds could be greatly increased.

GENERAL SIGNIFICANCE

Bioreactor systems have the ability to control many key differentiation stimuli including mechanical stress and oxygen content. The further integration of cell signaling investigations within dynamic culture systems will lead to a quicker realization of the promise of tissue engineering and regenerative medicine. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Co-culture of mesenchymal stem cells with umbilical vein endothelial cells under hypoxic condition

By co-culturing humm mesenchymal stem cells (hMSCs) and human umbilical rein endothelial cells (HUVECs) under hypoxia and creating a microenvironment similar to that of transplanted hMSCs for the treatment of avascular ni ANFH, the effect of hMSCs on survival, apoptosis, migration and angiogenesis of human umbilical vein endothelial cells (HUVECs) under the hypoxic condition were investigated in vitro. hMSCs and HUVECs were cultured and identified in vitro. Three kinds of conditioned media, CdM-CdM(NOR), CdM-CdM(HYP) and HUVEC-CdM(HYP) were prepared. HUVECs were cultured with these conditioned media under hypoxia. The survival rate, apoptosis rate, migration and angiogenesis of HUVECs were respectively detected by CCK-8, flow cytometry, Transwell and tube formation assay. The content of SDF-1α, VEGF and IL-6 in CdM was determined by ELISA. Our results showed that hMSCs and HUVECs were cultured and identified successfully. Compared with MSC-CdM(NOR) and HUVEC-CdM(HYP) groups, the survival rate, migration and angiogenesis of HUVECs in MSC-CdM(HYP) group were significantly increased while the apoptosis rate was declined (P<0.05). Moreover, the expression of SDF-1α, VEGF and IL-6 in MSC-CdM(HYP) group was up-regulated. Under hypoxia, the apoptosis of HUVECs was inhibited while survival, migration and angiogenesis were improved by co-culture of hMSCs and HUVECs. The underlying mechanism may be that hMSCs could secrete a number of cytokines and improve niche, which might be helpful in the treatment of femoral head necrosis.

Combination of low O(2) concentration and mesenchymal stromal cells during culture of cord blood CD34(+) cells improves the maintenance and proliferative capacity of hematopoietic stem cells

The physiological approach suggests that an environment associating the mesenchymal stromal cells (MSC) and low O(2) concentration would be most favorable for the maintenance of hematopoietic stem cells (HSCs) in course of ex vivo expansion of hematopoietic grafts. To test this hypothesis, we performed a co-culture of cord blood CD34(+) cells with or without MSC in presence of cytokines for 10 days at 20%, 5%, and 1.5% O(2) and assessed the impact on total cells, CD34(+) cells, committed progenitors (colony-forming cells-CFC) and stem cells activity (pre-CFC and Scid repopulating cells-SRC). Not surprisingly, the expansion of total cells, CD34(+) cells, and CFC was higher in co-culture and at 20% O(2) compared to simple culture and low O(2) concentrations, respectively. However, co-culture at low O(2) concentrations provided CD34(+) cell and CFC amplification similar to classical culture at 20% O(2) . Interestingly, low O(2) concentrations ensured a better pre-CFC and SRC preservation/expansion in co-culture. Indeed, SRC activity in co-culture at 1.5% O(2) was higher than in freshly isolated CD34(+) cells. Interleukin-6 production by MSC at physiologically low O(2) concentrations might be one of the factors mediating this effect. Our data demonstrate that association of co-culture and low O(2) concentration not only induces sufficient expansion of committed progenitors (with respect to the classical culture), but also ensures a better maintenance/expansion of hematopoietic stem cells (HSCs), pointing to the oxygenation as a physiological regulatory factor but also as a cell engineering tool.

Low oxygen tension positively influences cardiomyocyte progenitor cell function

Previously we observed that cardiomyocyte progenitor cells (hCMPCs) isolated from the human heart differentiate spontaneously into cardiomyocytes and vascular cells when transplanted after myocardial infarction (MI) in the ischemic heart. After MI, deprivation of oxygen is the first major change in the cardiac environment. How cells handle hypoxia is highly cell type dependent. The effect of hypoxia on cardiac stem or progenitor cells remains to be elucidated. Here, we show for the first time that short- and long-term hypoxia have different effects on hCMPCs. Short-term hypoxia increased the migratory and invasive capacities of hCMPCs likely via mesenchymal transformation. Although long-term exposure to low oxygen levels did not induce differentiation of hCMPCs into mature cardiomyocytes or endothelial cells, it did increase their proliferation, stimulated the secretome of the cells which was shifted to a more anti-inflammatory profile and dampened the migration by altering matrix metalloproteinase (MMP) modulators. Interestingly, hypoxia greatly induced the expression of the extracellular matrix modulator thrombospondin-2 (TSP-2). Knockdown of TSP-2 resulted in increased proliferation, migration and MMP activity. In conclusion, short exposure to hypoxia increases migratory and invasive capacities of hCMPCs and prolonged exposure induces proliferation, an angiogenic secretion profile and dampens migration, likely controlled by TSP-2.

Human mesenchymal stem cell position within scaffolds influences cell fate during dynamic culture

Cell-based tissue engineering is limited by the size of cell-containing constructs that can be successfully cultured in vitro. This limit is largely a result of the slow diffusion of molecules such as oxygen into the interior of three-dimensional scaffolds in static culture. Bioreactor culture has been shown to overcome these limits. In this study we utilize a tubular perfusion system (TPS) bioreactor for the three-dimensional dynamic culture of human mesenchymal stem cells (hMSCs) in spherical alginate bead scaffolds. The goal of this study is to examine the effect of shear stress in the system and then quantify the proliferation and differentiation of hMSCs in different radial annuli of the scaffold. Shear stress was shown to have a temporal effect on hMSC osteoblastic differentiation with a strong correlation of shear stress, osteopontin, and bone morphogenic protein-2 occurring on day 21, and weaker correlation occurring at early timepoints. Further results revealed an approximate 2.5-fold increase in cell number in the inner annulus of TPS cultured constructs as compared to statically cultured constructs after 21 days. This result demonstrated a nutrient transfer limitation in static culture which can be mitigated by dynamic culture. A significant increase (P < 0.05) in mineralization in the inner and outer annuli of bioreactor cultured 4 mm scaffolds occurred on day 21 with 79 ± 29% and 53 ± 25% mineralization area, respectively, compared to 6 ± 4% and 19 ± 6% mineralization area, respectively, in inner and outer annuli of 4 mm statically cultured scaffolds. Surprising lower mineralization area was observed in 2 mm bioreactor cultured beads which had the highest levels of proliferation. These results may demonstrate a relationship between scaffold position and stem cell fate. In addition the decreased proliferation and matrix production in statically cultured scaffolds compared to bioreactor cultured constructs demonstrate the need for bioreactor systems and the effectiveness of the TPS bioreactor in promoting hMSC proliferation and differentiation in three-dimensional scaffolds.

Potential for osteogenic and chondrogenic differentiation of MSC

The introduction of mesenchymal stem cells (MSC) into the field of tissue engineering for bone and cartilage repair is a promising development, since these cells can be expanded ex vivo to clinically relevant numbers and, after expansion, retain their ability to differentiate into different cell lineages. Mesenchymal stem cells isolated from various tissues have been intensively studied and characterized by many research groups. To obtain functionally active differentiated tissue, tissue engineered constructs are cultivated in vitro statically or dynamically in bioreactors under controlled conditions. These conditions include special cell culture media, addition of signalling molecules, various physical and chemical factors and the application of different mechanical stimuli. Oxygen concentration in the culture environment is also a significant factor which influences MSC proliferation, stemness and differentiation capacity. Knowledge of the different aspects which affect MSC differentiation in vivo and in vitro will help researchers to achieve directed cell fate without the addition of differentiation agents in concentrations above the physiological range.

Hypoxia impedes vasculogenesis of in vitro engineered bone

To ensure the survival of engineered bone after implantation, we combined human endothelial colony forming cells (ECFCs) and multipotent stromal cells (MSCs) as a proof of concept in a co-culture model to create in vitro prevascularized bone constructs. We hypothesized that a hypoxic stimulus will contribute to prevascularization of engineered bone. Bone marrow-derived MSCs and ECFCs from human adult peripheral blood were allowed to form co-culture pellets containing ECFCs and MSCs (1:4) or MSCs only in controls. After culture under normoxia or hypoxia (5%), pellets were harvested and processed for immunohistochemistry of CD31, α-smooth muscle actin, and osteocalcin. Expression of vascular endothelial growth factor and SDF-1α was analyzed by PCR to elucidate their involvement in hypoxic stimulation of prevascularization. The normoxic condition in co-cultures of MSCs and ECFCs supported the formation and maintenance of prevascular structures, including organized CD31-positive cells embraced by differentiated mural cells. These structures failed to form in hypoxic conditions, thereby rejecting the hypothesis that hypoxia stimulates prevasculogenesis in three-dimensional engineered bone constructs. Further, the formation of prevascular structures was paralleled by increased SDF-1α expression. It is suggested that actual oxygen levels were below 5% in the hypoxic co-cultures, which prevented prevascular structure formation. In conclusion, our normoxic co-culture model containing cells from clinically relevant sources sustained simultaneous endothelial, smooth muscle, and osteogenic differentiation.

Hypoxia induces osteogenesis-related activities and expression of core binding factor α1 in mesenchymal stem cells

Mesenchymal stem sells (MSCs) have received much attention in the field of bone tissue engineering due to their biological capability to differentiate into osteogenic lineage cells. Hypoxia-inducible factor 1alpha (HIF-1α) plays an important role in the MSC-related bone regeneration during hypoxia, while core binding factor alpha 1 (Cbfα1) is a transcription regulator that is involved in the chondrocyte differentiation and ossification. In the present study, we investigated the effects of hypoxia on biological capability of MSCs. MSCs were isolated from adult rabbit bone marrow, and were cultured in vitro under normoxia (air with 5% CO(2)) or hypoxia (5% CO(2) and 95% N(2)). The proliferation of MSCs, alkaline phosphatase (ALP) activity, and production of collagens type I and type III (Col I/III) were examined. The expression levels of HIF-1α and Cbfα1 were measured by real-time PCR and western blot analyses. We found that hypoxia significantly induced the proliferation of MSCs and increased ALP activity and the production of Col I/III. Moreover, hypoxia increased the expression of Cbfα1 mRNA after 12 h, whereas the expression of HIF-1α mRNA was increased after 1 h of hypoxia. Knockdown of HIF-1α expression with a small interfering RNA significantly increased the expression levels of Cbfα1 protein either under the normoxia or hypoxia condition. Our results indicate that hypoxia enhances MSCs to differentiate into osteogenic lineage cells and suggest that Cbfα1 may be negatively regulated by HIF-1α.

Mesenchymal stem cells as a therapeutic approach to glomerular diseases: benefits and risks

Most studies using adult stem cells (ASCs) and progenitor cells as potential therapeutics for kidney disorders have been conducted in models of acute kidney injury, where the damage mainly affects the tubulointerstitium. The results are promising, whereas the underlying mechanisms are still being discussed controversially. Glomerular diseases have not received as much attention. Likely reasons include the often insidious onset, rendering the choice of optimal treatment timing difficult, and the fact that chronic diseases may require long-term therapy. In this mini review, we summarize current strategies in adult stem cell-based therapies for glomerular diseases. In addition, we focus on possible side effects of stem cell administration that have been reported recently, that is, profibrotic actions and maldifferentiation of mesenchymal stem cells.

Adipose tissue and bone marrow-derived stem cells react similarly in an ischaemia-like microenvironment

Mesenchymal stem cells (MSCs) from adipose tissue and bone marrow are promising cell sources for autologous cell therapy of nerve injuries, as demonstrated by their intrinsic neurotrophic potential. However, extensive death of transplanted cells limits their full benefits. This study investigated the effects of ischaemia (metabolically induced by sodium azide and 2-deoxyglucose) and serum-derived mitogens on the viability and functional profile of MSCs in vitro. MSCs were more susceptible to combined, rather than individual, blockade of glycolysis and oxidative phosphorylation. Apoptosis and autophagy were involved in ischaemia-induced cell death. Chemical ischaemia alone and serum withdrawal alone induced a similar amount of cell death, with significantly different intracellular ATP maintenance. Combined ischaemia and serum deprivation had additive effects on cell death. Expression of the extracellular matrix (ECM) molecules laminin and fibronectin was attenuated under ischaemia and independent of serum level; however, BDNF and NGF levels remained relatively constant. Strong upregulation of VEGF and to a lesser extent angiopoietin-1 was observed under ischaemia but not in serum withdrawal conditions. Importantly, this study demonstrated similar reactions of MSCs derived from adipose and bone marrow tissue, in ischaemia-like and mitogen-deprived microenvironments in terms of viability, cellular energetics, cell death mechanisms and expression levels of various growth-promoting molecules. Also, the results suggest that ischaemia has a larger impact on the ability of MSCs to survive transplantation than withdrawal of mitogens.

A concerted HIF-1α/MT1-MMP signalling axis regulates the expression of the 3BP2 adaptor protein in hypoxic mesenchymal stromal cells

Increased plasticity, migratory and immunosuppressive abilities characterize mesenchymal stromal cells (MSC) which enable them to be active participants in the development of hypoxic solid tumours. Our understanding of the oncogenic adaptation of MSC to hypoxia however lacks the identification and characterization of specific biomarkers. In this study, we assessed the hypoxic regulation of 3BP2/SH3BP2 (Abl SH3-binding protein 2), an immune response adaptor/scaffold protein which regulates leukocyte differentiation and motility. Gene silencing of 3BP2 abrogated MSC migration in response to hypoxic cues and generation of MSC stably expressing the transcription factor hypoxia inducible factor 1alpha (HIF-1α) resulted in increased endogenous 3BP2 expression as well as cell migration. Analysis of the 3BP2 promoter sequence revealed only one potential HIF-1α binding site within the human but none in the murine sequence. An alternate early signalling cascade that regulated 3BP2 expression was found to involve membrane type-1 matrix metalloproteinase (MT1-MMP) transcriptional regulation which gene silencing abrogated 3BP2 expression in response to hypoxia. Collectively, we provide evidence for a concerted HIF-1α/MT1-MMP signalling axis that explains the induction of adaptor protein 3BP2 and which may link protein binding partners together and stimulate oncogenic MSC migration. These mechanistic observations support the potential for malignant transformation of MSC within hypoxic tumour stroma and may contribute to evasion of the immune system by a tumour.

Modulating endochondral ossification of multipotent stromal cells for bone regeneration

For years it has been recognized that engineering of large bone constructs will be feasible only if the hurdle of vascularization is overcome. Attempts to engineer bone tissue have predominantly focused on intramembranous (direct) bone formation. A relatively new and most likely more physiological approach in this line is endochondral bone formation, comprising an intermediate cartilaginous stage. Cartilage in nature is an avascular tissue and its cells are equipped to survive the poor oxygenation and nutritional conditions inherent to implanted tissues. Subsequent terminal differentiation (hypertrophy) of the chondrocytes initiates the formation of a mineralized matrix that will then be converted into bone. Through this mechanism, our long bones grow and most fractures heal through the process of secondary fracture healing. The feasibility of the attractive concept of endochondral bone tissue engineering has already been shown. Most emphasis has gone to the multipotent stromal cells because of their great potential for expansion and differentiation and immunoprivileged nature. This review will focus on the promises and current status of this new field. Further, potent modulators of endochondral bone tissue engineering, including oxygen tension and mechanical stimuli, will be discussed.

A link between the accumulation of DNA damage and loss of multi-potency of human mesenchymal stromal cells

Human mesenchymal stromal cells (hMSCs) represent an attractive cell source for clinic applications. Besides being multi-potent, recent clinical trials suggest that they secrete both trophic and immunomodulatory factors, allowing allogenic MSCs to be used in a wider variety of clinical situations. The yield of prospective isolation is however very low, making expansion a required step toward clinical applications. Unfortunately, this leads to a significant decrease in their stemness. To identify the mechanism behind loss of multi-potency, hMSCs were expanded until replicative senescence and the concomitant molecular changes were characterized at regular intervals. We observed that, with time of culture, loss of multi-potency was associated with both the accumulation of DNA damage and the respective activation of the DNA damage response pathway, suggesting a correlation between both phenomena. Indeed, exposing hMSCs to DNA damage agents led to a significant decrease in the differentiation potential. We also showed that hMSCs are susceptible to accumulate DNA damage upon in vitro expansion, and that although hMSCs maintained an effective nucleotide excision repair activity, there was a progressive accumulation of DNA damage. We propose a model in which DNA damage accumulation contributes to the loss of differentiation potential of hMSCs, which might not only compromise their potential for clinical applications but also contribute to the characteristics of tissue ageing.

Osteogenic differentiation of miniature pig mesenchymal stem cells in 2D and 3D environment

Mesenchymal stem cells (MSCs) have been repeatedly shown to be able to repair bone defects. The aim of this study was to characterize the osteogenic differentiation of miniature pig MSCs and markers of this differentiation in vitro. Flow-cytometrically characterized MSCs were seeded on cultivation plastic (collagen I and vitronectin coated/uncoated) or plasma clot (PC)/plasma-alginate clot (PAC) scaffolds and differentiated in osteogenic medium. During three weeks of differentiation, the formation of nodules and deposition of calcium were visualized by Alizarin Red Staining. In addition, the production of alkaline phosphatase (ALP) activity was quantitatively detected by fluorescence. The expression of osteopontin, osteonectin and osteocalcin were assayed by immunohistochemistry and Western Blot analysis. We revealed a decrease of osteopontin expression in 2D and 3D environment during differentiation. The weak initial osteonectin signal, culminating on 7(th) or 14(th) day of differentiation, depends on collagen I and vitronectin coating in 2D system. The highest activity of ALP was detected on 21(th) day of osteogenic differentiation. The PC scaffolds provided better conditions for osteogenic differentiation of MSCs than PAC scaffolds in vitro. We also observed expected effects of collagen I and vitronectin on the acceleration of osteogenic differentiation of miniature pig MSC. Our results indicate similar ability of miniature pig MSCs osteogenic differentiation in 2D and 3D environment, but the expression of osteogenic markers in scaffolds and ECM coated monolayers started earlier than in the monolayers without ECM.

Mesenchymal stromal cells promote tumor growth through the enhancement of neovascularization

Mesenchymal stromal cells (MSCs), also called mesenchymal stem cells, migrate and function as stromal cells in tumor tissues. The effects of MSCs on tumor growth are controversial. In this study, we showed that MSCs increase proliferation of tumor cells in vitro and promote tumor growth in vivo. We also further analyzed the mechanisms that underlie these effects. For use in in vitro and in vivo experiments, we established a bone marrow-derived mesenchymal stromal cell line from cells isolated in C57BL/6 mice. Effects of murine MSCs on tumor cell proliferation in vitro were analyzed in a coculture model with B16-LacZ cells. Both coculture with MSCs and treatment with MSC-conditioned media led to enhanced growth of B16-LacZ cells, although the magnitude of growth stimulation in cocultured cells was greater than that of cells treated with conditioned media. Co-injection of B16-LacZ cells and MSCs into syngeneic mice led to increased tumor size compared with injection of B16-LacZ cells alone. Identical experiments using Lewis lung carcinoma (LLC) cells instead of B16-LacZ cells yielded similar results. Consistent with a role for neovascularization in MSC-mediated tumor growth, tumor vessel area was greater in tumors resulting from co-injection of B16-LacZ cells or LLCs with MSCs than in tumors induced by injection of cancer cells alone. Co-injected MSCs directly supported the tumor vasculature by localizing close to vascular walls and by expressing an endothelial marker. Furthermore, secretion of leukemia inhibitory factor, macrophage colony-stimulating factor, macrophage inflammatory protein-2 and vascular endothelial growth factor was increased in cocultures of MSCs and B16-LacZ cells compared with B16-LacZ cells alone. Together, these results indicate that MSCs promote tumor growth both in vitro and in vivo and suggest that tumor promotion in vivo may be attributable in part to enhanced angiogenesis.

Angiogenic properties of aged adipose derived mesenchymal stem cells after hypoxic conditioning

Background

Mesenchymal stem cells derived from adipose tissue (ADSC) are multipotent stem cells, originated from the vascular-stromal compartment of fat tissue. ADSC are used as an alternative cell source for many different cell therapies, however in ischemic cardiovascular diseases the therapeutic benefit was modest. One of the reasons could be the use of autologous aged ADSC, which recently were found to have impaired functions. We therefore analysed the effects of age on age markers and angiogenic properties of ADSC. Hypoxic conditioning was investigated as a form of angiogenic stimulation.

Methods

ADSC were harvested from young (1-3 month), adult (12 month) and aged (18-24 month) mice and cultured under normoxic (20%) and hypoxic (1%) conditions for 48 h. Differences in proliferation, apoptosis and telomere length were assessed in addition to angiogenic properties of ADSC.

Results

Proliferation potential and telomere length were decreased in aged ADSC compared to young ADSC. Frequency of apoptotic cells was higher in aged ADSC. Gene expression of pro-angiogenic factors including vascular endothelial growth factor (VEGF), placental growth factor (PlGF) and hepatic growth factor (HGF) were down-regulated with age, which could be restored by hypoxia. Transforming growth factor (TGF-β) increased in the old ADSC but was reduced by hypoxia.

Expression of anti-angiogenic factors including thrombospondin-1 (TBS1) and plasminogen activator inhibitor-1 (PAI-1) did increase in old ADSC, but could be reduced by hypoxic stimulation. Endostatin (ENDS) was the highest in aged ADSC and was also down-regulated by hypoxia. We noted higher gene expression of proteases system factors like urokinase-type plasminogen activator receptor (uPAR), matrix metalloproteinases (MMP2 and MMP9) and PAI-1 in aged ADSC compared to young ADSC, but they decreased in old ADSC. Tube formation on matrigel was higher in the presence of conditioned medium from young ADSC in comparison to aged ADSC.

Conclusions

ADSC isolated from older animals show changes, including impaired proliferation and angiogenic stimulation. Angiogenic gene expression can be partially be improved by hypoxic preconditioning, however the effect is age-dependent. This supports the hypothesis that autologous ADSC from aged subjects might have an impaired therapeutic potential.

Hypoxia and amino acid supplementation synergistically promote the osteogenesis of human mesenchymal stem cells on silk protein scaffolds

Tailoring tissue engineering strategies to match patient- and tissue-specific bone regeneration needs offers to improve clinical outcomes. As a step toward this goal, osteogenic outcomes and metabolic parameters were assessed when varying inputs into the bone formation process. Silk protein scaffolds seeded with human mesenchymal stem cells in osteogenic differentiation media were used to study in vitro osteogenesis under varied conditions of amino acid (lysine and proline) concentration and oxygen level. The cells were assessed to probe how the microenvironment impacted metabolic pathways and thus osteogenesis. The most favorable osteogenesis outcomes were found in the presence of low (5%) oxygen combined with high lysine and proline concentrations during in vitro cultivation. This same set of culture conditions also showed the highest glucose consumption, lactate synthesis, and certain amino acid consumption rates. On the basis of these results and known pathways, a holistic metabolic model was derived which shows that lysine and proline supplements as well as low (5%) oxygen levels regulate collagen matrix synthesis and thereby rates of osteogenesis. This study establishes early steps toward a foundation for patient- and tissue-specific matches between metabolism, repair site, and tissue engineering approaches toward optimized bone regeneration.

Osteogenic proliferation and differentiation of canine bone marrow and adipose tissue derived mesenchymal stromal cells and the influence of hypoxia

The aim of this study was to compare the osteogenic and proliferative potential of canine mesenchymal stromal cells (cMSCs) derived from bone marrow (BM-cMSCs) and adipose tissue (AT-cMSCs). Proliferation potential was determined under varying oxygen tensions (1%, 5%, and 21% O(2)). Effects of reduced oxygen levels on the osteogenic differentiation of AT-cMSCs were also investigated. AT-cMSCs proliferated at a significantly faster rate than BM-cMSCs, although both cell types showed robust osteogenic differentiation. Culture in 5% and 1% O(2) impaired proliferation in cMSC from both sources and osteogenic differentiation in AT-cMSCs. Our data suggests that AT-cMSCs might be more suitable for use in a clinical situation, where large cell numbers are required for bone repair, due to their rapid proliferation combined with robust osteogenic potential. Our data also suggests that the inhibitory effects of hypoxia on both cell proliferation and differentiation should be considered when using MSCs in a potentially hypoxic environment such as a fracture site.

Oxygen in stem cell biology: a critical component of the stem cell niche

The defining hallmark of stem cells is their ability to self-renew and maintain multipotency. This capacity depends on the balance of complex signals in their microenvironment. Low oxygen tensions (hypoxia) maintain undifferentiated states of embryonic, hematopoietic, mesenchymal, and neural stem cell phenotypes and also influence proliferation and cell-fate commitment. Recent evidence has identified a broader spectrum of stem cells influenced by hypoxia that includes cancer stem cells and induced pluripotent stem cells. These findings have important implications on our understanding of development, disease, and tissue-engineering practices and furthermore elucidate an added dimension of stem cell control within the niche.

Bioengineering strategies for regeneration of craniofacial bone: a review of emerging technologies

Although advances in surgical techniques and bone grafting have significantly improved the functional and cosmetic restoration of craniofacial structures lost because of trauma or disease, there are still significant limitations in our ability to regenerate these tissues. The regeneration of oral and craniofacial tissues presents a formidable challenge that requires synthesis of basic science, clinical science, and engineering technology. Tissue engineering is an interdisciplinary field of study that addresses this challenge by applying the principles of engineering to biology and medicine toward the development of biological substitutes that restore, maintain, and improve normal function. This review will explore the impact of biomaterials design, stem cell biology and gene therapy on craniofacial tissue engineering.

Increased proliferation and analysis of differential gene expression in human Wharton's jelly-derived mesenchymal stromal cells under hypoxia

Multipotent mesenchymal stromal cells (MSCs) from Wharton's jelly (WJ) of umbilical cord bear higher proliferation rate and self-renewal capacity than adult tissue-derived MSCs and are a primitive stromal cell population. Stem cell niche or physiological microenvironment plays a crucial role in maintenance of stem cell properties and oxygen concentration is an important component of the stem cell niche. Low oxygen tension or hypoxia is prevalent in the microenvironment of embryonic stem cells and many adult stem cells at early stages of development. Again, in vivo, MSCs are known to home specifically to hypoxic events following tissue injuries. Here we examined the effect of hypoxia on proliferation and in vitro differentiation potential of WJ-MSCs. Under hypoxia, WJ-MSCs exhibited improved proliferative potential while maintaining multi-lineage differentiation potential and surface marker expression. Hypoxic WJ-MSCs expressed higher mRNA levels of hypoxia inducible factors, notch receptors and notch downstream gene HES1. Gene expression profile of WJ-MSCs exposed to hypoxia and normoxia was compared and we identified a differential gene expression pattern where several stem cells markers and early mesodermal/endothelial genes such as DESMIN, CD34, ACTC were upregulated under hypoxia, suggesting that in vitro culturing of WJ-MSCs under hypoxic conditions leads to adoption of a mesodermal/endothelial fate. Thus, we demonstrate for the first time the effect of hypoxia on gene expression and growth kinetics of WJ-MSCs. Finally, although WJ-MSCs do not induce teratomas, under stressful and long-term culture conditions, MSCs can occasionally undergo transformation. Though there were no chromosomal abnormalities, certain transformation markers were upregulated in a few of the samples of WJ-MSCs under hypoxia.

Oxygen and oxygenation in stem-cell therapy for myocardial infarction

Myocardial infarction (MI) is caused by deprivation of oxygen and nutrients to the cardiac tissue due to blockade of coronary artery. It is a major contributor to chronic heart disease, a leading cause of mortality in the modern world. Oxygen is required to meet the constant energy demands for heart contractility, and also plays an important role in the regulation of heart function. However, reoxygenation of the ischemic myocardium upon restoration of blood flow may lead to further injury. Controlled oxygen delivery during reperfusion has been advocated to prevent this consequence. Monitoring the myocardial oxygen concentration would play a vital role in understanding the pathological changes in the ischemic heart following myocardial infarction. During the last two decades, several new techniques have become available to monitor myocardial oxygen concentration in vivo. Electron paramagnetic resonance (EPR) oximetry would appear to be the most promising and reliable of these techniques. EPR utilizes crystalline probes which yield a single sharp line, the width of which is highly sensitive to oxygen tension. Decreased oxygen tension results in a sharpening of the EPR spectrum, while an increase results in widening. In our recent studies, we have used EPR oximetry as a valuable tool to monitor myocardial oxygenation for several applications like ischemia-reperfusion injury, stem-cell therapy and hyperbaric oxygen therapy. The results obtained from these studies have demonstrated the importance of tissue oxygen in the application of stem-cell therapy to treat ischemic heart tissues. These results have been summarized in this review article.

Hair growth stimulated by conditioned medium of adipose-derived stem cells is enhanced by hypoxia: evidence of increased growth factor secretion

Adipose-derived stem cells (ADSCs) and their secretomes mediate diverse skin-regeneration effects, such as wound-healing and antioxidant protection, that are enhanced by hypoxia. We investigated the hair-growth-promoting effect of conditioned medium (CM) of ADSCs to determine if ADSCs and their secretomes regenerate hair and if hypoxia enhances hair regeneration. If so, we wanted to identify the factors responsible for hypoxia-enhanced hair-regeneration. We found that ADSC-CM administrated subcutaneously induced the anagen phase and increased hair regeneration in C(3)H/NeH mice. In addition, ADSC-CM increased the proliferation of human follicle dermal papilla cells (HFDPCs) and human epithelial keratinocytes (HEKs), which are derived from two major cell types present in hair follicles. We investigated the effect of hypoxia on ADSC function using the same animal model in which hypoxia increased hair regrowth. Forty-one growth factors in ADSC-CM from cells cultured under hypoxic or normoxic conditions were analyzed. The secretion of insulin-like growth factor binding protein (IGFBP)-1, IGFBP-2, macrophage colony-stimulating factor (M-CSF), M-CSF receptor, platelet-derived growth factor receptor-beta, and vascular endothelial growth factor was significantly increased by hypoxia, while the secretion of epithelial growth factor production was decreased. It is reasonable to conclude that ADSCs promote hair growth via a paracrine mechanism that is enhanced by hypoxia.

The role of hypoxia in bone marrow-derived mesenchymal stem cells: considerations for regenerative medicine approaches

Bone marrow-derived mesenchymal stem cells (MSCs) have demonstrated potential for regenerative medicine strategies. Knowledge of the way these cells respond to their environment in in vitro culture and after implantation in vivo is crucial for successful therapy. Oxygen tension plays a pivotal role in both situations. In vivo, a hypoxic environment can lead to apoptosis, but hypoxic preconditioning of MSCs and overexpression of prosurvival genes like Akt can reduce hypoxia-induced cell death. In cell culture, hypoxia can increase proliferation rates and enhance differentiation along the different mesenchymal lineages. Hypoxia also modulates the paracrine activity of MSCs, causing upregulation of various secretable factors, among which are important angiogenic factors such as vascular endothelial growth factor and interleukin-6 (IL6). Finally, hypoxia plays an important role in mobilization and homing of MSCs, primarily by its ability to induce stromal cell-derived factor-1 expression along with its receptor CXCR4. This article reviews the current literature on the effects of hypoxia on MSCs and aims to elucidate its potential role in regenerative medicine strategies.

Hypoxia preconditioned mesenchymal stem cells improve vascular and skeletal muscle fiber regeneration after ischemia through a Wnt4-dependent pathway

Mesenchymal stem cells (MSC) are multipotent postnatal stem cells, involved in the treatment of ischemic vascular diseases. We investigate the ability of MSC, exposed to short-term hypoxic conditions, to participate in vascular and tissue regeneration in an in vivo model of hindlimb ischemia. Transplantation of hypoxic preconditioned murine MSC (HypMSC) enhanced skeletal muscle regeneration at day 7, improved blood flow and vascular formation compared to injected nonpreconditioned MSC (NormMSC). These observed effects were correlated with an increase in HypMSC engraftment and a putative role in necrotic skeletal muscle fiber clearance. Moreover, HypMSC transplantation resulted in a large increase in Wnt4 (wingless-related MMTV integration site 4) expression and we demonstrate its functional significance on MSC proliferation and migration, endothelial cell (EC) migration, as well as myoblast differentiation. Furthermore, suppression of Wnt4 expression in HypMSC, abrogated the hypoxia-induced vascular regenerative properties of these cells in the mouse hindlimb ischemia model. Our data suggest that hypoxic preconditioning plays a critical role in the functional capabilities of MSC, shifting MSC location in situ to enhance ischemic tissue recovery, facilitating vascular cell mobilization, and skeletal muscle fiber regeneration via a paracrine Wnt-dependent mechanism.

Tissue harvest by means of suction-assisted or third-generation ultrasound-assisted lipoaspiration has no effect on osteogenic potential of human adipose-derived stromal cells

BACKGROUND

Human adipose-derived stromal cells readily undergo osteogenic differentiation in vitro and in vivo. Thus, interest in their potential role in skeletal tissue engineering continues to escalate. Very little is known regarding the effects that energy delivered by means of third-generation ultrasound-assisted lipoaspiration may have on the osteogenic potential of these cells. The authors investigated whether differences in adipose-derived stromal cell yield, and the in vitro proliferation and osteogenic potential of these cells obtained by suction-assisted lipoaspiration or third-generation ultrasound-assisted lipoaspiration, exist.

METHODS

Adipose-derived stromal cells were harvested from lipoaspiration specimens of patients undergoing elective suction-assisted lipoaspiration and third-generation ultrasound-assisted lipoaspiration. Harvested cells were seeded to evaluate proliferative capacity and in vitro osteogenic potential. Alkaline phosphatase and alizarin red staining were performed to evaluate early and terminal osteogenic differentiation, respectively. Quantitative real-time polymerase chain reaction analysis was used to examine osteogenic gene expression patterns of RUNX2/CFBA1 (early differentiation) and osteocalcin (late differentiation).

RESULTS

No significant differences in the proliferative capacity (n = 3), alkaline phosphatase staining (n = 3), or extracellular matrix mineralization (n = 3) of suction-assisted lipoaspiration- or third-generation ultrasound-assisted lipoaspiration-derived cells were appreciated. Transcript levels of markers of early and terminal osteogenic differentiation were not significantly different (n = 3).

CONCLUSIONS

These findings suggest that exposure of adipose-derived stromal cells to ultrasound energy during tissue harvest by means of third-generation ultrasound-assisted lipoaspiration does not impart a negative consequence toward their proliferative capacity or osteogenic potential. Thus, the cells harvested using third-generation ultrasound-assisted lipoaspiration are comparable to those obtained by means of suction-assisted lipoaspiration for use in the study of osteogenic differentiation and skeletal tissue engineering.

Hypoxia increases Sca-1/CD44 co-expression in murine mesenchymal stem cells and enhances their adipogenic differentiation potential

Mesenchymal stem cells (MSCs) are usually cultured under normoxic conditions (21% oxygen). However, in vivo, the physiological "niches" for MSCs have a much lower oxygen tension. Because of their plasticity, stem cells are particularly sensitive to their environments, and oxygen tension is one developmentally important stimulus in stem cell biology and plays a role in the intricate balance between cellular proliferation and commitment towards differentiation. Therefore, we investigated here the effect of hypoxia (2% oxygen) on murine adipose tissue (AT) MSC proliferation and adipogenic differentiation. AT cells were obtained from the omental fat and AT-MSCs were selected for their ability to attach to the plastic dishes, and were grown under normoxic and hypoxic conditions. Prior exposure of MSCs to hypoxia led to a significant reduction of ex vivo expansion time, with significantly increased numbers of Sca-1(+) as well as Sca-1(+)/CD44(+)double-positive cells. Under low oxygen culture conditions, the AT-MSC number markedly increased and their adipogenic differentiation potential was reduced. Notably, the hypoxia-mediated inhibition of adipogenic differentiation was reversible: AT-MSCs pre-exposed to hypoxia when switched to normoxic conditions exhibited significantly higher adipogenic differentiation capacity compared to their pre-exposed normoxic-cultured counterparts. Accordingly, the expression of adipocyte-specific genes, peroxisome proliferator activated receptor gamma (Ppargamma), lipoprotein lipase (Lpl) and fatty acid binding protein 4 (Fabp4) were significantly enhanced in hypoxia pre-exposed AT-MSCs. In conclusion, pre-culturing MSCs under hypoxic culture conditions may represent a strategy to enhance MSC production, enrichment and adipogenic differentiation.

Proteomic profiling of human bone marrow mesenchymal stem cells under shear stress

Mesenchymal stem cells (MSCs) are promising seed cells for tissue engineering of blood vessels. As seed cells, MSCs must endure blood fluid shear stress after transplantation. It has been shown that fluid shear stress can regulate the proliferation and differentiation of MSCs. However, the effects of fluid shear stress on MSCs including the types of proteins modulated are still not well understood. In this study, we exposed human mesenchymal stem cells (HMSCs) to 3 dyn/cm(2) shear stress for 6 h and compared them to a control group using proteomic analysis. Thirteen specific proteins were affected by shear stress, 10 of which were up-regulated. Shear stress especially induced sustained increases in the expression of Annexin A2 and GAPDH, which have been specifically shown to affect HMSCs function. We present here the first comparative proteome analysis of effect of shear stress on HMSCs.

Two-photon microscopy for non-invasive, quantitative monitoring of stem cell differentiation

Background

The engineering of functional tissues is a complex multi-stage process, the success of which depends on the careful control of culture conditions and ultimately tissue maturation. To enable the efficient optimization of tissue development protocols, techniques suitable for monitoring the effects of added stimuli and induced tissue changes are needed.

Methodology/Principal Findings

Here, we present the quantitative use of two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) as a noninvasive means to monitor the differentiation of human mesenchymal stem cells (hMSCs) using entirely endogenous sources of contrast. We demonstrate that the individual fluorescence contribution from the intrinsic cellular fluorophores NAD(P)H, flavoproteins and lipofuscin can be extracted from TPEF images and monitored dynamically from the same cell population over time. Using the redox ratio, calculated from the contributions of NAD(P)H and flavoproteins, we identify distinct patterns in the evolution of the metabolic activity of hMSCs maintained in either propagation, osteogenic or adipogenic differentiation media. The differentiation of these cells is mirrored by changes in cell morphology apparent in high resolution TPEF images and by the detection of collagen production via SHG imaging. Finally, we find dramatic increases in lipofuscin levels in hMSCs maintained at 20% oxygen vs. those in 5% oxygen, establishing the use of this chromophore as a potential biomarker for oxidative stress.

Conclusions/Significance

In this study we demonstrate that it is possible to monitor the metabolic activity, morphology, ECM production and oxidative stress of hMSCs in a non-invasive manner. This is accomplished using generally available multiphoton microscopy equipment and simple data analysis techniques, such that the method can widely adopted by laboratories with a diversity of comparable equipment. This method therefore represents a powerful tool, which enables researchers to monitor engineered tissues and optimize culture conditions in a near real time manner.

Effect of hyperbaric oxygen on mesenchymal stem cells for lumbar fusion in vivo

Background

Hyperbaric oxygen (HBO) therapy has been proved in improving bone healing, but its effects on mesenchymal stem cells (MSCs) in vivo is not clear. The aims of this study are to clarify whether the HBO therapy has the same enhancing effect on MSCs with regard to bone formation and maturation and to ascertain whether the transplanted MSCs survive in the grafted area and contribute to new bone formation.

Methods

Twenty-three adult rabbits underwent posterolateral fusion at L4-L5 level. The animals were divided into three groups according to the material implanted and subsequent treatment: (1) Alginate carrier (n = 6); (2) Alginate-MSCs composite (n = 11); and (3) Alginate-MSCs composite with HBO therapy (n = 6). After 12 weeks, spine fusion was examined using radiographic examination, manual testing, and histological examination. Using a PKH fluorescence labeling system, whether the transplanted MSCs survived and contributed to new bone formation in the grafted area after HBO therapy was also examined.

Results

The bilateral fusion areas in each animal were evaluated independently. By radiographic examination and manual palpation, union for the Alginate, Alginate-MSCs, and Alginate-MSCs-HBO groups was 0 of 12, 10 of 22, and 6 of 12 respectively. The difference between the Alginate-MSCs and Alginate-MSCs-HBO groups was not significant (P = 0.7997). The fluorescence microscopy histological analysis indicated that the transplanted PKH67-labeled MSCs survived and partly contributed to new bone formation in the grafted area.

Conclusions

This study demonstrated that the preconditioned MSCs could survive and yield bone formation in the grafted area. HBO therapy did not enhance the osteogenic ability of MSCs and improve the success of spine fusion in the rabbit model. Although there was no significant effect of HBO therapy on MSCs for spine fusion, the study encourages us to research a more basic approach for determining the optimal oxygen tension and pressure that are required to maintain and enhance the osteogenic ability of preconditioned MSCs. Further controlled in vivo and in vitro studies are required for achieving a better understanding of the effect of HBO treatment on MSCs.

Mechanical Stimulation Mediates Gene Expression in MC3T3 Osteoblastic Cells Differently in 2D and 3D Environments

Successful bone tissue engineering requires the understanding of cellular activity in three-dimensional (3D) architectures and how it compares to two-dimensional (2D) architecture. We developed a perfusion culture system that utilizes fluid flow to mechanically load a cell-seeded 3D scaffold. This study compared the gene expression of osteoblastic cells in 2D and 3D cultures, and the effects of mechanical loading on gene expression in 2D and 3D cultures. MC3T3-E1 osteoblastlike cells were seeded onto 2D glass slides and 3D calcium phosphate scaffolds and cultured statically or mechanically loaded with fluid flow. Gene expression of OPN and FGF-2 was upregulated at 24 h and 48 h in 3D compared with 2D static cultures, while collagen 1 gene expression was downregulated. In addition, while flow increased OPN in 2D culture at 48 h, it decreased both OPN and FGF-2 in 3D culture. In conclusion, gene expression is different between 2D and 3D osteoblast cultures under static conditions. Additionally, osteoblasts respond to shear stress differently in 2D and 3D cultures. Our results highlight the importance of 3D mechanotransduction studies for bone tissue engineering applications.

Resident and bone marrow-derived mesenchymal stem cells in head and neck squamous cell carcinoma

Head and neck squamous cell carcinoma (HNSCC) is a major healthcare problem worldwide affecting more than half a million patients each year. Despite considerable advances in the treatment of HNSCC, a high rate of recurrences aggravates the clinical situation and disease outcomes have only modestly improved. Recent insights show that cancer is not only a disease of the transformed epithelium but is also influenced and dependent on its stromal environment. In this review we suggest that resident and bone marrow (BM)-derived mesenchymal stem cells (MSCs) are precursors of the stroma associated with HNSCC and contribute to blood- and lymph angiogenesis, modulate the immune system and produce tumor-associated myofibroblasts. In addition, the impact of radiation therapy on the stromal reaction in HNSCC is discussed. Understanding the mechanisms of how MSCs promote invasive growth and metastasis in HNSCC and respond to cancer management strategies is of profound medical importance and will help us to design improved therapeutic protocols.

Human adipose-derived stromal cells respond to and elaborate bone morphogenetic protein-2 during in vitro osteogenic differentiation

BACKGROUND

Interest in the potential application of adipose-derived stromal cells in cell-mediated tissue engineering of bone and other mesenchymal-derived tissues is growing. This study aimed to investigate the hypothesis that human adipose-derived stromal cells respond to and elaborate bone morphogenetic protein (BMP) 2, which could represent an important target of molecular manipulation to enhance the osteogenic potential of human adipose-derived stromal cells.

METHODS

Human adipose-derived stromal cells were differentiated for 10 days toward the osteogenic lineage in osteogenic differentiation media alone or supplemented with recombinant human BMP2 (rhBMP2). Alizarin red staining was quantified by spectrophotometry. Gene expression analyses were performed using quantitative real-time polymerase chain reaction. BMP2 levels in conditioned media were titered by enzyme-linked immunosorbent assay daily during osteogenic differentiation. Human adipose-derived stromal cells were cultured in complete or partially (50 percent) changed osteogenic differentiation media, or unchanged osteogenic differentiation media, to assay for pro-osteogenic secreted factors. In addition, human adipose-derived stromal cells were cultured in osteogenic differentiation media supplemented with BMP2/BMP4-neutralizing antibody.

RESULTS

Exogenous rhBMP2 significantly augmented the in vitro osteogenic potential of human adipose-derived stromal cells in a dose-dependent fashion, and significantly increased transcript levels of RUNX2 and osteocalcin. BMP2, BMP4, BMPR1B, and SMAD1/5 expression was significantly increased during differentiation. Enzyme-linked immunosorbent assay demonstrated significantly increased BMP2 elaboration during differentiation. Culture in conditioned osteogenic differentiation media led to significantly increased matrix mineralization. Mineralization was significantly decreased when osteogenic differentiation media was supplemented with a BMP2/BMP4-neutralizing antibody.

CONCLUSIONS

These data strongly support that BMP signaling is dynamic and important during normal in vitro osteogenic differentiation of human adipose-derived stromal cells. Thus, BMP2 may be used to enhance the osteogenic differentiation of human adipose-derived stromal cells for bone tissue engineering. Future studies will examine the effect of rhBMP2 on osteogenic differentiation of human adipose-derived stromal cells in vivo.

Low oxygen tension and synthetic nanogratings improve the uniformity and stemness of human mesenchymal stem cell layer

A free-standing, robust cell sheet comprising aligned human mesenchymal stem cells (hMSCs) offers many interesting opportunities for tissue reconstruction. As a first step toward this goal, a confluent, uniform hMSC layer with a high degree of alignment and stemness maintenance needs to be created. Hypothesizing that topographical cue and a physiologically relevant low-oxygen condition could promote the formation of such an hMSC layer, we studied the culture of hMSCs on synthetic nanogratings (350 nm width and 700 nm pitch) and either under 2 or 20% O(2). Culturing hMSCs on the nanogratings highly aligned the cells, but it tended to create patchy layers and accentuate the hMSC differentiation. The 2% O(2) improved the alignment and uniformity of hMSCs, and reduced their differentiation. Over a 14-day culture period, hMSCs in 2% O(2) showed uniform connexon distribution, secreted abundant extracellular matrix (ECM) proteins, and displayed a high progenicity. After 21-day culture on nanogratings, hMSCs exposed to 2% O(2) maintained a higher viability and differentiation capacity. This study established that a 2% O(2) culture condition could restrict the differentiation of hMSCs cultured on nanopatterns, thereby setting the foundation to fabricate a uniformly aligned hMSC sheet for different regenerative medicine applications.

Hypoxic preconditioning of human mesenchymal stem cells overcomes hypoxia-induced inhibition of osteogenic differentiation

Osteogenic differentiation of human mesenchymal stem cells (hMSCs) into osteoblasts is a prerequisite for subsequent bone formation. Numerous studies have explored osteogenic differentiation under standard tissue culture conditions, which usually employ 21% of oxygen. However, bone precursor cells such as hMSCs reside in stem cell niches of low-oxygen atmospheres. Furthermore, they are subjected to low oxygen concentrations when cultured on three-dimensional scaffolds in vitro, and even more so after transplantation when vascularization has yet to be established. Similarly, hMSCs are exposed to low oxygen in the fracture microenvironment following bony injury. Recent studies revealed that hypoxic preconditioning improves cellular engraftment and survival in low-oxygen atmospheres. In our study we investigated the osteogenic differentiation potential of hMSCs under 2% O(2) (hypoxia) in comparison to a standard tissue culture oxygen atmosphere of 21% (normoxia). We assessed the osteogenic differentiation of hMSCs following hypoxic preconditioning to address whether this pretreatment is beneficial for subsequent differentiation processes as well. To validate our findings we carefully characterized the extent of hypoxia exerted and its effect on cell survival and proliferation. We found that hMSCs proliferate better if cultured under 2% of oxygen. We confirmed that osteogenic differentiation of hMSCs is indeed inhibited if osteogenic induction is carried out under constant hypoxia. Finally, we showed for the first time that hypoxic preconditioning of hMSCs prior to osteogenic induction restores osteogenic differentiation of hMSCs under hypoxic conditions. Collectively, our results indicate that maintaining constant levels of oxygen improves the osteogenic potential of hMSCs and suggest that low oxygen concentrations may preserve the stemness of hMSCs. In addition, our data support the hypothesis that if low-oxygen atmospheres are expected at the site of implantation, hypoxic pretreatment may be beneficial for the cells' subsequent in vivo performance.

Adult human bone marrow- and adipose tissue-derived stromal cells support the formation of prevascular-like structures from endothelial cells in vitro

Inadequate vascularization of in vitro-engineered tissue constructs after implantation is a major problem in most tissue-engineering applications. In this study we evaluated whether adipose tissue-derived stromal cells (ASCs), similar to bone marrow-derived stromal cells (BMSCs), can support the organization of endothelial cells into prevascular-like structures using an in vitro model. In addition, we investigated the mechanisms leading to the support of endothelial organization by these cells. We cultured human umbilical vein endothelial cells (HUVECs), ASCs, and BMSCs either alone or in combination in fibrin-embedded spheroids for 14 days. We found that BMSCs and ASCs formed cellular networks that expressed alpha smooth muscle actin and, in the case of ASCs, also CD34. Further, BMSCs and ASCs secreted hepatocyte growth factor and tissue inhibitor of metalloproteinase 1 and 2. In addition, ASC-conditioned medium induced HUVEC outgrowth, whereas BMSC-conditioned medium and hepatocyte growth factor-supplemented medium did not. Finally, both BMSCs and ASCs supported HUVEC organization into prevascular-like structures when cocultured. Our results suggest that both BMSCs and ASCs can support the formation of prevascular-like structures in vitro. Further, our findings indicate that cell-cell contacts and reciprocal signaling play an important role in the formation of these prevascular structures.

Inhibition of histone deacetylase activity in reduced oxygen environment enhances the osteogenesis of mouse adipose-derived stromal cells

Recent studies suggest that oxygen tension has a great impact on the osteogenic differentiation capacity of mesenchymal cells derived from adipose tissue: reduced oxygen impedes osteogenesis. We have found that expansion of mouse adipose-derived stromal cells (mASCs) in reduced oxygen tension (10%) results in increased cell proliferation along with induction of histone deacetylase (HDAC) activity. In this study, we utilized two HDAC inhibitors (HDACi), sodium butyrate (NaB) and valproic acid (VPA), and studied their effects on mASCs expanded in various oxygen tensions (21%, 10%, and 1% O(2)). Significant growth inhibition was observed with NaB or VPA treatment in each oxygen tension. Osteogenesis was enhanced by treatment with NaB or VPA, particularly in reduced oxygen tensions (10% and 1% O(2)). Conversely, adipogenesis was decreased with treatments of NaB or VPA at all oxygen tensions. Finally, NaB- or VPA-treated, reduced oxygen tension-exposed (1% O(2)) ASCs were grafted into surgically created mouse tibial defects and resulted in significantly increased bone regeneration. In conclusion, HDACi significantly promote the osteogenic differentiation of mASCs exposed to reduced oxygen tension; HDACi may hold promise for future clinical applications of ASCs for skeletal regeneration.

Low physiologic oxygen tensions reduce proliferation and differentiation of human multipotent mesenchymal stromal cells

Background

Human multipotent mesenchymal stromal cells (MSC) can be isolated from various tissues including bone marrow. Here, MSC participate as bone lining cells in the formation of the hematopoietic stem cell niche. In this compartment, the oxygen tension is low and oxygen partial pressure is estimated to range from 1% to 7%. We analyzed the effect of low oxygen tensions on human MSC cultured with platelet-lysate supplemented media and assessed proliferation, morphology, chromosomal stability, immunophenotype and plasticity.

Results

After transferring MSC from atmospheric oxygen levels of 21% to 1%, HIF-1α expression was induced, indicating efficient oxygen reduction. Simultaneously, MSC exhibited a significantly different morphology with shorter extensions and broader cell bodies. MSC did not proliferate as rapidly as under 21% oxygen and accumulated in G₁ phase. The immunophenotype, however, was unaffected. Hypoxic stress as well as free oxygen radicals may affect chromosomal stability. However, no chromosomal abnormalities in human MSC under either culture condition were detected using high-resolution matrix-based comparative genomic hybridization. Reduced oxygen tension severely impaired adipogenic and osteogenic differentiation of human MSC. Elevation of oxygen from 1% to 3% restored osteogenic differentiation.

Conclusion

Physiologic oxygen tension during in vitro culture of human MSC slows down cell cycle progression and differentiation. Under physiological conditions this may keep a proportion of MSC in a resting state. Further studies are needed to analyze these aspects of MSC in tissue regeneration.

Bone regeneration: the stem/progenitor cells point of view

After bone injuries, several molecular mechanisms establish bone repair from stem/progenitor cells. Inflammation factors attract regenerative cells which expand and differentiate in order to build up a bone highly similar to that before injury. Bone marrow (BM) mesenchymal stem cells (MSCs) as skeletal stem cells and endothelial progenitors (EPCs) are at the origin of such reparation mechanisms. However, discrepancies exist about their identities. Although cultured MSCs are extensively described, their in vivo native forms are poorly known. In addition, recent experiments show that several types of EPC exist. We therefore review up-to-date data on the characterization of such stem/progenitor cells and propose a new point of view of their function in bone regeneration.