The metabolism of human mesenchymal stem cells during proliferation and differentiation

The effect of oxygen tension on human articular chondrocyte matrix synthesis: integration of experimental and computational approaches

Significant oxygen gradients occur within tissue engineered cartilaginous constructs. Although oxygen tension is an important limiting parameter in the development of new cartilage matrix, its precise role in matrix formation by chondrocytes remains controversial, primarily due to discrepancies in the experimental setup applied in different studies. In this study, the specific effects of oxygen tension on the synthesis of cartilaginous matrix by human articular chondrocytes were studied using a combined experimental-computational approach in a “scaffold-free” 3D pellet culture model. Key parameters including cellular oxygen uptake rate were determined experimentally and used in conjunction with a mathematical model to estimate oxygen tension profiles in 21-day cartilaginous pellets. A threshold oxygen tension (pO₂ ≈ 8% atmospheric pressure) for human articular chondrocytes was estimated from these inferred oxygen profiles and histological analysis of pellet sections. Human articular chondrocytes that experienced oxygen tension below this threshold demonstrated enhanced proteoglycan deposition. Conversely, oxygen tension higher than the threshold favored collagen synthesis. This study has demonstrated a close relationship between oxygen tension and matrix synthesis by human articular chondrocytes in a “scaffold-free” 3D pellet culture model, providing valuable insight into the understanding and optimization of cartilage bioengineering approaches. Biotechnol. Bioeng. 2014;111: 1876–1885.

A multicompartment holder for spinner flasks improves expansion and osteogenic differentiation of mesenchymal stem cells in three-dimensional scaffolds

In the tissue engineering field dynamic culture systems, such as spinner flasks, are widely used due to their ability to improve mass transfer in suspension cell cultures. However, this culture system is often unsuitable to culture cells in three-dimensional (3D) scaffolds. To address this drawback, we designed a multicompartment holder for 3D cell culture, easily adaptable to spinner flasks. Here, the device was tested with human mesenchymal stem cells (MSCs) seeded in 3D porous chitosan scaffolds that were maintained in spinner flasks under dynamic conditions (50 rpm). Standard static culture conditions were used as control. The dynamic conditions were shown to significantly increase MSCs proliferation over 1 week (approximately 6-fold) and to improve cell distribution within the scaffold. Moreover, they also promoted osteogenic differentiation of MSCs, inducing an earlier peak in alkaline phosphatase (ALP) activity, and a more homogenous ALP staining and matrix mineralization in the whole scaffolds, but particularly in the center. Overall, this study shows a new multicompartment holder to culture 3D scaffolds that can broaden the application of spinner flasks.

Effects of hyperbaric oxygen on the osteogenic differentiation of mesenchymal stem cells

Background

Hyperbaric oxygenation was shown to increase bone healing in a rabbit model. However, little is known about the regulatory factors and molecular mechanism involved.We hypothesized that the effect of hyperbaric oxygen (HBO) on bone formation is mediated via increases in the osteogenic differentiation of mesenchymal stem cells (MSCs) which are regulated by Wnt signaling.

Methods

The phenotypic characterization of the MSCs was analyzed by flow cytometric analysis. To investigate the effects of HBO on Wnt signaling and osteogenic differentiation of MSCs, mRNA and protein levels of Wnt3a, beta-catenin, GSK-3beta, Runx 2, as well as alkaline phosphatase activity, calcium deposition, and the intensity of von Kossa staining were analyzed after HBO treatment. To investigate the effects of HBO on Wnt processing and secretion, the expression of Wntless and vacuolar ATPases were quantified after HBO treatment.

Results

Cells expressed MSC markers such as CD105, CD146, and STRO-1. The mRNA and protein levels of Wnt3a, β-catenin, and Runx 2 were up-regulated, while GSK-3β was down-regulated after HBO treatment. Western blot analysis showed an increased β-catenin translocation with a subsequent stimulation of the expression of target genes after HBO treatment. The above observation was confirmed by small interfering (si)RNA treatment. HBO significantly increased alkaline phosphatase activity, calcium deposition, and the intensity of von Kossa staining of osteogenically differentiated MSCs. We further showed that HBO treatment increased the expression of Wntless, a retromer trafficking protein, and vacuolar ATPases to stimulate Wnt processing and secretion, and the effect was confirmed by siRNA treatment.

Conclusions

HBO treatment increased osteogenic differentiation of MSCs via regulating Wnt processing, secretion, and signaling.

Cyclic mechanical loading enables solute transport and oxygen supply in bone healing: an in vitro investigation

Bone healing is a complex process with an increased metabolic activity and consequently high demand for oxygen. In the hematoma phase, inflammatory cells and mesenchymal stromal cells (MSCs) are initially cut off from direct nutritional supply via blood vessels. Cyclic mechanical loading that occurs, for example, during walking is expected to have an impact on the biophysical environment of the cells but meaningful quantitative experimental data are still missing. In this study, the hypothesis that cyclic mechanical loading within a physiological range significantly contributes to oxygen transport into the fracture hematoma was investigated by an in vitro approach. MSCs were embedded in a fibrin matrix to mimic the hematoma phase during bone healing. Construct geometry, culture conditions, and parameters of mechanical loading in a bioreactor system were chosen to resemble the in vivo situation based on data from human studies and a well-characterized large animal model. Oxygen tension was measured before and after mechanical loading intervals by a chemical optical microsensor. The increase in oxygen tension at the center of the constructs was significant and depended on loading time with maximal values of 9.9%±5.1%, 14.8%±4.9%, and 25.3%±7.2% of normal atmospheric oxygen tension for 5, 15, and 30 min of cyclic loading respectively. Histological staining of hypoxic cells after 48 h of incubation confirmed sensor measurements by showing an increased number of normoxic cells with intermittent cyclic compression compared with unloaded controls. The present study demonstrates that moderate cyclic mechanical loading leads to an increased oxygen transport and thus to substantially enhanced supply conditions for cells entrapped in the hematoma. This link between mechanical conditions and nutrition supply in the early regenerative phases could be employed to improve the environmental conditions for cell metabolism and consequently prevent necrosis.

Role of SIRT1 and AMPK in mesenchymal stem cells differentiation

The differentiation capabilities of mesenchymal stem cells (MSCs) compromise with age and with in vitro passages which could impair the efficacy of cell therapy and tissue engineering. However, how to maintain these capabilities is not fully understood. Calorie restriction (CR, decreasing caloric intake by 30-40%) could extend longevity and reduce aging-related diseases. Recent studies revealed that CR could influence the lineage determination of stem cells including MSCs. Two important mediators of CR might be silent mating type information regulation 2 homolog 1 (SIRT1), a NAD(+)-dependent deacetylase, and AMP-activated protein kinase (AMPK), an energy-sensing kinase. Evidences are mounting that both SIRT1 and AMPK play important roles in cell fate determination of MSCs. Herein, we intend to sum up our understanding about the role of SIRT1 and AMPK in osteogenic and adipogenic potential of MSCs. Metabolic process of MSCs differentiation and the putative interplay of SIRT1 and AMPK in this process was also discussed.

Isolation and ex vivo expansion of synovial mesenchymal stromal cells for cartilage repair

BACKGROUND AIMS

Hyaline articular cartilage is a highly specialized tissue that offers a low-friction and wear-resistant interface for weight-bearing surface articulation in diarthrodial joints, but it lacks vascularity. It displays an inherent inability to heal when injured in a skeletally mature individual. Joint-preserving treatment procedures such as mosaicplasty, débridement, perichondrium transplantation and autologous chondrocyte implantation have shown variable results, and the average long-term result is sub-standard. Because of these limitations of the treatment methods and lack of intrinsic repair capacity of mature cartilage tissue, an alternative treatment approach is needed, and synovial mesenchymal stromal cells (SMSCs) represent an attractive therapeutic alternative because of their ex vivo proliferation capacity, multipotency and ability to undergo chondrogenesis.

METHODS

SMSCs were isolated from tissues obtained by arthroscopy using two types of biopsies. Ex vivo cell expansion was accomplished under static and dynamic culture followed by characterization of cells according to the International Society for Cellular Therapy guidelines. Kinetic growth models and metabolite analysis were used for understanding the growth profile of these cells.

RESULTS

For the first time, SMSCs were expanded in stirred bioreactors and achieved higher cell density in a shorter period of time compared with static culture or with other mesenchymal stromal cell sources.

CONCLUSIONS

In this study we were able to achieve (8.8 ± 0.2) × 10(5) cells within <2 weeks in dynamic culture under complete xeno-free conditions. Our results also provided evidence that after dynamic culture these cells had an up-regulation of chondrogenic genes, which can be a potential factor for articular cartilage regeneration in clinical settings.

Combined use of platelet rich plasma and vitamin C positively affects differentiation in vitro to mesodermal lineage of adult adipose equine mesenchymal stem cells

Repair of injured soft and hard tissues in horses can benefit greatly from the use of regenerative therapies with mesenchymal stem cells (MSC). Vitamin-C and platelet-rich-plasma had been used for in vitro differentiation of MSC. This study was aimed to evaluate the effect of vitamin-C, platelet-rich-plasma and their combination on the in vitro differentiation of adipose horse MSC. We isolated MSC from horse fat and differentiated them in vitro into osteogenic and chondrogenic lineages, as demonstrated by specific staining and RT-qPCR of selected genes. Combining vitamin-C and plasma-rich-platelet positively affected the ability of MSC to differentiate in vitro into mesodermal lineages during 14 days of culture; this effect was not as marked when differentiation was attempted for 21 days. This provides valuable information on the effect of combined use of these molecules in regenerative therapies and their potential application along stem cells for lesions of musculoskeletal tissue in sport horses.

Gas chromatography-mass spectrometry analysis of human mesenchymal stem cell metabolism during proliferation and osteogenic differentiation under different oxygen tensions

Bone marrow derived human mesenchymal stem cells (hMSC) are the primary cell type in bone tissue engineering, and their life span during osteogenic differentiation is associated with changes in oxygen tension. As a ubiquitous regulator of cellular metabolic activity, oxygen tension influences the profiles of metabolites in the entire metabolic network and plays an important role in hMSC survival, function, and osteogenic differentiation. In the current study, we hypothesize that hMSC have a metabolic phenotype that supports growth in low oxygen environments and that this phenotype changes upon differentiation, leading to differential responses to oxygen tension. We developed a gas chromatography-mass spectrometry (GC-MS) based metabolic profiling approach to analyze the metabolic fate of (13)C-glucose in glycolysis and the tricarboxylic acid cycle (TCA) in undifferentiated hMSC and hMSC-derived osteoblasts (hMSC-OS) in response to perturbation in oxygen tension; specifically we compared changes induced by culture under 20% vs. 2% O2. The isotope enrichments in the metabolites were calculated and used to infer activities of specific metabolic enzymes and the associated pathways. The results revealed contrasting metabolic profiles for hMSC and the hMSC-OS in both 20% and 2% O2 states, with the most significant differences involving coupling of glycolysis to the TCA cycle, glutaminolysis, and the malate-aspartate shuttle. The results have important implications in defining the optimal culture conditions for hMSC expansion and osteogenic differentiation.

Expansion of mesenchymal stem cells under atmospheric carbon dioxide

Stem cells are needed for an increasing number of scientific applications, including both fundamental research and clinical disease treatment. To meet this rising demand, improved expansion methods to generate high quantities of high quality stem cells must be developed. Unfortunately, the bicarbonate buffering system - which relies upon an elevated CO2 environment - typically used to maintain pH in stem cell cultures introduces several unnecessary limitations in bioreactor systems. In addition to artificially high dissolved CO2 levels negatively affecting cell growth, but more importantly, the need to sparge CO2 into the system complicates the ability to control culture parameters. This control is especially important for stem cells, whose behavior and phenotype is highly sensitive to changes in culture conditions such as dissolved oxygen and pH. As a first step, this study developed a buffer to support expansion of mesenchymal stem cells (MSC) under an atmospheric CO2 environment in static cultures. MSC expanded under atmospheric CO2 with this buffer achieved equivalent growth rates without adaptation compared to those grown in standard conditions and also maintained a stem cell phenotype, self-renewal properties, and the ability to differentiate into multiple lineages after expansion.

Stem cell metabolism in tissue development and aging

Recent advances in metabolomics and computational analysis have deepened our appreciation for the role of specific metabolic pathways in dictating cell fate. Once thought to be a mere consequence of the state of a cell, metabolism is now known to play a pivotal role in dictating whether a cell proliferates, differentiates or remains quiescent. Here, we review recent studies of metabolism in stem cells that have revealed a shift in the balance between glycolysis, mitochondrial oxidative phosphorylation and oxidative stress during the maturation of adult stem cells, and during the reprogramming of somatic cells to pluripotency. These insights promise to inform strategies for the directed differentiation of stem cells and to offer the potential for novel metabolic or pharmacological therapies to enhance regeneration and the treatment of degenerative disease.

Modulating gradients in regulatory signals within mesenchymal stem cell seeded hydrogels: a novel strategy to engineer zonal articular cartilage

Engineering organs and tissues with the spatial composition and organisation of their native equivalents remains a major challenge. One approach to engineer such spatial complexity is to recapitulate the gradients in regulatory signals that during development and maturation are believed to drive spatial changes in stem cell differentiation. Mesenchymal stem cell (MSC) differentiation is known to be influenced by both soluble factors and mechanical cues present in the local microenvironment. The objective of this study was to engineer a cartilaginous tissue with a native zonal composition by modulating both the oxygen tension and mechanical environment thorough the depth of MSC seeded hydrogels. To this end, constructs were radially confined to half their thickness and subjected to dynamic compression (DC). Confinement reduced oxygen levels in the bottom of the construct and with the application of DC, increased strains across the top of the construct. These spatial changes correlated with increased glycosaminoglycan accumulation in the bottom of constructs, increased collagen accumulation in the top of constructs, and a suppression of hypertrophy and calcification throughout the construct. Matrix accumulation increased for higher hydrogel cell seeding densities; with DC further enhancing both glycosaminoglycan accumulation and construct stiffness. The combination of spatial confinement and DC was also found to increase proteoglycan-4 (lubricin) deposition toward the top surface of these tissues. In conclusion, by modulating the environment through the depth of developing constructs, it is possible to suppress MSC endochondral progression and to engineer tissues with zonal gradients mimicking certain aspects of articular cartilage.

A quantitative approach for understanding small-scale human mesenchymal stem cell culture - implications for large-scale bioprocess development

Human mesenchymal stem cell (hMSC) therapies have the potential to revolutionise the healthcare industry and replicate the success of the therapeutic protein industry; however, for this to be achieved there is a need to apply key bioprocessing engineering principles and adopt a quantitative approach for large-scale reproducible hMSC bioprocess development. Here we provide a quantitative analysis of the changes in concentration of glucose, lactate and ammonium with time during hMSC monolayer culture over 4 passages, under 100% and 20% dissolved oxgen (dO2 ), where either a 100%, 50% or 0% growth medium exchange was performed after 72h in culture. Yield coefficients, specific growth rates (h(-1) ) and doubling times (h) were calculated for all cases. The 100% dO2 flasks outperformed the 20% dO2 flasks with respect to cumulative cell number, with the latter consuming more glucose and producing more lactate and ammonium. Furthermore, the 100% and 50% medium exchange conditions resulted in similar cumulative cell numbers, whilst the 0% conditions were significantly lower. Cell immunophenotype and multipotency were not affected by the experimental culture conditions. This study demonstrates the importance of determining optimal culture conditions for hMSC expansion and highlights a potential cost savings from only making a 50% medium exchange, which may prove significant for large-scale bioprocessing.

Response of sheep chondrocytes to changes in substrate stiffness from 2 to 20 Pa: effect of cell passaging

AIM

The influence of culture substrate stiffness (in the kPa range) on chondrocyte behavior has been described. Here we describe the response to variations in substrate stiffness in a soft range (2-20 Pa), as it may play a role in understanding cartilage physiopathology.

METHODS

We developed a system for cell culture in substrates with different elastic moduli using collagen hydrogels and evaluated chondrocytes after 2, 4, and 7 days in monolayer and three-dimensional (3D) cultures. Experiments were performed in normoxia and hypoxia in order to describe the effect of a low oxygen environment on chondrocytes. Finally, we also evaluated if dedifferentiated cells preserve the capacity for mechanosensing.

RESULTS

Chondrocytes showed less proliferating activity when cultured in monolayer in the more compliant substrates. Expression of the cartilage markers Aggrecan (Acan), type II collagen (Col2a1), and Sox9 was upregulated in the less stiff gels (both in monolayer and in 3D culture). Stiffer gels induced an organization of the actin cytoskeleton that correlated with the loss of a chondrocyte phenotype. When cells were cultured in hypoxia, we observed changes in the cellular response that were mediated by HIF-1α. Results in 3D hypoxia cultures were opposite to those found in normoxia, but remained unchanged in monolayer hypoxic experiments. Similar results were found for dedifferentiated cells.

CONCLUSIONS

Chondrocytes respond differently according to the stiffness of the substrate. This response depends greatly on the oxygen environment and on whether the chondrocyte is embedded or grown onto the hydrogel, since mechanosensing capacity was not lost with cell expansion.

Noninvasive real-time monitoring by alamarBlue(®) during in vitro culture of three-dimensional tissue-engineered bone constructs

Bone tissue engineering (TE) aims to develop reproducible and predictive three-dimensional (3D) TE constructs, defined as cell-seeded scaffolds produced by a controlled in vitro process, to heal or replace damaged and nonfunctional bone. To control and assure the quality of the bone TE constructs, a prerequisite for regulatory authorization, there is a need to develop noninvasive analysis techniques to evaluate TE constructs and to monitor their behavior in real time during in vitro culturing. Most analysis techniques, however, are limited to destructive end-point analyses. This study investigates the use of the nontoxic alamarBlue(®) (AB) reagent, which is an indicator for metabolic cell activity, for monitoring the cellularity of 3D TE constructs in vitro as part of a bioreactor culturing processes. Within the field of TE, bioreactors have a huge potential in the translation of TE concepts to the clinic. Hence, the use of the AB reagent was evaluated not only in static cultures, but also in dynamic cultures in a perfusion bioreactor setup. Hereto, the AB assay was successfully integrated in the bioreactor-driven TE construct culture process in a noninvasive way. The obtained results indicate a linear correlation between the overall metabolic activity and the total DNA content of a scaffold upon seeding as well as during the initial stages of cell proliferation. This makes the AB reagent a powerful tool to follow-up bone TE constructs in real-time during static as well as dynamic 3D cultures. Hence, the AB reagent can be successfully used to monitor and predict cell confluence in a growing 3D TE construct.

Oxygenated environment enhances both stem cell survival and osteogenic differentiation

Osteogenesis of mesenchymal stem cells (MSCs) is highly dependent on oxygen supply. We have shown that perfluorotributylamine (PFTBA), a synthetic oxygen carrier, enhances MSC-based bone formation in vivo. Exploring this phenomenon's mechanism, we hypothesize that a transient increase in oxygen levels using PFTBA will affect MSC survival, proliferation, and differentiation, thus increasing bone formation. To test this hypothesis, MSCs overexpressing bone morphogenetic protein 2 were encapsulated in alginate beads that had been supplemented with an emulsion of PFTBA or phosphate-buffered saline. Oxygen measurements showed that supplementation of PFTBA significantly increased the available oxygen level during a 96-h period. PFTBA-containing beads displayed an elevation in cell viability, which was preserved throughout 2 weeks, and a significantly lower ratio of dead cells throughout the experiment. Furthermore, the cells from the control group expressed significantly more hypoxia-related genes such as VEGF, DDIT3, and PKG1. Additionally, PFTBA supplementation led to an increase in the osteogenic differentiation and to a decrease in chondrogenic differentiation of MSCs. In conclusion, PFTBA increases the oxygen availability in the vicinity of the MSCs, which suffer oxygen exhaustion shortly after encapsulation in alginate beads. Consequently, cell survival is increased, and hypoxia-related genes are downregulated. In addition, PFTBA promotes osteogenic differentiation over chondrogeneic differentiation, and thereby can accelerate MSC-based bone regeneration.

A review of the important central role of altered citrate metabolism during the process of stem cell differentiation

Stem cells are highly proliferating cells that have the potential for differentiation leading to the development of specialized functional cell types. The process of stem cell differentiation requires an increase in the recruitment and population of the undifferentiated stem cells, which are then differentiated to specific functional cell types. Genetic/metabolic transformations in the cellular intermediary energy metabolism are required to provide the bioenergetic, synthetic, and catabolic requirements of the stem cells during this process. However, the identification of the intermediary energy metabolism pathways and their alterations during the proliferation and differentiation of stem cells remain largely unknown; mainly due to the lack of attention and/or required research that focuses on this relationship. In the absence of such information, a full understanding of the factors and conditions required to promote stem cell differentiation leading to development of normal functional metabolic specialized cells cannot be achieved. The purpose of this review is to provide the background and bring attention to the essential relationship of altered cellular intermediary metabolism in the context of the process of stem cell proliferation and differentiation. Citrate metabolism is central to the genetic and metabolic transformation leading to the development of the specialized functional cells. This review identifies the involvement of altered citrate metabolism and the associated genetic alterations of key pathways, enzymes, and transporters; as well as the bioenergetic implications. The importance is emphasized for identification and employment of required conditions to insure that the process of experimental stem cell differentiation results in the development of specialized cells that represent the functional metabolic characteristics and capabilities of their native specialized cells. This is an essential requirement for the successful application of stem cell therapy and regenerative medicine for many pathological conditions.

The Important Role of Osteoblasts and Citrate Production in Bone Formation: "Osteoblast Citration" as a New Concept for an Old Relationship

It has been known for about seventy years that bone, in all vertebrates, contains uniquely high citrate levels. However, the role of citrate, its source, its regulation, and its implication in normal bone formation and in bone disorders have remained largely unknown. For the past thirty-five years, the relationship of citrate in bone has been a neglected area of attention and research. It has recently been discovered that citrate is critical for the structure of the apatite nanocrystal, and is required to impart the important properties of bone such as its stability, strength, and resistance to fracture. This brings to focus the need for a renewed interest and research into the relationships of citrate in bone formation. A most fundamental question that must be resolved is "What is the source of citrate in bone?". This presentation provides a historical review of the early research to the present status of citrate implications in bone. This leads to a new concept of the role of osteoblasts as specialized citrate producing cells that provide the source of citrate in bone formation; i.e. the "osteoblast citration" process. This also brings into focus a new insight into the role of zinc in bone in relation to osteoblast citrate production. The genetic/hormonal/metabolic relationships of "net citrate production" are described. The intent of this presentation is to provide the background for a new perspective of the important implications of osteoblasts and citrate in bone formation; which, hopefully, will stimulate a renewed interest and essential research.

Continuous and uninterrupted oxygen tension influences the colony formation and oxidative metabolism of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) are an attractive cell source for tissue engineering applications due to their multipotentiality and increased expansion potential compared to mature cells. However, the full potential of MSCs for cellular therapies is not realised, due, in part, to premature proliferative senescence and impaired differentiation capacity following expansion under 20% oxygen. Bone marrow MSCs reside under reduced oxygen levels (4%-7% oxygen), thus this study investigates the effects of uninterrupted physiological oxygen tensions (2%, 5%) on MSC expansion and subsequent differentiation. Expansion potential was evaluated from colony formation efficiency, population-doubling rates, and cellular senescence. Colony formation was significantly reduced under 5% oxygen compared to 2% and 20% oxygen. Population-doubling time was initially shorter with 20% oxygen, but subsequently no significant differences in doubling time were detected between the oxygen conditions. MSCs expanded with 20% oxygen contained a greater proportion of senescent cells than those under physiological oxygen levels, indicated by a three to fourfold increase in β-galactosidase staining. This may be related to the approximately twofold enhanced mitochondrial oxygen consumption under this culture condition. Chondrogenic differentiation was achieved following expansion at each oxygen condition. However, osteogenesis was only achieved for cells expanded and differentiated at 20% oxygen, indicated by alkaline phosphatase activity and alizarin red staining. These studies demonstrate that uninterrupted hypoxia may enhance long-term MSC expansion, but results in a population with impaired osteogenic differentiation potential. Thus, novel differentiation conditions are required to enable differentiation to nonchondrogenic lineages using hypoxia-cultured MSCs.

Metabolic labeling of human bone marrow mesenchymal stem cells for the quantitative analysis of their chondrogenic differentiation

Human mesenchymal stem cells (hMSCs), residing in bone marrow as well as in the synovial lining of joints, can be triggered to differentiate toward chondrocytes. Thus, hMSCs harbor great therapeutic potential for the repair of cartilage defects in osteoarthritis (OA) and other articular diseases. However, the molecular mechanisms underlying the chondrogenesis process are still in part unknown. In this work, we applied for the first time the stable isotope labeling by amino acids in cell culture (SILAC) technique for the quantitative analysis of protein modulation during the chondrogenic differentiation process of hMSCs. First, we have standardized the metabolic labeling procedure on MSCs isolated from bone marrow (hBMSCs), and we have assessed the quality of chondrogenesis taking place in these conditions. Then, chondrogenic differentiation was induced on these labeled cells, and a quantitative proteomics approach has been followed to evaluate protein changes between two differentiation days. With this strategy, we could identify 622 different proteins by LC-MALDI-TOF/TOF analysis and find 65 proteins whose abundance was significantly modulated between day 2 and day 14 of chondrogenesis. Immunohistochemistry analyses were performed to verify the changes on a panel of six proteins that play different biological roles in the cell: fibronectin, gelsolin, vimentin, alpha-ATPase, mitochondrial superoxide dismutase, and cyclophilin A. All of these proteins were increased at day 14 compared to day 2 of chondrogenic induction, thus being markers of the enhanced extracellular matrix synthesis, cell adhesion, metabolism, and response to stress processes that take place in the early steps of chondrogenesis. Our strategy has allowed an additional insight into both specific protein function and the mechanisms of chondrogenesis and has provided a panel of protein markers of this differentiation process in hBMSCs.

High mesenchymal stem cell seeding densities in hyaluronic acid hydrogels produce engineered cartilage with native tissue properties

Engineered cartilage based on adult mesenchymal stem cells (MSCs) is an alluring goal for the repair of articular defects. However, efforts to date have failed to generate constructs with sufficient mechanical properties to function in the demanding environment of the joint. Our findings with a novel photocrosslinked hyaluronic acid (HA) hydrogel suggest that stiff gels (high HA concentration, 5% w/v) foster chondrogenic differentiation and matrix production, but limit overall functional maturation due to the inability of the formed matrix to diffuse away from the point of production and form a contiguous network. In the current study, we hypothesized that increasing the MSC seeding density would decrease the required diffusional distance, and so expedite the development of functional properties. To test this hypothesis bovine MSCs were encapsulated at seeding densities of either 20,000,000 or 60,000,000 cells ml(-1) in 1%, 3%, and 5% (w/v) HA hydrogels. Counter to our hypothesis the higher concentration HA gels (3% and 5%) did not develop more rapidly with increased MSC seeding density. However, the biomechanical properties of the low concentration (1%) HA constructs increased markedly (nearly 3-fold with a 3-fold increase in seeding density). To ensure that optimal nutrient access was delivered, we next cultured these constructs under dynamic culture conditions (with orbital shaking) for 9 weeks. Under these conditions 1% HA seeded at 60,000,000 MSCs ml(-1) reached a compressive modulus in excess of 1 MPa (compared with 0.3-0.4 MPa for free swelling constructs). This is the highest level we have reported to date in this HA hydrogel system, and represents a significant advance towards functional stem cell-based tissue engineered cartilage.

Substrate stiffness and oxygen as regulators of stem cell differentiation during skeletal tissue regeneration: a mechanobiological model

Extrinsic mechanical signals have been implicated as key regulators of mesenchymal stem cell (MSC) differentiation. It has been possible to test different hypotheses for mechano-regulated MSC differentiation by attempting to simulate regenerative events such as bone fracture repair, where repeatable spatial and temporal patterns of tissue differentiation occur. More recently, in vitro studies have identified other environmental cues such as substrate stiffness and oxygen tension as key regulators of MSC differentiation; however it remains unclear if and how such cues determine stem cell fate in vivo. As part of this study, a computational model was developed to test the hypothesis that substrate stiffness and oxygen tension regulate stem cell differentiation during fracture healing. Rather than assuming mechanical signals act directly on stem cells to determine their differentiation pathway, it is postulated that they act indirectly to regulate angiogenesis and hence partially determine the local oxygen environment within a regenerating tissue. Chondrogenesis of MSCs was hypothesized to occur in low oxygen regions, while in well vascularised regions of the regenerating tissue a soft local substrate was hypothesised to facilitate adipogenesis while a stiff substrate facilitated osteogenesis. Predictions from the model were compared to both experimental data and to predictions of a well established computational mechanobiological model where tissue differentiation is assumed to be regulated directly by the local mechanical environment. The model predicted all the major events of fracture repair, including cartilaginous bridging, endosteal and periosteal bony bridging and bone remodelling. It therefore provides support for the hypothesis that substrate stiffness and oxygen play a key role in regulating MSC fate during regenerative events such as fracture healing.

Regulation of the cell cycle via mitochondrial gene expression and energy metabolism in HeLa cells

Human cervical cancer HeLa cells have functional mitochondria. Recent studies have suggested that mitochondrial metabolism plays an essential role in tumor cell proliferation. Nevertheless, how cells coordinate mitochondrial dynamics and cell cycle progression remains to be clarified. To investigate the relationship between mitochondrial function and cell cycle regulation, the mitochondrial gene expression profile and cellular ATP levels were determined by cell cycle progress analysis in the present study. HeLa cells were synchronized in the G0/G1 phase by serum starvation, and re-entered cell cycle by restoring serum culture, time course experiment was performed to analyze the expression of mitochondrial transcription regulators and mitochondrial genes, mitochondrial membrane potential (MMP), cellular ATP levels, and cell cycle progression. The results showed that when arrested G0/G1 cells were stimulated in serum-containing medium, the amount of DNA and the expression levels of both mRNA and proteins in mitochondria started to increase at 2 h time point, whereas the MMP and ATP level elevated at 4 h. Furthermore, the cyclin D1 expression began to increase at 4 h after serum triggered cell cycle. ATP synthesis inhibitor-oligomycin-treatment suppressed the cyclin D1 and cyclin B1 expression levels and blocked cell cycle progression. Taken together, our results suggested that increased mitochondrial gene expression levels, oxidative phosphorylation activation, and cellular ATP content increase are important events for triggering cell cycle. Finally, we demonstrated that mitochondrial gene expression levels and cellular ATP content are tightly regulated and might play a central role in regulating cell proliferation.

Transient exposure to TGF-β3 improves the functional chondrogenesis of MSC-laden hyaluronic acid hydrogels

Tissue engineering with adult stem cells is a promising approach for the restoration of focal defects in articular cartilage. For this, progenitor cells would ideally be delivered to (and maintained within) the defect site via a biocompatible material and in combination with soluble factors to promote initial cell differentiation and subsequent tissue maturation in vivo. While growth factor delivery methods are continually being optimized, most offer only a short (days to weeks) delivery profile at high doses. To address this issue, we investigated mesenchymal stem cell (MSC) differentiation and maturation in photocrosslinkable hyaluronic acid (HA) hydrogels with transient exposure to the pro-chondrogenic molecule transforming growth factor-beta3 (TGF-β3), at varying doses (10, 50 and 100 ng/mL) and durations (3, 7, 21 and 63 days). Mechanical, biochemical, and histological outcomes were evaluated through 9 weeks of culture. Results showed that a brief exposure (7 days) to a very high level (100 ng/mL) of TGF-β3 was sufficient to both induce and maintain cartilage formation in these 3D constructs. Indeed, this short delivery resulted in constructs with mechanical and biochemical properties that exceeded that of continuous exposure to a lower level (10 ng/mL) of TGF-β3 over the entire 9-week time course. Of important note, the total TGF delivery in these two scenarios was roughly equivalent (200 vs. 180 ng), but the timing of delivery differed markedly. These data support the idea that acute exposure to a high dose of TGF will induce functional and long-term differentiation of stem cell populations, and further our efforts to improve cartilage repair in vivo.

Mechanisms underlying the osteo- and adipo-differentiation of human mesenchymal stem cells

Human mesenchymal stem cells (hMSCs) are considered a promising cell source for regenerative medicine, because they have the potential to differentiate into a variety of lineages among which the mesoderm-derived lineages such adipo- or osteogenesis are investigated best. Human MSCs can be harvested in reasonable to large amounts from several parts of the patient's body and due to this possible autologous origin, allorecognition can be avoided. In addition, even in allogenic origin-derived donor cells, hMSCs generate a local immunosuppressive microenvironment, causing only a weak immune reaction. There is an increasing need for bone replacement in patients from all ages, due to a variety of reasons such as a new recreational behavior in young adults or age-related diseases. Adipogenic differentiation is another interesting lineage, because fat tissue is considered to be a major factor triggering atherosclerosis that ultimately leads to cardiovascular diseases, the main cause of death in industrialized countries. However, understanding the differentiation process in detail is obligatory to achieve a tight control of the process for future clinical applications to avoid undesired side effects. In this review, the current findings for adipo- and osteo-differentiation are summarized together with a brief statement on first clinical trials.