Decreased oxygen tension lowers reactive oxygen species and apoptosis and inhibits osteoblast matrix mineralization through changes in early osteoblast differentiation

Synovial Membrane Is a Major Producer of Extracellular Inorganic Pyrophosphate in Response to Hypoxia

Calcium pyrophosphate dehydrate (CPPD) crystals are found in the synovial fluid of patients with articular chondrocalcinosis or sometimes with osteoarthritis. In inflammatory conditions, the synovial membrane (SM) is subjected to transient hypoxia, especially during movement. CPPD formation is supported by an increase in extracellular inorganic pyrophosphate (ePPi) levels, which are mainly controlled by the transporter Ank and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). We demonstrated previously that transforming growth factor (TGF)-β1 increased ePPi production by inducing Ank and Enpp1 expression in chondrocytes. As the TGF-β1 level raises in synovial fluid under hypoxic conditions, we investigated whether hypoxia may transform SM as a major source of ePPi production. Synovial fibroblasts and SM explants were exposed to 10 ng/mL of TGF-β1 in normoxic or hypoxic (5% O2) culture conditions. Ank and Enpp1 expression were assessed by quantitative PCR, Western blot and immunohistochemistry. ePPi was quantified in culture supernatants. RNA silencing was used to define the respective roles of Ank and Enpp1 in TGF-β1-induced ePPi generation. The molecular mechanisms involved in hypoxia were investigated using an Ank promoter reporter plasmid for transactivation studies, as well as gene overexpression and RNA silencing, the respective role of hypoxia-induced factor (HIF)-1 and HIF-2. Our results showed that TGF-β1 increased Ank, Enpp1, and therefore ePPi production in synovial fibroblasts and SM explants. Ank was the major contributor in ePPi production compared to ENPP1. Hypoxia increased ePPi levels on its own and enhanced the stimulating effect of TGF-β1. Hypoxic conditions enhanced Ank promoter transactivation in an HIF-1-dependent/HIF-2-independent fashion. We demonstrated that under hypoxia, SM is an important contributor to ePPi production in the joint through the induction of Enpp1 and Ank. These findings are of interest as a rationale for the beneficial effect of anti-inflammatory drugs on SM in crystal depositions.

Gene Expression Analyses in Models of Rosiglitazone-Induced Physiological and Pathological Mineralization Identify Novel Targets to Improve Bone and Vascular Health

Clinical studies revealed detrimental skeletal and vascular effects of the insulin sensitizer rosiglitazone. We have shown earlier that rosiglitazone accelerates osteoblast differentiation from human mesenchymal stem cells (hMSC) at the expense of increased oxidative stress and cell death. In calcifying human vascular cells, rosiglitazone stimulates pathological mineralization, an effect diminished by the antioxidant resveratrol. Here, we aimed to elucidate transcriptional networks underlying the rosiglitazone-enhanced mineralization phenotype. We performed genome-wide transcriptional profiling of osteogenic hMSCs treated with rosiglitazone for short-term periods of 1 up to 48 h during the first two days of differentiation, a phase that we show is sufficient for rosiglitazone stimulation of mineralization. Microarray-based mRNA expression analysis revealed 190 probes that were differently expressed in at least one condition compared to vehicle-treated control. This rosiglitazone gene signature contained well-known primary PPAR targets and was also endogenously regulated during osteogenic hMSC differentiation and osteoblast-like differentiation of vascular smooth muscle cells (VSMCs) into calcifying vascular cells (CVCs). Comparative analysis revealed rosiglitazone targets that were commonly enriched in osteoblasts and CVCs or specifically enriched in either osteoblasts or CVCs. Finally, we compared expression patterns of CVC-specific genes with patient expression data from carotid plaque versus intact adjacent tissue, and identified five rosiglitazone targets to be differentially regulated in CVCs and carotid plaque but not osteoblasts when compared to their non-mineralizing counterparts. These targets, i.e., PDK4, SDC4, SPRY4, TCF4 and DACT1, may specifically control extracellular matrix mineralization in vascular cells, and hence provide target candidates for further investigations to improve vascular health.

Extracellular Vesicles Derived from Osteogenic-Differentiated Human Bone Marrow-Derived Mesenchymal Cells Rescue Osteogenic Ability of Bone Marrow-Derived Mesenchymal Cells Impaired by Hypoxia

In orthopedics, musculoskeletal disorders, i.e., non-union of bone fractures or osteoporosis, can have common histories and symptoms related to pathological hypoxic conditions induced by aging, trauma or metabolic disorders. Here, we observed that hypoxic conditions (2% O2) suppressed the osteogenic differentiation of human bone marrow-derived mesenchymal cells (hBMSC) in vitro and simultaneously increased reactive oxygen species (ROS) production. We assumed that cellular origin and cargo of extracellular vesicles (EVs) affect the osteogenic differentiation capacity of hBMSCs cultured under different oxygen pressures. Proteomic analysis revealed that EVs isolated from osteogenic differentiated hBMSC cultured under hypoxia (hypo-osteo EVs) or under normoxia (norm-osteo EVs) contained distinct protein profiles. Extracellular matrix (ECM) components, antioxidants and pro-osteogenic proteins were decreased in hypo-osteo EVs. The proteomic analysis in our previous study revealed that under normoxic culture conditions, pro-osteogenic proteins and ECM components have higher concentrations in norm-osteo EVs than in EVs derived from naïve hBMSCs (norm-naïve EVs). When selected for further analysis, five anti-hypoxic proteins were significantly upregulated (response to hypoxia) in norm-osteo EVs. Three of them are characterized as antioxidant proteins. We performed qRT-PCR to verify the corresponding gene expression levels in the norm-osteo EVs' and norm-naïve EVs' parent cells cultured under normoxia. Moreover, we observed that norm-osteo EVs rescued the osteogenic ability of naïve hBMSCs cultured under hypoxia and reduced hypoxia-induced elevation of ROS production in osteogenic differentiated hBMSCs, presumably by inducing expression of anti-hypoxic/ antioxidant and pro-osteogenic genes.

Hypoxia mimetics restore bone biomineralisation in hyperglycaemic environments

Diabetic patients have an increased risk of fracture and an increased occurrence of impaired fracture healing. Diabetic and hyperglycaemic conditions have been shown to impair the cellular response to hypoxia, via an inhibited hypoxia inducible factor (HIF)-1α pathway. We investigated, using an in vitro hyperglycaemia bone tissue engineering model (and a multidisciplinary bone characterisation approach), the differing effects of glucose levels, hypoxia and chemicals known to stabilise HIF-1α (CoCl₂ and DMOG) on bone formation. Hypoxia (1% O₂) inhibited bone nodule formation and resulted in discrete biomineralisation as opposed to the mineralised extracellular collagen fibres found in normoxia (20% O₂). Unlike hypoxia, the use of hypoxia mimetics did not prevent nodule formation in normal glucose level. Hyperglycaemic conditions (25 mM and 50 mM glucose) inhibited biomineralisation. Interestingly, both hypoxia mimetics (CoCl₂ and DMOG) partly restored hyperglycaemia inhibited bone nodule formation. These results highlight the difference in osteoblast responses between hypoxia mimetics and actual hypoxia and suggests a role of HIF-1α stabilisation in bone biomineralisation that extends that of promoting neovascularisation, or other system effects associated with hypoxia and bone regeneration in vivo. This study demonstrates that targeting the HIF pathway may represent a promising strategy for bone regeneration in diabetic patients.

Long non‑coding RNA XIST inhibits osteoblast differentiation and promotes osteoporosis via Nrf2 hyperactivation by targeting CUL3

Osteoporosis (OP) is a common skeletal disorder characterized by a low bone mass and the deterioration of bone structure. Long non‑coding (lnc)RNA X inactive‑specific transcript (XIST) is highly expressed in the serum and monocytes of patients with OP. Thus, the purpose of the present study was to explore the mechanisms underlying the role of XIST in the progression of OP. To establish animal models of OP, female rats underwent a bilateral ovariectomy. The bone mineral density of individual rats was measured using dual‑energy X‑ray absorptiometry. The combination of XIST and cullin‑3 (CUL3) was analyzed using a dual‑luciferase reporter assay. Bone histopathological changes were assessed by hematoxylin and eosin staining. Alkaline phosphatase activity was examined by ALP staining. Finally, a series of functional experiments were performed to examine the effects of XIST on cellular behaviors. In the present study, XIST promoted OP and inhibited bone formation by regulating the expression levels of CUL3 and nuclear factor erythroid 2‑related factor 2 (Nrf2) in the rats with OP. Moreover, XIST directly targeted CUL3 and negatively regulated its expression. Of note, CUL3 downregulation reversed the effects of XIST silencing on cell viability, differentiation and mineralization, as well as the expression of Nrf2 and CUL3 in MC3T3‑E1 cells. Collectively, XIST was demonstrated to inhibit the differentiation of osteoblasts and promote OP by inhibiting the degradation of Nrf2 via targeting CUL3.

Superoxide dismutase 3 is expressed in bone tissue and required for normal bone homeostasis and mineralization

Superoxide dismutase 3 (SOD3) is an extracellular protein with the capacity to convert superoxide into hydrogen peroxide, an important secondary messenger in redox regulation. To investigate the utility of zebrafish in functional studies of SOD3 and its relevance for redox regulation, we have characterized the zebrafish orthologues; Sod3a and Sod3b. Our analyses show that both recombinant Sod3a and Sod3b express SOD activity, however, only Sod3b is able to bind heparin. Furthermore, RT-PCR analyses reveal that sod3a and sod3b are expressed in zebrafish embryos and are present primarily in separate organs in adult zebrafish, suggesting distinct functions in vivo. Surprisingly, both RT-PCR and whole mount in situ hybridization showed specific expression of sod3b in skeletal tissue. To further investigate this observation, we compared femoral bone obtained from wild-type and SOD3^-/- mice to determine whether a functional difference was apparent in healthy adult mice. Here we report, that bone from SOD3^-/- mice is less mineralized and characterized by significant reduction of cortical and trabecular thickness in addition to reduced mechanical strength. These analyses show that SOD3 plays a hitherto unappreciated role in bone development and homeostasis.

Superoxide Dismutase 2 (SOD2) in Vascular Calcification: A Focus on Vascular Smooth Muscle Cells, Calcification Pathogenesis, and Therapeutic Strategies

Vascular calcification (VC) describes the pathophysiological phenotype of calcium apatite deposition within the vascular wall, leading to vascular stiffening and the loss of compliance. VC is never benign; the presence and severity of VC correlate closely with the risk of myocardial events and cardiovascular mortality in multiple at-risk populations such as patients with diabetes and chronic kidney disease. Mitochondrial dysfunction involving each of vascular wall constituents (endothelia and vascular smooth muscle cells (VSMCs)) aggravates various vascular pathologies, including atherosclerosis and VC. However, few studies address the pathogenic role of mitochondrial dysfunction during the course of VC, and mitochondrial reactive oxygen species (ROS) seem to lie in the pathophysiologic epicenter. Superoxide dismutase 2 (SOD2), through its preferential localization to the mitochondria, stands at the forefront against mitochondrial ROS in VSMCs and thus potentially modifies the probability of VC initiation or progression. In this review, we will provide a literature-based summary regarding the relationship between SOD2 and VC in the context of VSMCs. Apart from the conventional wisdom of attenuating mitochondrial ROS, SOD2 has been found to affect mitophagy and the formation of the autophagosome, suppress JAK/STAT as well as PI₃K/Akt signaling, and retard vascular senescence, all of which underlie the beneficial influences on VC exerted by SOD2. More importantly, we outline the therapeutic potential of a novel SOD2-targeted strategy for the treatment of VC, including an ever-expanding list of pharmaceuticals and natural compounds. It is expected that VSMC SOD2 will become an important druggable target for treating VC in the future.

Effects of normobaric cyclic hypoxia exposure on mesenchymal stem-cell differentiation-pilot study on bone parameters in elderly

BACKGROUND

Mesenchymal stem cells (MSC) of bone marrow are the progenitor of osteoblasts and adipocytes. MSC tend to differentiate into adipocytes, instead of osteoblasts, with aging. This favors the loss of bone mass and development of osteoporosis. Hypoxia induces hypoxia inducible factor 1α gene encoding transcription factor, which regulates the expression of genes related to energy metabolism and angiogenesis. That allows a better adaptation to low O2 conditions. Sustained hypoxia has negative effects on bone metabolism, favoring bone resorption. Yet, surprisingly, cyclic hypoxia (CH), short times of hypoxia followed by long times in normoxia, can modulate MSC differentiation and improve bone health in aging.

AIM

To evaluate the CH effect on MSC differentiation, and whether it improves bone mineral density in elderly.

METHODS

MSC cultures were induced to differentiate into osteoblasts or adipocytes, in CH (3% O2 for 1, 2 or 4 h, 4 d a week). Extracellular-matrix mineralization and lipid-droplet formation were studied in MSC induced to differentiate into osteoblast or adipocytes, respectively. In addition, gene expression of marker genes, for osteogenesis or adipogenesis, have been quantified by quantitative real time polymerase chain reaction. The in vivo studies with elderly (> 75 years old; n = 10) were carried out in a hypoxia chamber, simulating an altitude of 2500 m above sea level, or in normoxia, for 18 wk (36 CH sessions of 16 min each). Percentages of fat mass and bone mineral density from whole body, trunk and right proximal femur (femoral, femoral neck and trochanter) were assessed, using dual-energy X-ray absorptiometry.

RESULTS

CH (4 h of hypoxic exposure) inhibited extracellular matrix mineralization and lipid-droplet formation in MSC induced to differentiate into osteoblasts or adipocytes, respectively. However, both parameters were not significantly affected by the other shorter hypoxia times assessed. The longest periods of hypoxia downregulated the expression of genes related to extracellular matrix formation, in MSC induced to differentiate into osteoblasts. Interestingly, osteocalcin (associated to energy metabolism) was upregulated. Vascular endothelial growth factor an expression and low-density lipoprotein receptor related protein 5/6/dickkopf Wnt signaling pathway inhibitor 1 (associated to Wnt/β-catenin pathway activation) increased in osteoblasts. Yet, they decreased in adipocytes after CH treatments, mainly with the longest hypoxia times. However, the same CH treatments increased the osteoprotegerin/receptor activator for nuclear factor kappa B ligand ratio in both cell types. An increase in total bone mineral density was observed in elderly people exposed to CH, but not in specific regions. The percentage of fat did not vary between groups.

CONCLUSION

CH may have positive effects on bone health in the elderly, due to its possible inhibitory effect on bone resorption, by increasing the osteoprotegerin / receptor activator for nuclear factor kappa B ligand ratio.

In vitro temporal HIF-mediated deposition of osteochondrogenic matrix governed by hypoxia and osteogenic factors synergy

Musculoskeletal interfaces are naturally hypoxic. An understanding of key interactions occurring between different cell populations and their environment is critical for native tissue recapitulation. Here, an enthesis coculture model (preosteoblasts and tendon cells) was used to understand the influence of hypoxia (5% O₂ ) and osteogenic medium (OM) supplementation in cells' phenotype modulation. In single cultures, preosteoblasts were found to undergo osteogenic impairment, while tendon cells underwent a maturation process through extracellular matrix (ECM) rescue. When in co-culture, hypoxia and osteoinduction promoted a temporal chondro/osteogenic pathway activation, as observed by an early deposition of cartilaginous ECM associated with HIF1A stabilization and RUNX2 activation, and later hypertrophic differentiation resulting from HIF2A translocation and SOX9 activation. Moreover, the presence of OM under hypoxia was shown to influence the extracellular ROS/HIF1A interplay. Overall, this study revealed a link between biochemical factors and cell-cell crosstalk, providing a molecular framework for hypoxic control and modulation of cells' fate toward enthesis-like phenotypes.

A New Non-invasive Technique for Measuring 3D-Oxygen Gradients in Wells During Mammalian Cell Culture

Oxygen tension plays an important role in overall cell function and fate, regulating gene expression, and cell differentiation. Although there is extensive literature available that supports the previous statement, little information is to be found about accurate O2 measurements during culture. In fact, O2 concentration at the cell layer during culture is commonly assumed to be equal to that of the incubator atmosphere. This assumption does not consider oxygen diffusion properties, cell type, cell density, media composition, time in culture nor height of the cell culture medium column. In this study, we developed a non-invasive, optical sensor foil-based technique suitable for measuring the 3D oxygen gradient that is formed during cell culture as a result of normal cell respiration. For this propose, we created a 3D printed ramp to which surface an oxygen optode sensor foil was attached. The ramps were positioned inside the culture wells of 24 well plate prior cell seeding. This set up in conjunction with the VisiSens TD camera system allows to investigate the oxygen gradient formation during culture. Cultivation was performed with three different initial cell densities of the cell line A549 that were seeded on the plate containing the ramps with the oxygen sensors. The O2 gradient obtained after 96 h of culture showed significantly lower O2 concentrations closer to the bottom of the well in high cell density cultures compared to that of lower cell density cultures. Furthermore, it was very interesting to observe that even with low cell density culture, oxygen concentration near the cell layer was lower than that of the incubator atmosphere. The obtained oxygen gradient after 96 h was used to calculate the oxygen consumption rate (OCR) of the A549 cells, and the obtained value of ~100 fmol/h/cell matches the OCR value already reported in the literature for this cell line. Moreover, we found our set up to be unique in its ability to measure oxygen gradient formation in several wells of a cell culture plate simultaneously and in a non-invasive manner.

The essential role of osteoclast-derived exosomes in magnetic nanoparticle-infiltrated hydroxyapatite scaffold modulated osteoblast proliferation in an osteoporosis model

Magnetic hydroxyapatite (MHA) scaffolds promoted osteoblast proliferation in a model of osteoporosis through altering the osteoclast-derived exosomal cargo and decreasing the efficiency of exosome uptake by osteoblasts. Noticeably, certain proteins including ubiquitin, ATP and reactive oxygen species decreased in the osteoclast-derived exosomal cargo with MHA stimulation, while Rho kinase increased.

Immediate hyperbaric oxygen after tooth extraction ameliorates bisphosphonate-related osteonecrotic lesion in rats

BACKGROUND

This study aims to assess whether hyperbaric oxygen (HBO) applied immediately after tooth extraction could ameliorate medication-related osteonecrosis of the jaw in rats.

METHODS

To evaluate whether osteonecrosis could be successfully induced, healing of extraction maxillary molars was examined in 40 female Sprague Dawley rats received zoledronic acid (7.5 µg/kg) plus dexamethasone (1 mg/kg). Rats were divided into four groups, receiving zero, two, four, or seven injection(s) for 7 days, respectively. Effect of HBO, pressurized to 2.5 atmospheres absolute (ATA) at rate of 0.15 ATA/min with 100% oxygen for 90 minutes, applied immediately after tooth extraction, on the development of osteonecrosis was evaluated. Lesions among groups were compared by size of ulceration, exact area (mm² ) or relative area (%), and by histology.

RESULTS

Unhealed ridge was observed in all nine rats in four and seven injection groups, but none of 10 rats in the control (non-injection) group. Immediate HBO significantly reduced the lesions in rats that received four injections, regardless of the distribution and the total/relative areas of lesions (P <0.01). Histological findings showed the lesions were uncovered epithelium and severe tissue inflammation.

CONCLUSION

This is the first in vivo study demonstrating the HBO applied immediately after tooth extraction effectively decreases the development of medication-related osteonecrosis.

Peroxiredoxin II negatively regulates BMP2-induced osteoblast differentiation and bone formation via PP2A Cα-mediated Smad1/5/9 dephosphorylation

Peroxiredoxin II (Prx II), an antioxidant enzyme in the Prx family, reduces oxidative stress by decreasing the intracellular ROS levels. Osteoblast differentiation is promoted by bone morphogenetic protein 2 (BMP2), which upregulates the expression of osteoblast differentiation marker genes, through Smad1/5/9 phosphorylation. We found that Prx II expression was increased by a high dose of lipopolysaccharide (LPS) but was not increased by a low dose of LPS. Prx II itself caused a decrease in the osteogenic gene expression, alkaline phosphatase (ALP) activity, and Smad1/5/9 phosphorylation induced by BMP2. In addition, BMP2-induced osteogenic gene expression and ALP activity were higher in Prx II knockout (KO) cells than they were in wild-type (WT) cells. These inhibitory effects were mediated by protein phosphatase 2A Cα (PP2A Cα), which was increased and is known to induce the dephosphorylation of Smad1/5/9. The overexpression of Prx II increased the expression of PP2A Cα, and PP2A Cα was not expressed in Prx II KO cells. Moreover, PP2A Cα reduced the level of BMP2-induced osteogenic gene expression and Smad1/5/9 phosphorylation. LPS inhibited BMP2-induced Smad1/5/9 phosphorylation and the suppressed phosphorylation was restored by the PP2A inhibitor okadaic acid (OA). Bone phenotype analyses using microcomputed tomography (μCT) revealed that the Prx II KO mice had higher levels of bone mass than the levels of the WT mice. We hypothesize that Prx II has a negative role in osteoblast differentiation through the PP2A-dependent dephosphorylation of Smad1/5/9.

An antioxidant enzyme actively works to reduce bone synthesis under oxidative stress conditions in order to protect bone cells from damage and cell death. Bone is generated by cells called osteoblasts, which differentiate from stem cells. In osteoporosis and diabetes, excessive reactive oxygen species (ROS) within cells can disrupt osteoblast differentiation. South Korean researchers led by Eun-jung Kim at Kyungpook National University, Daegu, and Won-Gu Jang at Daegu University, Gyeongbuk, have shown that an antioxidant enzyme, peroxiredoxin II (PrxII), helps regulate bone formation under oxidative stress. The team generated PrxII-deficient mice and compared them with healthy normal mice. Under oxidative stress conditions, the mice had higher bone mass and higher expression of genes related to bone formation than the normal mice. PrxII limits osteoblast differentiation during elevated ROS by influencing associated protein activity and signalling pathways.

Culture surfaces induce hypoxia-regulated genes in human mesenchymal stromal cells

Culturing human Mesenchymal stromal cells (hMSCs) in vitro in hypoxic conditions resulted in reduced senescence, enhanced pluripotency and altered proliferation rate. It has been known that in vitro hypoxia affects expression of cell surface proteins. However, the impact of culture surfaces on the hypoxia-regulated genes (HRG) have not yet been reported. This study utilized Next-Generation sequencing to analyse the changes in the gene expression levels of HRG for hMSCs cultured on different culture surfaces. The samples, which were cultured on four different synthesized surfaces (treatments) and tissue culture plate (control), resulted in a difference in growth rate. The sequencing results revealed that the transcription of a number of key genes involved in regulating hypoxic functions were significantly altered, including HIF2A, a marker for potency, differentiation, and various cellular functions. Significant alternations in the expression levels of previously reported oxygen-sensitive surface proteins were detected in this study, some of which closely correlate with the expression levels of HIF2A. Our analysis of the hMSCs transcriptome and HRG mapped out a list of genes encoding surface proteins which may directly regulate or be regulated by HIF2A. The findings from this study showed that culture surfaces have an impact on regulating the expression profile of HRG. Therefore, novel culture surfaces may be designed to selectively activate HIF2A and other HRG and pathways under in vitro normoxia. The understanding of the crosstalk between the regulating genes of hypoxia and culture surfaces may be utilized to strengthen desired hypoxic functions.

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

Mathematical and computational models for bone tissue engineering in bioreactor systems

Research into cellular engineered bone grafts offers a promising solution to problems associated with the currently used auto- and allografts. Bioreactor systems can facilitate the development of functional cellular bone grafts by augmenting mass transport through media convection and shear flow-induced mechanical stimulation. Developing successful and reproducible protocols for growing bone tissue in vitro is dependent on tuning the bioreactor operating conditions to the specific cell type and graft design. This process, largely reliant on a trial-and-error approach, is challenging, time-consuming and expensive. Modelling can streamline the process by providing further insight into the effect of the bioreactor environment on the cell culture, and by identifying a beneficial range of operational settings to stimulate tissue production. Models can explore the impact of changing flow speeds, scaffold properties, and nutrient and growth factor concentrations. Aiming to act as an introductory reference for bone tissue engineers looking to direct their experimental work, this article presents a comprehensive framework of mathematical models on various aspects of bioreactor bone cultures and overviews modelling case studies from literature.

Mathematical modelling of interacting mechanisms for hypoxia mediated cell cycle commitment for mesenchymal stromal cells

Background

Existing experimental data have shown hypoxia to be an important factor affecting the proliferation of mesenchymal stromal cells (MSCs), but the contrasting observations made at various hypoxic levels raise the questions of whether hypoxia accelerates proliferation, and how. On the other hand, in order to meet the increasing demand of MSCs, an optimised bioreactor control strategy is needed to enhance in vitro production.

Results

A comprehensive, single-cell mathematical model has been constructed in this work, which combines cellular oxygen sensing with hypoxia-mediated cell cycle progression to predict cell cycle commitment as a proxy to proliferation rate. With oxygen levels defined for in vitro cell culture, the model predicts enhanced proliferation under intermediate (2–8%) and mild (8–15%) hypoxia and cell quiescence under severe (< 2%) hypoxia. Global sensitivity analysis and quasi-Monte Carlo simulation revealed that within a certain range (+/− 100%), model parameters affect (with varying significance) the minimum commitment time, but the existence of a range of optimal oxygen tension could be preserved with the hypothesized effects of Hif2α and reactive oxygen species (ROS). It appears that Hif2α counteracts Hif1α and ROS-mediated protein deactivation under intermediate hypoxia and normoxia (20%), respectively, to regulate the response of cell cycle commitment to oxygen tension.

Conclusion

Overall, this modelling study offered an integrative framework to capture several interacting mechanisms and allowed in silico analysis of their individual and collective roles in shaping the hypoxia-mediated commitment to cell cycle. The model offers a starting point to the establishment of a suitable mechanism that can satisfactorily explain the different existing experimental observations from different studies, and warrants future extension and dedicated experimental validation to eventually support bioreactor optimisation.

Electronic supplementary material

The online version of this article (10.1186/s12918-018-0560-3) contains supplementary material, which is available to authorized users.

Spontaneous up-regulation of SIRT1 during osteogenesis contributes to stem cells' resistance to oxidative stress

Osteogenic differentiation of bone marrow-derived mesenchymal stem cells (BM-MSCs) is a central event in bone formation. However, oxidative stress has a deleterious impact on BM-MSC osteogenesis. In this study, we hypothesized that oxidative stress influenced BM-MSC osteogenesis differently in the early or late stages, in which silent information regulator type 1 (SIRT1) played a critical role. A continuous exposure to sublethal concentrations of hydrogen peroxide (H₂ O₂ ), ranging from 25 to 100 µM for 21 days, resulted in the complete inhibition of BM-MSC osteogenesis. We found that a 7-day treatment with H₂ O₂ inhibited the lineage commitment of BM-MSCs toward osteoblasts, as evidenced by a significant reduction of alkaline phosphatase activity (a typical marker for early osteogenesis). However, moderate oxidative stress did not affect late-differentiated BM-MSCs, as there were comparable levels of matrix mineralization (a typical marker for late osteogenesis). In addition, we observed a spontaneous up-regulation of SIRT1 and intracellular antioxidant enzymes such as superoxide dismutase 2, catalase, and glutathione peroxidase 1, which accounted for the enhanced resistance to oxidative stress upon osteogenic differentiation. Activation of SIRT1 by resveratrol rescued the effect of H₂ O₂ on early-differentiated BM-MSCs and inhibition of SIRT1 by nicotinamide intensified the effect of H₂ O₂ on late-differentiated BM-MSCs, indicating that the SIRT1-mediated pathway was actively involved in MSC osteogenesis and antioxidant mechanisms. Our findings uncovered the relationship between SIRT1 and resistance to H₂ O₂ -induced oxidative stress during BM-MSC osteogenesis, which could provide a new strategy for protecting MSCs from extracellular oxidative stress.

Oxygen-distribution within 3-D collagen I hydrogels for bone tissue engineering

Tissue engineering (TE) approaches typically envisage the structural and functional reconstitution of previously damaged tissue in situ. An adequate three-dimensional environment is therefore of fundamental importance for the designated cells associated to the scaffold material. The sufficient supply with nutrients and oxygen in vitro and in vivo mark thereby critical challenges of TE. In this study, we intended to analyse the level of locally dissolved oxygen within 3-D cell-loaded collagen I gels in vitro. For the analysis of the oxygen levels in situ, we employed an optical fibre-based micro sensor setup, as well as a camera supported non-invasive optical sensor foil based technique. These complementary analytical tools enable the identification, localization, and temporal follow-up investigation of specified regions of interest within TE constructs. Human adipose-derived mesenchymal stem cells (hAdMSCs) cultured in collagen I gels under normoxic conditions were analysed periodically and kinetically up to 70 days - thereby revealing dynamic changes of the level of dissolved oxygen inside the gel constructs. Dependent on the applied cell concentration, the in vitro oxygen concentration (cO₂) within the gels reached physiological ranges (7-9%) after 21 days, or 35 days of culture. The minimal cO₂ was measured after 35 days in vitro, featuring an oxygen level of 4.8 ± 1.3%. Upon prolonged culture, a plateau-like status of the cO₂ around 8-9% established, indicating a change in the physiological activity of the cells under investigation. The expression patterns of BCL2, CASP3 and MCM5 revealed significant differences among the proliferative and apoptotic stages of the cell-loaded samples at the investigated time points of 7 and 70 days in culture. In summary, these data show the temporary dynamic nature of the oxygen distribution in cell-loaded gel constructs. The applied technique is an ideal tool for the evaluation of multiple parameters affecting the oxygen distribution in vitro. We conclude that it takes 5 weeks for establishing an equilibrium of cO₂. Levels reached in a 3-D gel construct are comparable with physiological oxygenation ranges in bone-associated tissues.

Osteogenic Differentiation of Periosteal Cells During Fracture Healing

Five to ten percent of fractures fail to heal normally leading to additional surgery, morbidity, and altered quality of life. Fracture healing involves the coordinated action of stem cells primarily coming from the periosteum which differentiate into the chondrocytes and osteoblasts, forming first the soft (cartilage) callus followed by the hard (bone) callus. These stem cells are accompanied by a vascular invasion that appears critical for the differentiation process and which may enable the entry of osteoclasts necessary for the remodeling of the callus into mature bone. However, more research is needed to clarify the signaling events that activate the osteochondroprogenitor cells of periosteum and stimulate their differentiation into chondrocytes and osteoblasts. Ultimately a thorough understanding of the mechanisms for differential regulation of these osteochondroprogenitors will aid in the treatment of bone healing and the prevention of delayed union and nonunion of fractures. In this review, evidence supporting the concept that the periosteal cells are the major cell sources of skeletal progenitors for the fracture callus will be discussed. The osteogenic differentiation of periosteal cells manipulated by Wnt/β-catenin, TGF/BMP, Ihh/PTHrP, and IGF-1/PI3K-Akt signaling in fracture repair will be examined. The effect of physical (hypoxia and hyperoxia) and chemical factors (reactive oxygen species) as well as the potential coordinated regulatory mechanisms in the periosteal progenitor cells promoting osteogenic differentiation will also be discussed. Understanding the regulation of periosteal osteochondroprogenitors during fracture healing could provide insight into possible therapeutic targets and thereby help to enhance future fracture healing and bone tissue engineering approaches. J. Cell. Physiol. 232: 913-921, 2017. © 2016 Wiley Periodicals, Inc.

Effects of hypoxia on osteogenic differentiation of mesenchymal stromal cells used as a cell therapy for avascular necrosis of the femoral head

BACKGROUND AIMS

Avascular necrosis of the femoral head (AVN) occurs as common result of various conditions or develops as a primary entity, with a high freqency in young adults. Because of its tendency toward osteoarthritis requiring total hip arthroplasty, alternative treatments are being advocated, including cell therapy with mesenchymal stromal cells (MSCs). Because osteonecrotic bone is a severely hypoxic tissue, with a 1-3% oxygen tension, the survival and function of multipotent cells is questionable.

METHODS

In this study, the proliferative, immunophenotypic and osteogenic properties of bone marrow (BM)-derived MSCs from a clinical series of patients with AVN were evaluated under in vitro conditions mimicking the hypoxic milieu of AVN to verify the rationale for cell therapy. MSCs retrieved from the iliac crest (BM-MSC) were isolated, expanded and induced to osteogenic differentiation under a 2% pO2 atmosphere (hypoxia) in comparison with the standard 21% pO2 (normoxia) that is routinely used in cell culture assays.

RESULTS

Both proliferation and colony-forming ability were significantly enhanced in hypoxia-exposed BM-MSCs compared with BM-MSCs under normoxia. The expression of bone-related genes, including alkaline phosphatase, Type I collagen, and osteocalcin was significantly increased under hypoxia. Moreover, mineral deposition after osteogenic induction was not hampered, but in some cases even enhanced under low oxygen tension.

CONCLUSIONS

These findings support autologous cell therapy as an effective treatment to stimulate bone healing in the hypoxic microenvironment of AVN.

Titanium Oxide: A Bioactive Factor in Osteoblast Differentiation

Titanium and titanium alloys are currently accepted as the gold standard in dental applications. Their excellent biocompatibility has been attributed to the inert titanium surface through the formation of a thin native oxide which has been correlated to the excellent corrosion resistance of this material in body fluids. Whether this titanium oxide layer is essential to the outstanding biocompatibility of titanium surfaces in orthopedic biomaterial applications is still a moot point. To study this critical aspect further, human fetal osteoblasts were cultured on thermally oxidized and microarc oxidized (MAO) surfaces and cell differentiation, a key indicator in bone tissue growth, was quantified by measuring the expression of alkaline phosphatase (ALP) using a commercial assay kit. Cell attachment was similar on all the oxidized surfaces although ALP expression was highest on the oxidized titanium alloy surfaces. Untreated titanium alloy surfaces showed a distinctly lower degree of ALP activity. This indicates that titanium oxide clearly upregulates ALP expression in human fetal osteoblasts and may be a key bioactive factor that causes the excellent biocompatibility of titanium alloys. This result may make it imperative to incorporate titanium oxide in all hard tissue applications involving titanium and other alloys.

HIF-1α-induced microRNA-210 reduces hypoxia-induced osteoblast MG-63 cell apoptosis

To better understand the ischemic-hypoxia-induced fracture healing impairment, we determined in this study the microRNA-210 expression in broken bone specimens and in osteoblasts under hypoxia and then determined the influence of microRNA-210 overexpression on the osteoblast cell proliferation and apoptosis. Results demonstrated that microRNA-210 expression was upregulated with an association with HIF-1α overexpression in clinical human catagmatic tissues and was upregulated HIF-1α-dependently in response to hypoxia in osteoblast MG-63 cells. CCK-8 assay indicated that microRNA-210 upregulation by microRNA-210 mimics reduced the chemotherapeutic 5-FU-induced osteoblast cell death, and colony formation assay demonstrated that microRNA-210 mimics promoted osteoblast cells growth. Moreover, the microRNA-210 mimics transfection inhibited the hypoxia-induced MG-63 cell apoptosis via inhibiting the activation of caspase 3 and caspase 9. Therefore, our research indicated a protective role of microRNA-210 in response to hypoxia. And microRNA-210 might serve as a protective role in bone fracture healing.

A pilot study of change in fracture risk in patients with acute respiratory distress syndrome

Introduction

Acute skeletal muscle wasting is a major contributor to post critical illness physical impairment. However, the bone response remains uncharacterized. We prospectively investigated the early changes in bone mineral density (BMD) and fracture risk in critical illness.

Methods

Patients were prospectively recruited ≤24 hours following intensive care unit (ICU) admission to a university teaching or a community hospital (August 2009 to April 2011). All were aged >18 years and expected to be intubated for >48 hours, spend >7 days in critical care and survive ICU admission. Forty-six patients were studied (55.3% male), with a mean age of 54.4 years (95% confidence interval (CI): 49.1 to 59.6) and an APACHE II score of 23.9 (95% CI: 22.4 to 25.5). Calcaneal dual X-ray absorptiometry (DXA) assessment of BMD was performed on day 1 and 10. Increase in fracture risk was calculated from the change in T-score.

Results

BMD did not change between day 1 and 10 in the cohort overall (0.434 (95% CI: 0.405 to 0.463) versus 0.425 g/cm² (95% CI: 0.399 to 0.450), P = 0.58). Multivariable logistical regression revealed admission corrected calcium (odds ratio (OR): 1.980 (95% CI: 1.089 to 3.609), P = 0.026) and admission PaO₂-to-FiO₂ ratio (OR: 0.916 (95% CI: 0.833 to 0.998), P = 0.044) to be associated with >2% loss of BMD. Patients with acute respiratory distress syndrome had a greater loss in BMD than those without (−2.81% (95% CI: −5.73 to 0.118%), n = 34 versus 2.40% (95% CI: 0.204 to 4.586%), n = 12, P = 0.029). In the 34 patients with acute respiratory distress syndrome, fracture risk increased by 19.4% (95% CI: 13.9 to 25.0%).

Conclusions

Patients with acute respiratory distress syndrome demonstrated early and rapid bone demineralisation with associated increase in fracture risk.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-015-0892-y) contains supplementary material, which is available to authorized users.

Stimulators of mineralization limit the invasive phenotype of human osteosarcoma cells by a mechanism involving impaired invadopodia formation

BACKGROUND

Osteosarcoma (OS) is a highly aggressive bone cancer affecting children and young adults. Growing evidence connects the invasive potential of OS cells with their ability to form invadopodia (structures specialized in extracellular matrix proteolysis).

RESULTS

In this study, we tested the hypothesis that commonly used in vitro stimulators of mineralization limit the invadopodia formation in OS cells. Here we examined the invasive potential of human osteoblast-like cells (Saos-2) and osteolytic-like (143B) OS cells treated with the stimulators of mineralization (ascorbic acid and B-glycerophosphate) and observed a significant difference in response of the tested cells to the treatment. In contrast to 143B cells, osteoblast-like cells developed a mineralization phenotype that was accompanied by a decreased proliferation rate, prolongation of the cell cycle progression and apoptosis. On the other hand, stimulators of mineralization limited osteolytic-like OS cell invasiveness into collagen matrix. We are the first to evidence the ability of 143B cells to degrade extracellular matrix to be driven by invadopodia. Herein, we show that this ability of osteolytic-like cells in vitro is limited by stimulators of mineralization.

CONCLUSIONS

Our study demonstrates that mineralization competency determines the invasive potential of cancer cells. A better understanding of the molecular mechanisms by which stimulators of mineralization regulate and execute invadopodia formation would reveal novel clinical targets for treating osteosarcoma.

Can we enhance fracture vascularity: What is the evidence?

Angiogenesis is a vital component of bone healing. The formation of the new blood vessels at the fracture site restores the hypoxia and nutrient deprivation found at the early stages after fracture whilst at a later stage facilitates osteogenesis by the activity of the osteoprogenitor cells. Emerging evidence suggests that there are certain molecules and gene therapies that could promote new blood vessel formation and as a consequence enhance the local bone healing response. This article summarizes the current in vivo evidence on therapeutic approaches aiming at the augmentation of the angiogenic signalling during bone repair.

Stratified control of IGF-I expression by hypoxia and stress hormones in osteoblasts

Bone cells respond to the integrated effects of local and systemic regulation. Here we show that hypoxia and the stress hormones PGE2 and glucocorticoid interact in complex ways in osteoblasts, converging on insulin like growth factor I (IGF-I) expression. Whereas hypoxia alone rapidly increased transcription factor HIF activity, it suppressed DNA synthesis, had no significant effects on protein synthesis or alkaline phosphatase activity, and drove discrete changes in a panel of osteoblast mRNAs. Notably, hypoxia increased expression of the acute phase response transcription factor C/EBPδ which can induce IGF-I in response to PGE2, but conversely prevented the stimulatory effect of PGE2 on IGF-I mRNA. However, unlike its effect on C/EBPδ, hypoxia suppressed expression of the obligate osteoblast transcription factor Runx2, which can activate an upstream response element in the IGF-I gene promoter. Hypoxic inhibition of IGF-I and Runx2 were enforced by glucocorticoid, and continued with prolonged exposure. Our studies thus reveal that IGF-I expression is stratified by two critical transcriptional elements in osteoblasts, which are resolved by the individual and combined effects of hypoxic stress and stress hormones. In so doing, hypoxia suppresses Runx2, limits the enhancing influence of PGE2, and interacts with glucocorticoid to reduce IGF-I expression by osteoblasts.

Angiopoietin-1 peptide QHREDGS promotes osteoblast differentiation, bone matrix deposition and mineralization on biomedical materials

Bone loss occurs as a consequence of a variety of diseases as well as from traumatic injuries, and often requires therapeutic intervention. Strategies for repairing and replacing damaged and/or lost bone tissue include the use of biomaterials and medical implant devices with and without osteoinductive coatings. The soluble growth factor angiopoietin-1 (Ang-1) has been found to promote cell adhesion and survival in a range of cell types including cardiac myocytes, endothelial cells and fibroblasts through an integrin-dependent mechanism. Furthermore, the short sequence QHREDGS has been identified as the integrin-binding sequence of Ang-1 and as a synthetic peptide has been found to possess similar integrin-dependent effects as Ang-1 in the aforementioned cell types. Integrins have been implicated in osteoblast differentiation and bone mineralization, processes critical to bone regeneration. By binding integrins on the osteoblast surface, QHREDGS could promote cell survival and adhesion, as well as conceivably osteoblast differentiation and bone mineralization. Here we immobilized QHREDGS onto polyacrylate (PA)-coated titanium (Ti) plates and polyethylene glycol (PEG) hydrogels. The osteoblast differentiation marker, alkaline phosphatase, peaked in activity 4-12 days earlier on the QHREDGS-immobilized PA-coated Ti plates than on the unimmobilized, DGQESHR (scrambled)- and RGDS-immobilized surfaces. Significantly more bone matrix was deposited on the QHREDGS-immobilized Ti surface than on the other surfaces as determined by atomic force microscopy. The QHREDGS-immobilized hydrogels also had a significantly higher mineral-to-matrix (M/M) ratio determined by Fourier transform infrared spectroscopy. Alizarin Red S and von Kossa staining and quantification, and environmental scanning electron microscopy showed that while both the QHREDGS- and RGDS-immobilized surfaces had extensive mineralization relative to the unimmobilized and DGQESHR-immobilized surfaces, the mineralization was more considerable on the QHREDGS-immobilized surface, both with and without the induction of osteoblast differentiation. Finally, treatment of cell monolayers with soluble QHREDGS was demonstrated to upregulate osteogenic gene expression. Taken together, these results demonstrate that the QHREDGS peptide is osteoinductive, inducing osteoblast differentiation, bone matrix deposition and mineralization.

TRPV4 deficiency causes sexual dimorphism in bone metabolism and osteoporotic fracture risk

We explored the role of transient receptor potential vanilloid 4 (TRPV4) in murine bone metabolism and association of TRPV4 gene variants with fractures in humans. Urinary and histomorphometrical analyses demonstrated reduced osteoclast activity and numbers in male Trpv4(-/-) mice, which was confirmed in bone marrow-derived osteoclast cultures. Osteoblasts and bone formation as shown by serum procollagen type 1 amino-terminal propeptide and histomorphometry, including osteoid surface, osteoblast and osteocyte numbers were not affected in vivo. Nevertheless, osteoblast differentiation was enhanced in Trpv4(-/-) bone marrow cultures. Cortical and trabecular bone mass was 20% increased in male Trpv4(-/-) mice, compared to sex-matched wild type (Trpv4(+/+)) mice. However, at the same time intracortical porosity was increased and bone matrix mineralization was reduced. Together, these lead to a maximum load, stiffness and work to failure of the femoral bone, which were not different compared to Trpv4(+/+) mice, while the bone material was less resistant to stress and less elastic. The differential impacts on these determinants of bone strength were likely responsible for the lack of any changes in whole bone strength in the Trpv4(-/-) mice. None of these skeletal parameters were affected in female Trpv4(-/-) mice. The T-allele of rs1861809 SNP in the TRPV4 locus was associated with a 30% increased risk (95% CI: 1.1-1.6; p=0.013) for non-vertebral fracture risk in men, but not in women, in the Rotterdam Study. Meta-analyses with the population-based LASA study confirmed the association with non-vertebral fractures in men. This was lost when the non-population-based studies Mr. OS and UFO were included. In conclusion, TRPV4 is a male-specific regulator of bone metabolism, a determinant of bone strength, and a potential risk predictor for fractures through regulation of bone matrix mineralization and intra-cortical porosity. This identifies TRPV4 as a unique sexually dimorphic therapeutic and/or diagnostic candidate for osteoporosis.

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.

The role of oxygen during fracture healing

Oxygen affects the activity of multiple skeletogenic cells and is involved in many processes that are important for fracture healing. However, the role of oxygen in fracture healing has not been fully studied. Here we systematically examine the effects of oxygen tension on fracture healing and test the ability of hyperoxia to rescue healing defects in a mouse model of ischemic fracture healing. Mice with tibia fracture were housed in custom-built gas chambers and groups breathed a constant atmosphere of 13% oxygen (hypoxia), 21% oxygen (normoxia), or 50% oxygen (hyperoxia). The influx of inflammatory cells to the fracture site, stem cell differentiation, tissue vascularization, and fracture healing were analyzed. In addition, the efficacy of hyperoxia (50% oxygen) as a treatment regimen for fracture nonunion was tested. Hypoxic animals had decreased tissue vascularity, decreased bone formation, and delayed callus remodeling. Hyperoxia increased tissue vascularization, altered fracture healing in un-complicated fractures, and improved bone repair in ischemia-induced delayed fracture union. However, neither hypoxia nor hyperoxia significantly altered chondrogenesis or osteogenesis during early stages of fracture healing, and infiltration of macrophages and neutrophils was not affected by environmental oxygen after bone injury. In conclusion, our results indicate that environmental oxygen levels affect tissue vascularization and fracture healing, and that providing oxygen when fractures are accompanied by ischemia may be beneficial.

Cell death, non-invasively assessed by intrinsic fluorescence intensity of NADH, is a predictive indicator of functional differentiation of embryonic stem cells

BACKGROUND INFORMATION

Continued advances in stem cell biology and stem cell transplantation rely on non-invasive biomarkers to characterise cells and stem cell aggregates. The non-invasive quality of such biomarkers is essential because exogenous labels, probes or reporters can unintentionally and dramatically alter stem cell state as can disruption of cell-cell and cell-matrix interactions. Here, we investigate the utility of the autofluorescent metabolite, nicotinamide adenine dinucleotide (NADH), as a non-invasive, intrinsic biomarker of cell death when detected with multi-photon optical-based approaches. To test this possibility, cell death was induced in murine embryoid bodies (EBs) at an early stage (day 3) of differentiation using staurosporine, an ATP-competitive kinase inhibitor of electron transport. Several hours after staurosporine treatment, EBs were stained with a single-colour, live/dead probe. A single-cross-sectional plane of each EB was imaged to detect the fluorescence intensity of the live/dead probe (extrinsic fluorescence) as well as the fluorescence intensity of NADH (intrinsic fluorescence). EBs were assessed at subsequent time points (days 6-12) for the formation of beating areas as an indicator of functional differentiation.

RESULTS

Statistical comparison indicated a strong positive correlation between extrinsic fluorescence intensity of the live/dead stain and intrinsic fluorescence of NADH, suggesting that the intensity of NADH fluorescence could be used to reliably and non-invasively assess death of cells of EBs. Furthermore, EBs that had high levels of cell death soon after aggregate formation had limited ability to give rise to functional cardiomyocytes at later time points.

CONCLUSIONS

We demonstrate the utility of NADH fluorescence intensity as a non-invasive indicator of cell death in stem cell aggregates when measured using multi-photon excitation. In addition, we show that the degree of stem cell death at early stages of differentiation is predictive for the formation of functional cardiomyocytes.