Oxygenated environment enhances both stem cell survival and osteogenic differentiation

Oxygen tension regulating hydrogels for vascularization and osteogenesis via sequential activation of HIF-1α and ERK1/2 signaling pathways in bone regeneration

Angiogenesis plays a crucial role in bone regeneration. Hypoxia is a driving force of angiogenesis at the initial stage of tissue repair. The hypoxic microenvironment could activate the hypoxia-inducible factor (HIF)-1α signaling pathway in cells, thereby enhancing the proliferation, migration and pro-angiogenic functions of stem cells. However, long-term chronic hypoxia could inhibit osteogenic differentiation and even lead to apoptosis. Therefore, shutdown of the HIF-1α signaling pathway and providing oxygen at later stage probably facilitate osteogenic differentiation and bone regeneration. Herein, an oxygen tension regulating hydrogel that sequentially activate and deactivate the HIF-1α signaling pathway were prepared in this study. Its effect and mechanism on stem cell differentiation were investigated both in vitro and in vivo. We proposed a gelatin-based hydrogel capable of sequentially delivering a hypoxic inducer (copper ions) and oxygen generator (calcium peroxide). The copper ions released from the hydrogels significantly enhanced cell viability and VEGF secretion of BMSCs via upregulating HIF-1α expression and facilitating its translocation into the nucleus. Additionally, calcium peroxide promoted alkaline phosphatase activity, osteopontin secretion, and calcium deposition through the activation of ERK1/2. Both Cu2+ and calcium peroxide demonstrated osteogenic promotion individually, while their synergistic effect within the hydrogels led to a superior osteogenic effect by potentially activating the HIF-1α and ERK1/2 signaling pathways.

Glucose Metabolism: Optimizing Regenerative Functionalities of Mesenchymal Stromal Cells Postimplantation

Mesenchymal stromal cells (MSCs) are considered promising candidates for regenerative medicine applications. Their clinical performance postimplantation, however, has been disappointing. This lack of therapeutic efficacy is most likely due to suboptimal formulations of MSC-containing material constructs. Tissue engineers, therefore, have developed strategies addressing/incorporating optimized cell, microenvironmental, biochemical, and biophysical cues/stimuli to enhance MSC-containing construct performance. Such approaches have had limited success because they overlooked that maintenance of MSC viability after implantation for a sufficient time is necessary for MSCs to develop their regenerative functionalities fully. Following a brief overview of glucose metabolism and regulation in MSCs, the present literature review includes recent pertinent findings that challenge old paradigms and notions. We hereby report that glucose is the primary energy substrate for MSCs, provides precursors for biomass generation, and regulates MSC functions, including proliferation and immunosuppressive properties. More importantly, glucose metabolism is central in controlling in vitro MSC expansion, in vivo MSC viability, and MSC-mediated angiogenesis postimplantation when addressing MSC-based therapies. Meanwhile, in silico models are highlighted for predicting the glucose needs of MSCs in specific regenerative medicine settings, which will eventually enable tissue engineers to design viable and potent tissue constructs. This new knowledge should be incorporated into developing novel effective MSC-based therapies. Impact statement The clinical use of mesenchymal stromal cells (MSCs) has been unsatisfactory due to the inability of MSCs to survive and be functional after implantation for sufficient periods to mediate directly or indirectly a successful regenerative tissue response. The present review summarizes the endeavors in the past, but, most importantly, reports the latest findings that elucidate underlying mechanisms and identify glucose metabolism as the crucial parameter in MSC survival and the subsequent functions pertinent to new tissue formation of importance in tissue regeneration applications. These latest findings justify further basic research and the impetus for developing new strategies to improve the modalities and efficacy of MSC-based therapies.

Enzyme-Mediated Alleviation of Peroxide Toxicity in Self-Oxygenating Biomaterials

Oxygen releasing biomaterials can facilitate the survival of living implants by creating environments with a viable oxygen level. Hydrophobic oxygen generating microparticles (HOGMPs) encapsulated calcium peroxide (CPO) have recently been used in tissue engineering to release physiologically relevant amounts of oxygen for several weeks. However, generating oxygen using CPO is mediated via the generation of toxic levels of hydrogen peroxide (H2 O2 ). The incorporation of antioxidants, such as catalases, can potentially reduce H2 O2 levels. However, the formulation in which catalases can most effectively scavenge H2 O2 within oxygen generating biomaterials has remained unexplored. In this study, three distinct catalase incorporation methods are compared based on their ability to decrease H2 O2 levels. Specifically, catalase is incorporated within HOGMPs, or absorbed onto HOGMPs, or freely laden into the hydrogel entrapping HOGMPs and compared with control without catalase. Supplementation of free catalase in an HOGMP-laden hydrogel significantly decreases H2 O2 levels reflecting a higher cellular viability and metabolic activity of all the groups. An HOGMP/catalase-laden hydrogel precursor solution containing cells is used as an oxygenating bioink allowing improved viability of printed constructs under severe hypoxic conditions. The combination of HOGMPs with a catalase-laden hydrogel has the potential to decrease peroxide toxicity of oxygen generating tissues.

Photosynthetic microorganisms for the oxygenation of advanced 3D bioprinted tissues

3D bioprinting technology has emerged as a tool that promises to revolutionize the biomedical field, including tissue engineering and regeneration. Despite major technological advancements, several challenges remain to be solved before 3D bioprinted tissues could be fully translated from the bench to the bedside. As oxygen plays a key role in aerobic metabolism, which allows energy production in the mitochondria; as a consequence, the lack of tissue oxygenation is one of the main limitations of current bioprinted tissues and organs. In order to improve tissue oxygenation, recent approaches have been established for a broad range of clinical applications, with some already applied using 3D bioprinting technologies. Among them, the incorporation of photosynthetic microorganisms, such as microalgae and cyanobacteria, is a promising approach that has been recently explored to generate chimerical plant-animal tissues where, upon light exposure, oxygen can be produced and released in a localized and controlled manner. This review will briefly summarize the state-of-the-art approaches to improve tissue oxygenation, as well as studies describing the use of photosynthetic microorganisms in 3D bioprinting technologies. STATEMENT OF SIGNIFICANCE: 3D bioprinting technology has emerged as a tool for the generation of viable and functional tissues for direct in vitro and in vivo applications, including disease modeling, drug discovery and regenerative medicine. Despite the latest advancements in this field, suboptimal oxygen delivery to cells before, during and after the bioprinting process limits their viability within 3D bioprinted tissues. This review article first highlights state-of-the-art approaches used to improve oxygen delivery in bioengineered tissues to overcome this challenge. Then, it focuses on the emerging roles played by photosynthetic organisms as novel biomaterials for bioink generation. Finally, it provides considerations around current challenges and novel potential opportunities for their use in bioinks, by comparing latest published studies using algae for 3D bioprinting.

Elimination of Induced Hypoxic Regions in Depth of 3D Porous Silk Scaffolds by the Introduction of Channel Configuration

Development of large, clinically sized tissue constructs with efficient mass transport is a tremendous need in tissue engineering. One major challenge in large tissue-engineered constructs is to support homogeneous delivery of oxygen and nutrients throughout the tissue scaffold while eliminating induced hypoxic regions in depth. To address this goal, we introduced an especial channeled architecture on porous silk-based tissue scaffolds to improve supplying of oxygen to the cells in central regions of the scaffolds. Oxygen gradients were measured and evaluated in three scaffold prototypes, namely, one unchanneled and two channeled scaffolds with different channel diameters (500 μm and 1000 μm). The channels were introduced into the constructs using stainless-steel rods arranged uniformly in stainless-steel mold, a fabrication method that enables precise control over channel diameter and the distance between channels. During 2-week culture of G292 cells, the 1000 μm channeled scaffolds demonstrated higher oxygen concentration at the center compared to 500 μm channeled prototype; however, the oxygen concentration approached the same level around the last days of culture. Nevertheless, homogenous oxygen distribution throughout the 1000 μm channeled constructs and the consequence of higher cell proliferation at day 14 postseeding corroborate the efficient elimination of induced hypoxic regions; and therefore, it holds promise for clinically relevant sized scaffold especially in bone tissue engineering.

3D-printed oxygen-releasing scaffolds improve bone regeneration in mice

Low oxygen (O₂) diffusion into large tissue engineered scaffolds hinders the therapeutic efficacy of transplanted cells. To overcome this, we previously studied hollow, hyperbarically-loaded microtanks (μtanks) to serve as O₂ reservoirs. To adapt these for bone regeneration, we fabricated biodegradable μtanks from polyvinyl alcohol and poly (lactic-co-glycolic acid) and embedded them to form 3D-printed, porous poly-ε-caprolactone (PCL)-μtank scaffolds. PCL-μtank scaffolds were loaded with pure O₂ at 300-500 psi. When placed at atmospheric pressures, the scaffolds released O₂ over a period of up to 8 h. We confirmed the inhibitory effects of hypoxia on the osteogenic differentiation of human adipose-derived stem cells (hASCs and we validated that μtank-mediated transient hyperoxia had no toxic impacts on hASCs, possibly due to upregulation of endogenous antioxidant regulator genes. We assessed bone regeneration in vivo by implanting O₂-loaded, hASC-seeded, PCL-μtank scaffolds into murine calvarial defects (4 mm diameters × 0.6 mm height) and subcutaneously (4 mm diameter × 8 mm height). In both cases we observed increased deposition of extracellular matrix in the O₂ delivery group along with greater osteopontin coverages and higher mineral deposition. This study provides evidence that even short-term O₂ delivery from PCL-μtank scaffolds may enhance hASC-mediated bone tissue regeneration.

3D-Printed Oxygen-Carrying Nanocomposite Hydrogels for Enhanced Cell Viability under Hypoxic and Normoxic Conditions

Insufficient and heterogeneous oxygen (O₂) distribution within engineered tissues results in hypoxic conditions. Hypoxia is one of the characteristics of solid tumors. To date, very few studies have used an O₂-deliverable injectable hydrogel for cancer treatment under hypoxic conditions. In this field, we describe a new O₂-carrying nanomaterial and an injectable nanocomposite hydrogel (PMOF and AlgL-PMOF, respectively) that can provide extended oxygen levels for cell survival under hypoxia. Particularly, PMOF and AlgL-PMOF enhance cell viability under hypoxic and normoxic cell culturing conditions. Moreover, sustained oxygen availability in the presence of an anticancer drug within the 3D network of AlgL-PMOF results in a decrease in the viability of malignant and immortal cells, while the viability of healthy cells increases.

A hydrogel/particle composite with a gradient of oxygen releasing microparticle for concurrent osteogenic and chondrogenic differentiation in a single scaffold

In the present study, a hydrogel/particle scaffold with a gradient of the oxygen releasing microparticles was developed. Hydrogel component was composed of the oxidized pectin and silk fibroin, whereas the microparticles were constituted from polylactic acid (PLA) and calcium peroxide (CPO). A controlled mixing of the suspensions with different content of the PLA/CPO microparticles conferred a gradient of microparticles in scaffold thickness in a manner that the microparticle content increased with moving from lower to upper face of the composite. Measurement of the scaffold mechanical properties corroborated that with moving from lower to upper face, the compressive modulus increased by 78 %. The measurement of the oxygen and calcium release from the successive sections of the composite revealed that the gradient of microparticle concentration resulted in the gradient of the released oxygen and calcium. MTT analysis proved that the gradient oxygen releasing composite did not induce any toxic effect on human adipose-derived mesenchymal stem cells (hAd-MSCs). Moreover, the cell culture on successive sections of the gradient composite confirmed that oxygen releasing composite substantially improved the cell viability and density comparing the pristine hydrogel and the non-oxygen releasing counterpart. The increase in microparticle content conferred a positive impact on the number of viable cells. The study of osteogenic (ALP, OCN and OPN) and chondrogenic (SOX9, AGG and COL ⅠⅠ) gene expression proved that the gradient composite parts with high microparticle content promoted osteogenesis, whereas the parts with low microparticle content encouraged chondrogenesis of mesenchymal stem cells.

Effect of synthetic oxygen-generating system on cell survival under hypoxic condition in vitro

A significant challenge in the tissue engineering of injured sites is the lack of vascularization in the engineered sites due to insufficient oxygen supply. A scaffolding system is required to support seeded cells as vascularization develops. In this study, we examined the effects of hypoxic conditions and oxygen release on cell survival in a synthetic system. We developed a three-dimensional system using CaO2/poly(lactic-co-glycolic acid) microspheres suspended in a hydrogel. The system material was evaluated using stem cells under hypoxic conditions alongside controls to evaluate its oxygen-generating potential over a period of 21 days. The hydrogel acted as a flexible carrier supporting cell attachment and growth, protecting microspheres, and prolonging oxygen release. The system generated oxygen and supported cell growth, which are together expected to promote stem cell survival and growth in the weeks following implantation.

Effects of oxygen generating scaffolds on cell survival and functional recovery following acute spinal cord injury in rats

Persistent local oxygen delivery is crucial to create a microenvironment for cell survival and nerve regeneration in acute spinal cord injury (SCI). This study aimed to fabricate calcium peroxide-based microspheres incorporated into a 3-D construct scaffold as a novel oxygen release therapy for SCI. The scaffolds were able to generate oxygen over the course of 21 days when incubated under hypoxic conditions. In vitro, GFP-labeled bone marrow-derived mesenchymal stem cells (MSCs) were planted into the scaffolds. We observed that scaffolds could enhance MSC survival under hypoxic conditions for more than 21 days. Oxygen generating scaffolds were transplanted into spinal cord injury sites of rats in vivo. Twelve weeks following transplantation, cavity areas in the injury/graft site were significantly reduced due to tissue regeneration. Additionally, the oxygen generating scaffolds improved revascularization as observed through vWF immunostaining. A striking feature was the occurrence of nerve fiber regeneration in the lesion sites, which eventually led to significant locomotion recovery. The present results indicate that the oxygen generating scaffolds have the property of sustained local oxygen release, thus facilitating regeneration in injured spinal cords.

Mesenchymal stem cells modifications for enhanced bone targeting and bone regeneration

In pathological bone conditions (e.g., osteoporotic fractures or critical size bone defects), increasing the pool of osteoblast progenitor cells is a promising therapeutic approach to facilitate bone healing. Since mesenchymal stem cells (MSCs) give rise to the osteogenic lineage, a number of clinical trials investigated the potential of MSCs transplantation for bone regeneration. However, the engraftment of transplanted cells is often hindered by insufficient oxygen and nutrients supply and the tendency of MSCs to home to different sites of the body. In this review, we discuss various approaches of MSCs transplantation for bone regeneration including scaffold and hydrogel constructs, genetic modifications and surface engineering of the cell membrane aimed to improve homing and increase cell viability, proliferation and differentiation.

Hypoxia reduces the osteogenic differentiation of peripheral blood mesenchymal stem cells by upregulating Notch-1 expression

Purpose: Mesenchymal stem cells (MSCs) seeded on biocompatible scaffolds have therapeutic potential for bone defect repair. However, MSCs can be affected by hypoxia and nutritional deficiency due to a lack of blood vessels in the scaffolds. Here, we explored the effects of hypoxia on MSC differentiation to clarify these mechanisms. Methods: Peripheral blood mesenchymal stem cells (PBMSCs) were cultured in small individual chambers with oxygen concentrations of 1%, 9%, and 21%. Cell proliferation was evaluated by Cell Counting Kit 8 assays, and cell survival was determined using live/dead assays. Scratch assays were performed to evaluate cell migration. Ca²⁺ deposition/mineralization experiments, reverse transcription quantitative real-time polymerase chain reaction, and Western blotting were performed to assess the osteogenic differentiation of cells. Notch1 expression was downregulated by lentivirus-transfected PBMSCs to observe the effects of Notch1 knockdown on osteogenic gene and protein expression. Results: PBMSCs exposed to hypoxia (1% O₂) demonstrated accelerated proliferation, increased migration, and reduced survival in the absence of serum. Although 9% oxygen promoted osteogenic differentiation, the osteogenic differentiation of PBMSCs was significantly reduced by 1% O₂, and this effect was associated with increased Notch1 expression. Reducing Notch1 expression using small interfering RNA significantly restored the osteogenic differentiation of PBMSCs. Conclusions: Hypoxia accelerated proliferation, increased migration, and reduced PBMSC differentiation into osteoblasts by increasing Notch1 expression. These findings may contribute to the development of appropriate cell culture or in vivo transplantation conditions to maintain the full osteogenic potential of PBMSCs.

Pharmaceutical design and development of perfluorocarbon nanocolloids for oxygen delivery in regenerative medicine

Perfluorocarbons (PFCs) have been investigated as oxygen carriers for several decades in varied biomedical applications. PFCs are chemically and biologically inert, temperature and storage stable, pose low to no infectious risk, can be commercially manufactured, and have well established gas transport properties. In this review, we highlight design and development strategies for their successful application in regenerative medicine, transplantation and organ preservation. Effective tissue preservation strategies are key to improving outcomes of extremity salvage and organ transplantation. Maintaining tissue integrity requires adequate oxygenation to support aerobic metabolism. The use of whole blood for oxygen delivery is fraught with limitations of poor shelf stability, infectious risk, religious exclusions and product shortages. Other agents also face clinical challenges in their implementation. As a solution, we discuss new ways of designing and developing PFC-based artificial oxygen carriers by implementing modern pharmaceutical quality by design and scale up manufacturing methodologies.

Mechanism of Action of Icariin in Bone Marrow Mesenchymal Stem Cells

Osteoporosis, femoral head necrosis, and congenital bone defects are orthopedic disorders characterized by reduced bone generation and insufficient bone mass. Bone regenerative therapy primarily relies on the bone marrow mesenchymal stem cells (BMSCs) and their ability to differentiate osteogenically. Icariin (ICA) is the active ingredient of Herba epimedii, a common herb used in traditional Chinese medicine (TCM) formulations, and can effectively enhance BMSC proliferation and osteogenesis. However, the underlying mechanism of ICA action in BMSCs is not completely clear. In this review, we provide an overview of the studies on the role and mechanism of action of ICA in BMSCs, to provide greater insights into its potential clinical use in bone regeneration.

Recent Advances and Future of Gene Therapy for Bone Regeneration

PURPOSE OF REVIEW

The purpose of this review is to discuss the recent advances in gene therapy as a treatment for bone regeneration. While most fractures heal spontaneously, patients who present with fracture nonunion suffer from prolonged pain, disability, and often require additional operations to regain musculoskeletal function.

RECENT FINDINGS

In the last few years, BMP gene delivery by means of electroporation and sonoporation resulted in repair of nonunion bone defects in mice, rats, and minipigs. Ex vivo transfection of porcine mesenchymal stem cells (MSCs) resulted in bone regeneration following implantation in vertebral defects of minipigs. Sustained release of VEGF gene from a collagen-hydroxyapatite scaffold to the mandible of a human patient was shown to be safe and osteoinductive. In conclusion, gene therapy methods for bone regeneration are systematically becoming more efficient and show proof-of-concept in clinically relevant animal models. Yet, on the pathway to clinical use, more investigation is needed to determine the safety aspects of the various techniques in terms of biodistribution, toxicity, and tumorigenicity.

Fibroblast-Cytophilic and HeLa-Cytotoxic Dual Function Carbon Nanoribbon Network Platform

Carbon nanomaterials have emerged as a promising material in cancer diagnosis and therapy. Carbon nanomaterials/nanostructures (C-C molecular structure) act as a carrier/skeleton and require further surface modification through functionalization with chemicals or biomolecules to attain cell response. We report the synthesis of a novel carbon nanoribbon network (CNRN) platform that possesses a combination of C-C and C-O bond architecture. The bioactive CNRN showed enhanced ability for cell adhesion. Most importantly, it induced opposite cell responses from healthy cells and cancerous cells, cytophilic to fibroblasts but cytotoxic to HeLa cells. Ultrafast laser ionization under ambient conditions transforms nonbioresponsive C-C bond of graphite to C-C and C-O bonds, forming a self-assembled CNRN platform. The morphology, nanochemistry, and functionality on modulating fibroblast and HeLa adhesion and proliferation of the fabricated CNRN platforms were investigated. The results of in vitro studies suggested that the CNRN platforms not only attracted but also actively accelerated the adhesion and proliferation of both fibroblasts and HeLa cells. The proliferation rate of fibroblasts and HeLa cells is 91 and 98 times greater compared with that of a native graphite substrate, respectively. The morphology of the cells over a period of 24 to 48 h revealed that the CNRN platform induced an apoptosis-like cytotoxic function on HeLa cells, whereas fibroblasts experienced a cytophilic effect and formed a tissuelike structure. The degree of cytotoxic or cytophilic effect can be further enhanced by adjusting parameters such as the ratio of C-C bonds to C-O bonds, the nanoribbon width, and the nanovoid porosity of the CNRN platforms, which could be tuned by careful control of laser ionization. In a nutshell, for the first time, pristine carbon nanostructures free from biochemical functionalization demonstrate dual function, cytophilic to fibroblast cells and cytotoxic to HeLa cells.

Cellularizing hydrogel-based scaffolds to repair bone tissue: How to create a physiologically relevant micro-environment?

Tissue engineering is a promising alternative to autografts or allografts for the regeneration of large bone defects. Cell-free biomaterials with different degrees of sophistication can be used for several therapeutic indications, to stimulate bone repair by the host tissue. However, when osteoprogenitors are not available in the damaged tissue, exogenous cells with an osteoblast differentiation potential must be provided. These cells should have the capacity to colonize the defect and to participate in the building of new bone tissue. To achieve this goal, cells must survive, remain in the defect site, eventually proliferate, and differentiate into mature osteoblasts. A critical issue for these engrafted cells is to be fed by oxygen and nutrients: the transient absence of a vascular network upon implantation is a major challenge for cells to survive in the site of implantation, and different strategies can be followed to promote cell survival under poor oxygen and nutrient supply and to promote rapid vascularization of the defect area. These strategies involve the use of scaffolds designed to create the appropriate micro-environment for cells to survive, proliferate, and differentiate in vitro and in vivo. Hydrogels are an eclectic class of materials that can be easily cellularized and provide effective, minimally invasive approaches to fill bone defects and favor bone tissue regeneration. Furthermore, by playing on their composition and processing, it is possible to obtain biocompatible systems with adequate chemical, biological, and mechanical properties. However, only a good combination of scaffold and cells, possibly with the aid of incorporated growth factors, can lead to successful results in bone regeneration. This review presents the strategies used to design cellularized hydrogel-based systems for bone regeneration, identifying the key parameters of the many different micro-environments created within hydrogels.

Oxygen mapping: Probing a novel seeding strategy for bone tissue engineering

Bone tissue engineering (BTE) utilizing biomaterial scaffolds and human mesenchymal stem cells (hMSCs) is a promising approach for the treatment of bone defects. The quality of engineered tissue is crucially affected by numerous parameters including cell density and the oxygen supply. In this study, a novel oxygen-imaging sensor was introduced to monitor the oxygen distribution in three dimensional (3D) scaffolds in order to analyze a new cell-seeding strategy. Immortalized hMSCs, pre-cultured in a monolayer for 30-40% or 70-80% confluence, were used to seed demineralized bone matrix (DBM) scaffolds. Real-time measurements of oxygen consumption in vitro were simultaneously performed by the novel planar sensor and a conventional needle-type sensor over 24 h. Recorded oxygen maps of the novel planar sensor revealed that scaffolds, seeded with hMSCs harvested at lower densities (30-40% confluence), exhibited rapid exponential oxygen consumption profile. In contrast, harvesting cells at higher densities (70-80% confluence) resulted in a very slow, almost linear, oxygen decrease due to gradual achieving the stationary growth phase. In conclusion, it could be shown that not only the seeding density on a scaffold, but also the cell density at the time point of harvest is of major importance for BTE. The new cell seeding strategy of harvested MSCs at low density during its log phase could be a useful strategy for an early in vivo implantation of cell-seeded scaffolds after a shorter in vitro culture period. Furthermore, the novel oxygen imaging sensor enables a continuous, two-dimensional, quick and convenient to handle oxygen mapping for the development and optimization of tissue engineered scaffolds. Biotechnol. Bioeng. 2017;114: 894-902. © 2016 Wiley Periodicals, Inc.

Oxygen Delivering Biomaterials for Tissue Engineering

Tissue engineering (TE) has provided promising strategies for regenerating tissue defects, but few TE approaches have been translated for clinical applications. One major barrier in TE is providing adequate oxygen supply to implanted tissue scaffolds, since oxygen diffusion from surrounding vasculature in vivo is limited to the periphery of the scaffolds. Moreover, oxygen is also an important signaling molecule for controlling stem cell differentiation within TE scaffolds. Various technologies have been developed to increase oxygen delivery in vivo and enhance the effectiveness of TE strategies. Such technologies include hyperbaric oxygen therapy, perfluorocarbon- and hemoglobin-based oxygen carriers, and oxygen-generating, peroxide-based materials. Here, we provide an overview of the underlying mechanisms and how these technologies have been utilized for in vivo TE applications. Emerging technologies and future prospects for oxygen delivery in TE are also discussed to evaluate the progress of this field towards clinical translation.

Selective Accelerated Proliferation of Malignant Breast Cancer Cells on Planar Graphene Oxide Films

Graphene nanomaterials have been actively investigated for biomedical and biological applications, including that of cancer. Despite progress made, most of such studies are conducted on dispersed graphene nanosheets in solution. Consequently, the use of planar graphene films, especially in cancer research, has not been fully explored. Here, we investigate the cellular interactions between the graphene material films and breast cancer cell lines, specifically the effects these films have on cellular proliferation, spreading area, and cytotoxicity. We demonstrate that the graphene oxide (GO) film selectively accelerates the proliferation of both metastatic (MDA-MB-231) and nonmetastatic (MCF-7) breast cancer cells, but not that of noncancer breast epithelial cells (MCF-10A). Contrastingly, this accelerated proliferation is not observed with the use of graphene (G) film. Moreover, GO induces negligible cytotoxicity on these cells. We suggest that the observed phenomena originate from the synergistic effect resulted from the high loading capacity and conformational change of cellular attachment proteins on the GO film, and the high amount of oxygenated groups present in the material. We anticipate that our findings can further shed light on the graphene-cancer cellular interactions and provide better understanding for the future design and application of graphene-based nanomaterials in cancer research.

Monitoring/Imaging and Regenerative Agents for Enhancing Tissue Engineering Characterization and Therapies

The past three decades have seen numerous advances in tissue engineering and regenerative medicine (TERM) therapies. However, despite the successes there is still much to be done before TERM therapies become commonplace in clinic. One of the main obstacles is the lack of knowledge regarding complex tissue engineering processes. Imaging strategies, in conjunction with exogenous contrast agents, can aid in this endeavor by assessing in vivo therapeutic progress. The ability to uncover real-time treatment progress will help shed light on the complex tissue engineering processes and lead to development of improved, adaptive treatments. More importantly, the utilized exogenous contrast agents can double as therapeutic agents. Proper use of these Monitoring/Imaging and Regenerative Agents (MIRAs) can help increase TERM therapy successes and allow for clinical translation. While other fields have exploited similar particles for combining diagnostics and therapy, MIRA research is still in its beginning stages with much of the current research being focused on imaging or therapeutic applications, separately. Advancing MIRA research will have numerous impacts on achieving clinical translations of TERM therapies. Therefore, it is our goal to highlight current MIRA progress and suggest future research that can lead to effective TERM treatments.

Synergistic effects of hypoxia and morphogenetic factors on early chondrogenic commitment of human embryonic stem cells in embryoid body culture

Derivation of articular chondrocytes from human stem cells would advance our current understanding of chondrogenesis, and accelerate development of new stem cell therapies for cartilage repair. Chondrogenic differentiation of human embryonic stem cells (hESCs) has been studied using supplemental and cell-secreted morphogenetic factors. The use of bioreactors enabled insights into the effects of physical forces and controlled oxygen tension. In this study, we investigated the interactive effects of controlled variation of oxygen tension and chondrocyte-secreted morphogenetic factors on chondrogenic differentiation of hESCs in the embryoid body format (hESC-EB). Transient hypoxic culture (2 weeks at 5 % O2 followed by 1 week at 21 % O2) of hESC-EBs in medium conditioned with primary chondrocytes up-regulated the expression of SOX9 and suppressed pluripotent markers OCT4 and NANOG. Pellets derived from these cells showed significant up-regulation of chondrogenic genes (SOX9, COL2A1, ACAN) and enhanced production of cartilaginous matrix (collagen type II and proteoglycan) as compared to the pellets from hESC-EBs cultured under normoxic conditions. Gene expression profiles corresponded to those associated with native cartilage development, with early expression of N-cadherin (indicator of cell condensation) and late expression of aggrecan (ACAN, indicator of proteoglycan production). When implanted into highly vascularized subcutaneous area in immunocompromised mice for 4 weeks, pellets remained phenotypically stable and consisted of cartilaginous extracellular matrix (ECM), without evidence of dedifferentiation or teratoma formation. Based on these results, we propose that chondrogenesis in hESC can be synergistically enhanced by a control of oxygen tension and morphogenetic factors secreted by chondrocytes.

Environmental manipulation to promote stem cell survival in vivo: use of aggregation, oxygen carrier, and BMP-2 co-delivery strategies

While mesenchymal stem cell (MSC)-based strategies for critically-sized bone defect repair hold promise, poor cell survival in vivo remains a significant barrier to the translation of these therapeutics.

Hypoxia Inhibits De Novo Vascular Assembly of Adipose-Derived Stromal/Stem Cell Populations, but Promotes Growth of Preformed Vessels

Vascularization is critical for cell survival within tissue-engineered grafts. Adipose-derived stromal/stem cells (ASCs) are widely used in tissue engineering applications as they are a clinically relevant source of stem cells and endothelial progenitor cells. ASCs have previously been shown to self-assemble into pericyte-stabilized vascular networks in normoxic (20% O2) cultures. This capacity for de novo vascular assembly may accelerate graft vascularization in vivo rather than relying solely on angiogenic ingrowth. However, oxygen depletion within large cell-seeded grafts will be rapid, and it is unclear how this worsening hypoxic environment will impact the vascular assembly of the transplanted cells. The objectives of this study were to determine whether ASC-derived vessels could grow in hypoxia and to assess whether the vessel maturity (i.e., individual cells vs. preformed vessels) influenced this hypoxic response. Utilizing an in vitro vascularization model, ASCs were encapsulated within fibrin gels and cultured in vitro for up to 6 days in either normoxia (20% O2) or hypoxia (0.2% or 2% O2). In a subsequent experiment, vessels were allowed to preform in normoxia for 6 days before an additional 6 days of either normoxia or hypoxia. Viability, vessel growth, pericyte coverage, proliferation, metabolism, and angiogenic factor expression were assessed for each experimental approach. Vessel growth was dramatically inhibited in both moderate and severe hypoxia (47% and 11% total vessel length vs. normoxia, respectively), despite maintaining high cell viability and upregulating endogenous expression of vascular endothelial growth factor in hypoxia. Bromodeoxyuridine labeling indicated significantly reduced proliferation of endothelial cells in hypoxia. In contrast, when vascular networks were allowed to preform for 6 days in normoxia, vessels not only survived but also continued to grow more in hypoxia than those maintained in normoxia. These findings demonstrate that vascular assembly and growth are tightly regulated by oxygen tension and may be differentially affected by hypoxic conditions based on the maturity of the vessels. Understanding this relationship is critical to developing effective approaches to engineer viable tissue-engineered grafts in vivo.

Characterization of secretomes provides evidence for adipose-derived mesenchymal stromal cells subtypes

Introduction

This study was aimed at deciphering the secretome of adipose-derived mesenchymal stromal cells (ADSCs) cultured in standard and hypoxic conditions to reveal proteins, which may be responsible for regenerative action of these cells.

Methods

Human ADSCs were isolated from 10 healthy donors and cultured for 3–4 passages. Cells were serum deprived and cell purity was assessed using multiple cell surface markers. Conditioned media was collected and analyzed using LC-MS with a focus on characterizing secreted proteins.

Results

Purity of the ADSC assessed as CD90+/CD73+/CD105+/CD45-/CD31- cells was greater than 99 % and viability was greater than 97 %. More than 600 secreted proteins were detected in conditioned media of ADSCs. Of these 100 proteins were common to all cultures and included key molecules involved in tissue regeneration such as collagens and collagen maturation enzymes, matrix metalloproteases, matricellular proteins, macrophage-colony stimulating factor and pigment epithelium derived factor. Common set of proteins also included molecules, which contribute to regenerative processes but were not previously associated with ADSCs. These included olfactomedin-like 3, follistatin-like 1 and prosaposin. In addition, ADSCs from the different subjects secreted proteins, which were variable between different cultures. These included proteins with neurotrophic activities, which were not previously associated with ADSCs, such as mesencephalic astrocyte-derived neurotrophic factor, meteorin and neuron derived neurotrophic factor. Hypoxia resulted in secretion of 6 proteins, the most prominent included EGF-like repeats and discoidin I-like domains 3, adrenomedullin and ribonuclease 4 of RNase A family. It also caused the disappearance of 8 proteins, including regulator of osteogenic differentiation cartilage-associated protein.

Conclusions

Human ADSCs with CD90+/CD73+/CD105+/CD45-/CD31-/PDGFRβ+/NG2+/CD146+(−) immunophenotype secrete a large array of proteins, the most represented group is comprised of extracellular matrix components. Number of secreted proteins is largely unaffected by prolonged hypoxia. Variability in the secretion of several proteins from cultured ADSCs of individual subjects suggests that these cells exist as a heterogeneous population containing functionally distinct subtypes, which differ in numbers between donors.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-015-0209-8) contains supplementary material, which is available to authorized users.

Strategies to engineer tendon/ligament-to-bone interface: Biomaterials, cells and growth factors

Integration between tendon/ligament and bone occurs through a specialized tissue interface called enthesis. The complex and heterogeneous structure of the enthesis is essential to ensure smooth mechanical stress transfer between bone and soft tissues. Following injury, the interface is not regenerated, resulting in high rupture recurrence rates. Tissue engineering is a promising strategy for the regeneration of a functional enthesis. However, the complex structural and cellular composition of the native interface makes enthesis tissue engineering particularly challenging. Thus, it is likely that a combination of biomaterials and cells stimulated with appropriate biochemical and mechanical cues will be needed. The objective of this review is to describe the current state-of-the-art, challenges and future directions in the field of enthesis tissue engineering focusing on four key parameters: (1) scaffold and biomaterials, (2) cells, (3) growth factors and (4) mechanical stimuli.

Exposure to 100% Oxygen Abolishes the Impairment of Fracture Healing after Thoracic Trauma

In polytrauma patients a thoracic trauma is one of the most critical injuries and an important trigger of post-traumatic inflammation. About 50% of patients with thoracic trauma are additionally affected by bone fractures. The risk for fracture malunion is considerably increased in such patients, the pathomechanisms being poorly understood. Thoracic trauma causes regional alveolar hypoxia and, subsequently, hypoxemia, which in turn triggers local and systemic inflammation. Therefore, we aimed to unravel the role of oxygen in impaired bone regeneration after thoracic trauma. We hypothesized that short-term breathing of 100% oxygen in the early post-traumatic phase ameliorates inflammation and improves bone regeneration. Mice underwent a femur osteotomy alone or combined with blunt chest trauma 100% oxygen was administered immediately after trauma for two separate 3 hour intervals. Arterial blood gas tensions, microcirculatory perfusion and oxygenation were assessed at 3, 9 and 24 hours after injury. Inflammatory cytokines and markers of oxidative/nitrosative stress were measured in plasma, lung and fracture hematoma. Bone healing was assessed on day 7, 14 and 21. Thoracic trauma induced pulmonary and systemic inflammation and impaired bone healing. Short-term exposure to 100% oxygen in the acute post-traumatic phase significantly attenuated systemic and local inflammatory responses and improved fracture healing without provoking toxic side effects, suggesting that hyperoxia could induce anti-inflammatory and pro-regenerative effects after severe injury. These results suggest that breathing of 100% oxygen in the acute post-traumatic phase might reduce the risk of poorly healing fractures in severely injured patients.

The role of hypoxia in stem cell potency and differentiation

Regenerative medicine relies on harnessing the capacity of stem cells to grow, divide and differentiate safely and predictably. This may be in the context of expanding stem cells in vitro or encouraging their expansion, mobilization and capacity to regenerate tissues either locally or remotely in vivo. In either case, understanding the stem cell niche is fundamental to recapitulating or manipulating conditions to enable therapy. It has become obvious that hypoxia plays a fundamental role in the maintenance of the stem cell niche. Low O2 benefits the self-renewal of human embryonic, hematopoietic, mesenchymal and neural stem cells, as well as improving the efficiency of genetic reprogramming to induced pluripotency. There is emerging evidence that harnessing or manipulating the hypoxic response can result in safer, more efficacious methodologies for regenerative medicine.

Stem Cells toward the Future: The Space Challenge

Astronauts experience weightlessness-induced bone loss due to an unbalanced process of bone remodeling that involves bone mesenchymal stem cells (bMSCs), as well as osteoblasts, osteocytes, and osteoclasts. The effects of microgravity on osteo-cells have been extensively studied, but it is only recently that consideration has been given to the role of bone MSCs. These live in adult bone marrow niches, are characterized by their self-renewal and multipotent differentiation capacities, and the published data indicate that they may lead to interesting returns in the biomedical/bioengineering fields. This review describes the published findings concerning bMSCs exposed to simulated/real microgravity, mainly concentrating on how mechanosignaling, mechanotransduction and oxygen influence their proliferation, senescence and differentiation. A comprehensive understanding of bMSC behavior in microgravity and their role in preventing bone loss will be essential for entering the future age of long-lasting, manned space exploration.

Vascularization strategies for bone regeneration

The functional regeneration of thick vascularized tissues such as bone and muscle is complicated by the large volume of lost tissue, challenging biomechanical environment, and the need to reproduce the highly organized structure of both the native tissue extracellular matrix and its vascular support system. Stem cell or progenitor cell delivery approaches, for example, continue to be plagued by low viability and engraftment in part due to the initial absence of a vascular supply. Recognition of diffusion limitations in thick tissues has prompted regenerative strategies that seek to accelerate establishment of a functional vasculature. The successful design of robust regeneration strategies for these challenging clinical scenarios will rely on a thorough understanding of interactions between construct design parameters and host biological and biomechanical factors. Here, we discuss the critical role of vascularization in normal bone tissue homeostasis and repair, vascular network adaptation to the local biomechanical environment, and the future directions of revascularization approaches being developed and integrated with bone regeneration strategies.

Human bone marrow-derived mesenchymal stem cells display enhanced clonogenicity but impaired differentiation with hypoxic preconditioning

Stem cells are promising candidate cells for regenerative applications because they possess high proliferative capacity and the potential to differentiate into other cell types. Mesenchymal stem cells (MSCs) are easily sourced but do not retain their proliferative and multilineage differentiative capabilities after prolonged ex vivo propagation. We investigated the use of hypoxia as a preconditioning agent and in differentiating cultures to enhance MSC function. Culture in 5% ambient O(2) consistently enhanced clonogenic potential of primary MSCs from all donors tested. We determined that enhanced clonogenicity was attributable to increased proliferation, increased vascular endothelial growth factor secretion, and increased matrix turnover. Hypoxia did not impact the incidence of cell death. Application of hypoxia to osteogenic cultures resulted in enhanced total mineral deposition, although this effect was detected only in MSCs preconditioned in normoxic conditions. Osteogenesis-associated genes were upregulated in hypoxia, and alkaline phosphatase activity was enhanced. Adipogenic differentiation was inhibited by exposure to hypoxia during differentiation. Chondrogenesis in three-dimensional pellet cultures was inhibited by preconditioning with hypoxia. However, in cultures expanded under normoxia, hypoxia applied during subsequent pellet culture enhanced chondrogenesis. Whereas hypoxic preconditioning appears to be an excellent way to expand a highly clonogenic progenitor pool, our findings suggest that it may blunt the differentiation potential of MSCs, compromising their utility for regenerative tissue engineering. Exposure to hypoxia during differentiation (post-normoxic expansion), however, appears to result in a greater quantity of functional osteoblasts and chondrocytes and ultimately a larger quantity of high-quality differentiated tissue.

The effect of synthetic oxygen carrier-enriched fibrin hydrogel on Schwann cells under hypoxia condition in vitro

Schwann cell (SC), which plays a key role in peripheral nerve regeneration, is one of the most classic supportive cells in neural tissue engineering. However, the biological activity of SCs seeded in nerve scaffolds decays subsequently due to local hypoxia induced by ischemia. Thus, we aimed to investigate whether a synthetic oxygen carrier-enriched fibrin gel would provide a sustained oxygen release to cultured SCs in vitro for overcoming a temporary (48 h) oxygen deprivation. In this study, perfluorotributylamine (PFTBA)-based oxygen carrying fibrin gel was prepared to provide oxygen for SCs under normoxic or hypoxic conditions. The dissolved oxygen within the culture media was measured by a blood-gas analyzer to quantify the time course of oxygen release from the PFTBA-enriched fibrin gel. SCs were cultured in the presence or absence of PFTBA-enriched fibrin gel under normoxic or hypoxic conditions. The tolerance of SCs to hypoxia was examined by a cell apoptosis assay. The growth of cells was characterized using S-100 staining and a CCK-8 assay. The migration of cells was examined using a Transwell chamber. The mRNA of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), glial cell derived neurotrophic factor (GDNF), neural cell adhesion molecule (N-CAM) and vascular endothelial growth factor (VEGF) in SCs were assayed by RT-PCR. In addition, SCs cultured in 3D PFTBA-enriched hydrogel were characterized by Live/Dead staining and the mRNA levels of BDNF, NGF, GDNF, N-CAM and VEGF were assayed by RT-PCR. The results showed that the PFTBA-enriched fibrin hydrogel was able to promote cell adhesion, migration, and proliferation under hypoxic conditions. Interestingly, PFTBA applied through the fibrin hydrogel dramatically enhanced the mRNA of BDNF, NGF, GDNF, N-CAM and VEGF under hypoxic condition. These findings highlight the possibility of enhancing nerve regeneration in cellular nerve grafts through PFTBA increased neurotropic secretion in SCs.